Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 117

mineral chemistry and relationshi ps
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the rock suite and as phenocrysts in all rock groups, except one; the Mg poor
basalts called aphyric basalts. The phenocrysts occur as anhedral to euhed-
ral and range in size up to more than 1 cm.
These clinopyroxenes are Ca rich and plot mostly in the diopside and
salite fields of the pyroxene trapezium (Deer et al. 1966). The diopsides are
usually rich in Cr while the salites are usually rich in Ti and Al. These are
here termed chromian diopside and titan-salite respectively. There is a
compositional continuum between the extreme Cr rich diopsides and Ti rich
salites. Thus there are no natural boundaries, which can be set between
these two extremes. Titan poor salites occur as well in the rock suite. Small
amounts of relatively iron poor ferroaugite pyroxenes occur and faintly
greenish, slightly sodic, iron rich salite is found in one sample in small
amounts.
The pyroxenes are fresh and unaltered except in the gabbro xenoliths,
where they may show fme lamellae of exsolved opaque oxides.
II. The composition of the clinopyroxenes
When the pyroxenes of alkaline and tholeiitic rock suites are compared,
four characteristic diíferences are most frequently found: a) a lack of Ca
poor and Ca free pyroxenes in alkaline rocks, b) a trend ofslightly higher Ca
content in alkaline rock pyroxenes than the most Ca rich pyroxene trend of
the tholeiitic rocks, c) a trend towards much less iron enrichment of
pyroxenes along with evolution of rock suites in the case of alkaline rocks
and d) higher A1 and Ti contents of the pyroxenes of alkaline rocks. All these
characteristics of alkaline rock pyroxenes are found in the Jan Mayen rock
suite.
The overall range of the composition of the clinopyroxenes of the rock
suite is from Cr rich diopside through Ti rich to I'i poor salite and to Fe
poor, Ca rich ferroaugite. The xenoliths occurring in the rock suite contain
clinopyroxenes of compositions which are within the range of the rock suite
examples. The compositional trend is shown in Fig. 75, where the analyses
are plotted in the pyroxene trapezium. Representative analyses are given in
Table 18.
Most of the clinopyroxene phenocrysts of the ankaramites are anhedral
relatively large chromian diopside crystals. These may be mantled by a
zoned rim of titan-salite, which also may occur as independant, euhedral,
zoned phenocrysts and microphenocrysts. In the case of the basalts the
volume relations are reversed, phenocrysts and microphenocrysts of titan-
salite, euhedral and zoned, are most common, while the chromian diopside
is relatively scarce. Most of the intermediate rocks contain salites, which are
poorer in both Ti and A1 and richer in Mn and Na than the titan-salites of
the basalts. Some trachytes contain iron poor ferroaugite clinopyroxene, still
poorer in Ti and Al. Only a few groundmass clinopyroxene crystals have