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Monzonite suites: the innermost Cordilleran plutonism of Patagonia

Published online by Cambridge University Press:  03 November 2011

C. W. Rapela  and
R. J. Pankhurst
C. W. Rapela
Affiliation:
Carlos W. Rapela, Centro de Investigaciones Geólogicas, Universidad de La Plata, Calle 1 No 644, 1900 La Plata, Argentina.
R. J. Pankhurst
Affiliation:
Robert J. Pankhurst, British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, U.K..
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Abstract:

In Patagonia a Triassic-Early Jurassic Cordilleran interior magmatic belt preceded the widespread eruption of Middle Jurassic syn-extensional rhyolites. Two plutons (La Calandria and La Leona) represent the easternmost plutonic rocks of this belt, > 750 km east of the present oceanic trench. They define a high-K calc-alkaline monzonite series in contrast with the main Andinotype arc magmatism of the Pacific margin: they 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-level fractionation of plagioclase, hornbleńde, biotite, K-feldspar and accessories (sphene, apatite and zircon).

Initial 87Sr/86Sr ratios, average εNdt 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 attributed to partial melting of Mesoproterozoic mafic lower crust. The preferred model for the origin of the monzonites is remelting of an amphibole- + garnet-bearing, plagioclase-poor, high-K mafic source (?underplating). This occurred in a distal sector of a dying oblique subduction regime, immediately preceding the extensional silicic volcanism.

Keywords

granitesJurassicgeochemistryisotopespetrogenesismodellingsubductionSouth AmericaAndesDeseado Massif
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 87 , Issue 1-2: Third Hutton Symposium. The Origin of Ganites and Related Rocks , 1996 , pp. 193 - 203
Copyright
Copyright © Royal Society of Edinburgh 1996

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