2016 Vol. 41, No. 6
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2016, 41(6): 901-918.
doi: 10.3799/dqkx.2016.077
Abstract:
Today we are facing a series of climatic problems such as elevation of PCO2, global warming, and ocean acidification, which may lead to the sixth mass extinction. The similar extreme climate has been happened repeatedly in the Earth history. Therefore, taking history as mirror will help us to better understand the nature of these problems and resolve them. The most severe extinction happened during the Permian-Triassic transition (252 Ma), eliminating over 90% species. In the past few decades, the process and cause about this extinction event have become the key scientific questions that need to be uncovered. Recent studies show the extreme climatic events we are facing now have been occurred near the Permian-Triassic boundary. The main purpose of this paper is to summarize recent studies on biotic and environmental events during the Permian-Triassic transition, including extinction process, pattern, and causes of the extinction, and survival mechanisms in its aftermath.
Today we are facing a series of climatic problems such as elevation of PCO2, global warming, and ocean acidification, which may lead to the sixth mass extinction. The similar extreme climate has been happened repeatedly in the Earth history. Therefore, taking history as mirror will help us to better understand the nature of these problems and resolve them. The most severe extinction happened during the Permian-Triassic transition (252 Ma), eliminating over 90% species. In the past few decades, the process and cause about this extinction event have become the key scientific questions that need to be uncovered. Recent studies show the extreme climatic events we are facing now have been occurred near the Permian-Triassic boundary. The main purpose of this paper is to summarize recent studies on biotic and environmental events during the Permian-Triassic transition, including extinction process, pattern, and causes of the extinction, and survival mechanisms in its aftermath.
2016, 41(6): 919-930.
doi: 10.3799/dqkx.2016.078
Abstract:
Girvanella filaments in the microbialite are widely distributed in Cambrian, which provide more visual evidences for the participation of microorganism in the formation of the microbialite. Abundant oncoids, a kind of microbialite, occur in the carbonates from the Second Member of the Mantou Formation (Cambrian Series 3), Mianchi area, western Henan. The Girvanella filaments which are preserved in the nuclei, cortices and surrounding rocks exhibit large density and high integrity. The microscopic analysis on the oncoids shows that the Girvanella filaments are preserved in both of the nucleus and cortex of oncoids. The filaments appear as a flexuous, unsegmented and unbranched tubular colony. Each individual is about 20 μm in diameter and 400 μm in maximum extended length. They are mainly densely winding, being intertwined with each other with irregular distribution. The nucleus of oncoids is dominated by filaments surrounding lime clumps and debris, occasionally the bioclasts. The filaments in the cortex of oncoids mainly occur in dark laminae and exhibit four kinds of growth patterns including winding and intertwined, trans-laminar, convex and bedding-arrangement. Girvanella filaments share the similar growth pattern no matter where they occur in oncoids, showing that they have similar growth condition. The mass occurrence of Girvanella filaments in oncoids has much to do with their growth condition of low-energy coastal oolitic inter-shoal or back-shoal facies. This kind of low-energy condition faclitates the growth and preservation of Girvanella filaments. Further studies on the distribution characteristics and micro-fabrics of the Girvanella filaments which were well preserved in the oncoids in the study area are expected to offer significant evidences for the genesis analysis of other microbialite in Cambrian and for the interaction between microorganism and environment.
Girvanella filaments in the microbialite are widely distributed in Cambrian, which provide more visual evidences for the participation of microorganism in the formation of the microbialite. Abundant oncoids, a kind of microbialite, occur in the carbonates from the Second Member of the Mantou Formation (Cambrian Series 3), Mianchi area, western Henan. The Girvanella filaments which are preserved in the nuclei, cortices and surrounding rocks exhibit large density and high integrity. The microscopic analysis on the oncoids shows that the Girvanella filaments are preserved in both of the nucleus and cortex of oncoids. The filaments appear as a flexuous, unsegmented and unbranched tubular colony. Each individual is about 20 μm in diameter and 400 μm in maximum extended length. They are mainly densely winding, being intertwined with each other with irregular distribution. The nucleus of oncoids is dominated by filaments surrounding lime clumps and debris, occasionally the bioclasts. The filaments in the cortex of oncoids mainly occur in dark laminae and exhibit four kinds of growth patterns including winding and intertwined, trans-laminar, convex and bedding-arrangement. Girvanella filaments share the similar growth pattern no matter where they occur in oncoids, showing that they have similar growth condition. The mass occurrence of Girvanella filaments in oncoids has much to do with their growth condition of low-energy coastal oolitic inter-shoal or back-shoal facies. This kind of low-energy condition faclitates the growth and preservation of Girvanella filaments. Further studies on the distribution characteristics and micro-fabrics of the Girvanella filaments which were well preserved in the oncoids in the study area are expected to offer significant evidences for the genesis analysis of other microbialite in Cambrian and for the interaction between microorganism and environment.
