Metasedimentary Belt Research Articles

Varicolored Marble in the Central Metasedimentary Belt, Grenville Supergroup. I. Circumstantial Evidence of Melting

Abstract Large expanses of granulite-facies white marble occur in the Central Metasedimentary Belt of the Grenville orogen in western Quebec and in contiguous areas of eastern Ontario and New York. Locally, the white rock is transformed into “varicolored” marble, ranging from pink, orange, yellow, green, gray to blue marble, in some cases closely juxtaposed. We have dissected specimens from three target areas in western Quebec. A powerful analytical workflow including high-resolution ZEISS Atlas 5 large-area imaging was employed in combination with detailed energy-dispersive spectroscopy analyses to create digital map-like online datasets of entire samples to display the textures and characteristics in their entirety. We describe in detail the texture and mineralogy of marble specimens collected from exposures created during the construction of Autoroute 5 near Wakefield and of Autoroute 50 near Grenville-sur-la-Rouge and two specimens collected near Bryson, Quebec. At the inferred pressure and temperature and in the presence of H2O, the white marble seems to have melted. A carbonate melt, a powerful flux, can dissolve silicate wallrocks along a mutual contact. In this way, a marble-derived melt has assimilated wallrocks ranging from peridotite to granite. This melting event may have occurred more than once in the 350-million-year lifespan of the Grenville event; the thermal event associated with the Rigolet stage, during gravitational collapse of the orogen approximately one billion years ago, was the most intense. The marble samples contain no vestiges of older deformed assemblages. Whereas the varicolored marble undoubtedly is metasedimentary, a coeval influx of mantle-derived carbonatitic melt and related fluids from a subjacent complex may have caused localized metasomatism of the marble prior to and after melting.

Isotope evidence for Archean accordion-tectonics in the Superior Province

Archean cratons preserve the oldest continental crust, but their tectonic mode of formation and assembly remain key unknowns. The Superior Province is Earth’s largest Archean craton and comprises Eo-Neoarchean plutonic-gneiss terranes separated by Meso-Neoarchean granite-greenstone terranes and metasedimentary belts. Resolving whether the plutonic-gneiss terranes represent dismembered fragments of a once contiguous proto-craton or a series of unrelated accreted crustal fragments is critical to understanding the early evolution of the Superior Province and Archean tectonics. We present zircon U-Pb-Hf isotopic data from the Tannis and Cedar Lake TTG gneisses, which record the early crustal evolution of the Winnipeg River plutonic-gneiss terrane and are some of the oldest rocks in the western Superior Province. The gneisses yield a large range of zircon εHf(t) signatures (−6 to +4) at igneous formation (ca. 3.3–3.25 Ga) indicating coeval crustal growth and reworking of isotopically depleted and evolved sources. Crustal reworking of Eo-Paleoarchean sources is supported by sub-chondritic ca. 3.5–3.4 Ga zircon xenocrysts in the Cedar Lake gneiss. The early crustal evolution of the central Winnipeg River terrane is similar to the Hudson Bay and Minnesota River Valley terranes, on the northern and southern margins of the Superior Province, respectively. Correlation of the early history amongst the three plutonic-gneiss terranes supports a tectonic model in which a once coherent Eo-Paleoarchean proto-craton disaggregated into three fragments during formation of intervening granite-greenstone terranes in the Mesoarchean. These fragments reaggregated in the Neoarchean. The complex history of the Superior Province highlights that Archean cratons are not simple entities that formed in a single event, but may have experienced times of cratonic breakup and reassembly.

Recrystallisation and Trace-Element Mobility in Zircons: Implications for U-Pb Dating

Complex microstructures of zircon recrystallisation have been discovered in igneous and metamorphic zircons from the Jack Hills Metasedimentary Belt in Western Australia, and the granitic injection complex of Harris in north-west Scotland. Cathodoluminescence (CL), electron backscatter diffraction (EBSD) and wavelength dispersive spectroscopy (WDS) analysis reveal that recrystallisation in the studied zircons is often characterised by (1) translation, bending and fading of the oscillatory zoning; (2) structural recovery of the zircon; (3) an enrichment in Hf and depletion in Y in recrystallised zircon; and (4) the formation of a recrystallisation interface with a minor (<2°) misorientation. A new, composite recrystallisation model in which trace element and dislocation migration occurs shortly after crystallisation during magmatic cooling and pipe diffusion along dislocation arrays formed by the accumulation of dislocations allows enhanced diffusion to enrich Hf and leach U, Pb and Y. After the recrystallisation event, subsequent metamictization of primary zircon (typically with oscillatory zoning) creates zones that can structurally recover through a diffusion-reaction re-equilibration mechanism, resulting in further trace-element mobility. These mechanisms can create complex microstructures in zircons, suggesting that a detailed understanding of the crystallisation and recrystallisation history of a zircon may be required to accurately interpret its U-Pb ages.

