P-T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (D2). The syn-D2 P-T path of the Lower Unit of the TGD was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the TWEEQU thermobarometric technique, from 6.4-8.1 kbar at 735-750 DC (upper structural levels) and 7.2 kbar at 770 DC ( lower structural levels) to 3.3-3.9 kbar and 645-680 DC. This evolution is consistent with the observed sequence of meIting reactions and the generation of garnet- and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P-T path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P-T path is recorded by assemblages with And + Bt + Ms and Ms + ChI + Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions

The use of heavy mineral correlation for determining the source of impact ejecta: A Manicouagan distal ejecta case study

Due to the nature of distal impact ejecta layers on Earth, preserved deposits are rare and are commonly altered by diagenetic/weathering processes. To establish the source of ejecta, trace element signatures of impact glasses have typically been used to establish the source crater of a deposit. However, in many cases, impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition. This is the case for the Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. This Late Triassic ejecta deposit was originally believed to be derived from two possible sources, the Rochechouart or Manicouagan impact structures. To accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion Ar-Ar dating of shocked biotite (U-Pb dating of zircon was also used but proved inconclusive). Radiogenic dating of shocked biotites (observed exclusively in the ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan and excluded Rochechouart as a potential source. Garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the Manicouagan target rocks and impactite rocks. A strong garnet composition signature correlation between the samples from Manicouagan and the ejecta deposit provides convincing evidence that Manicouagan is the source of the SW Britain ejecta deposit. Furthermore, we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta

P–T Paths Derived from Garnet Growth Zoning in an Extensional Setting: an Example from the Tormes Gneiss Dome (Iberian Massif, Spain)

The Tormes Gneiss Dome (NW Iberian Massif, Variscan Belt of Spain), comprises a metamorphic core complex (Lower Unit) bounded by a major extensional detachment. Despite metamorphic temperatures in the upper amphibolite facies (∼700–740°C), metapelites from the highest levels of the Lower Unit contain garnet with preserved growth zoning. These rocks were used for reconstruction of quantitative P–T paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning. The results are consistent with a two-stage tectonothermal evolution under high-grade conditions: (1) an early compressional phase of deformation that led to upper amphibolite facies Barrovian-type metamorphism and to P increase and T rise to approximately 9 kbar and 700–725°C; (2) a subsequent major extensional phase of deformation that led to quasi-isothermal decompression from 8–9 to ∼3 kbar at T conditions between 700 and 740°C. Several lines of structural, textural and petrological evidence suggest that up to 15–20 km of overburden was removed from the Lower Unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions. A final stage of quasi-isobaric cooling to greenschist facies conditions is locally recorded in late low-grade detachments

Core-Rim thermobarometry in cliffusional zoned garnets: An example from the Tormes Gneissic Dome, NW Salamanca

Metapelites and add orthogneisses from the Lower Unit of the Tormes Gneissic Dome, NW Salamanca, contain high-grade mineral assemblages with characteristics diffusional zoned garnets, The P-T path deduced for this unit from combined studies based on TWEEQU thermobarometry in core-rim garnets, retrograde reaction analysis and mineral zoning, shows an important isothermal decompression of several kilobars. This decompression is the record of the tectonic exhumation of the unit, caused by a D2 extenslonal event related with the collapse of the previously thickened Variscan crus

Phase equilibria modelling of kyanite-bearing anatectic paragneisses from the central Grenville Province

Kyanite-bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high-P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K-feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub-aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P–T conditions of the metamorphic peak. These P–T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high-T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P–T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P–T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P–T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P–T conditions of 12.5–13 kbar and 815–830 °C and flatter P–T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P–T evolution of high-P anatectic quartzo-feldspathic rocks

Lithoprobe line 55: integration of out-of-plane seismic results with surface structure, metamorphism, and geochronology, and the tectonic evolution of the eastern Grenville Province

Abstract: Lithoprobe line 55, in the Grenville Province of eastern Quebec, provides unusually good control on the three-dimensional (3-D) geometry and structural relationships among the major lithological units there. Archean basement underlies the exposed Proterozoic rocks, along the entire seismic line, and there is a lateral ramp in this basement immediately behind a lobate stack of thrust slices of high-pressure metamorphic rocks comprising the Manicouagan Imbricate Zone (MIZ). Integration of the 3-D geometry with P-T and geochronological data allows derivation of a tectonic model for the region. The MIZ was buried to depths >60 km at 1050 Ma. Preservation of its high-pressure assemblages, and the absence of metamorphism at 990 Ma, which is characteristic of lower pressure metamorphic rocks that tectonically overlie them, indicates the MIZ rocks were rapidly unroofed, early in the tectonic history. There were two discrete pulses of crustal thickening during the Grenvillian Orogeny in this region. The first, involving imbrication of Labradorian and Pinwarian rocks that comprised part of southeast Laurentia, culminated in the Ottawan pulse at ca. 1050 Ma, and produced the high-pressure metamorphism of the MIZ. Its effects were rapidly reversed, with extrusion of the MIZ rocks to shallow crustal levels at ca. 1020 Ma. The crust was again thickened, with the Moho subsiding to depths >60 km, in the Rigolet pulse at ca. 990 Ma. The site of extrusion of the MIZ was probably controlled by the subsurface lateral ramp. High geothermal gradients indicate that extrusion may have been aided by lithospheric delamination in the crustal-thickening zone. Résumé : La ligne 55 du projet Lithoprobe, dans la Province de Grenville du Québec oriental, procure un contrôle exceptionnellement utile pour préciser la géométrie tridimensionnelle et établir les relations structurales parmi les plus importantes unités lithologiques de cette région. Le socle archéen est sous-jacent aux roches protérozoïques qui sont exposées tout le long de la ligne sismique; ce socle comprend une rampe latérale qui apparaît immédiatement derrière un empilement d'écailles divisé en lobes de roches métamorphiques de haute-pression, incluant la zone imbriquée de Manicouagan (ZIM). La combinaison de la géométrie tridimensionnelle avec les données de P-T et la géochronologie permet l'élaboration d'un modèle tectonique adapté à cette région. Il y a 1050 Ma, la ZIM était enfouie à des profondeurs >60 km. La préservation des assemblages de haute-pression, et l'absence du métamorphisme daté de 990 Ma qui caractérise les roches métamorphiques de plus basse-pression les recouvrant, révèlent que les roches de la ZIM furent rapidement dénudées, et dès le début de l'histoire tectonique. Deux pulsions d'épaississement crustal distinctes ont surgi durant l'orogénie de Grenville dans cette région. La première est une imbrication des roches du Labradorien et du Pinwarien incorporant une portion du sud-est de Laurentia, et dont la culmination date d'environ 1050 Ma lors de la pulsion d'Ottawan responsable du métamorphisme de haute-pression dans la ZIM. Ses effets ont rapidement rétrogra-dés, avec l'extrusion vers 1020 Ma des roches de la ZIM à des niveaux crustaux peu profonds. Un nouvel épaississe-ment de la croûte a suivi, il était accompagné de la subsidence du Moho à des profondeurs >60 km vers 990 Ma lors de la pulsion de Rigolet. Il est probable que la rampe latérale de subsurface ait joué un rôle important dans la détermi-nation du site d'extrusion de la ZIM. Les gradients géothermiques élevés suggèrent que l'extrusion a pu être facilitée par une délamination lithosphérique dans la zone d'épaississement de la croûte. [Traduit par la Rédaction] Hynes et al. 35