Field trip to the Vosges

June 21st - 27th, 2015

Elisabeth sets off in a quite car
Elisabeth sets off in a quite car

Day 1 of the OUGS ME spring trip to the Vosges / Monday, June 22nd

Monday dawned clear and bright. Our leader was Dr Françoise Chalot-Prat from Nancy and our drivers were the redoubtable sisters, Béatrice and Elizabeth, who knew all the localities backwards, and so always found localities together; marvellous.

We went first to look at the Thalhorn ophiolite, through Kruth to Fellering.

Pale gabbro with large augite phenocrystal
Fig. 2: Pale gabbro with large augite phenocrystal

Up a fairly steep hillside to the first outcrop. This was a rather obscure conglomerate, with quartz, feldspar and pebbles of basic igneous rock. Then to a gneiss, finely foliated in which sillimanite had been found. The suggestion was that this was an original sediment, which had found its way to the lower crust. Climbing the hill we found a variety of gabbros, including small ‘pods’ of pale gabbro with large augite phenocrysts up to 1 cm, indicating either very slow cooling or, more probably, wet conditions of crystallisation (figure 2)

Higher up the hill we found a variety of altered peridotites, and crossing a fairly tricky boulderfield and several electric fences, we found a decent track leading to an old quarry. Here it was just like being back at the Lizard in Cornwall, lots of sepentinised peridotite, with large olivines now altered to micas and excellent lizard-skin surfaces. Brigitte even found a few thin seams of fibrous white talc.

The suggestion was that this was the remains of a back-arc ophiolote.

Cliff of Eocene conglomerate at Turckheim, with a small cave at the base
Fig. 3: Cliff of Eocene conglomerate at Turckheim, with a small cave at the base

After lunch we went to see something completely different: the gravels and limestone on the edge of the Rhine rift valley at Turckheim on the outskirts of Colmar. The first locality was a steep cliff alongside a vineyard with about 10 m of poorly sorted conglomerate, with pebbles mainly 4 to 20 cm, virtually all of pale brown Jurassic limestone, with just a few red sandstone and granite pebbles (figure 3).

Commander [Mike] Molloy, even with Prussian support [Gisela], still can't make much sense of the conglomerate
Commander [Mike] Molloy, even with Prussian support [Gisela], still can't make much sense of the conglomerate
Upper surface of a Jurassic limestone block with horizontal burrows - Thalassinoides?
Upper surface of a Jurassic limestone block with horizontal burrows - Thalassinoides?

A few of the pebbles showed smooth pressure solution features where smaller pebbles had left a dished impression in an adjacent one, but most of the deposit was matrix supported, without ‘dished’ pebbles. This conglomerate is of Eocene age, and Eocene beds form the uppermost beds of the Rhine graben.

There were also larger blocks of Jurassic limestone up to 3 m long. Examples of a fine-grained oolite, shelly and burrowed limestone were seen, indicating a fairly shallow-water deposit.

Fig. 6: Françoise and her multi-coloured whiteboard
Fig. 7: From vineyard looking east to the Rhine valley

The limestone and probably the Eocene conglomerates were probably formed as the step faulting developed to form the Rhine graben, here expertly sketched by Françoise on her multi-coloured whiteboard (figure 6).

Our final locality was in another vineyard, where both the conglomerate and Jurassic limestone were again seen. From here we got a good view east across the Rhine valley to show how the land in Françoise’s sketch looks, with the Black Forest in the background

At the end of a most interesting introductory day, we began to prepare ourselves for more challenging geological ideas which were to follow later in the week

Dave Williams

South West branch

Day 2 / Tuesday, June 23rd

Under the guidance of Dr Maryse Ohnenstetter, director of research at CNRS, we headed for the site of Le Fossard, La Charme, an important landmark in the metamorphosed parts of the Moldanubian Hercynian zone.

At La Charme stands one of the best bodies of serpentinised garnet peridotite in the Central Vosges as witnesses of deep metamorphic events. The unit is a wall within a fault plane, and a succession of ridges where subspherical garnet grains of varying sizes, mostly between 50 and 150 mm, protrude on a dark brown to green ground mass which is often striated. Peridotite outcrops in 17 locations, often showing subvertical contacts in the field. The fault follows a reactivated early carboniferous fault.

On a large scale, migmatite separates an upper granulitic gneiss unit formed at a pressure of 5 to 6 GPa, from a lower tectonic unit of eclogitic facies. This is spinel peridotite with veins of mafic and ultramafic minerals.

The pressure and temperature conditions of these two units do not relate to a stratigraphic sequence.

Layered serpentinised garnet peridotite
Fig. 1: Layered serpentinised garnet peridotite
Some garnets show a clear corona structure
Fig. 2: Some garnets show a clear corona structure

On a large scale, migmatite separates an upper granulitic gneiss unit formed at a pressure of 5 to 6 GPa, from a lower tectonic unit of eclogitic facies. This is spinel peridotite with veins of mafic and ultramafic minerals.

