M2 Metamorphism Research Articles

Blue amphiboles and their significance for the metamorphic history of the Pan‐African Gariep belt, Namibia

ABSTRACTThe metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan‐African Gariep orogenic belt in south‐western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro‐) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio‐riebeckite with little ferro‐glaucophane component in a highly metasomatized albite‐rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low‐P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio‐riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.

Variscan very low‐grade metamorphism in southwest England: a diastathermal and thrust‐related origin

Abstract Regional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low‐grade metamorphism whilst in the southern region the very low‐grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High‐Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late‐stage structural up‐doming of the higher grade areas over the roof of the batholith.

Structural and thermal evolution of the Karakoram crust

Prior to the Eocene (c. 50 Ma) collision of the Indian and Asian plates, the southern margin of Asia along the Karakoram plate was an Andean-type margin dominated by tonalitic-granodioritic magmatism of Jurassic––Lower Cretaceous age (Hushe gneiss, Muztagh Tower gneiss and K2 ortho-gneiss) and associated low pressure andalusite, staurolite and garnet grade metamorphism (M1). Following India-Asia collision, crustal shortening, thickening and regional Barrovian metamorphism (M2) occurred between 50–37 Ma. Thermobarometry of kyanite-grade metapelites indicate burial to depths of around 30–35 km. Simultaneous solution of the garnet-biotite geothermometer with the garnet-muscovite-biotite-plagioclase and garnet-AI 2 , SiO 5 ,-quartz-plagioclase geobarometers indicates peak M2 P - T conditions of 696±20°C at 8.6±0.7kbar (860 MPa). Temperatures may have exceeded 700 °C in sillimanite-grade metapelites to produce in situ partial melting and leucogranitic melt pods. Peak M2 metamorphism occurred prior to 37 ±0.8 Ma, the crystallization age of the Mango Gusar two-mica granite pluton which cross-cuts syn-metamorphic deformation fabrics. Post-M2 thermal relaxation followed from 37–25 Ma, after which localized high heat concentrations at the base of the thickened crust caused widespread crustal melting and intrusion of the Baltoro granite batholith at 25–21 Ma. A high temperature-low pressure thermal aureole (M3) along the northern contact is synchronous with the 21 ±0.5 Ma zircon age of the Baltoro granite. Andalusite hornfels along the northern contact of the batholith (Mitre thermal aureole) indicates maximum pressures of 3.75 kbar (375 MPa). A 75 °C increase of temperature in kyanite-sillimanite grade gneisses approaching the southern granite contact of the Baltoro granite is interpreted as the thermal upwarping of pre-37 Ma Barrovian metamorphic M2 isograds around the 21 Ma contact aureole M3 isotherms.

Isotopic and petrological evidence for the infiltration of water-rich fluids during the Miocene M2 metamorphism on Naxos, Greece

A detailed carbon and oxygen isotopic study, in conjunction with petrological work, has been carried out across calcite-dolomite marble layers in the high-grade metamorphic sequence on the island of Naxos, Greece. The purpose of this study is to examine fluid flow during the Miocene Barrovian metamorphic event. Individual marble bands show two dominant styles of isotopic alteration from core values of 22 to 29‰ in δ 18 O and 1 to 3‰ in δ 13 C. Firstly, contacts between the marble bands and surrounding pelitic rocks have altered isotopic ratios across a metre wide boundary layer. The isotopic values of the marble along the contacts drop to 15 to 17‰ in δ 18 O and 1 to −5‰ in δ 13 C. Secondly, there is a drop in the isotopic composition of the marbles along vein networks associated with the development of calc-silicates. The isotopic compositions in these veins drop to 14 to 16‰ in δ 18 O and −3 to −4‰ in δ 13 C. Cross-cutting relationships observed between these two alterations allow the relative timing of infiltration to be determined. The development of boundary layers is shown to result from the infiltration of fluids with isotopic signatures of 12 to 16‰ in δ 18 O and −3 to −12‰ in δ 13 C, and an X CO 2 less than 0.3. These fluids are derived from dehydration of graphite-bearing pelites during the prograde metamorphism. These conclusions are at odds with those of previous workers who suggested that the prograde metamorphism was a result of the pervasive influx of mantle-derived fluids with X CO 2 greater than 0.5. The alteration associated with vein networks results from peak to post-peak infiltration of fluids which drive decarbonation reactions. At least some of these fluids have an X CO 2 less than 0.05, but this is obscured by the production of CO 2 during the decarbonation reactions. These fluids were most probably produced from the crystallising partial melts formed during the prograde metamorphism. These infiltrating fluids are thought to have δ 18O ≤ 10‰ and δ 13 C of −5 to −7‰.