Stable isotope (C, O, S, and Zn) geochemistry of marble-hosted exhalative zinc deposits in the Central Metasedimentary Belt, Grenville Province, Canada: Insights into ore deposition and tectonic setting

Mesoproterozoic marbles of the southwestern Grenville Province contain numerous stratiform zinc deposits, which include the world-class Balmat-Edwards Zn-Pb district in the Adirondack Lowlands and the Franklin-Sterling Zn-Fe-Mn district in the New Jersey Highlands. Ten small deposits in the Central Metasedimentary Belt (CMB) of Ontario and Quebec were selected for stable isotope study (C and O in carbonates and S and Zn in sulfides) to help constrain depositional conditions, ore genesis, and tectonic setting. These deposits and the Balmat-Edwards and Franklin-Sterling districts are interpreted to have formed in contemporaneous back-arc basins associated with rifting of the Andean-style margin of Laurentia. Stable isotope systematics and the geology of these deposits indicate two distinct sedimentary exhalative (Sedex) settings: marble-hosted deposits associated with volcanic rocks, and marble-hosted deposits that are not associated with volcanic rocks and sometimes occur with evaporites. Volcanic-associated Sedex deposits (the Long Lake, Slave Lake, Wilkinson, Lafontaine, Bouchette, and Leitch deposits, and the Franklin-Sterling district) have COS isotope systematics that are consistent with water/rock interaction between marine carbonate protoliths and fluids having an igneous component at ca. 150 ± 25 °C. In contrast, Sedex deposits not associated with volcanic rocks (the Cadieux, Spry, Thirty Island Lake, and Salerno Lake deposits, and the Balmat-Edwards district) have C and O isotope ratios that are consistent with formation at ambient or low (<80 °C) temperatures. Sulfur isotopes of sulfides in some evaporite-associated deposits are similar to those of other Proterozoic Sedex deposits (δ34S = 5–15 ‰ VCDT), with the Cadieux and Spry having lower values (δ34S ≈ 0‰) which might reflect differences in depositional settings. Zinc isotope ratios of CMB Zn deposits are relatively constrained (δ66Zn = 0.17 ± 0.04 ), and are similar to other seafloor deposits. An exception to this is the Thirty Island Lake deposit in the Frontenac terrane, which has δ66Zn = 0.46 ± 0.05 ‰, which may indicate an exotic origin relative to other CMB deposits. The recognition of the volcanic association for some Grenville Sedex deposits and not for others (and the relationship with evaporites for this group) has implications for Zn exploration programs in this region.

How old are the Jack Hills metasediments really?: The case for contamination of bedrock by zircon grains in transported regolith

Abstract The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains in some samples of metasedimentary rocks from the Jack Hills, Western Australia, that otherwise appear to have been deposited before 3.0 Ga is explained by contamination with embedded zircon grains sourced from transported regolith. Zircon U-Pb age spectra obtained by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of modern alluvium in creek beds draining the metasedimentary belts of Mount Narryer and Jack Hills are dominated by these exotically sourced younger components, as is the red soil that partly covers and infiltrates fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The consistency of the age patterns found in superficial deposits sampled over some 100 km implies a dominance of colluvium transported from the wider Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather than material produced from the local (Mesoarchean and older) bedrocks. The potential for similar cases of “environmental contamination” of surface rocks by exogenous zircons needs to be considered more widely, particularly in situations where interpreted maximum deposition ages are based on only a small number of outlying youngest analyzed grains.

