Late Cretaeous Magmatism In The Central Gangdese Arc,Tibet,and Its Implication For The Growth Of Continental Crust

The formation of andesitic continental crust has been the subject of vigorous discussion in the field of solid earth research.Understanding the generations of intermediate-felsic rocks in continental arc is highly relevant to understanding the continental crust formation.The Gangdese belt,as the product of northward subduction of Neo-Tethyan slab,has geochemical characters similar to juvenile continental crust.It thus provides an ideal laboratory to investigate the growth and evolution of continental crust.While the Late Cretaceous was a key period for crustal growth in the Gangdise arc,the dynamic process and genesis of magmatic rocks during this period are less certain.This study focuses on Late Cretaceous Nyemo Igneous Complex（NIC）in the central gangdese belt,southern Tibet.Detailed field and petrographic observation,zircon geochronology,whole-rock and mineral geochemistry,Sr-Nd-Hf isotopic studies for the NIC are primary presented.After these works,the process of fractional crystallization of the NIC has been identified;The magma mush origin for the high-Sr/Y intermediate-felsic rocks from the NIC and its significance for the formation of continental crust have been revealed.Geodynamic background of the Late Cretaceous arc magmatism in the Gangdise arc has also been discussed in this study.Following results are shown below:（1）In the NIC,gabbros,monzogabbros,monzodiorites,monzonites,monzogranites are exposed sequentially from southeast to northwest.The gradational relationships and petrographic observation suggest that they can be ascribed to a magma fractionation sequenc:olivine→clinopyroxene+orthopyroxene+apatite+Fe-Ti oxides→plagioclase+biotite→K-feldspar→amphibole.From southeast to northwest,the crystallization depths of the NIC decrease from 4.3 to 1.4 kbar,representing a middle to upper crust of Gangdese arc during Late Cretaceous（Paleodepth=16-5 km）.（2）The zircon U-Pb ages（91-89 Ma）of vorious lithologies in the NIC are consistent within error,indicating that the NIC was formed during the Late Cretaceous magmatic"flare-up".The whole-rock major and trace elements components of varios lithologies from the NIC are highly various（SiO₂=45.4-68.8 wt.%）,and follow a calc-alkaline trend.A/CNK values of the NIC gradually increase from 0.71 to 1.08.The whole-rock geochemical compositions are dominantly controlled by the cumulative minerals.The identical Sr-Nd-Hf isotopic compositions((⁸⁷Sr/⁸⁶Sr)_i=0.70399-0.70480,ε_Nd（t）=+2.3 to+4.7,ε_Hf（t）=+8.2 to+15.4)is indicative of negligible crustal contamination.The parental magma of the NIC was sourced from partial melting of a mantle wedge metasomatized by subduction fluids/melts.The present of plagioclase and biotite without amphibole at early stage of differentiation indicate that the parental melt of the NIC is water-unsaturated.The metaluminous to weak-peraluminous intermediate-felsic rocks from the NIC evolve similar to the array of natural continental arc magmas.Therefore,a key observation for this study is that fractional crystallization of low-H₂O magma with plagioclase and biotite fractionations may be essential for the continental crust formation.（3）The monzodiorites and monzogranites from the NIC have high Sr（460-1026ppm）,low Y（6.63-25.4 ppm）and Yb（0.53-1.68 ppm）contens,and thus high Sr/Y ratios（16-140）.Integrating the petrography,mineral chemistry,and whole-rock geochemistry,this study believes that the magma source of the high-Sr/Y intermediate-felsic rocks from the NIC would not necessarily relate to garnet±hornblende residues and high-pressure fractional crystallization.Instead,these rocks are magma mush built by plagioclase+interstitial metls and the plagioclase cumulate dominate their signatures of high-Sr/Y.The geochemical similarity between high-Sr/Y intermediate-felsic rocks from the NIC and Archean TTG imply that parts of TTG are magma mush as well,that is of significance for understanding the formation and evolution of Earth’s early continental crust.（4）The magmatic“flare-up”event during Late Cretaceous is probably triggered by the Neo-Tethyan slab roll-back.The various compositions of the arc magma along the Gangdese arc probably were caused by spatial heterogeneity of the age and serpentinization degree of the subducted Neo-Tethys oceanic lithosphere.