Granulites

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamor-phism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

The H.U. Sverdrupfjella is part of the high-grade Maud Belt in Dronning Maud Land (East Antarctica), which was located in a central position of the Gondwana supercontinent. Here we study high-pressure granulites from the eastern H.U. Sverdrupfjella and present a detailed reconstruction of the P–T–t history based on a combination of Zr-in-rutile and Ti-in-zircon thermometry, zircon U–Pb dating, monazite chemical dating, garnet diffusion modelling and petrological modelling. Peak metamorphic conditions of $930 C and 1Á45 GPa persisted for less than 6 Myr and were attained at 570 6 7 Ma, embedded in a well-documented clockwise loading, heating, and decom-pression path. The rocks had already been rapidly exhumed to a crustal depth of $30 km at 556 6 7 Ma. In addition to the very short-lived ultrahigh-temperature peak, zircon preserves evidence for protracted granulite-facies conditions with temperatures above 800 C from as early as c. 590 Ma, persisting for c. 40 Myr. Constraints on prograde metamorphism are recorded in zircon and in rutile inclusions in garnet. Zr-in-rutile thermometry using rutile included in different generations of garnet is used to reconstruct the prograde P–T path, documenting burial followed by heating to ultrahigh temperatures at peak pressures. Complementary Ti zonation in prograde cores of zircon grains documents and dates heating and peak temperatures, whereas younger zircon rims show lower Ti-in-zircon temperatures and date the retrograde stages of metamorphism. Our results provide the first evidence for Neoproterozoic high-pressure granulite-facies metamorphism and ultrahigh-temperature conditions for this region. The clockwise loading–heating path and the peak P–T conditions strongly indicate that the rocks preserved in Dronning Maud Land were part of the lower plate during a continent–continent collision event related to Gondwana assembly at c. 570 Ma. The metamorphic evolution determined in this study and the correlation with similar P–T evolutions documented in adjacent terranes favour the continuation of the c. 580–560 Ma Mozambique Belt into Dronning Maud Land. Furthermore, the striking contemporaneity of the metamorphism in the different parts of central Gondwana suggests that the Coats Land Block was part of greater India prior to this collision.

An online library of high resolution, full color scans of close to 430 "exotic" igneous, metamorphic and metasomatic rocks in thin section, displayed under both natural (unpolarized) light and under crossed polarizers.  The minerals featured in the three volume optical mineralogy series (see "Books" section, below) are sourced from the initial ~100 or so of these samples.  The scan library is indexed and fully searchable by locality, minerals, geologic environment, and atypical element enrichments.  The featured samples and accompanying information are routinely updated, and I'm now in the process of adding tabulated mineral composition data.  Note: this online resource is a website, so the "download" here (a scan of a thin section of serendibite-bearing marble under crossed polarizers, fortuitously shaped roughly like Western Australia :-) ) is just a single low resolution image for illustrative purposes.  Look for the collection to be continually expanding, with a new batch of 20-40 additional thin sections added once or twice per year.

See: https://www.rockptx.com/thin-section-scans/

Abstract Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293 Ma. Peak metamorphic conditions at c. 1290 Ma were c. 850 °C at pressures of 7–9 kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9 kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290 Ma, essentially coincident with the emplacement of mafic rocks at 1292 Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268 Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400 °C) occurred at 1260 Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234 ± 17 Ma.

Existence of a possible detachment zone at Elampillai region, NW margin of Kanjamalai Hills, located in the northern part of Cauvery Suture Zone (CSZ), Southern India, is reported here for the first time. Detailed structural mapping provides anatomy of the zone, which are rarely preserved in Precambrian high grade terranes. The detachment surface separates two distinct rock units of contrasting lithological and structural characters: the upper and lower units. The detachment zone is characterized by a variety of fold styles with the predominance of tight isoclinal folds with varied plunge directions, limb rotations and the hinge line variations often leading to lift-off fold like geometries and deformed sheath folds. Presence of parasitic folding and associated penetrative strains seem to be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and the structural relief of the underlying basement. Our present study in conj...

We present evidence for K-feldspar metasomatism in charnockitic granulites from two well-known terranes: the Shevaroy Hills Massif, S. India (750°C, 8 kbars) and the Bamble Sector, S.E. Norway (790°C, 7.5 kbars) in the form of K-feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of K-feldspar in an uneven scattering of

