Petrogenesis Of Potassium-rich Volcanic Rocks In Lhasa Terrane, Tibetan Plateau: Magnesium And Potassium Isotopic Constraints

Located in the Himalaya-Tibet orogenic belt,the Tibetan Plateau is an ideal place to study the structure and evolution of the lithosphere and the deep processes.The potassium,ultrapotassic and magnesium-rich ultrapotassic volcanic rocks in the Lhasa block are one of the main products of the evolution of the plateau lithosphere,which provides an important entry point for the study of the structure and evolution of the Tibetan Plateau lithosphere,the deep magmatism process and the plateau uplift mechanism.It has great research significance.The petrogenesis,source of metasomatic material and geodynamic background of potassium-rich volcanic rocks in Lhasa Block are still controversial.Based on this,this paper studies the potassium,ultrapotassic and magnesium-rich ultrapotassic volcanic rocks in the Lhasa block.Based on the Mg and K isotopic compositions of the samples,the origin characteristics,petrogenesis and geodynamic background of the volcanic rocks are discussed in detail.The results show that the Mg isotopic composition of the three potassian-rich volcanic rocks in the Lhasa block is relatively uneven,with theδ²⁶Mg values ranging from-0.44‰to-0.03‰and the averageδ²⁶Mg values of-0.30‰±0.20‰（2SD）.The K isotopic compositions of the three potassian-rich volcanic rocks are uniform,but significantly lower than that of the mantle.Theδ⁴¹K values of the three potassian-rich volcanic rocks range from-0.67‰to-0.39‰,with an averageδ⁴¹K value of-0.47‰±0.10‰（2SD）.On the whole,the Mg and K isotopic compositions of the three types of rocks are lighter than those of the mantle.The source characteristics of the three potassium-rich volcanic rocks in the Lhasa block are discussed.The Mg and K isotopic characteristics of the three rocks are not caused by the metasomatism of the Indian continental crust,and are not related to the plate fluid and altered oceanic crust,but to the metasomatism of the sediments.The Sr-Nd-Pb-Mg-K isotopes of potassium,ultrapotassic and magnesium-rich ultrapotassic volcanic rocks in the Lhasa block were simulated.The results show that the three potassium-rich volcanic rocks are derived from the enriched mantle and are formed by the mixture of sediments and mantle.The mantle source region is subject to about 10%sediment metasomatism.The potassic volcanic rocks in Lhasa block received terrigenous clastic sediments and turbidites during metasomatism,while the ultrapotassic and magnesium-rich ultrapotassic volcanic rocks received high magnesium carbonate（dolomite and magnesite）during metasomatism from the mantle source.The potassium,ultrapotassic and magnesium-rich ultrapotassic volcanic rocks in the Lhasa block have a close genetic relationship,and the three types of rocks all originate from the same parent magma.Firstly,the lithospheric mantle source zone with metasomatic enrichment of about 10%of the sediments produced its parent magma through partial melting,and then underwent low degree partial melting to form potassium,ultrapotassic magma and magnesium-rich ultrapotassic magma.Finally,potassium,ultrapotassic and magnesium-rich ultrapotassic volcanic rocks were formed in Lhasa block.Combined with the provenance characteristics and petrogenesis of the potassium-rich volcanic rocks in the Lhasa block,we propose a four-stage geodynamic model,which suggests that the convective thinning/delamination of the thickening lithospheric mantle resulted in postcollusive magmatism.At 26～10 Ma,the residual lithospheric mantle undergoing sediment metasomatism was partially melted to form the potassian-rich volcanic rocks in the Lhasa block.