Alexander Proyer

The preservation of high-pressure rocks during exhumation: metagranites and metapelites

Metagranites in the NKFMASH system require external hydration during prograde high-pressure metamorphism in order to equilibrate to ambient HP conditions by producing more siliceous muscovite. Any lack of external fluid or the disappearance of biotite stops re-equilibration and thus prevents recording of high-pressure conditions. The same hydration reactions cause dehydration during exhumation. Orthogneiss from shear zones or adjacent to metapelites and metabasites will take up external fluid during subduction and record the highest P–T conditions, but will also be the first to dehydrate upon exhumation, now hydrating other lithologies and probably refuelling shearzones. The (de)hydration behavior of Ca-bearing metagranitoids is similar to that in the Ca-free system. However, the anorthite component of plagioclase decomposes with increasing pressure to form either (clino)zoisite or a grossular-rich garnet. Both reactions are fluid-consuming. If H2O is supplied from an external source, the garnet-bearing assemblage can record P–T conditions up to very high pressures, but dehydrates again during heating and/or decompression to form a more Fe-rich garnet and Al-rich mica(s). The garnet compositions observed in natural HP-metagranites are mostly too Fe-rich to be formed in the presence of an H2O-rich fluid. N(C)KFMASH metapelites generally have a more complex mineralogy and succession of mineral assemblages along a P–T path. The H2O contained in hydrous silicates like chlorite and chloritoid is only partly released, but partly transferred to other minerals like paragonite, glaucophane or phengite during subduction and further dehydration during exhumation is common. The mineral assemblage preserved by the rock may then record P–T conditions way below those of the actual pressure and temperature peak of the path. Contouring of the P–T pseudosection of a specific metapelite composition with mode isopleths for H2O shows which P–T conditions along a given path are the ones most likely recorded by the rock.

Relics of high-pressure metamorphism from the Grossglockner region, Hohe Tauern, Austria: Paragenetic evolution and PT-paths of retrogressed eclogites

Retrogressed eclogites occur embedded in mostly calcareous micaschists and greenschists of the Upper Schieferhulle in the Grossglockner region of the central Tauern Window, east of the Eclogite Zone. A four-stage meta-morphic evolution has been derived from textural and mineral chemical observations: Relics of early pre-eclogitefacies events (stage I: chlorite, actinolite, plagioclase and glaucophane, paragonite, clinozoisite) are preserved mainly in the cores of gamets. The peak-metamorphic paragenesis (stage II) of garnet, omphacite, paragonite, glaucophane, (clino)zoisite, quartz and rutile ± phengite and dolomite records conditions of around 17 kbar and 570°C, slightly below those reported from the Eclogite Zone (around 600°C, 20 kbar). In several instances, growth of coarse-grained barroisitic to actinolitic amphibole occurred during uplift, apparently still within the eclogite facies (stage III). As it is impossible to reconcile the observed amphibole growth textures at the expense of omphacite, glaucophane, garnet, paragonite and quartz with a closed-system reaction, metasomatic interaction must have played a role in the formation of these rocks. The final emplacement in the present tectonic setting and co-metamorphism with the surrounding metasediments (stage IV) occurred under conditions of about 5-6 kbar and 500-530°C. It caused severe hydration and retrograde alteration (symplectite formation), transformed most of the eclogite bodies into garnet amphibolites and even green-schists, with a new, strong stage-IV foliation parallel to that of the country rocks, and erased most evidence of the earlier PT-path. Evidence of eclogite-facies metamorphism of the country rocks has been found only in the westernmost eclogite occurrence. Since the lower tectonic units, the gneiss domes and the Lower Schieferhulle, have not yet produced any evidence of Alpine eclogite-facies metamorphism either, we prefer the interpretation that one major tectonic slice -the Eclogite Zone—and several minor ones -those found within the Upper Schieferhulle nappes today—have been tectonically injected into the evolving Penninic nappe stack during underthrusting of the Penninic units underneath the Austroalpine.

High-pressure/low-temperature metamorphism of basalts in Lavrion (Greece): implications for the preservation of peak metamorphic assemblages in blueschists and greenschists

The Upper Tectonic Unit of the Lavrion area is part of the Attic-Cycladic blueschist belt and was affected by high-pressure, low-temperature metamorphism. Blueschists and greenschists occur in the same outcrop and are believed to have experienced the same pressure-temperature (P-T) history which has been quantified using geothermobarometry and pseudosections for specific bulk-rock compositions. Calculated P-T conditions indicate minimum pressure of ∼ 0.9 GPa and temperature of ∼ 370 °C for the peak of metamorphism. The prograde and retrograde paths followed a very similar low geothermal gradient (10-12 °C/km) with cooling during decompression. Pseudosections show that both blueschists and greenschists can exist stably at the metamorphic peak, the dominant amphibole being a function of bulk composition: the blueschists, on average, have lower Mg# than the greenschists, which results in a larger P-T stability field of blue amphibole. A pseudosection analysis of the dehydration behaviour indicates that blueschists and some greenschists can preserve their peak assemblages (no dehydration along the retrograde path), whereas greenschist assemblages, in general, are rather prone to undergo dehydration and hence re-equilibration to lower P-T conditions during exhumation.

