GEOLOGY AND GEOCHRONOMETRY OF THE COGBURN CREEK-SETTLER CREEK AREA, NORTHEAST OF HARRISON LAKE, B.C. by JANET ELIZABETH GABITES B.Sc, Victoria University Of Wellington, New Zealand, 1973, B.Sc.(Honours), Victoria University Of Wellington, New Zealand, 1975 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department Of Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1985 © Janet Elizabeth Gabites, 1985 in presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 n.t„ 8 October 1985 DE-6(3/81) ii Abstract Metamorphic supracrustal rocks in the Cogburn Creek area belong to the Cogburn Creek Group and the Settler Schist. These are separated by a melange zone, which has been correlated with the Shuksan thrust zone and contains Baird Metadiorite and ultramafic rocks, and intruded by the Spuzzum batholith and minor younger granodiorite. Three phases of folding are recognised in the schist units: fi is associated with contact metamorphism that preceded regional metamorphism, f2 produced pervasive mica foliation and tight folds, and kinks and broad warps are associated with f3, which was locally pervasive approaching pluton margins. Mineral assemblages indicate increasing metamorphic grade from west to east from garnet to garnet-staurolite, andesine-epidote amphibolite, staurolite- kyanite, fibrolite, and coarse sillimanite zones. Metamorphic conditions vary from 300 to 500 °C in Cogburn Creek Group rocks to 550 to 700 °C at 6 to 8 kbar for pelites in the Settler Schist. Conditions deduced for metamorphism of the ultramafic rocks are consistent with those for enclosing pelitic schists. Geochronometry indicates that the Baird Metadiorite is probably Precambrian and equivalent to the Yellow Aster Complex of the North Cascade Mountains, Washington. The Cogburn Creek Group was dated as Late Paleozoic (296 ± 58 Ma, Rb-Sr WR isochron), and is provisionally correlated with the Bridge River Group. The protolith of the Settler Schist was deposited around 210 ± 27 Ma (Rb-Sr WR isochron), and it contains 2450 ± 230 Ma detrital zircon indicating partial ultimate derivation from iii Precambrian basement rocks. The Spuzzum batholith was intruded at 95 to 110 Ma, before the culmination of regional metamorphism. Rb-Sr biotite dates from all units and K-Ar Hb isochron dates in the range 66 to 88 Ma are metamorphic cooling dates. The youngest intrusive rocks, granodiorite dated at 32 ± 2 Ma to 42 ± 14 Ma, postdate the regional metamorphic and intrusive event. Movement on the Shuksan Thrust is bracketed as Albian, after regional blueschist metamorphism of the Shuksan Suite in the North Cascade Mountains and before intrusion of Spuzzum batholith and regional metamorphism east of Harrison Lake. iv Table of Contents Abstract ii List of figures vii List of plates xi List of tables xii Acknowledgements xiii 1 Introduction 1 1.1 Geographic location and access 1 1.2 Previous work 1 1.3 Regional geology 4 2 Geology of the Cogburn Creek area 9 2.1 Baird Metadiorite 12 2.2 Ultramafic rocks 14 2.3 Cogburn Creek Group 15 2.4 Settler Schist 18 2.5 Premetamorphic intrusive rocks 21 2.6 Foliated diorite in melange zone 22 2.7 Spuzzum Batholith ' 23 2.8 Younger Intrusives 24 2.9 Breakenridge Formation gneiss 25 3 Metamorphism 27 3.1 Baird Metadiorite 27 3.2 Ultramafic rocks 27 3.3 Cogburn Creek Group 28 3.4 Settler Schist 31 3.5 Premetamorphic intrusive rocks 35 3.6 Discussion 36 V Ultramafic rocks 36 Contact metamorphism 36 Regional metamorphism 39 4 Structure 50 5 Geochronometry 59 5.1 Previous Geochronometry 59 5.2 Baird Metadiorite 64 5.3 Cogburn Creek Group 66 5.4 Settler Schist 67 5.5 Premetamorphic intrusive rocks 70 5.6 Foliated diorite in fault zone 72 5.7 Spuzzum batholith 72 5.8 Agmatised quartz diorite" 77 5.9 Breakenridge Formation 77 5.10 Discussion 78 6 Regional Synthesis 107 Plates 115 References 125 Appendix A. Isotopic dating methods. K-Ar, Rb-Sr, zircon U-Pb 134 Appendix B. Rb-Sr isotopic data ; 136 Appendix C. Dating sample descriptions, locations 147 Zircon descriptions 149 Appendix D. Thin section location map 150 Geochronometry sample location map 151 Appendix E. Rb-Sr data and isochron for Chilliwack batholith in the North Cascades Mountains 152 Maps • in pocket vi List of Figures Figure 1.1 Area Location Map 2 Figure 1.2 Regional Geology of the North Cascade Mountains and southern Coast Mountains 5 Figure 1.3 Geographical location of the Settler Schist and Chiwaukum Schist 8 Figure 2.1 Generalised geology of the Cogburn Creek Area .. 10 Figure 3.1a Timing of mineral growth with respect to deformation, Cogburn Creek Group 29 Figure 3.1b Timing of mineral growth with respect to deformation, Settler Schist 29 Figure 3.2 Schematic T-X(C02) diagram for the system MgO- Si02-H20-C02 at elevated pressures and temperatures, adapted from Johannes (1969) 37 Figure 3.3 Pressure-temperature conditions in the pelitic assemblages, from Pigage (1973) 38 Figure 3.4 Metamorphic mineral assemblages in pelites and greenschist from the Cogburn Creek Group ... 