Geology and petrology of the coesite-bearing terrain, Dora Maira massif, Western Alps.
Chopin, Christian; Henry, Caroline; Michard, Andre
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 263 - 292
published: Apr 18, 1991
manuscript accepted: Dec 19, 1990
manuscript received: Nov 29, 1990
ArtNo. ESP147050302005, Price: 29.00 €
Abstract The southern Dora Maira massif consists of several continental crustal slices differing in their metamorphic grade, separated and themselves dissected by major low-angle faults. In the entire pile, the main structural imprint is a regional foliation which roughly parallels the lithologic and tectonic boundaries. This mylonitic foliation contains a widespread ENE-WSW stretching lineation, parallel to isoclinal-fold axes and with clear top-to-the-west kinematic indicators in most units. This strong Alpine ductile imprint developed under greenschist-facies conditions and may be related to the piling up of the units ; it postdates and overprints earlier Alpine high-pressure (HP) assemblages. From bottom to top, the pile consists of: 1) the Pinerolo Unit, an allegedly Carboniferous detrital sequence, which probably reached epidote-blueschist conditions during Alpine times ; 2) polymetamorphic basement units, including the coesite-bearing one, and 3) Upper Palaeozoic units (a Permo-Carboniferous to Permo- Triassic detrital sequence with abundant igneous acidic products) with intercalated slices of polymetamorphic material. These uppermost units (3) were metamorphosed under HP conditions (∼10-12 kbar, ∼500°C), with epidote-garnet-glaucophane as the critical assemblage in metabasite, and chloritoid-almandine or chloritoidkyanite ± chlorite in phengite-paragonite schist. The polymetamorphic basement (2) contains abundant granitic gneiss, with the assemblage quartz-phengite (Si3.2 p.f.u.)-albite-microcline-biotite-clinozoisite-titanite, in which only the core composition of large phengites (Si3.5) and, in the coesite-bearing unit, the additional presence of garnet (Gro50-Alm50) suggest early HP conditions. On the other hand, intercalated metapelite and metabasite preserve early Alpine HP assemblages of very contrasted grade. On the basis of these HP assemblages and of lithology, we distinguish in this 'basement' several units, all eclogite-bearing. The lower one, directly overlying the Pinerolo Unit, is the coesite-bearing unit, made up of two formations. A varied formation, up to 300 m thick, consists of metapelite (kyanite-almandine-phengite schist with coesite relics), metabasite (kyanite-eclogite with pseudomorphs after coesite, KDFe-Mggt-cpx ∼ 10 ), and marble, all interlayered within fine-grained gneiss. The other formation, up to 500 m thick, encloses the varied formation and is made of gneiss varying from fine-grained blastomylonite to augen gneiss to nearly undeformed porphyritic metagranite ; at least the latter probably represents Hercynian (∼300 Ma old) intrusives into a metamorphic basement. Detailed mapping shows that the magnesian quartzite (with pyrope-kyanite-talc-phengite-coesite) occurs within the gneiss series as one or several boudin trails, on either side of the varied formation. Similar formations occur in the next overlying unit, at much lower grade. 'Cold' glaucophane eclogite (KD ∼ 30) contains paragonite but no kyanite ; chloritoid is common in metapelite, talc-chloritoid (XMg = 0.55)-chlorite and talc-kyanite-chlorite occur with quartz and phengite in magnesian schist. This suggests conditions near 15 kbar, 550°C for these rocks, against about 30 kbar, 700-750°C for the coesite-bearing ones. Several lines of evidence suggest that the country-rock gneisses shared the same HP and very-HP evolution as the enclosed rock-types. In the gneiss of the coesite-bearing unit, the early coexistence of Ca-rich garnet (Gro80-85) with rutile (both preserved as armoured relics) shows that the coesite stability field must have been reached by the country-rock. We are thus dealing with coherent metamorphic terrains, now juxtaposed with considerable pressure gaps. Reconstructed P-T paths suggest continuous cooling during uplift. There is still conflicting evidence as to radiometric ages. In the hypothesis of a Cretaceous HP and very-HP metamorphism, the main structural, low-pressure event is dated at 40 Ma. This implies for the coesite-bearing unit an initial rapid exhumation (from about 120 to 100 Ma) with uplift and cooling rates of 1.5 to 4 mm/y. and 7 to 15°C/m.y., respectively, then a long period characterised by low rates of 0.1-0.3 mm/y. and 2°C/m.y., and from 40 Ma to the present a period of faster uplift and cooling (0.3-0.5 mm/y., 10°C/m.y.).