Coupling thermodynamic modeling and high-resolution in situ LA-ICP-MS monazite geochronology: evidence for Barrovian metamorphism late in the Grenvillian history of southeastern Ontario

The Flinton Group is a greenschist to upper amphibolite facies package of metasediments in southeastern Ontario that was metamorphosed during the Ottawan Orogeny. Thermodynamic modeling of metapelitic mineral assemblages suggests an increase in peak conditions of metamorphism across the 40 km wide study area from 3.5 to 7.9 kbar and 540 to 715 °C. Garnet isopleth thermobarometry applied to the cores of compositionally zoned porphyroblasts reveals remarkably similar P-T conditions of initial crystallization at approximately 3.7–4.0 kbar and 512–520 °C, corresponding to a relatively high geothermal gradient of ca. 34–45 °C km^?1. It is inferred from modeling and reaction textures that metamorphism was along Barrovian P-T paths. Major and trace element zoning in garnet from one sample records a complex growth history as evidenced by major and trace element zoning and the distribution of xenotime, allanite and monazite inclusions. High-resolution (6 μm) LA-ICP-MS U-Pb geochronology performed on monazite in the rock matrix and included in the outer 150 μm of garnet rim-ward of a Y annulus revealed an age of 976?±?4 Ma. The age is interpreted to reflect monazite growth at the expense of allanite and apatite late in garnet’s growth history over the P-T interval 4.5–6.8 kbar and 540–640 °C. This new age estimate for near peak metamorphism fits well into the regional framework but is significantly younger than previously reported ages for Ottawan metamorphism. Based on microstructures this new age suggests that compressional tectonics were operating much later in the history of the Grenville of southeastern Ontario than previously thought.     

Very low-grade metamorphism of sedimentary rocks of the Meliata unit,Western Carpathians,Slovakia: implications of phyllosilicate characteristics

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.     

P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton

The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane (SW Siberian Craton). Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution. The first stage of regional metamorphism (M1) occurred at high-amphibolite facies conditions at T ​= ​780–800 ​°C and P ​= ​8–9 ​kbar. The second stage (M2) belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600–750 ​°C and pressure 2–4 ​kbar. Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses. Both of them correspond to the Neoarchean age: the age of M1 falls into the interval of ca. 2489–2496 ​Ma, the age of M2 – ca. 2446–2456 ​Ma. The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.     

New finding of sillmanite in sapphirine-bearing granulites from Pedapalli,NE part of the Eastern Dharwar Craton,India

We report the occurrence of sillimanite in sapphirine-bearing granulites near Pedapalli town in the north eastern part of the Eastern Dharwar Craton (EDC). The sapphirine-bearing granulites of the Pedapalli area display evidence of metamorphic reactions involved in the formation of diverse mineral assemblages documented in different types of reaction textures. The common stable assemblage contains sapphirine-spinel-orthopyroxene-cordieriteplagioclage-corundum-sillimanite. The P-T evolution of these sapphirine granulites has been constrained by using the TWEEQU program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 °C, and pressure ca. 8 kbar. The assemblage reported in this study has important bearing on the high-temperature metamorphism and exhumation history of the EDC.     

Dehydration melting, fluid buffering and decompressional P–T path in a granulite complex from the Eastern Ghats, India

A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.     

Petrology of Precambrian metaultramafites of the Gridino high-pressure complex (Karelia)

Along with eclogitized gabbro and gabbronorite bodies, boudinaged metaultramafites such as garnet-pyroxene rocks and orthopyroxenites were revealed in the Archean plagiogneiss strata of the Gridino complex. The garnet-pyroxene rock crope out as a boudin on Vysokii Island. The early stage of the rock evolution is documented by inclusions of diabantite (Fe-Si chlorite), a mineral that occurs in metasomatized peridotites. Diabantite was found in all rock-forming minerals in paragenesis with mineral phases enriched in REE (Ce, Nd, La, etc.), U, and Th. The confinement of ore phases to the inclusion rims and the development of two systems of cracks, radial and concentric, around the inclusions in pyroxenes point to the transformation of the inclusions after their trapping. Thermobarometric studies of the crystal cores revealed that the anhydrous paragenesis garnet + orthopyroxene + clinopyroxene, which replaced the chlorite-bearing rock, formed at ~690 ºC and ~17 kbar. The rims of the rock-forming minerals reflect isothermal decompression to ~12 kbar, which was followed by decompression cooling to ~650 ºC and ~9 kbar with the formation of regressive amphibole-garnet-pyroxene paragenesis. The giant-grained orthopyroxenites compose chains of boudinaged bodies on Izbnaya Luda Island. The orthopyroxene crystals host abundant amphibole, quartz, biotite, and pyrite inclusions pointing to amphibolite metamorphism at the early stage of the rock evolution. There are two types of amphibole: magnesian hornblende and anthophyllite. The hornblende is a primary mineral, whereas the low-temperature anthophyllite forming rims around the quartz inclusions was produced at the regressive stage of metamorphism. There are no indicators of the PT-conditions of the peak metamorphism in the orthopyroxenite. The reaction enstatite + quartz + H₂O = anthophyllite allows deciphering water activity of anthophyllite formation, a ≤ 0.5.     

