Field trip in the Vosges 22nd - 27th October 20011
Day 0 : Saturday - arrival
After dinner on the day we arrived, I walked out through our French window overlooking the valley of Xonrupt-Longemer to breathe the clean air. There were stars glittering in the sky above and a bright band of mist hid the little town below. Beautiful! The start of another memorable field trip.
We were staying in a youth hostel. In my younger days, youth hostels were welcoming but austere places. Here, our modern room had four beds (no bunks), en-suite shower with separate toilet, and under-floor heating. Breakfast and four-course evening meals were served at table, and we were provided with very generous packed lunches every day. No cooking or washing up for us to do - how things have changed!
Day 1, Sunday morning : crust and mantle metamorphism
The first excursion the next morning was to the Fossard massif. We had two guides, Benoît Marconnet, a doctor of geology from the University of Nancy, who had studied under Professor Claude Gagny, locally renowned for his work on the magmatic rocks of the Vosges, and Jean Paul Gremilliet, manager of the Terrae Genesis centre (more of that later).
We started near the little town of Cleurie, by examining a quite pale rock face of migmatite. It was a patchy, jumbled rock where the boundaries between granite and gneiss/schist were hard to distinguish. The granite had been dated by zircons to 330 ma, during the Variscan orogeny. The rock had been 5 km beneath the surface when it started to melt. We were told that it was packed full of rotten cordierite, a ferro-magnesian mineral typical of high temperature metamorphic grade. See Photo 1.
The OU had taught me about the British geologist George Barrow who identified the Barrovian series of metamorphic grades for regional metamorphism along the north Aberdeenshire coast in the late nineteenth/early twentieth centuries. However, now we were in the Vosges and here we were told about the Alsatien geologist Rosenbusch who was also a pioneer in identifying metamorphic rocks in the nineteenth century. Rosenbusch wrote a paper in 1877 on the contact rocks in the aureole of Alsace granites, providing an interpretation of hornfels and other contact rocks.
A short walk into the forest took us to a steep valley edge, a Variscan fault, above which we found a loose pile of dark stones. The stones came from a lamprophyre dyke that had been intruded into the migmatite during the Variscan, at 5 km depth. Further on we found the dyke in situ and examined the contact. There were spherulitic feldspar porphyroblasts in the dyke rock that were larger in the interior and smaller in the chilled margin. The porphyroblasts were about 0.5cm in diameter and had weathered proud, so that the rock was covered in ‘bobbles’. The dyke contained phlogopite and biotite micas with vosgesite and alkaline feldspar. Prof. Gagny had tried to give this lamprophyre the name bussangite, although the name is not now widely used. There are four to five other contact sites in the region. See Photo 2.
We walked on to our next stop, known as ‘La Charme’ in the Corbelières wood, where we found a rock whose origin was more difficult to explain. It was ultramafic, a serpentinite, but it was full of pyrope garnets. There are only four other known locations in the world where you get such a combination, we were told – in Bohemia, Spain, Uzbekistan and China.
This serpentinite started out as peridotite in the mantle. It originated from depths of around 150 km, where there are very high temperatures, around 1200˚C, and high pressure. These are the conditions needed to form diamonds (but I don’t know whether graphite has been found in the serpentinite). The peridotite came up to the surface slowly and in several stages, changing as it rose. Any diamonds would have been destabilised. Likewise, the peridotite had been hydrated to serpentinite during uplift. The pyrope garnets, too, had been altered, to kelyphite, but some of them had fresh centres. Kelyphite is produced when solid garnet and olivine react together while peridotite is rising from the mantle. Some of the elements found in this rock are platinum, chromium and sulphur. See Photo 3.
Another rare mineral, passaveite, was discovered here: the second find in the world after one in Siberia.
The outcrop was called an écaille in French, which I think roughly translates to a scale, shell or slice. There are nine of these ‘slices’ in the region, each having a different composition and each having been emplaced into the migmatite. The largest of them is 1.6km long and 100m across. There were different stages of uplift; one during the Caledonian and one during the Variscan.
