Zircon U-Pb geochronological, geochemical, and Sr-Nd isotope data for Early Cretaceous mafic dykes in the Tancheng-Lujiang Fault area of the Shandong Province, China: Constraints on the timing of magmatism and magma genesis

摘要

The timing and source of magmatism that formed Early Cretaceous dolerite dykes in the Tancheng-Lujiang (Tan-Lu) Fault area of the southeastern North China Craton was determined using geochronological, geochemical, and whole-rock Sr-Nd isotopic data. Laser ablation-inductively coupled plasma-mass (LA-1CP-MS) spectrometry U-Pb analysis of zircon yielded consistent ages of 129.6 ±0.7, 126.8 ±0.7, 125.5 ± 0.7, 124.9 ± 0.9, 126.4 ± 0.7, and 125.5 ± 0.7 Ma for six samples of the mafic dykes within the NCC. The K_2O + Na_2O concentrations (5.02-5.21 wt.%) of the dykes indicate they are alkaline and these dykes have K_2O concentrations (2.35-2.48 wt.%) that indicate they are shoshonitic. These dolerites are also characterized by high and wide ranging (La/Ybfo (14.5-36.0), have slightly negative Eu anomalies (δEu = 0.70-0.91) and positive Ba, U, K, and Pb anomalies, and are depleted in the high field strength elements (Nb, Ta, P, and Ti). In addition, these mafic dykes are characterized by high radiogenic Sr [(~(87)Sr/~(86)Sr)_1 - 0.7099-0.7100] and negative ε_(Nd) (t) values (-14.4 to -13.7). These data suggest that the magmas that formed the dykes were derived through the partial melting (12.0-15.0%) of an enriched region of the mantle that was hybridized during interaction with subducted sedimentary rocks from the Yangtze Craton. The parental magmas then fractionated olivine and Fe-Ti oxides during ascent and underwent negligible crustal contamination during magma emplacement. These mafic magmas were finally emplaced as dyke swarms associated with lithospheric extension.