The pressure and temperature conditions of these two units do not relate to a stratigraphic sequence.

The serpentinized peridotites belong to Leptynitic Granulites, a metamorphic acid rock with quartz and feldspar, olivine, orthopyroxene, clinopyroxene and garnet, equilibrated in high metamorphic grade for both temperature and pressure.

The pyrope garnets have been kelyphitised, many with a visible corona structure, and serpentinisation further dims their paragenesis. Decay of the original inner crystalline structure through kelyphitisation has resulted in fine grained aggregates of brown amphibole, enstatite, diopside, spinel and plagioclase.

In the olivine, the forsterite content is high, with a high Mg/Mg+Fe ratio. Pyroxene is present as diopside (clinopyroxene) and enstatite (orthopyroxene) with some lineation of the pyroxenes lying subparallel to the garnets. The amount of pyroxene and garnet is variable. The layering, partly due to serpentinization, disappears where boudinage or partial melting occurred.

A red tinge is due to the oxidation of iron to hematite in the presence of abundant oxygen
Fig. 3: A red tinge is due to the oxidation of iron to hematite in the presence of abundant oxygen. On this early summer day a lush vegetation and ubiquituous moss confirm a high moisture level typical of a fault zone. "Ya faut ya eau" (where there is fault there is water, too) is a convinient reminder to French geology students.

Though this site has been researched by many geologists, it is still a work in progress. Altherr and Kalt did petrographic research in 1996, and link the site’s emplacement to exhumation of continental rather than oceanic mantle. This massif would have resulted from

First: thickening of continental crust during thrusting or collision

then: resulting delamination of thickened crust

finally: exhumation of delaminated mantle fragments; exhumation must have been fast enough for this high level of conservation of garnet grains.

There is gneiss and upward foliated migmatite around the peridotite, begging the question of how migmatite was emplaced and affected by subsequent faulting. There are also varying signs of limited partial melting and metasomatic potassic enrichment. The origin of this potassium is also unclear.

Other minerals present in the peridotite include phlogopite and apatite.

At a nearby exposure, on a 20 meter block, the garnet grains are finer overall. The size is determined either from initial crystallization or locally partial melting of the peridotite, at great depth, with recrystallization in the spinel facies producing Al Cr Spinel, in which chromium was probably introduced by partial melting.

Remiremont granite quarry
Fig. 4: Remiremont granite quarry

We left La Charme and travelled to Le Syndicat St Amé to see an active quarry of Remiremont granite. This granite body which extends 30 x 10 km from Remiremont to Gerardmer was emplaced by injection of magma in a N70 direction. Its generation is linked to partial melting of thickened crust in palaeozoic tectonic processes (395 Ma). The granite is anatexic, syntectonic or tardytectonic, and the orientation of its emplacement is NE- SW, a similar orientation to the foliation direction for the above described peridotite. This suggests a reactivation of a deep-seated fault link for both the granite body and the peridotite.

Remiremont granite is a fine-grained leucogranite of Upper Carboniferous age. It is an aluminous sodic potassic granite composed of Qz for 26%, 43 % anorthite, 2 micas: biotite (5%) and muscovite (3.5%) and traces of andalusite.

It shows some variations in colours and patina. With regard to its economic use, it was observed that pink granite is more brittle and therefore not as strong as its light grey counterpart. It is pink where tectonically juxtaposed to migmatite (or possibly intruded into migmatite) and affected by metasomatism. There is in fact a cross-cutting relation between granite and migmatite. Its exploitation is facilitated by a characteristic lamellar structure.

There are many granite bodies in the central Vosges, such as the most ancient Granite Fondamental and Granite des Cretes as well as later intrusive granites such as Remiremont and microgranites, all witnesses of a long tectonic history.

 

Terrae Genesis: Outdoor museum
Fig. 5: Terrae Genesis: Outdoor museum
Terrae Genesis: Stone samples on external walls
Fig. 6: Terrae Genesis: Stone samples on external walls

In the vicinity, there is the “Terrae Genesis” Geology Museum (Peccavillers, Le Syndicat St Amé), where we were welcome over lunch.

The museum provides geological information to visitors, schools and colleges, with resource centre and research instruments. There is a vast collection of thin-section listed on the website. The scope is the geological history of the Vosges, with particular emphasis on the Remiremont granite and other plutonic rocks of the Vosges, as well as its sedimentary rocks (sandstone and conglomerate). Landscape interpretation includes quaternary glaciations and resulting features.

Behind the Terrae Genesis main building is the former site of exploitation for a blue Remiremont granite. Heavy machinery, pulleys and chains, large circular saws are displayed as an outdoor museum.

