2018 Vol. 43, No. 4
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2018, 43(4): 933-951.
doi: 10.3799/dqkx.2018.701
Abstract:
It is important to clarify the relationship between the eastern and western boundary faults of the eastern Himalayan syntaxis (EHS), and tectonic evolution of the eastern Himalayan syntaxis (EHS) since the collision of the Indian and Eurasian plates. Combined with field mapping, structure observations, zircon U-Pb geochronological investigations and previous studies of the EHS, this study identifies and describes the architecture and kinematics of the two boundary faults, discusses their multi-stage of deformation, also the evolution of the study area since the collision of Indian and Eurasian plates. The results show that the two boundary shear zones are similar in architecture and kinematics, and their tectonic deformation is obviously contemporaneous and in same stage, having experienced a series of important geological events, including the collision, subduction-exhumation and collapse-uplift.
It is important to clarify the relationship between the eastern and western boundary faults of the eastern Himalayan syntaxis (EHS), and tectonic evolution of the eastern Himalayan syntaxis (EHS) since the collision of the Indian and Eurasian plates. Combined with field mapping, structure observations, zircon U-Pb geochronological investigations and previous studies of the EHS, this study identifies and describes the architecture and kinematics of the two boundary faults, discusses their multi-stage of deformation, also the evolution of the study area since the collision of Indian and Eurasian plates. The results show that the two boundary shear zones are similar in architecture and kinematics, and their tectonic deformation is obviously contemporaneous and in same stage, having experienced a series of important geological events, including the collision, subduction-exhumation and collapse-uplift.
2018, 43(4): 952-974.
doi: 10.3799/dqkx.2018.702
Abstract:
The tectonic setting and genesis of the ophiolites in the southern belt (SB) of the western Yarlung Zangbo ophiolitic belt are still controversial.They occur much larger peridotite massifs in contrast with those in the northern belt that are made discontinuously of lensoidal ophiolitic bodies in serpentinite matrix mélanges. The Dongbo ophiolite in the SB has been investigated and mapped, especially a 1 002.06 m dominant peridotite core has been drilled in the northwestern margin in 2015. Geochemical and geochronological (U-Pb zircon age) data from isotropic gabbros are presented in this paper. Dongbo ophiolite consists dominantly of harzburgite, minor dunite and mafic intrusions, associated with thin oceanic crust.Dismembered Late Juarassic to Early Cretaceous volcanic-sedimentary sequences of seamounts overlie the peridotites in the margins.Detachment and ductile shear faults, mylonite and mylonitic serpentinite and ophiolitic breccia are found in the mantle peridotites. In-situ LA-ICP-MS analysis of zircon grains from isotropic gabbros yields 129.0±1.8 Ma.The geochemical data of these gabbros are characterized by low Si, K, P, Fe, Ti and high Ca, Mg, showing N-MORB-like chondrite-normalized REE patterns and remarkable Th, Nb, Sr and Pb negative anomalies in N-MORB normalized spider diagrams. It is proposed that part of Dongbo peridotite and isotropic gabbro formed in a slow-spreading mid-ocean ridge which exhumed along detachment faults as an oceanic core complex.
The tectonic setting and genesis of the ophiolites in the southern belt (SB) of the western Yarlung Zangbo ophiolitic belt are still controversial.They occur much larger peridotite massifs in contrast with those in the northern belt that are made discontinuously of lensoidal ophiolitic bodies in serpentinite matrix mélanges. The Dongbo ophiolite in the SB has been investigated and mapped, especially a 1 002.06 m dominant peridotite core has been drilled in the northwestern margin in 2015. Geochemical and geochronological (U-Pb zircon age) data from isotropic gabbros are presented in this paper. Dongbo ophiolite consists dominantly of harzburgite, minor dunite and mafic intrusions, associated with thin oceanic crust.Dismembered Late Juarassic to Early Cretaceous volcanic-sedimentary sequences of seamounts overlie the peridotites in the margins.Detachment and ductile shear faults, mylonite and mylonitic serpentinite and ophiolitic breccia are found in the mantle peridotites. In-situ LA-ICP-MS analysis of zircon grains from isotropic gabbros yields 129.0±1.8 Ma.The geochemical data of these gabbros are characterized by low Si, K, P, Fe, Ti and high Ca, Mg, showing N-MORB-like chondrite-normalized REE patterns and remarkable Th, Nb, Sr and Pb negative anomalies in N-MORB normalized spider diagrams. It is proposed that part of Dongbo peridotite and isotropic gabbro formed in a slow-spreading mid-ocean ridge which exhumed along detachment faults as an oceanic core complex.
2018, 43(4): 975-990.
doi: 10.3799/dqkx.2018.703
Abstract:
The Daba ophiolite is located in the southern sub-belt of the western segment of the Yarlung Zangbo suture zone (YZSZ) and mainly composed of mantle peridotite, mafic dikes and siliceous rocks. Its formation time and tectonic setting are not clear. This is the first report of zircon U-Pb age, petrologic and whole-rock geochemical data from the Daba ophiolite. The major elements of Daba gabbro and dolerite are characterized by high contents in Al, Mg and low contents in Ti, K and P, which belong to subalkaline basalt. The REE distribution patterns are very similar to those of the normal mid-ocean ridge basalt (N-MORB). However, the normalized spider diagram shows significant depletion of Nb, suggesting the influence of fluids from downgoing slab.The dolerites yielded zircon U-Pb ages of 120.0±1.7 Ma, and geochemical features show partial melting from the low degree spinel-bearing mantle source. Combining with previous studies, it is suggested that the intrusion of the mafic rocks into the Daba peridotites may be interpreted as a result of mixing process between MORB-like melts and arc-derived fluids, which may have occurred during the stage of subduction initiation in an extending forearc region.
The Daba ophiolite is located in the southern sub-belt of the western segment of the Yarlung Zangbo suture zone (YZSZ) and mainly composed of mantle peridotite, mafic dikes and siliceous rocks. Its formation time and tectonic setting are not clear. This is the first report of zircon U-Pb age, petrologic and whole-rock geochemical data from the Daba ophiolite. The major elements of Daba gabbro and dolerite are characterized by high contents in Al, Mg and low contents in Ti, K and P, which belong to subalkaline basalt. The REE distribution patterns are very similar to those of the normal mid-ocean ridge basalt (N-MORB). However, the normalized spider diagram shows significant depletion of Nb, suggesting the influence of fluids from downgoing slab.The dolerites yielded zircon U-Pb ages of 120.0±1.7 Ma, and geochemical features show partial melting from the low degree spinel-bearing mantle source. Combining with previous studies, it is suggested that the intrusion of the mafic rocks into the Daba peridotites may be interpreted as a result of mixing process between MORB-like melts and arc-derived fluids, which may have occurred during the stage of subduction initiation in an extending forearc region.
2018, 43(4): 991-1010.
doi: 10.3799/dqkx.2018.704
Abstract:
As the characteristic ore deposits in ophiolites, podiform chromitite deposits mainly formed in the ultramafic section of an ophiolite complex. However, the following issues on podiform chromitite remain controversial:(1) crystal-melt separation; (2) residual of mantle partial melting; (3) melt-rock interaction. It is of particular importance to study the formation processes of podiform chromitite when large amounts of unusual minerals are found in podiform chromitite and its host rock peridotite. Petrogeneses of chromitite are reviewed in this paper. The pMELTS thermodynamic software was used to calculate the contribution of shallow mantle processes to the formation of podiform chromitite. Preliminary simulation results show that it is impossible to explain the formation of large-scale podiform chromitite deposits by individual mantle partial melting, mantle melt separation or tholeiitic melt-depleted mantle interaction processes, which implies that the podiform chromitite deposits may have been formed by multi-stage or deep mantle processes. Further researches are needed to be conducted on the origin of podiform chromitite deposits.
As the characteristic ore deposits in ophiolites, podiform chromitite deposits mainly formed in the ultramafic section of an ophiolite complex. However, the following issues on podiform chromitite remain controversial:(1) crystal-melt separation; (2) residual of mantle partial melting; (3) melt-rock interaction. It is of particular importance to study the formation processes of podiform chromitite when large amounts of unusual minerals are found in podiform chromitite and its host rock peridotite. Petrogeneses of chromitite are reviewed in this paper. The pMELTS thermodynamic software was used to calculate the contribution of shallow mantle processes to the formation of podiform chromitite. Preliminary simulation results show that it is impossible to explain the formation of large-scale podiform chromitite deposits by individual mantle partial melting, mantle melt separation or tholeiitic melt-depleted mantle interaction processes, which implies that the podiform chromitite deposits may have been formed by multi-stage or deep mantle processes. Further researches are needed to be conducted on the origin of podiform chromitite deposits.
