The Geology of the Early Precambrian Rocks of the Jasper Lake Area, Cook County, Northeastern Minnesota

Abstract

The rocks of the Jasper Lake area, located within the eastern Vermilion district in Cook County, northeastern Minnesota, represent the basal portion of a thick (3500-6400 meters) metavolcanic-metasedimentary sequence. The area contains three dominantly igneous units: a greenstone unit, a pyroclastic unit, and an andesite intrusive unit. The rocks within the district were shown to have been complexly faulted and isoclinally folded by Gruner (1941). All units have been metamorphosed to greenschist facies. The oldest unit consists of predominantly massive metavolcanics (including basalt, diabase, andesite, and minor dacite) and herein is referred to as the Jasper Lake greenstone. The unit is linear in outline, 1000-1500 meters thick, vertical, and trends east-west. Based upon the presence of pillow structures and quench textures observed in the basalts at several localities, these rocks are interpreted as subaqueous lava flows. Hypabyssal diabase and andesite-dacite intrusions within the flows were contemporaneous and probably consanguineous with them. The Jasper Lake pyroclastic unit, and the associated Jasper Lake andesite, are believed to overlie the greenstone conformably, and are approximately vertical in attitude trending west-northwest. The pyroclastic unit consists of volcanic breccias, tuffs, and lesser amounts of conglomerate and greywacke-argillite. Clasts range from 0.1 mm to 1.2 meters in diameter and are composed dominantly of porphyritic andesite with very minor amounts of basalt, dacite, and tuff. Some of the basaltic clasts may have been derived from the older greenstone unit. The presence of unsorted, angular to sub-rounded fragments within the unit suggest deposition by volcanic mudflows or lahars. The Jasper Lake andesite unit trends west-northwest with largely vertical contacts, and is composed predominantly of porphyritic augite andesite with lesser amounts of massive, porphyritic hornblende andesite-dacite. The rock is typically fine-grained to aphanitic, and locally vesicular to amygdaloidal, thereby representing a shallow, hypabyssal intrusion which may have reached the surface locally. It exhibits chilled margins up to 50 meters wide and is subconcordant with bedding indications in the surrounding pyroclastic unit. These rocks are conformably overlain by a well-bedded, graded graywacke-slate unit at least 1.6 km thick. The Saganaga tonalite batholith, dated at 2.7-2.75 billion years old (Goldich, 1968), intrudes the greenstone unit along its northern margin. Locally, along the contact with the greenstone, the intrusion raised the grade of metamorphism to amphibolite facies along a zone 30-60 meters wide. Late retrograde prehnite-pumpellyite facies metamorphism also affected the rocks in this zone. The units of the Knife Lake Group, including the Ogishke conglomerate described by Gruner (1941), are vertical in attitude and trend northeastward, truncating the rocks of the Jasper Lake area on the west. These rocks contain Saganaga tonalite detritus, unlike the Jasper Lake units. The Ogishke conglomerate also locally overlies the Jasper Lake greenstone along the northwest margin of the greenstone. Two periods of folding have affected the area. Initial isoclinal folding of the Jasper Lake units along west-northwest-trending fold axes occurred contemporaneously with the Saganaga tonalite intrusion. A second period of folding produced deformation in the eastern Vermilion district, but apparently not within the Jasper Lake units. This episode involved folding along steep northeast-trending fold axes due to later rise of the Saganaga tonalite, after deposition of the Knife Lake units. Two periods of faulting, which post-date folding, have affected the area. Faulting within the Jasper Lake units along dominantly west-northwest trends occurred after the first folding episode during Saganaga intrusion. The second period of faulting, trending northeastward, affected the entire eastern Vermilion district, and truncated the faults within the Jasper Lake units. Detailed study in the area indicates that the basalt-andesite-dacite suite of volcanic rocks at Jasper lake represent the oldest part of the regional volcanic pile, because of the lack of Saganaga detritus as in younger units, and suggest deposition in a setting similar to modern island-arc tectonic environments.