Proterozoic Mantle Research Articles

Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northernmost part of the Archean Wyoming craton

Minettes exposed in southern Alberta near the Milk River are the northern outliers of the Eocene Sweet Grass Hills igneous complex of the Montana alkalic igneous province. These minettes often contain coarse-grained xenoliths of phlogopite + clinopyroxene ± apatite. The parent magmas of the minettes were generated at pressures [Formula: see text]17 kbar in equilibrium with clinopyroxene + phlogopite ± olivine. Fractional crystallization and mixing provided a spectrum of evolved minettes and cumulates, the latter of which were sampled by subsequent minette magmas as xenoliths. Two xenoliths were dated at 49.0 ± 0.8 Ma and 52 ± 1.7 Ma. The host dyke of the latter xenolith gave an age of 50 ± 0.3 Ma. The minettes and their xenoliths have overlapping values of 87Sr/86Sri, εNdT, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, similar to those of alkaline igneous rocks from farther south in the Montana alkalic igneous province. The Sweet Grass Hills lie north of the Great Falls Tectonic Zone, previously interpreted as a Proterozoic suture zone separating the Archean Medicine Hat block from the Archean Wyoming craton to the south. Geochemical data for the Milk River minettes provide evidence for a history of the mantle underneath the Medicine Hat block, similar to that found previously for mantle-derived rocks of the Wyoming craton, including a significant Proterozoic mantle enrichment event. Given this similarity, we suggest that the Wyoming craton extends into southern Alberta, and that the Great Falls Tectonic Zone does not represent a Proterozoic suture of two Archean blocks.

Crustal-contaminated komatiitic basalts in Southern China: products of a Proterozoic mantle plume beneath the Yangtze Block

Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the Proterozoic Sibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiitic basalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with NiCu(PGE) sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and diorite in their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt%. The Zhongkui Flow is highly fractionated to form a spinifex zone with lower MgO (5.3–5.9 wt%) and a cumulate zone with higher MgO (17.3–17.9 wt%). Overall the rocks have TiO 2=0.44–0.74 wt%. Relative to primitive mantle, they are enriched in Th and LREE, but exhibit negative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminated komatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments, which caused sulfide-segregation resulting in the formation of NiCu(PGE) sulfide deposits. The occurrence of the komatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental rift environment, and implies that the Yangtze Block may have had an Archean basement through which the Sibao komatiitic basalts erupted.

The Siberian lithosphere traverse: mantle terranes and the assembly of the Siberian Craton

The kimberlite fields scattered across the NE part of the Siberian Craton have been used to map the subcontinental lithospheric mantle (SCLM), as it existed during Devonian to Late Jurassic time, along a 1000-km traverse NE–SW across the Archean Magan and Anabar provinces and into the Proterozoic Olenek Province. 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe (major elements) and proton microprobe (trace elements). These data, and radiometric ages on the kimberlites, have been used to estimate the position of the local (paleo)geotherm and the thickness of the lithosphere, and to map the detailed distribution of specific rock types and mantle processes in space and time. A low geotherm, corresponding approximately to the 35 mW/m 2 conductive model of Pollack and Chapman [Tectonophysics 38, 279–296, 1977], characterised the Devonian lithosphere beneath the Magan and Anabar crustal provinces. The Devonian geotherm beneath the northern part of the area was higher, rising to near a 40 mW/m 2 conductive model. Areas intruded by Mesozoic kimberlites are generally characterised by this higher, but still `cratonic' geotherm. Lithosphere thickness at the time of kimberlite intrusion varied from ca. 190 to ca. 240 km beneath the Archean Magan and Anabar provinces, but was less (150–180 km) beneath the Proterozoic Olenek Province already in Devonian time. Thinner Devonian lithosphere (140 km) in parts of this area may be related to Riphean rifting. Near the northern end of the traverse, differences in geotherm, lithosphere thickness and composition between the Devonian Toluopka area and the nearby Mesozoic kimberlite fields suggest thinning of the lithosphere by ca. 50–60 km, related to Devonian rifting and Triassic magmatism. A major conclusion of this study is that the crustal terrane boundaries defined by geological mapping and geophysical data (extended from outcrops in the Anabar Shield) represent major lithospheric sutures, which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in composition and rock-type distribution between 100 and 250 km depth. The presence of significant proportions of harzburgitic and depleted lherzolitic garnets beneath the Magan and Anabar provinces is concordant with their Archean surface geology. The lack of harzburgitic garnets, and the chemistry of the lherzolitic garnets, beneath most of the other fields are consistent with the Proterozoic surface rocks. Mantle sections for different terranes within the Archean portion of the craton show pronounced differences in bulk composition, rock-type distribution, metasomatic overprint and lithospheric thickness. These observations suggest that individual crustal terranes, of both Archean and Proterozoic age, had developed their own lithospheric roots, and that these differences were preserved during the Proterozoic assembly of the craton. Data from kimberlite fields near the main Archean–Proterozoic suture (the Billyakh Shear Zone) suggest that reworking and mixing of Archean and Proterozoic mantle was limited to a zone less than 100 km wide.

