MINNESOTA GEOLOGICAL SURVEY MATT WALTON, Director PETROLOGY OF SOME LOGAN DIABASE SILLS, COOK COUNTY, MINNESOTA Norris W. Jones Report of Investigations 29 l5TI ISSN 0076-9177 UNIVERSITY OF MINNESOTA Saint Paul - 1984 PETROLOGY OF SOME LOGAN DIABASE SILLS, COOK COUNTY, MINNESOTA CONTENTS Abstract •••• Introduction. • Field relations 2 Structure 2 Sills . 3 Sill A 4 Sill B 4 Sill C 6 Sill D 6 Dikes • • 7 Petrography • 7 Lower chilled margins 7 Plagioclase. • • 7 Mafic silicates. 7 Opaques •• 9 Quartz • • • • • • 9 Xenoliths. • • • 9 Lower fine-grained zones. 9 Sill A 9 Sill B 10 Sill C 11 Sill D • 12 Medium-grained zones. 12 Plagioclase. 13 Pyroxene •• 15 Olivine ••• 15 Fe-Ti oxides 15 Quartz and quartz-feldspar intergrowths. 18 Accessory minerals 18 Coarse-grained zones. 18 Plagioclase. 18 Pyroxene • • • • • 18 Fe-Ti oxides • • • 19 Quartz and quartz-feldspar intergrowths. 19 Accessory minerals • • • 19 Upper medium-grained zones. • 19 Porphyry or porphyritic zones 19 Plagioclase. 21 Pyroxene • • • • • 21 Fe-Ti oxides • • • 21 Quartz and quartz-feldspar intergrowths. • 21 Accessory minerals • 21 Upper fine-grained zones. 21 Upper chilled margins 22 Inclusions • • • • • 22 Pyroxene alteration • • • 23 Marginal alteration. 23 Pervasive or cleavage-related alteration 23 Patch alteration • • • • • • • 24 Relation to sill stratigraphy. 25 Rock chemistry •• 25 Mineral chemistry • 27 Plagioclase 27 Pyroxene •• 29 Olivine. 29 Biotite • 29 Amphibole 29 Fe-Ti oxides. 29 Quartz-feldspar intergrowths. 29 Discussion. • • • 3-2 Development of textural zones 33 Conclusions • • 34 Acknowledgments 34 References cited •• 34 Appendix - Chemical and microprobe analyses • 37 iii ILLUSTRATIONS Figure 1 • Location map. • • • • • • • • • • • • • • • • • 2 2. Map showing distribution of Logan intrusions. • • • • 3 3. Geologic map, northern part of South Lake quadrangle. 5 4. Block diagram • • • • • 5 5. Sill stratigraphy • • • • • 6 6. Grain size vs height in sills • • 8 7. Photomicrograph of chilled margin 10 8. Photomicrographs of Fe-Ti oxides in chilled margins • 10 9. Photomicrographs of textures of lower fine-grained zones. 11 10. Photomicrographs of textures of medium-grained zones. 14 1 1 • Photomicrograph of composite augite ophite. • • • • • 14 12. Photomicrograph of cluster of plagioclase crystals. • 14 13. Photomicrographs of olivine and alteration of olivine 16 14. Ppotomicrographs of textural types of ilmenite. • • 17 15. Photomicrograph of micrographic material. • • • • • 20 16. Photomicrograph of intergranular texture in coarse- grained zone. • • • • • • • • • • • • • • • • • 20 1 7. Photograph and photomicrograph of diabase porphyry •• 20 18. Photographs of upper chilled margin of sill C and inclusions. .. .. .. .. .. .. .. .. .. .. .. .. 22 19. Photomicrographs of pyroxene alteration • 24 20. Diagrammatic comparison of sills and MORB 25 21 • Alkali-silica plot. • • • • 26 22. Ne'-Q'-Ol' plot ••••••••••• 26 23. Plot of normative An-Ab-Or •••••• 26 24. Normative color index vs normative An • 27 25. Weight percent oxides vs Si02 and MgO 27 26. Weight percent oxides vs height in sills A and B. 28 27. Normative mineralogy vs height in sills A and B • 28 28. An-content vs height in sills • • • • • • • • • • • 29 29. Pyroxene compositions on the pyroxene quadrilateral 30 30. Pyroxene compositions on Ti-AIIV -NaM2 diagram ••• • 31 31 • Analyses of different textural types of pyroxene plotted on ~yroxene quadri lateral • • • • • • • • • 31 32. Fe2+/Fe + + Mg in pyroxenes vs height in sills ••••••• 32 TABLES Table 1. Stratigraphic succession for Keweenawan rocks in northeastern Minnesota. • • • • 2 2. Textural zones and observed fabrics 6 3. Modal analyses. • • • • • • • • • • • 13 The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, creed, color, sex, national origin, or handicap. iv PETROLOGY OF SOME LOGAN DIABASE SILLS, COOK COUNTY, MINNESOTA by Norris W. Jones ABSTRACT Four diabase sills (Logan intrusions; middle Proterozoic; Keweenawan) between South and Birch Lakes, South Lake quadrangle, northeastern Minne­ sota, have the following general sequence of textural zones: lower chilled margin, lower fine-grained, medium-grained, coarse-grained, porphyry or porphyritic, upper fine-grained, and upper chilled margin. Chilled margins contain scattered phenocrysts of plagioclase in a matrix of plagioclase, amphibole and/or biotite, quartz, and granular, acicular, or skeletal ilmenite. The fine- and medium-grained zones consist of plagioclase, augite, interstitial quartz, acicular apatite, some K­ feldspar, and embayed, skeletal, or lath-shaped ilmenite; pigeonite and partially resorbed iron-rich olivine are present in some samples. Coarse­ grained zones are distinguished by