TiO2 exsolution from garnet by open-system precipitation: evidence from crystallographic and shape preferred orientation of rutile inclusions

We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt<111> or describes cones with opening angle 27.6° around Grt<111>. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with microchemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as “open-system precipitation” (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of Fe2+ to Fe3+ and transfer from the dodecahedral to the octahedral site just vacated by

Ti- and Zr-minerals in calcite-dolomite marbles from the ultrahigh-pressure Kimi Complex, Rhodope mountains, Greece: Implications for the P-T evolution based on reaction textures, petrogenetic grids, and geothermobarometry

{\text{Ti}}^{ 4+ }: 6\,{\text{M}}^{ 2+ }_{ 3} {\text{TiAl}}\left[ {{\text{AlSi}}_{ 2} } \right]{\text{O}}_{ 1 2} + 6\,{\text{M}}^{ 2+ }_{ 2, 5} {\text{TiAlSi}}_{ 3} {\text{O}}_{ 1 2} = 10\,{\text{M}}^{ 2+ }_{ 3.0} {\text{Al}}_{ 1. 8} {\text{Fe}}_{0. 2} {\text{Si}}_{ 3} {\text{O}}_{ 1 2} + {\text{M}}^{2+} + 2 {\text{e}}^{-} + 1 2\,{\text{TiO}}_{ 2} .

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

OSP is likely to occur at conditions where the transition of natural systems to open-system behaviour becomes apparent, as in the granulite and high-temperature eclogite facies.

Pitfalls in geothermobarometry of eclogites: Fe3+ and changes in the mineral chemistry of omphacite at ultrahigh pressures

The overall metamorphic reaction in an impure calcite marble from the ultra-high pressure (UH P ) zone at Changpu, eastern Dabie Shan, China, is partitioned into local reaction domains defined by isolated grains of omphacite, garnet, epidote, quartz and ilmenite within the calcite matrix. These reaction domains witness different stages of the metamorphic evolution. Chemically homogeneous omphacite in some instances has partial or complete coronas of quartz which are interpreted as a prograde phenomenon, i.e . caused by replacement of a plagioclase precursor by omphacite with increasing pressure. Rare relics of talc in epidote indicate that the first breakdown reaction during exhumation was talc + omphacite + garnet = amphibole + epidote ± quartz (still within the eclogite facies). During this early retrogression stage, omphacite was partially or completely rimmed by chemically heterogeneous amphibole. Most garnets developed a thick corona of epidote plus some quartz and calcite during this early retrogression or are completely pseudo-morphed. Where garnet occurs near or next to omphacite, this corona consists partly or entirely of amphibole. The second retrograde stage is commonly characterized by the breakdown of omphacite + quartz to albite + diopside + amphibole, often in the form of symplectite. Localized reactions become evident as omphacite at its site can decompose to three distinct types of symplectite (albite + diopside, albite + amphibole, albite + diopside + amphibole). Matrix quartz grains develop rims of individual diopside crystals at distances up to 2–3 millimetres from symplectites; additional outer rims of albite develop adjacent to garnet or corona epidote. Matrix quartz remote from omphacite sites is locally transformed into albite, where an Al-bearing mineral (mostly epidote) is found nearby. Epidote in contact with quartz develops albite coronas as it decomposes into albite + minute grains of Ce-rich epidote (allanite). Primary epidote and corona epidote remote from any quartz develop a complex compositional zoning with enrichment of mainly Ce 3+ at the outer rim, indicating exposure to a fluid undersaturated in alumina. Finally, large ilmenite grains in the matrix develop thick overgrowths of titanite, but in several instances an intermediate step of rutile formation is preserved. All these features indicate a direct correlation of textures with transport distances of dissolved chemical species in a non-pervasively infiltrating pore fluid. Simplified open-system reactions were derived for each site at each metamorphic stage and show that Al must have been quite mobile during prograde quartz corona formation, whereas both Si and Al were relatively immobile during the two retrograde stages – the relative sequence of mobility being Na + > Mg 2+ , Ca 2+ , Fe 2+ >> Si 4+ > Al 3+ , Ti 4+ . Mass-balance between sites is consistent and no significant gain or loss of cations is indicated.

The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15 GPa with possible implications for the formation of oriented SiO2-inclusions in omphacite

Abstract Rutile, titanite, and zircon formed as relatively coarse-grained accessory minerals in several samples of high-grade calcite-dolomite marble with an early ultrahigh-pressure history. These minerals decomposed to a texturally complex set of secondary minerals during subsequent stages of retrograde metamorphism. The reactions involve several generations of geikielite-ilmenite as well as zirconolite [(Ca,Th,U)Zr(Ti,Fe,Nb,Ta)2O7], kassite/cafetite [CaTi2O4(OH)2/CaTi2O5·H2O], Ti-bearing humite group minerals, thorianite, and sometimes euxenite [(Ca,U,Th,REE)(Nb,Ta,Ti)2(O,OH)6]. Stable coexistence of zircon and olivine is observed and stably coexisting titanite with olivine and/or humite-group minerals is reported here for the first time outside of carbonatites, kimberlites, or lamprophyres. Petrogenetic grids constructed for Ti- and Zr-bearing olivine/antigorite-saturated calcite-dolomite marbles show that geikielite is stable at highest pressures, followed by titanite and rutile, and that baddeleyite + diopside replaces zircon + calcite to higher pressures. The observed reaction textures are consistent with an earlier derived P-T path for the Kimi Complex. They corroborate a period of heating during decompression from 25 to 20 kbar and ca. 800 °C, where the assemblage olivine-diopside-spinelrutile- zircon formed. This assemblage partially re-equilibrated during subsequent decompression and cooling, thus forming the observed reaction textures. Even though no memory of the UHP path is preserved in the accessory minerals, their reaction relationships turn out to be potentially very useful for geothermobarometry over a large range of metamorphic conditions.