Petrology and Geochemistry of the Eocene Volcanic Rocks in the West of Sechangi, Lut Block

The Eocene volcanic rocks exposed in the NW Sechangi area of Lut block, include pyroclastic deposits and lavas. These volcanic rocks are basalt, basaltic andesite, andesite, andesite-trachyandesite, trachydacite, dacite and rhyolite-ignimbrite in composition. Based on our field observations, the volcanic rocks erupted in four stages in the aquous to subaerial environments. The volcanic rocks are exposed as alternation of acidic and basic-intermediate rocks resembling bimodal volcanism. According to our petrographical investigations the acidic rocks include pyroxenite microenclaves which probably originated from the remains of a sub-continental lithospheric mantle. In the discriminating magmatic diagrams the volcanic lavas show shoshonitic to high-K calc-alkaline trends. In the normalized REE patterns and spider diagrams they display enrichments of LREE and LILE relative to HFSE, comparable to subduction-related volcanism. In the tectonomagmatic diagrams they cluster within the continental margin field. Interpretation of the geochemical diagrams indicates that the basic-intermediate volcanic rocks were produced by partial melting of lithospheric mantle which were previously enriched by the subduction components (fluids-melt) effects of which may not be distinguished from the crustal effects. The acidic rocks record some crustal elements which indicate involving of the crust in the generation of these rocks. It appears that after the closure of the Neotethys branches in Late Cretaceous and subsequent lithospheric thickening in the Lut block, the Eocene volcanic rocks in the Sechangi area were formed in collision extensional environment.