2018, 43(4): 1011-1024.
doi: 10.3799/dqkx.2018.705
Abstract:
Genesis of chromittie deposit in ophiolites has been debated for a long time. It is commonly related to host ophiolite, chromite occurrence, simple mineral assemblage and simple mineral chemistry, and unusual and complicated mineral inclusions. To constrain the genesis of chromite, a study on Fe and Mg isotopes of mantle peridotite and chromitite from Purang and Luobusa ophiolites in Tibet and Kızıldaǧ and Kop ophiolites in Turkey is conducted. The results reveal that (1) mantle peridotite of ophiolites has uniform Fe and Mg isotope compositions, falling in the same range of worldwide peridotite; (2) chromite and olivine in chromitite show large Fe and Mg isotope fractionations with overall lower δ56Fe and higher δ26Mg in chromite than olivine; (3) Fe and Mg isotope fractionations between chromite and olivine are mainly controlled by fractional crystallization and subsolidus Fe and Mg exchanges in two opposite processes. Therefore, Fe and Mg isotopes have great potentials to trace origin and nature of parental magmas of chromitite and crystallization and precipitation of chromite.
Genesis of chromittie deposit in ophiolites has been debated for a long time. It is commonly related to host ophiolite, chromite occurrence, simple mineral assemblage and simple mineral chemistry, and unusual and complicated mineral inclusions. To constrain the genesis of chromite, a study on Fe and Mg isotopes of mantle peridotite and chromitite from Purang and Luobusa ophiolites in Tibet and Kızıldaǧ and Kop ophiolites in Turkey is conducted. The results reveal that (1) mantle peridotite of ophiolites has uniform Fe and Mg isotope compositions, falling in the same range of worldwide peridotite; (2) chromite and olivine in chromitite show large Fe and Mg isotope fractionations with overall lower δ56Fe and higher δ26Mg in chromite than olivine; (3) Fe and Mg isotope fractionations between chromite and olivine are mainly controlled by fractional crystallization and subsolidus Fe and Mg exchanges in two opposite processes. Therefore, Fe and Mg isotopes have great potentials to trace origin and nature of parental magmas of chromitite and crystallization and precipitation of chromite.
2018, 43(4): 1025-1037.
doi: 10.3799/dqkx.2018.706
Abstract:
The titanium-bearing alloy and titanium-bearing oxide inclusions in corundum collected from Luobusa podiform chromitite in Tibet are studied. The EDS and EPMA analyses show that the Ti-N alloys are rounded or forming a quincunx with the size of 17 μm×35 μm; the Ti-B alloys are long columnar in size of 10 μm×58 μm; the authigenic and exnotopic Ti-C alloys are 40 μm×50 μm; the Ti-Si-P and Ti-Si-Fe alloys occur as highly heterogeneous melt in the corundum and the Ti-Al-Zr oxides are pretty pure in composition. Considering the unusual ultra-high pressure mantle mineral groups like diamond and moissanite recovered from the Luobusa chromitite, it is suggested that the corundum and its special mineral inclusions might form at the high-pressure deep mantle environment.
The titanium-bearing alloy and titanium-bearing oxide inclusions in corundum collected from Luobusa podiform chromitite in Tibet are studied. The EDS and EPMA analyses show that the Ti-N alloys are rounded or forming a quincunx with the size of 17 μm×35 μm; the Ti-B alloys are long columnar in size of 10 μm×58 μm; the authigenic and exnotopic Ti-C alloys are 40 μm×50 μm; the Ti-Si-P and Ti-Si-Fe alloys occur as highly heterogeneous melt in the corundum and the Ti-Al-Zr oxides are pretty pure in composition. Considering the unusual ultra-high pressure mantle mineral groups like diamond and moissanite recovered from the Luobusa chromitite, it is suggested that the corundum and its special mineral inclusions might form at the high-pressure deep mantle environment.
2018, 43(4): 1038-1050.
doi: 10.3799/dqkx.2018.707
Abstract:
The origin and mechanisms involved in the formation of chromitite deposit in ophiolites remain a controversial subject of continuous debate. One of the important ways to address this issue is to investigate the nature and composition of parental magmas of chromitite, which may be revealed by mineral inclusions in chromite interpreted to have crystallized contemporaneously with or earlier than the host chromite. Various types of inclusions have been found in chromite ores with different textures in the Pozantı-Karsantı ophiolite in Turkey, which include (1) anhydrous silicate type such as olivine and clinopyroxene, (2) hydrous silicate type such as amphibole and phlogopite, (3) composite type such as the association of serpentine, wollastonite and clinopyroxene, (4) and uncommon mineral type such as apatite and platinum group element sulfide. The occurrence of hydrous mineral and the high Mg# of some minerals (e.g., olivine Mg#=95.4-97.1, clinopyroxene Mg#=92.0-99.0, amphibole Mg#=88.9-99.8) suggest that parental magmas of the chromite are rich in Mg and water contents. Besides the inclusions of apatite and uvarovite, it is reported, for the first time, that calcite and wollastonite inclusions in chromite, which, together with high-CaO features in silicate minerals, indicate Ca-enrichment. The elevated Ca contents in melts are favorable in stabilizing Cr3+ in silicate melt, while crystallization of Ca-bearing minerals could result in Cr enrichment in the melts. The Ca-rich component was probably derived from Ca-enriched rocks in subducting slab.
The origin and mechanisms involved in the formation of chromitite deposit in ophiolites remain a controversial subject of continuous debate. One of the important ways to address this issue is to investigate the nature and composition of parental magmas of chromitite, which may be revealed by mineral inclusions in chromite interpreted to have crystallized contemporaneously with or earlier than the host chromite. Various types of inclusions have been found in chromite ores with different textures in the Pozantı-Karsantı ophiolite in Turkey, which include (1) anhydrous silicate type such as olivine and clinopyroxene, (2) hydrous silicate type such as amphibole and phlogopite, (3) composite type such as the association of serpentine, wollastonite and clinopyroxene, (4) and uncommon mineral type such as apatite and platinum group element sulfide. The occurrence of hydrous mineral and the high Mg# of some minerals (e.g., olivine Mg#=95.4-97.1, clinopyroxene Mg#=92.0-99.0, amphibole Mg#=88.9-99.8) suggest that parental magmas of the chromite are rich in Mg and water contents. Besides the inclusions of apatite and uvarovite, it is reported, for the first time, that calcite and wollastonite inclusions in chromite, which, together with high-CaO features in silicate minerals, indicate Ca-enrichment. The elevated Ca contents in melts are favorable in stabilizing Cr3+ in silicate melt, while crystallization of Ca-bearing minerals could result in Cr enrichment in the melts. The Ca-rich component was probably derived from Ca-enriched rocks in subducting slab.
2018, 43(4): 1051-1069.
doi: 10.3799/dqkx.2018.709
Abstract:
LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd-Pb isotopic data analysis were conducted on the granite porphyry in Mariaicuo area in order to determine its formation time, petrogenesis, structural setting and geological significance. LA-ICP-MS zircon U-Pb dating of the granite porphyry in Mariaicuo yields a crystallization age of 78.3±0.4 Ma, indicating that the rock formed in the late of Late Cretaceous. Petrogeochemically, the samples of the rocks are high in SiO2 (72.41%-74.06%) and rich in Na2O+K2O of 6.66%-7.14%, which belong to calc-alkaline series. Its A/CNK ratios of 1.01-1.07 are of characteristics of weakly peraluminous granites. They are enriched in the large-ion lithospile elements of Rb, Ba, U, K and Th, and strongly depleted in high field strength elements Nb, Ta, P, and Ti. The REE distribution mode is obvious right-leaning, indicating that it remains garnet-amphibolite facies in source area. Their isotopic compositions are characterized by low initial (87Sr/86Sr)i of 0.705 2-0.706 0 and low positive εNd(t) (1.5-2.3) values and relatively young second-stage model age (tDM2=692-758 Ma) of crust. It has relatively homogeneous Pb isotope composition with the evolution of orogenic characteristics. Comprehensive analysis shows that the granite porphyry in Mariaicuo area may originate from the new partial melting of crust within extensional background, and its source region is enriched in fluid and remains garnet-amphibolite facies, excluding plagioclase. It formed in the post-collision extensional environment, which occurred after the closure of Bangonghu-Nujiang Tethys Ocean (BTO) and collision between tarranes on both sides, when the middle section of Bangonghu-Nujiang suture zone has entered the extension phase in Late Cretaceous.
LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd-Pb isotopic data analysis were conducted on the granite porphyry in Mariaicuo area in order to determine its formation time, petrogenesis, structural setting and geological significance. LA-ICP-MS zircon U-Pb dating of the granite porphyry in Mariaicuo yields a crystallization age of 78.3±0.4 Ma, indicating that the rock formed in the late of Late Cretaceous. Petrogeochemically, the samples of the rocks are high in SiO2 (72.41%-74.06%) and rich in Na2O+K2O of 6.66%-7.14%, which belong to calc-alkaline series. Its A/CNK ratios of 1.01-1.07 are of characteristics of weakly peraluminous granites. They are enriched in the large-ion lithospile elements of Rb, Ba, U, K and Th, and strongly depleted in high field strength elements Nb, Ta, P, and Ti. The REE distribution mode is obvious right-leaning, indicating that it remains garnet-amphibolite facies in source area. Their isotopic compositions are characterized by low initial (87Sr/86Sr)i of 0.705 2-0.706 0 and low positive εNd(t) (1.5-2.3) values and relatively young second-stage model age (tDM2=692-758 Ma) of crust. It has relatively homogeneous Pb isotope composition with the evolution of orogenic characteristics. Comprehensive analysis shows that the granite porphyry in Mariaicuo area may originate from the new partial melting of crust within extensional background, and its source region is enriched in fluid and remains garnet-amphibolite facies, excluding plagioclase. It formed in the post-collision extensional environment, which occurred after the closure of Bangonghu-Nujiang Tethys Ocean (BTO) and collision between tarranes on both sides, when the middle section of Bangonghu-Nujiang suture zone has entered the extension phase in Late Cretaceous.
