Petrogenesis And Geodynamic Implications Of Neoproterozoic Granitoids In The Western Yangtze Block,South China

As a significant part of the South China,the western Yangtze Block preserves voluminous granitic rocks and associated mafic-ultramafic rocks.These igneous rocks were considered as the magmatic response to the assembly and breakup of Rodinia supercontinent during the Late Mesoproterozoic to Neoproterozoic,and recorded the important information about the nature of mantle source,crustal growth and reworking as well as crust-mantle interaction.Therefore,they are ideal media to probe the evolution of Rodinia supercontinent.The researches on the petrogenesis of different kinds of granitoids is significant for understanding the crustal pressure-temperature(P-T)condition and crust-mantle interaction.Although the nature of mantle source and geodynamic implication of Neoproterozoic mafic rocks have been systematically studied in the western Yangtze Block,the systematic studies on different kinds of Neoproterozoic granitoids are unsubstantial.Previous studies have suggested that partial melting of Neoproterozoic subducted oceanic slab,metasomatized mantle,juvenile mafic crust,and thickened lower crust have been presented in the western Yangtze Block,whether there is mature crust-derived magma?Based on subducting setting,the subducted slab fluids-and slab melts-related mantle metasomatism have been reported,whether there exists the subducting sediment melts-related mantle metasomatism?Furthermore,how is the subduction-related tectonic transition process in the western Yangtze Block during the Neoproterozoic?Whether there are co-evolution relationships between subduction processes and mantle metasomatism or crustal growth and reworking processes?In this dissertation,we present the integrated data of petrography,LA-ICP-MS zircon U-Pb ages,whole-rock major and trace elements,whole-rock Sr-Nd isotopes,and in-situ zircon Lu-Hf isotopes for the Neoproterozoic Shuilu high-Mg^#diorites,Kuanyu and Cida peraluminous granites,Dalu I-type granodiorites-granites,and Panzhihua-Yanbian gabbro-diorites–adakitic granites–A-type granites association in the western Yangtze Block,South China.Based on the previous results on regional geology and experimental petrology,we conduct the detailed researches on different types of Neoproterozoic granitoids and rocks association,in order to reveal the systematic magmatic response of different depth(metasomatized mantle source-juvenile mafic low crust source-mature continental crust source)during the Neoproterozoic,and present the subduction fluids-and sediment melts-related mantle metasomatism and mature continetal crusal magmatism.We also provide further constraints on the Neoproterozoic subduction setting and subduction-related tectonic transition.The predominant opinions are presented as follow:1.Neoproterozoic subduction fluids-and sediment melts-related mantle metasomatism in the western Yangtze Block,South China:Constraints from the ca.850-835 Ma Shuilu high-Mg^#dioritesThe voluminous Neoproterozoic subduction fluids-and slab melts-related mantle metasomatism in the western Yangtze Block have been reported,the subducting sediment melts-related mantle metasomatism has rarely been studied in detail.High-Mg^#diorites can provide significant insights on the mantle metasomatism under the subduction zone.Here we investigate the genesis of Neoproterozoic Shuilu high-Mg^#diorites in the Miyi region along the western Yangtze Block,to evaluate the role of subduction-related fluids and sediment melts acting on the mantle sources during the subduction process.Zircon U-Pb geochronology display that the Shuilu high-Mg^#diorites were formed at ca.850-835 Ma.They are metaluminous and calc-alkaline rocks,and characterized by moderate Si O₂(57.08–61.12 wt.%),high Mg O(3.36–4.30 wt.%)and Mg^#values(56–60).The relatively low initial ⁸⁷Sr/⁸⁶Sr ratios(0.703406–0.704157),highly positive whole-rock?Nd(t)(+3.3?+4.3)and zircon?Hf(t)values(+8.43?