Mineral Isochron Ages Research Articles

Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects

Petrographic, petrological and geochemical data for 16 mafic meta-igneous, granulite facies lower crustal xenoliths from the East Eifel were collected in order to develop a model for the lower crustal history for this region. The xenoliths consist of plagioclase±amphibole±clinopyroxene±garnet±orthopyroxene±scapolite + opaque minerals±apatite±rutile±zircon. Garnet has reacted to a variable extent with plagioclase and clinopyroxene to form a corona of plagioclaseII+ amphibole + orthopyroxeneII. Pyroxenes and plagioclases show complex zoning patterns with regard to Al and Ca which can be interpreted in terms of P, T history. Decreasing temperature and pressure conditions are recorded by decreasing Al in clinopyroxene rims coexisting with increasing anorthite contents in plagioclase rims and the breakdown of garnet. In addition, a young heating event that affected the granulites to different degrees is inferred from the complementary Ca-zoning patterns in clino- and orthopyroxenes. Rare earth element (REE) patterns of whole rocks together with the trends displayed and fractionated liquids. REE analyses of the mineral separates display equilibrium partitioning patterns for amphibole and clinopyroxene, although isotopic data show that amphibole contains externally-derived Sr and Nd components not recognized in other minerals. At least a 4-stage history for the granulites is recorded: (1) intrusion and crystal fractionation of basaltic magmas in the lower crust, probably accompanied by crustal assimilation, (2) granulite facies metamorphism, (3) a decrease in temperature and pressure, and (4) a later heating event. The complicated thermal history is reflected in Sm−Nd mineral isochron ages which range from about 170 Ma down to about 100 Ma and cannot be assigned to distinct geological events. These ages correlate with inferred temperatures; the low ages are measured for xenoliths with the highest temperatures. In some cases the young heating event is likely to be responsible for partial resetting of the mineral isochrons.

U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma?

Research Article| November 01, 1990 U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma? W. R. Premo; W. R. Premo 1U.S. Geological Survey, M.S. 963, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar R. T. Helz; R. T. Helz 2U.S. Geological Survey, M.S. 959, National Center, Reston, Virginia 22092 Search for other works by this author on: GSW Google Scholar M. L. Zientek; M. L. Zientek 3U.S. Geological Survey, West 920 Riverside Avenue, Spokane, Washington 99201 Search for other works by this author on: GSW Google Scholar R. B. Langston R. B. Langston 4Stillwater Mining Company, HC 54-Box 365, Nye, Montana 59061 Search for other works by this author on: GSW Google Scholar Author and Article Information W. R. Premo 1U.S. Geological Survey, M.S. 963, Federal Center, Denver, Colorado 80225 R. T. Helz 2U.S. Geological Survey, M.S. 959, National Center, Reston, Virginia 22092 M. L. Zientek 3U.S. Geological Survey, West 920 Riverside Avenue, Spokane, Washington 99201 R. B. Langston 4Stillwater Mining Company, HC 54-Box 365, Nye, Montana 59061 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (11): 1065–1068. https://doi.org/10.1130/0091-7613(1990)018<1065:UPASNA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. R. Premo, R. T. Helz, M. L. Zientek, R. B. Langston; U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma?. Geology 1990;; 18 (11): 1065–1068. doi: https://doi.org/10.1130/0091-7613(1990)018<1065:UPASNA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seven analyses from two samples of the Lower Banded series of the Stillwater Complex, Montana, yielded a U-Pb zircon-baddeleyite age of 2705 ±4 Ma. A more precise age of 2704 ±1 Ma is obtained if we regress only five of seven analyses. In either case, the age is in very good agreement with the previously reported Sm-Nd mineral isochron age of 2701 ±8 Ma for a gabbro from the Lower Banded series.Noncumulate mafic sills and dikes that immediately underlie the Stillwater Complex are separated into five compositional groups. U-Pb zircon ages for a group 1 gabbronorite (2711 ±1 Ma) and a group 3 mafic norite (2703 ±10 Ma) are comparable to previously reported U-Pb zircon ages for a group 1 diabase (2713 ±3 Ma) and a group 4 high-Ti norite (2712 ±3 Ma). Although group 2 high-Mg gabbronorite and group 6 olivine gabbro have not yet yielded significant amounts of zircon, baddeleyite, or sphene, mineral separates from group 2 ielded a four-point Pb-Pb internal isochron age of 2704 ±25 Ma, and separates from group 6 yielded a Sm-Nd internal isochron age of 2706 ±64 Ma, indicating that these groups are at least contemporaneous with intrusion of the Stillwater Complex.These results indicate that groups 1 and 4 of the associated sills and dikes are perhaps slightly older than the complex and that groups 2, 3, and 6 may be coeval with it, supporting the idea that these three groups are the best candidates for a Stillwater parent magma, a conclusion previously presented on the basis of detailed mineralogic and geochemical evidence. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The influence of recent lead loss on the interpretation of disturbed UPb systems in zircons from igneous rocks in East Greenland

