New Zealand Journal of Geology and Geophysics abstracts
Age and isotopic characterisation of geological terranes in Marlborough Schist,
Nelson/Marlborough, New Zealand
C. J. ADAMS
I. J. GRAHAM
M. R. JOHNSTON*
Institute of Geological & Nuclear Sciences
P.O. Box 31 312
Lower Hutt, New Zealand
*Present Address: Soils & Foundations Ltd., P.O.Box 1085, Nelson, New
Zealand.
Abstract K-Ar and Rb-Sr ages of Marlborough Schist and
Pelorus Group metasediments show coherent regional metamorphic/cooling age
patterns, irrespective of their terrane protolith, in which ages are primarily
inversely related to metamorphic grade and, more locally, with distance from
the Picton Fault Zone. The oldest ages, from the lowest grade
(prehnite-pumpellyite facies, textural grade I/IIA) Pelorus Group are mainly in
the range 175-200 Ma, indicating regional metamorphism occurred at least
in earliest Jurassic or latest Triassic times. Progressive cooling of
Marlborough Schist resulted in a spectrum of cooling ages, mainly
110-180 Ma, which reflects slow regional uplift during the Jurassic and
Early Cretaceous. Some disruption of this pattern occurred at the Picton Fault
Zone where post-Early Cretaceous movement has juxtaposed high and low
structural levels. Initial 87Sr/86Sr isotopic ratios (i)
at the time of metamorphism (t), derived from Rb-Sr whole-rock isochron data,
further characterise terrane protoliths identified by field, petrographic, and
geochemical evidence. The (t)-(i) data largely confirm these prior regional
groupings but do not entirely confirm the terrane identifications. A
less radiogenic group, (i) < 0.7055, termed Western Marlborough
Schist, comprises the Pelorus Group, and schist equivalents, west of a
postulated suture/terrane boundary in the schists, and is correlated with
Caples Terrane rocks in Otago. "Torlesse"-type schists are more radiogenic, (i)
> 0.7062, but do not match Torlesse (Rakaia) Terrane (t)-(i) data from
Wellington or Otago. They correlate better with Kaimanawa Schist of the central
North Island. An intermediate group, (i) = 0.7042-0.7062, is separate
from the "Torlesse" group, but partly overlaps the Caples Terrane group. This
correlates well with Waipapa Terrane of the central North Island and Haast
Schist of uncertain terrane affinity in Otago. The combined radiogenic and
intermediate groups, termed Eastern Marlborough Schist, define a
lineage which thus includes Kaimanawa Schist, Pahau Subterrane rocks,
and both Waipapa-type and Torlesse-type schists in Marlborough. It is
possible that the Moonlight Fault Zone of Otago has a continuation in Eastern
Marlborough Schist as the Picton Fault Zone.
Keywords Marlborough; Nelson; Caples Terrane; Torlesse
Terrane; Waipapa Terrane; K-Ar geochronology; Rb-Sr geochronology; strontium
isotopes; Picton Fault Zone
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42: 33-55
0028-8306/4201-0033 $7.00/0 (c) The Royal Society of New Zealand 1999
PDF file of entire paper: medium quality (3584K); (scanned from paper original: notes about this process)
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