Gondwanaland plutonism in central Patagonia: Granitoids in the Gastre area (42°15', 69° 13')

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

Denver

Congreso; Geological Society of America, Centennial Meeting; 1988

Geological Society of America

The Gastre plutonic suite Ls part of the oblique late Paleozoic (?) to early Jurassic Central Patagonian batholith that extends discontinuously for about 1000 1cm, from the Panguipulli Lake in Chile (39°40’) to the Atlantic Ocean at 48S. These plutonic rocks, together with early Jurassic high-K calc-alkaline volcanic rocks, were formed in the youngest magmatic arc of the Gondwana supercontinent which predated the opening of the south Atlantic in late Jurassic early Cretaceous times. A detailed geochemical and petrographical study of the granitoid rocks in the Gastre area shows that these plutonic rocks can be divided into two suites, both of which are intrusive into an oldier Carboniferous metamorphic basement. The older suite is composed of foliated hornblende-biotite granodiorites and tonalites. The younger suite, known as the Lipetren suite, consists of small to intermediate size plutons of massive granite which intrude both the metamorphic basement and the older foliated suite. The Lipetren granites are leucocratic and can be divided into a biotite and a hornblende-bearing facies. A Rb-Sr isochron on the Lipetren suite yields a late Triassic-early Jurassic age of 208+/-1 Ma and an initial 7Sr/ Sr ratio of 0.7057. Major and trace element chemistry indicate that all of the Gastre granitoids are calc-alkaline and are similar to those found in magmatic arcs, consistent with the interpretation of the central Patagonian batholith as a magmatic arc. In detail, when compared with typical 1-type Andean granites, the central Patagonian granitoids have higher average Sto2 contents, higher levels of the LIL elements and higher 87Sr/ 86Sr initial ratios.