Near-isothermal exhumation of lower crust in the Caledonian Orogen: Metamorphic path of kyanite eclogite from the Danmarkshavn area, North-East Greenland Caledonides

<p>Kyanite eclogite from the North-East Greenland Caledonides – the upper plate of the Caledonian orogeny – preserves a mineral assemblage and petrographic texture that are consistent with an initial near-isothermal exhumation path. Two medium-grained kyanite eclogites from the Danmarkshavn area (76°46’N, 18°40’W) located west of the Germania Land shear zone contain the peak assemblage of garnet + omphacite + kyanite + phengite + amphibole + rutile. Subhedral garnet encloses monomineralic omphacite and polymineralic inclusions of clinopyroxene + plagioclase ± quartz ± amphibole ± K-feldspar ± kyanite. X-ray mapping of garnet indicates a homogenous core with a composition of Py<sub>51–52</sub>Alm<sub>28–29</sub>Gr<sub>19–20</sub>Sp<sub>0–1</sub>, along with a slightly zoned rim of Py<sub>54</sub>Alm<sub>31</sub>Gr<sub>15</sub>Sp<sub>1</sub> that is replaced by a corona of symplectitic amphibole + plagioclase. Omphacite (X<sub>Na</sub> up to 0.41), rarely present in the matrix, is indicated by symplectite of clinopyroxene + amphibole + plagioclase. Symplectites of corundum + plagioclase, spinel + plagioclase and sapphirine + plagioclase replace former kyanite. These symplectites are typically surrounded by a plagioclase corona with decreasing Ca (from X<sub>An</sub> = 92–97 to X<sub>An</sub> = 47–53) from the symplectite to the matrix. Isochemical phase equilibrium modeling along with homogenous garnet core and peak omphacite compositions yielded a peak metamorphic pressure-temperature (<em>P-T</em>) condition at 1.9 GPa, 840 ˚C. Assuming local equilibrium at the microscopic scale, an attempt to model a symplectite of spinel + sapphirine + plagioclase after kyanite using a pseudosection yielded estimated <em>P-T</em> conditions at 0.8–1.3 GPa and 700–900 ˚C. Integrating the calculated <em>P-T</em> conditions and previous geochronological results, an initial exhumation path from 1.9 GPa to ~1.0 GPa from ~415–390 Ma to ~375 Ma is nearly isothermal at around 800 ˚C.</p>

Oxidized slab fluids revealed in metasomatized eclogites: A case study from Syros, Greece

<p>Garnet-epidote oxybarometry, major element mineral compositions, and textural analysis of eclogites from Syros, Greece reveal the presence of high fO<sub>2</sub> slab fluids. The four investigated samples from blocks hosted in the serpentinite matrix mélange on Syros fall into three categories: unmetasomatized eclogite (Type I, n = 1), heavily metasomatized garnet-clinopyroxene bearing rocks (Type II, n = 2), and an eclogite which hosts veins of Andr + Acm + Ep + Hem + Chl (Type III, n = 1). Type I samples of metagabbroic eclogite are characterized by a peak assemblage of Grt + Cpx + Rt with abundant clinozoisite after lawsonite. In addition to Grt + Cpx, Type II samples contain Chl + Ilm + Py + Ap ± Ep ± Cam. Clinopyroxene compositions within Type I samples display a prograde trend of increasing jadeite and decreasing acmite. In contrast, the matrix clinopyroxene in one Type II sample exhibits compositions up to 60 % acmite component. Alternatively, clinopyroxene in the second Type II sample investigated exhibit an increase in acmite component during metasomatism. Type II samples also contain epidote and hematite-rich ilmenite as opposed to clinozoisite and rutile. The association of Fe<sup>3+</sup>-rich phases with sulfides, such as inclusions of acmitic pyroxene in pyrite, in Type II samples suggests a temporal link between sulfide deposition and oxidation of Fe<sup>2+</sup> to Fe<sup>3+</sup>. In contrast, the Adr + Acm + Ep + Chl + Hem veins in the Type III sample are sulfide absent and suggest fluids with <em>f</em>O<sub>2</sub> above the Hem-Mag and Hem-Py buffers.</p><p>Garnet-epidote oxybarometry revealed elevated <em>f</em>O<sub>2</sub> in metasomatically altered samples. Calculations were performed using a new oxybarometry Matlab code written by the authors. Our code utilizes the latest thermodynamic database, <em>A-X</em> models, and equations of state implemented in THERMOCALC. The code was also implemented in the XMapTools software package for quantitative visualization of <em>f</em>O<sub>2</sub> using EPMA X-ray maps. Fugacity calculations were conducted at 550 °C, 2.0 GPa, and an <em>a</em>H<sub>2</sub>O of unity, unless otherwise stated. Oxygen fugacities for clinozoisite-garnet pairs in the Type I sample were calculated in XMapTools and fall within 0.5 log units of the quartz-fayalite-magnetite (QFM) buffer. Inclusions of garnet in epidote and epidote overgrowths on garnet were selected for <em>f</em>O<sub>2</sub> calculation in the Ep-bearing Type II sample. These garnet-epidote pairs exhibit <em>f</em>O<sub>2</sub> of QFM+1.9 to +2.0. A minimum <em>f</em>O<sub>2</sub> of QFM+4 calculated from the Hem-Mag buffer is estimated for Type III veins. The remarkably high <em>f</em>O<sub>2</sub> of Type III veins contrasts with prograde <em>f</em>O<sub>2</sub>conditions of QFM+1 to +2 estimated for epidote inclusions in garnet cores from the same sample at 400-450 °C and 1.0-1.5 GPa. While elevated <em>f</em>O<sub>2</sub> and acmite inclusions in pyrite are consistent with a SO<sub>x</sub>(aq)-Fe<sup>2+</sup> redox pair in Type II samples, <em>f</em>O<sub>2</sub> above the Hem-Mag buffer require the bulk addition of Fe<sup>3+</sup> or Mn<sup>3+</sup> as a carrier of oxidation. These data demonstrate that slab fluids may impose <em>f</em>O<sub>2</sub> well above the sulfur-sulfur oxide buffer.</p>

