Sm-Nd Isotope Geochemistry Research Articles

Provenance of metasiliciclastic rocks at the northwestern margin of the East Gabonian Block: Implications for deposition of BIFs and crustal evolution in southwestern Cameroon

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Reply to comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Hubert Mvondo and Thierry Bineli Betsi

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Volcanic, sedimentary, and deformation history of Winter Lake greenstone belt, Slave craton, Northwest Territories: preliminary results from the 2019 field season

The Archean Winter Lake greenstone belt (WGB) in the Slave craton, located ~250 km northeast of Yellowknife in the Northwest Territories, has been underexplored relative to other supracrustal belts in the Slave craton, but shows potential for base-metal mineralization. It consists of lower greenschist to upper amphibolite grade mafic to felsic volcanic rocks and sedimentary rocks that are surrounded by ca. 3.3 to 2.9 Ga granitoids of the Central Slave Basement Complex (CSBC). The overall objective of this study is to better understand the origin and geodynamic evolution of the WGB and to evaluate its economic potential. The project will examine the tectonic setting of volcanic and sedimentary rocks of the WGB and their relationships to the CSBC granitoids.
 In 2019, three weeks were spent bedrock mapping near Newbigging Lake and one week near Big Bear Lake (at 1:5,000 and 1:10,000 scales, respectively) to generate a lithostratigraphic scheme for the southern WGB. This mapping evaluated the nature of contacts, previously interpreted as unconformities, between the CSBC, the ca. 2734-2924 Ma Central Slave Cover Group (CSCG) volcanic and sedimentary rocks, and post-volcanic sedimentary and granitoid rocks (Hrabi et al., 1995). Mapping near Big Bear Lake also allowed the examination of a previously mapped ~ 1.3-1.7 km wide rhyolite interpreted to be part of the ca. 3.3 Ga Newbigging Formation (Hrabi et al., 1995). Based on field observations, the formation does not include a rhyolite sensu stricto but rather a succession of weathered mafic to intermediate volcanic and intrusive rocks. This field season, the first of three, also led to the discovery of several semi-massive to massive sulfide showings within the mafic volcanic sequence, and the identification of sulfides (interpreted as remobilised) and multiple younging indicators within the younger conglomerate unit that are necessary for our lithostratigraphy study.
 Future U-Pb geochronological analysis of detrital zircon and Sm-Nd isotope geochemistry of metasedimentary rocks will enable us to determine their sources and age. Together with absolute timing of thermotectonic events, an Archean oceanic and continental crust geodynamic evolution model will be produced. An additional goal of this study is to identify the ore-forming environment for the sulfide mineralization and to generate an ore-deposit model for the mineralisation observed in the WGB.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Genesis and evolution of a Paleoproterozoic basement inlier within West Gondwana addressed by Sm-Nd isotopic geochemistry and Zr saturation thermometry

This paper presents new isotopic Sm-Nd ID-TIMS data and applies the Zr saturation thermometry on orthogneisses of the Quirino Complex to address the mechanisms of magma genesis, evolution and tectonic setting. Quirino Complex is considered a Paleoproterozoic basement inlier within a Neoproterozoic belt, comprised of two distinct calcalkaline magmatic suites: a Siderian to Rhyacian high-K suite (2.30–2.19 Ga); and a Rhyacian medium-K suite (2.17–2.16 Ga). High zircon solubility and εNd(t) values (+2.7 to −5.8) indicate that the protoliths of the high-K suite gneisses can be related to high-temperature (TZr = ca. 810 °C) I-type granites. The gneisses of the medium-K suite display isotopic signature for crustal residence (εNd(t) = −17.9 and −8.1). Their protoliths, rich in inherited zircons (ca. 3.0 Ga) and other restitic/refractory components (hornblende clots and mafic enclaves), were derived from partial melting of Archean crust at relatively low temperature (TZr = ca. 750 °C) within a collisional tectonic setting.

Provenances of the paleozoic terrigenous sequences of the Oldoi terrane of the Central Asian orogenic belt: Sm-Nd isotope geochemistry and U-Pb geochronology (LA-ICP-MS)

The results of Sm-Nd isotope geochemical investigations of the Paleozoic terrigenous sequences of the Oldoi terrane and U-Pb dating of detrital zircons by the LA-ICP-MS technique showed that the clastic material was mainly derived from Late Precambrian granitoids and Early to Middle Paleozoic granitoids and volcanics, which were formed owing to the reworking of the Late Precambrian continental crust. During the Silurian, the main source of terrigenous material in the sedimentation basin was the erosion of Late Precambrian and Early Paleozoic granitoids. In the Devonian, provenances became more diverse, and the sedimentation basin was additionally supplied by the decomposition products of Middle Paleozoic granitoids and silicic volcanics. The age ranges obtained for detrital zircons from the sandstones of the Middle-Late Devonian Oldoi Formation and the Early Carboniferous Tipara Formation are almost identical to the stratigraphic ages of the formations, which indicates that terrigenous sedimentation in the Oldoi terrane had been accompanied, at least since the Middle Devonian, by vigorous magmatic activity and occurred in an environment of a mature island arc or an active continental margin.