Secular evolution of subcontinental lithospheric mantle

Chemical and petrographic observations on mantle-derived xenoliths and xenocrysts define a fundamental distinction between Archean cratonic mantle and Phanerozoic circumcratonic mantle. Archean xenoliths are more depleted on average, and those from South Africa and Siberia, the best-studied areas, have higher Si/Mg than Phanerozoic xenoliths of similar Mg#; subcalcic harzburgites are wellrepresented in Archean xenolith and xenocryst suites, but rare in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. Typically, garnet suites from volcanics penetrating Archean crust have higher Cr#, lower Y, Y/Ga and HREE/Sc, and higher Zr/Y than those from volcanic rocks in Proterozoic and Phanerozoic terranes. These differences, and the rarity of subcalcic garnets in mantle beneath Proterozoic and Phanerozoic crust, are consistent with a decrease in mean modal cpx/gnt and (cpx+gnt) from Archean to Proterozoic to Phanerozoic mantle. Temperatures calculated from the Ni content of peridotitic garnets (TNi) can be referred to a local paleogeotherm to map the distribution of garnet types with depth. These maps show that Archean mantle sections are typically strongly stratified, with concentrations of harzburgitic rocks and depleted lherzolites in the middle to lower parts of the sections, and more fertile lherzolites at depth. Proterozoic mantle sections are less depleted and typically less strongly stratified, while the garnet-facies parts of Phanerozoic sections are still less depleted and essentially homogeneous. Inversion of TNi and composition data on garnets allows mapping of the vertical distribution of the XFo of coexisting olivine in mantle sections. Archean sections typically show decreasing XFo with depth; Proterozoic sections are less regular, and many show the lowest XFo in the upper parts of the sections. These differences imply changes in the processes that have produced subcontinental lithospheric mantle (SCLM) at different times in Earth history. In xenolith suites, the Cr content of peridotitic garnet correlates well with the Al content of the host peridotite, and Al is closely related to the contents of other major and minor elements. These correlations allow calculation of the mean composition of a mantle section, given the Cr content of garnet xenocrysts from that section. The calculated mean composition of SCLM beneath Archean, Proterozoic and Phanerozoic terrains shows a secular evolution in all measures of depletion, such as Al, Ca, Mg#, and Fe/Al; Proterozoic SCLM is intermediate in composition between Archean and Phanerozoic SCLM. Cenozoic SCLM, exemplified by garnet peridotites from young extensional areas (China, Siberia, Australia) is only mildly depleted (²10% melt extraction) relative to Primitive Mantle. Similar compositions are found in some exposed spinel lherzolite massifs in extensional settings, such as Zabargad Island. Shallow spinel-facies SCLM beneath some Phanerozoic terrains, especially in Europe (both xenolith suites and exposed massifs), is more depleted and may represent reworked Proterozoic SCLM; this is consistent with some Re- Os depletion ages. Archean xenolith suites show positive correlations between Al, Fe and Cr that are not present in younger mantle samples; these correlations indicate that no Cr-Al phase was present on the liquidus during the melting event that produced Archean mantle. We suggest that most Archean SCLM was derived by high-degree melting at depths ³150 km, and that subduction processes have not been significant in its genesis. Phanerozoic circumcratonic xenolith suites are typically much less depleted than ophiolitic and abyssal peridotites; this suggests that subducted oceanic mantle is not a major component of Phanerozoic SCLM. Instead, the fertile composition of circumcratonic xenolith suites, and comparison with the Zabargad situation, suggests that most existing Phanerozoic SCLM probably has been generated in extensional post-orogenic environments. Density modelling shows that Phanerozoic SCLM, once it has cooled to typical conductive geotherms, will be gravitationally unstable with respect to the asthenosphere. This instability provides a mechanism for removing such lithosphere and replacing it with upwelling fertile material on a short time scale. In contrast, typical sections of Archean and Proterozoic SCLM are gravitationally stable, allowing their long-term preservation. The broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crust-mantle coupling over periods of aeons beneath Archean and Proterozoic terranes; it also requires an evolution in fundamental processes involved in the formation of continents and their roots. This evolution may be related to the secular cooling of Earth, and a consequent decrease in the melting temperature and depth of melting in mantle plumes.

Evidence for Interaction of Proterozoic (2 Ga) Sub-Continental Mantle with an Enriched Mantle Plume from Re-Os Isotope Systematics in French Massif Central Peridotite Xenoliths

Evidence for Interaction of Proterozoic (2 Ga) Sub-Continental Mantle with an Enriched Mantle Plume from Re-Os Isotope Systematics in French Massif Central Peridotite Xenoliths

High-Mg andesites from the Gila Bend Mountains, southwestern Arizona: Evidence for hydrous melting of lithosphere during Miocene extension

Miocene andesites and basaltic andesites from the eastern Gila Bend Mountains have high Mg# values (most 66–72), high concentrations of Ni and Cr, and olivine phenocrysts as Mg rich as Fo90. Cores of euhedral olivine phenocrysts in some andesites are in equilibrium with the whole-rock Mg# values, and the compositions of chromian spinel inclusions in olivine phenocrysts are similar to those from boninites and other primitive magnesian silicic volcanic rocks. The Gila Bend lavas are interpreted to have evolved from a variety of primary mantle-derived melts, some as silicic as approximately 56% SiO2. All the lavas have strong lithospheric geochemical characteristics: Ba/Nb ratios range from 60 to 170, initial 87Sr/86Sr is from 0.7063 to 0.7073, and ϵNd is from −5 to −8. Previous experimental studies have established that the generation of primary andesitic magmas from mantle peridotite requires low-pressure (<15 kbar), hydrous conditions, and a refractory source. The high-Mg andesites erupted near the end of Miocene extension in the Gila Bend region. Because of their primitive nature and tectonic setting, the Gila Bend lavas can be used to test three general types of models that have been proposed recently to relate magmatism, extension, and lithospheric geochemical character of mid-Cenozoic volcanic rocks of the Basin and Range. Models that require crustal contamination of the magmas or that produce the magmas by melting of basaltic material within mantle lithosphere are unsuitable for the origin of the Gila Bend lavas. However, some elements of a model involving hydrous melting of mantle lithosphere during extension are consistent with the chemistry and tectonic setting of the lavas. The high-Mg andesites are interpreted to reflect hydrous melting of harzburgite near the base of mantle lithosphere at unusually low pressure. The low pressure was the result of tectonic thinning of the lithosphere during extension, and the source of the volatiles was most likely contemporaneous subduction. Although the Nd isotopic composition of the Gila Bend lavas originated from Proterozoic mantle lithosphere, the Pb isotope ratios mainly reflect the influence of Mesozoic and Cenozoic subduction.

Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin

Nd and Pb isotopes indicate significant direct involvement of older continental crust during the generation of the Penokean synorogenic intrusive suite in Wisconsin. Initial values for plutons show a clear trend of increasing ϵ Nd , from −6 to +4 southward from the Superior Province to the suture between the Pembine-Wausau and Marshfield terranes, indicating decreasing amounts of contamination by older crust. Crossing into the Archean Marshfield terrane, there is a rapid shift back down to lower ϵ Nd values (−1 to −7). Feldspar common Pb from synorogenic plutons generally defines a mixing array between Archean crust and primitive arc crust, which has Pb isotope compositions that are similar to those of the Proterozoic mantle. A mixing model based on Nd and Pb concentrations and isotopic ratios indicates that synorogenic intrusive rocks acquired their isotopic ratios through interaction with Archean crust, rather than through contamination of a mantle source region by sediment subduction. In addition, direct dating of exposed basement in the Pembine-Wausau terrane indicates, for the first time, that Archean crust is present here (U/Pb zircon age of 2607 ± 22 Ma). The combined Nd and Pb isotope data demonstrate the importance of distinguishing between contamination of the mantle and remelting crust basement for the origin of old Nd model ages in Precambrian orogens. Such distinctions are critical for evaluating crustal growth estimates in Precambrian orogenic terranes. The preferred tectonic model for the Penokean orogeny involves back-arc rifting of the southern Superior Province during a major period of Penokean orogenesis, followed by collision of the Marshfield terrane to the south, which was also intruded by Penokean-age magmas. In addition to the isotopic and geochronology data presented here, this model is supported by gravity data which indicate the presence of low-density material (probably Archean crust) south of the Niagara fault; this feature is interpreted to be a buried extension of the southern margin of the Superior Province beneath the Pembine-Wausau terrane. During the Penokean orogeny, synorogenic intrusive rocks acquired their isotope compositions through direct interaction with Archean crust.

Petrology and geochemistry of crustally contaminated komatiitic basalts from the Vetreny Belt, southeastern Baltic Shield: Evidence for an early Proterozoic mantle plume beneath rifted Archean continental lithosphere

New isotope and trace element data are presented for komatiitic basalts and a related peridotite Vinela Dike from the large Vetreny Belt in the southeastern Baltic Shield. The MgO contents of the erupted and intruded magmas are inferred to increase from 13 to 17% towards the center of the belt, which implies the respective increase in liquidus temperatures from 1370 to 1440°C. The elevated liquidus temperatures suggest that the source of the komatiite magmas had a substantially higher potential temperature (1630°C) than the ambient mantle (1480°C) and are regarded as evidence for the existence of a mantle plume underlying the region at ∼2.45 Ga. Parental magmas to the lavas and the Vinela Dike were shown to have komatiite composition and were derived from a long-term LREE-depleted mantle source with ϵNd( T) of ca. +2.6. The evolution of these magmas en route to the surface was mainly controlled by 4–15% contamination with older felsic crustal rocks, which resulted in substantial changes in incompatible trace element and isotope ratios. The obtained SmNd internal isochron ages of 2449 ± 35 and 2410 ± 34 Ma for the lavas, 2430 ± 174 Ma for the Vinela Dike, and a whole-rock PbPb age of 2424 ± 178 Ma for the lavas together with a UPb zircon age of 2437 ± 3 Ma are identical to the reported UPb zircon and baddeleyite ages for numerous mafic-ultramafic layered intrusions in central and northern Karelia. From their chemical and isotope similarities, it is likely that these rocks had allied parental magmas. These magmas may have been emplaced in a continental rift setting during the interaction of a mantle plume and continental crust. Impinging of a plume head beneath the continental lithosphere resulted in its thinning, stretching, and rifting but failed to open a new ocean. This extensive magmatic event was responsible for a substantial contribution of early Proterozoic juvenile material to the Archean continental crust in the Baltic Shield.

Thermal perturbation during charnockitization and granulite facies metamorphism in southern India

Abstract We have deduced the steady‐state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P‐T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m2 at 1–2.5 Ga. Comparison of the calculated steady‐state geotherm with the available P‐T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km.Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO2 influx from a deep‐seated source, possibly the mantle. In this work, we have evaluated both the transient and steady‐state thermal effects of the heat convected by CO2 outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady‐state geotherm, and that a flux of CO2 of ±90 mol/m2 yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO2 flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value.Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35‐km‐thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre‐uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non‐erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO2 fluxing from a deep‐seated source as the cause for the formation of charnockites in this zone. The required focusing of CO2 in this case is c. 40% of that estimated in the model where CO2 fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO2 fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.

