The geochemistry and age of ophiolitic strata of the Xinglongshan Group: Implications for the amalgamation of the Central Qilian belt

The Xinglongshan Group comprises a metabasalt and metasedimentary rock (quartz–mica schist) assemblage representative of low-grade metamorphic basement strata in the Central Qilian belt. Metabasaltic compositions include MORB-like tholeiites and alkaline basalts of axial seamount or ocean island origin. Depending on assumed origin, the latter yield Sm–Nd model ages between 824 and 490 Ma. Whole rock Rb–Sr, and Sm–Nd systematics of acid leached metasedimentary rock samples define a Late Cambrian isochron of 513±17 Ma with initial an 87Sr/86Sr ratio of 0.70930±0.00050 which is interpreted to mark the timing of basin closure. High abundances of trace elements (Cr, Ni, Sc) associated with ferromagnesian minerals, relative to elements (Th, Zr) associated with resistant minerals, precludes derivation of the sediment assemblage solely from gneisses of the Central Qilian belt. Rather, the Nd–Sr isotope systematics of the metasedimentary rock samples indicate mixing between a mafic or arc component and an ancient felsic crustal composition. Th–Sc abundances suggest the felsic component was not the supracruastal sequences of the North China block. Isotopic and geochemical data thus suggest low-grade metamorphic basement strata in the Central Qilian belt are of likely NeoProterozoic age, assembly of the belt took place during the early stages of the Caledonian orogenic cycle before closure of ocean basins in the North Qilian belt, and that the Central Qilian belt likely bears an allochthonous relationship to North China.