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

Media type: E-Book

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

Contributor: Piantone, Patrice [Author]

Published: [Erscheinungsort nicht ermittelbar]: HAL CCSD, 1980

Language: French

Origination:

University thesis: Dissertation, HAL CCSD, 1980

Footnote:

Description: 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. ; L'étude des roches intrus ives de la zone houillère briançonnaise concerne deux phénomènes indépendants permettant d'expliquer leur évolution minéralogique et géochimique : (1) le magmatîsme hercynien (2) et le métamorphisme alpin. Le magmatisme hercynien : Il concerne presque exclusivement les roches intrusives de la série gréso-pélitique du Carbonifère et se traduit par des phénomènes cryptovolcaniques et plutoniques: sills, dykes (peu) et un pluton d'extension kilométrique, de roches surtout microgrenues porphyriques. Les liquides mis en place sont de nature dioritique et granodioritique (9/10) accompagnés d'un faible volume de granite et d'une quantité mineure de roches à tendance basaltique (dolérites s.l. ). La pétrologie et la géochimie des éléments majeurs (hormis Ca, Na et K) et en trace montrent indiscutablement l'appartenance de ce magmatisme à la lignée calco-alcaline et l'unicité de la série de différenciation à partir d'un seul magma parental. L'absence de pyroxène et d'olivine, ainsi que le comportement de V, Ni et Cr, témoignent d'une différenciation par cristallisation fractionnée intratellurique de titanomagnétite, hornblende et plagioclase; en outre, l'absence de minéraux ferromagnésiens pyrogènes anhydres (hormis la titanomagnétite) indique la très grande richesse en fluide de ce magmatisme (plus de 6 % d'eau en poids). Dans les zones fortement injectées, un autométamorphisme apparaît dans les roches intrusives, affectant ces minéraux pyrogènes ou donnant de la biotite ou de l'actino-hornblende et un métamorphisme de contact se développe dans les grès et les pélites encaissantes donnant des cornéennes. Des comparaisons géochimiques (éléments majeurs seulement) effectuées avec d'autres gisements régionaux permiens (Vallon de la Ponsonnière) font apparaitre des divergences notables portant sur le Ti et les alcalins. Le volcanisme de la zone houillère briançonnaise replacé dans le contexte géostructural européen fini-hercynien où des phénomènes volcaniques de nature très diverse (calco-alcalin, alcalin, tholéiitique )se côtoient ou se téléscopent (Vosges), est difficilement interprétable malgré ses caractères géochimiques tranchés. Le comportement géochimique peu contrasté du Ti situe ce magmatisme dans un contexte géostructural compressif et sa position paléogéographique le long de fractures profondes qui ont joué un rôle prépondérant dans l'Europe hercynienne et alpine. Le métamorphisme alpin: Les paragenèses métamorphiques se développent presque exclusivement au sein des porphyres verts (microgranodiorites - microdiorites) qui constituent un milieu chimiquement privilégié, compatible avec la cristallisation des aluminosilicates calciques typomorphes. Deux tendances principales sont mises en évidence dans les porphyres verts: (1) des associations à lawsonite exceptionnelle (région de Briançon) (2) des associations à lawsonite dominante (région de la Bissorte Arc) auxquelles s'associent à divers degrés la pumpellyite, la prehnite, l'albite, le mica blanc, la chlorite et le sphène. La différenciation chimique due au métamorphisme a surtout affecté les alcalins et le Ca. Elle se manifeste par une destruction de la molécule d'anorthite contrebalancée par une albitisation et/ou une muscovitisation qui se traduisent par un gain en Na ou en K et/ou perte substantielle en Ca. Au contraire, l'Al, élé...

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