Neodymium Model Ages Research Articles

Direct evidence for crust-mantle differentiation in the late Hadean

Constraints on the evolution of the silicate Earth between 4.5 and 3.8 billion years ago are limited by the scarcity of pristine geological material from that period. The geodynamic evolution of the early Earth, prior to the preserved rock record, is thus mainly inferred from numerical modelling. To evaluate the geological significance of these simulations, geochronological constraints pertaining to the evolution of the Hadean crust are required. Here we show using Neodymium isotope variations generated by decay of now-extinct 146-Samarium that Paleoarchean rocks from the Singhbhum Craton, India derived from a Hadean depleted mantle reservoir that differentiated {4.19}_{-0.12}^{+0.06} billion years ago. The event postdates Neodymium model ages of mantle depletion inferred from other Archean rocks by 200 million years. This geochronological record is mirrored in the Hafnium isotope composition of the oldest zircon grains, suggesting that Hadean mantle differentiation proceeded via distinct pulses of large-scale magmatic activity and crustal rejuvenation.

The Congo deep-sea fan: Mineralogical, REE, and Nd-isotope variability in quartzose passive-margin sand

ABSTRACT The Congo deep-sea fan, the largest on Earth fed entirely with anorogenic detritus, is characterized by quartzose to pure quartzose sand, reflecting multiple recycling coupled with extreme chemical weathering in cratonic equatorial Africa. The very youthful lower course of the Congo River connects directly to a steep canyon, where detritus including quartz grains up to a few millimeters in diameter is funneled towards Atlantic Ocean floors and deposited at abyssal depths more than a thousand kilometers away from shore. This article illustrates for the first time in detail the mineralogical and geochemical signatures of Congo Fan sands and discusses the factors controlling their intersample and intrasample variability as a key to understand how sediment is generated, recycled, and finally transferred to the deep sea. Compositional variability is largely grain-size-dependent. Combined petrographic and Raman spectroscopy analyses demonstrate that quartz increases in coarser samples and size classes, whereas feldspars are concentrated in finer sizes, plagioclase relative to K-feldspar and orthoclase relative to microcline, defining an order of mechanical and chemical durability among detrital tectosilicates. Because of overwhelming quartz abundance and very low heavy-mineral concentration, quartz contributes significantly to the REE budget and up to 40–50% of Nd in coarser samples, characterized by εNd values as low as –21. The strong grain-size-dependent variability of εNd suggests that quartz carries a markedly more negative εNd signature than monazite and other detrital components. This is chiefly ascribed to the durability of quartz grains, able to survive repeated cycles of weathering and diagenesis through Proterozoic and Phanerozoic time better than all other minerals. Neodymium model ages are influenced less by grain size and quartz abundance but more by the Sm/Nd ratio of different detrital components, and samples hydrodynamically enriched in LREE-rich minerals display TNd,CHUR and TNd,DM ages 1.2–1.4 Ga younger than samples enriched in HREE minerals. Not all detritus in the Congo Fan is supplied transversally by the Congo River. Forward-mixing calculations based on mineralogical data indicate that sand entrained northward by longshore currents mixes progressively with Congo River sand along the northernmost Angola coast, penetrates in the Soyo estuary, and is eventually captured in the canyon and transferred to the deep-sea fan, where it is estimated to represents 7 ± 2% of turbidite deposits.

The Cuyano proto-ocean between the Chilenia and Cuyania terranes: rifting and plume interaction during the Neoproterozoic – early Palaeozoic evolution of the SW Gondwana margin

