2009 Vol. 34, No. 1
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2009, 34(1): 1-10.
Abstract:
Comprehensive geochemical investigations on the Late Mesozoic volcanic rocks from Tahe and Xiwu areas, Great Xing'an range reveal that the mantle sources for these rocks are of attributes of old continent blocks, while the mantle sources for the coeval volcanic rocks from other area show juvenile features.The detrital zircons from Tahe volcanic rocks have records of Late Proterozoic and Early Paleozoic.Trace elements and Sr, Nd, Pb and Hf isotopic features imply that the mantle sources for these two volcanic fields were inherited from enriched lithospheric mantle beneath old blocks with overprint of subducted juvenile island arc materials.This study provides solid deep geochemical constraints on the existence of Precambrian blocks among the Paleozoic Xingmeng orogen.During the Late Mesozoic, with the closure of the Mongol-Okhotsk and Mesozoic global event, regional extensional geological setting developed in these regions, which trigged decompressional melting of the lithospheric mantle and caused the voluminous eruption of volcanic rocks.
Comprehensive geochemical investigations on the Late Mesozoic volcanic rocks from Tahe and Xiwu areas, Great Xing'an range reveal that the mantle sources for these rocks are of attributes of old continent blocks, while the mantle sources for the coeval volcanic rocks from other area show juvenile features.The detrital zircons from Tahe volcanic rocks have records of Late Proterozoic and Early Paleozoic.Trace elements and Sr, Nd, Pb and Hf isotopic features imply that the mantle sources for these two volcanic fields were inherited from enriched lithospheric mantle beneath old blocks with overprint of subducted juvenile island arc materials.This study provides solid deep geochemical constraints on the existence of Precambrian blocks among the Paleozoic Xingmeng orogen.During the Late Mesozoic, with the closure of the Mongol-Okhotsk and Mesozoic global event, regional extensional geological setting developed in these regions, which trigged decompressional melting of the lithospheric mantle and caused the voluminous eruption of volcanic rocks.
2009, 34(1): 11-16.
Abstract:
301 Pb isotopic data of mantle-derived basalts for Cenozoic (contain few Late Mesozoic samples) were collected in this work.The variety trend (contour) maps of mantle Pb isotopic in China continent for Cenozoic were draw up systematically and the "variety trend of 206Pb/204Pb ratio of Cenozoic basalts derived from mantle in continent of China" is submitted in this paper only.The result shows that there is a difference between North China and South China on 206Pb/204Pb ratio.The geographic boundary between two regions is from Hefei-Zhengzhou-Yinchuan-Hantenggeli peak and the 206Pb/204Pb ratio boundary is believed to 18-18.5 approximately.The 206Pb/204Pb ratios in North China are < (18-18.5) and vice versa.Combining with Nd isotopic data, three mantle types can be identified, they are: mantle of orogenic belt; mantle of continental rifting; and mantle of non-typical region.The map of "variety trend of 206Pb/204Pb ratio of lithosphere in continental of China" is also showed in this paper.This is a comprehensive map, and compiled by the Pb isotopic data of samples which collected from different periods and different type of rocks.By comparison to these two maps, some differences and problems can be found such as: The values for 206Pb/204Pb ratio in northwestern of Northeast China; the intensity of Pb isotopic-anomaly in "Bohai basin"; whether the high-value area of Pb isotopic appears in 104°E and adjacent or not; a problem about DUPAL anomaly in South China.Based on the differences, the compositions of mantle at present-day did not inherit the total characteristics of lithosphere before and injects the new asthenospheric material.It is referred that a new mantle convection system appears in China continent for Late-Mesozoic-Cenozoic.However, the difference of 206Pb/204Pb ratio between North and South China still exist in both maps implying that the effect of subduction from Pacific slab for China continent is not very important.
301 Pb isotopic data of mantle-derived basalts for Cenozoic (contain few Late Mesozoic samples) were collected in this work.The variety trend (contour) maps of mantle Pb isotopic in China continent for Cenozoic were draw up systematically and the "variety trend of 206Pb/204Pb ratio of Cenozoic basalts derived from mantle in continent of China" is submitted in this paper only.The result shows that there is a difference between North China and South China on 206Pb/204Pb ratio.The geographic boundary between two regions is from Hefei-Zhengzhou-Yinchuan-Hantenggeli peak and the 206Pb/204Pb ratio boundary is believed to 18-18.5 approximately.The 206Pb/204Pb ratios in North China are < (18-18.5) and vice versa.Combining with Nd isotopic data, three mantle types can be identified, they are: mantle of orogenic belt; mantle of continental rifting; and mantle of non-typical region.The map of "variety trend of 206Pb/204Pb ratio of lithosphere in continental of China" is also showed in this paper.This is a comprehensive map, and compiled by the Pb isotopic data of samples which collected from different periods and different type of rocks.By comparison to these two maps, some differences and problems can be found such as: The values for 206Pb/204Pb ratio in northwestern of Northeast China; the intensity of Pb isotopic-anomaly in "Bohai basin"; whether the high-value area of Pb isotopic appears in 104°E and adjacent or not; a problem about DUPAL anomaly in South China.Based on the differences, the compositions of mantle at present-day did not inherit the total characteristics of lithosphere before and injects the new asthenospheric material.It is referred that a new mantle convection system appears in China continent for Late-Mesozoic-Cenozoic.However, the difference of 206Pb/204Pb ratio between North and South China still exist in both maps implying that the effect of subduction from Pacific slab for China continent is not very important.
2009, 34(1): 17-27.
Abstract:
This paper discusses the composition and structure of the lithosphere of the Tibetan Plateau, especially of the main collision zone in southern Tibet upon the basis of petrological and geochemical studies, combining with geological and geophysical researches.The Indo-Asia main collision zone possess the thickest crust of the Tibetan Plateau, which consists of two different types of the crust, juvenile crust and recycled crust.The thicken crust formed by two mechanism, both structural thickening and the inputs of the mantle materials into the crust via magmatism.The lithospheric mantle underneath the Tibetan Plateau is inhomogeneous in petrology and geochemistry.At least three mantle isotopic reservoirs may be distinguished from the heterogeneity of Tibetan magmatic sources: (1) a Neo-Tethyan, Indian Ocean (DUPAL-like) component, (2) an EM2-rich Indian subcontinental lithospheric mantle component, and (3) a primordial Tibetan lithospheric mantle component generated prior to the India-Asia collision, which can also be considered the pre- (India-Asia) collisional Asian lithospheric mantle component.Also, some mantle-and lower crust-derived xenoliths carried by volcanics, and the outcrops of high pressure-ultrahigh pressure mineral assemblages have been found on the Tibetan Plateau.Three structural types of the lithosphere of the Tibetan Plateau can be distinguished, i.e., thickened lithosphere (Pamirs-type), thinned lithosphere (Gangdese-type) and thickened-thinned-rethickened lithosphere (Qiangtang-type).The temporal relations among these three structural types of the lithosphere, however, is unclear so far.The post-collisional potassic-ultrapotassic volcanism along the Gangdese was presumably related to slab break-off of the subducting Neo-Tethyan plate and the subduction of Indian continental lithosphere beneath the Lhasa Block, with different mechanism in western, middle and eastern segments, respectively.In the northern part of the plateau (the Qiangtang, the Hoh Xil, etc.), however, volcanism could be related to a wavelike outward propagation of upwelling asthenosphere.In the northern margin of the plateau (western Kunlun, Yumen, etc.), volcanism might be as a result of decompressive melting induced by large-scale strike-slip faulting.Migration of collisional and postcollisional volcanism with time shows a highly distinctive pattern.Initially, as an initial response to the India-Asia collision, igneous activity migrated northward between ca.65 and 45Ma, away from the Tsangpo collision suture.Between ca.45 and 6Ma, volcanic activity migrated outward from the plateau interior, implying wavelike outward propagation of upwelling asthenosphere.A third stage, still in progress, is marked by the migration of activity to northwestern, northeast-eastern, and southeastern peripheral regions of the plateau between 6Ma and the present.Overall, such a highly distinctive pattern of activity can be interpreted to reflect lateral asthenospheric mantle flow or lower crust flow induced by the approach, and ensuing collision, of relatively thick (India and Eurasia) continental plates.
This paper discusses the composition and structure of the lithosphere of the Tibetan Plateau, especially of the main collision zone in southern Tibet upon the basis of petrological and geochemical studies, combining with geological and geophysical researches.The Indo-Asia main collision zone possess the thickest crust of the Tibetan Plateau, which consists of two different types of the crust, juvenile crust and recycled crust.The thicken crust formed by two mechanism, both structural thickening and the inputs of the mantle materials into the crust via magmatism.The lithospheric mantle underneath the Tibetan Plateau is inhomogeneous in petrology and geochemistry.At least three mantle isotopic reservoirs may be distinguished from the heterogeneity of Tibetan magmatic sources: (1) a Neo-Tethyan, Indian Ocean (DUPAL-like) component, (2) an EM2-rich Indian subcontinental lithospheric mantle component, and (3) a primordial Tibetan lithospheric mantle component generated prior to the India-Asia collision, which can also be considered the pre- (India-Asia) collisional Asian lithospheric mantle component.Also, some mantle-and lower crust-derived xenoliths carried by volcanics, and the outcrops of high pressure-ultrahigh pressure mineral assemblages have been found on the Tibetan Plateau.Three structural types of the lithosphere of the Tibetan Plateau can be distinguished, i.e., thickened lithosphere (Pamirs-type), thinned lithosphere (Gangdese-type) and thickened-thinned-rethickened lithosphere (Qiangtang-type).The temporal relations among these three structural types of the lithosphere, however, is unclear so far.The post-collisional potassic-ultrapotassic volcanism along the Gangdese was presumably related to slab break-off of the subducting Neo-Tethyan plate and the subduction of Indian continental lithosphere beneath the Lhasa Block, with different mechanism in western, middle and eastern segments, respectively.In the northern part of the plateau (the Qiangtang, the Hoh Xil, etc.), however, volcanism could be related to a wavelike outward propagation of upwelling asthenosphere.In the northern margin of the plateau (western Kunlun, Yumen, etc.), volcanism might be as a result of decompressive melting induced by large-scale strike-slip faulting.Migration of collisional and postcollisional volcanism with time shows a highly distinctive pattern.Initially, as an initial response to the India-Asia collision, igneous activity migrated northward between ca.65 and 45Ma, away from the Tsangpo collision suture.Between ca.45 and 6Ma, volcanic activity migrated outward from the plateau interior, implying wavelike outward propagation of upwelling asthenosphere.A third stage, still in progress, is marked by the migration of activity to northwestern, northeast-eastern, and southeastern peripheral regions of the plateau between 6Ma and the present.Overall, such a highly distinctive pattern of activity can be interpreted to reflect lateral asthenospheric mantle flow or lower crust flow induced by the approach, and ensuing collision, of relatively thick (India and Eurasia) continental plates.
2009, 34(1): 28-36.
Abstract:
The formation and destruction of the continental root is one of the most important going topics on the solid earth sciences.A series of deep-seated xenoliths, including mafic to felsic granulite, eclogite, metagabbro, pyroxenite and peridotite, from the Xinyang area with unique temporal and spatial meaning, at southern margin of the North China, were used to dating in ages and properties.The geometrical model of the zonal lithosphere structure in age and composition were suggested for the conjoint position of multi-blocks in Mesozoic (~160Ma).Basing on the zonal lithosphere, the authors also discussed the dynamic processes for its formation.The exposed rocks up to ca.2.85Ga old are underlain by felsic granulites and rare pyroxenites with zircon ages of 3.6-3.4Ga (to ca.30km depth).Deeper (ca.30-45km) parts of the lower crust consist of high-pressure mafic to felsic granulite and meta-gabbro, which give Paleoproterozoic (2.0-1.8Ga) zircon ages.Our data show the significance of underplating and vertical crustal growth in the Paleoproterozoic, which was related to the amalgamation of the eastern and western block of this craton and a global (Columbia) supercontinent assembly.Hf-isotope data indicate that both juvenile material and remelting of older (3.8-3.0Ga) crustal rocks were involved in this important event.Paleozoic (440-260Ma) and Early Mesozoic (228-219Ma) zircons are also found in xenoliths from the deeper part of the lower crust and the uppermost mantle (i.e., eclogite and peridotite).They are interpreted as reflecting geodynamic processes related to the continental collision between this craton and the Yangtze craton, respectively.The xenoliths from Xinyang diatremes thus record the growth and modification of the old (Paleo-Mesoarchean?) continental lithosphere by magma underplating during several tectonic events: assembly of the southern and northern parts of the Eastern Block in Neoarchean time, collision of the western and eastern blocks along the Trans-China orogen in the Paleoproterozoic, and subduction and collision of the Yangtze craton with the North China craton in early Paleozoic and Triassic times, respectively.
The formation and destruction of the continental root is one of the most important going topics on the solid earth sciences.A series of deep-seated xenoliths, including mafic to felsic granulite, eclogite, metagabbro, pyroxenite and peridotite, from the Xinyang area with unique temporal and spatial meaning, at southern margin of the North China, were used to dating in ages and properties.The geometrical model of the zonal lithosphere structure in age and composition were suggested for the conjoint position of multi-blocks in Mesozoic (~160Ma).Basing on the zonal lithosphere, the authors also discussed the dynamic processes for its formation.The exposed rocks up to ca.2.85Ga old are underlain by felsic granulites and rare pyroxenites with zircon ages of 3.6-3.4Ga (to ca.30km depth).Deeper (ca.30-45km) parts of the lower crust consist of high-pressure mafic to felsic granulite and meta-gabbro, which give Paleoproterozoic (2.0-1.8Ga) zircon ages.Our data show the significance of underplating and vertical crustal growth in the Paleoproterozoic, which was related to the amalgamation of the eastern and western block of this craton and a global (Columbia) supercontinent assembly.Hf-isotope data indicate that both juvenile material and remelting of older (3.8-3.0Ga) crustal rocks were involved in this important event.Paleozoic (440-260Ma) and Early Mesozoic (228-219Ma) zircons are also found in xenoliths from the deeper part of the lower crust and the uppermost mantle (i.e., eclogite and peridotite).They are interpreted as reflecting geodynamic processes related to the continental collision between this craton and the Yangtze craton, respectively.The xenoliths from Xinyang diatremes thus record the growth and modification of the old (Paleo-Mesoarchean?) continental lithosphere by magma underplating during several tectonic events: assembly of the southern and northern parts of the Eastern Block in Neoarchean time, collision of the western and eastern blocks along the Trans-China orogen in the Paleoproterozoic, and subduction and collision of the Yangtze craton with the North China craton in early Paleozoic and Triassic times, respectively.
