Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

Abstract

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tec- tonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be de- scribed as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar + plagioclase + quartz + kyanite + garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphismforthemaficHPgranuliteat434±3to435±3Maandpeakmetamorphismandpartialmelting for thefelsicHP granulite at 433 ± 5 to 438 ± 4 Ma, which overlaps the ages of UHP metamorphismfor adjacent eclogite (430–446 Ma). 40 Ar/ 39 Ar amphibole ages of 423 to 432 Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamor- phism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsicHPgranulites(leucosome) formedfromanadakiticmeltderivedfrom partial melting ofmafic HPgranulite intheoverridingplateinarelativelyhighergeothermalgradient(15–18 °C/km),leavinggarnet-cumulateand/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10 °C/km). Penecontemporaneousmetamorphic agesbut different geothermalgradientsbetweenHPgranulitesand related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.