Whole-rock chemistry

Compositions of the different plutonic and volcanic units are presented in Table D.1 and Table D.7. Bulk rock compositions of the Takidani Pluton and coexisting volcanic deposits are sub-alkaline and range from dacitic to rhyolitic compositions (Figure 5.3).

Mafic enclaves in the Takidani Pluton cover the range from alkaline to subalkaline with (basaltic) andesite and (basaltic) trachy-andesite compositions (Figure 5.3, Ta-ble D.5, D.6). The Hotaka Andesite and Nyukawa PFD form the relatively low silica end-members with silica contents of pumice and whole rock ranging between 63 to 65 wt.% SiO2and 62 to 68 wt.% SiO2, respectively (Figure 5.5). The composition of

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FIGURE5.4: Major and trace element binary diagrams of the volcanic and plutonic deposits. All data are recalculated to anhydrous condi-tions. (a,b) Al2O3and MgO content decrease with increasing silica content along liquid line of descent from enclave to high-silica aplites.

(c) Variations in Na2O content with silica content. Nyukawa PFD and Chayano–Ebsiutoge PD are depleted in Na2O relative to composi-tions of the Takidani Pluton and Hotaka Andesite. (d) Incompatible behaviour and enrichment of K2O with from andesite to rhyolite Liq-uid line of descent from Hotaka Andesite, Nyukawa PFD, Takidani Pluton to Ebisutoge PD compositions. (e,f) Variations of Sr and Rb concentrations with increasing silica content. Andesitic enclaves are enriched in Rb content with respect to dacites of the Hotaka Andesite and Nyukawa PFD, and show large variations in Sr content. (g) Con-centrations of Zr from andesite to rhyolite form M-shaped patter with volcanic units being relatively enriched in Zr. (h) Increase in Ba con-tent from low to high-silica compositions. Aplites are depleted in Ba

content.

5.7. Geochemistry 91

FIGURE5.5: Spider diagrams of the Takidani Pluton (a) and volcanic units (b). Granodiorite and granites of the Takidani Pluton show similar trace element variations. Granites are slightly more enriched in heavy REE relative to granodiorite concentrations. Aplites are depleted in light and heavy REE, while enclaves in the Takidani Pluton are in enriched in REE. Trace element trends are comparable between different volcanic units and consistent with Takidani granodiorites and granites. Dacites and granodiorite trends are consistent and do not display a Sr or Eu anomaly. Rhyolite and granite trace element variations are also consistent showing negative Sr and Eu variations.

All values are normalised to Chondrite concentration of McDonough

& Sun (1995).

the Takidani Pluton varies from granodiorite to high-silica granite from about 64 to 78 wt.% SiO2. Whole rock analyses of the Ebisutoge PD (including Units B, C and D) show silica contents of about 73 to 74 wt.% and are consistent with published analyses of pumice (Figure 5.3; Kimura & Nagahashi 2007). Bulk rock analysis of Unit C (i.e. pumice fall out) displays a lower silica content of 71 wt.% SiO₂. Takidani granites contain between 73 and 76 wt.% SiO₂and overlap with glass and pumice of the Ebisutoge PD (Kimura & Nagahashi 2007). Aplitic dikes cross cutting the Takidani Pluton (Figure 5.2d) form a high-silica endmember of the intrusive portion of the system with composition similar to matrix glass of the Nyukawa PFD (Kimura

& Nagahashi 2007). Glass analyses of the Chayano–Ebisutoge PD and Nyukawa PFD from Kimura & Nagahashi (2007) show lower Na2O contents with respect to the Takidani pluton. Our new glass analyses for the Ebisutoge glass matrix (i.e. Units B to D), however, are consistent with compositions from the Takidani Pluton, which may suggest that previously reported matrix glass analyses were affected by Na₂O loss.

Spider diagrams overall show compositional similarities between plutonic (Fig-ure 5.5a) and volcanic units (Fig(Fig-ure 5.5b). The granites of the Takidani Pluton are consistent with granodiorites with exception of a negative Sr and Eu anomaly. Aplitic dikes are strongly depleted in REE. Enclaves are enriched in REE. Nyukawa PFD and Hotaka Andesite show similar compositional variability consistent with Takidani

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granodiorites. Pumice and whole rock compositions of Chayano–Ebisutoge PD also have negative Sr and Eu anomalies, similar to Takidani granites. Major and trace element variations for selected elements show distinct trends for the volcanic and plutonic rocks (Figure 5.4). Concentration in major elements of Al₂O₃ and MgO generally decrease with increasing silica content from about 18 to 12 wt.% and 4.5 to 0.02 wt.%, respectively (Figure 5.4a,b). Pumice bulk-analyses of Chayano–Ebisutoge PD display lower MgO and higher Al2O3 with respect to Takidani compositions (Figure 5.4a,b). Parallel trends of Na₂O vs SiO₂contents are observed for volcanic and plutonic rocks (Figure 5.4c). The composition of Ebisutoge PD splits into two groups: Unit B (i.e. unconsolidated pumice flow deposit) and Unit C (i.e. pumice fall outs) plot together with published pumice analysis from Kimura & Nagahashi (2007).

Bulk analysis of Unit D (i.e. welded ignimbrite) display higher Na₂O and lower K₂O contents (Figure 5.4d) and are consistent with compositions of the Takidani Pluton.

K2O content is consistent between volcanic and plutonic units and generally increases from about 2.0 to 5.5 wt.% from andesite to rhyolite compositions. Concentrations of Rb are elevated in andesitic enclaves with respect to the dacitic composition of the Ho-taka Andesite and Nyukawa PFD (Figure 5.4e). A sharp increase in Rb concentrations at about 73 wt.% SiO2is observed in the volcanic and plutonic rocks (i.e. granites) followed by a drop at about 75 wt.% SiO₂(Figure 5.4e). A wide range of Sr content is observed in the mafic enclaves ranging from about 200 to 530 ppm (Figure 5.4f).

A continuous decrease in Sr content from about 400 ppm is observed from the Ho-taka Andesite to aplitic dikes in the Takidani Pluton. Matrix glass of the Nyukawa PFD contains significantly more Sr than the matrix of the Chayano–Ebisutoge PD (Figure 5.4f; Kimura & Nagahashi 2007). Zirconium concentrations are distributed over a M-shaped pattern showing different trends for volcanic and plutonic rocks (Figure 5.4g). Barium behaves incompatible in all units with concentrations reaching nearly 1000 ppm in the volcanic glasses. Aplitic dykes become progressively more depleted in Ba with increasing silica content consistent with fractionation of alkali feldspar and biotite (Figure 5.4h).