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The compositional zoning of the major divalent cations in metamorphic garnet is a useful tool in reconstructing the pressure–temperature path. However, trace elements can provide a better-preserved record of petrogenetic evolution due to their strong affinity in garnet and slow diffusion rates. In this study, three high-pressure micaschist samples of varying composition and garnet textures from the Krušné hory Mountains (Saxothuringian zone, Bohemian Massif) were examined. By utilizing electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry, three distinct types of compositional zoning in garnet were identified by compositional mapping. The zoning types were classified as continuous core-to-rim change, concentric annular changes, and overprinting of a pre-existing distribution; all three provide information on the original mineral composition and texture before garnet overgrowth. The transition from overprint to annular zoning shows relation to temperature increment. The annular zoning allowed the identification of several coupled substitutions, including alkali (sodium and lithium) + yttrium and the alkali + phosphorus substitution which is typical of high- to ultra-high-pressure conditions. The formation of annuli zoning was interpreted to originate not only from the decomposition of trace element bearing phases, but also to be related to the availability of fluid medium during garnet growth. Two samples contained atoll texture garnets, interpreted to be originated from the dissolution of the garnet central part, chemically distinct from the new garnet growing coevally on the rim or replacing the original central part. This proposed process is evidenced by the mass balance calculation of yttrium and heavy rare earth elements between the dissolved garnet and newly formed parts. [ABSTRACT FROM AUTHOR]

This paper presents and discusses the results of phase diagram (Perple_X) and diffusion modelling (CZGM, or Compositional Zoning and its Modification by diffusion) to constrain the P–T path of metamorphism. The approach is based on the best fits between the zoning profile in measured garnet and that obtained by the intersections of garnet isopleths calculated by phase diagram modelling using whole rock bulk composition. The model was applied to garnets in natural rocks of various metamorphic grades, which were formed within different geotectonic environments. To compare the sequence of compositional change during Barrovian-type metamorphism, well-studied pelitic rocks from garnet–staurolite, kyanite–sillimanite, and sillimanite-K-feldspar metamorphic zones were selected. Garnets with two-stepped core and rim profiles that were formed during two different metamorphic stages or events were used for pressure–temperature (P–T) path constraint of each stage or event. For high-grade rocks, in which the original zoning profile in garnet was severely modified, the diffusion of the initial zoning profile was quantified to estimate the timescale of the metamorphic event. These rocks include high- to ultra-high-pressure rocks, which were subjected to thermal overprinting during collisional orogenesis. The results of the application of this approach allow for deciphering the reason why the calculated profile by phase diagram modelling does not fit with that of the measured garnet from low-grade rocks, in which garnet has preserved the original compositional zoning. This includes garnets whose nucleation was shifted from the garnet-in boundary to higher temperatures and pressures, as well as garnet crystallised during different metamorphic stages or events. Finally, the P–T paths in high-grade rocks were constrained after the multicomponent diffusion in garnet was quantified, and this was used for further P–T-time path constraint of metamorphism in the rocks. [ABSTRACT FROM AUTHOR]

This paper presents the results of petrological observations and diffusion modelling on garnet from high‐pressure to ultrahigh‐pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite‐bearing micaschist preserves two generations of garnet growth that are related to the Pre‐Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre‐Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky‐shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short‐time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet. [ABSTRACT FROM AUTHOR]

Basement rocks from the Western Hindu Kush preserve evidence of multiple metamorphic and magmatic events that occurred along the boundary between the Archean–Proterozoic Afghan Central and Afghan–Tajik Blocks. To verify the different metamorphic stages or events, mineral textures and phase equilibria in metamorphic basement rocks and their age relations to magmatic episodes have been investigated. Quartzofeldspathic gneiss and migmatite with lenses of amphibolite (with assumed Proterozoic age for their metamorphism) are intruded by the Triassic Hindu Kush granitoid batholith and small Cretaceous and Oligocene granite intrusions. The age of thermal overprint (210–170 Ma) by the Triassic batholith is confirmed by new monazite data. Both Triassic and Cretaceous granitoids and surrounding basement rocks underwent subsequent metamorphism up to epidote–amphibolite facies. The degree of this metamorphism increases southward at the contact to the Kabul Block, which under‐plates the Western Hindu Kush from the south. An early Miocene age was obtained by Pb–Th analyses in thorite and huttonite, which are close or slightly younger than the Oligocene granite in this area. The Cretaceous meta‐granodiorite near the border with the Kabul Block contains xenoliths of granulite facies rocks that could come from the Neoarchean granulite facies basement of the Kabul Block. The multi‐stage metamorphic and magmatic evolution classifies the Hindu Kush mountain belt as a long‐lived suture zone that was active since the early Palaeozoic. The results of this study support the interpretation about possible relations of the Afghan Central Blocks to the southern margin of Eurasia during the evolution of Para‐ and Neotethys. [ABSTRACT FROM AUTHOR]

