Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd–Sr–Pb–O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ¹⁸O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (⁸⁷Sr/⁸⁶Sr: 0.7031–0.7035, ε Nd: ca. + 1, δ¹⁸O: 7‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (⁸⁷Sr/⁸⁶Sr: 0.7034, ε Nd: ca. + 1, δ¹⁸O: 6.5‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.59) whereas more differentiated samples have higher ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.714), slightly higher δ¹⁸O values (7.0–7.1‰), less radiogenic ε Nd values (− 1.1 to − 1.4) and more radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.     

Sr–Nd–Pb isotopic compositions of the Kovdor phoscorite–carbonatite complex, Kola Peninsula, NW Russia

The Sr, Nd and Pb isotopic compositions for the Kovdor phoscorite–carbonatite complex (PCC), Kola Peninsula, NW Russia, have been determined to characterize the mantle sources involved and to evaluate the relative contributions of a plume and subcontinental lithospheric mantle in the formation of the complex. The Kovdor PCC is a part of the Kovdor ultramafic–alkaline–carbonatite massif, and consists of six intrusions. The initial isotopic ratios of the analyzed samples, calculated at 380 Ma, display limited variations: ε_Nd, + 2.0 to + 4.7; ⁸⁷Sr/⁸⁶Sr, 0.70319 to 0.70361 (ε_Sr, − 12.2 to − 6.2); ²⁰⁶Pb/²⁰⁴Pb, 18.38 to 18.74; ²⁰⁷Pb/²⁰⁴Pb, 15.45 to 15.50; ²⁰⁸Pb/²⁰⁴Pb, 37.98 to 39.28. The Nd and Sr isotope data of the Kovdor PCC generally fit the patterns of the other phoscorites and carbonatites from the Kola Alkaline Province (KAP), but some data are slightly shifted from the mixing line defined as the Kola Carbonatite Line, having more radiogenic ⁸⁷Sr/⁸⁶Sr ratios. However, the less radiogenic Nd isotopic compositions and negative Δ7/4 values of Pb isotopes of the analyzed samples exclude crustal contamination, but imply the involvement of a metasomatized lithospheric mantle source. Isotopic variations indicate mixing of at least three distinct mantle components: FOZO-like primitive plume component, EMI-like enriched component and DMM-like depleted component. The isotopic nature of the EMI- and DMM-like mantle component observed in the Kovdor samples is considered to be inherited from metasomatized subcontinental lithospheric mantle. This supports the previous models invoking plume–lithosphere interaction to explain the origin of the Devonian alkaline carbonatite magmatism in the KAP.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U–Pb ages, oxygen isotopes and REE data

Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ¹⁸O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma.     

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ¹⁸O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648.

The initial ¹⁴³Nd/¹⁴⁴Nd ratios, expressed by ε_Nd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ¹⁸O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of ¹⁸O/¹⁶O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ¹⁸O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ¹⁸O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.     

Geochemical and Sr–Nd–Pb isotopic compositions of the Eocene Dölek and Sariçiçek Plutons, Eastern Turkey: Implications for magma interaction in the genesis of high-K calc-alkaline granitoids in a post-collision extensional setting

The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at  43 Ma at shallow depths ( 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO₂ = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO₂ = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb)_N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are ε_Nd(43 Ma) = − 0.6 to 0.8 and mostly I_Sr = 0.70482–0.70548. The Nd model ages (T_DM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.60–18.65, (²⁰⁷Pb/²⁰⁴Pb) = 15.61–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with I_Sr ranging from 0.70485 to 0.70517, ε_Nd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of (²⁰⁶Pb/²⁰⁴Pb) = 18.58–18.64, (²⁰⁷Pb/²⁰⁴Pb) = 15.60–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.64–38.77. The MMEs have T_DM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that  77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime.     

Isotopic equilibrium/disequilibrium in granites, metasedimentary rocks and migmatites (Damara orogen, Namibia)—a consequence of polymetamorphism and melting

