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The Hidden Monzogranite of Soultz-Sous-Forêts

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The Hidden Monzogranite of Soultz-Sous-Forêts The hidden monzogranite Jean-Marc STUSSI (1) of Soultz-sous-Forêts Alain CHEILLETZ (2, 3) (Rhine Graben, France). Jean-Jacques ROYER (2, 3) Philippe CHÈVREMONT (4) Mineralogy, petrology and genesis * Gilbert FÉRAUD (5) Le monzogranite sous couverture de Soultz-sous-Forêts (graben rhénan, France). Minéralogie, pétrologie et genèse Géologie de la France, 2002, n° 1, pp. 45-64, 10 fig., 7 tabl. Key-words: Monzogranites, Major-element analyses, Trace-element analyses, Rare earths, K-Ar, Ar40-Ar39, Crustal origin, Variscan orogeny, Bas-Rhin France, Rhine Graben. Mots-clés : Monzogranite, Analyse éléments majeurs, Analyse élément trace, Terre rare, K-Ar, Ar40-Ar39, Origine crustale, Orogénie varisque, Bas-Rhin, Fossé Rhénan. Abstract Soultz monzogranite had a mainly crustal nite et par une composition calco-alcaline origin with a possible contribution of potassique, enrichie en Ba, K, Th, U, Y, et Drilling at Soultz-sous-Forêts, France, mantle-derived components represented by terres rares. Les variations chimiques le conducted as part of the European Hot Dry the enclaves. Emplacement was related to long du sondage mettent en évidence deux Rock (HDR) Project, penetrated granitic collision tectonics during the transition zones ne présentant pas de solution de basement down to 5,080 m and revealed a between Early and Late Carboniferous. continuité. Elles se distinguent par les porphyritic monzogranite that has no abondances de minéraux (hornblende, known equivalent in the granitic outcrops Résumé biotite, plagioclase, orthose), par les of adjacent basement areas. The EPS-1 Retenu comme l’une des cibles du teneurs en Ba, Sr, Th et terres rares (biotite, borehole core (from 980 to 2,230 m) shows projet Europe Hot Dry Rock (HDR), le orthose, Ba Th et terres rares plus élevées a monzogranite that has a high-K calc- socle sous couverture secondaire et en dessous de la cote 1 800 m) et par leurs alkaline composition and is enriched in K, tertiaire du site de Soultz-sous-Forêts modalités de variation (plus régulières en Th, U, and REE. Petrological and miner- (graben rhénan, France) a été atteint à profondeur qu’au-dessus de 1 800 m). Ces alogical characteristics indicate that the 1 430 m, foré en carotté continu jusqu’à variations sont rapportées à des fraction- monzogranite formed through two crystal- 2 230 m (sondage EPS-1) et en destructif nements solide-liquide lors de l’ascension lization stages: a first, deep-seated, stage jusqu’à 5 080 m (sondage GPK2). Ce socle et de la mise en place du monzogranite. À was characterized by the crystallization of est constitué d’un monzogranite à méga- ces variations se superposent des litages liquidus phases in the 755-790 °C range, at cristaux de feldspath potassique dont on ne magmatiques formés par l’accumulation 3.5 kb (11-12 km depth). This was connaissait pas d’équivalent de surface de mégacristaux de feldspath potassique followed, during ascent and at emplace- dans les régions hercyniennes voisines. Les (jusqu’à 60 % d’orthose) et quelquefois de ment level, by consolidation at 665-715 °C, variations des caractéristiques pétrolo- minéraux ferro-magnésiens. D’après la 1.5-2.0 kb (4.5-5.5 km depth) associated giques le long de la colonne carottée du course de cristallisation des minéraux et with the development of K-feldspar monzogranite ont été déterminées à partir les paramètres intensifs obtenus (pression layering. Published Rb/Sr ages (325 Ma), du prélèvement de 14 échantillons, ainsi de cristallisation calculée à partir de la new 40Ar/39Ar cooling ages (319 Ma), a que des analyses effectuées antérieurement composition zonée de la hornblende, U/Pb on zircon emplacement age of par le BRGM (dont 5 enclaves). Le monzo- température de cristallisation, fO2), le 331 Ma, and petrological arguments such granite de Soultz se caractérise par une monzogranite aurait cristallisé en deux as the presence of monzogabbrodiorite to paragenèse minérale à quartz, plagioclase, stades : (i) un stade profond à 3,5 kb (11- quartz-syenite enclaves, indicate that the orthose, biotite, amphibole, sphène, alla- 12 km) et à 755-790 °C (minéraux au * Manuscrit déposé le 12 octobre 2001, accepté le 18 décembre 2001 (1) 5, rue des Glières 54500 Vandoeuvre-lès-Nancy, France. (2) Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501 Vandoeuvre-lès-Nancy Cedex, France. (3) Ecole Nationale Supérieure de Géologie, BP 40, 54501 Vandoeuvre-lès-Nancy Cedex, France. Fax 33 (0)3 83 51 17 98. (4) Bureau de Recherches Géologiques et Minières, BP 6009, 45060 Orléans cedex 2, France. Fax 33 (0)2 38 64 33 34. (5) Université de Nice, UMR Geoscience Azur EP 125, Parc Valrose, Sophia Antipolis, 06108 Nice, France, Fax 33 (0)4 92 07 68 16. GÉOLOGIE DE LA FRANCE, N° 1, 2002 45 THE HIDDEN MONZOGRANITE OF SOULTZ-SOUS-FORÊTS liquidus : hornblende, rare sphène, alla- loppement du litage magmatique et monzodiorites quartzifères, monzonites