2016, 41(6): 931-947.
doi: 10.3799/dqkx.2016.079
Abstract:
The nature of early Paleozoic Caledonian orogeny (Guangxi movement), as one of the key geolocial problems of tectonic evolution in South China, has been controversial for a long time. In this study we report a suite of early Paleozoic basaltic andesite-andesite volcanic rocks located at the Dashuang in the Cenxi area of southeastern Guangxi in South China. Petrogeochemical data of the basaltic andesite-andesite show the contents of SiO2=47.8%-58.8%, Al2O3=13.35%-14.25%, CaO=3.79%-5.61% and TiO2=0.46%-0.56%. It also shows high contents of MgO=4.39%-9.96%, Ni=141×10-6-210×10-6 and Cr=427×10-6-750×10-6, indicating that they belong to high magnesian (HM)-magnesian (M) basaltic andesite-andesite from calc-alkaline to transitional series. The volcanic rocks exhibit enriched-LREE, weak negative δEu abnormally (0.73-0.92), (La/Yb)N=3.4-7.9, (La/Sm)N=3.15-4.95. Basaltic andesite-andesite is characterized by pronounced enrichment of LILEs (e.g. Rb, Ba, Th, U, K, Pb), relative depletion of HFSEs (e.g. Nb, Ta, Ti), and low Ti/V=14.7-21.8. All these show typical characteristics of high magnesian (HM)-magnesian (M) basaltic andesite-andesite (sanukite). The LA-ICP-MS zircon U-Pb age of the magnesian andesite is 443±4 Ma, showing that the formation ages of the volcanic rock series should be between the Late Ordovician and Early Silurian epoch. In conclusion, we suggest that the Dashuang volcanic rocks are the suite of high magnesian (HM)-magnesian (M) basaltic andesite-andesite series formed in the forearc tectonic environment in Late Ordovician-Early Silurian epoch, which provides key evidence for Early Paleozoic oceanic crust subduction between Yangtze Craton and Cathaysia block along Qinzhou-Hangzhou tectonic suture belt in South China.
The nature of early Paleozoic Caledonian orogeny (Guangxi movement), as one of the key geolocial problems of tectonic evolution in South China, has been controversial for a long time. In this study we report a suite of early Paleozoic basaltic andesite-andesite volcanic rocks located at the Dashuang in the Cenxi area of southeastern Guangxi in South China. Petrogeochemical data of the basaltic andesite-andesite show the contents of SiO2=47.8%-58.8%, Al2O3=13.35%-14.25%, CaO=3.79%-5.61% and TiO2=0.46%-0.56%. It also shows high contents of MgO=4.39%-9.96%, Ni=141×10-6-210×10-6 and Cr=427×10-6-750×10-6, indicating that they belong to high magnesian (HM)-magnesian (M) basaltic andesite-andesite from calc-alkaline to transitional series. The volcanic rocks exhibit enriched-LREE, weak negative δEu abnormally (0.73-0.92), (La/Yb)N=3.4-7.9, (La/Sm)N=3.15-4.95. Basaltic andesite-andesite is characterized by pronounced enrichment of LILEs (e.g. Rb, Ba, Th, U, K, Pb), relative depletion of HFSEs (e.g. Nb, Ta, Ti), and low Ti/V=14.7-21.8. All these show typical characteristics of high magnesian (HM)-magnesian (M) basaltic andesite-andesite (sanukite). The LA-ICP-MS zircon U-Pb age of the magnesian andesite is 443±4 Ma, showing that the formation ages of the volcanic rock series should be between the Late Ordovician and Early Silurian epoch. In conclusion, we suggest that the Dashuang volcanic rocks are the suite of high magnesian (HM)-magnesian (M) basaltic andesite-andesite series formed in the forearc tectonic environment in Late Ordovician-Early Silurian epoch, which provides key evidence for Early Paleozoic oceanic crust subduction between Yangtze Craton and Cathaysia block along Qinzhou-Hangzhou tectonic suture belt in South China.
2016, 41(6): 948-970.
doi: 10.3799/dqkx.2016.080
Abstract:
Exsolution lamellae-bearing minerals preserve information on the physicochemical conditions of the precursor homogeneous host and are helpful in understanding the subduction depth as well as the processes of decompression recorded in the host rocks during exhumation. Oriented silica precipitates in clinopyroxene have been reported widely in eclogite and garnet pyroxenite from high pressure and ultrahigh pressure metamorphic terranes around the world.Most of such silica precipitates are identified as α-quartz which in part coexist with hydrous minerals such as calcic amphibole.Such oriented precipitates are elongated parallel to the c-axis of host clinopyroxene, while the long axes of quartz being either c[0001]or a[1120]. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (96%) of quartz precipitates have topotactic relationships with their host clinopyroxenes.Three types of crystallographic topotactic relationships have been identified between quartz and host clinopyroxene: (1) 50% quartz precipitates share the same orientation for the c-axes with [0001]Qz//[001]Cpx; (2) 35% quartz precipitates share the same orientation for the a-axes with [1120]Qz//[001]Cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (1121)Qz//(100)Cpx.Other quartz axes and planes disperse in large or small girdles around the shared axes or planes.Calcic amphibole precipitates also have a strong crystallographic relationship with host clinopyroxene, i.e., (100)Amp//(100)Cpx, [010]Amp//[010]Cpx, [001]Amp//[001]Cpx, [100]Amp∧[100]Cpx≈32°. The results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite.Integrated analyses of published high pressure and high temperature experiments show that the cation vacancy and Ca-Eskola (CaEs) component in clinopyroxene are affected by bulk chemistry, pressure and temperature. The solubility of SiO2 in clinopyroxene are sensitive to bulk chemistry, and the CaEs content in clinopyroxene at high pressure conditions is buffered by free silica phase and kyanite. The CaEs contents in clinopyroxene depend strongly on pressure, which shows rapid increase with pressure up to 6 GPa and then decrease with pressure. By contrast, the CaEs contents in clinopyroxene decrease slightly with temperature which indicates that the effect of temperature is relatively week.On the basis of the above integrated analyses on high pressure experiments and observations on mineral association and microstructural results from natural samples, we suggest that the formation mechanism of the oreiented silica in clinopyroxene is more complicated than we might initially assume. The oriented precipitates of α-quartz and calcic amphiboles in host clinopyroxene are probably formed during multi-stage mechanism involving exsolution, diffusion and exchange of multiple substances, nucleation and growth, recrystallization and some retrograded reactions, which are probably promoted by supercritical fluid or partial melting during exhumation. This study suggests that the texture of oriented quartz precipitates in clinopyroxene is neither necessary nor sufficient for UHP rocks, i. e., it cannot be used as an indisputable UHP-indicator.