2018, 43(4): 1070-1084.
doi: 10.3799/dqkx.2018.710
Abstract:
The Dongcuo ophiolite is located at the western part of Bangong-Nujiang River suture zone, Gaize County. In spite of previous studies on its spacious distribution, tectonic environment and genesis, the key problem remains unresolved. Taking the gabbro of Dongcuo ophiolite as the research object, a detailed study of petrography, geochemistry, petrology and geochronology is carried out to understand the formation of Bangong-Nujiang ocean basin, the tectonic settings of the Dongcuo ophiolite. Compared with N-MORB, the major and trace elements of Dongcuo ophiolite are characterized by high content in Mg# and low content in K, Na, P, and Ti, the depletion of high field-strength elements (HFSE) of Hf, Zr, Nb and light rare earth element depletion. Geochemistry characteristics of the Dongcuo ophiolite indicate that it is originated from~30% partial melting of depleted spinel lherzolite formed in the tectonic environment of subduction-related island arc or fore arc, belonging to super subduction zone (SSZ) ophiolite. The LA-ICP-MS of zircon U-Pb dating yields the age of 222±4.8 Ma (MSWD=0.5), providing the age constraint on the Dongcuo ophiolite (Late Triassic), and showing that it belongs to the Tethyan system of the Bangong-Nujiang River suture zone, which indicates the intra-oceanic subduction process.
The Dongcuo ophiolite is located at the western part of Bangong-Nujiang River suture zone, Gaize County. In spite of previous studies on its spacious distribution, tectonic environment and genesis, the key problem remains unresolved. Taking the gabbro of Dongcuo ophiolite as the research object, a detailed study of petrography, geochemistry, petrology and geochronology is carried out to understand the formation of Bangong-Nujiang ocean basin, the tectonic settings of the Dongcuo ophiolite. Compared with N-MORB, the major and trace elements of Dongcuo ophiolite are characterized by high content in Mg# and low content in K, Na, P, and Ti, the depletion of high field-strength elements (HFSE) of Hf, Zr, Nb and light rare earth element depletion. Geochemistry characteristics of the Dongcuo ophiolite indicate that it is originated from~30% partial melting of depleted spinel lherzolite formed in the tectonic environment of subduction-related island arc or fore arc, belonging to super subduction zone (SSZ) ophiolite. The LA-ICP-MS of zircon U-Pb dating yields the age of 222±4.8 Ma (MSWD=0.5), providing the age constraint on the Dongcuo ophiolite (Late Triassic), and showing that it belongs to the Tethyan system of the Bangong-Nujiang River suture zone, which indicates the intra-oceanic subduction process.
2018, 43(4): 1085-1109.
doi: 10.3799/dqkx.2018.711
Abstract:
The study on the petrogenesis and tectonic setting of the Early Cretaceous magmatic rocks in the northern Lhasa is important to define the geodynamic evolution for the Lhasa terrane. In this paper, it is reported of petrology, petrogeochemistry, zircon U-Pb ages and zircon Hf isotopic comopositions of Early Cretaceous magmatic rocks from Yongzhu area in the northern Lhasa terrane. Zircon U-Pb ages for biotite-monzonitic granite, rhyolite and andesite are 118±1.0 Ma, 121±0.8 Ma and 115±0.8 Ma respectively, representing their intrusion and eruption period. Biotite-monzonitic granite, granite porphyry and rhyrolite show similar geochemical characteristics. They are high K calc-alkaline and weakly peraluminous-strongly peraluminous granites (A/CNK=1.01-1.35). In primitive mantle-normalized spider diagrams, these rocks are characterized by enriched large ion lithophile elements Rb, K and radioactive elements U, Th, and negative anomalies in Nb, P, Ti, Ba and Sr. Chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE and negative Eu anomalies. Above chemical natures suggest that they are island-arc igneous rocks and formed in continental margin arc setting. The Hf isotopic compositions in the biotite-monzonitic granite and rhyolite are -1.21 to 3.01 and -0.68 to 5.35, respectively, and two stage model ages are 0.99-1.26 Ga and 0.84-1.22 Ga, respectively, which suggests mixed source of crust and mantle. In contrast, the andesite shows slightly different geochemical characteristics. They are characterized by (1) high K calc-alkaline; (2) negative anomalies in Nb, Ta, P, Ti, U and Sr, and enrichment of Rb, K and Th in primitive mantle-normalized spider diagrams; (3) chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE, and slight negative Eu anomalies; (4) formation in the continental margin arc setting. It is proposed that the Early Cretaceous magmatic rocks in Yongzhu were formed in the continental margin arc setting. During southern subduction of Bangonghu-Nujiang Tethyan oceanic basin, dehydration melting of the subduction oceanic plate produced the high thermal molten mass, which induced partial melting of the mantle wedge and formation of mafic magma. Then upwelling of mafic magma induced partial melting of the lower crust material and formation of acidic magma. During ascent process of the mafic magma and acidic magma, the two types of magma mixed in different proportion, and formed volcanic and plutonic rocks.
The study on the petrogenesis and tectonic setting of the Early Cretaceous magmatic rocks in the northern Lhasa is important to define the geodynamic evolution for the Lhasa terrane. In this paper, it is reported of petrology, petrogeochemistry, zircon U-Pb ages and zircon Hf isotopic comopositions of Early Cretaceous magmatic rocks from Yongzhu area in the northern Lhasa terrane. Zircon U-Pb ages for biotite-monzonitic granite, rhyolite and andesite are 118±1.0 Ma, 121±0.8 Ma and 115±0.8 Ma respectively, representing their intrusion and eruption period. Biotite-monzonitic granite, granite porphyry and rhyrolite show similar geochemical characteristics. They are high K calc-alkaline and weakly peraluminous-strongly peraluminous granites (A/CNK=1.01-1.35). In primitive mantle-normalized spider diagrams, these rocks are characterized by enriched large ion lithophile elements Rb, K and radioactive elements U, Th, and negative anomalies in Nb, P, Ti, Ba and Sr. Chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE and negative Eu anomalies. Above chemical natures suggest that they are island-arc igneous rocks and formed in continental margin arc setting. The Hf isotopic compositions in the biotite-monzonitic granite and rhyolite are -1.21 to 3.01 and -0.68 to 5.35, respectively, and two stage model ages are 0.99-1.26 Ga and 0.84-1.22 Ga, respectively, which suggests mixed source of crust and mantle. In contrast, the andesite shows slightly different geochemical characteristics. They are characterized by (1) high K calc-alkaline; (2) negative anomalies in Nb, Ta, P, Ti, U and Sr, and enrichment of Rb, K and Th in primitive mantle-normalized spider diagrams; (3) chondrite-normalized REE patterns show that these rocks are enriched in LREE, nearly flat HREE, and slight negative Eu anomalies; (4) formation in the continental margin arc setting. It is proposed that the Early Cretaceous magmatic rocks in Yongzhu were formed in the continental margin arc setting. During southern subduction of Bangonghu-Nujiang Tethyan oceanic basin, dehydration melting of the subduction oceanic plate produced the high thermal molten mass, which induced partial melting of the mantle wedge and formation of mafic magma. Then upwelling of mafic magma induced partial melting of the lower crust material and formation of acidic magma. During ascent process of the mafic magma and acidic magma, the two types of magma mixed in different proportion, and formed volcanic and plutonic rocks.
2018, 43(4): 1110-1124.
doi: 10.3799/dqkx.2018.712
Abstract:
In order to deepen our knowledge and understanding about the tectonic evolution of the Bangonghu-Nujiang suture zone(BNS), zircon U-Pb dating was conducted for granitic gneiss and granodioritic gneiss of Nyainrong microcontinent by LA-ICP-MS in this study. Results show two groups of average zircon U-Pb ages, with one of 453.7±2.5 Ma, and the other of 176.6±1.1 Ma and 178.04±0.8 Ma. 40Ar-39Ar dating was conducted for biotite from granitic gneiss and muscovite from granodioritic gneiss in Nyainrong microcontinent.The 40Ar-39Ar plateau ages yield 161.8±1.1 Ma and 178.9±1.2 Ma, respectively, showing that the study area experienced the Late Ordovician and the Early Jurassic magmatic events. Combined with field condition and study results, it is suggested that the Ordovician magmatic event was related to the Early Paleozoic orogeny which could be caused by proto-Tethyan oceanic subduction along Gondwana continental margin, and the Jurassic magmatic event was related to the subduction and closure of the Bangong-Nujiang oceanic crust.