+13.6)imply that they were predominantly sourced from depleted lithospheric mantle.These high-Mg^#diorites show the enrichment of light rare earth elements and large ion lithophile elements as well as depletion of high field strength elements,resembling an typical arc magma affinity.Considering the insignificant crustal continuation and slightly Nd-Hf isotopic decoupling,the highly variable Ba,Rb/Y,Th/Ce,Th/Sm,Ba/La,and Th/Yb values indicate a significant incorporation of subduction-related fluids and sediment-derived melts into the primary mantle source prior to its partial melting.We therefore propose that the ca.850-835 Ma Shuilu high-Mg^#diorites were formed by the partial melting of metasomatized mantle source enriched by subduction fluids and sediment melts.Our new data,in conjunction with numerous studies of Neoproterozoic metasomatized mantle magmatism,suggest that the Neoproterozoic mantle sources beneath the western Yangtze Block were gradually metasomatized by the subduction-related compositions involvement of slab fluids,sediment melts,and oceanic slab melts during subduction process.2.Neoproterozoic disequilibrium melting of mature continental crust in the western Yangtze Block,South China:Insights from the ca.840-835 Ma Kuanyu and Cida peraluminous granitesSignificant widespread melting of the metasomatized mantle and juvenile mafic lower crust is known to have occurred along the western margin of the Yangtze Block during the Neoproterozoic,but melting of the mature continental crust remains poorly understood.Peraluminous granites can provide vital insights on the reworking of mature continental crustal materials.We therefore present zircon U-Pb-Hf isotopic,whole-rock geochemical,and Sr-Nd isotopic data from the Neoproterozoic peraluminous granites in the western Yangtze Block,in order to decipher their petrogenesis and provide significant insights into the mature continental crust-derived magmatism.Zircon U-Pb dating results display that the Kuanyu and Cida peraluminous granites were formed at ca.840-835 Ma.These peraluminous granites have high Si O₂(66.88–75.56 wt.%),K₂O(4.61–7.29 wt.%)contents,as well as high K₂O/Na₂O(1.44–3.25)and A/CNK(1.04–1.18)ratios.They are enriched in Rb,K,Th,U,and Pb,and depleted in Nb,Ta,Ba,Sr,and Ti,indicating a middle-upper crustal affinity.They are also characterized by high(⁸⁷Sr/⁸⁶Sr)_i(0.709893–0.721704)and negative?Nd(t)values(-5.1?-2.9),which resemble the isotopic features of an evolved continental crust source.Furthermore,these peraluminous granites possess variable Ca O/Na₂O(0.09–0.65)and Al₂O₃/Ti O₂(25.3–88.4)ratios,moderate Rb/Ba(1.68–3.86)and Rb/Sr(0.32–0.85)ratios,as well as high molar Al₂O₃/(Mg O+Fe OT)(2.04–5.23)and low molar Ca O/(Mg O+Fe OT)(0.15–0.48)ratios,implying that they predominantly originate from heterogeneous metasedimentary sources(metapelites+metagreywackes).Scarce evidence of hybridization processes between crust-and mantle-derived components indicates that their heterogeneous zircon Hf isotopic compositions(?Hf(t)=-7.75?+3.31)may be caused by disequilibrium partial melting of heterogeneous metasedimentary sources.In combination with previously reported results,we suggest that the Kuanyu and Cida peraluminous granites represent disequilibrium melting of mature crustal material in an evolved middle-upper crust source during the early stages of the Neoproterozoic subduction process.The western margin of the Yangtze Block underwent not only melting of the juvenile mafic lower crust but also reworking of the mature continental crust during the Neoproterozoic.3.Neoproterozoic different crust-derived magmatic response in the western Yangtze Block,South China:Evidence from the ca.780 Ma Dalu I-type granodiorites-granites I-type granites can provide important insights into the continental crustal growth and melting processes.We present detailed studies on the Neoproterozoic Dalu I-type granodiorites and granites in the western Yangtze Block,in order to reveal different magmatic response of crustal depth and discuss the geochemical diversity of I-type composite granitic pluton.Zircon U-Pb geochronology show that the Dalu I-type granodiorites-granites were formed at ca.780 Ma.Whole-rock geochemistry display that the Dalu I-type granodiorites are Na-rich,calc-alkaline,metaluminous to slightly peraluminous rocks.They are characterized by moderate Si O₂(60.88–68.07 wt.%)contents and high Na₂O/K₂O(2.27–3.65)ratios as well as various A/CNK(0.94–1.08)ratios.The Dalu I-type granodiorites display positive whole-rock?Nd(t)(+1.1?+2.3)and zircon?Hf(t)(+2.16?+7.39)values,indicating that they were derived from the juvenile mafic lower crust source.The Dalu I-type granites are high-K calc-alkaline,peraluminous rocks.They have negative whole-rock?Nd(t)(-0.8?-0.6)and variable zircon?Hf(t)(-4.65?