Abstract
We report picritic porphyrites from the Jianchuan area in western Yunnan, within the Sanjiang–Tethyan Tectonic Domain (STTD), forming part of the the southeastern segment of the East Tethyan tectonic domain, east of the Himalayan–Tibetan Orogen. Feldspar 40Ar39Ar plateau ages from the porphyrites range from 201.7Ma to 190.6Ma. Geochemically, the picritic porphyrites are characterized by relatively low SiO2 (43.9–46.1wt.%) and high MgO (18.9–26.7wt.%), and display ocean island basalt (OIB)-like signatures, characterized by the enrichment of light rare earth element (LREE) and large ion lithophile elements (LILE) relative to high rare earth element (HREE: LaN/YbN=7.12–10.24). These rocks also show the absence of prominent Eu anomalies and lack of Nb, Ta and Ti troughs. On the basis of the olivine-melt equilibrium, the parental magma that produced the porphyrites is estimated to be a picritic magma with MgO content of 18.5–20.5wt.%. The rocks show relatively high initial (87Sr/86Sr)i ratios (0.705253 to 0.705640), negative εNd(t) values (−1.28 to −1.69) and radiogenic Pb ((206Pb/204Pb)t=18.165 to 18.343, (207Pb/204Pb)t=15.558 to 15.523, and (208Pb/204Pb)t=38.547 to 38.741). Trace element modeling suggests that the picritic magma was generated by a relatively low degree of partial melting (4–6%) of a peridotite source of the spinel to garnet transition, but closer to garnet-facies peridotite at high temperature (~1580°C) and pressure (~2.88Gpa) conditions. In combination with the regional geology, we infer that the picritic porphyrites were generated in the transitional phase between the termination of the Paleo-Tethys Ocean evolution and the onset of the Neo-Tethys Ocean evolution, when the break-off of the Jinshajiang oceanic slab that subducted westward underneath the India plate during the late Triassic to early Jurassic opened a slab window allowing asthenospheric upwelling and partial melting.
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