In order to deepen our knowledge and understanding about the tectonic evolution of the Bangonghu-Nujiang suture zone(BNS), zircon U-Pb dating was conducted for granitic gneiss and granodioritic gneiss of Nyainrong microcontinent by LA-ICP-MS in this study. Results show two groups of average zircon U-Pb ages, with one of 453.7±2.5 Ma, and the other of 176.6±1.1 Ma and 178.04±0.8 Ma. 40Ar-39Ar dating was conducted for biotite from granitic gneiss and muscovite from granodioritic gneiss in Nyainrong microcontinent.The 40Ar-39Ar plateau ages yield 161.8±1.1 Ma and 178.9±1.2 Ma, respectively, showing that the study area experienced the Late Ordovician and the Early Jurassic magmatic events. Combined with field condition and study results, it is suggested that the Ordovician magmatic event was related to the Early Paleozoic orogeny which could be caused by proto-Tethyan oceanic subduction along Gondwana continental margin, and the Jurassic magmatic event was related to the subduction and closure of the Bangong-Nujiang oceanic crust.
2018, 43(4): 1125-1141.
doi: 10.3799/dqkx.2017.613
Abstract:
Duolong district has more than 20 million tons of copper resources, which is the top large-scale Cu-Au mineral district in China. However, the prospecting potential in its peripheral region is still huge. This paper reports the geochronology and geochemical test results of the granite porphyry and diorite porphyry in the Senadong area for the first time, providing data for regional metallogenic regularity. In this study, the accurate zircon isotopic dating, Hf isotopic and whole rock major and trace elements reveal that emplacement ages of granitic and dioritic porphyries 123±2 Ma, respectively; the formation of rocks is related to partial melting of subducted oceanic plate, and the magma contains significant mantle source component. Based on these results, it is proposed that the intrusions of the Senadong area adjacent to ore-bearing porphyry in Duolong district formed during the period of the Early Cretaceous magmatic activity, suggesting that they possibly have the same geodynamic background. All these intrusions are the products of Bangongco-Nujiang ocean arc-arc "soft" collision, which occurred in the end of subduction transform to the beginning of the collision. These magmas provide heat and fluid for mineralization, which suggests that the Senadong area could be a potential district to find the new porphyry deposits.
Duolong district has more than 20 million tons of copper resources, which is the top large-scale Cu-Au mineral district in China. However, the prospecting potential in its peripheral region is still huge. This paper reports the geochronology and geochemical test results of the granite porphyry and diorite porphyry in the Senadong area for the first time, providing data for regional metallogenic regularity. In this study, the accurate zircon isotopic dating, Hf isotopic and whole rock major and trace elements reveal that emplacement ages of granitic and dioritic porphyries 123±2 Ma, respectively; the formation of rocks is related to partial melting of subducted oceanic plate, and the magma contains significant mantle source component. Based on these results, it is proposed that the intrusions of the Senadong area adjacent to ore-bearing porphyry in Duolong district formed during the period of the Early Cretaceous magmatic activity, suggesting that they possibly have the same geodynamic background. All these intrusions are the products of Bangongco-Nujiang ocean arc-arc "soft" collision, which occurred in the end of subduction transform to the beginning of the collision. These magmas provide heat and fluid for mineralization, which suggests that the Senadong area could be a potential district to find the new porphyry deposits.
2018, 43(4): 1142-1163.
doi: 10.3799/dqkx.2018.713
Abstract:
The petrogenesis of Miocene porphyries is still under debate, involving four different genetic models, such as, partial melting of residual oceanic crust, partial melting of thickened lower crust, partial melting of sub-continental lithospheric mantle and partial melting of metasomatized mafic lower crust related to subducted fluids. In order to clarify the petrogenesis and tectonic setting, petrological analyses and zircon LA-ICP-MS U-Pb dating in this study were carried out. In addition, whole-rock major and trace elements were analyzed by means of XRF and ICP-MS methods. The results demonstrate that the porphyries formed at 16.7-14.4 Ma, showing adakitic geochemical features. Geochemical characteristics, trace elemental ratios and discrimination diagrams suggest that magma source of the granitoid porphyries was derived from partial melting of the lower juvenile crustal material, whereas magma source of gabbro diorite porphyrite was sourced from the enriched lithospheric mantle. Together with published data, it is proposed that the Qinghai-Tibetan plateau was in the tectonic transformation from compression to extension or strike-slip during 18-13 Ma in the early stage of Miocene. The ore-bearing porphyries ascended through vertical fractures or faults and mingled with mid-upper crustal material generating granite porphyry, monzonitic granite porphyry, granodiorite porphyry and quartz diorite porphyrite. On the contrary, the coeval magma derived from the lithosphere mantle formed gabbro diorite porphyrite. Ore-bearing hydrothermal fluid was driven by tectonic stress and heat flow from magma took place at the sandstone slate and hornfel of the Linbuzong Formation, and marble and limestone of the Duodigou Formation characterized by inter-bedded tectonic fracture belts and collapse locations of folds, forming skarn-type copper polymetallic ore deposits.
The petrogenesis of Miocene porphyries is still under debate, involving four different genetic models, such as, partial melting of residual oceanic crust, partial melting of thickened lower crust, partial melting of sub-continental lithospheric mantle and partial melting of metasomatized mafic lower crust related to subducted fluids. In order to clarify the petrogenesis and tectonic setting, petrological analyses and zircon LA-ICP-MS U-Pb dating in this study were carried out. In addition, whole-rock major and trace elements were analyzed by means of XRF and ICP-MS methods. The results demonstrate that the porphyries formed at 16.7-14.4 Ma, showing adakitic geochemical features. Geochemical characteristics, trace elemental ratios and discrimination diagrams suggest that magma source of the granitoid porphyries was derived from partial melting of the lower juvenile crustal material, whereas magma source of gabbro diorite porphyrite was sourced from the enriched lithospheric mantle. Together with published data, it is proposed that the Qinghai-Tibetan plateau was in the tectonic transformation from compression to extension or strike-slip during 18-13 Ma in the early stage of Miocene. The ore-bearing porphyries ascended through vertical fractures or faults and mingled with mid-upper crustal material generating granite porphyry, monzonitic granite porphyry, granodiorite porphyry and quartz diorite porphyrite. On the contrary, the coeval magma derived from the lithosphere mantle formed gabbro diorite porphyrite. Ore-bearing hydrothermal fluid was driven by tectonic stress and heat flow from magma took place at the sandstone slate and hornfel of the Linbuzong Formation, and marble and limestone of the Duodigou Formation characterized by inter-bedded tectonic fracture belts and collapse locations of folds, forming skarn-type copper polymetallic ore deposits.
2018, 43(4): 1164-1182.
doi: 10.3799/dqkx.2018.714
Abstract:
The junction of the East Kunlun orogen and the West Qinling orogen is a key area for contrasting the tectonic evolution of the East Kunlun orogen with West Qinling orogen and determining the south margin of "Qinling-Qilian-Kunlun" proto-Tethys ocean. Kuhai mafic rocks, situated in the junction, were structurally placed into a Late Paleozoic foliated turbidites, indicating typical characteristics of mélange. The LA-ICP-MS zircon U-Pb dating indicates that the gabbros in Xueqiong were crystallized at 610-630 Ma, and the mafic rocks in Yari were crystallized with an age of about 504 Ma. Geochemical and isotopic studies show that Xueqiong gabbros were derived from subcontinent lithosphere mantle in a rift setting. Yari mafic rocks were derived from enrichment mantle wedge modified by subduction metasomatic fluids in subduction, representing the existence of the ocean. According to regional data analysis, the situation that proto-tethys materials mixed in paleo-tethys mélange belt may demonstrates a subduction-accretion process occurred in the junction of the East Kunlun orogen and the West Qinling orogen. From Late Neoproterozoic to the Early Palaeozoic, A'nyemaqen ocean had similar tectonic evolution with Mianlüe ocean. The margin of proto-Tethys ocean reached A'nyemaqen and Mianlüe mélange.
The junction of the East Kunlun orogen and the West Qinling orogen is a key area for contrasting the tectonic evolution of the East Kunlun orogen with West Qinling orogen and determining the south margin of "Qinling-Qilian-Kunlun" proto-Tethys ocean. Kuhai mafic rocks, situated in the junction, were structurally placed into a Late Paleozoic foliated turbidites, indicating typical characteristics of mélange. The LA-ICP-MS zircon U-Pb dating indicates that the gabbros in Xueqiong were crystallized at 610-630 Ma, and the mafic rocks in Yari were crystallized with an age of about 504 Ma. Geochemical and isotopic studies show that Xueqiong gabbros were derived from subcontinent lithosphere mantle in a rift setting. Yari mafic rocks were derived from enrichment mantle wedge modified by subduction metasomatic fluids in subduction, representing the existence of the ocean. According to regional data analysis, the situation that proto-tethys materials mixed in paleo-tethys mélange belt may demonstrates a subduction-accretion process occurred in the junction of the East Kunlun orogen and the West Qinling orogen. From Late Neoproterozoic to the Early Palaeozoic, A'nyemaqen ocean had similar tectonic evolution with Mianlüe ocean. The margin of proto-Tethys ocean reached A'nyemaqen and Mianlüe mélange.