ザクロス山脈Sanandaj-Sirjan Zone北部のフェルシック深成岩のRb－Sr、Sm－Nd同位体分析

ザクロス山脈Sanandaj-Sirjan Zone北部のフェルシック深成岩のRb－Sr、Sm－Nd同位体分析

Elemental and Sm-Nd isotopic geochemistry on detrital sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains

Systematic results of major and trace element geochemistry and Sm-Nd isotopic geochemistry on detrital sedimentary rocks of Precambrian to Triassic in the Ganzi-Songpan block and Longmen Mountains are presented. The rocks are classified into greywackes or feldspar sandstones, grains of which are the mixtures of mafic rocks, felsic rocks, and quartz+calcite. Total rare earth elements (REE) contents of the rocks increase gradually and negative Eu anomalies become more obvious from Precambrian to Triassic, which may indicate intensifying crustal anatexis. Tectonic setting was stable during the Late Paleozoic, therefore there are obvious negative Ce anomalies. Nd model ages are between 1.6 Ga and 2.4 Ga, which are very similar to those of the Yangtze craton, South Qinling and North Qinling belts and quite different from those of the North China craton. Therefore, provenance of the sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains was the Yangtze craton and/or the Qinling orogen, which evolved on the basis of the Yangtze craton. The correlation between provenances and tectonostratigraphic strata of the western Yangtze craton shows that the source materials should be primarily from Neoproterozoic. Secondary sources were Archean and Paleoproterozoic strata. Triassic clastic sedimentary rocks contain Late Paleozoic mantle-derived materials, represented by the Emeishan Permian flood basalts. Spatial distribution of initial Nd isotopic compositions indicates that denudating areas were in the east and the north and depositing areas of deep water were in the west and the south for the Ganzi-Songpan basin during Triassic.

Evolution of the Cordilleran orogen (southwestern Alberta, Canada) inferred from detrital mineral geochronology, geochemistry, and Nd isotopes in the foreland basin

The provenance of sedimentary strata that accumulate in foreland basins record the growth and denudation of the adjacent orogen. We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trace- and rare earth element geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Foreland basin clastic rocks comprise three major pulses of sediment delivery: (1) upper Fernie Formation-Kootenay Group (154-142 Ma), (2) Blairmore Group (115-103 Ma), and (3) Milk River Group-Porcupine Hills Formation (78-58 Ma). Nd isotope data are dominated by e N d values of-7 to -12, interpreted to represent a well-mixed provenance from Devonian through Triassic strata and subordinate contributions from thrust-imbricated pre-Devonian strata of the Cordilleran miogeocline. Significant deviations to less negative (more juvenile) values between -5 and +1 represent periods when the foreland was flooded by juvenile detritus from oceanic arc sources such as Quesnel terrane and from syndepositional continental magmatic arcs of mid- and Late Cretaceous ages. Detrital zircon and monazite from the Fernie-Kootenay elastic pulse (pulse 1) indicate derivation from Triassic-Ordovician sand-stones imbricated within the thrust-and-fold belt, consistent with the Nd tracer results and petrography. U-Pb zircon ages from the Blairmore Group (pulse 2) confirm a provenance from Triassic and Jurassic arc rocks of Quesnel terrane with only minor contributions from older miogeoclinal rocks; they also record the presence of syndepositional magmatic material. The upper part of the Blairmore Group shows a transition to less juvenile Nd isotopic signatures and the reappearance of detrital zircons of miogeoclinal derivation. A similar pattern occurs in the Milk River-Porcupine interval (pulse 3) with juvenile material occurring early in the sequence, accompanied by zircon grains from syndepositional volcanic sources and by more continental material in the upper part of the sequence. Pulse 1 records the erosion of thrust-imbricated miogeoclinal rocks during the creation and erosion of the foreland thrust-and-fold belt with no detectable material derived from the deeper parts of the hinterland. A significant unconformity of ∼27 m.y. duration led to redistribution of the foreland basin fill and erosion of the adjacent thrust-and-fold belt and corresponds to a period of magmatic and tectonic quiescence in the southern Canadian Cordillera. Renewed contraction within the erosionally modified thrust wedge led to development of out-of-sequence thrust structures which allowed juvenile terranes (Quesnel terrane) to become the dominant source for foreland (pulse 2) to the exclusion of miogeoclinal material. Reappearance of the miogeoclinal signature in upper pulse 2 is interpreted to record eastward propagation of the thrust-and-fold belt into miogeoclinal strata. The third pulse of sediment records significant input of windblown ash from juvenile sources in the Coast Belt mixed with bedload components derived from more local sources in the eastern Cordillera. The youngest deposits in the basin (ca. 58 Ma) are characterized by a cosmopolitan provenance that likely records cannibalization of older parts of the foreland as previously deposited foreland strata became important components of the thrust wedge.

