Reaction textures and P–T conditions of Opx-Crd gneisses from Karimnagar, Andhra Pradesh, India

Summary The Karimnagar granulite terrain is an integral part of the Eastern Dharwar Craton (EDC). It has received much interest because of the only reported granulite facies rocks in the EDC. These granulites contain quartz-free sapphirine-spinel-bearing granulites, kornerupine – bearing granulites, mafic granulites, orthopyroxene-cordierite gneisses, charnockites, amphibolites, dolerite dykes, granite gneisses, quartzites and banded magnetite quartzite. The orthopyroxene-cordierite gneisses occur as enclaves within granite-gneiss in association with banded magnetite quartzites, charnockites and amphibolites. The observed reaction textures, spectacular as they are, have an extraordinary information content within a tiny domain. Coronas, symplectites and resorption textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The main mineral assemblages encountered in these gneisses are orthopyroxene – cordierite – biotite – plagioclase – perthite – quartz and garnet – orthopyroxene – cordierite – biotite – quartz – plagioclase – perthite ± sillimanite. Multiphase reaction textures in conjunction with mineral chemical data in the KFMASH system indicate the following reactions: Based on chemographic relationships and petrogenetic grids in the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH) system, a sequence of prograde (early stage), isothermal decompression (middle stage) and retrograde (late stage) reactions (‘back reactions’ and hydration reactions) are inferred. Relatively lower P–T estimates (0.35 GPa/550–750 °C) obtained from the different geothermobarometers are attributed to late Fe–Mg re-equilibration during cooling. Therefore, the convergence method has been applied to retrieve simultaneously the P–T conditions of the thermal peak of metamorphism. The near thermal peak condition of metamorphism estimated by the convergence method are 850 °C/0.62 GPa. The P–T estimates define a retrograde trajectory with substantial decompression.     

P–T evolution and tectonic significance of lawsonite-bearing schists from the eastern segment of the southwestern Tianshan,China

Robust quantification of pressure (P)–temperature (T) paths for subduction-related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down-going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur in voluminous host metasedimentary schists, information about the P–T evolution of these rocks in the eastern segment is limited, precluding a full understanding of the development of the belt as a whole. In this study at Kekesu in the eastern segment of the SW Tianshan, we use microstructural evidence and phase equilibrium modelling to quantify the peak and retrograde P–T conditions from two lawsonite-bearing micaschists and an enclosed garnet–epidote blueschist; for two of the samples we also constrain the late prograde P–T path. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, polymineralic aggregates included in garnet are interpreted as pseudomorphs after lawsonite. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, titanite and ilmenite, the maximum P–T conditions for the peak stage are 18.0–19.0 kbar at 480–485°C. During initial exhumation, the retrograde P–T path passed through metamorphic conditions of 15.0–17.0 kbar at 460–500°C. For garnet–glaucophane micaschist TK33, which is mainly composed of garnet, glaucophane, phengite/paragonite, albite, chlorite, quartz, relict lawsonite and minor epidote, with accessary titanite, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are >24.0 kbar at 400–500°C. During exhumation, the P–T path passed through metamorphic conditions of 17.5–18.5 kbar at 485–495°C and 14.0–17.5 kbar at 460–500°C. For garnet–epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary titanite, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of ~20.0 kbar at ~445°C to P_max conditions of ~21.5 kbar at 450–460°C and T_max conditions of 19.5–21.0 kbar at 490–520°C. During exhumation, the rock passed through metamorphic conditions of 17.5–19.0 kbar at 475–500°C, before recording P–T conditions of <17.5 kbar at <500°C. These results demonstrate that maximum recorded pressures for individual samples vary by as much as 6 kbar in the eastern segment of the SW Tianshan, which may suggest exhumation from different depths in the subduction channel. Furthermore, the three samples record similar P–T paths from ~17.0 to 15.0 kbar, which suggests they were juxtaposed at a similar depth along the subduction interface. We compare our new results with published information from eclogites in the same area before considering the wider implications of these data for the orogenic development of the belt as a whole.     

Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.     