2018, 43(4): 1183-1206.
doi: 10.3799/dqkx.2018.715
Abstract:
The East Kunlun orogenic belt in the northeast of Qinghai-Tibet plateau contains large-scale crust-mantle mixed granitoid in the Late Permian and Late Triassic periods. It is an ideal region to investigate crust-mantle mixing and continental crust growth-recycling.In this paper, detailed petrography, LA-ICP-MS zircon U-Pb isotopic geochronology, mineral chemistry, rock geochemistry and Sr-Nd isotopes from the Naomuhungou diorite-porphyrites are reported. Zircon U-Pb ages indicate that the diorite-porphyrites were emplaced at~215.4±3.6 Ma.Plagioclase phenocrysts mainly comprise andesine and a small amount of labradorite, characterized by normal zoning and reverse zoning texture. Amphibole phenocrysts belong to the endenite. Naomuhungou diorite-porphyrites are characterized by the metaluminous to peraluminous, relatively rich in magnesium (Mg#~51.2) and middle K-high K calc-alkaline. The rock samples display marked enrichment in LREE and LILE (e. g. Rb, Th, U and K), and depleted in some HREE and HFSE (e.g., Nb, Ta, Ti), showing affinities similar to continental arc granites. In addition, the rocks have a homogeneous ISr value (0.708 6-0.708 8) and relatively low εNd(t) ((-5.3)-(-4.8)), and have a relatively older model age of TDM2 (1.38-1.43 Ga). The above mineralogical and geochemical characteristics suggest that diorite-porphyrite might originate from the enriched mantle-derived magmatic underplating of the ancient basement crust and homogeneous mixing with crustal melts, with obvious MASH features. In combination with regional geology studies, it is concluded that the Naomuhungou diorite-porphyrites may have formed during the transitional period of Late A'nemaqen Paleo-Tethys ocean subduction and collision with the continent, and may represent the post-magmatic-response to the suturing of the Bayan Har-Songpan-Ganzi-East Kunlun terrane.This study is also helpful for understanding the origin of the continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.
The East Kunlun orogenic belt in the northeast of Qinghai-Tibet plateau contains large-scale crust-mantle mixed granitoid in the Late Permian and Late Triassic periods. It is an ideal region to investigate crust-mantle mixing and continental crust growth-recycling.In this paper, detailed petrography, LA-ICP-MS zircon U-Pb isotopic geochronology, mineral chemistry, rock geochemistry and Sr-Nd isotopes from the Naomuhungou diorite-porphyrites are reported. Zircon U-Pb ages indicate that the diorite-porphyrites were emplaced at~215.4±3.6 Ma.Plagioclase phenocrysts mainly comprise andesine and a small amount of labradorite, characterized by normal zoning and reverse zoning texture. Amphibole phenocrysts belong to the endenite. Naomuhungou diorite-porphyrites are characterized by the metaluminous to peraluminous, relatively rich in magnesium (Mg#~51.2) and middle K-high K calc-alkaline. The rock samples display marked enrichment in LREE and LILE (e. g. Rb, Th, U and K), and depleted in some HREE and HFSE (e.g., Nb, Ta, Ti), showing affinities similar to continental arc granites. In addition, the rocks have a homogeneous ISr value (0.708 6-0.708 8) and relatively low εNd(t) ((-5.3)-(-4.8)), and have a relatively older model age of TDM2 (1.38-1.43 Ga). The above mineralogical and geochemical characteristics suggest that diorite-porphyrite might originate from the enriched mantle-derived magmatic underplating of the ancient basement crust and homogeneous mixing with crustal melts, with obvious MASH features. In combination with regional geology studies, it is concluded that the Naomuhungou diorite-porphyrites may have formed during the transitional period of Late A'nemaqen Paleo-Tethys ocean subduction and collision with the continent, and may represent the post-magmatic-response to the suturing of the Bayan Har-Songpan-Ganzi-East Kunlun terrane.This study is also helpful for understanding the origin of the continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.
2018, 43(4): 1207-1218.
doi: 10.3799/dqkx.2018.716
Abstract:
A large amount of granites widely distributes in Halasen area in the East Kunlun Mountains. The study of the granites not only help us to understand the tectono-magmatic evolution history of East Kunlun orogenic belt in the Late Paleozoic and Early Mesozoic, but also provides constraints for the subduction and closed time limit of the Paleo-Tethys Ocean. In this paper, we constrain geochronology and petrogenesis of the Halasen granites by zircon U-Pb dating and geochemical analyses. The LA-ICP-MS U-Pb analyses show that the formation ages of K-feldspar granite and fine-grained monzogranite are 239.2±1.7 Ma (MSWD=0.19) and 232.4±1.2 Ma (MSWD=0.76), respectively, which are the products of the Middle Triassic granitic magmatism. The geochemistry data show that granites are high-potassium calcalkaline-shoshonite, peraluminous rock which is characterized by high silicon and aluminum, enriched alkali, low titanium, as well as enriched light rare earth elements (LREE) and large ion lithophile elements (LILE, e.g., K, Th and Rb), and depleted in high field-strength elements (HFSE, e.g., Nb, Ti, P, and Ta) with obvious Eu negative anomaly (the value of δEu is between 0.27 and 0.65). Halasen granites belong to highly fractionated Ⅰ-type granite. It is suggested that the Halasen granites were most likely derived from parental magma by mixing of depleted mantle-derived magma and induced crustal-melted felsic magma in the deep crust, and then suffered further differentiation during magma ascent. The subduction of the East Kunlun Paleo-Tethys Ocean lasted to the Early Triassic, and the Middle Triassic witnessed the intracontinental collision.
A large amount of granites widely distributes in Halasen area in the East Kunlun Mountains. The study of the granites not only help us to understand the tectono-magmatic evolution history of East Kunlun orogenic belt in the Late Paleozoic and Early Mesozoic, but also provides constraints for the subduction and closed time limit of the Paleo-Tethys Ocean. In this paper, we constrain geochronology and petrogenesis of the Halasen granites by zircon U-Pb dating and geochemical analyses. The LA-ICP-MS U-Pb analyses show that the formation ages of K-feldspar granite and fine-grained monzogranite are 239.2±1.7 Ma (MSWD=0.19) and 232.4±1.2 Ma (MSWD=0.76), respectively, which are the products of the Middle Triassic granitic magmatism. The geochemistry data show that granites are high-potassium calcalkaline-shoshonite, peraluminous rock which is characterized by high silicon and aluminum, enriched alkali, low titanium, as well as enriched light rare earth elements (LREE) and large ion lithophile elements (LILE, e.g., K, Th and Rb), and depleted in high field-strength elements (HFSE, e.g., Nb, Ti, P, and Ta) with obvious Eu negative anomaly (the value of δEu is between 0.27 and 0.65). Halasen granites belong to highly fractionated Ⅰ-type granite. It is suggested that the Halasen granites were most likely derived from parental magma by mixing of depleted mantle-derived magma and induced crustal-melted felsic magma in the deep crust, and then suffered further differentiation during magma ascent. The subduction of the East Kunlun Paleo-Tethys Ocean lasted to the Early Triassic, and the Middle Triassic witnessed the intracontinental collision.
2018, 43(4): 1219-1236.
doi: 10.3799/dqkx.2018.717
Abstract:
Late Silurian A-type granite is reported for the first time in the Wulonggou gold district, eastern segment of the Kunlun orogenic belt. This paper presents (1) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age for the Wulonggou A-type granite to determine precisely the time of the magmatism, (2) geochemical, Nd and Hf isotope data for the Wulonggou A-type granite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon from the Wulonggou A-type granite yielded a weighted mean 206Pb/238U age of 420±3 Ma, indicating that it was emplaced in the Late Silurian. All rock samples are enriched in SiO2 (76.0%-78.4%), K2O (4.64%-5.22%) and Na2O (2.93%-3.25%), but with relatively low concentrations of FeOT (0.98%-1.45%), MgO (0.11%-0.22%) and CaO (0.27%-0.79%). Samples are also enriched in large ion lithophile elements (Rb, K, La) and light rare earth elements, but depleted in high field strength elements (Nb, P, Ti) and heavy rare earth elements with strong negative Eu anomalies (Eu/Eu*=0.09-0.12). The 104×Ga/Al ratios of the Wulonggou A-type granites vary from 3.09 to 3.15. Mineralogy and geochemistry of the rocks show an affinity with aluminous A-type granite. The εNd(t) values of whole-rock range from -2.5 to -2.2 with corresponding two-stage Nd model ages ranging from 1 339 to 1 365 Ma. The εHf(t) values of zircon vary from -2.8 to +2.1 and two-stage Hf model ages range from 1 269 to 1 583 Ma. The Nd and Hf isotopic data imply that different source materials have contributed to the magma genesis. Integrated geological, geochemical and isotopic data suggest that Wulonggou A-type granite is most likely generated via the mixture of asthenosphere-derived magma and the ancient crustal materials. The A-type granite was marked by A2 type and was formed in post-collisional extensional tectonic environment. In combination with the A-type granites such as those in the Helegangnaren and Binggou, it is concluded that the eastern Kunlun area had begun its post-orogenic extension stage since Late Silurian, and this stage continued at least about 34 Ma.