The first hypothesis we were given for its formation was that this complex was part of an ophioloite. There are some other ophiolites in the Vosges. However, we were told that you don’t usually find garnets in ophiolites.
The second hypothesis was that it was a deep mantle rock that had been brought to the surface by some other means. How, it wasn’t known; a hot spot that hadn’t erupted, maybe, but had lifted mantle rock into the deep continental crust. It could be called a ‘window’ into the mantle. 5-6km of uplift would have occurred as a result of erosion. Sadly, there has been no funding to study these rocks.
We left the woods and walked back into the sunshine. Beside a road, we came across the ruins of an old farmhouse, which had been made of blocks of peridotite – surely a very rare building material. There were also blocks of other local rocks from the Vosges scattered on the ground: blocks of Triassic sandstone (buntsandstein) and blocks of Remiremont granite. The sandstone was laid down in beds on top of the basement during the Variscan orogeny. Most of the mountains in the Vosges appear rather flat-topped because of these horizontal sandstones.
Further along the road there was a school named ‘Ecole la Serpentine’, which had a large block of serpentinite on its boundary wall above an information board about the rock. See Photo 4.
We drove higher into a large granite quarry to eat our packed lunches. The granite was the Remiremont granite, relatively fine grained and used for paving stones in Paris and the north of France in the 1870s.
The weather was clear and sunny, and there was a superb view over a flat-bottomed valley called the Cleurie plain and the little town of Saint-Amé. The valley had once witnessed the collision, rather than the confluence, of two glaciers. One of the glaciers had been forced around and pushed up the valley side by the other glacier. Evidence was in the moraines and the peridotite erratics. Although the valley had been glaciated, we were actually observing a Variscan peneplain that once contained an enormous lake.
Day 1 : Sunday afternoon: in Terrae Genesis
We spent that afternoon in the Terrae Genesis centre. It is a dream combination of natural history museum and rock shop, with rooms where rocks and minerals are displayed, both local and from around the world. There are workshops in the basement where rocks are sliced and polished and thin sections prepared. My special memories are of giant orbicular rocks, a dark room containing rocks that fluoresce under UV light, rare maps and a spectacularly zoned labradorite. The gardens, too, contain carved granite sculptures and the external walls are hung with racks of polished stone samples. See Photo 5.
For more about the centre, see their website: http://www.terraegenesis.org
After dinner that evening, some people went into Gerardmer to see a replay of the rugby match between France and New Zealand that had taken place earlier in the day. It was a sad end for the French, but they were very magnanimous in their defeat and said that it wouldn’t have felt right for France to have won against the powerful All Blacks. Rugby was new to me, but it seemed that both sides had played superbly.
Day 2, (not) the Rhine graben: a day of mists and U-turns … is that it?
I’m driving today. While I do like a bit of a challenge, I do hope it won’t be as exciting a drive as yesterday – lunch took us effectively “off road”, but in normal cars. The day’s supposed to be about the Rhine graben, in which I have quite a vested interest since I live on a branch of it.
We start off by looking at a rhyolite dyke in schist and are told that this does not relate to the opening of the Rhine graben/rift, as the intrusion would then have been basaltic. This is therefore probably from a late phase of the Variscan orogeny. Closer inspection of the rocks shows some great dendrites, which allows Dave to get his hammer out and bash us off a few small pieces.
There then followed the first U-turn of the day.
We were then taken up to the top of the Hohneck (1,362m) where we were again told that, after the Variscan orogeny, the mountains were worn down to a peneplain, which was subsequently incised by glaciation. This time, and from this perspective, it was blindingly obvious as the tops of all of the Vosges mountains reach approximately the same height. It’s a lovely view, with mist hanging in the valleys.
