Intense Metamorphism Research Articles

Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic–Rif arc

Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP–LT) metamorphism (at c . 7–9 kbar and 330–430 °C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east–west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP–LT Temsamane units: (1) an internal origin related to the subduction and the HP–LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic–Rif region.

Lithostratigraphic and petrographic analysis of ICDP drill core LB‐07A, Bosumtwi impact structure, Ghana

Abstract— Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB‐07A in the Bosumtwi impact structure revealed two sequences of impactites below the post‐impact crater sediments and above coherent basement rock. The upper impactites (333.38–415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67–470.55 m depth) consists essentially of monomict impact breccia formed from meta‐graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55–545.08 m depth) is composed of lower greenschist‐facies metapelites (shale, schist and minor phyllite), meta‐graywacke, and minor meta‐sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric‐textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre‐impact granitoid complexes.Impact melt fragments are not as prevalent in LB‐07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within‐crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB‐07A appears to have been derived from the <35 GPa shock zone of the transient crater.

Trace elements in primitive meteorites—VII Antarctic unequilibrated ordinary chondrites

We report RNAA results for Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl and Cd (in increasing order of metamorphic mobility) in 22 Antarctic unequilibrated ordinary chondrites (UOC). This brings to 38 the number of UOC for which data for highly volatile elements are known. For elements of lesser mobility (Co to Se, omitting Cs) overall variability in UOC are low, relative standard deviations (one sigma) being no more than a factor of two. For Ag, Te and Zn, relative standard deviations are 2–4×, while for Cs and the four most volatile elements, the variabilities are 8–110×. Elemental abundances do not vary with chemical type (H, L and LL) nor with UOC subtype (3.0–3.9). Contents of all elements reach levels up to, even exceeding, cosmic and all but Cd and the two alkalis, seem unaffected by post-accretionary processes. Contents of highly volatile elements are consistent with the idea that source regions producing contemporary falls and older Antarctic UOC differed in thermal histories. The presence or absence of carbide magnetite assemblages (CMA) generally accords with high or low Cd contents, respectively. This relationship accords with the prior suggestion that CMA formed by alteration of Fe–Ni metal by C–O–H-containing fluids at temperatures <700 K, generated by thermal metamorphism in parent body interiors. The absence of CMA in most UOC (and OC), may indicate that they were subsequently destroyed as metamorphic intensity increased. The high, often supercosmic, Rb and Cs levels in UOC may result from their high solubility in liquid water signalling their redistribution by C–O–H-containing fluid while in the liquid water field. Because of its uniquely high mobility, Cd could have been enriched by the C–O–H fluids and should have been lost from parent regions during later, higher temperature anhydrous metamorphism at temperatures in the 500–600 °C range.

Evolution of the Snow Area Index of the Subarctic Snowpack in Central Alaska over a Whole Season. Consequences for the Air to Snow Transfer of Pollutants

The detailed physical characteristics of the subarctic snowpack must be known to quantify the exchange of adsorbed pollutants between the atmosphere and the snow cover. For the first time, the combined evolutions of specific surface area (SSA), snow stratigraphy, temperature, and density were monitored throughout winter in central Alaska. We define the snow area index (SAI) as the vertically integrated surface area of snow crystals, and this variable is used to quantify pollutants' adsorption. Intense metamorphism generated by strong temperature gradients formed a thick depth hoar layer with low SSA (90 cm(2) g-1) and density (200 kg m(-3)), resulting in a low SAI. After snowpack buildup in autumn, the winter SAI remained around 1000 m(2)/m(2) of ground, much lower than the SAI of the Arctic snowpack, 2500 m(2) m-(2). With the example of PCBs 28 and 180, we calculate that the subarctic snowpack is a smaller reservoir of adsorbed pollutants than the Arctic snowpack and less efficiently transfers adsorbed pollutants from the atmosphere to ecosystems. The difference is greater for the more volatile PCB 28. With climate change, snowpack structure will be modified, and the snowpack's ability to transfer adsorbed pollutants from the atmosphere to ecosystems may be reduced, especially for the more volatile pollutants.

