Geochronoloy and Geochemical Characteristics of Lundu Mafic Rocks in Kuching Area, Sarawak

Kalimantan Island is the largest island in the south of South China Sea that has undergone complex tectonic evolution, resulting from convergence of the Indian-Australian, Pacific and Philippine Sea plates. The Kuching area of Kalimantan Island has a series of Late Cretaceous magmatic rocks, but the distribution of basic rocks is less. The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period. In this study, it presents new petrographic, geochronological and geochemical data for the Lundu gabbroic pluton in the Kuching area. The basic rocks are predominantly made of dolerite and gabbro. Zircon U-Pb dating result shows that the crystallization age of the gabbro is 83.4±0.9 Ma, suggesting that the basic rocks intruded in the Late Cretaceous. These samples have low SiO2 ranging from 52.01% to 52.38%, K2O from 0.37% to 0.53% and TiO2 ranging from 0.81% to 0.92% with high Al2O3 of 14.00%-14.54% and MgO of 7.40%-7.86%. These samples are geochemically characterized by enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) and flat distribution of heavy rare earth elements (HREE) with light low REE abundance (∑REE= 43.96×10-6-48.19×10-6). The representative samples show low initial 87Sr/86Sr ratios (0.705 1 to 0.705 3) and positive εNd(t) values (2.1 to 3.3). Combination of trace elemental and isotopic results suggest that the parental magmas were likely derived from the mantle source modified by subduction-related fluid and sediments in the back-arc basin tectonic setting in the response of the subduction of the Paleopacific plate, and may link with Southeast China, Hainan island and Vietnam. © 2021, Editorial Department of Earth Science. All right reserved.