Evolution of the Lüliangshan garnet peridotites in the North Qaidam UHP belt, Northern Tibetan Plateau: Constraints from Re–Os isotopes

Abstract

Garnet peridotite, interlayered with lesser amounts of dunite and olivine websterite and interwoven with garnet pyroxenite dikes, makes up a 0.5 km 2 massif near Lüliangshan in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, Northern Tibetan Plateau. Previous studies have suggested that the garnet peridotites represent Alaskan-style magmatic cumulates or oceanic serpentinites that were subducted to depths of > 200 km. Re–Os isotopic data provide new constraints on the origin and formation of the peridotites. Most dunites (Mg# 92–> 95) have high Os contents (2.90–4.25 ppb) and low 187Os/ 188Os (minimum 0.1108), while garnet pyroxenites have high 187Os/ 188Os (up to 0.2847) and low Os contents (0.03–1 ppb); garnet lherzolites lie between these extremes. The Archean model ages (TRD = 2.6 Ga, TMA = 2.8 Ga) of the most refractory dunites represent minimum estimates for the age of the original melt depletion. In an Os– 187Os/ 188Os plot, almost all garnet peridotites fall along two mixing lines: one is defined by mixing between the most refractory dunite and a garnet-rich pyroxenite, and the other by mixing between the dunite and a garnet-poor pyroxenite. The extremely depleted nature of the dunites, the high Ca and Al contents of the lherzolites, the variable enrichment in incompatible elements, and the Re–Os isotopic data suggest that the Lüliangshan garnet peridotites represent a fragment of subcontinental mantle, in which ancient residues after melt extraction (e.g. dunite) were refertilised by mafic melts similar to the garnet pyroxenites. Modelling of the garnet peridotite data suggests that at least some of the metasomatism is Paleoproterozoic in age; Re also may have been introduced into some of the garnet peridotites during the Paleozoic UHP Qaidam orogeny. The Lüliangshan peridotites are interpreted as refertilised Archean subcontinental lithospheric mantle, probably derived from beneath the North China Craton and entrained in the surrounding gneisses during their subduction into the mantle; the garnet peridotite assemblages may have formed during the original refertilisation event, or during the subduction.