Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

High-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P−T conditions of 15.8−18.2 kbar/625−690 °C (M1), 11.8−14.5 kbar/788−806 °C (M2), and 5.4−6.4 kbar/613−668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240−244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation

Electron back scattered diffraction data of garnet crystals from the Nelson Aureole, British Columbia and the Mosher’s Island formation, Nova Scotia, reveals that 22 garnet crystals are all oriented with one of three crystal directions parallel to the trace of the foliation plane in thin section. Structural models suggest that these relationships are due to preferential garnet nucleation onto muscovite, with the alignment of repeating rows of Al octahedra and Si tetrahedra in each leading to inheritance of garnet orientation from the muscovite. These results highlight that epitaxial nucleation may be a prevalent process by which porphyroblast minerals nucleate during metamorphism and carry implications for the role that non-classic nucleation pathways play in the crystallization of metamorphic minerals, the distribution of porphyroblasts in metamorphic rocks, and, in cases in which nucleation is the rate limiting step for crystallization, the energetics of metamorphic reactions.

Documenting isobaric cooling in the lower crust using cordierite breakdown textures (Mont Mary nappe, Western Alps)

<p>Intracrystalline diffusion is an efficient mechanism in high-grade rocks. Therefore, growth zoning in garnet is erased and the evidence for prograde path is lost. However, information recorded by the textures may store significant clues for deciphering part of the P-T path. An example is provided here from the migmatitic paragneisses from the Mont Mary nappe (Western Alps).</p><p>The latter is made of a pre-Alpine basement consisting of an upper and a lower unit. The upper unit is made of paragneisses, marbles and amphibolites similar to those of the Valpelline Unit and of the Ivrea Zone. The lower unit displays granitic orthogneisses, paraschists (with muscovite, biotite, garnet with local occurrences of staurolite, kyanite and andalusite) (Dal Piaz et al. 2015). In this unit, we discovered a hectometre-sized volume with no Alpine overprint, preserving migmatitic paragneisses, the topic of this study.</p><p>The paragneisses display quartzo-feldspathic leucocratic layers interpreted as crystallized melts. The leucosomes are separated by biotite- and sillimanite-rich layers, with conspicuous garnet porphyroblasts. In addition, fresh cordierite crystals are found in these layers. Sillimanite included in garnet rims has the same orientation than the one in the matrix. There, the foliation is defined by the shape fabric of biotite and sillimanite, wrapping both garnet and cordierite crystals.</p><p>Such textures may be used to propose a P-T path. A sequence of prograde reactions, including dehydration-melting of muscovite, then biotite, result in the production of a large amount of sillimanite. Garnet growth was continuing during incongruent melting. However, intracrystalline diffusion has erased the prograde chemical zoning, as well as the distribution and shape of mineral inclusions. The late replacement of garnet and cordierite by biotite and sillimanite indicates near-isobaric cooling, also recorded by chemical zoning along garnet rims.</p><p>Chemical data on coexisting minerals will be used to provide quantitative constraints on the P-T path. In addition, preliminary geochronological data suggest that detrital zircons grains were significantly reset during the HT metamorphism, which could have taken place c. 270 Ma ago. To conclude, the studied paragneisses offer another example of Permian near-isobaric cooling in the middle crust of the Adriatic plate.</p><p>Dal Piaz G.V., Bistacchi A., Gianotti F., Monopoli B., Passeri L., Schiavo A. & collaboratori (2015) – Note illustrative della carta Geologica d’Italia alla scala 1:50.000. Foglio 070, Monte Cervino. ISPRA, Servizio Geologico d’Italia, 070, 1-431.</p>

Role of P-T conditions and bulk rock composition in the mineralogical variations in millimeter scale: a study from South Maharashtra Shear Zone, western India

<p>The pressure-temperature conditions are transient in time and space during tectonic processes. To understand the complete P-T history of crustal domains examining the mineral paragenetic sequences and zoning profiles of minerals from diverse lithologies in the domain is necessary. But in highly tectonised crustal domains establishing time equivalence between far-spaced samples is difficult. To overcome this, a mylonite sample with closely spaced layers of different mineralogy collected from the South Maharashtra Shear Zone located along the north of Western Dharwar Craton (Rekha and Bhattacharya, 2014) was studied. The mylonite has four mineralogically distinct layers of few millimeters width containing garnet porphyroblasts of distinct zoning pattern separated by quartz layers. Layer-1 has two domains on the basis of the relative abundance of quartz; Layer-1A with more quartz and less flaky minerals and Layer-1B with less quartz and more flaky minerals. Layer-1A is composed of quartz>biotite>plagioclase>chlorite>K-feldspar with syn- to post-tectonic garnet porphyroblasts and the fabric is defined by shape preferred biotite-chlorite aggregates, recrystallized plagioclase and quartz ribbons.Layer-1B is relatively quartz poor and plagioclase>biotite>chlorite>K-feldspar aggregates rich domain as compared to L1A with biotite-chlorite aggregates and recrystallized plagioclase defined fabric.Prehnite elongated parallel to schistosity present but not very common. Layer-2 is very thin with amphibole-biotite±chlorite defined foliation and consists of plagioclase-K-feldspar-quartz with large garnet porphyroblast ofsyn to post-tectonic origin. Chlorites are mainly present near to garnet. Layer-3 is composed of biotite-calcite-plagioclase-chlorite-quartz with syn/post-tectonic garnet porphyroblast and the foliation is defined by biotite-chlorite aggregates, recrystallize plagioclase, calcite grains aligned parallel to the foliation and elongated quartz grains.Layer-3 is separated from the quartz layers on both sides by the formation of thin hornblende layers arranged parallel to the foliation. Very few hornblende grains found within the layer aligned parallel with the fabric defining minerals. Large pre-tectonic muscovite grains are preserved in Layer-3 and are altered to epidote along the margins of the grain. Layer-4 consists of hornblende, calcite, quartz with few plagioclase, K-feldspar and post tectonic garnet porphyroblast. The fabric is defined by the long axis of amphibole and calcite grains aligned parallel to it. Later biotite-prehnite grains formed at high angle to the fabric defining minerals. Conventional geothermobarometers were used for P-T estimation and it varies from 450-560°C and 6 kbar for Layer-1A, 445-550°C and 7 kbar for Layer-1B, 475-570°C and 6 kbar for Layer-2, 450-575°C and 7-8 kbar for Layer-3 and 450-5500°C and 7-9 kbar for Layer-4 at reference temperature of 500°C and pressure of 6kbar. Though different layers have distinctly different mineral assemblages there is hardly any variation in the P-T conditions which implies the original bulk rock composition was different for different layers not the P-T conditions of deformation.</p><p><strong>Keywords:</strong> Mylonite, Western Dharwar Craton, Geothermobarometry</p>

