Methane-rich fluid inclusions from ophiolitic dunite and post-collisional mafic–ultramafic intrusion: The mantle dynamics underneath the Palaeo-Asian Ocean through to the post-collisional period

Abstract

The protracted development history of the Palaeo-Asian Ocean starting from ∼1000 Ma to ∼320 Ma, and continuation of mantle-derived magmatism right through the post-collisional period as manifested by the widespread mafic–ultramafic intrusion and alkaline–peralkaline granite suggest a vigorous, long-running mantle dynamo. Using the micro-laser Raman spectrometer, we analyzed olivine- and plagioclase-hosted fluid inclusions from ophiolitic dunite, Kudi of the western Kunlun Range, and post-collisional mafic–ultramafic intrusions of the Xiangshan and the Huangshandong, eastern Chinese Tienshan Mountains. Our results show that fluids brought from the mantle by these rocks are rich in H 2 O and CH 4 with variable amounts of N 2 . Estimation of the redox state, pressure and depth of formation for the melt–CH 4 –H 2 O + N 2 fluid systems from the 3 rock suites of the Xinjiang, and comparison of these parameters with those of the peridotite–fluid parageneses for the sub-cratonic upper mantle give meaningful results. These fluids were formed in the asthenosphere, were originally CH 4 –N 2 -rich, primordial mantle fluid, but have been progressively diluted with H 2 O and become oxidized by repeated subduction of oceanic slab. We propose that repeated redox melting in the mantle due to dilution of CH 4 with H 2 O and oxidation caused formation of the long-lived Palaeo-Asian Ocean, brought about extensive intrusion of mafic–ultramafic complexes and alkaline–peralkaline granites right through the post-collisional period, and, consequently, resulted in significant Phanerozoic continental growth in the Central Asian Orogenic Belt. Rise of atmospheric O 2 and the Earth's major oxidation occurred at about 2.4 to 2.2 Ga. Oxidation of the Earth's mantle promoted the mantle melting and magma input to the crust, resulting in major continental growth after 2.5 Ga. However, much-postponed oxidation of the primordial mantle underneath the Central Asian Orogenic Belt led to significant continental growth in the Phanerozoic times.