Geological Implications for the Deep-Derived Mafic Enclaves from Cenozoic Shoshonitic Rocks in Jianchuan-Dali Area, West Yunnan

摘要

There exist intermediate to intermediate-acid shoshonitic rocks dated at 26.3- 36.7Ma in the Early Tertiary extensional basins along the NE flank of the Dali-Jianchuan section of the Honghe-Ailaoshan left-lateral strike-slip ductile shear zone, where some high- and medium-high grade metamorphosed mafic enclaves have been found. According to the P-T conditions and mineral assemblage characteristics, the enclaves are grouped into three types: type-Ⅰ, garnet diopsidite, formed at lower crust or in the transitional section between the mantle and the crust with the metamorphic condition P=(10.47-11.51)×108 Pa and T=771-932℃; type-Ⅱ, garnet- and diopside-bearing amphibolite, formed at the depth of middle to lower crust with the metamorphic condition: P=(6-10) ×108 Pa and T=780-830℃; type-Ⅲ, metamorphosed gabbro, formed at relatively higher levels than the above 2 types with lower metamorphic condition. Type-Ⅰand type-Ⅱenclaves had experienced a decompressional retrograde metamorphism caused by tectonic elevation associated with the metasomatism of SiO 2, Na 2O-rich fluid phase with the new equilibrated temperature at T=761-778℃. Geochemical evidence indicates that both deep-derived mafic enclaves and host rocks have similar EM Ⅱ properties, which are related to the involvement of subducted materials. The sustaining subduction, compression and strike-slip ductile shearing between the India plate and the Yangtze craton are the main constraints on the subcontinental mantle properties, interaction between crust and mantle, and the formation and evolution of alkali-rich magma.