Significance of MORB-derived Amphibolites from the Aracena Metamorphic Belt, Southwest Spain

Abstract

The Aracena metamorphic belt, in the southwest Iberian Massif, is characterized by the presence of MORB-derived amphibolites and continental rocks deformed and metamorphosed during the Hercynian orogeny. Geochemical relationships of these amphibolites indicate the existence of a multiple fractionation process from a set of parental magmas, implying the existence of a multi-chamber system beneath the ridge where the basalt protolith was extruded. Neodymium isotopic ratios are typical of MORB, and oxygen isotopes indicate that these amphibolites have been derived from the uppermost part of the oceanic crust Thermal evolution, revealed from the study of chemical variations in the amphibole chemistry, is interpreted as resulting from subduction in a low-pressure regime in which the thermal structure of the continental hanging-wall played an important role. This continental wall was previously heated by subduction of a slab window resulting from migration of a triple junction along the continental edge during plate convergence. Three petrologic arguments support this tectonic model These are: (1) the low-pressure inverted metamorphic gradient of amphibolites of the oceanic domain; (2) the high-temperaturelow-pressure metamorphism of the continental hanging wall; (3) the early intrusion of boninites into the continental domain.

Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif

“…The production of methane would indicate low f O 2 conditions. Structural, thermal and chemical characteristics of graphite in the high-grade rocks of the Fuente del Oro series correspond to highly crystalline graphite (Rodas et al 2000) and they are compatible with the high temperature estimated for the southern part of the Aracena Metamorphic Belt (Castro et al 1996b). However, the higher content of graphite in these rocks with respect to the concentrations found in the La Umbría series needs further consideration.…”

Age constraints to the relationships between magmatism, metamorphism and tectonism in the Aracena metamorphic belt, southern Spain

“…However, an anastomosing network of thinner shear bands traverses the entire amphibolite pile, isolating fish-shaped portions of less deformed rock. The kinematics of the second deformational phase is similar to that of the first one, but with a stronger strike-slip component (Castro et al 1996b). The OD-D 2 develops a greenschist facies retrometamorphism over the amphibolites (mafic schists with actinolite-chlorite).…”

“…The intensity of this fabric decreases towards the top, where undeformed igneous textures are easily recognizable. Structural analysis points to a non-coaxial deformation regime for OD-D 1 , and reveals a complex kinematic frame, with a top-to-the-south thrust component, and a sinistral strike-slip component (Castro et al 1996b). Coeval to OD-D 1 is an inverted metamorphic gradient.…”

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careón Ophiolite (Northwest Spain)

“…Suprasubduction zone ophiolites crop out along the Variscan belt from the Iberian to the Bohemian massifs, including the Careó n (northwest Iberia; Sá nchez Martínez et al 2007), Lizard (Cornwall;Nutman et al 2001), Ś lęż a (Bohemian Massif; Dubiń ska et al 2004) and, probably, Beja-Acebuches (southwest Iberia; Castro et al 1996). These ophiolites are key elements to understanding the final stages of evolution of the Rheic Ocean as well as the early stages of convergence between Laurussia and Gondwana (Sá nchez Martínez et al 2007;Arenas et al 2007aArenas et al , 2007bMartínez Catalá n et al 2009).…”