A Geochemical Study Of Mesozoic-Cenozoic Granitoids From The Gangdese Orogen,Southern Tibet

The Neo-Tethys oceanic lithosphere has been northwardly subducted beneath the southern Lhasa subbelt in the southern margin of Asia since the Late Triassic,which resulted in the Mesozoic continental arc magmatism and formed a series of magmatic rocks with granitoid as the dominant rock type.With the subduction and clousre of the Neo-Tethys oceanic lithosphere,the Indian continental crust started to subduct at the early Cenozoic,which was acompanied with large scale syn-collisional magmatism.The two-stage magmatism formed the famous Gangdese batholith in the southern Lhasa subbelt,which consists of a large number of granitoids.Therefore,it is an ideal target area for studing granite petrogenesis at convergent plate boundaries.This dissertation has performed an integrated study of petrological,zircon U-Pb chronology,and geochemistry for the Late Cretaceous and Eocene granitoids from the Gangdese batholith,the results indicate that the fomer is the product from reworking of the juvenile Gangdese arc crust,the later is the product of reworking of the juvenile Gangdese arc crust/the sunducted ancient Indian continental crust.Although arc magmatism was the major way for the growth of continental crust since Phanerozoic,and the continental arc and continent are characterized by the widespread occurrence of granitoids,the causal relationship between continental crust growth and continental arc granitic magmatism remains enigmatic.The fractional crystallization of basaltic magmas(with or without crustal contamination)and partial melting of mafic lower crust are two main mechanisms for the production of granitoids in continental arcs.Due to the similarity of major-trace elements and isotope compositional effects during melting and fractionation,whole-rock geochemistry cannot discriminate the two mechanisms effectively.However,considering that the two processes are essentially different,whole-rock geochemistry combined with mineral in-situ micro-zone geochemistry especially zircon U-Pb geochronology,could distinguish the two mechanisms.In this regard,this dissertation has performed a combined study of zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Sr-Nd-Pb isotopes for the Langxian Late Cretaceous granitoids from the Gangdese orogen.The Langxian granitoids exhibit high contents of SiO2(65.76-70.75 wt%)and Na2O+K2O(6.38-8.15 wt%)but low contents of MgO(0.19-0.98 wt%),Fe2O3T(0.88-3.13 wt%),CaO(2.00-3.82 wt%),Ni(<5.8 ppm),and Cr(?10 ppm).They are enriched in large ion lithophile elements,Pb,and light rare earth elements but depleted in high field strength elements.The granitoids have relatively depleted whole-rock Sr-Nd isotope compositions with(87Sr/86Sr)i ratios of 0.7043-0.7048 and ?Nd(t)values of 0.5-2.6,and have relatively low 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios.Laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)and secondary ion mass spectrometry(SIMS)U-Pb dating on syn-magmatic zircons yield ages of 77± 2 to 81± 1 Ma for their emplacement.Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic,respectively.The whole-rock Sr-Nd isotopes of the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith.In addition,both syn-magmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks.All these observations combined with previous reports of the earlier Late Cretaceous mafic lower crustal rocks exist contemporaneous petrographic evidences of partial melting with granitic magmatism indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust.Therefore,isotopically depleted granitoids in the Gangdese orogen were derived from reworking of the juvenile mafic arc crust rather than fractional crystallization of basaltic magmas,these granitoids do not necessarily contribute to the crustal growth,but it is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic crust in the continental arc.Due to the superposition of different stage orogenies and overlapping of rocks with different properties,the granitoids in continental collision zones may have more complex formation mechanism than in continental arcs.They can be formed by either partial melting of obducted continental crust,or partial melting of subducted continental crust,or partial melting of subducted oceanic crust or fractional crystallization of mantle-derived mafic magmas.Whereas the first two scenarios cause crustal reworking,the latter two scenarios lead to crustal growth.On the basis of the depleted radiogenic isotope compositions of the syn-collisional granitoids and the relatively weak crustal recycling rate at collisional orogens,recent studies argue that continental collision orogens are the important sites for continental crust growth.Such a hypothesis needs to be tested regarding the complex origin of the depleted radiogenic isotopes of the syn-collisional granitoids.Here we report an integrated study of in-situ zircon U-Pb ages and Hf isotopes,whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as mineral O isotopes for the Eocene syn-collisional granitoids from the Quxu pluton in the Gangdese orogen,southern Tibet.Combined with the previous data and research results,we have done a further discussion on the genetic mechanism of the syn-collisional granitoids.The results show that these granitoids can be divided into two groups according to their emplacement ages and geochemical features.The early Eocene granitoids were emplaced at 50-48 Ma,and the late Eocene granitoids were emplaced at 42 Ma.The early Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope compositions with(87Sr/86Sr)i?0.7044-0.7048,?Nd(t)=0.6-3.9 and ?Hf(t)=6.5-10.5,and zircon ?Hf(t)values of 1.6-12.1,and comparable to slightly higher zircon ?18O values of 5.28 to 6.26‰relative to normal mantle zircons.These geochemical characteristics are quite similar to those of the Late Cretaceous mafic arc igneous rocks in the Gangdese orogen,indicating that they were derived from partial melting of the juvenile mafic arc crust.In comparison,the late Eocene granitoids have lower MgO,Fe2O3,HREE contents but higher K2O,Rb,Sr,Th,U,Pb contents and Sr/Y,(La/Yb)N ratios.They also exhibit more enriched whole-rock Sr-Nd-Hf isotope compositions with(87Sr/86Sr)i?0.7070-0.7085,?Nd(t)=-5.2 to-3.9 and ?Hf(t)=0.9-2.3,and zircon ?Hf(t)values of-2.8-8.0,and slightly higher zircon ?18O values of 6.25 to 6.68‰ than the early Eocene ones.An integrated interpretation of these geochemical features is that both the ancient Indian continental crust and the juvenile arc crust were partially melted to produce the late Eocene granitoids.In this regard,the compositional evolution of syn-collisional granitoids from the early Eocene to the late Eocene indicates the temporal change of their magma sources from the juvenile arc crust to the mixed source due to subduction of the ancient Indian continent beneath the juvenile Gangdese arc.Therefore,the syn-collisional granitoids are the reworked products of both the juvenile arc crust and the subducted ancient continental crust,and they have no contribution to the crustal growth in the continental collision zone.