Bulletin of the Geological Society of Malaysia, 43, pp: 49-61
Geological Research and Development Centre
JI. Diponegoro 57 Bandung 40122
Abstract: New geochemical and K/Ar data of the plutonic and volcanic rocks from the Meratus Range are presented. The data show that three main periods of magmatic activities occurred in this area, i.e. , Lower Cretaceous (131-103 Ma) represented by granitoid rocks, Upper Cretaceous (82-66 Ma) characterised by volcanic/subvolcanic and granitoid rocks, and Upper Paleocene-Lower Miocene (62-19 Ma) represented by volcanic and subvolcanic rocks. Although some samples show plagiogranites, which may have been derived from an oceanic basaltic magma, the Lower Cretaceous granitoid rocks are mostly calc-alkaline, I-type, subducted related volcanic arc granite (VAG). Preliminary geochemical studies on the Upper Cretaceous and Tertiary volcanics and subvolcanics confirm that they were produced in an island arc environment. The Upper Cretaceous volcanics vary from medium-K basalt to medium- and high-K andesite and dacite and the Tertiary volcanics, except one sample oflow-K basalt, are medium Kbasalt, andesite and dacite. Except the low-K Tertiary basalt, those two volcanics are enriched in LILE (Ba, Rb, Th and K), LREE (La, Ce and Nd) and depleted Nb relative to K and La. However, unlike many other subducted related rocks, part of the Upper Cretaceous andesite and most of the Tertiary volcanics have high MgO concentrations. While the high MgO Upper Cretaceous volcanics may have been produced in an immature island arc, the origin of the Tertiary high MgO volcanics is still open for discussion. Considering the tectonic evolution of the Meratus Range, the Tertiary high MgO volcanics might have resulted from a reaction between ascending melts and hot mantle peridotite.