Gondwanide continental collision and the origin of Patagonia

A review of the post-Cambrian igneous, structural and metamorphic history of Patagonia, largely revealed by a five-year programme of U–Pb zircon dating (32 samples), geochemical and isotope analysis, results in a new Late Palaeozoic collision model as the probable cause of the Gondwanide fold belts of South America and South Africa. In the northeastern part of the North Patagonian Massif, Cambro-Ordovician metasediments with a Gondwana provenance are intruded by Mid Ordovician granites analogous to those of the Famatinian arc of NW Argentina; this area is interpreted as Gondwana continental crust at least from Devonian times, probably underlain by Neoproterozoic crystalline basement affected by both Pampean and Famatinian events, with a Cambrian rifting episode previously identified in the basement of the Sierra de la Ventana. In the Devonian (following collision of the Argentine Precordillera terrane to the north), the site of magmatism jumped to the western and southwestern margins of the North Patagonian Massif, although as yet the tectonics of this magmatic event are poorly constrained. This was followed by Early Carboniferous I-type granites representing a subduction-related magmatic are and Mid Carboniferous S-type granites representing crustal anatexis. The disposition of these rocks implies that the North Patagonian Massif was in the upper plate, with northeasterly subduction beneath Gondwana prior to the collision of a southern landmass represented by the Deseado Massif and its probable extension in southeastern Patagonia. This ‘Deseado terrane’ may have originally rifted off from a similar position during the Cambrian episode. Intense metamorphism and granite emplacement in the upper plate continued into the Early Permian. Known aspects of Late Palaeozoic sedimentation, metamorphism, and deformation in the Sierra de la Ventana and adjacent Cape Fold Belt of South Africa are encompassed within this model. It is also compatible with modern geophysical and palaeomagnetic data that do not support previous hypotheses of southward-directed subduction and collision along the northern limit of Patagonia. Subsequent Permian break-off of the subducted plate, perhaps with delamination of the lower part of the upper plate, allowed access of heat to the overlying Gondwana margin and resulted in voluminous and widespread silicic plutonism and volcanism throughout Permian and into Triassic times. Thus the new model addresses and attempts to explain three long-standing geological enigmas—the origin of the Gondwanide fold belts, the origin of Patagonia, and the cause of widespread Permian silicic magmatism (Choiyoi province) in southern South America. Differing significantly from previous models, it has new implications for the crustal structure, mineral resources, and plant and animal distribution in this part of Gondwana, since the southern landmass would have had an independent evolution throughout the Early Palaeozoic.

Tectonic Evolution of the Dabie-Sulu UHP and HP Metamorphic Belts, East-Central China: Structural Record in UHP Rocks

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.

Alkylation of polycyclic aromatic hydrocarbons in carbonaceous chondrites

Using microprobe laser-desorption, laser-ionization mass spectrometry (μL 2MS), we measured the distributions of alkylated and unalkylated polycyclic aromatic hydrocarbons (PAHs) in the free organic material of 20 carbonaceous chondrites. These meteorites represent a variety of meteorite classes and alteration histories, including CI, CK, CM, CO, CR, CV, and Tagish Lake. This work provides information on free organic compounds that is complementary to studies of the structure and composition of meteoritic macromolecular content. For the nine CM2 meteorites analyzed, we observe that higher relative abundances of alkylated PAHs correlate with more intense aqueous activity. We attribute this correlation to the differences in solubility and volatility between unalkylated and alkylated PAHs. Naphthalene and its alkylation series are more susceptible to the effects of aqueous exposure than the less-soluble PAH phenanthrene and its alkylated derivatives. These observations are consistent with the possibility of chromatographic separations on the meteorite parent bodies. We identify six CM2 meteorites with similar PAH distributions that may represent the original, unaltered organic composition of the parent body. Increased metamorphic intensity reduces the abundance of all PAHs. The thermally metamorphosed CK chondrites had no detectable levels of typical meteoritic PAHs. This observation might be explained either by a loss of PAHs caused by volatilization or by a significantly different organic content of the CK parent body.

Petrology and structure of the Palaeoproterozoic (1.9�Ga) Rytky nickel sulphide deposit, Central Finland: a comparison with the Kotalahti nickel deposit

