Mylonites in the Dabie Mountain orogen
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Abstract:
Mylonites in the Dabie orogen are fault and shear zone related rocks formed under metamorphic conditions.They can be divided into greenschist,amphibolite,granulite,and eclogite facies mylonites.The main criterion for identifying the mylonites in the Dabie Mountain is the ductile(plastic)deformation of index minerals of the constituent metamorphic facies.This is clearly different from the former definition and description of mylonite.Formation epoch of the mylonites in the Dabie Mountain was approximately the same as(or a little later than)that of peak metamorphism of their host rocks.From older to younger,the series follows the order eclo-gite facies mylonite→granulite facies mylonite →amphibolite facies mylonite→greenschist facies mylonite.According to lithology and texture,the mylonites can be further divided into second and third subclasses if necessary.Former mylonites of eclogite and granulite facies are always superimposed by the later facies and show the appearance of amphibolite or even greenschist facies mylonites;the earlier formed mylonites are thus preserved as relics in their retrograded products.Mylonites in the low-grade metamorphic rocks in southern and northern Dabie Mountain only experienced the metamorphic history of greenschist and amphibolite facies mylonitization during the Paleozoic-Triassic period.Keywords:
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Greenschist
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The Grenville Front in eastern Labrador coincides with the Benedict Fault, a south-dipping zone of heterogeneously deformed rocks. In the Smokey archipelago, the Benedict Fault transects Paleohelikian plutonic rocks of the Benedict Mountains Intrusive Suite (BMIS). Various members of the BMIS locally contain a north- to northeast-trending pre-Grenvillian planar fabric known as the Makkovik trend. Makkovik trend fabrics are defined by amphibolite-facies mineral assemblages, and their presence characterizes the BMIS as a pre-Grenvillian lithostructural domain, one of three identified in the area. The age of the Makkovik trend is constrained by Rb-Sr (whole rock) dates determined for granulite-facies gneisses (≥ ca. 1.9Ga) intruded by the BMIS, and by a 1676 ± 77Ma age determined for ferrodiorite-ferrosyenite which intrudes the BMIS. -- The White Bear Islands Granulite Complex (WBIGC), a second lithostructural domain, comprises high-grade ortho- and paragneisses, which show evidence of a period of passive retrogression to amphibolite-facies predating the development of Grenvillian fabrics. Relatively high PT-estimates of 830-860 ± 75ﾟC and 7-8 ± 1Kbar were determined for the granulite-facies event using the two-pyroxene geothermometer and the Al content of orthopyroxene coexisting with garnet. Relatively low PT-estimates of ca. 685ﾟC and 4.6Kbar were derived using various calibrations of the garnet-orthopyroxene (-plagioclase-quartz) geothermobarometer. -- Conditions of pre-Grenvillian retrogression of the WBIGC are estimated at 650 ± 50ﾟC and ca. 5.5 ± 1.5Kbar (garnet-cordierite geothermobarometry) assuming P(H2O) ~ 0.4P(total). The garnet-biotite and two-feldspar geothermometers give variable results which are interpreted to record points on the cooling curve of the WBIGC. -- The third lithostructural domain, known as the Bluff Head orthogneiss, consists predominantly of amphibolite-facies granodioritic gneisses typical of the northern Groswater Bay Terrane. Age relations of these gneisses with the BMIS and the WBIGC are uncertain. Both gneissic domains, however, locally contain pre-migmatitic metabasites bearing clinopyroxenes of similar composition, suggesting equilibration under broadly similar metamorphic conditions. The WBIGC and the Bluff Head orthogneiss are thus inferred to have shared a common, early tectono-metamorphic history prior to the emplacement of the BMIS. Pre-Grenvillian retrogression of the WBIGC is most pronounced in the western portion of the study area, and may be transitional into the amphibolite-facies assemblages near Bluff Head. Alternatively, the Bluff Head orthogneiss may record the effects of significant recrystallization during the Grenvillian orogeny; garnet-biotite and two-feldspar temperature estimates for the gneisses are consistent with conditions determined for Grenvillian metamorphism in the area. -- A zonal distribution of structural and metamorphic features attributed to the Grenvillian orogeny is apparent from north to south across the Smokey archipelago. Three east-west trending Grenvillian structural domains, separated by major Grenvillian high-strain zones, have been recognized on the basis of contrasting fabric development and metamorphic grade. North of the Benedict Fault, Makkovik trend fabrics are locally retrograded to greenschist-facies assemblages. Greenschist- to lower amphibolite-facies assemblages characterize south-dipping Grenvillian fabrics in a ~3km wide transitional domain to the south of the Benedict Fault. The Cut Throat Island Fault (CTIF), a south-dipping zone of mylonites, defines the southern extent of the transitional domain. Garnetiferous assemblages of the epidote amphibolite- to lower amphibolite-facies characterize Grenvillian fabrics south of the CTIF. Grenvillian L-S fabrics are most extensively developed in this domain and formed during the second of three phases of folding attributed to the Grenvillian orogeny. Temperatures of Grenvillian metamorphism south of the CTIF are best estimated by low Mn-Ca garnet-bearing mineral assemblages at ca. 550 ± 30ﾟC (garnet-biotite, garnet-amphibole thermometery). Mineral assemblages suitable for pressure determinations are lacking; however the composition of amphiboles together with the presence of garnet in domain G indicate a southerly increase in lithostatic pressure in the map area. -- The Grenville Front zone on the Labrador coast is thus characterized by a southward-increasing Grenvillian metamorphic gradient which has been telescoped across major thrust (or high angle reverse) faults, and by the heterogeneous development of L-S fabrics which overprint pre-Grenvillian structural and metamorphic features. Similar features characterize the Grenville Front zone elsewhere in Labrador, and are consistent with the characterization of this portion of the Grenville Province as a region of significant crustal thickening about 1Ga ago.
