Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

Anatectic xenoliths from the volcanic fields of the Eifel (Germany) and the Chaine des Puys (Massif Central, France) contain a glass phase (quenched in-situ melt) along mineral grain boundaries. Microanalytical studies show that on a mm- to cm-scale melt chemistry is strongly controlled by the mineral phases involved in anatexis. Domains rich in mafic phases (biotite, orthopyroxene, spinel, garnet) contain brown glass with much higher abundances of FeO, MgO and TiO₂ than colourless glass in quartzofeldspathic layers. By contrast, K₂O abundances are similar. Differences in melt composition in turn strongly control the crystallization of minerals from the melt as well as minor reactions between the melt and the xenolith phase assemblage.

In comparison, textural features of anatectic HT-LP migmatites from SW Finland provide evidence of both prograde and retrograde reactions between minerals and melt. They reveal, that the modal and also the chemical composition of migmatites is more strongly affected by retrograde processes than in xenoliths. Hence, microanalytical and textural studies of partially melted crustal xenoliths can provide important insights in prograde processes operating in migmatitic terranes.

Plutonic and high-grade metamorphic rocks from the crystalline complexes of the Saxonian Erzgebirge and Granulitgebirge in Germany were used for experiments up to 0.5 GPa at laboratory temperature to measure the elastic wave velocities (v_p) and (v_s), under the minimized influence of pore space and cracks in an oil pressure chamber. The obtained high pressure data were also used to select samples for further experiments under the conditions of mineral dehydration and partial melting. We also applied a high performance gas pressure apparatus to investigate v_p and v_s on cylindrical samples up to 2 GPa and temperatures up to 1200°C. This paper shows the results of the elastic wave velocities v_p and v_s, measured simultaneously with encapsulated samples of granite, pyroxene granulite and pyroxenite under different partial melting conditions. Post-experimental microscopic analyses of the quenched samples including digital image processing were undertaken to determine the material and/or structural cause of a change in the physical properties, resulting in empirical relationships between elastic wave velocities and mineral content, dissipation and orientation of melt along grain boundaries. In a granite melt starts to form at 650°C with an amount of about 4 % at 900°C and reaching massive melting at temperatures above 1000°C. For basic to ultrabasic rocks at a pressure of 2 GPa a much smaller amount of melt has formed in the temperature interval above 900°C. At lower pressures the Poisson's ratio increases in the range of partial melting dramatically from 0.25 to about 0.4. For basic to ultrabasic rocks at 2 GPa pressure it keeps nearly unchanged up to 1000°C.

The Sondalo gabbroic complex is related to the magmatic event that postdated the Variscan orogeny. The complex mainly consists of gabbronorites, associated with subordinate amounts of olivine-gabbros, and minor quartz-diorites. These rocks can be related to tholeiitic liquids that underwent a differentiation process controlled by fractional crystallisation and concomitant assimilation of crustal material. Basement country rocks are mostly made of two-mica amphibolite-facies metapelites that are locally sillimanite-bearing. Close to the contact with the gabbroic complex, the metapelites display migmatitic fabric. Melanosomes show the consumption of muscovite and the development of sillimanite and anhedral garnet. Leucosomes are quartz-feldspathic in composition and locally characterised by the growth of porphyroblastic garnet. Contact migmatites are in place characterised by the presence of peraluminous granitoid pods. Large xenolith blocks (up to hundreds of meters in size) showing granulite-facies assemblages, as well as peraluminous granitoid dykes, crop out within the gabbroic complex. The xenolith blocks are volumetrically dominated by cordierite- and sillimanite-bearing garnet-rich rocks. These rocks are characterised by low silica and alkalis, and high Al₂O₃, Fe₂O₃^tot, MgO, TiO₂ and MnO. These chemical features are interpreted to reflect a restitic origin, related to the extraction of “granitic” liquids from metapelitic sources. The garnet-rich xenolith blocks are discontinuosly wrapped by bodies of garnet-bearing mafic rocks. The Sondalo gabbroic complex resulted from the intrusion of mantle-derived liquids into intermediate levels of the continental crust. Such intrusion did not lead to regional granulite-facies metamorphism and triggered melting processes in contact metapelites. Extraction of anatectic liquids gave rise to dense restitic blocks that could sink into the magma chamber, where they possibly underwent further melting.