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.

Eighty galena samples from different mineral deposits within the Chilean Andes from the Mesozoic and Cenozoic eras were analyzed for lead isotopes. The results show a restricted dispersion between 18.65 to 18.28, 15.68 to 15.56, and 38.72 to 38.11 on 2ø6Pb/2ø4pb, 2ø7pb/•ø4pb, and •øspb/•ø4pb ratios, respectively. Galenas from deposits hosted by preoJurassic to Jurassic rocks are relatively enriched in radiogenic Pb, reflecting the involvement of old crustal Pb. Galenas from Lower Cretaceous-hosted deposits are less radiogenic and may reflect a mantlelike component. Those from Tertiary rocks are isotopically more variable, reflecting the influence of a more evolved crust. There is no obvious correlation between the isotopic composition of the galena leads and the age of the hydrothermal event that gave rise to the particular mineral deposit (when such ages can be determined from independent geologic evidence). It is concluded that the galena leads were derived from a mixture of different lead reservoirs (as defined in Zartman and Doe, 1981). The correlation of isotopic composition with age of host rocks suggests either that the concentration of lead to form galena was more or less contemporaneous with the formation of the host rock or that the galena lead was essentially derived from the country rocks and inherited their original isotopic signature. Other workers have reported an overall similarity between the isotopic composition of galena leads and whole-rock leads from the Chilean Andes. However, the isotopic compositions of galena leads from the Argentinian Andes show different characteristics from those of the Chilean Andes, but the data are insufficient to allow firm conclusions.

With 3 figures ZusammenfassungBasische Magmen spielten eine bedeutende Rolle bei der friihen Entwicklung eines Abschnittes des Patagonischen Batholithen (141-34 Mio. J.), der s/.idlich Tierra del Fuego, Chile, aufgeschlossen ist. Der basische Magmatismus dauerte 40 bis etwa 60 Millionen Jahre (141-84 Mio. J.). Er besteht aus frfihen Gabbros und Hornblenditen (141-103 Mio. J.) und aus synplutonischen G~ingen und einigen der zahlreichen basischen Einschliisse in dem Granitkomplex der kretazischen Plutonit-Gruppe (113-84 Mio. J.). Einige der Einschlfisse werden als isolierte Anteile von synplutonischen G~ngen gedeutet.Die zur Verffigung stehenden Daten belegen, dai~ der basaltische bis rhyolitische Vulkanismus, der sich in diesem Gebiet w~ihrend der friihen Kreide ereignete, gleichzeitig mit dem hier vorgestellten basischen Magmatismus und in dessen letztem Stadium zusammen mit granitischen Magmen ereignete. Daher waren offensichtlich Gabbros an den Wurzeln des Inselbogens w~hrend der friihen Entwicklungsstadien beteiligt, w~ihrend Granotioide mit gr6igerem Anteil an dem sp~iterem Stadium des Wurzelsystems beteiligt waren. ResumenMagmas b~tsicos tuvieron un importante rol en la historia temprana del segmento det Batolito Patag6nico (141 a 34 M a) que se expone al sur de Tierra del Fuego, Chile. E1 magmatismo b~sico tuvo una duraci6n de unos 40 May hasta unos 60 Ma (141 a 84 Ma). Este magmatismo est~ representado por gabros y hornblenditas (141 a 103 Ma), por diques sinplut6nicos y por algunas de las numerosas inclusiones b~tsicas en los granitoides pertenecientes a un grupo plut6nico del Cret~cico (113 a 84 Ma). Algunas de las inclusiones se interpretan como partes de diques sinplut6nicos.La informaci6n existente permite indicar que el volcanismo bas~ltico y riol~tico que tuvo lugar en la regi6n, durante el Cretficico Inferior, rue contempor~meo con el magmatismo bfisico del que se informa en este artlculo. Aparentemente, la etapa final de este volcanismo habrla sido coetfineo con un importante magmatismo granltico. En consecuencia, aparentemente, en las etapas iniciales de la evoluci6n de la cadena volc~nica, los gabros habrlan constituldo parte de su ralz, mientras que posteriormente habrla habido mayor participaci6n de granitoides. Available data indicate that basaltic to rhyolitic volcanism which took place in the area during the Early Cretaceous was contemporaneous with the basic magmatism reported here and during its latest stage, with granitoid magmas. Therefore, gabbros apparently formed part of the roots of the island arc during the early stages of its evolution, whilst granitoids may have had a greater participation at a later stage in the root system. Abstract

No abstract

No abstract