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

Taylor, Hugh P., Jr. and Magaritz, Mordeckai and Wickham, Stephen M. (1991) Application of stable isotopes in identifying a major Hercynian synplutonic rift zone and its associated meteoric-hydrothermal activity, southern Schwarzwald, Germany. In: Stable isotope geochemistry : a tribute to Samuel Epstein. Special publication (Geochemical Society). No.3. Geochemical Society , San Antonio, TX, pp. 355-371. ISBN 0-941809-02-1.

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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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Additional Information:This paper is dedicated to Samuel Epstein, whom we honor in this volume for a lifetime of creative work. He has been a valued asset, friend, inspirational teacher and colleague to all of us over the years. His continued research creativity at a very high level even at the age of 72 is an inspiration to everyone lucky enough to be able to interact with him. We also wish to thank a number of others with whom we have had important discussions about these kinds of problems over the years, principally Ron Oxburgh, Al Hofmann, and particularly to Prof. Dr. W. Wimmenauer who provided some samples and who also aided us in collecting others. We also would like to recommend the southern Schwarzwald as an outstanding and interesting area in which to do field work. This research was supported by the National Science Foundation, Grants No. EAR-83-13106, EAR-88-16413, and EAR-90-19190.
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Caltech Division of Geological and Planetary Sciences5069
Series Name:Special publication (Geochemical Society)
Issue or Number:3
Record Number:CaltechAUTHORS:20221012-154010586
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ID Code:117370
Deposited By: Tony Diaz
Deposited On:12 Oct 2022 22:28
Last Modified:12 Oct 2022 22:28

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