Southern Louisiana salt dome xenoliths: First glimpse of Jurassic (ca. 160 Ma) Gulf of Mexico crust Robert J. Stern1*, Elizabeth Y. Anthony2, Minghua Ren2, Brian E. Lock3, Ian Norton4, Jun-Ichi Kimura5, Takashi Miyazaki5, Takeshi Hanyu5, Qing Chang5, and Yuka Hirahara5 1Geosciences Department, University of Texas at Dallas, Richardson, Texas 75083-0688, USA 2Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA 3Department of Geology, Box 44530, University of Louisiana, Lafayette, Louisiana 70504, USA 4Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758, USA 5Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan ABSTRACT Salt that was deposited on top of the Late Juras- No direct information about the age and composition of rift-related igneous activity associ- sic crust, occasionally bring up samples of igne- ated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are ous rocks (Lock and Duex, 1996; Ren et al., deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples 2009). The Five Islands of southern Louisiana of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie are part of seven uniformly spaced salt diapirs an isolated magnetic high, which may mark the position of an ancient volcano. Three samples that defi ne a linear northwest trend (Fig. 1). The from two domes were studied; they are altered but preserve relict igneous minerals including Five Islands trend overlies transitional crust strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. thought to have formed during Gulf of Mexico 40Ar/39Ar ages of 158.6 ± 0.2 Ma and 160.1 ± 0.7 Ma for Ti-rich biotite and kaersutite from two opening (Dobson and Buffl er, 1997; Harry and different salt domes are interpreted to represent the time the igneous rock solidifi ed. Trace Londono, 2004). The diapirs containing mafi c element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks (Jefferson, Avery, and Weeks) igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of rise over a magnetic high (Fig. 1), as might be Nd and Hf indicate derivation from depleted mantle. This information supports the idea that expected from a signifi cant volume of buried crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern mafi c igneous rocks. Salt mine exposures reveal fl ank of the Gulf of Mexico ca. 160 Ma. These results also confi rm that some magnetic highs highly deformed bedding defi ned by interlay- mark accumulations of mafi c igneous rocks buried beneath thick sediments around the Gulf ered halite and anhydrite with inclusions of of Mexico margins. Oligocene sandstone, shale, and igneous rocks, as well as pockets of water, oil, and gas (Lock INTRODUCTION lack of correlatable, spreading-related magnetic and Duex, 1996). Structure in the salt domes The Gulf of Mexico opened as the western- anomalies. Indirect evidence indicates opening is essentially vertical, with multiphase isocli- most arm of Tethys, related to breakup of Pangea between ca. 165 and 139 Ma (Late Jurassic) and nal folding. The original stratigraphic position and synchronous with opening of the Central that the transitional crust ranges from a narrow, of the igneous samples is not known, but must Atlantic (Pindell, 1985). In spite of this general magma-rich volcanic rifted margin beneath the have been immediately beneath, interbedded understanding about when and how it opened, Texas coast to a broader, magma-poor rifted with, and/or intruded into or above the salt the Gulf of Mexico is a rare example where the passive margin beneath Louisiana (Mickus et (Fig. 2). The age of the Louann Salt is bracketed origin of a sizable oceanic basin at low latitudes al., 2009; Stern and Dickinson, 2010). We have on stratigraphic grounds (Salvador, 1991) as is unclear, due largely to thick blanketing sedi- no direct way to sample and study this Jurassic post–Early Jurassic to pre–late Oxfordian, prob- ments (to 16 km; Muehlberger, 1992) and the seafl oor, but salt diapirs, sourced from Louann ably mostly Callovian (165–161 Ma; Walker and Geissman, 2009). We report here mineral chemical, whole-rock chemical, Nd and Hf iso- Figure 1. Magnetic anom- topic compositions, and radiometric ages for Mississippi aly map of Louisiana Ala (United States) and en- Avery to Weeks ~12 km Louisiana virons. Red is magnetic 32°N high, blue is magnetic Texas low. Seven black dots are Cenozoic sediments Av salt domes of Five Islands Fl trend. Note that three salt J W domes containing mafi c 30°N xenoliths (J—Jefferson, Late Jurassic and Cretaceous 2. Alkalic mafics Av—Avery, W—Weeks) sediments intruded into salt? 3 . Lava lt? are above magnetic high s plucked above sa interpreted to mark pres- Louann Salt (176–158 Ma) ence of buried mafi c lavas. Magnetic anoma- 1. Alkalic mafics plucked from beneath salt? 28°N Gulf of Mexico lies are from Maus et al. (2007). Fl—Florida, Ala— Figure 2. Possible relationship between salt Alabama. 