New Zealand Journal of Geology and Geophysics abstracts
Age of metamorphism of Otago Schist in eastern Otago and determination of
protoliths from initial strontium isotope characteristics
C. J. ADAMS
I. J. GRAHAM
Institute of Geological & Nuclear Sciences
P.O. Box 31 312
Lower Hutt, New Zealand
Abstract Geochronological studies of the eastern Otago Schist
and adjacent metagreywacke sequences are applied to their
structural/metamorphic history and schist protoliths. Rb-Sr whole-rock isochron
and K-Ar total-rock ages are inversely correlated with metamorphic grade (both
in terms of mineral facies and textural zones), dating the time of peak
metamorphism and/or postmetamorphic cooling. On the northern flank of the
schists, older ages (200-230 Ma) date initial burial metamorphism and
deformation in late Triassic - early Jurassic times. Younger age patterns (to
115 Ma) reflect either (1) higher grade metamorphism during a single event
followed by long-continued uplift, or (2) early burial metamorphism at
>200 Ma with a second, regional metamorphism between then and
115 Ma followed by rapid uplift. Initial Sr isotopic ratios derived from
isochrons indicate that protoliths originate in two terranes. North of Dunedin,
initial 87Sr/86Sr ratios range from 0.7064 to 0.7092,
similar to Torlesse Terrane (Rakaia Subterrane) analogues in the Wellington
region, and indicate a granitoid source dominated by I-types. South of Dunedin,
along the south Otago coast, schists have lower initial
87Sr/86Sr ratios, 0.7052 to 0.7064, unlike Caples-type
rocks elsewhere in Otago. They are thus of "uncertain" terrane affinity but
indicate a dominantly silicic-intermediate volcanic source.
Keywords Rb-Sr; K-Ar; absolute age; Sr isotopes; Otago
Schist; metagreywacke; Torlesse; Caples
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 275-286
0028-8306/97/4003-0275 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (2331K); (scanned from paper original: notes about this process)
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