Triassic "Gondwana" granites of the Gastre district, North Patagonian Massif

RAPELA, C.W.; PANKHURST, R.J; HARRISON, S.M.

EARTH AND ENVIRONMENTAL SCIENCE TRANSACTIONS OF THE ROYAL SOCIETY OF EDINBURGH

ROYAL SOC EDINBURGH

Año: 1992 vol. 83 p. 291 - 304

1755-6910

Granites of the Batholith of Central Patagonia were emplaced into late Precambrian metamorphic basement rocks and Palaeozoic orthogneisses (here dated by Rb-Sr whole-rock errorchrons at 346 ¡À35 Ma and 267 ¡À27 Ma), and are in fault-contact with younger volcanic rocks, mostly andesites. Two main suites of granites are recognized: both are much younger than their previously-supposed Late Palaeozoic age. The Gastre Suite is composed predominantly of hornblende-biotite granodiorite and monzogranite, often slightly foliated, and has yielded a Rb-Sr whole-rock isochron age of 220 ¡À3 Ma. The Lipetr¨¦n Suite includes biotite monzogranite and leucogranite, grading into quartz-feldspar porphyries and felsites, and has been dated at 208 ¡À1 Ma. A minor granodioritic unit yields an apparently Middle Jurassic age of 172 ¡À15 Ma. Textural evidence and hornblende geobarometry confirm that these are high-level sub-volcanic intrusions. Metaluminous compositions are common in the Gastre Suite, but are subordinate to highly siliceous (>70% Si02) and peraluminous varieties in the Lipetr¨¦n Suite. Despite this compositional bias, the granites are almost entirely calcalkaline and I-type, and have volcanic-arc rather than intraplate or collisional trace element characteristics. Initial 87Sr/86Sr ratios of O.706 and ¦ÅNdt, values of ¨C 2.5 are also relatively primitive and are thought to indicate a juvenile crustal contribution (Nd ¡°depleted mantle¡± model ages are Iess than 1,000 Ma). The Triassic ¡°Gondwana¡± magmatic episode is thus not an expression of Permo-Triassic collision of an allochthonous Patagonian terrane with the rest of southern S America, but may be related in the initial stages of supercontinent rifting.