Deformation and reaction fabrics in eclogites from the Western Gneiss Region (Norway) - evidence of dehydration reactions attributed to episodic deformation

<p>The eclogites from Vårdalneset, Western Gneiss Region, Norway, show an exceptional large variety of reaction and deformation microfabrics that document the processes and conditions during burial and exhumation. Coarse grained eclogites comprise about 35% omphacite, 25% garnet and 20% amphibole with various amounts of white mica, zoisite, kyanite, rutile, zircon and pyrite. Their fabric is characterized by few mm long and several hundred µm wide amphibole and omphacite grains aligned in the foliation plane with zoned garnet porphyroblasts up to several mm in diameter. In contrast, finer-grained mylonitic eclogites with grain diameters of few hundred µm comprise systematically higher amounts of garnet (45%) and omphacite (35%) and generally less amphibole (< 5%) but similar amounts of zoisite, white mica, rutile and quartz. In the coarse-grained eclogite, amphibole shows evidence of dislocation creep as indicated by undulatory extinction, subgrains and recrystallized grains in necks of boudinaged coarse amphibole layers as well as in contact to garnet. The large garnet porphyroblasts generally show a complex zonation with an inclusion-rich Fe-poor and Mg-rich inner core surrounded by a zone with Fe- and Ca-rich patches and a broad Mg-rich, Ca- and Fe-poor rim. Only at contact to coarse amphibole an additional, a few tens of µm thin serrated rim further enriched in Mg can occur. At the direct contact to such serrated Mg-rich rims, amphibole is partly replaced by a fine-grained quartz-kyanite ± rutile aggregate, indicating dehydration reactions of amphibole. Quartz - kyanite ± rutile aggregates are surrounding garnet also in contact to omphacite, zoisite and to other garnet crystals. The microstructures suggest that deformation and dehydration of amphibole are coupled and played an important role during deformation of the eclogites finally leading to the mylonitic eclogites with higher amounts of garnet and omphacite. Deformation is suggested to have triggered the dehydration reaction by a slight and local increase in temperature. Furthermore, deformation provided additional pathways for the escaping fluids along the increased grain and phase boundary area, as indicated by commonly present quartz within interstitials between recrystallized amphibole grains. In all samples, few µm wide amphibole rims replacing garnets document restricted rehydration-reactions at a later stage. The large variety of the deformation and reaction microfabrics exemplarily show that both deformation and metamorphic reactions did not proceed at long-term continuous conditions, but that both are coupled and occurred episodically.</p>

Microstructure characteristics of the ganet-bearing schist from Nam Co formation, Son La area, Song Ma suture zone, Northwestern Vietnam

The garnet-bearing schists of the Nam Co formation have an identical mineral assemblage consisting of garnet, chlorte, albite, quartz and muscovite, together with accessory apatite, zircon, monazite, xenotime, and ilmenite. An aggregate of muscovite and chlorite defines the major foliations (Sn). Both albite and garnet occur as a porphyroblast, ranging in size 0.2÷1 mm and 0.5÷1.2 mm, respectively. Albite porphyroblasts commonly have the curved to sigmoidal inclusion trails defined by graphitic materials (Sn-1). Garnet porphyroblasts in the sample is generally characterized by paucity of inclusions and retrograde corona of bitotite and chlorite. Garnet also occurs as an inclusion within albite porphyroblast. Porphyroblastic garnet shows the compositional zonation typified by a bell-shaped spessartine profile balanced by increasing almandine from core to rim. Whereas, inclusion garnet is homogeneous compositions with rich in almandin and poor in spessatin, pyrop and grossula. All the above microstructures suggest two deformation and metamorphic stages (M1 and M2) that were affected to politic rocks of the Nam Co formation, Song Ma suture zone.

Preliminary data for melt inclusions in garnet porphyroblasts from Ograzhden metapelites, SW Bulgaria

Micro-inclusions in garnet porphyroblasts from high-grade Ograzhden metapelites, SW Bulgaria, have been studied by SEM and micro-Raman Spectroscopy. Micro-inclusions are presented by single grains with facetted outlines parallel to rational crystallographic orientations of the host garnet or by multiphase aggregates with negative crystal shape. Many of studied micro-inclusions can be formed by the presence of melt. The morphology of some of them suggests formation under high pressure metamorphism.