caldera-forming eruptions
5.3 The Volcanic Units
monotonous magmas; Huber et al. 2012 and references therein) and a nearly 2 km thick section of its potential magma reservoir (i.e. Takidani Pluton; Harayama 1992) are exposed. The eruptive units include the Nyukawa Pyroclastic Flow Deposit (PFD) and the Chayano-Ebisutoge Pyroclastic Deposit (PD), some of the largest eruptions in Central Japan near the Plio-Pleistocene boundary (1.75–1.76±0.17 Ma; fission track;
Harayama et al. 1991, Nagahashi et al. 2000). The volcanic units comprise pyroclastic material of about 500 km3DRE (Kimura & Nagahashi 2007, Oikawa 2003), and are preserved in stratigraphic order in the Takayama Basin (Figure 5.1a; Nagahashi 1995, Nagahashi et al. 2000). The deposits change from orthopyroxene-clinopyroxene-bearing crystal-rich dacite (Nyukawa PFD) to orthopyroxene-hornblende orthopyroxene-clinopyroxene-bearing crystal-poor rhyolites (Chayano-Ebisutoge PD) in 10 kyr (Kimura & Nagahashi 2007).
The welded ignimbrites and co-ignimbrite ash fallout of the three deposits spatially overlap and distribute concentrically around what is supposed to be a common eruption centre, located about 40 km north-east Hotake-Dake (Figure 5.1b; Kataoka et al. 2001, Nagahashi et al. 2000), where the young Takidani Pluton is located.
Here we explore the relationships between the volcanic and plutonic units of the Takidani Pluton and Nyukawa PFD and Chayano-Ebisutoge PD using major and trace element chemistry of whole rock, glass and minerals. The fundamental questions that we address in this contribution include: (1) is Takidani Pluton the exhumed magma reservoir that fed the Nyukawa and Chayano-Ebisutoge eruptions?
(2) How were the Nyukawa dacite the Chayano-Ebisutoge rhyolites generated? (3) Was the Chayano-Ebisutoge PD generated by melt extraction from the Takidani Pluton? Our study provides new insights into silicic magmatism and the generation of caldera-forming eruptions.
5.3 The Volcanic Units
5.3.1 The Nyukawa PFD
The Nyukawa Pyroclastic Flow Deposit (PFD) is a thick welded dacite flow unit exposed in the Takayama basin (Figure 5.2a) and Matsumoto basin, of up to 100 m thickness (Kimura & Nagahashi 2007, Nagahashi 1995, Nagahashi et al. 2000). The erupted volume has been estimated to exceed 400 km3 DRE (Oikawa 2003). The chemical signature and spatial distribution of correlated tephra beds (Hotaka-Kd 39) point towards an eruption centre in the Hotaka area in the central Northern Japan Alps (Kataoka et al. 2001, Nagahashi et al. 2000). The age of the Nyukawa PDF is estimated at 1.76 ± 0.17 Ma using fission track dating (Harayama 1999, Nagahashi et al. 2000). The Nyukawa PDF is crystal-rich (about 30–40%) and contains plagioclase, orthopyroxene, clinopyroxene, iron oxides and trace amounts of quartz and hornblende. Zircon and apatite are common accessory phases.
eruptions
250 0 250 500 7501000 m
(b) (a)
FIGURE5.1: Location (a) and geological map (b) of the Takidani Pluton and associated volcanic deposits. Inlet shows overview location in Central Japan. Active volcanoes of the Norikura Volcanic Chain are highlighted in white. Sample locations are shown with white circles.
Faults are indicated with black lines in (b). Grey lines indicate contours of 50 m.
