Age and origin of the gabbros in the Jilotlán pluton, Jalisco: primitive magmatic rocks in the southern part of the Guerrero terrane

Abstract

An outstanding feature of the Mexican Cordilleran magmatic arc between Sonora and Chiapas is the presence of numerous plutonic and batholitic bodies of Cretaceous to Miocene ages. In this contribution, we investigated the petrogenesis and age of the gabbros from Jilotlán, Jalisco, in order to better understand their genetic relation with the intruding and surrounding Late Cretaceous calc-alkaline mantle-type granitoids. Gabbros from Manzanillo (Colima), Tepalcaltepec, and Aquila (Michoacán) were also studied. The objective of this work is to contribute new findings to the tectonic setting and magma generation processes of the Cretaceous magmatic arc in the coastal Zihuatanejo terrane, which is part of the Guerrero terrane.

All gabbros are hornblende rich, which indicates hydrated con- ditions. Clinopyroxene crystals and hornblende pseudomorphs have plagioclase inclusions, suggesting exsolution by a rapid uplift. Geochemically, most gabbros are calc-alkaline. The relatively flat trace element patterns display incompatible element (LILE and LREE) en- richment relative to the less incompatible elements (HFSE and HREE), which show minor enrichment or are even depleted with respect to N-MORB. Positive Pb and Sr anomalies as well as LILE enrichment in comparison to HFSE and REE suggest a magmatic arc origin.

The Sr and Nd isotopic compositions indicate that the Jilotlán, Tepalcaltepec, and Manzanillo gabbros have a common source, comparable to N-MORB. Depleted mantle model ages (TDM) are similar for the gabbros and a quartz monzodiorite (averaging ~500 Ma) reflecting participation of a crustal component in their genesis. Isotopic data indicate that the gabbros could have been generated from a combination of mantle and crustal components. The existence of a Cretaceous, Barremian to Albian (~130–100 Ma), extensional arc proposed by other studies suggests that the gabbros were generated in a thinned crust setting.

Hornblende 40Ar/39Ar ages and U-Pb zircon ages of Jilotlán gabbros are undistinguishable at ~114 Ma, which indicates that they crystallized ~40 Ma earlier than the surrounding granitoids dated at ~70 Ma. The identical ages for both isotopic systems in the gabbros give evidence for their shallow emplacement and/or rapid uplift. The ~118 Ma (K-Ar hornblende) age of an andesite from the Tecalitlán Formation host rock suggests that the gabbros may represent the plutonic counterpart of this volcanosedimentary sequence.

The crystalline rocks of the study area are magmatically very primitive (initial epsilon Nd (εNdi) up to ~+7, TDM max. ~0.7 Ga), without evidence of evolved crustal components in comparison to the Puerto Vallarta batholith, located NW of the study area, which contains Precambrian crustal components (εNdi as low as ~-7, TDM up to ~1.55 Ga). This evidences different tectonic settings for these two areas. This study supports the hypothesis that a long-lived subduction zone (at least since the Early Cretaceous) generated poorly evolved arc rocks in a continental arc setting in the Guerrero terrane.