Evidence For High-pressure Metamorphism Research Articles

Zircon petrochronology and mineral equilibria of the eclogites from western Tasmania: Interrogating the early Palaeozoic East Gondwana subduction record

Evidence for high-pressure metamorphism along the Cambro-Ordovician East Gondwana margin is limited to occurrences of eclogite-facies rocks in the central Transantarctic Mountains, northern Victoria Land, western Tasmania, and eastern Australia. The western Tasmanian eclogites in the Franklin Metamorphic Complex are enclosed by high-grade metapelitic lithologies and contain a high-pressure mineral assemblage of garnet + omphacitic clinopyroxene + pargasitic amphibole + phengite + quartz + rutile + clinozoisite. Mineral relationships and compositions in the eclogites, combined with mineral equilibria forward modelling, indicate the eastern margin of Tasmania was subducted along a relatively cold thermal gradient to pressures between 18 and 21.5 kbar at temperatures of 650–700 °C. LA–ICP–MS U-Pb analysis of zircon yields middle Cambrian ages, consistent with previous geochronology, as well as older Mesoproterozoic ages. Rare earth element concentrations in Cambrian-aged zircon suggest they formed at high pressures whereas Mesoproterozoic-aged zircon formed at lower pressures. Garnet preserves elemental concentrations consistent with prograde growth, with middle and heavy rare earth element concentrations resolving a complex growth history involving the consumption of clinozoisite and apatite. In-situ rutile LA–ICP–MS U-Pb ages are early to middle Cambrian and indicative of prograde to peak growth during subduction, with Zr concentrations in matrix rutile yielding upper-amphibolite-facies temperatures. In this contribution, the pressure-temperature (P–T) results for the western Tasmanian eclogites revise existing conventional thermobarometric P–T data. The updated thermal gradient information can be more precisely allied with subduction zone thermal models, which provide predictions of the thermal structure of the subduction geodynamic system. Furthermore, when integrated with the mineral equilibria and P–T data from other eclogites along the margin, these results expand upon the existing interpretations of the geodynamic character of the Cambro-Ordovician East Gondwana margin.

High-grade metamorphism during Archean–Paleoproterozoic transition associated with microblock amalgamation in the North China Craton: Mineral phase equilibria and zircon geochronology

Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth. The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recent models propose that the cratonic architecture was built through the assembly of several Archean microcontinental blocks into larger crustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery of the Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblage of garnet+clinopyroxene+orthopyroxene+magnetite+plagioclase+quartz±biotite±ilmenite. Mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak P–T condition of 800–830°C and 7–8kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in another sample containing the peak mineral assemblage of garnet+orthopyroxene+quartz+magnetite±fluid yield peak P–T conditions of 860–920°C and 11–14kbar. Geochemical data show tonalitic to granodioritic composition and arc-related tectonic setting for the magmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562±20Ma (MSWD=0.94) and 2539±21Ma (MSWD=0.59) which is correlated with the timing of emplacement of the magmatic protolith. A younger group of zircons with upper intercept ages of 2449±41Ma (MSWD=0.83); N=6 as 2449±41Ma (MSWD=0.83; N=6) and 2480±44Ma (MSWD=1.2; N=9) constrains the timing of metamorphism. Zircon Lu–Hf data show dominantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC) in the range of 2529 to 2884Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction–collision tectonics associated with microblock amalgamation in the NCC at the end of Archean. Together with the evidence for high pressure metamorphism from an adjacent locality, our results correlate with models that predict paired metamorphism at the Archean–Proterozoic transition with the onset of modern style plate tectonics.

