Early cretaceous crust–mantle interaction in the Middle-lower Yangtze River Metallogenic Belt, east China: Li–Nd–Sr isotopic and elemental constraints

Abstract

The petrogenesis of adakitic rocks and their implications for the crust–mantle interaction remain enigmatic. To provide important insights into these issues, we carried out a detailed investigation of Li–Nd–Sr isotopes and elements for the Early Cretaceous intermediate rocks and related mafic microgranular enclaves (MMEs) from the southwestern Middle–Lower Yangtze River Metallogenic Belt, central eastern China. The intermediate rocks in the Edong district are characterized by high SiO2, MgO, and K2O and low Na2O concentrations of 61.5–66.1 wt%, 0.4–2.3 wt%, 2.8–7.4 wt%, and 2.3–5.5 wt%, respectively, enrichment in LREEs and LILEs (e.g., Sr and La concentrations of 414.99–1619.45 ppm and 20.41–53.88 ppm), and relative depletion in Y (7.84–19.20 ppm) and Yb (0.59–1.73 ppm), endowed with both the high silica adakitic and high-K calc-alkaline affinities. The MME-bearing adakitic rocks exhibit wide ranges of (87Sr/86Sr)i, εNd(t) and δ7Li values (0.7028–0.7087, −8.9 to −3.0 and − 2.0‰ to +9.9‰, respectively), indicating a mixing process between felsic melts from a juvenile lower crust and mafic melts from an enriched mantle source. They were subsequently subjected to significant fractional crystallization of pyroxene and hornblende and minor crustal contamination during the ascent. High Sr/Y and La/Yb ratios, the salient geochemical peculiarities of adakitic rocks, might be inherited from magma sources and then be strengthened by fractionation of ferromagnesian phases. Hybrid magmas with high water content, and high oxygen and sulfur fugacity facilitate Cu ± Mo ± Au polymetallic mineralization. The adakitic rocks and related deposits in central eastern China could be related to upwelling of asthenospheric mantle caused by the subduction of the Paleo-Pacific plate during the Early Cretaceous and its induced reactivation of sub-continental lithosphere. The crust–mantle interaction model is probably applicable to formation of the adakitic rocks worldwide.