LATE JURASSIC LAMPROPHYRES FROM LOUISIANA SALT DOMES

Thick sediments preclude direct sampling of crust beneath the Gulf of Mexico (GoM) and its flanks but Late Jurassic lamprophyres are brought up by salt diapirs in southern Louisiana. The Five Islands of southern Louisiana are part of seven uniformly spaced salt domes that define a linear NW-trend; these overly transitional crust formed during GoM opening. Diapirs containing mafic igneous rocks are associated with a magnetic anomaly, as might reflect deeply buried mafic igneous rocks. The Louann Salt is mostly Callovian (164.7-161.2 Ma). Sample 2EO is from Weeks Island dome and W25 and W26 are from Avery Island dome. All samples are altered; W26 is the least altered, followed by 2EO and then W25. Alteration resulted in an assemblage of quartz-orthoclase (Or90) – calcite-pyrite-hematite-gypsum. Mcgillite, a manganous hydroxycholorosilicate, is intergrown with calcite. We interpret the alteration assemblage as sea-floor/hydrothermal phenomena. We rely on fresh phenocrystic phases and immobile trace element abundances to infer original magma compositions. Phenocrysts include vermiculite (pseuodomorphic after biotite), diopside, Mg-Al chromite, titanite, and less kaersutite and Ti-rich (5 – 7 % TiO2) biotite. Phenocrystic biotite (W26) and kaersutitic amphibole (2EO) both gave well-defined Ar plateau ages: 158.6±0.2 Ma and 160.1±0.7 Ma. Diopside exhibits core/overgrowth textures with two distinct core compositions: high Cr, Si and low Cr, Si. Rims for both core types exhibit titanopyroxene (MgSi2 = TiAl2) and Ca-tschermaks (MgSi = AlviAliv) substitution and are identical to matrix diopside. Chromite and diopside compositions are typical of alkaline mafic to ultramafic magmas. Mafic composition, elevated abundances of high field strength elements (HFSE e.g., Ti, Zr, Hf, Nb, Y) and rare earth elements (REE), and fresh phenocrystic biotite and amphibole suggest that the appropriate name for the rocks is lamprophyre. This composition is typical of rift initiation magmas from strongly metasomatized mantle. Alteration affected fluid-mobile major and trace element compositions but not. Isotopic compositions of Nd and Hf are indistinguishable for the 3 samples, confirming the immobility of REE and HFSE. Isotopes indicate derivation of lamprophyric magma from depleted mantle.