Abstract This paper describes the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones and is indicative of multistage fracturing and metamorphism in the lower crust, by studying the microstructural and chemical characteristics of plagioclase in sparsely fractured anorthosites and anorthositic mylonites from the Eidsfjord shear zone, Langøya, northern Norway. Based on the field relationship between sparsely fractured anorthosite and anorthositic mylonite, the fracturing of anorthosite occurred before the formation of mylonite. In sparsely fractured anorthosites, transgranular fractures are observed; hydration-reaction products, including Na-rich plagioclase, occur within cracks and fractures, suggesting that hydration reactions occurred during or after fracturing. The hydration reactions in sparsely fractured anorthosites are estimated to have occurred at higher-pressure (P) amphibolite-facies conditions (~0.9–1.0 GPa and ~550–700 °C). In anorthositic mylonites, which are considered to have initiated by fracturing and subsequent hydration metamorphism at lower-P amphibolite-facies conditions (~0.7 GPa and ~600 °C), recrystallized plagioclase grains often show compositional zoning with an Na-rich core and a Ca-rich rim. Because the compositions of metamorphic plagioclase grains in the sparsely fractured anorthosites and those of the Na-rich cores of the reversely zoned plagioclase in anorthositic mylonites are similar to each other, the Na-rich cores of the matrix plagioclase in the anorthositic mylonites have recrystallized under higher-P amphibolite-facies conditions and then been overgrown or replaced by the Ca-rich rims under lower-P conditions. Consequently, the reversely zoned plagioclase observed frequently in lower crustal shear zones is an indicator of multistage brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.

The Jalal Abad magmatic rocks are located at the southeastern edge of the Kashmar-Kerman tectonic zone, which includes Cadomian magmatic rocks and sedimentary strata intruded by several Silurian alkaline plutons and associated dikes. They have typical alkaline kaersutite and alkaline to per-alkaline pyroxene and are characterized geochemically by enrichment in LREE, Nb and Ta, and high concentrations of incompatible trace elements, demonstrating alkaline features with typical ocean island basalt signatures. Chondrite-normalized REE and multi-element spider diagrams along with High 206 Pb/ 204 Pb ratios (18.46-19.83) for the Jalal Abad gabbro, diorite, and dibasic dikes indicate involvement of an OIB-like source during the formation of these rocks. Modeling of bulk-rock trace elements and Sr-Nd isotopes suggest that magmas were generated by a pair of HIMU-EM1- like mantle source, consistent with a plume mantle origin in a within-plate rift zone. The melting took place in garnet stability field and the fractional crystallization played a major role in magmatic evolution of the mantle-derived parental magma. The Jalal Abad mafic rocks have U-Pb zircon ages of 425.5 + 8.6 Ma and along with other Ordovician to Silurian rocks in different parts of the Iranian plate are related to the extensional tectonic regime responsible for the rifting of Cadomian fragments from northern Gondwana and the opening of Paleo-Tethys. Our findings indicate that the rifting and seafloor spreading of Paleo-Tethys and the formation of its oceanic crust were intensely influenced by a mantle plume activity in the early Paleozoic.

The alkaline tephra U-Oki is widely dispersed in and around the Japanese Islands. U-2 tephra, among Holocene tephras (U-2, U-3 and U-4 teplums) on Ulleung Island, was considered to be the first candidate to be correlative with alkaline tephra. Recent studies on stratigraphy, eruption age, and chemical composition of Ulleung Island tephras and Ulleung Island-derived tephras show that U-3 and U-4 tephras, not U-2 tephra, reached the Japanese Islands. Therefore, it is necessary to confirm whether Ulleung Island-derived tephras, such as U-Oki, can be correlative with U-3 or U-4 tephras based on reliable chemical and chronological data. Such reliable correlations provide information on eruption magnitude and dispersal. Alkaline tephra (Hm-2 tephra), which is petrographically similar to U-Oki, is found as an intercalated layer along with numerous other intercalated layers of Hakusan Volcano tephra in a peat layer formed during the last 13,000 years in the summit area of Hakusan Volcano, central Japan. EPMA analyses of major element glass composition and AMS radiocarbon dating have been performed for Hm-2 tephra, showing a correlation between Hm-2 tephra and tephra on Ulleung Island. Volcanic glass shards from Hm-2 tephra have a distinctly high alkali content with high alumina and intermediate silica contents. These chemical characteristics accord with those of Ulleung Island tephra and Ulleung Island-derived tephras. AMS C-14 ages obtained for peat layers just under Hm-2 tephra are 7600 BP and 8490 BP, which is consistent with the eruption age of U-3 tephra on Ulleung Island. The ages of the Hm-2 tephra indicate that Hm-2 is correlative with U-3 tephra. The correlation supports the view that tephras correlative with U-3 tephra are distributed widely in and around the Japanese Islands, as well as those correlative with U-4 tephra. A comparison of chemical compositions between Hm-2 tephra and three Units (U-3a, U-3b, and U-3c) of U-3 tephra shows that Hm-2 tephra is most similar to Unit U-3b among the three Units, indicating that a correlation is highly probable. This result suggests that Unit U-3b, in addition to Unit U-3c, also reached central Japan, which is approximately 550 km from Ulleung Island, with the eruption of U-3 tephra.

