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P. A. Cawood - One of the best experts on this subject based on the ideXlab platform.
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detrital zircon record and Tectonic Setting
Geology, 2012Co-Authors: P. A. Cawood, Bruno Dhuime, C J HawkesworthAbstract:ABSTRACTDetrital zircon spectra refl ect the Tectonic Setting of the basin in which they are deposited. Convergent plate margins are charac-terized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic Settings contain greater proportions with older ages that refl ect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sedi-ment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the Tectonic Setting in which the sediment was deposited.INTRODUCTION Detrital zircons are a minor constituent of clastic sedimentary rocks, yet their physiochemical resilience and high concentrations of certain key trace elements means that they have become an important phase in sedimentary provenance analysis and in crustal evolution studies (e.g., Cawood et al., 2007b; Hawkesworth et al., 2010). Large numbers of in situ, high precision analyses of both igneous and detrital zircons are now available, and a striking feature of the zircon record is that it clusters into peaks of crystallization ages (Condie et al., 2009). Compilations of crys-tallization ages for detrital and igneous zircons show remarkably similar patterns of peaks and troughs, although with some variation in the rela-tive amplitude of the peaks (Condie et al., 2009). This coincidence sug-gests that the sedimentary record is a valid representation of the magmatic record (Hawkesworth et al., 2010).We establish that detrital zircon spectra have distinctive age distribu-tion patterns that refl ect the Tectonic Setting of the basin in which they are deposited. These patterns are principally controlled by (i) the volumes of magma generated in each Tectonic Setting and their preservation poten-tial, (ii) the ease with which magmatic and detrital zircons of various ages and origins become incorporated into the sedimentary record, and (iii) the record of old zircons incorporated into the sediment. These in turn provide a framework that can be used to constrain the Tectonic Setting of sedimen-tary packages. This approach distinguishes between three Tectonic Settings (i.e., convergent, collisional, and extensional), and it is most sensitive when the depositional age of the sediment investigated is well constrained. Basin Setting will evolve with Tectonic regime; for example, arc-continent or continent-continent collision will result in the evolution of convergent and extensional basins into collisional foreland basins. Hence the three Settings distinguished herein are end-members, and the zircon age patterns associ-ated with each show a spectrum of distributions that merge and overlap rather than defi ne discrete fi elds. Discriminant plots developed for igneous rock geochemistry (e.g., Pearce and Cann, 1973) or sediment framework modes (e.g., Dickinson and Suczek, 1979) often have diffuse boundaries or overlap between fi elds, but remain important approaches in understanding and constraining Tectonic Setting. Equally important, exceptions to simple end-member classifi cations can provide insight into subtleties of Tectonic process, such as outlined below for Avalonia in eastern North America.
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Detrital zircon record and Tectonic Setting
Geology, 2012Co-Authors: P. A. Cawood, C J Hawkesworth, Bruno DhuimeAbstract:Detrital zircon spectra reflect the Tectonic Setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic Settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the Tectonic Setting in which the sediment was deposited.
