The Quaternary Kurobegawa Granite: an example of a deeply dissected resurgent pluton

The Quaternary Kurobegawa Granite, central Japan, is not only the youngest known granitic pluton exposed on the Earth’s surface, it is one of few localities where both Quaternary volcanics and related plutons are well exposed. Here, we present new zircon U–Pb ages together with whole rock and mineral geochemical data, revealing that the Kurobegawa Granite is a resurgent pluton that was emplaced following the caldera-forming eruption of the Jiigatake Volcanics at 1.55 ± 0.09 Ma. Following the eruption, the remnant magma chamber progressively cooled forming the voluminous Kurobegawa pluton in the upper crust (~ 6 km depth) until ~ 0.7 Ma when resurgence caused rapid uplift and erosion in the region. This is the first study to document the detailed spatiotemporal evolution of resurgent pluton for a Quaternary caldera system. Our new findings may contribute significantly to understanding the fate of active caldera systems that can produce supereruptions.

reduction was done with the SHRIMPTOOLS software (available from www.ugr.es/~fbea), which is a new implementation of the PRAWN software originally developed for the SHRIMP. Errors are reported at the 95% confidence interval (C.I.: ~2 σ). Standard errors (95% C.I.) on the 37 replicates of the TEMORA standard measured during the analytical session were ± 0.35% for 206 Pb/ 238 U and ± 0.83% for 207 Pb/ 206 Pb.
Due to the extremely young ages of KWR sample, we applied the 230 Th correction 67,68 considering a Th/U ratio of 29.0 from whole-rock composition of sample KWR-3 (Supplementary Table S4 Table S3). All analyzed data are plotted along a discordant line with a lower interception providing an age of 0.79 ± 0.12 Ma (MSWD = 19.26). The weighted means (errors reported at 2σ) of the 207-corrected 238 U/ 206 Pb age yielded an age of 0.77 ± 0.04 Ma (MSWD = 2.25) ( Supplementary Fig. S3).

Mineral chemistry
Major element analyses of minerals were obtained by SEM and EDS with a Zeiss DSM-950 scanning microscope equipped with a Link Isis series 300 Analytical Pentafet system, operated at 20 kV and 1-2 nA bean current. Natural and synthetic standards were employed in the analyses of mineral major elements. Precision was better than ± 1.5 proportion for analyte concentrations of 1 wt% proportion. Minerals were normalized to total 100 wt% and 8 oxygen atoms for plagioclase and alkali feldspar, total 95 wt% and 22 oxygen atoms for biotite, total 97 wt% and 23 oxygen atoms for amphibole and total 100 wt% and 5 oxygens atoms for sphene (The results are shown in Supplementary Tables S6-S10).
Three populations of amphiboles are distinguished according to textural-chemical features in MME: Amp-1 has magnesiohastingsite compositions and it forms core and mantle of large crystals characterized by low content in total Fe (1.606-2.284 atoms per formula unit: apfu) and high total Al (1.878-2.352 apfu) and Ti (0.196-0.309 apfu). Amp-2, hastingsite, overgrowths Amp-1 and it trends away from the composition of Amp-1, being richer in total Fe (2.362-2.784 apfu) and similar content in Ti (0.211-0.308 apfu) and total Al Amphibole in host granite is very scarce and it is restricted to small grains included in plagioclase, with compositional features of Amp-2, and small interstitial grains, the latter matching with Amp-3. Biotite is the main hydrated ferromagnesian mineral in granite with a homogeneous composition (Mg# = 42-45) and is richer in total Al (2.526-2.604 apfu) and poorer in Ti (0.393-0.510 apfu) than biotite from MMEs. There are two kinds of plagioclases in host granites: Pl-4 represents large megacryts with slightly zonation from core to rim from andesine to oligoclase with An11-37. Pl-5 are small grains with a large compositional zonation from labradorite to oligoclase with An16-62. K-feldspar is present as anhedral inclusions in Pl-4 and as interstitial grains with same composition of Or80-100.

Whole-rock composition
Whole rock major element determinations were performed by XRF, after fusion with lithium tetraborate (Supplementary Table S4). Typical precision was better than ±1.5% for an analyte concentration of 10 wt%.
Zirconium was determined by X-ray fluorescence on the same disks, with a precision better than ± 4% for 100 ppm Zr. Trace element determinations were done by ICP-mass spectrometry (ICP-MS) after HNO3+HF digestion of 0.1000 g of sample powder in a Teflon-lined vessel at ~180 °C and 200 psi for 30 min, evaporation to dryness, and subsequent dissolution in 100 ml of 4 vol% HNO3. Instrument measurements were carried out in triplicate with a PE SCIEX ELAN-8000 spectrometer using rhodium as an internal standard. Precision, as determined from standards WS-E, BR and AGV run as unknowns, was better than ± 2% and ± 5% for analyte concentrations of 50 and 5 ppm, respectively. Samples for Sr and Nd isotope analysis (0.1000 g) were digested with HNO3+HF in a Teflon-lined vessel at 200 psi. The elements were separated with ion-exchange resins, and the Sr and Nd isotope ratios were determined by thermal ionization mass spectrometry with a Finnigan Mat 262. All reagents were ultra clean. Normalization values were 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219. Blanks were 0.6 and 0.09 ng for Sr and Nd, respectively. The external precision (2σ), estimated by analyzing 10 replicates of the standard WS-E 69 , was better than ± 0.003% for 87 Sr/ 86 Sr and ± 0.0015% for 143 Nd/ 144 Nd. 87 Rb/ 86 Sr and 147 Sm/ 144 Nd were directly determined by ICP-MS following the method developed by Montero and Bea 70 , with a precision better than ± 1.2% and ± 0.9% (2σ), respectively. Simple binary magma mixing model for major, trace elements and isotopes were calculated according to Faure and Mesning 71 . We consider sample KRW-1 as mafic end-member and sample KRW-3 as felsic end-member. Mixing products were calculated for every 10% of weight fractions for each end-member (Supplementary Table S4 In the intermingled zone it is possible to recognize complex textural relationship of main phases from MME´s and host granite, well indicated amphibole, biotite and plagioclase. It suggests an interaction and mixing of contrasted magmas and subsequent reequilibration process of main phases and to crystallise new phases derived from hybrid magma.

Thermobarometric estimations
The crystallization temperatures were determined using: 1) the zircon-saturation thermometer of Borisov and Aranovich 72 for MMEs and Bohenke et al. 73 for intermediate to felsic compositions, and 2) the apatite-saturation thermometer 74 , with a correction proposed by Bea et al. 75 for the peraluminous compositions for whole-rock data (Fig. 4a).
Due to complex textural and compositional variations observed in amphibole and plagioclase, we carefully selected Amp-Pl pairs to ensure the equilibrium between these two phases. We consider four kinds of pairs: Amp-4 (Al: 0.862-1.015 apfu)-Pl-3 (An: 0.16-0.23) (Fig. 4b). We used the Al-in-hornblende barometer 29 as MMEs samples bear the buffering assemblage of quartz + plagioclase + K feldspar + amphibole + titanite.
However, groups 3 and 4 have progressive lower pressure range of 2.6-3.6 to 1.5-1.9 kbar, respectively (Fig.   4b). The high pressure and temperature conditions of groups 1 and 2 can be interpreted as early crystallized phases when MMEs magma was rising up towards shallow levels. By contrast, the lower pressure range of groups 3 and 4 indicates the depth conditions, 5-7 km, where MMEs magma interacted with host granite magma chamber.