Exsolution lamellae-bearing minerals preserve information on the physicochemical conditions of the precursor homogeneous host and are helpful in understanding the subduction depth as well as the processes of decompression recorded in the host rocks during exhumation. Oriented silica precipitates in clinopyroxene have been reported widely in eclogite and garnet pyroxenite from high pressure and ultrahigh pressure metamorphic terranes around the world.Most of such silica precipitates are identified as α-quartz which in part coexist with hydrous minerals such as calcic amphibole.Such oriented precipitates are elongated parallel to the c-axis of host clinopyroxene, while the long axes of quartz being either c[0001]or a[1120]. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (96%) of quartz precipitates have topotactic relationships with their host clinopyroxenes.Three types of crystallographic topotactic relationships have been identified between quartz and host clinopyroxene: (1) 50% quartz precipitates share the same orientation for the c-axes with [0001]Qz//[001]Cpx; (2) 35% quartz precipitates share the same orientation for the a-axes with [1120]Qz//[001]Cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (1121)Qz//(100)Cpx.Other quartz axes and planes disperse in large or small girdles around the shared axes or planes.Calcic amphibole precipitates also have a strong crystallographic relationship with host clinopyroxene, i.e., (100)Amp//(100)Cpx, [010]Amp//[010]Cpx, [001]Amp//[001]Cpx, [100]Amp∧[100]Cpx≈32°. The results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite.Integrated analyses of published high pressure and high temperature experiments show that the cation vacancy and Ca-Eskola (CaEs) component in clinopyroxene are affected by bulk chemistry, pressure and temperature. The solubility of SiO2 in clinopyroxene are sensitive to bulk chemistry, and the CaEs content in clinopyroxene at high pressure conditions is buffered by free silica phase and kyanite. The CaEs contents in clinopyroxene depend strongly on pressure, which shows rapid increase with pressure up to 6 GPa and then decrease with pressure. By contrast, the CaEs contents in clinopyroxene decrease slightly with temperature which indicates that the effect of temperature is relatively week.On the basis of the above integrated analyses on high pressure experiments and observations on mineral association and microstructural results from natural samples, we suggest that the formation mechanism of the oreiented silica in clinopyroxene is more complicated than we might initially assume. The oriented precipitates of α-quartz and calcic amphiboles in host clinopyroxene are probably formed during multi-stage mechanism involving exsolution, diffusion and exchange of multiple substances, nucleation and growth, recrystallization and some retrograded reactions, which are probably promoted by supercritical fluid or partial melting during exhumation. This study suggests that the texture of oriented quartz precipitates in clinopyroxene is neither necessary nor sufficient for UHP rocks, i. e., it cannot be used as an indisputable UHP-indicator.
2016, 41(6): 971-981.
doi: 10.3799/dqkx.2016.081
Abstract:
Although modern sediment dating method has been widely used in coastal areas, applications in the open tidal flat are still challenged. Especially, 210Pbexc dating results given by CIC (constant initial concentration) model and CRS (constant rate of supply) model in the same core are sometimes very different, which is difficult to interpret even compared with the 137Cs data. In this paper, one typical short core collected from open tidal flat was chosen for a comparison study to facilitate future applications of 210Pbexc methods in open coastal areas. The 210Pbexc CIC and CRS models were employed for the calculation, and the two 210Pbexc dating models were analyzed and compared with 137Cs method on the basis of sedimentary history data. It is found that the results given by CIC model are more reliable than those by CRS model, which is attributed to the fact that it is easier for the sediments transported by currents to meet the prerequisites for CIC model, in other words, the initial concentration of 210Pbexc is constant in most instances in coastal areas. As data users generally are expecting to obtain the age of each layer by 210Pbexc dating method (e.g. CRS model), its applications in open coastal area require prudence.
Although modern sediment dating method has been widely used in coastal areas, applications in the open tidal flat are still challenged. Especially, 210Pbexc dating results given by CIC (constant initial concentration) model and CRS (constant rate of supply) model in the same core are sometimes very different, which is difficult to interpret even compared with the 137Cs data. In this paper, one typical short core collected from open tidal flat was chosen for a comparison study to facilitate future applications of 210Pbexc methods in open coastal areas. The 210Pbexc CIC and CRS models were employed for the calculation, and the two 210Pbexc dating models were analyzed and compared with 137Cs method on the basis of sedimentary history data. It is found that the results given by CIC model are more reliable than those by CRS model, which is attributed to the fact that it is easier for the sediments transported by currents to meet the prerequisites for CIC model, in other words, the initial concentration of 210Pbexc is constant in most instances in coastal areas. As data users generally are expecting to obtain the age of each layer by 210Pbexc dating method (e.g. CRS model), its applications in open coastal area require prudence.
