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Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile

DOI: 10.4067/S0716-02082001000100002

Keywords: back-arc volcanism, 40ar-39ar ages, magma source, crustal extension, chilean patagonia.

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Abstract:

eighteen new radiometric ages (fourteen 40ar-39ar, four k-ar), combined with previously published ages, confirm the existence of three main extensional back-arc volcanic events, previously defined by stratigraphic relationships, in chilean patagonia (aysén region). these three events developed during the middle jurassic -early cretaceous (160-130 ma). cretaceous (114-75 ma), and eocene (55-46 ma). based on distinct geochemical data and sr-nd isotopic characteristics of the back-arc volcanic rocks collected north and south of 46°30's, two mesozoic-eocene magmatic domains are recognized: northern magmatic domain (nmd) and southern magmatic domain (smd). most analyzed basalts and intermediate volcanic rocks of the nmd have alkaline affinities and depleted to slightly depleted sr-nd isotopic values similar to those derived from an asthenosphere-dominated source. the smd mafic volcanic rocks have a subalkaline character and more enriched sr-nd isotopic signatures, comparable to those derived from a lithospheric source. the felsic volcanic rocks of the smd have lower end values and slightly higher initial 87sr/86sr ratios than the nmd felsic rocks, suggesting a larger crustal contribution in the magma sources. the geochemical and isotopic distinction between nmd and smd felsic rocks could be influenced by the presence of paleozoic metamorphic rocks as basement of the volcanic rocks of the smd. moreover, the compositional distinction between basalts of both domains may correspond to differences in magnitude of extension, the nmd being the one where the extension would have been greater and, consequently, the lithosphere thinner

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