Late Cretaceous magmatism and metamorphism records and its tectonic significance of garnet amphibolite from the eastern end of the Gangdese magmatic arc

The Gangdese magmatic arc, located at the south Tibet, was formed during the evolution of the Andean-type orogeny derived from the northward subduction of the Neo-Tethyan oceanic lithosphere and the Himalayan-type orogeny related to India-Asia collision, and therefore representing a natural experimental laboratory for studying the Mesozoic to Cenozoic tectonic evolution of the Tibetan Plateau. In this paper, we report the petrology, phase equilibria modeling and zircon U-Pb geochronology of the garnet amphibolite from the eastern Gangdese magmatic arc. The studied rock is composed of garnet, amphibole, plagioclase, epidote, muscovite and quartz, with minor rutile and ilmenite, showing chemical compositions of typical mafic rocks. Phase equilibria modeling indicates that the garnet amphibolite experienced high-pressure granulite facies metamorphism and significantly partial melting under peak metamorphic conditions of similar to 820 degrees C and similar to 1. 67GPa, corresponding to the formation depth of similar to 55km. The zircons from the garnet amphibolite have inherited magmatic core and metamorphic rim. The magmatic core of zircon has a banded zoning, relatively high HREE content ( with a mean value is 1150 x 10(-6)) and Th/U ratio (0. 51 similar to 0. 95), and yields a protolithic crystallization age of 90. 4 +/- 1. 7Ma ( MSWD = 0. 32). The zircon rim shows no zoning, has relatively low HREE content ( with a mean value is 58. 5 x 10(-6)) and low Th/U ratio (0. 29-0. 40), and yields a metamorphic age of 86. 7 +/- 5. 7Ma ( MSWD = 0. 15). The present study shows that the Late Cretaceous gabbros in the eastern Gangdese arc experienced nearly syn-intrusive high-grade metamorphism and associated partial melting, and that the underplating and accretion of voluminous mantle-derived magma resulted in significantly growth and thickening of juvenile crust.