Metasedimentary Belt Research Articles

Modelling 3D architecture of western Grenville from surface geology, xenoliths, styles of magma emplacement, and Lithoprobe reflectors

Information derived from regional geology, styles of magma emplacement, xenoliths, and western Grenville Lithoprobe reflectors is integrated to model the architecture of the Central Metasedimentary Belt in Quebec. The belt comprises a thin-skinned western marble domain that projects structurally to the east above a quartzite domain; both domains are underlain by a series of gneiss domes. In contrast, the seismic reflectors dip systematically to the east. They stem from exposed high-strain zones and tectonic boundaries hosting concordant, 1.7-1.16 Ga, postmetamorphic sheetlike plutons. Though discontinuous, the reflectors reach highly reflective crust segments near the Moho. The reflections are interpreted as series of plutons positioning crustal-scale structures rooted in underplated, felsic, and mafic magmas reservoirs. These structures served as magma conduits and ponding sites. The nonreflective segments of the inferred structures are interpreted as through-going pathways for magmas in gneissose crust. Based on the thousands of felsic to ultramafic xenoliths brought up by a 1.07 Ga minette dyke, the nature of the intermediate and lower crust is interpreted as interleaving of nonexposed quartzite-bearing supracrustal assemblages with mylonites, gneisses, and a series of mafic to ultramafic intrusive bodies, many of them inferred to be underplated material. In western Quebec, crustal growth along the pre-Grenvillian Laurentian margin involved pre-1.2 Ga stacking of allochthons including nonexposed metasedimentary and intrusive components during an early stage of Grenvillian collision. Subsequent, 1.7-1.6 Ga magma emplacement along high-strain zones shaped much of the seismic reflectivity of the belt.

Docking of the Central Metasedimentary Belt to Laurentia in geon 12: evidence from the 1.17-1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec

The Chevreuil intrusive suite (1.17-1.16 Ga) represents a chronological field marker of regional extent that intruded the Central Metasedimentary Belt in the western Grenville Province of Quebec after peak metamorphism. Style and site of magma emplacement, and extent of deformation of Chevreuil plutons and dykes permit unravelling of the early Grenvillian evolution of the belt with respect to cratonal North America. The suite comprises a series of vertically layered gabbro stocks and monzonite-diorite-gabbro sheet intrusions, and a swarm of microdiorite dykes that cut across gneisses. The dykes display systematic variations in extent of deformation across the belt. We targeted U-Pb geochronology on gneisses within the identified strain windows; they preserve the record of a ca. 1.20 Ga high pressure-temperature (P-T) metamorphic event. The sheet intrusions define magmatic corridors all along, and concordant with, the western, northern, and eastern tectonic boundaries of the belt. The concordant and elongate shape of these bodies results from emplacement, not deformation. Chevreuil magmas thus sealed the belt boundaries largely in their current positions, with the implication that docking of Elzevirian and pre-Elzevirian terranes with cratonal North America predates 1.17 Ga. We interpret the 1.20 Ga metamorphism as evidence for the initiation of Grenvillian continent-continent collision during the culmination of the Elzevirian orogeny at ca. 1.22 Ga. Emplacement-related fabrics indicate that the Chevreuil suite and the coeval Morin anorthosite suite intruded during renewed orogenesis. This orogenic pulse (Shawinigan) is not accretionary, but represents a strongly partitioned, compressive, intraplate reactivation event.

Convergent margin on southeastern Laurentia during the Mesoproterozoic: tectonic implications