2016, 41(6): 982-998.
doi: 10.3799/dqkx.2016.082
Abstract:
Situated in the middle of Tethyan Himalaya, northern Himalayan granites not only better our understanding of melting behaviors and mechanism of the crust during the collisional orogenic processes, but also facilitate the investigation and evaluation of tectonic-magmatic evolution of the Himalayan orogen.In this paper, we present geochronological, geochemical and Sr-Nd isotopic data of the Paiku granites in the northern Himalaya.LA-MC-ICP-MS zircon U-Pb dating reveals that Paiku granites were crystallized from 23.9 Ma to 16.5 Ma and have experienced at least two episodes of anatexis at 22.3±0.6 Ma and 17.3±0.3 Ma, respectively. The age of 17.3±0.3 Ma probably represents the final crystallized timing.Bulk-rock major and trace elements analyses indicate that Paiku granites were characterized by high SiO2(71.87% to 75.56%), Al2O3(13.57% to 15.49%), K2O(3.34% to 4.59%), and high values of K2O/Na2O(1.02 to 1.39), A/CNK(1.21 to 1.23), and enrichment in Rb, Th, U, depletion in Ba, Nb, Sr, Zr and no or weak negative Eu anomalies ((La/Yb)N=10.76-16.60), and strong fractionation between LREE and HREE (δEu=0.76-0.97). These features demonstrate that Paiku granites belong to high potassium Calc-alkaline and peraluminous granites. The values of (87Sr/86Sr)i and εNd(t) range from 0.736 184 to 0.741 258 and from -14.6 to -14.3, and can compare well with the metasedimentary rock in the Greater Himalaya Crystalline complex (GHC). It is inferred that the Paiku granites were generated from partial melting of the GHC metasedimentary rocks. The granites show relatively high Sr, but low Rb and Rb/Sr which are nearly constant relative to large variations in Ba concentrations. These features are concordant with the trend of fluxed muscovite partial melting. Based on above evidences, we suggest the Paiku granites were derived from fluxed partial melting of the GHC metasedimentary rock, possibly related closely with the structural activity along the South Tibet detachment system (STDS).
Situated in the middle of Tethyan Himalaya, northern Himalayan granites not only better our understanding of melting behaviors and mechanism of the crust during the collisional orogenic processes, but also facilitate the investigation and evaluation of tectonic-magmatic evolution of the Himalayan orogen.In this paper, we present geochronological, geochemical and Sr-Nd isotopic data of the Paiku granites in the northern Himalaya.LA-MC-ICP-MS zircon U-Pb dating reveals that Paiku granites were crystallized from 23.9 Ma to 16.5 Ma and have experienced at least two episodes of anatexis at 22.3±0.6 Ma and 17.3±0.3 Ma, respectively. The age of 17.3±0.3 Ma probably represents the final crystallized timing.Bulk-rock major and trace elements analyses indicate that Paiku granites were characterized by high SiO2(71.87% to 75.56%), Al2O3(13.57% to 15.49%), K2O(3.34% to 4.59%), and high values of K2O/Na2O(1.02 to 1.39), A/CNK(1.21 to 1.23), and enrichment in Rb, Th, U, depletion in Ba, Nb, Sr, Zr and no or weak negative Eu anomalies ((La/Yb)N=10.76-16.60), and strong fractionation between LREE and HREE (δEu=0.76-0.97). These features demonstrate that Paiku granites belong to high potassium Calc-alkaline and peraluminous granites. The values of (87Sr/86Sr)i and εNd(t) range from 0.736 184 to 0.741 258 and from -14.6 to -14.3, and can compare well with the metasedimentary rock in the Greater Himalaya Crystalline complex (GHC). It is inferred that the Paiku granites were generated from partial melting of the GHC metasedimentary rocks. The granites show relatively high Sr, but low Rb and Rb/Sr which are nearly constant relative to large variations in Ba concentrations. These features are concordant with the trend of fluxed muscovite partial melting. Based on above evidences, we suggest the Paiku granites were derived from fluxed partial melting of the GHC metasedimentary rock, possibly related closely with the structural activity along the South Tibet detachment system (STDS).
2016, 41(6): 999-1015.
doi: 10.3799/dqkx.2016.083
Abstract:
The Lakang'e Cu-Mo deposit is a typical porphyry deposit in the eastern Gangdese metallogenic belt, on which previous geochemical studies are not adequate and systematic.In this paper, we present the results of the systematic logging of drillholes and provide new petrological zircon U-Pb age, and Hf isotope data for the deposit that allow the identification of the timing of formation and mineralization of the Lakang'e porphyry Cu-Mo deposit and the relationship between this deposit and other porphyry-type deposits in the Gangdese belt.Chemical analyses show that the ore-bearing granodiorite-porphyry has high SiO2, low Mg and Ca (SiO2=62.51% to 72.41%, MgO=0.59% to 1.30%, CaO=0.95% to 3.44%), highalkaline (Na2O=3.51% to 4.75%, K2O=3.30% to 4.97%), metaluminous (A/CNK=0.90 to 1.01), enriched in LILES such as Rb, Ba, Th, U, Sr, and depleted in HFSES such as Nb, Ta, Ti, P and Zr. The rocks also display low total REEcontents (∑REE=82.80×10-6 to 132.09×10-6), enriched LREE and differentiate obviously, with weakly negative Eu and Ce anomalies. LA-ICP-MS zircon U-Pb dating indicates that the Lakang'e ore-bearing granodiorite-porphyry emplaced at 13.58±0.42 Ma. εHf(t) values and two-stage model ages of zircons are -3.99 to 4.49 and 808 to 1349 Ma respectively. The results indicate that the ore-bearing granodiorite-porphyry in the Lakang'e deposit has adakitegeochemical characteristics, and the parental magmas were mainly derived from a juvenile crust and partly mixed with old continental crust rock during the evolution. The porphyry in Lakang'edeposit was formed in post-collisional extension setting of India-Asia continental collision orogenic belt.
