OIB-type picritic porphyrite from Jianchuan in tne western Yunnan Province: Results of break-off of the subducting plate?

Small scale OIB-type picritic porphyrite, comprising olivine, clinopyroxene and plagioclase, outcrops in Jianchuan area, western Yunnan, and the olivine is predominantly chrysolite with the Mg# up to 88.8. The bulk-rock composition is characterized by relatively low SiO 2 (43% - 48% ) but high MgO ( 12% - 24% content On the basis of the principle of the olivine-melt equilibrium, we conclude that their parental magma is picritic (MgO = 13% , FeO = 10% ). The normalized rare-earth element ( REE) patterns of picritic porphyrite is similar to that of ocean island basalts ( OIB) , rich in light rare-earth element ( LREE) and large ion lithophile element (LILE) as well In addition, melt and spinel inclusions have been recognized in the phenocryst olivine. The spinel inclusions are relatively enriched in Cr with the maximum Cr# of 69.0, and have low Al 2O 3 ( 12.5% - 18.6% ) and high TiO 2 (0.2% -1.7% ) contents. The result of modelling of the trace elements implied that the picritic magma was derived from the asthenospheric garnetbearing peridotite which had experienced relatively low degree of partial melting (4% - 7% ) under the condition of high temperature and pressure (T = 147°C, P = 2.7GPa). Taking the regional geological setting into consideration, we infer that the deep dynamic mechanism refers to the partial melting of upwelling asthenosphere due to the open of slab window, which probably had been triggered by the break-off of the India slab subducted at a high-angle bebeath the Eurasia plate.