Oxidation state of lithospheric mantle along the northeastern margin of the North China Craton: implications for geodynamic processes

摘要

Quaternary volcanic rocks in the Kuandian (KD), Longgang (LG), Changbaishan (CBS), Wangqing (WQ), and Jilin (JL) volcanic centres in eastern Liaoning and southern Jilin provinces contain mantle xenoliths of spinel-facies lherzolites and minor harzburgites. Among the study sites, the KD, LG, and CBS volcanic fields are located on the northeastern margin of the North China Craton (NCC), whereas the WQ and JL fields lie on the southern margin of the Xing'an-Mongolia Orogenic Belt (XMOB). The (Fo) components of olivine (01) and Cr# (=Cr/(Cr + Al)) of spinel, together with trace element abun dance of clinopyroxene, suggest that the subcontinental lithospheric mantle (SCLM) in the study area has undergone a low degree (4-6%) of partial melting. The rocks do not show modal metasomatism, but clinopyroxene grains in selected sam ples show elevated large ion lithophile element compositions, suggesting that the mantle xenoliths underwent minor cryptic metasomatism by exchange with a silicate melt. Two-pyroxene thermometry yielded equilibration temperatures ranging from 740°C to 1210°C. The corresponding oxygen fugacity (fO_2) was calculated to range from FMQ -2.64 to +0.39 with an average of-0.59 (n = 53). The oxidation state is comparable to that of abyssal peridotites and the asthenospheric man tle. We failed to discover differences in equilibration temperatures and oxidation state between lherzolites and harzburgites, suggesting that partial melting did not affect fO_2 values. In addition, similar f_2 of non-metasomatized and metasomatized samples suggest that metasomatism in the region did not affect fO_2. Our data suggest that the present SCLM beneath the northeastern margin of the NCC and the southern margin of the XMOB are very similar and likely formed from a fertile asthenosphere after delamination of an old lithospheric keel below the NCC in response to the west-dipping subduction of the Pacific oceanic plate since early to middle Mesozoic time.