40 research outputs found
U-Pn geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny
No full textWithin the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D2 have been dated using SHRIMP UâPb geochronology. Zircon ages of 424 8 Ma (Vagastie Bridge granite), 420 6 Ma (Klibreck granite) and 429 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D2 deformation, during
mid- to late Silurian time. Titanite ages of 413 3 Ma (Vagastie Bridge granite) and 416 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550â 500 8C. A mid- to late Silurian age for D2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of forelandpropagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arcâcontinent orogeny. The mid-to late Silurian D2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia
Variations in the concentration of radon in parts of the Ogof Ffynnon Ddu system, Penwyllt, South Wales and estimates of doses to recreational cavers
No full textTectono-stratigraphic terranes in Archaean gneiss complexes as evidence for plate tectonics: The Nuuk region, southern West Greenland
No full textU–Pb zircon study of tectonically bounded blocks of 2940–2840 Ma crust with different metamorphic histories, Paamiut region, South-West Greenland: implications for the tectonic assembly of the North Atlantic craton
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Field recognition of rocks totally retrogressed from granulite facies: an example from Archaean rocks in the Paamiut region, South-West Greenland
No full textRelict Plutonic Textures in Archaean Ultramafic Rocks from the Fiskenaesset Region, Southern West Greenland; Implications for Crustal Thickness
No full textDunites from Isua, Greenland: A ca. 3720 Ma window into subcrustal metasomatism of depleted mantle
No full textAbstract
The chemistry of surviving pieces of Eoarchaean mantle together with related crust helps us determine early crust-forming mechanisms. Two lenses of high-Mg, low-Al dunite within a ca. 3720 Ma part of the Isua supracrustal belt in Greenland are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses are within altered, higher Al, Ca ultramafic schists and are intercalated with amphibolitized pillow basalts and gabbros with island arc chemical signatures, all intruded by 3715–3710 Ma tonalites. One variety of dunite is dominated by forsterite (Fo90–92) olivine with accessory chromite and rare clinopyroxene, which does not show high field strength element (HFSE) anomalies. Another variety contains olivine (Fo96–98), some intergrown with Ti-humite group minerals with strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. We propose that these dunites are tectonic slivers of ca. 3720 Ma subarc mantle that preserve evidence for varying interaction with mafic magmas in a ≥850 °C, 1.7–2.0 GPa subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone–like environments on the early Earth.</jats:p
Conflicting mineral and whole-rock isochron ages from the Late-Archaean Lewisian complex of northwestern Scotland: Implications for geochronology in polymetamorphic high-grade terrains
No full textGeochemistry of the Archaean gneiss complex and associated rocks of the Kanakapura area, Karnataka, South India.
No full textThe area investigated forms part of the Archaean high-grade gneiss terrain of Karnataka. The rocks comprise both acid and basic lithologies and include the late Archaean Closepet granite. Petrographically the acid rocks cover the range tonalite to granite, whilst the basic rocks are amphibolites and two-pyroxene granulites. The acid rocks have calc-alkaline affinities and resemble those from the transition zone to the east. Trace elements show them to be relatively undepleted with K/Rb ratios >500. Simulated REE patterns of gneisses reveal that there are three broad groups: two have steep distributions but different REE abundances, suggesting high-P evolution; one group has a relatively flat distribution suggesting low-P evolution. The basic rocks have a tholeiitic affinity, but are clearly separate, the granulite-facies types being slightly more magnesian. The trace-element chemistry of these basic rocks has been disturbed by migmatization. The REE patterns of Closepet granite are steep and may be related to the partitioning of accessory minerals such as allanite. An attempt is made to establish the relative position of the granulite-facies metamorphism with respect to the regional fabric and retrogression.-J.M.H
