Crustal units and role of the Mylonite zone system in the Varberg‐Horred region, SW Sweden
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Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to moderate fabric‐forming deformation and only localised formation of migmatitic leucosomes. The final Gothian orogenic episode at c. 1.58 Ga and three distinct anorogenic events between 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus supporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpreted to include eastward thrusting on the Mylonite Zone, followed by dominantly static metamorphism prior to 0.98 Ga, due to the thickened crust. Subsequent uplift and rapid cooling preserved granulite‐facies assemblages in the southern Eastern Segment. Late Sveconorwegian extensional movements occurred until c. 0.92 Ga along the largely west‐dipping Mylonite Zone system. Åhäll, K.‐L, 1995: Crustal units and role of the Mylonite Zone system in the Varberg‐Horred region, SW Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 185–198. Stockholm. ISSN 1103–5897.Keywords:
Mylonite
The rocks of the Holsteinsborg district were subjected to conditions of granulite facies metamorphism throughout and for some time after the period of ductile Nagssugtoqidian deformation. Within this span of time the pressure and temperature conditions did not remain stable and may be shown to have varied considerably. Zoning of Al in orthopyroxene coexisting with garnet, and of Mg and Fe in coexisting orthopyroxene and garnet, and clinopyroxene and garnet are interpreted as evidence of gradual cooling folIowing the peak of metamorphism. Zoning of Ca and AI in coexisting c1inopyroxene and plagioclase is interpreted as an indication of decreasing pressure. The assemblage orthopyroxene, sillimanite and quartz was stable during the peak of Nagssugtoqidian metamorphism and is considered to indicate extreme conditions, approaching 900·C, 9.5 kbar. Pyroxene thermometry suggests that peak temperatures exceeded 800 C.
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Early Precambrian rock units in the Urals are present in several polymetamorphic complexes, which are exposed in the Urals in the form of small (<1500 km2) tectonic blocks. Their ages are Archaean (as old as 3.5 Ga) and Palaeoproterozoic. During the formation of these complexes in the early Precambrian, two stages of ultra-high-temperature (granulite) metamorphism occurred. The maximum age of the early Neoarchaean stage of metamorphism is 2.79 Ga. Evidence of this metamorphic event includes the dating of the Taratash gneiss-granulite complex of the South Urals. Gneiss-migmatite complexes, which dominate the lower Precambrian section of the Urals, were formed in the Palaeoproterozoic during the sequential appearance of granulite facies metamorphism followed by amphibolite facies metamorphism and accompanying granitization. The maximum age of the Palaeoproterozoic stage of granulite metamorphism in the Alexandrov gneiss-migmatite complex, the most well-studied complex in the South Urals, is 2.08 Ga.
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Journal Article Zoned Ultrabasic and Basic Gneiss Masses in the Early Lewisian Metamorphic Complex at Scourie, Sutherland Get access M. J. O'HARA M. J. O'HARA Grant Institute of Geology, University of EdinburghScotland Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 2, Issue 2, 1961, Pages 248–276, https://doi.org/10.1093/petrology/2.2.248 Published: 01 June 1961
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Granulite-facies metamorphism affecting the Slishwood Division was extreme. Three samples yielded P-T conditions of 15.8, 14, 14.9kbar at 810, 750 and 880°C, respectively. Four Sm-Nd mineral isochrons, defined by granulite-facies basic and pelitic metamorphic assemblages, yield ages of 544 ± 52 Ma, 539 ± 11 Ma, 596 ± 68 Ma and 540 ± 50 Ma. These ages confirm that granulite- and earlier eclogitefacies metamorphism took place before the c. 470Ma Grampian Orogeny. Detailed chronological interpretation is inhibited by microscopic inclusions within, and isotope disequillbnum between, the dated minerals It is possible that the ages record crystallisation of either the granulite or eclogite-facies assemblages. However, it is more likely that they record post-metamorphic cooling. Relict pre-granulite-facies igneous minerals from a metagabbro body possibly date its intrusion at 580 ± 36 Ma. Extreme metamorphism in the late Neoproterozoic to Early Cambrian suggests that the Slishwood Division is exotic to Laurentia.
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The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed.
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Granulite-facies metamorphism affecting the Slishwood Division was extreme. Three samples yielded P-T conditions of 15.8, 14, 14.9kbar at 810, 750 and 880°C, respectively. Four Sm-Nd mineral isochrons, defined by granulite-facies basic and pelitic metamorphic assemblages, yield ages of 544 ± 52 Ma, 539 ± 11 Ma, 596 ± 68 Ma and 540 ± 50 Ma. These ages confirm that granulite- and earlier eclogitefacies metamorphism took place before the c. 470Ma Grampian Orogeny. Detailed chronological interpretation is inhibited by microscopic inclusions within, and isotope disequillbnum between, the dated minerals It is possible that the ages record crystallisation of either the granulite or eclogite-facies assemblages. However, it is more likely that they record post-metamorphic cooling. Relict pre-granulite-facies igneous minerals from a metagabbro body possibly date its intrusion at 580 ± 36 Ma. Extreme metamorphism in the late Neoproterozoic to Early Cambrian suggests that the Slishwood Division is exotic to Laurentia.
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