Genesis of porphyry Mo deposits linked to gradually dehydrating subcontinental lithospheric mantle metasomatised by previous subduction in northeastern China

Abstract

Many Mesozoic Mo-bearing granites, present along the northern margin of the Precambrian North China Block (NCB) and Phanerozoic Xing'an-Mongolian Belt (XMB) within the Central Asian Orogen (CAO) have hydrated sources. These granites are temporally and spatially related to biotite-rich mafic, alkali syenitic plutons, are highly evolved, have an arc-like trace element geochemical pattern, and were emplaced between ca. 245–129 Ma bracketed by A2- and A1-type granites. This conflicts temporally with the current model involving the westward subduction of the Pacific oceanic plate. We use Sm/Nd and Th/Yb ratios of Mesozoic nepheline-bearing pyroxene-biotite syenites (biotite-syenites), mantle-peridotite xenoliths from Palaeozoic diamondiferous kimberlites, and Mesozoic to Cenozoic alkali basalts to define a hydrous melt-extraction trend. The results indicate a Palaeozoic mantle-peridotite source that is more metasomatised than the lower continental crust and the source of the Mesozoic (<129 Ma) to Cenozoic mantle-peridotite xenoliths (Th/Yb < 1). This indicates that the sub-continental lithospheric mantle (SCLM) beneath northeast China has been gradually dehydrating since the Palaeozoic, and the lower continental crust and Cenozoic SCLM in northern NCB are too depleted to be the magmatic source for the Mo-bearing granites. The results of the study also indicate that the Palaeozoic SCLM beneath the northern margin of the NCB was more metasomatised than beneath its central zone. The development of Mo-bearing granites in both the Precambrian northern margin of NCB and Phanerozoic XMB indicate that the presence of an ancient SCLM is not necessary for the genesis of Mo mineralisation. From these observations, it is proposed that the genesis of Mo mineralisation in the region is attributed to a hydrated SCLM metasomatised by the subduction of the Paleo-Asian oceanic plate rather than the younger Pacific oceanic plate.