Geochemical Compositions Of Cenozoic Basalts, Peridotites And Gem Zircons From Indochina Peninsula

Geochemical composition of continental basalts and their mantle xenoliths and xenocrysts can provide the best proxy record available for study of chemical-physical evolution of deep continental lithosphere and underlying mantle. Cenozoic continental basalts are widely distributed in the Indochina Peninsula of Southeast Asia. They are rich in mantle xenoliths and zircon megacrysts. Zircon megacrysts found in basalts may provide a window into metasomatic processes and element transport in the upper mantle. However, little is known about the formation conditions of these zircons. Furthermore, basalts contain a huge amount of iron, bauxite, and gem resources, which have drawn strong interesting for research. However,geological research on Cenozoic basalts and mantle xenoliths is still limited in Laos and Cambodia.In view of the important scientific significance of rocks mentioned above, and considering increasing competition of mineral resources and research imbalance of this area, we plan to study geochemistry of Cenozoic basalts and mantle xenoliths and gem zircons in Laos and Cambodia. This proposal aims to genesis of basalts and gem zircons by means of geochemical techniques. Taking basalts and mantle xenoliths and gem zircons as "rock probe", we try to clarify the properties and geological processes in the mantle source region and further, to constrain mechanism of deep mantle dynamics behind basalt genesis in the framework of geological evolution and spatial-temporal distribution of basalts in Indochina Peninsula. Geochemical data obtained in this proposal are extremely necessary for understanding tectonic setting of Cenozoic basalts in Indochina Peninsula and adjacent regions.Here we report major elements, trace elements, Sr-Nd-Pb isotopes data of the tholeiitic and alkali basalts from Laos and Cambodia, and major and trace elements data of minerals in spinel lherzolite from SE Cambodia, and U-Pb ages, Hf-O isotope compositions, trace element data and mineral inclusion analyses for six large fragments of gem zircons from the Gem village in NE Cambodia. Obviously, there is an decoupling between radiogenic isotopes and incompatible trace elements in the Cenozoic basaltic volcanism, with the incompatible element enriched, yet isotopically depleted (?Nd >0), which is similar to OIB. A recently and low-degree mantle metasomatism is required to explained the nature of the Cenozoic basalts. Basalt is derived from the decompression melting of asthenosphere mantle. The dynamic mechanism is the joint action of Indian-Eurasian plate collision and the wedge suction of western Pacific Ocean and Indian Ocean plate, that induce the asthenosphere mantle south-east extrusion below the Indochina Peninsula. The ring olivine and clinopyxene in the peridotite from SE Cambodia have the characteristics of gradually increasing trace elements content from the core to the rim. As well as, more incompatible elements have more enrichment trace elements content in the rim of ring minerals. That indicating the peridotite is subjected to the metasomatism of the silicate melt. The zircons have low contents in U, Th, Y, P, and rare earth elements(REE; 130 to 288 ppm). The normalized REE pattern is characterized by a steeply-rising slope from LREE to HREE and positive Ce-anomalies. SIMS isotopic analyses on the zircon megacrysts yield a mean U-Pb age of 0.98 ±0.04 Ma (2a) and homogeneous oxygen isotopic composition with a mean ?18O value of 5.0 ±0.02‰(2?), identical to the ?18O mantle value. A mean 176Hf/177Hf value of 0.283007±0.0000026 (2?), with ?Hf (0.98 Ma) value of 7.9, obtained by the LA-MC-ICPMS technique, further points to the origin of the grains in the depleted mantle. The zircon megacrysts contain apatite inclusions, and reaction rim composed of baddeleyite and Al-Fe-Zr-silicate glass. This leads us to conclude that mantle zircon crystallized during a metasomatic event from phosphate rich fluids and/or silicate melts enriched in Zr.