Metamorphic evolution of the Attic Cycladic Metamorphic Belt on Naxos (Cyclades, Greece) utilizing 40Ar/39Ar age spectrum measurements

Abstract 40Ar/39Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high‐pressure, low‐ to medium‐temperature metamorphism, M1, occurred about 50 Ma ago. Phengite has 40Ar* gradients that suggest that part of the scatter observed in conventional K–Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent 40Ar/39Ar plateaus at 15.0 ± 0.1 Ma, 11.8 ± 0.1 Ma, and 11.4 ± 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to M2. Toward lower M2 metamorphic grade, 40Ar/39Ar plateau ages of hornblendes increase to 19.8 ± 0.1 Ma; concomitantly the proportion of excess 40Ar in the spectra increases as well. We propose that the peak of M2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K–Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K–Ar ages from the same samples are in the range 12.1–13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M2 metamorphism.The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900–1500 MPa. Transition to normal crustal thicknesses ended the M1 metamorphism about 50 Ma ago. The M2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic ar°C in a southerly direction through the area.

Thermal evolution of the Greater Himalaya, Garhwal, India

The hanging wall of the Main Central Thrust (MCT) in Garhwal, India (roughly 79°N–80°E; 30°N–31°N), exhibits an inverted metamorphic gradient: sillimanite ± potassium feldspar assemblages near the top of the hanging wall, or Greater Himalayan sequence, are underlain by kyanite grade rocks near the fault. Textural relationships in pelitic samples from the Alaknanda and Dhauli river valleys indicate that the “inversion” is the product of two distinct metamorphic events: an early Harrovian event (M1), which affected the entire Greater, Himalayan sequence and a later Buchan event (M2), the effects of which are most obvious in the upper part of the sequence. Rim thermobarometry, garnet inclusion thermobarometry, and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak M1 conditions of >900 K and >960 MPa (roughly 36 km depth) before following an “erosion controlled” uplift path (e.g., England and Richardson, 1977). M2 metamorphic temperatures in the upper part of the sequence also exceeded 900 K, but maximum pressures (317–523 MPa) indicate paleodepths of only 12–19 km. Calculated pressure‐temperature paths indicate that M2 was characterized by temperature increases of >80 K and roughly 5 km of tectonic burial We attribute M1 to tectonic burial of the Greater Himalayan sequence during the early stages of India‐Eurasia collision. We believe that the uplift and cooling path of the sequence was interrupted in late Oligocene(?) ‐ Miocene time by a second burial and heating event (M2) related to thrust imbrications in southern Tibet. This burial was coincident with the generation of leucogranites, which are abundant near the top of the Greater Himalayan sequence but are virtually absent near the MCT. Field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the Greater Himalayan sequence and were injected at their present structural level, or were melted in situ. If the granites were injected, then they may have provided some of the heat necessary for M2 metamorphism. Although our data suggest no direct relationship between the Main Central Thrust (as mapped in Garhwal) and metamorphism in the Greater Himalaya, anatectic melting of an unexposed portion of the Greater Himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped MCT in central Nepal (cf. Le Fort, 1981). If the granites were produced by in situ M2 melting, then we must appeal to a heat source within the upper part of the Greater Himalayan sequence such as locally high concentrations of heat‐producing elements (cf. Pinet and Jaupart, 1987).

Isotopic dating of the mylonitization of the Azuaga Group in the Badajóz-Córdoba belt, SW Spain

On the basis of Rb-Sr muscovite dating an age of about 355 Ma (Late Tournaisian/Early Visean) is assigned to the M1 metamorphism and associated mylonitization of the ultramylonites of the Las Grullas Formation in the Azuaga Group, Badajoz-Cordoba belt. This age may possibly be related to the accretion process (suturing of the Ossa-Morena Zone to the Central Iberian Zone) thought to be responsible for the metamorphism and mylonitization. Muscovite from the Valencia de Las Torres Formation displays a somewhat lower Rb-Sr age of about 337 Ma. The corresponding K-Ar ages indicate that after the termination of the metamorphism it took some 20 Ma before the rocks had cooled below the closure temperature of muscovite to K-Ar. Whole-rock Rb-Sr data indicate that the Las Grullas ultramylonites were probably derived from Ordovician granitoids.

The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40 Ar/ 39 Ar mineral dates of the Penmynydd schists

The 'Penmynydd' schists of SE Anglesey include several high P /low T lithologies including: (1) metasedimentary quartz–phengite schist; and (2) mafic blueschist with barroisite/crossite assemblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengite and three amphibole concentrates have been analysed by incremental-release 40 Ar/ 39 Ar dating methods and display discordant age spectra with anomalously young apparent ages recorded in low T increments. Ages increase systematically throughout intermediate T portions of the analyses to define high T plateaux of c . 550–560 Ma (barroisite/crossite-rich concentrates) and c. 580–590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discordant spectrum with intermediate T increments defining ages similar to those recorded by barroisite/crossite and high T increments giving ages similar to those of actinolite. The amphibole and phengite ages are interpreted as dating post-metamorphic cooling following a regional low- P Ml greenschist metamorphism ( c . 580–590 Ma) and a high- P M2 metamorphism ( c . 550–560 Ma). 40 Ar/ 39 Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of c. 550–560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.