AbstractThe Precordillera mafic–ultramafic belt (PMUB), located in central-western Argentina, comprises mafic and ultramafic bodies interlayered and/or in tectonic contact with marine siliciclastic units. Whole-rock, mineral geochemistry and Nd–Sr isotope analyses performed in magmatic rocks suggest a relatively different spatial and temporal evolution along the belt. The southern PMUB (south of 32° S) evolved as an intra-continental rifted margin with an enriched mid-ocean-ridge basalt (E-MORB) tholeiitic to alkaline magmatism, to a proto-ocean basin (the Cuyano proto-ocean) with tholeiitic normal-MORB geochemical signature. Based on neodymium model ages (TDM), the magmatic activity started during the late Neoproterozoic Era and continued into the early Palaeozoic Era. Instead, the northern PMUB (28–32° S) evolved as an intra-continental rifted margin with dominant tholeiitic E-MORB to continental flood basalt (CFB) magmatism during the early Palaeozoic Era. ϵNd values (+3.4 to +8.4), rare earth element trends and high-field-strength element systematics, together with an estimated potential mantle temperature of c. 50–100°C above ambient mantle, suggest the PMUB magmatism derived from an enriched mantle source related to the effect of a rising plume linked to the Iapetus Ocean opening. In particular, TDM estimations of 600–550 Ma agree with reported magmatism in central to southern Appalachians. The magmatism in the PMUB, and those registered in the Neoproterozoic Catoctin Formation and in the Southern Oklahoma Aulacogen in the conjugated Laurentian margin, seem to be contemporaneous, sharing a similar plume-enriched mantle source. In this context, the E-MORB signature identified along the PMUB can be described as a plume-distal ridge tectonic setting over an extended margin.

Mapping Neodymium Model Ages of the Quebecia Terrane Near Saguenay, Quebec

The geological evolution of the Grenville Province remains a subject of confusion among geologists. Orogenic events have deformed the original features, making the geology of the area challenging to delineate. This study maps the distribution of crustal formation ages within the Quebecia terrane of the Grenville Province. This provides insight into the crustal provenance of the geological units present. Previous research suggested the presence of slivers of Paleoproterozoic crust (>1.65 Ga) within Pinwarian crust (1.5 Ga). Four geological samples were analyzed from the southern side of the Saguenay graben, where the Paleoproterozoic crustal slivers were thought to extend. Analysis through TDM model ages derived from Sm-Nd radiogenic dating aimed to identify the boundaries of these slivers. Determining the model age distribution within the terrane allows for further delineation of the geological history of the region. The samples analyzed in this study yielded Pinwarian TDM model ages, indicating that slivers of old crust are not present within the study area. These results provide further constraints in the detailed structure of the Quebecia composite arc belt and the geological events preserved within the Grenville Province.

Does the metavolcanic-sedimentary Rio do Coco Group, Araguaia Belt, Brazil, represent a continuity of the Quatipuru ophiolitic complex? – Constraints from U-Pb and Sm-Nd isotope data

Serpentinized peridotites and basaltic volcanic rocks from the Quatipuru, Serra do Tapa, and Morro do Agostinho complexes are part of dismembered ophiolite bodies hosted by metasedimentary units of the Neoproterozoic Araguaia Belt, Tocantins Province, central Brazil. Approximately 10 lenticular ultramafic bodies, which contain primitive (serpentinite) and more evolved (actinolite schist, actinolite-talc schist, actinolite-serpentinite schist, talc schist, talc-serpentine schist, actinolitite, cloritite) rocks, were identified in the Rio do Coco Group (RCG), a metavolcanic-sedimentary sequence in the southern portion of the belt. These bodies are intercalated with metasedimentary rocks. All lithologies are metamorphosed to upper greenschist and lower amphibolite facies. The sequence was initially characterized as a classic Archean greenstone belt, but the present study suggests that it should be considered an ophiolite suite. On the basis of the ƐNd(t) values the RCG rocks can be subdivided into two groups: Group 1, with positive ƐNd(t) values, neodymium model ages (TDM) of ca. 1.0 to 1.1 Ga, with ocean-island-basalt (OIB) affinity, and being less contaminated with metasedimentary rocks of the Baixo Araguaia Supergroup; and Group 2, with negative ƐNd(t) values, TDM ranging from 1.2 to 1.7 Ga, with a variable ocean island basalt (OIB), plume or enriched mid-ocean-ridge basalt (P- and E-MORB) affinity, and being more contaminated with metasedimentary rocks. Two samples were analyzed by the U-Pb method on zircon (LA-MC-ICPMS). The weighted average age from a serpentinite (715 ± 26 Ma) and an actinolite schist (702 ± 24) indicates a maximum emplacement age for this sequence at 708 ± 35 Ma, which is similar to the previously published age for the Quatipuru Complex (Sm-Nd isochron age of 757 ± 49 Ma). This may suggest that the metavolcanic-sedimentary sequence of the Rio do Coco Group and the Quatipuru ophiolite system could be part of the same dismembered oceanic lithosphere.

