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

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.

Granulite terranes from Central Spain are Hercynian anatectic areas of mid-crustal levels (P-T conditions: 750–800° C and 4–6 kb). The granulite xenolith suite of the same sector comes from the lower crust as deduced from their P-T estimates (850–950° C and 8–11 kb). Chemical differences between granulites from terranes and those from xenoliths are less marked in Central Spain than in other places as this xenolith suite lack mafic lithologies. Granulitic xenoliths show U and Rb depletion, and occasionally a subtle enrichment in Ba, Fe, Mg, Ti, (V) and HREE. Contents in Zr, Th, Y, and REE in both granulite types might be explained as a consequence of the minor role of accessory phases in controlling the geochemistry of the residual rocks in the lower crust. Migmatitic leucosomes and other felsic anatectic melts show the same trend of Rb and K enrichment and Fe, Mg, Ti, Cr, Ni, V and Th, Y, Zr, Nb, REE impoverishment (compared to protoliths) than granitic plutons. Although plutonic granites show higher Ca (Na), Fe and Mg contents a genetic link with anatectic melts is suggested. Isotopic differences (Sr, O) between granitic plutons and anatectic melts might be a reflection of their derivation from a different crustal level in an isotopically stratified crust.

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.