For a long time, the granite was transported to Paris, where it had been highly valued as paving material. The introduction of macadam had brought this to an end though both world wars were to create another need for the stone: War memorials. At the height of activity, these works had employed up to 3000 staff.

Terrae Genesis has moved in where a granite and general stone trading company had its offices, leaving its stones samples on the walls. The museum web address is www.terraegenesis.org

Entering Gabe Gottes Mine
Fig. 7: Entering Gabe Gottes Mine

Our next locality was the Gabe Gottes mine, Sainte-Marie-aux-Mines.

The drive from the Col du Bonhomme and approach to Sainte-Marie-aux-Mines offers a plunging view over the reactivated Hercynian faults and metamorphic complex which gave rise to intense mining activities in the past.

A witness to the local mining history is the 15th to 16th century Maison des Mineurs which we passed

on the way to the mine entrance. Among other functions the Maison des Mineurs had, in the past, served as a prison for violators of mine rules.

At the Gabe Gottes mine, five main mineralized galleries have been exploited, yielding silver, copper, iron, antimony, arsenic, and more.

Regarding the mining activities we were told of Silver extraction by monks to a depth of 60 meters between the 8th and 15th century. At that point it employed up to 3 000 miners and the village of

Marienkirch was designed to house 12 000 peasants, all involved in mine-related activities such as tools and wood production. .

The mine had expanded considerably from 1549 to 1640. 300 small galleries of that period have been identified, dedicated to the extraction of silver and copper. After bankruptcy and a 300 years interval, activities resumed in 1935 specifically for arsenic extraction, making it the first arsenic mine in the world. Arsenic had mineralized as a component of tennantite and tetrahedrite. This came to a halt in 1940.

Records show a total 300 tons of pure silver extracted from these mines.

Conjugate fault in the Adit Gallery (Gabe Gottes Mine)
Fig. 8: Conjugate fault in the Adit Gallery

Records show a total 300 tons of pure silver extracted from these mines.

The complex networks of faults of different sizes and orientations are related to distinct tectonic

events. For mining purposes, the NNE-SSW trending Gabe Gottes fault, its conjugate faults, and E-W Saint Jacques fault offered the most valuable infilling of polymetallic veins.

Fractures filled by veins of calcite and quartz. Hematite, chalcopyrite and tetrahedrite mineralised along these veins (Gabe Gottes mine)
Fig. 9: Fractures filled by veins of calcite and quartz. Hematite, chalcopyrite and tetrahedrite mineralised along these veins

Veins are made visible by the pale calcite or / and quartz filling, or the purple or red colour resulting

from hematisation of iron.

The ore wealth was due to polymetallic mineralization, with copper, Iron, antimony, silver, arsenic. Along the St Jacques vein, we can observe the mineralization along veins of calcite, dolomite, quartz and/or feldspar filling fractures linked to reactivated faults of the Hercynian orogeny. Some veins of chalcopyrite and crystals of tetrahedrite are still visible, as well as lenses of albite

Migmatitic gneiss and albitite (Gabe Gottes Mine)
Fig. 10: Migmatitic gneiss and albitite

Much of the country rock is a greenish gneiss due to chloritisation and sericitisation. There is also a greened metabasalt south of the Adit gallery. A migmatitic gneiss in the Adit Gallery shows large bands of pink albite. Pearl gneiss, a garnet-bearing gneiss, is a characteristic host rock of the E-W gallery south of the Adit gallery. Hydrothermal alteration of this metamorphic complex is striking in many locations along the galleries.

Brigitte Revol Macdonald

 

Geological map of the Northern Vosges
Geological map of the Northern Vosges

Day 3 / Wednesday, June 24th

On day 3, we focused on volcanism and magmatism in the Northern Vosges, looking at Upper  Palaeozoic volcano-sedimentary rocks later cut by Hercynian granitoids.

Our guide was Etienne Feutscher whose extensive field notes helped to place what we saw in the context of regional and global tectonics, and whose local knowledge of where to find exposures in this heavily overgrown terrain was invaluable. Françoise Chalot-Prat and Elisabeth d’Eyrames also helped us to ‘read’ the outcrops at a number of localities.

The volcanic and plutonic rocks of the Northern Vosges lie along a nearly linear axis trending SW-NE, becoming increasingly acid-alkaline over 70 Ma years of evolution (c. 360-290 Ma). In the 1970s, initial mapping followed by a number of local studies suggested a subduction/collision tectonic setting, but the chronology and structural details were not clearly understood. Recent work (Edel and Schulmann, 2009; Skrzypek et al., 2014; Tabaud, A-M. et al, 2014) using current methods of isotopic and chemical analysis has supported interpretation in terms of regional/global tectonics. A simplified summary (linked to the localities that we visited) is presented below.

(It must be noted that the history of this area is complex with successive magmatic suites generated from contrasting sources in different geodynamic environments).