The Lakang'e Cu-Mo deposit is a typical porphyry deposit in the eastern Gangdese metallogenic belt, on which previous geochemical studies are not adequate and systematic.In this paper, we present the results of the systematic logging of drillholes and provide new petrological zircon U-Pb age, and Hf isotope data for the deposit that allow the identification of the timing of formation and mineralization of the Lakang'e porphyry Cu-Mo deposit and the relationship between this deposit and other porphyry-type deposits in the Gangdese belt.Chemical analyses show that the ore-bearing granodiorite-porphyry has high SiO2, low Mg and Ca (SiO2=62.51% to 72.41%, MgO=0.59% to 1.30%, CaO=0.95% to 3.44%), highalkaline (Na2O=3.51% to 4.75%, K2O=3.30% to 4.97%), metaluminous (A/CNK=0.90 to 1.01), enriched in LILES such as Rb, Ba, Th, U, Sr, and depleted in HFSES such as Nb, Ta, Ti, P and Zr. The rocks also display low total REEcontents (∑REE=82.80×10-6 to 132.09×10-6), enriched LREE and differentiate obviously, with weakly negative Eu and Ce anomalies. LA-ICP-MS zircon U-Pb dating indicates that the Lakang'e ore-bearing granodiorite-porphyry emplaced at 13.58±0.42 Ma. εHf(t) values and two-stage model ages of zircons are -3.99 to 4.49 and 808 to 1349 Ma respectively. The results indicate that the ore-bearing granodiorite-porphyry in the Lakang'e deposit has adakitegeochemical characteristics, and the parental magmas were mainly derived from a juvenile crust and partly mixed with old continental crust rock during the evolution. The porphyry in Lakang'edeposit was formed in post-collisional extension setting of India-Asia continental collision orogenic belt.
2016, 41(6): 1016-1030.
doi: 10.3799/dqkx.2016.084
Abstract:
The Early Paleozoic granitic pluton has a closely genetic relationship with the Alkali metasomatism type uranium mineralization in the middle Longshou Moutains area, which is an important activity zone of Longshoushan granitic magma. However, people do not know more about the cause of Jiling rock mass, properties of magma source area and the relationship between Jiling rock mass and uranium mineralization. The Sr-Nd-Pb isotopic composition characteristics of granitic pluton show that the initial Sr isotope ratios (87Sr/86Sr)i of granite in different periods (with first intrusion of gray monzonitic granites in the Early Paleozoic, the pink monzonitic granites of the second intrusion, and pink fine-grained (potassium) granites of the intrusion in Late Paleozoic) fall in the range of that of the continental crust (from 0.706 to 0.718); at the same time, the average initial Sr isotope ratios (87Sr/86Sr)i has a trend of falling after rising (0.707 12→0.710 00→0.707 89), and the average of εNd(t) shows the trend of rising after falling (-7.00→-8.09→-4.65). tDM2 average of granites in different periods is 1 735.50 Ma, 1 814.66 Ma and 1 737.50 Ma respectively, which is very close to the age of residual crust, indicating that the main material source of the granitic pluton should be the group strata of paleoproterozoic Longshou mountains, with the addition of mantle or young crustal materials. Its Pb Isotope ratios are high: 206Pb/204Pb range from 18.328 to 19.240, 207Pb/204Pb range from 15.549 to 15.619, 208Pb/204Pb range from 38.390 to 39.075, μ values range from 9.37 to 9.43, with an average of 9.40, 206Pb/204Pb range from 30.209 to 43.529, 207Pb/204Pb range from 16.097 to 25.076, 208Pb/204Pb range from 39.107 to 39.42, μ values range from 18.47 to 30.24, with an average of 9.40;206Pb/204Pb range from 19.071 to 19.767, 207Pb/204Pb range from 15.577 to 25.438, 208Pb/204Pb range from 38.682 to 42.593, μ values range from 9.36 to 9.49, with an average of 9.41. All samples show high radioactive Pb isotopic characteristics with a mixed source, but the crust is the main source. The comparison of Sr-Nd-Pb Isotope shows that the initial Sr isotope ratios (87Sr/86Sr)i and εNd(t) of sodium metasomatic rock (ore) are very similar to those of pink monzonitic granites of the second intrusion in the Early Paleozoic, and the subpoint of (87Sr/86Sr)i-εNd(t) diagram is also consistent, which indicates that alkali metasomatism type uranium mineralization of the study area is mainly related to the second invasion in Early Paleozoic. The high uranium background values of granitic pluton in other periods show that it may also provide a source of uranium.