Hornblende
Geothermobarometry
Charnockite
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Blueschist
Omphacite
Greenschist
Basement
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Abstract Metamorphic rocks associated with ophiolitic rocks occur on the eroded surface of a NW–SE-trending anticline in the Allahyarlu area, NW Iran, between the Caucasus and Zagros orogenic belts. Metapelitic rocks consist mainly of quartz, muscovite chlorite, altered biotite and garnet. S 1 is the pervasive schistosity, wrapping garnet, which is folded by the second schistosity (S 2 ). The amphibolite records only one phase of deformation as the main lineation. The rocks experienced metamorphism up to the amphibolite facies, then overprinted by greenschist facies condition. Thermobarometry indicates an average pressure of c. 5 kbar and an average temperature of c. 600 °C for the amphibolite facies metamorphism, corresponding to a ∼33 °C km −1 geothermal gradient in response to a thick magmatic arc setting. Greenschist facies metamorphism shows re-equilibration of the rocks during exhumation. Amphibolites whole rock geochemistry shows trace elements patterns similar to both island arc and back-arc basin basalts, suggesting that the protolith-forming magma of the amphibolites was enriched at shallow to medium depth of a subduction system. Negative Nb anomaly and slight enrichment in light rare earth elements (LREE) and large-ion lithophile elements (LILE) of the amphibolites indicate arc-related magmatism for their protolith and a back-arc sialic setting for their formation. 40 Ar– 39 Ar dating on muscovite separated from two gneiss samples, and hornblende separated from three amphibolite samples, documents a Variscan (326–334 Ma) age. The magmatic and metamorphic rock association of the Allahyarlu area suggests the existence of an active continental margin arc during the Variscan orogeny, without clear evidence for a continental collision.
Protolith
Greenschist
Hornblende
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Mylonite
Shearing (physics)
Overprinting
Metamorphic core complex
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The Vardalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five-stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T= 490±63°C) is inferred from garnet cores with amphibole inclusions and bell-shaped Mn profile. (2) Formation of L>S-tectonite eclogite (T=680±20°C, P=16±2 kbar) related to the subduction of conti- nental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial. (3) Formation of sub-horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T=690±20°C, P=15±1.5 kbar) with top-W kinematics may indicate, however, that non- coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T=564±44°C, P<10.3-8.1 kbar), quartz veins and development of conju- gate shear zones indicate that coaxial vertical shortening and sub-horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non-coaxial top-W movement are related to large-scale movement on the extensional detachments active during the late-orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vardalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.
Mylonite
Tectonite
Greenschist
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Greenschist
Overprinting
Basement
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Mylonite
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Abstract Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐ T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression ( c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions.