2009, 34(1): 37-42.
Abstract:
China' s two major natural diamond-producing mines-Shandong Mengyin and Liaoning Fuxian produce a large quantity of high-quality diamonds. EA-IRMS (Isotope Ratio Mass Spectrometry connected with Elemental Analyzer) analyses reveal that these single-crystal diamonds ubiquitously bear carbon isotope zonation. And in both localities, the carbon isotope zonation is more apparent in solid inclusion-bearing diamonds than in non-inclusion diamonds. Carbon isotope nature in Chinese diamonds also demonstrates that these diamonds-forming carbons were derived from mantle, with no equivocal evidence for the involvement of crust carbons derived from the recycling of the surface sediments.
China' s two major natural diamond-producing mines-Shandong Mengyin and Liaoning Fuxian produce a large quantity of high-quality diamonds. EA-IRMS (Isotope Ratio Mass Spectrometry connected with Elemental Analyzer) analyses reveal that these single-crystal diamonds ubiquitously bear carbon isotope zonation. And in both localities, the carbon isotope zonation is more apparent in solid inclusion-bearing diamonds than in non-inclusion diamonds. Carbon isotope nature in Chinese diamonds also demonstrates that these diamonds-forming carbons were derived from mantle, with no equivocal evidence for the involvement of crust carbons derived from the recycling of the surface sediments.
2009, 34(1): 43-55.
Abstract:
As a burgeoning tracer of stable isotopes, lithium geochemistry has attracted increasing attention from the international geological community recently.Its application field covers the interactions between fluids and minerals from the surface to the mantle.During weathering, the lighter isotope 6Li is preferentially retained in the solid phase while 7Li goes into solution.Thus river waters have heavy Li isotopic compositions compared to the original bedrock.The heavy Li isotopic compositions of river water feed the oceans.Low-temperature alteration of oceanic crust then makes seawater heavier than river water.In a subduction zone, heavy Li of fluids escaping the slab at low temperature likely enrich the forearc mantle wedge and hydrate the adjacent mantle and make them isotopically heavier than before.Meanwhile, an isotopically light component of slab dehydrated is subducted deeply and may form a locally light-Li mantle reservoir.There are three main factors that can result Li isotopic fractionation in mantle peridotites.They are temperature, diffusion-driven fractionation and mixing with exotic melts.At high temperatures of magmatic processes, Li isotopes do not show per mil-level mass fractionation, and simple diffusive process can not account for the Li isotopic fractionation between the minerals in peridotitic xenoliths from the Hannuoba area in the North China Craton, thus the reaction between peridotites and melts with light Li isotopic compositions maybe a reasonable explanation.It should point out that diffusion of Li isotopes during peridotite-melt reaction makes some contributions to the isotopic fractionation between the mantle minerals.
As a burgeoning tracer of stable isotopes, lithium geochemistry has attracted increasing attention from the international geological community recently.Its application field covers the interactions between fluids and minerals from the surface to the mantle.During weathering, the lighter isotope 6Li is preferentially retained in the solid phase while 7Li goes into solution.Thus river waters have heavy Li isotopic compositions compared to the original bedrock.The heavy Li isotopic compositions of river water feed the oceans.Low-temperature alteration of oceanic crust then makes seawater heavier than river water.In a subduction zone, heavy Li of fluids escaping the slab at low temperature likely enrich the forearc mantle wedge and hydrate the adjacent mantle and make them isotopically heavier than before.Meanwhile, an isotopically light component of slab dehydrated is subducted deeply and may form a locally light-Li mantle reservoir.There are three main factors that can result Li isotopic fractionation in mantle peridotites.They are temperature, diffusion-driven fractionation and mixing with exotic melts.At high temperatures of magmatic processes, Li isotopes do not show per mil-level mass fractionation, and simple diffusive process can not account for the Li isotopic fractionation between the minerals in peridotitic xenoliths from the Hannuoba area in the North China Craton, thus the reaction between peridotites and melts with light Li isotopic compositions maybe a reasonable explanation.It should point out that diffusion of Li isotopes during peridotite-melt reaction makes some contributions to the isotopic fractionation between the mantle minerals.
2009, 34(1): 56-74.
Abstract:
The small porphyry plutons or dikes in the Baogutu area, western Junggar, have attracted wide attentions owing to the close association between them and Cu-Au mineralization.This paper presents new LA-ICP-MS zircon U-Pb age and geochemical data of ore-bearing porphyries in the Baogutu area.The quartz diorite porphyry bodies Ⅱ and Ⅴ and diorite porphyry body Ⅲ have crystallization ages of 314.9±1.7Ma, 309.9±1.9Ma, and 313.9±2.6Ma, respectively, suggesting they were generated in Late Carboniferous.They are characterized by high Na2O/K2O and high Sr values but low Y and Yb contents, and negligible Eu anomalies, similar to adakites.In addition, some samples have high MgO (3.93%-4.78%) and Mg# (47-74) values, similar to high-Mg andesite.Taking into account the data of regional geology and magmatic rocks, we suggest that (1) The Baogutu intrusive rocks were possibly formed in an island-arc setting linking to ocean ridge suduction in Late Carboniferous, the adakitic magmas have likely formed by partial melting of the leading edge of the subducted ridge, and high-Mg diorites possibly originated from the interaction between adakitic melts and mantle peridotite; (2) The Baogutu Cu-Au mineralization might occur above a slab window during ocean ridge suduction, and the interaction between high oxygen fugacity slab melt and mantle peridotite caused the decomposition of metal sulfides and the Cu and Au mineralization.
