The role of changing geodynamics in the progressive contamination of Late Cretaceous to Late Miocene arc magmas in the southern Central Andes

Rosemary E Jones,Linda A Kirstein,Simone A Kasemann,Vanesa D Litvak,Stella Poma,Ricardo N Alonso,Richard Hinton

doi:10.1016/j.lithos.2016.07.002

Abstract

The tectonic and geodynamic setting of the southern Central Andean convergent margin changed significantly between the Late Cretaceous and the Late Miocene, influencing magmatic activity and its geochemical composition. Here we investigate how these changes, which include changing slab-dip angle and convergence angles and rates, have influenced the contamination of the arc magmas with crustal material. Whole rock geochemical data for a suite of Late Cretaceous to Late Miocene arc rocks from the Pampean flat-slab segment (29–31 °S) of the southern Central Andes is presented alongside petrographic observations and high resolution age dating. In-situ U–Pb dating of magmatic zircon, combined with Ar–Ar dating of plagioclase, has led to an improved regional stratigraphy and provides an accurate temporal constraint for the geochemical data.A generally higher content of incompatible trace elements (e.g. Nb/Zr ratios from 0.019 to 0.083 and Nb/Yb from 1.5 to 16.4) is observed between the Late Cretaceous (~72Ma), when the southern Central Andean margin is suggested to have been in extension, and the Miocene when the thickness of the continental crust increased and the angle of the subducting Nazca plate shallowed. Trace and rare earth element compositions obtained for the Late Cretaceous to Late Eocene arc magmatic rocks from the Principal Cordillera of Chile, combined with a lack of zircon inheritance, suggest limited assimilation of the overlying continental crust by arc magmas derived from the mantle wedge. A general increase in incompatible, fluid-mobile/immobile (e.g., Ba/Nb) and fluid-immobile/immobile (e.g., Nb/Zr) trace element ratios is attributed to the influence of the subducting slab on the melt source region and/or the influx of asthenospheric mantle.The Late Oligocene (~26Ma) to Early Miocene (~17Ma), and Late Miocene (~6Ma) arc magmatic rocks present in the Frontal Cordillera show evidence for the bulk assimilation of the Permian–Triassic (P–T) basement, both on the basis of their trace and rare earth element compositions and the presence of P–T inherited zircon cores. Crustal reworking is also identified in the Argentinean Precordillera; Late Miocene (12–9Ma) arc magmatic rocks display distinct trace element signatures (specifically low Th, U and REE concentrations) and contain inherited zircon cores with Proterozoic and P–T ages, suggesting the assimilation of both the P–T basement and a Grenville-aged basement. We conclude that changing geodynamics play an important role in determining the geochemical evolution of magmatic rocks at convergent margins and should be given due consideration when evaluating the petrogenesis of arc magmas.

Highlights

In order to obtain overall U–Pb ages for the Late Cretaceous–Late Miocene samples the data has been plotted on Tera-Wasserburg plots, where the U–Pb data is uncorrected for common Pb and the mixing line is anchored to the 207Pb/206Pb ratio for modern day common Pb (0.84) (Section 2, Supplementary Material)
How the trace element (TE) compositions have changed over time, due to what processes, and how this relates to the changing geodynamic setting of the southern Central Andean margin, is discussed below
On this basis it is suggested that the observed enrichments in fluid-mobile, incompatible elements (e.g., LILE) and depletions in HFSE and heavy rare earth elements (HREE) reflect processes occurring in the source region, rather than resulting from crustal contamination and/or differentiation processes

Summary

Introduction

The contamination of the melt source region with subducted sediments (e.g., Kilian and Behrmann, 2003; Lucassen et al, 2010; Sigmarsson et al, 1990), crustal material from subduction erosion (e.g., Goss et al, 2013; Kay et al, 2005; Stern, 1991), melts derived from the subducting oceanic plate (e.g., Reich et al, 2003; Stern and Kilian, 1996), as well as the contamination of arc magmas during ascent through the continental crust (e.g., Davidson et al, 1991; Hildreth and Moorbath, 1988; James, 1982; Wörner et al, 1992), has all previously been invoked to explain the petrological and geochemical characteristics of arc magmatic rocks from the Andean margin

Results

Discussion

Conclusion