Geochemistry of pre-Taconian mafic volcanism in the Humber Zone of the northern Appalachians, Québec, Canada

There are two types of pre-Taconian mafic volcanics in the Humber Zone of the northern Appalachians: (1) relatively small scattered volumes of transitional to alkaline basalts and hypabyssal intrusives; and (2) localized, though more voluminous, subalkaline tholeiitic volcanics. They formed during a Late Proterozoic to Early Paleozoic period of continental lithospheric extension which led to the break-up of Laurentia and to the opening of the Iapetus ocean, and were subsequently deformed and metamorphosed during the Ordovician Taconian and Late Silurian to Middle Devonian Acadian Orogenies. Subalkaline tholeiitic basalts have major-element and REE abundances similar to that of modern N-MORB. However, they are further characterized by variable [ La Sm ] N (0.7–1.7) and ϵ Nd values (+2.8 to +7.0, at 540 Ma), and Ti Zr , Ta La , Th Ta and P Ce ratios forming more-or-less developed Ti, Ta and P troughs on mantle-normalized diagrams. Primitive subalkaline tholeiitic basaltic melts were most likely derived from a depleted asthenospheric source, while heterogeneity in the ϵ Nd , La Sm , Ti Zr , Ta La , Th Ta and P Ce values is attributed to variable degrees of assimilation and fractional crystallization of both lower- and upper-crustal material during ascent and eruption. The transitional to alkaline basalts are enriched in moderately to highly incompatible trace elements (LREE, Ti, P, Hf, Ta, Th), but unlike tholeiites, they do not show evidence of crustal assimilation. Transitional basalts have less fractionaed REE ( 3<-[ La Yb ] N ≤9 ) and lower ϵ Nd values (+3.3 to +4.9) than alkaline basalts ( 12≤ La Yb ] N ≤20 ; ϵ Nd = +5.9 to +6.4). The enriched character of the transitional to alkaline basalts is either explained by extraction from a heterogeneous enriched asthenospheric mantle source, or a metasomatized lithospheric mantle source previously enriched by OIB veins. Although the available geochemical data cannot discriminate between contributions from the lithospheric and asthenospheric mantles, relatively high and positive ϵ Nd values indicate that the source of transitional and alkaline basalts had not suffered long-term enrichment, and that contribution from the Late Archean and Early to Middle Proterozoic mantle lithosphere is unlikely.

Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granite area, southeastern Finland

Abstract The rapakivis in southeastern Finland and Russian Karelia cover a total area of more than 20,000 km 2 and comprise the Proterozoic (1.6-1.3 Ga) anorogenic Wiborg, Ahvenisto, Suomenniemi, and Onas batholiths, and associated mafic rocks. A model of the crustal and upper mantle structure in this area based on gravity, aeromagnetic, deep seismic sounding, and petrophysical data is presented. The average density (2625 kg/m 3 ) of the batholiths is lower than that of the surrounding Svecofennian rocks by about 80 kg/m 3 . The density histogram of the rapakivi granites is divided into two normally distributed components with mean densities of 2625 kg/m 3 (major) and 2690 kg/m 3 (minor), respectively. There are related local gravity anomalies. The susceptibility histogram of the rapakivi granites is decomposed into five log-normally distributed components. The two major components have mean susceptibilities of (165 and 1305) × 10 −6 SI units. The former component (40%) is virtually paramagnetic, and the latter (60%) is weakly ferromagnetic. The two components with mean susceptibilities of (6000 and 13,5000) × 10 −6 SI units constitute only 3% of all the samples. Associated gabbro-anorthosites also have high susceptibilities (1450-30,370) × 10 −6 SI units. These ferrimagnetic rocks cause noticeable local aeromagnetic anomalies. An extensive Bouguer anomaly low is associated with the batholiths. A pronounced regional high surrounds this low. The range of values is nearly 60 mGal. The anomaly pattern is primarily the result of superposition of the gravity low, which is due to the relatively low-density upper and middle crust, onto the broader gravity high, that is due to the uplifted Moho. According to the published deep seismic sounding results, the crust is 41 km thick in the rapakivi region. Thus it is 6–20 km thinner than to the west, north and south of the rapakivi batholiths. The refraction model also implies a 6 km thick high-velocity body at a depth of 10 km. This body has a high v P v S -ratio and is interpreted as gabbro-anorthosite. The magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but lower than the values in the metavolcanic belt to the west. Within the Wiborg batholith there is a zone of positive regional magnetic anomalies with a separate, well defined magnetic high of about 400 nT in the western part. This anomaly is interpreted as a pipe-like magnetic body with an abnormally strong magnetization. The upper surface of the body lies at depths between 12 and 15 km. The body extends from the upper to the lower crust. The model derived from geophysical data supports a Proterozoic mantle upwelling, which provided excess heat to cause uplift and partial melting of both the upper mantle and the lower crust. Partial melting of the upper mantle produced intrusions of gabbro-anorthosites and diabases, while extensive partial melting of the lower crust produced rapakivi magmas and caused their emplacement into the upper crust.

