| Summary: | Many layered igneous rocks consist of two main parts: the cumulus (plus adcumulus and heteradcumulus) material and the pore material (or 'mesostasis') produced by the crystallisation of the trapped liquid which necessarily had the composition of the contemporary magma. Part One of this thesis is primarily concerned with the geochemistry of the mesostasis of some layered igneous cumulates. In the past no workable method, has been developed for deducing the amount or composition of the mesostasis, for igneous cumulates in general. Wager (1963) established a method for the Skaergaard intrusion, Hast Greenland, by assuming that all the phosphorus in a rock of the Lower and Middle Zones (i.e. before the incoming of cumulus apatite) of the Layered Series existed in the mesostasis, so that the phosphorus content of any rock gave a relative measure of the amount of mesostasis. Furthermore, the phosphorus contents of the successive fractions of the Skaergaard magma were known from another approach and so it was possible to deduce the absolute amount of mesostasis. For most other layered intrusions the successive compositions of the fractionated magma are not known and the method cannot be applied to them. knowledge about the geochemistry of the mesostasis (and hence the contemporary magma) would be most useful in straight comparative geochemical studies; in deducing fractionation trends; and in the determination of solid/liquid trace element partition coefficients, which in turn could be useful in considering the petrogenesis of certain other fractionated igneous rocks. Thus, an attempt has been made to produce a geochemical method for deducing the amount of mesostasis in igneous cumulates. For this method, elements of contrasting geochemical behaviour are required: those that show a strong preference to remain in the residual liquid of a fractionating basic magma (the so called. 'low-k elements') and those which tend to preferentially enter one of the cumulus phases (the 'high-k elements'). Two methods are considered: ...
|
|---|