Nature and processes of the lithospheric mantle beneath the western Qinling: Evidence from deformed peridotitic xenoliths in Cenozoic kamafugite from Haoti, Gansu Province, China

The Cenozoic Haoti kamafugite field (23 Ma) is situated at the western Qinling Orogen, Gansu Province in China, which is a conjunction region of the North China Craton, the Yangtze Craton and the Tibetan Plateau. Fresh peridotitic xenoliths entrained in these volcanic rocks provide an opportunity to study the nature and processes of the lithospheric mantle beneath the western Qinling. These xenoliths can be divided into two groups based on the petrological features and mineral compositions, type 1 and type 2. Type 1 xenoliths with strongly deformed texture have higher Fo (90–92.5) contents in olivines, Mg# (91–94) and Cr# (15–35) of clinopyroxenes, and Cr# (36–67) of spinels than the weakly deformed type 2 xenoliths, which have the corresponding values of 89–90, 89–91.5, 10–15 and 5–15 in minerals, respectively. CaO contents in fine-grained olivines are slightly higher than 0.10 wt% compared with coarse-grained ones (less than 0.10 wt%). Fine-grained clinopyroxenes have low Al2O3 + CaO contents (generally 23 wt%). Fo contents in fine-grained olivines mainly in the melt pocket of the type 1 xenoliths are higher than those in coarse-grained ones, which is somewhat contrary to the type 2 xenoliths without melt pocket. Clinopyroxenes of the type 2 display higher Na2O contents (1.7–1.9 wt%) than those of the type 1 ( 10%), modal metasomatic and deformed characteristics, and may represent the old refractory lithospheric mantle. In contrast, the type 2 peridotites show fertile features with low degree of partial melting (