Late Silurian A-type granite is reported for the first time in the Wulonggou gold district, eastern segment of the Kunlun orogenic belt. This paper presents (1) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age for the Wulonggou A-type granite to determine precisely the time of the magmatism, (2) geochemical, Nd and Hf isotope data for the Wulonggou A-type granite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon from the Wulonggou A-type granite yielded a weighted mean 206Pb/238U age of 420±3 Ma, indicating that it was emplaced in the Late Silurian. All rock samples are enriched in SiO2 (76.0%-78.4%), K2O (4.64%-5.22%) and Na2O (2.93%-3.25%), but with relatively low concentrations of FeOT (0.98%-1.45%), MgO (0.11%-0.22%) and CaO (0.27%-0.79%). Samples are also enriched in large ion lithophile elements (Rb, K, La) and light rare earth elements, but depleted in high field strength elements (Nb, P, Ti) and heavy rare earth elements with strong negative Eu anomalies (Eu/Eu*=0.09-0.12). The 104×Ga/Al ratios of the Wulonggou A-type granites vary from 3.09 to 3.15. Mineralogy and geochemistry of the rocks show an affinity with aluminous A-type granite. The εNd(t) values of whole-rock range from -2.5 to -2.2 with corresponding two-stage Nd model ages ranging from 1 339 to 1 365 Ma. The εHf(t) values of zircon vary from -2.8 to +2.1 and two-stage Hf model ages range from 1 269 to 1 583 Ma. The Nd and Hf isotopic data imply that different source materials have contributed to the magma genesis. Integrated geological, geochemical and isotopic data suggest that Wulonggou A-type granite is most likely generated via the mixture of asthenosphere-derived magma and the ancient crustal materials. The A-type granite was marked by A2 type and was formed in post-collisional extensional tectonic environment. In combination with the A-type granites such as those in the Helegangnaren and Binggou, it is concluded that the eastern Kunlun area had begun its post-orogenic extension stage since Late Silurian, and this stage continued at least about 34 Ma.
2018, 43(4): 1237-1252.
doi: 10.3799/dqkx.2018.500
Abstract:
The blueschist is lenticularly wrapped in the lawsonite-bearing phengite schist in Lanling area, central Qiangtang. The lawsonite included in quartz, which, on its part, is enclosed in porphyroblastic garnet in the phengite schist, is discovered for the first time. Studying on the metamorphic P-T paths of the blueschist and lawsonite-bearing phengite schist is important for the further research of the formation and evolution of the Longmu Co-Shuanghu low-temperature and high-pressure metamorphic belts. The detailed petrography and phase equilibria modeling reveal that the blueschist experienced a more obvious thermal relaxation process (the peak pressure conditions are 490℃ and 2.4 GPa, and the peak temperature conditions are 540℃, 2.1 GPa) while the lawsonite-bearing phengite schist has barely experienced a thermal relaxation process (the peak temperature and pressure conditions are 530℃ and 2.2 GPa). The similarity of the retrograde P-T paths of the blueschist and lawsonite-bearing phengite schist reveals that the exhumation of the blueschist from Lanling area, central Qiangtang, may have resulted from the exhumation of the low-density lawsonite-bearing phengite schist, which carried the denser blueschist to the earth's crust levels.
The blueschist is lenticularly wrapped in the lawsonite-bearing phengite schist in Lanling area, central Qiangtang. The lawsonite included in quartz, which, on its part, is enclosed in porphyroblastic garnet in the phengite schist, is discovered for the first time. Studying on the metamorphic P-T paths of the blueschist and lawsonite-bearing phengite schist is important for the further research of the formation and evolution of the Longmu Co-Shuanghu low-temperature and high-pressure metamorphic belts. The detailed petrography and phase equilibria modeling reveal that the blueschist experienced a more obvious thermal relaxation process (the peak pressure conditions are 490℃ and 2.4 GPa, and the peak temperature conditions are 540℃, 2.1 GPa) while the lawsonite-bearing phengite schist has barely experienced a thermal relaxation process (the peak temperature and pressure conditions are 530℃ and 2.2 GPa). The similarity of the retrograde P-T paths of the blueschist and lawsonite-bearing phengite schist reveals that the exhumation of the blueschist from Lanling area, central Qiangtang, may have resulted from the exhumation of the low-density lawsonite-bearing phengite schist, which carried the denser blueschist to the earth's crust levels.
2018, 43(4): 1253-1265.
doi: 10.3799/dqkx.2018.718
Abstract:
The North Qilian orogenic belt underwent a complete Wilson Cycle through opening and closing of an ocean basin. Deepening the research of granite within can provide pivotal clues to reconstruct the subduction-accretion/collision orogenic framework. On the basis of systematic petrological and petrographical research, this paper conducts electron microprobe analysis on the main rock-forming minerals, aiming to determine the physicochemical conditions during rock formation and provide further constraints on the petrogenesis and tectonic setting. Results show that Qingshan monzogranite is mainly composed of K-feldspar (orthoclase), plagioclase (andesine-oligoclase), biotite(magnesian biotite), amphibole (magnesiohornblende) and quartz. Zircon saturation thermometer shows that the average temperature of initial magma is 750℃. Meanwhile, mineral chemistry analysis reveals that the average crystallization temperature for biotite is 647℃ and the oxygen fugacity during the rock formation is -15, corresponding to solidification depth of 6.73 km and solidification pressure of 1.85×108 Pa. The data in this paper indicates that Qingshan monzogranite is Ⅰ-type subsolvus granite, with the features of crust-mantle mixed source. Thus, it is proposed that Qingshan monzogranite belongs to calc-alkaline granite and produced under water-bearing condition.
The North Qilian orogenic belt underwent a complete Wilson Cycle through opening and closing of an ocean basin. Deepening the research of granite within can provide pivotal clues to reconstruct the subduction-accretion/collision orogenic framework. On the basis of systematic petrological and petrographical research, this paper conducts electron microprobe analysis on the main rock-forming minerals, aiming to determine the physicochemical conditions during rock formation and provide further constraints on the petrogenesis and tectonic setting. Results show that Qingshan monzogranite is mainly composed of K-feldspar (orthoclase), plagioclase (andesine-oligoclase), biotite(magnesian biotite), amphibole (magnesiohornblende) and quartz. Zircon saturation thermometer shows that the average temperature of initial magma is 750℃. Meanwhile, mineral chemistry analysis reveals that the average crystallization temperature for biotite is 647℃ and the oxygen fugacity during the rock formation is -15, corresponding to solidification depth of 6.73 km and solidification pressure of 1.85×108 Pa. The data in this paper indicates that Qingshan monzogranite is Ⅰ-type subsolvus granite, with the features of crust-mantle mixed source. Thus, it is proposed that Qingshan monzogranite belongs to calc-alkaline granite and produced under water-bearing condition.
2018, 43(4): 1266-1277.
doi: 10.3799/dqkx.2018.719
Abstract:
Yemaquan monzogranite pluton is the largest outcrop in North Altyn, with a medium-coarse grain and porphyritic texture. This paper aims to discuss the genesis and diagenesis environment of the monzogranite, and the tectonic evolution of North Altyn. Thus, it is necessary to study petrology, geochemistry, zircon U-Pb chronology and Hf isotope of the pluton. The zircon U-Pb age data suggest that the monzogranite was generated at 450-453 Ma. The rock shows significant features of Ⅰ-Type granite with high ratio of Na2O/K2O (1.72-2.29), aluminum saturation index (A/CNK=0.99-1.10) and negative correlation between P2O5% and SiO2%. The REE pattern shows depletion of HREE, relative enrichment of LREE, and slightly negative Eu anomaly. In the primitive mantle-normalized trace-element diagram, the rock shows strong enrichment of Rb, Ba, Th, U, K and depletion of Nb, Ta, P, Ti. The values of εHf(t) range in 5.52-10.75. The two-stage model ages (tDM2) vary from 0.75 to 1.09 Ga. These characteristics indicate that the source rock was formed from the juvenile crust (basites). Considering the regional tectonic setting, it is concluded that the monzogranite was generated by partial melting of the basites at the root of the orogenic belt in the syn-collision to post-collision environment.
Yemaquan monzogranite pluton is the largest outcrop in North Altyn, with a medium-coarse grain and porphyritic texture. This paper aims to discuss the genesis and diagenesis environment of the monzogranite, and the tectonic evolution of North Altyn. Thus, it is necessary to study petrology, geochemistry, zircon U-Pb chronology and Hf isotope of the pluton. The zircon U-Pb age data suggest that the monzogranite was generated at 450-453 Ma. The rock shows significant features of Ⅰ-Type granite with high ratio of Na2O/K2O (1.72-2.29), aluminum saturation index (A/CNK=0.99-1.10) and negative correlation between P2O5% and SiO2%. The REE pattern shows depletion of HREE, relative enrichment of LREE, and slightly negative Eu anomaly. In the primitive mantle-normalized trace-element diagram, the rock shows strong enrichment of Rb, Ba, Th, U, K and depletion of Nb, Ta, P, Ti. The values of εHf(t) range in 5.52-10.75. The two-stage model ages (tDM2) vary from 0.75 to 1.09 Ga. These characteristics indicate that the source rock was formed from the juvenile crust (basites). Considering the regional tectonic setting, it is concluded that the monzogranite was generated by partial melting of the basites at the root of the orogenic belt in the syn-collision to post-collision environment.
2018, 43(4): 1278-1292.
doi: 10.3799/dqkx.2018.720
Abstract:
The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt. Numerous Neoproterozoic granites outcrops in the South Altyn continental block, and are mainly located in Paxialayidang-Yaganbuyang-Kekesayi area. These granites provide indispensable dynamics information of the Rodinia supercontinent aggregation in Neoproterozoic. Therefore, the study of granites can help us to understand the formation and evolution history of the Altyn Tagh orogenic belt. In this paper, Kekesayi granitic pluton was studied by means of petrography, geochemistry, zircon U-Pb chronology and Hf isotopic analyses. The results are as follows. (1) Main minerals of Kekesayi monzonitic granite are:quartz+K-feldspar+plagioclase+biotite+muscovite. Zircon U-Pb dating shows that the granite was emplaced in 947-945 Ma. (2) Geochemistry characteristics show high SiO2 (71.54%-74.69%), K2O+Na2O (6.33%-7.40%) contents and low CaO (1.59%-2.00%), MgO (0.43%-0.61%) and TiO2 (0.25%-0.37%) contents, with K2O/Na2O ratios of 1.02-1.71 and A/CNK ratios of 1.10-1.14, showing a typical high-K calc-alkaline series with peraluminous features. Meanwhile, the granite is also enriched in Rb, K, Th and La, and depleted in Nb, Ta, Sr and Ba, with negative Eu anormalies and relative enrichment in LREE. (3) εHf(t) values range from -4.09 to +3.87 while two-stage model ages (tDM2) vary in 1.6-2.0 Ga. It is argued that the Kekesayi monzonitic granites were derived from partial melting of the meta-grey wackes of Late Paleoproterozoic to Early Mesoproterozoic ancient crustal materials. In combination with other Neoproterozoic granite features, the petrogenesis and isotopic geochronology indicate that the Kekesayi monzonitic granite was formed in collisional orogeny setting and may have been triggered by the assemblage of Rodinia supercontinent in Neoproterozoic.
The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt. Numerous Neoproterozoic granites outcrops in the South Altyn continental block, and are mainly located in Paxialayidang-Yaganbuyang-Kekesayi area. These granites provide indispensable dynamics information of the Rodinia supercontinent aggregation in Neoproterozoic. Therefore, the study of granites can help us to understand the formation and evolution history of the Altyn Tagh orogenic belt. In this paper, Kekesayi granitic pluton was studied by means of petrography, geochemistry, zircon U-Pb chronology and Hf isotopic analyses. The results are as follows. (1) Main minerals of Kekesayi monzonitic granite are:quartz+K-feldspar+plagioclase+biotite+muscovite. Zircon U-Pb dating shows that the granite was emplaced in 947-945 Ma. (2) Geochemistry characteristics show high SiO2 (71.54%-74.69%), K2O+Na2O (6.33%-7.40%) contents and low CaO (1.59%-2.00%), MgO (0.43%-0.61%) and TiO2 (0.25%-0.37%) contents, with K2O/Na2O ratios of 1.02-1.71 and A/CNK ratios of 1.10-1.14, showing a typical high-K calc-alkaline series with peraluminous features. Meanwhile, the granite is also enriched in Rb, K, Th and La, and depleted in Nb, Ta, Sr and Ba, with negative Eu anormalies and relative enrichment in LREE. (3) εHf(t) values range from -4.09 to +3.87 while two-stage model ages (tDM2) vary in 1.6-2.0 Ga. It is argued that the Kekesayi monzonitic granites were derived from partial melting of the meta-grey wackes of Late Paleoproterozoic to Early Mesoproterozoic ancient crustal materials. In combination with other Neoproterozoic granite features, the petrogenesis and isotopic geochronology indicate that the Kekesayi monzonitic granite was formed in collisional orogeny setting and may have been triggered by the assemblage of Rodinia supercontinent in Neoproterozoic.
2018, 43(4): 1293-1306.
doi: 10.3799/dqkx.2018.721
Abstract:
The Shuangfengshan mafic intrusion crops out in the southern margin of the Songnen-Zhangguangcai range. It is predominantly made of olivine gabbro, which yielded a crystallization age of 279±4 Ma (Early-Middle Permian). These rocks consist mainly of olivine, pyroxene, high-An (80.1-87.9) plagioclase and hornblende as reaction rim. The features of these minerals indicate a high-H2O or water-saturated parental magma which experienced fluid metasomatism in the mantle source. Major and trace elements studies suggest that the mafic magma was derived from partial melting (~20%) of depleted garnet-lherzolite mantle source. Variable degrees of plagioclase accumulation and orthopyroxene crystal fractionation occurred in the process of magma evolution. They are also characterized by relatively low Sr isotopic ratio (0.705 2-0.706 1), positive εNd(t) (0.9-1.3) and εHf(t) (0-10.2) values, confirming that the Shuangfengshan mafic magma was derived from depleted mantle source and apparent crustal contamination was not found during magma upwelling. Combining trace elemental and isotopic results suggest that the parental magmas were likely derived from mantle wedge metasomatized by sedimentary melt and fluid from the subducted Paleo-Asian slab. Moreover, these rocks display transitional properties of MORB and arc basalt, conforming that the studied olivine gabbros were probably formed in back-arc extensional environment. And the mantle heterogeneity beneath the studied area was probably reduced by the multi-stage Paleozoic subduction of the Paleo-Asian Ocean.
The Shuangfengshan mafic intrusion crops out in the southern margin of the Songnen-Zhangguangcai range. It is predominantly made of olivine gabbro, which yielded a crystallization age of 279±4 Ma (Early-Middle Permian). These rocks consist mainly of olivine, pyroxene, high-An (80.1-87.9) plagioclase and hornblende as reaction rim. The features of these minerals indicate a high-H2O or water-saturated parental magma which experienced fluid metasomatism in the mantle source. Major and trace elements studies suggest that the mafic magma was derived from partial melting (~20%) of depleted garnet-lherzolite mantle source. Variable degrees of plagioclase accumulation and orthopyroxene crystal fractionation occurred in the process of magma evolution. They are also characterized by relatively low Sr isotopic ratio (0.705 2-0.706 1), positive εNd(t) (0.9-1.3) and εHf(t) (0-10.2) values, confirming that the Shuangfengshan mafic magma was derived from depleted mantle source and apparent crustal contamination was not found during magma upwelling. Combining trace elemental and isotopic results suggest that the parental magmas were likely derived from mantle wedge metasomatized by sedimentary melt and fluid from the subducted Paleo-Asian slab. Moreover, these rocks display transitional properties of MORB and arc basalt, conforming that the studied olivine gabbros were probably formed in back-arc extensional environment. And the mantle heterogeneity beneath the studied area was probably reduced by the multi-stage Paleozoic subduction of the Paleo-Asian Ocean.
2018, 43(4): 1307-1320.
doi: 10.3799/dqkx.2018.722
Abstract:
The study of weathered ores is relatively limited in the Mianning-Dechang REE ore belt, western Sichuan. According to the geological survey in recent years, ore types and genesis of suspected weathered ore from Dalucao, Maoniuping and Muluozhai deposits were studied through EPMA, XRD and whole rock analysis. The results show that a large number of clay minerals (up to 40%) and ore minerals (up to 60%) are developed in the weathered ore of the Dalucao deposit, and the weathering degree is relatively high. On the contrary, suspected weathered ore types almost have not developed clay minerals from Maoniuping and Muluozhai deposits, and the weathering degree is relatively low. Through the geochemical characteristics and mineral composition analysis of the suspected weathered ore in Maoniuping and Muluozhai deposits, combining the compositions and occurrences of the wall rock and veins, it is obvious that the suspected weathed ore has been formed by mainly mechanically weathering from the nearby veins and wall rock under the strong tectonic action. Frequent breccia activities promote the strong mechanically weathering of weathered ores, and accelerate the water-rock reaction in Dalucao deposit. At present, there is no obvious biological weathering in the ore belt. The weathered ores in the belt have lower weathering degree than that of the Nanling ion adsorption type rare earth elements ores, and the age of the formation of the belt is younger (11 Ma) and a large amount of bastnaesite preservation, so it has not formed the ion adsorption type deposit.
The study of weathered ores is relatively limited in the Mianning-Dechang REE ore belt, western Sichuan. According to the geological survey in recent years, ore types and genesis of suspected weathered ore from Dalucao, Maoniuping and Muluozhai deposits were studied through EPMA, XRD and whole rock analysis. The results show that a large number of clay minerals (up to 40%) and ore minerals (up to 60%) are developed in the weathered ore of the Dalucao deposit, and the weathering degree is relatively high. On the contrary, suspected weathered ore types almost have not developed clay minerals from Maoniuping and Muluozhai deposits, and the weathering degree is relatively low. Through the geochemical characteristics and mineral composition analysis of the suspected weathered ore in Maoniuping and Muluozhai deposits, combining the compositions and occurrences of the wall rock and veins, it is obvious that the suspected weathed ore has been formed by mainly mechanically weathering from the nearby veins and wall rock under the strong tectonic action. Frequent breccia activities promote the strong mechanically weathering of weathered ores, and accelerate the water-rock reaction in Dalucao deposit. At present, there is no obvious biological weathering in the ore belt. The weathered ores in the belt have lower weathering degree than that of the Nanling ion adsorption type rare earth elements ores, and the age of the formation of the belt is younger (11 Ma) and a large amount of bastnaesite preservation, so it has not formed the ion adsorption type deposit.
2018, 43(4): 1321-1332.
doi: 10.3799/dqkx.2018.723
Abstract:
Hainan Island is located in the convergence zone of the Pacific and Tethys tectonic domains and is a key area for investigating the Pacific and Tethyan evolution of the Indochina and South China blocks. In this paper, a set of new zircon U-Pb and Lu-Hf isotopic data, along with whole-rock elemental and Sr-Nd isotopic analytical results for the Changzheng and Lingshui granitoid plutons in Southeast Hainan Island are presented. The Changzheng and Lingshui granodiorites yield Late Permian LA-ICP-MS zircon U-Pb ages of 251±2 Ma and 256±1 Ma, respectively. They show K2O/Na2O=0.2-0.9, FeOT/MgO ratios of 0.90, A/CNK=0.74-0.96 with 0.00-0.18% corundum. Their εNd(t) values range from -6.0 to -3.2 and in-situ zircon εHf(t) values from -4.4 to +0.2. Such signatures, in combination with the absence of muscovite, cordierite and garnet, suggest that these Late Permian granodiorites are I-type granites. The Changzheng and Lingshui granites were derived from the subalkaline metabasites in response to the closure of the Jinshajiang-Ailaoshan-Songma-Bangxi-Chenxing branch/back-arc basin during Paleotethyan evolution.
Hainan Island is located in the convergence zone of the Pacific and Tethys tectonic domains and is a key area for investigating the Pacific and Tethyan evolution of the Indochina and South China blocks. In this paper, a set of new zircon U-Pb and Lu-Hf isotopic data, along with whole-rock elemental and Sr-Nd isotopic analytical results for the Changzheng and Lingshui granitoid plutons in Southeast Hainan Island are presented. The Changzheng and Lingshui granodiorites yield Late Permian LA-ICP-MS zircon U-Pb ages of 251±2 Ma and 256±1 Ma, respectively. They show K2O/Na2O=0.2-0.9, FeOT/MgO ratios of 0.90, A/CNK=0.74-0.96 with 0.00-0.18% corundum. Their εNd(t) values range from -6.0 to -3.2 and in-situ zircon εHf(t) values from -4.4 to +0.2. Such signatures, in combination with the absence of muscovite, cordierite and garnet, suggest that these Late Permian granodiorites are I-type granites. The Changzheng and Lingshui granites were derived from the subalkaline metabasites in response to the closure of the Jinshajiang-Ailaoshan-Songma-Bangxi-Chenxing branch/back-arc basin during Paleotethyan evolution.
2018, 43(4): 1333-1349.
doi: 10.3799/dqkx.2018.724
Abstract:
The rocks of Emeishan large igneous province (ELIP) are mainly composed of mafic dykes, layered and ring mafic complex, high Ti and low Ti basalts. The major elements, trace elements, Sr-Nd isotopes and zircon SHRIMP U-Pb ages of mafic dykes from South Guizhou Province were studied in order to further determine the temporal link between ELIP and related mass extinction events and research the genesis of ELIP in this paper. Results show ∑REE=135.66×10-6-280.59×10-6, LREE/HREE=6.42-7.54, (La/Yb)N=7.94-9.85, relatively enriched LREE to HREE, δEu=1.0-1.3, enriched LILE, such as Ba, K, Sr, depleted HFSE, such as Nb, Ta, Zr, Hf, displaying similar geochemical characteristics of Emeishan high Ti basalts. The similar ratios of Th/Ta(1.80-1.94), Nb/U(30.8-39.88), Th/La(0.08-0.10), Nb/Th(7.89-8.40) to primitive mantle, low (87Sr/86Sr)i(0.705 278-0.706 052)), εNd(t)(-0.5~+1.6) and Th/Ta(< 2.13) imply no obvious crustal contamination, and the mafic rocks may have been caused by 10%-12% partial melting of the garnet peridotite in their source region that interacted with the upwelling Emeishan plume at the periphery of the plume. The 261.2±2.6 Ma age of zircon SHRIMP U-Pb may imply that the ELIP formed by a mantle plume at~260 Ma and was one of the likely causes of the end-Guadalupian mass extinction.
The rocks of Emeishan large igneous province (ELIP) are mainly composed of mafic dykes, layered and ring mafic complex, high Ti and low Ti basalts. The major elements, trace elements, Sr-Nd isotopes and zircon SHRIMP U-Pb ages of mafic dykes from South Guizhou Province were studied in order to further determine the temporal link between ELIP and related mass extinction events and research the genesis of ELIP in this paper. Results show ∑REE=135.66×10-6-280.59×10-6, LREE/HREE=6.42-7.54, (La/Yb)N=7.94-9.85, relatively enriched LREE to HREE, δEu=1.0-1.3, enriched LILE, such as Ba, K, Sr, depleted HFSE, such as Nb, Ta, Zr, Hf, displaying similar geochemical characteristics of Emeishan high Ti basalts. The similar ratios of Th/Ta(1.80-1.94), Nb/U(30.8-39.88), Th/La(0.08-0.10), Nb/Th(7.89-8.40) to primitive mantle, low (87Sr/86Sr)i(0.705 278-0.706 052)), εNd(t)(-0.5~+1.6) and Th/Ta(< 2.13) imply no obvious crustal contamination, and the mafic rocks may have been caused by 10%-12% partial melting of the garnet peridotite in their source region that interacted with the upwelling Emeishan plume at the periphery of the plume. The 261.2±2.6 Ma age of zircon SHRIMP U-Pb may imply that the ELIP formed by a mantle plume at~260 Ma and was one of the likely causes of the end-Guadalupian mass extinction.
2018, 43(4): 1350-1366.
doi: 10.3799/dqkx.2018.725
Abstract:
Andesite is a typical rock type in subduction-related environment, and can reveal important information on the subduction processes. In this paper, it is presented of zircon U-Pb geochronology, whole-rock major and trace element and Sr-Nd isotope data for the Wuwamen andesites in the southern margin of Central Tianshan block, to provide constraints on the evolution processes of the South Tianshan ocean and Central Tianshan block. The andesites are high-K, calc-alkaline trachyandesite composed of augite, calcic amphibole, plagioclase and K-Na feldspar, with SiO2=56.23%-59.28%, K2O=2.70%-3.37%, and Na2O=3.32%-4.11%. They are characterized by fractionated rare earth element patterns with (La/Yb)N=18.5-20.9, and show negligible Eu anomalies with δEu=0.82-0.88; they are highly enriched in Rb, Ba and Sr, and depleted in Nb, Ta and Ti. Their initial 87Sr/86Sr ratios are in the range of 0.706 2-0.707 5 and εNd(t) in the range of 2.96-3.01. The data suggest that the Wuwamen andesites are typical Andean-type continental arcs, which originated from the partial melting of the mantle wedge that had been previously metasomatized by subduction-related fluids; the parental magma has been modified by the old crustal rocks during the magmas ascent. The origin and tectonic affinity of the Wuwamen andesites indicate that the South Tianshan ocean has subducted northward beneath the southern margin of the Central Tianshan block at the end of Early Silurian, forming a mature oceanic-continental subduction system; the southern margin of the Central Tianshan block has been a typical active continental margin characterized by development of Andean-type continental arc rocks.
Andesite is a typical rock type in subduction-related environment, and can reveal important information on the subduction processes. In this paper, it is presented of zircon U-Pb geochronology, whole-rock major and trace element and Sr-Nd isotope data for the Wuwamen andesites in the southern margin of Central Tianshan block, to provide constraints on the evolution processes of the South Tianshan ocean and Central Tianshan block. The andesites are high-K, calc-alkaline trachyandesite composed of augite, calcic amphibole, plagioclase and K-Na feldspar, with SiO2=56.23%-59.28%, K2O=2.70%-3.37%, and Na2O=3.32%-4.11%. They are characterized by fractionated rare earth element patterns with (La/Yb)N=18.5-20.9, and show negligible Eu anomalies with δEu=0.82-0.88; they are highly enriched in Rb, Ba and Sr, and depleted in Nb, Ta and Ti. Their initial 87Sr/86Sr ratios are in the range of 0.706 2-0.707 5 and εNd(t) in the range of 2.96-3.01. The data suggest that the Wuwamen andesites are typical Andean-type continental arcs, which originated from the partial melting of the mantle wedge that had been previously metasomatized by subduction-related fluids; the parental magma has been modified by the old crustal rocks during the magmas ascent. The origin and tectonic affinity of the Wuwamen andesites indicate that the South Tianshan ocean has subducted northward beneath the southern margin of the Central Tianshan block at the end of Early Silurian, forming a mature oceanic-continental subduction system; the southern margin of the Central Tianshan block has been a typical active continental margin characterized by development of Andean-type continental arc rocks.
