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POST-LOWER CAMBRIAN PORPHYRY INTRUSIONS

Granophyre and Quartz-feldspar Porphyry

Small distinctive bodies of granophyre and feldspar-quartz porphyry of
dacitic and rhyolitic composition cut the Tenakihi and Ingenika group
strata in several places. They are most abundant in Tenakihi Range, where
they form an irregular stock about 2,000 feet in diameter south of Jim
May Creek, and many sills and dykes, mostly less than 50 feet thick, in the
surrounding rocks. North of Mesilinka River rocks of this type occur as
dykes and small irregular masses in the grey granodiorite stock northeast
of Blackpine Lake, and as sills up to 30 feet thick and small stocks less than
10 acres in exposed area in the Tenakihi group rocks on Chase Mountain.

All of these rocks are very similar in appearance and composition. They
are medium grey, weathering a light pinkish grey to grey-brown, almost all
porphyritic, with grey to buff-coloured feldspar phenocrysts up to 2 mm.
long. The proportion of feldspar, phenocrysts ranges from about 10 to
nearly 100 per cent. About half of these rocks contain scattered grains of
quartz up to 1 mm. in diameter. Some rocks contain up to 15 per cent
of smaller phenocrysts of a ferromagnesian mineral, which in some speci-
mens is hornblende, and in others biotite, partly to completely altered to
chlorite. The groundmass is light grey, with a dull, earthenware-like
appearance, and is seen in thin section to be a fine-grained aggregate of
quartz and feldspar, much of it in a granophyric intergrowth. All the rocks
are somewhat altered, despite their fresh appearance in outcrop; the feld-
spars are so clouded with sericitic and kaolinitic material that it is difficult
to determine their composition. All the feldspars examined have a mean
refractive index of less than 1-544: many exhibit good polysynthetic twin-
ning, and a few appear to show perthitic intergrowths. The phenocrysts
are probably oligoclase, orthoclase or microcline, and perthite. The
granophyric intergrowths in the groundmass are commonly exceedingly fine
grained, and where best developed form more or less spherical shells with
a semi-radiating structure around rounded grains of quartz. Ferromag-
nesian minerals, represented by hornblende and biotite, make up less than
10 per cent of any of the specimens examined, and are generally found only
as small phenocrysts; the groundmass of most. specimens appears to be
free of dark minerals, except for a little magnetite and secondary chlorite.
Secondary minerals include sericite, chlorite, clinozoisite, and unidentified
cloudy clay-like material in the feldspars.

None of the rocks examined showed any preferred orientation or

flowage textures. All exhibit sharp, commonly chilled borders against the
rocks they intrude.

The feldspar-quartz porphyries and granophyres cut the grey grano-
diorite northeast of Blackpine Lake, and were, therefore, intruded later than
the period of formation of the Wolverine complex rocks. Though altered,
they show no evidence of metamorphic recrystallization, and are thus pre-
sumably later than the metamorphism of the Tenakihi and Ingenika group
rocks. They are younger than some of the shear zones that cut the meta-
morphosed rocks. The rocks resemble some of the early Tertiary, Kastberg