Generation Of Post-collisional Granitoids In The Qinling-tongbai Orogen And Crust-mantle Magma Interaction

Granitoid is the most important constituent of continental crust,determining its genesis hold the key to depicter the formation and evolution of the continental crust and crust-mantle interaction through the Earth's history.The orogenic belt is one of the most important sites where a variety of granitoid are developed and the mass and energy exchange between the crust and mantle have commonly occurred.Exploring the petrogenesis of granitoid in the orogen,along with their related intermediate-mafic rocks,can help us not only to probe the nature and lithological configuration of the deep crust but also to retrieve the crust-mantle magma interaction during the magmatism.The Qinling-Tongbai contains a great amount of post-collisional granitic intrusions,some of which contain abundant MMEs and are associated with intermediate to mafic stocks.It thus provides us good natural samples to investigate these issues and to advance our understanding of granitoid petrogenesis.In this dissertation,we present an integrated study of petrology,mineral chemistry,whole-rock geochemistry,in-situ zircon and titanite U-Pb dating,zircon Hf-O isotopes,as well as titanite trace elements and Nd isotope compositions for the late Mesozoic post-collisional Mangling complex and Liangwan pluton in the North Qinling terrane of the Qinling-Tongbai orogen.The major achievements are summarized below:(1)The Mangling complex is composed mainly of monzogranites with some mafic microgranular enclaves(MMEs)and subordinate diorites.Zircon U-Pb dating yields identical crystallization ages of 141-143 Ma for the diorites and monzogranites,suggesting that they are formed simultaneously and belonging to post-collisional magmatism.The Mangling diorites have relatively low Si O2 but high Mg O,Cr,and Ni contents and Mg#values.They are characterized by enrichment of large ion lithophile elements(LILEs)but depletion of high field-strength elements(HFSEs).They show initial Sr isotopic(ISr)of 0.706491-0.706622,?Nd(t)of-10.82 to-10.38,and zircon?_Hf(t)of-10.8 to-7.9 and?¹⁸O values of 6.48-7.18‰.These geochemical features indicate that the diorites were derived from an enriched lithospheric mantle source.In contrast,the Mangling monzogranites have high silica and low Mg#values.They exhibit more variable isotopic compositions:Isr=0.707191-0.708310,?Nd(t)=-13.74 to-8.10,and zircon?_Hf(t)=-17.2 to-7.0 and?¹⁸O=5.83-7.08‰.Some inherited zircon cores have been observed in the monzogranite samples with U-Pb ages of 1773-1791 Ma.These inherited cores have?¹⁸O values of 5.42-5.69‰and?Hf(t)values of-7.8 to-5.9.Their U-Pb ages and Hf isotope compositions are similar to those of the Xiong'er Group in the southern margin of the North China Block,which have underthrusted the juvenile(in terms of isotopes)Kuanping unit where the Mangling complex has been emplaced.Isotopic modeling shows that the monzogranites were likely produced by fusion of the Kuanping unit and Xiong'er Group,with the involvement of enriched mantle-derived materials.The crustal rocks as well as the enriched sub-lithospheric mantle were both contributed to the Mangling complex,and some of the crustal components show more juvenile isotopic fingerprints.Therefore,it is not mandatory to involve a depleted juvenile mantle-derived component to recognize mantle-crust interaction during granitoid generation,especially in which the preceded juvenile arcs were superimposed on the ancient crust and/or enriched mantle.The‘inverse'isotopic compositions in crust and mantle components imply that a detailed petrochemical and isotopic study should be conducted to address the issue of crust-mantle interaction during granitoid generation.(2)The Liangwan pluton is composed of monzogranites and abundant MMEs enclosed within it.Zircon and titanite U-Pb dating yielded identical ages of ca.130Ma for the host monzogranites and MMEs,suggesting they were coevally formed and belonging to post-collisional magmatism.The MMEs have chilled margins and display fine-grained igneous textures.They contain acicular apatite and amphibole and exhibit none signature of deformations.Petrographic observations as well as titanite and amphibole compositions reveal that the MMEs represent magmatic globules quenched within the host granitic magmas.There were only minor materials exchange between the MMEs and the host monzogranites.The MMEs have relatively low silica but high K2O contents and Mg#values,compared to the host granites.They are characterized by enrichment of large ion lithophile elements but depletion of high field strength elements.The MMEs exhibit enriched Sr-Nd-Hf and normal mantle-like zircon isotopic compositions,with whole-rock(⁸⁶Sr/⁸⁷Sr)i ratios of 0.707290 to0.707314,?_Nd(t)values of-14.53 to-14.01,and?_Hf(t)values of-20.28 to-17.72.Their zircon?_Hf(t)values are of-18.5±0.3,zircon?¹⁸O values of 5.65±0.15‰,and titanite?Nd(t)values of-14.38±0.15.These features are distinct from the ambient Erlangping juvenile arc crust but similar to those of the post-collisional I-type granites which were sourced from the subducted South China Block(SCB).Accordingly,the MMEs could be stemmed from an enriched mantle that metasomatized by the subducted SCB.By comparison,the host monzogranites have relatively high Si O2 and K2O,but low Mg#values.They show similar Sr-Nd-Hf-O isotopic signatures to the MMEs.The monzogranites cannot be produced by the differentiation of mafic magmas counterparts of the MMEs.Taking previously geophysical observations into account,the monzogranites are likely formed by fusion of the subducted SCB,which might have been located underneath the Erlangping unit by the Triassic continental subduction.The Liangwan pluton with high K2O content and enriched isotopic compositions are distinct from those of the Paleozoic rocks in the Erlangping unit,suggesting that the basement of the Erlangping paleo-oceanic arc might have been replaced before the late Mesozoic.The comparable Sr-Nd-Hf-O isotopic composition of the MMEs and host monzogranites could inherit from the subducted SCB.As such,it is not a mandate to envisage diffusive equilibrium during the magma mixing for the similar isotopic composition between the MMEs and the host rocks.