Quick
Search: 		  
advanced search
 GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help 
  Geological Magazine   Signup for GSW Email News
JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Geological Magazine; September 2008; v. 145; no. 5; p. 677-684; DOI: 10.1017/S0016756808004998
© 2008 Cambridge University Press (CUP)
This Article
Right arrow Figures Only
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by NUONG, N. D.
Right arrow Articles by NISHIMURA, Y.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation

Age (K–Ar phengite)–temperature–structure relations: a case study from the Ishigaki high-pressure schist belt, southern Ryukyu Arc, Japan

N. D. NUONG*, T. ITAYA*,{dagger} and Y. NISHIMURA{ddagger}

* Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan
{ddagger} Miyanoshimo 1648-1, Yamaguchi 753-0011, Japan

{dagger} Author for correspondence: itaya{at}rins.ous.ac.jp

The Ishigaki high-pressure schist belt in the southern Ryukyu Arc is correlated with the Suo high-pressure schist belt in southwest Japan. The former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones, the lower-grade zone A, the medium-grade zone B and the high-grade zone C, based on the mineral assemblages of the basic schists. The K–Ar phengite age gives 188–205 Ma for zone A, 196–206 Ma for zone B and 208–220 Ma for zone C, while the apparent d002 spacing of carbonaceous materials is 3.590–3.437 Å, 3.415–3.390 Å and 3.387–3.364 Å, respectively. The age–d002 relationships suggest that the ages become older with increasing metamorphic temperature. This positive age–temperature relationship in the Ishigaki area contrasts with a negative relationship in the Nishiki area in the Suo belt. The two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure. These contrasting age–temperature–structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences. We suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature (500 °C) during exhumation of the host rocks. The duration from the beginning of exhumation to the apparent resetting of the phengite K–Ar system was different between the two metamorphic sequences, and significantly longer in the Ishigaki than the Nishiki.

Key Words: Suo metamorphic belt • Ishigaki high-pressure schists • K-Ar phengite age • d002 spacing of carbonaceous materials • ductile deformation • exhumation tectonics • age-temperature-structure relations






JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2009 by Cambridge University Press (CUP)