nite, biotite, orthose, plagioclase), (2) un consolidation de la matrice quartzo-feld- quartzifères et syénites quartzifères forte- stade d’ascension et de mise en place du spathique. La mise en place du monzogra- ment enrichies en K, Th, U, et terres rares. magma à 1,5-2,0 kb (4,5-5,5 km) et 665- nite est accompagnée d’un magmatisme Les rapports initiaux du strontium (87/86Sr 715 °C, marqué par la fin de cristallisation shoshonitique synchrone représenté par = 0.7058), ainsi que les simulations par de la hornblende et de la magnétite, déve- des enclaves de monzogabbrodiorites, bilans de masse et fractionnement d’élé- ments traces, il est suggéré que le monzo- granite aurait une origine essentiellement crustale impliquant un constituant NORTH mantello-dérivé représenté le plus proba- SEA blement par les monzodiorites quartzifères rencontrées en enclaves. VFL D’après les datations isotopiques RH Rb/Sr à 325 Ma, K/Ar (321 ± 8 Ma), 40Ar/39Ar (319,8 ± 0,6 à 322,4 ± 0,4 Ma ; âges de refroidissement) et U/Pb sur ARMORICAN MASSIF ST zircon (331 ± 9 Ma ; âge de mise en place), le monzogranite s’est mis en place BM à la limite du Viséen supérieur et du MASSIF BF MO CENTRAL Namurien. Le monzogranite de Soultz constitue de ce fait l’une des dernières manifestations plutoniques actuellement connues dans le domaine de la ride cris- talline médio-germanique. Par ses caractéristiques pétrologiques (Th, U, Y, Zr, terres rares), il s’apparente davantage aux monzogranites shoshonitiques de Senones et Natzwiller (Vosges septentrio- 100 km nales) qu’aux granites ultimes de l’unité HARZ - MTS - HERCYNIAN III de l’Odenwald (Heidelberg, Tromm). RHENO L’âge de mise en place et la structure magmatique fruste indiquent pour le RHEIN monzogranite de Soultz une mise en place SCHIEFER- C R GEBIRGE Meissen syn- à post-D4 par rapport aux phases de Zone te Giessen-Harz nappes Elbe - Va KAK déformation de l’Odenwald. Celle-ci lley s’inscrirait dans un contexte collisionnel ZGEBIRGE Graben thern phylli or ER ger - N à post-collisionnel des zones externes Terl E SPESSART septentrionales de l’orogène varisque SAXO - THURINGIAN G M FICHTELGEB. centre-ouest européen. RE ODENWALD BOHEMIAN Introduction MASSIF W S MOLD The Soultz-sous-Forêts site in the ANUBIAN REG luton Rhine Graben (Alsace, France) has been Strasbourg P CF selected as one of the three main areas in Se ION Europe for demonstrating the feasibility of BLACK FOREST Bohemian the Europe Hot Dry Rock (HDR) concept B (Bresee, 1992), because of its high geot- VOSGES hermal gradient (Gérard and Kappelmeyer, 1987) that probably results from the Fig. 1.- Geological sketch map and location of the Soultz monzogranite in the Variscan Saxo-Thuringian upwelling of hot fluids from depth zone (after Franke, 1989a, simplified). (Clauser, 1989). It is located along the Abbreviations: VFL:Variscan Front Line; RH: Rheno-Hercynian zone; ST: Saxo-Thuringian zone; MGCR: Mid-German Crystalline Rise; MO: Moldanubian zone; BM: Bohemian massif; V: Vosges; BF: Rhine Graben where several geothermal Black Forest; RE: Rhine Graben escarpment intrusions; W: Windstein; S: Soultz; CF: Champ-du-Feu; anomalies are known. The physical char- Se: Senones; B: Ballons. acteristics of the heat exchanger have been Fig. 1.- Le site de Soultz dans son cadre géologique Saxo-Thuringien (d’après Franke (1989a) simplifié). investigated through structural, mechan- Abréviations : VFL : front varisque ; RH : zone rhéno-hercynienne ; ST : Saxo-Thuringien ; MGCR : ical, and thermal studies, as well as model- Ride cristalline médio-germanique ; MO : Moldanubien ; BM : Massif de Bohême ; BF : Forêt-Noire ; ling based on logging of the three wells RE : intrusions du talus rhénan occidental. Autres abréviations, cf. légende anglaise. GPK1, GPK2 and EPS-1 (Traineau et al., 46 GÉOLOGIE DE LA FRANCE, N° 1, 2002 THE HIDDEN MONZOGRANITE OF SOULTZ-SOUS-FORÊTS 1991; Genter and Traineau, 1992; Kappelmeyer et al., 1991; Dezayes et al., 350 1995, 1996; Genter et al., 1995; Schulmann 322.8 ± 0.6 Ma et al., 1997; Ledésert et al., 1999). The heat exchanger is located within a buried granitoid intrusion of the Variscan 322.0 ± 0.8 Ma basement beneath Mesozoic and Cenozoic Apparent Age (Ma) 300 rocks in the Rhine Graben. Outcrops of this basement occur in the Odenwald, the Northern Vosges and locally along the a Western Rhine Graben escarpment. This study focuses on the petrology, miner- K204 biotite (-2207.5 m) b alogy and genesis of the blind Soultz 250 monzogranite intrusion. Three deep holes 350 penetrate the basement, GPK-1, GPK-2 and EPS-1. GPK-2 was recently deepened to 5,080 m depth, while GPK-1 reached 3,580 m (Elsass et al., 1995) providing an exceptional cross section through the 319.8 ± 0.6 Ma (P) granitic basement of the Rhine Graben. 300 Only EPS-1 was cored continuously from 980 to 2,230 m; in addition to previous analyses (including five enclaves) by BRGM, 14 new samples collected from this drill-core have been analysed in order GPK2 biotite (-3876 m) to define the origin and emplacement of 250 the Soultz monzogranite.
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