Acknowledgments

The Editorial Board would like to thank all the scientists who served as referees in 2019.

The metamorphic pressure–temperature (P–T) conditions of high–grade pelitic gneisses from Akarui Point, Skarvsnes, Skallen, and Rundvågshetta in the Lützow–Holm Complex (LHC), East Antarctica are re–examined by applying the Zr–in–rutile geothermometer to rutile inclusions in garnet enclosing Al₂SiO₅ minerals and to matrix rutile grains. By utilizing the P zoning of garnet to indicate isochronous surface, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P–T conditions around the kyanite/sillimanite transition boundary (~ 830–850 °C/~ 11 kbar). From Rundvågshetta, higher–T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) was confirmed from rutile inclusions in garnet enclosing sillimanite. Matrix rutile yielded similar temperature as inclusion rutile for Akarui Point, Skarvsnes, and Skallen samples. Therefore, the traditional metamorphic zone mapping based on matrix mineral assemblages, which classified Akarui Point as belonging to the transitional zone between the upper–amphibolite and the granulite facies zones, does not reflect the highest metamorphic conditions attained. The P–T–t evolution of the LHC needs to be re–evaluated utilizing detailed petrochronological approaches.

The viscosity of alkali titanosilicate melt with a composition of K₂TiSi₄O₁₁ (KTS4) was measured at pressures up to of 7.41 GPa and temperatures up to of 2133 K. The aim of this study is to determine the effect of pressure on the viscosity change and to estimate the relation between structural change and the viscosity change under high pressures. The viscosity decreased up to 3.30 GPa, whereas it increased from 3.30 to 7.41 GPa. The viscosity change above 3.30 GPa is likely caused by the dominance of six–fold coordinated titanium.

We carried out zircon U–Pb dating of the Ryoke granitoids exposed in the Takanawa Peninsula to determine their magmatic ages. The granitoids are mainly comprised of tonalite, granodiorite, and granite. The new zircon U–Pb ages are: 93.68 ± 0.79 Ma for a tonalite sample, 88.90 ± 0.61 Ma for a granodiorite sample, and 98.8 ± 1.0 Ma, 96.29 ± 0.98 Ma, 96.0 ± 1.0 Ma, and 93.86 ± 0.93 for granite samples. The new zircon U–Pb ages suggest multiple intrusive episodes with granitoid magmas of variable compositions in the Takanawa Peninsula area during ~ 99–89 Ma. Contrary to previous geological studies that postulated a three–stage intrusive sequence starting with tonalite intrusion, followed by granodiorite, and culminating by granite intrusions, our results indicate that early magmatism in the study area was granitic in composition.

The antigorite–grade serpentinite and Late Paleozoic high–pressure schists are main components of a serpentinite–matrix mélange in the Itoigawa–Omi area, Hida–Gaien Belt, Japan. Based on the petrologic characteristics of the high–pressure schists, the mélange is divided into two units, namely an ‘eclogitic unit’ and a ‘non–eclogitic unit’. Our preliminary in–situ boron isotope analyses of five serpentinites using a laser–ablation multiple–collector inductively–coupled–plasma mass spectrometry (LA–MC–ICPMS) found a systematic difference of boron isotopic trends among the two units in the same mélange. The ‘eclogitic unit’ serpentinites from Yunotani and Kotagi–gawa are characterized by lower δ¹¹B value (mostly lower than +10‰), whereas the non–eclogitic unit serpentinite from Omi–gawa is higher than +10‰. Although the δ¹¹B value of <0‰ was not measured from the eclogitic unit serpentinites, the relatively low δ¹¹B values of <+10‰ might have recorded the signature of fluids released from deep subducted dehydrating slab. In contrast, the non–eclogitic unit serpentinite might have been affected by fluids released from shallower portion. Our new data confirmed the potential sensitivity of the boron isotope signature of serpentinites reflecting variation of high–pressure metamorphism.

Lawsonite, jadeite, and glaucophane are iconic minerals within a Pacheco Pass metagraywacke of the Franciscan Complex, California. Those minerals and the associated quartz form the distinctive very low–temperature and high–pressure metamorphic lawsonite–jadeite–glaucophane assemblage, which is diagnostic of ‘cold’ oceanic subduction zones. In this paper, we evaluate the ability of lawsonite geochemistry to trace protoliths with in–situ trace element and Sr–Pb isotope analyses in lawsonite from the Pacheco Pass blueschist–facies metagraywacke, a classical example of trench–fill sediments in subduction zones. Initial Sr isotope ratios are relatively high (⁸⁷Sr/⁸⁶Sr = 0.7071–0.7074), and initial Pb isotope ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.74–19.66, ²⁰⁷Pb/²⁰⁴Pb = 15.58–15.70, and ²⁰⁸Pb/²⁰⁴Pb = 38.41–39.34, which range from a MORB trend to a cluster on the EMII component. These geochemical signatures suggest the protolith of the metagraywacke mainly contained material derived from continental volcaniclastic rocks and quartzofeldspathic sediments. There is also a possibility that the protolith contains plume–related oceanic island basalt that reached or intruded into the fore–arc sedimentary sequence of California. Considering the maximum depositional age of the metagraywacke at ~ 102 Ma, the subduction of the Farallon Plate beneath the continental crust of the North American Plate might have carried alkali basalt with OIB–like isotopic signatures to the Franciscan trench.

 Our study proves the advantage of in–situ lawsonite Sr–Pb isotope analyses to characterize protoliths of metamorphic rocks. The results would manifest that the Sr–Pb isotopic signature of Ca–Al silicate minerals, such as lawsonite, and possibly epidote and pumpellyite, in various types of metamorphic/metasomatic rocks, would be an effective tool for investigating convergent margins.

Late Paleozoic Yunotani eclogites in the Itoigawa–Omi area of the Hida–Gaien Belt were subjected to a blueschist–facies recrystallization and deformation after the peak eclogite–facies metamorphism. We studied fluid inclusions in quartz domains of different metamorphic stages in the retrograde eclogite. Coarse–grained quartz filling pressure shadow of porphyroblastic garnets contains transgranular fluid inclusions (Group–I) with high salinity [6.0–8.0 eq mass% NaCl, with a mean value 7.2 ± 0.6 eq mass% NaCl (n = 12)]. In contrast, transgranular fluid inclusions in later stage quartz–rich band (Group–II) in parallel to a penetrative foliation developed in matrix are characterized by lower salinity [1.1–5.8 eq mass% NaCl, with a mean value 3.4 ± 1.2 eq mass% NaCl (n = 12)]. The occurrences of fluid inclusion trails suggest that each group corresponds to the fluids entrapped prior to the blueschist–facies recrystallization and the fluids entrapped during the blueschist–facies stage, respectively. The Group–II fluid inclusions formed at blueschist–facies conditions support the idea that aqueous fluids in the subduction channel have a salinity similar to seawater (~ 3.5 eq mass% NaCl). Moreover, the Group–I fluid inclusions indicate a possibility that aqueous fluids at the eclogite–facies depth in subduction channels possess a salinity higher than seawater.

Index to Journal of Mineralogical and Petrological Sciences Vol.114 No. 1-6

Author Index

Keyword Index

Journal of Mineralogical and Petrological Sciences

Volume 114, Number 1-6

February-December, 2019