We find a large-grained pegmatite seam in a lump of fairly fine grained granite. The discussion on how it came about included a fascinating suggestion about putting a bottle of beer into the freezer, taking it out, seeing how it is still liquid, then giving the bottle a tap and watching the beer flash-freeze. Hmm – I think I have better things to do with bottles of beer, but it was a good illustration. Basically, something caused some melt left in the cracks of the cooling granite to crystallise quickly, possibly over as little as a few weeks, giving the larger crystals of the pegmatite.
Does going up a mountain and coming back down count as a U-turn? If so, that was number two.
The road took us down into the mists, giving a very atmospheric drive through autumnal vineyards (did we go round a roundabout twice? I’m sure it wasn’t direct) to Turckheim, where we should have been able to look up to the granite massif, if it hadn’t been veiled in low cloud. Although there was a very steep cliff face, we were again told that this was not due to the Rhine rift. Instead we were told about the vineyards, which are always situated on the slopes (can be up to 40% in gradient, necessitating hand picking and ropes), never on top of the massif nor on the valley floor because the requisite minerals are not present in those soils.
We are promised that we will, definitely, see evidence of the opening of the Rhine graben at the next stop, which turns out to be at Riquewihr. We park the cars and head off on foot – oops, not this way, lets try that road, oh, no, not there either, turn round ... So we are taken off to see something else: an outcrop of alternating clays and gypsum just outside the town walls . Interestingly, we are informed that the clays were laid down in turbulent water (mer agitée). Most of us look at each other in astonishment, though maybe it is just lost in translation, and means in contrast to the enclosed lagoons where the gypsum would have crystallised out as the seas evaporated.
After lunch outside, sat around a large tree, a coffee/comfort stop in the town and yet another U-turn – or two – we were finally taken to the vineyard where there was “proof” of the opening of the Rhine rift. Only in this one field can you find lumps of basalt dating to around 30-plus ma, when it is believed that the rift started to open. The volcano that would have extruded this has been eroded away; indeed, we were told that the basalt outcrop in the field has not yet been located, but no other fields around have this basalt in them, which rather made us question the validity of the “proof”.
Given the rather vague information imparted over the day, it was obvious that today’s leader had neither read the details that Elisabeth had sent in advance about our level of knowledge, nor been speaking about his area of expertise. Elisabeth therefore asked him to take us somewhere where he could comfortably talk to us.
We ended up in what seemed like primeval forest, with lush, ankle-deep mosses, ferns, mushrooms, rotting trees, evidence of wild animals (deer? wild boar? both?) … and an incredibly steep hill, which was even harder to climb as you couldn’t rely on the trees to haul yourself up. (I kept expecting a rather cuddly Disney dinosaur in a red check shirt to show up, but maybe I watch too much telly.) We eventually – he explained that he didn’t normally approach the workings from the direction he brought us – found a old manganese mine that was not fully worked out.
Did any of us go into the mine workings? I couldn’t possibly say, not least because I started back down before the rest ;0)
Thus ended a rather frustrating day, since we didn’t really learn much at all to do with the Rhine graben, but the drive was fun even with all of the U-turns, we saw an awful lot of the Alsace, and the scampering through the forest was definitely an experience I for one won’t be forgetting in a hurry.
Day 3, Excursion next to the Massif du Champ du Feu
On Tuesday morning we were joined by our leader, Benoît Marconnet who proposed to visit four different places on and around the Massif du Champ du Feu. ( see A, B, C , D on the geological map).
The Vosges can broadly be divided into two different terranes that collided during the Variscan Orogeny. In Devonian and Carboniferous times, Gondwana cought up with Armorica and then collided with Laurasia which was a patchwork of previously collided Northern Terranes, the main ones bearing the poetic names of Laurentia, Avalonia and Baltica. All that built the super-continent Pangea.
The Northern Vosges relate to the Saxo-Thuringian Zone and is the domain of the crystalline nappes. The Southern part is in the Moldanubian Zone, which is the internal zone of the Variscan Mountain Belt with the highest-grade metamorphic rocks and extends southwards over the Massif Central.
