Late Paleozoic Granitoid Gneisses of Northeastern Venezuela and the North America‐Gondwana Collision Zone

Abstract

We present here a new model for the origin and tectonic significance of quartz‐feldspar gneiss bodies that occur in restricted clusters within an elongate belt of low‐rank schist lining the present‐day northern coast of South America in eastern Venezuela (Paria Peninsula) and Trinidad. Conflicting origins have been ascribed to these gneisses, owing partly to faulty earlier dating. Pb/U data from abraded fractions of zircons from the Dragon Gneiss indicate discordia with a lower intercept of 321 (+13, ‐29) Ma. From petrography, zircon morphology, and rock composition, this intercept date is interpreted as the age of magmatic crystallization of a late Paleozoic granitoid plutonic protolith of the Dragon Gneiss. Petrography and 40Ar/39Ar and Rb/Sr dates on metamorphic white mica from the Dragon Gneiss and 40Ar/39Ar data from plutonic biotite of the El Mango Gneiss indicate transformation of protolith to gneiss occurred in mid‐Cenozoic time. This transformation was caused by protomylonitization and pressure solution, which varied in intensity from place to place. The gneisses evolved concurrently with the enveloping low‐rank schists whose precursors were sedimentary rocks of continental provenance and deposition. The gneiss protoliths are interpreted to come from Paleozoic continental basement, perhaps in a horst in the early Mesozoic passive margin of northern South America. The schist protoliths are Mesozoic and Paleogene strata of a passive margin wedge that lay above and around the horst. Mid‐Cenozoic convergence in the Caribbean‐South America plate boundary zone caused detachment of basement fragments and the passive margin wedge from sites to the NW or N of the current position of the gneisses. The deformation and metamorphism of the gneisses and schists were in an accretionary wedge, propelled continentward in the collision zone. Such inferences suggest that the continental edge is or was far to the north of the present coastline. The pluton or plutons that gave rise to the gneiss bodies presumably represent late Paleozoic orogenic activity in northern Venezuela. A late Paleozoic orogenic belt, the Apure allochthon, already recognized in western Venezuela, can now be extended to northern eastern Venezuela. This belt is the southern fringe of a very broad collision zone between Gondwana and southern North America that formed during the amalgamation of Pangea. Comparison of Ouachita and Apure allochthons that flank the collision zone suggests the Apure allochthon is a product of minor backthrusting at the back of the late Paleozoic continental arc of northern Gondwana.