Timing and sources of pre-collisional Neoproterozoic sedimentation along the SW margin of the Congo Craton (Kaoko Belt, NW Namibia)

Isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium- to high-grade part of the Kaoko Belt in Namibia provides first robust constraints on the age of Neoproterozoic sedimentation along the southwestern margin of the Congo Craton. Dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c. 1.00 and c. 1.45Ga, and age populations comparable with known protolith ages from the gneisses of the Congo/Kalahari cratons. Dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c. 740–710Ma.Detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the Congo Craton. These samples show dominance of c. 1.00Ga, c. 750Ma and c. 650Ma old zircon grains that are probably derived from the Punta del Este – Coastal Terrane (Dom Feliciano and Kaoko belts) that acted as an arc/back-arc domain at c. 650–630Ma. Neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the Congo Craton.Recognition of the Punta del Este – Coastal Terrane crust as a source region for the youngest pre-collisional sediments of the Kaoko Belt suggests that this terrane must have been in close proximity to the Congo Craton passive margin already some time prior to their mutual collision at c. 570–550Ma. This is in accord with an interpretation that the c. 650–630Ma arc/back-arc Punta del Este – Coastal Terrane has developed directly on top of, or very close to, the attenuated Congo Craton passive margin.

Geochemical, Sr–Nd–Pb isotope, and zircon U–Pb geochronological constraints on the origin of Early Permian mafic dikes, northern North China Craton

Dolerite dike swarms are widespread across the North China Craton (NCC) of Hebei Province (China) and Inner Mongolia. Here, we report new geochemical, Sr–Nd–Pb isotope, and U–Pb zircon ages for representative samples of these dikes. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb analysis yielded consistent Permian ages of 274.8 ± 2.9 and 275.0 ± 4.5 Ma for zircons extracted from two dikes. The dolerites have highly variable compositions (SiO2 = 46.99–56.18 wt.%, TiO2 = 1.27–2.39 wt.%, Al2O3 = 14.42–16.20 wt.%, MgO = 5.18–7.75 wt.%, Fe2O3 = 8.03–13.52 wt.%, CaO = 5.18–9.75 wt.%, Na2O = 2.46–3.79 wt.%, K2O = 0.26–2.35 wt.%, and P2O5 = 0.18–0.37 wt.%) and are light rare earth element (LREE) and large ion lithophile element (LILE, e.g. Rb, Ba, and K, and Pb in sample SXG1-9) enriched, and Th and high field strength element (HFSE, e.g. Nb and Ta in sample SXG1-9, and Ti) depleted. The mafic dikes have relatively uniform (87Sr/86Sr)i values from 0.7031 to 0.7048, (206Pb/204Pb)i from 17.77 to 17.976, (207Pb/204Pb)i from 15.50 to 15.52, (208Pb/204Pb)i from 37.95 to 38.03, and positive ϵNd(t) (3.6–7.3), and variable neodymium model ages (TDM1 = 0.75–0.99 Ga, TDM2 = 0.34–0.74 Ga). These data suggest that the dike magmas were derived from partial melting of a depleted region of the asthenospheric mantle, and that they fractionated olivine, pyroxene, plagioclase, K-feldspar, and Ti-bearing phases without undergoing significant crustal contamination. These mafic dikes within the NCC formed during a period of crustal thinning in response to extension after Permian collision between the NCC and the Siberian Block.