The small porphyry plutons or dikes in the Baogutu area, western Junggar, have attracted wide attentions owing to the close association between them and Cu-Au mineralization.This paper presents new LA-ICP-MS zircon U-Pb age and geochemical data of ore-bearing porphyries in the Baogutu area.The quartz diorite porphyry bodies Ⅱ and Ⅴ and diorite porphyry body Ⅲ have crystallization ages of 314.9±1.7Ma, 309.9±1.9Ma, and 313.9±2.6Ma, respectively, suggesting they were generated in Late Carboniferous.They are characterized by high Na2O/K2O and high Sr values but low Y and Yb contents, and negligible Eu anomalies, similar to adakites.In addition, some samples have high MgO (3.93%-4.78%) and Mg# (47-74) values, similar to high-Mg andesite.Taking into account the data of regional geology and magmatic rocks, we suggest that (1) The Baogutu intrusive rocks were possibly formed in an island-arc setting linking to ocean ridge suduction in Late Carboniferous, the adakitic magmas have likely formed by partial melting of the leading edge of the subducted ridge, and high-Mg diorites possibly originated from the interaction between adakitic melts and mantle peridotite; (2) The Baogutu Cu-Au mineralization might occur above a slab window during ocean ridge suduction, and the interaction between high oxygen fugacity slab melt and mantle peridotite caused the decomposition of metal sulfides and the Cu and Au mineralization.
2009, 34(1): 75-88.
Abstract:
Nominally anhydrous minerals, clinopyroxene (Cpx), orthopyroxene (Opx) and olivine (Ol), in peridotite xenoliths hosted by Cenozoic basalts from Panshishan, Lianshan and Fangshan of the Subei basin, eastern China have been investigated by Micro-FTIR for their water contents.All Cpx and Opx grains contain a certain amount of "water" as hydroxyl defect in the crystal structure.Water contents (expressed as H2O wt.) of Cpx and Opx from Panshishan, Lianshan and Fangshan peridotites are 64×10-6-183×10-6, 37×10-6-102×10-6, 41×10-6-177×10-6 and 16×10-6-61×10-6, 13×10-6-45×10-6, 21×10-6-74×10-6, respectively.OH peaks can not be detected for almost all olivine grains, indicating that the water content is below the detection limit (~2×10-6) of FTIR.The whole rock water contents calculated according to the water contents of minerals and their volume proportions are 12×10-6-52×10-6, 7×10-6-25×10-6 and 13×10-6-44×10-6 for Panshishan, Lianshan and Fangshan respectively.Based on the new results from the Subei basin and the data reported in literatures, the partition coefficient of H2O between Cpx and Opx in the continental lithosphere mantle is estimated to be 2.2±0.4.Peridotite xenoliths from Nüshan, Hannuoba and the Subei basin of the North China block have much lower water contents than those from other localities of the world (South Africa craton, New Mexico, Colorado plateau and basin and Range province of USA, South Mexico, French Central Massif and West Kettle of Canada).This difference is probably induced by the thermal erosion of asthenosphere during Mesozoic-Cenozoic period in the North China block.If this explanation is correct, the present lithosphere mantle in the North China block is mainly the relict after the Mesozoic lithospheric thinning, rather than the new accreted mantle.
Nominally anhydrous minerals, clinopyroxene (Cpx), orthopyroxene (Opx) and olivine (Ol), in peridotite xenoliths hosted by Cenozoic basalts from Panshishan, Lianshan and Fangshan of the Subei basin, eastern China have been investigated by Micro-FTIR for their water contents.All Cpx and Opx grains contain a certain amount of "water" as hydroxyl defect in the crystal structure.Water contents (expressed as H2O wt.) of Cpx and Opx from Panshishan, Lianshan and Fangshan peridotites are 64×10-6-183×10-6, 37×10-6-102×10-6, 41×10-6-177×10-6 and 16×10-6-61×10-6, 13×10-6-45×10-6, 21×10-6-74×10-6, respectively.OH peaks can not be detected for almost all olivine grains, indicating that the water content is below the detection limit (~2×10-6) of FTIR.The whole rock water contents calculated according to the water contents of minerals and their volume proportions are 12×10-6-52×10-6, 7×10-6-25×10-6 and 13×10-6-44×10-6 for Panshishan, Lianshan and Fangshan respectively.Based on the new results from the Subei basin and the data reported in literatures, the partition coefficient of H2O between Cpx and Opx in the continental lithosphere mantle is estimated to be 2.2±0.4.Peridotite xenoliths from Nüshan, Hannuoba and the Subei basin of the North China block have much lower water contents than those from other localities of the world (South Africa craton, New Mexico, Colorado plateau and basin and Range province of USA, South Mexico, French Central Massif and West Kettle of Canada).This difference is probably induced by the thermal erosion of asthenosphere during Mesozoic-Cenozoic period in the North China block.If this explanation is correct, the present lithosphere mantle in the North China block is mainly the relict after the Mesozoic lithospheric thinning, rather than the new accreted mantle.
2009, 34(1): 89-104.
Abstract:
It can better understand the geological evolution of the Tethys and construction of the whole Tibet plateau through studying Mesozoic volcanic rocks widely exposed in Lhasa block.In present study, we report new major trace of elemental compositions and zircon U-Pb dating results of the Duoni Formation volcanic rocks in north Lhasa block.Duoni Formation volcanic rocks in Daguo and Mayuexiang areas mainly contain acid rocks and a few basalts, andesitic-basalts (intermediate-basic group).The intermediate-basic rocks with high Cr (184×10-6 in average) and Ni (43×10-6 in average), showing geochemical characteristics similar to the volcanic rocks that in arcs or active continental margins, such as LILE's enrichment and HFSE's depletion, which thought to be the melting products of continental marginal mantle wedge below a thickened crust due to southward subduction of the Bangongco-Nujiang ocean.The acid rocks have relatively high SiO2, K2O, K2O/Na2O and very low Mg# (the maxim is 32.4), enrich in Rb, Ba, Th, U and LREE, high Rb/Sr (1.06-4.47, the average is 2.73), and have spidergram patterns similar to the acid rocks of the Linzizong Group derived from a crust source.Thus, the acid rocks in Duoni Formation were came from a magmatic source different from those of the intermediate-basic rocks.Zircons from two acid rocks in Duoni Formation were dated by LA-ICPMS, obtaining age of 116±1Ma and 115±1Ma, respectively.We suggest that there are probably two possibilities for the formation tectonic setting of Duoni Formation: the southward subduction of the Bangongco-Nujiang ocean, or the tectonic extension in Early-Cretaceous in the Lhasa terrane.
It can better understand the geological evolution of the Tethys and construction of the whole Tibet plateau through studying Mesozoic volcanic rocks widely exposed in Lhasa block.In present study, we report new major trace of elemental compositions and zircon U-Pb dating results of the Duoni Formation volcanic rocks in north Lhasa block.Duoni Formation volcanic rocks in Daguo and Mayuexiang areas mainly contain acid rocks and a few basalts, andesitic-basalts (intermediate-basic group).The intermediate-basic rocks with high Cr (184×10-6 in average) and Ni (43×10-6 in average), showing geochemical characteristics similar to the volcanic rocks that in arcs or active continental margins, such as LILE's enrichment and HFSE's depletion, which thought to be the melting products of continental marginal mantle wedge below a thickened crust due to southward subduction of the Bangongco-Nujiang ocean.The acid rocks have relatively high SiO2, K2O, K2O/Na2O and very low Mg# (the maxim is 32.4), enrich in Rb, Ba, Th, U and LREE, high Rb/Sr (1.06-4.47, the average is 2.73), and have spidergram patterns similar to the acid rocks of the Linzizong Group derived from a crust source.Thus, the acid rocks in Duoni Formation were came from a magmatic source different from those of the intermediate-basic rocks.Zircons from two acid rocks in Duoni Formation were dated by LA-ICPMS, obtaining age of 116±1Ma and 115±1Ma, respectively.We suggest that there are probably two possibilities for the formation tectonic setting of Duoni Formation: the southward subduction of the Bangongco-Nujiang ocean, or the tectonic extension in Early-Cretaceous in the Lhasa terrane.
