Origin of Igneous Rock Fragments from South Louisiana Salt Domes

Minghua Ren¹, Robert Stern², Brian E. Lock³, Randy Griffin²,

Elizabeth Y. Anthony¹, and Ian Norton⁴

¹Department of Geology, University of Texas at El Paso,

500 W. University Ave., El Paso, Texas  79968-0555

²Geosciences Department, University of Texas at Dallas,

800 W. Campbell Rd., Richardson, Texas  75080-3021

³Department of Geology, University of Louisiana,
P.O. Box 44530, Lafayette, Louisiana  70504

 ⁴Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin,

4412 Spicewood Springs Rd., Bldg. 600, Austin, Texas  78759-8500

ABSTRACT

Several years ago, samples of mafic igneous rock were found incorporated in salt diapirs within Avery Island and Weeks Island mines, Louisiana.  At the time, several alternative hypotheses were proposed for the source of this material.  More recently, an additional, slightly larger sample was recovered from the Avery Island mine which has been re-examined, together with the original samples.  The rock types have been identified as high-Mg ultrabasic lavas that plot as “continental-rift basic rocks” with volcanic textures.  Xenolithic quartz grains within the igneous rocks suggest contamination from metamorphic continental crust.  A newly obtained ⁴⁰Ar/³⁹Ar date of 160.1 ± 0.7 Ma from separated amphiboles indicates an Oxfordian (early Late Jurassic) age.  This age helps eliminate several of the early hypotheses and enables a more confident interpretation for the origin of the igneous rocks.

Weeks Island and Avery Island salt domes are part of a linear trend of shallow domes, known as the Five Islands, striking northwest-southeast.  This particular lineament is, in turn, a component of a sub-regional system of northeast-southwest and  northwest-southeast features that may be a surface expression of basement faults and fractures dating from crustal extension (transitional crust) during the early opening of the Gulf of Mexico Basin.  It is now possible to propose that igneous activity accompanied the opening and that magmatic rocks were picked up by the salt as it started to form diapirs.  The Five Island trend is believed to consist of diapirs whose initial movement was triggered by salt thickness irregularities along a basement fault, and the fault may also have provided a pathway for the picritic magma through the stretched continental crust.

It is noted that the end of salt deposition was essentially coeval with the start of formation of oceanic crust in the central Gulf of Mexico.  The new radiometric date fits closely with this event. 

Ren, M., R. Stern, B. E. Lock, R. Griffin, E. Y. Anthony, and I. Norton, 2009, Origin of igneous rock fragments from South Louisiana salt domes:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 641-651.

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana