Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 17

GENERAL INTRODUCTION
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presented in Fig. 1. The present work is a direct continuation and the field
knowledge used in the discussions and interpretations of the petrological
and mineralogical problems is rooted there. For the few local names
mentioned in the text, the reader is also referred to the earlier work. The
field knowledge and the rock samples were collected in two short summer
excursions to the island, organized by the Norwegian Polar Institute in
Oslo, while the analytical work and the data processing took place at the
Nordic Volcanological Institute in Reykjavík.
This work is a shortened version of a previous publication (Imsland,
1980), and deals with the petrology, mineralogy, and evolution of the rock
suite. Both whole rocks and minerals are thoroughly analysed. On the basis
of these analyses the compositional range of individual mineral suites, as
well as their internal relations, are depicted. The crystallization history of
individual rock groups and the rock suite as a whole is traced and the
temperature and pressure conditions of the crystallization followed. The
origin of the compositional diversity of the rock suite is tested against the
traditional fractional crystallization concept. It is shown how the evolution-
ary history of the crust of the area, viewed in the light of global tectonic
evolution, is able to explain the presence of the Jan Mayen volcanism and
how it would effect the rock compositions.
The rocks of Jan Mayen are entirely igneous in origin. Here they are
divided into two major groups:
1) A continuous rock suite, extending from ankaramites to trachytes. They
occur as lavas, domes, hyaloclastites, tephra and a few dykes, of which
the island is built up. The rock suite is potassic alkaline and ranges from
highly porphyritic, Mg rich ankaramites with nepheline in the norm,
through ne-normative basalts and low-silica intermediate rocks of
varying mineralogical characteristics, to trachytes, some of which con-
tain a small amount of qz in the norm, while others are still ne-
normative. The various basalt types are dominant in the rock mass.
2) Xenoliths occurring in the other rocks. These are of several types. Their
volume is insignificant, but they give information which is important in
connection with the mineralogy, petrogenesis and stratigraphy.
From all over the island, 96 samples have been taken for analyses. Of
these, 88 are extrusive rocks and 8 are xenoliths formed by subsurface
crystallization. From a selection of these, — 1200 microprobe, mineral- and
glass analyses have been made.