Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mi...Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mineral for provenance studies, is also a hindrance as zircon can be recycled through numerous sedimentary basins, thus obscuring the first cycle source to sink relationship. An elegant approach to addressing this potential issue is to compare the Pb isotope composition of detrital K-feldspar, a mineral which is unlikely to survive more than one erosion-transport-deposition cycle, with that of magmatic K-feldspar from potential basement source terranes. Here we present new in situ Pb isotope data on detrital K-feldspar from two Proterozoic arkosic sandstones from Western Australia, and magmatic K-feldspar grains from potential igneous source rocks, as inferred by the age and Hf isotope composition of detrital zircon grains. The data indicate that the detrital zircon and K-feldspar grains could not have been liberated from the same source rocks, and that the zircon has most likely been recycled through older sedimentary basins. These results provide a more complete understanding of apparently simple source to sink relationships in this part of Proterozoic Western Australia.展开更多
The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate...The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values(from -4.7 to +10.8) in contrast to a limited range of εNd(t) values(from -5.8 to -1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values(from -34.3 to -6.5) and positive d7 Li values(from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nde Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of177 Hf in the source, thereby elevating the^(176) Hf/^(177) Hf ratio of the melt, and decoupling from the^(143) Nd/^(144) Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.展开更多
As a result of recycling, the mineralogical and chemical compositions of riverine sediments may reflect the combined effects of the present-day weathering regime as well as previous weathering and diagenetic alteratio...As a result of recycling, the mineralogical and chemical compositions of riverine sediments may reflect the combined effects of the present-day weathering regime as well as previous weathering and diagenetic alteration history. River sediments can be interpreted as a mixture of non-weathered bedrock—of igneous, metamorphic, or sedimentary origin—and solids formed by the modern weathering system. The correlation between the weathering proxies chemical index of alteration and weathering index of Parker offers an approach to distinguish fine suspended particles, coarse bedload sediments, and recycled sediments under the influence of quartz dilution. Recycling of cation-depleted source rocks formed during past geological weathering episodes may have great impacts on the weathering indices of sediments from the Changjiang(Yangzte) and Zhuoshui Rivers. Special caution is required when using chemical weathering indices to investigate the intensity of chemical weathering registered in fluvial sediments. To minimize the effect of hydrodynamic sorting or sediment recycling, we suggest that the fine sediments(e.g.suspended particles and ﹤2 lm fractions of bedload sediments) in rivers better reflect the average of weatheredcrust in catchments and the terrigenous end-member in marginal seas.展开更多
The study was carried out to delineate the geochemistry of unstudied meta-sedimentary rocks in the Igangan sheet 240 NW. Geological field mapping was undertaken to delineate the different metasedimentary rocks;three d...The study was carried out to delineate the geochemistry of unstudied meta-sedimentary rocks in the Igangan sheet 240 NW. Geological field mapping was undertaken to delineate the different metasedimentary rocks;three different metasedimentary rocks were observed and 17 samples were analyzed using ICP-AES/MS to obtain elemental concentrations. Geochemical analytical results revealed average concentration (%) of 74.5, 12.66, 1.99, 4.23, and 3.59 for SiO2, Al2O3, Fe2O3, Na2O and K2O respectively. Trace elemental average concentrations (ppm) for selected trace elements in the metasediments are as: U (0.4 - 4.8 ppm, 2.39), Th (0.9 - 25.60 ppm, 10.38), Ni (0.1 - 17.70 ppm, 7.40), Sr (37 - 124 ppm, 81.75), Ba (55 - 1228 ppm, 620.31), Sn (1.4 - 4.8), Be (1 - 11 ppm, 3.31), Li (2.6 - 48.50 ppm, 23.28), Rb (32 - 376.3 ppm, 140.16), Ta (0.5 - 65.20 ppm, 12.48), Nb (7.76 - 47.5 ppm, 23.82) and Cs (0.8 - 33.30 ppm, 8.33). REE patterns of all samples are parallel to sub-parallel, LREE-enriched, with weak to distinct negative Eu anomalies and weakly fractionated HREE segments. Source rocks of the metsediments underwent low to moderate chemical weathering and metasediments are derivatives of felsic igneous sources.展开更多
基金funded via an Australian Geophysical Observing System grant provided to Au Scope Pty Ltd.the AQ44 Australian Education Investment Fund programpartly funded by the Western Australian Exploration Incentive Scheme
文摘Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mineral for provenance studies, is also a hindrance as zircon can be recycled through numerous sedimentary basins, thus obscuring the first cycle source to sink relationship. An elegant approach to addressing this potential issue is to compare the Pb isotope composition of detrital K-feldspar, a mineral which is unlikely to survive more than one erosion-transport-deposition cycle, with that of magmatic K-feldspar from potential basement source terranes. Here we present new in situ Pb isotope data on detrital K-feldspar from two Proterozoic arkosic sandstones from Western Australia, and magmatic K-feldspar grains from potential igneous source rocks, as inferred by the age and Hf isotope composition of detrital zircon grains. The data indicate that the detrital zircon and K-feldspar grains could not have been liberated from the same source rocks, and that the zircon has most likely been recycled through older sedimentary basins. These results provide a more complete understanding of apparently simple source to sink relationships in this part of Proterozoic Western Australia.
