Spilites, keratophyres, and the igneous spectrum

Abstract

SummaryAlthough the structures and textures of spilites and keratophyres are comparable to those of fresh lavas, recent work has established that they did not acquire their present mineralogy at the time of extrusion. Furthermore, the compositions of many analyzed samples of spilites and keratophyres are shown to lie outside a ‘spectrum’ of compositions of all comparable fresh igneous rocks. A later metamorphism has therefore been responsible for their formation and has commonly been accompanied by metasomatism. This possibility of later metasomatism must be appreciated before conclusions are drawn that are based upon analysis of rocks of a spilitic or keratophyric mineralogy. This applies to age-dating and to analytically based petrographic and petrogenetic conclusions.

Rare earth and other trace element mobility accompanying albitization in a Proterozoic granite, W. Bergslagen, Sweden

“…Ca is variable, gained by one sample, lost from the other. The effects of albitization on the alkali elements is best shown by the 'igneous spectrum' of Hughes (1972), where the samples are displaced out of the normal range of igneous values into the Naenriched field ( fig. 3).…”

Tectonized actinolite-albite rocks from the Outokumpu district, Finland: Field and geochemical evidence for mafic extrusive origin

“…The average alkaline contents of 3.1 ®7o Na 2 0 and 0.4 % K 2 0 do not give reason to suppose that the bulk composition has changed much in pre-metamorphic or metamorphic alteration (cf. Aumento et al 1976, Hughes 1972). The contents of CaO and P 2 0 5 are slightly lower than the average given for basalts.…”

Geochronology and lithogeochemistry of granitoid rocks from the central part of the Central plutonic belt, New Brunswick, Canada: implications for Sn-W-Mo exploration

“…Average compositions for individual plutons are given in Table 3; these averages were determined by supplementing newly acquired analyses (i.e., the data in Appendix B) with data compiled from Whalen (1993;23 similar to those reported by Whalen (1993); furthermore, in most cases, felsic plutonic rocks of all ages display overall geochemical similarity. Post-emplacement alteration and alkali mobility has been relatively minor based on the Hughes (1973) (not shown), as almost 90% of samples plot within the "igneous spectrum" and most of the others are only marginally altered using this criterion. Nevertheless, these slightly altered samples were not included in the geochemical plots discussed herein.…”