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Rb-Sr resetting ages and chemical characterization of turbidites in an accretionary wedge; Torlesse Complex, Otaki Gorge, New Zealand; with Suppl. Data 89-07

Ian J. Graham and R. J. Korsch
Rb-Sr resetting ages and chemical characterization of turbidites in an accretionary wedge; Torlesse Complex, Otaki Gorge, New Zealand; with Suppl. Data 89-07
Geological Society of America Bulletin (March 1989) 101 (3): 355-363

Abstract

Whole-rock geochemical and Rb-Sr isotopic analyses have been undertaken on turbidites from three discrete localities, each about 2 km apart across strike, within the Torlesse Complex, a Permian-Cretaceous accretionary wedge in New Zealand. Overall, the three suites show similar petrographic and geochemical compositions to turbidites from elsewhere in the Torlesse Complex, but between the suites, there are detectable differences for most major and trace elements, particularly Sr, MgO, and Na (sub 2) O. The individual suites define three parallel Rb-Sr whole-rock isochrons with identical dates of about 205 Ma. The present of Monotis bivalves provides a depositional age of Norian (219-225 Ma). Hence, the isochrons suggest that the Rb-Sr clock was reset some 15-20 m.y. after deposition, during low-grade metamorphism associated with accretion and deformation of the sediment pile. There is a wide variation in the initial (super 87) Sr/ (super 86) Sr ratios from 0.70683 to 0.70955, which is best explained by changes in the composition of the detritus being provided from the source area, as detected by the chemical differences. Sea water is not considered to have played a significant role in contributing to the initial ratios. The failure of some samples collected away from the main sample localities to plot on their respective isochrons suggests that domains of Sr isotopic equilibration are small, in the order of a few cubic meters. As a result of this study, we have extended the Rb-Sr whole-rock dating method of sediments, now widely applied to shales and cherts, to the more complex turbidite sequences from accretionary wedges and have shown that meaningful dates can be obtained. This should provide an invaluable method of dating these sparsely fossiliferous terranes.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 101
Serial Issue: 3
Title: Rb-Sr resetting ages and chemical characterization of turbidites in an accretionary wedge; Torlesse Complex, Otaki Gorge, New Zealand; with Suppl. Data 89-07
Affiliation: DSIR, Inst. Nucl. Sci., Lower Hutt, New Zealand
Pages: 355-363
Published: 198903
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 70
Accession Number: 1989-028765
Categories: GeochronologyGeochemistry of rocks, soils, and sedimentsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
S47°30'00" - S34°30'00", E166°30'00" - E178°30'00"
Secondary Affiliation: Victoria Univ. Wellington, NZL, New Zealand
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by Institute of Geological and Nuclear Sciences Limited (GNS Science), Lower Hutt, New Zealand
Update Code: 1989
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