Geochronology of diamond-bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: A record of complex histories from oceanic lithosphere subduction to continental collision

We report the results of a comprehensive study of zircons separated from the Luliangshan peridotite massif within the 400-km-long North Qaidam UHPM belt, northern Tibetan Plateau, NW China. The peridotite massif is dominated by garnet lherzolite with minor amounts of interlayered garnet-bearing dunite and cross-cutting garnet pyroxenite dikes. Most zircons from the garnet lherzolite show rather complex zoning. One diamond and a few graphite inclusions are identified in some zircons by Raman spectroscopy. SHRIMP dating on these zircons show four major age groups: (a) 484-444 Ma (weighted mean age, 457 22 Ma) for cores of most crystals, whose morphology and rare earth element (REE) systematics (i.e., very high [Lu/Sm](CN) = 88-230) suggest a magmatic origin, consistent with the protolith being magmatic cumulate; (b) 435-414 Ma with a mean of 423 5 Ma, which, given by mantle portions of zircon crystals, is interpreted to record the event of ultrahigh-pressure metamorphism (UHPM) at depths greater than 200 km in an Andean-type subduction zone; (c) 402-384 Ma (mean age 397 6 Ma) for near-rim portions of zircon crystals; and (d) 368-349 Ma for outermost rims, which is interpreted as representing some post-orogenic thermal events. Inherited cores in two zircon crystals were identified using CL and found to be Proterozoic. Morphology and CL images show that zircons from dunite and garnet pyroxenite are of metamorphic origin. The mean age of dunite zircons is 420 +/- 5 Mat which overlaps the mantle age of the garnet lherzolite zircon (see (b) above). The mean age of garnet pyroxenite zircons is 399 +/- 8 Ma, which overlaps ages of near-rim domains in garnet lherzolite zircons (see (c) above). Some garnet pyroxenite zircons also recorded a retrograde event at 358 +/- 7 Ma. All these data suggest that the Luliangshan garnet peridotite massif is not a fragment of ancient lithospheric mantle, but a peridotite body with long and complex histories from Early Ordovician to Late Devonian, including the emplacement of ultramafic cumulate probably in the shallow section of a mantle wedge, deep (> 200 km) subduction of the cumulate induced by the subducting slab, and ultimate exhumation genetically associated with continental collision. (c) 2005 Elsevier B.V. All rights reserved.