Paleozoic Granitoids In The Chinese South Tianshan And Its Implications For Geological Evolution Of The Region

The Chinese South Tianshan is located in the southwestern margin of the giganticCentral Asian Orogenic Belt (CAOB). Tectonically, the region is composed of SouthTianshan Collisional Belt and the Northern Margin of Tarim Block. During Paleozoictimes, the Chinese South Tianshan was intensively affected by regionaltectono-thermal events. In this doctoral thesis, I mainly report the U-Pb ages as wellas geology, petrology and geochemistry of some Paleozoic plutons, including Tie’rekepluton, Chuanwulu complex, Halajun granitic intrusions and Boziguo’er pluton.The Tie’reke monzonite pluton was emplaced during late Silurian at~419.9±2.1Ma. Whole-rock geochemical and isotopic data reveal that the magma was deriveddominantly from partial melting of the Paleoproterozoic continental crust with inputfrom juvenile materials derived from a depleted mantle wedge. In addition, rocks ofthis pluton display typical characteristics of continental arc-related igneous rocks,implying a subduction-related tectonic setting.The Chuanwulu complex mainly consists of a gabbro–diorite unit (unit I) and amonzonite–syenite unit (unit II), which are both cut by some granite dykes (unit III).Despite of difference in rock types, the mafic and felsic rocks were likelysimultaneously emplaced, with U-Pb zircion ages of unit I and II of287.8±4.3Ma and286.4±2.5Ma, respectively. The magma mixing process during the formation of thecomplex is proposed based on considerable petrographic and geochemical evidence.Mafic rocks of unit I seems derived from an incompatible element-enrichedlithospheric mantle and a spinel lherzolite source that contains a little amount ofgarnet could account for the REE features, implying that parental magma of maficrocks could be generated at depth between70km and80km. Besides, felsic rocks(units II and III) have a “C-type” adakite-like geochemical signature and arecomparable with those of rocks derived from an ancient garnet-bearing amphibolitefacies lower crust.Halajun granitic intrusions consistently show petrological and geochemical featureswith an A-type affinity. Four intrusions, including Huoshibulake, Tamu, Kezi’ertuoand Halajun II intrusions were focused herein. Those intrusions were emplaced in latest Late Permian at~275Ma. Our elemental and isotopic data suggest that the fourgranitic intrusions were generated by the partial melting of a common Neoproterozoicgabbroic source, probably as a result of the~275Ma underplating of theasthenosphere mantle-derived magmas. Subsequent to the generation of the originalmagma, alkali feldspar, arfvedsonite, biotite, Fe–Ti oxides and zircon seem to havefractionated prior to the final emplacement of the granitic magmas.The Boziguo’er Nb-Ta and REE bearing granitic pluton was emplaced during EarlyPermian at290.1±1.4Ma, based on LA-ICP-MS zircon dating. Geochemical andisotopic results reveal that the original magma was formed by the moderate-degreepartial melting in a previously re-fertilized crustal source, triggered by theunderplating of the asthenosphere-derived magmas. The re-fertilized process, prior tothe partial melting, of the source region was likely related to the supercritical fluidsreleased from the asthenosphere-derived magmas. Besides, the presence of substantialfluorine in granitic melts would not only prolong the duration of the magmaticdifferentiation but can also act as complexing agents with HFSE and REE. Thus, thefluorine in the magmatic system was likely the key factor contributing to the raremetal enrichment.Based upon studies present in this doctoral thesis, in combination with previouslypublished geological data of the region, a new tectonic evolution model for theChinese South Tianshan region is suggested. According to the model, the closure ofthe Paleozoic South Tianshan Ocean was associated with the double subductionduring the Late Ordovician to Middle Devonian, and was related to the northwardsubduction during Late Devonian to Middle Carboniferous. The final closure of theocean is proposed to have occurred in the Late Carboniferous, followed by thecollision of the Tarim and Central Tianshan blocks. Igneous rocks exposed in theSouth Tianshan Collisional Belt, with early to middle Early Permian ages, arebelieved to formed in the post-collisional setting, whereas the generation of Halajungranitic intrusions, as well as ones in the northern and inner parts of Tarim Block, wasgenetically related to the upwelling of the Permian Tarim mantle plume.