Epidote-amphibolite Facies Metamorphism Research Articles

Newly recognized blueschist-facies metamorphism (glaucophane-omphacite-garnet), Belvidere Mountain Complex, northern Appalachians

Abstract An occurrence of blueschistfacies metamorphism in the Appalachian orogen is newly recognized in northwestern New England, United States. Inclusions of glaucophane and omphacite occur in a relict garnet core from a retrogressed garnetbarroisite amphibolite of the Belvidere Mountain Complex in Vermont. Pressuretemperature pseudosection and mineral composition isopleth calculations demonstrate that the Belvidere Mountain Complex blueschistfacies mineral assemblage of glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnetabsent amphibolite with barroisite and chlorite inclusions in clinozoisite records highpressure epidote-amphibolite–facies metamorphism at ~1.0–1.4 GPa and ~515–550 °C. These new findings quantify deep subduction of the Belvidere Mountain Complex during the Cambrian to Ordovician Taconic orogenic cycle and suggest that more blueschistfacies mineral assemblages could be revealed in the Appalachians with detailed analysis of retrogressed rocks.

Reappraisal of the oldest high-pressure type schist in Japan: New zircon U-Pb age of the Kitomyo Schist of the Kurosegawa Belt

The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K–Ar dating of white mica. The schist consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite ± phengite ± rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = ~0.8–1.5 GPa and T = ~500–570 °C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U–Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of ~440 Ma. Also the U–Pb data confirmed ~360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono–Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the ‘Greater South China’ margin that was extensively eroded during later tectonic processes.

Geochronology of metamorphism, deformation and fluid circulation: A comparison between Rb-Sr and Ar-Ar phyllosilicate and U-Pb apatite systematics in the Karagwe-Ankole Belt (Central Africa)

This work presents an integrated geochronology study combining step-heating Ar-Ar and in-situ Rb-Sr phyllosilicate geochronology with EPMA elemental maps and U-Pb apatite geochronology in order to determine the timing and duration of regional deformation and fluid circulation processes in the Karagwe-Ankole Belt in Central Africa. The Kibuye-Gitarama-Gatumba study area (West Rwanda) forms part of the Western Domain of this Mesoproterozoic belt, where the metamorphic and deformation history, geothermometric evolution and mineralization processes (both genesis and geochronology) have been well-documented in previous studies.Our geochronological results and interpretations show the importance of thermal diffusion throughout the elevated temperature regime present in the Neoproterozoic. During the Ediacaran, metamorphism and fluid alteration in the Gatumba area was associated with deformation, as independently demonstrated by U-Pb apatite and Ar-Ar phyllosilicate geochronology. From the end of the Ediacaran until the Late Cambrian, saline fluid activity is recorded and fluid-induced (re)crystallization processes dominate the Gatumba biotite isotope record. The direct comparison between and Ar-Ar geochronology suggests a parentless 40Ar-component is preserved during this fluid-induced (re)crystallization due to its saline, (earth) alkali-rich composition. Additionally, this multi-method approach allows us to place the history of the Western Domain of the Karagwe-Ankole belt in relation to that of the surrounding areas, i.e. the Eastern Domain and the Congo and Tanzania Cratons. Deformation and epidote-amphibolite facies metamorphism (at ~590–570 Ma), and fluid fluxes (at ~590–495 Ma) occurred in the Kibuye-Gitarama-Gatumba area after the West Gondwana Orogeny but from the onset of the East African Orogeny onwards till after the culmination of the Gondwana amalgamation, demonstrating the progressively eastward-moving orogenic front in this supercontinent assembly.

Deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions

We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1GPa, 400°C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500°C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust.

