Metamorphic evolution and tectonic significance of garnet-bearing amphibolites from the upper valley of the Hatu River, East Kunlun Orogenic Belt

Abstract

The East Kunlun Orogenic Belt (EKOB), traditionally divided into northern (or Qimantagh Belt), central, and southern segments, has undergone a long and complex evolution. Eclogites exhumed from subduction zones are significant for reconstructing the tectonometamorphic histories of these continental blocks. However, they have not yet been reported from the southern EKOB. In this study, we first identified a suite of garnet-bearing metabasic riverbed cobbles in the upper Hatu valley, southern EKOB, some of which contain calcic clinopyroxene–plagioclase (Cpx–Pl) and/or amphibole–quartz (Amp–Qz) intergrowths typical of omphacite pseudomorphs. The absence of Cpx–Pl symplectite in strongly foliated amphibolites likely indicates complete amphibolitization of clinopyroxene, which may have been greatly enhanced by ductile deformation. The Cpx–Pl symplectite-bearing amphibolites experienced four metamorphic stages: (1) a prograde epidote amphibolite facies stage indicated by primary garnet inclusions, (2) a peak eclogite facies stage with the assemblage of garnet + omphacite + rutile + quartz + amphibole + biotite, (3) a symplectitization stage forming the Cpx–Pl intergrowth, and (4) an amphibolite facies stage forming a kelyphitic rim around garnet. Phase equilibrium modeling supports the eclogite facies conditions of 660 °C–750 °C and 13–16 kbar. Fine-grained garnet and amphibole in thin sodic plagioclase films between quartz grains may have formed as a result of peritectic reactions during partial melting at the eclogite facies stage rather than during decompression. Porphyroblastic epidote embayed by plagioclase and amphibole in the epidote-rich amphibolites likely indicates an interaction between epidote and the intergranular melt. UPb ages of the metamorphic zircons at ∼460 Ma probably correspond to the Early Paleozoic subduction of the Proto-Tethys Ocean, which may have contributed to the formation of the back-arc basaltic rocks of the Nachitai Group in the southern EKOB. This study supports the Mid-Ordovician as an important growth episode for the multiple subduction–accretionary system of the entire EKOB.