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Tectonic Setting of the south china block in the early paleozoic resolving intracontinental and ocean closure models from detrital zircon u pb geochronology
Tectonics, 2010Co-Authors: Yuejun Wang, P. A. Cawood, Feifei Zhang, Guowei Zhang, Shiyue Chen, Aimei ZhangAbstract:[1] Zircon U-Pb geochronological data on over 900 zircon grains for Cambrian to Silurian sandstone samples from the South China Block constrain the pre-Devonian Tectonic Setting of, and the interrelationships between, the constituent Cathaysia and Yangtze blocks. Zircons range in age from 3335 to 465 Ma. Analyses from the Cathaysia sandstone samples yield major age clusters at ∼2560, ∼1850, ∼1000, and 890–760 Ma. Zircons from the eastern and central Yangtze sandstone samples show a similar age distribution with clusters at ∼2550, ∼1860, ∼1100, and ∼860–780 Ma. A minor peak at around 1450 Ma is also observed in the Cathaysia and central Yangtze age spectra, and a peak at ∼490 Ma represents magmatic zircons from Middle Ordovician sandstone in the eastern Yangtze and Cathaysia blocks. The Cambrian and Ordovician strata show a transition from a carbonate-dominated succession in the central Yangtze Block, to an interstratified carbonate-siliciclastic succession in the eastern Yangtze Block, to a neritic siliciclastic succession in the Cathaysia Block. Paleocurrent data across this succession consistently indicate directions toward the W-NNW, from the Cathaysia Block to the Yangtze Block. Our data, together with other geological constraints, suggest that the Cathaysia Block constitutes a fragment on the northern margin of east Gondwana and both Cathaysia and east Gondwana constituted the source for the analyzed early Paleozoic samples. The similar age spectra for the Cambrian to Silurian sandstone samples from the Yangtze and Cathaysia blocks argue against the independent development and spatial separation of these blocks in the early Paleozoic but rather suggest that the sandstone units accumulated in an intracontinental basin that spanned both blocks. Subsequent basin inversion and Kwangsian orogenesis possibly at 400–430 Ma also occurred in an intracontinental Setting probably in response to the interaction of the South China Block with the Australian-Indian margin of east Gondwana.
Bruno Dhuime - One of the best experts on this subject based on the ideXlab platform.
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detrital zircon record and Tectonic Setting
Geology, 2012Co-Authors: P. A. Cawood, Bruno Dhuime, C J HawkesworthAbstract:ABSTRACTDetrital zircon spectra refl ect the Tectonic Setting of the basin in which they are deposited. Convergent plate margins are charac-terized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic Settings contain greater proportions with older ages that refl ect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sedi-ment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the Tectonic Setting in which the sediment was deposited.INTRODUCTION Detrital zircons are a minor constituent of clastic sedimentary rocks, yet their physiochemical resilience and high concentrations of certain key trace elements means that they have become an important phase in sedimentary provenance analysis and in crustal evolution studies (e.g., Cawood et al., 2007b; Hawkesworth et al., 2010). Large numbers of in situ, high precision analyses of both igneous and detrital zircons are now available, and a striking feature of the zircon record is that it clusters into peaks of crystallization ages (Condie et al., 2009). Compilations of crys-tallization ages for detrital and igneous zircons show remarkably similar patterns of peaks and troughs, although with some variation in the rela-tive amplitude of the peaks (Condie et al., 2009). This coincidence sug-gests that the sedimentary record is a valid representation of the magmatic record (Hawkesworth et al., 2010).We establish that detrital zircon spectra have distinctive age distribu-tion patterns that refl ect the Tectonic Setting of the basin in which they are deposited. These patterns are principally controlled by (i) the volumes of magma generated in each Tectonic Setting and their preservation poten-tial, (ii) the ease with which magmatic and detrital zircons of various ages and origins become incorporated into the sedimentary record, and (iii) the record of old zircons incorporated into the sediment. These in turn provide a framework that can be used to constrain the Tectonic Setting of sedimen-tary packages. This approach distinguishes between three Tectonic Settings (i.e., convergent, collisional, and extensional), and it is most sensitive when the depositional age of the sediment investigated is well constrained. Basin Setting will evolve with Tectonic regime; for example, arc-continent or continent-continent collision will result in the evolution of convergent and extensional basins into collisional foreland basins. Hence the three Settings distinguished herein are end-members, and the zircon age patterns associ-ated with each show a spectrum of distributions that merge and overlap rather than defi ne discrete fi elds. Discriminant plots developed for igneous rock geochemistry (e.g., Pearce and Cann, 1973) or sediment framework modes (e.g., Dickinson and Suczek, 1979) often have diffuse boundaries or overlap between fi elds, but remain important approaches in understanding and constraining Tectonic Setting. Equally important, exceptions to simple end-member classifi cations can provide insight into subtleties of Tectonic process, such as outlined below for Avalonia in eastern North America.
