Metamorphic Petrology

Alpine metamorphic and tectonic evolution of the Inzecca-Ghisoni area (southern Alpine Corsica, France)

by Florence ReSeT

Francesca Garfagnoli, Francesco Menna, Enrico Pandeli, Gianfranco Principi

In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units,... more In the Inzecca-Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units, affected by Alpine deformations and metamorphism, crops out. Among them, there are some tectonic units (Parautochthonous Units, i. e. parautochtone of previous studies), that represent fragments of the continental Corse basement (Palaeozoic granitoids and associated volcanic and metamorphic pre-Carboniferous rocks) and of its Mesozoic to Tertiary sedimentary cover, that are tectonically sliced between the allochthonous Ligurian-Piedmontese Units (Schistes Lustrés) and the autochthonous basement (Variscan Corsica). The reconstructed polyphase deformation and metamorphic evolution of such units and the finding of high-pressure/low-temperature mineral assemblages in the continental-derived tectonic slices, points to the involvement of the south-eastern border of the European basement of Corsica in the tectonic processes linked to the Alpine subduction

New experimental data on the antigorite dehydration in silica enriched serpentinite

by José Alberto Padrón-Navarta

published in 'Contributions to Mineralogy and Petrology'

Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure... more Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, southern Spain). At 630–660C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The
second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al2O3) and chromium contents (0.59 wt% Cr2O3) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680–710C and 1.6–1.9 GPa.

Fluid transfer into the wedge controlled by high-pressure hydrofracturing in the cold top-slab mantle

by José Alberto Padrón-Navarta

Published in Earth and Planetary Science Letters, 2010

Before attaining the mantle wedge, where they trigger partial melting, volatiles released from dehydration reactions... more Before attaining the mantle wedge, where they trigger partial melting, volatiles released from dehydration reactions in the slab have to migrate across a relatively cold (<750 °C), peridotite-layer above the incoming slab. In order to unravel the mechanisms allowing for this initial stage of fluid transport, we performed a detailed field and microstructural study of metamorphic prograde peridotites in the Cerro del Almirez ultramafic massif (Betic Cordillera, Spain), where evidences of one of the most important dehydration reactions in subduction zones, the high-pressure antigorite breakdown (P=1.6–1.9 GPa and T≈680 °C), can be mapped in the field. This reaction led to arborescent growth of centimeter-size olivine and orthopyroxene, producing a chlorite–harzburgite with a spinifex-like texture. Microstructural observations and crystal preferred orientations (CPO) mapping show no evidences of solid-state deformation during the prograde growth of olivine and orthopyroxene at the expenses of antigorite. However, a few tens to a hundred meters away from the reaction front, the metamorphic texture is partially obliterated by grain-size reduction in roughly planar conjugate zones, a few mm to meters wide. Grain size reduction zones (GSRZ) are characterized by (1) sharp contacts with undeformed spinifex-like texture domains, (2) important reduction of the olivine grain size (60–250 μm), (3) olivine color change from brownish to colorless, (4) decrease in the modal amount of orthopyroxene, and (5) at the mm- to cm scale, irregular shapes and abrupt terminations. Field and microstructural observations exclude that relative displacement took place across these GSRZ. Changes in modal composition imply reactions with fluids undersaturated in silica. Analysis of olivine crystal-preferred orientations (CPO) in GSRZ shows patterns similar, but more dispersed, than those in neighboring spinifex-like domains. It also reveals mm- to cm-scale discrete domains with rather homogeneous crystallographic orientations suggesting inheritance from the preexisting spinifex-like olivines in the host peridotite. Misorientation angles between neighboring grains in the GSRZ show peaks at 5–10° and 20°, but rotations are not crystallographically controlled. Based on these observations, we rule out the formation of the GSRZ by dynamic recrystallization during dislocation creep and propose that they record brittle deformation (microcraking) of the spinifex-like chlorite–harzburgite, probably induced by hydrofracturing at high pressure and relative low temperature conditions (680–710 °C). High-pressure hydrofracturing can, thus, be invoked as an efficient mechanism for fluid flow across the cold top-slab mantle layer, hence allowing the slab-derived fluids to ingress in the wedge.

H2O and the dehydroxylation of phyllosilicates: An infrared spectroscopic study

by Simon Redfern

Petrology, fluid inclusions and metamorphic history of Bhopalpatnam granulites, central India

by Hari K R

For reprint request, please send a mail to krharigeology@gmail.com

New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine …

by Reia M. Chmielowski

This is an additional collaboration I was involved in with a colleague from India, who requested my assistance with the geothermobarometric calculations.

The Cambrian Metamorphic History of Tasmania

by Reia M. Chmielowski

This is my PhD research, completed in 2009 at the University of Tasmania.

