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

Latest Devonian to early plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For similar to 362 Ma old gabbroic rocks from the northern tectonometamorphic unit II initial isotopic compositions (epsilon(Nd)=+34 to +3.8; Sr-87/Sr-86 =0.7035-0.7053; delta(18)O = 6.8-8.0 parts 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 LII) Odenwald. syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340-335 Ma 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 (epsilon(Nd)=0.0 to -2.5; Sr-87/Sr-86 =0.7044-0.7056) but different oxygen isotopic compositions (delta(18)O=7.3-8.7 parts per thousand in mafic vs 9.3-9.5 parts 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 (epsilon(Nd)=-1.1 to -1.8; Sr-87/Sr-86 =0.7054-0.7062; delta(18)O=7.2-7.6 parts 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.