Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt

Abstract

A B S T R A C TAnalysis of the Careó n Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age ( Ma, U-Pb) was obtained in a leucocratic gabbro. The 395 ‫ע‬ 2 whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole ( ЊC; kbar) suggested that imbrications T ≈ 650 P ≈ 11.5 developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

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

“…This episode took place under conditions typical of the high-pressure and medium-to-high-temperatures part of the amphibolite facies (ca. , 650Њ ‫ע‬ 55ЊC ; Díaz García et al 1999;Pin et al 9.5 ‫ע‬ 1.5 kbar 2002). Garnet-and corundum-bearing assemblages represent the maximum P-T conditions that were reached, and they are spatially linked to high-strain zones where a mylonitic fabric developed (figs.…”

“…Based on Díaz García et al (1999), Gó mez Barreiro et al (2006), and Arenas et al (2007a). zone ophiolite are variably preserved between ductile shear zones. A complete transition from ultramafic rocks to isotropic metagabbros is found in tectonic sheets (Díaz García et al 1999). Figure 3 summarizes structural and lithological relationships in one of these sheets.…”

“…3, 4). These structures are considered intraoceanic thrusts that led to the initial imbrication of the suprasubduction oceanic lithosphere and the formation of metamorphic soles during the closure of the Rheic Ocean (Díaz García et al 1999;Sá nchez Martínez et al 2007). A retrograde stage followed, resulting in low-amphibolite to greenschist facies synkinematic mineral assemblages.…”

“…Tracking the evolution of deformation fabrics in old ophiolites is not a trivial matter since we commonly deal with partially preserved sections, tectonically reorganized, with several stages of retrograde metamorphism (Hacker and Mosenfelder 1996;Díaz García et al 1999). It has been shown that crystallographic-preferred orientation or texture analysis can provide independent and valuable evidence about the conditions of deformation and its kinematics.…”

“…Structural and petrological features suggest that early intraoceanic subduction fabrics generated during the closure of the Rheic Ocean have been preserved in the ophiolite (Díaz García et al 1999;Arenas et al 2007a;Sán-chez Martínez et al 2007). Textural analyses on selected amphibolites from a high-temperature shear zone, corresponding to a metamorphic sole, and a mylonitic/ultramylonitic shear zone within one of the imbricate fault slices of the Careó n ophiolite were carried out with TOF neutron diffraction and EBSD techniques to determine crystallographic-preferred orientation of the principal phases.…”