A continental-margin magmatic arc is inferred to have existed on the southeastern (present coordinates) margin of Laurentia from Labrador to Texas from ~1500-1230 Ma, with part of the arc subsequently being incorporated into the 1190-990 Ma collisional Grenville Orogen. Outside the Grenville Province, where the arc is known as the Granite-Rhyolite Belt, it is undeformed, whereas within the Grenville Province it is deformed and metamorphosed. The arc comprises two igneous suites, an inboard, principally quartz monzonitic to granodioritic suite, and an outboard tonalitic to granodioritic suite. The quartz monzonite-granodiorite suite was largely derived from continental crust, whereas the tonalitic-granodiorite suite is calc-alkaline and has a juvenile isotopic signature. Available evidence from the Grenville Province suggests that the arc oscillated between extensional and compressional settings several times during the Mesoproterozoic. Back-arc deposits of several ages, that formed during relatively brief periods of extension, include (1) mafic dyke swarms subparallel to the arc; (2) continental sediments, bimodal volcanics and plateau basalts; (3) marine sediments and volcanics formed on stretched continental crust; and (4) ocean crust in a marginal basin. Closure of the back-arc basins occurred during the accretionary Pinwarian (~1495-1445 Ma) and Elzevirian (~1250-1190 Ma) orogenies, as well as during three pulses of crustal shortening associated with the 1190-990 Ma collisional Grenvillian Orogeny. During the Elzevirian Orogeny, closure of the Central Metasedimentary Belt marginal basin in the southeastern Grenville Province was marked by subduction-related magmatism as well as by imbrication of back-arc deposits. The presence of a continental-margin magmatic arc on southeastern Laurentia during the Mesoproterozoic implies that other coeval magmatism inboard from the arc took place in a back-arc setting. Such magmatism was widespread and chemically diverse and included large volume "anorogenic" anorthosite-mangerite-charnockite-granite (AMCG) complexes as well as small volume alkaline, quartz-saturated and -undersaturated "within-plate" granitoids. Recognition of the ~300 million year duration of the Mesoproterozoic convergent margin of southeastern Laurentia suggests that there may be useful parallels with the evolution of the Andes, which has been a convergent margin since the early Paleozoic.

GENESIS OF CORDIERITE - GEDRITE GNEISSES, CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO, CANADA

Cordierite- and gedrite-bearing lithologies crop out along the base of the Central Metasedimentary Belt boundary thrust zone (CMBbtz) in Ontario, the boundary between the Central Metasedimentary Belt and the Central Gneiss Belt. It has been proposed that these rocks represent metamorphosed and partially melted Al- and Mg-rich sedimentary rocks. Whole-rock oxygen isotope values (δ18O in the range 6–8‰) from these rocks are not consistent with closed-system metamorphism of either soils or pelitic sediments, and cannot be explained solely by extraction of partial melt from pelitic rocks. The whole-rock δ18O and chemical composition of cordierite–gedrite rocks in the CMBbtz indicate that these rocks represent volcanic rocks hydrothermally altered by seawater. After hydrothermal alteration, these rocks were metamorphosed to the upper amphibolite facies, stabilizing the cordierite + gedrite assemblages. Diffusion modeling of quartz–garnet fractionations suggests intermineral exchange of oxygen in a closed system at moderate fugacity of H2O during slow cooling after regional metamorphism. We believe that cordierite–gedrite lithologies in the CMBbtz mark the site of Mid-Proterozoic rifting at the margin of Laurentia.

GRENVILLIAN METAMORPHISM OF MONOCYCLIC ROCKS, GEORGIAN BAY, ONTARIO, CANADA: IMPLICATIONS FOR CONVERGENCE HISTORY

The Parry Sound and Shawanaga domains of the Central Gneiss Belt along Georgian Bay, Ontario, contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their deformation and metamorphism are entirely attributable to Grenvillian orogenesis. Metamorphic assemblages, fabrics, and P–T–t paths from these rocks therefore provide important constraints on Grenvillian thermal and tectonic history. Rocks in the interior and basal Parry Sound assemblages of the northern Parry Sound domain were metamorphosed to granulite-facies conditions during an early phase of Grenvillian tectonism, i.e. , at ca. 1161 and 1163 Ma, respectively. The most likely setting for high-P – high-T granulite-facies metamorphism in the interior Parry Sound assemblage was at or near the base of crust that was underplated by voluminous mafic magma. In contrast, heat advected from anorthosite could account for intermediate-P granulite-facies metamorphism in the basal Parry Sound assemblage. In the upper part of the basal Parry Sound assemblage, retrogression from the granulite to upper-amphibolite facies likely occurred in response to thrust emplacement of the interior Parry Sound assemblage onto this part of the basal Parry Sound assemblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblage, thrust deformation and re-equilibration at lower-amphibolite-facies conditions took place at ca. 1120 Ma. In the southern Parry Sound domain, a highly attenuated sequence of quartzites and mafic rocks, deposited some time after 1140–1120 Ma, was affected by upper-amphibolite-facies metamorphism before or at 1080 Ma. Northwest of and structurally below the Parry Sound domain, rocks in the Shawanaga domain were affected by eclogite-facies metamorphism at ca. 1090–1085 Ma, suggesting deep burial or partial subduction of the Laurentian margin beneath the Central Metasedimentary Belt at this time. Widespread upper-amphibolite-facies metamorphism after ca. 1080 Ma was associated with a major phase of northwest-directed thrusting and crustal thickening. Sillimanite-grade conditions in the Shawanaga domain were maintained until at least ca. 1020 Ma, the time of major extensional deformation. Data on metamorphic grade and age are consistent with progressive northwest-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and may thus record progressive or multistage convergence at the southeastern margin of Laurentia. Evidence for multiple phases of Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga domains suggests that construction and interpretation of P–T–t paths from these rocks require careful assessment of timing and overprinting relationships. Derived P–T–t paths suggest that exhumation of interior Parry Sound granulites resulted from thrusting soon after peak metamorphism, whereas those for rocks of the Shawanaga domain suggest that exhumation was likely associated with both thrusting and extension.

