New Evidence Of Plume Origin For The Comei Fragmented Large Igneous Province In Southeastern Tibet

The extensive Early Cretaceous Comei igneous rocks consisting of mafic dikes and sills, basaltic lavas, gabbroic intrusions and a few subordinate layered ultramafic rocks and silicic volcanic rocks are exposed in the Tethyan Himalaya, southeastern Tibet. All these rocks were synchronously emplaced at ca.132 Ma with a short magmatic duration (< 5 Ma). Such feature, together with its large spatial extent, presence of bimodal volcanic suite, and paleographical position at ca.132 Ma, have been associated with the activity of Kerguelen Plume. These rocks have been referred as to the Comei fragmented large igneous province (CFLIP) preserved in the present-day SE Tibet that has been located around the triple junction of NE Great Indian continent, SW Australia, and Antarctica in Eastern Gondwana during the Early Cretaceous. To further constrain the origin of the Comei LIP, we performed petrographical, geochemical, and mineral geochemical analysis for the igneous rocks from the province. Our new results show that:1) Most of the mafic rocks are alkaline-subalkaline, similar to oceanic island basalt (OIB) in composition, indicating a mantle plume origin. However, some samples also display significant negative Nb-Ta anomalies with negative (?)Nd(t) values, implying that these rocks have been contaminated by crust. A small amount of mafic rocks are subalkaline basaltic/andesitic with geochemical compositions similar to normal mid-ocean ridge basalt (N-MORB), showing that they are likely originated from the asthenosphere. Some mafic rocks are transitional between OIB and N-MORB, and are geochemically comparable to enriched mid-ocean ridge basalt (E-MORB).2) A picrite porphyrite collected from Gujue, Cona in the Comei LIP has been dated to be 125±11 Ma by whole-rock K-Ar dating, which is coeval with the emplacement of the CFLIP within error. The Fo in olivine from picrite porphyrite in Cona range from 82.0-89.6. The Cr-spinel from the Cona picrite-porphyrite display enrichment of TiO2 and depletion of A12O3. The MgO and FeO contents of primitive magma calculated by olivine-melt equilibrium method from the picrite porphyrites and associated basalts are 17.1-22.2 wt.% and 10.9-11.5 wt.%, respectively, yielding mantle potential temperatures of 1500-1615℃and initial melting pressure of 3.8-6.3 GPa. These results indicate that the picrite porphyrites and associated basalts are derived from partial melting of a deep mantle source under a phase of garnet stability, consistent with an origin of an abnormal hot mantle like mantle plume activity. The olivines from the ultramafic rocks in Zhegu Tso are most likely accumulated from lithospheric mantle, as indicated by their low Fo (< 80) and low CaO contents. Our filed and petrographical observations indicate the possible presence of pyroxene inclusion in the Cona basalt, but further verification is required. The complexity of rock types, whole-rock geochemistry, and olivine composition likely indicates the presence of multi-stage magmatic system in the CFLIP. We inferr that magma chambers may exist in different levels from lithosphere mantle, lower crust, and also middle to upper crust, ultimately resulting in the emplacement of materials from shallow chambers accompanied with the ascend of magmas from deep magma chambers. We therefore propose a reasonable model for the generation of the CFLIP involving the formation of multi-stage magmatic systems in different levels that can be attributed to lithosphere-mantle plume at about 132 Ma.