The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania.... more The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania. Chemical U-Th-Pb monazite dating reveals that the peak episode of metamorphism took place in the Cambrian, with a weighted mean age for all units analysed of 505 ± 1 Ma. However, variations in the results by region range from ~ 511 to ~ 497 Ma. The pelitic schists of the Franklin Metamorphic Complex contain garnet porphyroblasts which record a rapid, nearly isothermal, pressure increase; the garnet cores formed at ~ 600o C, 6,000 bars and the rims at ~ 700o C, 14,000 bars at 511 ± 3 Ma. Likewise, the eclogite from the same region records a change from ~ 550o C, 6,250 bars to ~ 650o C, 19,000 bars. The whiteschist, which was obtained from the opposite side of a major local fault, formed garnet cores at ~ 545o C, 19,600 bars; its garnet rims and matrix minerals formed at 506 ± 5 Ma after an increase in temperature of at least 30-90o C. All of these units show evidence of very rapid isothermal exhumation. Other Franklin Metamorphic Complex fault blocks record P/T for peak conditions at ~ 570o C, 8,600 bars (Mt. Mary), and ~ 700o C, 11,400 bars (Raglan Range). The Forth Metamorphic Complex achieved peak metamorphism at 509 ± 7 Ma, at conditions of 670o C, 16,900 bars, and the nearby Settlers Schist gives results of 513 ± 8 Ma. The garnet porphyroblasts of the Port Davey Metamorphic Complex record a single episode of metamorphism which took place at 505 ± 2 Ma at ~ 550 to 570o C and ~ 6,000 bars during which a dehydration event resulted in both a change of garnet composition and texture. The regional geology indicates metamorphism predated post-collisional extension and associated eruption of the Mount Read volcanics at 506 to 500 Ma. Most of the monazite dating is consistent with this observation. However, the Mersey River Metamorphic Complex gives very consistent results of 497 ± 3 Ma. This is problematical, as it would have been at depth undergoing metamorphism after that extension took place. This could be the result of an unknown analytical problem, but the Mersey River monazite grains are indistinguishable chemically from monazite in the other units, and this sample has undergone repeated analysis. Alternatively, this sample reflects a different metamorphic event than that recorded in all other samples studied.

Petrology of the eclogites from western Tasmania: Insights to the Cambro-Ordovician evolution of the paleo-Pacific margin of Gondwana. Lithos

by Reia M. Chmielowski

Palmeri, R., Chmielowski, R. M., Sandroni, S., Talarico, F. and Ricci, C. A. in-press: Lithos (available on line as of July 2008)

Eclogite-facies rocks along the Paleozoic active margin of Gondwana are rare. They are limited to isolated segments of... more Eclogite-facies rocks along the Paleozoic active margin of Gondwana are rare. They are limited to isolated segments of northern Victoria Land (Antarctica), western Tasmania and south-eastern Australia. New petrological data for mafic rocks and their host garnet-kyanite schists from the Franklin Metamorphic Complex (western Tasmania) permit reconstruction of six main stages of mineral growth for the eclogite.  Stage I and II occurred at greenschist/amphibolite-facies conditions (ca. 500-600 °C; 0.55-0.7 GPa for stage II) before attaining high pressure conditions (at ≈600-650 °C; > 1.5 GPa - stage III). The following stages, IV and V, record the decompression from high pressure conditions to amphibolite facies (ca. 500-600 °C; 0.4-1.0 GPa). Finally, stage VI represents the late greenschist-facies retrogression. However, the pelitic schist, surrounding the eclogite, records only the medium-pressure amphibolite-facies stage. The P-T evolution over time outlines a clockwise path which is quite steep both in the prograde and retrograde segments. The latter shows a nearly isothermal decompression between the eclogite and the high pressure amphibolite-facies stage IV which was achieved at deep crustal levels (≈ 30 km), and a final decrease in both pressure and temperature from deep/intermediate to shallow crustal levels with a typical cooling-unloading path. The lack of a complete re-equilibration during the different stages and the high dP/dT for both the prograde and retrograde paths are indicative of a rapid burial and initially rapid exhumation. The similarity of the mafic whole rock chemical composition including N, T to E – MORB and of the peak metamorphic age (≈ 500 Ma) between the Tasmanian eclogites and the UHP rocks from northern Victoria Land supports the idea that they formed in the context of the same contractional event. However, the different P-T conditions and dP/dT point to different tectono-metamorphic settings for the two sectors of the paleo-Pacific margin of Gondwana during the Ross/Tyennan orogeny.

Citation: Xypolias, P., I. Iliopoulos, V. Chatzaras, and S. Kokkalas (2012), Subduction-and exhumation-related structures in the Cycladic Blueschists: insights from …

by Ioannis Iliopoulos

Metamorphic Record of High-pressure Dehydration of Antigorite Serpentinite to Chlorite Harzburgite in a Subduction Setting (Cerro del Almirez, Nevado–Filábride Complex, Southern Spain)

