Magma mixing: origin of intermediate rocks and “enclaves” from volcanism to plutonism

Abstract

Intermediate magmatic rocks are rarely homogeneous; a common example is the presence of magmatic “enclaves” Volcanic examples show that they are indications of magma mixing processes. Mixing events in the trachyandesitic suites from the Sancy Volcano (France). The volcanic activity from Sancy consists of several brief trachyandesitic cycles. Each of them begins with the eruption of highly heterogeneous benmoreites, followed by more homogeneous mugearites. Light porphyritic trachyandesites enclose numerous inclusions of varied darker lavas. The crenulated geometry of the contacts, the presence of chilled margins, the vesiculation of the core of the basic parts, suggest that these different rock types were magmatic at the same time although under marked thermal disequilibrium. Xenocrysts are common: partly resorbed sanidine rimmed with plagioclase in a basic matrix, reactional olivines in tridymite-bearing trachytes. This shows that the mixing occurred between partially crystallized and fractionated magmas. Chemical compositions are continuously variable from basalts to trachytes within the same eruptive cycle. All these facts might be interpreted as the result of a mechanical intermingling, more or less achieved, between two end-members of contrasting composition in the reservoir. In the Sancy Volcano this phenomenon occurred at least, 4 or 5 times during a 600 000 y span. Mixing evidence in Hercynian granodiorites from France. Similar observations may be described in granodioritic rocks. The more widespread “enclaves” exhibit the same characteristics as their volcanic equivalents. Although most of them are small, more important volumes of basic rocks may be associated in the same massif. In the neighbourhood of such large bodies, swarms of smaller enclaves recall the generation of basic pillows. The nature of the contact between small “microgranular enclaves” and their host rocks, the occurrence of xenocrysts with reaction rims (rapakivi feldspars and quartz ocelli in a basic surrounding, amphibole-pyroxene clots in a granitic matrix) and geochemical variations, again suggest thermal and mineralogical disequilibrium. Such observations indicate the comagmatic character of enclaves with regard to their host rocks. They cannot be interpreted as restites and are best described as co-igneous inclusions resulting from non achieved mixing between coexisting magmas. Photonic rocks are less suitable to demonstrate such petrogenetic processes because they result from slow cooling and crystallization during a longer residence time in a magmatic chamber. The successive mixing episodes, giving rise to different generations of co-igneous inclusions are also difficult to recognize. On the other hand, in volcanism this process is quenched and periodically sampled by eruptions. In conclusion the heterogeneity and the variability of many intermediate rocks cannot be explained only in terms of simple partial fusion and fractional crystallization processes in a closed system. Volcanic examples point to a complex evolution of magmatic reservoirs that were continuously fractionated, periodically tapped, periodically refilled and mixed.