Petrogenesis of meta-sedimentary rocks in the deep crust of the eastern Gangdese arc

The production of new crustal material at magmatic arcs is dominated by juvenile continental crust growth through the emplacement of voluminous mantle-derived magmatic rocks. There is evidence, however, that such settings can also accommodate the burial of supracrustal material to mid/lower crustal depths resulting in the production of the new crust through metamorphism and anatexis. Investigation of migmatitic garnet- and sillimanite-bearing schists and paragneisses demonstrate that rocks exposed within the exhumed, former middle to lower crust of eastern Gangdese arc, southern Tibet, underwent sillimanite-grade metamorphism and partial melting. Monazite in the studies rocks yield 208Pb/232Th ages of ca. 72–57 Ma, which overlaps with 206Pb/238U dates from zircon rim overgrowths of ca. 73–61 Ma. Inherited zircon cores in paragneisses and adjacent low-metamorphic grade Carboniferous sedimentary rocks of the Lhasa terrane yield similar UPb dates, with main populations at 1220–1070 Ma, 570–470 Ma, and 370–320 Ma. When combined with previously published studies, these new data demonstrate that meta-sedimentary rocks are common in the middle/lower crust of the eastern Gangdese arc and were mainly derived from Carboniferous strata of the Lhasa terrane. The Carboniferous protolith rocks were buried to deep crustal levels via thrusting during crustal shortening during Late Cretaceous (ca. 90–70 Ma) flat subduction of the Neo-Tethyan slab. Subsequent (ca. 70–60 Ma) mantle-derived magmatism induced by slab rollback, likely facilitated metamorphism and anatexis of the sedimentary rocks in the arc. This study provides important new information on the origin of the meta-sedimentary rocks and on how they affected the evolution of the eastern Gangdese arc during the Late Cretaceous to Early Cenozoic.