Peak PT Conditions Research Articles

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

MetamorphicP‒Tpaths and Zircon U–Pb ages of Archean ultra‐high temperature paragneisses from the Qian’an gneiss dome, East Hebei terrane, North China Craton

AbstractThe East Hebei terrane of North China Craton is characterized by the dome‐and‐keel structure, a common feature in most Archean cratons, where supracrustal rocks of granulite facies commonly occur as enclaves or rafts in tonalite–trondhjemite–granodiorite (TTG) gneisses. The metamorphicP–Tpaths of the granulites are significant for addressing the Archean tectonic regimes. Two types of granulite facies paragneiss with pelitic and greywacke compositions from the western margin of Qian'an gneiss dome are documented for their petrography, mineral chemistry, phase equilibria modelling usingthermocalc,and zircon dating. AnticlockwiseP–Tpaths involving the pre‐peak pressure increase to the ultra‐high temperature peak conditions and post‐peak cooling and decompression processes were recognized. The pre‐peak pressure increase process was constrained for a pelitic granulite mainly based on the spinel and cordierite inclusions in garnet and rutile corona around ilmenite, where the transition from spinel to garnet is modelled at 6–7 kbar at a fixedT = 1,000°C. For greywacke granulite, the pre‐peak pressure increase evolution can be ascertained from the textural relation that orthopyroxene is surrounded by garnet, and the outwards increasing grossular (from 0.03 to 0.05) in the core of the atoll‐like garnet (Grt‐A), to occur from ~7 kbar at ~1,000°C. The peakP–Tconditions for pelitic granulite are roughly limited to 7–11 kbar/890–1,050°C on the basis of the stability of the inferred peak assemblage involving garnet, perthite, sillimanite, rutile/ilmenite, and quartz. The peakP–Tcondition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (XGrs = 0.045–0.050) in the core of subhedral garnet (Grt‐B) and the mantle of Grt‐A together with an average re‐integrated anorthite content (XAn = 0.07) in K‐feldspar. The peak temperature condition is consistent with the ternary feldspar thermometer results mostly of 950–1,020°C for antiperthite and perthite in greywacke granulite, and in accordance with the development of oriented needle‐like exsolution of Ti±Fe oxides in garnet from pelitic granulite. The post‐peak cooling and decompression process was consistent with the decreasingXGrsin the mantle of Grt‐A and core of Grt‐B in greywacke sample, and the final‐stage cooling conditions can be well constrained from the stability of final assemblages marked by the later growth of biotite, as 8–9 kbar/820–880°C for pelitic granulite and 6–9 kbar/840–890°C for greywacke granulite. Zircon dating yields provenance ages from 3.34 to 2.57 Ga and metamorphic ages ofc.2.50 Ga for the two types of granulite. The metamorphic ages overlap the final pulse of the Neoarchean magmatic activity of TTGs that ranges fromc.2.56 toc.2.48 Ga with a peak atc.2.52 Ga. Combining the development of dome‐and‐keel structures, the penecontemporaneity between the metamorphism of supracrustal rocks and TTG magmatic activity, and also the unique anticlockwiseP–Tpaths, we prefer a vertical sagduction regime to interpret the tectonic evolution of the East Hebei terrane, which may be also significant for other Archean cratons.

Timanide (Ediacaran-Early Cambrian) Metamorphism at the Transition from Eclogite to Amphibolite Facies in the Beloretsk Complex, SW-Urals, Russia

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages.

Mineral phase equilibria and zircon geochronology constrain multiple metamorphic events of high‐pressure pelitic granulites in south‐easternTibetan Plateau

