Enriched subcontinental lithospheric mantle (SCLM) in NE China: geochemical evidence from mafic volcanic rocks

Ming ZHANG and Suzanne Y. O'Reilly (GEMOC, Macquarie Univ., Australia)

Northeast China consists of two tectonic terranes: Sino-Korean Craton (formed during Archaean - mid-Proterozoic times) in the south and Xing'an-Mongolian Fold Belt (a Palaeozoic - early Mesozoic (?) orogeny between the Sino-Korean and Siberian Cratons) in the North. The nature and geodynamic evolution of the SCLM beneath the two blocks can be constrained from the geochemistry of widespread Cenozoic volcanic rocks (from olivine leucitites and nephelinites to quartz tholeiites). The diversified compositional range of these basaltic rocks indicates a significant role of the SCLM as a source component and provides constraints upon the nature of SCLM evolution.

Potassic rocks (6.5 Ma-Present) occur in five separate volcanic provinces over an area of ~300x300 km in the Fold Belt. Their distinct chemistry requires an EM1-type lithospheric mantle source with very low 143Nd/144Nd (eNd ²-12) and 206Pb/204Pb (² 16.30) ratios and exhibits noticeable fractionation between HFSE and LILE (eg U+Th vs Rb+K) and among various HFSE (eg Nb+Ta vs Zr+Hf). The inferred SCLM source is chemically similar to the Archaean mantle source for potassic rocks in Montana, NW USA. The evolution of such as an enriched mantle source requires ancient (mid-Proterozoic or older ages) metasomatic event(s) that resulted in high LREE/HREE and Pb/U ratios, high LILE abundances and HFSE fractionation in the source and preservation of such an enriched SCLM block beneath the Phanerozoic Fold Belt.

The Jingbo Lake province is located ³500 km southeast of the potassic provinces within the Fold Belt, at a second-order tectonic boundary between Palaeozoic and Neoproterozoic terranes and along one of the northern branches of the Tan-Lu deep fault. Small volume of quartz tholeiites erupted during Early Tertiary (45-42 Ma) are isotopically similar to the potassic rocks, suggesting possible extension of the EM1-type lithospheric mantle and overlying lower crust into this area. In contrast, chemical characteristics of the new episodes of alkali basaltic volcanism (16-0.01 Ma), such as the enriched smooth incompatible element patterns, vertical trends in 206Pb/204Pb vs 207Pb/204Pb variations and positive correlation of 206Pb/204Pb and 87Sr/86Sr, probably reflect interactions between two source components: a Dupal OIB-type deep mantle source common in NE China and another lithospheric component dominated by recycled terrestrial sediments.

Temporal chemical variations in basalts from Changbai Mts province at the northeastern margin of the Sino-Korean craton manifest gradual increase in trapping an enriched shallow EM2-type SCLM source in the last 20 Ma. With the transition from Miocene alkaline basalts to Quaternary tholeiites, the basalts are more enriched in terms of Sr-Nd isotope systematics (particularly high 87Sr/86Sr), decreased in 206Pb/204Pb and 208Pb/204Pb ratios but increased in 207Pb/204Pb. Relative depletions in Th, U, Nb, and LREE and enrichments in Ba, K and Sr also become more prominent with decreasing eruption age. This EM2-like SCLM source could have formed at the craton margin during the final Mesoproterozoic consolidation of the block. Alternatively, the enrichment may have resulted from subduction processes of the Pacific plate before the opening of the Japan sea.

Inferred chemical signatures of the garnet peridotite source for the Chinese potassic rocks are consistent with those observed from entrained shallow spinel peridotite xenoliths. The geochemical correspondence between the deep and shallow SCLM for the other provinces warrants further investigation.