Zircon U-Pb age, geochemical, and Sr-Nd-Pb isotopic constraints on the origin of alkaline intrusions in eastern Shandong Province, China

Alkaline intrusions in the eastern Shandong Province consist of quartz monzonite and granite. U-Pb zircon ages, geochemical data, and Sr-Nd-Pb isotopic data for these rocks are reported in the present paper. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses yielded consistent ages ranging from 114.3 ± 0.3 to 122.3 ± 0.4 Ma for six samples of the felsic rocks. The felsic rocks are characterised by a wide range of chemical compositions (SiO2 = 55.14–77.63 wt. %, MgO = 0.09–4.64 wt. %, Fe2O3 = 0.56–7.6 wt. %, CaO = 0.40–5.2 wt. %), light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (i.e., Rb, Pb, U) enrichment, as well as significant rare earth elements (HREEs) and heavy field strength (HFSEs) (Nb, Ta, P and Ti) depletion, various and high (87Sr/86Sr) i ranging from 0.7066 to 0.7087, low e Nd (t) values from −14.1 to −17.1, high neodymium model ages (TDM1 = 1.56–2.38Ga, TDM2 = 2.02–2.25Ga), 206Pb/204Pb = 17.12–17.16, 207Pb/204Pb = 15.44–15.51, and 208Pb/204Pb = 37.55–37.72. The results suggested that these rocks were derived from an enriched crustal source. In addition, the alkaline rocks also evolved as the result of the fractionation of potassium feldspar, plagioclase, +/− ilmenite or rutile and apatite. However, the alkaline rocks were not affected by crustal contamination. Moreover, the generation of the alkaline rocks can be attributed to the structural collapse of the Sulu organic belt due to various processes.

Geochemistry and neodymium model ages of Precambrian charnockites, Southern Granulite Terrain, India: Constraints on terrain assembly

The Southern Granulite Terrain (SGT) of southern India comprises a collage of Archean and Neoproterozoic high-grade metamorphic terrains, where the regional tectonic and high-grade metamorphic events culminated at ca.2.52Ga in the northern granulite blocks and ca.0.55Ga in the Neoproterozoic Madurai and Trivandrum Blocks. These domains were presumed to be separated either by the crustal-scale Cauvery shear zone system (CSZ) or the less known Karur–Kambam–Painavu–Trichur shear zone (KKPT). The Neoproterozoic granulite domains comprise mainly the Madurai Block (MB) to the north and Trivandrum Block (TB) to the south separated by the Achenkovil shear zone (ASZ) system. The SGT has been central to several studies focusing on the amalgamation of Gondwana supercontinent. A favored view has been that the Neoproterozoic terrains of the SGT represent segments of orogen(s) that accreted to an Archean continental margin during the early Cambrian although whether the tectonic regime was collisional, accretionary or intracratonic remains enigmatic. This article presents a large data set of major, trace element, Rb–Sr and Sm–Nd isotopic compositions focusing on Sm–Nd model ages (TDM) signifying the crustal residence age of charnockitic orthogneisses from both Archean and Neoproterozoic domains of the SGT, especially from the data deficient MB and TB domains. We present a regional Sm–Nd model age map for the SGT that delineates four distinct isotopic provinces with ages in the range of ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga. Clearly, the KKPT shear zone separates two isotopic provinces, extending the Archean terranes further south across the CSZ. The Archean and Neoproterozoic charnockites show distinct compositional characteristics indicating contrasting conditions of magma genesis and tectonic setting. Most importantly, an overwhelming majority of the Neoproterozoic charnockite gneisses show Sm–Nd model ages significantly older than their crystallization age constrained by zircon dating. The charnockite gneisses bear a strong affinity to calc-alkaline magmas and compositions consistent with mixing of Neoproterozoic arc magmas and older crustal components prompting a close analogy with magmatism at convergent margin-collisional tectonic settings.

