U–Th–Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada

1989 ◽  
Vol 26 (12) ◽  
pp. 2636-2644 ◽  
Author(s):  
L. Schiøtte ◽  
W. Compston ◽  
D. Bridgwater
Keyword(s):  
Field Model ◽  
Main Phase ◽  
High Grade ◽  
Positive Evidence ◽  
Zircon Growth ◽  
Ion Probe ◽  
Igneous Origin ◽  
Minimum Age ◽  
Younger Generation ◽  
Textural Evidence

Ion-probe U–Pb dating of igneous zircons from an acid metavolcanite belonging to the Nulliak supracrustal association of the Saglek area, northern Labrador, suggests an original age of 3776 ± 8 Ma (2σ), interpreted as the deposition age of the volcanite. In accordance with the field model, this is slightly but significantly older than the previously obtained ion-probe U–Pb age of igneous zircons from the main phase of the Uivak gneisses, which carries inclusions of the supracrustal association. A secondary event of zircon growth recorded in the metavolcanite has a maximum age of 3065 ± 8 Ma (2σ).A metapelite belonging to the Weekes amphibolite supracrustal association of the Hopedale area south of Nain contains rounded, low-U zircons of 3258 ± 24 Ma (2σ) age. Occasional textural evidence suggests that some of these zircons are most likely detrital grains of igneous origin, in which case 3258 ± 24 Ma is a minimum age of the source from which the sediment was derived and a maximum age for the deposition of the sediment. However, the possibility that the age represents recrystallization of the sediment under high-grade conditions cannot be ruled out. A 2841 ± 5 Ma (2σ) age obtained on a younger generation of euhedrally zoned high-U zircons from the metapelite is within error of the published bulk zircon U–Pb age of the tonalitic component of the Kanairiktok plutonic suite that intrudes the older orthogneiss and supracrustal units in the area. These younger zircons are most likely to have grown during migmatization of the metasediment at the time of emplacement of the suite.The zircon data give no positive evidence to support correlation between the Nulliak supracrustals and the Weekes amphibolite association.

The Pikwitonei Granulite Domain, Manitoba: a giant Neoarchean high-grade terrane in the northwest Superior ProvinceThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.N. Machado, T.E. Krogh, and W. Weber are deceased.

2011 ◽  
Vol 48 (2) ◽  
pp. 205-245 ◽  
Author(s):  
L. M. Heaman ◽  
Ch. O. Böhm ◽  
N. Machado ◽  
T. E. Krogh ◽  
W. Weber ◽  
...  
Keyword(s):  
Granulite Facies ◽  
Superior Province ◽  
High Grade ◽  
Metamorphic Zircon ◽  
Special Issue ◽  
Metamorphic History ◽  
Zircon Growth ◽  
The World ◽  
Northwestern Margin ◽  
History Of

The Pikwitonei Granulite Domain located at the northwestern margin of the Superior Province is one of the largest Neoarchean high-grade terranes in the world, with well-preserved granulite metamorphic assemblages preserved in a variety of lithologies, including enderbite, opdalite, charnockite, and mafic granulite. U–Pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities; Natawahunan Lake, Sipiwesk Lake, and Cauchon Lake. The U–Pb age results indicate that some of the layered enderbite gneisses are Mesoarchean (3.4–3.0 Ga) and the more massive enderbites are Neoarchean. The high-grade metamorphic history of the Pikwitonei Granulite Domain is complex and multistage with at least four episodes of metamorphic zircon growth identified: (1) 2716.1 ± 3.8 Ma, (2) 2694.6 ± 0.6 Ma, (3) 2679.6 ± 0.9 Ma, and (4) 2642.5 ± 0.9 Ma. Metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent, corresponding to M1 amphibolite- and M2 granulite-facies events, respectively, consistent with previous field observations. The youngest metamorphic episode at 2642.5 Ma is only recognized at southern Cauchon Lake, where it coincides with granite melt production and possible development of a major northeast-trending deformation zone. The timing and multistage metamorphic history recorded in the Pikwitonei Granulite Domain is similar to most Superior Province high-grade terranes and marks a fundamental break in Archean crustal evolution worldwide at the termination of prolific global Neoarchean greenstone belt formation.

