Rubidium–strontium ages of Archean and Proterozoic rocks in the Nejanilini and Great Island domains, Churchill Province, northern Manitoba, Canada

1988 ◽  
Vol 25 (2) ◽  
pp. 246-254 ◽  
Author(s):  
G. S. Clark ◽  
D. C. P. Schledewitz
Keyword(s):  
Granitic Rocks ◽  
Granitic Magma ◽  
Initial Ratio ◽  
Early Proterozoic ◽  
Metasedimentary Rocks ◽  
Field Evidence ◽  
Southern District ◽  
Island Group ◽  
Minimum Age ◽  
Archean Basement

Rubidium–strontium whole-rock ages are reported from the Nejanilini – Great Island area in northeastern Manitoba. This area is part of an extensive zone of Archean basement that was metamorphosed and intruded by granitic magma during the Proterozoic; it extends into Saskatchewan and southern District of Keewatin, Northwest Tertitories. An age of 2577 ± 42 Ma (1σ error) for the extensive Nejanilini granulite massif (Nejanilini domain), considered one of the oldest rock units in the area, is interpreted as a minimum age for late Archean granulite-facies metamorphism. A minimum age of 2052 ± 41 Ma (initial ratio 0.7150) for quartz–feldspar porphyry that intrudes the Seal River volcanic suite constrains the age of these volcanics and could represent a partially reset Archean age. Early Proterozoic quartzite and metagreywacke of the Great Island Group unconformably overlies the quartz–feldspar porphyry. These metasedimentary rocks, which are probably correlative with the Daly Lake Group (Saskatchewan) or the Hurwitz Group (southern District of Keewatin), give an age of 1885 ± 85 Ma, with an initial ratio of 0.7093. The age records the time of closure of the Rb–Sr isotopic system subsequent to early Proterozoic metamorphism. The age and initial ratio are consistent with published results for other, possibly correlative, metasedimentary rocks in this zone. Modelling the Rb–Sr isotopic data constrains the time of sedimentation to between ca. 2100 and 2000 Ma ago. Syn- to late-kinematic, early Proterozoic granite to granodiorite batholiths, which intruded metasedimentary rocks of the Great Island Group, may largely be the product of melting of Archean basement, based on field evidence and high initial 87Sr/86Sr ratios. The Caribou Lake porphyritic quartz monzonite gives an age of 1795 ± 35 Ma, with an initial 87Sr/86Sr ratio of 0.7084. High initial ratios seem to typify early Proterozoic granitic rocks in this remobilized craton.

Geology and Ages of Buried Precambrian Basement Rocks, Manitoulin Island, Ontario

1975 ◽  
Vol 12 (7) ◽  
pp. 1175-1189 ◽  
Author(s):  
W. R. Van Schmus ◽  
K. D. Card ◽  
K. L. Harrower
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Aeromagnetic Data ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Basement Rocks ◽  
Metavolcanic Rocks ◽  
Northern Half ◽  
East End ◽  
Archean Basement

The geology of the buried Precambrian basement under Manitoulin Island in northern Lake Huron, Ontario, has been re-evaluated on the basis of aeromagnetic data, well cuttings, core samples, and rubidium–strontium and uranium–lead geochronologic data on some of the subsurface samples. We conclude that the northern half of the island is underlain in part by Huronian metasedimentary rocks, but that these are absent from the southern part of the island, which is underlain by granitic, gneissic, and metavolcanic rocks. Granitic and gneissic rocks are also present under the northern half of the island.Geochronologic data show that rocks underlying major positive aeromagnetic anomalies are quartz-monzonitic composite plutons which are about 1500 ± 20 m.y. old. Surrounding metasedimentary. gneissic, and granitic rocks are at least 1700 m.y. old. No evidence was found for extrapolation of the pre-Huroman Archean basement beneath Manitoulin Island; if it is present it has been affected by younger metamorphic overprinting.The south west ward extension of the boundary zone between the Grenville Province and rocks to the west can he traced along the east end of Manitoulin Island on the basis of aeromagnetic data.

U–Pb geochronology in the Trans-Hudson Orogen, northern Saskatchewan, Canada

1987 ◽  
Vol 24 (3) ◽  
pp. 407-424 ◽  
Author(s):  
W. R. Van Schmus ◽  
M. E. Bickford ◽  
J. F. Lewry ◽  
R. Macdonald
Keyword(s):  
Granitic Rocks ◽  
Early Proterozoic ◽  
Tectonic Events ◽  
Northern Saskatchewan ◽  
High Grade Metamorphism ◽  
The One ◽  
Relationship Of ◽  
The Relationship ◽  
Orogenic Events ◽  
Archean Basement

We have obtained U–Pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern Trans-Hudson Orogen in northern Saskatchewan. These data constrain the timing of early Proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the Trans-Hudson Orogen to other early Proterozoic orogens in North America.With one exception, all units studied so far yield zircon ages of 1890–1835 Ma, most of which are systematically earlier than previously reported Rb–Sr isochron ages on the same or similar units, suggesting open-system behavior in the Rb–Sr systems. Five metarhyolites, from volcanic sequences in the La Ronge domain, Glennie domain, and Hanson Lake block, give ages ranging from 1888 to 1876 Ma. Most of the plutons we dated, ranging from gneissic syntectonic tonalites and granodiorites to less-deformed late intrusions such as the Wathaman batholith and other smaller bodies, yield ages of 1870–1850 Ma, apparently constraining peak plutonic activity to about 1860 ± 10 Ma ago. The youngest unit found is a small discordant pluton with an age of 1836 ± 7 Ma. The concordance of ages of volcanics on the one hand and of plutons on the other suggests that domainal distinctions are mainly lithotectonic rather than temporal.Zircons from the Sahli charnockitic granite in the Hanson Lake block yield equivocal results. Discordia upper and lower intercepts for the Sahli granite suggest that granitic rocks at least 2500 Ma old were subjected to high-grade metamorphism about 1800–1900 Ma ago, with substantial resetting of zircons. Reworked Archean basement is thus present in this domain, supporting previously reported Rb–Sr isochron data from the Sahli granite. No other indications of Archean basement in the Trans-Hudson Orogen are documented, although one sample from the adjacent Peter Lake domain shows that it consists of Archean continental crust.Zircon ages in the range 1890–1835 Ma from this part of the Trans-Hudson Orogen are similar to those obtained from igneous units of the Penokean and Wopmay orogens, in North America, and from the Svecofennian Orogen, suggesting essential synchroneity of igneous and tectonic events in these four major orogens during major Proterozoic continental assembly.

The geology and geochronology of the basement complex of the central Transantarctic Mountains

1968 ◽  
Vol 5 (3) ◽  
pp. 555-560 ◽  
Author(s):  
Gunter Faure ◽  
J. G. Murtaugh ◽  
R. J. E. Montigny
Keyword(s):  
Granitic Rocks ◽  
Basement Complex ◽  
Metasedimentary Rocks ◽  
Late Cambrian ◽  
Field Evidence ◽  
Metavolcanic Rocks ◽  
Transantarctic Mountain ◽  
Quartz Monzonite ◽  
Transantarctic Mountains ◽  
Mountain Chain

The central part of the Transantarctic Mountain chain consists of a basement complex of igneous and metamorphic rocks overlain nonconformably by sedimentary rocks of late Paleozoic age, including Permian tillites and coal measures. The basement complex is exceptionally well exposed in the Wisconsin Range of the Horlick Mountains, which are located about 500 km from the South Pole. In this area clastic metasedimentary rocks and overlying metavolcanic rocks of probable pyroclastic origin are intruded by a variety of granitic rocks of the Wisconsin Range batholith, including rapakivi granites and quartz diorites, which are in turn cut by bodies of quartz monzonite, and aplite and pegmatite dikes. The basement complex elsewhere in the central Transantarctic Mountains also includes Cambrian limestones overlain by acid volcanic pyroclastic rocks and lava flows.Age determinations by the Rb–Sr method applied to suites of total rock specimens suggest the presence of two orogenic events accompanied by intrusions of granitic plutons. The first of these occurred about 630 ± 25 m.y. ago and was accompanied by the formation of rapakivi granites. The second took place during the Ordovician Period about 480 ± 10 m.y. ago and involved the intrusion of quartz monzonite and pegmatite dikes. The metasedimentary rocks, which were intruded by both of the granitic rocks, give an apparent age of 460 ± 16 m.y., while the overlying pyroclastic unit was dated at 633 ± 13 m.y. A Precambrian age for the metasedimentary and metavolcanic rocks is consistent with the dating and the field evidence. Rhyolites from localities in the Byrd Mountains and the Long Hills were dated at 489 ± 30 and 498 ± 45 m.y. and are late Cambrian to early Ordovician in age.

Rb–Sr and U–Pb ages of volcanism and granite emplacement in the Michipicoten belt—Wawa, Ontario

1982 ◽  
Vol 19 (8) ◽  
pp. 1608-1626 ◽  
Author(s):  
A. Turek ◽  
Patrick E. Smith ◽  
W. R. Van Schmus
Keyword(s):  
Greenstone Belt ◽  
Granitic Rocks ◽  
Iron Formation ◽  
Superior Province ◽  
Initial Ratio ◽  
Metasedimentary Rocks ◽  
Granite Emplacement ◽  
Time Period ◽  
Metavolcanic Rocks ◽  
Volcanic Belts

The Michipicoten greenstone belt at Wawa, Ontario is typical of Archean volcanic belts in the Superior Province. The supracrustal rocks are divisible into lower, middle, and upper metavolcanic sequences, which are separated by iron formation and clastic metasedimentary rocks. These are intruded by granitic stocks and embayed by granitic batholiths.This study reports whole rock Rb–Sr and zircon U–Pb ages for the lower and upper metavolcanics, for the granitic rocks that are physically within the greenstone belt (internal granites), and for the granitic rocks that embay the greenstone belt (external granites). The apparent Rb–Sr ages for the lower metavolcanics are 2530 ± 90, 2285 ± 70, and 2680 ± 490 Ma. The U–Pb ages are 2749 ± 2 and 2744 ± 10 Ma. The internal granites give an Rb–Sr age of 2560 ± 270 Ma and a U–Pb age of 2737 ± 6 Ma. The external granite at Hawk Lake indicates an Rb–Sr age of 2550 ± 175 Ma and a U–Pb age of 2747 ± 7 Ma. It is possible that this unit contains elements older than 2812 Ma as it contains xenocrystic zircons. The upper volcanics give a U–Pb age of 2696 ± 2 Ma, which indicates that the belt evolved over a time period in excess of 53 Ma. The Rb–Sr ages are significantly younger than the U–Pb zircon ages and have very large uncertainties in age; hence it is unlikely that they have any stratigraphic significance. They probably reflect the Kenoran orogeny at about 2560 Ma. The 2285 ± 70 Ma Rb–Sr isochron age has an initial ratio of 0.7275 ± 0.0052, which is interpreted as a rotational isochron defining a younger post-Kenoran event in the area. The zircon ages appear to be correct chronostratigraphically. Furthermore, it appears that the granitic rocks are coeval and may also be cogenetic with the lower acid metavolcanic rocks.

scholarly journals Ore Genesis of the Takatori Tungsten–Quartz Vein Deposit, Japan: Chemical and Isotopic Evidence

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 765
Author(s):  
Yuichi Morishita ◽  
Yoshiro Nishio
Keyword(s):  
Oxygen Isotope ◽  
Early Stage ◽  
Isotope Ratios ◽  
Equilibrium Temperature ◽  
Granitic Rocks ◽  
Granitic Magma ◽  
Lithium Isotope ◽  
Vein Deposit ◽  
Pressure Independent ◽  
Isotope Equilibrium

The Takatori hypothermal tin–tungsten vein deposit is composed of wolframite-bearing quartz veins with minor cassiterite, chalcopyrite, pyrite, and lithium-bearing muscovite and sericite. Several wolframite rims show replacement textures, which are assumed to form by iron replacement with manganese postdating the wolframite precipitation. Lithium isotope ratios (δ7Li) of Li-bearing muscovite from the Takatori veins range from −3.1‰ to −2.1‰, and such Li-bearing muscovites are proven to occur at the early stage of mineralization. Fine-grained sericite with lower Li content shows relatively higher δ7Li values, and might have precipitated after the main ore forming event. The maximum oxygen isotope equilibrium temperature of quartz–muscovite pairs is 460 °C, and it is inferred that the fluids might be in equilibrium with ilmenite series granitic rocks. Oxygen isotope ratios (δ18O) of the Takatori ore-forming fluid range from +10‰ to +8‰. The δ18O values of the fluid decreased with decreasing temperature probably because the fluid was mixed with surrounding pore water and meteoric water. The formation pressure for the Takatori deposit is calculated to be 160 MPa on the basis of the difference between the pressure-independent oxygen isotope equilibrium temperature and pressure-dependent homogenization fluid inclusions temperature. The ore-formation depth is calculated to be around 6 km. These lines of evidence suggest that a granitic magma beneath the deposit played a crucial role in the Takatori deposit formation.

Isotopic data bearing on the origin of Mesozoic and Tertiary granitic rocks in the western United States

1984 ◽  
Vol 310 (1514) ◽  
pp. 743-753 ◽  
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
Regional Survey ◽  
Regional Patterns ◽  
Peraluminous Granite ◽  
Isotopic Compositions ◽  
Geographic Coverage ◽  
Magma Flux ◽  
Archean Basement ◽  
Mantle Magma

A regional survey of initial Nd and Sr isotopic compositions has been done on Mesozoic and Tertiary granitic rocks from a 500 000 km 2 area in California, Nevada, Utah, Arizona, and Colorado. The plutons, which range in composition from quartz diorite to monzogranite, are intruded into accreted oceanic geosynclmal terrains in the west and north and into Precambrian basement in the east. Broad geographic coverage allows the data to be interpreted in the context of the regional pre-Mesozoic crustal structure. Initial Nd isotopic compositions exhibit a huge range, encompassing values typical of oceanic magmatic arcs and Archean basement. The sources of the magmas can be inferred from the systematic geographic variability of Nd isotopic compositions. The plutons in the accreted terrains represent mantle-derived magma that assimilated crust while differentiating at deep levels. Those emplaced into Precambrian basement are mainly derived from the crust. The regional patterns can be understood in terms of: (1) the flux of mantle magma entering the crust; (2) crustal thickness; and (3) crustal age. The mantle magma flux apparently decreased inland; in the main batholith belts purely crustal granitic rocks are not observed because the flux was too large. Inland, crustal granite is common because mantle magma was scarce and the crust was thick, and hot enough to melt. The values of peraluminous granite formed by melting of the Precambrian basement depend on the age of the local basement source.

scholarly journals Gold-hosting supracrustal rocks on Storø, southern West Greenland: lithologies and geological environment

2007 ◽  
Vol 13 ◽  
pp. 41-44 ◽  
Author(s):  
Christian Knudsen ◽  
Jeroen A.M. Van Gool ◽  
Claus Østergaard ◽  
Julie A. Hollis ◽  
Matilde Rink-Jørgensen ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
West Greenland ◽  
Metasedimentary Rocks ◽  
Quartz Veins ◽  
Rock Types ◽  
Minimum Age ◽  
Arc Setting ◽  
Basic Volcanic ◽  
Back Arc ◽  
Different Sources

A gold prospect on central Storø in the Nuuk region of southern West Greenland is hosted by a sequence of intensely deformed, amphibolite facies supracrustal rocks of late Mesoto Neoarchaean age. The prospect is at present being explored by the Greenlandic mining company NunaMinerals A/S. Amphibolites likely to be derived from basaltic volcanic rocks dominate, and ultrabasic to intermediate rocks are also interpreted to be derived from volcanic rocks. The sequence also contains metasedimentary rocks including quartzites and cordierite-, sillimanite-, garnet- and biotite-bearing aluminous gneisses. The metasediments contain detrital zircon from different sources indicating a maximum age of the mineralisation of c. 2.8 Ga. The original deposition of the various rock types is believed to have taken place in a back-arc setting. Gold is mainly hosted in garnet- and biotite-rich zones in amphibolites often associated with quartz veins. Gold has been found within garnets indicating that the mineralisation is pre-metamorphic, which points to a minimum age of the mineralisation of c. 2.6 Ga. The geochemistry of the goldbearing zones indicates that the initial gold mineralisation is tied to fluid-induced sericitisation of a basic volcanic protolith. The hosting rocks and the mineralisation are affected by several generations of folding.

Geochronology of the Belt Series, Montana

1968 ◽  
Vol 5 (3) ◽  
pp. 737-747 ◽  
Author(s):  
J. D. Obradovich ◽  
Z. E. Peterman
Keyword(s):  
Average Rate ◽  
Sedimentary Rocks ◽  
Time Interval ◽  
Depositional History ◽  
Middle Cambrian ◽  
Basement Rocks ◽  
Field Evidence ◽  
Minimum Age ◽  
History Of ◽  
Central Montana

