Significance of “gneissic” rocks in the Liscomb Complex, Nova Scotia

2010 ◽  
Vol 47 (6) ◽  
pp. 927-940 ◽  
Author(s):  
J. V. Owen ◽  
R. Corney ◽  
J. Dostal ◽  
A. Vaughan
Keyword(s):  
Igneous Rocks ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
High Grade ◽  
Peraluminous Granite ◽  
Isotopic Signatures ◽  
Conventional View ◽  
Basement Rocks ◽  
Mineral Assemblages ◽  
Intrusive Rocks

The Liscomb Complex comprises Late Devonian intrusive rocks (principally peraluminous granite) and medium- to high-grade metamorphic rocks (“gneisses”) that collectively are hosted by low-grade (greenschist facies) metasediments of the Cambro-Ordovician Meguma Group. The conventional view that these “gneisses” contain high-grade mineral assemblages and represent basement rocks has recently been challenged, and indeed, some of the rocks previously mapped as gneisses, particularly metapelites, have isotopic compositions resembling the Meguma Group. Amphibole-bearing enclaves in the Liscomb plutons, however, are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the Popes Harbour lamprophyre dyke, south of the Liscomb area. Metasedimentary enclaves with Meguma isotopic signatures can contain garnets with unzoned cores (implying high temperatures) that host high-grade minerals (prismatic sillimanite, spinel, and (or) corundum) and are enclosed by retrograde-zoned rims. These features are interpreted here as having formed during and following the attainment of peak temperatures related to Liscomb magmatism. The amphibole-bearing meta-igneous rocks described here contain cummingtonite or hornblendic amphibole and occur as enclaves in granodioritic to tonalitic plutons. They are mineralogically, texturally, and isotopically distinct from Meguma metasediments and at least some of the plutonic rocks that enclose them, so remain the most likely candidate for basement rocks in the Liscomb Complex.

Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands

1993 ◽  
Vol 5 (2) ◽  
pp. 193-206 ◽  
Author(s):  
P. D. Kinny ◽  
L. P. Black ◽  
J. W. Sheraton
Keyword(s):  
Granulite Facies ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
High Grade ◽  
Ion Microprobe ◽  
Rock Interaction ◽  
Mafic Rocks ◽  
Grade Fluid ◽  
Vestfold Hills ◽  
Intrusive Suite

The application of zircon U-Pb geochronology using the SHRIMP ion microprobe to the Precambrian high-grade metamorphic rocks of the Rauer Islands on the Prydz Bay coast of East Antarctica, has resulted in major revisions to the interpreted geological history. Large tracts of granitic orthogneisses, previously considered to be mostly Proterozoic in age, are shown here to be Archaean, with crystallization ages of 3270 Ma and 2800 Ma. These rocks and associated granulite-facies mafic rocks and paragneisses account for up to 50% of exposures in the Rauer Islands. Unlike the 2500 Ma rocks in the nearby Vestfold Hills which were cratonized soon after formation, the Rauer Islands rocks were reworked at about 1000 Ma under granulite to amphibolite facies conditions, and mixed with newly generated felsic crust. Dating of components of this felsic intrusive suite indicates that this Proterozoic reworking was accomplished in about 30–40 million years. Low-grade retrogression at 500 Ma was accompanied by brittle shearing, pegmatite injection, partial resetting of U-Pb geochronometers and growth of new zircons. Minor underformed lamprophyre dykes intruded Hop and nearby islands later in the Phanerozoic. Thus, the geology of the Rauer Islands reflects reworking and juxtaposition of unrelated rocks in a Proterozoic orogenic belt, and illustrates the important influence of relatively low-grade fluid-rock interaction on zircon U-Pb systematics in high-grade terranes.

