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

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 ofthe 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 andmineral assemblage characteristics, the enclaves are grouped into three types: type- Ⅰ , garnetdiopsidite, formed at lower crust or in the transitional section between the mantle and the crustwith 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 themetamorphic condition: P ＝ (6 - 10) × 108 Pa and T = 780 - 830℃; type- Ⅲ, metamorphosedgabbro, formed at relatively higher levels than the above 2 types with lower metamorphic condi-tion. Type- Ⅰ and type-Ⅱ enclaves had experienced a decompressional retrograde metamorphismcaused by tectonic elevation associated with the metasomatism of SiO2, Na2O-rich fluid phasewith the new equilibrated temperature at T = 761 -778℃. Geochemical evidence indicates thatboth deep-derived mafic enclaves and host rocks have similar EMⅡ properties, which are relatedto 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 onthe subcontinental mantle properties, interaction between crust and mantle, and the formationand evolution of alkali-rich magma.