by José Alberto Padrón-Navarta

Published in 'Journal of Petrology', 2011

The antigorite dehydration front preserved in the Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, SE... more The antigorite dehydration front preserved in the Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, SE Spain) offers a unique opportunity to investigate the high-pressure prograde breakdown of antigorite serpentinite (Atg-serpentinite) to chlorite harzburgite (Chl-harzburgite; olivine + orthopyroxene + chlorite + tremolite) in a subduction setting. Our detailed mapping of the antigorite dehydration front shows that in the transition between Atg-serpentinite and prograde Chl-harzburgite a narrow band of transitional lithologies occurs that consists of chlorite–antigorite–olivine-serpentinite (Chl-serpentinite), grading through antigorite–chlorite–orthopyroxene–olivine (Atg–Chl–Opx–Ol) rocks and, subsequently, to Chl-harzburgite. Just beneath the reaction front, Chl-harzburgite shows everywhere a granofelsic texture; however, down section, it is interspersed in decameter-sized domains with spinifex-like Chl-harzburgite. Field and textural relationships, mineral compositions and chemographic phase relations indicate that the prograde breakdown of Atg-serpentinite to Chl-harzburgite occurred through a series of continuous metamorphic reactions forming transitional lithologies. Chl-harzburgites with granofelsic and spinifex-like textures were formed under similar P–T conditions (1·6 – 1·9 GPa and 680–710°C). We ascribe their differing textures to shifts in the growth rate of the products caused by temporal and spatial fluctuations in the affinity of the antigorite-breakdown reaction driven by cyclic variations of the fluid pressure. Crystallization at a low affinity of the antigorite-breakdown reaction, producing granofelsic Chl-harzburgite, was attained by a slow drain of fluids from the dehydrating system. With the advancement of the dehydration front overpressured domains were left behind, preserving serpentinite that was highly metastable under lithostatic pressure. Brittle failure of rocks surrounding these overpressured domains resulted in an increase in their permeability, a fast drop in fluid pressure towards hydrostatic pressure, and sudden stabilization of the prograde reaction products. This process resulted in the crystallization of the prograde assemblage under a high affinity of the antigorite-breakdown reaction, accounting for disequilibrium fast growth of spinifex-like Chl-harzburgite. Prograde textures in the Cerro del Almirez hence witness the feedback between the rate of metamorphic reactions and the cyclic dynamics of metamorphic fluid expulsion during high-pressure serpentinite dehydration in a subduction setting.

Nb–Ta fractionation by partial melting at the titanite–rutile transition

by J. Elis Hoffmann

Timm John, Reiner Klemd, Stephan Klemme, Jörg A. Pfänder, J. Elis Hoffmann, Jun Gao (2011)
Contributions to Mineralogy and Petrology 161, 35-45.

During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean... more During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean granitoids of the tonalite–trondhjemite–granodiorite (TTG) suite, which represent the Earth’s early continental crust, show larger Nb/Ta variations than any other Earth reservoir. This implies that significant Nb–Ta fractionation must have occurred during early crust formation, while the underlying mechanism behind is still unclear. Here, we present a new model on how Nb may be fractionated from Ta during partial melting of subducted oceanic crust. Our data show that Nb/Ta ratios in melts derived from rutile- and titanite-bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source. High modal ratios of titanite over rutile generate melts with very high Nb/Ta (>60), whereas low modal titanite/rutile produces melts with much lower Nb/Ta (≤30). Very low Nb/Ta (<16) occur when all Ti-phases are consumed at very high degrees of melting. As the modal ratio of titanite to rutile is a function of pressure, the Nb/Ta of melts is a function of melting depth. Our new model helps to explain the extreme variation of Nb/Ta observed in many TTGs and thus how Nb and Ta were fractionated during the early evolution of the Earth. Furthermore, the model also indicates that simple one-stage melting models for mafic crust are not sufficient to explain the formation of TTGs.

Extensional Flow during Gravitational Collapse: A Tool for Setting Plate Convergence (Padrón Migmatitic Dome, Variscan Belt, NW Iberia)

by Juan Gomez Barreiro

Early Mesozoic evolution of the western U.S. Cordillera

by Cathy Busby

Saleeby, J. B., and Busby-Spera, C. J., 1992, Early Mesozoic evolution of the western U.S. Cordillera, In B. C. Burchfiel, P. W. Lipman and M. L. Zoback (eds.) The Cordilleran Orogen:  Conterminus United States:  Decade of North American Geology, Geological Society of America, v. G-3, p. 107-168.

Age constraints on Lower Paleozoic convection system: Magmatic events in the NW Iberian Gondwana margin

by Juan Gomez Barreiro

A rootless suture and the loss of the roots of a mountain chain: the Variscan belt of NW Iberia

by Juan Gomez Barreiro

Power and pitfalls of trace element geochemistry in zircon from high-temperature–high-pressure rocks: some examples from NW Spain

by Juan Gomez Barreiro

REE-assisted U-Pb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of the A Silva granodiorite, Iberian Allochthonous Complexes

by Juan Gomez Barreiro

Thrust and detachment systems in the Ordenes Complex (northwestern Spain): Implications for the Variscan-Appalachian geodynamics

by Juan Gomez Barreiro

Paleozoic ophiolites in the Variscan suture ofGalicia (northwest Spain): Distribution, characteristics, and meaning

by Juan Gomez Barreiro

Careón Ophiolite, NW Spain: Suprasubduction zone setting for the youngest Rheic Ocean floor

by Juan Gomez Barreiro