Comparison of petrogenetic signatures between mantle-derived alkali silicate intrusives with and without associated carbonatite, Namibia

We compare the petrogenetic and chemical signatures of two alkali silicate suites from the Cretaceous Damaraland igneous province (Namibia), one with and one without associated carbonatite, in order to explore their differences in terms of magma source and evolution. The Etaneno complex occurs in close spatial proximity to the Kalkfeld bimodal carbonatite–alkali silicate complex, and is dominated by nepheline (ne)-monzosyenites and ne-bearing alkali feldspar syenites. The Etaneno samples have isotopic compositions of ⁸⁷Sr/⁸⁶Sr(i)=0.70462–0.70508 and Nd=−0.5 to −1.5, with the highest ⁸⁷Sr/⁸⁶Sr(i) and lowest Nd values observed in evolved samples. The magma differentiated via olivine, feldspar, clinopyroxene, and nepheline (ne) fractionation in a F-rich system, which fractionated Zr from Hf, and Y from Ho. Partly glassy, recrystallized inclusions in some samples are less evolved than their host rocks and contain a cumulate component (nepheline, plagioclase). The Kalkfeld ne-foidites (ijolites) and ne-syenites have ⁸⁷Sr/⁸⁶Sr(i)=0.70285–0.70592 and Nd=0.5 to 1.1. The isotope ratios show no consistent variation with rock composition, and they are in the same range as the associated carbonatites. The Kalkfeld silicate magma fractionated nepheline and alkali-feldspar in a CO₂-dominated, F- and Ca-poor system. As a result, the rocks display some major and trace element trends distinctly different from those of the Etaneno samples.

We suggest that the Etaneno and the Kalkfeld magmas represent different melt fractions of a heterogeneous mantle source, resulting in different compositions especially with respect to CO₂ contents of the primitive, parental magmas. In this scenario, the carbonated alkali silicate Kalkfeld parental melt contained a critical CO₂ concentration and underwent liquid separation of carbonate and silicate melt fractions at crustal depths. The resulting silicate melt fraction experienced a very different mode of differentiation than the carbonate-poor Etaneno parental magma. Thus, the Kalkfeld rocks are depleted in Ca and other divalent cations, as well as F, rare-earth elements (REE), Ba, and P relative to the Etaneno syenites. We interpret these differences to reflect the partitioning of these elements into the carbonate melt fraction during immiscible separation.     

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy,and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K₂O/Na₂O, K₂O/TiO₂ and intermediate CaO/Al₂O₃ that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K₂O/Na₂O, K₂O/TiO₂ and CaO/Al₂O₃ that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K₂O/TiO₂ similar to those from VF, and intermediate CaO/Al₂O₃ as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO₂–eclogites) whereas the less important EM component to recycled marine or continental sediments.     

Fluid evolution in the nepheline syenites of the Ditrău Alkaline Massif, Transylvania, Romania

The Ditrău Alkaline Massif is an intrusion into the Bucovina nappe system that is part of the Mesozoic crystalline zone located in Transylvania, Romania, in the Eastern Carpathians. Nepheline syenites are the most abundant rocks in the central and eastern part of the Massif, and represent the last major intrusion of the complex. Fluid inclusions in nepheline, aegirine and albite were trapped at magmatic conditions on or below the H₂O-saturated nepheline syenite solidus at about 400–600 °C and 2.5–5 kbars. Early nepheline, and to a lesser extent albite, were altered by highly saline fluids to produce cancrinite, sodalite and analcime, during this process cancrinite also trapped fluid inclusions. The fluids, in most cases, can be modeled by the H₂O–NaCl system with varying salinity; however inclusions with more complex fluid composition (containing K, Ca, CO₃, etc., in addition to NaCl) are common. Raman spectroscopic analyses of daughter minerals confirm the presence of alkali-carbonate fluids in some of the earliest inclusions in nepheline, aegirine and albite.

During crystallization, the melts exsolved a high salinity, carbonate-rich magmatic fluid that evolved to lower salinity as crystallization progressed. Phases that occur early in the paragenesis contain high-salinity inclusions while late phases contain low-salinity inclusions. The salinity trend is consistent with experimental data for the partitioning of chlorine between silicic melt and exsolved aqueous fluid at about 2.0 kbars. The activity of water (a_H2O) in the melt increases during crystallization, resulting in the formation of hydrous phases during late-stage crystallization of the nepheline syenites.     

Petrology of nepheline syenite gneiss from Amazonian Brazil

Nepheline syenite gneiss is exposed in three localities near the village of Boca Nova (47·04′W, 1°51′S) in northeastern Pará, Brazil. Isotopic and chemical evidence for an igneous premetamorphic history include: (1) bulk compositions which plot in the Q–Ne–Ks system on a peralkaline fractionation curve; (2) bulk compositions which fall on the Hviddal igneous trend in the SiO₂–Al₂O₃–Na₂O + K₂O molecular diagram; (3) an average K/Rb ratio identical to the average for nepheline syenites of the Khibina massif; (4) K/Rb ratios which lie on the ideal igneous fractionation curve; (5) Nb(Ta) contents far in excess of average crustal abundance; and (6) a low Sr₈₇/Sr⁸⁶ value (0·7034) characteristic of mantle derivation. Post-kinematic lit-par-lit injections, discordant veins, and concordant pods of leucocratic nepheline–alkali feldspar pegmatite partially replaced the metamorphic assemblage and destroyed any microscopic equilibrium texture that may have been present. The final disequilibrium assemblage consists of Fe-rich biotite and two generations each of sodic plagioclase, microcline, and nepheline. Some minor metasomatic chemical effects may be present in the rocks but a case for nephelinization of crustal rocks cannot be documented. Field, petrographic, and chemical constraints indicate that the Boca Nova gneiss is an early-stage migmatite formed by anatexis of metamorphosed igneous nepheline syenite or its volcanic equivalent. Regional metamorphism and partial melting opened the Rb-Sr system so that the whole-rock Rb/Sr isochron age of 724 m.y. has no unambiguous interpretation. A K/Ar age of 580 ± 10 m.y. from biotite denning the metamorphic foliation is regarded as a minimum date for the regional metamorphism and anatexis. No other alkaline rocks of comparable age are known in the Sao Luis Craton.