Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 313
10. SUMMARY AND CONCLUSIONS
In this chapter the main features and conclusions of this work are
summarized. It is arranged in four major units. The first one comprises
some general chemical and petrological features, mostly concerning the rock
suite and xenoliths and their spatial relationships. The second one summa-
rizes the mineralogical study. The combined pattern of the petrology,
mineralogy, and petrochemistry; i.e. the rock suite fractionation, is then
briefly treated in the third unit. Finally the evolution of the Jan Mayen
crustal segment and the origin of the magma system of the island are
summarized, as well as the consequences of this evolution on the petro-
genesis.
A. CHEMICAL- AND GENERAL PETROLOGY
1) The volcanic rock suite ofjan Mayen is classified as a potassic alkaline
one. The rocks are ne-normative all the way up to trachytes of over 60 per
cent Si02.
2) General alteration of the rocks is quite small. Vesicles in the lavas are
empty and no smectite minerals occur in the groundmass. The hyaloclast-
ites may contain xenoliths of subsurface lavas (eruption conduit material).
These are unrecognizeable from the rock suite, except for their alteration.
They show a low-grade metamorphism or alteration, characterized by
calcite and zeolite fillings of vesicles and smectite minerals as alteration
products of groundmass minerals and glasses.
3) The rock types form a pattern in their distribution on the island. This
pattern may be described as a decrease in maficity from northeast to
southwest, i.e. along the length of the island. In accordance with this,
ankaramites and wehrlites only occur on Nord-Jan and the mafic minerals,
the kink-banded olivine, the chromian diopside, and the chrome-spinel,
primarily characterizing the ankaramites, decrease in amount in the basalts,
as these are erupted further towards the southwest. As these highly mafic
materials decrease in amount, an increase in the highly felsic material
occurs. Trachytes and tristanites, and the accompanying syenite xenoliths
are thus most prominent on Sör-Jan.
4) The rock types on Jan Mayen form a bimodal distribution pattern
regarding composition. Low-silica intermediate rocks are extremely scarce,
when compared to the more basic and more silicic rocks. This bimodal
distribution is present in the number of erupted lavas. Whether it is present
in the volume relations as well is uncertain.