The Ailao Shan (ALS) metamorphic belt separates the Indochina Block to the west from the South China Block and is an important region for understanding the history of evolution of the Palaeo‐Tethys in relation to the collision of the Indian and Eurasian plates and the formation of the Tibetan Plateau. The multiple metamorphic events andP–T–tpaths of the ALS belt remain debated. Here, we present the results of an integrated study of the petrography, mineral chemistry, phase equilibria modelling, and zircon U–Pb geochronology of high‐grade metasedimentary rocks from this belt. Abundant metamorphic zircon grains with mineral inclusions of Sil/Ky + Bt + Pl + Qz ± Mnz record two metamorphic events, the first one during the Triassic (227–220 Ma), although intense shearing overlapped during the Cenozoic and mineral assemblages of this thermal event were not preserved. The second metamorphic event occurred during the Palaeogene (49 –23 Ma) and involved three stages: (a) a peak high‐pressure (HP) granulite‐facies metamorphism (M1) with a mineral assemblage of Ky + Grt1 + Bt1 + Pl + Qz ± Kfs; (b) a postpeak near‐isothermal decompression (M2) characterized by Sil1–2 + Grt2 + Bt1 + Pl + Qz + Kfs + Ilm and the decompression reactions of Grt + Ky + Qz = 3An and Ky = Sil; and (c) a late decompressional cooling (M3) with a mineral assemblage of Sil3 + Grt3 + Bt2 + Pl + Qz + Mnz ± Kfs, followed by a series of hydration reactions. The metamorphic evolution occurred along a clockwiseP–T–tpath with peakP–Tconditions of 13.5–14.0 kbar and 810–820°C. Our new data, combined with those from previous studies, reveal that the Diancang Shan–Ailao Shan Complex Belt is not a single suture that represents the closure of the Palaeo‐Tethyan Ocean during the Triassic, but that it also represents a typical continent–continent collisional orogenic belt that formed during the Eocene–Oligocene prior to the strong left‐lateral, ductile deformation along the Ailao Shan–Red River shear zone.

Phase equilibrium modelling and implications forP–Tdeterminations of medium‐temperature UHP eclogites, North Qaidam terrane, China

AbstractIn contrast to low‐Teclogites (garnet growth zoning preserved) or high‐Teclogites (garnet diffusionally homogenized at peak conditions), medium‐temperature eclogites pose additional challenges toP–Tdeterminations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh‐pressure terrane, exposes minor eclogites hosted by ortho‐ and paragneisses. Four fresh, medium‐temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph±Ky±Zo. In all samples, garnetXMgshows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine aP–Tpath. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a progradeP–Tpath. This approach yields a path from ~17 kbar and ~410°C to ~35 kbar and ~625°C with a gradient of ~5–9°C/km through the lawsonite stability field. PeakP–Tconditions determined from the intersection between Si pfu in phengite and Zr‐in‐rutile isopleths are ~26–33 kbar and ~625–700 °C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re‐equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50°C) with garnet–omphacite–phengite (±kyanite) thermobarometry in three of the four samples. We regard the phengite–rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at ~60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison ofP–Testimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the progradeP–Tpath of medium‐temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peakP–Tconditions.

Tectonic slicing and mixing processes along the subduction interface: The Sistan example (Eastern Iran)

Suture zones preserve metamorphosed relicts of subducted ocean floor later exhumed along the plate interface that can provide critical insights on subduction zone processes. Mélange-like units are exceptionally well-exposed in the Sistan suture (Eastern Iran), which results from the closure of a branch of the Neotethys between the Lut and Afghan continental blocks. High pressure rocks found in the inner part of the suture zone (i.e., Ratuk complex) around Gazik are herein compared to previously studied outcrops along the belt. Detailed field investigations and mapping allow the distinction of two kinds of subduction-related block-in-matrix units: a siliciclastic-matrix complex and a serpentinite-matrix complex. The siliciclastic-matrix complex includes barely metamorphosed blocks of serpentinized peridotite, radiolarite and basalt of maximum greenschist-facies grade (i.e., maximum temperature of 340 °C). The serpentinite-matrix complex includes blocks of various grades and lithologies: mafic eclogites, amphibolitized blueschists, blue-amphibole-bearing metacherts and aegirine-augite-albite rocks. Eclogites reached peak pressure conditions around 530 °C and 2.3 GPa and isothermal retrogression down to 530 °C and 0.9 GPa. Estimation of peak PT conditions for the other rocks are less-well constrained but suggest equilibration at P < 1 GPa. Strikingly similar Ar–Ar ages of 86 ± 3 Ma, along ~70 km, are obtained for phengite and amphibole from fourteen eclogite and amphibolitized blueschist blocks. Ages in Gazik are usually younger than further south (e.g., Sulabest), but there is little age difference between the various kinds of rocks. These results (radiometric ages, observed structures and rock types) support a tectonic origin of the serpentinite-matrix mélange and shed light on subduction zone dynamics, particularly on coeval detachment and exhumation mechanisms of slab-derived rocks.

Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China

The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx+Grt+Pl+Kfs+Qtz and a retrograde P-T trajectory. Peak PT conditions are 845–860°C and 0.83–0.86Gpa, corresponding to a thermal gradient of ~30°C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710–720°C, 0.57–0.58Gpa for garnet-biotite gneiss and 760–780°C, 0.60–0.62Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430–450Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79Ga and 0.83Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the source rocks involved juvenile crustal components with Hf model ages of 1.1–1.5Ga. Sedimentation of such a continental shelf sequence as well as the high thermal gradient of ~30°C/km confirm the previously proposed continental arc formation and maturation model in the Chencai region.

Repeated slip along a major decoupling horizon between crustal-scale nappes of the Central Western Carpathians documented in the Ochtiná tectonic mélange

The Ochtiná Unit is situated in the ENE–WSW-trending contact zone between two crustal-scale nappes, the upper Gemer Unit and the lower Vepor Unit, in the Central Western Carpathians, Slovakia. The Ochtiná Unit consists mainly of Carboniferous phyllitic schists and sandstones enclosing lenses of diverse lithological nature and contrasting metamorphic history. Peak PT conditions obtained by means of phase equilibrium modelling from lenses of amphibolite and chloritoid schist in this unit indicate 500–600°C and 4–6.5kbar and 500–520°C and 9–11kbar, respectively. These PT conditions contrast not only with the greenschist-facies metamorphism of dominant phyllite but also with each other documenting two distinct metamorphic field gradients related to Variscan and Alpine metamorphic events. Geochemical data reveal an affinity of the amphibolite lenses similar to Variscan rocks in the basement of the upper Gemer Unit and of the chloritoid schist similar to Alpine rocks in the cover of the lower Vepor Unit. Such heterogeneous lithological and metamorphic record is consistent with a block-in-matrix rock arrangement and the Ochtiná Unit is interpreted as deep seated tectonic mélange. The mélange evolved via repeated slip along the rheologically weak sediments of the Ochtiná Unit during the building and collapse of the Eo-Alpine orogenic wedge of the Central Western Carpathians. Deformation record indicates that the mélange separates two distinct structural domains marked by a decoupled behaviour, i.e. the orogenic suprastructure represented by the Gemer Unit and the infrastructure represented by the Vepor Unit. With this respect, the Ochtiná Unit represents an unusual example of a suprastructure–infrastructure transition zone with its position being controlled by the mechanical weakness of this sedimentary horizon and not by the temperature-dependent rheological transition.

Alteration patterns of chromian spinels from La Cabaña peridotite, south-central Chile

La Cabana peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabana peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabana peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO ( 0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabana reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.

Magnetic fabric in amphibolized eclogites and serpentinized ultramafites in the Mariánské Lázně Complex (Bohemian Massif, Czech Republic): Product of exhumation-driven retrogression?

The largest body of metabasic rocks in the Bohemian Massif, the Mariánské Lázně Complex (MLC), represents an Early Paleozoic ophiolite complex, which experienced poly-metamorphic evolution with an early high-pressure amphibolite, followed by eclogite and subsequent amphibolite facies overprint. This study aims to investigate the fabric relationships among the rock bodies and to analyze the deformation–recrystallization history of the MLC using the anisotropy of magnetic susceptibility (AMS).Magnetic susceptibility of the MLC rocks varies according to the presence of para- and ferromagnetic minerals as well as according to the primary lithologies. In weakly magnetic rocks (Km<1×10−3 [SI]), the main carrier of susceptibility is paramagnetic mafic silicates, amphibole and biotite in amphibolites, clinopyroxene and garnet in eclogites, olivine and serpentine minerals in serpentinized ultramafite, and amphibole in gabbro. In highly magnetic rocks (Km≫1×10−3), ferromagnetic minerals, such as magnetite, titanomagnetite, general Fe–Ti oxides, and subordinately also pyrrhotite can play an important role.The results of AMS indicate that the orientation of magnetic minerals in most rocks is related to the last exhumation and retrogression event that occurred in amphibolite facies conditions. However, there are some well preserved eclogites possessing the magnetic fabrics that are at least partially related to prograde or peak PT conditions during metamorphism. In addition, massive metagabbros that suffered only weak deformation partially preserve the intrusive magnetic fabrics.

