Geological study in Tal - Talekhu section of Manang District along the Besisahar – Chame Road

The section between Tal to Talekhu of Manang District lacks the detailed geological study. The geological mapping in the scale of 1:50,000 followed by the preparation of geological cross-section and lithostratigraphic column has been done in the present study. The studied area lies partially in the Higher Himalayan Crystalline and the Tibetan Tethys Sequence. The units of the Higher Himalayan Group from Tal to Talekhu consists mainly of vigorous to faintly calcareous gneiss, migmatitic gneiss, quartzite, granite, etc. They are named as the Calc. Silicate Gneiss and Paragneiss and the Orthogneiss and Granite units. The lowermost part of the Tibetan Tethys consisted of metamorphosed calcareous rocks containing silicates and feldspar, so this unit is termed as the Marble and Calc. Gneiss. The section is about 9 km in thickness and is highly deformed with presence of igneous rocks at many places.

Fluid-Present Partial Melting of Paleoproterozoic Okbang Amphibolite in the Yeongnam Massif, Korea

The waning stage of a long-lived collisional orogeny is commonly governed by an extensional regime in association with high-temperature metamorphism, anatexis, and magmatism. Such a late-orogenic process is well-recorded in the Okbang amphibolite, Yeongnam Massif, Korea, where thin layers or irregular patches of tonalitic leucosomes are widespread particularly in association with ductile shear zones. Various microstructures including interstitial felsic phases and former melt patches indicate that leucosomes are the product of partial melting. These leucosomes are aligned en echelon and contain large (up to ~2 cm) grains of peritectic hornblende, suggesting synkinematic fluid-present anatexis. The leucosomes are enriched in Na2O and Sr contents compared to the amphibolite but depleted in rare earth and high field-strength elements. P-T conditions of the anatexis were estimated at 4.6–5.2 kbar and 650–730°C, respectively, based on hornblende-plagioclase geothermobarometry. Sensitive high-resolution ion microprobe U-Pb analyses of zircon from an amphibolite and a leucosome sample yielded weighted mean 207Pb/206Pb ages of 1866±4 Ma and 1862±2 Ma, which are interpreted as the times for magmatic crystallization and subsequent anatexis of mafic protolith, respectively. The latter is consistent with the time of partial melting determined from a migmatitic gneiss and a biotite-sillimanite gneiss at 1861±4 Ma and 1860±9 Ma, respectively. The leucosomes are transected by an undeformed pegmatitic dyke dated at 1852±3 Ma; by this time, extensional ductile shearing has ceased. Initial εHft values of zircon from the amphibolite range from 4.2 to 6.0, suggesting juvenile derivation of basaltic melt from the mantle. In contrast, lower εHft values (–0.1 to 3.5) in leucosome zircons indicate a mixing of crust-derived melt. Taken together, the Okbang amphibolite has experienced synkinematic fluid-present melting during the waning stage of Paleoproterozoic hot orogenesis prevalent in the Yeongnam Massif as well as the North China Craton.

HT–LP crustal syntectonic anatexis as a source of the Permian magmatism in the Eastern Southern Alps: evidence from xenoliths in the Euganean trachytes (NE Italy)

Oligocene trachytes from the Euganean Hills include various regionally metamorphosed gneissic and granulitic xenoliths. These xenoliths provide the unique opportunity to investigate South Alpine intermediate to deep crustal levels that are not at present exposed in the Eastern Alps. The estimated P–T conditions are in the range of 780–850°C and 0.45–0.55 GPa for a migmatitic gneiss xenolith. Sensitive high-resolution ion microprobe (SHRIMP II) U–Pb analyses on zircon from this xenolith provide concordant ages around 259.7 ± 3.5 Ma, consistent with a proton-induced X-ray emission (PIXE) U–Th–Pb age on monazite of 262 ± 12 Ma. The Sr–Nd–Pb isotopic compositions, and major and trace element data show distinct origins for the different types of xenoliths. Mafic granulite xenoliths have an isotopic signature close to mantle-derived rocks and to Permian gabbroic rocks from the Western Southern Alps. Metapelite xenoliths have high Sr and low Nd initial ratios like those of acid crustal rocks and could possibly represent the source of the crustal component that is dominant in the acid Permian supervolcanoes. The migmatitic xenolith provides the first documented evidence for a Permian thermal event associated with crustal thinning in the Eastern Southern Alps. Here the South Alpine basement escaped most of the Alpine crustal shortening and still preserves most of the original Permian extension under thick Mesozoic cover.Supplementary material: Microprobe analyses of mineralogical phases and Ti-in-biotite geothermometric calculations are available at https://doi.org/10.6084/m9.figshare.c.5032337