Along-strike variations of P–T conditions in accretionary wedges and syn-orogenic extension,the HP–LT Phyllite–Quartzite Nappe in Crete and the Peloponnese

Syn-orogenic detachments in accretionary wedges make the exhumation of high-pressure and low-temperature metamorphic rocks possible with little erosion. The velocity of exhumation within the subduction channel or the accretionary complex, and thus the shape of P–T paths, depend upon the kinematic boundary conditions. A component of slab retreat tends to open the channel and facilitates the exhumation. We document the effect of slab retreat on the shape of P–T paths using the example of the Phyllite–Quartzite Nappe that has been exhumed below the Cretan syn-orogenic detachment during the Miocene in Crete and the Peloponnese. Data show a clear tendency toward colder conditions at peak pressure and during exhumation where the intensity of slab retreat is larger. This spatial evolution of P–T gradient is accompanied with an evolution from a partly coaxial regime below the Peloponnese section of the detachment toward a clearly non-coaxial regime in Crete.     

Cretaceous sedimentary–volcanogenic complexes of the Kamchatka Isthmus: Structure,composition and geodynamic formation conditions

New data on the chemical and rare-element composition and age of the rocks referred earlier to the Iruney suite of the Kamchatka Isthmus are received. In the recent structure these rocks compose the structural–strata complexes of the nappe-folded Lesnovsky Rise. Radiolarian analysis data substantiate that the deposits belonging to the Ening series and the middle and upper parts of the Iruney suite were formed in a single sedimentation basin in the Campanian time. The discovery of a new occurrence of Prunobrachidae representatives on the Kamchatka Peninsula allows us to draw wide interregional correlations and reconstruct the sedimentation conditions. The studied volcanites relate to different igneous series and were formed in geodynamic conditions of the marginal sea and the volcanic arc. The igneous rocks of the Ening stratum are similar to the N-MOR and OI basalts that were formed within the marginal sea (Iruney Marginal Sea) basin. The Upper Cretaceous formations of the eastern slope of the Sredinny Range were formed within the volcanic rise with the island-arc type of volcanism. The younger Eocene igneous rocks of the neo-autochthon (granites and granodiorites) and the volcanic rocks of the Kinkil suite mark a new orogenic stage of development of the Kamchatka margin.     

Composition and conditions of crystallization of zircon from the rare-metal ores of the Gremyakha–Vyrmes Massif,Kola Peninsula

The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.     

Time calibration of a P–T path from a Variscan high-temperature low-pressure metamorphic complex (Bayerische Wald, Germany), and the detection of inherited monazite

A temperature–time path was constructed for high-temperature low-pressure (HT–LP) migmatites of the Bayerische Wald, internal zone of the Variscan belt, Germany. The migmatites are characterised by prograde biotite dehydration melting, peak metamorphic conditions of approximately 850 °C and 0.5–0.7 GPa and retrograde melt crystallisation at 800 °C. The time-calibration of the pressure–temperature path is based on U–Pb dating of single zircon and monazite grains and titanite separates, on ⁴⁰Ar/³⁹Ar ages obtained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and on ⁴⁰Ar/³⁹Ar spot fusion ages of biotite determined in situ from sample sections. Additionally, crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns, suggesting that peak temperatures of 850 °C cannot have prevailed much longer than 2.5 Ma. The temperature–time paths obtained for two areas approximately 30 km apart do not differ from each other considerably. U–Pb zircon ages reflect crystallisation from melt at 850–800 °C at 323 Ma (southeastern area) and 326 Ma (northwestern area). The U–Pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance. The preservation of inherited monazite and the presence of excess ²⁰⁶Pb resulting from the incorporation of excess ²³⁰Th in monazite formed during HT–LP metamorphism suggest that monazite ages in the migmatites of the Bayerische Wald reflect crystallisation from melt at 850–800 °C and persistence of older grains at these temperatures during a comparatively short thermal peak. The U–Pb ages of titanite (321 Ma) and ⁴⁰Ar/³⁹Ar ages of hornblende (322–316 Ma) and biotite (313–309 Ma) reflect cooling through the respective closure temperatures of approximately 700, 570–500 and 345–310 °C published in the literature. Most of the feldspars' ages (305–296 Ma) probably record cooling below 150–300 °C, while two grains most likely have higher closure temperatures. The temperature–time paths are characterised by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 °C/my at temperatures between 850–800 and 700 °C to 11–16 °C/my at temperatures down to 345–310 °C. Further cooling to feldspar closure for Ar was probably even slower. The lack of decompressional features, the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating, rapid uplift and extension due to lithospheric delamination as proposed elsewhere. Instead, the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material. Received: 7 July 1999 / Accepted: 28 October 1999     

P–T conditions of mineralization in the Jonnagiri granitoid-hosted gold deposit,eastern Dharwar Craton,southern India: Constraints from fluid inclusions and chlorite thermometry

Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H₂O–CO₂–CH₄ + salt) fluid that underwent phase separation and gave rise to gaseous (CO₂–CH₄), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O₂), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems.     