Metamorphism of the Amdo metamorphic complex,Tibet: implications for the Jurassic tectonic evolution of the Bangong suture zone

AbstractTibet consists of several terranes that progressively collided with the southern margin of Asia during the Mesozoic following the closure of intervening ocean basins. This Mesozoic amalgamation history, as well as the extent to which it may have contributed to crustal thickening prior to the Cenozoic Indo‐Asia collision, remains poorly constrained and strongly debated. Here, we present a metamorphic petrological and U‐Pb zircon geochronological study of the Amdo metamorphic complex, one of the few exposures of high‐grade metamorphic rocks in central Tibet, located along the Bangong suture between the Qiangtang terrane to the north and the Lhasa terrane to the south. U‐Pb ages of metamorphic zircon in gneiss constrain the timing of peak metamorphism atc. 178 Ma, prior to the Early Cretaceous collision between the two terranes. PeakP–Tconditions of gneiss within the metamorphic complex are constrained by conventional as well as multi‐equilibrium (THERMOCALCv.3.21 and v.3.33) geothermobarometry of two samples of garnet‐amphibolite. WhereasTHERMOCALCv.3.21 yields similar results as conventional geothermobarometry,THERMOCALCv. 3.33 yields dramatically lower pressures, mostly due to the change in the amphibole activity model used. UsingTHERMOCALCv.3.21, the two garnet‐amphibolite samples yield similarP–Tconditions of 0.83 ± 0.06 GPa at 646 ± 33 °C and 0.97 ± 0.06 GPa at 704 ± 35 °C. Plagioclase coronas on the garnet‐amphibolite sample with lower peakP–Tconditions indicate a period of isothermal decompression. Additional geothermometry on two garnet‐free amphibolites yielded similar temperatures of 700–750 °C and suggests similarP–Tconditions across most of the complex. However, two exposures of garnet‐kyanite schist located along the southern edge of the metamorphic complex yielded slightly lower peak conditions of 0.75–0.85 GPa and 550–610 °C. Petrographic and field relations suggest the difference in metamorphic grade between the schist and gneiss is due to an intervening thrust fault. The existence of the thrust fault indicates that at least part of the exhumation of the complex was due to contractional deformation, possibly during the Lhasa‐Qiangtang collision. OurP–T–tresults indicate the occurrence of a significant Early Jurassic tectonothermal event along the southern, active margin of the Qiangtang terrane that deeply buried the Amdo rocks. We suggest that the metamorphism is a result of arc‐related tectonism that may have been regionally extensive along the southern Qiangtang terrane; geological records of this tectonism may be rarely exposed along strike because of a lack of exhumation or subsequent depositional and structural burial.

P–Tevolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31–32°S)

AbstractIn the late Palaeozoic fore-arc system of north-central Chile at latitudes 31–32°S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strike-slip event (derived peakP–Tconditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350–430°C, 6–9 kbar), (2) the frontally accreted Arrayán Formation (AF; 280–320°C, 4–6 kbar) and (3) the retrowedge basin of the Huentelauquén Formation (HF; 280–320°C, 3–4 kbar). In the CMC, Ar–Ar spot ages locally date white-mica formation at peakP–Tconditions and during early exhumation at 279–242 Ma. In a local garnet mica-schist intercalation (570–585°C, 11–13 kbar) Ar–Ar spot ages refer to the ascent from the subduction channel at 307–274 Ma. Portions of the CMC were isobarically heated to 510–580°C at 6.6–8.5 kbar. The age of peakP–Tconditions in the AF can only vaguely be approximated at ≥ 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below ~ 280°C at ~ 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar–Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at ~ 245–240 Ma, ~ 210–200 Ma, ~ 174–159 Ma and ~ 142–127 Ma. The zircon fission-track ages are locally reset at 109–96 Ma. All resetting of Ar–Ar white-mica ages is proposed to have occurred byin situdissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn- and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.

Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador

The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu–Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600°C, corresponding to a maximum burial depth of ~60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence.

Peak and post-peak PT conditions and fluid composition for scapolite-clinopyroxene-garnet calc-silicate rocks from the Takab area, NW Iran

The Takab calcareous rocks of northwest Iran crop out in association with a variety of metamorphic rocks including mafic granulites, amphibolites, granitic gneisses, pelitic schists and meta-ultramafic rocks. They can be divided into marbles and calc-silicate rocks on the basis of the dominance of calcite/dolomite and silicate minerals. Dominant peak metamorphic granulite facies assemblage of calc-silicate rocks is Scp + Grt(I) + Cpx + Cal + Qtz +/- Hbl(I). The decrease of temperature and pressure during exhumation produced post-peak metamorphic assemblages. Coronal garnet (Grt II) in the calc-silicate rocks was produced by retrograde reactions consuming plagioclase and clinopyroxene, while peak metamorphic garnet (Grt I) occurs as preserved xenoblastic grains in calcite and/or plagioclase (Pl II). Regional metamorphism took place at 740 degrees C and X-CO2 similar to 0.9. Garnet-clinopyroxene-plagioclase-quartz (GADS) barometry yields a pressure of 8-9 kbar, corresponding to a depth of ca. 24-27 km. This was followed by decompression and hydration during exhumation of the crustal rocks up to the surface. Secondary phases such as garnet (II) hornblende (II), plagioclase (II), zoisite and titanite (II) constrain the temperature and pressure of post-peak metamorphism as similar to 600 degrees C and similar to 6 kbar respectively and a fluid with XCO2 as low as 0.4. Halogens were near-absent during the peak metamorphic stage. The scapolite and hornblende crystallized underpeak metamorphic conditions contain very low fluorine and chlorine, whereas relatively high fluorine (similar to 0.8 wt%) in the titanite (II) and hornblende (II) suggests a possible infiltration of F-rich fluids into the calc-silicate rocks during retrogression. It is interpreted to be related to external fluids released during crystallisation of granitoid magmas and/or leucosome patches in the adjacent migmatites.

Three metamorphic monazite generations in a high-pressure rock from the Bohemian Massif and the potentially important role of apatite in stimulating polyphase monazite growth along a PT loop

Based on microstructural and compositional criteria, three generations of monazite can be identified in a peraluminous, garnet-rich, high-pressure metamorphic rock from the Bohemian Massif. The first monazite generation (M1) formed probably on the prograde part of the clockwise PT loop, a second (M2) during the high-pressure stage (26 ± 3 kbar and 830 ± 30 °C) and a third generation (M3) during subsequent near-isothermal decompression of the rock to 8 ± 2 kbar. The high-pressure, M2 monazites are volumetrically dominant and form 100–300 μm grains. They are characterised by an unusual Sr–brabantite component (SrO up to 2 wt.%) and grew together with accessory Sr-rich fluorapatites (SrO up to 33 wt.%). The Sr contents of both minerals are indicative of high-pressure conditions and result from the breakdown of plagioclase and the release of Sr that occurred during this process. M3 monazites occur exclusively as small grains (10–100 μm) within and at the rims of apatite. Microstructures show that they grew at the expense of the apatite extracting LREEs from the latter under retrograde conditions. These “parasite monazites” in apatite are characterised by very low Th contents, which is consistent with having been derived primarily from an apatite source. M1 monazites are represented by resorbed, irregular shaped cores, which are occasionally preserved within the Sr–brabantite rich, high-pressure monazites (M2). These M1 cores are generally Sr-poor, indicating that they should have formed prior to the high-pressure stage, in the presence of plagioclase. The dissolution of M1 monazite and the reprecipitation of a Th, U-enriched M2 monazite during the high-pressure stage implies that LREEs were taken up by the high-pressure Sr-rich fluorapatite to some extent, and not completely processed for the formation of the M2 monazite. This would also explain the widespread retrograde growth of M3 monazite within the apatite. The ability of apatite to act as an interim host for LREEs at peak-PT conditions could be one of the reasons why monazite growth during regional metamorphism is often periodic.

