The Rand Granite in the southern Schwarzwald and its geodynamic significance in the Variscan belt of SW Germany

Abstract

The Rand Granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the Moldanubian Zone at the southern margin of the Central Schwarzwald Gneiss Complex. It is a largely mylonitized elongated body and is thrust over the Badenweiler-Lenzkirch Zone forming a nappe with shear zones along its northern and southern boundaries. It comprises meta-granites, meta-trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites. Mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions. Nappe thrusting, which also affected the South Schwarzwald Gneiss Complex, occurred in Visean time during high-temperature / low-pressure metamorphism. Kinematic indicators in the mylonites document E- to ESE-directed nappe transport, highly transpressive relative to the trend of the nappe boundaries and the foliation. The trondhjemites formed at 351 +5/-4 Ma, predating the Variscan HT metamorphism. They have initial ε_Nd-values of +6.6 to +6.7 and relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7042 to 0.7063), indicating a predominant mantle origin. The granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 Ma, suggested by U-Pb zircon model ages. They are different from the trondhjemites with low initial ε_Nd-values (−4.7 to −3.3) and higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7068–0.7077), indicating that large part of the Rand Granite originated from anatexis of continental crust. Internal structure of zircons from the Rand Granite reveals mixing of magmas derived from both mantle and crust sources. These new data support an interpretation that the Rand Granite developed along an active continental margin and therefore represents a possible root of a Variscan magmatic arc.

Appendix

Analytical methods

Whole-rock powder was obtained from ca. 15 kg sample material; it was analysed for major and some trace elements on twelve samples by XRF technique. Loss on ignition (LOI) was determined after heating sample powder to 1,000 °C for one hour. Zircon mineral was separated from crushed rocks using a Wilfley table, a Frantz isodynamic separator and heavy liquids and finally handpicked under a stereo microscope. Zircon grains studied by the cathodoluminescence (CL) investigation were mounted in epoxy and polished down to expose the grain centres. CL images were obtained with a microprobe JEOL JXA-8900RL working at 15 kV. All isotopic measurements were made on a Finnigan MAT 262 mass spectrometer.

For conventional U-Pb analyses, single zircons or populations consisting of a few morphologically identical grains (up to 4 grains) were shortly washed in warm 7 N HNO₃ and warm 6 N HCl prior to dissolution to remove surface contamination. A mixed ²⁰⁵Pb-²³⁵U-tracer solution was added to the grain(s). Dissolution was performed in PTFE vessels in a Parr acid digestion bomb (Parrish 1987) using the vapour digestion method. The bomb was placed in an oven at 210 °C for one week in 22 N HF and for one day in 6 N HCl to assure re-dissolution of the fluorides into chloride salts. Separation and purification of U and Pb were carried out on Teflon columns with an about 40-μl bed of AG1-X8 (100–200 mesh) anion exchange resin. Pb was loaded with Si-gel onto Re-filament and measured at 1,300 °C in single-filament mode, while U was loaded with 1 N HNO₃ onto Re-filament and measured in double-filament configuration. Total procedural blanks were <10 pg for Pb and U. A factor of 1‰ per atomic mass unit for instrumental mass fractionation was applied to all Pb analyses, using NBS 981 as reference material. Common Pb contribution remaining after correction for tracer and blank was corrected using values of Stacey and Kramers (1975). Discordia intercept ages were calculated using regression treatment of Wendt (1986). Our own repeated measurements on natural zircons from the Phalaborwa Igneous Complex (South Africa) and from Kuehl Lake/Canada (Zircon 91500; Wiedenbeck et al. 1995) are published in Chen et al. (2002a). Using the U-Pb method, we obtained a ²⁰⁷Pb/²⁰⁶Pb age of 2,053.5±1.2 Ma for Phalaborwa zircon, similar to the age of 2,051.8±0.4 Ma obtained by Pb-Pb evaporation method (Kröner et al. 2001). Analysis of zircon 91500 gave concordant U-Pb ages of 1,065.6±2.2 Ma, consistent with the reported U-Pb age of 1,065.4±0.3 Ma obtained in different laboratories (Wiedenbeck et al. 1995).

For Nd-Sr isotope analyses, Rb-Sr and light rare-earth elements were isolated on quartz columns by conventional ion exchange chromatography with a 5-ml resin bed of AG 50W-X12 (200–400 mesh). Nd and Sm were separated from other rare-earth elements on quartz columns using 1.7-ml Teflon powder coated with HDEHP, di(2-ethylhexyl)orthophosphoric acid as cation exchange medium. Sr was loaded with Ta-HF activator solution on W-filament and measured in single-filament mode. Sm and Nd were loaded as phosphate on Re-filament and measured in double-filament configuration. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios are normalized to ⁸⁶Sr/⁸⁸Sr=0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd=0.7219, respectively. Repeating analyses of Ames metal and NBS987 Sr standard gave mean values of 0.512125±0.000010 for the ¹⁴³Nd/¹⁴⁴Nd ratio (n=24) and 0.710259±0.000012 for the ⁸⁷Sr/⁸⁶Sr ratio (n=28). Total procedural blanks were <300 pg for Sr and <50 pg for Nd. Further details on analytical techniques are given in Chen et al. (2000, 2002b).