Thermal and redox states of subcontinental lithospheric mantle: Constraints from basalt-borne mantle xenoliths in East China

Xisheng XU,a,b S.Y. O'REILLY,b Xinmin ZHOU,a W.L. GRIFFIN,c Ming ZHANGb

(a, Department of Earth Sciences, Nanjing University, Nanjing 210093, China;

b, GEMOC, School of Earth Sciences, Macquarie University, NSW 2109, Australia; c, CSIRO Exploration and Mining, North Ryde, Sydney 2113, Australia)

Basalt-borne ultramafic xenoliths are widespread within the four tectonic blocks from Archaean to Phanerozoic in East China, namely Xing-Mong block, Sino-Korean block, Yangtze block and Cathaysia block from North to South. These deep-seated xenoliths include a large variety of rock types, mainly garnet lherzolite, spinel-garnet lherzolite, spinel lherzolite, spinel-garnet pyroxenite, spinel pyroxenite and granulite. Mineral equilibria observed in these xenoliths provide records of geothermal and redox states in the lithospheric mantle among the different Chinese tectonic terranes. This information, in combination with detailed geophysical data available in China, can provide rigorous constraints upon the physical nature of subcontinental lithospheric mantle and tectonic evolution.

Xenolith suites with suitable mineral assemblages from several key volcanic provinces representative of various tectonic settings, including Wudalianchi (Xing-Mong block), Nüshan (Yangtze block), Xinchang, Mingxi and Qilin (Cathaysia block), are chosen for this study. Paleogeotherms have been established for Nüshan and Qilin localities using garnet-bearing mantle xenoliths (particularly some composite xenoliths) and appropriate geothermobarometers which are consistent with experimentally determined spinel-garnet phase transition boundaries for particular mineral assemblages. Mantle oxygen fugacities (relative to the fayalite-magnetite-quartz buffer) have been calculated for spinel peridotite xenoliths in these provinces.

The established geotherms show different features at shallow levels which are corresponding to different degrees of 'underplating' of basaltic magmas at the crust / mantle boundary (CMB), resulting in advective heat transfer. However, all the geotherms converge below 60 km, and would intersect the asthenospheric adiabat at depths of ca 80-100 km. As in eastern Australia, the lower crust / uppermost mantle beneath an old craton represents a thermal regime distinct from that beneath the younger fold belt. A large amount of spinel-bearing peridotites are detailed studied for understanding both crust/mantle boundary and redox states. It is indicated that different regions have different characteristics of CMB, either as a sharp boundary or as a transitional zone. The calculated oxygen fugacities show that there are distinct differences between the different tectonic settings and/or mantle domains. For instance, the high oxygen fugacity calculated for spinel peridotite xenoliths in the Wudalianchi potassic volcanic provinces is associated with their metasomatised nature.