Delineation of Archean and Mesoproterozic crustal terranes in the Parent-Clova region of south-central Quebec: Evidence of crustal mixing, cryptic suturing and ensialic arc formation
The true evolution of the Grenville Province still remains one of the most problematic questions regarding the nature of crustal assembly in the Canadian Shield. Due to multiple phases of metamorphic episodes, voluminous magmatism and the terminal uplift of the Grenville orogeny (1.1 Ga), traditional means of geologic interpretation have fallen short when delineating distinct pre-Grenvillian terranes within the province. Isotope geochemistry has shown its validity in unraveling geologic history, as early contributions demonstrated that the metamorphic resistant Sm-Nd isotopic system could be used to effectively determining crustal formation ages from granitoids. Currently, hundreds of Sm-Nd analyses have been used to characterize the provenance of distinct first-order crustal terranes in most of the Grenville Province, in order to produce a comprehensive crustal formation map. The Parent-Clova region of south central Quebec comprises of a 50,000 km2 area and remains one of the last locations to be studied in this manner, as complex geology, lack of mineral resources and limited access have hampered even reconnaissance mapping efforts. A new crustal formation map for the Parent-Clova region of south central Quebec is proposed, by mapping boundaries of terranes with distinct isotopic signatures. Over 90 samples have been analyzed using the Nd isotopic system in conjunction with some major element analysis, and have identified three crustal blocks in the region with distinct Nd model age ranges. Pristine Archean crust resides in the north, while allochthonous Mesoproterozoic crust encompasses much of the south. The central block shows an isotopic signature reflective of crustal mixing events, and is proposed to be a continuation of the ensialic arc identified to the northeast. This interpretation unifies arguments in support of a failed rift propagating through the region, with contributions from cryptic suturing between Archean and Paleoproterozoic crust.
Mesoproterozoic Meta-Anorthosite Sheets and Megacrystic Amphibolites, New Jersey Highlands: Evidence for AMCG Magmatism in the North Central Appalachians, USA
Megacrystic amphibolites and meta-anorthositic sheets from several locations in the Mesoproterozoic New Jersey Highlands appear to be related based on field relations and geochemistry. Field relationships suggest that megacrystic amphibolites and meta-anorthosites were emplaced into supracrustal rocks after 1200 Ma and then penetratively deformed during Ottawan orogensis (ca. 1050 Ma). The megacrystic amphibolites outcrop as 0.5 to 2 m thick, <100 m long, conformable layers and likely represent metamorphosed dikes. They contain weakly zoned, subhedral plagioclase megacrysts (An29-44) up to 13 cm long in a groundmass of magnesio-hastingsite and plagioclase (An18-38). Megacrystic amphibolites are geochemically similar to mafic rocks associated with anorthosite in the Adirondack Highlands. Both have high TiO2 (2.5-3.5%), Al2O3 (14-19%), FeOT (10-16%) and comparable trace and REE abundances that form a distinct Al-Fe mafic magma type characteristic of anorthosite associations. In the New Jersey Highlands, meta-anorthositic sheets (60-240 m thick) are mainly medium-grained, well-foliated leucocratic gneisses that contain abundant plagioclase (60-80%) with minor hornblende and/or biotite. Whole-rock geochemistry of the meta-anorthositic sheets are characterized by high Al2O3 (20-25%) and CaO (8-11%); low MgO (1-3%) and K2O (<1.5%); SiO2 between 50-55%, and significant positive Eu anomalies (Eu/Eu* up to 3.8). Megacrystic amphibolites are interpreted to have formed coeval with meta-anorthosite in the New Jersey Highlands through plagioclase separation from a common mafic magma ponded at the base of the crust. Premature tapping of this magma by extensional fractures may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions. Instead, this process resulted in emplacement of small volumes of anorthosite and megacrystic basalt as thin sill and dikes, respectively. Based on similar field relations, geochemistry, and spatial associations of anorthositic rocks in New Jersey and the Adirondacks, we interpret the former to be coeval with AMCG magmatism in the Adirondack Highlands.
