The Petrogenesis And Tectonic Significance Of Mafic-Ultramafic Rocks From Mangxin,Changning-Menglian Belt,Southwest Yunnan

Mafic-ultramafic lavas of the Early Carboniferous Tournaisian,such as basalts,picrites and accumulative peridotites,outcrop in Mangxin,southwestern Yunnan,China.Picrites with 26-31%MgO and a quenched texture are the most significant component of this rock association.This article divides the Mangxin picrites into three types according to their petrological and geochemical characteristics.The mantle potential temperature(T_p)is about 1465-1513°C for Type-1,1437°C for Type-2 and 1497-1502°C for Type-3,which is higher than the Tp range of mid-ocean ridges and reaches that of mantle plumes.The difference is,the T_p calculated based on the matrix of Type-2 picrites is 1388-1413°C,belong to the T_p range of mid-ocean ridges,while the Tp calculated based on the matrix of Type-3 picrites is 1588-1605°C,obviously similar to the Tp range of hot spots.Also,Type-1 and Type-2 picrites have similar rare earth element(REE)characteristics of mid-ocean ridge basalts(MORBs),while Type-3 picrites have similar REE characteristics of oceanic island basalts(OIBs).Thus it can be seen that the seemingly similar picrites actually represent different eruption environments.According to the geochemical characteristics,the Type-1 and Type-2 picrites probably formed from the melting of the mantle plume head and were contaminated by ambient depleted mantle,while the Type-3 picrites formed from the melting of hot mantle plume tails.In addition,the basalts of Mangxin can be devided into high-Ti group and low-Ti group.According to the analyses of high-field-strength elements,the formation of high-Ti basalts and Type-3 picrites are all related to the melting of the mantle plume tail,however,the formation of low-Ti basalts are related to the melting of the mantle plume head.After the comprehensive analysis of three types of picrites,accumulative peridotites and two types of basalts outcropped in Mangxin area,this study builds a model of an oceanic plateau associated with mantle plume activity that can be used to explain the basic tectono-palaeogeographic features of the Palaeotethys.