Takeshi Imayama

Carboniferous inherited grain and age zoning of monazite and xenotime from leucogranites in far-eastern Nepal: Constraints from electron probe microanalysis

Abstract Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO2 and Y2O3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

Paleoproterozoic high-pressure metamorphic history of the Salma eclogite on the Kola Peninsula, Russia

The Precambrian Salma eclogites on the Kola Peninsula, Russia, represent some of the oldest eclogites in the world; however, there has been much debate regarding whether the timing of their eclogite facies metamorphism is Archean (2.72–2.70 Ga) or Paleoproterozoic (1.92–1.88 Ga). New microstructural observations, pressure-temperature (P-T ) analyses, zircon inclusion analyses, and U-Pb zircon dating performed in this study suggest that eclogite facies metamorphism occurred at ca. 1.87 Ga under P-T conditions of 16–18 kbar and 750–770 °C. Metamorphic zircons with the age of 1.87 Ga have inclusions of garnet (Grt) + omphacite (Omp) + Ca-clinopyoxene (Cpx) + amphibole (Amp) + quartz (Qz) + rutile (Rt) ± biotite (Bt), as well as flat heavy rare earth element (HREE) patterns due to the presence of abundant amounts of garnet during peak eclogite facies metamorphism. The Paleoproterozoic ages (1.92–1.88 Ga) presented in previous studies are reinterpreted to represent prograde ages, rather than peak ages, because these ages have been inferred from U-Pb dating in zoisite-bearing zircon and Sm-Nd and Lu-Hf geochronologic analyses of garnet showing growth zoning. In contrast, the 2.73–2.72 Ga unzoned zircons with dark cathodoluminescence contain inclusions of Grt + Amp + plagioclase (Pl) + Qz + rutile (Rt) ± Bt and are relatively enriched in HREEs, suggesting that an initial amphibolite facies metamorphic event occurred during the Archean. This study also proposes that the Salma eclogites underwent granulite facies retrograde metamorphism at 10–14 kbar and 770–820 °C, with rapid decompression occurring soon after peak metamorphism ca. 1.87 Ga. The final period of retrograde amphibolite facies metamorphism occurred at 8–10 kbar and 590–610 °C. Whole-rock chemical analyses indicate that the Salma eclogites were originally tholeiitic basalts formed at a mid-ocean ridge. The occurrence of eclogite facies metamorphism ca. 1.87 Ga suggests that the collision between the Kola and Karelian continents occurred during the Paleoproterozoic, rather than the Archean. These results, as well as those of previous studies, imply that the subduction required to form eclogites may have begun during or before the Paleoproterozoic. LITHOSPHERE; v. 9; no. 6; p. 855–873; GSA Data Repository Item 2017317 | Published online 14 September 2017 https://doi.org/10.1130/L657.1

1.74 Ga crustal melting after rifting at the northern Indian margin: investigation of mylonitic orthogneisses in the Kathmandu area, central Nepal

ABSTRACT Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the ‘syn-collisional’ and ‘post-collisional’ fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent.