Juvenile subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt

摘要

Mineral compositions, clinopyroxene trace-element and Sr-Nd-Hf isotopic compositions, whole-rock major-element and Platinum Group Element (PGE) contents, and Re-Os isotopes of peridotite xenoliths in the Cenozoic alkali basalts from Abaga, Inner Mongolia (China) help to constrain the nature of the subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt. The mantle xenoliths are mainly spinel lherzolites, with minor olivine websterite and spinel harzburgite. Clinopyroxene-orthopyroxene equilibration temperatures are higher than 840°C. An olivine websterite (sample A2-2) has the lowest MgO and highest Al _2O _3, CaO and TiO _2 contents, whereas a harzburgite (sample A2-1) has the highest MgO and low Al _2O _3, CaO and TiO _2 contents. The spinel lherzolites have major-element concentrations transitional between the harzburgites and websterites. According to the chemical characteristics and Sr-Nd-Hf isotopic compositions of clinopyroxenes, two groups of spinel lherzolites can be identified. Group 1 spinel lherzolites are moderately depleted in Light Rare Earth Elements (LREEs) and with flat Heavy Rare Earth Element (HREE) patterns, indicating a low degree of melt extraction and minimal metasomatism. They have clinopyroxenes with homogeneous ~(87)Sr/ ~(86)Sr (0.7030-0.7033), ~(143)Nd/ ~(144)Nd (0.5131-0.5132) and 176Hf/ 177Hf (0.2834-0.2835) ratios. They have flat chondrite-normalized PGE patterns and radiogenic ~(187)Os/ ~(188)Os ratios (0.1232-0.1256), within the range of modern convective mantle. Group 2 spinel lherzolites have low HREE contents with variable LREE concentrations, suggesting complex melt extraction and mantle metasomatism processes. They have relatively heterogeneous Sr-Nd-Hf isotopic compositions with ~(87)Sr/ ~(86)Sr of 0.7040-0.7054, ε _(Nd) values of +6.1 to +12.1 and ε _(Hf) values of +19.2 to +474. The Group 2 spinel lherzolites have flat chondrite-normalized PGE patterns and radiogenic ~(187)Os/ ~(188)Os ratios (0.1231-0.1252). The websterite has a magmatic origin with weakly decoupled Nd-Hf isotopes and the lowest ~(187)Os/ ~(188)Os ratio of 0.1219. The harzburgite has a ~(187)Os/ ~(188)Os ratio of 0.1280 similar to that of the modern convective mantle. All these characteristics indicate that the Group 2 spinel lherzolites represent ancient lithospheric mantle that experienced high-degree melt extraction and subsequent extensive silicate metasomatism. Thus, it is proposed that the SCLM beneath the Abaga region of the eastern Central Asian Orogenic Belt (CAOB) is mostly juvenile with minor relicts of old depleted lithospheric mantle, which cooled from near-adiabatic to near-lithospheric temperatures to form new lithospheric mantle during the formation of the eastern CAOB.