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Detrital zircon record and Tectonic Setting
Geology, 2012Co-Authors: P. A. Cawood, C J Hawkesworth, Bruno DhuimeAbstract:Detrital zircon spectra reflect the Tectonic Setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic Settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the Tectonic Setting in which the sediment was deposited.
Surendra P Verma - One of the best experts on this subject based on the ideXlab platform.
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critical evaluation of six Tectonic Setting discrimination diagrams using geochemical data of neogene sediments from known Tectonic Settings
Sedimentary Geology, 2005Co-Authors: John S Armstrongaltrin, Surendra P VermaAbstract:Abstract An attempt is made to evaluate 6 Tectonic Setting discrimination diagrams (1 discriminant function and 5 bivariate diagrams) frequently used by many researchers. For this purpose, an extensive database was established for major element geochemistry derived from Miocene to Recent sand and sandstone (medium to fine-grained) samples collected from a variety of Tectonic Settings including (1) passive margin (PM) Setting, (2) active continental margin (ACM) Setting, and (3) oceanic island arc (OIA) Setting. Our results suggest that the discrimination fields proposed to infer Tectonic Settings for six commonly used discrimination diagrams do not work properly for the analyzed Miocene to Recent sediments. The % success for these diagrams varies from 0% to about 62%. We therefore recommend that these diagrams be used with prudence.
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geochemistry of sandstones from the upper miocene kudankulam formation southern india implications for provenance weathering and Tectonic Setting
Journal of Sedimentary Research, 2004Co-Authors: John S Armstrongaltrin, Surendra P Verma, S RamasamyAbstract:ABSTRACT Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, Tectonic Setting, and weathering conditions. All sandstone samples are highly enriched in quartz (Q) but poor in feldspar (F) and lithic fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the Kudankulam sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA values (chemical index of alteration; mean value 44.5) for these sandstones and the A-CN-K diagram suggest their low-weathering nature. Similarly, their Fe2O3* + MgO (mean 2.7), Al2O3/SiO2 ( 0.09), K2O/Na2O ( 2.2) ratios and TiO2 contents ( 0.3) are consistent with a passive-margin Setting. The Eu/Eu* ( 0.5), (La/Lu)cn ( 21), La/Sc ( 5.9), Th/Sc ( 1.9), La/Co ( 5.7), Th/Co ( 1.8), and Cr/Th ( 5.3) ratios support a felsic source for these sandstones. Chondrite-normalized REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly also are attributed to felsic source-rock characteristics for Kudankulam sandstones. Total REE concentrations of these sandstones reflect the variations in their grain-size fractions. The source rocks are probably identified to be Proterozoic gneisses, charnockites, and granites of the Kerala Khondalite Belt, which must have been exposed at least since the late Miocene. Finally, the unusual Ni enrichment in the Kudankulam sandstones, unaccompanied by a similar enrichment in Cr, Co, and V, may be related to either the presence of pyrite in the sandstones or, more likely, the fractionation of garnet from the source rocks during transportation.
John S Armstrongaltrin - One of the best experts on this subject based on the ideXlab platform.
