Abstract
White mica (phengite and paragonite) K-Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84-89 Ma (Seba, central Shikoku), 78-80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82-88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K-Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K-Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K-Ar ages are attributed to excess 40Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high 40Ar/36Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme 40Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K-Ar closure temperature.
| Original language | English |
|---|---|
| Pages (from-to) | 1014-1022 |
| Number of pages | 9 |
| Journal | International Geology Review |
| Volume | 57 |
| Issue number | 5-8 |
| DOIs | |
| Publication status | Published - Jun 11 2015 |
Fingerprint
Keywords
- eclogites
- excess argon
- Pacific-type orogen
- phengite K-Ar geochronology
- Sanbagawa metamorphic belt
ASJC Scopus subject areas
- Geology
Cite this
White mica K-Ar geochronology of Sanbagawa eclogites from Southwest Japan : Implications for deformation-controlled K-Ar closure temperature. / Itaya, Tetsumaru; Tsujimori, Tatsuki.
In: International Geology Review, Vol. 57, No. 5-8, 11.06.2015, p. 1014-1022.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - White mica K-Ar geochronology of Sanbagawa eclogites from Southwest Japan
T2 - Implications for deformation-controlled K-Ar closure temperature
AU - Itaya, Tetsumaru
AU - Tsujimori, Tatsuki
PY - 2015/6/11
Y1 - 2015/6/11
N2 - White mica (phengite and paragonite) K-Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84-89 Ma (Seba, central Shikoku), 78-80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82-88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K-Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K-Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K-Ar ages are attributed to excess 40Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high 40Ar/36Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme 40Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K-Ar closure temperature.
AB - White mica (phengite and paragonite) K-Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84-89 Ma (Seba, central Shikoku), 78-80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82-88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K-Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K-Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K-Ar ages are attributed to excess 40Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high 40Ar/36Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme 40Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K-Ar closure temperature.
KW - eclogites
KW - excess argon
KW - Pacific-type orogen
KW - phengite K-Ar geochronology
KW - Sanbagawa metamorphic belt
UR - http://www.scopus.com/inward/record.url?scp=84928542834&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84928542834&partnerID=8YFLogxK
U2 - 10.1080/00206814.2014.973915
DO - 10.1080/00206814.2014.973915
M3 - Article
AN - SCOPUS:84928542834
VL - 57
SP - 1014
EP - 1022
JO - International Geology Review
JF - International Geology Review
SN - 0020-6814
IS - 5-8
ER -