Ultrahigh pressure metamorphism and tectonic evolution of southwestern Tianshan orogenic belt, China: a comprehensive review

AbstractRecently, a huge ultrahigh-pressure (UHP) metamorphic belt of oceanic type has been recognized in SW Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well-preserved coesite inclusions are commonly found in eclogites, garnet phengite schists and marbles. Ti-clinohumite and Ti-chondrodite have been identified in UHP metamorphic serpentinites. Based on the P–T pseudosection calculation and combined U–Pb zircon dating, the P–T–t path has been outlined as four stages: cold subduction to UHP conditions before c. 320 Ma whose peak ultrahigh pressure is about 30 kbar at 500°C; heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600°C at 22 kbar; then the early cold exhumation from amphibolite eclogite facies to epidote-amphibolite facies metamorphism before 220 Ma; and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn-subduction arc-like 333–326 Ma granitic rocks and 280–260 Ma S-type granites in the coeval low-pressure and high-temperature metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

Reactivation and strain localisation in Bhavani shear zone, South India

Shear markers and shear sense indicators from the Bhavani shear zone, a member of the shear system separating the northern Archaean and southern Pan African granulite terranes of the South Indian shield, with differently exhumed crustal blocks, suggest multiphase reactivation. This is revealed by textural and geochemical characteristics of mylonites which indicate an event of prograde epidote-amphibolite facies metamorphism of the previously retrograded protolith of shear zone rocks. The amphibole varies progressively from an initial magnesio-hornblende to ferrotschermakitic hornblende with increasing Al and K while the plagioclase maintains oligoclase composition (An21-25). P-T estimates suggest metamorphic conditions of 634°C - 720°C at 3.6±0.5 to 5.8±0.5 kbar. Strain partitioning during the reactivation of the shear zone prior to the Pan African event, is responsible for the gradation in the intensity of mylonitic fabric as well as some of the opposing shear sense indicators.

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.

Geochemistry, age, and petrogenesis of rocks from the Garevka metamorphic complex, Yenisey Ridge

The mineralogical, petrological, geochemical and geochronological data on the Garevka metamorphic complex (GMC) of the Yenisey Ridge were used to evaluate the age, nature, and provenance of their protoliths. The evolution of the GMC occurred in two stages with different ages, thermodynamic regimes, and metamorphic field gradients. The final emplacement of granitoids was marked by high-pressure (HP) amphibolite facies regional metamorphism (970 Ma). At the second stage, these rocks experienced Late Riphean (900–870 Ma) retrograde epidote-amphibolite facies metamorphism accompanied by the formation of blastomylonitic complexes within narrow zones of brittle-ductile deformation. The metamorphism of migmatites (850 Ma) is coeval with the collisional medium-pressure metamorphism of the kyanite-sillimanite type. The GMC is different from the other rock complexes of the Yenisey Ridge in the presence of rapakivi-type granites. The geochemistry of these rocks, which is characterized by stronger enrichment in K2O, FeO, Y, Th, U, Zr, Hf, Nb, Ta, and REE relative to the other mineral assemblages of the GMC, is typical of anorogenic (A-type) within-plate granites. Among other distinctive features of these rocks are the strong iron enrichment of the melanocratic minerals, the presence of ilmenite as the sole Fe-Ti oxide, and crystallization from higher temperature (T = 825°C vs. T = 750°C) water-poor magmas under reducing conditions below the FMQ buffer. Significant variations in the geochemical and petrological characteristics of the GMC rocks suggest that they could not be derived from a single source. The main volume of the high-K rocks varying in composition from A-type to S-type granites was generated by melting of mixed mantlecrustal sources. The products of melting of the Late Archean-Early Proterozoic infracrustal gneisses of the Siberian Craton could be a possible source for the least oxidized rocks.