Early Cenozoic resetting of potassium–argon dates and geothermal history of north Okanagan area, British Columbia

1981 ◽  
Vol 18 (8) ◽  
pp. 1310-1319 ◽  
Author(s):  
Wm. H. Mathews
Keyword(s):  
Vitrinite Reflectance ◽  
Volcanic Rocks ◽  
Blocking Temperature ◽  
Low Grade ◽  
High Grade ◽  
Basement Rocks ◽  
Thermal Changes ◽  
History Of ◽  
The Trinity ◽  
Early Cenozoic

Unmetamorphosed Early Eocene sediments and volcanic rocks of the Trinity Hills and Enderby Cliffs yield K–Ar dates of 42–49 Ma. These overlie high-grade gneisses yielding K–Ar ages on biotites, muscovites, and hornblende ranging from 47 to 60 Ma. The Eocene sediments and volcanics rest nearby on low-grade phyllites, greenstones, and schists yielding dates from 83 to 155 Ma. The gneiss dates are regarded as reset by some Late Cretaceous to earliest Cenozoic thermal event that did not affect, at least to the same degree, the nearby less metamorphosed basement rocks. A thermal history has been constructed to account for the decreasing apparent ages of biotite (assumed blocking temperature of 250 °C) with increasing depth below the sub-Eocene unconformity, for the greater ages of hornblende and muscovite in the same rocks (blocking temperatures of 500 and 350 °C), as well as for thermal changes associated with high vitrinite reflectance from coal at one site in the covering sediments. Very rapid stripping (something like 5 km in 12 Ma) is inferred for the areas of reset gneisses, but not for the schist areas, in early Cenozoic time.

A model for the origin of the Lac St. Jean anorthosite massif

1976 ◽  
Vol 13 (2) ◽  
pp. 389-399 ◽  
Author(s):  
R. A. Frith ◽  
K. L. Currie
Keyword(s):  
Experimental Data ◽  
Partial Melting ◽  
Metamorphic Rocks ◽  
High Grade ◽  
Calc Alkaline ◽  
Grenville Province ◽  
Thermal Event ◽  
Anorthosite Massif ◽  
Mineral Assemblages ◽  
Potassic Rocks

An ancient tonalitic complex becomes migmatitic around the Lac St. Jean massif, ultimately losing its identity in the high grade metamorphic rocks surrounding the anorthosite. Field relations suggest extreme metamorphism and anatexis of tonalitic rocks. Experimental data show that extensive partial melting of the tonalite leaves an anorthositic residue. The same process operating on more potassic rocks would leave monzonitic or quartz syenitic residues. Synthesis of experimental data suggests that the process could operate at pressures of 5–8 kbar and temperatures of 800–1000 °C, which are compatible with mineral assemblages around the anorthosite massif. Slightly higher temperatures at the end of the process could generate magmatic anorthosite.Application of the model to the Grenville province as a whole predicts generation of anorthosite during a long-lived thermal event of unusual intensity. Residual anorthosite would occur as a substratum in the crust, overlain by high-grade metamorphic rocks intruded by anorthosite and syenitic rocks, while higher levels in the crust would display abundant calc-alkaline plutons and extrusives.

scholarly journals APATITE-ALBITE METASOMATITES (ACEITES) OF THE EASTERN ALDAN-STANOVOY SHIELD

2021 ◽  
Vol 40 (5) ◽  
pp. 59-73
Author(s):  
V.E. Kirillov ◽  
Keyword(s):  
Rare Earth ◽  
Structural Control ◽  
Earth Element ◽  
Rare Metal ◽  
Igneous Rocks ◽  
Metamorphic Rocks ◽  
Intrusive Rocks ◽  
Characteristic Features ◽  
Metasomatic Zoning ◽  
Geochemical Specialization

The paper summarizes the findings of research on Riphean ore-bearing apatite-albite metasomatites (aceites) identified in metamorphic, volcanic and intrusive rocks in the eastern Aldan-Stanovoy shield. The characteristic features of lithological and structural control of aceites, their mineral and petrochemical composition, geochemical associations, ontogeny, metasomatic zoning, and geochemical specialization are outlined. Aceites in metamorphic rocks are assigned to the albite-chlorite-apatite facies and in igneous rocks to the albite-apatite facies. Apatite-albite metasomatites host mineralization of two types: uranium (in aceites after metamorphic rocks) and uranium – rare earth element – rare metal (in aceites after volcanic and intrusive rocks).

scholarly journals The ammonium content in the Malayer igneous and metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)

