Crustal accretion in a slow-spreading center of Meso-Tethyan Ocean: Constraints from cumulates in the Dongco ophiolite (Central Tibet)

Lower ocean crustal rocks are important for deciphering magmatic processes in the magma chamber during the generation of the ophiolite. Gabbroic cumulates are volumetrically outcropped in the Dongco ophiolite in the Bangong-Nujiang Suture, which are pervasively intruded by diabase dikes. This study presents mineralogical, geochemical, and geochronological data of the Dongco cumulates to constrain their parental magma compositions and the tectonic setting of the Dongco ophiolite. Zircons and titanites from six gabbros yielded U-Pb ages of 159-166 Ma, suggesting that the Dongco ophiolite was generated during Middle to Late Jurassic. Petrological and major element data of the Dongco cumulates suggest that they were derived from water-poor magmas (similar to 0.5-1.0 wt% H2O) via low-pressure crystallization. Clinopyroxenes of the Dongco cumulates have lower Ti, Zr, and REE but higher Ba abundances compared to those in mid-ocean ridge cumulates. Melts in equilibrium with clinopyroxene of the Dongco cumulates are estimated to compositionally resemble the diabase dikes, both of which share similarities with island arc basalts, e.g., enrichments in Ba, Sr and Pb but depletions in REE and Nb. Moreover, the Dongco cumulates have been inferred to experience rapid exhumation shortly after its emplacement. Therefore, we propose that the Dongco cumulates were generated at a slow-spreading center in a backarc basin. The Dongco ophiolite in the Bangong-Nujiang suture and the Lagkorco ophiolite in the Shiquanhe-Yunzhug-Namco suture share great similarities in their formation ages, spreading rate and tectonic setting, which supports that the western Shiquanhe-Yunzhug-Namco suture is the southward-thrusted nappe of the Bangong-Nujiang suture.