Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 95

ROCK CHEMISTRY
91
CaO decreases evenly from almost 16 per cent in the most mafic ankara-
mites with a relatively broad scatter in the Mg poorer basalts. In the
intermediate rocks it decreases to about 1 per cent in the most silicic
trachytes.
Na20 increases from around 1 per cent in the most mafic ankaramites to
about 4 per cent in the more Mg poor basalts and then continues to increase
towards around 6 per cent in the trachytes.
K20 has a similar pattern as NazO. It increases from 0.6 per cent in the
most mafic ankaramites to scattered values around 2.5—3 per cent in the
more Mg poor basalts. In the intermediate rocks it increases relatively more
than Na or from around 3 per cent to over 7 per cent, as Mg decreases.
P20s increases from 0.22 per cent in the most mafic ankaramite to a
maximum of 1.5 per cent in the Mg poor basalts and Mg rich intermediate
rocks, but shows a wider scatter in the Mg poor basalts than any other of the
oxides; or from 0.5 to 1.5 per cent. In the intermediate rocks P205 decreases
as Mg decreases from the 1.5 per cent maximum down to almost 0.1 per cent
in the most silicic trachytes.
H20. None of the water components (H20+, H20^ or H2Ol) shows any
trend, when plotted against Mg or other elements, but are widely scattered.
The trends of all major elements change slope or direction at ca. 4 per
cent MgO. At this point Si starts to increase from an almost constant value
in the basic rocks, the rate ofincrease of A1 diminishes, but accelerates in the
case of both Na and K. The rate of decrease of Ca becomes faster. At the
same point there is a maximum for P, Ti, and total iron. Mn is also at a
maximum. At lower Mg concentrations the Mn concentrations do not
increase but stay at this upper level, though with a considerable scatter of
lower values. This point thus marks a major change in the overall composi-
tion of the rock suite. The significance of this change is discussed in chapter
9.
The overall composition described here leads to the following main
normative characters. All the ankaramites, basalts and basaltic tristanites of
the rock suite are silica undersaturated. Only two of the basalts have hy in
the norm, otherwise both the ankaramites and basalts are ol-ne-normative.
Two of the three analysed basaltic tristanites are hy-normative, while one is
ne-normative. Similarly two of the three analysed tristanites are ol-ne-
normative, while one is qz-hy-normative. This is the first appearance of
normative qz in the rock suite and occurs at 57 per cent Si02. The trachytes
are mostly qz-hy-normative. Two ol-ne-normative trachytes though occur
(Si02=62—63 per cent). One of these also contains ac in its norm and is the
only peralkaline trachyte analysed. One analysed trachyte is peraluminous
containing 0.59 per cent normative co. The other trachytes are “normal” in
this respect, i.e. neither containing ac nor co in the norms. The range of
normative ol, ne and qz in the rock groups of the suite is shown in Fig. 9.