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petrography and geochemistry of the siliciclastic araba formation cambrian east sinai egypt implications for provenance Tectonic Setting and source weathering
Geological Magazine, 2017Co-Authors: Hossam A Tawfik, Ibrahim M Ghandour, Wataru Maejima, John S Armstrongaltrin, Abdelmonem T AbdelhameedAbstract:Combined petrographic and geochemical methods are utilized to investigate the provenance, Tectonic Setting, palaeo-weathering and climatic conditions of the Cambrian Araba clastic sediments of NE Egypt. The ~ 60 m thick Araba Formation consists predominantly of sandstone and mudstone interbedded with conglomerate. Petrographically the Araba sandstones are mostly sub-mature and classified as subarkoses with an average framework composition of Q 80 F 14 L 6 . The framework components are dominated by monocrystalline quartz with subordinate K-feldspar, together with volcanic and granitic rock fragments. XRD analysis demonstrated that clay minerals comprise mixed-layer illite/smectite (I/S), illite and smectite, with minor kaolinite. Diagenetic features of the sandstone include mechanical infiltration of clay, mechanical and chemical compaction, cementation, dissolution and replacement of feldspars by carbonate cements and clays. The modal composition and geochemical parameters (e.g. Cr/V, Y/Ni, Th/Co and Cr/Th ratios) of the sandstones and mudstones indicate that they were derived from felsic source rocks, probably from the crystalline basement of the northern fringe of the Arabian–Nubian Shield. The study reveals a collisional Tectonic Setting for the sediments of the Araba Formation. Palaeo-weathering indices such as the chemical index of alteration (CIA), chemical index of weathering (CIW) and plagioclase index of alteration (PIA) of the clastic sediments suggest that the source area was moderately chemically weathered. On the northern margin of Gondwana, early Palaeozoic weathering occurred under fluctuating climatic conditions.
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geochemistry of sands along the san nicolas and san carlos beaches gulf of california mexico implications for provenance and Tectonic Setting
Turkish Journal of Earth Sciences, 2014Co-Authors: John S Armstrongaltrin, R NagarajanAbstract:The weathering conditions, provenance, and Tectonic Setting of sands from the San Nicolas (SN) and San Carlos (SC) beaches along the Gulf of California, Mexico, have been studied using mineralogy, major element, and trace element data. The compositional similarity among 4 independent groups (each beach area consists of 2 grain-size groups, i.e. medium- and fine-grained sands) was tested statistically by the application of analysis of variance at the 99% confidence level to avoid misinterpretation. The X-ray diffraction and SEM-EDS data revealed that the fine-grained SN sands were abundant in rutile and zircon minerals. The higher SiO2/Al2O3 ratio of the SN sands than the medium- and fine-grained SC sands indicated that the compositional maturity was greatest for the SN sands (Fcalc = 366.756151 and (Fcrit)99% = 5.065158, where Fcalc > (Fcrit)99% indicates that the data populations are significantly different at 99% confidence level). The chemical index of alteration values for the SN (ca. 41-45) and SC (ca. 48-51) sands indicated low to moderate weathering intensity in the source region. The significant enrichment of the low rare earth element and the flat heavy rare earth element patterns of the SN sands indicated that the sources were largely felsic rocks. The low positive Eu anomaly in the SC sands was probably due to the contribution of sediments from intermediate rocks between felsic and mafic compositions. The comparison of rare earth element data of the sands with rocks located relatively close to the study areas revealed that the SN sands received a major contribution from felsic rocks and SC sands from intermediate rocks. The compositional difference between the SN and SC beach areas indicated that longshore currents played a less significant role. Discriminant function-based major element diagrams for the Tectonic discrimination of siliciclastic sediments revealed a rift Setting for the Gulf of California, which is consistent with the general geology of Mexico.
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critical evaluation of six Tectonic Setting discrimination diagrams using geochemical data of neogene sediments from known Tectonic Settings
Sedimentary Geology, 2005Co-Authors: John S Armstrongaltrin, Surendra P VermaAbstract:Abstract An attempt is made to evaluate 6 Tectonic Setting discrimination diagrams (1 discriminant function and 5 bivariate diagrams) frequently used by many researchers. For this purpose, an extensive database was established for major element geochemistry derived from Miocene to Recent sand and sandstone (medium to fine-grained) samples collected from a variety of Tectonic Settings including (1) passive margin (PM) Setting, (2) active continental margin (ACM) Setting, and (3) oceanic island arc (OIA) Setting. Our results suggest that the discrimination fields proposed to infer Tectonic Settings for six commonly used discrimination diagrams do not work properly for the analyzed Miocene to Recent sediments. The % success for these diagrams varies from 0% to about 62%. We therefore recommend that these diagrams be used with prudence.