Nature and timing of Franklin igneous events, Canada: Implications for a Late Proterozoic mantle plume and the break-up of Laurentia

U Pb baddeleyite/zircon ages have been determined for gabbro sills and diabase dykes emplaced during Franklin igneous events in northern Canada. Results for seven widely separated sills from Victoria Island, Coronation Gulf and Bathurst Inlet, and for a single diabase dyke from southeastern Baffin Island, indicate that most of the Franklin igneous activity was synchronous and occurred at 723 + 4/−2 Ma. A U Pb baddeleyite/ zircon age of 718 ± 2 Ma for one of the Victoria Island sills suggests that the magmatism continued for at least 5 m.y. after the initial outburst. The short-duration and large volume of the intrusive tholeiitic magmatism and its association with the Natkusiak flood basalts suggest a link with a mantle plume-generated hotspot. The onset of magmatism may have coincided with the arrival of the plume at the base of the lithosphere. Stratigraphic relationships on Victoria Island indicate that differential uplift occurred just prior to the initial outbreak of igneous activity, which is interpreted as a response to doming above the upwelling plume. The exposed record provides no indication of significant pre-volcanic extension and rifting, although some NE-SW extension occurred after the initial eruptive phase. The 779 Ma mafic magmatism in the Mackenzie Mountains, the 723 Ma Franklin events and the 615 Ma Long Range dykes in Labrador are three precisely dated mafic events in North America that may be contemporaneous with various phases of Late Proterozoic break-up of Laurentia. These ages establish key chronological constraints for global correlation of Late Proterozoic epicontinental basins and for Precambrian plate reconstructions.

Geochemistry of two metavolcanic arc suites from the Central Metasedimentary Belt of the Grenville Province, southeastern Ontario, Canada

Samples of two subalkaline metavolcanic suites, the Tudor formation (ca. 1.28 Ga) and the overlying Kashwakamak formation, have been analysed for major elements and 27 trace elements (including rare-earth elements). The Tudor formation is tholeiitic and contains mainly basaltic flows, whereas the Kashwakamak formation is calc-alkaline and contains mainly andesitic rocks with minor felsic rocks. The succession has been regionally metamorphosed to upper greenschist – lower amphibolite facies. Trace-element abundances and ratios indicate that rocks of the Tudor and Kashwakamak formations are island-arc type. Geochemical modelling using rare-earth elements, Zr, Ti, and Y indicates that the Tudor volcanic rocks are not derived from a single parental magma through simple fractional crystallization. Equilibrium partial melting of a heterogeneous Proterozoic upper mantle can explain the trace-element abundances and ratios of Tudor formation volcanic rocks. The intermediate to felsic rocks of the Kashwakamak formation appear to have been derived from a separate partial melting event. The data are consistent with an origin of the arc either on oceanic crust or on thinned continental crust, and with accretion of the arc to a continental margin between the time of extrusion of Tudor volcanic rocks and that of Kashwakamak volcanic rocks.

Nd and Pb isotopic constraints on the origin of the Purtuniq ophiolite and Early Proterozoic Cape Smith Belt, northern Québec, Canada