Intermediate orthogranulites were collected on the western flank of the Galicia bank during the Galinaute II cruise in 1995. The petrography of these rocks reveals two types of granulites. The first type is hydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + hornblende + garnet + biotite + opaque + zircon + apatite assemblage. Both hornblende and orthopyroxene define a weak foliation plane. A late deformation event is expressed by some fractures cross-cutting the foliation. The second is anhydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + clinopyroxene + opaque + zircon + apatite assemblage. The rocks display a granoblastic texture and are affected by brittle deformation as testified by the development of numerous microfractures. The P–T conditions (7 ± 1 Kbar, 750 ± 50 °C) calculated from two representative samples demonstrate that the rocks equilibrated under granulite facies conditions. Ar-Ar dating gives Precambrian ages ranging between ca. 2500–2000 Ma for the amphibole from the hydrous granulite and 1600–1500 Ma for the core of the K-feldspar from the anhydrous and hydrous granulites. A younger age of 900 Ma is obtained from the recrystallized rims of the K-feldspar from the two samples. These data indicate that the granulitic rocks in the Galicia Bank had already been exhumed and cooled below ca. 140–400 °C (blocking T° for K-feldspar) in Precambrian times (900 Ma). Given the very well preserved granulitic minerals assemblage of the rocks, the granulites behaved as competent and metastable boudins during their exhumation. The granulitic samples were previously interpreted as fragments of the lower continental crust sampled by the main detachment fault during Cretaceous rifting, but they were part of an upper continental crust from the Precambrian. Geochronological data and petrological assemblages suggest that the granulite blocks in the Galicia Bank probably were derived from the North Armorican Domain (northern part of France) where a Precambrian terrain outcrops. The opening of the Bay of Biscay could be responsible for the scattering of the Precambrian terrain and may explain the presence of the granulitic blocks on both sides of the Bay of Biscay. During the subduction of Europe below the Iberian peninsula the granulite blocks were transported southward and incorporated into a Cretaceous conglomerate forming the accrecionary prism on the Northern Iberia Margin. The granulite facies blocks found on the Galicia Bank represent another example of Gondwanian relics supporting the idea that the West European plate belonged to the West African craton during the Proterozoic.Des orthogranulites de composition intermédiaire ont été prélevées sur le flanc ouest du banc de Galice au cours de la campagne de plongée Galinaute II en 1995. L’étude pétrographique de ces roches révèle deux types de granulites. Le premier se compose de granulites hydratées qui sont formées d’un assemblage à feldspath potassique + plagioclase + quartz + orthopyroxène + hornblende + grenat + biotite + opaque + zircon + apatite. Dans ces roches l’amphibole et l’orthopyroxène soulignent le plan de foliation, sur cette déformation ductile se surimpose une déformation fragile qui s’exprime par le développement de microfractures recoupant le plan de foliation. Le deuxième type se compose de granulites anhydres qui sont formées d’un assemblage à feldspath potassique + plagioclase + quartz + orthopyroxène + clinopyroxène + opaque + zircon + apatite. Ces roches se caractérisent par une texture granoblastique, on ne note aucune orientation préférentielle des minéraux, en revanche, une déformation froide s’exprime par l’apparition de microfractures qui recoupent les roches. Deux échantillons représentatifs de ces deux types de granulites ont été datés et étudiés du point de vue pétrographique. Les conditions pression température calculées pour les deux échantillons (7 ± 1 kbar et 750 ± 50 °C pour la granulite hydratée et, 5 ± 1 kbar et 800 ± 50 °C pour la granulite anhydre) attestent que ces roches ont été équilibrées dans les conditions du faciès des granulites. Des datations Ar-Ar sur minéraux donnent un âge précambrien compris entre 2 500–2 000 Ma pour l’amphibole de la granulite hydratée et 1 600–1 500 Ma pour le cœur du feldspath potassique pour les granulites hydratée et anhydre. Un âge plus récent de 900 Ma a été obtenu à partir de la bordure recristallisée du feldspath potassique des deux échantillons. Ces données montrent que les blocs de granulites prélevés sur le banc de Galice étaient déjà refroidis en dessous de 140–400°C (T° de fermeture de l’Ar pour le feldspath potassique) et exhumés au Précambrien (900 Ma). Le bon degré de préservation de la paragenèse granulitique des roches suggèrent une exhumation rapide, après leur mise en place en position structurale haute dans la croûte continentale, les granulites se comportent comme des boudins compétent (non déformés) et métastables (non rétromorphosés). Ces échantillons de granulite étaient initialement interprétés comme des fragments de la partie inférieure de la croûte continentale hercynienne, prélevées en base de croûte par le détachement majeur au cours du rifting Crétacé, les données géochronologiques démontrent que ces blocs faisaient partie de la croûte supérieure dès le Précambrien. Les données géochronologiques et pétrologiques suggèrent que les granulites du banc de Galice dérivent du domaine nord-armoricain où des terrains Précambriens affleurent. L’ouverture du Golf de Gascogne pourrait être responsable du démantèlement et de la dispersion des terrains Précambriens que l’on retrouve sous la forme de pierres volantes de part et d’autre de la baie de Biscaye. Puis au cours de la convergence entre l’Europe et l’Ibérie les granulites ont été transportées vers le sud et certaines ont été incorporées dans les conglomérats Crétacé–Éocène (par exemple sur le banc Le Danois) qui constituent le prisme d’accrétion lié à la subduction sur la marge nord de la Galice. Les granulites du banc de Galice constituent des reliques précambriennes, identiques à celles du banc Le Danois. Ces données renforcent l’hypothèse selon laquelle l’Europe occidentale faisait partie du craton ouest africain au cours du Protérozoïque.