+5.80)values.Considering the coexist of Dalu I-type granodiorites and granites,the Dalu I-type granites were formed by the partial melting of mature metasediment rocks induced by the newly formed mafic lower crust-derived melts.Therefore,the Dalu I-type granodiorites-granites are the magmatic response from different crustal levels,which were induced by the heat anomaly due to the upwelling of asthenosphere mantle in the subduction-related setting.Furthermore,the geochemical diversity observed in Dalu granitic pluton is controlled not only by the distinct source rocks but also by the different melting temperatures.4.Neoproterozoic tectonic transition from regionally crustal thickening to thinning under the subduction setting in the western Yangtze Block,South China:Evidence from the ca.810-750 Ma gabbro-diorites–adakitic granites–A-type granites associationThe occurrence of A-type granites following adakitic granites can provide unique insights into the tectonic transition from regionally crustal thickening to thinning under the subduction zone.We present a comprehensive study of zircon U-Pb-Hf isotopes and whole-rock geochemistry on the Neoproterozoic gabbro-diorites,adakitic granites,and A-type granites in the Panzhihua-Yanbian region along the western Yangtze Block,South China,in order to evaluate their petrogenesis and geodynamic implications.The Dajianshan gabbro-diorites were formed at ca.810 Ma.These gabbro-diorites are sodic and calc-alkaline rocks with low Si O₂(52.62–53.87 wt.%),medium Mg O(2.67–3.41wt.%),high Fe₂O₃T(7.18–7.49 wt.%)and Ca O(5.68–7.50 wt.%)contents.They display high Th/Zr and Rb/Y ratios but low Nb/Zr and Nb/Y ratios,indicating subduction fluids-related mantle metasomatism.Together with their positive whole-rock?Nd(t)(+1.0?+1.5)and zircon?Hf(t)values(+3.66?+8.18),we suggest that the Dajianshan gabbro-diorites were derived from the partial melting of depleted lithospheric mantle modified by subduction-related fluids.The Dajianshan adakitic granites were formed at ca.800 Ma.They display relatively high Si O₂(74.08–74.82 wt.%),Na₂O(4.76–5.60 wt.%),Sr contents(335–395 ppm),Sr/Y ratios(38.9–54.3),low Y(7.04–9.71 ppm)and Yb contents(0.78–1.08 ppm).Their low Mg O(0.25–0.30 wt.%)contents,Mg^#(36–41)values,Cr(2.94–3.59 ppm)and Ni(1.32–1.55 ppm)contents as well as positive whole-rock?Nd(t)(+0.5?+0.6)and zircon?Hf(t)values(+1.62?+8.07)indicate that they were formed by the partial melting of thickened juvenile mafic lower crust.The Panzhihua A-type granites were formed at ca.750 Ma.They have extremely high Si O₂(76.61–77.14 wt.%)and alkalis(Na₂O+K₂O=8.55–9.69 wt.%)contents as well as high 10000*Ga/Al(2.56–2.80)ratios and differentiation index(95–97).We suggest that these A₂-type granites were mainly generated by the partial melting of felsic crustal rocks under low pressure condition,subsequently underwent the extensive fractionation.Taking into account previous studies from the western Yangtze Block,the ca.810 Ma Dajianshan gabbro-diorites imply that the western Yangtze Block underwent the subduction process during the Neoproterozoic.The widespread metasomatized mantle-derived magma(>810Ma)formed the mafic-ultramafic intrusions and simultaneously thicken the mafic lower crust in the western Yangtze Block.The occurrence from ca.800 Ma Dajianshan adakitic granites to ca.750 Ma Panzhihua A-type granites indicate the geodynamic transition process from regionally crustal thickening to thinning under subduction background.Based on the systematic researches on the four kinds of typical granitoids and rocks association,we have presented the systematic magmatic response of different depth(metasomatized mantle source–juvenile mafic lower crust source–mature continental crust source)in the western Yangtze Block during the Neoproterozoic,and provided a further constraint on the Neoproterozoic subduction setting in the western Yangtze Block.Considering the subduction setting,we proposed that the Neoproterozoic mantle sources beneath the western Yangtze Block were gradually metasomatized by the subduction slab fluids,sediment melts,and oceanic slab melts.Furthermore,we proposed the Neoproterozoic tectonic transition process:the Early-Middle Neoproterozoic metasomatized mantle-derived magmatism formed the mafic-ultramafic intrusions and simultaneously thicken the mafic lower crust in the western Yangtze Block.The occurrence from thickened juvenile mafic lower crust-derived adakitic granites to A-type granites indicate the geodynamic transition from regionally crustal thickening to thinning under subduction background.The regionally crustal thinning stand for the back-arc extension during the waning stage of subduction process.