scholarly journals Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

Minerals ◽  
2019 ◽  
Vol 9 (9) ◽  
pp. 508 ◽  
Author(s):  
Jeffrey Chiarenzelli ◽  
Marian Lupulescu ◽  
George Robinson ◽  
David Bailey ◽  
Jared Singer
Keyword(s):  
Fluid Flow ◽  
Shear Zone ◽  
Isotopic Ratios ◽  
Large Crystal ◽  
Grenville Province ◽  
Orogenic Collapse ◽  
Metasedimentary Rocks ◽  
Granitic Magmatism ◽  
Oxygen Isotopic ◽  
Sedimentary Carbonates

Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (<10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.

Crustal evolution along a seismic section across the Grenville Province (western Quebec)

2000 ◽  
Vol 37 (2-3) ◽  
pp. 291-306 ◽  
Author(s):  
J Martignole ◽  
A J Calvert ◽  
R Friedman ◽  
P Reynolds
Keyword(s):  
Shear Zone ◽  
Large Scale ◽  
Seismic Profile ◽  
Detrital Zircons ◽  
Seismic Section ◽  
Grenville Province ◽  
Good Definition ◽  
Metasedimentary Rocks ◽  
Thin Crust ◽  
Front Zone

Results of deep seismic reflection survey along a 375 km long transect of the Grenville Province in western Quebec are combined with a review of geological observations and published isotopic ages. The seismic profile offers a remarkably clear image of the crust-mantle boundary and a good definition of the various crustal blocks. Crust about 44 km thick beneath the Grenville Front zone thins abruptly to ca. 36 km southeastward, perhaps the result of extension on southeast-dipping surfaces extending to the Moho. Other zones of relatively thin crust, although less pronounced, occur where Proterozoic crust overlies Archean crust, and beneath the Morin anorthosite complex. The thickest crust is found at the extreme southeast of the transect, east of the Morin anorthosite. From northwest to southeast, three main crustal subdivisions are (1) deformed Archean rocks with southeast-dipping reflectors in the Grenville Front zone, (2) an Archean parautochthon with northwest-dipping reflectors extending to the lower crust, and (3) an overlying three-layer crust interpreted as accreted Proterozoic terranes. The boundary between (2) and (3) is a major, southeast-dipping, crustal-scale ramp (Baskatong ramp) interpreted to have accommodated strain during and after accretion. U-Pb and Pb-Pb ages on detrital zircons show that metasedimentary rocks of the allochthons (Mont-Laurier, Réservoir Cabonga, and Lac Dumoine terranes) range from Archean to as young as 1.21 Ga. A single zone with 1.4 Ga old Sm-Nd model ages appears to lack Archean components and may be considered as a fragment of juvenile Mesoproterozoic crust pinched in a shear zone (Renzy shear zone) that could be raised to the status of terrane (Renzy terrane). In the allochthons, U-Pb ages of metamorphic zircon and monazite cluster around 1.17 Ga (Mont-Laurier and Réservoir Cabonga terrane) and 1.07 Ga (Renzy and Lac Dumoine terrane) and are interpreted to record late and post-accretion crustal reworking, a common feature of the Grenville orogen. A final high-grade metamorphic event (ca. 1.0 Ga) documented only in the parautochthon and the Grenville Front zone records large-scale, piggyback-style thrusting of allochthonous slabs onto the parautochthon. The age of transcurrent displacement following peak metamorphism affecting both the allochthons and the parautochthon decreases northwestward from 1.07 to 1.00 Ga. Dating thus shows that Grenvillian deformation in western Quebec occurred in pulses over an interval of 180 million years, with a tendency to propagate from the inner part of the orogen toward the Grenville Front. Reworked migmatites from the parautochthon cooled from the ca. 1.0 Ga peak of metamorphism through about 450°C (Ar closure in hornblende) at ca. 0.96 Ga with calculated cooling rates of about 6°C per million years, and unroofing rates of 0.33 km per million years. The cooling-unroofing history of the allochthons is not so straightforward, probably due to tectonic disturbances related to allochthon emplacement. Cooling through 450°C occurred between 1.04 and 1.01 Ga, at least 50 million years earlier than cooling in the parautochthon; this contrast agrees with the northwestward propagation of the orogen.

Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen

2000 ◽  
Vol 137 (1) ◽  
pp. 39-51 ◽  
Author(s):  
ALAN S. COLLINS ◽  
THEODORE RAZAKAMANANA ◽  
BRIAN F. WINDLEY
Keyword(s):  
Shear Zone ◽  
Tectonic Evolution ◽  
Structural Phase ◽  
East African ◽  
Metasedimentary Rocks ◽  
The North ◽  
Basement Rocks ◽  
Granitic Magmatism ◽  
Shear Sense ◽  
East African Orogen

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest.σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved.The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.

