CHARACTERISTICS OF THREE MEIMICHITE-PICRITE-FOIDITE XENOLITHS FROM TWO LOUISIANA SALT DOMES

Three mafic-ultramafic igneous xenoliths have been collected from the “Five Islands” salt domes of southern Louisiana. These xenoliths have chemical compositions that are similar to mid- to late Cretaceous alkaline igneous rocks of the Arkansas Alkalic Province and the Balcones Igneous Province of Texas; however, the Louisiana xenoliths are remarkably silica undersaturated (normalized SiO2 37-39 wt%), and are richer in TiO2 (3.2-4.5 wt%) and MgO (16-25 wt%) than any previously-reported igneous rocks from the Cretaceous Gulf Coast Alkaline Magmatic Province, and could represent magmatic activity associated with extensional episode in the Gulf of Mexico. These three xenoliths have compositions of meimechite, picrite, and foidite. The xenoliths all plot as “continental-rift basic rocks” in major and trace element discrimination diagrams. The xenoliths are porphyritic and strongly altered. Relict mineralogy reflects the extreme composition of the magmas. The matrices are composed of anhedral micro-scale sphene, clinopyroxene, and amphibole that are inside vermiculite masses. The surviving phenocrysts are clinopyroxene, Cr spinel, and amphibole; and almost all have sphene rims. Clinopyroxene phenocrysts are subhedral to euhedral, some preserved as cores inside amphibole. Spinels are anhedral with corroded rims. Large amphibole phenocrysts have rounded edges. The majority of phenocrysts have been altered to vermiculite that retain euhedral to subhedral pseudomorph shapes of phlogopite. There are some quartz grains in the xenoliths from Weeks Island salt diapir. These have a weak crystal shape with sphene rims. Each grain is composed of polygonalized quartz mosaic crystals. Some have anhedral clinopyroxene pieces in the quartz. The mineral chemistry of these clinopyroxenes within quartz has higher Si and Mg contents than the clinopyroxene phenocrysts. The texture and composition indicate these grains might come from some metamorphosed rocks. The Five Islands are on a trend that may reflect a pre-existing basement fault scarp which led to initiation and localization of salt diapirism. We have separated amphiboles from one of the xenoliths and hope to have a 40Ar/39Ar age by the time of the meeting.