Spatial-Temporal Pattern, Petrogenesis And Tectonic Implications Of The Triassic Granitoids From The Western Kunlun Orogen, Northwestern China

Granite is a major component of Earth's continental crust,and the study of petrogenesis of granite is an important issue in geology.Granitoid plutons are widely exposed in the west Kunlun orogen,the northwest edge of the Tibetan Plateau,and their petrogenesis are related to the evolution of the Paleo-Tethys Ocean.In this PHD thesis,a comprehensive study of the Triassic granitoids in weat Kunlun orogen was carried out in field geology,petrology,mineralogy,mineral chemistry,geochronology,elemental and isotopic geochmistry.The purpose of this work is to clarify the rock types and spatio-temporal distribution of the Triassis granitoids in the west Kunlun orogen,and to explore the sources nature,petrogenetic mechanism and tectonic setting of these granitoids.The major conculusions of this thesis are as follow:1.Accurate zircon U-Pb dating re-determines the Sanshiliyingfang-Dahongliutan magmatic belt,which was previously considered to belong to the Jurassic to the Late Triassic.Moreover,the ages of the three granites in the western section of the orogen were determined as the Late Triassic.This indicates that the Triassic granites were widely distributed in the western kunlun orogen,weste to the Uzbel pass,and east to the Qitai pass,runs through the entire Western Kunlun orogen.2.Yuqikapa pluton?²43 Ma?mainly consists of monzonite,and from the east to the west of the pluton the SiO₂ content?from 67.11 wt%to 77.8 wt%?and the degree of differentiation increased.The samples are mainly weak peraluminous?A/CNK=0.98¹.09?,with typical characteristics of I-type granite.The major and trace elements of Yuqikapa granitiod indicate the fractionation of K-feldspar,plagioclase,allanite,apatite,and zircon durring the fomation of the pluton.The Yuqikapa pluton have the same Sr-Nd-Hf isotopic compositions as the Middle-Late Triassic I-type granties in the middle and eastern sections of the orogen,indicating that they have the same source and genesis.3.The Triassic I-type granites widely exposed in the west Kunlun orogen,and they are mainly distributed along the Kangxiwa suture.This thesis investigated the three plutons in the eastern section of the orogen,namely the QTDB,SSL and SSLW,and the two plutons in the western section of the orogen,namely QTD and KZLJK.These plutons usually have mafic magmatic enclaves?MMEs?,except for the SSLW pluton.The MMEs have identical zircon U-Pb age to their host rock,the zircon U-Pb ages of the five pluton are concentrated in 222²13 Ma.The host rocks of this pluton are mainly quartz diorite,granodiorite and monzogranite,with SiO₂ of 60.87⁶8.97 wt%,and the samples are mainly metaluminous and weak peraluminous,and belong to high-K calc-alkaline and Shoshonite series,and contain hornblende,implying they are I-type granite.The MMEs of these pluton arte mainly consists of diorite and quartz diorite,with SiO₂of 51.03⁶1.73 wt%and Mg#of 41.8⁵6.7,and the minerals in MMEs are mainly hornblende and biotite.The samples of MMEs usually show enriched MREE and HREE and depleted LREE with strong negative Eu anomalies.Electron microprobe and La-ICP-MS analyses of its main minerals revealed that the REE pattern of MMEs are mainly affected by hornblende,as hornblende contains high REE.The MMEs usually have identical isotopic compositions with their host rocks.The MMEs of the these plutons were the crystal accumulations of the host granitoid magma,and assimilation and contamination changes the composition of magmas at the top of the chamber where the mineral of the MMEs crystaled.Elemental and isotopic data indicate that the Triassic I-type granites are partial melting of the Paleo-Tethys ocean curst and continental materials during continental collision.And Triassic granites in the middle section of the western kunlun orogen are older than those in the estern scetion of the orogen,suggesting that the collisions in the middle section of of the orogen was earlier than that in the eastern section of the orogen.4.Two Late Triassic S-type granites?²10Ma?were discovered in the Western Kunlun Orogen?DHLT and MJX pluton?,the zircon U-Pb age of them are 210Ma and209 Ma respectively.All samples of them are strong peraluminous?A/CNK>1.1?,contain a lots of primative muscovite and belongs to the Shoshonite series,implying they are S-type granite.Both of them have advanced isotope composition.DHLT two-mica granite show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7165⁰.7167,?Nd?t?of-10.4^-10.1,and?Hf?t??in-situ zircon?of-14.01^-9.02;and the MJX two-mica granite have initial⁸⁷Sr/⁸⁶Sr ratios of 0.7118⁰.7162,?Nd?t?of-10.2^-9.63,and?Hf?t??in-situ zircon?of-16.6^-0.6.Both of them contain lots of inherited zircons.The U-Pb age of inherited zircons in the DHLT two-mica granite are 231²485 Ma,and the U-Pb age of inherited zircons in the MJX two-mica granite are 385³200 Ma.The two plutons have low HREE and Y,showing the fractionation between LREE and HREE,implying the magma come from the source that had garnet in its melting residue.Elemental and isotopic data indicate that the DHLT and MJX two-mica granites are partial melting of sedimentary rocks during continental collision.5.Two Late Triassic high-Mg diorite rocks were discovered in the east part of Western Kunlun Orogen,namely the AKSY and the FLYT pluton.The zircon U-Pb age of the two plutons are 217 Ma and 214 Ma repectively,and the Mg#of them are59.6⁶0.3 and 50.2⁵1.5 respectively.AKSY high-Mg diorite show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7073⁰.7074,?Nd?t?of-1.7^-2.2,and?Hf?t??in-situ zircon?of 4.3⁶.9.Compare with AKSY,the FLYT quartz diorite have more advanced isotope composition((⁸⁷Sr/⁸⁶Sr)_i=0.7079⁰.7080,?Nd?t?=-5.4,?Hf?t?=-2.1^-4.6).Elemental and isotopic data indicate that the AKSY diorite and the FLYT quartz diorite both were derived from partial melting of partial melting of mantle wedge peridotite,fluxed by fluids or melts liberated from the subducting slab,and the differences of the degree of mantle enrichment and partial melting,and the depth of the partial melting result in different elemental and isotopic geochemical characteristics of them.