Upper to intermediate levels of continental crust, as shaped by Late Hercynian metamorphism and magmatism, are exposed in the Sila nappe (Calabria, S. Italy) after Tertiary tectonics. Direct observation of the crustal section reveals a composite metamorphic block dominated by low- to high-grade metapelites and meta-arenites, intruded centrally by granitoids. The average thickness of granitoids, estimated on cross-sections and by geobarometry, reaches 9 ± 2 km. Their composition changes with depth from leucogranite to tonalite with increasing colour index. The estimated bulk composition of the Sila nappe is characterised by higher contents of A1203, REE and Th and lower contents of CaO and Sr than is typical of upper continental crust, reflecting abundant metapelite in the Sila nappe. Comparisons between metasedimentary rocks from the upper and intermediate levels indicate that intracrustal differentiation was determined by anatexis, which favoured A1203 , REE and Th enrichment in medium- to high-grade metapelites. Distribution of REE, Th and Zr was connected to the fate of monazite and zircon during crustal anatexis. Melting reactions involved mainly muscovite whereas biotite, enclosing most accessory phases, often remained stable in the residue. Consequent passive enrichment of Th in medium- to high-grade restitic metapelite determined an unusual increase in radiogenic heat production with depth. Compositional data suggest a connection between leucogranite genesis and anatexis of intermediate crust. The composition of leucogranites is similar to that of crustal melts generated by muscovite dehydration melting reactions. The extremely low concentrations of Sr and Ba and high Rb contents are comparable with those of Himalayan leucogranites.

An exposed cross-section of the late Hercynian upper and intermediate continental crust in the Sila nappe (Calabria)

PROSSER, Giacomo
2001

Abstract

Upper to intermediate levels of continental crust, as shaped by Late Hercynian metamorphism and magmatism, are exposed in the Sila nappe (Calabria, S. Italy) after Tertiary tectonics. Direct observation of the crustal section reveals a composite metamorphic block dominated by low- to high-grade metapelites and meta-arenites, intruded centrally by granitoids. The average thickness of granitoids, estimated on cross-sections and by geobarometry, reaches 9 ± 2 km. Their composition changes with depth from leucogranite to tonalite with increasing colour index. The estimated bulk composition of the Sila nappe is characterised by higher contents of A1203, REE and Th and lower contents of CaO and Sr than is typical of upper continental crust, reflecting abundant metapelite in the Sila nappe. Comparisons between metasedimentary rocks from the upper and intermediate levels indicate that intracrustal differentiation was determined by anatexis, which favoured A1203 , REE and Th enrichment in medium- to high-grade metapelites. Distribution of REE, Th and Zr was connected to the fate of monazite and zircon during crustal anatexis. Melting reactions involved mainly muscovite whereas biotite, enclosing most accessory phases, often remained stable in the residue. Consequent passive enrichment of Th in medium- to high-grade restitic metapelite determined an unusual increase in radiogenic heat production with depth. Compositional data suggest a connection between leucogranite genesis and anatexis of intermediate crust. The composition of leucogranites is similar to that of crustal melts generated by muscovite dehydration melting reactions. The extremely low concentrations of Sr and Ba and high Rb contents are comparable with those of Himalayan leucogranites.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11563/3859
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