Mapping the basement architecture using magnetotelluric data across a coastal part of the Borborema structural province, Ceará – Brazil

The Borborema Province (BP) constitutes part of the west Gondwana supercontinent formed by several tectonic collisions during Neoproterozoic, widely known as Brasiliano - Pan-African orogeny. It consists of a complex tectonic mosaic composed mainly by Archean to Paleoproterozoic migmatite-gneiss basement complexes, Proterozoic metasedimentary belts, a large volume of Neoproterozoic granitic intrusions, and Phanerozoic covers that represent ca. 25 % of the total area of BP. This work presents the results from 21 Magnetotelluric (MT), and electromagnetic time-domain (TDEM) soundings interpretations carried out along a 120 km profile in a coastal area of the northeast Borborema Province covered by Phanerozoic sedimentary rocks and sediments. The final geoelectric model allows delineating the architectural geometry of the subsurface and variations in physical properties of different crustal blocks bounded by major electrical discontinuities. The interpretation was based on a 2D and 3D model from the inversion of the MT data. The 2D model has a visually satisfactory agreement with the region's geological framework and a reasonable resolution of the upper crust compared to the 3D model. The geoelectric section allows delineating the variation in physical properties of different upper crust blocks bounded by major electrical discontinuities. The result enables the characterization of 1) the uppermost structures correlated to the Mesozoic to mid-Cenozoic sedimentary formations; 2) covered lithotectonic framework represented by lithotypes and main shear zones related to Ceará Central and Rio Grande do Norte crustal tectonic domains; 3) distinctive anomalies corresponding to the Senador Pompeu, Orós, and Jaguaribe Neoproterozoic main shear zones. Our interpretations agree with the results and interpretation presented by previous MT works in the Borborema Province, which reveals the existence of an important conductive zone, interpreted as a suture zone, spatially related to the Senador Pompeu shear zone and in smaller proportion to the Orós shear zone. The results presented here are a starting point to further MT long-period investigation to understand this complex part of the Borborema Province.

Neodymium isotope mapping a polygenetic TTG batholith: failed back-arc rifting in the Central Metasedimentary Belt, southwestern Grenville Province

Fifty-five new Nd isotope analyses are presented for plutonic orthogneisses from the Grimsthorpe domain in the marble-rich segment of the Grenvillian Central Metasedimentary Belt (CMB) to test the back-arc aulacogen model for its origin. Nd isotope analyses from the Weslemkoon batholith, Elzevir batholith, Lingham Lake complex, and Canniff tonalite are used to probe the crustal formation age of their source rocks. Despite its concentric foliation, the Weslemkoon batholith displays a complex geochemical pattern consisting of several northeast-trending domains with older TDM ages, surrounded by juvenile crustal material. The new Nd isotope results, coupled with geochemistry for the Weslemkoon and Elzevir batholiths, depict the fragmentation of a block of old crust that formed a screen between en echelon segments of a mid-Mesoproterozoic back-arc rift zone. The isotope boundaries identified within the Weslemkoon batholith delineate magma pulses sampling two distinct sources, interpreted as Laurentian basement and juvenile basaltic underplate. Underplating could be attributed to slab rollback under the pre-Grenvillian continental margin arc. The intensification of rift-related magmatism in the CMB is demonstrated by its bimodal petrological character. A modern analogue for the tectonic context of the CMB is the Gulf of California, where subduction-related magmatism has transitioned to rift-related magmatism. However, the Gulf of California exhibits more transcurrent motion than is evidenced by the geometry of the CMB rift. A geometrical analogue for the break-up of the Elzevir block between two rift segments is provided by the Danakil block of the Red Sea, which is currently undergoing similar tectonic fragmentation.

Nd isotope mapping of the Frontenac Terrane in southwestern Grenville Province

Nd isotope analyses are presented for granitoid rocks from the western part of Frontenac Terrane in the Grenville Province of Ontario. The depleted mantle model (TDM) ages show no correlation with the silica content of the rocks, but instead correlate with geographical location, suggesting that the TDM ages are indicative of regional crustal formation age and do not result from mixing between sources with different provenance ages. Based on these observations, we identify a new crustal age boundary that follows the Desert Lake – Canoe Lake fault and the Rideau Lake fault and, hence, a new juvenile crustal block (Westport domain). This domain is identified as part of the ensimatic back-arc rift zone that formed the juvenile segment of the Central Metasedimentary Belt in Ontario. However, additional sampling along the Ottawa River suggests that the juvenile Westport domain does not extend into Quebec. Instead, a narrower ensialic rift zone is represented by the Marble domain in Quebec. Based on comparison with the Taupo volcanic zone and the northern Red Sea as modern analogues, we suggest that the transition from a wide ensimatic rift zone in Ontario to a narrow ensialic rift in Quebec was accommodated by transtensional motion along a zone of diffuse shear east of Ottawa.