2009, 34(1): 105-116.
Abstract:
According to previous research achievements of Mesozoic volcanics and mineralizations in the middle and lower reaches of Yangtze River, the less studied volcanics in Lishui basin have been focused on their geochronology, and detailed U-Pb chronological study of zircons in trachyte has been carried out.Combining with geochemical data and lithostratigraphy for the four separated Mesozoic volcanic basins (i.e., Luzong basin, Fanchang basin, Ningwu basin and Lishui basin), volcanics from each basin with four volcanic cycles has been studied comparatively.It shows that, trachytes from the Longwangshan Formation of Lishui basin were formed in the Early Cretaceous with the crystallization age of about 128Ma, which implies the incipience of extensive eruption of volcanics along the middle and lower reaches of Yangtze River.The ages of four volcanic cycles are: before 125Ma, during 125-120Ma, ranging from 120 to 110Ma and from 110 to 90Ma, respectively.It is more reasonable that the Fushan Formation of Luzong basin should be considered as the product of third volcanic cycle, and corresponding to the Gushan and Niangniangshan Formation of Ningwu basin (i.e., Guanshan and Jiashan formations of Lishui basin).The geochemical characteristics of Cretaceous volcanics in this area suggest that the mantle derived alkali magma had been contaminated by crustal materials during its eruption, and early volcanics (e.g., the Longwangshan Formation of Lishui basin) could be the source rock of Sr deposit.
According to previous research achievements of Mesozoic volcanics and mineralizations in the middle and lower reaches of Yangtze River, the less studied volcanics in Lishui basin have been focused on their geochronology, and detailed U-Pb chronological study of zircons in trachyte has been carried out.Combining with geochemical data and lithostratigraphy for the four separated Mesozoic volcanic basins (i.e., Luzong basin, Fanchang basin, Ningwu basin and Lishui basin), volcanics from each basin with four volcanic cycles has been studied comparatively.It shows that, trachytes from the Longwangshan Formation of Lishui basin were formed in the Early Cretaceous with the crystallization age of about 128Ma, which implies the incipience of extensive eruption of volcanics along the middle and lower reaches of Yangtze River.The ages of four volcanic cycles are: before 125Ma, during 125-120Ma, ranging from 120 to 110Ma and from 110 to 90Ma, respectively.It is more reasonable that the Fushan Formation of Luzong basin should be considered as the product of third volcanic cycle, and corresponding to the Gushan and Niangniangshan Formation of Ningwu basin (i.e., Guanshan and Jiashan formations of Lishui basin).The geochemical characteristics of Cretaceous volcanics in this area suggest that the mantle derived alkali magma had been contaminated by crustal materials during its eruption, and early volcanics (e.g., the Longwangshan Formation of Lishui basin) could be the source rock of Sr deposit.
2009, 34(1): 117-126.
Abstract:
This paper reports LA-ICP-MS U-Pb dates of detrital zircons from the Nanhua clastic sedimentary rocks in the Yangtze gorges. Neoproterozoie U-Pb ages show two peaks at 833 Ma and 785 Ma,assumably corresponding to two large-scale periods of magmatism.εHf(t) values for the 910-890Ma zircons are positive (≈10,similar to the value of the coeval depleted mantle),while those for the 890-840Ma zircons tend to decrease to negative values,and there shows age peaks at 890Ma. These data allow us to infer that transformation of oceanic subduction into continental collision or continental-arc collision occurred at about 890 Ma via amalgamation of the Yangtze and Cathaysia blocks. The high εHf(t) values prior to 890 Ma resulted from the subduction of oceanic crust. The subsequent drop of εHf(t) values was caused by the crust melting and crustal collision. εHf(t) values for the 840-800Ma zircons are either negative or positive,whereas εHf(t) values for the 800-780Ma zircons are all negative and εHf(t) values for the 780-750Ma zircons are mostly positive. These data are in coincidence with two stages of the mantle plume beneath the Yangtze craton at 830-795Ma and 780-745Ma.
This paper reports LA-ICP-MS U-Pb dates of detrital zircons from the Nanhua clastic sedimentary rocks in the Yangtze gorges. Neoproterozoie U-Pb ages show two peaks at 833 Ma and 785 Ma,assumably corresponding to two large-scale periods of magmatism.εHf(t) values for the 910-890Ma zircons are positive (≈10,similar to the value of the coeval depleted mantle),while those for the 890-840Ma zircons tend to decrease to negative values,and there shows age peaks at 890Ma. These data allow us to infer that transformation of oceanic subduction into continental collision or continental-arc collision occurred at about 890 Ma via amalgamation of the Yangtze and Cathaysia blocks. The high εHf(t) values prior to 890 Ma resulted from the subduction of oceanic crust. The subsequent drop of εHf(t) values was caused by the crust melting and crustal collision. εHf(t) values for the 840-800Ma zircons are either negative or positive,whereas εHf(t) values for the 800-780Ma zircons are all negative and εHf(t) values for the 780-750Ma zircons are mostly positive. These data are in coincidence with two stages of the mantle plume beneath the Yangtze craton at 830-795Ma and 780-745Ma.
2009, 34(1): 127-136.
Abstract:
The major element composition of late Cenozoic basalts from Leiqiong area,South China is analyzed by wet chemical method. Coupled with some previous data,Fe3+/∑Fe ratios of these basalts can be divided into two groups,0.20-0.35 (named as normal samples) and 0.45-0.66 (named as abnormal samples). The oxygen fugacity in the basalt formation is calculated by the empirical equation which linked fO2 to Fe2O3/FeO. The relative fO2 of normal samples are about FMQ+1.1 to FMQ+2.4,slight higher than that of the mantle xenoliths found by previous researchers (FMQ-0.943 to FMQ+ 1.235). The difference may be attributed to the calculation methods,but the relative fO2 of abnormal samples are around FMQ+3.3 to FMQ+4.8,2log units higher than that of their source region,and that difference can not be explained in the same way as above. Through discussing the effect of magmatic processes on fO2,it indicates fractional crystallization,degassing and oxygen exchange between ascending melts and crust material and lithospheric material can not result in so large fO2 and high Fe3+/∑Fe ratio in abnormal samples. Many abnormal samples collected from craters and the vicinity of craters are always related to unusual high Fe3+/∑Fe ratios. Thus we believe that the local structure resetting in lavas due to slowly cooling rate,which will oxidize Fe^2+ to a smaller Fe3+ ion for size consideration during the densification process,has led to elevated Fe3+/∑Fe ratios of the basaltic melts. One important conclusion is: to estimate the intrinsic fO2 of the source region from which the basalts derived by Fe oxidation state of these basalts,sampling should be cautious and samples far away from the craters should be selected.
