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Journal of the Geological Society; 1999; v. 156; issue.4; p. 673-694;
DOI: 10.1144/gsjgs.156.4.0673
© 1999 Geological Society of London

Article

Mesozoic-Cenozoic evolution of the North Patagonian Batholith in Aysen, southern Chile

R. J. PANKHURST1, S. D. WEAVER2, F. HERVÉ3 & P. LARRONDO3

1 British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Keyworth, U.K.(e-mail: r.pankhurst{at}bas.ac.uk)
2 Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
3 Departamento de Geologia, Universidad de Chile, Casilla 13518, Santiago, Chile

A detailed Rb-Sr geochronological and geochemical study has been carried out on granitoids of the North Patagonian batholith in Aysén. The results for 25 plutonic bodies reveal a complex age pattern. The principal zones defined are, from west to east: Late Cretaceous (a single 76 Ma pluton), Early Cretaceous (c. 135 Ma), Eocene (c. 45 Ma), and Early Miocene (25-15 Ma), reverting to mid-Cretaceous (120-90 Ma) in the Main Cordillera. The igneous suite is typically metaluminous and calc-alkaline, with hornblende-biotite granodiorite and tonalite dominant, although small bodies of Late Miocene/Pliocene (c. 10-5 Ma) peraluminous leucogranite occur sporadically. Tertiary plutonism extends to gabbroic compositions and is concentrated in the vicinity of the dextral strike-slip Liquine-Ofqui fault zone. The highest initial 87Sr/86Sr ratios (c. 0.7050) occur in the Early Cretaceous group, with a sharp decrease to 0.7034–0.7045 that persists until a Late Miocene reversal to higher values (0.7040–0.7048). These variations are reflected by Nd isotopes, but trends in the {varepsilon}Sr, v. {varepsilon}Nd, diagram show that this is not due to contamination from the accretionary complex into which the batholith was emplaced. An origin by melting of mafic crustal underplate and lower crust is suggested for the main magma suite. The discrete episodes of magmatism correlate with significant changes in subduction kinematics.

Key Words: granites • absolute age • isotopes • petrogenesis • tectonics




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