the abundance and variety of intergrowths between quartz and K-feldspar or sodic plagioclase; pyroxene is typically more altered than in finer grained zones. Porphyritic zones contain pla­ gioclase phenocrysts which appear to be slightly more calcic than the matrix plagioclase; other aspects of the mineralogy are similar to the medium­ grained zones. Deuteric alteration is extensive in all zones, but the intensity, distribution, and exact nature of the alteration are variable, even on the scale of a thin section. Rock analyses for major oxides and microprobe analyses of feldspar, pyroxene, and olivine show that: (1) there is little variation in bulk or mineral chemistry in the fine- and medium-grained zones; (2) the composition of cores of groundmass plagioclase crystals is An4S-S2' except in the coarse­ grained zones in which it is An4-9; phenocryst compositions range from AnS4 to AnS9; (3) augite compositions are similar throughout, mostly in the range W030-40En3S-4SFs20-30; (4) chilled margins are notably richer in Si02 and poorer in CaO and alkalis than other zones; (S) coarse-grained zones are enriched in Si02 and alkalis, depleted in CaO and MgO, have high Fe203/FeO ratios, and contain plagioclase which is notably more sodic than other zones; (6) porphyritic zones are lower in Fe, MgO, and Ti02 and richer in alkalis and A1 203 , reflecting the high percentage of plagioclase. These data suggest emplacement of a magma containing plagioclase phenocrysts, minor assimilation of adjacent Rove Formation, concentration of plagioclase phenocrysts in the upper parts of the sills, crystallization from the margins inward with little or no gravity segregation of newly crystallizing phases, and migration of silica, alkalis, and water toward the coarse-grained zones, which were the last to solidify. INTRODUCTION Northeastern Minnesota is characterized by informally referred to as the "Logan sills" intrusive and extrusive igneous rocks of middle (Lawson, 1893) or the "Logan intrusives" (Grout Proterozoic (Keweenawan) age (fig. 1). Largest and Schwartz, 1933). The term has been used in volume are the Duluth Complex (see ~'ieiblen rather loosely to describe most of the Keweenaw­ and Morey, 1980, for a recent summary) and the an diabase sills and dikes in northeastern North Shore Volcanic Group (Green, 1972). NU­ Minnesota and adjacent Ontario. However, Geul merous smaller mafic intrusive complexes also (1970) has demonstrated that all "Logan" dia­ have been delineated (Sims, 1970, and fig. 1). bases in the adjoining Thunder Bay District, Age relationships of the Keweenawan rocks are Ontario, are not of the same age; he recognized given in Table 1. two major types: (1) Early Mafic intrusions of tholeiitic diabase, and (2) younger pigeon River The best known and most extensively exposed intrusions of olivine diabase. The paleomagnet­ small intrusions are diabases which have been ic data of Du Bois (1962), Palmer (1970), and weeks were spent in the field collecting samples r-c:77"'::;=-________---=9:;..lo _________ 9:;O:...o ____ • and mapping the stratigraphy of these sills. Samples were collected at height intervals of about 3 m along one traverse through each of the f'i----;:;------,;=-----;----;,--,-------1 sills and at other selected locations. A Jacob's staff was used to determine sill thicknesses and r-~~EE~~~~~~~~~~ stratigraphic positions within sills • .- Thin sections of 135 rocks were examined. Grain sizes were determined as they appeared in / thin sections and average sizes were estimated; therefore, grain sizes are useful for intrasill / but not intersill comparisons. Shore Volcanic Group / FIELD RELATIONS ,,~-.---_-....L1' The northern third of the South Lake quad­ ,,/' rangle is characterized by east-trending hills /' which slope steeply to the north and gently to the south. Southward-dipping Logan diabase sills underlie the hills, and less-resistant o 5 10 20 30 Miles [f argi llite and graywacke of the Rove Formation 6 '10 '20 30' 40 50 Kilometers ''--'-.......-'--'- .... underlie the valleys between hills. Many of the valleys contain lakes. The southern two-thirds of the quadrangle is underlain by the northern Figure 1 • Generalized geologic map of north­ prong of the Duluth Complex, which consists of a eastern Minnesota (after Sims, 1970). Most layered series composed of 27 different units Logan intrusions are in the area labeled Rove (Nathan, 1969; Morey and Nathan, 1977). Field Formation and diabase. and paleomagnetic data (Beck, 1970; Beck and Lindsley, 1969) indicate that these rocks are older than other parts of the Duluth Complex, Table 1. Stratigraphic succession for Keweenawan and the layered series appears to be struc­ rock~ in northeastern Minnesota