Archean mantle contributes to the genesis of chromitite in the Palaeozoic Sartohay ophiolite, Asiatic Orogenic Belt, northwestern China

Podiform chromitites in ophiolite usually are interpreted as the crystallization products of mafic magmas, contemporaneous with the generation of the ophiolite at mid-oceanic ridges or back-arc spreading centres. However, the real ages of the chromitites can rarely be determined directly, because their chemistry provides few opportunities for geochronology. Here we employ the ¹⁸⁷Re- ¹⁸⁸Os decay system (N-TIMS) to date the chromitite and a cross-cutting troctolite in the Sartohay ophiolite, northwestern China, and we have dated zircons separated from the troctolite by SHRIMP U-Pb methods. Inherited zircons from the troctolite yield a lower intercept age of 0.48±0.08Ga and an upper intercept age of 2.28±0.11Ga; two concordant grains give ages >2.4Ga. Whole-rock Re-Os data for the troctolites and chromites plot between 2.45Ga and 0.43Ga reference isochrons. Plots of T _MA vs ¹⁸⁷Os/ ¹⁸⁸Os suggest mixing between ultra-depleted chromitite and suprachondritic troctolite, mainly affecting the ¹⁸⁷Os/ ¹⁸⁸Os of the troctolites; subsets of troctolite samples scatter around reference isochrons with ages of 0.4-0.5Ga. The zircon data and the Re/Os data are consistent with published Sm-Nd evidence that the troctolites, and hence the Sartohay ophiolite, were formed in Palaeozoic time. However, the ¹⁸⁷Os/ ¹⁸⁸Os compositions of the chromites (0.1109±4 to 0.1249±5), give Neo-Proterozoic to Paleo-Archean model ages (T _MA=0.8-3.5Ga), indicating extraction from the primitive mantle as early as 3.5Ga, some 3Ga before the formation of the Sartohay ophiolite. A Re-Os apparent isochron age (2.45Ga), the old T _Ma model ages and the ancient zircon ages (>2.4Ga) all are consistent with part formation of the Sartohay chromitite in Archean time, and then reworked in the Palaeozoic time. We suggest that a volume of early Archean depleted mantle remained within the Siberian lithospheric mantle for 2.5-3.0Ga; it later became portions of the mantle wedge above the Paleozoic subduction zone of the Paleo-Asian Ocean, and ultimately was incorporated into the overthrust ophiolite.