A series of Caledonian igneous rocks from East Greenland have been dated by means of UPb isotope determinations on zircons. All discordias obtained give too low intercept ages, in one case more than 200 Ma younger than the RbSr mineral isochron age from the same sample. Leach experiments show that recent or subrecent loss of lead is responsible for the geologically meaningless ages obtained. Further it is shown that the recent lead loss is not caused by laboratory treatment, but must be related to natural processes. X-ray investigations combined with transmission electron microscope analyses show a correlation between recent lead loss and the chemistry of the investigated zircons.

Strontium isotopic geochemistry of Pan-African/Brasiliano rocks, Chapada copper deposit, Goiás, Brazil

Whole rock and mineral Rb-Sr isochrons from the upper amphibolite grade metamorphic host rocks of the Chapada copper deposit in Goiás, Brazil, yield an age of 561±9 Ma, with a87Sr/86Sr initial (I) of 0.70414±0.00005, for the crystallization of pre-metamorphic volcanic and igneous rocks and 532 ±1 Ma for their metamorphism during the Pan-African/Brasiliano orogeny (700-450 Ma). A porphyritic diorite stock which intruded the host schists yields a mineral isochron age of 534±16 Ma (I=0.70415±0.00004). The low I values determined from these isochrons suggest that the host schists at Chapada were derived from upper mantle or lower crust material and are characteristic of magmatic products associated with island arcs. The Early Cambrian age of the host rocks at Chapada contradicts the previously assigned mid-Proterozoic age and suggests that Pan-African/Brasiliano magmatic activity may have been more extensive in central Brazil than previously thought.

Rb-Sr ages of granitic rocks from the Tsukuba district, Japan.

The Rb-Sr isochron ages were determined for the granitic rocks in the Tsukuba district, Japan. The Coarse-grained granite gives a mineral isochron age of 60.2 ± 0.4 Ma and whole rock isochron age of 61.6 ± 4.3 Ma. The Porphyritic granodiorite has a mineral isochron age of 59.5 2.7 Ma. Pegmatite dikes in the Yamanoo area, which are the products of final stage granitic activity in the Tsukuba district, yield whole rock and mineral isochron ages of 59.9 ± 4.0 Ma and 58.3 ± 2.3 Ma, respectively. The Rb-Sr ages in this study and published K-Ar mineral ages for the granitic rocks in the Tsukuba district are concentrated in very narrow range, mostly 59-62 Ma, irrespective of intrusive stages, indicating that the magmatic activities in the Tsukuba district occurred during short time interval (less than 3 Ma). Concordance of the Rb-Sr whole rock age and K-Ar mineral ages suggests that the granitic magma cooled rapidly after emplacements.

The age and origin of felsic intrusions of the Thetford Mines ophiolite, Quebec

The Thetford Mines ophiolite in southern Quebec was obducted in Early Ordovician time during the closing of the proto-Atlantic. The tectonized peridotite lower unit of the ophiolite is intruded by felsic dikes and pods including isolated lenses of massive rodingite, small bodies of strongly deformed diorite, and younger, less deformed quartz monzonite. These intrusions are found only near the base of the ophiolite, do not intrude the surrounding country rock, and are rootless; for these reasons they are considered to have been emplaced in the ophiolite before it reached its present location.The younger group of intrusions consists of biotite–muscovite quartz monzonite and leuco–quartz monzonite. Analyzed samples have high K2O contents, high (K2O × 100)/(Na2O + K2O) ratios, and high initial strontium ratios (0.7171–0.7179), indicating that the magma source region was continental and that these felsic rocks formed by partial melting of continental sediments. Whole-rock and mineral isochron ages suggest that the felsic intrusions are about 456 ± 4 Ma old and that they were metamorphosed about 418 ± 7 Ma ago.The detachment of the ophiolite occurred about 491 ± 3 Ma ago and is recorded by the age of the metamorphic aureole beneath the ophiolite. The felsic dikes were intruded some 35 Ma years later during the Taconic Orogeny. The lengthy time between detachment and final nappe emplacement recorded by the felsic dikes may be a requirement for the formation of abundant asbestiform chrysotile.