The major element composition of late Cenozoic basalts from Leiqiong area,South China is analyzed by wet chemical method. Coupled with some previous data,Fe3+/∑Fe ratios of these basalts can be divided into two groups,0.20-0.35 (named as normal samples) and 0.45-0.66 (named as abnormal samples). The oxygen fugacity in the basalt formation is calculated by the empirical equation which linked fO2 to Fe2O3/FeO. The relative fO2 of normal samples are about FMQ+1.1 to FMQ+2.4,slight higher than that of the mantle xenoliths found by previous researchers (FMQ-0.943 to FMQ+ 1.235). The difference may be attributed to the calculation methods,but the relative fO2 of abnormal samples are around FMQ+3.3 to FMQ+4.8,2log units higher than that of their source region,and that difference can not be explained in the same way as above. Through discussing the effect of magmatic processes on fO2,it indicates fractional crystallization,degassing and oxygen exchange between ascending melts and crust material and lithospheric material can not result in so large fO2 and high Fe3+/∑Fe ratio in abnormal samples. Many abnormal samples collected from craters and the vicinity of craters are always related to unusual high Fe3+/∑Fe ratios. Thus we believe that the local structure resetting in lavas due to slowly cooling rate,which will oxidize Fe^2+ to a smaller Fe3+ ion for size consideration during the densification process,has led to elevated Fe3+/∑Fe ratios of the basaltic melts. One important conclusion is: to estimate the intrinsic fO2 of the source region from which the basalts derived by Fe oxidation state of these basalts,sampling should be cautious and samples far away from the craters should be selected.
2009, 34(1): 137-147.
Abstract:
A-type granite is anorogenic, anhydrous and alkaline granite and has unique mineralogical, petrological and geochemical characteristics, which is considered to be emplaced in post-orogenic, within-plate or plume-related extensional setting.A-type granites and related alkaline rocks imply important information of geodynamics and are the key to understand the growth of continental crust, the lithospheric evolution and the development of regional tectonics.Late Mesozoic A-type granites and related alkaline rocks are widespread in the eastern North China craton.Previous studies and our recently published data show that they formed via a complex process involving mixing of magmas derived from mantle and upper and lower crust sources, crystal fractionation and infracrustal melting during Early Cretaceous (130-110Ma).On the basis of the study of regional geology, it is suggested that the A-type granites and related alkaline rocks were formed in an extensional setting and were the shallow responses to the lithospheric thinning and decratonization, which indicate the peak time of lithospheric thinning and destruction of the North China craton.
A-type granite is anorogenic, anhydrous and alkaline granite and has unique mineralogical, petrological and geochemical characteristics, which is considered to be emplaced in post-orogenic, within-plate or plume-related extensional setting.A-type granites and related alkaline rocks imply important information of geodynamics and are the key to understand the growth of continental crust, the lithospheric evolution and the development of regional tectonics.Late Mesozoic A-type granites and related alkaline rocks are widespread in the eastern North China craton.Previous studies and our recently published data show that they formed via a complex process involving mixing of magmas derived from mantle and upper and lower crust sources, crystal fractionation and infracrustal melting during Early Cretaceous (130-110Ma).On the basis of the study of regional geology, it is suggested that the A-type granites and related alkaline rocks were formed in an extensional setting and were the shallow responses to the lithospheric thinning and decratonization, which indicate the peak time of lithospheric thinning and destruction of the North China craton.
2009, 34(1): 148-164.
Abstract:
The study on the chronology, geochemistry and zircon Hf isotopes of the Zhuangzili and Mopanshan K-feldspar granites in Bengbu uplift was carried out in order to constrain their petrogenesis.Zircons from the intrusions display oscillatory zoning and high Th/U ratios (0.13-1.47), implying their magmatic origin.Zircon LA-ICP-MS, U-Pb dating results of the Zhuangzili and Mopanshan K-feldspar granites indicate that they formed in Paleoproterozoic with the upper intercept ages of 2104±20Ma and 2196±190Ma, respectively.Their SiO2 and K2O contents range from 69.65 to 77.95% and 4.98 to 5.17%, respectively.Moreover, the K-feldspar granites are enriched with light rare earth elements as well as Zr, Hf, Rb, Th, and U, while markedly depleted in Ba, Sr, Eu, P and Ti.Their εNd(t) values vary from-3.4 to +3.2 and Nd model ages range from 2.31 to 2.79Ga.Their initial Hf isotope ratios and Hf two-stage model ages range from-5.1 to +7.8 and 2.26 to 2.83Ga, respectively.Taken together, it is suggested that the primary magma for the K-feldspar granites could be mainly derived from partial melting of the Paleoproterzoic and Neoarchean juvenile crust and that a small amount of ancient crustal material was involved in their origin.Chemically, these K-feldspar granites are A-type ones, implying that they formed under an extensional tectonic setting.
The study on the chronology, geochemistry and zircon Hf isotopes of the Zhuangzili and Mopanshan K-feldspar granites in Bengbu uplift was carried out in order to constrain their petrogenesis.Zircons from the intrusions display oscillatory zoning and high Th/U ratios (0.13-1.47), implying their magmatic origin.Zircon LA-ICP-MS, U-Pb dating results of the Zhuangzili and Mopanshan K-feldspar granites indicate that they formed in Paleoproterozoic with the upper intercept ages of 2104±20Ma and 2196±190Ma, respectively.Their SiO2 and K2O contents range from 69.65 to 77.95% and 4.98 to 5.17%, respectively.Moreover, the K-feldspar granites are enriched with light rare earth elements as well as Zr, Hf, Rb, Th, and U, while markedly depleted in Ba, Sr, Eu, P and Ti.Their εNd(t) values vary from-3.4 to +3.2 and Nd model ages range from 2.31 to 2.79Ga.Their initial Hf isotope ratios and Hf two-stage model ages range from-5.1 to +7.8 and 2.26 to 2.83Ga, respectively.Taken together, it is suggested that the primary magma for the K-feldspar granites could be mainly derived from partial melting of the Paleoproterzoic and Neoarchean juvenile crust and that a small amount of ancient crustal material was involved in their origin.Chemically, these K-feldspar granites are A-type ones, implying that they formed under an extensional tectonic setting.
2009, 34(1): 165-178.