First episode

Devonian-Dinantian magmatism dominated by the tholeitic volcanism ((I-type granite derived from the mantle) of the Schirmek Massif. (Localities 1, 2, 3)

Second episode

(a) Mafic magmas (Bande Mediane) followed by calc-alkaline plutons (granodiorites of Hohwald, south of the Champ du Feu Massif) (Localities 6, 7, 8)

(b) K-rich calc-alkaline magmatism (the old granites north of the Champ du Feu, Waldersbeck, Fouday Baremback) (Locality 6)

(c) Mg-K magmatism (younger granites, Natzwiller, Andlau, Senones) (Localities 5 and 8)

Third episode

(a) Permian peraluminous alkaline granites (Kagenfels (some have placed it earlier), Kreuzweg)

(b) Acid rhyolite volcanics (Nideck- Donon) (Localities 4, 8)

volcanic rocks in the Northern Vosges
volcanic rocks in the Northern Vosges
ages of volcanic rocks in the Northern Vosges
ages of volcanic rocks in the Northern Vosges

In terms of tectonics, he Northern Vosges complex shows evidence of polyphase subduction, collision and rifting and polymetamorphism spanning a range of Periods; as the evidence for these tectonic processes has only been recently published (see above), they are still under discussion. A simplified account is as follows:

1. Middle Devonian (335-330 Ma): rocks of the Schirmeck-Rabedeau massifs evolve from tholeitic to calc-alkaline composition, possible due to anatexis of Saxothuringian crust during their ascent in a continental back arc setting.

2. c 330 Ma: the mafic rocks of the Bande Mediane and diorite and granodiorite of the Hohwald are generated by partial melting of an enriched mantle wedge metasosmatised by fluids expelled from the Saxothuringian subduction zone. The evolution to acid composition here may be due to fractional crystallisation (dominated by amphibole) and/or crustal assimilation.

3. 318 (±3) Ma: The Belmont granite is emplaced; the composition here is due to magma mixing of enriched mantle and felsic magma derived from dehydration of the subducted Avalonian crust.

4. 312±2 Ma: the younger granite (derived by a similar mechanism) is emplaced

5. The Permian Kagenfels granite (S-type derived from crustal material) and the Nidock-Donon volcanic rocks are emplaced; these emerge from progressive subduction of the Rheno-Hercynian continental crust.

Those who want more detail should consult the material referenced above. An intergrated synthesis is offered in the schematic cross-sections in Scrzypek et al. (2014), p. 63. The relevant map is the BGRM 1:50000 Molsheim Sheet.

 

Locality 1: Blanc Rupt

Here we examine two outcrops at the roadside. The upper body of red/purple fractured rock has traces of cross-of bedding, and some columnar jointing.

Françoise explained that this may be the end of a cooling pyroclastic flow, where the pumice has flattened and the rock has become welded and is massive enough for such jointing to develop as it cools. A lower bed of lighter rock with phenocrysts of feldspar, quartz and biotite is part of a circular body, formerly interpreted as an intrusive rhyolitic dome. However close examination revealed pumice fragments in a glassy matrix, with some evidence of sorting. An alternative interpretation was offered: this is a fall deposit, an altered ignimbrite.

rock sample from a pyroclastic flow
rock sample from a pyroclastic flow
exposure at Blanc Rupt
exposure at Blanc Rupt

We than looked at two further pyroclastic flows; the second of these had cross bedding, formed as the base of each successive flow carved out its own channel eroding the top of its predecessor.

ignimbrite
altered ignimbrite
rock sample from a pyroclastic flow
rock sample from a pyroclastic flow

Locality 2: Col du Donon

At this locality we see the most northerly part of the Vosges Variscan basement. here mostly volcanic and composed of two volcanic series: Schirmeck and Nidoch-Donon. The first outcrop is a massive dark rock, a amygdaloidal basalt altered by spilitisation .The original basalt was tholeitic – REE analysis suggests that it derives from partial melting of depleted mantle. This facies has been interpreted as a subaquatic flow (pillow structures have been observed at nearby Rothau). However, close examination of the rock revealed angular fragments (including sedimentary basement material) which suggests classic explosive volcanism – this might be as another pyroclastic flow.

 

Casner quarry
Casner quarry

Locality 3: Casner Quarry climbing site

After a brief stop to examine outcrops of volcanic breccia (possibly sub-hyaloclastic) by the path, we proceed to the Casner Quarry where a massive extrusive rhyolite is exposed that has undergone partial devitrification and silicification. The rock is heavily fractured, and has been co-opted as a training site for rock climbers.

Casner Quarry climbing site routes
Casner Quarry climbing site routes

This rock is thought to have co-erupted with the tholeitic basalt (Giventian volcanism is bi-modal).

