Nature and evolution of the lithospheric mantle beneath the eastern Central Asian Orogenic Belt: Constraints from peridotite xenoliths in the central part of the Great Xing'an Range, NE China

摘要

Our knowledge of the lithospheric mantle beneath the Central Asian Orogenic Belt is still sparse. Petrologic, major- and trace-element studies on the peridotite xenoliths from the Cenozoic volcanic fields in the Aershan area, the central part of the Great Xing'an Range, NE China, provide insights into the nature and evolution of the lithospheric mantle beneath the eastern part of the belt. According to the REE patterns of clinopyroxene, these peridotites can be divided into three groups which show clear differences in microstructure, geochemistry and equilibration temperature. Group 1 xenoliths (LREE-depleted patterns of Cpx) are lherzolites, with protogranular microstructure and high modal Cpx (8-13 wt.%), low Cr-# in spinet (<21.1), high whole-rock CaO and Al2O3 contents and estimated temperatures of 834-849 degrees C. Group 2 xenoliths (flattened REE patterns of Cpx) are harzburgites, with microstructures transitional between mosaic and tabular and low Cpx content (2-3 wt.%); they have high Cr-# in spinet (41.1-49.6), low whole-rock CaO and Al2O3 levels and equilibration temperatures of 1183-1244 degrees C. Group 1 peridotites represent the newly accreted fertile mantle which was not significantly affected by post-melting enrichment; while Group 2 xenoliths may be older relics of moderately refractory mantle that underwent H2O bearing silicate-melt metasomatism (Ti/Eu > 3300, (La/Vb)(N) < 1 and occurrence of amphibole). Group 3 peridotites (convex-up REE patterns of Cpx) comprise both lherzolite and harzburgite; they have porphyroclastic microstructures, and show a broad range of Cpx modes (0-7 wt.%), spinel-Cr-# (26.5-71.1), bulk rock CaO and Al2O3 contents and temperatures (941-1239 degrees C). The high TiO2 contents (up to 1.42 wt.%) in spinels of Group 3 imply the involvement of melt/rock reactions. We suggest that the upwelling of asthenospheric material played a key role in modifying the lithospheric mantle underneath the eastern Central Asian Orogenic Belt, and resulted in mantle heterogeneity, which is characterized by juvenile fertile lithospheric material (e.g. Group 1) mixed with older, moderately refractory relics (e.g. Group 2), with the fertile type dominating the shallower levels. (C) 2015 Elsevier B.V. All rights reserved.