scholarly journals Neodymium isotopic evidence for early Proterozoic units in the Watersmeet gneiss dome, northern Michigan. Temperature-pressure estimates of dynamically recrystallized rocks in the early Proterozoic Mountain shear zone, northeastern Wisconsin

1991 ◽  
Keyword(s):  
Shear Zone ◽  
Gneiss Dome ◽  
Early Proterozoic ◽  
Isotopic Evidence ◽  
Northern Michigan

On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan

1996 ◽  
Vol 33 (7) ◽  
pp. 1053-1053 ◽  
Author(s):  
David Schneider ◽  
Daniel Holm ◽  
Daniel Lux
Keyword(s):  
Gravity Anomaly ◽  
Gneiss Dome ◽  
Near Surface ◽  
Dome Formation ◽  
Crustal Shortening ◽  
Gneiss Domes ◽  
History Of ◽  
The Republic ◽  
Different Origins ◽  
Northern Michigan

Biotite 40αr/39αr cooling ages from medium-pressure (500–600 MPa) rocks in the Watersmeet district, northern Michigan, suggest significant cooling–uplift and concomitant deformation during gneiss dome formation at~1755 Ma, well after the close of the 1870–1830 Ma Penokean orogeny. However, an 1822 Ma hornblende plateau date indicates that the isograds surrounding the dome are Penokean in age. We attribute gneiss dome formation and doming of Penokean-aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening. This contrasts with the compressional origin for gneiss domes preserved in the low-pressure (200–300 MPa) Republic district. The different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt. In contrast to the Watersmeet area, hornblende and biotite 40Ar/39Ar ages obtained from the Republic area are 1720–1680 Ma. Given the relatively shallow depth of this region, it is unlikely that temperatures remained above 500 °C for over 100 Ma following collision. We interpret these ages to reflect a major thermal event that may have been responsible for formation of the Republic metamorphic node. This interpretation is supported by the recent identification of an ~1730 Ma pluton that is likely the cause of a large, near-surface, negative gravity anomaly coincident with the node, and by the fact that the metamorphic node crosscuts Penokean structures.

U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

1991 ◽  
Vol 28 (6) ◽  
pp. 899-911 ◽  
Author(s):  
George E. Gehrels ◽  
William C. McClelland ◽  
Scott D. Samson ◽  
P. Jonathan Patchett ◽  
David A. Brew
Keyword(s):  
Shear Zone ◽  
Late Cretaceous ◽  
Detrital Zircons ◽  
Northern Coast ◽  
Coast Mountains ◽  
Western Margin ◽  
Early Proterozoic ◽  
Isotopic Signatures ◽  
Metasedimentary Rocks ◽  
Early Tertiary

U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.

Striding-Athabasca mylonite zone: Complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan

1994 ◽  
Vol 31 (8) ◽  
pp. 1287-1300 ◽  
Author(s):  
Simon Hanmer ◽  
Randy Parrish ◽  
Michael Williams ◽  
Chris Kopf
Keyword(s):  
Flow Pattern ◽  
Shear Zone ◽  
Crustal Deformation ◽  
Principal Axis ◽  
Shear Zones ◽  
Granulite Facies ◽  
Strike Slip ◽  
Tectonic Zone ◽  
Early Proterozoic ◽  
Lower Deck

The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed mylonites and late Archean ribbon mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean mylonites is compatible with that of the late Archean mylonites, and suggests that the crustal-scale lozenge influenced deformation since the mid-Archean. In the interval ca. 2.62–2.60 Ga, deformation in the upper and lower decks evolved from a granulite facies pervasive regime to a more localized amphibolite facies regime. With further cooling, deformation was localized within very narrow greenschist mylonitic faults at the lateral limits of the lower deck. By the late Archean, the East Athabasca mylonite triangle was part of a deep-crustal, intracontinental shear zone. This segment of the Snowbird tectonic zone was not the site of an Early Proterozoic suture or orogen.

