MONZONITE SUITES - THE INNERMOST CORDILLERAN PLUTONISM OF PATAGONIA

In Patagonia a Triassic-Early Jurassic Cordilleran interior magmatic b elt preceded the widespread eruption of Middle Jurassic syn-extensiona l rhyolites. Two plutons (La Calandria and La Leona) represent the eas ternmost plutonic rocks of this belt, >750 km east of the present ocea nic trench. They define a high-K calc-alkaline monzonite series in con trast with the main Andinotype are magmatism of the Pacific margin: th ey are enriched in large ion lithophile elements (K, Rb, Ba, Sr and Th ), LREE and P2O5 and depleted in HREE and Y, with low FeO/MgO ratio. The range of observed compositions (56-76% SiO2) resulted from high-le vel fractionation of plagioclase, hornblende, biotite, K-feldspar and accessories (sphene, apatite and zircon). Initial Sr-87/Sr-86 ratios, average epsilon Nd-t and mean depleted-mantle Nd model ages of the two plutons are 0.70487, -0.5 and 1050 Ma for La Calandria and 0.70509, - 1.4 and 1125 Ma for La Leona, respectively. They are thus isotopically more primitive than the Middle Jurassic rhyolites, previously attribu ted to partial melting of Mesoproterozoic mafic lower crust. The prefe rred model for the origin of the monzonites is remelting of an amphibo le- + garnet-bearing, plagioclase-poor, high-K mafic source (?underpla ting). This occurred in a distal sector of a dying oblique subduction regime, immediately preceding the extensional silicic volcanism.