The origin of enriched mantle beneath North China block: Evidence from young carbonatites

Abstract

A swarm of Late Triassic (220Ma) carbonatite dykes is emplaced into the deformed southern margin of the North China block (NCB) at Lesser Qinling, discontinuously extending for about 10km. The carbonatites are volumetrically minor, and their formation is related to collision between the South China block (SCB) and Qinling orogen, which led to the amalgamation of the NCB and SCB. The carbonatites are intruded into different Archean and Mesoproterozoic wall-rocks, but are characterized by remarkably similar isotopic compositions [(87Sr/86Sr)i=0.7048–0.7057; εNd=−4.3 to −10.1; 207Pb/206Pb=0.878–0.889 and 208Pb/206Pb=2.136–2.160], which approach, and trend toward slightly less radiogenic Sr and Nd values than, the enriched mantle component EM1. Proterozoic oceanic crust recycled through deep mantle is interpreted to be the principal source of carbon for the Lesser Qinling carbonatites. In comparison with most other young carbonatites (<200Ma) emplaced in a rift setting, the Lesser Qinling suite contains appreciably lower εNd and higher 207Pb/206Pb and 208Pb/206Pb values, which suggest the presence of an isotopically distinct additional component in its mantle source. The Pb isotopic signature of these carbonatites is significantly distinct from that of the Precambrian rocks in the North China block, but is similar to that of basement rocks in the South Qinling. On the basis of the available isotopic, geophysical and tectonic constraints, we suggest that the southern margin of the North China block was underthrust by crustal material derived from the South Qinling during their collision. The underthrusting contributed to thickening of the lower crust beneath the North China block and its conversion to dense eclogite. This process culminated in brittle delamination of the eclogitized material into the mantle and its metasomatic reworking by carbonate-rich melts derived from the EM1-type recycled Proterozoic crust. Carbonate metasomatism could produce an enriched sub-continental lithospheric source capable of yielding a variety of magma types.