This paper presents new radiometric data that permit some qualified statements to be made on the depositional history of the Belt sedimentary rocks. The period of deposition of sedimentary rocks of the Precambrian Belt Series has been placed within a broad time interval, for they rest on metamorphosed basement rock dated at ~ 1800 m.y. and are overlain by the Middle Cambrian Flathead Quartzite (circa 530 m.y.). Prior geochronometric data gathered during the last decade indicate most of the Belt Series to be older than ~ 1100 m.y.K–Ar and Rb–Sr techniques have been applied recently to a variety of samples selected from the whole gamut of the Belt Series. Glauconite from various formations in the sequence McNamara Formation down to the uppermost beds of the Empire Formation in the Sun River area has been dated at 1080 ± 27 m.y. by the K–Ar method and at 1095 ± 22 m.y. by the Rb–Sr mineral isochron method. A Rb–Sr whole-rock isochron based on argillaceous sedimentary rocks from this 5000-ft section gives an age of 1100 ± 53 m.y. The concordance of the preceding results and the K–Ar ages (1075 to 1110 m.y.) on Purcell sills and lava imply that this age represents the time of sedimentation of these units.A Rb–Sr isochron based on whole-rock samples stratigraphically far below the Umpire Formation— the Greyson Shale, Newland Limestone, Chamberlain Shale, and Neihart Quartzite in the Big Belt and Little Beit Mountains—yields an age of 1325 ± 15 m.y. This result is interpreted as indicating a substantial unconformity beneath the Belt Series, at least in central Montana; it also suggests a major hiatus, unsuspected from field evidence, between the uppermost part of the Empire Formation and the Greyson Shale.The results for the youngest of Belt rocks—the Pilcher Quartzite and the Garnet Range Formation, which are exposed in the Alberton region—are equivocal in that there is widespread dispersion. A large component of detrital muscovite in some of the samples could readily account for the magnitude and sense of this dispersion. A maximum age of ~930 m.y. based on an isochron of minimum slope through the various points may be inferred for this sequence. A K–Ar age of 760 m.y. obtained on biotite from a sill in the Garnet Range Formation provides a minimum age for these younger Belt rocks.Three distinct periods of sedimentation for Belt rocks sampled are suggested at ≥ 1300, 1100, and ≤ 900 m.y., with two substantial hiatuses of 200 m.y. or more. In addition the data for the sequence in the Big and Little Belt Mountains suggest that sedimentation may not have commenced for a period of possibly 400 m.y. after the metamorphism that affected basement rocks, while the data for the Garnet Range and Pilcher sequence suggest that sedimentation ceased some 200 to 400 m.y. prior to the deposition of the Middle Cambrian Flathead Quartzite.To suggest that the Belt sediments were deposited continuously over a period of 400 m.y. or more would imply an unusually low average rate of deposition of ≤ 0.1 ft/1000 yr, and this for the thickest part of the Belt Series. As a realistic expression of the depositional history of the Belt Series, both viewpoints are open to question, but the viewpoint that the Belt basin has been characterized by discontinuous sedimentation would be more in keeping with the principle of uniformity.

The oxygen isotope composition of the surface crystalline rocks of the Canadian Shield

1978 ◽  
Vol 15 (11) ◽  
pp. 1773-1782 ◽  
Author(s):  
Yuch-Ning Shieh ◽  
Henry P. Schwarcz
Keyword(s):  
Isotope Composition ◽  
Crystalline Rocks ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Baffin Island ◽  
Grenville Province ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Northern District ◽  
Basic Rocks

The average 18O/16O ratios of the major rock types of the surface crystalline rocks in different parts of the Canadian Precambrian Shield have been determined, using 47 composite samples prepared from 2221 individual rock specimens. The sampling areas include Baffin Island, northern and southwestern Quebec, Battle Harbour – Cartwright, northern District of Keewatin, Fort Enterprise, Snowbird Lake, Kasmere Lake, and Saskatchewan, covering approximately 1 400 000 km2. The granitic rocks from the Superior, Slave, and Churchill Provinces vary only slightly from region to region (δ18O = 6.9–8.4‰) and are significantly lower in 18O than similar rock types from the younger Grenville Province (δ = 9.2–10.0‰). The sedimentary and metasedimentary rocks have δ18O = 9.0–11.7‰ and hence are considerably lower than their Phanerozoic equivalents, possibly reflecting the presence of a high percentage of little-altered igneous rock detritus in the original sediments. The basic rocks in most regions fall within a δ18O range of 6.8–7.6‰, except in northern and southwestern Quebec where the δ-values are abnormally high (8.5–8.9‰). The overall average 18O/16O ratio of the surface crystalline rocks of the Canadian Shield is estimated to be 8.0‰, which represents an enrichment with respect to probable mantle derived starting materials by about 2‰.

Early Aphebian basaltic volcanism in the southern District of Keewatin, Northwest Territories

1976 ◽  
Vol 13 (7) ◽  
pp. 1003-1005 ◽  
Author(s):  
Roy V. Beavon
Keyword(s):  
Northwest Territories ◽  
Geological Survey ◽  
Basaltic Volcanism ◽  
Dike Swarm ◽  
Southern District ◽  
Hurwitz Group ◽  
Textural Evidence ◽  
Archean Basement

A thin formation of folded mafic flows resting unconformably on Archean basement is informally named the Spi Lake basalt. Stratigraphic, structural, and textural evidence suggests that these lavas were probably fed from a diabase dike swarm dated at 2250+ m.y. by the Geological Survey of Canada. The Spi Lake basalt is overlain by conglomerate that may represent the base of the Hurwitz Group.

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