Abstract:To better understand the compositions of enriched mantle source of ultrapotassic rocks in southern Tibet, here we report Ca and Mg isotopic compositions of eight ultrapotassic rock samples from the Mibale and Maiga areas in the Lhasa Terrane. Excluding the effect of weathering, crustal contamination, fractional crystallization and partial melting on ultrapotassic rocks’ Ca and Mg isotopic compositions, it is determined that some carbonate sediments from subducted NeoThethy oceanic crust may join into the mantle source of ultrapotassic rocks. The ultrapotassic rocks show significantly lower δ44Ca values ranging from 059 to 075 (average=068±004), which are distinctly lower than the upper mantle (105±004), bulk silicate earth (094± 005) and igneous rocks (080±010). All these features indicate that the mantle source of ultrapotassic rocks might contain materials with low δ44Ca value. The δ26Mg values of ultrapotassic rocks display limited varication from -033 to -024 (mean=-029±003), which are similar to the upper mantle (δ26Mg=-025±007) within the analytical uncertainty. The CaMg isotopic compositions of ultrapotassic rocks are positively correlated, indicating their source might be metasomatized by the materials with low δ44Ca and δ26Mg value. Considering the tectonic evolution of Lhasa Terrane, we suggest that the low CaMg isotopic matericals are very likely marine carbonate sediments derived from subducted NeoTethyan oceanic crust.