The overwhelming part of the continental crust in the high-grade part of the Damara orogen of Namibia consists of S-type granites, metasedimentary rocks and migmatites. At Oetmoed (central Damara orogen) two different S-type granites occur. Their negative ε_Nd values (− 3.3 to − 5.9), moderately high initial ⁸⁷Sr/⁸⁶Sr ratios (0.714–0.731), moderately high ²⁰⁶Pb/²⁰⁴Pb (18.21–18.70) and ²⁰⁸Pb/²⁰⁴Pb (37.74–37.89) isotope ratios suggest that they originated by melting of mainly mid-Proterozoic metasedimentary material. Metasedimentary country rocks have initial ε_Nd of − 4.2 to − 5.6, initial ⁸⁷Sr/⁸⁶Sr of 0.718–0.725, ²⁰⁶Pb/²⁰⁴Pb ratios of 18.32–18.69 and ²⁰⁸Pb/²⁰⁴Pb ratios of 37.91–38.45 compatible with their variation in Rb/Sr, U/Pb and Th/Pb ratios. Some migmatites and residual metasedimentary xenoliths tend to have more variable ε_Nd values (initial ε_Nd: − 4.2 to − 7.1), initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr: 0.708–0.735) and less radiogenic ²⁰⁶Pb/²⁰⁴Pb (18.22–18.53) and ²⁰⁸Pb/²⁰⁴Pb (37.78–38.10) isotope compositions than the metasedimentary rocks. On a Rb–Sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c. 550 Ma which is interpreted to indicate major fractionation of the Rb–Sr system at that time. However, initial ⁸⁷Sr/⁸⁶Sr ratios of the melanosomes of the stromatic migmatites (calculated for their U–Pb monazite and Sm–Nd garnet ages of c. 510 Ma) are more radiogenic (⁸⁷Sr/⁸⁶Sr: 0.725) than those obtained on their corresponding leucosomes (⁸⁷Sr/⁸⁶Sr: 0.718) implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the Rb–Sr system. However, the leucosomes have similar Nd isotope characteristics than the inferred residues (melanosomes) indicating the robustness of the Sm–Nd isotope system during high-grade metamorphism and melting. On a Rb–Sr isochron plot residual metasedimentary xenoliths show residual slopes of c. 66 Ma (calculated for an U–Pb monazite age of 470 Ma) again indicating major fractionation of Rb/Sr at c. 540 Ma. However, at 540 Ma, these xenoliths have unradiogenic Sr isotope compositions of c. 0.7052, indicating depleted metasedimentary sources at depth. Based on the distinct Pb isotope composition of the metasedimentary rocks and S-type granites, metasedimentary rocks similar to the country rocks are unlikely sources for the S-type granites. Moreover, a combination of Sr, Nd, Pb and O isotopes favours a three-component mixing model (metasedimentary rocks, altered volcanogenic material, meta-igneous crust) that may explain the isotopic variabilty of the granites. The mid-crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre-existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central Damara orogen of Namibia.     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

Pb/Pb age determinations on the Newania and Sevattur carbonatites of India: evidence for multi-stage histories

The ages of Indian carbonatites are still controversial. Most of the earlier datings were done by K/Ar methods. We therefore analysed Pb/Pb ratios in carbonatites from carbonatite-alkalic complexes of Newania (NW India, Rajasthan State) and Sevattur (SW India, Tamil Nadu State) to constrain the age and geological history of these rocks. Newania carbonatites are intrusive into Precambrian Untala granite-gneiss and mainly dolomitic in composition (rauhaugite) followed by a later phase of ankerite carbonatite, while thin calcite carbonatite (sövite) dykelets are the youngest in the sequence. The analysed whole-rock samples are characterised by ²⁰⁶Pb/²⁰⁴Pb ratios between 60 and 176 and ²⁰⁷Pb/²⁰⁴Pb ratios between 22 and 40, which are extremely high in comparison to common igneous rocks and even for carbonatite compositions. One sample, New 37, shows the extreme ratios of ²⁰⁶Pb/²⁰⁴Pb = 574 and ²⁰⁷Pb/²⁰⁴Pb = 73. This requires a μ-value of about 2000 for the last 1550 Ma. If the samples are classified according to their petrographic/geochemical characteristics this results in an isochron age of 1551 ± 46 Ma for the ankerite carbonatites (six samples). The dolomites (6 samples) yield an isochron age of 2.27 Ga. Although these results fit quite well into the geological evolution scheme of the area, the extreme long age hiatus between dolomite carbonatite and ferrocarbonatite formation events raises severe problems for their petrologic interpretation.

The Proterozoic Sevattur carbonatite complex (SCC, Tamil Nadu) was emplaced contemporaneously with a large number of carbonatite complexes within the Precambrian gneissic terrane of the Eastern Ghats Mobile Belt. The main mass is composed of dolomite carbonatite (rauhaugite) with a few dikes of calcite carbonatite (sövite) and ankerite carbonatite within it. All eight samples together yield an isochron of 805 ± 10 Ma. This isochron is mainly determined on ankerite carbonatites with μ-values up to 1900 for the last 800 Ma. Taking only ankerite carbonatites into account, the resulting age is 801 ± 11 Ma. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios of these samples are similar to the main group of Newania and far beyond the isotopic composition of common igneous rocks.