Kinematics of the Grenville Front Tectonic Zone
Field mapping of the Grenville Front Tectonic Zone (GFTZ) near Sudbury has enabled us to identify 4 generations of deformation. D1 deformation is characterized by isoclinal folds in high grade migmatitic gneissic rocks. Regional compositional layerings were developed due to the transposition process of D1 deformation. This transposition foliation is the main form surface that defines F2 folds and younger generation folds. D2 deformation is characterized by large-scale (amplitude: ∼5km) folds. They are trending at high angle to the Grenville Front, with one limb dipping toward SEE, and the other dipping toward S. These folds are tight to open folds, and do not have axial plane foliations. They can be easily identified in aerial photos. Centimeter- to meter-scale NEE-trending folds overprinted on F2 with fold axes plunging toward SW on E- trending limbs of F2 and toward NE on NNE-trending limbs of F2 during D3 deformation. NEE-trending mylonite zones also developed during the transposition process of D3 deforamtion, but become more common towards the northwest boundary of the GFTZ. The fold axial planes of NEE-trending folds and the foliations within the NEE-trending mylonite zones vary in space, and this is due to the gentle folding of D4 deformation. Considering that the F2 folds plunge at a high angle relative to the strike of the Grenville Front, we believe that the D1 and D2 deformations are pre-Grenvillian deformations. Based on the orientations and kinematics of the D3 fabrics, we believe that D3 deformation belongs to the Grenville Orogeny (1000-980Ma). Within the NEE- trending mylonite zones, small scale folds are common. Structural analysis of these folds suggests that they were initially F3 bulking folds, and were progressively modified by D3 shear zones which are a consequence of the strain localization and the thursting. The kinematics of the D3 deformation suggests that the deformation during Grenville Orogeny at the GFTZ is a convergent process which began with the bulking along NW-SE direction and followed by the top-to-the-NW thrusting.
Regional Geology of the Eastern Seal Lake Area, Central Labrador
The Mesoproterozoic Seal Lake Group in central Labrador comprises a sequence of argillaceous and arenaceous sedimentary rocks, intercalated with basalt flows and intruded by gabbro sills. The entire group is folded into an east-to north-east-trending, doubly plunging syncline of which the southern limb has been overturned and overridden by older rocks transported northwards along Grenvillian thrust faults. The group is host to numerous copper sulphide and native copper occurrences and minor, localized uranium mineralization is associated with the basal sedimentary rocks. Regional mapping in the eastern Seal Lake area recognized two periods of deformation. The first regional event produced a dominant east-northeast-striking fabric, resulted in folding of the main syncline and some fault development along the limbs and hinge area. A later deformation resulted in folding of the main fabric into small-scale folds and open warps. A dominant south-southeast plunge of mesoscopic, first generation fold axes define the plunge of the main syncline. Prevalent south-plunging mineral lineations and small-scale dextral shearing suggests a north-directed thrust component is present in east-trending faults. Metamorphic grade ranges from chlorite-tremolite assemblages in the southern map area to pumpellyite facies in the north. Examination of known mineral occurrences in the eastern Seal Lake area confirm native copper, malachite and bornite mineralization hosted by quartz + carbonate veins associated with fractures and small-scale shear zones proximal to basalt-slate contacts. Bornite, chalcocite, Cu-carbonates and chalcopyrite are also hosted in gabbro sill margins. Several new copper occurrences and minor uranium mineralization have been identified in the eastern Seal Lake area.
