Ancient sub-continental lithospheric mantle (SCLM) beneath the eastern part of the Central Asian Orogenic Belt (CAOB): Implications for crust-mantle decoupling

摘要

Potassic basaltic lavas erupted during the Cenzoic are widespread in Wudalianchi, Erkeshan and Keluo (WEK) areas, northeast China, and contain abundant mantle xenoliths. Mantle xenoliths in this study are selected from the Keluo potassic lavas, and mainly comprise spinel lherzolites and harzburgites with minor dunites and wehrlites. Modal metasomatism is evident in some Keluo xenoliths from the presence of phlogopite and rutile. Mineral compositions indicate that the Keluo xenoliths represent mantle residues after variable degrees of melting, i.e., 1-5% for lherzolites and 3-11% for harzburgites, with subsequent refertilization. Both bulk-rock and clinopyroxene data show enrichment in light rare earth elements (LREE) and some other incompatible elements (e.g. Ba, U). Clinopyroxenes from some of the Keluo xenoliths have radiogenic Sr isotopes, but unradiogenic Nd-Hf isotopes relative to the Cenozoic mantle xenoliths from other localities of the NE China. The Keluo mantle xenoliths have variable ~(187)Os/~(188)Os ratios ranging from 0.11458 to 0.13194. Although most Keluo mantle xenoliths have been affected by Re addition, their Os isotope compositions have not been significantly modified. The unradiogenic ~(187)Os/~(188)Os ratios of the refractory harzburgites give Re depletion ages (T_(RD)) of ～1.9-2.1 Ga. This suggests that the sub-continental lithospheric mantle (SCLM) beneath the Keluo area was formed during the Paleoproterozoic, which is older than the age of the overlying crust that was mainly formed since the Neoproterozoic. Therefore, the SCLM beneath the Kelou area is temporally decoupled from the overlying crust. It is unlikely that the Cenozoic SCLM beneath the Keluo area was newly accreted from the asthenosphere, in which ancient mantle domains were preserved. We propose that the ancient mantle beneath the Keluo region is extraneous and has been emplaced from other locations.