Géochimie et minéralogie comparées d'associations magmatiques acide-basique de type magnésiopotassique et calco-alcalin. Exemple du massif de l'Agly (Pyrénées Orientales)

par Ahmed Touil

Thèse de doctorat en Terre, océan, espace

Sous la direction de Daniel Garcia.

Soutenue en 1994

à Paris, ENMP .

mots clés

• Magmatisme -- Pyrénées-Orientales (France)
• Cycle hercynien -- Pyrénées-Orientales (France)
• Granite -- Pyrénées-Orientales (France)
• Roches -- Analyse -- Pyrénées-Orientales (France)

Résumé

Le massif de l'Agly (Pyrénées Orientales) comporte deux associations acide-basiques d’âge hercynien mises en place respectivement dans la catazone et dans la mésozone: le massif d'Ansignan et le massif de St-Arnac. Cinq familles basiques et intermédiaires ont été distinguées dans l'association catazonale: le groupe A (Tholéitique), B Calco-Alcalin, C Calco-Alcalin Potassique, C1 Chimiquement analogue à certaines vaugnérites et D qui ne contient que des termes intermédiaires et qui est transitionnel entre le calco-alcalin et le tholéitique. Dans l'association mésozonale les deux groupes chimiques distingués sont de type calco-alcalin avec un caractère plus ou moins potassique. Seul le groupe B montre un équivalent du point de vue des rapports d'incompatibles avec le groupe le moins potassique mésozonal. Les deux faciès acides du massif d'Ansignan (La granodiorite charnockitique et le granite blanc à grenat) sont alumineux et présentent certaines caractéristiques des granites magnesiopotassiques. La charnockite et le granite blanc ne peuvent être obtenus par différenciation des roches basiques A, B, C ou D. Les variations linéaires observées dans le groupe C1 rejoignent le domaine de composition de la charnockite suggérant une certaine relation génétique entre les deux faciès. Dans le massif de St-Arnac les trois faciès acides (la Granodiorite, le Granite porphyroide et le Granite clair de Lansac) présentent les caractéristiques minéralogiques géochimiques des facies calco-alcalins. L'absence de termes intermédiaires entre les facies basiques et acides font que les relations génétiques entre ces deux faciès ne sont pas bien établies.

Titre traduit

Comparative Geochemistry and mineralogy of a magnesiopotassic and calc-alkalic magmatic acid basic associations: Exemple from Agly massif (Eastern Pyrenees)

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Résumé

The Agly massif (Eastem Pyrenees) contains two hercynian acid-basic associations emplaced respectively in the catazone and the mesozone : the Ansignan massif and the Saint Arnac massif. These associations show a great chemical diversity of the basic inclusions ; interestingly, this diversity is neither simply related to the spatial distribution nor to the degree of deformation of the basic and intermediate rocks. In spite of this difficulty, the ratios between incompatible elements evidence four distinct groups in the catazonal association: a tholeiitic group A ; a calc-alkaline group B ; a potassic calc-alkaline group C and a magnesiopotassic group (C1 ) similar to some vaugnerites. A last group D, which contains only intermediate rocks (2<MgO<4. 5%), is transitional between the calc-alkaline and the tholeiitic series. Ln the mesozonal association, two groups are evidenced ; both are calc-alkaline but variably enriched in potassium. The comparison of different groups in the two associations show the identity of the incompatible ratios between the group B in the catazone and the less potassic one in the mesozone. The mesozonal potassic group has no strict equivalent in the catazone. It shows however some similarities with C and C1 which are characterized respectively by a potassic calc-alkaline chemistry and high content in incompatible (REE, Th and Zr) and in transition elements, compared to calc-alkaline rocks. Ln the basic and intermediate rocks, whole rocks CaO contents are variably depleted due to partial or complete retrogression of clinopyroxene. This is evidenced, in the catazonal basic rocks, by variations in the chemistry of orthopyroxenes : in spite of the absence of clinopyroxene, alumina content of orthopyroxenes, which lies to the activities of anorthite and Al203, remain at a low level like in clinopyroxène-bearing rocks. Two acidic rock types are distinguished in the Ansignan massif: a magnesian granodiorite which contain biotite and/or orthopyroxene and/or garnet ("chamockitic granite") and a garnet-rich granite (white granite). These rocks are aluminous (A/CNK~1. 1) and display some characteristics of magnesiopotassic granites like low Fe/Mg, low Ca/Mg ratios (CaO/MgO # 1) and a high content of Ni, Cr, LREE, Zr and Th. The chamockite composition shows little internal variations which are mainly related to the accumulation of plagioclase, K-feldspath and biotite. The white granite shows some differentiation. The variations of refractory (Ni, Cr and MgO) and incompatibles (Th and Ce) elements indicate that the chamockite and the white granite cannot result from the differentiation of A, B, C or D basics groups. The relation between C1 -group enclaves and the chamockitic granite is more complex. Linear variation in the C1 -group is interpreted as a mixing trend between an enriched basic source and an acidic melt resembling the garnet-bearing white granite in composition. This acidic end member may be purely anatectic or, alternatively, a differenciated product by assimilation and crystallization of the basic melt. The bulk composition of the orthopyroxene bearing chamockitic granite belong to the C1 mixing trend and is interpreted as an hybridized rock. Internal variations within the chamockitic granite indicate that this hybrid material was vigourously convected and homogeneized and then further differenciated through crystallisation and accumulation. Ln the Saint-Arnac massif, three acidic rocks type are distinguished : a granodiorite, a porphyritic granite and a leucocratic granite containing secondary muscovite (Lansac granite). These rocks show the mineralogical and geochemical characteristics of common calc-alkaline granites, i. E. High Fe/Mg and Ca/Mg ratios, as well as moderate Ni, Cr, LREE, Zr and Th contents. Their high CaO/MgO favours the stability of albanite instead of monazite and the abundant crystallisation of apatite ; so that differentiated, leucocratic members of this suite are phosphorus depleted. While the granodiorite and porphyritic granite are cogenetic, the Lansac granite must have a distinct source. Crystallisation and accumulation control the differentiation of these granitoids ; the segregation between Iiquid and solids occurs through the selective extraction of residual liquids. In the Lansac granite, accumulation is less important and the rock composition is closer to that of the Iiquid. The relation between the basic and acid cannot be established because intermediate rocks are lacking. The geochemical results obtained on the Agly magmatism reveal the chemical diversity in both the basic and acidic rock types ; these acid-basic association cannot be simply considered as bimagmatic. While geochemistry is remarkably efficient to reveal this complexity, the mineralogical study fails to show such primary features. Minerais are often unzoned or only slightly zoned. In the catazonal association, for example, the chemistry of pyroxenes is strictly determined by the chemistry of the whole rocks through its mineral paragenesis (and the related variations of the activity of alumina) ; in most case mineral compositions seem to be acquired at a late to postmagmatic stage.