Magmatisme et métamorphisme des roches intrusives calco-alcalines du Carbonifére briançonnais entre Arc et Durance : minéralogie, pétrographie, géochimie. Alpes françaises

Abstract : This study of the intrusive rocks of the «zone houillère briançonnaise» deals with two independant phenomena, viz. (1) Hercynian magmatism and (2) Alpine rnetamorphism, by means of which the mineralogical and geochimical evolution of these rocks can be explained. Hercynian magmatism : It almost exclusively consists of the intrusive rocks of the Carboniferous sanstone pelite series, with resulting cryptovolcanic and plutonic phenomena such as sills, a few dykes, and a kilometer-size pluton; the rocks are mostly microcrystalline and porphyritic. The liquids emplaced are chiefly (about 9/10) dioritic and granodioritic, together with a small volume of granite and with minor amounts of basaltic-trending rocks (dolerites s. l. ). The petrology and geochemistry of the major elements (excepting Ca, Na and K) and of those of the trace elements both unquestionably show that this magmatism comes from calco-alkaline stock, and also that a single source magma yielded a single differenciation series. The lack of pyroxene and olivine, together with the behaviour of V, Ni and Cr, are both evidence of differenciation through intratelluric fractional crystallization of titanomagnetite, of hornblende and of plagioclase. Furthermore, the lack of anhydrous pyrogenetic ferromagnesian minerals (but for titanornagnetic), is evidence of the very high fluid contents of this rnagmatism (over 6 %water by weight). In the strongly-injected zones, autometarnorphism appears in the intrusive rocks. It acts upon the pyrogenetic mineraIs or yields either biotite or actinolite-hornblende. Contact metamorphism of the standstones and pelites gives hornfelses. Geochemical comparisons (major elements only) with other regional Permian deposits (Vallon de la Ponsonnière) bring out notable discrepancies in the Ti and alkalis contents. Despite its clear-cut geochemical features, the volcanism of the « zone houillère briançonnaise » is hard to explain when put back into the European geostructural frame at the end of the Hercynian, where very diverse volcanic phenomena (calco-alkaline, alkaline, tholeitic) occur side by side or mutually interpenetrate. The littIe-contrasted geochimical behaviour of Ti shows that this magmatism took place in a compressive geostructural environment and was paleographically located along deep-seated fractures which played a leading role in Hercynian an Alpine Europe. Alpine metamorphism : The metarnorphic parageneses almost exclusively appear in the green porphyries (microgranodiorites, microdiorites) : such rocks providded a chemically privileged environment, allowing crystallization of typomorphous calcic aluminosilicates. Two major trends are recognized in the green porphyries : (1) parageneses with little or no lawsonite (Briançon area) (2) parageneses with major lawsonite (Bissorte - Arc region), with more or less pumpellyite, prehnite, albite, white mica, chlorite and titanite. The chemical differenciation caused by metamorphism mostly involved the alkalis and calcium. The anorthite molecule is destroyed, with con comitant albitization and/or muscovitization resulting in increase of the Na and/or K contents and by substantial loss of Ca. On the contrary, the proportion of little-mobile Al is makedly increased with respect to the initial composition, as evidenced by the presence of normative corundum. For the other elements, variations are negligible ; they are diminished by the increase of the loss on ignition. Compared with the Li contents of some non-metarnorphic andesites (Southern Peru, Sardinia - X = 14.62 and X = 19.04 respectively), the Li contents of the intermediaite green porphyries are abnormaly high; the Li – FeO correlation is highly significant. Indeed, taken together with the abundance of Fe2+ in these rocks, the Li – FeO correlation and the results yielded by vector projection and by A. C. P. N. all lead to believe that the Li occurs in chlorite. Spot microprobe analyses on the chlorite-pumpellyite pair do not reveal any Fe/Fe + Mg correlation between these two phases. Contrary to pumpellyite, which is quite independent of the chemical composition of the host-rock, chlorite is highly dependent. The chemistry of pumpellyites varying from Fe - pumpellyite to Al - pumpellyite, is highly dependent of FeO, which governs the Fe3+ Al3+ and Fe2+ - Mg2+. Both the complexity of the metamorphic parageneses and their succession show that the calcic aluminosilicates are highly sensitive to small variations of the physical parameters. As shown by the ubiquity of the calcite - chlorite - albite - white mica parageneses, a CO2 plays a leading part in the stability of the calcic aluminosilicates ; the omnipresence of titanite or of leucoxene in association with typomorphous parageneses allow to suggest a value of about 0.04 for x CO2. The two major parageneses thus recognized allow the albite - lawsonite zone of Saliot (1970) to be divided into two sub-zones, on both sides of a line making a large angle with the Alpine structures trend. These differences in the appearance of typomorphous silicates are closely linked with the variations of the P parameter. The Southern framework (Briançon area) may have been submitted to a small temperature increase (from 250 to 300° C), with a concomitant moderate pressure increase (p 2. 5 kb), whereas for a similar temperature, the Northern framework would have undergone a sharper, more hornogeneous pressure increase. Furthermore, in the North, both the porphyries and the sedimentary framework exhibit more intensive deformations and recrystallizations, together with a more pronounced schistosity.
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Theses
Geochemistry. Universite Scientifique et Medicale de Grenoble, 1980. French


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Patrice Piantone. Magmatisme et métamorphisme des roches intrusives calco-alcalines du Carbonifére briançonnais entre Arc et Durance : minéralogie, pétrographie, géochimie. Alpes françaises. Geochemistry. Universite Scientifique et Medicale de Grenoble, 1980. French. <tel-00558740>

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