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.

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.

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.

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‰.

Rb–Sr geochronology and metamorphic history of Proterozoic to early Archean rocks north of the Cape Smith Fold Belt, Quebec

1987 ◽  
Vol 24 (4) ◽  
pp. 813-825 ◽  
Author(s):  
Ronald Doig
Keyword(s):  
Volcanic Rocks ◽  
Granitic Rocks ◽  
Iron Formation ◽  
Fold Belt ◽  
Metasedimentary Rocks ◽  
Metamorphic History ◽  
The North ◽  
Basement Rocks ◽  
History Of ◽  
Wide Zone

The Churchill Province north of the Proterozoic Cape Smith volcanic fold belt of Quebec may be divided into two parts. The first is a broad antiform of migmatitic gneisses (Deception gneisses) extending north from the fold belt ~50 km to Sugluk Inlet. The second is a 20 km wide zone of high-grade metasedimentary rocks northwest of Sugluk Inlet. The Deception gneisses yield Rb–Sr isochron ages of 2600–2900 Ma and initial ratios of 0.701–0.703, showing that they are Archean basement to the Cape Smith Belt. The evidence that the basement rocks have been isoclinally refolded in the Proterozoic is clear at the contact with the fold belt. However, the gneisses also contain ubiquitous synclinal keels of metasiltstone with minor metapelite and marble that give isochron ages less than 2150 Ma. These ages, combined with low initial ratios of 0.7036, show that they are not part of the basement, as the average 87Sr/86Sr ratio for the basement rocks was about 0.718 at that time.The rocks west of Sugluk Inlet consist mainly of quartzo-feldspathic sediments, quartzites, para-amphibolites, marbles, and some pelite and iron formation. In contrast to the Proterozoic sediments in the Deception gneisses, these rocks yield dates of 3000–3200 Ma, with high initial ratios of 0.707–0.714. These initial ratios point to an age (or a provenance) much greater than that of the Archean Deception gneisses. The rocks of the Sugluk terrain are intruded by highly deformed sills of granitic rocks with ages of about 1830 Ma, demonstrating again the extent and severity of the Proterozoic overprint. The eastern margin of this possibly early Archean Sugluk block is a discontinuity in age, lithology, and geophysical character that could be a suture between two Archean cratons. It is not known if such a suturing event is of Archean age, or if it is related to the deformation of the Cape Smith Fold Belt.Models of evolution incorporating both the Cape Smith Belt and the Archean rocks to the north need to account for the internal structure of the fold belt, the continental affinity of many of the volcanic rocks, the continuity of basement around the eastern end of the belt, and the increase in metamorphism through the northern part of the belt into a broad area to the north. The Cape Smith volcanic rocks may have been extruded along a continental rift, parallel to a continental margin at Sugluk. Continental collison at Sugluk would have thrust the older and higher grade Sugluk rocks over the Deception gneisses, produced the broad Deception antiform, and displaced the Cape Smith rocks to the south in a series of north-dipping thrust slices.

Composition, age, and origin of granitoid rocks in the Davin Lake area, Rottenstone Domain, Trans-Hudson Orogen, northern Saskatchewan

2005 ◽  
Vol 42 (4) ◽  
pp. 599-633 ◽  
Author(s):  
D Barrie Clarke ◽  
Andrew S Henry ◽  
Mike A Hamilton
Keyword(s):  
Granitic Rocks ◽  
Lake Area ◽  
Granitoid Magmatism ◽  
Metasedimentary Rocks ◽  
Metavolcanic Rocks ◽  
Granitic Magmatism ◽  
Deformation Features ◽  
High Grade Metamorphism ◽  
Granitoid Rocks ◽  
Host Rocks