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm Nd mineral isochron for a plagioclase lherzolite yields an age, T = 427 ± 32 Ma and initial ε Nd = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm Nd model age of the plagioclase lherzolite totalrock is T CHUR Nd = 3.4 AE . This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm Nd mineral isochron age of T = 435 ± 21 Ma and ε Nd = + 6.7 ∓ 0.3 . A pyroxenite dike yields an initial ε Nd = + 7.3 ± 0.4 . The initial ε Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ε Sr ⩽ −21 for the plagioclase lherzolite and ε Sr ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.

Rb-Sr ages of the gneiss and metamorphosed intrusive rocks of the Hida metamorphic belt in the Urushiyama area, Gifu Prefecture, central Japan.

Metamorphosed intrusive rocks (metagabbro and metatonalite), and nonmetamorphosed aplite dyke, all of which intrude into the coarse-grained biotite-hornblende gneiss, are exposed in the Urushiyama area along the Takahara River, the main part of the Hida metamorphic belt. RbSr mineral isochron ages of 216±10 Ma and 222±2 Ma are given for the coarse-grained biotitehornblende gneiss and metagabbro, respectively. On the contrary, the whole rock isochrons of the metagabbro and metatonalite show 415±144 Ma and 332±74 Ma, respectively. These whole rock ages correspond to the ages of intrusion of the gabbro and tonalite and they were re-equilibrated with respect to the Rb-Sr system approximately 220 Ma by the regional metamorphism. Whole rock samples of aplite dyke yield an isochron age of 172±7 Ma. The Hida gneiss in the Urushiyarna area seems to have been formed by the main stage of metamorphism and the deformation having been already finished before the intrusion of gabbro.

船津花こう岩類の同位体年代

Rb-Sr age determinations were carried out on the Funatsu Granitic Rocks from the area east of Kamioka and on the Mizunashi Granite from the Amo area, Hida mountains. The Funatsu and Shimonomoto type rocks, which are two major rock types of the Funatsu Granitic Rocks, give Rb-Sr whole-rock isochron age of 188.9±4.4 Ma and 197.7±15.4 Ma, with initial 87Sr/86Sr ratios of 0.70480±0.00009 and 0.70481±0.00016, respectively. The age difference between the two types is insignificant because of a large error in age of the Shimonomoto type, and thus 22 samples combined by the both types give a more probable isochron age of 188.8±4.4 Ma and an initial ratio of 0.70486±0.00006 for the Funatsu Granitic Rocks as a whole. The result suggets that the Funatsu Granitic Rocks were emplaced in Early Jurassic, within a short period of time between the deposition of the Kuruma and Tetori Groups. Minerals from the Funatsu Granitic Rocks give Rb-Sr isochron and K-Ar ages ranging from 172 to 181 Ma. The Mizunashi Granite from the Amo area gives Rb-Sr whole-rock and mineral isochron ages of 296.7±25.6 Ma and 210.9±1.8 Ma, respectively. The age results indicate that the Mizunashi Granite is older than the Funatsu Granitic Rocks.

Sm-Nd direct dating of the Collins Bay hydrothermal uranium deposit, Saskatchewan

Primary pitchblende mineralization from the Collins Bay unconformity-type uranium deposit in northern Saskatchewan has been dated using the Sm-Nd method. The mineral isochron age of 1281 +/- 80 Ma is in good agreement with other geologic and geochronological data, and demonstrates the potential of the Sm-Nd technique for the direct dating of hydrothermal mineral deposits. 23 references, 5 figures, 7 tables.

Rb‐Sr isotopic evidence for Archaean‐Proterozoic crustal evolution of part of the central Yilgarn Block, Western Australia: Constraints on the age and source of the anorthositic Windimurra Gabbroid

