Geochemical characteristics of Xiangtaohu metamorphic cumulate gabbro in the Hongjishan area of Qiangtang, north Tibet and their geological significance

Xiangtaohu metamorphic cumulate gabbro is the Early Paleozoic basic rock resulting from disintegration of "Maoershan Formation", and it is important to comprehend the tectonic evolution of the Lungmu Co-Shuanghu-Lancang River suture zone. Petrographic studies indicate that the cumulate gabbro suffered from high amphibolite facies and subsequent greenschist facies metamorphism, and geochemical studies yielded the following data: SiO2=44.7%~46.2%, MgO=7.05%~8.48%, Mg#=51.02~53.90, compatible element Cr=168×10-6~288×10-6, Ni=60×10-6~103×10-6, by far lower than the corresponding values of the primary basaltic magma, which suggests that Xiangtaohu metamorphic cumulate gabbro has undergone a certain degree of fractional crystallization of mafic minerals. The REE and trace elements of some samples are similar to those of OIB, and the other elements are similar to those in E-MORB. It is thus concluded that the cumulate gabbro was formed in a mid-ocean range, with the superposition of mid-ocean ridge axis type mantle plume, thus probably belonging to the fragments of the Eopaleozoic Ocean.