Nd isotope mapping of a tectonic window near Chalk River, Ontario: Implications for large-scale Grenvillian tectonics

Over forty new Nd isotope determinations are presented for granitoid orthogneisses from the Chalk River area, on the border between Ontario and Quebec in the SW Grenville Province. TDM model ages provide evidence for a tectonic window through the allochthonous belt into the Grenvillian parautochthon in the Chalk River area. This tectonic window constrains the location of the main ramp of the Allochthon Boundary Thrust southeast of the Chalk River area. However, the structure of the tectonic window has been complicated by down-faulting on the Ontario side of the Ottawa River, due to reactivation of late Precambrian normal faulting associated with the Ottawa–Bonnechere graben. Thus, on the Ontario side of the border, the surface expression of the tectonic window is represented by a strongly sheared tectonic duplex that underlies the main allochthon. However, the lower section of a 1 km-deep borehole drilled by Canadian Nuclear Laboratories has TDM model ages indicating penetration into the underlying parautochthon. The new data, combined with previous Nd isotope mapping and Lithoprobe seismic reflection results, point to a relatively constant thickness for the exhumed Muskoka allochthon. By showing that this exhumation occurred immediately below the Central Metasedimentary Belt boundary, this evidence strongly supports a ramp-flat thrusting model for the SW Grenville province. This model greatly reduces the likelihood of major crustal exhumation to the NW of Parry Sound domain, thus removing the theoretical basis for pervasive southeast directed orogenic collapse across the Ontario gneiss belt.

Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

AbstractTerrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle.

The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 °C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P–T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction.

Geodynamic significance of Neoarchean metasedimentary belts in the Superior Province: Detrital zircon U-Pb LA-ICP-MS geochronology of the Opinaca and La Grande subprovinces

Metasedimentary subprovinces within the Superior Province and many Archean cratons worldwide occur as prominent belts separating volcano-plutonic subprovinces. The stages of their development mark major, distinct geodynamic events. Deposition ubiquitously follows a restricted period of voluminous, regional, felsic pluton emplacement. Their geodynamic setting of deposition is not well understood, and despite similar sedimentological, structural and post-deposition metamorphic characteristics, metasedimentary subprovinces have been variously interpreted as developing in accretionary prism, syn-orogenic, and back-arc basin settings. We present the first synthesis of detrital zircon U-Pb LA-ICP-MS geochronology from paragneiss and migmatite of the Opinaca and La Grande subprovinces. This study constrains detrital zircon age populations of the sedimentary protoliths, assesses their provenance, and evaluates the geodynamic setting of deposition. Including detrital geochronology compiled from regional literature, nine paragneiss samples from the Laguiche Complex (Opinaca) and eleven from the Rivière Salomon, Thor, Keyano, Magin, Marbot, Low and Ekomiak formations (La Grande) provide dominant modal age peaks similar to regional Neoarchean felsic plutonic suites, and lesser age peaks consistent with regional Neoarchean and Mesoarchean greenstone belts and Mesoarchean gneiss. The units have comparable zircon age populations; therefore we interpret a shared provenance from the La Grande and Minto subprovinces, suggesting that none of the metasedimentary units represent exotic accreted terranes. Mean ages of youngest LA-ICP-MS zircon age clusters range from 2716 to 2695 Ma, thus defining the maximum ages of deposition. ID-TIMS analyses of select youngest grains confirm the efficacy of the LA-ICP-MS constraints. Regional intrusive suites ranging from ca. 2710 to 2678 Ma further constrain deposition to a short time interval. Our detrital geochronology results are similar to all other Superior Province metasedimentary subprovinces, which consistently exhibit large proportions of detrital zircon ages close to their depositional age. This would conventionally lead to the interpretation that the extensional environment is a back-arc basin to an active convergent margin. However, as the sequence of events preceding formation of the Opinaca subprovince may not reflect uniformitarian tectonic processes, we discuss alternate models of basin formation and how the similar patterns observed in Superior Province detrital geochronology could be consistent with a variety of geodynamic environments.

A Silicocarbonatitic Melt and Spinel-Bearing Dunite of Crustal Origin at the Parker Phlogopite Mine, Notre-Dame-du-Laus, Quebec, Canada

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier–Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found “frozen” in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.

