Peridotite Xenoliths And Zircon Megacrysts In Cenozoic Basalts From Se China——keys To The Nature And Modification Of Subcontinental Lithospheric Mantle

Refractory subcontinental lithospheric mantle (SCLM) is produced by the removal of partial melts from mantle rocks; this process includes the removal of H2O. The nature and evolution of the SCLM is also strongly influenced by hydrous melts and fluids, which affect the physical and chemical properties of mantle minerals and rocks. Like some LREEs (La, Ce) or LILs (K), H2O behaves as an extremely incompatible component (Dperidotite/melt=ca0.007-0.009) in a melt/solid system, which makes it a sensitive tracer of melt-extraction and metasomatism in the SCLM. Thus an understanding of the H2O inventory of the SCLM provides another tool to study the nature and the evolution of the SCLM, and the role H2O has played during lithospheric modification.Peridotite xenoliths in alkali basalts consist mainly of nominally anhydrous minerals (NAMs, e.g., olivine, pyroxene) and are direct samples that may reflect the actual H2O budget of SCLM. This study has focussed on determining the H2O contents of peridotite xenoliths from four localities (Mingxi, Anyuan, Niutoushan and Qilin) in the Cathaysia block, SE China using Fourier Transform Infrared Spectroscopy (FTIR). Considering (1) the homogeneity of water distribution within single pyroxene grains;(2) the partitioning of water between cpx and opx with an average Dcpx/opx of2.3; and (3) the correlations between the H2O contents and major element concentrations in cpx, it is suggested that the pyroxenes have largely preserved the water content of their mantle source.The whole-rock water contents calculated from mineral modes range from12to94ppm (average60±20ppm). This is much higher than the previously-reported water contents of xenoliths from the North China Craton (NCC)(average26±17ppm). However, it is still quite low compared to those of continental lithospheric mantle worldwide, as inferred from analyses of typical cratonic (122±54ppm) and off-craton (81±40ppm) peridotites. The present relatively low water budget has evolved through multiple geological events over the long history of this SCLM, e.g., hydration due to paleo-Pacific plate subduction, dehydration by melt extraction during Yanshanian magmatism and subsequent rehydration due to the fluxing by low-degree astheno spheric melts after lithospheric thinning. This is evidenced by young basaltic volcanism at the surface. Water itself plays an important role during the modification of the subcontinental lithospheric mantle. The garnet Iherzolites, which represent the deepest portion of the SCLM (-1.9GPa) sampled in this study, have the highest water contents (>80ppm). This fertile garnet-facies layer with almost no extraction of water at depth may be young lithosphere accreted at the bottom of the pre-existing lithosphere. It marks the completion of the lithospheric thinning episode and the upwelling of asthenosphere.A negative correlation between pyroxene water contents and the oxygen fugacity has been found only in xenoliths from Niutoushan (Mg#<90), which lies on the Changle-Nan'ao fault zone. In contrast with the xenoliths from the other three localities, the Niutoushan xenoliths also have lowest water content (<60ppm) and Mg#<90). The fault may have facilitated the infiltration of the Niutoushan peridotites by oxidized fluids (or hydrous melts) rising from the subducting Pacific plate.Besides the abundant peridotites in the Cenozoic basalts in eastern China, there are also garnet, pyroxene and corundum megacrysts. As a key holder of Lu-Hf isotopic system, zircon megacrysts might be another probe into the SCLM. Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown-maroon, and some of them are up to16mm long. U-Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are19.2±0.7Ma,1.2±0.1Ma and4.1±0.2Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05-18.87Ma,0.9-2.2Ma,3Ma). εHf(t) values of zircon megacrysts are9.02±0.49,6.83±0.47,4.46±0.48for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.