Permian back-arc extension in central Inner Mongolia, NE China: Elemental and Sr-Nd-Pb-Hf-O isotopic constraints from the Linxi high-MgO diabase dikes

摘要

Early Permian (272 +/- 2Ma) diabase dikes from the Linxi area in central Inner Mongolia of NE China have high MgO (10.4-12.3wt%), Cr (301-448ppm) and Ni (167-233ppm) concentrations, and show enrichments in large ion lithophile element (LILE) and light rare earth elements (REE) but depletions in high field strength element (HFSE, e.g., Nb and Ta), with depleted mantle-type Sr [Sr-87/Sr-86 (i)=0.70315-0.70362], Nd [epsilon(Nd) (t)=+6.8-+7.4], Pb [Pb-206/Pb-204 (i)=18.10-18.16] and zircon Hf [epsilon(Hf) (t)=+14.7-+19.1] isotopic compositions, but slightly higher zircon O-18 (5.2-6.0 parts per thousand with an average of 5.7 parts per thousand) than normal mantle. The combined geochemical data indicate their derivation from a depleted mantle metasomatized by recycled crustal component. Elemental and isotopic modeling results suggest that the primary magma was produced through 5 % to 10 % melting of a depleted mantle, which contained approximately1 % sediment fluid released from the subducted paleo-Asian Ocean. Considering the widespread distribution of contemporaneous mafic rocks across the central Inner Mongolia, which show REE patterns from E-MORBs to normal MORBs, we propose a petrogenetic link between the Early Permian mafic magmatism and a back-arc extension in response to northward subduction of the paleo-Asian Ocean. The Permian mafic magmatism and the new age constraints from the metamorphic and sedimentary records in this area tend to indicate the ultimate closure of the paleo-Asian Ocean by the end of Paleozoic.