Clinopyroxene-hosted melt inclusions in seamount basalts in the Muli melange in Sichuan, SW China : Tracks of hotspot-ridge interaction in the Ganzi-Litang Paloetethys Ocean

The Muli melange in Southwest Sichuan is located on the western margin of the Yangtze plate and plays an important role in understanding the relationship between the Yangtze plate and Ganzi-Litang Paleotethys Ocean. Our careful field investigation shows that the Muli melange is composed of different types of oceanic plate strata, in which two seamount associations (basalt-limestone type of melange) are relatively intact and composed of the basalt base and overlying carbonate cap with olistostromes occurred on the seamount slopes. The seamount basalts display the typical porphyritic texture that consists of clinopyroxene phenocrysts and subordinate plagioclase phenocrysts in a matrix of acicular plagioclase. The primary melt inclusions (MIs) , 20 similar to 50 mu m in size, are hosted in clinopyroxene phenocrysts. The internal component and texture of the MIs are simple and clear, and composed of opaque glass with circular or oval contraction bubbles. A few of MIs have crystallized the daughter minerals along the rims. Using LA-ICP-MS and electron microprobe techniques, the major and trace elements of the MIs and chemical compositions of their hosted clinopyroxene phenocrysts were analyzed. The results reveal that the seamount basalts in the Muli melange gave the geochemical features similar to OIB and E-MORB. The OIB-like geochemistry indicates that the seamount basalts in the Muli melange were derived from magma source of low degree melting of garnet lherzolite within the oceanic plate. While the E-MORB-like feature indicates some degree of hotspot-ridge interaction, suggesting the mantle plume occurred in the Ganzi-Litang Paleotethys Ocean. Apatite U-Pb analyses indicate that Ganzi-Litang Paleotethys Ocean has been developed in the duration of the end of the Carboniferous to Early Permian (302 +/- 11 Ma).