Early Proterozoic tectonics of the Lake Superior region: Tectonostratigraphic terranes near the purported collision zone

1983 ◽  
pp. 33-48 ◽  
Author(s):  
D. K. Larue
Keyword(s):  
Lake Superior ◽  
Early Proterozoic ◽  
Collision Zone

Geochemistry and petrogenesis of the early Proterozoic Hemlock volcanic rocks and the Kiernan sills, southern Lake Superior region

1988 ◽  
Vol 25 (4) ◽  
pp. 528-546 ◽  
Author(s):  
W. C. Ueng ◽  
T. P. Fox ◽  
D. K. Larue ◽  
J. T. Wilband
Keyword(s):  
Trace Elements ◽  
Rare Earth ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Lake Superior ◽  
Volcanic Rocks ◽  
Wall Rock ◽  
Major Elements ◽  
Chemical Contamination ◽  
Early Proterozoic

During the early Proterozoic, the 2 km thick differentiated gabbroic Kiernan sills were emplaced into a thick accumulation of pillow basalt and associated deep-water strata, the Hemlock Formation, in the southern Lake Superior region. On the basis of major elements and trace elements (including rare-earth-element data), the Kiernan sills and the hosting volcanic rocks of the Hemlock Formation were determined to be comagmatic in origin, and both evolved from assimilation – crystal fractionation processes. The major assimilated components in these igneous rocks are identified as terrigenous sedimentary rocks. Assimilation affected the abundance of Nb, Ta, light rare-earth elements, and most likely P, Rb, Th, and K in the magma. The effect of chemical contamination from wall-rock assimilation accumulates with increasing differentiation.With wall-rock contamination carefully evaluated, a series of tectonic discriminating methods utilizing immobile trace elements indicates that the source magma was a high-Ti tholeiitic basalt similar to present-day mid-ocean-ridge basalts (MORB). It is suggested from this study that most of the enriched large-ion lithophile elements and LREE of the magma were not inherited from the mantle but from assimilation of supracrustal rocks. Chemical signatures of these rocks are distinctively different from those of arc-related volcanics. A rifting tectonic regime analogous to the opening of the North Atlantic Ocean and extrusion of North Atlantic Tertiary volcanics best fits the criteria revealed by this study.

Early Proterozoic Sedimentary Basins of the Lake Superior Region

1980 ◽  
Vol 91 (8_Part_II) ◽  
pp. 1836-1874 ◽  
Author(s):  
D. K. Larue ◽  
L. L. Sloss
Keyword(s):  
Lake Superior ◽  
Sedimentary Basins ◽  
Early Proterozoic

Postcollisional cooling of the Penokean orogen in east-central Minnesota

1993 ◽  
Vol 30 (5) ◽  
pp. 913-917 ◽  
Author(s):  
Daniel K. Holm ◽  
Timothy B. Holst ◽  
Daniel R. Lux
Keyword(s):  
Lake Superior ◽  
Tectonic Activity ◽  
East Central ◽  
Early Proterozoic ◽  
Granite Emplacement ◽  
Internal Zone ◽  
Basement Gneiss ◽  
Rhyolitic Volcanism ◽  
Archean Basement ◽  
Collisional Orogeny

New 40Ar/39Ar thermochronologic data from the internal zone of the Penokean orogen in east-central Minnesota indicate rapid cooling from mid-crustal temperatures. Early Proterozoic metamorphosed cover rocks yield concordant hornblende and biotite plateau ages of ~ 1755 Ma. Underlying Archean basement gneiss yields concordant muscovite and biotite plateau ages of ~ 1705 Ma. These are some of the oldest cooling ages recorded in rocks buried during the 1830–1860 Ma Penokean collisional orogeny. Cooling was coeval with regional rhyolitic volcanism and granite emplacement, suggesting a period of tectonic activity in this region during mid to late 1700 Ma. Based on similarities with Phanerozoic extensional regimes, we suggest, preliminarily, that these data might be explained by a previously unrecognized major extensional episode in the Lake Superior region.

Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario

1983 ◽  
Vol 20 (4) ◽  
pp. 622-638 ◽  
Author(s):  
D. L. Southwick ◽  
W. C. Day
Keyword(s):  
Hot Spot ◽  
Lake Superior ◽  
Volcanic Rocks ◽  
North Central ◽  
Early Proterozoic ◽  
Iron Range ◽  
Tectonic Model ◽  
Group A ◽  
Archean Crust ◽  
Mesabi Iron Range

Proterozoic diabase and gabbro dikes (~2120 Ma old) form a major northwest-oriented swarm extending about 300 km from the Mesabi iron range in Minnesota to the vicinity of Kenora, Ontario. The dikes were emplaced into Archean crust at about the time that the Early Proterozoic basin of the Lake Superior region was opening by rifting, and the swarm and the basin may therefore be tectonically related. The dikes are overlain unconformably by the Animikie Group, the upper sedimentary sequence in the Proterozoic basin in Minnesota, but may be approximately coeval with mafic volcanic rocks in the pre-Animikie Mille Lacs Group. A two-stage tectonic model involving (1) regional right-lateral crustal shear in the late Archean and (2) hot-spot rifting in the Early Proterozoic is proposed to account for the swarm.The dikes are iron-rich quartz tholeiites that are differentiated toward dioritic compositions. Late alteration to hydrous phases, including blue–green amphibole, chlorite, and sericite, together with lesser amounts of prehnite and epidote, is ubiquitous but variable in intensity, and is regarded as a deuteric phenomenon. The interior portions of some large dikes are compositionally layered parallel to contacts; the layers differ from each other in the proportions of primary hornblende, clinopyroxene, and plagioclase and thus range in composition across the gabbro–diorite boundary.Chilled margins of the dikes contain flow-aligned phenocrysts of plagioclase, clinopyroxene, titanomagnetite, and ilmenite. The clinopyroxenes occur as three morphotypes that have distinct compositions, indicating a complex intratelluric history. The dike magma was emplaced into cool Archean crust at an inferred temperature of about 1085 °C and was quenched in a matter of minutes at the dike walls. Complete solidification at the centers of dikes wider than 100 m appears to have taken more than 40 years.

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