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.

Macro- and microstructural analysis of the North Tea Lake Mylonite Zone: an extensional shear zone in the Central Gneiss Belt, Grenville Province, Ontario

The North Tea Lake Mylonite Zone is a late extensional ductile fault that is concordant with and has reworked fabrics of the North Tea Lake Shear Zone, the frontal thrust shear zone of the upper amphibolite–granulite facies Kiosk domain within the interior of the Central Gneiss Belt. North Tea Lake Mylonite Zone fabric is an anomalously fine-grained mylonite compared to Central Gneiss Belt gneisses, and consists of three microstructural domains that display progressive recrystallization and grain size refinement of the protolith granitoid. On the basis of petrography and electron backscatter diffraction, these microdomains are inferred to represent a transition from dominantly dislocation creep to diffusion creep and diffusion-accommodated grain boundary sliding at elevated stress (>100 MPa), low fluid activity, and temperatures ∼500 °C. The North Tea Lake Mylonite Zone is interpreted to mark a step in the progressive transition in deformation mode during late- to post-Ottawan extension and cooling of the Grenville orogen from weak, wide, wet, and warm shear zones to Rigolet-phase cooler, narrow, ultrafine, high-stress shear zones reworking dry protoliths.

Nd isotope mapping of the Allochthon Boundary Thrust on the shores of Georgian Bay, Ontario: significance for Grenvillian crustal structure and evolution

Nearly 50 new Nd isotope analyses are presented for the Shawanaga region of Georgian Bay, Ontario, to study crustal evolution in the Grenvillian Central Gneiss Belt. Depleted mantle (TDM) Nd model ages are used to map a major Grenvillian tectonic boundary, the Allochthon Boundary Thrust (ABT), which in the Shawanaga area separates gneisses with TDM ages above and below 1.65 Ga. This is lower than the 1.8 Ga age cut-off observed further north, and is attributed to a southward increase in Mesoproterozoic magmatic reworking of an original Palaeoproterozoic continental margin, causing a progressive southward decrease in Nd model ages. Between Shawanaga Island and Franklin Island, Nd isotope mapping yields an ABT trajectory that closely matches published geological mapping, and passes within 100 m of four retrogressed eclogite bodies typically associated with the thrust boundary. This validation of the method gives confidence in the mapped trajectory south of Snake Island, where sparse outcrop inhibits lithological mapping. The new results suggest that published 1.7–1.9 Ga TDM ages in the Lower Go Home domain of the Central Gneiss Belt further south are also indicative of parautochthonous crust. Hence, we propose that the main ramp of the ABT is located in the immediate hangingwall of the Go Home domain, much further south than generally recognized. This has important implications for the large-scale crustal structure of the SW Grenville Province, suggesting that the ABT ramp has a similar curved trajectory to the Grenville Front and the Central Metasedimentary Belt boundary thrust.

Post-convergent structures in lower parts of the 1090–1050 Ma (early-Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield

Remnants of the early-Ottawan thrust-sheet stack are exposed in the Central Gneiss Belt (CGB, lower portion of stack) and the Composite Arc Belt (upper portion of stack). Post-collisional vertical thinning and associated horizontal extension of the stack produced structures ranging over eight orders of magnitude in horizontal length, and both orogen-parallel and orogen-perpendicular in orientation. At the 100 km scale, the fold-induced constriction in the northern Parry Sound domain appears to have been enhanced, and lineation trend lines in its footwall locally deflected, by a component of NW–SE (i.e., orogen-perpendicular) flattening and a component of NE–SW (i.e., orogen-parallel) ductile extension. At the 10 km scale, four non-cylindrical lenticular bodies of gabbro–anorthosite gneiss within the domain, inferred to be triaxial mega-boudins or heterogeneously strained plutons, are separated by large extensional bending folds, the complementary structures attesting to a component of NW–SE flattening and a component of NE–SW extension. Non-cylindrical lenticular structures in other domains of the CGB, interpreted as triaxial foliation mega-boudins, exceed 30 km in length. Their moderately strained granulite-facies interiors give way to highly strained amphibolite-facies margins, thus documenting subvertical ductile flattening and multi-lateral extension during retrogression. Well-layered, highly strained gneiss is commonly deformed by steep NE–SW-trending extensional faults and associated monoclinal fault-propagation folds (FPFs). The short limbs of the FPFs bend the regional elongation lineation and host a set of fault-parallel, unstrained to slightly deformed, granite–pegmatite dikes. Dilation vectors of most dikes are oblique to the granite–pegmatite contacts, and the sense of their tangential components attests to orogen-perpendicular extension. The fault-parallel dikes and associated FPFs are cut by a set of unstrained dikes. Collectively these observations document a prolonged history of post-collisional extension of the mid crust, from ductile structures indicative of a significant component of orogen-parallel extension shortly after the metamorphic peak at mid-crustal depths, to brittle–ductile structures indicative of a component of orogen-perpendicular extension and associated magmatic dilation following its exhumation and cooling in the upper crust.

Wall-rock structure at the present contact surfaces between repeatedly deformed thrust sheets, Grenville Orogen of central Ontario, Canada

In the Central Gneiss Belt of the Grenville Orogen (Ontario), ca. 1020 Ma, extensional shearing, disharmonic buckle folding, and seismic faulting at middle to upper crustal levels affected the geological structure of pre-1040 Ma, ductile-thrust sheets. Because much of the repeated in situ deformation was mechanically discontinuous, the present contacts between thrust sheets may not coincide at all localities with the original thrust surfaces. We focused special attention on the basal contact of the Parry Sound domain, whose synformal structure may have resulted from gravitational subsidence of its dense rocks immediately after ductile thrusting. East of Wahwashkesh Lake, a transverse gradient of total strain is absent on horizontal scales of 100–1000 m in lithologically uniform granite gneiss comprising the uppermost western footwall of the northern Parry Sound domain. This contrasts with the steep transverse-strain gradients documented by others, on the same scale, in the wall rocks of Phanerozoic ductile thrusts. We hypothesize that ductile or brittle extension faulting may have removed a 10–20 km long sole-thrust segment at the western flank of the northern Parry Sound domain, together with severely strained rocks of the original uppermost footwall, from the level of the current erosion surface. Within the Parry Sound domain, by contrast, most if not all of the original footwall of the 1160 Ma Mill Lake thrust seems to be preserved at the presently exposed contact surface between the allochthonous basal and interior Parry Sound assemblages.

Buckle folding and deep-crustal shearing of high-grade gneisses at the junction of two major high-strain zones, Central Gneiss Belt, Grenville Province, Ontario

Low-plunging, transport-parallel F3 folds are common at all scales in the Central Gneiss Belt of the Grenville Province, but few of these folds are sheath folds. Where the D1–D2 Parry Sound shear zone intersects the D3 Shawanaga shear zone (SSZ) at a high angle, F3 folds formed at several scales (centimetre to greater than outcrop scale) in layered D1–D2 "straight" gneisses. At the start of their evolution, the F3 folds formed just beyond the SSZ with hinges near orthogonal to the D3 shear direction and with typical buckle features, e.g., wavelengths vary with layer thickness, and hinges are discontinuous and bifurcate. The buckle folds evolved within the SSZ by rotation of hinges towards the shear direction. Even though hinges initiated at a high angle to the shear direction, sheath folds were not produced. In addition to tightening the buckles, the ductile reorientation produced thin–thick (extended–shortened) limb pairs and very straight, ridge-like fold hinges and removed small folds from the extended limbs of larger folds. Such features may serve as criteria to distinguish transport-parallel folds that initiated in layering at high angles to the shear direction from those formed in layers containing the shear direction. A general shear parallel to the SSZ can reproduce several features inferred to mark stages in the progressive reorientation of the folds; the pure shear component of the general shear is inferred to have had a positive stretch direction down the dip of the shear zone, at a high angle to the transport (simple shear) direction. The interplay of buckling and shearing in the study area is, plausibly, the expression of deformation at the upper boundary of a channel-like flow that succeeded initial crustal thickening.

Timing and duration of melting in the mid orogenic crust: Constraints from U–Pb (SHRIMP) data, Muskoka and Shawanaga domains, Grenville Province, Ontario

The Central Gneiss Belt in the Grenville Province, Ontario, exposes metaplutonic rocks, orthogneisses, and minor paragneisses that were deformed and metamorphosed at crustal depths of 20–35 km during the Mesoproterozoic Grenvillian orogeny. We present sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon data from eight samples of migmatitic orthogneiss, granite, and pegmatite from the Muskoka and Shawanaga domains that constrain the age and duration of partial melting in the mid orogenic crust. Our results support earlier interpretations that the protoliths to these migmatitic orthogneisses formed at ca. 1450 Ma. Emplacement and crystallization of granite and pegmatite in the Shawanaga domain took place at ca. 1089 Ma, apparently coevally with deformation and high-grade metamorphism. Leucosomes in the Muskoka and Shawanaga domains yield ages of 1067 and 1047 Ma, respectively, interpreted as the ages of melt crystallization. The geochronological data and field observations suggest that melt was present at the mid-crustal level of the Grenville orogen during a significant part of its deformational history, probably at least 20–30 million years. By analogy with modern orogens, the amount and duration of melting observed in the Muskoka and Shawanaga domains may have had an impact on the orogenic evolution of the area.