Occurrence of Neoproterozoic low-grade metagranite in the western Beihuaiyang zone, the Dabie orogen

Based on detailed field investigations and petrographic observations, we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone, the Dabie orogen. This study reports the results of zircon U-Pb dating and preliminary petrographic observations on two metagranite samples. The studied rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their U-Pb ages are 726 ± 6 and 758 ± 12 Ma, respectively, similar to those for the Luzhenguan complex in the eastern segment of the Beihuaiyang zone. In combination with previously determined 635 ± 5 Ma low-grade metagabbro, this study suggests the occurrence of at least two types of Neoproterozoic intrusive rocks in the Beihuaiyang zone, the northern margin of the South China Block (SCB): 726–758 Ma metagranite and 635 Ma metagabbro. These rocks occur within the metamorphosed Ordovician volcanic zone (originally named the Dingyuan Formation) and are in tectonic contact to each other, but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid- and late-Neoproterozoic mafic and felsic magmatism in the Suizhou and Zaoyang area, Hubei Province, but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks, it appears that these Neoproterozoic low-grade rocks are exotic and they may have been detached and scraped from subducting SCB crust in the early Triassic during the initial continental subduction, and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.

Structure and petrology of the Dadeldhura Group, far western Nepal, Himalaya

The granites, phyllites, schists, and gneisses of the Dadeldhura Group exhibit a significant Himalayan metamorphic imprint. The rocks of the group constitute a synform and the group is delimited by thrusts. The North Dadeldhura Thrust (NDT) zone is dominated by granitic mylonite with subordinate quartzite, quartz-chlorite schist, and amphibolite. In the quartz-chlorite schist from the NDT zone, relict kyanite is observed, which together with recrystallisation textures in the granitic mylonite indicate that low-temperature syn-tectonic retrogression has affected the thrust zone. Prograde garnets with spiral structures from a zone 2–4 km structurally above the base of the NDT are associated with mylonite-like rocks, and indicate distinct prograde shear zones in this area. This suggested that prograde thrust stacking has affected about 4 km wide north belt of the Dadeldhura Group. Rim thermobarometry from the garnet holding rocks shows that the minerals were re-equilibrated at 440–550 °C and 6.5–9.5 kbar. In the southern part of the Dadeldhura Group, textures in quartz and feldspar from the Saukhark Granite-Gneiss indicate that temperatures during recrystallisation were around 450–550 °C. These P–T estimates suggest that most of the now exposed rocks in the Dadeldhura Group experienced prograde epidoteamphibolite facies metamorphism during early orogenic build up and thrusting of the Dadeldhura Nappe. A subsequent syntectonic retrograde phase is observed in the NDT zone as low temperature recrystallisation of quartz and feldspar in mylonitic rocks, and chlorite and quartz in chlorite-grade rocks holding relict kyanite. Retrograde trusting was also focused along a distinct “back thrusting” zone within the northern part of the group.

Polymetamorphism of Aktyuz eclogites (northern Kyrgyz Tien-Shan) deduced from inclusions in garnets

Garnet and omphacite in eclogites from the Aktyuz area, northern Kyrgyz Tien-Shan, preserve evidence of two distinct metamorphic events. Relic polyphase inclusions of Fe-rich staurolite (XMg < 0.29), Mg-taramite, paragonite, and oligoclase (An < 16) contained in garnet cores represent a separate pre-eclogitic amphibolite or epidote-amphibolite facies metamorphism (T = 560-650 °C, P = 4-10 kbar). This relatively low- to medium-pressure metamorphic event probably resulted from the heat of the hot hanging wall at the inception of subduction. Inclusions in the garnet cores and rims and matrix omphacite (glaucophane, sodic-calcic and calcic amphiboles, paragonite, phengite, epidote, albite, rutile, hematite, and quartz) suggest that epidote-blueschist facies conditions (T = 330-570 °C, P = 8-16 kbar) were experienced during the prograde stage of the eclogitic metamorphism. This event occurred within a subduction zone. The estimated peak metamorphic conditions of the eclogites are T = 600-710 °C and P = 15-25 kbar. The subsequent retrograde path through epidote-amphibolite and greenschist facies suggests a clockwise P-T path, in contrast to the anticlockwise P-T path proposed by previous work.

