The Grenville Orogen Explained? Integrating Numerical Models With Geological and Geophysical Data
Numerical models offer powerful insights into tectonic processes when their validity can be tested against geological and geophysical observations from natural orogens. Although model designs must be simplified by comparison with nature, this simplification allows isolation and testing of key tectonic parameters. While ancient orogens lack constraints on key geodynamic factors such as convergence and erosion rates, this is partly offset by the ability to observe features formed in the syn-orogenic mid-crust. We define a "working" geodynamic model as one that successfully accounts for a wide range of diverse and independent observations in a physically consistent manner. Some criteria for successful integration of models with data are explained with reference to the western Grenville Orogen. Grenville-style model orogens involve convergence of variable-strength lower crust representing an inherited, laterally heterogeneous, accretionary margin. Results from crustal-scale, kinematic-subduction models are presented for both syn- and post-convergent ductile flow, and compared with results from equivalent upper- mantle-scale, dynamic-subduction models. Model feasibility is assessed by comparison with data from the Georgian Bay transect, focusing on the Ottawan (post-ca.1100 Ma) stage of Grenvillian convergence. The tectonic style can be explained by activation of ductile flow in thick, hot, lower and middle crust (Central Gneiss Belt) in response to underthrusting by a strong lower crustal block (Superior Province). Model geometries and PTt paths compare well with the data, although better age constraints are needed to refine model-data comparisons further. Post-convergent ductile flow is required to explain the positions and timing of observed normal-sense ductile shear zones. Terminal Rigolet (ca.1000 Ma) tectonism is interpreted as the result of post-convergent gravitational spreading, which produced coeval thrusting in the Grenville Front Tectonic Zone coeval and ductile extension and flattening in the orogenic core. The models suggest that variations in inherited pre-orogenic crustal strength may profoundly influence subsequent along-strike tectonic style.
Field Testing the Synconvergent Ductile Flow Hypothesis
Stacking and deformation of sheets of lower crustal gneiss in the Central Gneiss Belt (CGB) of the Grenville orogen may be explained by synconvergent ductile flow of variable-strength crust. Numerical models testing this hypothesis predict two stages of deformation and metamorphism: initial crustal-scale quasi- homogeneous thickening and assembly of lower-crustal panels along steep shear zones, followed tens of My later by ductile flow of heterogeneously deformed, thermally weakened, lower and middle crustal nappes, activated by underthrusting of a lower crustal indentor. Heterogeneous second phase flow should locally preserve relicts of first phase metamorphism and structures. Recognition of such relicts would in principle allow characterization of the processes associated with both early crustal thickening and later softening and ductile flow. Large first-phase relicts should be recognizable by map patterns indicating older gneissosity at a high angle to transposed, often migmatitic, fabrics formed during second phase flow. The Parry Sound and Kiosk domains of the CGB in Ontario are possible relicts of first-phase Grenvillian tectonism. The Parry Sound domain is a granulite klippe with well established first phase (ca. 1160 Ma) metamorphism and structures. High-strain amphibolite gneiss defines a ductile sheath traceable over much of the SW margin of the klippe; it is interpreted to have formed during second phase flow that was initiated by hydration of granulite by fluid released from pegmatites emplaced into brittle fractures. The pegmatite filled fractures evolved to individual shear zones which then linked to form shear zone systems and ultimately developed to swaths of transposed gneiss. The Kiosk domain consists of moderately SE-dipping granulite facies gneisses marked by penetrative L=S fabrics, and is deformed at its margins by ductile structures assigned to the second phase. New data on P-T conditions (P = 14 kbar, T = 800 °C) and age of metamorphism (from recent LA-MC-ICPMS U-Pb zircon results for a corundum-bearing sample) ca. 1100-1160 Ma are comparable to those reported from the Parry Sound domain, and suggest that this formerly enigmatic region also represents a map-scale (ca. 40x30 km) relict of first-phase Grenvillian structures.