Pressure–Temperature Evolution of a Late Palaeozoic Paired Metamorphic Belt in North–Central Chile (34°–35°30′S)

In the Chilean Coastal Cordillera, two units, the Western and Eastern Series, constitute coeval parts of a Late Palaeozoic paired metamorphic belt dominated by siliciclastic metasediments. The Western Series also contains rocks from the upper oceanic crust and represents an accretionary prism. Omnipresent high-pressure conditions are reflected by Na–Ca-amphibole and phengite in greenschists. Peak PT conditions of 7 0–9 3 kbar, 380–420 C point to a metamorphic gradient of 11–16 C/km. Three unique occurrences of blueschist yield deviating conditions of 9 5–10 7 kbar, 350– 385 C and are interpreted as relics from the lowermost part of the basal accretion zone preserving the original gradient of 9–11 C/km along the subducting slab. Pervasive ductile deformation related to basal accretion occurred near peak PT conditions. Deformation and PT evolution of the metapsammopelitic rocks is similar to that of the metabasites. However, a rare garnet mica-schist yields peak PT conditions of 9 6–14 7 kbar, 390–440 C reflecting a retrograde stage after cooling from a high-temperature garnet-forming stage. It is considered to be an exhumed relic from the earliest siliciclastic rocks subducted below a still hot mantle wedge. A retrograde overprint of all rock types occurred at 300–380 C. Continuous reactions caused crystal growth and recrystallization with abundant free water mostly under strain-free conditions. They record a pressure release of 3–4 kbar without erasing peak metamorphic mineral compositions. The Eastern Series lacks metabasite intercalations and represents a less deformed retro-wedge area. In the study area it was entirely overprinted at a uniform depth at 3 0 5 kbar with temperatures progressively rising from 400 C to 720 C towards the coeval Late Palaeozoic magmatic arc batholith. The interrelated pattern of PT data permits a conceptual reconstruction of the fossil convergent margin suggesting a flat subduction angle of 25 with continuous basal accretion at a depth of 25–40 km and a short main intrusion pulse in the magmatic arc. The latter was accompanied by the formation of a thermal dome in the retro-wedge area, which remained stable relative to the vertically growing accretionary prism characterized by cyclic mass flow.

The orogenic wedge in the central Scandinavian Caledonides: Scandian structural evolution and possible influence on the foreland basin

The orogenic wedge of the central Scandinavian Caledonides - overlying the Baltic Shield without a substantial foreland basin - is built up of a stack of nappes, which are divided here into three mechanically distinct tiers: A: Lower Allochthon with exclusively Scandian (Middle to Late Silurian) structures and metamorphism, B: Middle Allochthon and Seve units of Upper Allochthon with traces of Finnmarkian (Early Ordovician) events, and C: overlying exotic terranes (Köli units of Upper Allochthon, Uppermost Allochthon). Detailed structural work documented shear criteria at the tier B-C (Seve-Köli) boundary, which consistently indicate a down-dip, top-to-the-west movement at a regional scale and immediately after peak PT conditions (c. Late Wenlockian). Subsequently, the Seve-Köli boundary was deformed by regional folds, caused by the stacking of thrust systems in the underlying Lower Allochthon. Due to lithology and early cooling, the tier B of the Caledonian orogenic wedge is dominated by strong rocks, whereas the lower tier (A), containing a number of horizons with low friction, is of relatively weaker strength. For the orogenic wedge as a whole, the contrast in strength between tiers A and B gave rise to a relatively flat toe of the wedge and a small frontal load. The model of a tiered wedge with a toe is proposed here as an explanation for the small frontal depression and foreland basin along the eastern margin of the north-central Scandinavian Caledonides.