To clarify the rheological properties and deformational mechanisms that operate within lower-crustal shear zones, we analyzed deformation microstructures and fabrics in plagioclase grains from high-strain regions of intrafolial drag folds of an anorthositic ultramylonite from Langøya, Vesterålen, northern Norway. These grains have developed weak crystallographic preferred orientations (CPOs). Other than deformation twinning, they do not show any indications of intracrystalline plasticity or dynamic recrystallization. In some domains of the drag fold, plagioclase CPOs are more distinct, but the majority of inferred slip systems are not consistent with the main slip systems in plagioclase. These suggest that neither dislocation creep nor dislocation-accommodated grain boundary sliding (GBS) produced the plagioclase CPOs but rather grain rotation via GBS along specific grain boundaries parallel or subparallel to crystallographic planes of (010), (100) or (110), which led to an alignment of the easy-slip grain boundaries in the flow direction that produced the observed CPO patterns. The anisotropy of dissolution and growth rates may have been responsible for the development of the specific grain boundaries.

We report on the evidence of the pulverization in a deep-seated meta-anorthosite in the Eidsfjord shear zone, Vesteralen, northern Norway. Some plagioclase porphyroclasts comprise a few large relict clasts and many fine grains that preserve the outlines of the original grains. The fine-grained plagioclase does not show any plastic-deformation microstructures and has strong crystallographic preferred orientations, which are inherited from the twinned porphyroclast. Misorientation-axis distributions indicate that the grains have rotated randomly, so that the misorientation axes are not aligned with either the crystallographic or kinematic axes. The observed grain-size distribution has a fractal dimension, suggesting their fracturing/fragmentation origin. The microstructures are characterized by the fracturing/fragmentation with a very low shear strain, indicating that it may be associated with pulverization at similar to 20-25 km depth.

We examine ultramafic and olivine-rich troctolite blocks of the East Taiwan Ophiolite (ETO) in the Lichi Mélange. Although ultramafic rocks are extensively serpentinized, the primary minerals, such as olivine, orthopyroxene, clinopyroxene, spinel and plagioclase can be identified. The ultramafic rocks are classified into harzburgite (± clinopyroxene), dunite, and olivine websterite. Major and trace element compositions of the primary minerals in harzburgites, such as the Cr# [= Cr/(Cr + Al) atomic ratio] of chromian spinel (0.3–0.58) and incompatible elements-depleted trace element patterns of clinopyroxenes, indicate their residue origin after partial melting with less flux components. These compositions are similar to those from mid-ocean ridge peridotites as well as back-arc peridotites from the Philippine Sea Plate. The olivine websterite contains discrete as well as occasional locally concentrated plagioclase grains. Petrological characteristics coupled with similarity in trace element patterns of clinopyroxenes in the harzburgite and olivine websterite samples indicate that the olivine websterite is likely formed by clinopyroxene addition to a lherzolitic/harzburgitic peridotite from a pyroxene-saturated mafic melt. Dunite with medium Cr# spinels indicates cumulus or replacement by melt-peridotite reaction origins. Mineral composition of olivine-rich troctolite cannot be explained by simple crystallization from basaltic magmas, but shows a chemical trend expected for products after melt-peridotite interactions. Mineral compositions of the dunite and olivine-rich troctolite are also within chemical ranges of mid-ocean ridge samples, and are slightly different from back-arc samples from the Philippine Sea Plate. We conclude that peridotites in the ETO are not derived from the northern extension of the Luzon volcanic arc mantle. Further geochronological study is, however, required to constrain the origin of the ETO ophiolite, because peridotites are probably indistinguishable in petrology and mineralogy between the Philippine Sea and the South China Sea/Eurasian Plates.

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.

The Amami Plateau, Daito Ridge, and Oki-Daito Ridge of the northwestern Philippine Sea Plate are remnants of Mesozoic island arcs. We have newly recovered samples of peridotite and peridotite-derived minerals from the Daito Ridge. The peridotite samples are composed of serpentinized/ altered olivines, orthopyroxene porphyroclasts, small clinopyroxenes, and spinels, indicating a harzburgitic origin. Chondrite-and primitive mantle-normalized trace-element patterns for clinopyroxenes are characterized by a steep positive slope from middle rare earth elements to heavy rare earth elements (HREEs) plus yttrium. The light rare earth elements (LREEs) and Sr and Zr contents of clinopyroxenes vary in abundance, and some crystals have high LREE/HREE ratios coupled with positive Sr and Zr anomalies. These petrological and geochemical characteristics are not consistent with the Daito peridotites being the residue of a single partial melting event including melt extraction expected for mid-ocean ridge mantle. Instead, the peridotite source must have been enriched with slab-derived components, which are associated with arc-related magma. Thus, it is concluded that the studied peridotite fragments belong to an exhumed mantle section of a remnant proto-Philippine Sea island arc.