The Massif du Champ du Feu is located just north of the suture line which is a fault system active in the Devonian and Visean ( 340 Ma). The movement was a dextral shearing and thrusting of the Moldanubian over the Saxothuringian zone. Locally it is called by the names of two towns in the Vosges “Lalaye Lubine fault” (Lalaye is at the bottom of the map, south of the Schists of Villé). This fault can be traced under the Parisian Basin towards south of Cornwall, changing its name a few times on its way. Eastwards the fault can be traced into Bohemia, where we had a great post-exam field trip looking amongst other things at Variscan features and tertiary rifts.
Our first stop (A) on Tuesday morning was in a granite quarry next to a village called Natzwiller which gave the name to that pinkish porphyritic granite. It is one of the plutons that was emplaced at the end of the Variscan orogeny and is part of what is called the Massif of the Champ du Feu. It encompasses also the different granodiorites and volcanics, starting in the south with the granodiorite from the Hohwald (in light pink) intruding the Silurian schists of Steige (in beige) and intrudes at the northern edge the Devonian-Dinantian sediments and volcanics (in green on the map).
Second Stop (B). We drove then to a place called “La Rothlach” which was next to a Ferme Auberge. It started to rain; we contemplated the promise of a hot drink, and still we walked on a path towards a small abandoned quarry. The rocks were identified as andesite. Dark coloured rock with small plagioclase crystals. We were right within what is called “bande médiane volcanique” on the map. Don’t you feel that you get a grip of French by now? This middle band is the most recent formation in the massif and originates from the most evolved magma in this area. (Deschamps 1985, University H. Poincarré Nançy)
Third stop (C). We walked back to the Auberge, it stopped raining and we decided to go for another walk through the forest. It took us a while to discover an outcrop at a place called Neuntelstein. And guess how that rock was called? Neuntelstein diorite! The name doesn’t appear on the map, as it is the darker green spot in contact with the granodiorite of the champ du Feu. It is a porphyritique diorite whith Ca-plagioclase and hornblend amphibole.
Studies of the chemical composition of the different diorites and granodiorites by M. Deschamps show that they are all composed of the same minerals (plagioclase, Kfeldspaths, amphibole quartz and biotite), but not in the same proportions. The granites show also a similar composition and form thus with all the other plutonic and volcanic rocks a calc-alkaline magma series, which is consistent with subduction zone magmatism.
The study of the relation between the different bodies and their basic enclaves, revealed also that they were all emplaced sub-simultaneously with the most basic ones slightly later; which is opposite to what you might expect.
This outcrop was next to a “mountain view” location from where we had a look over the Rhine graben, “Fossé (rift) rhénan” on the map, which was in the mist that day. But we could still see the Black Forest as a thin dark line on the horizon. Before us the foothills of the Vosges peaked out of the fog; it is a very fractured, transitional zone on the margins of the rift. We were then standing at the top of the ridge flanking the graben.
Why all these granitic rocks outcrop are at that altitude is not related at all to the Variscan orogeny. At the end of the Permian this area was completely flat and dry. During the Triassic large rivers laid down a thick cover of sandstone, up to 500 m in the northern Vosges ( grès poudingue , buntsandstein) and eventually the Muschelkalk Sea transgressed . Bulging of the area started upper Jurassic and the down faulting of the central part was well active during the Eocene and Oligocene. The Jurassic and Triassic cover have been eroded away uncovering the crystalline massif. The thesis of M. Michon (2000, University Blaise Pascal, Clemont) shows a relation between the West European Rift systems, their associated volcanism and the Alpine orogeny.
Before driving to the last place of the day, we relaxed a while in the Auberge and had what ever warmed each one up.
Fourth stop (D). We started at the quarry in Eftermatten, a small place where we could see rhyolite embedded in schists. The rhyolite is located at the southern end, fingershaped, of the Kagenfels boddy that filled a large fissure of the massif at the end of the orogeny. It has been dated -331 Ma (Hess, Lippolt, Kover 1994). So gradually, the granite changed into an extrusive acidic volcanic rock. A slight contact metamorphism changed some minerals in the surrounding schists giving it a spotty look. The very first time studies were carried out on contact-metamorphism, was in this area, more precisely along the river Andlau, starting from the granitic pluton up through the shists, by Pr Rosenbush, a German geologist ,at the end of the 19th century.