Southward extrusion of eclogite-bearing mafic–ultramafic bodies in the Sanbagawa belt, central Shikoku, Japan

Several mafic rock masses, which have experienced eclogite facies metamorphism, are distributed in flat-lying non-eclogitic schists in an intermediate structural level (thermal core) of the Sanbagawa belt. The largest, Iratsu mass, and an associated peridotite, the Higashi-Akaishi mass, extend E–W for about 8 km, and N–S for about 3 km, and are surrounded by pelitic, basic and quartz schists. The Iratsu mass consists of metabasites of gabbroic and basaltic origin, with intercalations of ultramafic rocks, felsic gneiss, quartz schist and metacarbonate. The Iratsu mass can be divided into two layers along a WNW-trending metacarbonate layer. The Higashi-Akaishi mass consists of peridotite with intercalations of garnet clinopyroxenite. It is situated beneath the western half of the Iratsu mass, and their mutual boundary dips gently or steeply to the N or NE. These masses underwent eclogite, and subsequent epidote-amphibolite facies metamorphism as has been reported elsewhere. The Iratsu–Higashi-Akaishi masses and the surrounding rocks underwent ductile deformation under epidote-amphibolite facies (or lower P– T) metamorphic conditions. Their foliation generally trends WNW and dips moderately to the NNE, and the mineral lineation mostly plunges to the N and NE. In non-eclogitic schists surrounding the Iratsu–Higashi-Akaishi masses, the foliation generally trends WNW and dips gently or steeply to the N or S and the mineral lineation mostly plunges to the NW, N and NE. Kinematic analysis of deformation structures in outcrops and oriented samples has been performed to determine shear senses. Consistent top-to-the-north, normal fault displacements are observed in peridotite layers of the Higashi-Akaishi mass and eclogite-bearing epidote amphibolite layers of the Iratsu mass. Top-to-the-northeast or top-to-the-northwest displacements also occur in non-eclogitic pelitic–quartz schists on the northern side of the Iratsu mass. In the structural bottom of the Iratsu–Higashi-Akaishi masses and to the south, reverse fault (top-to-the-south) movements are recognized in serpentinized peridotite and non-eclogitic schists. These observations provide the following constraints on the kinematics of the rock masses: (1) northward normal displacement of Iratsu relative to Higashi-Akaishi, (2) northward normal displacement of non-eclogitic schists on the north of the Iratsu mass and (3) southward thrusting of the Iratsu–Higashi-Akaishi masses upon non-eclogitic schists in the south. The exhumation process of the Iratsu–Higashi-Akaishi masses can be explained by their southward extrusion.

Metamorphic evolution of kyanite–staurolite‐bearing epidote–amphibolite from the Early Palaeozoic Oeyama belt, SW Japan

AbstractEarly Palaeozoic kyanite–staurolite‐bearing epidote–amphibolites including foliated epidote–amphibolite (FEA), and nonfoliated leucocratic or melanocratic metagabbros (LMG, MMG), occur in the Fuko Pass metacumulate unit (FPM) of the Oeyama belt, SW Japan. Microtextural relationships and mineral chemistry define three metamorphic stages: relict granulite facies metamorphism (M1), high‐P (HP) epidote–amphibolite facies metamorphism (M2), and retrogression (M3). M1 is preserved as relict Al‐rich diopside (up to 8.5 wt.% Al2O3) and pseudomorphs after spinel and plagioclase in the MMG, suggesting a medium‐P granulite facies condition (0.8–1.3 GPa at &gt; 850 °C). An unusually low‐variance M2 assemblage, Hbl + Czo + Ky ± St + Pg + Rt ± Ab ± Crn, occurs in the matrix of all rock types. The presence of relict plagioclase inclusions in M2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite‐bearing M2 assemblage during cooling. The corundum‐bearing phase equilibria constrain a qualitative metamorphic P–T condition of 1.1–1.9 GPa at 550–800 °C for M2. The M2 minerals were locally replaced by M3 margarite, paragonite, plagioclase and/or chlorite. The breakdown of M2 kyanite to produce the M3 assemblage at &lt; 0.5 GPa and 450–500 °C suggests a greenschist facies overprint during decompression. The P–T evolution of the FPM may represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a Pacific‐type orogen.

