Metamorphic history of Nuvvuagittuq greenstone belt, Northeastern Superior Province, Northern Quebec, Canada

Transcription

1 Metamorphic history of Nuvvuagittuq greenstone belt, Northeastern Superior Province, Northern Quebec, Canada By: Majnoon, M., Supervisor: Minarik, W.G., Committee members: Hynes, A., Trzcienski, W.E. 1. Introduction The Nuvvuagituq greenstone belt (NGB) is located on the eastern coast of Hudson Bay, in the Northeastern Superior Province (NESP) of Canada, approximately 30 km south of the municipality of Inukjuak. NESP dominantly consists of Neoarchean plutonic suites in which amphibolite- to granulite-grade greenstone belts occur as relatively thin keels (1-10 km) that can be traced continuously for up to 150 km along strike (Leclair, 2005). However, the NGB reveals volcano-sedimentary succession that takes place as a tight to isoclinal synform refolded into a more open south-plunging synform, with bedding largely parallel to the main schistosity (Fig. 1). It is surrounded by a 3.6 Ga tonalite, itself encompassed by a younger 2.75 Ga tonalite (Stevenson and Bizzarro, 2006). 142 Nd work on the NGB (O Neil et al., 2008) suggests that it formed at ~4.28 Ga, making it the only know remnant of Hadean crust preserved on Earth. Regarding this age, petrology, mineral chemistry study and determination of the pressure-temperature-time (P-T-t) paths of different metamorphic events of the NGB will help us to understand the meaning of the obtained ages. 2. Research objectives The main objective of the present project is to study the preserved metamorphic history of the NGB. I will determine the minerals present, their zoning and composition,

2 and find evidence for reactions. The petrography will be combined with bulk composition (mainly from O Neil thesis, 2009) and thermodynamics to estimate temperature, pressure and timing of assemblage crystallization. 3. General description My study focuses on two of the mapped units within the NGB: the Faux Amphibolite and the Greenschist (as mapped in O Neil et al., 2007). The Faux Amphibolite will be referred to as the amphibolite in the rest of the proposal since it contains various amphibole minerals and was metamorphosed within the amphibolite facies. The mineral assemblage in the amphibolite is mainly consisting of amphibole (cummingtonitehornblende- anthophylite- actinulite- tremolite) / quartz / mica (biotite- phlogopite) / plagioclase / garnet / spinel / chlorite (Mg-rich Chlorite- Fe-rich Chlorite) / epidote (epidote- clinozoisite) / pyrite / zircon / allanite / monazite / rutile / titanite. Its composition ranges from basalt to basaltic andesite (SiO 2 = wt.%), and is divided into two distinct geochemical groups that are stratigraphically separated by a BIF. Lower Al2O3 and higher TiO2 recognize one group, whereas high Al 2 O 3 and low TiO 2 recognize another one (O Neil et al., 2007). The different geochemical groups of amphibolite appear to be cogenetic with ultramafic sills intruding the NGB following the same stratigraphic order (O Neil, 2009). The NGB preserved four structural events, which from oldest to youngest are: (1) Intense, layer-parallel shearing accompanied by sheath-fold development, with a uniform vergence; (2) Isoclinal folding of the shear fabric of first metamorphic event; (3) Development of the regional SE-plunging synform; and (4) Sinistral shearing (Hynes, 2009). 1

