Formation Mechanism And Tectonic Implications Of The Metamorphic Rocks In Nimu, Tibet

Metamorphic rocks are widely distributed in Qinghai-Tibet Plateau, but these rocks have not been studied until recently. The mineral chemistry and geochemistry as well as geochronology of such metamorphic rocks in Nimu, exposed on the north side of Yarlung suture zone, southern Tibet, are studied to help understanding the formation and evolution of these metamorphic rocks.To synthesize the studies of both field and mineralogy and texture, the metamorphic rocks are mainly composed of garnet hornfels and granofels. Our studies show that the rocks have metamorphosed to pyroxene-hornfels facies. Garnet porphyroblasts show growth zoning. Amphiboles are calcic amphiboles. Most of the biotites are ferrobiotites and siderophyllites, while feldspars consist of oligoclases and andesines, and minor K-feldspar. Calculations based on garnet-biotite thermometer, garnet-biotite-plagioclase-quartz geobarometry and plagioclase- hornblende thermobarometry suggest metamorphic temperature of 619~661℃and 695~702℃, and pressure of 1.86~1.94 kbar and 3.69~4.56 kbar for garnet hornfels and granofels. Results of petrologic and geochemical studies, indicate that these rocks underwent middle-high temperature contact metamorphism.Geochemistry characteristics of metamorphic rocks in Nimu show that the garnet hornfels are parametamorphic rocks, with the protoliths being the Fe-shale or shale, and the granofels are orthometamorphic rocks. The orthometamorphic rocks are enriched in LREE relative to HREE, with slightly positive or negative Eu anomalies. They are all enriched in large ion lithophile elements (LILE) relative to high field strength elements (HFSE), depleted Ti, P, Nb and Ta. Because of the later metamorphism, the contents of large ion lithophile elements (LILE) such as K, Rb, Ba etc., in orthometamorphic rocks are very unstable, whereas the high field strength elements (HFSE) are more stable. The orthometamorphic rocks show very similar geochemistry to the typical island arc volcanic rocks, such as Yeba volcanic rocks.Three groups ages are obtained by U-Pb zircon LA-ICP-MS technique from Nimu parametamorphic rocks, in which 163±3~203±3Ma, 134±2~157±2Ma may be of inherited magmatic origin while the latest group (92.1Ma) may represent the time when the protoliths stop accepting sediments. We infer metamorphism would not happened earlier than the minimum deposition age of the protolith.Based on this study, the protoliths were considered to be Yeba volcanic rocks and their upper sedimentary rocks developed in the southern margin of the Gangdese. With the large continental margin magmatic activities of south Gangdese, the protoliths underwent middle-high temperature contact metamorphism.