2011 ◽  
Vol 62 (2) ◽  
pp. 171-180 ◽  
Author(s):  
Vahid Ahadnejad ◽  
Ann Hirt ◽  
Mohammad-Vali Valizadeh ◽  
Saeed Bokani
Keyword(s):  
Colorimetric Method ◽  
Igneous Rocks ◽  
Granitic Rocks ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
High Concentration ◽  
Western Iran ◽  
Metasedimentary Rocks ◽  
Sanandaj Sirjan Zone ◽  
External Sources

The ammonium content in the Malayer igneous and metamorphic rocks (Sanandaj-Sirjan Zone, Western Iran)The ammonium (NH4+) contents of the Malayer area (Western Iran) have been determined by using the colorimetric method on 26 samples from igneous and metamorphic rocks. This is the first analysis of the ammonium contents of Iranian metamorphic and igneous rocks. The average ammonium content of metamorphic rocks decreases from low-grade to high-grade metamorphic rocks (in ppm): slate 580, phyllite 515, andalusite schist 242. In the case of igneous rocks, it decreases from felsic to mafic igneous types (in ppm): granites 39, monzonite 20, diorite 17, gabbro 10. Altered granitic rocks show enrichment in NH4+(mean 61 ppm). The high concentration of ammonium in Malayer granites may indicate metasedimentary rocks as protoliths rather than meta-igneous rocks. These granitic rocks (S-types) have high K-bearing rock-forming minerals such as biotite, muscovite and K-feldspar which their potassium could substitute with ammonium. In addition, the high ammonium content of metasediments is probably due to inheritance of nitrogen from organic matter in the original sediments. The hydrothermally altered samples of granitic rocks show highly enrichment of ammonium suggesting external sources which intruded additional content by either interaction with metasedimentary country rocks or meteoritic solutions.

The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism

1986 ◽  
Vol 50 (357) ◽  
pp. 399-415 ◽  
Author(s):  
David C. Rubie
Keyword(s):  
Grain Boundaries ◽  
Fluid Phase ◽  
Aqueous Fluid ◽  
Western Alps ◽  
Metamorphic Rocks ◽  
Time Interval ◽  
Basement Rocks ◽  
Mineral Assemblages ◽  
Intergranular Diffusion ◽  
Adula Nappe

AbstractThe problems of characterizing inter-granular regions and of estimating rates of intergranular diffusion in metamorphic rocks are discussed. Inter-granular regions can be anhydrous, hydrated but under-saturated with H2O, or saturated with H2O, but only in the latter case can a free aqueous fluid phase be present. Estimates of intergranular diffusion coefficients (DIGR) at 550°C derived from a variety of published experimental work, vary from ∼ 10−8 m2 s−1 for diffusion of species through an intergranular fluid film to ⩽ 4 × 10−24 m2 s−1 for diffusion of SiO2 or O in anhydrous grain boundaries in quartzite. Estimates of DIGR for hydrated grain boundaries vary from ∼ 10−13 m2 s−1 to ∼ 10−21 m2 s−1; the concentration of H2O in the grain boundaries and the identity of the diffusing species (generally unknown) may be important controlling factors, and there exists the possibility of a spectrum of values between these two extremes.Using available kinetic data it is shown that a free aqueous fluid could never have been present in parts of the basement terrane of the Sesia Zone (Western Alps) during uplift from the eclogite facies, except possibly late in the cooling history. The breakdown of sodic pyroxene + quartz occurred in response to the localized infiltration of catalytic aqueous fluid, possibly over a time interval as short as 6–6000 a, and possibly under conditions remote from equilibrium. H2O-present conditions during a dehydration reaction in metapelites of the Adula nappe (central Alps) could also have been of short duration. These examples are consistent with a model in which basement rocks at deep crustal levels are dry for long periods of time and in which the development of equilibrium mineral assemblages and microstructures generally occurs over relatively short periods of time under transitory fluid-present conditions (caused by devolatilization and/or infiltration).

scholarly journals On the High-grade Metamorphic Rocks associated with the Yakuno Intrusive Rocks of the Maizuru Zone

