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Comparative geochemical interpretation of Permian-Triassic plutonic complexes of the Coastal Range and Altiplano (25°30′ to 26°30′S), northern Chile

By
Michael Brown
Michael Brown
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Published:
January 01, 1991

In the Chanaral–El Salvador area of northern Chile, Permian-Triassic plutonic complexes, emplaced into upper Paleozoic metasedimentary basement, occur in two parallel belts in the Coastal Range and Altiplano. Geochronological data show that these two belts of magmatic activity are coeval. In the Coastal Range, rock types present vary from tonalite, through granodiorite and granite, to leucogranite; in the Altiplano, leucogranite is not represented and granite is less common. To investigate whether the two magmatic belts could have been part of the same arc system and to discriminate their tectonic setting, 72 samples from the Coastal Range complexes and 55 samples from the Altiplano complexes have been analyzed for a range of major, minor, and trace elements, and a subset of 31 samples has been analyzed for the REE, Hf, Ta, Th, and U.

The geochemistry of the Permian-Triassic complexes from both belts is similar. Both are calcic and metaluminous to mildly peraluminous with TiO2 and Ni decreasing with increasing SiO2. Rb increases, Sr decreases, and Ba first increases and then decreases with increasing SiO2. K/Rb is variable around 200, and Ba/La may be constant or decrease with increasing SiO2. In general, the Altiplano complexes have similar total REE contents, degree of fractionation, and variably negative Eu anomalies to those from the Coastal Range, but without the flat REE patterns with extreme negative Eu anomalies characteristic of the leucogranites of the Coastal Range. The phases required in any fractional crystallization model to explain the REE concentrations and patterns are consistent with petrographic observations and petrogenetic modeling of other trace element distributions, such as Rb, Sr, and Ba. A model of mantle-derived magma that underwent modification by interaction with the crust followed by fractional crystallization is preferred, but the leucogranites could represent either extreme fractionation at high-silica contents or crustal melts. Trace element discrimination diagrams for tectonic interpretation of granitic rocks suggest a volcanic arc origin for both magmatic belts.

It is concluded that the Permian-Triassic plutonic complexes of the Coastal Range and Altiplano could have been part of the same magmatic arc system located on the western edge of South America. This arc was split by late Triassic to early Cretaceous behind-arc extension and basin development while South America was essentially static in mantle reference frame.

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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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