CARBONATITES AND ALKALINE ROCKS IN THE SOUTHERN CANADIAN CORDILLERA Leo J. Millonig1*, Axel Gerdes2, Lee A. Groat1 1. DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, BC, CANADA, V6T 1Z4 [*CORRESPONDENCE: LMILLONIG@EOS.UBC.CA] 2. INSTITUT FUER GEOWISSENSCHAFTEN, ALTENHOEFERALLEE 1, D-60438 FRANKFURT AM MAIN, GERMANYSCOPE AND GEOLOGICAL SETTINGIn British Columbia (B.C.) carbonatites, nepheline and sodalite syenite gneisses and related alkaline rocks are found in a zone which is parallel to,and on either side of the Rocky Mountain Trench. From the 14 known carbonatite occurrences in B.C. (Fig. 1) 7 have been dated with reportedintrusion ages between 356 Ma (Ice River Complex) and 202 Ma (Howard Creek). Associated syenite complexes are either ~coeval (Trident Moun-tain) or significantly older (Mount Copeland Syenite). However, most of the intrusion ages cluster around Mississippian time. Subsequently allcarbonatites and alkaline rocks were affected by (multiple?) greenschist to amphibolitefacies metamorphism. The scope of this study is to providereliable age data through LA-ICP-MS U-Pb age dating of zircons of previously undated and/or unknown carbonatites and accompanying alkalinerocks in British Columbia in order to better understand the mineralogical and chemical differences between the individual magmatic bodies and thegeodynamic setting during their emplacement. 10 carbonatite, 2 mafic and 4 syenite samples are currently being processed in this study. FIG. 1: a) Carbonatites (black) and associated alkaline rocks (blue) in B.C. with their interpreted intrusion ages (modified after Woolley & Kjarsgaard, 2008). b) Distribution of miogeoclinal sediments in B.C. in which the carbonatite and alkaline rocks intruded (Lund et al., 2010).How old are the undated carbonatites/alkaline rocks?Is the spread in interpreted intrusion ages (202-356 Ma) real? GEODYNAMIC BACKGROUND • Miogeoclinal sediments of the Windermere Supergroup were deposited during the initial break-up of Rodinia in the Neoproterozoic at <750 Ma (Fig. 2). • Final separation of Siberia and Laurentia occurred during the Early Cambrian at ~540 Ma and during the Middle Devonian the passive continental margin of western Laurentia became an active margin (Fig. 3). • Extensional tectonics in the Early Carboniferous lead to back-arc basin formation at ~355-320 Ma (Fig. 3). FIG. 2: The Windermere Supergroup FIG. 3: Geodynamic evolution of the western margin of Laurentia during Early (Nelson & Colpron, 2007). Paleozoic to Permian time (modified after Monger, 2008).
CARBONATITES AND ALKALINE ROCKS IN THE SOUTHERN CANADIAN CORDILLERA Leo J. Millonig1*, Axel Gerdes2, Lee A. Groat1 1. DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, BC, CANADA, V6T 1Z4 [*CORRESPONDENCE: LMILLONIG@EOS.UBC.CA] 2. INSTITUT FUER GEOWISSENSCHAFTEN, ALTENHOEFERALLEE 1, D-60438 FRANKFURT AM MAIN, GERMANYSAMPLE LOCALITIES AND LA-ICP-MS U-PB DATINGBlue River area Perry River area FIG 4.: Carbonatites in the Blue River area.Carbonatites in the Blue River area (Fig. 4) are numerous and widely dispersed withinterpreted intrusion ages between 202 Ma (Howard Creek) and 351 Ma (Paradise Lake).In the Perry River Area (Fig. 5) carbonatites and alkaline rocks intruded the mantlinggneiss succession of the Frenchman Cap Dome and are either of Cryogenian age FIG 4.: Carbonatites and syenites in the(Mount Copeland syenite) or Cambrian age (Mount Grace carbonatite) based on Perry River area.stratigraphic relationships.Serpentine Creek carbonatite – Blue River Area FIG 7.: Concordia diagram of U-Pb zircon analyses from the Serpentine Creek carbonatite with aninterpreted intrusion age of FIG 6.: Zircons from the Serpentine Creek ~340 Ma and carbonatite. a) Image of euhedral transparent short Pb-loss during a prismatic zircons. b+c ) CL - images of the zircons metamorphic revealing oscillatory magmatic zoning in some cores overprint at ~80 and alteration/zircon growth along the rims. Ma.Little Chicago carbonatite – Blue River Area FIG 9.: Concordia diagram of U-PB zircon analyses from the Little Chicago carbonatite with an interpreted intrusion age of FIG 8.:Zircons from the Little Chicago carbonatite ~490 Ma and a) Image of euhedral transparent to yellow short Pb-loss during prismatic zircons. b+c) CL - images of the zircons metamorphism at revealing oscillatory magmatic zoning in some cores and ~156 Ma. various forms and degrees of alteration and/or zircon growth.
