Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 128

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PÁLLIMSLAND
In the literature there are rather few Cr determinations in analyses of
clinopyroxenes of common rock suites and even these give incomplete data.
No attempt will therefore be made to compare the Cr contents of the Jan
Mayen clinopyroxenes to other clinopyroxenes than those of the Tertiary
alkaline rocks ofHocheifel, Germany, reported by Huckenholz (1965a & b),
which show decreasing Cr with increasing iron as do the Jan Mayen
examples.
The general Cr behaviour during clinopyroxene crystallization seems to
be an entrance of Cr into the early formed clinopyroxenes and accordingly
Cr poor later ones. A small amount of clinopyroxene which has crystallized
as the very last clinopyroxenes of each rock may though be enriched in Cr,
relative to the bulk of the late clinopyroxene and thus seem to pick up the Cr
rejected by late crystallizing minerals and accordingly enriched by this
process in the final liquid.
MnO is low in both the chromian diopsides and the titan-salites, ranging
up to 0.2 and 0.3 wt. per cent respectively. In the titan poor salites it
increases considerably, exceeding 1 wt. per cent and the increase continues
in the ferroaugites up to >3 wt. per cent. The titan-salites of the gabbro
xenoliths show a tendency to be higher in Mn than the titan-salites of the
lavas. The ferroaugites of the syenite xenolith are almost 1 wt. per cent
higher in MnO, than those of the lavas. The titan poor salites of the hydrous
mineral xenolith tend to be lower in Mn than the titan poor salites of the
lavas.
That Mn in Ca rich clinopyroxenes increases with differentiation in the
rock suites seems to be a general rule, regardless of the nature of the rock
suite. Brown & Vincent (1963) and Carmichael (1967) found this increase
in the tholeiitic rock suites of Skaergaard and Thingmuli respectively.
Sigurdsson (1970) found it in tholeiitic, transitional and alkaline rock suites
of Snæfellsnes, Iceland and Fodor et al. (1975) found it in the tholeiitic,
alkalic and nephelinic rock suites of Hawaii. The trend is clear in the few
clinopyroxene analyses reported by Bryan et al. (1972) from the calc-
alkaline rocks of the Tonga Islands.
Na20 is rather constant in the chromian diopsides, ranging between 0.2
and 0.35 wt. per cent. It shows a slight increase in the titan-salites and titan
poor salites, approaching 1 wt. per cent as a maximum. The ferroaugites are
slightly poorer in Na^O, grouping around 0.5 wt. per cent. The syenitic
xenolith ferroaugites show slightly lower Na content than the ferroaugite of
the lavas.
Le Bas (1962) found Na in clinopyroxenes to increase with differentiation
of rocks in his “calc-alkaline, tholeiitic, normal-alkaline and per-alkaline”
rock suites but to decrease in high-alumina rock suites. Brown & Vincent
(1963) found Na in clinopyroxenes of the rocks of the Skaergaard intrusion,
usually classified as tholeiitic, to decrease with differentiation. This is
probably not a contrasting result with the one of Le Bas (op.cit.) because