The schists are called “Schiste de Steige”, another local name. Thin sections revealed fossils, a few millimetres long of a type of protozoa that only lived in the Paleozoic seas, and trace fossils of trilobites and arthropods. They have been dated as Silurian.
The aim of our next stroll along a path up a river valley was to walk across a thrust fault, called “ ligne de chevauchement vers le nord” on the map, where odler schists, Schiste de Villé, have been pushed northwards over the Steige schists. At some point we could observe the change, but we had to trust our guide’s explanations.
These schists of Villé have different facies according to the sediments initially deposited. Although they experienced after deposition, moderate, green shist facies metamorphism, and folding, the most ancient traces of live in Alsace have been found in them: millimetre long sponge skeletons, green algae related organisms and some traces that are thought faecal. (Pr. Jean-Claude Gall, University de Strasbourg, Alsace des fossiles).
This dates vaguely these rocks as Precambrian to Ordovician and gives a picture of what that area looked like 500 Ma ago: a sea with its floor colonised by siliceous sponges, muds and sands accumulated on it, and plankton drifting in the upper layer.
I took a picture of that rock in the museum where it was at is best. The “chevron” folds characterises this rock. In situ they are consistent with a south-north compression and might be related to the Caledonian orogeny.
Day 4, granite and glacial valley
The last full day (Wednesday, 26 October 2011) of our geological exploration of the Vosges Mountains started with a guided tour of the area just north of Chevre-Roche. Our guide was M. Jean-Paul Gremilliet The topography was typical of much of the Vosges highlands, steep sided hills, mostly over 800 m, with a vegetation of mainly mature conifer. M. Gremilliet led us to a granite quarry dating back to the late nineteenth century (abandoned since the nineteen fifties) that had produced the granite widely seen in Paris and northern France.
Over 90% of the curb stones in Paris originated in the Vosges. The granite was also valued for tombstones; the fine grain and easily split rock has a specific direction that makes carving unproblematic. Drill holes for the black powder used to split the granite during excavation (dynamite was too violent, Jean-Paul explained), were clearly visible in some slabs. He also pointed-out decompression joints that occurred as erosion removed the top of the granite pluton. Markings on the rocks enabled us to deduce the direction of the last glacier.
Our guide showed us a 2 m deep trench that he and other local geologists had dug to expose ~300 Ma sandstone rock above ~320 Ma aerated granite. A layer was missing from the gap, an unconformity, between the two formations
The remains of trenches from the First World War were still clearly visible near the summit of the hill. We eat our pack lunches overlooking a landscape that 4 000 years earlier has been a lake dammed by a moraine to the west of the valley. The flat tops of the surrounding mountains are the remains of the pene-plaine that existed prior to the graben formation.
Historians have established that the lake had disappeared by the time of the Roman occupation. On our descent we crossed a grass covered peat field once the source of fuel for local farmers.
In the afternoon Jean-Paul Andre and Claudine Jeannerot, both enthusiastic local geologists, lead us over a glaciated terrace, between the Fossard Mountains and Saint-Nabord Longuet, to the River Mosel. Several moraines were evident on the wide flat valley. Small rounded pebbles left by the last retreating glacier were could be found in the cultivated fields; they included basanite (a smooth, black siliceous rock also known as Lydian or black jasper), remiremont granite, and microgranite. Madame Jeannerot said that migmatite, sandstone and diatomite pebbles were also present in the fields. Continuing towards the Mosel we stopped to examine a disused quarry where siltite had been excavated. The siltite (sedimentary rock of clastic particles silt size of 0.05 to 0.005 mm) was thought to have formed at the bottom of an 11 000 year old lake, this layered formation is also called varves