SP-08 The Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota

Abstract

The Isaac Lake quadrangle lies between the Mesabi and Vermilion ranges, and includes a segment of each. The Lower Precambrian rocks that characterize the Vermilion range underlie the northern 2/3 of the quadrangle, and include both the Ely Greenstone (with interbedded iron-formation) and an overlying sequence of interlayered metasedimentary and meta-volcanic rocks. These rocks strike northwestward and dip steeply northeast or southwest; they were folded and intruded by the Giants Range Granite during the Algoman orogeny (2.5 b. y.). The grade of regional metamorphism increases southeastward along strike from Tower into the Isaac Lake quadrangle; the Ely Greenstone and meta-sedimentary and meta-volcanic rocks within the quadrangle contain mineral assemblages characteristic of the upper amphibolite facies. The well known Middle Precambrian Animikie Group of the Mesabi range uncomformably overlies the Giants Range Granite in the southeastern corner of the quadrangle and these rocks have been intruded by dikes of Keweenawan (?) age and metamorphosed by the Middle Keweenawan Duluth Complex. The basalts and tuffaceous sedimentary rocks of the Ely Greenstone are metamorphosed to massive or schistose amphibolites, and contain thin lenses of cherty iron-formation. The overlying highly metamorphosed sedimentary and volcanic rocks have been informally divided into two units, the layered gneiss and the (overlying ?) Argo gneiss. The Argo gneiss consists of fine-grained weakly foliated biotite gneisses with rare thin mafic layers; these gneisses are metamorphic equivalents of greenschist facies slates and graywackes which lie along strike to the northwest. The layered gneiss comprises amphibolite and leucocratic biotite and hornblende gneisses, interlayered on all scales and well-foliated parallel to the compositional layering. Thick layers of very coarse-grained biotite gneiss within the layered gneiss apparently were partially mobile during metamorphism. The layered gneisses are correlative with less-metamorphosed volcanic and volcaniclastic rocks to the northwest. The higher-grade parts of the layered gneiss are migmatized by fme-grained leucotrondhjemites that were foliated by the regional metamorphism, recrystallized to gneissic textures, and cut by granitic dikes. The Algoman Giants Range Granite intrudes the older rocks and underlies the southern half and the northeastern corner of the quadrangle. It is predominantly a coarse-grained hornblende granodiorite or monzonite. The granite is strongly foliated parallel to its contacts and is generally conformable to the structure of the country rocks. The batholith is interpreted as a late-kinematic forceful intrusion, emplaced at relatively shallow depths. Two major transcurrent faults strike northeastward across the quadrangle. The western one, here named the Waasa fault, offsets the granite-gneiss contact three and three-fourth miles. The eastern, or Camp Rivard fault offsets this contact about two miles. Contacts and fold axes within the gneisses appear to be offset even further, and other evidence also suggests that the faults were active both before and after emplacement of the granite. Numerous smaller faults are sub-parallel to the larger ones. Rocks north of the Waasa fault dip steeply northeast; those between the two faults generally dip steeply south west. Statistical analysis of foliations and lineations suggests non-cylindrical folding about a northwest-trending axis. In the northwestern corner of the quadrangle there has been minor later crossfolding about an axis perpendicular to the axis of the major folding. The Middle Precambrian Animikie Group, in the East Mesabi district of the Mesabi range consists of three comformable sedimentary formations, the Pokegama Quartzite at the base, the Biwabik Ironformation, and the Virginia Formation at the top. They are exposed in the southeastern part of the quadrangle where they lie unconformably on the Giants Range Granite. The Pokegama Quartzite fills minor topographic irregularities in the older granite surface; accordingly, it is variable in thickness, ranging from near zero to approximately 30 feet. The Biwabik Iron-formation is approximately 400 feet thick and is subdivided into seven cartographic units on the basis of bedding characteristics, texture, and gross mineralogy. These units are readily correlated with those previously recognized by Wolff (1917) and Gundersen (1960). The Virginia Formation is not exposed in the quadrangle and is known only through diamond drilling. The structure of the Animikie Group is relatively simple. The beds dip 5° - 15° SE, but the lower part of the Biwabik Iron-formation is warped by several small-scale folds whose axes trend north-northwest and plunge south-southeast at low angles. Structural closure on each fold seems to decrease upward so that near the top of the iron-formation the small-scale folds are no longer apparent. The Animikie Group was intruded by gabbro dikes of possible Middle Keweenawan age, and metamorphosed by the Middle Keweenawan Duluth Complex. The formation is characterized by abundant prograde and retrograde cummingtonite with minor amounts of prograde fayalite, orthopyroxene, hedenbergite, and diopside.