Early cretaceous gabbroic complex from Yinan, Shandong Province: Petrogenesis and mantle domains beneath the North China Craton

摘要

Sensitive high resolution ion microprobe ( SHRIMP) zircon U - Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO2, low FeO and TiO2 contents and a strong enrichment of light rare earth elements [(La/Yb)(n) = 11 - 50], but no Eu anomaly. They have low Nb/La (0.07 - 0.29), radiogenic Sr-87/Sr-86 (0.710) and unradiogenic epsilon(Nd)(t) (-15 to -13). These "crustal fingerprints'' cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (> 20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low Sr-87/Sr-86) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high Sr-87/Sr-86) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros ( 127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.