Metamorphism of the Scourian Complex, NW Scotland

The rocks of the Scourian Complex have been intensively studied, but there is still no consensus as to the conditions of the granulite-facies metamorphism preserved in these rocks. Recent estimates of these conditions fall into two groups, one at 820-920°C and ca. 11 kbar and the second at ca. 1000°C and >12 kbar. Investigation of a variety of rocks shows that the recorded conditions vary with grain-size, with higher-grade conditions recorded by the cores of coarser ( ca. 10 mm) crystals, and lower-grade conditions recorded by the rims of coarser grains and by finer grains. This observation suggests that re-equilibration during recovery of these rocks to the surface has been important which may account for the discrepancy in estimated P-T conditions. Revised estimates of the equilibration conditions of the Scourian Complex of T > 1000°C and P > 8.5 kbar are presented. The conditions suggested for the peak of metamorphism mean that the role of anatexis in the genesis of these rocks must be considered and the nature of the fluid phase thoroughly investigated.     

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region,east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12 kbar and 825–845 °C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30 Ma and lasted to ca. 20 Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.     

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.     

Genesis of garnet-bearing rocks at the Berezitovoe deposit,Upper Amur Region,Russia

The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.     

柴北缘绿梁山高压基性麻粒岩的变质演化历史：岩石学及锆石SHRIMP年代学证据

高压基性麻粒岩出露在柴北缘HP/UHP变质带的绿梁山地区,它主要呈透镜体状分布在石榴蓝晶(夕线)黑云片麻岩中。岩石学和矿物学数据显示高压基性麻粒岩经历了多阶段变质历史,早期可能经历了榴辉岩相变质作用(p>15kbar),以石榴子石中保留的少量绿辉石为特征;高压麻粒岩组合(Grt-Cpx-Pl-Qtz±Amp±Rt-Ilm)为退变质作用产物,其形成的变质条件为p=9.6~13.5kbar,T=730~870℃。晚期的变质反应以围绕石榴子石和后成合晶生成斜方辉石的为特征,形成的p-T条件为6.2~8.5kbar和720~860℃。高压基性麻粒岩中的锆石SHRIMP测定共获得两组年龄,分别为(448±3)Ma和(421±5)Ma。结合锆石阴极发光和矿物包体研究,前者代表高压麻粒岩阶段的变质年龄,后者代表晚期与斜方辉石形成有关的中低压麻粒岩阶段的变质年龄。这些年龄结果显示麻粒岩相变质作用持续了大约27Ma,这可能与早古生代祁连地块与柴达木地块碰撞作用所引起的地壳加厚和后来的热松驰有关。     

Partial melting and retrogression during exhumation of high-grade metapelites,the Tatra Mountains,Western Carpathians

Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO₂-N₂ fluids (5–30 mol. % N₂) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.     

Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex,North China Craton: Evidence from thermodynamic modeling and isotope geochronology

Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of X_ca (grt) isopleths. The P_max (M2) is distinguished by the X_ca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The T_max metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques.LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga.     

Metamorphism of Greater and Lesser Himalayan rocks exposed in the Modi Khola valley, central Nepal

Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks. This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust. Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata, juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about 15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades, although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic metamorphism.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block,Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ～720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ～750 °C and ～9 kbar and decompression to ～650 °C and ～8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ～620 °C and 6 kbar, followed by further burial to ～590 °C and 8.5 kbar. The monazite data yield a continuum of ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ～ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.     

Early Paleozoic UHT/LP Metamorphism in the Sangilen Block of the Tuvino-Mongolian Massif

The P–T conditions of Early Paleozoic metamorphism in the Sangilen block of the Tuvino-Mongolian Massif (southeastern part of the Central Asian Mobile Belt) achieved a value of 910–950°C and 3–4 kbar, which corresponded to the conditions of ultrahigh temperature–low pressure (UHT/LP) metamorphism. During retrograde metamorphism, cooling down to 850°C was accompanied by compression (up to 5.5–7 kbar), and then cooling down to 580–650°C took place at nearly the same pressure (5.5–6.5 kbar). UHT metamorphism was related to the elevated heat flow from the mantle, leading also to an intensive basite magmatism. The “counter-clockwise” P–T evolution was evoked by underthrusting of the hot tectonic slab (Erzin complex) beneath the colder one (Moren complex).     

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero:Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks.The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades.The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny,making it unique among dome-and-keel provinces.However,a lack of appropriate lithologies,datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province.A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak P-T conditions of 5-7 kbar and 700-750 ℃ and a published age of ca.2730 Ma based on U-Pb ages of zircon from leucosomes,suggesting that this age represents the migmatisation event.A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole,giving P-7 conditions of 8-9 kbar and 550 ℃ with an associated titanite age of ca.2050 Ma.A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700 ℃,and texturally late titanite from this sample produces an age of ca.2060 Ma.Three additional samples were collected from the edges of the dome.A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution.This sample has a U-Pb zircon age of ca.2775 Ma,which could date metamorphism or be the age of its volcaniclastic protolith.Texturally unconstrained titanite from the sample gives an age of ca.2040 Ma.A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca.2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700 ℃ on a near isobaric P-T path.An amphibolite dike,observed to cross-cut the felsic schist produces a zircon U-Pb age of ca.2760 Ma.Altogether this data suggests that the samples were metamorphosed in the Archean(ca.2775-2730 Ma)and again during the Transamazonian event.The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time.The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures,with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles.The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature,but they are susceptible to reworking,resulting in an enigmatic record of formation.     

High-pressure metamorphism in the Western Cordillera of North America

The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid-Cretaceous(?) to early Tertiary high-P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8–10 kbar at 650–725° C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist. Mineral reactions continued during nearly isothermal decompression from 8–10 kbar to c. 3–5 kbar. Early high-P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower-grade, staurolite-bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature. These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low-P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression. Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25–30 km.