Differentiated Mafic-Ultramafic Sills in the Archean Vermilion District, Northeastern Minnesota

Abstract

Detailed mapping of part of the Newton Lake Formation north of Ely, Minnesota has shown the presence of numerous sill-like mafic-ultramafic intrusions in the Archean volcanic sequence. Three types of intrusions have been found: layered, differentiated mafic-ultramafic sills ranging from 400 to 1500 feet thick and at least 4 miles long; gabbroic sills of variable thickness and e xtent; and ultramafic lenses ranging from 75 to 300 feet thick and up to 3000 feet long. The layered sills were studied in detail. They have a well-developed internal stratigraphy which consists of a lower chilled-margin, peridotite, pyroxenite, bronzite gabbro, and gabbro units and an upper chilled-margin. Petrographic studies indicate that the units formed by gravitational accumulation of mineral grains, The stratigraphic succession reflects the general order of crystalization: 1) chromite, 2) chromite + olivine, 3) clinopyroxene + orthopyroxene, 4) clinopyroxene + orthopyroxene + plagioclase, 5) clinopyroxene + plagioclase. Microprobe analyses of cumulus pyroxenes has shown cryptic variations exist between units. The gabbroic sills are generally similar to the upper gabbroic parts of the layered sills. The ultramafic lenses consist of a central peridotite unit surround by a complex chilled-margin. Textural and structural features of the ultramafic lenses suggest intrusion of an olivine bearing liquid, with flowage differentiation forming the peridotite unit and rapid chilling of a crystal free liquid forming the chilled-margin. Differentiation trends of whole rock samples and calculated liquid compositions of the layered intrusions are generally in close agreement and show a general iron but minimal alkali enrichment with differentiation. Chemical analyses of chilled-margins are low in Al2O3 (10 wt%) and high in MgO (11 wt%) with a high CaO/Al2O3 ratio (0.85). The calculated bulk composition is distinctly ultrabasic in nature (MgO = 17 wt%). Mixing calculations utilizing least square methods suggest that the magma was partially differentiated at the time of intrusion. The bulk composition for the layered sills is very similar to high-MgO basalts from the Archean of Western Australia. An inital magma temperature for the layered intrusions of between 1200°C and 1100°C is inferred from element partitioning between coexisting mineral phases. Textural relations and phase equilibria suggest that crystallization of both sills and lenses probably did not take place under pressures greater than about 1 kb. (2.7 km) and may have been even lower. Metamorphism of the lower grades of the greenschist facies has affected all rocks of the Newton Lake Formation. The ultramafic rocks are serpentinized to varying degrees with olivine often the only altered phase. Serpentinization appears to have occurred along small microfractures which cut the peridotite. The mafic rocks typically have actinolite, sausseritized plagioclase, chlorite, epidote, and rare sphene. Relict grains are common in both ultramafic and mafic units suggesting that metamorphic reactions were retarded perhaps by low H2O and CO2 availability and pressure, The layered sills and lenses of the Newton Lake Formation apparently formed penecontemporaneously with the surrounding basic volcanic rocks. The layered sills probably formed as high level synvolcanic intrusions, some of which may have also acted as magma chambers for gabbroic liquids which formed sills and/or flows. The ultramafic lenses could be smaller injections of the same magma which formed the larger layered sills.