Geochronology and geochemistry of zircon for plutonic complexes of the Coastal Cordillera in Northern Chile

Abstract

The Coastal Cordillera represents the first stage of the Andean orogeny that started nearly 200 Ma ago with subduction reactivation after an anorogenic period. However, recent studies have questioned the tectonic regime of the pre-Andean cycle, proposing an uninterrupted extensional subduction since Early Carboniferous. Consequently, the nature of the pre-Andean tectonic cycle and the onset of the Andean orogeny are under debate. Moreover, even though there is strong evidence that support a multistage development of the early Andean continental arc, few works attempted to understand the petrological and geochemical changes experienced by this arc through time. The main goal of this work is to contribute to the understanding of the evolution of the early Andean Cordillera of northern Chile by identifying magmatic compositional changes using key petrological and geochemical signatures from intrusive rocks and correlate them with the tectonic changes that have been reported in previous studies to provide new insights on the Mesozoic evolution of the southwestern margin of Gondwana. In Chapter 2, I present the first zircon U-Pb/trace element dataset of plutonic complexes from the Coastal Cordillera of northern Chile, providing new constraints on the early evolution of the Andean orogeny. The oldest event in the study area (Cerros del Vetado pluton; ~247 Ma) is interpreted as a transitional magmatism belonging to the pre-Andean tectonic cycle; a controversial result respect to recently published articles. In contrast, the following event (Quebrada Quiscuda stock; ~211 Ma) presents typical characteristics of I-type, subduction related magmatism and similar zircon geochemical signatures with Jurassic to Early Cretaceous plutons. Based on these results, I propose that the Andean orogeny started at least 10 Ma before previous estimates, i.e., at 210 Ma during the Norian stage. Our results also indicate that plutons from the first stage of the Andean orogeny (latest Triassic-Early Cretaceous) were formed from a rather depleted mantle sources in a changing subduction regime, with less evolved signatures before the transition from extensional to transtensional settings (~150-130 Ma). Following this transition, magmas became enriched possibly from subduction contributions. In Chapter 3 a more detail sampling over a larger area provides additional constraints on the magmatic evolution of the early Andean continental arc and by inference the tectonic regime that controls the magmatic activity. Expanding the study area and the findings of the previous chapter, the chemistry and ages of the zircon from the plutonic complexes allowed us to: i) reject a stable tectonic framework for this period; ii) correlate the changes in whole-rock and zircon chemistry of the samples over time with the tectonic regime; and iii) characterize the magmatic conditions of each substage in the tectonic cycle. As a result, six plutonic episodes were identified between Late Triassic and earliest Late Cretaceous (215-94 Ma), each related to particular substages in the evolution of the arc. These episodes were defined based on their geochronological and geochemical features, are better identified and characterized by their zircon petrochronology, and could be associated with tectonic changes in the continental margin. Therefore, an external forcing model with mantle-derived inputs is argued for the episodic plutonism of this extensional continental arc. Finally, Chapter 4 briefly discusses and summarizes the main findings of this research and succinctly explores its potential implications for future studies.