New Zealand Journal of Geology and Geophysics abstracts
Relationships between the Brook Street Terrane and Median Tectonic Zone (Median
Batholith): evidence from Jurassic conglomerates
A. J. TULLOCH
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
D. L. KIMBROUGH
Department of Geological Sciences
San Diego State University
San Diego, CA 92182, USA
C. A. LANDIS
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
N. MORTIMER
Institute of Geological & Nuclear Sciences
Dunedin, New Zealand
M. R. JOHNSTON
395 Trafalgar Street
Nelson, New Zealand
Abstract U-Pb zircon ages of 237-180 Ma and c.
280 Ma of seven granitoid clasts from the Rainy River Conglomerate which
lies within the eastern Median Tectonic Zone (Median Batholith) in Nelson, and
the Barretts Formation of the Brook Street Terrane in Southland, constrain the
depositional ages of both units to be no older than c. 180-200 Ma
(Early Jurassic). The minimum age of the Rainy River Conglomerate is
constrained by the 147 +2 -1 Ma (latest Jurassic) emplacement age of the
One Mile Gabbronorite (new name: previously western Buller Diorite). The
ages and chemistry of five of the granitoid clasts are broadly compatible with
derivation from rocks that are now represented by Triassic plutons of the
Median Tectonic Zone (Median Batholith), although ages as young as 180 Ma
are slightly outside the range of the latter as currently exposed in New
Zealand. The age (273-290 Ma, 237 +/- 3 Ma) and chemistry of the
other two clasts (one each from Rainy River Conglomerate and Barretts
Formation) suggest derivation from the Brook Street Terrane. Similarity in
stratigraphic age, depositional characteristics, granitoid clast ages and
composition between Rainy River Conglomerate and Barretts Formation suggests
that they are broadly correlative and collectively overlapped a combined Brook
Street Terrane - Median Batholith (MTZ) before the Late Jurassic (147 +2
-1 Ma). Sedimentary overlap may also have continued across to Middle
Jurassic conglomeratic strata in the Murihiku Terrane to the east of the Brook
Street Terrane. A U-Pb zircon age of 261 +/- 2 Ma is reported for
Pourakino Trondhjemite of the Brook Street Terrane.
Keywords conglomerate; granite clasts; U-Pb dating;
geochronology; Jurassic; provenance; Brook Street Terrane; Median Tectonic
Zone
New Zealand Journal of Geology and Geophysics, 1999, Vol. 42: 279-293
0028-8306/99/4202-0279 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (1446K); (scanned from paper original: notes about this process)
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