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geochemistry of sandstones from the upper miocene kudankulam formation southern india implications for provenance weathering and Tectonic Setting
Journal of Sedimentary Research, 2004Co-Authors: John S Armstrongaltrin, Surendra P Verma, S RamasamyAbstract:ABSTRACT Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, Tectonic Setting, and weathering conditions. All sandstone samples are highly enriched in quartz (Q) but poor in feldspar (F) and lithic fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the Kudankulam sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA values (chemical index of alteration; mean value 44.5) for these sandstones and the A-CN-K diagram suggest their low-weathering nature. Similarly, their Fe2O3* + MgO (mean 2.7), Al2O3/SiO2 ( 0.09), K2O/Na2O ( 2.2) ratios and TiO2 contents ( 0.3) are consistent with a passive-margin Setting. The Eu/Eu* ( 0.5), (La/Lu)cn ( 21), La/Sc ( 5.9), Th/Sc ( 1.9), La/Co ( 5.7), Th/Co ( 1.8), and Cr/Th ( 5.3) ratios support a felsic source for these sandstones. Chondrite-normalized REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly also are attributed to felsic source-rock characteristics for Kudankulam sandstones. Total REE concentrations of these sandstones reflect the variations in their grain-size fractions. The source rocks are probably identified to be Proterozoic gneisses, charnockites, and granites of the Kerala Khondalite Belt, which must have been exposed at least since the late Miocene. Finally, the unusual Ni enrichment in the Kudankulam sandstones, unaccompanied by a similar enrichment in Cr, Co, and V, may be related to either the presence of pyrite in the sandstones or, more likely, the fractionation of garnet from the source rocks during transportation.
Yanjing Chen - One of the best experts on this subject based on the ideXlab platform.
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mo deposits in northwest china geology geochemistry geochronology and Tectonic Setting
Ore Geology Reviews, 2017Co-Authors: Yanshuang Wu, Yanjing Chen, Kefa ZhouAbstract:Abstract Northwest China, covering northern Xinjiang, northern Gansu and westernmost Inner Mongolia, mainly includes Junggar Basin and its surrounding mountains such as Chinese Altay, Junggar, Chinese Tianshan and Beishan. It lies at the junction of Siberia, Tarim and Kazakhstan plates, and is a key sector of the Central Asian Orogenic Belt (CAOB), characterized by multistage Phanerozoic continental growth. Herein at least nine Mo-only or Mo-dominated, fourteen Cu-Mo, two W-Mo and one Be-Mo deposits have been discovered. These 27 deposits occur in Altay, West Jungar, West Tianshan, East Tianshan and Beishan areas, and have been formed during accretionary or collisional orogenies. The majority of the deposits are porphyry type, followed by the skarn and quartz vein types. The orebodies occur mainly as veins, lens, pods in the positions from inner intrusions through contact zones to the hostrocks distal to causative intrusions. The host-rocks are variable in lithologies, including granites, porphyries, volcanic breccias and tuffs, and sedimentary rocks. Outward from orebodies to hostrocks, the wallrock alteration is zoning from potassic (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alterations, with skarn specifically occurring in skarn-type systems. Hydrothermal mineralization generally includes four stages, from early to late, represented by (1) potassic feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-polymetallic sulfide stockworks, and (4) quartz ± carbonate ± fluorite veins or veinlets. The ore-forming fluids were initially magmatic in origin and shew high-temperature and high-salinity, containing daughter mineral- and/or CO 2 -bearing fluid inclusions; and eventually evolved to low-temperature, low-pressure, low-salinity and CO 2 -poor meteoric water. The porphyry Mo deposits can be further subdivided into two subtypes, i.e., Dabie- and Endako-types. The Endako-type Mo deposits, e.g., Suyunhe and Hongyuan, together with all the Cu-Mo systems, were formed in the Palaeozoic subduction-related magmatic arcs. The Dabie-type porphyry Mo deposits, represented by giant Donggebi and Baishan, together with the Mo-only, Mo-dominated and W-Mo or Be-Mo deposits were formed in syn- to post-collisional Tectonic Setting, with isotope ages ranging 260–213.2 Ma, with the Kumutage skarn-type Mo system being an exception aged 319 Ma. The Dabie-type porphyry Mo deposits are characterized by the CO 2 -bearing fluid inclusions that cannot be observed in the Endako-type porphyry Mo systems. The Re contents in molybdenites from porphyry and porphyry-skarn Cu-Mo systems are mainly > 100 ppm, suggesting a source significantly contributed by the mantle; whereas the Re contents in molybdenites from the Mo-only or W-Mo deposits are mainly
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besshi type mineral systems in the palaeoproterozoic bryah rift basin capricorn orogen western australia implications for Tectonic Setting and geodynamic evolution
Geoscience frontiers, 2016Co-Authors: Franco Pirajno, Yanjing Chen, Nuo Li, Chao Li, Limin ZhouAbstract:Abstract In this contribution we use VMS mineral systems in the Bryah rift-basin to constrain the Tectonic Setting of the widespread mafic and ultramafic magmatism that characterises the rift-basin in question. Two distinct, but temporally closely associated, lithostratigraphic sequences, Narracoota and Karalundi Formations, are discussed. The Karalundi Formation is the main host of VMS mineral systems in the region. The Karalundi Formation consists of turbiditic and immature clastic sediments, which are locally intercalated with basaltic hyaloclastites, dolerites and banded jaspilites. We propose that the basaltic hyaloclastites, dolerites and clastics and jaspilites rocks, form a distinct unit of the Karalundi Formation, named Noonyereena Member. The VMS mineral systems occur near the north-east trending Jenkin Fault and comprise the giant and world-class DeGrussa and the Red Bore deposits. The nature of these deposits and their intimate association with terrigenous clastic rocks and dominantly marine mafic volcanic and subvolcanic rocks, as well as the common development of peperitic margins, are considered indicative of a Besshi-type environment, similar to that of present-day Gulf of California. Our Re-Os age data from a primary pyrite yielded a mean model age of 2012 ± 48 Ma, which coincides (within error) with recent published Re-Os data (Hawke et al., 2015) and confirms the timing of the proposed geodynamic evolution. We propose a geodynamic model that attempts to explain the presence of the Narracoota and Karalundi Formations as the result of mantle plume activity, which began with early uplift of continental crust with intraplate volcanism, followed by early stages of rifting with the deposition of the Karalundi Formation (and Noonyereena Member), which led to the formation of Besshi-type VMS deposits. With on-going mantle plume activity and early stages of continental separation, an oceanic plateau was formed and is now represented by mafic-ultramafic rocks of the Narracoota Formation.
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geochronology and geochemistry of the dashui adakitic granitoids in the western qinling orogen central china implications for triassic Tectonic Setting
Geological Journal, 2014Co-Authors: Yanjing ChenAbstract:The Dashui porphyry granitoids in the western Qinling Orogen are spatially associated with the Dashui gold deposit, but their ages and Tectonic Setting have not been well defined yet in previous studies. This paper reports the results and understandings obtained from our original study in geochronology and isotope and element geochemistry. SHRIMP zircon U–Pb dating yielded ages of 213.7 ± 2.7 Ma for granitic stock and 215.1 ± 2.5 and 210.2 ± 1.6 Ma for granitic dikes. The granitoids are characterized by high LREE but low HREE and HFSE. They have (87Sr/86Sr)i ratios of 0.706 to 0.709 and eNd(t) values of −5.3 to −7.6. The majority of zircons in the Dashui granitoids have eHf(t) values ranging from −6.2 to 6.7 and TDM2(Hf) ages from 743 to 1464 Ma. The isotope signatures, together with La/Y and Mg# values, show that the magmas are mainly sourced from partial melting of a subducted slab, and possibly assimilated immature sediments during upward migration. Thus, the Dashui granitoids most likely formed in a volcanic arc where supra-subduction zone melts assimilated lower crustal material no later than 215 Ma. Copyright © 2014 John Wiley & Sons, Ltd.