The tectonic story here is complex: subduction has created two back arcs – one at the northern margin above the subduction zone of the Rheic; under the southern margin, there was a second back arc which never proceeded to full oceanisation –there is evidence only of shallow marine incursion.

 

Locality 4: Forest path at Bambois

After lunch, the group undertake a practical exercise, simple mapping of highly fragmented alternating tuffs and granites (Kagenfels) on outcrops along a forest trail. These were interpreted as ash deposits (very fine, sub-aerial tongues of tuff) infiltrated by granite – the granite was interpreted as a networked sill, emplaced by lateral movement across a lattice of fractures.

Locality 5: Fouday-Gare

Here we return to the road to examine contact metamorphism where a large porphyroidal microgranite dyke crosses schistose sedimentary country rock. The dyke is oriented SW-NE and is part of the swarms in the Natzwiller granite. At the contact there are clusters of hornfels with white rims.

Locality 6: Downstream of Waldersbach

Here we examine a mix of felsic and mafic rock, formerly thought to be the interpenetration of Hercynican volcanics by a CDF type granite. More recently, the reverse has been proposed. We look at some of the evidence: the diorite has intruded the granite and formed droplets; owing to the temperature difference (1000 °C and 700 °C respectively), these have developed a cortex, while amphibole continues to grow in the centre. While some members of our group were not convinced by this scenario (the temperature differential was not compatible with the proposed process), the presence of a diorite dyke cutting the whole complex at nearby Trouchy Magel appears to support the idea of intrusion into granite by a later diorite.

Locality 7: Le Bambois – Freudeneck sous Belmont

The first stop is an outcrop at Le Bambois-Freudeneck sous Belmont to examine volcanics of the Bande Mediane (c. 334 Ma) – a complex of lava flows, pyroclastic deposits, ignimbrite, tuffs and breccia. These were formerly thought to be Devonian; however, recent dating of the ignimbrite gives an age of 334 Ma.

board explaining the creation of spotted schist
board explaining the creation of spotted schist
spotted schist
spotted schist

Locality 8: Lilsbach

As befits a site of historical importance (in this area contact metamorphism was first described by Rosenbusch (1875-7), a rather neglected information board marks the contact between intrusions and country rock.

Here the country rock (Ordovician/Silurian quartzphyllite – the Steige schists) has been altered to spotted schist with porphyroblasts of altered cordierite and andalusite. The rock belongs to different facies at its two different contacts, with the younger granite of Andlau and the granodiorite of the Hohwald. A further intrusive rock, a Permian rhyolite, passes over the schist (Southern part of the Kagenfels granite), though it is not responsible for the metamorphism.

Elisabeth Davenport

East Of Scotland Branch

REFERENCES:

Edel, J-B. and Schulmann, K. (2009) Geophysical constraints and model of the “Saxothuringian and Rhenocynian subductions – magmatic arc system” in NE France and SW Germany. Bulletin Societe Geologique de la France, 180(6), 545-558.

Skrzypek, E., et al. (2014) Palaeozoic evolution of the Variscan Vosges Mountains. Geological Society, London, Special publications, vol. 405 (1), 45-75. DOI:10.1144/SP405.8

A.-S. Tabaud et al. (2014) Devonian-Permian magmatic pulses in the northern Vosges Mountains (NE France): result of continuous subduction of the Rhenohercynian Ocean and Avalonian passive margin. Geological Society London Special Publications 06/2014; 405(1):197-223.DOI:10.1144/SP405.12

 

Day 4  / Thursday, June 25th

The Vosges are part of this huge Variscan orogeny that took place in the Carboniferous and can be traced from from Spain to Germany and from Irland to Bohemia, and beyond. Because of Tertiary tectonics, which amongst other events resulted in the Rhine Graben formation, we have access to the Variscan basement. It is thus a small window in a large tectonic event. The scientific community is still working, by using recent investigating technics on a large comprehensive framework integrating these various terranes scattered all over Europe, to get a broader picture of what happened at a time when large dragonflies were roaming in bracken forests.

Panorama from the Col de Baganelles down the valley of Liepvrette
Fig. 1: Panorama from the Col de Baganelles down the valley of Liepvrette
From the peneplain to today's topography
Fig. 2: From the peneplain to today's topography

The central part of the Vosges Massif represents a part of the deep root of the Variscan mountain chain. During day 3 we have looked at rocks in the Northern Vosges, which is bounded southwards by the west-east trending Lalaye-Lubine dislocation, named after two local villages. South of this dislocation the terrane is totally different: we enter a zone of highgrade metamorphism and granitic bodies. These rocks tell deep burial and very fast exhumation, accompanied by migmatization and emplacement of granitic plutons. Etienne Feuchter and Françoise Chalot-Prat took us along the various outcrops and provided back-ground on what we observed.

