Early Devonian suprasubduction-zone ophiolite related to incipient collisional processes in the Western Variscan Belt: The Sierra de Careón unit, Ordenes Complex, Galicia

Abstract

The Careón unit consists of imbricate tectonic sheets of serpentinite, metagabbro, and diabase dikes interpreted as a dismembered ophiolitic sequence. Zircons from leucogabbro intruding serpentinites point to an igneous crystallization at 395 ± ± 3 Ma, in accord with previous results. Combined with a reported 377 ± ± 1 Ma Ar/Ar cooling age, this date implies that ophiolite generation occurred shortly before its tectonic emplacement. Most samples are enriched in light rare earth elements (REE) relative to heavy REEs and show distinct fractionation of the heavy REEs. Enrichments in Th and La relative to Nb are ubiquitous, while negative anomalies of Zr and Ti occur in most samples. Initial ε ε Nd values (+ +6.4 to + +9.1), even for samples with high light REE/heavy REE ratios, point to a time-integrated mantle source strongly depleted in Nd relative to Sm, and preclude signiµcant contamination of maµc melts during their ascent through the crust. Combined trace element and Nd-isotope data favor an intraoceanic, suprasubduction-zone setting, where hydrous ×uids and silicate melts metasomatized a wedge of depleted mantle and triggered its partial melting. On the basis of magmatic afµnities, age constraints, and broad tectonometamorphic context, ophiolite generation is interpreted to re×ect oceanic spreading above a subduction zone dipping away from a passive continental margin. Suprasubduction extension might have been associated with subduction hinge retreat caused by instability of old, cold oceanic lithosphere entering the subduction zone. It is suggested that this occurred in the context of early stage arc collision documented at that epoch in the Variscan realm.

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

“…Results suggest P-T ranges of and . 7 ‫ע‬ 2 kbar 650Њ-450ЊC Mineral relationships correlate with previous P-T estimations and interpretations (Díaz García et al 1999;Pin et al 2002), where the synkinematic growth of epidote, actinolitic hornblende rims, and oligoclase rims is linked to lower amphibolite facies assemblages (fig. 4).…”

“…395 Ma (U-Pb, zircon; Díaz García et al 1999;Pin et al 2002), whereas the regional tectonic foliation that was developed under amphibolite facies conditions yields 40 Ar/ 39 Ar ages between 390 and 376 Ma in this and other upper ophiolitic units (hornblende concentrates; Dallmeyer and Gil Ibarguchi 1990;Peucat et al 1990;Dallmeyer et al 1991Dallmeyer et al , 1997. The amphibolite-facies fabric is related to ophiolite imbrication (Díaz García et al 1999;Arenas et al 2007a), and the narrow age interval between oceanic lithosphere generation and thrusting is consistent with a rapid deformation in a suprasubduction zone context.…”

“…Pressure-temperature space for metabasites of the Careó n ophiolite. Thermobarometric conditions as calculated for FACAR-3 type garnet-bearing amphibolites (Díaz García et al 1999; 1), hornblende-plagioclase assemblages in FACAR-2 type amphibolites (this work; Pin et al 2002;2), retrograde conditions in FACAR-2 type amphibolites (this work; 3); suggested P-T path for sample FACAR-2 (4). Aluminum silicate phase diagram after Holdaway (1971). shape fabric (Lm; fig.…”

“…Semiquantitative estimations on these rocks support conditions below 650ЊC and 5 kbar ( fig. 4) during this second stage (Pin et al 2002).…”

A peri-Gondwanan arc in NW IberiaI: Isotopic and geochemical constraints on the origin of the arc—A sedimentary approach

“…The basal units are considered to be the most external margin of Gondwana subducted beneath Laurussia at the onset ofVariscan deformation (c. 370 Ma; Arenas et al, 1995;Martlnez Catalan et al, 1996;Rodrtguez et al, 2003;Abati et al, 2010), whereas the upper units are interpreted to be an arc-derived terrane. This arc also has a peri-Gondwanan provenance (Fernandez Suarez et al, 2003), but left the main continent during the Middle Cambrian-Early Ordovician and drifted north contemporaneously with the opening of the Rheic Ocean, which is represented within the stack of allochthonous units by different types of ophiolites (Dlaz Garda et al, 1999;Pin et al, 2002;Arenas et al, 2007;Sanchez Martlnez et al, 2007a,b). This rifted arc was finally accreted to the southern margin of Laurussia during the Lower or Middle Devonian (G6mez Barreiro et al, 2007).…”