Triassic lithospheric modification of the northern North China Craton: Evidences from the composite Kalaqin Batholith and ultramafic-mafic Heilihe Intrusive Complex in Inner Mongolia

Abstract

Whole rock major and trace elements and Sr-Nd-Hf isotopes and zircon in situ UPb ages and HfO isotopes have been determined for the Kalaqin granitic batholith and Heilihe Intrusive Complex (HIC) on the northern margin of the North China Craton (NCC), to investigate their magmatic sources and petrogenesis and further to reveal the lithospheric evolution of the craton. Zircon UPb dating, combined with the field observations and petrographic features, shows that at least 5 episodes of granites can be identified in the Kalaqin Batholith, i.e., 246 Ma monzogranite in the north, 234–227 Ma medium-grained monzogranite in the southwest, 224–221 Ma syenogranites in the north, 211 Ma granites in the south-central part and 206 Ma coarse-grained to megacrystic monzogranites intruding into the early batholith, and 238–231 Ma for the HIC ultramafic to mafic rocks. The 246 Ma granites have high SiO2 contents, negative whole rock εNd(t) and εHf(t) values, and negative zircon εHf(t) values and high δ18O values (up to 6.55‰), with ancient Nd and Hf model ages, indicating that they were derived from an ancient continental crustal source. However, the 234–221 Ma granites have positive whole rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating that they were derived from a relatively young crustal source. The granites of later episodes own some transitional isotopic feature and are suggested to be derived from a mixed source. The 238–231 Ma HIC rocks have high MgO contents, primitive mantle-like whole rock εNd(t) and εHf(t) values and zircon εHf(t) values and slightly higher zircon δ18O values than mantle. Zircon HfO isotopic constraints have further pointed to a mixed origin by depleted mantle and enriched lithospheric mantle for the HIC rocks. Calculation of the two-pyroxene thermobarometer yields crystallizing conditions of gabbronorites at around 1080 °C and 5 kbar, suggesting the emplacement of hot mantle-derived magmas into mid-crustal depth. All of the observations show that the lower continental crust beneath the northern margin of the NCC had been modified from ancient to relatively young by mantle-derived magma underplating during 246–221 Ma, although no solid evidences supporting the direct magmatic linkage were found between the batholith and the HIC. It is suggested that the lithosphere of the northern NCC had been modified and became weakened during 246–221 Ma, which makes the NCC easy to be destructed during late Mesozoic.