Geochemical comparison of Paleogene high-Mg potassic volcanic rocks in Sanjiang area and interior Tibetan Plateau:Compositional difference of the mantle sources

Paleogene high-Mg potassic volcanic rocks(MgO6%,K2O/Na2O1)are widely exposed in the Tibetan plateau,which are generally believed to be sourced from mantle. These volcanic rocks show some common characteristics such as enriched-K,LILE and depleted-HFSE and contemporary eruption,but the rocks from interior of the Tibetan Plateau have geochemical compositions clearly different from that in Sanjiang area. The high-Mg potassic volcanic rocks from interior of Tibetan Plateau have lower eNd (t) relatively to the volcanic rocks from Sanjiang area,which suggested the former was sourced from a relatively depleted enriched-mantle source. The high-Mg potassic volcanic rocks were probably derived from a K-rich enriched mantle which was related to metasomatism during earlier subduction events. However,difference of the high-Mg potassic rocks between the both areas possibly results from different enriched process,the mineral compositions in their mantle sources and the depth of magmatic sources. The break-off of the north-dipping Neo-Tethyan slab in southern Tibet during 50~40Ma and delaminated lower continental of northern Tibet in Eocene might triggered the Paleogene high-Mg potassic rocks in Tibetan plateau. The northward subduction and break-off of the Neo-Tethyan slab in Eocene likely contributed to enrichment of ore-bearing material and formation of porphyric deposits in Sanjiang area.