 |
| Titel |
Melt segregation evidence from a young pluton, Takidani Granodiorite (Japan) |
| VerfasserIn |
Eva Hartung, Luca Caricchi, David Floess, Simon Wallis, Satoru Harayama, Massimo Chiaradia, Kalin Kouzmanov |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128137
|
| Publikation (Nr.) |
 EGU/EGU2016-8090.pdf |
|
|
|
|
|
| Zusammenfassung |
| We are presenting new petrological data from one of the youngest exposed plutons in the
world, the Takidani Granodiorite (Japan), which has been suggested as a source for large
volume ignimbrites (> 300km3). Takidani Granodiorite (1.54 Ma ± 0.23 Ma) is located
within the active Norikura Volcanic Chain in the Northen Japan Alps and has been
previously linked to large andesitic (1.76 Ma ± 0.17 Ma) and rhyolitic eruptions
(1.75 Ma ± 0.17 Ma). The pluton is vertically zoned and consists of granites (67
to 68 wt.% SiO2) in the lower section, granodiorites (65 to 66 wt.% SiO2) in the
middle section, a chemically more evolved fine-grained porphyritic unit (67 to 71
wt.% SiO2) near the roof and a marginal granodiorite at the roof (67 to 68 wt.%
SiO2). The porphyritic texture of the more evolved unit near the roof indicates rapid
crystallisation, which could be the result of the late intrusion of this unit at the roof of
the magmatic system. However, no sharp contact is found between the underlying
granodiorite and the porphyritic unit. Instead, a gradual change in rock fabric, whole-rock
chemistry and mineralogy is observed suggesting that melt was extracted from the
granodiorite.
Electron microprobe analyses of plagioclases show three main crystal populations (Type
I, II and III) with distinct anorthite and Fe contents. Type I plagioclase (An30−40)
occurs dominantly within the marginal granodiorite at the roof. Type II plagioclase
(An40−45) are common in the granodiorite and porphyritic unit. Type III plagioclase
(An45−50) is predominantly present in the granite. All plagioclase populations
share a common sodic rim (An22) across the different units. Takidani Granodiorite
rocks are compared to crystallisation experiments from similar magmatic suites.
Emplacement conditions of the Takidani Granodiorite are obtained from the latter
as well as barometry, thermometry and hygrometry indicating that magmas were
ultimately emplaced at around 200 MPa, 850˚ C to 875˚ C and contained about 4 to 5
wt.% H2O. Major and trace elements whole rock analyses together with mineral
chemistry suggest that residual melt was extracted between 780˚ C and 730˚ C at
crystallinities between 40 to 60 vol.%. The fine-grained matrix of the porphyritic unit
indicates rapid crystallisation which may be the result of pressure quenching through
eruption.
We are currently working on the volcanic products putatively associated with the
Takidani pluton, to determine whether the porphyritic unit was actually the source
of large eruptions that produced more than 300km3 of volcano-clastic deposits. |
|
|
|
|
|
|