40Ar/39Ar whole-rock phyllite ages from late Precambrian rocks of the Avalon composite terrane, New Brunswick: evidence of Silurian–Devonian thermal rejuvenation

1994 ◽  
Vol 31 (5) ◽  
pp. 818-824 ◽  
Author(s):  
R. David Dallmeyer ◽  
R. Damian Nance
Keyword(s):  
New Brunswick ◽  
Low Grade ◽  
Fine Grained ◽  
Metasedimentary Rocks ◽  
Late Precambrian ◽  
Minimum Age ◽  
Tectonic Contact ◽  
Age Constraints ◽  
Gas Fractions ◽  
Thermal Overprint

Several variably deformed and metamorphosed, late Precambrian volcanic–sedimentary successions have been recognized within the Avalon composite terrane exposed in the Caledonian Highlands of southern New Brunswick. Whole-rock samples of metasedimentary phyllite and phyllitic metatuff from the oldest (ca. 600–635 Ma) Avalonian succession display similar, internally discordant 40Ar/39Ar age and apparent K/Ca spectra. Intermediate-temperature gas fractions were experimentally evolved solely from very fine grained, cleavage-aligned white micas. These yield apparent ages between ca. 430 and 410 Ma, and are interpreted to closely date a static Late Silurian – Early Devonian thermal rejuvenation.Evidence for a Silurian – Devonian thermal event has not been previously documented in Avalonian rocks of the Caledonian Highlands (Caledonia assemblage). However, a thermal overprint of similar age (ca. 400 Ma) is recorded by metamorphic muscovite in high-grade gneisses and platformal metasedimentary rocks (Brookville assemblage), which are in tectonic contact with the low-grade Caledonia assemblage. These potentially correlative thermal overprints may provide minimum age constraints on the juxtaposition of these contrasting tectono-stratigraphic assemblages, which are likely to have been palinspastically separate tectonic elements during the earliest Paleozoic.

40Ar/39Ar ages of detrital muscovite within early Paleozoic overstep sequences, Avalon composite terrane, southern New Brunswick: implications for extent of late Paleozoic tectonothermal overprint

1990 ◽  
Vol 27 (9) ◽  
pp. 1209-1214 ◽  
Author(s):  
R. D. Dallmeyer ◽  
R. D. Nance
Keyword(s):  
New Brunswick ◽  
Coarse Grained ◽  
Granitic Rocks ◽  
Late Paleozoic ◽  
Low Grade ◽  
Early Paleozoic ◽  
Late Precambrian ◽  
Pelitic Schist ◽  
Gas Fractions ◽  
Thermal Overprint

Concentrates of coarse-grained detrital muscovite from the Ratcliffe Brook Formation (lowermost Cambrian) display internally discordant 40Ar/39Ar age spectra. Gas fractions evolved at intermediate and high experimental temperatures record apparent ages of ca. 610–620 Ma. These are interpreted as dating initial cooling through temperatures appropriate for intracrystalline retention of 40Ar and may indicate derivation from mylonite zones developed within proximal late Precambrian granitic rocks. Gas fractions evolved at lower experimental temperatures record patterns of spectra discordance that suggest the constituent grains experienced partial, intracrystalline diffusive loss of 40Ar during a late Paleozoic, low-grade thermal overprint. A muscovite concentrate from pelitic schist beneath the allochthonous, latest Precambrian Cranberry Head granite records a 40Ar/39Ar plateau age of 318 ± 1 Ma. This is interpreted as closely dating Late Carboniferous thrust emplacement of the allochthon.

Tectonic implications of 40Ar/39Ar mineral ages from late Precambrian – Cambrian plutons, Avalon composite terrane, southern New Brunswick, Canada

1992 ◽  
Vol 29 (11) ◽  
pp. 2445-2462 ◽  
Author(s):  
R. David Dallmeyer ◽  
R. Damian Nance
Keyword(s):  
New Brunswick ◽  
Quartz Diorite ◽  
Head Group ◽  
Low Grade ◽  
Marine Strata ◽  
Metasedimentary Rocks ◽  
Late Precambrian ◽  
Sedimentary Sequences ◽  
Stratigraphic Record ◽  
Isotope Correlation