1959 ◽  
Vol 65 (764) ◽  
pp. 267-271 ◽  
Author(s):  
Hiroshi KANO ◽  
Keiji NAKAZAWA ◽  
Sachio IGI ◽  
Tsunemasa SHIKI
Keyword(s):  
Metamorphic Rocks ◽  
High Grade ◽  
Intrusive Rocks

scholarly journals Deconstructing the Infrastructure: A Complex History of Diachronous Metamorphism and Progressive Deformation during the Late Cretaceous to Eocene in the Thor-Odin–Pinnacles Area of Southeastern British Columbia

2016 ◽  
Vol 43 (2) ◽  
pp. 103 ◽  
Author(s):  
Deanne Van Rooyen ◽  
Sharon D. Carr
Keyword(s):  
British Columbia ◽  
Late Cretaceous ◽  
Normal Fault ◽  
Columbia River ◽  
Metamorphic Rocks ◽  
High Grade ◽  
Basement Rocks ◽  
History Of ◽  
Complex Structural ◽  
Structural Levels

The Thor-Odin dome is a basement-cored tectonothermal culmination in southern British Columbia containing high-grade metamorphic rocks that were polydeformed in the Late Cretaceous to Eocene. The rocks south of the Thor-Odin dome that extend ca. 20 km to the Pinnacles culmination and Whatshan batholith comprise a heterogeneous tract of polydeformed medium- to high-grade metamorphic rocks and host the South Fosthall pluton near the base of the structural section. They lie in the footwall of the Columbia River fault (CRF) zone, a moderately east-dipping, ductile-brittle, normal fault that was active after ca. 55 Ma and reactivated periodically up to 30 Ma. This tract of rocks has been interpreted as a mid-crustal zone that was exhumed and cooled during Eocene extension or, alternatively, a mid-crustal channel that was bounded at the top by the CRF and was active during the Late Cretaceous to Eocene. However, the timing of metamorphism, deformation, anatexis in basement rocks, and intrusion of leucogranite plutons reveals that there are four tectonothermal domains within the tract that each experienced metamorphism, deformation and cooling at different times. These rocks record Cretaceous metamorphism and cooling in the upper structural levels and three stages of progressive metamorphism and penetrative deformation that migrated into deeper crustal levels in the Paleocene and Eocene producing a complex structural section that was exhumed in part due to motion on the Columbia River fault zone, and in part due to NE-directed transport over a basement ramp.RÉSUMÉLe dôme de Thor-Odin correspond à une culmination tectonothermique d’un noyau de socle dans le sud de la Colombie-Britannique renfermant des roches métamorphiques de haute intensité polydéformées entre le Crétacé supérieur et l’Éocène. Les roches au sud du dôme de Thor-Odin qui s’étendent sur environ 20 km jusqu’à la culmination des Pinnacles et du batholite de Whatshan sont constituées d’une bande hétérogène de roches polydéformées à faciès métamorphique d’intensité moyenne à élevée qui constitue l’encaissant du pluton de South Fosthall près de la base de la colonne structurale. Elles se trouvent dans l'éponte inférieure de la zone de faille de la rivière Columbia (CRF), une faille normale à pendage modéré vers l’est, ductile-fragile, qui a été active après 55 Ma environ et a été réactivée périodiquement jusqu'à 30 Ma. Cette bande de roches a été interprétée comme une zone de mi-croûte qui a été exhumée et a refroidi durant l’extension éocène ou alors comme un canal mi-crustal qui a été limité au sommet par la CRF, et qui a été actif de la fin du Crétacé jusqu’à l’Éocène. Toutefois, la chronologie du métamorphisme, de la déformation, de l’anatexie dans les roches du socle, et de l'intrusion de plutons de leucogranite, montre qu'il existe quatre domaines tectonothermiques pour chaque bande qui ont subit du métamorphisme, de la déformation et du refroidissement à différents moments. Ces roches exhibent un métamorphisme et un refroidissement crétacé dans les niveaux structuraux supérieurs et trois stades de métamorphisme progressif et de déformation pénétrative qui ont migré dans les niveaux crustaux profonds au Paléocène et à l’Eocène constituant ainsi une colonne structurale complexe qui a été exhumée en partie en raison du mouvement de la zone de faille de Columbia River, et en partie en raison du transport vers le N.-E. sur une rampe de socle.

