Altyn Tagh Tectonic Belt The Western Part Of The Caledonian Yamane's Preliminary Study

At western segment of Altyn Tagh tectonic belt, The eclogites and khondalite series were recognized in Altyn Group (Early Proterozoic?) Which had been thought prviously that it consists of high greenschist facies and lower amphibolitic facies metamorphic rocks. They crop out at the northern slope and southern slope of Altyn Tagh Mountains respectively. The purpose of this thesis is twofold. One is to present the Petrology, geochemistry , geochronologyl, and ductile deformation on eclogites and Khondalite series. On the basis of these, the second objective is to propose that the eclogite and Khondalite series resulted from Caledonian continent-continent collision, and discuss the formation and evolution of Caledonian orogenic root, the regional tectonic significance of eclogite and khondalite series and possible displacement of Altyn Tagh fault.The eclogites at western segment of Altyn Tagh tectonic belt occur as lens or boundins within quratz-feldspathic gniesses characterized by amphibolite-facies parageneses. It mainly experienced the prograde amphibolite-facies stage (P=6.06-8.70kb, t=615-690℃) ,peak eclogite-facies stage (P＞15kb,T=700-850℃), granulitic facies stage (P=11-14kb,T=750℃) and ampbiolite-facies stage (P=6.3-9.5kb,T=619-738℃) during decompression. The trace element, rare earth element and Sm-Nd isotopic data suggest that most of the eclogites have protolith feature resembling T-type (transitional type) MORB (mid-ocean ridge basalt) . The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield an Whole rock-garnet-omphacite isochron of 500±10Ma age.The U-Pb isotopic measurements of zircons show that the four grain populations are near concordant and are well plotted on concordia curve, giving a weighted mean age 503.9±5.3Ma. Two kinds of methods obtain a similar age which reveals the peak metamorphic age of eclogites. Three stages of ductile deformation were recognized in eclogites and terrane related to eclogites . The first stage of deformation is characterized by thrusting from north to south with a deformation mechanism of simple shear, representing the deformation of orogenic root under the condition of eclogite-facies to high pressure granulite facies metamorphism. The second stage of deformation show coaxial deformation mechanism with SN compression and parallel-orogen EW stretching under the condition of middle-high pressure ampbiolite facies metamorphism. The third stage of deformation caused vertical extrusion of the under unit including eclogites and SN extension and thining of upper units.The Khondalite series are characterized by aluminum-rich gneisses(schists) consisting of sillimanite-garnet-biotite-monzonite gneiss, garnet-biotite-monzonite gniess, graphite-sillimanitebiotite schist, and garnet-amphibole two-pyroxene granulites occuring as lenses and layers within gneisses (schists). The petrology and geochemistry indicate that the protoliths of aluminum-rich gneisses(schists) are aluminum-rich pelitic and pelitic arenaceous sedimentary rocks, the protoliths of basic granulites are continental tholeiitic basalts. Therefore, the khondalite series may be produced at continental margin. They had suffered granulitic facies metamorphism with the peak temperatures of 700-850℃and the pressures of 8-12Kb.The U-Pb and Pb-Pb isotopic dating of zircons provided the ages of 447-462Ma representing the ages of peak granulitic metamorphism. The U-Pb dating of detrital zircons from aluminum-rich gneisses yielded older upper intercept ages which reflect the times of older materials derived from sourse rocks of the gneiss protoliths. Three stages of deformation can be observed in khondalite series.The first stage of deformation is characterized by co-axial SN compression and EW stretching under the condition of granulite facies metamorphism ; The second stage of deformation is represented by thrusting from NW to SE. The third stage of deformation may be related to sinistral strikeslipping of main Altyn Tagh fault.The ages of 447-503Ma for eclogite and khondalite series imply the existence of Caledonian orogenic root. The formation and evolution of orogenic root include: a: the subduction, continentcontinent collision and formation of orogenic root related to the formation of eclogite; b: SN compression and EW stretching related to early exhumation of eclogite and formation of khondalite series, and c: the vertical extrusion of orogenic root and simultaneous extension and spreading of upper crust.The tentative comparsion between eclogites and granulite facies rocks at western segment of Altyn Tagh, and North Qaidam Mountains imply that the western segment of Altyn Tagh tectonic belt is part of the Same Caledonian continent-continent collision orogenic belt as the North Qaidam tectonic belt, and is truncated by sinistral strike-slip Altyn Tagh fault. In combination with comparsion between other terraneain Altyn tagh Mountains and Qilian Mountains,the displacement of Altyn Tagh fault is about 400-500km.