基金financially supported by the National Natural Science Foundation of China(Grant No. 41502209)the Chinese State 973 Project (Grant No.2015CB250901)the Science Foundation of China University ofPetroleum, Beijing (Grant No. 2462014YJRC031)
文摘The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values(from -4.7 to +10.8) in contrast to a limited range of εNd(t) values(from -5.8 to -1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values(from -34.3 to -6.5) and positive d7 Li values(from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nde Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of177 Hf in the source, thereby elevating the^(176) Hf/^(177) Hf ratio of the melt, and decoupling from the^(143) Nd/^(144) Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.
基金supported by National Natural Science Foundation of China(Nos.41376049 and 41225020)National Programme on Global Change and Air-Sea Interaction(GASI-GEOGE-03)by Ao Shan Talents Program Supported by Qingdao National Laboratory for Marine Science and Technology(No.2015ASTP-OS11)
文摘As a result of recycling, the mineralogical and chemical compositions of riverine sediments may reflect the combined effects of the present-day weathering regime as well as previous weathering and diagenetic alteration history. River sediments can be interpreted as a mixture of non-weathered bedrock—of igneous, metamorphic, or sedimentary origin—and solids formed by the modern weathering system. The correlation between the weathering proxies chemical index of alteration and weathering index of Parker offers an approach to distinguish fine suspended particles, coarse bedload sediments, and recycled sediments under the influence of quartz dilution. Recycling of cation-depleted source rocks formed during past geological weathering episodes may have great impacts on the weathering indices of sediments from the Changjiang(Yangzte) and Zhuoshui Rivers. Special caution is required when using chemical weathering indices to investigate the intensity of chemical weathering registered in fluvial sediments. To minimize the effect of hydrodynamic sorting or sediment recycling, we suggest that the fine sediments(e.g.suspended particles and ﹤2 lm fractions of bedload sediments) in rivers better reflect the average of weatheredcrust in catchments and the terrigenous end-member in marginal seas.
文摘The study was carried out to delineate the geochemistry of unstudied meta-sedimentary rocks in the Igangan sheet 240 NW. Geological field mapping was undertaken to delineate the different metasedimentary rocks;three different metasedimentary rocks were observed and 17 samples were analyzed using ICP-AES/MS to obtain elemental concentrations. Geochemical analytical results revealed average concentration (%) of 74.5, 12.66, 1.99, 4.23, and 3.59 for SiO2, Al2O3, Fe2O3, Na2O and K2O respectively. Trace elemental average concentrations (ppm) for selected trace elements in the metasediments are as: U (0.4 - 4.8 ppm, 2.39), Th (0.9 - 25.60 ppm, 10.38), Ni (0.1 - 17.70 ppm, 7.40), Sr (37 - 124 ppm, 81.75), Ba (55 - 1228 ppm, 620.31), Sn (1.4 - 4.8), Be (1 - 11 ppm, 3.31), Li (2.6 - 48.50 ppm, 23.28), Rb (32 - 376.3 ppm, 140.16), Ta (0.5 - 65.20 ppm, 12.48), Nb (7.76 - 47.5 ppm, 23.82) and Cs (0.8 - 33.30 ppm, 8.33). REE patterns of all samples are parallel to sub-parallel, LREE-enriched, with weak to distinct negative Eu anomalies and weakly fractionated HREE segments. Source rocks of the metsediments underwent low to moderate chemical weathering and metasediments are derivatives of felsic igneous sources.
