Essai de Caractérisation Chimique des Associations Spilitiques

Abstract

The authors elucidate the chemical characteristics of the spilitic suite through a graphic representation of 344 published (or to be published) analyses according to the following scheme:

• a) data on different spilitic associations (147 analyses)

• b) preorogenic hercynian (varistic) volcanism in northern Europe (173 analyses)

• c) local variations between the cores and margins of pillows (24 analyses).

Two unconventional graphical representations have been used:

$$\begin{array}{*{20}c} {\left( {Al/3 - K} \right)f\left( {Al/3 - Na} \right)} \\ {\left[ {Si/3 - \left( {Na + K + 2Ca/3} \right)} \right]\left[ {K - \left( {Na + Ca} \right)} \right]} \\ \end{array} .$$

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The application of these to work concerning formation of volcanic rocks is not usual, but allows a clear division between the characteristic transition anorthite — albite in spilitization and that from basic to acidic rocks in magmatic differentiation. In this way, the diversity and uniqueness of the spilite suite have been brought out.

Besides, such a representation makes possible an attempt at an interpretation of spilitic rocks from a chemical point of view. Of the elements taken into consideration, the following values are unchangeable: Al, (Ca + Na) as well as (Si + Ca). Si and Na are thus introduced together, while Ca is released and Al remains in place. In pillow lavas such transformations depend mainly on the exchange of material between the pillows and their chloritic and carbonate matrix.

After a discussion of the problem of too selective a sampling in favor of pillow-cores, the authors reject the hypothesis of an overall composition similar to that of basalts. The spilitization of basalts necessitates a metasomatic introduction of Na and perhaps of Si, with a simultaneous release of Ca.

In the end the relationship of spilitic associations with tholeiitic and alkaline series of rocks is considered. The spilites form, just as the basalts, a continuum: certain spilitic associations have a clearly tholeiitic, others an alkaline character. The link with the paleogeo-graphic conditions of implacement, the presence or absence of a sialic crust, is beyond the scope of hercynian volcanism.