Abyssal and Suprasubduction Peridotites in the Lajishan Ophiolite Belt: Implication for Initial Subduction of the Proto-Tethyan Ocean

Several mantle peridotite massifs crop out along the northern margin of the South Qilian belt, but their origin remains controversial. The Xigou peridotite massif is the largest one within the Lajishan ophiolite belt and mainly consists of lherzolite, harzburgite, and serpentinized dunite, in association with anorthosite, amphibolite, dolerite dikes, and pillow basalt. Lherzolites show typical features of relatively fertile abyssal peridotites, with iron-rich olivine, Al-spinel, and aluminous pyroxene. Harzburgites and serpentinized dunites are characterized by U-shaped rare earth element patterns and low Pd-group platinum-group elements resembling the geochemical features of suprasubduction peridotites. Spinel Cr#'s (Cr/(Cr+Al)) range from 0.38 in harzburgite to 0.75 in dunite. The progressive increase in spinel Cr#'s is related to changing melt compositions from mid-ocean ridge basalt (MORB) like to boninitic during subduction initiation. The Xigou suprasubduction peridotites, MORB-like mafic rocks, and boninites probably constitute an incomplete forearc crust-mantle section. The abyssal lherzolite preserved in the forearc region might represent older oceanic lithosphere or younger fertile mantle residue along a trench-fracture zone intersection with a transform fault. Therefore, we suggest that the intraoceanic subduction of the Proto-Tethyan Ocean may have begun no later than 535 Ma, before the development of the Lajishan intraoceanic arc-trench system.