94°W 92° 90° 88° and alkalic mafi c rocks. Mafi c rocks could be plucked by rising salt from underneath or above, or could have intruded. Note distance *E-mail: [email protected]. between two salt domes containing xenoliths. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2011; 2011 v. 39; no. 4; p. 315–318; doi: 10.1130/G31635.1; 4 fi gures; 1 table; Data Repository item 2011108. 315 three samples of igneous rocks entrained in two (Fig. DR3). Diopside also exhibits core-over- 1 of these salt bodies. A third salt dome, Jeffer- growth textures with two distinct core composi- A 2EO kaersutitic amphibole son Island (Fig. 1), also contains altered mafi c tions: one with a high Cr, Si and a second with K/Ca I xenoliths (Balk, 1953), but samples of this were low Cr, Si (Fig. DR1). Rims for both core types 200 0.1 not available for study. This is the fi rst time that exhibit titanopyroxene (MgSi2 = TiAl2) and Ca- such information has been presented for igneous tschermaks (MgSi = AlviAliv) substitution and 180 Q rocks that formed when the Gulf of Mexico is are identical to matrix diopside, indicating that 160.1 ± 0.7 Ma (MSWD = 2.37) thought to have opened, in Jurassic time. We use the rim compositions were in equilibrium with 160 N P this information to further our understanding of the fi nal melt. The diopside cores and chromite LM O Gulf of Mexico formation. are probably xenocrysts and have compositions Apparent age (Ma) 140 similar to minerals in mantle rocks, e.g., abys- Integrated Age = 167.1 ± 0.8 Ma RESULTS sal peridotites from the Vulcan Fracture Zone 120 Methods are summarized in the GSA Data (Dick, 1989) or Samoan xenoliths (Wright, 100 Repository1 (Ar results and chemical and iso- 1987; Hauri and Hart, 1994). Kaersutitic amphi- 10 K/Ca topic analytical procedures). Three samples of bole occurs as small grains in W26 and as large B W26 biotitte porphyritic igneous rocks from two salt domes phenocrysts in 2EO (Fig. DR4). Similar min- 200 1 were studied. Sample 2EO is from the 1000′ eral associations have been reported for alkalic (~304 m) level of the Weeks Island dome, and magmas from the Jasper (Gee et al., 1991) and 180 W26 is from the 1300′ (~395 m) level and W25 Line Island seamounts (Natland, 1976). Ti- 158.6 ± 0.2 Ma (MSWD = 1.49) is from the 1600′ (~486 m) level of the Avery rich (~5%–7% TiO ) biotite is intergrown with 160 2 F K C E G H I J Island dome. The samples are altered, as is magnetite and quartz. Magnetite has substantial D obvious from petrographic examination, which jacobsite (Mn) and ulvospinel (Ti) components, Apparent age (Ma) 140 B reveals that primary igneous minerals such as typically (Mn0.5Fe1.1)(Fe0.9Ti0.5)O4. Relict igne- Integrated Age = 158.1 ± 0.3 Ma clinopyroxene are replaced by secondary quartz, ous feldspar was not found. This mineral assem- 120 vermiculite, calcite, hematite, and K-feldspar blage, i.e., mantle-like diopside and Cr-spinel 0 20 40 60 80 100 39 (Figs. DR2, DR8, and DR9 in the Data Reposi- with Ti-rich rims and Ti-rich hydrous phases of Cumulative % Ar Released tory). The K-feldspar assemblage is similar to biotite and amphibole, indicates involvement Figure 3. 40Ar/39Ar age and K/Ca spectra. A: Deep Sea Drilling Project Site 453 samples of moderately depleted mantle, overprinted by Weeks Island kaersutitic amphibole (sample described by Natland (1982). Intergrown with water-rich alkaline melts, probably low-degree 2EO). B: Avery Island biotite (sample W26). calcite is mcgillite, a manganous hydroxychol- partial melts. These melts were strongly alka- MSWD—mean square of weighted deviates. orosilicate (Stevenson et al., 1984). Because a line, most likely undersaturated basanite or oliv- signifi cant component of this alteration involves ine nephelinite (Anthony et al., 1989; Panina carbonate and water-rich vermiculite, a simple and Usoltseva, 2008). strength elements (HFSE, e.g., Ti, Zr, Hf, Nb, way to quantify alteration is with measured loss Biotite (W26) and kaersutitic amphibole Y; Fig. 4A). This interpretation is confi rmed on ignition (Table DR1 in the Data Repository) (2EO) with primary, igneous morphologies by elevated abundances of immobile incom- for whole-rock samples.