Search results for "AMOR"

The Nhlangano gneiss dome in south-west Swaziland – A record of crustal destabilization of the eastern Kaapvaal craton in the Neoarchaean

2015

Abstract The Nhlangano granite-gneiss in south-west Swaziland, dated at 2.98 Ga, forms an elliptical dome of remobilised, migmatitic crust. Strongly deformed supracrustal rocks wrap around or form synformal keels infolded with subdomes of Nhlangano Gneiss and represent Mesoarchaean Pongola Supergroup strata at amphibolite- to granulite-facies metamorphic grade. Amphibolites represent the metamorphosed equivalents of Nsuze Group basalts and andesites on the basis of similar trace element and Nd isotope data. Garnetiferous Mahamba Gneiss, dated at c. 2.96 Ga, represents the metamorphosed equivalents of felsic to intermediate volcanic rocks and minor pelitic interflow sedimentary units of the …

Zircon dating of North Bohemian granulites, Czech Republic: Further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

1996

U-Pb zircon and rutile multigrain ages and 207Pb/206Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in 207Pb/206Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of …

Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif

2021

The Velke Vrbno Dome crops out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19–0.82) combined with moderately positive Nd isotopic compositions (eNd(i) = +3.89 – +5.77) is used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios (87Sr/86Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphis…

Single zircon evaporation ages from Obudu Plateau: First evidence of archaean components in the schists of south-eastern Nigeria

1998

Quantifying the P-T-t conditions of north-south Lhasa terrane accretion: new insight into the pre-Himalayan architecture of the Tibetan plateau

2014

An integrated field, petrological and geochronological study of the Basong Tso region of south-eastern Tibet has constrained the timing and P–T conditions of north–south Lhasa terrane accretion and provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high-grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U–Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25…

Mineral chemical and geochronological constraints on the age and provenance of the eastern Circum-Rhodope Belt low-grade metasedimentary rocks, NE Gr…

2010

Abstract In north-eastern Greece the mid-greenschist facies Makri Unit and the anchizonal Melia Formation belong to the eastern Circum-Rhodope Belt that forms the uppermost tectonostratigraphic unit of the Rhodope metamorphic nappe pile. The two metasedimentary successions had different source areas, although they now lie in close proximity in the Rhodope Massif. The U–Pb isotopic ages of detrital zircons from a metasandstone of the Makri Unit analysed using LA–SF–ICP–MS and SHRIMP-II gave age clusters at ca. 310–290 Ma and at ca. 240 Ma for magmatic zircons, which may have been derived from Carboniferous–Permian basement rocks of the Thracia Terrane (Lower Tectonic Unit of the Rhodope Mass…

The Proterozoic P–T–t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites

2007

Integrated metamorphic and geochronological data place new constraints on the metamorphic evolution of a Neoproterozoic orogen in east Antarctica. Granulite-facies rocks from a 150 km stretch of the Kemp Land coast reflect peak conditions involving T � 870^9908 Ca t P� 7� 4^10 kbar, with pressure increasing westward towards an Archaean craton. Electron microprobe-derived (Th þ U)^Pb monazite ages from metapelitic assemblages indicate that the major mineral textures in these rocks developed during the c. 940 Ma Rayner Orogeny. Complex compositional zoning in monazite suggests high-T recrystallization over c. 25 Myr. Diversity in metapelitic reaction textures reflects silica and ferromagnesia…

Thermo-mechanical role of a Cambro-Ordovician paleorift during the Variscan collision: the NE margin of the Bohemian Massif

2001

Abstract The Stare Město (SM) belt (NE margin of the Bohemian Massif) represents a preserved example of an intracontinental Cambro-Ordovician rift that has been shortened during the Variscan orogeny. The rifted sequence consists of gabbroic intrusions and a leptyno-amphibolite complex of Cambro-Ordovician protolith age. The latter suffered medium pressure, granulite facies, Cambro-Ordovician metamorphism associated with extension in a continental protorift. The Variscan tectonometamorphic event is manifested by convergent orogenesis with syn-convergent intrusion of a Carboniferous tonalitic sill, high temperature–medium pressure compressional deformation of gabbros and high temperature meta…

High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa

2018

Abstract The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swazi…

Underplating-related finite-strain patterns in the Gran Paradiso massif, Western Alps, Italy: heterogeneous ductile strain superimposed on a nappe st…

2004

A finite-strain study in the central and northern Gran Paradiso massif of the Italian Western Alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the Alps. The R f /φ and Fry methods were used on feldspar porphyroclasts from 98 samples of the Gran Paradiso unit (upper tectonic unit of the Gran Paradiso massif) and 12 samples from the underlying Erfaulet unit (lower unit of the Gran Paradiso massif). Microstructures and thermobarometric data show that feldspar ductility at temperatures higher than c . 450 °C occurred only during high-pressure metamor…