The Early Paleozoic granitic pluton has a closely genetic relationship with the Alkali metasomatism type uranium mineralization in the middle Longshou Moutains area, which is an important activity zone of Longshoushan granitic magma. However, people do not know more about the cause of Jiling rock mass, properties of magma source area and the relationship between Jiling rock mass and uranium mineralization. The Sr-Nd-Pb isotopic composition characteristics of granitic pluton show that the initial Sr isotope ratios (87Sr/86Sr)i of granite in different periods (with first intrusion of gray monzonitic granites in the Early Paleozoic, the pink monzonitic granites of the second intrusion, and pink fine-grained (potassium) granites of the intrusion in Late Paleozoic) fall in the range of that of the continental crust (from 0.706 to 0.718); at the same time, the average initial Sr isotope ratios (87Sr/86Sr)i has a trend of falling after rising (0.707 12→0.710 00→0.707 89), and the average of εNd(t) shows the trend of rising after falling (-7.00→-8.09→-4.65). tDM2 average of granites in different periods is 1 735.50 Ma, 1 814.66 Ma and 1 737.50 Ma respectively, which is very close to the age of residual crust, indicating that the main material source of the granitic pluton should be the group strata of paleoproterozoic Longshou mountains, with the addition of mantle or young crustal materials. Its Pb Isotope ratios are high: 206Pb/204Pb range from 18.328 to 19.240, 207Pb/204Pb range from 15.549 to 15.619, 208Pb/204Pb range from 38.390 to 39.075, μ values range from 9.37 to 9.43, with an average of 9.40, 206Pb/204Pb range from 30.209 to 43.529, 207Pb/204Pb range from 16.097 to 25.076, 208Pb/204Pb range from 39.107 to 39.42, μ values range from 18.47 to 30.24, with an average of 9.40;206Pb/204Pb range from 19.071 to 19.767, 207Pb/204Pb range from 15.577 to 25.438, 208Pb/204Pb range from 38.682 to 42.593, μ values range from 9.36 to 9.49, with an average of 9.41. All samples show high radioactive Pb isotopic characteristics with a mixed source, but the crust is the main source. The comparison of Sr-Nd-Pb Isotope shows that the initial Sr isotope ratios (87Sr/86Sr)i and εNd(t) of sodium metasomatic rock (ore) are very similar to those of pink monzonitic granites of the second intrusion in the Early Paleozoic, and the subpoint of (87Sr/86Sr)i-εNd(t) diagram is also consistent, which indicates that alkali metasomatism type uranium mineralization of the study area is mainly related to the second invasion in Early Paleozoic. The high uranium background values of granitic pluton in other periods show that it may also provide a source of uranium.
2016, 41(6): 1031-1040.
doi: 10.3799/dqkx.2016.085
Abstract:
Okinawa Trough is a young and active back-arc basin located at active continental margin in the western Pacific, which is in the initial stage of spreading and characterized by widespread extensional faults and intense volcanic activities. Distribution and geochemistry of rocks show distinct differences of magmatism in the central and southern Okinawa Trough, but the reason is unclear so far. To explore this issue, we study comprehensively published petrologic and geochemical data, combined with Pb isotopic composition of basalts in Okinawa Trough in this paper. It is found that mantle source beneath Okinawa Trough and the trench-arc-basin system in which the Okinawa Trough is located belongs to Indian Ocean-type mantle, and basalts derived from southern Okinawa Trough are products of higher degree of mantle melting relative to basalts from middle Okinawa Trough. Basalts derived from Okinawa Trough are affected by the subduction component and extent of the effect of subduction component on magma source in the southern Okinawa Trough is higher than that in the middle Okinawa Trough, and the cause may be subducted depth of plate (southern: 100-150 km; middle: about 200 km); only a few basalts are similar to MORB-type, which is considered to be an important characteristic and evidence of spreading in the Okinawa Trough. On the basis of the above analysis, we suggest that the studies of magmatism in the Okinawa Trough shall be focused on the following aspects in the future: identification and distribution of basement rocks, the types and characteristics of magmatic mantle source, the restriction mechanism and contribution of subduction to magmatism, and the internal relation between magmatism and hydrothermal activities.
Okinawa Trough is a young and active back-arc basin located at active continental margin in the western Pacific, which is in the initial stage of spreading and characterized by widespread extensional faults and intense volcanic activities. Distribution and geochemistry of rocks show distinct differences of magmatism in the central and southern Okinawa Trough, but the reason is unclear so far. To explore this issue, we study comprehensively published petrologic and geochemical data, combined with Pb isotopic composition of basalts in Okinawa Trough in this paper. It is found that mantle source beneath Okinawa Trough and the trench-arc-basin system in which the Okinawa Trough is located belongs to Indian Ocean-type mantle, and basalts derived from southern Okinawa Trough are products of higher degree of mantle melting relative to basalts from middle Okinawa Trough. Basalts derived from Okinawa Trough are affected by the subduction component and extent of the effect of subduction component on magma source in the southern Okinawa Trough is higher than that in the middle Okinawa Trough, and the cause may be subducted depth of plate (southern: 100-150 km; middle: about 200 km); only a few basalts are similar to MORB-type, which is considered to be an important characteristic and evidence of spreading in the Okinawa Trough. On the basis of the above analysis, we suggest that the studies of magmatism in the Okinawa Trough shall be focused on the following aspects in the future: identification and distribution of basement rocks, the types and characteristics of magmatic mantle source, the restriction mechanism and contribution of subduction to magmatism, and the internal relation between magmatism and hydrothermal activities.