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200–1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (TDM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613–1634) and smallest epsilon Nd (eNd) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss–Rockport Granite suites (1155–1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

Chemical fingerprinting of multiple large-scale magmatic events in the Mesoproterozoic Bangemall Supergroup, Western Australia

Three major igneous events, dated at ∼1465, ∼1070 and ∼500 Ma, are represented in the Proterozoic of central Western Australia, yet their extent is poorly understood. The compositions of dated mafic rocks from the western Bangemall Supergroup of Western Australia have been used to establish a chemical fingerprint for the ∼1465 Ma and ∼1070 Ma intrusive events, and to assign sills of an unknown age to one of the two events. A similar approach has been used to identify the extent of ∼500 Ma magmatism. Low-pressure fractionation or accumulation has exerted a strong influence on the chemistry of rocks from all magmatic events, but distinctive trace-element ratios (e.g. Th/Nb, Nb/Zr) and rare-earth element (REE) chemistry (e.g. Eu/Eu*, (Gd/Yb)CN) can discriminate different events. These ratios remain constant regardless of the degree of fractionation or accumulation, and reflect the chemistry of the respective mantle sources. Based on chemistry, ∼500 Ma igneous rocks are not found in the Bangemall Supergroup. Neodymium model ages for ∼1465 Ma sills overlap with crystallisation ages of subduction-related felsic intrusive rocks from the adjacent Gascoyne Complex, and this, combined with trace-element and REE chemistry, suggests that the mantle source for these sills underwent ∼5% crustal contamination approximately 450 Ma prior to melting in a subduction zone environment. Unrealistically large amounts of contaminant are required to explain the chemistry of most ∼1070 Ma sills, and their chemistry is better explained by melting of a heterogeneous mantle source, consistent with the overlap of Nd model ages and crystallisation ages. However, the relatively low ϵNd(T) and high 87Sr/86Sr(T), elevated light REE, Rb and Zr concentrations, and high Th/Nb of some ∼1070 Ma eastern Bangemall Supergroup sills are indicative of crustal contamination. These sills are relatively depleted in chalcophile elements and platinum-group elements, consistent with coeval precipitation of sulfides and crustal contamination. The overlap in the maximum depositional age of host-rocks with the crystallisation age of some sills, the confining of most ∼1465 Ma sills to older parts of the stratigraphy, and field evidence showing that some sills belonging to both the ∼1465 and ∼1070 Ma events were intruded into wet sediments, indicate a close temporal relationship between sedimentation and sill intrusion.

Commentary on the position of higher Himalayan basement in Proterozoic Gondwanaland

