Jurassic (~160 Ma) Lamprophyric Xenoliths from Southern Louisiana Salt Domes: A Unique Perspective on Gulf of Mexico Crust (Invited)
Abstract
No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because these rocks are buried beneath several kilometers of sediments. Salt diapirs from southern Louisiana bring to the surface samples of the alkalic igneous rock lamprophyre; these salt domes rise from the base of the sedimentary pile to the surface. Two salt domes (Avery and Weeks) bring up igneous rocks that we studied; a third (Jefferson) brings up igneous rocks but these samples are lost. All three salt diapirs rise from an isolated magnetic high, which may mark the position of an ancient mafic volcano or intrusion in what appears to have evolved as a magma-starved rifted margin. Three lamprophyre samples from Weeks and Avery salt domes were studied, but we cannot tell whether these were lavas or shallow intrusions. The lamprophyres are altered but preserve relict igneous minerals including Cr-rich spinel Cr# ( = 100Cr/Cr+Al) ranging from 36 to 42 rimmed with titanite strongly zoned clinopyroxene (diopside to Ti-augite). Diopside cores have two distinct compositions: one with a high Cr, Si and a second with low Cr, Si. Rims for both core types exhibit titanopyroxene (MgSi2 = TiAl2) and Ca-tschermaks (MgSi = AlviAliv) substitution and are identical to matrix diopside, indicating that rim compositions were in equilibrium with the final melt. Excellent 40Ar/39Ar plateau ages of 158.6±0.2 Ma and 160.1±0.7 Ma for lamprophyric Ti-rich biotite and kaersutite from two different salt domes are interpreted to date when the lamprophyre solidified. Alteration disturbed primary igneous compositions, especially Si, alkali metals, alkaline earths, Pb and H2O, but not high field strength elements (HFSE, e.g., Ti, Zr, Hf, Nb, Y). Elevated abundances of immobile incompatible trace elements are one to two orders of magnitude enriched relative to N-MORB. However, isotopic compositions of Nd and Hf (ɛHf ~ +9, ɛNd ~ +7 at 160 Ma) indicate derivation from MORB-like depleted mantle. Trace element and isotopic data, considered together, imply that the lamprophyric magma was derived from low degree of partial melting (~2-3 %) of the mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern flank of the Gulf of Mexico ~160 Ma.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V13F..08S
- Keywords:
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- 1033 GEOCHEMISTRY / Intra-plate processes;
- 8415 VOLCANOLOGY / Intra-plate processes