P–T CONDITIONS, U/Pb AND 40Ar/39Ar ISOTOPIC AGES OF UHT GRANULITES FROM CAPE KALTYGEI, WESTERN BAIKAL REGION

The study is focused on metapelitic granulites of Cape Kaltygei (Western Baikal region) that contain a diagnostic mineral assemblage of ultrahigh temperature (UHT) metamorphic rocks (orthopyroxene+sillimanite+quartz). The pseudosection-based thermobarometry yields peak metamorphic temperature and pressure values (T=950 °C, P=~9 kbar) and suggests near-isobaric cooling (IBC) conditions during the retrograde evolution of the granulites. The U/Pb zircon age estimates for metamorphism (~1.87 Ga) support the data published by other researchers. The SHRIMP-II U-Pb dating of zircon cores yields a minimum protolith age of 1.94–1.91 Ga. Biotites and amphiboles from granulites of Cape Kaltygei show the 40Ar/39Ar isotopic ages that are close to the Early Paleozoic accretion-collision system of the Western Baikal region.

Generation of highly silicic magmas at ultra-high temperature conditions : evidence from melt inclusions in peritectic garnet

<p>Metamorphism at ultra-high temperature (UHT) conditions (i.e., T >900°C and pressures from 7 to 13 kbar) is now recognized as a fundamental process of Earth’s crust, and although progress has been achieved on its understanding, constraining melt generation and fluid regime at such extreme conditions is still poorly explored.</p><p>In this study we use former melt inclusions found in peritectic garnet to investigate anatexis and fluid regime of metapelitic granulites in samples from the Rundvågshetta area, the thermal axis of the Lützow-Holm Complex (East Antarctica). Peak P-T estimates are 925-1039°C at 11.5-15 kbar. The studied rock is a coarse-grained heterogeneous metapelitic granulite with a predominant mafic residual domain and a relatively more felsic, melt-rich domain. The mineral association in the mafic domain typically contains orthopyroxene (Al<sub>2</sub>O<sub>3</sub>6-8.1 wt.%) + sillimanite + quartz + garnet (Prp<sub>42-55</sub>Alm<sub>40-52</sub>Grs<sub>3-4</sub>Sps<sub>0.2-1</sub>; X<sub>Mg</sub>0.5) + K-feldspar (Kfs) + cordierite (X<sub>Mg</sub>0.86) + rutile ± sapphirine ±biotite (X<sub>Mg</sub>0.75; TiO<sub>2</sub>3.7-5.8 wt.%) ±plagioclase (An<sub>35-46</sub>). Interstitial Kfs and quartz with low dihedral angles are often present, in particular as thin films between sillimanite and quartz; these features are interpreted as evidence for the presence of former melt along the grain boundaries. In contrast, the more felsic, melt-rich domain is composed of mesoperthite + quartz + garnet + sillimanite + brown biotite (X<sub>Mg</sub>0.7; TiO<sub>2</sub>3.7-5.4 wt.%) + rutile, but is free of orthopyroxene. Cores of garnet porphyroblasts (0.2-0.8 cm, Prp<sub>54-57</sub>Alm<sub>39-42</sub>Grs<sub>3-4</sub>Sps<sub>0.2-0.6</sub>, X<sub>Mg</sub>0.57) in the melt-rich domains contain clusters of primary glassy inclusions (GI) and crystallized melt inclusions (nanogranitoids; NI) together with multiphase fluid inclusions (MFI) and accessory phases (mainly rutile and apatite).</p><p>The GI (5-20 µm) have negative crystal shapes and contain shrinkage bubbles with or without CO<sub>2</sub>and N<sub>2</sub>. In some cases, GI may have trapped apatite and rutile. Micro-Raman investigation suggest that the H<sub>2</sub>O contents of these glasses range from 0 to 3.4 wt.%. Glasses are weakly peraluminous (ASI=1-1.1), have high SiO<sub>2</sub>(76-78 wt.%), very high K<sub>2</sub>O (6.5-10 wt.%) and extremely low CaO and FeO+MgO contents.</p><p>The NI have variable sizes (10-150 µm) and often contains intergrowth of plagioclase + quartz, K-feldspar (Kfs) and biotite (Bt). Less frequently NI may have euhedral to subhedral grains of Kfs and Bt. Trapped phases are apatite and rutile, except for one inclusion that contains the sapphirine + quartz pair indicating that melt inclusions were trapped at UHT conditions.</p><p>The MFI are composed of CO<sub>2</sub>(with densities from 0.23 to 0.93 g/cm<sup>3</sup>) and step-daughter magnesite, pyrophyllite. Methane, N<sub>2</sub>or H<sub>2</sub>O were not detected.</p><p>Our results show that anatexis of metapelites at extremely hot conditions occurred in the presence of COHfluids and generated highly silicic, weakly peraluminous, mildly to strongly potassic magmas with low H<sub>2</sub>O contents. Additional trace element data will be acquired to shed light on further geochemical fingerprints of these peculiar magmas.</p>

Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides

In this study, we report the occurrence of zircon coronae textures in metapelitic granulites of the Ivrea–Verbano Zone. Unusual zircon textures are spatially associated with Fe–Ti oxides and occur as (1) vermicular-shaped aggregates 50–200 µm long and 5–20 µm thick and as (2) zircon coronae and fine-grained chains, hundreds of micrometers long and ≤ 1 µm thick, spatially associated with the larger zircon grains. Formation of such textures is a result of zircon precipitation during cooling after peak metamorphic conditions, which involved: (1) decomposition of Zr-rich ilmenite to Zr-bearing rutile, and formation of the vermicular-shaped zircon during retrograde metamorphism and hydration; and (2) recrystallization of Zr-bearing rutile to Zr-depleted rutile intergrown with quartz, and precipitation of the submicron-thick zircon coronae during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronae during stage (2). Formation of vermicular zircon (1) preceded ductile and brittle deformation of the host rock, as vermicular zircon is found both plastically and cataclastically deformed. Formation of thin zircon coronae (2) was coeval with, or immediately after, brittle deformation as coronae are found to fill fractures in the host rock. The latter is evidence of local, fluid-aided mobility of Zr. This study demonstrates that metamorphic zircon can nucleate and grow as a result of hydration reactions and mineral breakdown during cooling after granulite-facies metamorphism. Zircon coronae textures indicate metamorphic reactions in the host rock and establish the direction of the reaction front.

Interpretation of zircon corona textures from metapelitic granulites of the Ivrea-Verbano Zone, Northern Italy: Two-stage decomposition of Fe-Ti oxides

In this study, we report the occurrence of zircon coronae textures in metapelitic granulites of the Ivrea-Verbano Zone. Unusual zircon textures are spatially associated with Fe-Ti oxides and occur as (1) vermicular-shaped aggregates 50–200 µm long and 5–20 µm thick, and as (2) zircon coronae and fine-grained chains, hundreds of µm long and ≤ 1 µm thick, spatially associated with the larger zircon grains. Formation of such textures is a result of zircon precipitation during cooling after peak metamorphic conditions, which involved: (1) decomposition of Zr-rich ilmenite to Zr-bearing rutile and vermicular-shaped zircon during retrograde metamorphism and hydration; (2) recrystallization of Zr-bearing rutile to Zr-depleted rutile intergrown with quartz and submicron-thick zircon coronae during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronae during stage (2). Formation of vermicular zircon (1) preceded ductile and brittle deformation of the host rock, as vermicular zircon is found both plastically- and cataclastically-deformed. Formation of thin zircon coronae (2) was coeval with, or immediately after brittle deformation, as coronae are found to fill fractures in the host rock. The latter is evidence of local, fluid-aided mobility of Zr. This study demonstrates that metamorphic zircon can nucleate and grow as a result of hydration reactions and mineral breakdown during cooling after granulite-facies metamorphism. Zircon corona textures indicate metamorphic reactions in the host rock, and establishing the direction of the reaction front.