SPINEL-GARNET PHASE TRANSITION ZONE OF CENOZOIC LITHOSPHERIC MANTLE BENEATH THE EASTERN CHINA AND WESTERN QINLING AND ITS T-P CONDITION

Abstract

Coexistent five-phase (O1 + Opx + Cpx + Gt + Sp) mantle peridotitic xenoliths are direct samples from the spinel/garnet phase transition zone of the lithospheric mantle. Late Tertiary and Quaternary alkaline volcanic rocks, eastern China and western Qinling, contain few coexistent five-phase mantle peridotitic xenoliths, which provide an opportunity to directly study the phase transition zone of the lithospheric mantle beneath the regions. According to the spinel-garnet relationship, coexistent five-phase peridotitic xenoliths can be divided into three types. Type 1 is characterized by the granular spinel relicts surrounded with the garnet reaction rim. This type peridotite represents the upper limit of the phase transition zone, and can be called spinel-zone peridotites. Type 2 peridotite, in which garnet and spinel coexist without obvious phase transition relationship, represents the middle of the phase transition zone and can be called spinel-garnet transitional zone peridotite. Type 3 is typified by the garnet manded with the reaction rim of microcrystal pyroxene and spinel. This type peridotite represents the bottom of the phase transition zone and can be named garnet-zone peridotite. Therefore, the depth and thickness of the phase transition zone can be determined through mineral compositions of peridotitic xenoliths and estimated T and P conditions. This paper, based on T and P estimates of spinel-garnet lherzolites in Late Tertiary and Quaternary alkaline volcanic rocks from eastern China and western Qinling, further constrains the depth and thickness of the phase transition zone in the Cenozoic lithospheric mantle beneath the eastern China.