Within the Avalon composite terrane exposed in southern New Brunswick, late Precambrian, low-grade volcanic–sedimentary sequences are juxtaposed against late Precambrian gneisses (Brookville Gneiss) and older platformal metasedimentary rocks (Green Head Group) along the Caledonia Fault. Both assemblages host petrographically similar suites of calc-alkalic dioritic and granodioritic plutons. Those intruding volcanic–sedimentary sequences (Caledonia terrane) record ca. 615–625 Ma crystallization ages typical of arc-related magmatism throughout the Avalon composite terrane. However, 40Ar/39Ar age data from stocks intruding gneisses and platformal metasedimentary rocks (Brookville terrane) suggest significantly younger crystallization ages.36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende are interpreted to closely date postmagmatic cooling within six plutons: Fairville Granite (547 ± 1 Ma); French Village Quartz Diorite (539 ± 2 and 537 ± 1 Ma); Rockwood Park Granodiorite (529 ± 2 and 523 ± 3.5 Ma); Musquash Granite (526 ± 2 Ma); Milkish Head Granite (Red Bridge pluton, 520 ± 1.5 Ma); Lepreau Diorite (Talbot Road pluton, 519 ± 2 Ma and Hansen Stream pluton, 518 ± 1.5 Ma. A hornblende isotope correlation age of 530 ± 2 Ma from penetratively foliated amphibolite within the French Village Quartz Diorite suggests that the magmatic activity was locally accompanied by ductile shear. Muscovite within granitic pegmatite in the Brookville Gneiss records a 40Ar/39Ar plateau age of 510 ± 1 Ma interpreted to date final phases of associated magmatic activity.Arc-related magmatism extending into the Cambrian contrasts with the characteristic tectono-stratigraphic record in the Avalon composite terrane where late Precambrian igneous rocks are overstepped by Cambrian–Ordovician shallow-marine strata with only a local and minor record of rift-related volcanic activity. Although the Brookville terrane shows affinities with the Avalon composite terrane during the late Precambrian, the 40Ar/39Ar age data suggest that it was isolated as a distinct tectono-stratigraphic element by the Early Cambrian.

Deformation timing and strain in Neoproterozoic strata, Jebel Akhdar, northern Oman

2020 ◽  
Author(s):  
Christopher Bailey ◽  
Claire Rae
Keyword(s):  
Strain Analysis ◽  
Low Grade ◽  
Depositional History ◽  
Fine Grained ◽  
Metasedimentary Rocks ◽  
Aspect Ratios ◽  
Deformation Kinematics ◽  
History Of ◽  
Penetrative Foliation ◽  
Ar Geochronology

<p>Neoproterozoic rocks exposed in the Jebel Akhdar massif of northern Oman preserve glaciogenic deposits associated with multiple Cryogenian glaciations. Although the depositional history of these rocks is well understood, the significance of post-depositional deformation is poorly constrained. In this study, we examine low-grade metasedimentary rocks exposed in the Ghubrah Bowl, an erosional window in the Jebel Akhdar massif, in order to quantify the 3D finite strain, understand deformation kinematics, and determine the timing of deformation/metamorphism.</p><p>In the Jebel Akhdar massif, the older Ghubrah (Sturtian glaciation) and younger Fiq (Marinoan glaciation) formations comprise a >1 km thick sequence of diamictite interbedded with sandstone, siltstone, conglomerate, volcanic rock, and minor carbonate. Diamictites contain abundant clasts of siltstone and sandstone, with lesser amounts of granite and metavolcanic rock in a fine-grained quartz + sericite ± chlorite matrix. Clasts range from granules to boulders. Harder clasts tend to be subangular and poorly aligned with low aspect ratios, whereas fine-grained rock clasts are well-aligned with large aspect ratios. Bedding generally dips to the NW, but is gently folded in accord with the overall structure of the Jebel Akhdar massif. A penetrative foliation strikes E-W and dips to the S. At some locations, a prominent elongation lineation/pencil structure occurs and plunges gently to moderately to the S.</p><p>R<sub>f</sub>/phi strain analysis in the diamictites reveals a range of 3D strain geometries (apparent flattening to apparent constriction) with strain ratios up to 2.8 in XZ sections. Strain is strongly partitioned, as clasts of igneous rock have low aspect ratios and are only weakly aligned. Penetrative strain in clast-supported sandstones is negligible (XZ ratios of <1.2). Outsized clasts of granite and sandstone are mantled by distinctive symmetric pressure shadows (double-duckbill structures) that include more recrystallized minerals than elsewhere in the diamictite. <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of sericite in pressure shadows yields ages as young as 90 Ma, which are interpreted as mixed ages containing an older detrital component and a younger fraction formed during growth. Deformation is associated with southward emplacement and loading by the Oman ophiolite & Hawasina Group sediments over the autochthonous sequence in the late Cretaceous.</p>

