The petrogenesis of southern Yare composite pluton in western Lhasa terrane: Constraints from zircon U-Pb geochronology and geochemistry

Miocene porphyry systems, which are closely associated with porphyry Cu-Mo deposit, have been found widely spreading in Lhasa terrane, Tibetan Plateau and already well studied. However, the study of barren porphyry in western Lhasa terrane is rare. This paper, base on the LA-ICP-MS U-Pb dating for southern Yare composite pluton, presents that this pluton is composited from Eocene biotite monzonitic granite (age 49.4 +/- 0.9 Ma) and Miocene porphyry (age 16.3 similar to 16.5 Ma). The Eocene biotite monzonitic granite are shoshonite series, metaluminous-weakly peraluminous, low Sr/Y and (La/Yb)(N) high Y and Yb-N rocks, shows typically arc magma. Meanwhile, the Miocene porphyry and shows typical shoshonite series, metaluminous and like-adakitic feature. The source magma of those two kinds of rocks are also different; the Eocene biotite monzonitic granite's magma come from the middle-lower crust which has been contaminated by the mantle wedge; the Miocene porphyry is the product of the partial melting of ancient crust. Combined with geochronology and tectonic setting, this paper infers that petrogenesis of the Eocene biotite monzonitic granite is crystal fractionation of the melt which come from the partial melting of the contaminated middle-lower crust caused by Neo-Tethys slab break-off; petrogenesis of Miocene porphyry is magma from ancient lower crustal melting triggered by some dynamic mechanism invaded to the upper crust.