P–T composition and evolution of paleofluids in the Paleoproterozoic Mag Hill IOCG system,Contact Lake belt,Northwest Territories,Canada

The Echo Bay stratovolcano complex and Contact Lake Belt of the Great Bear Magmatic Zone, Northwest Territories, host a series of coalescing Paleoproterozoic hydrothermal systems that affected an area of several hundred square kilometers. They were caused by intrusion of synvolcanic diorite–monzodioritic plutons into andesitic host rocks, producing several characteristic hydrothermal assemblages. They include early and proximal albite, magnetite–actinolite–apatite, and potassic (K-feldspar) alteration, followed by more distal hematite, phyllic (quartz–sericite–pyrite), and propylitic (chlorite–epidote–carbonate±sericite±albite±quartz) alteration, and finally by late-stage polymetallic epithermal veins. These alteration types are characteristic of iron oxide copper–gold deposits, however, with distal and lower-temperature assemblages similar to porphyry Cu systems. Magnetite–actinolite–apatite alteration formed from high temperature (up to 560 °C) fluids with average salinity of 12.8 wt% NaCl equivalent. The prograde propylitic and phyllic alteration stages are associated with fluids with temperatures varying from 80 to 430 °C and a wide salinity range (0.5–45.6 wt% NaCl equivalent). Similarly, wide fluid temperature (104–450 °C) and salinity (4.2–46.1 wt% NaCl equivalent) ranges are recorded for the phyllic alteration. This was followed by Cu–Ag–U–Zn–Co–Pb sulfarsenide mineralization in late-stage epithermal veins formed at shallow depths and temperatures from 270 °C to as low as 105 °C. The polymetallic veins precipitated from high salinity (mean 30 wt% NaCl equivalent) dense fluids (1.14 g/cm³) with a vapor pressure of 3.8 bars, typical of epithermal conditions. Fluid inclusion evidence indicates that mixed fluids with evolving physicochemical properties were responsible for the formation of the alteration assemblages and mineralization at Mag Hill. An early high temperature, moderate salinity, and magmatic fluid was subsequently modified variably by boiling, mixing with cooler low-salinity meteoric water, and simple cooling. The evidence is consistent with emplacement of the source plutons and stocks into an epithermal environment within ~1 km of surface. This generated near-surface high-temperature alteration in a dynamic hydrothermal system that collapsed (telescoped) resulting in widespread evidence of boiling and epithermal mineralization superimposed on earlier stages of alteration.     

Podolian,Saxonian and baltic plates – Teisseyre–Tornquist Line and the edge of the East European Craton

In the area of the Central Europe three large continental scale tectonic units meet together, namely Precambrian East European Craton (EEC) to the northeast, Variscan West European Platform (WEP) terranes to the southwest, and younger Alpine Carpathian arc in the south. The reference structure of the Central Europe is a sharp edge of the East European Craton. In the area of Poland the south-western margin of the EEC is marked as Teisseyre–Tornquist Zone (TTZ), which continues to the north as Sorgenfrei–Tornquist Zone (STZ). Teisseyre–Tornquist Zone (TTZ) — earlier Teisseyre Line, Tornquist Line or Teisseyre–Tornquist Line (TTL), is a term created in commemoration of Polish geologist Wawrzyniec Teisseyre and German geologist and paleontologist Alexander Tornquist. At the turn of XIX and XX century, they noticed a fundamental difference in the geology of platform cover between the rigid East European Platform and its more mobile southwestern forefield (Teisseyre, 1893, 1903; Tornquist, 1908, 1910). From the very beginning the TTL was conceived as a linear feature (fault or fault zone) marking the southwestern boundary of the EEC. Contrarily, the Trans–European Suture Zone (TESZ) is a term coined by Asger Berthelsen for an assemblage of suspect terranes boarded by the East European Craton and the Variscan orogeny. It is not a linear structure, but a terrane accretion zone, 100–200 km wide. Both terms, TTL and TESZ, should not be mistaken, as is the case on many maps concerning the problem (Dadlez et al., 2005). The edge of the craton is a major lithospheric structure, which appears to be a deep-seated boundary reaching at least down to a depth of about 200 km as shown by tomographic analysis of shear wave velocity structure of the mantle under Europe. Another indication of the deep-seated nature of this zone was obtained from observations of earthquakes and explosions located in Europe. To explain the observed blockage of energy from regional seismic events by TTZ, the structural anomaly between eastern and western Europe must reach at least down to a depth of about 200 km. Continental scale tectonic units of the Central Europe are clearly visible in the crustal structure, Moho depth map, and also gravity, magnetic and heat flow maps.     

