Anatomy of a mid-crustal suture: Geology of the Central Metasedimentary Belt boundary thrust zone, Grenville Province, Ontario

Ontario2011_1What: This project focuses on the geology of the Bancroft area of Ontario, which is thought to have been the margin of North America during the Mesoproterozoic. This part of the Grenville Province of Ontario is made up of two tectonic elements: (1) High-grade gneisses that were part of the 1.7-1.4 Ga continental margin and (2) a package of volcanic, plutonic, and sedimentary rocks that are thought to be a collage of arc components accreted at ca. 1.17 Ga. This Keck project focuses on this arc assemblage and its collision + suture with North America. Because these rocks have enjoyed several mountain-building events at the margin of the craton, they are now highly-deformed gneisses, amphibolites and marbles. This project will examine the petrology and structural geology of these rocks to better understand the details of continental accretion, thrusting, and metamorphism in mid-crustal rocks.

When: July 1-30

Who: William Peck (Colgate University), Steve Dunn (Mt. Holyoke College), and Michelle Markley (Mt. Holyoke College)

Geologic Setting

The Composite Arc Belt of Carr et al. (2000) is a subdivision of the southern Grenville Province that includes a number of small terranes (Fig. 1) with distinct geology. These terranes contain greenschist- and amphibolite facies metasedimentary and 1.29-1.24 Ga metavolcanic and volcanoclastic rocks that are intruded by suites of gabbroic, tonalitic, and granitic plutons (Easton 1992). The volcanic rocks have arc geochemical signatures and are dominated by mafic tholeiitic suites, but include intermediate and felsic rocks and some with calc-alkaline signatures (see Easton 1992; Carr et al. 2000; Smith et al. 2001). Terrane boundaries are generally east-dipping, tops to the west shear zones. The youngest cross-cutting plutonic rocks may constrain terrane amalgamation in the Composite Arc Belt to 1.24-1.22 Ga (Carr et al. 2000).
Figure. 1. A. Location map of the Grenville Province. B. The southern Grenville Province of Ontario, after Hanmer and McEachern (1992). CMBbtz, Central Metasedimentary Belt boundary thrust zone; the Composite Arc Belt is sub-divided into the Harvey-Cardiff (H), Belmont (B), Grimsthorpe (G), Mazinaw (M), and Sharbot Lake (S) domains after Easton (1992).

The tectonic interpretation of the Composite Arc Belt is somewhat controversial. Some workers have postulated that these arcs formed on oceanic crust offshore of Laurentia (e.g., Brown et al. 1975; Windley 1989; Carr et al. 2000), while others have assigned these rocks to rifting and back-arc environments, some underlain by continental crust, and developed at the continental margin (Pehrsson et al. 1996; Hanmer et al. 2000; Smith et al. 2001).

The Composite Arc Belt is bounded on the west by the Central Metasedimentary Belt boundary thrust zone (CMBbtz; Fig. 2): a mid-crustal deformation zone that separates metasedimentary, metavolcanic, and metaplutonic rocks of the Composite Arc belt from the Central Gneiss Belt of the foreland. The CMBbtz is made up of annealed, heterogeneously deformed mylonitic and marble mélange tectonites surrounding metaigneous thrust sheets (Hanmer 1988, Hanmer and McEachern 1992, McEachern and van Breemen 1993; see Fig. 2).
Figure 3. Map of the Central Metasedimentary Belt boundary thrust zone after Hanmer and McEachern (1992). R, Redstone thrust sheet; D, Dysart thrust sheet; GL, Glamorgan thrust sheet; G, Grace thrust sheet; P, Papineau thrust sheet; F, Foymount thrust sheet; S, Stafford thrust sheet.

The timing of accretion of the Composite Arc Belt to the Central Gneiss Belt is controversial, as there is evidence for two deformation events in the CMBbtz at ca. 1.19 and 1.08-1.06 Ga (McEachern and van Breemen 1993; Carr and McMullen 2000). The 1.08-1.06 Ga event is associated with the most pervasive deformation, and Central Gneiss Belt rocks structurally below the CMBbtz only record the 1.08 Ga metamorphism, which is interpreted by Timmermann et al. (1997) as representing docking of the Composite Arc Belt. Pehrsson et al. (1996) proposed that the Raglan gabbro belt (Fig. 2; to the east of the CMBbtz) could have provided a rheological barrier that controlled the location of the top of the thrust zone, and Hanmer (1988) and Hanmer and McEachern (1992) proposed that the location of the lower boundary of the thrust zone was controlled by a weak (and probably discontinuous) horizon of aluminous (locally cordierite+gedrite) gneisses (Peck and Valley, 2000; Peck and Smith, 2005).

Keck Project Focus

The CMBbtz provides an ideal natural laboratory in which the interplay between terrane amalgamation, thrusting, metamorphism, and fluid-flow can be examined in mid-crustal rocks. This area has been well-characterized structurally (Hanmer 1988, Hanmer and McEachern 1992; Carr and McMullen 2000) has good geochronologic control (McEachern and van Breemen 1993; Pehrsson et al. 1996; Carr and McMullen 2000), and has been mapped in detail by the Ontario Geological Survey (e.g. Armstrong and Gittins 1968; Culshaw 1986; Bright 1987a,b; 1988; Easton 1990; 1986; 1987a,b; Breaks and Thivierge 2001; Lumbers and Vertolli 2003). However, igneous and metamorphic petrologic studies in this area are for the most part reconnaissance or regional in nature (e.g., Anovitz and Essene, 1990; Lumbers et al., 1990; Carr and Berman 1997). We plan to examine several outstanding petrologic and tectonic questions in the CMBbtz, with a goal of better understanding the mid-crustal levels of major thrust systems.

