Geochemistry and tectonomagmatic affinity of the Yungbwa ophiolite, SW Tibet

Abstract

The Yungbwa ophiolite is a thrust sheet of about 800 km 2 tectonically overlying an Upper Cretaceous melange south of the Indus–Tsangpo Suture Zone in SW Tibet. Both units have been thrust over the sedimentary series of the Tibetan Tethys zone in the course of the India–Eurasia collision. Harzburgite and clinopyroxene-poor lherzolite are the dominant lithologies. Occasional gabbronoritic and basaltic dikes crosscut the mantle tectonites, but plutonic and volcanic sections are notably absent. The majority of basaltic dikes are tholeiitic and similar to N-MORB with respect to LREE depletion, ratios of diagnostic trace elements and Sr and Nd isotope systematics. Sm–Nd isotope data of three tholeiitic samples yielded an isochron age of 147±25 Ma (MSWD=0.59), and an initial ε( t)Nd value of +8.8. In addition, magnesio-hornblende in a tholeiitic basaltic dike yielded a 40Ar/ 39Ar age of 152±33 Ma. The mineral chemistry of the spinel–peridotites is consistent with an origin in a tectonic setting similar to abyssal peridotites. The nearly chondritic initial 187Os/ 188Os value of a Re-poor orthopyroxene concentrate matches the composition of abyssal peridotites. The strongly LREE depleted trace element signature of the peridotite clinopyroxenes resembles that of clinopyroxenes from ocean floor peridotites. The Nd isotopic composition of a clinopyroxene concentrate is extremely depleted ( 143Nd/ 144Nd=0.514420), indicating an early Jurassic time of depletion ( T DM=187 Ma) and suggesting that this clinopyroxene represents residual mantle material. In contrast, the bulk peridotites are characterized by convex-downward REE patterns, coupled with low 143Nd/ 144Nd ratios and elevated 87Sr/ 86Sr ratios. We suggest that these geochemical signatures resulted from secondary processes during or after emplacement of the ophiolite complex.