Techniques and methodology used in a mineralogical and osmium isotope study of platinum group minerals from alluvial deposits in Colombia, California, Oregon and Alaska

1997 ◽  
Vol 61 (406) ◽  
pp. 367-375 ◽  
Author(s):  
I. C. Lyon ◽  
H. Tamana ◽  
D. J. Vaughan ◽  
A. J. Criddle ◽  
J. M. Saxton ◽  
...  
Keyword(s):  
Trace Element ◽  
Alluvial Deposits ◽  
Ion Probe ◽  
Platinum Group Minerals ◽  
Placer Deposits ◽  
Trace Element Contents ◽  
Isotope Study ◽  
Platinum Group ◽  
Textural Evidence ◽  
Rhenium Content

AbstractPlatinum-group minerals (PGM) from placer deposits in Colombia, California, Oregon and Alaska were investigated with the electron microprobe, proton microprobe (μ-PIXE) and ion probe to analyse their major and trace element contents and 187Os/186Os isotopic ratios. Most of the grains in the samples investigated proved to be essentially homogeneous alloys of Pt-Fe and Os-Ir-Ru although a few of them contained inclusions of other PGM such as cooperite and laurite. Detailed analyses were undertaken on the Os-Ir-Ru alloy phases.The 187Os/186Os isotope ratios fell into a range from 1.005 to 1.156 and are consistent with data published on PGM from other placer deposits from these regions. The ratios, together with the trace element data (and in particular the low rhenium content) determined by ion probe and μ-PIXE, indicate that crustal osmium was not incorporated in the grains and that no significant evolution of the 187Os/186Os ratios occurred during their history. These data, along with mineralogical and textural evidence, are consistent with a mantle origin for the grains through ultramafic intrusions, although the data do not entirely rule out alternative interpretations.

Gneissic Alkalic Rocks and Carbonatites in the Frenchman's Cap Gneiss Dome, Shuswap Complex, British Columbia

1974 ◽  
Vol 11 (2) ◽  
pp. 304-318 ◽  
Author(s):  
W. J. McMillan ◽  
J. M. Moore Jr.
Keyword(s):  
Regional Metamorphism ◽  
Lava Flows ◽  
High Grade ◽  
Metamorphic Complex ◽  
Sedimentary Deposits ◽  
Gneiss Domes ◽  
The Core ◽  
Igneous Origin ◽  
Eastern Border ◽  
Shuswap Metamorphic Complex

Frenchman's Cap dome is one of a series of gneiss domes along the eastern border of the high-grade Shuswap metamorphic complex. The granitic gneisses which compose the core of the dome are enclosed in an envelope of metasedimentary rocks.Before Shuswap metamorphism and deformation, the rocks of the sedimentary envelope were intruded by concordant bodies of alkalic rocks and carbonatite. Other carbonatite bodies appear to have formed at or extruded onto the surface. It is not certain whether these are exhalative sedimentary deposits, lava flows, or pyroclastic deposits.Criteria which can be used to distinguish igneous alkalic rocks from those of metasomatic origin were almost entirely destroyed by regional metamorphism. A few relict igneous textures show that at least some of the alkalic gneisses are of igneous origin.

Feldspars from Adamant pluton, British Columbia

1969 ◽  
Vol 6 (5) ◽  
pp. 1199-1209 ◽  
Author(s):  
Peter E. Fox ◽  
John M. Moore Jr.
Keyword(s):  
British Columbia ◽  
Distribution Coefficient ◽  
Structural State ◽  
Regional Metamorphism ◽  
Approach To Equilibrium ◽  
The Core ◽  
Igneous Origin ◽  
Quartz Monzonite ◽  
Textural Evidence ◽  
Alkali Feldspars

Adamant pluton is a zoned body of igneous origin, with a core of pyroxene monzonite transitional outward to hornblende quartz monzonite and biotite-hornblende granodiorite. On chemical and textural evidence, the lithologic zoning resulted from reaction with introduced water and oxygen during regional metamorphism to upper amphibolite facies. Feldspars in the monzonite are orthoclase and andesine of intermediate structural state. During transformation of the pluton, systematic changes in the feldspars comprised: (1) ordering of alkali feldspars toward intermediate microcline, with increase of 2V, decrease of Na and Ca content, and appearance of grid twins; (2) ordering and increase in Na content of plagioclase. The most ordered feldspars, with highest Na distribution coefficient, are found in peripheral granodiorite and in pegmatite cutting the quartz monzonite. The feldspar data support the model proposed for the evolution of the pluton, indicating approach to equilibrium in the peripheral rocks at lower temperatures than that attendant to the formation of the core.