Until recently the Higher Himalayan Crystalline Sequence (HHCS) was generally regarded to be the basement of the Himalayan Orogen. Our recent field work in the Kali Gandaki area of western Central Nepal showed that the HHCS is likely continuous with the overlying Tethyan Sedimentary Sequence (TSS) stratigraphically and structurally, and therefore does not form the basement of the Himalayan Orogen (Yoshida et al. 2004); a similar view was already mentioned in some earlier works (e.g., Le Fort 1975, Gehrels et al. 2003). Many workers have placed thrust or normal fault, and even unconformity between the HHCS and TSS; but based on our observation it is difficult to confirm any of these relationships. Our observation also conforms to the geochronologic work by DeCelles et al. (2000) that the HHCS is younger than ca 800 Ma and older than ca 480 Ma; this age-range naturally shows the geochronologic identity or continuation of the HHCS with the so far known base of the TSS. Although the basement of the Himalayan Orogen is not recognized anywhere, its paleoposition can be estimated by identifying the provenance of sediment deposited on the basement. Examination of zircon U-Pb data from HHCS gneisses by DeCelles et al. (2000) showed that the ages of detritus zircons have a range of ca 800~2700 Ma with a main peak at ca 954 Ma; based on these, they suggested the provenance to be the ArabianNubian Shield of the East-African Orogen lying to the west of the Proterozoic Himalayan Basin (Indian coordinate within the East Gondwana assembly, e.g., Yoshida et al. 2003). Robinson et al. (2001) showed the Nd model ages of HHCS to lie between 1.362.29 Ga (with an exceptional age of 2.85 Ga) and eNd (0) is moderately minus (-7.6~19.9, average–13.8); their neodymium isotopic data indicate the source area to have somewhat long crustal history. Recent compilation of geochronologic data from the Arabian-Nubian Shield (Johnson and Woldehaimanot 2003) shows zircon ages as well as neodymium model ages from the Arabian-Nubian Shield are a little too younger when compared with those of HHCS, and even bearing many positive eNd values. Although there are some exceptional data showing older neodymium model with highly minus eNd (0) values for rocks from some small areas of older crustal blocks within the ArabianNubian Shield, it is difficult to expect considerable amount of material supply from these blocks, since they are thought to lie, if any, below the younger geologic units and only a small amount could have exposed. Gehrels et al. (2003) recently reported somewhat older ages of detritus zircons from HHCS than those reported by DeCelles et al. (2000), showing a major peak range of ca 900-1300 Ma with a main peak of ca 1050 Ma. The wide range of Palaeoproterozoic and late Archaean zircons were also reported, although they form minor populations except those of ca 2500 Ma, which forms the second major peak. Thus, the ArabianNubian Shield may not constitute the source for the HHCS. We propose the Pinjarra Orogen (Western Australia, Fitzsimons 2003) lying to the east of the ancient Himalayan basin in the Proterozoic East Gondwana assembly, to be a more possible candidate for the source area that provided material to HHCS, based on their similar geochronologic and crustal signatures and suitable location near by the Himalayan basin. The Pinjarra Orogen suffered peak metamorphism of ca 1000–1100 Ma with later granitic activity of ca 800-650 Ma and superposed by ca 550– 500 Ma Pan-African deformation and metamorphism. Material of rocks in the orogen were derived from Mesoproterozoic Albany-Flaser Orogen of southwestern Australia, in which a variety of older crustal component were incorporated. The ages of rocks from the Pinjarra Orogen are quite conformable with recent zircon data from HHCS (Gehrels et al. 2003). Sporadically known Nd model ages from the Pinjarra Orogen lie between ca 1.6-2.2 Ga, with few exceptions of ca 1.1-1.6 Ga. Thus, it is most likely that the Pinjarra Orogen provided source material for the HHCS. Our conclusion does not support a recent model denoting the Pan-African assembly of East Gondwana (Pisarevsky et al. 2003) in which India and Western Australia do not juxtapose during the Neoproterozoic, but is in favour of the classical model of East Gondwana assembly through the Circum-East Antarctic Orogen of ca 1000 Ma (Yoshida et al. 2003) where Western Australia juxtaposes with northern India.

A test of detailed Nd isotope mapping in the Grenville Province: delineating a duplex thrust sheet in the Kipawa–Mattawa region

Over sixty new neodymium model ages were determined on orthogneisses from the Kipawa–Mattawa region of the Grenville Province to refine previous Nd isotope mapping work in this area. The combined Nd data sets support a tectonic model involving three major thrust sheets in the Kipawa area, separated by major shear zones. The uppermost sheet is correlated with the Allochthonous Polycyclic Belt, represented in the study area by the Lac Watson nappe, along with two allochthonous klippen. These have Nd model ages < 1.8 Ga, consistent with previous work. Within the underlying Parautochthonous Belt, previous workers identified a second major shear zone, separating rocks with Archean and Proterozoic crystallization ages, respectively. These two thrust sheets also have distinct Nd isotope signatures. The lowermost sheet consists of metamorphosed but otherwise relatively pristine Archean crust with Nd model ages > 2.6 Ga, whereas the overlying sheet consists of magmatically reworked Archean parautochthon with model ages from 1.8–2.6 Ga. A residual magnetic-field map developed from aeromagnetic data was compared with the terrane boundaries determined from isotopic data. The aeromagnetic data accurately reflect the margin of relatively pristine Archean crust in the study area, but this boundary does not correspond to the Allochthon Boundary Thrust. Instead, this boundary resulted from downcutting of the basal shear zone of the allochthon. This caused décollement of the strongly reworked Archean parautochthon to generate a duplex thrust sheet that was transported northwestwards over pristine Archean crust.

