Nature And Processes Of The Lithospheric Mantle Beneath The Western Qinling: Evidence From Deformed Peridotitic Xenoliths In Cenozoic Kamafugite From Haoti, Gansu Province, China

摘要

The Cenozoic Haoti kamafugite field (23 Ma) is situated at the western Qjnling 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 coarsegrained ones (less than 0.10 wt%). Fine-grained clinopyroxenes have low Al_2O_3 + CaO contents (generally < 23 wt%) relative to coarse-grained ones (> 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 Na_2O contents (1.7-1.9 wt%) than those of the type 1 (< 1.4 wt%). P-T estimations reveal that the type 1 xenoliths give temperature in range of 1106-1187 ℃ and pressure of 21-26 kbar and that relatively low temperature (907 and 1022 ℃) and pressure (19.0 and 18.5 kbar) for the type 2 xenoliths. The type 1 xenoliths are characterized by depletion due to high degree of partial melting (> 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 (< 5%) and may represent the newly-accreted lithospheric mantle. The lithospheric mantle beneath the western Qinling underwent partial melting, recrystallization, deformation and metasomatism due to asthenospheric upwelling and the latest decompression responding to the Cenozoic extensive tectonic environment. These processes perhaps are closely related to the evolution of Tibetan Plateau caused by the India-Asian collision.