3 Figure 1. Geological map of the NGB. Coordinates: UTM zone 18 NAD 27 (O Neil et al., 2007). 2

4 4. Research plan In order to reach the objective of this project, the following approaches are proposed: Petrography, mineral chemistry study and investigating the preserved reaction history in the amphibolite and identifying varying metamorphic sub-facies Geothermobarometry of the amphibolite (P-T paths) Accessory mineral chemical dating (P-T-t paths) Relate the timing of monazite growth to other reaction evidence in order to constrain the pressure temperature time paths followed by the Amphibolite 4.1. Petrography, mineral chemistry study and investigating the preserved reaction history in the amphibolite and identifying the varying metamorphic sub-facies To understand mineral changes driving the mineral reactions and forming specific mineral paragenesis, a detailed petrography and mineralogy of rocks is needed. This study is supplemented with whole-rock chemistry (O Neil, 2009) and will be supplemented with petrography and mineral chemistry in order to find possible mineral reactions. Through analyzing the results, it s possible to find the different metamorphic sub-facies and the reaction history of a specific metamorphic event Geothermobarometry of the amphibolite (P-T paths) Using classical thermobarometry, we will determine the amphibolite temperature and pressure conditions. Another convenient tool for modeling P-T conditions is to employ a thermodynamics program, e.g. Perple-X using whole-rock chemistry data. Resultant pseudosections will suggest pressure and temperature conditions for possible reactant s assemblages. 3

5 4.3. Accessory mineral chemical dating (P-T-t paths) Monazite chemical dating is one of the relatively recent techniques for U-Th-Pb chronology. Monazite is a light rare earth element (LREE)-bearing phosphate mineral that present in a wide variety of rock types that can preserve crystallization ages through a long history of geological events. Monazite crystals typically contain distinct compositional domains that represent successive generations of monazite, which in turn, can provide a detailed record of the geologic history of its host rocks. The Electron Microprobe (EMP) can be used to characterize the geometry of compositional domains, to analyze the composition of each domain, and, when carefully configured, to determine the U-Th-total Pb age for domains as small as 5 µm in width (Williams et al., 2007). Undertaking EMP technique to recognize different chemical composition domains of monazites is suggested to examine whether the observed zones are related to different metamorphic events (Fig. 2). A B Figure 2. A zoned monazite from Porpoise Cove amphibolite. A) Backscattered electron image. 2) X-Ray map of Th concentration. Notice to the three (at least) sharp zone boundaries in the monazite Distinguishing between metamorphic events (multiple P-T-t paths) These rocks have likely been deformed and metamorphosed many times. With petrography, we may find the traces of prior metamorphic events incompletely erased; 4

6 these traces will allow us to extend our knowledge of the metamorphic history of the amphibolite back towards previous metamorphic events. 5. Concluding remarks Our understanding of the Earth s early evolution is limited and initial geological processes remain poorly constrained. The NGB represents a great opportunity to study one of the oldest suites on the world. Focus on the petrology and geochemistry of these rocks and finding their P-T-t conditions are proposed in order to construct a picture of what happened in this area in the past. Estimating the pressure, temperature and timing of the rocks, it shall be possible to imagine how some of the geological processes effected Nuvvuagittuq in the past. 6. References Hynes, A. (2009): Porpoise Cove Structural Summary, unpublished. Leclair, A.D. (2005): Géologie du nord-est de la Province du Supérieur, Québec; Ministère des Ressources naturelles et de la Faune, DV , 19 pages, 1 carte (échelle 1: ). O'Neil, J., Maurice, C., Stevenson, R.K., Larocque, J., Cloquet, C., David, J. and Francis, D. (2007): The geology of the 3.8 Ga Nuvvuagittuq (Porpoise Cove) greenstone belt, northeastern Superior Province, Canada. In: M. van Kranendonk, H. Smithies and V. Bennett, Editors, Earth's Oldest Rocks, Developments in Precambrian Geology vol. 15, pp O'Neil, J., Carlson, R.W., Francis, D. and Stevenson, R.K. (2008): Neodymium-142 evidence for Hadean mafic crust, Science 321 (5897), pp O'Neil, J. (2009): Implications of the Nuvvuagittuq faux-amphibolite for the formation of Earth s early crust, unpublished. Stevenson, R.K. and Bizzarro, M. (2006): Petrology and petrogenesis of the Eoarchean Nuvvuagittuq tonalite suite: Abstract in Joint Annual Meeting; Geological Association of Canada, Mineralogical Association of Canada and Society of Economic Geologists, Montreal, Qc. Canada volume 31. Williams, M.L., Jercinovic, M.J. and Hetherington, C.J. (2007): Microprobe Monazite Geochronology: Understanding Geologic Processes by Integrating Composition and Chronology. Annu. Rev. Earth Planet. Sci. 35: