Eclogite facies relics in metabasites from the Sierra de Guadarrama (Spanish Central System): P-T estimations and implications for the Hercynian evolution

Abstract

Relics of HP-MT eclogitic assemblages related to the first metamorphic stage of the Hercynian nrr"TP'TlU in the Sierra de Guadarrama (Spanish Central SCS) are as boudins of IJi'-' -'JJLU-V metabasites enclosed by felsicTextures indicate a iHUUU--'H('Ll<.'-' metarnolrph the MT eclogite facies (as deduced from the presence of and rutile included in garnet) and through medium to Iow pressure and retrograde amphibolite-greenschist facies. Thermobarometric calculations in the ecIogitic yield pressures of -14 kbar for ternp,eraLtUlres in the range 725-775°C.for the indicates a Sl.ll� nlltlC,;m( drop in (P < 10 kbar) for similar temperatures vary from gab bro to lellCC)tonaJlltc:s showing the typical Fe enrichment of the tholeiitic series. Chemical characteristics indicate a derivation from low-pressure of tholeii tic melts more enriched than typical MORB compositions. Their original location far from continental margins as evidenced by the absence of ophiolitic material in the area and their association with platform sediments suggests that eclogitization was related to intracontinental crustal subduction and The P-T conditions estimated in the metabasites for the first metamorphic stage are similar to ones deduced for the metasediments and suggest that the Hercynian crust could have reached a thickness of -70-80 km, which is more than the double the present thickness.

Metaluminous pyroxene-bearing granulite xenoliths from the lower continental crust in central Spain: their role in the genesis of Hercynian I-type granites

“…Out cropping metamorphic rocks of Neoproterozoic to Lower Palaeozoic age are characterized by rare occurrences of meta-basites; minor continental-type tholeiitic meta basites were described by Barbero & Villaseca (2000). The lack of basic magmatic input associated with the Hercy nian collision is remarkable, estimated to be much less than 1 vol.…”

Thickening and exhumation of the Variscan roots in the Iberian Central System: Tectonothermal processes and 40Ar/39Ar ages

“…This is represented by a change in the P;T gradient of the garnet zone path during 01. The metamorphic gradient of the upper part of the thickened crust at the end of 01, calculated on the basis of the P-T con ditions of the garnet zone, is around 23 cC/km; while the mean crustal gradient after the M1 Pma.rTmax conditions calculated by Barbero and Villaseca (2000) for retroeclogite boudins found in the Lower Unit (14 kbar and 750 cC), is around 18 cC/km. However, the metamorphic field gradient in the BSZ calculated through the peak conditions of the garnet, staurolite 1 and the M1 kyanite (M2 sillimanite 1 zone) P-T conditions is arOll lld 33-36 cC/km, higher than the mean crustal gradient at the end of 01, and less than should Fig.…”

“…A first crustal thickening event (01; Perez-EstaUn et al, 1991) produced E-vergent, kilometer-scale, asymmetric isodinal folds, scarce and only preserved at upper structural levels, while the rod{s of the Lower Unit were buried at depths around 42 km (14 kbar; Barbero and Villaseca, 2000). 51 is a slaty cleavage associated with Fl folds, that constitutes the most pervasive fabric in the uppermost part of the Upper Unit (Fig.…”

“…Metamorphic-peak P-T conditions correspond to average values of rock samples near the top (circles) and near the base (squares) of the metamorphic zones. P-T conditions after Barbero and Villaseca (2000) for the eclogites from the Lower Unit are represented as (B&V 2000). The path of the SiI + Kfs zone is divided into a rapid cooling evolution of the top.…”

SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

“…1). In these mi gmatite areas, peak metamorphic conditions were at tained during or immediately after the first Hercynian tectonothermal stage (M1), reaching eclogite or gran ulite facies depending on the structural level (Villaseca et al 1999), followed by an exhumation stage with pressure differences of around 9-10 kb from peak pressure conditions of -14 kb and re-equilibration pressures of 4-5 kb (Escuder Viruete et al 1998;Barbero and Villaseca 2000). Available petrological data indicate that the SCS and nearby areas have re corded a clockwise P-T-time (t) evolution and that migmatization is related, to a certain extent, to the low P loop, when temperature was decreasing (Villaseca et al 2002;Villaseca and Ubanell 2005).…”