Precambrian muscovite–quartz (agalmatolite) palaeosols from Scotland and Canada

1992 ◽  
Vol 29 (12) ◽  
pp. 2523-2529 ◽  
Author(s):  
Iain Allison ◽  
Lindsay K. Ferguson ◽  
Fidel A. Cardenas ◽  
Barbara I. Kronberg
Keyword(s):  
Lower Cambrian ◽  
Low Grade ◽  
Marine Transgression ◽  
Fine Grained ◽  
Late Precambrian ◽  
Basement Rocks ◽  
The World ◽  
Northwest Ontario ◽  
Rock Lake ◽  
Middle Proterozoic

Weathering, during late Precambrian times, of pink granitic gneisses and pegmatites of the Middle Proterozoic Lewisian Complex in northwest Scotland has produced a characteristic pea-green palaeosaprolite called agalmatolite, which occurs from centimetres to metres beneath the planar unconformity with the overlying quartz sandstones of the Lower Cambrian marine transgression. Agalmatolite consists predominantly of massive, felted, very fine-grained muscovite of the form called pinite together with variable amounts of relict quartz. Pyrophyllite may also occur in the most altered parts, close to the unconformity, and the proportions of precursor minerals, mainly quartz and feldspar, increase as the parent gneisses are approached. Trends in whole-rock elemental variations are consistent with a weathering hypothesis, and the present mineralogy reflects the burial of the saprolite subjected to conditions of anchimetamorphism.Palaeosols developed on both granitic and basaltic basement rocks, with ages ranging up to about 3.0 Ga, have been described from a number of areas throughout the world. In all these palaeosols, muscovite, usually referred to as sericite, is the dominant mineral. In northwest Ontario weathering of the granitic basement beneath the Steep Rock Lake Group has produced a rock almost identical to the Scottish agalmatolite and consisting of muscovite, of the form pinite, relict quartz, and rutile. From published descriptions of other palaeosols, we conclude that the term agalmatolite may be applied to most Precambrian palaeosols, as it conveys more information than terms such as argillite and saprolith that have previously been in use.Compared with the less well-exposed alteration profiles of older palaeosols, the clear field relationships of agalmatolite in northwest Scotland allow more confidence to be placed on interpretations that such alteration profiles have been caused by surficial weathering, with subsequent changes due to burial and even low-grade metamorphism.

U–Pb geochronologic constraints on the cover sequence of the Monashee complex, Canadian Cordillera: Paleoproterozoic deposition on basement

1997 ◽  
Vol 34 (7) ◽  
pp. 1008-1022 ◽  
Author(s):  
James L. Crowley
Keyword(s):  
Hanging Wall ◽  
Crystalline Basement ◽  
Detrital Zircons ◽  
Canadian Cordillera ◽  
Metasedimentary Rocks ◽  
Monashee Complex ◽  
Minimum Age ◽  
Age Constraints ◽  
Unconformable Contact ◽  
Different Parts

The cover sequence in the Monashee complex is a platformal metasedimentary succession that occupies a nearly unique position in the Canadian Cordillera due to its unconformable contact with exposed crystalline basement. Zircon U–Pb data and field observations show that the lower part of the sequence contains Paleoproterozoic rocks, the oldest known metasedimentary rocks in the Cordilleran miogeocline, and the upper part of the sequence is Mesoproterozoic or younger. Maximum age constraints on the lower part are provided by 1.99 Ga detrital zircons from the basal unit and a 1862 ± 1 Ma orthogneiss upon which it was presumably deposited. Minimum age constraints are provided by rocks that intruded into the lower part: 1852 ± 4 Ma pegmatite, 1762 ± 6 Ma leucogranite, and 724 ± 5 Ma syenitic gneiss. The upper part of the sequence must be considerably younger than the lower part because it contains a detrital zircon dated at ~1.21 Ga. Other detrital zircons, dated at Neoarchean (2.95–2.86 Ga) and Paleoproterozoic (1.85–1.81, 1.75 Ga), suggest a source in the western Canadian Shield. These ages constrain the thickness of Mesoproterozoic and Neoproterozoic metasedimentary rocks in the cover sequence to be < 2 km. Combining these ages with previously interpreted Paleozoic deposition ages for the middle and upper parts of the sequence constrains the thickness to be <0.2 km, considerably less than that of coeval rocks above the Monashee complex in the hanging wall of the Monashee décollement. Such a contrast suggests that deposition above and below the décollement occurred in different parts of the Cordilleran miogeocline.