One of the best-preserved dykes of olivine minette among the lamprophyre dyke swarm in the Moldanubian Zone of the Bohemian Massif (Czech Republic) was investigated. The minette, exposed at Horní Kožlí Village (near Prachatice town), has porphyric texture with phenocrysts of olivine, clinopyroxene, orthopyroxene and biotite in a fine-grained matrix consisting of K-feldspar, biotite, clinopyroxene and minor plagioclase and quartz. Accessory minerals are apatite, Cr-rich spinel and iron sulphides. Olivine is mostly replaced by talc and rimmed by two zones (coronas) – a talc-rich inner zone and a biotite-rich outer zone. Rarely, larger grains of quartz with a corona of clinopyroxene are present. The clinopyroxene grows mostly perpendicular to the quartz rim and radially penetrates the quartz crystal. Three stages of mineral crystallization were distinguished. The first stage with apatite, olivine, biotite, spinel, orthopyroxene and part of the clinopyroxene occurred in the mantle position. During the second stage, felsic phases (K-feldspar, plagioclase, quartz) in the matrix were crystallized. The enrichment of the residual melt by silica and Na occurred as the result of both fractionation and contamination during magma ascent through the granulite facies crust during post-collision orogeny in the Bohemian Massif. Minerals related to the third stage were formed during filling of the vesicles (quartz with reaction rims of clinopyroxene) and subsequent alteration (talc after olivine). The origin of quartz with clinopyroxene reaction rims ('quartz ocelli') is explained by filling of cavities formed by the escape of volatiles. [ABSTRACT FROM AUTHOR]

Metamorphic basement rocks, that are exposed beneath the very low-grade to unmetamorphosed Upper Jurassic-Eocene formations north of the Torud fault zone within the Great Kavir Block, were investigated to elucidate the origin of their protoliths and the pressure and temperature conditions of metamorphism. The basement, previously assumed as a pre-Cambrian metamorphic complex, is mostly formed by amphibolite-facies orthogneisses (tonalite, granodiorite, and granite) with amphibolites and small amounts of metasediment-micaschists. Major- and trace-element geochemistry in combination with U-Pb age dating of zircon showed that the protoliths formed during Late Neoproterozoic continental arc magmatism that has also been identified in other tectonic blocks of Central Iran. In addition to quartz, feldspar(s), micas in orthogneisses, and amphibole + plagioclase in amphibolite, all rocks may contain garnet that shows prograde zoning. Kyanite was found only in some Al-rich amphibolite together with gedrite. The PT conditions of the rocks, based on conventional geothermobarometry and the pseudosection method, show a medium-pressure amphibolite-facies metamorphism. Ar-Ar age dating of muscovite reveals that this metamorphism occurred in the Middle Jurassic (166 Ma) and related to the closure of the Neotethyan basin. [ABSTRACT FROM AUTHOR]

Rocks from the Micaschist–Marble Complex of the Wölz Tauern, which are part of the middle Austroalpine unit, contain large (up to 2 cm) garnet crystals that show clear evidence of multistage growth. Isotopic dating indicates that a Variscan (∼270 Ma) garnet core was overgrown by new garnet formed during Eo-Alpine metamorphism at Cretaceous times. P-T paths for the Eo-Alpine metamorphism were obtained using the method of pseudosections (Powell and Holland in Metam Geol 16:309–343, 1998) and are consistent with earlier results from independent thermobarometry. Due to the large size of the garnets, growth zoning was preserved during amphibolite facies metamorphism at both Variscan and Alpine times. Full multicomponent diffusion modeling of compositional zoning at the interface of the Alpine and Pre-Alpine garnets in conjunction with the retrieved P-T paths allow average subduction/exhumation as well as heating/cooling rates to be retrieved. The modeling suggests that a minimum subduction/exhumation rate of ∼4 cm/a and heating/cooling rates on the order of 100–260°C/Ma for a 60°C subduction angle are required to preserve the observed compositional zoning overall while modifying the zoning at the interface between two garnets to the extent observed. Such rapid rates of burial/exhumation are consistent with the results of direct GPS measurements of convergence rates at several orogenic belts as well as with inferred rates from modeling in the Alps and other areas. In combination, this indicates that such rapid rates are commonplace during metamorphism in collisional orogens and places important constraints on the rheological behavior of crustal blocks in such orogens. [ABSTRACT FROM AUTHOR]