Potential Student Projects

There are a host of potential projects in this area depending on student background and interests. Please contact one of the project faculty if you are interested in any of these projects or some other aspect of petrology/structural geology not mentioned!

Metamorphic Petrology

Surprisingly little detailed metamorphic petrology has been done in the CMBbtz. Some students will focus on transects across-strike of the CMBbtz to constrain the meatamorphic field gradient in marbles. These projects will include attempts to map isograds in the field and use petrography and phase equilibria constraints on temperature and fluid evolution, possibly also using calcite-graphite carbon isotope thermometry or dating metamorphic minerals. This is important because Carr et al. (2000) have questioned the long-held assumption that the CMBbtz should be grouped with the Arc Belt in tectonic reconstructions, suggesting suggest that these metasediments were deposited on the Laurentian margin and not as part of Composite Arc Belt. Compositions, mineral assemblages, and stable isotopes of marbles across this proposed suture may help answer this question, and provide evidence for more accurate placement of the CAB-Laurentian suture zone.

Besides the constraints on mid-crustal fluid-flow from marbles, one student may examine fluid interaction between marbles and metaigneous rocks. The focus of this project will be the 40 km2 Allsaw anorthosite (larger anorthosite in Fig. 3, near the Dysart thrust sheet). This body has only been analyzed as part of geochemical reconnaissance, but is mapped in detail (see Easton, 1992). Interaction with fluids has produced spectacular vein replacement of igneous plagioclase with scapolite. This study may allow a better understanding of fluid-rock interaction and the control that rock-types exert on fluid infiltration in the CMBbtz. This project will involve field characterization of textures, petrography and petrology of metamorphic assemblages, and possibly whole-rock geochemistry to constrain igneous protoliths.

Igneous Petrology

Recent tectonic interpretations of the Composite Arc Belt have argued for a back-arc or rifted arc environment (e.g. Hanmer et al. 2000). One of the lines of evidence for this model is the distribution of 1.3 Ga calc-alkaline rocks, which are thought to represent a pre-rift continental arc that became fragmented by back-arc extension. Some of these rocks occur in the Adirondacks and Green Mountains (NY and VT), which are loosely correlated with tonalities in the Dysart thrust sheet in the CMBbtz (see Fig. 3). The CMBbtz tonalities haven’t been studied in detail, and this project would focus on the geochemistry and petrology of these rocks. This study will also be one in which obtaining zircon dates may be possible and refining the age of the Dysart intrusive rocks would be a useful constraint for the timing and spatial distribution of Grenville magmatism. It would be particularly significant if these rocks are indeed contemporaneous with those in New York and Vermont.

Structural Geology

Because this project focuses on the CMBbtz, structural geology projects will have an emphasis on deformation related to this structure and in adjacent terranes. One possible focus of study, the Bancroft shear zone, is a tectonically-late mainly marble-hosted deformation zone which has been interpreted by some as being the boundary between the CMBbtz and the Harvey-Cardiff domain of the Composite Arc Belt (e.g., Carlson et al., 1990; Cosca et al., 1992). In Carlson et al.’s tectonic reconstruction, the Bancroft shear zone represents the site of late extension after peak Grenville orogenesis, and the fundamental relationship between these adjacent terranes is one of tectonic collapse. The interpretation that this feature is a terrane boundary has been disputed by Carr and McMullen (2000) and Easton and Carr (2009). There are several field-based tests of the Carlson et al. (1990) tectonic model that can be applied, especially determining metamorphic conditions during shearing and constraining the age of deformation. This project will also bear on studies of the earlier Salerno Creek deformation zone (below).

The Salerno Creek deformation zone is the newly-proposed boundary between the CMBbtz and the Harvey-Cardiff domain of the Composite Arc Belt (Easton and Carr, 2009). This mylonite zone is well-mapped in part of the southern CMBbtz, so this project will focus on seeing if it is in fact a fundamental boundary across which rocks have experienced different histories. This zone separates two marble belts and appears to host plutonic rocks that do not appear to be present in adjacent terranes (Easton and Carr, 2009). Possible projects related to the Salerno Creek deformation zone include comparative igneous petrology of rocks within and outside of the zone and metamorphic petrology of similar lithologies astride the zone. This project includes enough questions and potential work that it could easily accommodate a second student, should that become necessary or desirable.

Field conditions and special logistics

Camping and hiking in at times moderately steep terrain. Possibility of wet + buggy conditions.

Course Preparation and special logistics

ALL STUDENTS MUST HOLD A U.S. PASSPORT FOR TRAVEL TO AND FROM CANADA!!  Students must have taken Mineralogy and Petrology and/or Structural Geology. Additional coursework in tectonics, geochemistry, or geologic field methods would be helpful.


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