Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 216

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PÁLL IMSLAND
from the same magma at higher levels, but occasionally became resorbed
and marginally zoned by more anorthitic plagioclase as the result of mixing
of diíferent basic magmas. These rocks are generally pl-ol-cpx porphyritic.
The pl-ol porphyritic rocks, on the other hand, generally seem to have
crystallized out much less plagioclase at an early magmatic stage and
contain only relatively small amounts of bigger phenocrysts. Just prior to
ascent of the magmas and during the ascent, these magmas crystallized
abundant plagioclase as microphenocrysts, which show slight but even
normal zoning, frequently enclosed in a thin marginal zone crystallized after
extrusion. In the case of the more evolved rocks the feldspar features
similarly indicate a complex crystallization history for some samples, while
it is relatively simple for others.
IV. Crystallization relationships of the feldspars
The temperature and pressure conditions in magmas crystallizing plagio-
clase cannot be interpreted from petrographical and chemical data in the
case of fully crystalline rocks. As no equilibrium related chemical para-
meters, which can be used to indicate or delimit the conditions, have been
found, some indirect means must be used to approach the conditions of
temperature and pressure in the case of the Jan Mayen plagioclase.
In experimental investigations on crystallization relations of basaltic
rocks, carried out at 1 atm, by a number of authors during the last two
decades, plagioclase starts to crystallize between 1100 and 1200°C in most
cases, regardless of the compositions of the basalts (see e.g. Bell &
Humphries, 1972; Humphries & Cox, 1972; Krishnamurthy, 1976; Thomp-
son & Tilley, 1969; Thompson & Flower, 1971; Tilley et al., 1964, 1965,
1967 and 1971; Tilley & Thompson, 1970 and Yoder & Tilley, 1962).
Temperatures above 1200 and below 1100°C seem to be very infrequent as
the first appearance temperatures of basaltic plagioclases compared to
temperatures between these values. In experiments carried out under
varying pressures (see e.g. Arculus, 1975; Cohen et al., 1967; Ford &
Macdonald, 1978; Green & Ringwood, 1967; Thompson, 1974 and 1975),
the temperatures for the first appearance of plagioclase at higher pressures
do not seem to be greatly different from the temperatures obtained at 1 atm.
In most experiments the first appearance occurs at slightly higher tempera-
tures, at the higher pressures, but in a few it occurs at similar or slightly
lower temperatures. The total pressure therefore does not seem to afiect the
crystallization temperature of the plagioclase drastically. The Ph20 is much
more efiective in this respect, as shown by e.g. Kudo & Weill (1970) and
Baxter (1978).
In a study of the Di-Fo-An system, at 1 atm to 20 kb, Presnall et al. (1978)
have shown how the An field decreases with increasing pressure and
disappears between 15 and 20 kb. In the same system, Herzberg (1972)