Alteration of magmatic monazite in granitoids from the Ryoke belt (SW Japan): Processes and consequences

The alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average 206Pb/238U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma. Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant 206Pb/238U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

Using the alteration of magmatic monazite to constrain the thermal history of the Ryoke metamorphic belt (SW Japan)

<p>The Ryoke plutono-metamorphic belt exposed in SW Japan is the type locality for low-Pressure/high-Temperature (LP/HT) metamorphism. The Ryoke metamorphic field gradient is, however, a complex object shaped by several deformation phases, multiple magmatic pulses and protracted metamorphism. In the western part of the Ryoke belt (Iwakuni-Yanai area), a petrological and geochronological study of two plutons emplaced before metamorphism is used to explore the behaviour of magmatic monazite along the LP/HT gradient and constrain the thermal history of the belt. We compare a massive granite adjoining schistose rocks affected by greenschist facies metamorphism with a gneissose granite adjoining migmatitic gneiss affected by upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization, and is variably replaced by allanite+apatite±xenotime±Th−U-rich phases. Primary domains yield an average 206Pb/238U date of 102 ± 2 Ma while Th−U phases show Th−U−Pb dates of ca. 58 and 15−14 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by inclusion-rich secondary domains enriched in Ca, Y, U, P. Primary domain analyses are commonly discordant (116−101 Ma) while secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma.</p><p>Despite alteration, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 Ma and 106 ± 5 Ma). In the massive granite, monazite replacement is ascribed to the influx of aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration below 500 °C. The oldest date (58 ± 5 Ma) obtained from the Th−U-rich alteration products is regarded as a minimum age for chloritization during final exhumation of the granite. In the gneissose granite a small amount of anatectic melt was responsible for a pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. The spread in 206Pb/238U dates for the secondary domains is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long HT history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.</p>

Th–U–total Pb monazite geochronology records Ordovician (444 Ma) metamorphism/partial melting and Silurian (419 Ma) thrusting in the Kåfjord Nappe, Norwegian Arctic Caledonides

The northern extent of the Scandinavian Caledonides includes the Skibotn Nappe Complex of still debated structural position. This paper is focused on part of this complex and presents new U–Th–total Pb monazite dating results for the migmatitic gneiss of the Kåfjord Nappe. The rocks show mineral assemblage of garnet + plagioclase + biotite + white mica + kyanite + rutile ± K-feldspar ± sillimanite. Thermodynamic modelling suggests that garnet was stable at P–T conditions of ca. 680–720 °C and 8–10 kbars in the stability field of kyanite and the rocks underwent partial melting during exhumation following a clockwise P–T path. This episode is dated to 444 ± 12 Ma using chemical Th–U–total Pb dating of the Y-depleted monazite core. Second episode highlighted by growth of secondary white mica resulted from subsequent overprint in amphibolite and greenschist facies. Fluid assisted growth of the Y-enriched monazite rim at 419 ± 8 Ma marks the timing of the nappe emplacement. Age of migmatization and thrusting in the Kåfjord Nappe is similar to the Kalak Nappe Complex, and other units of the Middle Allochthon to the south. Nevertheless, the obtained results do not allow for unambiguous definition of the tectonostratigraphic position of the Skibotn Nappe Complex.

In situ measurements of natural radioactivity in selected igneous rocks of the Opava Mountain region

In situ gamma-ray measurements of four igneous rocks were taken in the Opava Mountains (Eastern Sudetes, Poland). The activity of naturally occurring radionuclides was measured using a portable GX3020 gamma-ray spectrometry workstation. The activity concentrations of 40K varied from 914 ± 17 Bqkg-1 (gneiss, Kamienna Góra) to 2019 ± 37 Bqkg-1 (weathered granite, Sławniowice), while those of 232Th from 7.5 ± 0.6 Bqkg-1 (weathered granite, Sławniowice) to 68 ± 0.9 Bqkg-1 (migmatitic gneiss, Nadziejów). The activities associated with 238U decay series ranged from 10 ± 0.4 Bqkg-1 (weathered granite, Sławniowice) to 62 ± 1.6 Bqkg-1 (gneiss, Kamienna Góra). The results will be used in compiling Radiological Atlas of the Sudetes