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The petrological study of the southern part of the Cabo Ortegal area is a complement of Vogel’s (1967) investigation of the northern half. The present investigations include a structural as well as a petrofabric study. The rocks belong to an eugeosynclinal sequence which during the Precambrian underwent prograde metamorphism from the staurolite-almandine-muscovite and the kyanite-almandine-muscovite subfacies of the almandine-amphibolite facies through the clinopyroxene-pyralmandine (± hornblende) granulite facies into the eclogite facies (M1). Several of these zones, bounded by isogrades, which are sometimes tectonic in nature, have been mapped. From the fact that the banded gneisses are only metatexitic and from the jadeite content of omphacite, the P/T conditions for the eclogite facies are estimated as: T\u2248700°-750°C, P\u224811-13 Kb, and water vapour pressures: very low. In the gneisses isoclinal folding (F1) accompanied the metamorphism. The axial planes are subhorizontal and the fold axes plunge just west of north. The fabric analyses of eclogite (point-maximum for [010]) and basic granulite (point-maximum for [001]) show that the same stressfield which produced the F1-folds in the paragneisses, influenced the preferred orientation of clinopyroxene. A second Precambrian metamorphic phase retrograded the rocks into the hornblende granulite facies (M2). Before the onset of this phase pronounced cataclasis caused the formation of thick mylonitic horizons, followed by E-W trending drag folding (F2). The fabric diagram for clinopyroxene does not show a preferred orientation. During this deformation phase the M1 metamorphic zoning was turned upside down by a combined process of folding and thrusting. In the paragneisses the hornblende granulite metamorphism is marked by a second generation of kyanite. Gabbros, intruded along thrust planes, were partly metamorphosed into garnet-coronites. During this second metamorphic phase isoclinal folds (F3) with subhorizontal axial planes and N-S axial directions were formed. The fabrics of these folds show a marked orientation of the c-axes of (metastable) diopside and brown-green hornblende parallel to the fold axis direction. A third metamorphic phase caused further retrogradation of the rocks into the amphibolite facies (M3). The characteristic amphibole of this phase is a blue-green hornblende. The former metabasites were metamorphosed into (garnet-)amphibolites. Intruded gabbros were transformed along their margins into ‘flaser’ amphibolites. Folds with vertical axial planes and N-S axial directions reflect the synchronous F4-deformation. The large syn- and antiform structures are products of this phase. The fabric of the hornblende in the amphibolites is determined by the stress field of F4. Older hornblende orientations were destroyed. Whether a Hercynian age should be attributed to the amphibolite facies is not certain; if so, F4 is the first Hercynian deformation phase. After the overthrusting of the complex over its low-grade country rocks, a phase of chevron folding (F5) was active locally. On the thrust plane small folds of the second Hercynian folding phase can be discerned. A third Hercynian folding phase can be seen in the Paleozoic rocks but not in the Cabo Ortegal Complex proper. Local greenschist retrogradation (M4) and the emplacement of dolerite dykes are late Hercynian. The tectonic history ends with a phase of normal block faulting which caused the E-W faults.
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Almandine
Sillimanite
Cataclastic rock
Staurolite
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The southwestern French Massif central in western Rouergue displays an inverted metamorphic sequence with eclogite and amphibolite facies units forming the top of the nappe stack. They are often grouped into the leptyno-amphibolite complex included, in this area, at the base of the Upper Gneiss Unit. We sampled garnet micaschists and amphibolites to investigate their metamorphic history with isochemical phase diagrams, thermobarometry and U-Pb zircon dating. Our results demonstrate that two different tectono-metamorphic units can be distinguished. The Najac unit consists of biotite-poor phengite-garnet micaschists, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite orthogneisses. Pressure and temperature estimates on micaschists with syn-kinematic garnets yield a prograde with garnet growth starting at 380 °C/6–7 kbar, peak pressure at 16 kbar for 570 °C, followed by retrogression in the greenschist facies. The age of high pressure metamorphism has been constrained in a recent publication between ca. 383 and 369 Ma. The Laguépie unit comprises garnet-free and garnet-bearing amphibolites with isolated lenses, veins or dykes of leucotonalitic gneiss. Thermobarometry and phase diagram calculation on a garnet amphibolite yield suprasolidus peak P-T conditions at 710 °C, 10 kbar followed by retrogression and deformation under greenschist and amphibolite facies conditions. New U-Pb analyses obtained on igneous zircon rims from a leucotonalitic gneiss yield an age of 363 ± 3 Ma, interpreted as the timing of zircon crystallization after incipient partial melting of the host amphibolite. The eclogitic Najac unit records the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle Allochthon, sandwiched between the Lower Gneiss Unit and the Upper Gneiss Unit. Such an intermediate unit is still poorly defined in the French Massif central but it can be a lateral equivalent of the Groix blueschists in the south Armorican massif. The Uppermost Devonian, amphibolite facies Laguépie unit correlates in terms of P-T-t evolution to the Upper Gneiss Unit in the Western French Massif central. This Late Devonian metamorphism is contemporaneous with active margin magmatism and confirms that the French Massif central belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.
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Phengite
Blueschist
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