Geological map of the upper Gressoney Valley, Western Italian Alps

Please click here to download the map associated with this article. Detailed geological mapping of the upper Gressoney Valley in the Western Italian Alps has permitted us to reconstruct the lithostratigraphy and the structural evolution of three different tectonic units: the Monte Rosa Massif and the Zermatt-Saas Zone and The Combin Zone. The studied nappes can be grouped into a Lower Tectonic Element which underwent eclogite facies metamorphism (Monte Rosa Massif and Zermatt-Saas Zone) and an Upper Tectonic Element (Combin Zone) pervasively equilibrated under greenschist facies conditions and lacking mesoscale evidence of high pressure metamorphism. The reconstruction of the lithostratigraphy and the mapping of the different generations of tectonic contacts between the different units allowed us to infer the presence of different generations of tectonic contacts separating oceanic and continental crust units.

High-Pressure Metamorphism in the Texas Grenville Orogen: Mesoproterozoic Subduction of the Southern Laurentian Continental Margin

Collisional orogenesis in the Llano Uplift of central Texas during the late Mesoproterozoic drove metamorphism that comprised both an initial high-pressure (HP) phase (610-775°C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (~700°C at ~0.7 GPa). A later, low-pressure overprint (525-625°C at 0.3 GPa) took place under largely static conditions. Evidence for HP metamorphism is geographically widespread, but confined principally to boudins of mafic eclogite encased in felsic gneisses. The geographic distribution of P-T conditions for HP metamorphism inferred from the eclogites, combined with evidence from the relative degrees of homogenization of growth zoning in garnet from both eclogites and felsic gneisses, suggests a general increase from northeast to southwest in depths of burial during HP metamorphism. Exhumation of HP rocks to shallower depths prior to MP metamorphism appears to have been rapid: ages for the two phases of metamorphism overlap, on the basis of the very limited data now available. The region's early metamorphic history is best explained by southwestward subduction of the Laurentian continental margin during collision with a still-unidentified continental mass, followed by buoyancy-driven uplift to lower crustal levels while collisional contraction continued.

First evidence of high-pressure metamorphism in the “Cover Series” of the southern Menderes Massif. Tectonic and metamorphic implications for the evolution of SW Turkey

The Menderes Massif occupies a large part of western Turkey. It is tectonically overlain by nappes of the Izmir–Ankara Suture Zone (including the Bornova Flysch Zone) on its northern flank, the Afyon Zone on its eastern border, and the Lycian Nappes on its southern flank. The discovery of magnesiocarpholite–kyanite assemblages in the Mesozoic ‘cover’ sequence of the southern Menderes Massif leads to the very first consideration that a major high-pressure (HP) metamorphic event affected the massif during its Alpine evolution. Estimations of minimum pressure–temperature conditions about 10–12 kbar and 440 °C suggest a minimum burial of 30 km for the metasedimentary rocks of the Menderes Massif, similar to that for the Eocene Dilek Blueschists, which belong to the Cycladic complex. This finding of high-pressure rocks within the Menderes ‘cover’ series, as well as the recent description of low-grade high-pressure parageneses in the basal thrust sheets of the overlying Lycian nappe complex, has strong implications for the tectono-metamorphic evolution of these units. The characteristics of this high-pressure metamorphism and the deformation features observed in the southern Menderes Massif allow consideration of a fast burial during Alpine subduction and nappe stacking. Three high-pressure units were piled up during the Eocene collision of the Sakarya continent and the Tauride–Menderes block. The lowermost unit consists of the imbricated ‘core’ and ‘cover’ of the Menderes Massif. The intermediate unit is the Cycladic Blueschist complex (Dilek and Samos regions) and the uppermost unit is made of the Lycian Nappes. The retrogression of high-pressure parageneses from the Menderes Massif follows an isothermal decompression path and the associated deformation is characterized by a severe stretching oriented N–S to NE–SW during the high-pressure event and its exhumation. We propose a tectono-metamorphic model in three main stages: (1) the southward translation of the high-pressure Lycian Nappes and the burial of the Menderes (and Cycladic) metasediments to a depth of at least 30 km to allow their Eocene high-pressure metamorphism; (2) a northward continuum of deformation responsible for top-to-the-north fabrics associated with the Main Menderes Metamorphism peak (Paleocene–Eocene) and its progressive cooling (Late Eocene), north-verging folding, thrusting, imbrication of ‘core’ and ‘cover’ series, and N–S to NE–SW intense stretching associated with the Menderes high-pressure metamorphism and its exhumation. These top-to-the-north to top-to-the-northeast movements are probably related to the northward backthrusting of the Lycian Nappes, reactivating the contact between the Menderes Massif and the Lycian Nappes as a top-to-the-NE shear zone that allowed exhumation of the Lycian high-pressure rocks; (3) Oligo-Miocene N–S extension during which the final exhumation of the metamorphic rocks along kilometre-scale low-angle detachments occurred.

