Remagnetization in Keweenawan rocks. Part I: conglomerates

1981 ◽  
Vol 18 (3) ◽  
pp. 599-618 ◽  
Author(s):  
H. C. Palmer ◽  
H. C. Halls ◽  
L. J. Pesonen
Keyword(s):  
Igneous Rocks ◽  
Secondary Mineral ◽  
Native Copper ◽  
Late Precambrian ◽  
Normal Polarity ◽  
Keweenaw Peninsula ◽  
Partial Chemical

Conglomerate tests are used to show that Late Precambrian Keweenawan conglomerates on the Keweenaw Peninsula, Michigan and at Mamainse Point, Ontario have suffered a partial chemical remagnetization that predated tectonic tilting. At Mamainse Point the overprint was acquired during the final normal polarity stage of Keweenawan volcanism, whereas on the Keweenaw Peninsula it occurred after cessation of volcanism and has a maximum Late Freda Sandstone age. The extent of overprinting on the Keweenaw Peninsula increases westwards along a 40 km strike length in the Copper Harbor Conglomerate, and may at least in part be related to secondary mineral zonation (including native copper) in the underlying Portage Lake volcanics. It is shown that neither of the two overprinting episodes can be responsible for the reversal asymmetry observed in Keweenawan igneous rocks.

Paleomagnetism of the native copper mineralization, Keweenaw Peninsula, Michigan

2019 ◽  
Vol 56 (9) ◽  
pp. 932-947 ◽  
Author(s):  
D.T.A. Symons ◽  
K. Kawasaki
Keyword(s):  
Remanent Magnetization ◽  
White Pine ◽  
Native Copper ◽  
Time Limits ◽  
Clastic Rocks ◽  
Copper Mineralization ◽  
Lake Formation ◽  
World Class ◽  
Keweenaw Peninsula ◽  
Basaltic Flow

The age and genesis of Michigan’s world-class native copper deposits are poorly constrained. The copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 ± 2 Ma Keweenawan Portage Lake Formation. Progressive thermal demagnetization isolates stable hematite remanent magnetization components at 28 paleomagnetic sites. Paleomagnetic tilt tests show that magnetite in massive flow interiors is primary (1095 ± 2 Ma) and that hematite throughout the formation is syntectonic. The altered cupriferous deposits contain primary ∼1095 Ma and secondary ∼1053 Ma hematite in various proportions. The Caledonia Mine’s basaltic mineralization carries the ∼1053 Ma hematite dominantly whereas the Delaware Mine’s conglomeratic interbed mineralization carries the ∼1095 Ma hematite dominantly. The ∼1095 Ma hematite is attributed mostly to magnetite exsolution during flow extrusion and to weathering oxidation between extrusion events. An infusion of epigenetic hydrothermal fluids emplaced the native copper with additional hematite and polarity self-reversing titanohematite at 1053 ± 7 Ma. Importantly, paleomagnetic evidence supports a 1053 ± 7 Ma age also for the White Pine stratiform sedimentary copper mineralization, for the oxidation of the Oronto Group clastic rocks to red beds, and for the time limits of major tectonic uplift and deformation on the Keweenaw Peninsula.

Rb-Sr whole-rock isochron ages of late Precambrian to Cambrian igneous rocks from southern Britain

1980 ◽  
Vol 137 (5) ◽  
pp. 649-656 ◽  
Author(s):  
P. J. Patchett ◽  
N. H. Gale ◽  
R. Goodwin ◽  
M. J. Humm
Keyword(s):  
Igneous Rocks ◽  
Late Precambrian

scholarly journals Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA: A Comment

Minerals ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 506 ◽  
Author(s):  
Alex Brown
Keyword(s):  
Copper Isotope ◽  
Native Copper ◽  
Keweenaw Peninsula

Well-grouped δ65Cu values (−0.3 to +0.8‰)  from120 samples of native copper fromthe Keweenaw native copper district have been attributed [...]

scholarly journals Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA: Reply

Minerals ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 508
Author(s):  
Theodore Bornhorst ◽  
Ryan Mathur
Keyword(s):  
Copper Isotope ◽  
Native Copper ◽  
Opportunity To Respond ◽  
Keweenaw Peninsula

We appreciate the opportunity to respond to Brown’s [...]

Nd isotopic compositions of felsic igneous rocks in Cape Breton Island, Nova Scotia

1992 ◽  
Vol 29 (4) ◽  
pp. 650-657 ◽  
Author(s):  
Sandra M. Barr ◽  
Ernst Hegner
Keyword(s):  
New Brunswick ◽  
Small Variation ◽  
Igneous Rocks ◽  
Northwestern Part ◽  
Isotopic Data ◽  
Cape Breton Island ◽  
Late Precambrian ◽  
Cape Breton ◽  
Depleted Mantle ◽  
Juvenile Crust

Nd isotopic data from 18 felsic plutonic and volcanic units in Cape Breton Island show variations consistent with other geological and geophysical evidence for at least three distinct terranes. A ca. 1.2 Ga syenite considered to be part of Grenvilleage basement exposed in the northwestern part of the island yields an initial εNd value of +0.4 and a depleted-mantle model age (TDM) of 1.66 Ga, suggesting substantial involvement of older (Archean or Early Proterozoic) crust in its petrogenesis. A TDM of 1.38 Ga indicated by Nd isotopic data for a Devonian granite spatially associated with the syenite is also consistent with the presence of older crust in that part of Cape Breton Island. In contrast, Silurian rhyolite and Devonian granites from the Aspy terrane have TDM ranging from 0.7 to 1.1 Ga and initial εNd between +2.8 and −1.2, and overlap in isotopic characteristics with late Precambrian and Early Ordovician plutons in the Bras d'Or terrane that yield TDM of 0.9–1.1 Ga and initial εNd of +1.4 to −1.8. The relatively small variation in εNd and TDM in these terranes suggests that old crust like that under the Blair River Complex may not be present. Granitic plutons and rhyolite in the Mira terrane of southeastern Cape Breton Island have a range in TDM (0.8–1.2 Ga) similar to that of the Aspy and Bras d'Or terranes, but initial εNd values ranging from +0.8 to +5.0 indicate more juvenile crust. This juvenile crust also appears to be present under southern New Brunswick and eastern Newfoundland and may be characteristic of the Avalon terrane.

U-Pb zircon ages for Late Precambrian igneous rocks in southern Britain

1991 ◽  
Vol 148 (3) ◽  
pp. 435-443 ◽  
Author(s):  
R. D. TUCKER ◽  
T. C. PHARAOH
Keyword(s):  
Igneous Rocks ◽  
Late Precambrian
Sign in / Sign up

Export Citation Format

Share Document