Metamorphic P‒T paths and Zircon U–Pb ages of Archean ultra-high temperature paragneisses from the Qian’an gneiss dome,East Hebei terrane,North China Craton

The 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 metamorphic P–T paths 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 using thermocalc, and zircon dating. Anticlockwise P–T paths 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 fixed T = 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 peak P–T conditions 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 peak P–T condition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (X_Grs = 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 (X_An = 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 decreasing X_Grs in 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 of c. 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 from c. 2.56 to c. 2.48 Ga with a peak at c. 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 anticlockwise P–T paths, 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.     

Geochemical and SIMS U-Pb rutile and LA–ICP–MS U-Pb zircon geochronological evidence of the tectonic evolution of the Mudanjiang Ocean from amphibolites of the Heilongjiang Complex,NE China

This study presents new secondary-ion mass spectrometry (SIMS) rutile and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U-Pb geochronological and whole-rock geochemical data for amphibolites of the Heilongjiang Complex, located within the Yilan area of NE China, to constrain the tectonic evolution of the Mudanjiang Ocean between the Songnen–Zhangguangcai Range and Jiamusi massifs. Magmatic zircon from amphibolites collected from the Yilan Marble Quarry yields a weighted mean ²⁰⁶Pb/²³⁸U age of 274 ± 2 Ma, which is interpreted as the protolithic age. Amphibolites from the Longlangang and Tuanshanzi areas yield rutile U-Pb ages of 177 ± 11 Ma and 172 ± 5 Ma, respectively, which are interpreted to reflect the cooling of these rocks below the closure temperature of Pb diffusion in rutile. Amphibolites from the Yilan Marble Quarry are enriched in light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta and P) relative to large ion lithophile elements (LILEs). Amphibolites from the Longlangang and Tuanshanzi areas have relatively flat chondrite-normalized rare earth element (REE) patterns, and remarkable negative Nb and Ta anomalies. Moreover, all of the amphibolites from the Heilongjiang Complex in the Yilan area have tholeiitic and arc-type geochemical affinities. These amphibolites formed by similar petrogenetic processes, but from distinct mantle sources. The magmas that formed these units were generated by the partial melting of mantle sources metasomatized by subducted slab fluids, and the magma that formed the amphibolites within the Yilan Marble Quarry may have also incorporated sedimentary material. Mantle peridotite from the garnet-spinel transition zone is a possible source for the protolith of amphibolites in the Yilan Marble Quarry, and spinel-peridotites may have been the magma sources for the protoliths of amphibolites in the Longlangang and Tuanshanzi areas. Combining our data for amphibolites from the Heilongjiang Complex in the Yilan area with the results of previous research on Late Paleozoic–Early Mesozoic arc magmatism and metamorphism, we infer that the Late Paleozoic–Early Mesozoic tectonic evolution of the Mudanjiang Ocean was characterized by double-sided subduction. These data indicate that the Mudanjiang Ocean closed during the Jurassic (180–160 Ma).     

Diverse mineral compositions,textures, and metamorphic P–T conditions of the glaucophane-bearing rocks in the Tamayen mélange,Yuli belt,eastern Taiwan

This paper presents new petrologic data for high-pressure, low-temperature (HP–LT) metamorphic rocks at Juisui. We reinterpret the so-called “Tamayen block” (Yang and Wang, 1985) or “Juisui block” (Liou, 1981, Beyssac et al., 2008) as a tectonic mélange. It is not a coherent sheet but rather a mixture dominated by greenschist and pelitic schist with pods of serpentinite, epidote amphibolite, and rare blueschist. Four types of glaucophane-bearing rocks are newly recognized in this mélange. Type I is in contact with greenschist lacking glaucophane and garnet. Glaucophane is present only as rare inclusions within pargasite. This type records metamorphic evolution from epidote blueschists-, epidote amphibolite-, to greenschist-facies. Type II contains characteristic zoned amphiboles from barroisite core to Mg-katophorite mantle and glaucophane rim, implying an epidote amphibolite-facies stage overprinted by an epidote blueschists-facies one. Type III includes winchite and indicates P–T conditions of about 6–8 kbar, approaching 400 °C. Type IV contains paragonite but lacks garnet; amphibole shows a Na–Ca core surrounded by a glaucophane rim. This type shows a high-pressure (?) epidote amphibolite-facies stage overprinted by an epidote blueschists-facies one. Amphibole zoning trends and mineral assemblages imply contradictory P–T paths for the four types of glaucophane-bearing rocks—consistent with the nature of a tectonic mélange. The new P–T constraints and petrologic findings differ from previous studies (Liou et al., 1975, Beyssac et al., 2008).     

