RI-29 Petrology of Some Logan Diabase Sills, Cook County, Minnesota

Abstract

Four diabase sills (Logan intrusions; middle Proterozoic; Keweenawan) between South and Birch Lakes, South Lake quadrangle, northeastern Minnesota, 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 Kfeldspar, and embayed, skeletal, or lath-shaped ilmenite; pigeonite and partially resorbed iron-rich olivine are present in some samples. Coarsegrained 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 plagioclase phenocrysts which appear to be slightly more calcic than the matrix plagioclase; other aspects of the mineralogy are similar to the mediumgrained 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 An45-52, except in the coarsegrained zones in which it is An4-9; phenocryst compositions range from An54 to An59; (3) augite compositions are similar throughout, mostly in the range W030-40En35-45Fs20-30; (4) chilled margins are notably richer in Si02 and poorer in CaO and alkalis than other zones; (S) coarse-grained zones are enriched in SiO2 and alkalis, depleted in CaO and MgO, have high Fe2O3/FeO ratios, and contain plagioclase which is notably more sodic than other zones; (6) porphyritic zones are lower in Fe, MgO, and TiO2 and richer in alkalis and Al2O3 , 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.