3
.
3
. Felsic fragmental rocks unit F
3
.
3
.
1
. Field description This unit is composed of felsic pyroclastic rocks
and subordinate
reworked equivalents
Kozhevnikov, 1992. In this section we provide the field evidence for interpretation of the deposi-
tional environment of the unit. In the vicinity of location B, the sequence consists of metre scale
units of tuff breccia and lapilli tuff with occa- sional tuff intercalations. Primary textures and
structures indicate the presence of both pumice and lithic fragments. Each unit is defined by
distinct size ranges of clasts, texture of pumice, a distinct phenocryst content and a regular succes-
sion of size and density grading. Within individual units as defined above, reverse grading of pumice,
normal grading of lithic fragments, sporadic ex- amples of basal ground surge beds and thinly
laminated
fine ash
tuff beds
occur which
indicate the units represent ignimbrite deposition Sparks et al., 1973. Evidence for welding con-
sists of the presence of branching fumarolic struc- tures cf. Thurston, 1980 and the presence of
flattened silicified pumice fragments concentrated toward the middle of the stratigraphic unit. The
silicified
pumice ignores
depositional unit
boundaries and is thus interpreted as evidence of vapour phase recrystallization Ross and Smith,
1961 indicative of subaerial eruption and deposi- tion.
At location A, in a few exposures, the quartz arenite is overlain by a few metres of a fragmental
aluminous metaconglomerate with granitoid and possible metavolcanic clasts. This is overlain by
about a 100 m thickness of thin graded beds of sulfidic argillite and carbonate bearing silty
sandstones.
3
.
3
.
2
. Petrography The rhyolites at location B consist of varying
proportions of quartz, plagioclase and minor potassium feldspar with accessory biotite and
sericite. Primary textures are completely obliter- ated by metamorphic recrystallization but gross
grain size variation seen mesoscopically is present in thin section. Silicified pumice fragments contain
irregular polycrystalline plates of quartz whereas the matrix contains finer rained quartz, feldspar,
biotite and sericite.
3
.
3
.
3
. Geochemistry The sole analysis of the rhyolitic unit done for
the present study shows the unit to be a high silica rhyolite with potassium dominant over sodium
Table 3. In trace element terms, the unit displays a fractionated REE pattern similar to the FI
rhyolites of Lesher et al. 1986.
4. Summary and conclusions
The Hisovaara quartz arenites represent a mixed provenance involving contributions from
TTG suite granitoids and a mafic to ultramafic component with extensive weathering to explain
the lack of feldspar in the sandstones. Mature, quartzose, shallow-water sandstones are not com-
mon in Archean greenstones Thurston and Chivers, 1990; Lowe, 1994. The quartz-rich sand-
stones at Hisovaara are unusual in showing clearly the base of the sequence and evidence for
weathering of the andesitic basement seen in field, petrographic and geochemical evidence. Most
Archean quartz-rich sandstones are associated with platforms De Kemp, 1987; Thurston and
Chivers, 1990 with one example of a cannibalized platform within a submarine fan environment
Cortis, 1991. As an Archean quartz rich sand- stone sequence, the Hisovaara quartz arenites are
closely associated with subaerial arc andesites and subaerial rhyolites at the south end of the belt
similar to the ‘continental’ style assemblage type of Thurston 1994. However, at the north end of
the belt, the quartz arenites are succeeded upward by conglomeratic rocks, argillites, and an overly-
ing tholeiite unit with komatiites. This end of the unit is then comparable to some of the Superior
Province
platformal quartz
arenites in
that quartz-rich sedimentation is followed by volcan-
ism cf. De Kemp, 1987. Thus the Hisovaara quartz-rich sandstones demonstrate a relationship
in an Archean setting between subaerial volcan- ism in an arc setting and development of an
deepening basin and subsequent volcanism related to rifting.
Post Archean quartz-rich sandstones are con- ventionally
considered to
represent multiple
passes through the sedimentary cycle Pettijohn et al., 1972, p. 298 with the interplay of climate,
relief, and provenance influencing the composi- tion of the sands Basu, 1985. Recent work in the
Orinoco basin has demonstrated the production of single cycle quartz arenites in a regime of
intense chemical weathering Johnsson et al., 1988. The process involves either long soil resi-
dence times related to very low erosion and trans- port rates or storage of orogenically derived
sediments on alluvial plains enroute to the final depositional site. In spite of the variety of mecha-
nisms for production of quartz rich sandstones, we here use their presence in the Hisovaara green-
stone belt to indirectly indicate the presence of granitoid rocks in the source area. A granitoid
source area serves as a speculative indicator of at least unroofing of plutons if not a possible cra-
tonizing or orogenic event. If the latter is the case, the age constraints available in this greenstone
belt suggest the possibility of a pre-2.7 Ga oro- genic event in the Baltic shield. Much additional
work is required to validate such a concept, but the tantalizing indication seen in this project will
perhaps point the way to further work on this speculation.
5. Geochemical methods
