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Geochemistry of the Triassic to Jurassic plutonism of central Chile (30 to 33°S); Petrogenetic implications and a tectonic discussion

By
Miguel A. Parada
Miguel A. Parada
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Beatriz Levi
Beatriz Levi
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Jan O. Nystrom
Jan O. Nystrom
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Published:
January 01, 1991

Two north-south–trending Triassic-Jurassic plutonic belts are recognized in the Andes of central Chile (30 to 33°S). Both belts contain plutons of similar age and spatially temporally related mafic (in part lamprophyric) and felsic dikes. The High Andes Belt (HAB) comprises Triassic to Lower Jurassic hornblende-biotite leucogranite, two-mica leucogranite, leucogranodiorite, and granodiorite. The Coast Belt (CB) located 100 km west of the HAB, is composed of Upper Triassic to Lower Jurassic biotite-hornblende leucogranite, related gabbro, and Middle to Upper Jurassic granitoids that range from gabbro to granite, with a dominance of hornblende-biotite tonalités, granodiorites, and diorites.

The HAB leucogranites are metaluminous to slightly peralkaline and similar to A-type granites. The HAB two-mica plutons are distinctly peraluminous, similar to S-type granites, and have highly fractionated rare-earth element (REE) patterns. The CB leucogranites exhibit alumina saturation indexes, Na2O/K2O and Rb/Sr ratios, and moderately fractionated REE patterns with large negative Eu anomalies similar to those of the HAB leucogranites, but they are less alkaline.

The HAB and CB leucogranites could result from partial melting of a quartzo-feldspathic source, probably upper Paleozoic plutonic rocks. The two-mica HAB leuco-granitoids could be derived from a garnet-bearing metasedimentary source, and the mafic dikes could be derived from subcrustal sources under conditions of high PH2O.

The compositional differences in the two belts suggest different tectonic regimes, with a major change in the conditions of magma generation during the Early Jurassic. Whereas the Triassic to Lower Jurassic leucogranites and mafic dikes developed in a tensional environment that promoted fusion and emplacement of crustal- and mantle-derived magmas, the Middle to Upper Jurassic granitoids represent subduction-related magmatism.

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GSA Special Papers

Andean Magmatism and Its Tectonic Setting

Russell S. Harmon
Russell S. Harmon
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Carlos W. Rapela
Carlos W. Rapela
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Geological Society of America
Volume
265
ISBN print:
9780813722658
Publication date:
January 01, 1991

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