Petrology of the Cretaceous magmatic rocks from Meratus Range, southeast Kalimantan

Abstract

45 Crataceous magmatic rocks from the Meratus Range, southernmost Kalimantan, have been studied by petrographic, chemical (major- and trace-elements) and mineralogical (microprobe) analyses. They belong to the Alino Formation (Upper Aptian to Cenomanian), the Peridotitic Nappe overthrusted on the Alino Formation, and the Upper Turonian to Senonian Manunggul Formation, uncomformably overlying the Peridotitic Nappe. All the magmatic rocks studied have suffered hydrothermal/greenschist facies alteration; therefore petrologic studies are concerned with “immobile” or “less mobile” major and trace elements and with compositions of relicts of the primary minerals (Ca-clinopyroxene phenocrysts). With the exception of the Riam Andungan plagiogranites, which have been considered as original member of the Peridotitic Nappe, all the volcanic and plutonic rocks of the Manunggul Formation as well as the plutonic rocks intruding the Peridotitic Nappe are definitely of island-arc calc-alkaline affinity. This is also true for most volcanic rocks of the Alino Formation, with the exception of analcite-bearing basalts which prove to belong to an alkaline (K-rich, silica-undersaturated) island arc association. Thus a subduction-related tectonic environment is proposed for the Middle and Late Cretaceous evolution of the Meratus Range both before (Alino Formation) and after (Manunggul Formation) the obduction of the Peridotitic Nappe.