K-Ar Ages and Geochemical + Sr-Nd Isotopic Compositions of Adakitic Volcanic Rocks, Western Shandong Province, Eastern China: Foundering of the Lower Continental Crust

Abstract

K-Ar age, geochemical, and Sr-Nd isotopic data are reported for western Shandong volcanic rocks with the objectives of deciphering the origin and dynamic significance of Mesozoic magmatic events within the North China craton. K-Ar ages indicate eruption of the volcanic rocks between 116.2 ± 1.1 and 120.3 ± 1.1 Ma. The studied rocks include andesite and dacite that have SiO2 contents ranging from 59.3 to 65.9 wt% and Al2O3 contents from 15.3 to 16.8 wt%; K2O and Na2O contents range from 3.4 to 5.1 wt% and 3.1 to 3.8 wt%, respectively, indicating that they belong to the K-rich series. These rocks are depleted in heavy rare-earth elements (HREE) and variably enriched in light rare-earth elements (LREE). They have positive Ba, Sr, and K and negative Nb, Ta, and Ti anomalies. The rocks have high Sr ranging from 492 to 1665 ppm and relatively low Yb (0.77-1.75 ppm) and Y contents (9.5-17.1 ppm), resulting in high Sr/Y (52-102) and La/Yb ratios (28-45), characteristic of adakites. εNd(t) and initial Sr isotopic compositions range from -13.2 to -10.3 and from 0.7089 to 0.7098, respectively. These geochemical features are consistent with an origin from adakitic magmas that were likely derived from dehydration melting of a delaminated lower crust. The relatively high MgO, Cr, and Ni contents in the analyzed rocks suggest that interaction between melts and peridotite may have occurred before eruption. This study provides further evidence for foundering of lower crust accompanied by lithosphere thinning beneath the North China craton (NCC).