Stop 1: Panorama over the Sainte-Marie-aux-Mines Fault and granite

From the col des Bagenelles we had a good view over a major fault running north-northeast within the Central Vosges named "Sainte-Marie-aux-Mines Fault" (figure 1). This fault is a Variscan senestral transform fault that has been reactivated as a normal fault during the Tertiary rifting, lowering the eastern compartment. A small stream, La Liepvrette, runs along the fault down to the town of Sainte-Marie-aux-Mines. The fault follows the edge of the Granite des Crêtes in a straight line. From our viewpoint, we could observe a steeper slope of the valley on its western side (granite) and a gentler slope on the eastern side (gneisses).

Further away, in the background, outlayers of Triassic sandstone cup the Variscan basement.

In front of that panorama we discussed its geological history, starting with the Variscan peneplain that was well established at the end of the Carboniferous and on which Variscan sediments, transported by large rivers from the Southwest, have been deposited during lower Triassic forming the so-called Buntsandstein. Françoise took out her board and outlined in a few drawings this history. Figure 2 sums it up.

Geological simplified map of the Central Vosges, courtesy of Anne-Sophie Tabaud
Fig. 3: Geological simplified map of the Central Vosges, courtesy of Anne-Sophie Tabaud

Large boulders of the so-called granite des Crêtes are surrounding the parking place at the col des Bagenelles, as this granite occurs along some of the Vosges summits (“crêtes” in French). It is an Mg-K rich granite, with large K-feldspar phenocrysts, the matrix is composed of biotite, amphibole, K-feldspar, plagioclase and quartz. This granite is part of the Central Vosges Mg–K granite association (180 km2), which is composed of several bodies comprising amphibole–biotite porphiritic syenite, monzonite and granite. That large granitic complex appears in dark blue on the map (figure 3). This granite intrudes the Varied Gneiss unit of Sainte-Marie-aux-Mines and has been dated at 340 Ma. Its parental melt results from magma mixing between melts derived from peridotites and granulites belonging to the Moldanubian Bloc (A-S Tabaud 2013).

Stop 2: gneiss, granite, granulites and serpentinites in the vicinity of the Col des Bagenelles

From the col des Bagenelles we drove up a small road for 1.5 km, towards the Auberge du Haycot. We first walked passed the Auberge up to the pass du Brézouard along an outcrop of gneisses with sillimanite. It is the gneiss that is part of the Monotonous Gneiss unit which has the lowest metamorphic grade in the Central Vosges (7 kbar and 700 °C). After the pass, the so-called granite du Brézouard crops out along the forest path. All the granites in the Vosges have a local name that relates to the place where they occur, hence a plethora of names. Recently, Anne-Sophie Tabaud has studied them again using various geochemistry analysis and dating methods and came up with a new classification. In the Central Vosges and Southern Vosges, they can all be classified into only two groups: an Mg-K rich granite, which is an I-granite (of igneous origin) and an S-granite, that means that its origin is crustal melting (anatexis). Just to get more confusing this S-granite is also called Fundamental Granite or Central Vosges Granite; pick your name! It appears in purple on figure 3. Here on the Brézouard hill, the granite is an S-granite, more precisely a two-mica leucogranite rich in quartz, muscovite, K-feldspar, albite and with some cordierite, according to the geological map. The particularity of his granite is that it was emplaced within a strike-slip faulted zone (Bilstein fault) during the Carboniferous and the area bordering the gneiss has been subject to hydrothermalism, which resulted in iron oxide mineralisation. We could observe these deposits within the granite.

We walked back passed the Auberge along the road from where we came to look at outcrops of granulites and serpentinites. The granulites, serpentinites and the previous seen gneisses are all part of the so-called Gneiss Series of Sainte-Marie-aux-Mines. The granulites are fine-grained with elongated minerals (quartz, K-feldspar, plagioclase, garnet and some biotite, pyroxene and sillimanite) with a mylonitic texture. Some granulites are lighter in colour, other darker, depending on the initial rocks before metamorphism.

These rocks attest of high degree metamorphism (granulite facies : 800-900 °C, 12-15 kbar) and might indicate a 40-45 km deep burial.

They represent the lower part of the continental crust. Slices of black serpentinized peridotite are associated with the granulite and outcrop at several places along that forest road. The peridotite comprises garnets, which indicates an initial equilibration at 49 kbar and 100-1200 °C before been incorporated into the granulites.

In front of the migmatites of Orbey along D11
Fig. 4: In front of the migmatites of Orbey along D11

Stop 3: Migmatites next to Orbey

From the col des Bagenelles we drove eastwards towards our last morning stop. It was a roadcut out of the Village of Orbey. Here we could observe migmatites. In the Vosges, migmatites outcrop in the East and in the west flanking the S-granites (cf. figure 3). Weathering has blurred the features of the outcrop, thus the photo shows the people of the group in front of the rocks (figure 4).