Structural Significance of Mineral-Shape Fabrics and Mesoscopic Folds in Redeformed Walls of the Mill Lake Thrust, Parry Sound Domain, Grenville Orogen of Central Ontario
Marmolite, a Carbonate-Dominant Igneous Rock of Crustal Derivation, and its Importance in the Deep Crust in the Grenville Province and in Southern Madagascar
In the waning stages of a major collision-related event, at a time of regional relaxation and enhanced flow of heat into the lower crust, and perhaps owing to delamination of the lower crust, carbonate-dominant metasedimentary units become "remobilized" by melting. How does one call the resulting carbonate-dominant rocks that show intrusive relationships? The term "carbonatite" sends a wrong message, as the term implies a mantle source. The term "pseudocarbonatite" has been used, but is hardly satisfactory. We propose the term marmolite, from the Greek marmaron, marble, and lithos, stone. The term was proposed over 150 years ago for a variety of serpentinite used as an ornamental stone, but it is now considered obsolete by the IUGS subcommittee on the Systematics of Metamorphic Rocks. We propose to rejuvenate the term and give it a meaning consistent with its etymological roots: it refers to a rock resulting from a remelted carbonate-dominant metasedimentary assemblage. These carbonate-dominant rocks form what have been called "vein-dykes" in the Grenville Province in Canada. Perhaps because of the aggressive nature of a carbonate-dominant melt in the presence of silicate wallrocks, the melt also contained small quantities of dissolved silicate, phosphate, sulfate and fluoride, such that occurrences of marmolite contain strikingly coarse-grained euhedral olivine, pyroxene, amphibole, phlogopite, titanite, meionite, the feldspars, apatite-(CaF), anhydrite, and fluorite, and in addition, a host of exotic accessory phases, e.g., nioboaeschynite-(Y) at the Bear Lake Diggings, in Haliburton County, Ontario, documented in 2008. The carbonate "soup" had a very low viscosity, which favored very efficient gravitational settling of crystals. The cumulates could be dunitic where the activity of silica was low, or pyroxenitic where it was higher. Sheets of neoformed phlogopite up to tens of cm across did accumulate in this melt, and deposits have been exploited in the Gatineau district of Quebec in the past, and still are at Ampandandrava, southern Madagascar, where the post-Panafrican collision and ensuing uplift parallel the tectonic evolution of the Grenville Province. The presence of a carbonate-dominant intercumulus assemblage, the development of a pegmatitic texture, and intrusive contacts provide the only solid evidence for an igneous origin of marmolites and the associated suite; the subsolidus exchange of C and O isotopes likely partially obliterates the high-temperature equilibrium that existed among coexisting phases precipitating from the carbonate-dominant anatectic melt.
New insights into Composite Arc Belt geology based on recent mapping in the Harvey- Cardiff domain, Grenville Province, Ontario
The geologic history of the pluton-rich Harvey-Cardiff domain in the western Composite Arc Belt has been poorly understood, in part due to limited geochronology. New geochronological and geochemical data collected during a 2-year mapping program in southwestern Harvey-Cardiff domain indicates the presence of a long-lived, geochemically diverse, history ranging from 1290 to 1030 Ma. The oldest rocks are tonalite gneisses of the Anstruther and Burleigh gneiss complexes, previously dated at 1290 Ma. Mafic to felsic tectonites adjacent to these complexes may represent the basement into which these tonalite gneisses were emplaced. The next youngest units are the alkalic Glamorgan (Trooper Lake) gabbro, previously dated at 1246±3 Ma; and syenite and nepheline-bearing syenite suite rocks emplaced between 1242 and 1219 Ma. Two alaskite suite plutons in Harvey-Cardiff domain were dated as part of the present study: the syntectonic, monzogranitic Junction pluton yielded a U-Pb zircon age of 1221.0±6 Ma, whereas a K- feldspar megacrystic granodiorite body in the deformation zone that forms the boundary between the Harvey- Cardiff and Bancroft domains, have an age of 1211.3±6 Ma, which places a maximum age on domain boundary formation. Both ages are similar to a previously reported 1229 Ma age from a granite body in the Anstruther gneiss complex. Significantly, all are roughly 20 to 30 m.y. younger than typical alaskite suite ages from elsewhere in the Composite Arc Belt (1240-1245 Ma). Similarly, the newly recognized Salmon Burn diorite-gabbro intrusive complex in Harvey-Cardiff domain has many lithologic similarities to the Lingham Lake intrusive complex in Grimsthorpe domain, but is some 40 m.y. younger. The Composite Arc Belt is known to have generated compositionally similar magmas in similar tectonic settings at several distinct times, and it would appear that younger alaskite suite plutons and the Salmon Burn intrusive complex in Harvey-Cardiff domain provide yet another example of this phenomenon. Several small plutons of a newly recognized late granite suite are characterized by higher K2O (4.6-7.7 wt%) and Th (25-55 ppm) than alaskite suite plutons, and yield a U-Pb zircon age of 1066.8±3.7 Ma. A syenogranite pegmatite vein hosting the historic Cavendish U-Th mine has a U-Pb age of 1059.2±3.7 Ma, suggesting a temporal link between late granite and pegmatite emplacement. Although these ages are similar to those for pyroxenite to syenite plutons of the ultrapotassic Kensington-Skootamatta suite, chemically the late granites of the Cavendish area are most like the 1066 Ma Barbers Lake granite, as well as the Belmont Lake, Coe Hill, Elphin, Leggat Lake, McLean and Petroglyphs granites. These late granite plutons are spatially associated with earlier alaskite suite intrusions, but are slightly younger(10 m.y.) and have different magnetic and gamma-ray spectrometric signatures than the Kensington-Skootamatta suite plutons. These observations suggest that 2 distinct plutonic suites were emplaced into the Composite Arc Belt between 1085 and 1066 Ma, with emplacement of late granite suite plutons resulting from melting of older granitic rocks due to the influx of mafic magmas into the crust during emplacement of the ultrapotassic suite. Partial melting of the older alaskitic granites is most likely responsible for the higher concentration of radiogenic elements in the late granite suite and related pegmatite dikes and makes the late granite suite prospective for Rossing-style uranium mineralization.
