Plutonism in the Variscan Odenwald (Germany): from subduction to collision

• Published in: International journal of earth sciences : Geologische Rundschau 1999-10, Vol.88 (3), p.422-443
• Main Authors: Altherr, R., Henes-Klaiber, U., Hegner, E., Satir, M., Langer, C.
• Format: Article
• Language: English
• Subjects: Carboniferous; Dehydration; Devonian; Geochemistry; Geophysics; Isotopes; Magma; Melting; Plate tectonics; Rocks
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s005310050276

Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For 362Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions ([straight epsilon]^sub Nd^=+3.4 to +3.8;^sup 87^Sr/^sup 86^Sr =0.7035-0.7053;δ^sup 18^O=6.8-8.0[per thousand]) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340-335Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures ([straight epsilon]^sub Nd^=0.0 to -2.5;^sup 87^Sr/^sup 86^Sr=0.7044-0.7056) but different oxygen isotopic compositions (δ^sup 18^O=7.3-8.7[per thousand] in mafic vs 9.3-9.5[per thousand] in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic intrusives of the southern unit III have initial isotopic compositions ([straight epsilon]^sub Nd^=-1.1 to -1.8;^sup 87^Sr/^sup 86^Sr =0.7054-0.7062;δ^sup 18^O=7.2-7.6[per thousand]) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.[PUBLICATION ABSTRACT]