Chemical and isotopic evolution of the coastal batholith of southern Peru

Abstract

Southeast of Arequipa, the Coastal Batholith of southern Peru is composed of two segments (Arequipa and Toquepala) including five superunits which were emplaced in discrete magmatic pulses from the Jurassic to the Paleocene eras (190–61 Ma). Most superunits intruded a Precambrian basement dominated by granulitic and amphibolitic rocks showing a strong enrichment in large ion lithophile elements, low εNd p (−21 to −29) and 206Pb/204Pbp (16.11–17.03 (Tilton and Barreiro, 1980)), and high εSr p (+396 to +999) values. Major and trace element analyses reveal that each superunit is formed by distinct suites of calc‐alkaline plutons (i.e., “I” type) that range in composition from quartz gabbro to monzogranite. For the whole plutonic suite located in southern Peru, the evolution toward negative εNd i and positive εSr i values is followed by a significant decrease in 206Pb/204Pbi ratios but is also related to the density of Precambrian outcrops. This led us to classify the intrusives into three groups. Group 1 consists of intrusives carrying positive εNd i (+2.4 to +0.4) and generally negative εSr i values (−7.4 to +0.7). They are located in the Ilo‐Moquegua transect (17°22′–17°80′S), an area where Precambrian exposure is scarce. Group 2 consists of plutons with intermediate ε values (i.e., εNd i = +0.5 to −2.2 and εSr i = +7.1 to +55.7), which are found in the vicinity of Arequipa and Tarata where numerous Precambrian outcrops are present. Finally, group 3 is composed of intrusives showing negative εNd i (−4.4 to −8.0) and positive εSr i values (+27.1 to +56.1), including one anomalous granodiorite exposed near Tarata and two samples collected in the Arequipa quadrangle near the contact with the Charcani gneiss. There are several petrogenetic models which can explain the trace element, isotopic, and geographic correlations observed within the Coastal Batholith of southern Peru. One simple model advocates that the parental mafic magma(s) of the plutonic suites of each superunit must be ultimately derived from an isotopically depleted mantle wedge above the subduction zone. The gabbro‐diorite‐granodiorite‐monzogranite association is formed via fractional crystallization and crustal assimilation (i.e., AFC) during ascent in the Andean crust. If the exposed Precambrian crust represents a good compositional average of the upper crustal material assimilated, then the isotopic and some of the trace element variations observed in the three groups can be related to various amounts of crustal assimilation (i.e., small for group 1, but substantial for group 3), although surprisingly little assimilation is required when considering all southern Peruvian intrusives. More complex models advocate different parental mafic magmas formed by various mixtures of lower crustal and mantle derived melts, both having heterogenous but depleted Nd and Sr isotopic compositions. The parental magmas would then ascend, differentiate, and assimilate upper crustal materials of different compositions (i.e., a crust similar to the exposed Precambrian basement for groups 2 and 3 and a less enriched crust, not exposed in the area, for group 1). If the first model is correct, then the relatively small degree of crustal assimilation experienced during the formation of the Coastal Batholith of southern Peru is consistent with the hypothesis that the thick Andean crust was thinned during the prevailing extensional tectonic regime that led to the formation of a marginal basin in central Peru during the late Mesozoic. This allowed the basic magmas to encounter the base of the Precambrian section at shallower depth and spend less time ascending and differentiating in this enriched crust, effectively limiting the amount of assimilation.