Still, they show astounding features when looking closely and carefully. We could observe a very heterogenous material with oval-shaped enclaves up to several 10s of cm large of meta-sediments. The matrix looks like a gneiss or a granite with biotite surrounding minerals or in a trail.

These migmatites are the leftover (restite) result from partial melting of meta-sediments (some relate to the Visean Markstein series further south).

A short chronology to sum-up the story of the rocks we have seen in the morning:

• The gneiss unit around Sainte-Marie-aux-Mines comprises granulites with slices peridotite, and has been intruded by both the Mg-K I-granite and the S-granite. The peak of metamorphism of the gneisses is dated between 360 and 340 Ma and relates, according to a widespread model, to the southwards subduction of the Saxo-Thuringien (Northern Vosges Terrain) under the Moldanubian (Central Vosges and further South).

• The oldest granites, are the Mg-K granites (stop1) and are dated 340Ma, their chemical omposition tells a mantel origin that has been contaminated by crustal material from a subducted continental margin.

• The Central Vosges S-granites (stop2) are younger and dated 328-325Ma. They are the product of melting of the crust. We saw the migmatites (stop3), which are restites of that melting. The Western-central Vosges granite and the Brézouard granite are the product of the melting of the monotonous gneisses.

• By the end of the Carboniferous, 300Ma, this mighty mountain was completely eroded into a peneplain.

Around lunchtime, we left the weathered outcrops all overgrown by vegetation of the crystalline Variscan basement, and headed to Germany and the Kaiserstuhl, which is a much more recent story.

REFERENCES:

Tabaud, a.-S., Janou ek, V., Skrzypek, E., Schulmann, K., Rossi, P., Whitechurch, H., … Paquette, J.-L. (2014).Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the Southern-Central Variscan Vosges Mts (NE France). Journal of the Geological Society, 172(1), 87–102. doi:10.1144/jgs2013-123

Skrzypek, E., Schulmann, K., Tabaud, A., Edel, J., De, S., Umr, T., & Strasbourg, D. (2014). Palaeozoic evolution of the Variscan Vosges Mountains, 45–75.

Afternoon of Day 4  / Thursday, June 25th

At midday we crossed the Rhine, sat on its banks to have lunch and watched a rowing team preparing their boats. Then, of course, we had to go for Kaffee & Kuchen (coffee and cake), to keep up with German traditions. Eventually we made our way to the first stop, to look at the volcanics of the Kaiserstuhl with Françoise Chalot-Prat as leader.

This alkaline volcanic complex stands on its own in the middle of the southern Rhine Graben. Vogelsberg, the other volcanic complex in the Rhine Graben, is located 300 km northwards.It covers an area of 150 km2 and is part of a great European magmatic province, which is related to the peri-Alpine rifting that started during Eocene and affected the European lithosphere. With OUGS ME we have already had trips to the Eifel, the Cantal, Chaîne des Puys, and the Eger Valley Bohemia, which are all geological settings that are part of this magmatic province.

The volcanic rocks composing the Kaiserstuhl are all silica undersaturated. This means that when the magma evolves and differentiates, quartz would never crystallise, but feldspatoids like nepheline. In addition, in the middle of the complex are small carbonatites lava flows. This alkaline series is typical of intra-continental magmatism. The oldest eruption is a tephrite and has been dated at 17.5 Ma; the youngest is 16 to 14 Ma old and is the strato-volcano of Limberg. Erosion and loess hide most of the edifice, which still stands out on the plain.

Limburgite quarry (near Sasbach)
Fig. 1: Limburgite quarry

Stop 1: Quarry next to Sasbach

The Limberg is the name of a small hill next to the village of Sasbach. It is a small strato-volcano on the northwest flank of the Kaiserstuhl, and it is the type locality of the limburgite.

Like on most of the volcanic edifice, vegetation overgrows most of the features. Still, we could distinguish a lower layer of a massive lava flow of limburgite, capped by a thin lighter brownish layer of "baked marlstone" and above it a very heterogeneous layer with fragments of all sizes floating in a fine grained tephra matrix (figure 1).

The presence of sediments attests of a time gap between these two different volcanic events, and has been dated between 20-16 Ma, using fossilised remains of vertebrates.

The top pyroclastic layer is not sorted and thus it was deposited from an avalanche. It happens when the explosion propels material at medium height producing an ascending plume that collapses and runs down the flank of the volcano.

After having had a close look of the lava flow, everyone was looking for specific shape of rocks laying around: we were all looking for "boulets"(cannonballs) (see Figure 2 , next page).

Dense with little vesicles, they tell that the origin of the explosive event is phreatic and not just high gas content, which would have produced scoria (vesicular pyroclastic fragments). Here we see no scoria at all, thus the magma must have met an underground aquifer on its way up, to produce an explosive eruption. These boulets don't look like classic bombs, which form when lava cools when projected in the air. Françoise explained that when magma cools very fast, the contraction always develops curved fractures producing dense rounded shapes. The eruption then expels the magma previously fractured.