SW Grenville Province, Canada: the case against post–1.4 Ga accretionary tectonics

Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been identified by different authors in the Ontario–Quebec–Adirondack (OQA) segment of the Mesoproterozoic Grenville orogen in Canada. With one exception, the inferred accretionary terrane boundaries lie within, or at the margins of the Central Metasedimentary Belt (CMB), located between the Central Gneiss Belt and the Adirondack Highlands (Central Granulite Terrane). However, geological, geochronological, and petrological data suggest that the Grenville orogen on both sides of the proposed terrane boundaries (sutures) preserves a common 1.4–1.03 Ga tectonomagmatic history, inconsistent with its origin as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddle the proposed 1.16–1.03 Ga ‘sutures’, from the Central Gneiss Belt, via the Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic continental crust (1.5–1.4 Ga) forming the host and/or basement to younger magmatic and supracrustal suites. (2) A 1.35–1.3 Ga continental arc, remnants of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appalachians in the United States, built on the 1.5–1.4 Ga continental crust. (3) Intrusions of 1.17–1.13 Ga age in the Central Gneiss Belt (mafic suite), and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e. anorthosite massifs and related granitoids). (4) Relics of 1.18–1.14 Ga sedimentary basins in the northwestern CMB and the Mauricie area. We propose that an alternative model can adequately account for the observed geology of this part of the Grenville orogen wherein, the rocks of the OQA segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.35–1.3 Ga, a continental magmatic arc was built upon the southeastern margin of Laurentia represented by the 1.5–1.4 Ga Mesoproterozoic continental crust. The arc split at 1.3 Ga forming an ensialic back arc basin, relics of which now occur in the northwestern part of the CMB, and the back arc basin was flanked to the southeast by an active 1.28–1.25 Ga arc. Collision between the Laurentian margin and another continent (Amazonia?) occurred at 1.2 Ga, resulting in closure of the back arc basin and initiation of thrusting along the CMBBZ. Post-collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere, upper mantle melting, and extension of the overlying crust, resulting in widespread magmatic activity at 1.17–1.13 Ga, including emplacement of the AMCG massifs. Crustal extension generated sedimentary basins now represented by the St Boniface sediments in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penecontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisional, granulite facies shortening commenced at 1.12 Ga, manifested as nappes in the Central Gneiss Belt, and thrusting in the Mauricie area. Continued post-collisional shortening at 1.08–1.05 Ga was more localised, resulting in reactivation of thrusting in the CMBBZ, and initiation of the kinematically compatible Tawachiche shear zone along the eastern border of the Quebec extension of the Adirondack Highlands. The characteristics of the OQA segment of the Grenville orogen can all be accounted for in the context of: (1) a 1.4–1.2 Ga, southeast facing Andean-type margin to a Laurentian upper plate, associated with northwest dipping subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, continued, post-collisional shortening, without invoking accretion of exotic terranes between 1.4 and 1.0 Ga.

Docking of the Central Metasedimentary Belt to Laurentia in geon 12: evidence from the 1.17-1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec

Docking of the Central Metasedimentary Belt to Laurentia in geon 12: evidence from the 1.17-1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec

U-Pb ages of plutonism, sedimentation, and metamorphism of the Paleoproterozoic Kisseynew metasedimentary belt, Trans-Hudson Orogen (Manitoba, Canada)

