Geochemistry and Sm-Nd and Rb-Sr isotopic composition of eclogite in the Lhasa terrane, Tibet, and its geological significance

A new occurrence of eclogite has been discovered in the Lhasa terrane, northeast of Lhasa, Tibet along the northern margin of the Gangdese magmatic arc. The eclogite occurs as a 500-1000 m-wide belt that extends over 60 km in an E-W direction, and consists of coarse-grained, massive, rutile eclogite, quartz eclogite and phengite eclogite. Whole-rock geochemistry and relatively high εNd(t) for the eclogites suggest that their protoliths were low-K oceanic tholeiites, similar to typical N-MORB, derived from a depleted mantle source. However, all analyzed samples have negative Nb anomalies in MORB-normalized trace element patterns suggesting a suprasubduction zone influence. Zircon SHRIMP U-Pb ages of the eclogite ranging from 242 ± 15 Ma to 291 ± 13 Ma are interpreted as the age of peak metamorphism. a Sm-Nd isochron age of 306 ± 50 Ma (2σ) likely reflects the protolith age. These data suggest that the Paleo-Tethyan ocean existed along the northern margin of the Gangdese island arc until the early Permian.