Metamorphic structure and P–T conditions of the metapelitic constituent of the ZEV

Bounded by mylonite zones and imbrications the Zone of Erbendorf-Vohenstrauss is built up by medium- and low-pressure amphibolite-facies metapelites associated with amphibolites and biotite–hornblende gneisses, and intruded by the Leuchtenberg granite. Pelitic index assemblages define the staurolite– kyanite-, the garnet–kyanite-, the garnet–sillimanite- and the garnet–kyanite–sillimanite zone of medium pressure metamorphism, which is estimated at P=5–9.6 kbar and T=580–730 °C, and the cordierite zone of subsequent low-pressure metamorphism, which is estimated at P<4.5 kbar and T≤700 °C. Both facies series probably are tectonically juxtaposed. Staurolite–kyanite- and garnet zones are related by the reaction: staurolite+muscovite+quartz=garnet+ aluminium-silicate+biotite+H2O. This relation defines the only observable trace of a field gradient in the ZEV. An early sillimanite generation (sillimanite I) is found in all mineral zones as mineral inclusions in garnet rims. Post-P–Tmax evolution is indicated by the inversion kyanite⇒sillimanite (sillimanite II), the reaction: garnet+muscovite⇒sillimanite (sillimanite III)+biotite+quartz and by XFe decreasing in garnet rims. Garnet is resorbed under the conditions of stable kyanite+sillimanite+muscovite+quartz. Some staurolite–kyanite zone garnets show two stages separated by resorption which may be ascribed to published Ordovician and Late Devonian radiometric ages. The second garnet generation formed near 9.7 kbar/ 620–660 °C. No evidence for high-pressure metamorphism was found. Comparison of the medium-pressure part of the ZEV with the Zone of Tepla-Domazlice (Czech Republic) and with garnet–kyanite rocks at the northern edge of the Black Forest seems possible. The low-pressure cordierite rocks are similar to metapelites framing the Winklarn eclogites towards southeast of the ZEV, but are in contrast to the Moldanubian-type cordierite gneisses.

Equilibration conditions of eclogite lenses from Isla Margarita, Venezuela: Implications for the tectonic evolution of the metasedimentary Juan Griego Group

The constituent primary and secondary minerals in a suite of fifteen samples of eclogite, amphibole-eclogite and garnet-amphibolite, scattered as boudins and pods in metapelitic schists and gneisses on Isla Margarita, Venezuela, have been analysed by electron microprobe to augment existing bulk-rock chemical data. The large sample population available allows spurious effects of stoichiometric Fe 3+ calculation procedures and bulk-rock influence to be recognised and eliminated. All samples belong to one population with a relatively homogeneous character. The equilibration temperatures are derived from Fe 2+ Mg fractionation between garnet and clinopyroxene and range from 525–650 °C. Jadeite contents of clinopyroxene and the persistence of minor stable albite constrain pressures to between 13 and 19 kbar. Combined with new evidence for high-pressure metamorphism in the enclosing metapelites and with existing data on an eclogitic metabasic unit exposed in north-eastern Isla Margarita (La Rinconada Group), these data show convincingly that the Margarita crustal block is and has been a relatively coherent unit ever since the entire complex suffered high-pressure metamorphism in Late Mesozoic times.