| Abstract | dc.description.abstract | Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 33 degrees S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U-Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28.8-5.2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the NazcaFarallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)(N) between 3.6 and 7.0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18.358-19.023 for Pb-206/Pb-204, 15.567-15.700 for Pb-207/Pb-204 and 38.249-39.084 for Pb-208/Pb-204. Initial Sr-87/Sr-86 are mostly in the range of 0.70369-0.70505, with two more radiogenic values at 0.7066. Initial Hf isotopic compositions of zircons yield exclusively positive epsilon Hf-i ranging between +69 and +96. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile elementhigh field strengh element depletion and high U-238/Pb-204. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantlecontinental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of approximate to 0.039 and mu approximate to 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (>90%) basalticgabbroic to andesiticdioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet- and spinel-peridotite stability fields. The genesis of more differentiated magmas reaching rhyoliticgranitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material. | en_US |