Abstract The anorthositic Windimurra Gabbroid is the largest gabbroid body in the central Yilgarn Block of Western Australia, and is approximately equivalent in size to the Stillwater Complex. It contrasts with most other Yilgarn layered gabbroid bodies in its generally fresh mineralogy, anorthositic affinities, apparent lack of regional or static metamorphic effects, and unusual greenstone setting, wherein mafic and ultramafic volcanics are absent. The contacts with surrounding granitoids and greenstones are tectonic, and it has not been possible to establish direct age relationships. The age of the gabbroids cannot be directly determined using the Rb/Sr whole‐rock technique, because of the small range and low Rb/Sr ratios of the gabbroids, but a minimum age of 2669 ± 135 Ma is indicated from a Rb/Sr isochron for porphyry dykes—one of the three dyke sets herein dated that cut the complex; the other dykes give ages of about 2480 Ma. The Windimurra Gabbroid is thus Archaean in age. In contrast, a mineral isochron age of 1496 ±189 Ma, with an initial 87Sr/86Sr ratio of 0.7014 ± 0.0006, is obtained from gabbroid sample 66990. However, from the whole‐rock Sr‐isotopic data and constraints imposed by the ages of the dykes, an initial ratio range of 0.7007–0.7012 is indicated for the Windimurra Gabbroid. This is consistent with derivation of the gabbroid from an upper mantle source. The young mineral age probably results from isotopic re‐equilibration of Rb and Sr in the minerals, caused by a relatively mild thermal event. The old age and low initial 07Sr/86Sr ratio of the Windimurra Gabbroid set it apart from other large stratiform bodies (e.g. Bushveld, Sudbury, Dore Lake, Great Dyke, Stillwater); yet structurally it is dissimilar to the pre‐3000 Ma old ‘stratigraphic’ type anorthositic complexes (e.g. Fiskenaesset). The post‐gabbroid crustal history of the Windimurra region is similar to that of the Agnew‐Mt Keith region (about 190 km to the east), and a 2574 ± 74 Ma Rb‐Sr isotopic age of the granitoids is in agreement with the well‐documented period (2700–2550 Ma) of widespread emplacement and/or metamorphism of granitoid rocks elsewhere in the Yilgarn Block.

On the Age of Rhodesian Greenstone Belts

Rb-Sr isotopic analyses on metavolcanic rocks from some greenstone belts of the Rhodesian Archean craton yield the following isochron ages (T) and initial Sr87/Sr86 ratios (I): The three mineral isochron ages (about 2.4 b.y.) are tentatively suggested to represent the time of an important yet not widely recognized low-grade metamorphic event. The whole rock age of Midlands-Bulawayo belt is not well defined, but the eruption of the Bulawayan volcanic group might have taken place as early as 3.08 b.y. ago. The ages of Shabani and Salisbury belts are practically identical and are indistinguishable from the ages of Gwenoro gneisses and Sesombi Tonalite. They are preferably interpreted as metamoraphic ages, but primary ages interpretation cannot be ruled out. If the concept of “contemporaneity” of greenstone-granite terrain holds in the Rhodesian craton, then the “average” age (2.75 ± 0.09 b.y.) may represent the primary age for the Midlands-Bulawayo belt. The I value (0.7011) for the Midlands-Bulawayo belt and the lowest Sr87/Sr86 ratio (0.7012) measured for a tholeitic basalt (RH-154) define an upper limit for the Sr isotopic composition of the upper mantle from which the volcanic rocks were derived about 3 b.y. ago.

Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance

Rb-Sr age data and initial Sr 87 /Sr 86 ratios (I values) are reported for the Mesozoic granitic batholiths along the coast of southeastern China. Two major thermal episodes are recognized: 90 to 120 m.y. B.P., derived from the mineral isochrons, and 165 ± 13 (2σ) m.y. B.P., derived from a whole-rock isochron. These episodes appear to be correlative with periods of rapid spreading of the Mesozoic Pacific Ocean floor and embrace two orogenic phases of the Yenshan orogeny. The I values of granitic gneisses (0.7060 to 0.7159) vary systematically with the Rb/Sr ratios of their whole-rock samples. The high I value (0.7112) of a pegmatite dike cutting a granitic gneiss in Chinmen suggests that the pegmatite is derived from remelting of upper crustal rocks. The narrow range of I values for more mafic intrusive bodies in Matsu (0.7065 to 0.70695) suggests that they are genetically related and are probably derived from an upper mantle source and have some crustal contamination. An un-metamorphosed granitic sample from Changlo has a mineral isochron age of about 120 m.y., which is interpreted as the time of intrusion. Its rather low I value (0.70546) suggests that the magma source is likely in the upper mantle. The Yenshan orogenic belt in southeastern China may belong to the inner belt of the Pacific-type orogeny; it is characterized by polycyclic deformation, magmatism, and regional metamorphism. Unlike a classical geosynclinal regime, the magmatic activities in this region were not immediately preceded by a major marine geosynclinal subsidence. The apparent absence of marine deposits after Triassic time suggests that marine transgression is not a universal result of rapid sea-floor spreading. The extensive development of magmatic belts along the eastern margin of the Asian continent appears to require large-scale consumption of oceanic lithosphere at the edge of the continental plate. The age distribution and structural trend data of the magmatic rocks suggest that the presumed subduction zone may have dipped westward or northwestward in the western Pacific region during late Mesozoic time.

Variations in the Isotopic Composition of Volatilized Lead and the Age of the Western Granodiorite, Yellowknife, Northwest Territories

Large variations in the isotopic composition of Pb volatilized at different temperatures from rocks occur, and are linearly related. The line so obtained may be treated as a mineral isochron and interpreted in the same way as for the equivalent Rb/Sr case. For the western granodiorite at Yellowknife, we obtain a whole rock Pb/Pb age of 2675 m.y. and a series of mineral ages ranging from this upper limit down to about 2500 m.y. The whole rock age is in good agreement with other work and the range of mineral isochron ages is consistent with a cooling interval of about 150 m.y. as deduced by others. A comparison with other recent age determinations from the same area indicates that almost all the significant igneous activity and the emplacement of the ore minerals occurred in a very short time interval around 2650 m.y. ago.

Evaluation of Rb–Sr Dating Methods Applied to the Matachewan, Abitibi, Mackenzie, and Sudbury Dike Swarms in Canada

Re-evaluation of the age of Canadian dike swarms using Rb–Sr whole-rock data give the following results:[Formula: see text]Detailed studies of the Matachewan dike swarm show that in most cases Rb–Sr mineral isochron age values from individual samples are concordant with the Rb–Sr whole-rock age values for the dike swarm as a whole and are clearly greater than K–Ar age values on the same material. The fact that the mineral isochron ages and the whole-rock ages by the Rb–Sr method are similar suggests that there has been no significant thermal overprinting of these particular dikes since their emplacement and solidification or contamination by radiogenic 87Sr from the host environment. Furthermore, the general scatter and lack of significant grouping of the lower K–Ar values similarly suggests that there was no single period of thermal resetting of the K–Ar ratios. It is observed that the K–Ar age values are greater in samples containing mica, and are more scattered and lower in samples in which potassium is contained in late-stage or principal phases other than mica. Therefore, it seems likely that significant loss of argon may result at near or normal surface temperatures and that the rate of diffusion is dependent on the mineralogy of the sample. It is concluded that reasonably good age determinations can be made on diabase dike rocks of ancient age by the Rb–Sr whole-rock isochron method, if the mineral isochrons are concordant.

Rb–Sr Geochronology of the Hida metamorphic belt, Japan

Rb–Sr whole-rock and mineral isochron ages have been determined for metamorphic and granitic rocks of the Hida metamorphic belt. The results indicate that an extensive metamorphic event together with plutonic activity took place within the belt during the latest Paleozoic – early Mesozoic period. The older ages of 220–250 m.y. represent an earlier phase of the metamorphism, whereas the younger ages of 170–180 m.y. represent a later phase. The Funatsu granitic rocks yielded a whole-rock isochron age of 176 m.y. with an initial 87Sr/86Sr ratio of 0.7056. This age is believed to indicate the time of original emplacement, and the rocks are considered to represent late-kinematic intrusion in the Hida belt.Some information on the middle Paleozoic metamorphism in the Hida Mountains was obtained from the isochron study. The whole-rock isochron age of 412 m.y. for the metamorphic rocks of the Fujibashi area may be considered, although not confirmed, to indicate the time of older metamorphism. The Omi Schist of the Circum–Hida crystalline schist belt, which belongs to the glaucophanitic type of metamorphism, gave a mineral isochron age of 350 m.y. thereby providing evidence of mid-Paleozoic metamorphism.The initial 87Sr/88Sr ratios for the whole-rock samples of the Hida metamorphic belt are found to be generally low, i.e. 0.705–0.708. This is especially so for the metamorphic rocks from the northern part of the belt where the lowest values were found.

Geochronology of the Yellowknife area, Northwest Territories, Canada

New whole rock Rb–Sr isochron data indicate that the Archean greenstones of the Yellowknife area are 2625 ± 160 m.y. old (λ87Rb = 1.39 × 10−11 yr−1). The thick, complexly folded sequence of volcanics and overlying sediments is intruded by two syntectonic to late tectonic batholiths (Rb–Sr isochron ages 2610 ± 58 m.y. and 2590 ± 90 m.y.) and a large irregular mass of muscovite granite (Prosperous Lake granite), which yields a mineral isochron age of 2575 ± 25 m.y.Low initial 87Sr/86Sr ratios (0.702–0.703) suggest that the late tectonic batholiths were unlikely to have formed by granitization of a great thickness of sediments beneath the greenstones (as has been suggested). Since the initial ratios of the volcanics and intrusive rocks are similar, the basalt–dacite association is probably the early extrusive expression of the subcrustal process that leads to the formation of quartz diorite–granodiorite batholiths. Appreciable sialic contamination need not be invoked. On the other hand, the Prosperous Lake granite ((87Sr/86Sr)0 = 0.712) may be the product of partial anatexis of the newly derived sialic crustal segment.