Trace Element Partitioning Between Hydrous Minerals (Phengite, Zoisite, Amphibole) and Omphacite in Eclogites: Clues for Modelling Dehydration Reactions in Subduction Zones

The fluid composition emanating from dehydrating slabs below subduction zones can be strongly influenced by the presence of accessory and minor phases. A prominent example of this effect is the retention of HFSE by rutile. It is therefore important to evaluate if and how the so called fluid mobile elements (Cs, Rb, Ba, Th, U, Pb, Sr, LREE) can be retained in the downgoing slab due to the presence of certain phases. A plethora of phases can be present in dehydrating altered oceanic crust and pelagic sediments at the crucial PT conditions (e. 650~ and 3.0 GPa to 800~ and 5.0 GPa) including high pressure phases such as phengite, zoisite, lawsonite, chloritoid and possibly also talc, apatite, zircon. In order to evaluate the trace element budget in subducting altered oceanic crust we have focussed our study on eclogites from Trescolmen, Switzerland. High pressure hydrous phases are widespread and peak PT conditions approach these of subducting slabs beneath arcs (650~ and 2.2 GPa; Meyre et al.,

Garnet-bearing ultramafic rocks from the Erzgebirge, and their relation to other settings in the Bohemian Massif

Garnet-bearing ultramafic rocks (GBU) enclosed in high-grade gneisses are known from several parts of the Bohemian Massif. One of these is the high-pressure(HP)-unit 1 in the Erzgebirge Crystalline Complex, which is the subject of the present study. Hitherto, two different models have been put forward to explain the stabilisation of garnet in mantle-derived ultramafic rocks from the Bohemian Massif and their emplacement into the crust. (1) Garnetiferous assemblages were formed in the ultramafics before they came in contact with their crustal host rocks. (2) Garnet was formed in the ultramafics at the expense of spinel due to cooling caused by their tectonic emplacement in the crust. The PT-evolution revealed by the investigated GBU from the Erzgebirge Crystalline Complex, however, requires a third model. The reconstruction of the PT-paths for the Erzgebirge GBU is based on both conventional thermobarometry and phase relations. Thermodynamic calculations allowed the construction of a PT-phase diagram for the system Na2O-CaO-MgO-Al2O3-SiO2-H2O, which is the first quantitative petrogenetic grid in this model system relevant to ultramafic HP-rocks. The grid shows the uni-, di-, and tri-variant assemblages stable in peridotitic rocks at different PT-conditions, providing a tool to constrain PT-paths from the succession of mineral assemblages observed in a rock. The PT-path obtained for the Erzgebirge GBU suggests that the garnet-bearing assemblages formed by HP-metamorphism of spinel peridotite which was emplaced into the crust prior to or during the HP-compressional stage. This model is supported by peak PT-conditions around 900 °C and 30–35 kbar recorded by the ultramafic rocks, which are very similar to those attained in the eclogites sensu stricto which occur in the same tectonic unit (HP-unit 1) and for which an in situ metamorphism has been inferred (Schmadicke et al. 1992). On the other hand, the other two high-pressure units in the Erzgebirge, HP-units 2 and 3, which also contain eclogites sensu stricto but lack peridotites, record lower peak PT-conditions of 650–750 °C/24–26 kbar and 600–650 °C/20–24 kbar, respectively. Postulating an in situ HP-metamorphism for the garnet peridotites as result of continental collision during the Variscan orogeny, a crustal thickness of 90–110 km would be required. A comparison of the distribution of eclogites sensu stricto and mantle-derived rocks from the Bohemian Massif together with their reported PT-conditions reveals a correlation between peak PT-conditions in eclogites sensu stricto and the prevailing assemblages in the ultramafic rocks in the same unit. Furthermore, the Erzgebirge Crystalline Complex and the Snieznik Complex on one hand as well␣as the Granulitgebirge and the Sowie Gory on the other hand are thought to be genetically linked. The garnet peridotites from the Granulitgebirge, the Sowie Gory and the Gfohl unit seem to have experienced peak PT-conditions not recorded by their country rocks with non-eclogitic basic interlayers, inferring a formation of␣HP-assemblages in the ultramafics prior to their emplacement into the crust.