Meimechite, Picrite, and Foidite Xenoliths Entrained in Louisiana Salt Diapirs

Salt diapers in the Gulf of Mexico region transport sedimentary and igneous xenoliths. Examples of igneous xenoliths include those in the El Papolete diapir in the La Popa basin, Nuevo Leon, Mexico (Garrison and McMillan, 1999) and in the Gulf coast of Louisiana (Lock and Duex, 1996). Three samples of volcanic rocks from Louisiana diapirs (Avery and Weeks) were studied. These samples were previously identified as serpentinites but thin section examination reveals porphyritic textures, most consistent with a volcanic origin. These samples are highly altered (LOI=9-14%); absence of feldspar for two of the samples indicates alkaline affinities. Chemical analyses (recalculated 100% anhydrous) reveal the samples have remarkably low SiO2 (32 – 34 wt %), low Al2O3 (11.7-13.5%), high MgO (15-22 wt %), and high TiO2 (3 to 4 wt %). One sample is meimechite, the high TiO2 equivalent of komatiite; the others are picrite and foidite. Relict mineralogy reflects the extreme composition of the magmas: clinopyroxene and amphibole are strongly ferroan, mica is kaersutite, and titanite forms as reaction rims on many of the phenocrysts. The spinel phase is Mg-Al chromite with Cr#~39. Radiometric ages have not been determined, but these lavas share closest affinities with mid- to late Cretaceous alkaline igneous rocks of Arkansas and the Balcones Igneous Province of Texas; however, the Louisiana xenolith samples are much more Mg-rich than any previously-reported igneous rocks from the Gulf Coast Alkaline Magmatic Province.