High‐pressure micro‐inclusions and development of an inverted metamorphic gradient in the Santiago Schists (Ordenes Complex, NW Iberian Massif, Spain): evidence of subduction and syncollisional decompression

Abstract

The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low-to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin S i = S 1 foliation included in albite porphyroblasts, being composed of: albite + garnet4 + white mica-1 + chlorite-1 + epidote + quartz + rutile i ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 i 10 "C and 14.7 f 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 f 10 "C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-11, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation.The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character.The general metamorphic evolution of the Santiago Schists, with the development of h i g h 2 assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high?, low-to intermediate-T circum-Pacific belts.

REE-assisted U–Pb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of the A Silva granodiorite, Iberian allochthonous complexes

“…In a Th/U versus Yb/Gd plot (Fig. 7a), R33 (diamonds) shows a reversely correlated asymptotic trend, which is typical of a suite of zircons that have crystallized in an evolving magma ; i.e., zircon chemistry reflects the degree of its host rock fractionation (Belousova et al, 2002). Contrastingly, Yb/Gd 0.072 .c a.…”

“…These slices have been grouped into three main units: named from bottom to top; the basal units, the ophiolitic units (representing the Variscan suture), and the upper units. The basal and ophiolitic units record a westward oceanic subduction (Dlaz Garda et al, 1999a;Sanchez Martlnez et al, 2007;G6mez Barreiro et al, 2010) that evolved to continental subduction of the Gondwanan continental margin during Devonian to Early Carboniferous times (Arenas et al, 1995;Martlnez Catalan et al, 2007.…”

“…In the A Silva zircons, La contents are usually low, ranging from 0.003 to 0.5 ppm. Higher values (0.5-15 ppm) are commonly associated with high U contents and could reflect metamictization of the zircon (Belousova et al, 2002;Hoskin, 2005). Ce/Ce'" positive anomalies are variable, from 1 to 21, with the lowest values largely influenced by light (L) REE concentrations.…”

“…The latter is a large granodiorite to granite intrusion with a trapezoid shape (in map view), which crosscuts upright regional Variscan folds. Two main petrographic types were recognized on the basis of texture and modal composition, following a classical normal zoning pattern (Bellido et al, 1987). A porphyritic external facies occupies the largest area and is characterized by K-feldspar mega crysts (ACt in Fig.…”

The Corredoiras orthogneiss (NW Iberian Massif): Geochemistry and geochronology of the Paleozoic magmatic suite developed in a peri-Gondwanan arc

“…These units con verged at the onset of the Variscan Orogeny, and thereafter, during their accretion and exhumation, they were dismembered and thinned, reaching their present configuration as a consequence of late orogenic processes of folding and faulting (Martinez Catalan et al, 2009). As the ophiolitic units are considered to represent a colli sional suture sanchez Martinez et al, 2007asanchez Martinez et al, , 2007b, the basal units are interpreted as the most external continen tal margin of Gondwana (Abati et al, 2010;Arenas et al, 1995Arenas et al, , 1997Martinez Catalan et al, 1996;Rodriguez et al, 2003), whereas the upper units represent exotic terranes with Gondwanan affinity. These upper units can be divided in two sets with contrasting pres sure and temperature conditions; on one hand, the high-pressure and high-temperature units (HP-HT), occupying the lower structural position and, on the other hand, the intermediate pressure units (IP) on top of the orogenic pile (Abati et al, 1999Ordofiez Casado et al, 2001).…”