2016, 41(6): 1041-1054.
doi: 10.3799/dqkx.2016.086
Abstract:
The deepwater channel system is a large-scale deepwater sedimentary system which was newly found in abyssal mudstone of the upper Zhujiang Formation, eastern margin of Baiyun-Liwan sag in recent years. However, its sedimentary characteristics, sedimentary processes and main controlling factors are unclear yet.Based on substantial 2-D, 3-D seismic and well logging data, the depositional elements, geometry, internal architecture and sedimentary evolution of the deepwater channel system in Miocene Zhujiang Formation of Baiyun-Liwan sag were studied. In addition, the source supply, shelf break belt, palaeogeomorphology and their controlling effects on distribution, internal architecture and evolution of the deepwater channel system were discussed. Finally, the genetic mechanism of it was built.The results show that the deepwater channel system consists of channel, levee, and lobe, and channel is the primary depositional element. The deepwater channel can be subdivided into Baiyun sag segment, southern uplift segment and Liwan sag segment from north to south, which is mainly erosional channel, erosional-aggradational channel and aggradational channel, respectively. And it presents an evolutionary characteristic of "third-order channel" from north to south because the energy of gravity flow weakened by descending order. Two periods of channel evolution in vertical succession can be identified in each segment by identifying secondary erosional surfaces in the interior of channel. And there are distinctive internal filling properties in different channel evolution and different segments because of different fillings and transformation processes. The formation and development of deepwater channel system resulted from the interaction between the northern Pearl River Delta, shelf break belt and restricted paleotopography in eastern Baiyun-Liwan sag.
The deepwater channel system is a large-scale deepwater sedimentary system which was newly found in abyssal mudstone of the upper Zhujiang Formation, eastern margin of Baiyun-Liwan sag in recent years. However, its sedimentary characteristics, sedimentary processes and main controlling factors are unclear yet.Based on substantial 2-D, 3-D seismic and well logging data, the depositional elements, geometry, internal architecture and sedimentary evolution of the deepwater channel system in Miocene Zhujiang Formation of Baiyun-Liwan sag were studied. In addition, the source supply, shelf break belt, palaeogeomorphology and their controlling effects on distribution, internal architecture and evolution of the deepwater channel system were discussed. Finally, the genetic mechanism of it was built.The results show that the deepwater channel system consists of channel, levee, and lobe, and channel is the primary depositional element. The deepwater channel can be subdivided into Baiyun sag segment, southern uplift segment and Liwan sag segment from north to south, which is mainly erosional channel, erosional-aggradational channel and aggradational channel, respectively. And it presents an evolutionary characteristic of "third-order channel" from north to south because the energy of gravity flow weakened by descending order. Two periods of channel evolution in vertical succession can be identified in each segment by identifying secondary erosional surfaces in the interior of channel. And there are distinctive internal filling properties in different channel evolution and different segments because of different fillings and transformation processes. The formation and development of deepwater channel system resulted from the interaction between the northern Pearl River Delta, shelf break belt and restricted paleotopography in eastern Baiyun-Liwan sag.
2016, 41(6): 1055-1066.
doi: 10.3799/dqkx.2016.087
Abstract:
The Triassic Yanchang Formation, the most important oil and gas horizon in Ordos Basin, is a typical lithologic reservoir characterized by mass oil-bearing and complex hydrocarbon accumulation, and characteristics of its source rocks are the main controlling factors of lithologic reservoir distribution. Based on outcrops and well log data, this paper divides the Yangchang Formation into three sequences including SQ1, SQ2 and SQ3 upwards to research the characteristics and controlling factors of source rocks within sequence framework. The results show that the primary source rock occurred in expansive system tract, and is characterized by Chang7 source rock belonging to semi-deep lake and deep lake underfilled type deposits during the lake maximum flood period; meanwhile, the sapropel-type kerogen source rocks with a higher organic matter abundance are dominant due to the lack of terrigenous clastic supply. There are also source rocks in highstand system tract such as Chang9 and Chang6 to Chang 4+5 belonging to the deposition of lake-shore, shallow lake facies, in which terrigenous clastic supply makes humic type source rock dominant also with a relative high organic matter abundance. The crude oil of Yanchang Formation in the study area is primarily from Chang7 within expansive systems tract during maximum flooding period of Late Triassic according to crude oil correlation, which implies that contributions of source rocks in different system tracts to Yanchang reservoir vary. The vertical and lateral change of accommodation under sequence stratigraphic sedimentary dynamics setting not only decides the distribution of sedimentary facies types, but also directly controls the abundance and type of organic materials within system tracts. The difference of source rock occurrences between system tracts in sequence framework facilitates prediction and evaluation of distribution laws of source rocks in the sedimentary setting of basin.
