47

Thickness
Feet

10 per cent is quartzite, grey, relatively pure ..........-.0.e-ececeeceeece 700

DOrphyrODLasts™ pre sc Fate eee ee Ae ere ee ue er nana 2,000
Quartzite, grey to buff-coloured; about 40 per cent muscovite schist, red-
brown to golden, soft, coarse-grained; minor quartz-muscovite schist, grey 500

Quartzite, light creamy grey; about 20 per cent quartz-mica schist, golden brown,
typically in beds 1 foot to 10 feet thick; sections as much as 200 feet thick
of quartzite contain no interbedded schist ................0.ceeeeeeeeees 500
Muscovite-biotite schist, crumpled; quartz-biotite schist, platy; quartzite,
slightly micaceous; all three rock types intimately interbedded in about
Equalpropontionsmansmm nie aa rots ees Ee een eee nce 500

MerTAMORPHISM
SEQUENCE OF METAMORPHIC CHANGES

The Tenakihi group sedimentary strata, together with those of the
overlying Ingenika group, have been altered by regional metamorphism
that has produced remarkably uniform effects in all parts of the map-area
where these rocks are exposed. There is a gradation from slightly altered
sediments of the highest Ingenika group beds to relatively high-
grade metamorphic rocks of the lowermost beds of the Tenakihi group.
Throughout this sequence, the boundaries between metamorphic zones
approximately coincide with stratigraphic horizons.

It would appear that the Tenakihi group rocks were originally
argillaceous and arenaceous sediments, all relatively rich in silica. The
remarkably uniform regional metamorphism to which they have been sub-
jected has recrystallized and reconstituted their component minerals under
conditions in which temperature, hydrostatic pressure, and stress have
reached a given equilibrium at, so far as can be determined, very nearly the
same stratigraphic horizon over the whole area studied.

The schists, probably derived from shaly and silty sediments, are the
most sensitive to changes in metamorphic conditions. They demonstrate,
from highest to lowest stratigraphic levels in the Tenakihi group, changes
almost exactly corresponding to those regarded as typical of argillaceous
sediments under increasing temperature and hydrostatic pressure, subject
to the maximum possible shearing stress for each stage. The Tenakihi
group rocks range, under this scheme, from the ‘biotite zone’ of Barrow
(1912) and others, as shown by the stratigraphically highest beds, just
into the ‘sillimanite zone’ in the lowest horizons.

In the stratigraphically highest beds, the quartz has been almost
completely recrystallized (even the quartzites are crystalloblastic and show
a certain degree of schistosity) and the chlorite, which is a predominant