Antigorite is common in hydrated mantle domains and commonly shows strong alignment either due to deformation or growth in a preferred orientation. The alignment of antigorite imparts a strong anisotropy to the host rock affecting physical properties. A quantitative analysis of how antigorite affects these properties requires a reliable measurement of the crystallographic preferred orientation of antigorite, and EBSD analysis is the most widely used technique. Potential problems include (i) mis-indexing, which can add significant uncertainties to the results; and (ii) sample preparation, which may affect the measured CPO in particular for automated mapping. Combining results derived from X-ray goniometry, EBSD and U-stage techniques with FIB-TEM analysis leads to the following conclusions concerning these two issues. (i) There is a significant issue with mis-indexing a- and b-axes due to rotational similarities about the c-axis. Similar problems may also affect the c-axes measurements but this is less significant than the a- and b-axes when data are filtered using lower MAD values. Filtering using MAD values of <0.7° can significantly change the resulting CPO. (ii) Sample preparation can also affect the measured CPO: sections prepared parallel to the foliation show only minor differences with MAD values of <2.0°. Mis-indexing problems can be minimized by using an MAD value of <0.7° and analysing thin sections cut parallel to the foliation.

Titanium-bearing (hydro)andradites from hydrothermally altered mantle section of the Nagaland ophiolite, India have been investigated for mineral chemical study. In serpentinized peridotite they occur in association with accessory chromites or dusty magnetites and the TiO2 content in them is relatively low (<0.35wt%). (Hydro)andradites in magnetite pods, hosted within serpentinized peridotite have a range of TiO2 content (up to 13.77wt%) and occur typically within veins and veinlets intricately traversing the pods. The TiO2-rich variants of the (hydro)andradites within the veins occur either as large, polyhedral, margin-parallel zoned, mostly isolated grains within an andradite-rich porous matrix, or in clusters of equant, complexly zoned spherulites, resembling a fissure-fill. Besides (hydro)andradites as the most abundant component, the remaining of the veins consists mostly of serpentine and chlorite with relict titanites and perovskites. Magnetite pods are almost monomineralic, composed of large chromian magnetite crystals with accessory ilmenites occurring at the grain boundaries. Textural and mineralogical evidences suggest the transformation of these pods prior to the formation of (hydro)andradites from chromitite protoliths that have undergone extensive Ca-metasomatism together with host peridotites. We envisage at least two hydrothermal episodes for the mineralogical modifications where the formation of (hydro)andradites is related to the last event. The textural and mineral chemical characteristics of the entire assemblage as a whole demonstrate a gradual, temporal evolution in the composition of the metasomatic fluid from the onset of serpentinization to the end. This new occurrence of titanian andradites from chromitite protolith, hitherto not reported has major implications on the mobility of elements in hydrothermal environments.

The Gakkel Ridge is the world's slowest-spreading mid-ocean ridge varying from about 14 mm/year to 8 mm/year in full spreading rate (Cochran et al., 2003 JGR). It is widely accepted that the ultraslow-spreading ridge limits melting, leading to an idea that peridotites beneath the ultra-slow spreading ridges are relatively fertile in melt components. The ultraslow-spreading ridges, therefore, provide us unique opportunity to insight into original mantle heterogeneity before partial melting beneath the ocean ridge. Peridotite samples were recovered from the Gakkel Ridge during the international Arctic Mid-Ocean Ridge Expedition (AMORE) (Micael et al., 2003 Nature). Recently, D’Errico et al. (2016 GCA) reported a variety of peridotites corrected by the expedition. We also examined petrology and mineralogy of 12 abyssal peridotites from the Sparsely Magmatic Zones of the Gakkel Ridge. Our samples show a wide range of textures from protgranular to mylonitic textures. Based on trace element pattern of clinopyroxene, peridotites can be classified into three types: (Type-1: simple residue) systematic depletions in light rare earth elements (LREEs) from Heavy REEs (HREEs), (Type-2: residue after influx melting) concave-down REE pattern with highly enriched LREE, and (Type-3: unusual mantle) systematic depletions in LREEs from HREEs with no Zr negative anomaly. Type-1 peridotite can be explained as residue after partial melting and melt extraction, and are similar to other abyssal peridotites recovered from other mid-ocean ridges. Trace element pattern of clinopyroxene in Type-2 peridotite is similar trend to that in harzburgie sample of D’Errico et al. (2016). Type-2 peridotite can be explained as residue after influx melting in the melting column beneath the ridge. Type-3 peridotite has not been reported yet. We need further investing on origin of this sample: either reaction/influx melting with a Zr-rich fluid/melt or originally Zr-rich mantle source.

Cr-spinel is known as the most rigid mineral among constituents of mantle peridotite. However, it is often seen that it forms pull-apart structures in deformed peridotite as a result of brittle fracturing. Understanding of mechanical conditions for fracturing of Cr-spinel could give a clue for stress estimation. Therefore, comprehensive study on the cause of fractures in Cr-spinel is important. In this study, we performed petrological and microstructural analyses of Cr-spinel and host deformed dunite in Higashi-akaishi ultramafic body in Sanbagawa metamorphic belt. The occurrence can be related to physical and chemical conditions in subduction zones.