Petrogenesis of the late Cretaceous tholeiitic magmatism in the passive margins of northeastern Madagascar.

New chemical and Sr-Nd isotopic data on the Late Cretaceous mafic dike swarm intruding the Archean-Proterozoic crystalline basement in the Tamatave???Sainte Marie Island sector (northeast coast passive margin), and on lavas and dikes of the northeastern part of the Mahajanga sedimentary basin (passive margin after the opening of the Jurassic-Cretaceous Somali basin), allow better knowledge of the chemical variations observed in the northern part of the Madagascan igneous province. Two distinct basalt groups have been identified. Group 1 basalts have low light to heavy rare earth element (REE) ratios [(La/Yb)n = 2.2???2.9], low Zr/Y and Nb/Y (4???6 and 0.2???0.4, respectively), low (87Sr/86Sr)88 (0.7034???0.7042), and high to moderate E Nd(88) (+ 5.1 to +1.5). Subgroup 1a comprises basalts with the same light to heavy REE ratios [(La/Yb)n = 2.7???3], Zr/Y and Nb/Y (4.5???5.8 and 0.2???0.3, respectively), and slightly high (87Sr/86Sr)88 (0.7042???0.7048) at the same E Nd(88) (+ 5.4 to + 4.4) of the group 1 basalts. Group 2 basalts have high light to heavy REE ratios [(La/Yb)n = 5.3???7.8], high Zr/Y and Nb/Y (7???11 and 0.5???0.8, respectively), relatively high (87Sr/86Sr)88 (0.7045???0.7057), and low ENd(88) (+ 3.8 to + 1). The basalts of the groups 1, 1a, and 2 cannot be linked by closed-system magma differentiation processes, and require distinct mantle sources. The major and trace element variations of the Tamatave dikes of the group 1-1a are compatible with moderate degrees of crystal fractionation (~60%) from the least (MgO = 7.3 wt%) to the most evolved compositions (MgO = 4.2 wt%), involving the separation of plagioclase, augite, pigeonite, and minor oxides, perhaps accompanied by crustal contamination or differences in the 87Sr/86Sr ratios. The mantle sources of the group 1-1a basalts seem to be located well within the spinel stability field, whereas a larger contribution of melts derived from garnet-bearing residual mantle is observed in the geochemistry and in the melting models of the group 2 basalts. The chemical and isotopic composition of both rock groups indicate their ultimate provenance from variably enriched lithospheric mantle sources; there is no clear evidence of a hotspot component like that found in the present-day lavas of the Marion???Prince Edward archipelago. The sources of this volcanism seem to be significantly similar to those of the Mahableshwar and Ambenali basalts of the later erupted Deccan Traps, located on formerly contiguous parts of the Gondwana lithosphere

Petrogenesis of the late Cretaceous tholeiitic magmatism in the passive margins of northeastern Madagascar.

Special Paper 362, Magmatic rifted margin

The Late Cretaceous Flood Basalt Province of Madagascar: geochemistry of lavas and dykes in the northeastern passive margins, and regional correlations.

Special Pape

Geochemistry and petrogenesis of sodic and potassic mafic alkaline rocks in the Deccan Volcanic Province, Mumbai area (India)

Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres, tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12???2???wt%). The lamprophyres have high content of incompatible elements (e.g., TiO2???>???3.8 wt%, Nb???>???130???ppm, Zr???>???380???ppm, Ba???>???1200???ppm), and relatively high initial (at 65???Ma) 143Nd/144Nd (0.5128) and low 87Sr/86Sr (0.7038???0.7042). They are likely to be small-degree melts (2???3%) of volatile- and incompatible element-enriched mantle sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks have highly porphyritic textures (up to 57% phenocrysts of diopside???±???olivine), and anomalously low contents of incompatible elements (e.g., TiO2???<???1.3???wt%, Nb???<???24???ppm, Zr???<???100???ppm) indicating that they could not represent liquid compositions. Moreover, their very low initial 143Nd/144Nd ratios (0.5116???0.5120), at 87Sr/86Sr???=???0.7045???0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that could be identified as ???Lewisian-type??? lower crust and/or enriched mantle. The melting episode that generated these alkaline rocks likely occurred close to the base of the ca. 100???km-thick Indian lithosphere, very shortly after the main eruption of the Deccan tholeiites

Petrogenesis of the late Cretaceous tholeiitic magmatism in the passive matgins of northeastern Madagascar

Special paper

Geochemical prvinciality in the Cretaceous magmatism of northern Madagascar and mantle source implications.

Journal of the Geological Society, londo