Petrogenesis and tectonic setting of Late Paleozoic to Late Mesozoic igneous rocks in Cambodia

Abstract

Emplacement of igneous rocks from the Carboniferous to the Middle Jurassic of Cambodia was controlled by the Indosinian Orogeny, which was related to the sequence of suturing and collision between the continental fragments from Gondwana. To constrain magma genesis, tectonic setting, and relationship with igneous rocks in neighboring countries, we determined the magnetic susceptibility, chemical compositions, Rb-Sr isochron ages and Sr-Nd isotope ratios for igneous rock samples from Cambodia. Igneous rocks in Cambodia can be divided by the inferred Mae Ping fault into magnetite series in the NE region and ilmenite series in the SW region. The igneous rocks in the NE region showed I-type or A-type and metaluminous signatures, and strong involvement of mantle source materials. They are subdivided into older and younger adakitic rocks and non-adakitic rocks. The igneous rocks in the SW region showed non-adakitic and slightly metaluminous to peraluminous signatures. These have significant involvement of source materials with relatively uniform Nd isotope ratios but variable Sr isotope ratios, which may have originated from the lower to middle continental crust. The Late Triassic to Middle Jurassic igneous rocks were formed by the subduction of the Paleo-Tethys Ocean crust associated with the collision of the Sibumasu Terrane with the Indochina Terrane (the Indosinian Orogeny II). The younger granitic rocks showed A-type signature and are regarded as post-orogenic, within the plate or rifting zone that intruded in association with the subduction of the Paleo-Pacific Ocean crust beneath Indochina during the Cretaceous to the Paleogene.