Nature of the Precambrian metamorphic blocks in the eastern segment of Central Tianshan: Constraint from geochronology and Nd isotopic geochemistry

Granitoid gneisses are widespread in Precambrian metamorphic blocks of eastern segment of the Central Tianshan Tectonic Zone, and they have intrusive contact relationships with their metamorphic sedimentary country rocks of Proterozoic Xingxingxia and Kawabulag groups. Zircon U-Pb ages from a granodioritic gneiss (IW11-1) and a parametamorphic schist (W05-9) are determined at the Weiya area. Euhedral prismatic zircons from the granodioritic gneiss (IW11-1) provide U-Pb isotopic discordia intercept ages of 1218∓17 Ma and 426∓26 Ma, respectively, and euhedral prismatic zircons from the parametamorphic schist (W05-9) display U-Pb isotopic discordia intercept ages of 1216∓74 Ma and 290∓15 Ma, respectively. A whole-rock Sm-Nd isotopic isochron is determined in augen granitoid gneiss samples at the Gang-gou-Kumishi area and we obtain the isochron age of 1142∓120 Ma, and its eNd(t) = −4.3. These geochronological data suggest that these Precambrian metamorphic basement blocks within eastern segment of the Central Tianshan Tectonic Zone can be produced during 1140-1220 Ma, and occur a nearly homochronous metamorphism. Integrated to these geochronological data, Nd depleted mantle model ages (TDm) and epsilon Nd(t) values of these granitoid gneiss samples indicate that they can derive from mixing in various scales both magmas from mantle and crust sources at a late Mesoproterozoic active continental margin tectonic environment. Similarity in geochronology, Sm-Nd isotopic geochemistry between Weiya-Xingxingxia, Pargangtag and Ganggou-Kumishi areas suggests that they could be a bigger uniform metamorphic basement block, which could be formed by the assembly of the supercontinent Rodinia and be separated by late geological processes.

Grenvillian and Caledonian tectono-magmatic activity in northeasternmost Svalbard

Abstract The Nordaustlandet Terrane of NE Svalbard forms an exposed part of the Barentsia microcontinent. Augen gneisses, migmatites, granites and gabbros dominate the scattered outcrops along the northeastern coast of Nordaustlandet, and on the smaller islands to the north and east, as far as Kvitøya. These outcrops probably represent the deepest exposed crustal levels within the folded Caledonian basement of the Nordaustlandet Terrane. In the present study, a variety of rock types have been analysed by different U-Pb dating techniques (conventional, Pb-evaporation and ion microprobe) on zircon, titanite and monazite The major and trace element compositions and Sm-Nd isotope geochemistry of these rocks have also been investigated. The augen gneisses yield U-Pb ages of c. 950 Ma, indicating that they are deformed late Grenvillian granites, similar to the Grenville-age granites and augen gneisses of northwestern and central Nordaustlandet. Migmatites, grey granites, aplitic dykes and a syenite (boulder) yield U-Pb ages mainly falling in the 430–450 Ma range, slightly older than the 410–420 Ma late-tectonic Caledonian granites further west. Both the Grenvillian and Caledonian granites are of crustal anatectic origin, and the Caledonian granites and migmatites may have formed largely by remelting of Grenvillian crust. The ages of the mafic rocks are uncertain, but Sm-Nd data indicate a possible emplacement age of c. 700 Ma for two of the gabbros, suggesting that they may be the result of rift-related magmatism in connection with the opening of the Iapetus Ocean. A few enigmatic inherited zircons of similar late Neoproterozoic age found in younger granites may possibly be related to this event. No evidence for late Neoproterozoic orogenic activity, similar to that in the Timanides of northern Russia, is seen in eastern Svalbard. At this time, eastern Svalbard (Barentsia) was probably part of the Laurentian margin, and probably located far away from northern Baltica.