The Upper Neoproterozoic–Lower Cambrian of the Central Iberian Zone, Spain: chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source

The Upper Neoproterozoic–Lower Cambrian sedimentary succession in the Central Iberian Zone has recently been divided into 12 sedimentary units (I to XII). Units I to IV are of Late Neoproterozoic age, and units V to XII are Early Cambrian. Throughout the stratigraphic section, shales with similar ranges of SiO 2 and Al 2O 3 have TiO 2 and Zr contents that are coupled and vary gradually from the bottom (unit I, TiO 2 = 1.0–1.1%, Zr = 245–287 ppm) to the top (unit XII, TiO 2 = 0.74–0.86%, Zr = 141–192 ppm). These two parameters clearly distinguish Upper Neoproterozoic from Lower Cambrian compositions, as do certain elemental ratios (Al 2O 3/TiO 2, Rb/Zr, Nb/Ti), which vary from low values at the bottom of the section to high at the top. However, other elements (the rare earth elements [REEs], Y) do not fit this general trend, with heavy REEs and Y showing extreme concentrations in some Lower Cambrian units, which also have negative Ce anomalies. Units I to III and units XI to XII have uniform T DM neodymium model ages (∼1.1 and 1.7 Ga, respectively), but the units between these two groups have erratic values between 1.4 Ga (unit VII) and 3.4 Ga (unit VIII). Thus, the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions: a composition that records a juvenile contribution younger than 1.1 Ga and a composition mainly derived from an old basement. Some samples from Lower Cambrian units have disturbed REE abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in REE and Y redistribution. This alteration probably affected the material now preserved as unit IV during a period of relative sea level fall at the end of the Late Neoproterozoic. Both elemental and isotopic results support the suggestion that the Upper Neoproterozoic–Lower Cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement. Through time, the deeper crust gradually increased its contribution to the sediments in the basin from Late Neoproterozoic to Lower Cambrian times. These results are compatible with data from many other European zones.

Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr, Nd, Pb and O isotope geochemistry of quartz diorites (Damara orogen, Namibia)

Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K2O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO2) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P2O5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87Sr/86Sr: 0.70943–0.71285), Nd (initial e Nd: –9.1 to –15.2 ) and O (δ18O: 6.8–8.1‰) isotope compositions. Neodymium model ages (TDM) that range from 1.7 to 2.2 Ga and large variation in 207Pb/204Pb relative to 206Pb/204Pb indicates involvement of ancient crustal material. Lead (206Pb/204Pb: 17.08–17.23, 207Pb/204Pb: 15.53–15.62, 208Pb/204Pb: 37.71–38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.

Nd and Pb isotopic evidence for provenance and post-depositional alteration of the Paleoproterozoic Huronian Supergroup, Canada