We report Pb isotopic compositions for 35 samples and Sm-Nd isotopic data for 44 volcanic rocks from the Purtuniq ophiolite, the continental Povungnituk and oceanic Chukotat groups in the Cape Smith Belt. Pb-Pb whole-rock dating of the sheeted dike complex and cumulate rocks of the ophiolite, which are preserved in separate thrust sheets, yield identical isochron ages of 1.97 ± 0.04 and 1.97 ± 0.11 Ga, respectively, supporting a contemporaneous origin. Combined data for layered cumulates, sheets dikes and pillow basalts constrain the age of the ophiolite to 1.98 ± 0.03 Ga ( μ 1 = 8.0) in good agreement with a previously reported 1.998-Ga U-Pb zircon age. In contrast, Pb isotopic data of the ∼1.96−1.92-Ga continental Povungnituk and related transitional continental-oceanic Chukotat Group basalts yield young dates of 1.89 and 1.72 Ga, respectively, which may record tectonothermal events related to continental collision. Sm-Nd isotopic data for the ophiolite indicate highly heterogeneous sources ( ϵ Nd( t) + 1.0to + 6.1) and form a mixing array corresponding to an anomalously old age of ∼2.2 Ga. A similar Sm-Nd isotopic array with an age of ∼2.3 Ga is displayed by the data for LREE-enriched continental Povungnituk volcanic rocks and the associated, more depleted, oceanic basalts of the Chukotat Group. Magma contamination by Late Archean basement can be ruled out and we attribute the excessively old Sm-Nd ages to melting and mixing of two distinct mantle sources. The dominant magma source with ϵ Nd( t)-values of + 3 to + 5 is similar to the depleted Early Proterozoic mantle. A subordinate mantle component displays ϵ Nd( t)-values as low as + 1 and enrichment in LREE relative to the depleted Early Proterozoic mantle. It may have resulted from a relatively young enrichment event that produced a source with high Nd/Sm and moderately radiogenic Nd at 2 Ga. At the other extreme, it may reflect a much older event with little source enrichment in LREE. In this case the strong LREE enrichment in some volcanics would be due to LREE fractionation during melting, and it can be assumed that the isotopically evolved component had been stored outside the depleted mantle for a long time, in order to avoid its destruction by convection. The isotopic evidence is compatible with a magma origin in a plum pudding type mantle or a heterogeneous highly depleted mantle plume. The isotopic heterogeneity in the ophiolite suggests an origin at a slowly spreading ridge in an immature ocean basin.

Proterozoic mantle under Quesnellia: variably reset Rb–Sr mineral isochrons in ultramafic nodules carried up in Cenozoic volcanic vents of the southern Omineca Belt

Handpicked, acid-washed diopside, orthopyroxene, and olivine separates from 14 lherzolite nodules from four localities in southern British Columbia have been analyzed for 87Rb/86Sr and 87Sr/86Sr. The host lavas, of late Cenozoic to Pleistocene age, are nepheline-normative alkali basalt and basanite with Sr and Nd isotopic ratios (0.7025–0.7029; 0.51310–0.51326) close to those of mid-ocean ridge basalt. The present-day Sr isotopic compositions of nodule diopsides (9–239 ppm Sr) are 0.7021–0.7041, similar to those of the host rocks. Orthopyroxene and olivine, however, have 87Rb/86Sr ratios of 0.1 to &gt; 1.0 and much more radiogenic Sr (87Sr/86Sr commonly 0.708–0.720), and in several cases the three minerals form linear arrays in 87Rb/86Sr versus 87Sr/86Sr plots. If interpreted as mineral isochrons, these give dates from 99 to 1989 Ma; many cluster around 440–770 Ma. Four nodules from two localities (West Kettle River and Big Timothy Mountain) give dates of 1201–1989 Ma. Maximum dates for the other localities (Jacques Lake and Lassie Lake) are 600–650 Ma.87Rb/86Sr ratios for acid leaches of crushed and sieved nodule material are generally &lt; 0.706, and 87Sr/86Sr ratios for the host rocks are &lt; 0.703, so radiogenic interstitial material or host contamination cannot explain the high ratios observed in the low-Sr minerals. The acid leaches lie both above and below mineral isochrons, virtually never exactly on the isochrons, indicating that secondary material, not cogenetic with the nodules, is present along grain boundaries. Discordant mineral points lie as expected for partially reset old isochrons, with olivine most easily reequilibrated by Sr isotopic exchange with less radiogenic minerals, orthopyroxene next in changeability, and clinopyroxene least modified.The nodules cannot be cognate or related in any way to recent magma genesis. The radiogenic Sr in low-Sr minerals is due to in situ decay of Rb and thus the nodules are Proterozoic upper mantle, some of an age similar to the 1.7–2.3 Ga granitic basement that tectonically underlies southwestern Quesnellia in the same region containing the nodule-bearing vents. The 440–770 Ma isochrons are speculatively correlated with Late Proterozoic to early Paleozoic craton-margin-forming rift event(s). Pre-Phanerozoic mantle lithosphere extends as far west as the Okanagan Valley in southern British Columbia.