The Northbrook-Kaladar Formation: a microcosm of the Mazinaw Domain, Central Metasedimentary Belt
Detailed mapping of 2.5 km2 along the southern contact of the Northbrook Metatonalite documents a geographically compact record of igneous, sedimentary and tectonometamorphic events which appears to echo much of the geological evolution of the broader Mazinaw Domain. The oldest rock in the area is the 'Northbrook Tonalite' (NM, although commonly metagranoŽdioritic in composition). Its crystallization age has not been measured but Corfu and Easton (1995) suggest the same, ~ 1,250 ± 10 Ma age as that of the neighbouring Cross Lake Pluton to the north. Along its southern margin, the N Metatonalite was unconformably overlain, in the south-western half of the map area, by what we interpret to be a mafic volcanic edifice (VE), presently consisting of mafic to intermediate amphibolites with distinctive intermediate to large equant hornblende grains, which we believe to be pseudomorphs after coarse igneous pyroxene grains crystallized either within a volcanic edifice or in thick flows. On the east half of the map area, the NM is unconformably overlain by what we call the Northbrook-Kaladar formation (NK Fm). To the southwest, the NK Fm also overlies the mafic metavolcanic complex. The NK Formation consists of metamorphosed thin mafic volcanic flows, arkose, polymictic and arkosic conglomerate beds, and eventually pelitic rocks. The tonalite, volcanic edifice and NK Formation were all affected by a tectono-Žmetamorphic event leading to strong NE-SW horizontal extension and a corresponding vertical flattening foliation that grades southward into a reverse, south-over-north component of strain. The large horizontal extension can best be accounted for by horizontal transpression, but no convincing sense for the horizontal shear was found in the map area. The meta-igneous rock units (NM, VE) and the lower part of the NK Fm were initially relatively anhydrous, and their metamorphism is therefore mostly a hydration event, forming epidote and amphibole. Further south, the prograde pelitic assemblage includes quartz, muscovite, and staurolite breaking down to sillimanite, garnet and biotite. There is no indication of two stages of metamorphism, and although the mineral grains have a strong shape fabric, they exhibit no significant intracrystalline strain. A small circular stock of undeformed leucocratic granite punctures the rocks of the NK Fm. It does not seem to inprint any contact metamorphism on its host, and it is best interpreted as emplaced during the last - still hot but tectonically quiet - stage of the main metamorphic event. Immediately to the south, the 1,245 Ma (Van Breemen and Davidson 1988) Addington Granite, which was also metamorphosed and stretched horizontally at high temperature, is thrust over the NK Formation. While only the Addington has been dated, we might correlate the NK Fm with the Flinton Group (described in a narrow trough of metasediments to the west of our area) as others have done, and ascribe the strong transpression under medium-grade metamorphic conditions to the 1090-980 Ottawan orogeny. That transpression, common to much of the Mazinaw domain, would thus postdate the accretion of that domain to the Composite Arc Belt, but is apparently restricted to that domain.
The Ottawan Orogenic Lid: A New Tectonic Element in the Grenville Province
The term Ottawan Orogenic Lid (OOL) is introduced to the Grenvillian lexicon to describe parts of the interior Grenville Orogen situated structurally above the Allochthon Boundary Thrust that lack evidence for Ottawan (ca. 1090-1020 Ma) metamorphism and penetrative deformation. The OOL occurs in two discrete areas of the Grenville Province, in the northeast where it is largely underlain by Paleoproterozoic (ca. 1.65 Ga) crust, and in the southwest where it occupies part of the Late Mesoproterozoic (ca. 1.3-1.2 Ga) accreted terranes, and it may also underlie the Llano Uplift, a Grenvillian inlier in Texas. A characteristic attribute of the OOL is the presence of 40Ar/39Ar hornblende plateau ages > 1090 Ma, indicating it was not heated above 480-500 C during the Ottawan phase of the Grenvillian Orogeny. Given its location in the hinterland of the orogen and its juxtaposition against medium-pressure (MP) and low-pressure (LP) components of the Ottawan M-LP Belt, the OOL is interpreted as part of the orogenic suprastructure that was dropped down to deeper crustal levels on a system of normal-sense shear zones as a result of orogenic collapse. The shear zone system on which collapse occurred was initiated from ca. 1050-1020 Ma and rooted in the mid crust, with the presence of coeval contractional deformation elsewhere in the orogen implying that collapse of the mid-upper crust took place in an overall compressional orogen (fixed boundary collapse). Since plateaux are inherently unstable and prone to collapse on the thermally weakened mid crust, the presence of an orogenic lid is compatible with the former existence of an orogenic plateau in the hinterland under which some form of channel flow took place, although it is not in and of itself sufficient proof of one.