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

Abstract

Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For F362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions (ε Nd pc3.4 to c3.8; 87 Sr/ 86 Sr p0.7035-0.7053; d 18 Op6.8-8.0‰) 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 340-335 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 (ε Nd p0.0 to -2.5; 87 Sr/ 86 Srp0.7044-0.7056) but different oxygen isotopic compositions (d 18 Op7.3-8.7‰ in mafic vs 9.3-9.5‰ 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 (ε Nd p-1.1 to -1.8; 87 Sr/ 86 Sr p0.7054-0.7062; d 18 Op7.2-7.6‰) 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.

Careón ophiolite, NW Spain: Suprasubduction zone setting for the youngest Rheic Ocean floor

“…Important arc-related magrnatism ca. 360-335 Ma has been studied in the Mid-German Crystalline Rise (Saxo Thuringia zone), where it has been attributed to subduction toward the south (Altherr et al, 1999). The same age and tectonic setting have been suggested for the Late Devonian-Dinantian volcanism described in the French Massif Central .…”

Dynamic processes controlling foreland development – the role of mechanical (de)coupling of orogenic wedges and forelands

“…By the mid-Visean onset of the main Variscan orogenic cycle, this basin was closed by southward subduction beneath the Mid-German Crystalline High. Following late Visean detachment of the subducted oceanic slab, reflected by a change from I-type to shoshonitic magmatism in the MidGerman Crystalline High (Altherr et al, 1999), compressional stresses were projected into the Rhenohercynian Shelf, causing disruption of its carbonate platform and the development of a regional unconformity. From the early Namurian onwards, this shelf subsided rapidly under the loads of the advancing nappe systems that account for a total of 200 km of shortening (Oncken et al, 1999).…”

The Léon Domain (French Massif Armoricain): a westward extension of the Mid-German Crystalline Rise? Structural and geochronological insights

“…The southern boundary of the Rheno-Hercynian Zone is represented by the Northern Phyllite Zone (NPZ), where a Carboniferous high-pressure metamorphism is associated with north-directed thrusting. South of the NPZ, the Paleozoic rocks that crop out in the Spessart and Odenwald massifs are parts of a microcontinent called the Mid-German Crystalline Rise (MGCR, e.g., Holder and Leveridge 1986;Hirschmann and Okrusch 1988;Oncken 1997;Altherr et al 1999;Franke 2000;Will and Schmädicke 2001 and enclosed references).…”