Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 271

PETROGENETIC RELATIONSHIPS
267
analysed and calculated derivative liquid is quite small. The clinopyroxene
still dominates the olivine in the fractionate (>71 per cent). Producing these
primitive basalts from a Mg poor ankaramite involves the removal of 22—32
per cent of minerals and leaves a liquid amounting to 68—78 wt. per cent of
the original material (Jan 10).
Ifsome other samples are selected instead ofjan 10 or 26 (e.g. 30, 65, 66,
46) to represent this compositional interval in the crystal fractionation, the
misfit is increased and in some cases becomes even unacceptable, using
these same fractionating minerals. K, Ti and A1 are especially difficult to
account for in these cases.
It is thus clear that by subtracting the primitive mafic phenocryst phases
of the ankaramites (the wehrlite crystallization minerals) from “ankarami-
tic” liquids, more basaltic compositions are easily reached and these have
the whole rock chemical characteristics of the mafic rocks of the Jan Mayen
rock suite. The wehrlite fractionation, characterized by chromian diopside
and Fo rich olivine (~Fo90), is thus a possible process in the early evolution
of the magnesian Jan Mayen magmas. As these liquids start to rise from the
great depths of the wehrlite conditions, the ankaramite crystallization,
characterized by titan-salite and slightly more fayalitic olivine, may
apparently take over the fractionation and direct the evolution into the
compositional range of the primitive basalt category.
III. Crystal fractionation within the basalts
The basaltic rocks of the Jan Mayen rock suite are a voluminous group
and they show considerable compositional spectra. Crystal fractionation
within this group can thus be imitated in various ways guided by chemical
characteristics and mineral contents. In the fractionation within the basaltic
rocks, the misfit obtained is in numerous cases increased relative to that of
the early fractionation. This happens in spite of an increase of mineral
phases used in the calculations and in many cases relatively small com-
positional variations between “mother liquid and derived liquids”.
Here, four examples of fractionation within the basalts will be shown
(Tables 43—46). The first one considers fractionation within the ankarami-
tic basalts, which are characterized by the absence of plagioclase in nature.
Examples two and three consider the more magnesian basalts as (relatively)
K poor and K rich groups. The last example considers the possibilities of
fractionating within the low-magnesian basalts.
Usingjan 26, as the “mother liquid” in the case of fractionation within
ankaramitic basalts, the more magnesian ones (Table 43A) may be pro-
duced by subtracting a primitive titan-salite and an olivine of F083.5 com-
position. This choice of minerals is in reasonable agreement with petro-
graphic observations. The total fit is rather poor. The misfit is primarily
produced by a lack of A1 and Ca and an excess of the alkalies in the