Limburgite: to the left as a "boulet" , to the right a fresh dark fragment
Fig. 2: Limburgite: to the left as a "boulet" , to the right a fresh dark fragment

When fresh, the limburgite is a dark volcanic rock (figure 2) and is a type of basanite; that means that it is a silica undersaturate basalt, composed of the pyroxene augite, olivine and magnetite in a glassy matrix. Secondary white minerals, like calcite, aragonite and zeolite, have crystallised in vesicles. Alteration of that rock gives it a red-brownish tinge due to the formation of hematite.

Nephelinite lava flow with weathered peridotite enclaves
Fig. 3: Nephelinite lava flow with weathered peridotite enclaves

Stop 2: more limburgite

The second stop was at a quarry within the same small strato-volcano. We could see again from bottom to top the massive lava flow, a light-coloured layer of marls, a reddish layer of volcanic tuff. The whole complex is covered by loess.

Next to the road where we parked the car, was an outcrop of a nephelinite lava flow with peridotite (lherzolites) enclaves. More precisely, we could see the cavities left by the weathered peridotite nodules (Figure 3). These magmas are the most primitive ones and are mantle melts originating from a 100 km depth.

After that last stop we decided that it was the end of the day. It was time to drive back to our Gite du col du Bonhomme (our self-catering accommodation) and prepare our barbecue evening. Although the Kaiserstuhl would deserve that we spend more time, we were happy having had a glimpse of it. There is so much more to see, and to taste: the Kaisersthul is completely covered by vineyards! We will have to go back there on one of our future excursions.

Elisabeth d’Eyrames

(with Mike Molloy)

REFERENCES:

Wedepohl, K. Hans, Emil Gohn, and Gerald Hartmann. "Cenozoic alkali basaltic magmas of western Germany and their products of differentiation." Contributions to Mineralogy and Petrology 115.3 (1994): 253-278.

 

mineral fair at St-Marie-aux-mines
mineral fair at St-Marie-aux-mines
indoor exhibition at the mineral fair at St-Marie-aux-mines
indoor exhibition at the mineral fair at St-Marie-aux-mines

Day 5 / Friday, June 26th 2015

The last day we spent in the Vosges was dedicated to a visit of the mineral fair at St Marie-aux-Mines. Since several had been there before it was known that parking might be a problem. So we started early and were at St Marie shortly before the fair opened.

The mineral fair is an open-air affair - from afar looking a little bit like a mediaeval tournament - but there are indoor exhibitions as well in locations like a theatre, a high school and surprisingly even in a building with an indoor swimming pool. These indoor locations were quite welcome because it was a warm day and it was nice to be out of the sun once in a while. In some these buildings you could find the more valuable exhibitions but they were the locations for special activities as well. The fair is so huge, that it is split into two different, connected locations by a little shuttle train.

semi-precious stones for sale
semi-precious stones for sale
globes!
globes!

Each one of us got a bracelet giving us entrance for the day and after agreeing upon time and location for a meeting at lunch time we split off. Since an exhibition of microfossils in 3D was announced to begin soon in the theatre some of us decided to attend, but after that everyone went their own way and we hardly met again before lunch time.

The variety of exhibited items was amazing. Though I did not want to spend much money, I was sorely tempted sometimes. They did sell precious and semi-precious stones, on strings or single in all kind of appearances: raw or polished or shaped to balls or still attached to their host rock. Sometimes the stones were even worked into plates, vases and more surprisingly even into globes.

But there were fossils, tools to work minerals and boxes to store them for sale as well. And there was even a health section where they offered not only healing stones but also oils and incenses. Many exhibiting companies were from France or nearby countries but some came from far away countries, like Brazil. Not all owners were present. Some relied completely on employees and it struck me a strange that at a Danish stall - selling stuff from Greenland and Newfoundland - the selling was done by Germans.

local products at St-Marie-aux-mines
local products at St-Marie-aux-mines
celebrating the gifts
celebrating the gifts

We had lunch in a beer tent where schoolgirls were flitting around to remove the tablets. After lunch I went off to the gem area. Being to impatient to wait for the next shuttle train, I walked and discovered that along the road local products were for sale, including gigantic loafs of bread.

The others went off again as well - some of them with a special purpose: We had agreed the day before that we all are deeply indebted to Béatrice Will, Elisabeth d'Eyames and Françoise Chalot-Prat for this marvellous trip to the Vosges and a small present from the fair would be just right to express our gratitude. As Brigitte Revol MadDonald does know the most about jewellery (within our group) she was asked to see to the matter. Dee Edwards offered to go along and together they bought necklaces with pretty pendants - matching them nicely with each of the ladies.

Gisela Lunkwitz

thanks to Dee Edwards for the picture of the three ladies