Contrasting Enrichments in High- and Low-Temperature Mantle Xenoliths from Nushan, Eastern China: Results of a Single Metasomatic Event during Lithospheric Accretion?

Abstract

Distinct equilibration temperatures, deformation and trace element characteristics are observed in amphibole-bearing and amphibole-free peridotite xenoliths from Nushan, Sino-Korean Craton, eastern China. Amphibole-free peridotites are predominantly deformed, fine-grained ( 1 mm) and equilibrated at 990---1110 C. Their cpx are characterized by either light rare earth element (LREE)-depleted or relatively flat REE patterns with only a slight depletion in high field strength elements (HFSE). LREE enrichment is generally associated with Fe-rich samples, consistent with `wall-rock' metasomatism adjacent to basaltic veins. In contrast, amphibole-bearing peridotites are less deformed, coarse-grained (43 mm) and display chemical zonation in the pyroxenes suggesting cooling from 1050 to 850 C. Their cpx show a large variation in LREE (Cen ˆ 1 7---68) and almost constant heavy rare earth element (HREE) contents (Ybn ˆ 9 8---11 6). The highest LREE contents occur in cpx from amphibole-rich samples, coupled with Fe enrichment, strong enrichment in Th and U, and marked depletion in the HFSE. These characteristics may be accounted for by combined `wall-rock' and `diffuse' metasomatism involving an agent rich in volatiles and incompatible elements. As such the Nushan xenoliths could represent samples from two spatially separate metasomatic aureoles. Conversely, the cryptic and modal metasomatism could be genetically related, because the amphibole-peridotites and Fe-rich amphibole-free samples show similar Sr---Nd isotopic ranges that are indistinguishable from those of the Cenozoic basalts from eastern China. The different metasomatic assemblage and the trace element composition can be accounted for in terms of P---T control on amphibole stability and progressive chemical evolution of asthenosphere-derived melts during upward migration. Trace element signatures and metasomatic assemblages, together with the fertile composition of the Nushan peridotites, suggest an origin as newly accreted lithosphere rather than as relic cratonic mantle. Metasomatism may have occurred after late Mesozoic lithospheric thinning, which marked a dramatic change in lithospheric architecture beneath the Sino-Korean Craton.