Our investigations show that in carbonatitic rock systems extremely high lead isotopic ratios can be established due to the crystallization of uranium-rich mineral phases. In both cases the observed high to extremely high initial Pb isotope ratios require the residence of the lead in intermediate high-μ reservoirs either within the upper mantle or the crust prior to the carbonatite formation. A high-temperature event, which completely reset the Rb/Sr and K/Ar isotopic systems of Nevania carbonatites, seems to have no influence on the lead isotopic systematics.     

Sveconorwegian crustal underplating in southwestern Fennoscandia: LAM-ICPMS U–Pb and Lu–Hf isotope evidence from granites and gneisses in Telemark, southern Norway

Laser ablation ICPMS U–Pb and Lu–Hf isotope data on granitic-granodioritic gneisses of the Precambrian Vråvatn complex in central Telemark, southern Norway, indicate that the magmatic protoliths crystallized at 1201 ± 9 Ma to 1219 ± 8 Ma, from magmas with juvenile or near-juvenile Hf isotopic composition (¹⁷⁶Hf/¹⁷⁷Hf = 0.2823 ± 11, epsilon-Hf > + 6). These data provide supporting evidence for the depleted mantle Hf-isotope evolution curve in a time period where juvenile igneous rocks are scarce on a global scale. They also identify a hitherto unknown event of mafic underplating in the region, and provide new and important limits on the crustal evolution of the SW part of the Fennoscandian Shield. This juvenile geochemical component in the deep crust may have contributed to the 1.0–0.92 Ga anorogenic magmatism in the region, which includes both A-type granite and a large anorthosite–mangerite–charnockite–granite intrusive complex. The gneisses of the Vråvatn complex were intruded by a granitic pluton with mafic enclaves and hybrid facies (the Vrådal granite) in that period. LAM-ICPMS U–Pb data from zircons from granitic and hybrid facies of the pluton indicates an intrusive age of 966 ± 4 Ma, and give a hint of ca. 1.46 Ga inheritance. The initial Hf isotopic composition of this granite (¹⁷⁶Hf/¹⁷⁷Hf = 0.28219 ± 13, epsilon-Hf = − 5 to + 6) overlaps with mixtures of pre-1.7 Ga crustal rocks and juvenile Sveconorwegian crust, lithospheric mantle and/or global depleted mantle. Contributions from ca. 1.2 Ga crustal underplate must be considered when modelling the petrogenesis of late Sveconorwegian anorogenic magmatism in the region.     

Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area

Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ¹⁸O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ¹⁸O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ¹⁸O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the ¹⁸O/¹⁶O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Calcio-carbonatite melts and metasomatism in the mantle beneath Mt. Vulture (Southern Italy)

At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5–150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2–20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO₂ bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8–5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr–Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate–carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation.     

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO₂, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ¹⁸O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ¹⁸O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron (n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes).

The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization ( 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P_H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P_H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their ¹⁸O/¹⁶O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ¹⁸O (Fsp) values point to magmatic origin of fluids.

The stable and radiogenic isotope data [δ¹⁸O (Zrn) = 5.82 ± 0.06‰, I_Sr = 0.7022 ± 0.0064, ε_Nd (T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.     

Mixing of asthenospheric and lithospheric mantle-derived basalt magmas as shown by along-arc variation in Sr and Nd isotopic compositions of Early Miocene basalts from back-arc margin of the NE Japan arc

We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO₂ (> 1.5 wt.%; referred to as HT) and lower TiO₂ (< 1.5 wt.%; LT) basalts. HT basalt has higher Na₂O + K₂O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr (SrI) = 0.70389 to 0.70631, initial ¹⁴³Nd/¹⁴⁴Nd(NdI) = 0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na₂O + K₂O and K₂O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea.     

Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania

The minerals of Oldoinyo Lengai natrocarbonatite lavas are unstable under atmospheric conditions. Subsolidus mineral assemblages in natrocarbonatites were studied in 105 samples from contemporary eruptions ranging from present day to about 100 years old. The subsolidus minerals in natrocarbonatites were formed (i) along cracks on the lava surface from hot gases escaping during cooling, (ii) as atmospheric alteration by solution of water-soluble minerals, in particular halides and gregoryite, and by hydration of nyerereite under the influence of meteoric water and (iii) by reaction with fumarole gases. After solidification, the lavas were cut by a network of thin cracks, the edges of which are covered by polymineralic encrustations. Samples collected 2–24 h after eruption contain nahcolite, trona, sylvite, and halite with accessory kalicinite and villiaumite. Atmospheric humidity results immediately (≥ 2 h after eruption) in alteration of black lavas that is marked by the appearance of white powdery thermonatrite with nahcolite on the lava surface. Subsequent reaction (weeks, months, years) of natrocarbonatite with meteoric water and the atmosphere results in the formation of pirssonite, gaylussite, shortite, trona, thermonatrite, nahcolite and calcite. Generally, the first important step is the formation of pirssonite and the end-members are calcite carbonate rocks or loose aggregates. Fumarolic activity is common for the active northern crater of the volcano. Reaction of hot (54–141 °C) fumarolic gases with natrocarbonatite leads to the formation of sulphur, gypsum, calcite, anhydrite, monohydrocalcite, barite and celestine. Changes in mineralogy of the natrocarbonatite lead to substantial chemical transformation. The most obvious chemical changes in this process are the loss of Na, K, Cl and S, combined with an increase in H₂O, Ca, Sr, Ba, F and Mn. The oxygen and carbon isotopic composition of altered natrocarbonatites shows a significant shift from the primary “Lengai Box” to high values of δ¹⁸O and δ¹³C. Calcite exhibits δ¹³C values between − 2‰ and − 4‰ PDB and δ¹⁸O values of + 23‰ to + 26‰ SMOW. The observed assemblages of secondary minerals formed by reaction with atmosphere and meteoric water, the changes in chemical composition of the natrocarbonatite and field observations suggest that alteration of natrocarbonatite is an open-system low-temperature process. It takes place at temperatures between 8 and 43 °C with the addition of H₂O to the system and the removal of Na, K, Cl and S from the carbonatites. Low-temperature thermodynamic models developed for alkali carbonate systems can be used for the interpretation of Oldoinyo Lengai subsolidus mineralization.     

Upper Cambrian carbonate sequences of the Argentine Precordillera and the Steptoean C-Isotope positive excursion (SPICE)

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.

At Quebrada de La Angostura, upper part of the La Flecha Formation, δ¹³C_carb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.

The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ¹³C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.

At the Cerro La Silla section, the Zonda Formation exhibit δ¹³C values of  − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.

At the Quebrada de La Flecha, ⁸⁷Sr/⁸⁶Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of ⁸⁷Sr/⁸⁶Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.

The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m³ for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. ¹⁴³Nd/¹⁴⁴Nd_i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). ⁸⁷Sr/⁸⁶Sr_i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb_i isotope ratios of the enriched samples are rather radiogenic (²⁰⁶Pb/²⁰⁴Pb_i 18.8–20.6, ²⁰⁷Pb/²⁰⁴Pb_i 15.6–15.7, ²⁰⁸Pb/²⁰⁴Pb_i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of ²⁰⁸Pb/²⁰⁴Pb_i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.     

An evaporitic facies in Neoproterozoic post-glacial carbonates: The Gifberg Group, South Africa

A geological study of the hitherto poorly described Neoproterozoic Gifberg Group, with emphasis on lithogeochemistry and O, C and Sr isotopic composition of the carbonate-dominated Widouw Formation (Vredendal Outlier, westernmost South Africa) revealed that the entire group is an equivalent of the relatively well constrained Port Nolloth Group in the external, paraautochthonous part of the Pan-African Gariep Belt further north. Thus, the Vredendal Outlier can be regarded as the southern extension of the Port Nolloth Zone. Two diamictite units are recognised in the Vredendal Outlier, which can be correlated respectively with the c. 750 Ma Kaigas Formation diamictite and the 583 Ma, syn-Gaskiers Numees Formation diamictite in the Gariep Belt proper. The dominating carbonate unit in the studied area is post-glacial with respect to the older of the two diamictite units. The combined textural, structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds. Sedimentation in a restricted, very shallow and proximal basin led to a wide range in C isotope ratios (δ¹³C_PDB from − 4.2 to + 4.8‰), very high Sr concentrations (derived from original anhydrite) and initial ⁸⁷Sr/⁸⁶Sr ratios that are significantly higher (0.70785) than those of coeval seawater. As C and Sr isotopes are commonly used for chemostratigraphic correlation, and high Sr concentrations in Neoproterozic carbonates are often interpreted as evidence of former aragonite, the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition. Thus C and Sr isotope ratios alone in Neoproterozoic carbonates may be less powerful proxies of ancient seawater composition, and high Sr contents are not necessarily indicative of an “aragonite sea”, as previously inferred.