Petrography and phase equilibria modeling of mid-pressure aluminous gneisses derived from hydrothermally altered protoliths, Grenville Province, Canada

A large portion of the hinterland of the central Grenville Province is characterized by mid-pressure granulite-facies metamorphic rocks. In the Manicouagan – Lac du Milieu region of Quebec, aluminous gneisses derived from hydrothermally altered felsic protoliths provide a record of anatectic processes and of the pressure–temperature (P–T) evolution during orogenesis. Samples collected in areas separated by several tens of kilometres consist of garnet + biotite + quartz + K-feldspar ± plagioclase ± sillimanite, with retrograde cordierite in some, and most display microstructural evidence of partial melting. However, they have a wide range of bulk compositions and textures, and may be grouped into two types: sillimanite-rich rocks, with a high alumina index and sillimanite-poor rocks with low alumina index and in which sillimanite is not part of the peak assemblage. Phase equilibria modeling with THERMOCALC software constrained the P–T field of the peak mineral assemblage(s) at 800–900 °C and 6–11 kbar (1 kbar = 100 MPa), with melt solidification in the range of 800–865 °C and 6–8 kbar. The presence of sillimanite inclusions in garnet, and the scarcity of retrograde cordierite are consistent with moderate dP/dT gradient “hairpin” P–T paths. The data suggest that this part of the mid-P hinterland in the central Grenville experienced a rather uniform high-temperature metamorphic evolution, with large temperature variations relative to pressure. This is consistent with the thermal evolution of middle crust beneath an orogenic plateau.

Time–space relationships among structural and metamorphic aureoles related to granite emplacement: a case study from the Serre Massif (southern Italy)

Tectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10−14 s−1 within the country rocks during granodiorite intrusion.

Multi-stage dehydration–decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy)

Porphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx+Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl±Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. PreferredP–Testimates are: ∼900 °C and ∼1.1 GPa at the metamorphic peak; ∼850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. TheP–T–tpath agrees with the path shown by the uppermost migmatites of the Serre section, and theP–Testimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

Ultra-high pressure aluminous titanites in carbonate-bearing eclogites at Shuanghe in Dabieshan, central China

Petrographic features and compositions of titanites in eclogites within the ultra-high pressure metamorphic terrane in central Dabieshan are documented and phase equilibria and thermobarometric implications discussed. Carbonate-bearing eclogite pods in marble at Shuanghe contain primary metamorphic aluminous titanites, with up to 39 mol.% Ca(Al,Fe3+)FSiO4 component. These titanites formed as part of a coesite-bearing eclogite assemblage and thus provide the first direct petrographic evidence that AlFTi−1O−1 substitution extends the stability of titanite, relative to rutile plus carbonate, to pressures within the coesite stability field. However, it is emphasised that A1 and F contents of such titanites do not provide a simple thermobarometric index of P—T conditions but are constrained by the activity of fluorine, relative to CO2, in metamorphic fluids — as signalled by observations of zoning features in these titanites.These ultra-high pressure titanites show unusual breakdown features developed under more H2O-rich amphibolite-facies conditions during exhumation of these rocks. In some samples aluminous titanites have been replaced by ilmenite plus amphibole symplectites, in others by symplectitic intergrowths of secondary, lower Al and F, titanite plus plagioclase. Most other coesite-bearing eclogite samples in the central Dabieshan terrane contain peak assemblage rutile often partly replaced by grain clusters of secondary titanites with customary low Al and F contents.