The Kisseynew Domain is a metasedimentary belt in the central Reindeer Zone of the Trans-Hudson Orogen. It is bounded by 1.92-1.86 Ga volcanic-plutonic belts to the north and south, by an Archean terrane to the east (Superior Province), and by a volcanic-plutonic terrane underlain by an Archean terrane to the southwest (Glennie Domain). The Kisseynew Domain developed in an arc-related setting in the final stages of plate convergence involving the northward migration of arc-ocean floor complexes toward the Archean Hearne Craton. Terminal collision, involving also the Superior Craton, originated multiple fold-thrust systems and high-grade metamorphism. U-Pb ages of 1874-1860 Ma for pretectonic plutonic units in southern Kisseynew Domain are identical to ages of plutonism intruding the arc-ocean floor accretionary complex in the Flin Flon domain (Amisk collage) and indicate its northern extension. Deposition of the Burntwood Group turbidites started at ca. 1860 Ma, indicating uplift and erosion of the volcanic complexes and was coeval with arc magmatism that succeeded the Amisk collage. From 1848 Ma, Burntwood sedimentation was coeval with deposition of Missi Group continental sediments, with continental arc magmatism and early deformation. New and published ages for detrital zircon indicate that sediments were derived both from local 1.89-1.84 Ga units and also from 2.55-2.36 Ga sources. The latter suggest that a Neoarchean-Paleoproterozoic cratonic block was undergoing erosion, remnants of which occur in the Flin Flon Belt. Basin closure started after 1823 Ma and is marked by regional high-grade metamorphism lasting for ca. 30 million years from 1818 Ma to 1785 Ma; late- to posttectonic metamorphic activity lasted until ca. 1775 Ma.

Precise zircon geochronology in the Adirondack Lowlands and implications for revising plate-tectonic models of the Central Metasedimentary Belt and Adirondack Mountains, Grenville Province, Ontario and New York

New high-precision, single-grain dating of leucogranites from the Adirondack Lowlands, dated previously by multigrain zircon methods at ca. 1416 Ma (Wellesley Island) and ca. 1285-1230 Ma (Hyde School Gneiss), has yielded U-Pb zircon ages of ca. 1172 Ma, identical to that of Rockport granite of the Frontenac terrane. In addition, sensitive high resolution ion microprobe (SHRIMP) zircon dating of the intrusive Antwerp-Rossie suite in the Adirondack Lowlands indicates a maximum emplacement age of ca. 1207+26-11 Ma which fixes a minimum age for deposition of regional metasedimentary rocks that it crosscuts. These results remove apparent chronological discrepancies across the St. Lawrence River, thus expanding the significance of the Rockport granite and Hyde School Gneiss and requiring modification of plate-tectonic models for the Central Metasedimentary Belt and Adirondack Mountains in the interval ca. 1350-1125 Ma.

Archean continental assembly in the southeastern Superior Province of Canada

Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium‐ to high‐grade Opatica plutonic gneiss belt, the low‐grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65‐km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, whieh are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large‐scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5–8.2 kms−1, are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism.

Structure and tectonic evolution of the transitional region between the central Appalachian foreland and interior cratonic basins

The transitional region between the central Appalachian foreland basin and Michigan and Illinois interior cratonic basins is characterized by first and second order structures related to plate convergent processes at the Laurentian plate margin. The first order structures are the Waverly and Cincinnati arches believed to be forebulges associated with the Taconic and Alleghanian orogenies, respectively. The second order structures are basement faults that exhibit a protracted history of recurrent displacements. First and second order structures are linked in that the basement faults have had a major effect in controlling the location and uplift history of the arches. The Waverly and Cincinnati arches share many similarities that suggest a common process of arch development. The Waverly arch developed over the low density rocks in the Central Metasedimentary belt of the Grenville province near its structural contact with the Central Gneiss belt. Uplift on the arch commenced during passive margin development in the Cambrian, but reached its maximum development at the end of the Middle Ordovician Taconic orogeny. Reactivation of faults along the arch continued as late as the Pennsylvanian. The Cincinnati arch as expressed in the Precambrian basement surface is coincident with the underlying Grenville front on a regional scale. The front is a major crustal suture which served to localize and control the history of the Cincinnati arch. Local deviations between the location of the arch and front are explained by the presence of Proterozoic rift basins filled with low density sedimentary rocks, by interactions between the developing Appalachian, Michigan, and Illinois basins, and by a northward decrease in Alleghanian tectonic loading. In a pattern similar to the Waverly arch, the Cincinnati arch began its history with a period of regional uplift as early as the Middle to Late Ordovician followed by maximum arch development in the Pennsylvanian in response to the Alleghanian orogeny. Neither arch showed significant migration following initial development as predicted by general models of forebulge development. We attribute this to the effect of preexisting basement anisotropies, such as lithotectonic boundaries, which served to control the initial location of the forebulge and affect its subsequent history. A third major arch, the Kankakee arch, occurs in the western part of the transitional region and is oriented at right angles to the other arches. Unlike the Waverly and Cincinnati arches, this arch owes its development to subsidence in the adjacent Michigan and Illinois basins rather than orogenic activity in the Appalachians.

