Petrography and geochemistry of mafic igneous rocks from the South of Mune District, Vientiane Province, NW of Lao PDR

Abstract

The mafic igneous rocks from the south of the Mune District area, Vientiane Province, northwest of Lao PDR, is possibly a part of the central Loei sub-belt of the Loei Fold Belt in Thailand. These mafic rocks occur as shallow intrusive cover areas approximately 45 square kilometres and associated with sandstone, siltstone, and the Late Carboniferous shale to Early Permian. Based on petrographic studies, the south of Mune mafic samples show seriated texture except for sample numbers PK-2, PN-12, and PK-5, displaying porphyritic texture. They classify as gabbro and gabbronorite, mainly of plagioclase with subordinate clinopyroxene, orthopyroxene, and small amounts of opaque minerals. The plagioclase laths and clinopyroxene/orthopyroxene crystals may show disorderly orientation and ophitic/subophitic to plagioclase laths, respectively. Chemically, most representative mafic rock samples are comagmatic and show geochemical signatures analogous to MORB-type and have Zr/TiO2 and Nb/Y ranges from 0.00056 to 0.0127 and 0.1188 to 0.4481, respectively. They have typical chondrite normalized REE patterns of tholeiitic series, with (La/Yb)cn and (Sm/Yb)cn ranging from 0.94 to 3.69 and 1.11 to 1.58, respectively. The mafic igneous rocks from the south of the Mune District area have been probably formed as the major ocean basin or back-arc basin in the Late Devonian to Early Carboniferous.