Zoned garnet and amphibole occur in metabasites of the Kraubath Massif, Eastern Alps, that contain relic magmatic clinopyroxene. The amphibole composition gradually changes from core (XMg = 0·83) to rim (XMg = 0·6–0·7). A number of compositional varieties of garnet occur in the metabasite. An older porphyroblastic garnet (Py23–27, Alm41–43, Grs29–33) has two different compositional domains, one relatively rich in Mg (Py27–30) and the other rich in Ca (Grs35–38) with a low Mg (Py20–25) content. The youngest variety, which forms rims on, or microveins in, the porphyroblastic garnet, has high Ca and low Mg (Grs40–57, Py2–7, Alm46–51). The amphibole cores and garnet porphyroblasts are interpreted to represent minerals formed during Variscan regional metamorphism under amphibolite-facies conditions. Alpine metamorphism is represented by the most recent Ca-rich and Mg-poor variety of garnet that coexists with the amphibole rims, epidote and chlorite. Fracturing in the porphyroblastic garnet probably originated during retrogression of the Variscan amphibolite-facies assemblages. Textural relations suggest that the garnet in the microveins formed by dehydration of hydrous phases during an Alpine metamorphic overprint that reached P–T conditions of 550–583°C at 1·0 GPa. [ABSTRACT FROM AUTHOR]

The Meliata unit represents a mélange-like accretionary wedge, containing blueschist facies tectonic blocks and slices in a Triassic and Jurassic sedimentary matrix. The blueschist facies rocks are tectonic remnants of the subducted parts of the Meliata-Hallstatt branch of the Tethys. The phyllosilicate assemblages in very low-grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress. Therefore, temperature and pressure values of low-T metamorphism of the sedimentary series and the late stages of decompressional cooling of blueschist facies rocks, obtained by phyllosilicate "crystallinity", chlorite thermometric and white K-mica geobarometric methods, can be regarded as semiquantitative estimates. However, results of chlorite–white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale. For deciphering the age relations of prograde and retrograde events, K–Ar isotope geochronological methods were applied. The sedimentary series and related basalts of the Meliata unit experienced high-T anchizonal prograde regional metamorphism, the temperature and pressure of which can vary between ca. 280 and 350 °C and ca. 2.5 and 5 kbar. White K-mica b geobarometry suggests possible minimal pressures of ca. 1.5 to 3 kbar. The mylonitic retrogression of blueschist facies phyllites is characterised by 340 °C and 4 kbar (minimal P). The low-T prograde metamorphism was synchronous with the retrograde metamorphism of the blueschists. The ages of these two events may be between ca. 150 and 120 Ma, culminating most probably at around 140–145 Ma. Thus, the Upper Jurassic (lowermost Cretaceous) very low-grade metamorphism of the Meliata unit is younger than the subduction-related, 160–155 Ma blueschist facies event, and definitely older than the Cretaceous (100–90 Ma) metamorphism of the footwall Gemer Palaeozoic. [ABSTRACT FROM AUTHOR]

Scapolite and other halogen-rich minerals (phlogopite, amphibole, apatite, titanite and clinohumite) occur in some high-pressure amphibolite facies calc-silicates and orthopyroxene-bearing rocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan. The calc-silicates are subdivided into two groups: garnet-bearing and garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite, the amphibolite facies mineral assemblages in the calc-silicates include clinopyroxene, calcite, quartz and one or more of the minerals scapolite, plagioclase, K-feldspar, titanite, apatite and rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene, garnet, plagioclase, scapolite, amphibole, quartz, calcite and accessory dolomite and alumosilicate (kyanite?). Retrograde phases in the rocks are plagioclase, scapolite, calcite, amphibole, sodalite, haüyne, lazurite, biotite, apatite and dolomite. The clinopyroxene is mostly diopside and rarely also hedenbergite. Aegirine and omphacite with a maximum jadeite content of 29 mol % were also found. Garnet from the calc-silicates is Grs45–95Py0–2 and from the orthopyroxene-bearing rocks is Grs10–15Py36–43. Peak P–T metamorphic conditions, calculated using available exchange thermobarometers and the TWQ program, are 750°C and 1·3–1·4 GPa. Depending on the rock type, the scapolite exhibits a wide range of composition (from EqAn = 0·07, XCl =0·99 to EqAn = 0·61, XCl =0·07). Equilibria calculated for scapolite and coexisting phases at peak metamorphic conditions yield XCO2 = 0·03–0·15. XNaCl (fluid), obtained for scapolite, ranges between 0·04 and 0·99. Partitioning of F and Cl between coexisting phases was calculated for apatite–biotite and amphibole–biotite. Fluorapatite is present in calc-silicates, but orthopyroxene-bearing rocks contain chlorapatite. Cl preferentially partitions into amphibole with respect to biotite. All these rocks have suffered various degrees of retrogression, which resulted in removal of halogens, CO2 and S. Halogen- and S-bearing minerals formed during retrogression and metasomatism are fluorapatite, sodalite, amphibole, scapolite, clinohumite, haüyne, pyrite, and lazurite, which either form veins or replace earlier formed phases. [ABSTRACT FROM PUBLISHER]

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps. [ABSTRACT FROM AUTHOR]