Geochronology and geochemical characteristics of Late Triassic porphyritic rocks from the Zhongdian arc, eastern Tibet, and their tectonic and metallogenic implications

Abstract

Ore-bearing porphyritic rocks are widely distributed in the Zhongdian arc in the southern part of the Yidun arc, eastern Tibet. New U–Pb zircon dates, and previous results, show that the porphyritic rocks formed mainly between 221 and 211Ma, with a peak at 217–215Ma. These Late Triassic porphyritic rocks and associated volcanic rocks are primarily calc-alkaline igneous rocks, some of which have geochemical affinities with adakite, such as high SiO2 (≥56wt.%), Al2O3 (≥14wt.%), and Sr, and low Y and heavy rare earth element contents. However, moderate Sr/Y and La/Yb ratios of these rocks compared with typical adakites characterize them as being transitional between adakites and normal arc rocks. Those rocks that do not have adakitic affinities are typical normal arc volcanic rocks. The porphyritic and associated volcanic rocks occur in the eastern and western parts of the Zhongdian arc, and both have the same geochemical characteristics and ages. The new dates, geochemical data, and Sr–Nd isotopic ratios, combined with previous data on the Zhongdian arc (particularly the Xiaxiaoliu basalt that has enriched mid-ocean ridge basalt characteristics), suggest that these rocks are probably related to slab break-off or slab-tearing of the westward subducting Garze–Litang oceanic crust in the Late Triassic. The enriched mantle wedge metasomatized by subducted fluids and sediments was heated by ascending asthenosphere and underwent partial melting. These magmas then probably interacted with underplated mafic material and experienced a melting–assimilation–storage–homogenization process (MASH) in the lower crust and/or with slab-derived melts, resulting in formation of the porphyritic rocks and associated porphyry deposits in the Late Triassic Zhongdian arc.