文档共享_Beijing SHRIMP Center

Mafic dykes derived from Early Cretaceous depleted mantle beneath the Dabie orogenic belt_ implications for changing lithosphere mantle beneath eastern China [查看] LINGCHEN、CHANGQ.MA、JINY.ZHANG、ROGERMASON、CHAOZHANG

SHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxeneþhornblendeþplagioclase and yield a SHRIMP zircon U–Pb age of 111.65.3 Ma. They have a narrow range of SiO2 from 46.16% to 49.78%, and relative low concentrations of K2O (1.072.62%), Na2O (2.453.54%), Al2O3 (13.0414.07%), and P2O5 (0.420.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (87Sr/86Sr)i ratios from 0.704 to 0.705, eNd values from 3.36 to 4.33 and mantle-depletion Nd model ages (T2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative eNd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back-arc extension which resulted from the subduction of the Palaeo-Pacific Plate towards the Asian Continent.