5.3. The Volcanic Units 85
mafic enclave Nyukawa PFD
Chayano Tuff Ebisutoge PD
aplitic dike granodiorite
melt segregation
granodiorite
mafiic enclave granodiorite
Hotaka Andesite
quartz veins Unit B
Unit A Unit D
Unit C
(a) (b)
(c) (d)
(e) (f)
FIGURE5.2: Proximal deposits of the Nyukawa Pyroclastic Flow De-posit (a) and Chayano-Ebisutoge Pyroclastic DeDe-posit in the Takayama Basin. (c) Leucrocratic segregation with porphyritic plagioclase in granodioritic host rock observed on large boulders in the Shiradaashi-zawa valley. (d) Aplitic dike cross-cuts the granodiorite unit of the Takidani Pluton and embedded rounded enclave. (e) Mafic-felsic inter-action in the Takidani Pluton and break-up of a mafic injection in the granodiorite, observed in a large boulder in the Shiradashi-zawa val-ley. (f) Intrusion of quartz veins and fracturing of the Hotaka Andesite
welded-tuff at the roof of the Takidani Pluton.
eruptions
5.3.2 The Chayano-Ebisutoge PD
The Chayano Tuff and Ebisutoge Pyroclastic Deposit (PD) are a sequence of rhy-olitic voluminous ash fall and pyroclastic flow deposits (Figure 5.2b). The Chayano-Ebisutoge PD has an estimated eruptive volume of 380–490 km3(Kimura & Nagahashi 2007, Nagahashi et al. 2000) with a DRE of >100 km3(Kataoka et al. 2001). The outflow deposits are found in the Takayama Basin and Matsumoto area, and sit on top of the Nyukawa PDF (Kataoka et al. 2001, Nagahashi et al. 2000). The Chayano Tuff and Ebisutoge PD have been correlated with the Fukuda-Kd38 ash (Kataoka et al. 2001), a widespread marker bed distributed across Honshu Island. The stratigraphic location of the Fukuda tephra beds in sea sediments suggest that the rhyolites erupted only 10 kyr after the Nyukawa PFD at about 1.75 Ma (Harayama 1999, Nagahashi et al. 2000, Yoshikawa et al. 1996). The overlapping spatial distribution of pyroclastic deposits suggest that the Chayano–Ebisutoge PD was erupted from the same location of the Nyukawa PDF (Nagahashi et al. 2000).
The Chayano Tuff consists of pyroclastic fall (Chayano Tuff I) and non-welded flow deposits (Chayano I and II). The deposit is up to 4 m thick (Figure 5.2b) and associated with phreato-plinian activity (Kataoka et al. 2001). The Chayano Tuff contains plagioclase, iron oxides, hornblende, orthopyroxene and traces of clinopy-roxene, biotite and zircon (Nagahashi et al. 2000). The Ebisutoge PD comprises four main units (Figure 5.2b; Nagahashi 1995, Nagahashi et al. 2000): Unit-A, a 20 cm thick pumice fall out containing small amounts of rock fragments; Unit-B, a up to 10 m thick unconsolidated flow unit containing pumice and rock fragments; Unit-C, a 10 to 20 cm thick pumice fall out containing small amounts of lithics; and Unit-D a >30 m thick welded ignimbrite containing fiamme (Nagahashi et al. 2000). The Ebisutoge PD is characterised by the abundance of plagioclase, iron oxides (magnetite and trace amounts of ilmenite), orthopyroxene and trace amounts of amphibole, biotite, clinopyroxene and quartz. Zircon is present as accessory phase. Detailed stratigraphic description and mineral fractions of the proximal and distal deposits are presented in Kataoka et al. (2001), Nagahashi (1995) and Nagahashi et al. (2000).
Chemical and mineralogical data are presented in Kimura & Nagahashi (2007).
5.4 The Takidani Pluton
The Takidani pluton is one of the youngest exhumed intrusions on Earth (1.6 Ma
±0.1, U-Pb ages of in-situ and detrital zircons; Ito et al. 2017, Harayama 1992). It is located between Yake-Dake and Hotaka-Dake in the Hida Mountain Range in the central Northern Japan Alps (Figure 5.1). The Takidani Pluton is emplaced at the base of the Hotaka Andesite, a co-genetic series of tuffs, lavas and porphyritic sills constituting a caldera fill sequence. High exhumation rates of the Central Alps and eastward tilting of the pluton lead to an estimated 2 km vertical exposure of this zoned intrusion (Harayama et al. 2003).