Abstract:
Through the petrological and geochemical study carried out for the Zijingguan mafic pluton (North Taihang), this paper is to understand their magma sources and petrogenesis.The Zijingguan mafic rocks are mainly made up of olivine-bearing pyroxenites, hornblende gabbros and diorites.Cumulate textures can be seen in the olivine-bearing pyroxenites, suggesting a cumulus origin of them.Hornblende gabbros and diorites are probably crystallized products from the residual melts after the accumulation process.Plagioclases from diorites show complicated textures and compositional zoning, with relatively sodium-rich core mantled by calcium-rich plagioclase, the latter evolved to sodium-rich plagioclase towards the rim with descent of magma temperature.Similarly, clinopyroxenes from diorite show complicated compositional zoning, with the MgO abundances in the rim being slightly higher than that of immediate inner part of pyroxenes.These textures suggest that the magma chamber had experienced a process of magma recharge from the mantle source.The different rock types of the Zijingguan mafic pluton share many common geochemical features like highly fractionated REE patterns ((La/Yb)N=5.76-23.13) and high Sr/Y ratios (50.95-146.13), highly enriched LILE (K, Sr, Ba) and LREE, and depleted HFSE (Nb, Zr, Ti) and HREE, with minor Eu anomalies in the REE patterns.They show enriched Sr-Nd isotopic compositions, with ISr=0.7058-0.7066, εNd(t) =-13.8 to-18.2.These geochemical features suggest that the mafic pluton originated from partial melting of an EM1 type mantle source.Olivine-bearing pyroxenites have isotopic composition identical to that of hornblende gabbros (mostly εNd(t) =-15 to-16.6), while the later-crystallizing diorites have a slightly higher εNd(t) =-13.8 to-14.6, which is in agreement with the model of magma recharge during magma evolution.
Through the petrological and geochemical study carried out for the Zijingguan mafic pluton (North Taihang), this paper is to understand their magma sources and petrogenesis.The Zijingguan mafic rocks are mainly made up of olivine-bearing pyroxenites, hornblende gabbros and diorites.Cumulate textures can be seen in the olivine-bearing pyroxenites, suggesting a cumulus origin of them.Hornblende gabbros and diorites are probably crystallized products from the residual melts after the accumulation process.Plagioclases from diorites show complicated textures and compositional zoning, with relatively sodium-rich core mantled by calcium-rich plagioclase, the latter evolved to sodium-rich plagioclase towards the rim with descent of magma temperature.Similarly, clinopyroxenes from diorite show complicated compositional zoning, with the MgO abundances in the rim being slightly higher than that of immediate inner part of pyroxenes.These textures suggest that the magma chamber had experienced a process of magma recharge from the mantle source.The different rock types of the Zijingguan mafic pluton share many common geochemical features like highly fractionated REE patterns ((La/Yb)N=5.76-23.13) and high Sr/Y ratios (50.95-146.13), highly enriched LILE (K, Sr, Ba) and LREE, and depleted HFSE (Nb, Zr, Ti) and HREE, with minor Eu anomalies in the REE patterns.They show enriched Sr-Nd isotopic compositions, with ISr=0.7058-0.7066, εNd(t) =-13.8 to-18.2.These geochemical features suggest that the mafic pluton originated from partial melting of an EM1 type mantle source.Olivine-bearing pyroxenites have isotopic composition identical to that of hornblende gabbros (mostly εNd(t) =-15 to-16.6), while the later-crystallizing diorites have a slightly higher εNd(t) =-13.8 to-14.6, which is in agreement with the model of magma recharge during magma evolution.
2009, 34(1): 179-188.
Abstract:
The Huwan high-pressure (HP) metamorphic zone is a key region to investigate the evolution process and timing of the Qinling-Dabieshan-Sulu orogenic belt. However, the timing of eclogite-facies metamorphism in this zone has not been well constrained. In order to constrain the time of eclogite-facies metamorphism, zircons from a metasediment sample HXD98-9 near Xiongdian were selected for LA-ICPMS U-Pb dating and trace element analysis. Metamorphic zircons are characterized by weak or none zoning, low Th/U ratios, low and/or flat HREE patterns, and an insignificantly negative Eu anomaly. These reflect that the metamorphc zircons crystallized with the presence of garnet and the absence of plagioclase feldspar, which occurred in eclogite-facies conditions. The weighted mean 206Pb/238U age of 310±3 Ma for these metamorphic zircons is thus the best estimated age of the eclogite-faeies metamorphism in this HP metamorphic zone. Three groups of U-Pb age were obtained from the inherited magmatic zircons, ranging of (400±5 Ma)-(419±5Ma), (426±5Ma)-(449±5Ma), and (496±5 Ma)-(524±7 Ma), respectively. One inherited grain has a U-Pb age of 580.7±7.3 Ma. These indicate that the inherited zircons were mostly derived from southern part of the North China block, while some from the Yangtze block.
The Huwan high-pressure (HP) metamorphic zone is a key region to investigate the evolution process and timing of the Qinling-Dabieshan-Sulu orogenic belt. However, the timing of eclogite-facies metamorphism in this zone has not been well constrained. In order to constrain the time of eclogite-facies metamorphism, zircons from a metasediment sample HXD98-9 near Xiongdian were selected for LA-ICPMS U-Pb dating and trace element analysis. Metamorphic zircons are characterized by weak or none zoning, low Th/U ratios, low and/or flat HREE patterns, and an insignificantly negative Eu anomaly. These reflect that the metamorphc zircons crystallized with the presence of garnet and the absence of plagioclase feldspar, which occurred in eclogite-facies conditions. The weighted mean 206Pb/238U age of 310±3 Ma for these metamorphic zircons is thus the best estimated age of the eclogite-faeies metamorphism in this HP metamorphic zone. Three groups of U-Pb age were obtained from the inherited magmatic zircons, ranging of (400±5 Ma)-(419±5Ma), (426±5Ma)-(449±5Ma), and (496±5 Ma)-(524±7 Ma), respectively. One inherited grain has a U-Pb age of 580.7±7.3 Ma. These indicate that the inherited zircons were mostly derived from southern part of the North China block, while some from the Yangtze block.
2009, 34(1): 189-202.
Abstract:
A comprehensive U-Pb geochronological study of detrital zircons from the Cambrian and Devonian strata in Dayaos-han-Damingshan area, eastern Guangxi has been conducted. The strata contact through an unconformity surface, indicating an early Paleozoic depositing hiatus in the region. The Cambrian detrital zircons are characterized by age spectrums of 2.7-2.6,-1.8, 1.4-1.1 and-1.0 Ga with minor Meso-to Neoarchean grains as well as hydrothermal event records of 250 and 105 Ma. The Devonian detrital zircons, only few of which were formed in Paleoarcheozoic age, were dominated by-1.7 and 1.0 Ga grains and recorded a pronounced thermal event at-100 Ma. The geochronology suggests that the study area possessed a Cathaysian attribute in continental accretion and tectonic evolution during the Pre-Devonian, and thus infers that the continental boundary between the Yangtze and Cathaysian blocks was located northwestern to the Dayaoshan-Damingshan terrain. Contrasting characters in geochronology of the Cambrian and Devonian strata indicate a shift in sedimentary provenance after the depositing hiatus. However, zircons with timing intervening the Cambrian and Devonian are absent, which suggests that Cale donian magmatism was minimal or lacking in the region and its proximity. Combined with previous studies, it is inferred that Caledonian folding was likely resulted from an oblique collision between the Cathaysia and a continental block which had moved apart later. This collision also resulted in a development of passive basin in Dayaoshan-Damingshan area by extruding along the vertical direction of pressing. Isotopic resetting and hydrothermal reaction are thought to account for the Mesozoic ages recog-nized in a number of detrital zircons. This observation suggests that large scale eonvecting hydrothermal fluids were involved in the Indosinian and Yanshanian magmatic and tectonic activities. Thus the convecting hydrothermal liquids are regarded as one of the key factors resulting in the mineralization of the late Mesozoic Au-polymetallic ore deposits in this region and plausibly along the whole southeastern margin of the South China fold belt.
