Early Cretaceous highly positive ε_(Nd) felsic volcanic rocks from the Hinggan Mountains, NE China: origin and implications for Phanerozoic crustal growth

摘要

An early Cretaceous (135 ± 2 Ma) felsic volcanic suite of dacite and rhyolite from Huolinhe, NE China is characterized by large ion lithophile element and light REE enrichment and high field strength element (HFSE, e.g., Nb and Ta) and Ti-P depletion, and bulk silicate earth-like Sr L~(87)Sr/~(86)Sr(i) = 0.70409-0.70481], quite radiogenic Nd [ε_(Nd)(t) = +3.98 to +5.88], Pb [e.g., ~(206)Pb/~(204)Pb(i)=18.46-18.55] and Hf [ε_(Hf)(t)～+9.2] isotope compositions. Compared with contemporaneous mafic rocks in the region, these felsic rocks have even higher Nd and Hf isotopic ratios, precluding an origin through differentiation of coeval mantle-derived magmas. Isotope calculation results suggest that these magmas were derived from a preexistent mixture composed of mainly juvenile crust (70-80%), and a subordinate recycled crustal component (20-30%) having highly radiogenic Sr and Pb and unradiogenic Nd and Hf. About 25-30% melting of such a mixed source produced the primary dacitic magma. The rhyolites, which have relatively low MgO, FeO*, Al_2O_3, CaO, TiO_2, P_2O_5, Na_2O, Ba, Sr, REE, HFSE and Y, were differentiates of the dacites after removal of a fractional assemblage of hornblende + plagioclase + K-feldspar + apatite + zircon. Considering the prolonged events (from 262 to 130 Ma) that produced such highly positive ε_(Nd) felsicrnigneous rocks in the region, we prefer a pre-Mesozoic crustal growth model related to arc accretion associated with the Paleo-Asian Ocean subduction.