Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 258
9. PETROGENETIC RELATIONSHIPS
A. SUMMARY OF MINERAL RELATIONSHIPS
In the previous chapter the conditions and the compositions of the
minerals of the rock suite and xenoliths have been described and discussed.
The discussion i.a. attempted to approach restrictions to the temperature
and pressure range and the magma compositions at the time of crystalliza-
tion. This approach uses both direct and indirect means and the results
gained thus vary in reliability. It enables, however, construction of a
qualitative and generalized model for the evolution of both magmas and
mineral phases of the Jan Mayen rock suite. This evolution may be partly
shown by depicting the minerals as a function of temperature and pressure.
Fig. 123 shows the crystallization temperature of the minerals, while Fig.
124 shows the pressure at which this crystallization took place. In both cases
the minerals are divided into smaller categories, where convenient.
In Fig. 125 the data of Fig. 123 are combined to comprise the rock groups
and show their crystallization temperature range. As is to be expected, the
crystallization temperature range of highly porphyritic lavas is broad,
ranging from over 400°C in the case of the ankaramites to ~250°C in case of
more evolved rocks. The aphyric lavas are of three compositional types, the
primitive ankaramitic basalts, the more evolved aphyric basalts and basaltic
tristanites and the highly evolved tristanites to trachytes. In some cases the
magmas of these rocks have been taken to the surface to be quenched there
or rapidly crystallized to a practically phenocryst free aphanitic or interser-
tal lava. The ankaramitic basalt seems to exist as a liquid at >1200°C, while
the more evolved aphyric rocks seem to exist as liquids at ~1100°C or lower
temperatures, before they are taken to the surface. The crystallization of the
coarse-grained xenoliths takes place within variable temperature ranges. In
the case of the wehrlite the crystallization temperature is high but the range
narrow or less than 100°C. The range in the case of the gabbros is slightly
broader or just over 100°C. In the case of the syenitic xenoliths the range is
broader or ~300°C in some cases.
In Fig. 126 the general features of the evolution of the Jan Mayen rock
suite are summarized as they appear from the above discussions. According
to the experiments of Arculus (1975), made on rocks quite similar of those of
Jan Mayen, garnet is stable at ~30 kb at the high temperature of the early
spinel crystallization. The primitive liquids may originate at these depths,