Silicocarbonatitic melt inclusions in fluorapatite from the Yates prospect, Otter Lake, Québec: Evidence of marble anatexis in the central metasedimentary belt of the Grenville Province

Abstract We have investigated a locality very well known to mineral collectors, the Yates U-Th prospect near Otter Lake, Québec. There, dikes of orange to pink calcite enclose euhedral prisms of fluorapatite, locally aligned. Early investigators pointed out the importance of micro-inclusions in the prisms. We describe and image the micro-inclusions in two polished sections of fluorapatite prisms, one of them with a millimetric globule of orange calcite similar to that in the matrix. We interpret the globule to have been an inclusion of melt trapped during growth. Micro-globules disseminated in the fluorapatite are interpreted to have crystallized in situ from aliquots of the boundary-layer melt enriched in constituents rejected by the fluorapatite; the micro-globules contain a complex jigsawed assemblage of carbonate, silicate, and sulfate minerals. Early minerals to crystallize are commonly partly dissolved and partly replaced by lower-temperature phases. Such jigsawed assemblages seem to be absent in the carbonate matrix sampled away from the fluorapatite prisms. The pressure and temperature attained at the Rigolet stage of the Grenville collisional orogeny were conducive to the anatexis of marble in the presence of H2O. The carbonate melt is considered to have become silicocarbonatitic by assimilation of the enclosing gneisses, which were also close to their melting point. Degassing was important, and the melt froze quickly. The evidence points to a magmatic origin for the carbonate dikes and the associated clinopyroxenite, rather than a skarn-related association.

Three-dimensional visualization of top-down superimposed thrust sheets in the SW Grenville Province, Ontario

AbstractTo properly understand the tectonic history of the Grenville Province it is necessary to have a reliable, scientifically based understanding of the present-day three-dimensional (3D) structure of the orogen. Based on detailed Nd isotope mapping of surface boundaries and Lithoprobe seismic sections, this study provides the first detailed visualization of the 3D structure of the Grenville gneiss belt in Ontario using the SketchUp software package. The 3D visualization supports a model in which thrust geometry was imposed from the top downwards, controlled by the NW boundary of the Central Metasedimentary Belt that originated as a failed back-arc rift zone. The Central Metasedimentary Belt boundary controlled the trajectory of the Allochthon Boundary Thrust, its underlying tectonic duplex and, ultimately, the Grenville Front. This process of superimposed thrusting explains the large-scale change in the trajectory of the Grenville Front north of Georgian Bay that has been called the ‘Big Bend’. To assist in visualizing the 3D model, a fly-through animation is provided in the supplementary material.

Geothermometry of the western half of the Central Metasedimentary Belt, Grenville Province, Ontario, and its implications

Geothermometry of the western half of the Central Metasedimentary Belt, Grenville Province, Ontario, and its implications

Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

Deep crustal framework of the Borborema Province, NE Brazil, derived from gravity and magnetic data

The Borborema Province BP (NE Brazil) consists of a complex rock mosaic composed mainly by Proterozoic metasedimentary belts that amalgamate Archean-to-Paleoproterozoic gneiss-migmatite complexes. It is a key region to understand the West Gondwana amalgamation, during the Pan-African/Brasiliano orogeny because it is an important portion of the orogen located between the São Francisco-Congo and West African cratons. We present the deep-rooted crustal framework of the BP interpreting the lateral contrasts of density and magnetization using the first available regional coverage of gravity and aeromagnetic data. The external boundaries of the BP having deep-rooted crustal significance are the Transbrasiliano shear zone, to the west, and the limit with the São Francisco craton, to the south. The latter limit is marked by a gravity anomaly which is the typical gravity signature of collisional sutures. Inside the BP crustal block, the Pernambuco, Patos, and Jaguaribe shear zones are identified as the major structures, based on termination and/or truncation criteria of geophysical anomalies. The three shear zones correspond to geophysical lineaments which conform with the Transbrasiliano shear zone, forming a single splay that opens to the northeast/east direction. The external boundaries and major shear zones divide the BP crustal block into four internal major domains of contrasting geophysical characteristics, namely the Southern, Transversal Zone, Rio Grande do Norte, and Ceará geophysical domains in analogy with the division based on surface geology. However, compared with the delimitations based on surface geology, there are three differences: the Médio Coreau belt is not positioned over the BP crustal block, a portion of the Parnaíba basin baseament should be included in the BP crustal block, and the basement beneath the folded-and-thrusted metasedimentary rocks of the Casa Nova Group (which form the nappes of the Riacho do Pontal belt) is part of the São Francisco craton crustal block. A subdivision of each major domain is also presented. Of special relevance is a pattern of alternate-polarity, quasi-parallel aligned geophysical anomalies present in the Transversal Zone domain, suggesting a clear alternation of high-density/high-susceptibility with low-density/low-susceptibility deep-rooted crustal subdomains. This geophysical pattern is compatible with a tectonic model of terrane collage.