Application of cathodoluminescence to magmatic quartz in a tin granite – case study from the Schellerhau Granite Complex, Eastern Erzgebirge, Germany

A model of the cooling history of tin-bearing granitic magma forming the Schellerhau granites (Eastern Erzgebirge, Germany) is shown on the basis of quartz textures. Similar grain size, similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallization history before the melts of the Schellerhau granite varieties were intruded. Four nucleation events occurred during crystallization in different crustal levels between about 20 and 1 km depth. The parental melt of the Schellerhau granites is interpreted to have contained<2.5 wt% H₂O originally. The water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt% and characterizes highly evolved residual melts that enable the formation of tin deposits. This study contributes to a better understanding of the development and behaviour of fractionated tin-bearing granitic melts, and links quartz cathodoluminescence (CL) with microanalytical studies. Received: 28 October 1998 / Accepted: 18 August 1999     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.     

P–T conditions during skarn formation in the Ocna de Fier ore district, Romania

P–T conditions during skarn formation in the 75.5 Ma old Ocna de Fier-Dognecea (SW Romania) ore district are assessed in this work using a combination of petrogenetic grids, Berman's TWEEQU programme, and several independent geothermobarometers. These were applied both to hornfelses surrounding the skarn and to the granodiorite which caused the skarn and contact metamorphism. The results are consistent and point to a peak metamorphic temperature of 700 ± 50 °C, decreasing away from the contact, and to a pressure of 2.8 ± 1 kbar, equivalent to ∼10 km depth in the region. These results quantify the qualitative idea that skarn mineralisation normally forms in a high T, low P contact metamorphic environment. Received: 13 February 1998 / Accepted: 8 April 1999     

Re–Os isotopic ages of pyrite and chemical composition of magnetite from the Cihai magmatic–hydrothermal Fe deposit, NW China

The Eastern Tianshan Orogenic Belt of the Central Asian Orogenic Belt and the Beishan terrane of the Tarim Block, NW China, host numerous Fe deposits. The Cihai Fe deposit (>90 Mt at 45.6 % Fe) in the Beishan terrane is diabase-hosted and consists of the Cihai, Cinan, and Cixi ore clusters. Ore minerals are dominantly magnetite, pyrite, and pyrrhotite, with minor chalcopyrite, galena, and sphalerite. Gangue minerals include pyroxene, garnet, hornblende and minor plagioclase, biotite, chlorite, epidotite, quartz, and calcite. Pyrite from the Cihai and Cixi ore clusters has similar Re–Os isotope compositions, with ～14 to 62 ppb Re and ≤10?ppt common Os. Pyrrhotite has ～5 to 39 ppb Re and ～0.6 ppb common Os. Pyrite has a mean Re–Os model age of 262.3?±?5.6 Ma (n?=?13), in agreement with the isochron regression of ¹⁸⁷Os vs. ¹⁸⁷Re. The Re–Os age (～262 Ma) for the Cihai Fe deposit is within uncertainty in agreement with a previously reported Rb–Sr age (268?±?25 Ma) of the hosting diabase, indicating a genetic relationship between magmatism and mineralization. Magnetite from the Cihai deposit has Mg, Al, Ti, V, Cr, Co, Ni, Mn, Zn, Ga, and Sn more elevated than that of typical skarn deposits, but both V and Ti contents lower than that of magmatic Fe–Ti–V deposits. Magnetite from these two ore clusters at Cihai has slightly different trace element concentrations. Magnetite from the Cihai ore cluster has relatively constant trace element compositions. Some magnetite grains from the Cixi ore cluster have higher V, Ti, and Cr than those from the Cihai ore cluster. The compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures. Combined with regional tectonic evolution of the Beishan terrane, the Re–Os age of pyrite and the composition of magnetite indicate that the Cihai Fe deposit may have derived from magmatic–hydrothermal fluids related to mafic magmatism, probably in an extensional rift environment.