scholarly journals Sulfides in Metamorphic Rocks of the Fore Range Zone (Greater Caucasus). A New Type of Mineral Container for Peak Metamorphism Mineral Assemblages

Minerals ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 701 ◽  
Author(s):  
Kamzolkin ◽  
Konilov ◽  
Kulakova ◽  
Latyshev ◽  
Smulskaya ◽  
...  
Keyword(s):  
Metamorphic Rocks ◽  
Low Grade ◽  
Greater Caucasus ◽  
Mineral Inclusions ◽  
Mineral Assemblages ◽  
Peak Metamorphism ◽  
New Type ◽  
Formation Conditions ◽  
Si Content ◽  
Initial Rock

The rocks of the Armovka Formation (the Fore Range zone, Greater Caucasus) have undergone low-grade metamorphism that partially erased information about initial rock formation conditions. We discovered high-pressure mineral inclusions such as omphacite, phengite, garnet, and paragonite enclosed by pyrite and chalcopyrite. Mineral inclusions in sulfides may provide important information about metamorphic pressure−temperature conditions because they are shielded by the host minerals and isolated from significant low-grade overprinting. Calculations performed on phengite inclusions using the phengite Si-content barometry indicate a pressure ranging from 1.7 ± 0.2 to 1.9 ± 0.2 GPa for temperature of 600 ± 40 °C. These data are consistent with estimations obtained for eclogite bodies embedded in rocks of the Armovka Formation. Geothermobarometry of the latest yielded conditions of 680 ± 40 °C and a minimum pressure of 1.6 ± 0.2 GPa to upper pressure boundary at 2.1 GPa. This fact allows us to assume that the metamorphic rocks of the Armovka Formation were immersed in the subduction zone to the conditions of the eclogite facies of metamorphism, forming a coherent subduction complex together with eclogites.

New Constraints on the Timing and History of Breccia Dikes in the Western San Juan Mountains, Southwestern Colorado

2019 ◽  
Vol 56 (4) ◽  
pp. 397-420
Author(s):  
David Gonzales
Keyword(s):  
Spatial Relationship ◽  
Metamorphic Rocks ◽  
San Juan ◽  
San Juan Mountains ◽  
Basement Rocks ◽  
Intrusive Rocks ◽  
Close Spatial Relationship ◽  
History Of ◽  
Proterozoic Basement ◽  
Structural Levels

In the western San Juan Mountains, clastic (breccia) dikes crop out in Paleozoic to Cenozoic rocks. The dikes are tabular to bifurcating masses up to several meters thick and are exposed on northwest or northeast trends for up to several kilometers. They are matrix- to clast-supported with angular to rounded pebble- to boulder-sized fragments that in most dikes are dominated by Proterozoic igneous and metamorphic rocks. U-Pb age analyses (n = 3) reveal a range of zircon ages in all samples with several containing high proportions of 1820 to 1390 Ma zircons. The majority of Proterozoic zircons are interpreted as direct contributions from basement rocks during breccia dike formation and emplacement. Field relations and U-Pb zircon analyses reveal that breccia dikes formed in intervals from 65 to 30 Ma (Ouray) and 27 to 12 Ma (Stony Mountain); some dikes are closely allied with mineralization. The dikes formed at depths over 500 meters where Proterozoic basement was fragmented, entrained, and transported to higher structural levels along with pieces of Paleozoic to Cenozoic rocks. A close spatial relationship exists between breccia dikes and latest Mesozoic to Cenozoic plutons. This is best exemplified near Ouray where clastic dikes share similar trends with ~65 Ma granodiorite dikes, and there is a clear transition from intrusive rocks to altered-brecciated plutons, and finally to breccia dikes. The preponderance of evidence supports breccia dike formation via degassing and explosive release of CO2-charged volatiles on deep fractures related to emplacement of 70 to 4 Ma plutons or mantle melts. In addition to breccia dikes, several post-80 Ma events in the region involved explosive release of volatile-charged magmas: 29-27 Ma calderas, ~25 Ma diatremes, and ~24 Ma breccia pipes. Causal factors for production of these gas-charged magmas remain poorly understood, but partial melting or assimilation of altered and metasomatized lithospheric mantle could have played a role.

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