40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Overprinting of white micas from high pressure, low to medium temperature (M 1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M 2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M 1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M 2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M 2 metamorphism was caused by a shortlived heat source.

Metamorphism of pelitic rocks in the Snow Peak area, northern Idaho: Sequence of events and regional implications

Effects of two episodes of regional metamorphism are recorded in pelitic rocks in the Snow Peak area of northern Idaho. The first event (M1) produced a strong foliation and large porphyroblastic minerals. Metamorphic grade during M1 increased to the south. The second metamorphic episode (M2) occurred under relatively static conditions. Complete equilibration of metamorphic minerals to M2 conditions occurred, although pseudomorphs of M1 minerals are common. The progressive sequence of M2 mineral assemblages ranges from chlorite-biotite to kyanite grade via a low-variance transition zone in which some samples contain chlorite, biotite, garnet, staurolite, kyanite, muscovite, quartz, ilmenite, and graphite. This second metamorphic episode occurred after thrusting of Prichard over Wallace Formation rocks. M2 metamorphism occurred at moderate pressure but lacks evidence of penetrative deformation. It is suggested that formation of kyanite in the Goat Mountain area to the southwest was contemporaneous with Snow Peak M2, and that M2 was closely associated with deep-seated Cretaceous plutonic activity in the Idaho batholith. During Tertiary plutonic activity and uplift, rocks in the Goat Mountain area remained hot, and kyanite inverted to andalusite. The Snow Peak area cooled more rapidly and was affected only by minor retrograde hydration of staurolite. In the Snow Peak samples, therefore, assemblages produced during the moderate pressure event, M2, are preserved.

Metamorphism of the Thompson nickel belt gneisses: Paint Lake, Manitoba

Gneisses mapped in the Thompson nickel belt are the product of a Hudsonian metamorphic (M3) overprinting of an Archean granulite terrain. Pikwitonei region granulites may be partially preserved in amphibolite facies migmatite gneisses of the Thompson nickel belt. Temperatures attending the earlier Archean (M2) metamorphic event have been estimated to be 910 ± 70 °C. More detailed work on the younger overprinting suggests that the metamorphic conditions were relatively constant during the M3 metamorphism. Estimated temperature and pressure during the Hudsonian metamorphism are 650 ± 50 °C and 4–5 kbar (4–5 × 105 kPa), respectively.Although total pressure may have been constant, calculated values for [Formula: see text] during the retrograde overprinting indicate a variability due to lithologic control. This variability in [Formula: see text] is shown to be a controlling factor in the development of hydrous retrograde mineral assemblages. Metapelitic gneisses are characterized by [Formula: see text] values of 0.05Ptotal, which may explain the absence of muscovite as a stable M3 mineral phase. Enderbitic gneisses exhibit higher values of [Formula: see text] (0.25Ptotal) due to the emplacement of syn-M3 pegmatite dykes producing a more hydrous stable M3 mineral assemblage.

Modulation of optical constants of GaTe single crystals

The authors present the results obtained by wavelength and thermally modulating the reflectivity of the layer compound GaTe up to about 5 eV. The E1 and E3 structures are explained in terms of a two-dimensional M0+M1 metamorphism and an M2 two-dimensional critical point respectively. Furthermore a certain amount of information on the excitonic nature of some observed structure is presented and a clear relationship is found with the optical spectra of GaS and GaSe.

Experiments on the form of bellows nozzles, made at the Smithsonian Institute, June, 1855

The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic–Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Río Chañarito fault.The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc– silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M1 phase to the peak M3, characterized by the reactions: Qtz + Pl + Bt ± Ms → Grt + Bt2 + Pl2 ± Sil ± Kfs, Qtz + Bt + Sil → Crd + Kfs and Qtz + Grt + Sil→ Crd. The M3 assemblage is coeval with the dominant foliation related to a third deformational phase (D3).The QMMC, forming the foot wall, is made up of fine-grained banded quartz – biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M3 peak assemblage is quartz + biotite + plagioclase ± garnet ± sillimanite ± muscovite ± ilmenite ± magnetite ± apatite.The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S1 is nearly parallel to S0. In the EPMIC, D2 is represented by the S2 foliation, related to the F2 folding that overprints S1, with dominant strike NNW – SSE and high angles dip to the E. D3 in the EPMIC have F3 folds with axis oblique to S2; the S3 foliation has striking NW – SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S2 (D2) is a discontinuous cleavage oblique to S1 and transposed by S3 (D3), subparallel to S1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640°C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M3 are 710°C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.The schists from the QMMC suffered the early low grade M1 metamorphism with minimum PT conditions of ca 400°C and 0.35 GPa, comparable to the fine schists (M1) outcropping to the east. The D2 deformation is associated with the prograde M2 metamorphism. The penetrative D3 stage is related to a medium grade metamorphism M3, with peak conditions at ca 590°C and 0.55 GPa.The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.