A comprehensive U-Pb geochronological study of detrital zircons from the Cambrian and Devonian strata in Dayaos-han-Damingshan area, eastern Guangxi has been conducted. The strata contact through an unconformity surface, indicating an early Paleozoic depositing hiatus in the region. The Cambrian detrital zircons are characterized by age spectrums of 2.7-2.6,-1.8, 1.4-1.1 and-1.0 Ga with minor Meso-to Neoarchean grains as well as hydrothermal event records of 250 and 105 Ma. The Devonian detrital zircons, only few of which were formed in Paleoarcheozoic age, were dominated by-1.7 and 1.0 Ga grains and recorded a pronounced thermal event at-100 Ma. The geochronology suggests that the study area possessed a Cathaysian attribute in continental accretion and tectonic evolution during the Pre-Devonian, and thus infers that the continental boundary between the Yangtze and Cathaysian blocks was located northwestern to the Dayaoshan-Damingshan terrain. Contrasting characters in geochronology of the Cambrian and Devonian strata indicate a shift in sedimentary provenance after the depositing hiatus. However, zircons with timing intervening the Cambrian and Devonian are absent, which suggests that Cale donian magmatism was minimal or lacking in the region and its proximity. Combined with previous studies, it is inferred that Caledonian folding was likely resulted from an oblique collision between the Cathaysia and a continental block which had moved apart later. This collision also resulted in a development of passive basin in Dayaoshan-Damingshan area by extruding along the vertical direction of pressing. Isotopic resetting and hydrothermal reaction are thought to account for the Mesozoic ages recog-nized in a number of detrital zircons. This observation suggests that large scale eonvecting hydrothermal fluids were involved in the Indosinian and Yanshanian magmatic and tectonic activities. Thus the convecting hydrothermal liquids are regarded as one of the key factors resulting in the mineralization of the late Mesozoic Au-polymetallic ore deposits in this region and plausibly along the whole southeastern margin of the South China fold belt.
2009, 34(1): 203-219.
Abstract:
The compositions of the minerals and peridotites, especially trace elements of clinopyroxene (Cpx) can well reflect the characteristics of the subcontinental lithospheric mantle (SCLM). Based on the petrographical study of peridotite xenoliths in Yangyuan Cenozoic basalts and chemical study of minerals, the paper mainly analyzes the LAM-ICP-MS trace element com- positions of Cpx from the Yangyuan peridotitic xenoliths and discusses the nature and evolution of the Cenozoic SCLM beneath the center of the Trans-North China orogen. The SCLM beneath the Yangyuan area is heterogeneous, forming from the different partial melting of the primitive mantle: most melted less than 5% of the total, except a few among 15%-20%. The coex- istence of the fertile and refractory-transitional mantle was better explained by the effects of heterogeneous erosion, transformation and replacement of asthenospheric materials on the cratonic lithosphere mantle.
The compositions of the minerals and peridotites, especially trace elements of clinopyroxene (Cpx) can well reflect the characteristics of the subcontinental lithospheric mantle (SCLM). Based on the petrographical study of peridotite xenoliths in Yangyuan Cenozoic basalts and chemical study of minerals, the paper mainly analyzes the LAM-ICP-MS trace element com- positions of Cpx from the Yangyuan peridotitic xenoliths and discusses the nature and evolution of the Cenozoic SCLM beneath the center of the Trans-North China orogen. The SCLM beneath the Yangyuan area is heterogeneous, forming from the different partial melting of the primitive mantle: most melted less than 5% of the total, except a few among 15%-20%. The coex- istence of the fertile and refractory-transitional mantle was better explained by the effects of heterogeneous erosion, transformation and replacement of asthenospheric materials on the cratonic lithosphere mantle.
2009, 34(1): 220-224.
Abstract:
Phenocrystal olivines in mafic and ultramafic magmatic rocks are critical minerals for trace composition of primary mantle-derived magma and in turn to study composition and evolution of the mantle.We have developed two methods for accurate determination of Ni, Ca, Mn and other elements in olivine by EPMA and LA-ICP-MS.These elements are important indicators for the basalt source composition.The limits of detection for these elements are within the range of 7 to 57 (10-6) by using the proposed EPMA analytical method, which are lower than those of the routine EPMA method by a factor of 3-18.In this work, these elements in MPI-DING reference glasses (KL2-G (basalt), ML3B-G (basalt), StHs6/80-G (andesite) and T1-G (quartzdiorite)) were determined at a spatial resolution of 24μm by LA-ICP-MS.All the determined values of MPI-DING reference glasses agree within 8% with the reference values, and most of them agree within 5%, which demonstrated the accuracy of the established analytical methods.The EPMA and LA-ICP-MS were then successfully applied to the determination of Mg, Al, Ca, Ti, Cr, Mn, Fe, Co and Ni in olivine phenocrysts from Early Cretaceous Feixian (western Shandong) and Sihetun (western Liaonin) basalts in the North China craton.Most of the determined values in the same samples determined by EPMA and LA-ICP-MS agree within 10%.This demonstrates the accuracy of our developed EPMA and LA-ICP-MS methods.
Phenocrystal olivines in mafic and ultramafic magmatic rocks are critical minerals for trace composition of primary mantle-derived magma and in turn to study composition and evolution of the mantle.We have developed two methods for accurate determination of Ni, Ca, Mn and other elements in olivine by EPMA and LA-ICP-MS.These elements are important indicators for the basalt source composition.The limits of detection for these elements are within the range of 7 to 57 (10-6) by using the proposed EPMA analytical method, which are lower than those of the routine EPMA method by a factor of 3-18.In this work, these elements in MPI-DING reference glasses (KL2-G (basalt), ML3B-G (basalt), StHs6/80-G (andesite) and T1-G (quartzdiorite)) were determined at a spatial resolution of 24μm by LA-ICP-MS.All the determined values of MPI-DING reference glasses agree within 8% with the reference values, and most of them agree within 5%, which demonstrated the accuracy of the established analytical methods.The EPMA and LA-ICP-MS were then successfully applied to the determination of Mg, Al, Ca, Ti, Cr, Mn, Fe, Co and Ni in olivine phenocrysts from Early Cretaceous Feixian (western Shandong) and Sihetun (western Liaonin) basalts in the North China craton.Most of the determined values in the same samples determined by EPMA and LA-ICP-MS agree within 10%.This demonstrates the accuracy of our developed EPMA and LA-ICP-MS methods.
