MAGNETIC SUSCEPTIBILITY AND GEOCHEMISTRY OF GRANITIC ROCKS IN CAMBODIA

Abstract

Granitic rocks in Cambodia are divided into two groups, i.e. ilmenite-series in southern Cambodia and magnetite-series in northern Cambodia. Both groups belong to high-K calc-alkaline series, metaluminous to slightly peraluminous and display typical features of I-type granitic rocks. The ilmenite-series granitic rocks are characterized by high SiO₂ contents (70-75 wt. %) with abundance of quartz and K-feldspar, enrichment of LILEs, and strong negative anomalies of Ba, Sr, Eu (Eu/Eu*=0.1-0.5), Nb, and Ti. Chondrite-normalized REE patterns exhibit enrichment of LREE ([La/Yb]_N=1.4-17.6) with flat HREE patterns and flat to concave-upward MREE patterns. These granitic rocks have high CaO+FeO+MgO+TiO₂ and intermediate Al₂O₃/(FeO+ MgO+TiO₂) with intermediate magnesium number (Mg# = 22-38) (Mg# = 100 x MgO/[MgO+Total FeO]). Geochemical features of the granitic rocks suggest partial melting of crustal igneous rocks of intermediate composition where plagioclase was a major fractionating and/or residual phase. On the other hand, the magnetite-series granitic rocks show wide range of SiO₂ contents (59-70 wt. %), higher TiO₂, Al₂O₃, CaO, and MgO contents than ilmenite-series granitic rocks, and smallnegligible negative anomalies of Sr and Eu. The high CaO+FeO+MgO+TiO₂ and low Al2O3/(FeO+ MgO+TiO₂) coupled with high Mg# (32-48) suggest partial melting of amphibolite-type source with presence of plagioclase. The granites of Cambodia were formed in subduction-related tectonic setting. The ilmenite-series granites range from volcanic-arc granites to syn- and postcollision granites while the magnetite-series granitic rocks belong to volcanic arc granites.