Late Caledonian Ductile Thrusting Deformation in the Central East Kunlun Belt, Qinghai, China and Its Significance: Evidence from Geochronology

Abstract A high‐angle ductile thrusting deformation with top‐to‐the‐north movement penetratively developed in the Proterozoic‐Early Paleozoic metamorphic rocks along the Central East Kunlun belt. The deformed rocks suffered epidote‐amphibolite facies metamorphism. On the basis of our previous study, we present more data in this paper to further support that the ductile thrust deformation occurred in the later Caledonian and more detailed information about the deformation. A zircon U‐Pb concordant age of 446±2.2 Ma of a deformed granodiorite in the ductile thrust zone was obtained and can be interpreted as the lower limit of the deformation. A syntectonically crystallized and also strongly deformed hornblende Ar/Ar dating gives an Ar/Ar plateau age of 426.5±3.8 Ma, which represents the deformation age. A strongly orientated muscovite gives an Ar/Ar plateau age of 408±1.6 Ma, representing the cooling age after the peak temperature, constraining the upper limit of the ductile thrust deformation. This ductile thrust deformation can be interpreted as the result of the closing of the Central East Kunlun archipelago ocean. To the north, Ar/Ar plateau ages of 382.9±0.2 Ma and 386.8±0.8 Ma of muscovite in the deformed Xiaomiao Group represent the uplift cooling ages of deeper rocks after the thrusting movement. The original thrusting foliation has a low angle. A rotation model was put forward to explain the development of the foliation from the original low‐angle to present high‐angle dipping.

Timing of arc construction and metamorphism in the Slate Creek Complex, northern Sierra Nevada, California

Late Paleozoic to early Mesozoic rocks in the Sierra Nevada and Klamath Mountains, western United States, preserve a record of lateral growth of continental crust by incorporation of ophiolites, volcanic arcs, and associated sedimentary rocks. Deciphering the timing of arc construction and metamorphism is critical for elucidating tectonic and thermal evolution during this type of crustal growth, but is difficult in complex accretionary terranes. In this study, we address the timing of arc construction and regional metamorphism in the Slate Creek Complex, a volcano-plutonic terrane in the central part of the northern Sierra Nevada. New 40 Ar- 39 Ar ages from relict volcanic hornblende demonstrate that the youngest volcanic unit in the Slate Creek Complex is ca. 170 Ma, at least 30 m.y. younger than previous estimates of ca. 200 Ma for metaplutonic rocks in the lower part of the complex. Consequently, the Slate Creek Complex is polygenetic, with two distinct episodes of arc magmatism. The distribution of mineral assemblages and textures indicates that greenschist and epidote-amphibolite facies metamorphism is younger than 170 Ma and is associated with crosscutting plutons that yield cooling ages of 150 Ma and younger. Amphiboles from foliated rocks in the Slate Creek Complex yield plateau ages of 156.1 ± 0.6 and 152.0 ± 0.7 Ma that date cooling subsequent to or dynamic recrystallization during the metamorphic event. Age and lithologic similarities suggest that the lower parts of the Slate Creek Complex correlate with plutons (ca. 200 Ma) and the volcanic cover sequence of the Rattlesnake Creek terrane in the Klamath Mountains. The uppermost volcanic unit correlates with the ca. 170 Ma Western Hayfork terrane that structurally overlies the Rattlesnake Creek terrane. Metamorphism in the Slate Creek Complex correlates broadly with a similar late metamorphic event in the Klamath Mountains.

