Spilitic Magma. Characteristics and Mode of Formation

Abstract

It is suggested that the lapilli in the Middle Devonian tuff (schalstein) of the Lahn Basin throw light on the question of the existence of a primary spilitic magma. In a largely medium-grained type of lapilli, the mineralogical composition conforms to that of the massive weilburgites, giving evidence of a corresponding melt already in progress at the time of explosive activity and of its origin in depth. No other source or character is conceivable for the co-existing lapilli with deficient or only partially developed crystallization. Moreover, the refractive index of the amorphous material is considerably below that of basaltic glasses, which likewise points to the existence of a weilburgitic hydromagma before the explosion.

Judging from the capacity of forsterite, enstatite and diopside melts at high pressures to dissolve a noticeable amount of H₂O, it is suggested that hydromagmas might have been formed in the high Upper Mantle or in the lower crust zones. Hydrous solid phases, for instance in amphibole peridotites, are referred to as evidence of such an effect. Degassing and ascent are assumed to depend on the release of external pressure under suitable orogenic conditions, especially folding, in synclines.

Attention is called to the localized group of metasomatic spilites in the Lahn district, to the occasional appearance of weilburgites exhibiting semi-spilitic features, and to the characteristic differences between the weilburgites and diabases associated with them. In comparison with the weilburgites the variety is even greater in spilites resulting from burial metamorphism of andesite or basalt.