Use of oxygen isotopes to trace a zone of meteoric-hydrothermal fluid flow along a Hercynian fault system; Vosges, N.E. France

Abstract

Separated by the Tertiary Rhine Graben, the Schwarzwald (SW Germany) and the Vosges (NE France) are part of the Hercynian (Variscan) orogenic belt that extends across Europe. Taylor et al. (Geochem. Soc. Spec. Pub. No. 3, 1991) found ^(18)O-depleted samples along a 3-6 km wide, 50 km long, E-W zone in the southern Schwarzwald. This feature, called the Badenweiler-Lenzkirch zone (BLZ), consists of down-dropped Upper Devonian-Lower Carboniferous sediments and volcanics sandwiched between older granites and high-grade gneisses. The ^(18)O depletion was attributed to a major meteoric-hydrothermal event along a "pull-apart" associated with strike-slip faulting and granitic intrusion during the late Visean (330-340 Ma). The present work is an attempt to see if these ^(18)O effects can be traced westward into the Vosges, where there is a broad outcrop area of clastics and volcanics correlative with those in the BLZ. Our study of over 50 samples from the southern Vosges reveals that ^(18)O depletion is ubiquitous along a narrow, ENE-trending, N-dipping zone known as the Ligne des Klippes (LDK): this is a melange of slivers of granite, gneiss, and mafic and ultramafic rocks. Whole-rock at δ^(18)O values along a 3 km wide zone along the north side of the LDK fall between +2.6 and +9.4, whereas elsewhere in the Vosges these rocks have δ^(18)O = + 10.3 to + 15.5. These ^(18)O depletions, like those in the BLZ, can only have been caused by infiltration of large amounts of heated surface fluids along this zone. As is the case in the BLZ, the ^(18)O-depleted zone in the Vosges is cut by a late-stage, undeformed granite with "normal" δ^(18)O ≈ + 7, constraining the hydrothermal activity to mid-Hercynian time. Sediments near the zone are strongly silicified, indicating that the fluids were rich in silica. Because of the similar sediments and oxygen isotopic signatures in the two zones and because the LDK is near the projection of the BLZ, we conclude that this zone in the Vosges indeed represents an extension of the BLZ, probably an exposure at a higher structural level. Although kinematic indicators within the LDK define a complex history of faulting, our data require that in the late mid-Hercynian, heated meteoric ground waters were focused along this zone of high fracture permeability. This might be explained by oblique strike-slip movement along the LDK related to the analogous event in the BLZ (which was much more intense and involved major extension).

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