Petrogenesis of adamellites from eastern Shandong Province: geochronological, geochemical, and Sr-Nd-Pb isotopic evidence

Geochronology, geochemistry, and whole-rock Sr-Nd-Pb isotopes were studied on a suite of Mesozoic adamellites from eastern China to characterize their ages and petrogenesis. Sensitive high-resolution ion microprobe U-Pb zircon analyses were done, yielding consistent ages of 123.2 +/- 1.8 to 122.1 +/- 2.1 Ma for the samples. These rocks belong to the alkaline magma series in terms of K2O + Na2O contents (8.45-9.58 wt.%) and to the shoshonitic series based on their high K2O contents (5.23-5.79 wt.%). The adamellites are further characterized by high light rare earth element contents [(La/Yb)(N) = 14.96-45.99]; negative Eu anomalies (Eu = 0.46-0.75); positive anomalies in Rb, Th, Pb, and U; and negative anomalies in Sr, Ba, and high field-strength elements (i.e. Nb, Ta, P, and Ti). In addition, all of the adamellites in this study display relatively low radiogenic Sr [(Sr-87/Sr-86)(i) =0.7081-0.7089] and negative E-Nd(t) values from -16.70 to -17.80. These results suggest that the adamellites were derived from low-degree partial melting of an enriched lithospheric mantle below the North China Craton (NCC). The parent magmas likely experienced fractional crystallization of potassium feldspar, plagioclase and Fe-Ti oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during the ascent of alkaline rocks without crustal contamination.