Motohiro Tsuboi

Role of partial melting in the evolution of the Sulu (eastern China) ultrahigh-pressure terrane

Strongly deformed potassium feldspar–rich dikes are widely distributed in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. The fact that the crystallization ages of these dikes overlap with the age of peak UHP metamorphic conditions implies the presence of melt during metamorphism. Sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic Sulu gneiss. Partial melting may be the key to solving several unusual features of the Sulu and other UHP terranes, such as the almost complete lack of mineralogical evidence for UHP conditions and the limited growth of zircon during UHP conditions in the dominant felsic gneiss. In addition, because partial melting will cause a drastic reduction in the strength of the UHP gneisses, the most likely exhumation mechanism is diapiric rise of a low-viscosity, partially molten mass containing entrained blocks of eclogite, and not a thin sheet as usually proposed.

Heterogeneity of initial 87Sr/86Sr ratios within a single pluton: evidence from apatite strontium isotopic study

Abstract Magmatic apatite in samples of the unmetamorphosed undeformed Cretaceous pluton of the Habu Granodiorite, southwestern Japan, was examined to determine its viability as an indicator of initial 87Sr/86Sr heterogeneity in magmatic bodies. The Habu Granodiorite is a zoned pluton in the Ryoke metamorphic belt of southwestern Japan. A published Rb–Sr whole-rock isochron age of 124.2±10.8 Ma is significantly older than Th–U–total Pb monazite ages of ca. 86 Ma. A Rb–Sr mineral isochron of 90.0±2.5 Ma for a sample of granodiorite from the margin of the pluton has an initial 87Sr/86Sr (SrI) of 0.7056±0.0001, similar to a 87Sr/86Sr of 0.70539 for coexisting euhedral apatite. Similarly, a Rb–Sr mineral isochron of 91.4±2.5 Ma for a sample of adamellite from the center of the pluton has an SrI of 0.7068±0.0001, corresponding to a 87Sr/86Sr ratio of 0.70673–0.70684 for three fractions of coexisting euhedral apatite. The uniformity of 87Sr/86Sr values of apatite in individual samples and accordance of 87Sr/86Sr values of apatite and SrI from the mineral isochron suggest that magmatic apatite can be considered as a reliable indicator of SrI in granitic rocks. The Sr isotopic compositions of euhedral apatite, coupled with those determined through the mineral isochrons and whole-rock data, indicate that SrI varies between samples from 0.7054 to 0.7074. This provides direct evidence of SrI heterogeneity within a compositionally zoned pluton. Such heterogeneity can produce spurious whole-rock age estimates in granitic plutons, even where the true age of a pluton is uniform within measurable error. Assimilation cannot account for chemical differences between granodiorite and adamellite as well as Sr-isotopic heterogeneity. It is likely that the heterogeneity of SrI is caused by magma source materials.

Quaternary high-Nb basalts: existence of young oceanic crust under the Sanandaj–Sirjan Zone, NW Iran

Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO2, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO2 (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their 87Sr/86Sr values range from 0.7049 to 0.7053 and their ϵ0Nd values lie between –0.2 and 1.1. The small negative values of ϵ0Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.

Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan

Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The XFe [= Fe3+/(Al + Fe3+)] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe3+ (XFe = 0.08–0.29), Fe3+-poor epidote with XFe 0.20 and their host plagioclase. Epidote grains with XFe > 0.20 in plagioclase and the matrix are a magmatic phase that crystallized directly from the tonalite magma. The Fe3+-poor epidote (XFe < 0.20) and zoisite were probably formed by a local reaction between the trapped melt and its host plagioclase, and these are considered not to have been in equilibrium with the tonalite magma. Compositions of amphibole-plagioclase assemblages allowed for temperature estimates in the range of 730–770 °C and minimum pressures of 0.47–0.57 GPa for the epidote/zoisite-bearing tonalites of the Hazu area. Epidote/zoisite-free tonalites occur in other areas of the Ryoke belt. There may be several tonalite bodies that record different intrusion processes and solidification depths in the Ryoke belt.