Changes of Ionic and Oxygen Isotopic Composition of the Snowpack at the Glacier Austre Okstindbreen, Norway, 1995

1998 ◽  
Vol 29 (1) ◽  
pp. 1-20 ◽  
Author(s):  
Peter Raben ◽  
Wilfred H. Theakstone
Keyword(s):  
Isotopic Composition ◽  
Sea Level ◽  
Oxygen Isotopes ◽  
Oxygen Isotopic Composition ◽  
Isotopic Ratios ◽  
Vertical Variations ◽  
Oxygen Isotopic ◽  
The Difference ◽  
Liquid Precipitation ◽  
Different Altitudes

Marked vertical variations of ions and oxygen isotopes were present in the snowpack at the glacier Austre Okstindbreen during the pre-melting phase in 1995 at sites between 825 m and 1,470 m above sea level. As the first meltwater percolated from the top of the pack, ions were moved to a greater depth, but the isotopic composition remained relatively unchanged. Ions continued to move downwards through the pack during the melting phase, even when there was little surface melting and no addition of liquid precipitation. The at-a-depth correlation between ionic concentrations and isotopic ratios, strong in the pre-melting phase, weakened during melting. In August, concentrations of Na+ and Mg2+ ions in the residual pack were low and vertical variations were slight; 18O enrichment had occurred. The difference of the time at which melting of the snowpack starts at different altitudes influences the input of ions and isotopes to the underlying glacier.

Geology and age of the Precambrian basement in the Windsor, Chatham, and Sarnia area, southwestern Ontario

1982 ◽  
Vol 19 (8) ◽  
pp. 1627-1634 ◽  
Author(s):  
A. Turek ◽  
R. N. Robinson
Keyword(s):  
Oil And Gas ◽  
Precambrian Basement ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Gas Drilling ◽  
Arch System ◽  
Grenvillian Orogeny ◽  
Anomaly Map ◽  
Paleozoic Rocks ◽  
Basement Surface

Precambrian basement in the Windsor–Chatham–Sarnia area is covered by Paleozoic rocks that are up to 1300 m thick. The basement surface is characterized by a northeast–southwest arch system with a relief of about 350 m. Extensive oil and gas drilling has penetrated and sampled this basement, and an examination of core and chip samples from 133 holes and an assessment of the magnetic anomaly map of the area have been used to produce a lithologic map of the Precambrian basement. The predominant rocks are granite gneisses and syenite gneisses but also significant are gabbros, granodiorite gneisses, and metasedimentary rocks. The average foliation dips 50° and is inferred to have a northeasterly trend. The Precambrian basement has been regarded as part of the Grenville Province. An apparent Rb–Sr whole rock isochron, for predominantly meta-igneous rocks, yields an age of 1560 ± 140 Ma. This we interpret as pre-Grenvillian, surviving the later imprint of the Grenvillian Orogeny. Points excluded from the isochron register ages of 1830, 915, and 670 Ma, and can be interpreted as geologically meaningful.

scholarly journals Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

2018 ◽  
Vol 55 (9) ◽  
pp. 1063-1078 ◽  
Author(s):  
Michelle J. Markley ◽  
Steven R. Dunn ◽  
Michael J. Jercinovic ◽  
William H. Peck ◽  
Michael L. Williams
Keyword(s):  
Shear Zone ◽  
Crustal Thickening ◽  
Boundary Zone ◽  
Grenville Province ◽  
Metamorphic History ◽  
Crustal Thinning ◽  
Tectonic Significance ◽  
History Of ◽  
Central Gneiss Belt ◽  
Scale Shear

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

scholarly journals OXYGEN ISOTOPIC RATIOS IN COOL R CORONAE BOREALIS STARS

2010 ◽  
Vol 714 (1) ◽  
pp. 144-154 ◽  
Author(s):  
D. A. García-Hernández ◽  
David L. Lambert ◽  
N. Kameswara Rao ◽  
Ken H. Hinkle ◽  
Kjell Eriksson
Keyword(s):  
Isotopic Ratios ◽  
Oxygen Isotopic ◽  
R Coronae Borealis Stars

scholarly journals Oxygen isotopic ratios in intermediate-mass red giants

2015 ◽  
Vol 578 ◽  
pp. A33 ◽  
Author(s):  
T. Lebzelter ◽  
O. Straniero ◽  
K. H. Hinkle ◽  
W. Nowotny ◽  
B. Aringer
Keyword(s):  
Isotopic Ratios ◽  
Red Giants ◽  
Intermediate Mass ◽  
Oxygen Isotopic
Sign in / Sign up

Export Citation Format

Share Document