The Rottenstone Domain of the Trans-Hudson orogen is a 25-km-wide granitic–migmatitic belt lying between the La Ronge volcanic–plutonic island arc (1890–1830 Ma) to the southeast and the ensialic Wathaman Batholith (1855 Ma) to the northwest. The Rottenstone Domain consists of three lithotectonic belts parallel to the orogen: (i) southeast — gently folded migmatized quartzo-feldspathic metasedimentary and mafic metavolcanic rocks intruded by small concordant and discordant white tonalite–monzogranite bodies; (ii) central — intensely folded and migmatized metasedimentary rocks and minor metavolcanic rocks intruded by largely discordant, xenolith-rich, pink aplite-pegmatite monzogranite bodies; and (iii) northwest — steeply folded migmatized metasedimentary rocks cut by subvertical white tonalite–monzogranite sheets. Emplacement of granitoid rocks consists predominantly of contiguous, orogen-parallel, steeply dipping, syntectonic and post-tectonic sheets with prominent magmatic schlieren bands, overprinted by parallel solid-state deformation features. The white granitoid rocks have A/CNK (mol Al2O3/(mol CaO + Na2O + K2O)) = 1.14–1.22, K/Rb ≈ 500, ΣREE (sum of rare-earth elements) < 70 ppm, Eu/Eu* > 1, 87Sr/86Sri ≈ 0.7032, and εNdi ≈ –2. The pink monzogranites have A/CNK = 1.11–1.16, K/Rb ≈ 500, ΣREE > 90 ppm, Eu/Eu* < 1, 87Sr/86Sri ≈ 0.7031, and εNdi ≈ –2. The white granitoid rocks show a wider compositional range and more compositional scatter than the pink monzogranites, reflecting some combination of smaller volume melts, less homogenization, and less control by crystal–melt equilibria. All metavolcanic, metasedimentary, and granitic rocks in the Rottenstone Domain have the distinctive geochemical signatures of an arc environment. New sensitive high-resolution ion microprobe (SHRIMP) U–Pb geochronology on the Rottenstone granitoid rocks reveals complex growth histories for monazite and zircon, variably controlled by inheritance, magmatism, and high-grade metamorphism. Monazite ages for the granitoid bodies and migmatites cluster at ~1834 and ~1814 Ma, whereas zircon ages range from ~2480 Ma (rare cores) to ~1900–1830 Ma (cores and mantles), but also ~1818–1814 Ma for low Th/U recrystallized rims, overgrowths, and rare discrete euhedral prisms. These results demonstrate that at least some source material for the granitic magmas included earliest Paleoproterozoic crust (Sask Craton?), or its derived sediments, and that Rottenstone granitic magmatism postdated plutonism in the bounding La Ronge Arc and Wathaman Batholith. We estimate the age of terminal metamorphism in the Davin Lake area to be ~1815 Ma. Petrogenetically, the Rottenstone migmatites and granitoid rocks appear, for the most part, locally derived from their metasedimentary and metavolcanic host rocks, shed from the La Ronge Arc, Sask Craton, and possibly the Hearne Craton. The Rottenstone Domain was the least competent member in the overthrust stack and probably underwent a combination of fluid-present melting and fluid-absent decompression melting, resulting in largely syntectonic granitoid magmatism ~1835–1815 Ma, analogous to granite production in the High Himalayan gneiss belt.

Neoproterozoic I-type granites geochronology and geochemistry of the Chinese Central Tianshan Block

2021 ◽  
Author(s):  
Yujia Song ◽  
Xijun Liu ◽  
Zhiguo Zhang ◽  
Pengde Liu ◽  
Yao Xiao
Keyword(s):  
Orogenic Belt ◽  
Granitic Rocks ◽  
Eastern European ◽  
Precambrian Basement ◽  
Island Arcs ◽  
Basement Rocks ◽  
Central Asian ◽  
Altay Orogenic Belt ◽  
Rodinia Supercontinent ◽  
South Tianshan

&lt;p&gt;The Central Asian Orogenic Belt (CAOB), also known as the Altay orogenic belt, is the largest accretionary orogenic belt in the world. It is situated between the Eastern European, Siberian, Tarim, and North China cratons. The CAOB is a large and complex suture zone formed by amalgamation of diverse geologic units including several microcontinents, ophiolites, island arcs, seamounts and accretionary wedges. The evolution of the Precambrian basement in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB as well as the evolution of Rodinia supercontinent. The Tianshan block, an important part of the CAOB, is located in the southwestern CAOB, and subdivided from north to south into North Tianshan, Central Tianshan-Yili blocks, and South Tianshan. The Central Tianshan block, located between the Tarim block, the Junggar block and the Kazakhstan block, is one of numerous microcontinental block within the CAOB that overlie Precambrian basement rocks. Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB, and its place within the Rodinia supercontinent. However, to date, the timing and tectonic settings in which the basement rocks in the Central Tianshan formed are poorly constrained, with only sparse geochemical and geochronological data from granitic rocks within the central segment of the belt. Here, we present a systematic study combining U-Pb geochronology, whole-rock geochemistry, and the Sr-Nd isotopic compositions of newly-identified granites from the Bingdaban area of Central Tianshan. The analyzed samples yield a weighted mean Neoproterozoic &lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;238&lt;/sup&gt;U ages of 975-911 Ma. All have affinities with calc-alkaline, weakly-peraluminous, magnesian I-type granites. The samples are enriched in LREE, display relatively flat HREE patterns with negative Eu anomalies, and show a depletion in the high field strength elements (HFSEs) Nb, Ta, and Ti and enrichment in large ion lithophile elements (LILEs) Rb, U, Th and Nd geochemical characteristics indicative of subduction-related magmatism. All samples show initial (&lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr)&lt;sub&gt;(t)&lt;/sub&gt; ratios between 0.705136 and 0.706745. Values for &amp;#400;&lt;sub&gt;Nd(t)&lt;/sub&gt; in the granites are in the range -1.2 to -5.7, corresponding to Nd model ages of 1.6-2.1 Ga, indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths. The documented geochemical features indicate the protoliths for the granites had a similar petrogenesis and magmatic source, which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material. The Tianshan Block probably constituted part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the early Neoproterozoic, and which underwent a transition from subduction to syn-collision compression at 975-911 Ma. This study reveals that crustal reworking may played a key role in Neoproterozoic crustal evolution in the Central Tianshan block and this block has a tectonic affinity to the Yili block.&lt;/p&gt;&lt;p&gt;This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS &amp;#8220;Light of West China&amp;#8221; Program (2018-XBYJRC-003).&lt;/p&gt;

