Jurassic segmentation of the early Andean magmatic Province in southern central Chile (35–39°S): Petrological constrains and tectonic drivers

Abstract

Jurassic volcanismin Southern-Central Chile (35°–39°S) is represented mainly by two units. The first characterized by the volcanic and subvolcanic mostly andesitic deposits of the Altos de Hualmapu Formation, located in actual Coastal Cordillera (35°–35°30′S) and the second corresponding to the lower basaltic and upper andesitic to dacitic upper member of Nacientes del Biobio Formation, in actual Main Cordillera (39°S). Both units mark the transition between northern and patagonian segments of Early Andean Magmatic Province (EAMP) that reach its maximum magmatic activity in this area during late Middle Jurassic in Coastal Cordillera, after two minor pulses of activity between Upper Triassic and Lower Jurassic. No evidence of arc activity is recorded in this area after 155 Ma, when volcanic axis seems to shift to actual Main Cordillera until Lower Cretaceous when it is resumed again to the west. The discontinuity of the arc front suggest the presence of a major cutoff in axis at ≈36–37°. Whole rock geochemical and isotopical Sr-Nd-Pb data shows that this areal discontinuity coincides with an enrichment of the magmas that suggest ≈20–30% of participation of an enriched source in the genesis of the magmas. Given the mostly extensional to transtensional tectonic regime of Western Gondwana during Jurassic and Lower Cretaceous it is unlike to assume high degrees of assimilation at shallow levels, so the observed enrichment should reflect the addition of fertile asthenospheric mantle dragged by the slab as result of the massive roll back and tearing of the oceanic plate under the Arc in Patagonia during Upper Triassic and Middle Jurassic.