A mantle-derived bimodal suite in the Hercynian Belt: Nd isotope and trace element evidence for the subduction-related rift origin of the Late Devonian Brevenne metavolcanics, Massif Central (France)

Article Abstract:

A low-grade biomodal association of metabasalts and metarhyolites is exposed in the Brevenne Series in France, along with intrusive trondhjemite bodies. The relative depletion of high-field strength elements, particularly Nb, precludes an enriched-mantle source of ocean island basalt affinity. It is suggested that arc-like melts, depleted in Nb, were due to partial melting of a depleted-mantle source. The use of the Sm-Nd system with incompatible elements could provide diagnosis of the tectonic setting of bimodal metaigneous sites in ancient orogenic belts.

Observations, Mineralogical research

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The spatial distribution of grains and crystals in rocks

Article Abstract:

A technique involving the distance relationship between nearest neighbour grain centres in thin sections of rocks gives a useful estimate of the spatial distribution pattern (SDP) of grains or crystals in it. The SDP is characterized by the relationship between the R value and porosity on graph, which gives a random sphere distribution line (RSDL). Grain overgrowth, grain sorting and compaction give different porosity versus R plot, which is indicative of the relative importance of these processes.

Crystals, Porosity

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Contrasting P-T conditions recorded in ultramafic high-pressure rocks from the Variscan Schwarzwald (F.R.G.)

Article Abstract:

The ultramafic high-pressure rocks from the Variscan basement of the Schwarzwald (F.R.G.) are formed by an early high-temperature stage in the spinel periodite stability field, re-equilibration at 740-850 degree centigrade/3.2-4.3 GPa and finally recrystallization at lower pressures. Small discrete bulk are formed in low pressure/high-temperature gneisses and migmatites.

Natural history, Garnet, Black Forest

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