Controls of copper mineralization at Coniston, English Lake District

1977 ◽  
Vol 114 (3) ◽  
pp. 195-202 ◽  
Author(s):  
G. W. Dagger
Keyword(s):  
Regional Metamorphism ◽  
Fracture Pattern ◽  
Main Phase ◽  
Low Grade ◽  
Lake District ◽  
Granite Intrusion ◽  
Copper Mineralization ◽  
English Lake District ◽  
Textural Evidence ◽  
Volcanic Series

SummaryAnalysis of the fracture pattern in the Coniston area of the Lake District indicates that copper mineralization is localized in a series of fractures produced during the main phase of deformation affecting the Borrowdale Volcanic Series. Three events are recognized on the basis of field and textural evidence obtained from polished sections: an early haematite mineralization, correlated with the low grade regional metamorphism affecting the rocks; the main phase of sulphide mineralization, with zoning, which is correlated with a granite intrusion at depth; and a late renewed movement on the veins, with carbonate mineralization, believed to be related to uplift of the granite.

scholarly journals Textural evidence for high-grade ignimbrites formed by low-explosivity eruptions, Paraná Magmatic Province, southern Brazil

2018 ◽  
Vol 355 ◽  
pp. 87-97 ◽  
Author(s):  
Ana Carolina F. Luchetti ◽  
Darren M. Gravley ◽  
Guilherme A.R. Gualda ◽  
Antonio J.R. Nardy
Keyword(s):  
High Grade ◽  
Southern Brazil ◽  
Textural Evidence

Age limits for the Ubendian metamorphic episode in Northern Malawi

1975 ◽  
Vol 112 (4) ◽  
pp. 403-410 ◽  
Author(s):  
M. H. Dodson ◽  
B. J. Cavanagh ◽  
E. C. Thatcher ◽  
M. Aftalion
Keyword(s):  
High Grade ◽  
Zircon Age ◽  
Minimum Age ◽  
Age Limits

SummaryThe Nyika granite gives a U-Pb zircon age of 1970±30 Ma, which is a minimum age for the high-grade Ubendian metamorphism in Northern Malawi. Cordierite gneisses from the same region yield an Rb-Sr whole-rock isochron age of 1815±260 Ma. Because this age is believed to reflect the high-grade Ubendian metamorphism, the latter cannot be significantly older than about 2100 Ma.

scholarly journals Ultramafic Varėna Suite in the Precambrian crystalline basement of the Southern Lithuania – implications for the origin

Baltica ◽  
2016 ◽  
Vol 29 (2) ◽  
pp. 93-106
Author(s):  
Gediminas Motuza ◽  
Vaida Kirkliauskaitė
Keyword(s):  
Layered Intrusion ◽  
Crystalline Basement ◽  
Ultramafic Rocks ◽  
High Grade ◽  
Iron Ores ◽  
Metasomatic Alteration ◽  
Geochemical Characteristic ◽  
Form Complex ◽  
Igneous Origin ◽  
Mineral Commodities

The Varėna Suite comprises a set of ultramafic rocks: olivinic, pyroxenic, magnetitic, dolomitic, and apatite bearing rocks, which form complex bodies of few sq. km in the Precambrian crystalline basement near the Varėna town in Southern Lithuania. Occurrences of few mineral commodities are related to the Varėna Suite. Magnetitic rocks contain essential resources of high grade iron ores. Phlogopite, apatite, REE and Th mineralization is related to the Varėna Suite. The assessment of the potential for these commodities is primarily dependent on the origin of Varėna Suite, which is still disputed. The models of metasomatic (skarn), and igneous (layered intrusion) origin are proposed earlier. The article presents an overview and reinterpretation of the recent data on the Varėna Suite, its petrographic and geochemical characteristic, as well as the arguments for igneous origin of the Varėna Suite, as the polyphase intrusion with subsequent metasomatic alteration, with alkaline trend.

scholarly journals Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for Mesoarchean Continental Crustal Evolution in Kongling High-grade Metamorphic Terrane, Yangtze Craton, China

2019 ◽  
Author(s):  
Yunxu Wei ◽  
Wenxiao Zhou ◽  
Zhengxiang Hu ◽  
Xianxiao Huang ◽  
Haiquan Li ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Plate Tectonics ◽  
Volcanic Rocks ◽  
High Grade ◽  
Yangtze Craton ◽  
High K ◽  
Minimum Age ◽  
Subducted Oceanic Crust ◽  
Metamorphic Terrane ◽  
Basic Igneous Rock

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).

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