The Rı́o Lácteo Formation of Southern Chile. Late Paleozoic orogeny in the Andes of southernmost South America

The late Paleozoic Rı́o Lácteo Formation forms the metamorphic basement of the Mesozoic and Cenozoic Andean orogenic belt in the Lago General Carrera area (about 45°S) of southern Chile. The rocks were emplaced during a late Paleozoic orogeny on the active continental margin of southernmost South America. They form part of a SW to NE striking fold belt with dominant overthrusting towards the SE. The metamorphic grade and structural style are indicative of deformation in the crystalline core of an orogenic belt, probably resulting from microplate interactions, rather than fore-arc accretion. The Rı́o Lácteo Formation represents metamorphorphosed marine turbidites, limestones and pyroclastic rocks which accumulated in an intraplate setting. The strata were intensely deformed and subjected to medium grade, greenschist to epidote-amphibolite facies metamorphism in pre-late Carboniferous times. Four phases of deformation have been identified. The first phase produced a strong planar fabric. This was followed by subhorizontal SW to NE stretching and NW to SE compressive shortening. The third phase formed a spaced cleavage related to E-W extension. The fourth episode produced broad shear zones, characterised by chevron folding and intrusive dykes of crush breccia, related to thrusting towards the southeast.

Genesis and mobility of the H 2 O-CO 2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism: a petrological and stable isotope study in the Scottish Dalradian

During high-pressure, low-temperature greenschist and epidote-amphibolite facies metamorphism in Dalradian rocks of the SW Scottish Highlands, mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids; consequently, mineral assemblages capable of buffering the fluid phase composition were rare. Equilibrium prograde H 2 O-CO 2 fluids usually contained less or much less than about 1–2 mol% CO 2 . Three fluid infiltration events are recognized. During prograde greenschist facies metamorphism, metabasic sills were infiltrated by large volumes of CO 2 -bearing hydrous fluid; carbon isotope studies indicate that the CO 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments. This may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f O 2 , initiated by thermal expansion of water during heating, decompression and consequent hydraulic fracturing. In the epidote-amphibole facies (garnet zone), dehydration reactions in metabasites generated large quantities of water, which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular, diopside, K-feldspar, amphibole, clinozoisite and sphene. A late retrograde infiltration of CO 2 ,-bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing K-feldspar + chlorite + rutile ± dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation. Large-scale infiltration of fluid into greenschist-facies metadolerite sills was intimately related to, and possibly controlled by, penetrative deformation and, in the absence of a penetrative deformation, grain-boundary diffusion by itself was an ineffectual mechanism of fluid transport.

Geochronology of Archaean gneisses and tonalites from north of the Frederikshåbs isblink, S.W. Greenland

The main rock types in the area north of the Frederikshåbs isblink are streaky gneisses, massive tonalites and ‘supracrustals’. The gneisses are thought to be the parent rocks of the tonalite and can be seen to merge into tonalite across a narrow zone of nebulite. Rb-Sr whole rock points from samples of gneiss and tonalite fall on a common isochron with an age of 2662 ± 116 m.y. (2σ) and initial ratio of 0.7032 ± 0.0008 (2σ) (half-life of 87Rb = 50 b.y.). The uncertainties in the isochron could mask small age and initial ratio differences between the gneiss and tonalite. However, our present interpretation is that the isochron reflects a homogenization of Sr isotopes within and between the two rock types. The presence of two out of four K-feldspar points on the whole rock isochron is interpreted as evidence that the K-feldspar became closed to Sr isotope migration at the same time as the whole rocks. Subsequent local isotopic disturbance has resulted in a minor loss of radiogenic strontium from two of the samples. The interpretation of the K-feldspar as a product of the epidoteamphibolite facies metamorphism allows the conclusion that the whole rock-K-feldspar isochron is recording a Sr isotopic homogenization during this event and is not related to the formation of the gneiss or the tonalite. Rb-Sr closure ages of ca. 2515 m.y. for muscovite and ca. 1950 m.y. for biotite could be recording separate isotopic disturbances or the cessation of strontium isotope migration as the minerals cooled through their characteristic blocking temperatures. Zircons from both the gneiss and the tonalite have igneous morphological features. Their U-Pb systems are complex, however, and suggest a multistage history of isotopic disturbance. Whereas the zircon U-Pb and whole rock Rb-Sr results suggest a maximum age of approximately 3000 m.y. for the parent rocks of the gneiss and tonalite they do not entirely exclude the possibility that the rocks represent older crust in which the isotopic systems have been almost completely reset ca. 2700 m.y. ago.