Tectonic implications of 40Ar/ 39Ar ages from a pre-Mesozoic metamorphic basement penetrated on Leg 77 of the deep sea drilling project in the southern Gulf of Mexico

Abstract

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the south-eastern Gulf of Mexico. At Site 538A located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite records 40Ar/ 39Ar plateau dates of c. 500 Ma, indicating early Paleozoic metamorphism. Biotite from the gneiss yields a plateau age of c. 350 Ma which likely dates a superposed late Paleozoic thermal event. A whole-rock diabase sample records a crystallization date of 190.4 ± 3.6 Ma. A phyllitic metasedimentary sequence was penetrated at Site 537 drilled 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display variably discordant 240Ar/ 39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at c. 500 Ma. The age and lithologic character of the basement terrane penetrated at Sites 537 and 538A suggest that the drilled fault block are underlain by attenuated fragments of continental crust of “Pan-African” affinity. This support pre-Mesozoic tectonic reconstructions which locate Yucatan in the present Gulf recess during amalgamation of Pangea.