The Triassic Yanchang Formation, the most important oil and gas horizon in Ordos Basin, is a typical lithologic reservoir characterized by mass oil-bearing and complex hydrocarbon accumulation, and characteristics of its source rocks are the main controlling factors of lithologic reservoir distribution. Based on outcrops and well log data, this paper divides the Yangchang Formation into three sequences including SQ1, SQ2 and SQ3 upwards to research the characteristics and controlling factors of source rocks within sequence framework. The results show that the primary source rock occurred in expansive system tract, and is characterized by Chang7 source rock belonging to semi-deep lake and deep lake underfilled type deposits during the lake maximum flood period; meanwhile, the sapropel-type kerogen source rocks with a higher organic matter abundance are dominant due to the lack of terrigenous clastic supply. There are also source rocks in highstand system tract such as Chang9 and Chang6 to Chang 4+5 belonging to the deposition of lake-shore, shallow lake facies, in which terrigenous clastic supply makes humic type source rock dominant also with a relative high organic matter abundance. The crude oil of Yanchang Formation in the study area is primarily from Chang7 within expansive systems tract during maximum flooding period of Late Triassic according to crude oil correlation, which implies that contributions of source rocks in different system tracts to Yanchang reservoir vary. The vertical and lateral change of accommodation under sequence stratigraphic sedimentary dynamics setting not only decides the distribution of sedimentary facies types, but also directly controls the abundance and type of organic materials within system tracts. The difference of source rock occurrences between system tracts in sequence framework facilitates prediction and evaluation of distribution laws of source rocks in the sedimentary setting of basin.
2016, 41(6): 1067-1073.
doi: 10.3799/dqkx.2016.088
Abstract:
It is important to quantitatively characterize the microscopic structures of organic and inorganic shale pores making up shale matrix, since shale gas and oil show different transport mechanisms in them. In this paper, the typical shale organic pore and inorganic pore images are obtained from scanning electron microscope (SEM) respectively, and it is found that the image with relatively larger inorganic pores has a lower resolution, while the image with relatively smaller organic pores has a higher resolution. Then, image processing and Markov chain Monte Carlo (MCMC) method are used to reconstruct the corresponding inorganic pore digital rock and organic pore digital rock, and local superposition method is introduced to construct the shale matrix pore digital rock including inorganic pores and organic pores. At last, the structure properties are compared and analyzed among the three inorganic pore, organic pore and matrix pore digital rocks. Results show that the constructed shale matrix pore digital rock with local superposition method could describe the inorganic pore and organic pore structures simultaneously. In addition the inorganic pores have a poor connection while the organic pores have a better connection, and a higher local porosity and local permeability, which is important to the fluid flow in shale rocks.A reliable research platform is established for different pore structure analysis and gas & oil transport simulation in nanoscopic pores of shale rocks in this study.
It is important to quantitatively characterize the microscopic structures of organic and inorganic shale pores making up shale matrix, since shale gas and oil show different transport mechanisms in them. In this paper, the typical shale organic pore and inorganic pore images are obtained from scanning electron microscope (SEM) respectively, and it is found that the image with relatively larger inorganic pores has a lower resolution, while the image with relatively smaller organic pores has a higher resolution. Then, image processing and Markov chain Monte Carlo (MCMC) method are used to reconstruct the corresponding inorganic pore digital rock and organic pore digital rock, and local superposition method is introduced to construct the shale matrix pore digital rock including inorganic pores and organic pores. At last, the structure properties are compared and analyzed among the three inorganic pore, organic pore and matrix pore digital rocks. Results show that the constructed shale matrix pore digital rock with local superposition method could describe the inorganic pore and organic pore structures simultaneously. In addition the inorganic pores have a poor connection while the organic pores have a better connection, and a higher local porosity and local permeability, which is important to the fluid flow in shale rocks.A reliable research platform is established for different pore structure analysis and gas & oil transport simulation in nanoscopic pores of shale rocks in this study.
2016, 41(6): 1074-1080.
doi: 10.3799/dqkx.2016.089
Abstract:
Geometry and kinematics analysis of thrust-related fold system in piedmond is a frontier subject of structure and tectonics. These concepts and technical methods are relevant for the exploration and exploitation of hydrocarbon systems in piedmont basins worldwide. The geometry and kinematics models of the faults and folds are established by using the 2D seismic profiles, drilling well logging data, and geological mapping data in the eastern Kuqa depression in the piedmont of Tianshan Mountains, China. The fault throw maps of the Tuziluke fault and the eastern Qiulitag and the shorting map of the eastern Qiulitag anticline have been drew. The results reveal that there are two types of fault in the eastern Kuqa depression, i.e. the steeply basement-involved faults and the low dipping thrust faults. In Tertiary, the steeply faults made the basement uplift and the Meso-Cenozoic squence erosion in the piedmont of Tianshan. The low dipping faults develop in Pliocene, which was detachment in the lower of Mesozoic and formed the blind imbricate thrust-related fold system in the Kuqa basin
Geometry and kinematics analysis of thrust-related fold system in piedmond is a frontier subject of structure and tectonics. These concepts and technical methods are relevant for the exploration and exploitation of hydrocarbon systems in piedmont basins worldwide. The geometry and kinematics models of the faults and folds are established by using the 2D seismic profiles, drilling well logging data, and geological mapping data in the eastern Kuqa depression in the piedmont of Tianshan Mountains, China. The fault throw maps of the Tuziluke fault and the eastern Qiulitag and the shorting map of the eastern Qiulitag anticline have been drew. The results reveal that there are two types of fault in the eastern Kuqa depression, i.e. the steeply basement-involved faults and the low dipping thrust faults. In Tertiary, the steeply faults made the basement uplift and the Meso-Cenozoic squence erosion in the piedmont of Tianshan. The low dipping faults develop in Pliocene, which was detachment in the lower of Mesozoic and formed the blind imbricate thrust-related fold system in the Kuqa basin
