Application of stable isotopes in identifying a major Hercynian synplutonic rift zone and its associated meteoric-hydrothermal activity, southern Schwarzwald, Germany

Abstract

Whole-rock and mineral δ¹⁸O analyses were obtained on granites, gneisses, migmatites, and a variety of metamorphosed sedimentary and volcanic rocks from the Schwarzwald massif, focusing on Variscan-age (270-360 Ma) igneous, metamorphic, and hydrothermal events. The data display some remarkable geographical and temporal systematics; the extremely ¹⁸O-depleted samples (δ¹⁸O < +3) occur only in the southern Schwarzwald, confined to a 3-6 km wide, 50 km long, EW zone of down-dropped Upper Devonian-Lower Carboniferous sediments and volcanics sandwiched between older, high-grade gneisses on the north and south. This belt of rocks, the Badenweiler- Lenzkirch (B-L) tectonic line, was intruded all along its length by the 1-2 km wide Rand Granite pluton, which also exhibits extremely low δ¹⁸O values. The data imply that a very large meteorichydrothermal system was established here within a rift-zone setting, in Late Visean time (≈340 Ma). This date is fixed by paleontological evidence indicating that the B-L zone sediments changed from marine to terrestrial at this time, because the meteoric-hydrothermal event documented by the ¹⁸O/ ¹⁶O data obviously could not have occurred while marine sedimentation was taking place. The δD values of biotite, hornblende, and chlorite in 31 migmatite, schist, and gneiss samples from the B-L zone range from -60 to -111, but the most ¹⁸O-depleted of these samples (δ¹⁸O < +4) all have δD < -90. Assuming that these rocks were altered by hydrothermal fluids that originally had δ¹⁸O and δD on the Meteoric Water Line, we calculate that the ground waters in this area had δ¹⁸O ≈ -6 to -9 and δD ≈ -40 to -65 at that time, compatible with geological data suggesting a low latitude and sub-tropical climate. Previous geological models of the B-L zone emphasize collisional tectonics and conclude that it is a thrust-zone; however, such models are not compatible with the ¹⁸O/¹⁶O data. Our preferred model invokes formation of a dilatant zone along a major strike-slip fault at about 340 Ma. This pull-apart was intruded by the Rand Granite, which acted as the main "heat engine" that drove this large convective system. As the hydrothermal activity weakened, the B-L zone continued to deform, and the solidified pluton was stretched out and strongly attenuated. The hydrothermal episode and the strike-slip deformation both terminated prior to intrusion of some large, post-tectonic, two-mica granite plutons (which truncated the B-L zone and Rand Granite at about 300-315 Ma); these late plutons are not hydrothermally altered or ¹⁸O depleted. This work suggests that in complex geological terranes stable isotope data may be one of the best ways to identify fossil synplutonic rifts or pull-aparts associated with major strike-slip faults.

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