Evolution of Neoproterozoic Shillong Basin, Meghalaya, NE India: implications of supercontinent break-up and amalgamation

2021 ◽  
pp. 1-17
Author(s):  
Susobhan Neogi ◽  
Apoorve Bhardwaj ◽  
Amitava Kundu
Keyword(s):  
Structural Evolution ◽  
Low Grade ◽  
Peninsular India ◽  
Oblique Collision ◽  
Metasedimentary Rocks ◽  
Ne India ◽  
Fold Belts ◽  
Break Up ◽  
Tectonic Contact ◽  
Proterozoic Basement

Abstract Fragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia’s break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings.

scholarly journals Influence of Titaniferous Phases on Tungsten Mineralizing Processes at the Giant Sisson Brook W-Mo Deposit, New Brunswick, Canada: Mineral-Chemical and Geochronological Assessment

Minerals ◽  
2020 ◽  
Vol 10 (7) ◽  
pp. 637
Author(s):  
Aaron L. Bustard ◽  
Wei Zhang ◽  
David R. Lentz ◽  
Christopher R. M. McFarlane
Keyword(s):  
New Brunswick ◽  
Low Grade ◽  
Elemental Mapping ◽  
Metasedimentary Rocks ◽  
West Central ◽  
Icp Ms ◽  
Volcaniclastic Rocks ◽  
Acadian Orogeny ◽  
Host Rocks ◽  
Mineralizing Fluids

The Sisson Brook deposit is a low-grade, large-tonnage W-Mo deposit with notable Cu located in west-central New Brunswick, Canada, and is one of several W-Mo deposits in New Brunswick associated with fluids sourced from granitic plutons emplaced during the Devonian Acadian Orogeny. The younger Devonian-aged stockwork and replacement scheelite-wolframite-molybdenite (and chalcopyrite) mineralization straddles the faulted boundary between Cambro-Ordovician metasedimentary rocks with Ordovician felsic volcaniclastic rocks and the Middle Silurian Howard Peak Granodiorite, with dioritic and gabbroic phases. U-Pb dating of magmatic titanite in the host dioritic phase of the Howard Peak Granodiorite using LA ICP-MS resulted in a 204Pb-corrected concordant age of 432.1 ± 1.9 Ma. Petrologic examination of selected mineralization combined with elemental mapping of vein selvages using micro-XRF and metasomatic titanite and ilmenite grains using LA ICP-MS indicates that saturation of titaniferous phases influenced the distribution of scheelite versus wolframite mineralization by altering the aFe/aCa ratio in mineralizing fluids. Ilmenite saturation in Ti-rich host rocks lowered the relative aFe/aCa and led to the formation of scheelite over wolframite. Altered magmatic titanite and hydrothermal titanite also show increased W and Mo concentrations due to interaction with and/or saturation from mineralizing fluids.

Geometry and kinematics of a major crustal shear zone segment in the Appalachians of southern New Brunswick

1994 ◽  
Vol 31 (10) ◽  
pp. 1523-1535 ◽  
Author(s):  
Adrian F. Park ◽  
Paul F. Williams ◽  
Steven Ralser ◽  
Albert Léger
Keyword(s):  
New Brunswick ◽  
Shear Zone ◽  
Low Grade ◽  
Northern Boundary ◽  
Crystalline Complex ◽  
Late Precambrian ◽  
The North ◽  
Appalachian Orogen ◽  
Complex Forms ◽  
Geometry And Kinematics

In southern New Brunswick the Kennebecasis Fault follows the northern boundary of a crystalline portion of the late Precambrian – Cambrian Avalon terrane. This low-grade crystalline complex forms the basement to a series of Carboniferous through Triassic basins. This complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault, and that with it defines the Pocologan–Kennebecasis fault zone. The orientation of the main composite foliation (S1) and the included mineral (stretching) lineation (L1) indicate that this geometry is a primary feature of the shear zone, representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit. Kinematic indicators show that this allochthon moved parallel to the strike of the north Appalachian orogen, with top towards the west or west-southwest. The bounding shear zone is not uniform, but consists of a mylonite–phyllonite adjacent to the detachment. The upturned segment of the shear zone has been the site of later, brittle reactivation, one episode of which is represented by the Kennebecasis Fault. The main shear zone relic relates to more fundamental events, such as the accretion of the Avalon terrane.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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