The Jurassic magmatism of Southern South America: Origin and Geodynamic Implications

RAPELA, C.W.

Mendoza

Congreso; 4th International Congress on Jurassic Stratigraphy and Geology; 1994

Asociación Geológica Argentina

A distinctive characteristic of Patagonia compared with other segments of the Andean chain is the voluminous Jurassic magmatic products, which cover large areas of the foreland and cratonic massifs as well as the modern are and back-arc sectors. Latitude 39°30’S approximately marks a first segmentation of the orogene: South of this the Andean sector contains the Patagonian batholith and the Late Triassic-Early Jurassic oblique granitoid belts, whereas extra-Andean Patagonia contains the Jurassic Volcanic Province (JVP), one of the largest known silicic volcanic fields. Several major episodes that occurred from Late Triassic to Late Jurassic times are weII recorded in these rocks. Northern Patagonia is characterized by two Late Triassic-MiddIe Jurassic calcalkaline oblique batholitic complexes. An outer NNW trending Subcordilleran Belt (SB) cropping out from 41° 30’S is composed of zoned plutons that range from gabbros and quartz diorites to monzogranites, as well as scattered small pIutons of grabbro and related dykes. Reported ages for the plutons of the SB, which show typical metaluminous “l-type” chemistry, range from 206 to 162 Ma. Granitoids from drilling cores in the western sector of the basement of the San Jorge basin have K-Ar ages in the rango 218 -170 Ma indicating that this feature is not restricted to the Precordilleran sector. The southeasternmost outcrops of Early Jurassic granitoids occur in the Deseado Massif (c. 48° 00’S; 67° 20’W). Rb-Sr whole-rock isochron ages of 202-203 Ma have been obtained in these granitoid complexes. Isotopic homogeneity shows consanguinity over a wide range of lithologies from quartz gabbro to syenogranite and aplite. The high-K calc-alkaline chemistry and isotopic signatures of these monzonitic complexes siggests a deep origin and emplacement in a distal zone of the orogene. An Inner Cordilleran belt known as the Batholith of Central Patagonia (BCP) trends NW-SE between 40-43°S. The BCP is spatially related to the dextral Gastre Fault System, a prominent shear zone at Ieast 3O km wide in northern Patagonia. Granitoid complexes of the BCP have a Iithology dominated by calcalkaline Ieucomonzogranites and granodiorites emplaced at shallow levels, with Rb Sr ages in the range 186-220 Ma. The SB and BCP are interpreted as outer and inner Cordilleran belts respectively, produced during a Late Triassic-Early Jurassic episode of northeastward oblique subduction in southwestern Gondwana. This regime may have been inactive in the Central and Northern Andes, where scarce calcalkaline magmatism and an extensional environment have been interpreted as evidence of the Iack of subduction between the Pacific paleoplate and the supercontinent (transform relationships?). The JVP is dominated by high-K rhyolites but there is a trend to quartz-normative trachydacites defined as transalkaline, and dacite. SiO2 content varies from 53 to 80 % but in most areas there is a more restricted range with > 70 % SiO2. Metaluminous to  peraluminous compositions predominate, but in the case of Peninsula de Camarones here is a further trend to peralkaline subtype with high Ti02 and Zr. The most striking characteristic of these rhyolites is their very constant mean initial 87Sr/86Sr ratio of 0.7067 ± 0.0005. Rb-Sr isochron ages of the JVP are concordant with Iocal stratigraphic controls and imply that Iocally eruptions occurred during a relatively short interval, but there was a significant southward diachronism of activity from 188 Ma at 41° to 168 Ma at 47°30’S. Basic to intermediate rocks of the JVP (53 to 65% Si02) fall in he local Rb-Sr isochrons of the acid rocks and show no difference in Nd isotope systematics, suggesting that both groups are essentially cogenetic. Although precluding its derivation from anatexis of supracrustal sequences, the Nd and Sr isotope relationships of the JVP plot in more “crustal” cluster when compared with the BCP and SB. The rhyolite complexes of the JVP were erupted in an extensional environment of graben and hemigrabens. To the west these associations were roughly coeval in northern Patagonia with high-K arc, andesites erupted over the BCP, and with the younger plutons of the SB. In southern Patagonia, the rhyolite field extends westwards, meeting the SW deflection of the Cordilleran axis, where contemporaneous events produced the oldest plutons of the Patagonian batholith (160- 165 Ma) and related volcanic rocks. The upper Middle Jurassic start of CordiIleran rnagmatic activity in southern Patagonia and the Central Andes, which continue episodically to the present, marked the end of the JVP.