The Precambrian supracrustal rocks in the Naternaq (Lersletten) and Ikamiut areas, central West Greenland

2002 ◽  
pp. 24-32 ◽  
Author(s):  
Claus Østergaard ◽  
Adam A. Garde ◽  
Jeppe Nygaard ◽  
Jette Blomsterberg ◽  
Bo Møller Nielsen ◽  
...  
Keyword(s):  
Granitic Rocks ◽  
Precambrian Basement ◽  
West Greenland ◽  
The North ◽  
Basement Rocks ◽  
Outwash Plain ◽  
Supracrustal Belt ◽  
Sulphide Mineralisation ◽  
Hinge Zone ◽  
North Western

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article. Østergaard, C., Garde, A. A., Nygaard, J., Blomsterberg, J., Møller Nielsen, B., Stendal, H., & Thomas, C. W. (1). The Precambrian supracrustal rocks in the Naternaq (Lersletten) and Ikamiut areas, central West Greenland. Geology of Greenland Survey Bulletin, 191, 24-32. https://doi.org/10.34194/ggub.v191.5080 Naternaq, or Lersletten, in central West Greenland is an extensive Quaternary outwash plain characterised by light grey, silty sediments. Scattered low hills with outcrops of crystalline Precambrian basement rocks protrude from the outwash plain and form the northern part of the Nagssugtoqidian orogen (e.g. Connelly et al. 2000). The prominent Naternaq supracrustal belt, at least 25 km long and up to c. 2 km wide, occurs along the north-western margin of Lersletten, bordered on both sides by Archaean orthogneisses and granitic rocks; the supracrustal rocks outline a major fold structure with an irregular and sporadically exposed hinge zone (Fig. 1). The supracrustal rocks, including the fold closure, exhibit a negative signature on the regional aeromagnetic map (Fig. 2). The belt is known for its disseminated and massive iron sulphide mineralisation with minor copper and zinc, which is common in the south-eastern part of the belt.

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.

Authigenic K-feldspar in the Precambrian basement of Ohio and its effect on tectonic discrimination of the granitic rocks

1991 ◽  
Vol 28 (10) ◽  
pp. 1624-1634 ◽  
Author(s):  
Edward G. Lidiak ◽  
Vincent M. Ceci
Keyword(s):  
United States ◽  
Major Element ◽  
The United States ◽  
Granitic Rocks ◽  
Precambrian Basement ◽  
Grenville Province ◽  
Element Abundances ◽  
Ordinary Light ◽  
Alkaline Earths ◽  
Optic Angle

Authigenic K-feldspar of apparent low-temperature origin is widespread in the uppermost part of the two main Precambrian basement terranes of Ohio: the subsurface extension of the Grenville Province of Canada, and the anorogenic granite–rhyolite terrane of the Transcontinental Proterozoic Province of the United States. The authigenic K-feldspar occurs mainly as a replacement of both primary K-feldspar and albitic plagioclase in a variety of igneous and metamorphic rocks and can be identified by a salmon color in plane or ordinary light, moderate to low optic angle, monoclinic or triclinic symmetry, and Or-rich composition. Its origin has been ascribed to alkali exchange by potassic brines in early Paleozoic time.The K-feldspar authigenesis has changed whole-rock major-elemental compositions by increasing K2O, decreasing Na2O and CaO, and in general modifying SiO2 and Al2O3. These changes rule out using chemical parameters such as alkalis, alkaline earths, alumina, and silica as tectonic or petrologic discriminators on rocks affected by K-feldspar authigenesis. Of the 53 granitoid and felsic extrusive rocks in the basement of Ohio that we analyzed for major-element compositions, only 19 are unaffected by the authigenesis. Among the unaltered granitoids, 13 have major-element abundances and ratios indicative of orogenic paragenesis, and at least three of the granitoids have apparent anorogenic chemical characteristics. All but one of the orogenic granitoids occur east of the Grenville Front and within the Grenville Province. Anorogenic granitoids occur on both sides of the front, and they are probably part of the broad belt of anorogenic igneous rocks that extend from the southwestern United States to Labrador.

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.

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