Nd Isotope Evidence for the Extent of Juvenile Monocyclic Crust in the Southwest Grenville Province
Nd isotope data are used to generate a crustal formation age map for the southwest Grenville Province. These data show that juvenile monocyclic crust is restricted to part of the Central Metasedimentary Belt interpreted as an ensimatic back-arc rift zone. There is a clear isotopic distinction between juvenile crust in the rift zone and pre-Grenvillian crust on the flanks. However, the rift zone contains at least two blocks of older crust that appear to have rifted away from its walls. In addition, the rift has an ensialic extension in Quebec, suggesting that it dies out to the north. On the flanks of the rift zone, there is evidence for two crust-forming events, comprising an earlier (1.5 Ga) juvenile Pinwarian event, and a later 1.4-1.3 Ga continental margin arc (the Elzevirian orogeny). Based on the geographical distribution of Nd model ages in the SW Grenville Province, a revised tectonic model for the Grenville orogeny is developed. In this model, back-arc spreading created the rift zone behind a (pre-Grenvillian) Elzevirian subduction zone that obliquely straddled the edge of a Mesoproterozoic continental margin. As a result of this geometry, the rift zone died out to the north as it propagated into cold Archean crust. Elzevirian subduction was probably halted by the collision of the continental margin with an off-shore arc, and later with a major continental block, giving rise respectively to the Shawinigan and the Ottawan orogenic events. These collisional events can be grouped as parts of the Grenville Orogenic Cycle.
Neodymium Isotope Analysis to Constrain Terrane Boundries in the Saguenay and Baie- Commeau Regions of Quebec and the Tweed-Bancroft Area of Ontario: Back-arc Rifiting and Arc Accretion
This study combines isotope evidence and major element analysis in reconnaissance efforts to investigate interesting geological structures in three portions of the Grenville Province in Ontario and Quebec. The aim is to constrain terrane boundaries to explore the geologic environment encompassing the Saguenay and Baie- Comeau regions of Quebec and the Tweed - Bancroft area (Weslemkoon Batholith) of Ontario through spatial, temporal and tectonic relationships. The Grenville Orogeny (1.20 - 1.00 Ga) metamorphosed this area, obscuring boundaries between lithotectonic domains of pre-Grenvillian crust. Neodymium (Nd) isotope data have been successfully utilized in mapping many terrane boundaries in the Grenville Province, and will be applied to these three areas which are yet to be defined in detail. In each of these regions reconnaissance work has revealed portions of crust with older isotope signatures surrounded by more juvenile terranes. The Weslemkoon Batholith of the Elzevir Terrane is found within the Central Metasedimentary Belt (CMB). This area was formerly considered to be a composite arc belt but current evidence suggests that it represents a failed back-arc rift zone. The Weslemkoon batholith is located within a remnant block of older crust within the rift, but exhibits a diffuse terrane boundary to the east, with Nd model ages that are progressively more juvenile. The diffuse boundary is consistent with the rifting model, where magmatism sampled basement crust with varying formation ages. Within the Saguenay and Baie-Comeau regions of the Pinwarian-aged Quebecia Terrane, two older blocks of crust have recently been identified. These blocks are possible evidence that the Quebecia Terrane represents a composite arc belt similar to the model previously proposed for the CMB. Such circumstance would infer that these older blocks represent continent building through arc accretion at the eastern margin of Laurentia.
Nd-Isotope Mapping of Southern Labrador
The geology of Southern Labrador, in the eastern region of the Grenville Province, records the tectonic history of an important period of time in the formation of the North American Shield. Several major orogenic events occurred in Southern Labrador: the Makkovik orogeny (1890-1850 Ma), the Labradorian orogeny (1710-1600 Ma), the Pinwarian (1510-1450 Ma) and the Grenville orogeny (1050-985 Ma. In order to understand the geological evolution of this region through these successive events, it is critical to establish the extent of crust that was newly formed in each of these events, relative to areas of crustal reworking of pre-existing crust. Nd model age mapping is the most powerful and cost effective means of distinguishing between these different types of geological process, and is therefore being applied to N-S and E-W transects across Southern Labrador. Preliminary data largely support the conclusions of previous crustal formation age mapping in the Eastern Grenville Province, showing that the Hawke River and Mealy Mountain terranes contain significant areas of pre- Labradorian crust that may have formed during the Makkovik orogeny. In contrast, the Pinware terrane is shown to consist largely of juvenile Labradarian crust. The Lake Melville terrane appears to contain a mixture of Labradorian and pre-Labradorian elements that requires further investigation.