40 Figure 3.5 Metamorphic mineral assemblages in pelites from the Settler Schist 41 Figure 3.6 Equilibria calculated for P2 at a(H2O)=0.8l, plus the alumino-silicate equilibria, adapted from Bartholomew (1979) 44 Figure 3.7 Map of metamorphic mineral assemblages and isograds around Cogburn Creek 46 Figure 3.8 Regional isograds 47 Figure 4.1 Equal area projection for poles to compositional vii layering in the Settler Schist 51 Figure 4.2 Equal area projection for poles to f2 foliation for the Settler Schist south of Cogburn Creek .. 52 Figure 4.3 Equal area projection for poles to f2 foliation in Settler Schist north of Cogburn Creek 54 Figure 4.4 Equal area projection for poles to f2 foliation in Cogburn Creek Group 55 Figure 4.5 Equal area projection for poles to foliation and mineral lineations in Spuzzum batholith .... 58 Figure 5.1 Map showing geochronometry of the Harrison Lake - Fraser River region 63 Figure 5.2a Rb-Sr isochron plot for Baird Metadiorite 88 Figure 5.2b Rb-Sr isochron plot for Cogburn Creek Group .... 89 Figure 5.2c Rb-Sr isochron plot for Settler Schist 90 Figure 5.2d Settler Schist, expanded scale 91 Figure 5.2e Rb-Sr isochron plot for premetamorphic intrusive rocks 92 Figure 5.2f Rb-Sr isochron plot for small body of foliated granodiorite in imbricate zone (SD92) 93 Figure 5.2g Rb-Sr isochron plot for Spuzzum batholith, Hut Creek body 94 Figure 5.2h Rb-Sr isochron plot for Spuzzum batholith, Settler Creek body 94 Figure 5.2i Rb-Sr isochron plot for agmatitic quartz diorite (SD14), marginal to Cogburn Granodiorite 96 Figure 5.2j Rb-Sr isochron plot for Breakenridge Formation gneiss 97 Figure 5.2k Rb-Sr isochron plot for Chilliwack Group 98 viii Figure 5.21 Rb-Sr isochron plot for Bridge River Group 99 Figure 5.2m Rb-Sr isochron plots for Shuksan Suite and Darrington Phyllite 100 Figure 5.3 U-Pb concordia diagram for zircon dating 101 Figure 5.4a Plot of aoK/36Ar v. 40Ar/36Ar for hornblende separates from Spuzzum batholith 102 Figure 5.4b Plot of %K v. 40Ar nl/g for hornblende separates from Spuzzum batholith 102 Figure 5.5 U-Pb concordia diagram for Yellow Aster Complex, Skagit Gneiss and Swakane Gneiss, from Mattinson (1972) 103 Figure 5.6 Data-field diagram for Rb-Sr analyses from possible correlative stratigraphic units 104 Figure 5.7 Plot of %K v. *°Ar nl/g for regional data from Spuzzum batholith 105 Figure 5.8 Graph of eastward younging trend of K-Ar dates from Spuzzum batholith, from Bartholomew (1979).106 Figure 6.1 Tectono-stratigraphic terranes, modified from Monger and Berg (1984) 108 Figure 6.2 Reconstruction of Bridge River Group and Methow terrane, modified from Kleinspehn (1985) and Monger and Berg (1984) 109 Figure 6.3 Reconstructions of Settler Schist and Chiwaukum Schist, after Misch (1977) and Monger (1985) ...111 Figure 6.4 Partial reconstruction of the Methow-Tyaughton basin, modified from Kleinspehn (1985) 112 Figure D-1 Map of Cogburn Creek area showing locations of samples studied in thin section 150 ix Figure D-2 Map showing locations of geochronometry samples, Cogburn Creek area 151 Figure E-1 Rb-Sr isochron plot for Chilliwack Batholith ...153 Map 1 Topographic map showing rock units -i-R—pee-ket Map 2 Map showing planar structures y^^e^^A P©G-k-et Map 3 Map showing linear structures CalU^*^ in-_paql.ei Map 4 Topographic map showing station numbers in—pos-k-e-t V_ X List of Plates Frontispiece:01d Settler Mountain, looking south along Settler Creek xiv Plate 2.1 Imbricate zone: Baird Metadoirite, ultramafic rocks, Settler Schist 115 Plate 2.2 Mafic amphibolite pod in Settler Schist 115 Plate 2.3 Photomicrograph of sample SD101 116 Plate 2.4 Photomicrograph of sample SD66 116 Plate 3.1 Photomicrograph of sample HL30 117 Plate 3.2 Photomicrograph of sample HL16 117 Plate 3.3 Photomicrograph of sample HL15 118 Plate 3.4 Photomicrograph of sample HL80 118 Plate 3.5 Photomicrograph of sample HL142 119 Plate 3.6 Photomicrograph of sample SS110 119 Plate 3.7 Photomicrograph of sample SSI 35 120 Plate 3.8 Photomicrograph of sample SS66 120 Plate 3.9 Twinned staurolite in SS57, Settler Schist 121 Plate 3.10 Photomicrograph of sample SS114 121 Plate 3.11 Photomicrograph of sample SS53 122 Plate 3.12 Sillimanite porphyroblasts from ridge north of Cogburn Creek 122 Plate 3.13 Photomicrograph of sample SSl82a 123 Plate 4.1 Fault contact at the base of the imbricate zone .124 xi List of Tables Table 5.1 Table of dates and age estimates from previous studies between Harrison Lake and Fraser River .