nattanen-type - arkisto.gsf.fi - Geologian tutkimuskeskus

gneiss surroundings (Fig.1). They are discordant, rounded intrusions composed of
various leucocratic granite phases (Figs. 4, 8, 10 and 13) often exhibiting a zonal
arrangement. Erosion has exposed much of the upper parts of the intrusions, as is
indicated by the presence of rhyolite and quartz- porphyry dykes, miarolitic cavities,
country-rock screens and local greisening with MO mineralization. The mode of
occurrence of the Nattanen-type intrusions is characteristic of A-type, or anorogenic,
granites.
The oldest phases are coarse-porphyritic or coarse-grained granites and are followed
by porphyritic and/or medium-grained granite phases. These main types are monzo-
granites, which differ from each other in texture and geochemistry rather than in main
mineral composition. The youngest phases in the Nattanen-type complexes are abundant
cross-cutting quartz-porphyry and aplite dykes. Numerous texturally variable spherulitic
and flow-banded rhyolite dykes are found in the high-grade gneisses in the immediate
vicinity of the Nattanen stock (Fig. 6).
The granites are two-feldspar granites with biotite as the principal dark mineral
(Tables 2, 4, 7, 12 and 13). Hornblende is present only in coarse-porphyritic phases and
in certain fine-grained autoliths. Typical minor minerals are magnetite and sphene.
Accessory minerals include zircon, apatite, ilmenite, allanite, monazite and fluorite.
Common secondary minerals are muscovite, chlorite and epidote.
Alkali feldspar is cross-hatched microcline perthite and plagioclase is zoned oligoclase.
Plagioclase-mantled angular alkali feldspar grains are common, and phenocrysts or
perthite-mantled plagioclase grains have been recognized occasionally. The abundance
of biotite is usually less than 5%. The magnetite contents are generally high, decreasing
from the oldest granite phases to the youngest and allowing a first-order subdivision of
granite types on the basis of susceptibilities (Tables 3, 6, 8 and 15). The mineralogical
characteristics are those of I-type, or magnetite-series, granites.
Geochemistry
The Nattanen-type granites have high SiO, (> 70 wt.%), total alkalies and FeO*/
MgO, and a small but regular variation in major oxides (Table 16, Fig. 24). The silica
content increases, but that of TiO,, FeO*, MgO and CaO decreases towards the evolved
phases. The granites show alkaline affinities (Figs. 19 and 20) and are slightly
peraluminous, with the majority of A/CNK values being between 1.0 and 1.1 (Fig. 22).
Trace elements show a large variation and illustrate the evolution of the complexes in
greater detail than do the major oxides (Table 16). The content of Ba, Sr, Zr and La
decreases while that of Rb increases from the oldest granite phases to the youngest. In
terms of Rb, Ba and Sr, the oldest granites are a normal granite type. In terms of Rb
enrichment and Ba depletion, however, the youngest and most evolved phases are
specialized or strongly differentiated granites (Fig. 21).
Bulk geochemistry indicates that the Nattanen-type granites are largely of the I-type,
but toward the most evolved phases there is usually a scatter of the A-type and the
S-type (Fig. 22).
Alteration and MO mineralization
The cross-cutting and brecciating MO-bearing aplite granite is the youngest phase of
the Tepasto stock. In contrast to the other Nattanen-type complexes, the aplite granite
shows recrystallization and deformation textures. Alteration is encountered in varying
degrees of intensity and extent. Local quartz-muscovite greisens, small fissures and
patches filled with quartz and/or mica are enveloped in more wide-spread weak
sericitization (Fig. 17). Greisen bodies occur as small (0 - 10-30 cm) irregular zonal
spheres or pipes with quartz-rich margins transitionally changing to muscovite-rich
centres. The greisen bodies grade into relatively unaltered aplite granite within very short
distances.