Isotopic evidence for the extent of early Proterozoic basement in Scotland and northwest Ireland

1991 ◽  
Vol 128 (4) ◽  
pp. 385-388 ◽  
Author(s):  
A. P. Dickin ◽  
D. R. Bowes
Keyword(s):  
Nd Isotope ◽  
Early Proterozoic ◽  
Isotopic Evidence ◽  
Gneiss Complex ◽  
Proterozoic Basement ◽  
Western Ireland ◽  
Isotope Systematics

AbstractTightly clustered Sm–Nd model ages, with an average of 1.96±0.02Ga, for the gneiss complex of Inishtrahull indicate coeval development with the earlyProterozoic gneiss terrane of Islay. The extent of this terrane, largely beneath the Dalradian Supergroup, is argued to be 100×600 km, from northeast Scotland to western Ireland. This is based on the distribution of dated basement in conjunction with Pb, Sr and Nd isotope systematics and inherited zircons in Caledonian granites of the region.

Isotopic Evidence for Early Proterozoic Age of the Idono Complex, West-Central Alaska

1991 ◽  
Vol 99 (2) ◽  
pp. 209-223 ◽  
Author(s):  
Marti L. Miller ◽  
J. Y. Bradshaw ◽  
D. L. Kimbrough ◽  
T. W. Stern ◽  
T. K. Bundtzen
Keyword(s):  
Early Proterozoic ◽  
Isotopic Evidence ◽  
West Central ◽  
Proterozoic Age

U–Pb geochronology of deformation and metamorphism across a central transect of the Early Proterozoic Torngat Orogen, North River map area, Labrador

1993 ◽  
Vol 30 (7) ◽  
pp. 1470-1489 ◽  
Author(s):  
Jean-Michel Bertrand ◽  
J. Christopher Roddick ◽  
Martin J. van Kranendonk ◽  
Ingo Ermanovics
Keyword(s):  
Shear Zone ◽  
Stage Ii ◽  
Crustal Thickening ◽  
Stage Iii ◽  
Arc Magmatism ◽  
Oblique Collision ◽  
Early Proterozoic ◽  
Nappe Tectonics ◽  
Minimum Age ◽  
High Grade Metamorphism

The Early Proterozoic Torngat Orogen resulted from the oblique collision of the Archean Nain and southeastern Rae provinces and evolved in four stages: (0) deposition of platformal supracrustal assemblages followed by subduction-related arc magmatism in the margin of the Rae Province; (I) crustal thickening and nappe tectonics; (II) sinistral transpression and formation of the Abloviak shear zone; (III) uplift on steeply dipping, east-verging mylonites along the eastern orogenic front.U–Pb geochronology on zircon and monazite from major rock units and syntectonic intrusions indicates that arc magmatism at ca. 1880 Ma was followed by 40 Ma. of deformation and high-grade metamorphism from ca. 1860 to 1820. Subsequent uplift and final cooling occurred ca. 1795 – 1770 Ma. Several ages of mineral growth that correspond to distinct structural and metamorphic events have been recognized: (1) 1858 – 1853 Ma zircon and monazite dates are interpreted as the minimum age of stage I and peak metamorphic conditions; (2) 1844 Ma zircons from anatectic granitoids in the Tasiuyak gneiss complex (TGC), syntectonic with stage II deformation, are interpreted to date the formation of the Abloviak shear zone; (3) 1837 Ma magmatic zircons from an intrusive granite vein deformed along the western contact of the TGC represent a discrete intrusive event; (4) 1825 – 1822 Ma metamorphic overgrowths and newly grown zircons in granitic veins from the western portion of the orogen (Lac Lomier complex) represent a period of renewed transpressional deformation; (5) 1806 Ma magmatic zircons from a post-stage II granite emplaced along the eastern edge of the Abloviak shear zone defines the transition between stage II and stage III events; (6) 1794 – 1773 Ma zircons from leucogranites and pegmatites that are associated with uplift of the orogen (stage III). 1780 – 1740 Ma dates for monazite and a 40Ar/39Ar hornblende age correspond to the latest stages of uplift and cooling of the orogen.

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