Mantle metasomatism: Isotope and trace-element trends in xenoliths from Kimberley, South Africa

Modal metasomatism in peridotite xenoliths from Kimberley, South Africa, is observed in the development of phlogopite, K-richterite, diopside and LIMA. The average bulk-rock minor- and trace-element contents of different peridotite types range up to 100× primitive mantle, and the bulk-rock compositions reflect the modal proportions of metasomatic minerals in individual samples. Present-day whole-rock 87 Sr 86 Sr =0.7045–0.7118 and 206 Pb 204 Pb =17.48–21.24 , and they have been little affected by interaction with the host kimberlite which has 87 Sr 86 Sr =0.7042 and 206 Pb 204 Pb =18.6–19.2 . The preferred interpretation is that these isotope ratios reflect Rb/Sr and U/Pb fractionation during the modal metasomatism ∼150 Ma ago, i.e. after Karoo magmatism at ∼190 Ma and prior to emplacement of the kimberlite at 90 Ma. Present-day ϵ Nd= −9.3 to +2.4, reflecting both the presence of low-Sm Nd Proterozoic mantle prior to metasomatism at 150 Ma, and the inferred shift to relatively high ϵ Nd during that metasomatism. Rb/SrU/Pb fractionation during metasomatism has resulted in a distinctive negative correlation between present-day 87 Sr 86 Sr and 206 Pb 204 Pb . This contrasts with positive SrPb isotope trends in MORB glasses and selected OIB, but it is similar to that observed in certain CFB provinces. The trend to relatively high Rb/Sr at low U/Pb appears to be linked to the development of a potassic phase, typically phlogopite, and as such it arguably offers the first evidence for a style of minor- and trace-element enrichment which is restricted to shallow levels in the Earth's mantle.

Sm?Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Per�pohja schist belt in northern Finland

The Perapohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial ɛNd-values of the Penikat intrusion (ɛNd(2440) = −1.6) and the Runkausvaara sill (ɛNd(2200) ≈ 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during “late” greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial 207Pb/204Pb ratio (μ1 = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial ɛNd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial ɛNd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.

A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge

Major and trace element and Sr- and Nd-isotope analyses are presented on 186-0 Ma magmatic rocks along an east-west traverse across North Chile at 22°S. ε Sr ranges from −25 to +100 and ε Nd from +6 to −9, but the low ε Nd and high ε Sr values are in rocks generated in the last 15 Ma. It is argued that previous discussions of petrogenesis in North Chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one, but two processes. One results in a progressive shift of ε Sr from −25 to +20 and ε Nd from +6 to −6 in Jurassic to Recent rocks, which is accompanied by increasing Ta/Sm and Sr and decreasing Th/Ta. The second is largely confined to the younger rocks and it is characterised by ε Sr increasing up to +100 with increasing SiO 2 and decreasing Sr, and it results in relatively shallow trends on an ε Nd-ε Sr diagram. The preferred interpretation is that trend 1 is due to the mobilisation of old, late Proterozoic mantle lithosphere as magmatism migrated eastwards, and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust. It follows that the mantle wedge is the principal site of crust generation, and it is argued that < 20% of the Sr in the recent northern Chile rocks is derived from the subducted ocean crust.

Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping

Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.

The Geochemistry of Upper Proterozoic Lavas From the Red Sea Hills, NE Sudan

Summary The geochemistry of the Kadewēb and Homogar Volcanic Groups from the Red Sea Hills of north-east Sudan is discussed. Rb-Sr studies show that these lavas are Upper Proterozoic in age (720 Ma and 670 Ma respectively). The lavas of both groups display a calcalkaline character and are broadly similar in terms of their lithology, phenocryst mineralogy and major element chemistry. Geochemically the Kadawēb and Homogar volcanic rocks are similar to modern arc lavas associated with converging plate margins, although they tend to have higher concentrations of some HFS elements. Despite overall similarity, a distinction can be made between the two groups on the basis of the geochemistry of the basaltic components of the suites. The Homogar basalts are enriched in LIL and REE, having high La/Nb, (Ce/Yb) n and Ta/Yb ratios compared with those of the Kadawēb group. It is suggested that the older Kadawēb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved, possibly continental, arc margin. Differences between these Upper Proterozoic rocks and modern arc lavas may be explained as resulting from differences in the nature of the Proterozoic mantle.