Petrology, Genesis And Geological Significance Of Late Paleozoic-early Mesozoic Granitoids In East Kunlun Orogen

East Kunlun Orogen, situated in the northern margin of Qinghai-Tibet plateau and in thewestern section of Central Orogenic System, is an important part of Central Orogenic System.There have outcropped large quantities of granitoids in the East Kunlun Orogen, with theintruding and emplacement of which the Proto-and Paleo-Tethys oceanic crust subduction,continent-continent (arc) collision and post-collision events have been recorded in the EastKunlun region. Especially the Late Paleozoic-Early Mesozoic granitoids are major parts ofmagmatic rocks in the East Kunlun Orogen. Therefore, research for the granitoids in thecomposite East Kunlun Orogen has great scientific significance to reveal its tectonic evolutionand geodynamic background.Through detailed studies on the petrology, mineralogy, isotope chronology andgeochemistry of the Late Paleozoic-Early Mesozoic granitoids, the following cognitions areacquired in the thesis.1. Late Permian-early Middle Triassic continental marginal arc granitoids includeXiangjia’nanshan and Halagatu granitic intrusion. Xiangjia’nanshan granitic intrusionconsists of diorite, granodiorite, monzogranite, porphyritic monzogranite and moyite, andHalagatu granitic intrusion consists of tonalite, granodiorite, monzogranite, porphyriticmonzogranite and moyite. Both two intrusions are metaluminous-weakly peraluminousrocks, and belong to medium-K to high-K calc-alkaline series. The trace elementgeochemistries are characterized evidently by positively Rb, Th, Ba, U, La, etc. and negativeanomaly of high strength field elements Nb, Ta, Ti, etc., and show the characteristics of arcmagmatic rocks. With different regional tectonic stresses under subduction setting, the twointrusions were products of different extension degrees, different mantle-derived magmaunderplating positions, different magma mixing degrees and different magmatic evolution.The crystallization ages of the Xiangjia’nanshan and Halagatu intrusions are251.0~246.4Maand254.9~247Ma, respectively.2. Late Permian-early Middle Triassic Xiangjia’nanshan and Halagatu granitic intrusionshave developed large quantities of mafic microgranular enclaves, and include diorites in them.There exist imbalance minerals in the enclaves. Major elements and trace elements of theenclaves have wide ranges. MgO, Cr, Ni contents and Nb/Ta ratio of the enclaves are high.The enclaves are rich in LILE and depleted in HFSE. The trends of REE patterns and traceelement spider diagrams of some enclaves are similar to that of the host rocks. The enclaveswere produced by the mixing and mingling of basic magma and host rock magma, of whichthe basic magma was formed by partial melting of fluid metasomatic mantle. 3. Late Triassic Helegangxilikete pluton is mainly composed of granodiorite, in whichmafic microgranular enclaves are well developed. The pluton is metaluminous rock andbelongs to high-K calc-alkaline series. The pluton has high Sr/Y, La/Yb ratios and high Na2O,Sr contents, and is depleted in HREE relative to LREE and MREE and without Eu anomaly,and shows the characteristics of adakitic rock. Helegangxilikete pluton was the productformed by partial melting of high-mature lower crust, the partial melting was resulted fromthe underplating of basic magma which was caused by the delamination due to slab break-off,in which process the basic magma was formed by partial melting of enriched mantle. Thecrystallization ages of the host rock and the enclave are225±5Ma and224.9±4.1Ma,respectively.4. Late Triassic Kekeealong quartz diorite pluton has high Na2O, Sr contents (avg. Srcontent>400×10－6) and Sr/Y, La/Yb ratios, and is rich in LREE, depleted in HREE andwithout Eu anomaly, and has low Yb, Y contents and high Mg#value (avg.49) and Cr, Nicontents, which are similar to the characteristics of high magnesia adakitic rock. Thegeochemical and geochronological characteristics suggest that the pluton was generated in acondition of the appearance of primary thicken lower crust resulting from magmaunderplating under subduction setting, and was the product of the metasomatism of themagma formed by dehydration melting and its superjacent mantle, in which process thedehydration melting was caused by the primary thicken lower crust delaminated into theunderlying mantle. The crystallization age of the Kekeealong pluton is218.3±1.4Ma.5. Mineralogy researches on the rock forming minerals biotite, amphibole and plagioclaseselected from the Late Paleozoic-Early Mesozoic granitoids of eastern section of EastKunlun Orogen show that: The tested biotites in all rock types are magnesian, except for thebiotite from granodiorite of Xiangjia’nanshan intrusion is ferruginous, and the biotites havelow AlⅥ,(Fe3+/(Fe3++Fe2+)) and high (Mg/(Mg+Fe2++Mn)). The tested amphiboles aremagnesian types which have low Ti content and high MF value and high (FeO+Fe2O3),(Ca+AlⅣ) contents. The tested plagioclases from different rock types have wide compositionranges, and are oligoclase to andesine.6. Combining with previous researches, the formation of the Late Paleozoic-EarlyMesozoic granitoids of eastern section of East Kunlun Orogen is divided into three stages:Late Permian–early Middle Triassic continent marginal arc stage (260~237Ma), MiddleTriassic syn-collision stage (237~225Ma), Late Triassic post-collision stage (225~205Ma).