Studies Of Petrology And Geochemistry On Jianchuan Picritic Porphyrites Of Sanjiang District In Western Yunnan

The Paleo-Tethyan tectonic domain has a complex geological andtectonic history, including the opening, subduction and closure of thePaleo-Tethyan Ocean, subsequent opening of the Neo-Tethys Ocean,arc-continent collision during the Paleozoic-Mesozoic period, andcontinent-continent collision during the Himalayan period. We report picriticporphyrites from the Jianchuan area in western Yunnan, within theSanjiang-Tethyan Tectonic Domain （STTD）, forming part of the thesoutheastern segment of the Paleo-Tethyan tectonic domain, east of theHimalayan-Tibetan Orogen. Feldspar⁴⁰Ar-³⁹Ar plateau ages from theporphyrites range from201.7Ma to190.6Ma. The picritic porphyrites areN-S trending dykes or small stocks, having the area of hundreds of squarekilometres with the lagest area to0.1square kilometres. The OIB-type picriticporphyries, comprises olivine, clinopyroxene and plagioclase, outcrop inJianchuan area, western Yunnan, and the olivine is predominantly chrysolitewith the Mg^#maximum of88.8. The results show that most clinopyroxenes inthe Jianchuan area are augite, containing relatively lower TiO₂（0.62².84wt%）, Al₂O₃（1.86⁵.15wt%） and Na₂O （0.23⁰.61wt%）. Feldspar40Ar-39Arplateau ages from the porphyrites range from201.7Ma to190.6Ma.Geochemically, the picritic porphyrites are characterized by relatively lowSiO₂（43.9⁴6.1wt%） and high MgO （18.9²6.7wt%）, and display oceanisland basalt （OIB）-like signatures, characterized by the enrichment of lightrare earth element （LREE）and large ion lithophile elements （LILE） relative tohigh rare earth element （HREE: LaN/YbN=7.12¹0.24）. These rocks also showthe absence of prominent Eu anomalies and lack of Nb, Ta and Ti troughs. Onthe basis of olivine-melt equilibrium, the parental magma that produced theporphyrites is estimated to be picritic magma with MgO content of18.5²0.5wt%. The rocks show relatively high initial (⁸⁷Sr/⁸⁶Sr)iratios （0.705253to 0.705640）, negative εNd（t） values （-1.28to-1.69） and radiogenic Pb((²⁰⁶Pb/²⁰⁴Pb)t=18.165to18.343,(²⁰⁷Pb/²⁰⁴Pb)t=15.558to15.523, and(²⁰⁸Pb/²⁰⁴Pb)t=38.547to38.741). Trace element modeling suggests that thepicritic magma was generated by relatively low degree of partial melting（4⁶%） of a peridotite source of the garnet transition at high temperature（¹580°C） and pressure （².88Gpa） conditions. In combination with theregional geology, we infer that the picritic porphyrites were generated in thetransitional phase between the termination of the Paleo-Tethys Oceanevolution and onset of the Neo-Tethys Ocean evolution, when the break-off ofthe Jinshajiang oceanic slab that subducted westward underneath the Indiaplate during the late Triassic to early Jurassic opened a slab window allowingasthenospheric upwelling and partial melting.