CARBONATITES AND ALKALINE ROCKS IN THE SOUTHERN CANADIAN CORDILLERA Leo J. Millonig1*, Axel Gerdes2, Lee A. Groat1 1. DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, BC, CANADA, V6T 1Z4 [*CORRESPONDENCE: LMILLONIG@EOS.UBC.CA] 2. INSTITUT FUER GEOWISSENSCHAFTEN, ALTENHOEFERALLEE 1, D-60438 FRANKFURT AM MAIN, GERMANYPerry River syenite FIG 10.: Zircons from the Perry River syenite. a) Image of ±euhedral slightly elongated prismatic zircons. b+c) CL-images of the zircons revealing oscillatory magmatic zoning and different degrees of alterartion in some cores as well as alteration/zircon growth along the rims. d) Concordia diagram of U-Pb zircon analyses from the Perry River syenite with an interpreted intrusion age of ~800 Ma and Pb-loss during a metamorphic overprint at ~50 Ma.RESULTS AND DISCUSSION • The Serpentine Creek carbonatite was emplaced during extensional tectonics in Mississipian times at 340 Ma. • The intrusion of the Little Chicago carbonatite at 489 Ma is interpreted to be related to the separation of Laurentia and Siberia. No carbonatites of comparable age have been documented in the Canadian Cordillera yet, but are known from the Cordillera in the USA! • The Perry River syenite is interpreted to be part of the Mount Copeland syenite and intruded during the initial stages of the break-up of Rodinia at ~800 Ma. Carbonatitic and alkaline magmatism in the Canadian Cordillera spans ~460 Ma and is linked to several episodes of extensional tectonics.REFERENCESLund, K., Aleinikoff, J.N., Evans, K.V. duBray, E.A., Dewitt, E.H., Unruh, D.M., 2010: SHRIMP U-Pb dating of recurrent Cryogenian and LateCambrian–Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics. GSA Bulletin, v. 122, no. 3/4, p. 430–453.Monger, J.W.H., 2008: Evolution of Canada’s Western Mountains. Geological Survey of Canada, Open File 5804, 1 poster. Nelson, J., and Colpron,M., 2007: Tectonics and metallogeny of the British Columbia, Yukon and Alaskan Cordillera, 1.8 Ga to the present. In Goodfellow, W.D., ed.,Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and ExplorationMethods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 755-791. Woolley, A.R. & Kjarsgaard, B.A.,2008: Carbonatite occurrences of the world: Map and database. Geological Survey of Canada, Open File 5796.ACKNOWLEDGMENTSThe author is grateful to Zimtu Capital Corp. for funding this project and the geologist from Dahrouge Geological Consulting whose work in the BlueRiver area dicovered several new carbonatite occurences. Special thanks go to Rob Bergman who dicovered the Little Chicago carbonatite in 2008.Support and help during sample preparation from the geochronolgy unit at the Earth and Ocean Science Department of the UBC is gratefullyacknowledged.