Rapid maturation and stabilisation of middle Archaean continental crust: the Akia terrane, southern West Greenland

from the Akia terrane, southern West Greenland, supported by Sm-Nd isotope geochemistry, document its middle Archaean accretional history and provide new evidence about the location of its northern boundary. Zircon populations in grey gneiss and inherited zircons in granite show that magmatic accretion of new continental crust, dominated by intrusion of tonalite sheets in a convergent island arc setting, occurred between c. 3050 and 3000 Ma, around and within a c. 3220 Ma continental core. In the central part of the terrane, tonalite sheets were intercalated with older supracrustal rocks of oceanic affinity by intrusion, thrusting and folding during the Midterhøj and Smalledal deformation phases of Berthelsen (1960). Continued tonalite injection led to a thermal maximum with granulite facies conditions at c. 2980 Ma, dated by metamorphic zircons in grey gneiss. The metamorphic maximum was contemporaneous with upright, angular folds of the Pâkitsoq deformation phase. Within a few million years followed high-grade retrogression and intrusion of two large dome-shaped tonalite-granodiorite complexes, granites s.l. derived from remobilisation of grey gneiss, and post-kinematic diorite plugs. Whereas the relative chronology of these events is firmly established from field observations, zircons from the post-granulite facies intrusions all yielded statistically indistinguishable emplacement ages of c. 2975 Ma. These results show that crustal growth occurred in several short-lived events starting at c. 3220 Ma, and that final maturation and stabilisation of new, thick continental crust took place rapidly (within c. 20 Ma) at c. 2975 Ma.

Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada

Data from the Point Lake area, central Slave craton, suggest an intimate tectonic and paleogeographic association between volcano-sedimentary supracrustal rocks and adjacent gneisses. Granite plutons and orthogneisses yield U-Pb zircon crystallization ages ranging from ca. 3230 to 2818 Ma. Numerous mafic dykes cut the gneisses, and two have been dated by U-Pb zircon geochronometry at 2673 ± 3 and 2690 ± 3 Ma, ages similar to those of volcanic rocks in the Point Lake greenstone belt. Although high-strain zones form the greenstone-gneiss in most places, a structural repetition of granite about 4 km east of Keskarrah Bay is cut by numerous mafic dykes and apparently overlain depositionally(?) by pillow basalt. Mafic volcanic and plutonic rocks from Point Lake have initial (2.7 Ga) εNd values ranging from about +2.2 to -6.3, significantly lower than the depleted mantle at that time. The Nd data suggest either derivation from a more isotopically evolved reservoir, or assimilation of crust similar to the granite gneiss at Point Lake. We infer from the presence of mafic dykes of appropriate age in the basement and the low initial εNd values of some pillow basalts that the volcanic sequence developed on the older granitic crust. The supracrustal rocks may have been deposited in a back-arc basin floored at least in part by attenuated continental material. Closure of the basin, bulk east-west shortening, and sinistral oblique or strike-slip faulting then obscured the original relations between the volcanic and gneissic rocks.

Provenance analysis of lower Paleozoic cratonic quartz arenites of the North American midcontinent region: U-Pb and Sm-Nd isotope geochemistry

Lower Paleozoic supermature quartz arenites in Wisconsin and Michigan were derived from several Proterozoic and Archean terranes. Single-grain, detrital zircon populations from the Galesville and St. Peter Sandstones in Wisconsin yield very similar, closely concordant age distributions (Cambrian Galesville Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 6]). In contrast, most of the nine detrital zircons that were analyzed from the St. Peter Sandstone in the Michigan basin have high U contents (450‐2500 ppm) and are strongly discordant (60%‐90%). Six zircons from the Michigan basin that have ca. 1.0 Ga 207 Pb*/ 206 Pb* ages define a regression line that has an upper intercept of about 1100 Ma and a lower intercept of 15 Ma; one zircon has a 2.7 Ga 207 Pb*/ 206 Pb* age. The zircon data indicate that although a number of different terranes contributed detrital material to the Paleozoic quartz arenites in Wisconsin, 1.1 and 2.7 Ga terranes were the dominant sources, and not the local basement, which primarily consists of the ♢2.7 Ga Marshfield terrane, the 1.8 Ga Penokean orogen, and the 1.4 Ga Wolf River batholith. A terrane that has a 1.1 Ga age is probably the main source for the St. Peter Sandstone in the Michigan basin. Quartz separates were also analyzed for PbPb and Sm-Nd isotope variations, and the data do not indicate significant source differences between the heavy mineral fraction (zircons) and the quartz framework grains. Pb-Pb isochrons and Sm-Nd isotope data for quartz separates reflect mixing of the age groups and approximate relative proportions that are identified from the single zircon results. All isotope data on quartz separates and UPb zircon data indicate that the detrital constituents (zircons and quartz framework grains) that compose the lower Paleozoic quartz arenites in Wisconsin were primarily derived from the 2.7 Ga granite-greenstone terrane of the southern Superior Province and a 1.1 Ga terrane. The latter terrane is either the silicic volcanic rocks associated with the Midcontinent rift system, or, more likely, the voluminous granitic rocks that are associated with the Grenville Province on the eastern margin of North America. The Middle Proterozoic Grenville Province was the most important ultimate source of quartz and zircons to the St. Peter Sandstone in the Michigan basin; lesser amounts of material were contributed from the Archean Superior Province.

Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada

Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada