Abstract

Reconnaissance field studies on the Patagonian Batholith exposed S of Tierra del Fuego, together with 49 new K-Ar and Rb-Sr mineral dates, render new insights into two main aspects of the late Mesozoic geology: the geochronology and composition of the batholith, and time relations between the emplacement of the plutons and deformation. The envelope of the batholith in this area is mainly composed of late Jurassic-early Cretaceous sedimentary and volcanic rocks formed in an island arc-marginal basin system.

Three main plutonic groups have been recognized which have yielded mineral dates, interpreted as crystallization ages, ranging from 141 to 34 Ma. The earliest is a gabbroic assemblage, including gabbros, diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 Ma. The Canal Beagle Plutonic Group is composed of granitoids characterized by a penetrative syn-magmatic foliation and with dates of 113 to 81 Ma. The Seno Ano Nuevo Plutonic Group is formed of granitoids with dates of 60 to 34 Ma and is mainly exposed in the outer islands, indicating a local oceanwards shift of plutonism with time.

Lower Cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage, and, in part at least, of the Canal Beagle Plutonic Group. Folding of the marginal basin infill, which has been related to the closure of the marginal basin, occurred from about 100 to 93 Ma ago. The gabbroic assemblage was apparently mainly pre-tectonic, whereas the Seno Ano Nuevo Plutonic Group was clearly intruded after this deformational event. A post-tectonic or possibly late-tectonic emplacement for the youngest plutons of the Canal Beagle Plutonic Group and a pre- or syn-tectonic intrusion for the oldest is inferred.

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