DEEP CRUSTAL STRUCTURE AND MODEL OF NEORCHAEAN EVOLUTION OF THE NORTH AMERICAN CRATON

The structure of the Archaean crust of the North America has been studied based on the synthesis of geolo‐ gical and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the frag‐ ments of reworked Meso‐ and Paleoarchaean rocks, forms an asymmetric round‐oval‐shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of sou‐ thern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short‐term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island‐arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the oval‐ concentric zoning, the important role of high‐temperature magmatic and metamorphic processes and mainly in‐ tracontinental magmatism and sedimentation, indicates the leading role of the mantle‐plume type processes. The Neoarchaean evolution of the North American craton represents the plate‐tectonic processes initiated by a super‐ plume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval‐shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high‐temperature magmatism (usually with the participation of enderbite‐charnockites and gabbro‐anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high‐grade (granulite and high‐ temperature amphibolites facies) and low‐ or moderate‐grade (greenschist and epidote‐amphibolite facies) meta‐ morphic rocks; (6) the lower‐crust granulite‐basaltic layer that had formed and was deformed at the final stage of endogenic activity; (7) a thick lithosphere (the lithospheric keel reaches a depth of 250–350 km).

Natural and experimental fluorine substitution in biotite: Implications for fluid-rock thermochronometry and application to the Seridó Belt, northeastern Brazil

Abstract We modeled non-steady-state, fluid-assisted diffusion in a sheet plane, infinite cylinder and finite thin cylinder using analytical solutions of the diffusion equation and the experimental diffusivity of F-in-biotite. Diffusion experiments on natural biotite in the presence of hydrofluoric acid at 0.4 GPa and 650, 700, and 750 °C produced diffusive influx perpendicular to the c-axis. EPMA mapping and profiling allow us to define the following Arrhenius expression of F diffusion in biotite: D = 7.04 m 2 s × 10 − 4 × exp − 221 kJ mol RT − 1 The results yield the discrimination of a high-T homogenization domain where cooling paths continuously reequilibrate F-in-biotite, an intermediate-T zonation domain where cooling paths record F-in-biotite diffusion profiles and a low-T closure domain where F-in-biotite remains unchanged. The zonation domain significantly expands with increasing cooling rates and decreasing diffusion time. We systematically analyzed F-in-biotite with electron microprobe in samples collected along representative cross-sections within the Serido Belt, northeastern Brazil. Its metasedimentary units reached upper amphibolite grade metamorphic conditions, with constant F-in-biotite (F ≈ 0.33 wt.%) in grains from mica schist and paragneiss. The Ti-in-biotite geothermometer yielded nearly constant temperatures around 623 °C. The F-in-biotite regional background values remain unchanged at the contact with the major Ediacaran F-rich Acari pluton. It indicates the absence of magmatic fluid influx into the host rocks and is evidence for a fluid-absent nature of melts. By contrast, the F-in-biotite of the metric mica schist enclaves within dykes and sills of Cambrian pegmatitic granites was reequilibrated by interaction with exsolved fluids. The Ti-in-biotite geothermometer indicates temperatures around 642 °C, and our model suggests 100 kyr as a minimal duration for fluid-rock interaction. The preservation of intra-grain F-in-biotite homogeneity requires fluid flux cessation before cooling or fluid-present cooling rates below 100 °C Myr−1. Higher cooling rates would generate F-in-biotite intra-grain zonation. At the contact with the pegmatitic granites, the metasedimentary rocks show meter-scale F-in-biotite gradients due to thermal profiles setting the fluid (fH2O/fHF) gradient. The preservation of such gradients is related to a relatively fast fluid flux episode compared to the overall duration of the elevated thermal profile. The absence of intra-grain zonation indicates that F-in-biotite re-equilibration was fast relative to that of the fluid flux duration. We found intra-grain F-in-biotite zonation within an orthogneiss sample from the Paleoproterozoic Caico complex basement nearby the metasedimentary belt and intruded plutons. The fit of the natural profile by modeled profiles for cooling rates below 10 °C Myr−1, obtained from available Serido Belt U-Pb and 40Ar/39Ar ages, indicates that the orthogneiss cooled from 475 °C. It suggests the existence of a thermal gradient as the inner part of the belt cooled from 623 °C.