Mesozoic arc magmatism along the southern Peruvian margin during Gondwana breakup and dispersal

Abstract

A high-resolution U–Pb zircon geochronological study of plutonic units along the south Peruvian margin between 17° and 18°S allows the integration of the geochemical, geodynamic and tectonic evolution of this part of the Andean margin. This study focuses on the composite Jurassic–early Cretaceous Ilo Batholith that was emplaced along the southern Peruvian coast during two episodes of intrusive magmatism; a first period between 173 and 152 Ma (with a peak in magmatic activity between roughly 168 and 162 Ma) and a second period between 110 and 106 Ma. Emplacement of the Jurassic part of the composite Ilo Batholith shortly post-dated the accumulation of the volcanosedimentary succession it intruded (Chocolate formation), which allows to estimate a subsidence rate for this unit of ~ 3.5 km/Ma. The emplacement of the main peak of Jurassic plutonism of the Ilo Batholith was also closely coeval with widespread and repeated slumping (during deposition of the Cachíos Formation) in the back-arc region, suggesting a common causal link between these phenomena, which is discussed in the context of an observed 100 km trenchward arc migration at ~ 175 Ma, and the relation with extensional tectonics that prevailed along the Central Andean margin during Pangaea break-up.

Introduction

Changes in the temporal and spatial evolution of volcanic arcs partly reflect changes in the kinematics of subduction zones at plate margins. Intermittently magmatic arcs in Peru have been active since ~ 550 Ma (Cawood, 2005) as a consequence of subduction of Pacific plates beneath western South America. Therefore southern Peru can be considered as a type locality for long-lived, active continental margins (Chew et al., 2007, Mišković et al., 2009, Mukasa, 1986, Pindell and Tabbutt, 1995). Unlike other sections of the western South American margin, the Peruvian margin has not been modified by terrane accretion since the onset of subduction (Loewy et al., 2004, Mišković et al., 2009), mainly preserving original rock relationships, providing the opportunity to construct a chronostratigraphic framework for the evolution of the Jurassic arc system. This contribution presents geochronological and geochemical data from the Jurassic arc in southern Peru, which is used to constrain the tectonic evolution of the region during and subsequent to the fragmentation of Gondwana.

The Jurassic continental arc has been documented along almost the entire length of western South America, which formed during increased subduction-related magmatism after the fragmentation of Gondwana (Fig. 1). Plutonic, subduction-related rocks intrude Palaeozoic metamorphic rocks and Triassic high-temperature metamorphic rocks that formed during periods of extension associated with rifting in western Gondwana. Jurassic subduction is part of the active margin stage of the Pacific Wilson cycle along the Peruvian margin, although the exact timing of arc magmatism and the evolution of the continental margin have been poorly studied in southern Peru.

The composite Ilo Batholith fringes the coastline of southern Peru (17°–18° S), and forms the northern termination of the continuous Jurassic plutonic belt that extends southwards to central Chile (28°S). Jurassic plutonic rocks of the Ilo Batholith were emplaced as a large-scale tabular body within an extensional regime that has been documented during the Middle and Late Jurassic (Sempere et al., 2002). Slab roll-back and the formation of local pull-apart basins formed during southeast-directed oblique subduction of the Paleo-Pacific plate under the South American plate (Grocott et al., 1994, Jaillard et al., 1990, Scheuber and Gonzalez, 1999).

The Ilo Batholith intrudes through sparsely exposed, Grenvillian-aged basement (Loewy et al., 2004, Ramos, 2008b) of the Arequipa Massif (Fig. 1), which is defined by its radiogenic Pb isotopic compositions with low ²⁰⁶Pb/²⁰⁴Pb ratios, typical of high-grade rocks (Mamani et al., 2010), and is exposed along the coastal region of southern Peru and northern Chile. The highest levels of the Ilo Batholith intrude Late Triassic–Jurassic volcanic and volcaniclastic rocks of the Chocolate Formation (Romeuf et al., 1995).

We construct a detailed chronostratigraphy for the Jurassic arc along coastal southern Peru, and provide new age constraints for plutons of the Ilo Batholith, and volcanic and volcaniclastic rocks of the Chocolate Formation. Zircon U–Pb ages were obtained using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) analyses of magmatic and detrital zircons. The geochronological data have been combined with field observations, geochemical trace and major element analyses, and whole-rock Nd, Sr and Pb isotopes to investigate the evolution of Jurassic, plate-margin magmas, and the nature of the magmatic sources of the arc through time. The combination of these new data provides constraints on the temporal relationships between arc magmatism, back-arc extension and sedimentation along the southern Peruvian margin during the Jurassic.