Neodymium model ages for fine-grained formations of the Paleoproterozoic Huronian Supergroup (McKim, Pecors, Gowganda, Gordon Lake) range from 3.00 to 2.55 Ga and indicate a provenance dominated by the Late Archean Superior Province to the north and west. The stratigraphically highest unit (Gordon Lake Formation) has a distinctive Nd-isotopic composition, with T DM being 100-400 Ma younger than underlying mudstones. This suggests that the provenance changed, consistent with a previously documented change towards more negative Eu-anomalies, in the Gordon Lake. Lead isotopes are consistent with a Superior Province provenance and in addition provide evidence for two episodes of regional post-depositional disturbance of the U-Pb system. Lower Huronian (McKim, Pecors) samples align along 207Pb/ 204Pb– 206Pb/ 204Pb slopes equivalent to 2170±58 Ma (MSWD=92, n=9) and 2212±92 Ma (MSWD=9.1, n=5), respectively. These ages are at the minimum age limit on sedimentation and within uncertainty of the Nipissing Diabase (2219±4 Ma), a ubiquitous regional feature, parts of which may have intruded while much of the Huronian was unconsolidated. These Pb–Pb ages are interpreted to represent widespread diagenetic processes, possibly associated with an early phase of Nipissing intrusion. 206Pb/ 204Pb varies mostly from 19 to 34 (up to 59), whereas implied κ ( 232Th/ 238U) are mostly between 2 and 4, only slightly below the upper crustal value of 4. Changes in 206Pb/ 204Pb imply changes of μ ( 238U/ 204Pb) by factors of <1–5 and thus resetting of the U–Pb system likely involved Pb loss, with or without U gain. The upper Huronian displays more complex Pb-isotope systematics. Data align along 207Pb/ 204Pb– 206Pb/ 204Pb slopes of ca .1700 Ma, with regional variation in 207Pb/ 204Pb. For the Gowganda and Gordon Lake formations, 206Pb/ 204Pb ratios are 23–43 and 30–115, respectively, and imply changes in μ by factors commonly >2 (and up to 12.5). Values for κ are also in the range 2–4 and, accordingly, Pb loss appears to dominate this disturbance. Elevated 206Pb/ 204Pb correlates with post-depositional addition of potassium in the Gordon Lake and possibly the Gowganda formations. K-metasomatism has been demonstrated previously in the underlying Serpent Formation and in paleosols developed at the unconformity beneath the Huronian Supergroup, the latter being dated at 1690–1730 Ma. Widespread metasomatism, resulting in K-addition and Pb-loss, may have been related to northerly directed basin wide fluid movement in response to post-tectonic intrusions and erosion of the ca. 1.85 Ga Penokean Orogen to the south. Since Pb isotope systematics in the lower Huronian mudstones appear unaffected, apart from natural conduits provided by unconformity surfaces and possibly lower Huronian sandstone aquifers (that also may have been affected by K-metasomatism), it appears that fluid movement was more pervasive in the upper levels of this sedimentary sequence.

Formation of juvenile continental crust in the Arabian–Nubian shield: evidence from granitic rocks of the Nakasib suture, NE Sudan

Major and trace element data, U–Pb zircon ages, and initial isotopic compositions of Sr, Nd, and Pb are reported for ten granitic and one rhyolitic rock sample from the neo-Proterozoic Nakasib suture in NE Sudan. Chemical data indicate that the samples are medium- to high-K, "I-type" granitic rocks that mostly plot as "volcanic arc granites" on discriminant diagrams. Geochronologic data indicate that rifting occurred 790±2 Ma and constrain the time of deformation associated with suturing of the Gebeit and Haya terranes to have ended by approximately 740 Ma. Isotopic data show a limited range, with initial 87Sr/86Sr=0.7021 to 0.7032 (mean=0.7025), eNd(t) =+5.5 to +7.0 (mean=+6.4), and 206Pb/204Pb = 17.50–17.62. Neodymium model ages (TDM; 0.69–0.85 Ga; mean = 0.76 Ga) are indistinguishable from crystallization ages (0.79–0.71 Ga; mean=0.76 Ga), and the isotopic data considered together indicate derivation from homogeneously depleted mantle. The geochronologic data indicate that the terrane accretion to form the Arabian–Nubian shield began just prior to 750 Ma. The isotopic data reinforces models for the generation of large volumes of juvenile continental crust during neo-Proterozoic time, probably at intra-oceanic convergent margins. The data also indicate that crust formation was associated with two cycles of incompatible element enrichment in granitic rocks, with an earlier cycle beginning approximately 870 Ma and culminating approximately 740 Ma, and the second cycle beginning after pervasive high-degree melts – possibly hot-spot related – were emplaced approximately 690–720 Ma.

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia joint research project no. 1

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia joint research project no. 1

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia joint research project No. 1

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia joint research project No. 1

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia Joint Research Project No. 1

Neodymium model ages and isotopic signature of the Precambrian mobile belts of peninsular India: Japan-Russia Joint Research Project No. 1