Toshiaki Shimura

Linking granulites, silicic magmatism, and crustal growth in arcs: Ion microprobe (zircon) U-Pb ages from the Hidaka metamorphic belt, Japan

There is no consensus as to how the extreme metamorphic conditions required to form granulites are attained, or how these rocks relate to crustal growth and differentiation processes. Studying young granulites offers two advantages: the tectonic setting is likely to be well constrained, and the ambiguities that result from overprinting by younger metamorphic events are potentially avoided. We report the first ion microprobe U-Pb (zircon) ages for orthopyroxene-bearing granulites, tonalites, and gabbros of the Cenozoic Hidaka metamorphic belt (Hokkaido, Japan) to clarify the magmatic-metamorphic connection in this area. The data support a two-stage evolution for this terrane, which is attributed to episodes of supra-subduction zone magmatism (late Eocene) and back-arc extension (early Miocene). We relate granulite facies metamorphism and garnet-orthopyroxene tonalite generation to mafic magma under-accretion and lithosphere thinning due to the opening of the Japan Sea at 19 Ma. The Hidaka granulites are thus among the youngest exposed granulites on Earth, and manifest the thermal response to continental growth.

Magnesiohögbomite-2N4S: A new polysome from the central Sør Rondane Mountains, East Antarctica

Abstract Högbomite-group minerals are complex Fe-Mg-Zn-Al-Ti oxides related to the spinel group. Their polysomatic structure is composed of spinel (S) and nolanite (N) modules. The new polysome magnesiohögbomite-2N4S (IMA 2010-084) was found in the Sør Rondane Mountains, East Antarctica. It occurs in Mg-Al-rich, Si-poor skarns, characterized by a corundum-spinel-phlogopite-clinochlore assemblage. The new magnesiohögbomite polysome formed during the retrograde metamorphic stage. Magnesiohögbomite-2N4S appears macroscopically orange red, the streak is light orange colored. Euhedral crystals are hexagonal plates or prisms with cleavage planes on {001}. The mineral is optically uniaxial (-) and pleochroic with O = reddish brown and E = pale brown. The mean refractive index calculated from reflectance data in air at 589 nm is 1.85(3). The calculated density is 3.702(2) g/cm3. The Mohs hardness is 6.5-7, and VHN300 = 1020-1051, mean 1032 kg/mm2. The crystal structure of the new polysome magnesiohögbomite-2N4S has been solved and refined (R1 = 2.74%) from single-crystal XRD data. The crystal chemical formula is T10M24O46(OH)2 where T and M represent tetrahedral and octahedral sites. The mineral is hexagonal, space group P63mc (no. 186), a = 5.71050(10), c = 27.6760(4) Å, Z = 1, V = 781.60(2) Å3. The strongest lines in the powder XRD pattern [d (Å), I (%), hkl] are: 2.8561(4), 37, 110; 2.6120(3), 39, 109; 2.42818(16), 100, 116; 2.4160(4), 39, 1010; 2.01181(13), 50, 208; 1.54892(16), 35, 2110; 1.42785(6), 57, 220. Strongest peaks in Raman spectra are at 302, 419, 479, 498, 709, 780, and 872 cm-1, with a broad OH-characteristic absorption around 3400 cm-1. The mean chemical composition (wt%) is SiO2 0.05, TiO2 7.08, SnO2 0.15, Al2O3 66.03, Cr2O3 0.02, Fe2O3 0.50, FeO 4.87, MnO 0.06, MgO 18.71, CaO 0.01, ZnO 0.96, NiO 0.01, CoO 0.02, F 0.06, Cl 0.01, H2O 1.00, sum 99.51. The simplified formula is (Mg8.2Fe1.2Zn0.2)2+(Al22.7Fe0.1)3+ Ti4+1.6O46(OH)2 and ideal formula is Mg10Al22Ti2O46(OH)2. This mineral is a solid solution between the two ideal end-members, (Mg,Fe,Zn)102+(Al,Fe)223+Ti24+O46(OH)2 and (Mg,Fe,Zn)82+(Al,Fe)263+O46(OH)2.

Tetrahedral plot diagram: A geometrical solution for quaternary systems

Abstract The transformation from a tetrahedral four-component system to an XYZ-orthogonal coordinate axis system has been solved using the geometry of a tetrahedron. If a four component mixing ratio is described as t, l, r, and f (here, t + l + r + f = 1), the transforming equations can be written as A tetrahedral plot diagram can be easily constructed using the algorithms described in this paper. We present an implementation of these algorithms in a custom-designed Microsoft Excel spreadsheet, including adjustable viewing angles for the tetrahedral plot. This will be of general utility for petrological or mineralogical studies of quaternary systems.

Generation of I-type granitic rocks by melting of heterogeneous lower crust

[Extract]: Granite generation is a fundamental process for the growth and evolution of Earth’s continental crust. I-type granitic rocks, nominally derived from infracrustal sources, are the most common granite type and are voluminously emplaced in convergent margin settings. A puzzling feature is that many I-type granites show isotopic evidence for reworking of older supracrustal material, in conflict with the I-type designation. How the supracrustal component was incorporated by I-type magmas is a matter of deduction, particularly given difficulties in recognizing the putative infracrustal source region in the exposed geology. We report a case study of I-type granitic magma generation by hybridization between metasedimentary-derived partial melt and intercalated mafic granulite units during extraction of silicic magma from the lower crust in the Hidaka Metamorphic Belt (HMB), Japan (Hammerli et al., 2018). Isotopic data (Nd, Hf, O) obtained by microanalysis of accessory minerals in former melt networks (leucosomes) suggest that hybridization operates on a (sub-) grain scale, where repeated injections of externally derived melt attempt to approach local equilibrium with the host mafic granulites during transfer through complex melt pathways (see our figure 3; and also Hasalova et al. [2011] and references therein).