The thermal history of the Eastern Ghats Belt (India) as revealed by U–Pb and [formula omitted] dating of metamorphic and magmatic minerals: implications for the SWEAT correlation

The Eastern Ghats belt is a large high-grade terrane exposed along the east coast of India. The late Proterozoic orogen consists of N–S trending charnockite and metasedimentary belts. The younger E–W trending Godavari Rift divides the orogen into a northern and a southern segment. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt represents an important part of the reconstructed global Southwest-United-States–East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tectonism in this large orogenic belt plays a key role in this global reconstruction. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. In order to reconstruct parts of the regional thermal history of the Eastern Ghats belt, U–Pb ages were determined on monazite, allanite, zircon and sphene and 40Ar/ 39Ar ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of the Godavari Rift the last high-grade metamorphism occurred at ca. 960 Ma. In the most western unit, the Western charnockite zone, allanite and monazite from late pegmatites indicate a major thermal event around 1.6 Ga. This is the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the Eastern Ghats belt. The majority of the sphenes from the Eastern Ghats belt are discordant and lie along a reference line extending from ca. 935 Ma to 504 Ma. This discordance is interpreted to be due to a thermal disturbance during a Pan-African deformation phase, which is dated by the emplacement of apatite–magnetite veins that contain zircon with a concordant U–Pb age of 516±1 Ma. All hornblende 40Ar/ 39Ar ages from the central units of the belt provide Pan-African plateau ages. The partially reset sphene and completely reset hornblende ages indicate that the Pan-African metamorphism reached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogression of the high-grade mineral assemblages, but has so far only been recognized in the reset mineral ages and some late magmatic rocks. A hornblende 40Ar/ 39Ar age of ca. 1110 Ma from an amphibolite south of the Godavari rift indicates that the Pan-African thermal overprint was weaker in the Western charnockite zone, if it affected this area at all. The new ages presented for the Eastern Ghats belt are similar to published ages for the Rayner Complex and the Prydz Bay region of Eastern Antarctica. This similarity in ages is strong support for these areas in East Antarctica and the Eastern Ghats belt being complementary parts of an extensive orogenic belt formed during the Grenvillian Orogeny around 1 Ga. Both continental fragments also show evidence for a Pan-African overprint, however, this event is much more pronounced in Antarctica than in the Eastern Ghats belt.

U-Pb ages of plutonism, sedimentation, and metamorphism of the Paleoproterozoic Kisseynew metasedimentary belt, Trans-Hudson Orogen (Manitoba, Canada)

U-Pb ages of plutonism, sedimentation, and metamorphism of the Paleoproterozoic Kisseynew metasedimentary belt, Trans-Hudson Orogen (Manitoba, Canada)

Precise zircon geochronology in the Adirondack Lowlands and implications for revising plate-tectonic models of the Central Metasedimentary Belt and Adirondack Mountains, Grenville Province, Ontario and New York

Precise zircon geochronology in the Adirondack Lowlands and implications for revising plate-tectonic models of the Central Metasedimentary Belt and Adirondack Mountains, Grenville Province, Ontario and New York

Oxygen isotope ratios of Archean plutonic zircons from granite–greenstone belts of the Superior Province: indicator of magmatic source

Zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary δ 18O value by subsolidus exchange. Intrusive bodies from the Superior Province, Canada have an average δ 18O(Zircon) of 5.8±0.6‰ ( n=59 rocks). Those from granite–greenstone belts have an average δ 18O(Zrc)=5.8±0.6‰ ( n=55), in contrast to those from metasedimentary belts that average δ 18O(Zrc)=6.7±0.2‰ ( n=4). The δ 18O zircon values from granite–greenstone belts indicate a maximum allowable input of 14% supracrustal, high δ 18O material into Archean magmas of the Superior Province relative to mantle δ 18O values. Igneous bodies of the Superior Province have been separated into two groups based on geochemistry: unenriched plutons and sanukitoids. Magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of <1‰ between the two geochemical sets of rocks. This distinction is apparent only in the δ 18O(Zrc) and not in more commonly analyzed, less refractory minerals such as quartz and feldspar. Oxygen isotope data correlate with geochemical modeling of sanukitoids. The average δ 18O(Zrc) for sanukitoids is 6.4±0.2‰ in comparison to 5.5±0.4‰ for unenriched plutons. There is almost no overlap in values. The elevation of sanukitoid δ 18O(Zrc) indicates input from a high δ 18O source such as fluids from the subducting oceanic crust, melts of subducted sediments or continental crust. All of the plutons analyzed in this study have been dated by U–Pb zircon techniques, and the combination of U–Pb ages and δ 18O(Zrc) indicates little variability of δ 18O versus age during Archean plutonism in the Superior Province. Therefore, recycling of sediments and other high δ 18O sources was not a dominant process during crustal growth from 3003 to 2670 Ma in the Superior Province.

Bedrock Structure Control on Soil-Gas Radon-222 Anomalies in the Toronto Area, Ontario, Canada

Research Article| January 01, 1998 Bedrock Structure Control on Soil-Gas Radon-222 Anomalies in the Toronto Area, Ontario, Canada IMSHUN JE; IMSHUN JE Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario, M1C 1A4 Canada present address: Handex of Illinois Inc., 1701 West Quincy Avenue, Naperville, IL 60540. Search for other works by this author on: GSW Google Scholar NICHOLAS EYLES NICHOLAS EYLES Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario, M1C 1A4 Canada Search for other works by this author on: GSW Google Scholar Author and Article Information IMSHUN JE present address: Handex of Illinois Inc., 1701 West Quincy Avenue, Naperville, IL 60540. Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario, M1C 1A4 Canada NICHOLAS EYLES Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario, M1C 1A4 Canada Publisher: Association of Environmental & Engineering Geologists First Online: 09 Mar 2017 Online Issn: 1558-9161 Print Issn: 1078-7275 © Association of Engineering Geologists 1998 Environmental & Engineering Geoscience (1998) IV (4): 445–454. https://doi.org/10.2113/gseegeosci.IV.4.445 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation IMSHUN JE, NICHOLAS EYLES; Bedrock Structure Control on Soil-Gas Radon-222 Anomalies in the Toronto Area, Ontario, Canada. Environmental & Engineering Geoscience 1998;; IV (4): 445–454. doi: https://doi.org/10.2113/gseegeosci.IV.4.445 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Soil-gas radon is a newly recognized environmental hazard that has received much interest in the United States, but few studies have been conducted in Canada, particularly in urban areas. This paper describes the results of a 2-yr study of near-surface in situ radon soil-gas levels in the eastern end of the Greater Toronto Area (GTA: population 5.5 million) of southern Ontario, Canada. Transects up to 550 m in length, cross surface lineaments (stream valleys) associated with a prominent but deeply buried mid-Proterozoic shear zone (Central Metasedimentary Belt Boundary Zone: CMBBZ) that underlies the eastern GTA. Early Paleozoic and Pleistocene glacial strata that overlie the CMBBZ are extensively fractured.Regional background levels of radon identified in this study (100–200 pCi/L) are consistent with results of previous work in southern Ontario but repeat measurements also identified anomalously high radon levels (up to 1000 pCi/L). Anomalies occurred at or closely adjacent to the same locations along each transect over the duration of the study. Strata are extensively fractured and it is likely that fractures provide pathways for radon transport to the surface by carrier gases such as CO2, H2 and CH4 released from black shales. This transport mechanism has been recognized elsewhere in southwestern Ontario in regard to radon migration along faults and used to define hydrocarbon reservoirs.In light of these findings, hazardous or near-hazardous levels of radon may occur elsewhere above fractured strata that underlie the GTA. Mapping of fractures and related structures would appear to provide a useful means of delineating potentially hazardous radon levels across this heavily urbanized area. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A structural transect across the coastal mobile belt in the Brazilian Highlands (latitude 20°S): the roots of a Precambrian transpressional orogen

We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.

Rheological controls on Grenvillian intrusive suites: implications for tectonic analysis

Studies of plutonic suites in western Grenville, along with the recent advances in understanding magma ascent and emplacement, and interactions between magma and their country rock, provide insights on the use of intrusive rocks to infer the accretionary history of deeply-exposed Precambrian orogens. In the Grenvillian Central Metasedimentary Belt of Québec, terrane juxtaposition was inferred to be late based on the distribution of 1.08 Ga potassic alkaline plutons. This view was questioned, however, upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units, for example Grenvillian marble. In this context, preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons. The spatial association of plutons with marble and their contact relationships are, at the local scale, obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall-rock sequence. In contrast, plutons and dykes of an older, 1.17 Ga, less alkaline magma association are shown by field and remote-sensing studies to be evenly distributed across the various lithotectonic domains of the belt. Their sheet-like emplacement along the belt boundary constrains Grenvillian tectonic assembly in the region to be early. Contrasting loci, type and degree of deformation of intrusive bodies illustrate that host-rock rheological differences influence the final characteristics of an intrusive suite. Consequently, the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high-grade terranes.

The interpretation of complex zircon U–Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia

SHRIMP U–Pb ages of zircons from Archaean TTG granitoids and gneisses from the Jack Hills region of Western Australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon U–Pb systems under metamorphic and magmatic conditions. Strong light rare earth element (LREE) enrichment and similarities in the major and trace element chemistry suggest that TTG gneisses formed by fractional melting of a garnet-bearing amphibolitic source. Maximum ages, of 3.75 to 3.6 Ga, obtained from low U zircon cores are interpreted as the period of formation of the TTG parent rocks. However, most zircons in these populations are complex, consisting of zoned and unzoned parts, and have disturbed U–Pb systems which form a concordant to discordant array between 3.7 and 3.3 Ga on a concordia plot, suggesting these grains experienced one or more disturbance events within this time span. The range of complexity of U–Pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of Pb and possibly U has occurred within and from zircons, causing variable isotopic discordance and yielding a range of apparent ages within a single grain. Disturbance of the zircon isotopic systems appears to be due to a combination of early Archaean events together with recent Pb loss. The presence of 3.3 Ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite, together with the exclusive presence of 3.3 Ga zircons in one sample of porphyritic granite, is interpreted as evidence of a major magmatic and possibly metamorphic event at this time. The presence of ca 3.5 Ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization. Evidence for 3.3 Ga magmatism appears to be restricted to zircons from TTG rocks south of the Jack Hills Metasedimentary Belt, raising the possibility that the present position of the belt defines a major suture within the Narryer Gneiss Terrain. The post-tectonic monzogranites have LREE enriched patterns with strong negative Eu anomalies, similar to the older gneisses, but in contrast to these rocks, zircons from the monzogranites have relatively simple U–Pb systems, which define an age of 2654±7 Ma, and only rarely do zircons have inherited cores with nearly concordant ages of ca 3.6 Ga. The very low content of inherited zircon raises doubts about the formation of the granites by reworking of older TTG gneisses and an alternative explanation that the monzogranites formed from new 2.65 Ga crust, with minor 3.6 Ga crustal contamination, is seen as more likely.

The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India

The technique of single zircon dating from the thermal evaporation of 207Pb/206Pb (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages. In contrast Nd model ages may provide a measure of the period of crustal residency for the sample or its protolith. These two techniques have been combined to elucidate the tectonic history of the Proterozoic mobile belt of southern India, exposed south of the Palghat-Cauvery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt comprise the Madurai and Trivandrum Blocks, both of which are characterised by massive charnockite uplands and low-lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is restricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436 ± 4 Ma) are similar to model Nd ages from a range of lithologies suggesting crustal growth at that time. The Trivandrum Block, to the south of the Achankovil shear zone, is comprised of the Kerala Khondalite Belt, the Nagercoil charnockites and the Achankovil metasediments. Single zircon evaporation ages, together with conventional zircon and garnet chronometry, suggest that all three units underwent upper-amphibolite facies metamorphism at ∼1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This implies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil metasediments are much younger (1500–1200 Ma) than those from the adjacent Kerala Khondalite Belt and Madurai Blocks (3000–2100 Ma), but there is no evidence for zircon growth in these metasediments during the Mesoproterozoic. Hence the comparatively young model Nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth. Zircon overgrowths from the Madurai Block (547 ± 17 Ma) and Achankovil metasediments (530 ± 21 Ma) suggest that all tectonic units of the Proterozoic mobile belt of South India shared the same metamorphic history from the early Palaeozoic. This event has been recognised in the basement lithologies of Sri Lanka and East Antarctica, confirming that the constituent terrains of East Gondwana had assembled by this time.