The Palaeoproterozoic (1.9 Ga) Rytky and Kotalahti mafic-ultramafic intrusions are located in the contact zone between the Archaean craton and Proterozoic supracrustal rocks. During the second deformation event (D2) the surrounding country rocks were subjected to intensive metamorphism and deformation associated with the Svecofennian orogeny; the Archaean/Proterozoic boundary controlled both D2 thrusting and magma ascent. Emplacement of the Rytky and Kotalahti intrusions took place at the culmination of D2, as shown by the gneiss inclusions with S2 schistosity within the intrusions. Overthrusting continued after emplacement, with detached fragments of the bodies incorporated into the Archaean gneisses. During the third deformation event (D3) the originally subhorizontal intrusions were rotated into a subvertical position, so that they now have their stratigraphic top towards the west. The Rytky intrusion is composed mainly of medium- and coarse-grained lherzolite, websterite and gabbronorite. The nickel deposit with pentlandite as the main nickel mineral is associated with the lherzolite and websterite. The coarse-grained lherzolite, websterite and melagabbro represent the first rocks to form, and they contain the nickel sulphide mineralisation. Country rock contamination, as indicated by high TiO2, P2O5, Rb, Zr and light rare earth element contents (LREE), is most pronounced in the marginal part of the intrusion, which was the first to form. The variation in olivine composition (Fo 78.6-84.77 mole %; Ni 630–2386 ppm) and the metal ratio of the sulphide (Ni/Co 19.3 – 50.3) along with the internal stratigraphy of the intrusion indicate an in-situ process of sulphide ore formation.

A Tiny Piece of Basalt Probably from Asteroid 4 Vesta

Abstract Grove Mountains (GRV) 99018 is a new eucrite (0.23 g), consisting mainly of pyroxene (50.5 vol%) and plagioclase (37.2 vol%) with minor silica minerals (7.0 vol%) and opaque minerals (5.2 vol%). It was intensely shocked, leading to partial melting, formation of abundant tiny inclusions in pyroxenes and plagioclase, and heavy brecciation. Exsolution of most pyroxenes (1–3 μm in width of the lamellae), recrystallization of the shock‐induced melt pockets and veins (5–20 μm in size), and homogeneous compositions of pyroxenes of various occurrences suggest the intense thermal metamorphism of GRV 99018 in the asteroidal body Vesta. This new eucrite will bring additional constraints on the chemical composition and multi‐stage thermal and shock history of Vesta.

Reply to the comment: Mantle plume-, but not arc-related Neoproterozoic magmatism in South China

The Micangshan terrane is located along the NW margin of the Yangtze block and contains the second oldest Precambrian basement in South China. It comprises the Beiba and the Wangcang-Nanjiang segments, while the oldest suites (∼2.1 Ga Houhe complex) occur in the Beiba. The Hekou and the Wangjiaping metamorphic volcano-sedimentary sequences in the Wangcang are characterized by intensive metamorphism and deformation, long being regarded as the equivalent of the Houhe suite. This contribution presents an integrated study of zircon U-Pb geochronology and Hf isotope, whole rock elemental and Sr-Nd isotope geochemistry on volcanic rocks of the metastrata. It reveals that they formed at ∼ 865–860 Ma and underwent upper amphibolite facies metamorphism at ∼ 815 Ma with lower limits in P-T conditions of ∼ 5.0 kbar and ∼ 650–660 °C. The Hekou metavolcanics comprise dacitic-rhyolitic and alkaline basaltic rocks with minor tholeiitic mafics, whereas those of the Wangjiaping are composed of dacitic and tholeiitic mafic rocks. The Hekou alkaline metamafics show affinity to high-Nb to Nb-enriched basalts (HNB-NEB) with εNd(t) values of + 5.80 to + 6.25, whereas tholeiitic metamafics of the both sequences display characteristics typifying high-Al basalt (HAB) with εNd(t) value ∼+2.66. The Hekou and the Wangjiaping metafelsics have εNd(t) ranges of −2.33 to + 5.60 and −0.82 to + 0.99 (except one + 2.29), respectively, and the dacitic rocks show features akin to the high Mg-number andesites. The ∼ 865–860 Ma magmatism is suggested to have occurred in an extensional basin with a post-subduction affinity. It was induced by upwelling of convective asthenosphere succeed the ceased slab subduction: the Wangjiaping metafelsics were derived from partial melting of juvenile lower crust, the Hekou dacitic rocks were produced by partial melts of the relict of subducted oceanic crust interacted with mantle wedge; the Hekou HNB-BEB were sourced by metasomatized mantle wedge, whereas the HAB lavas were generated by anhydrous partial melts of the upwelling asthenosphere. The ∼ 815 Ma metamorphism was caused by an amalgamation between the South Qinling belt and the proto margin of the northwestern Yangtze block. This event was followed by an expanding rift from the Micangshan successively to the South Qinling.

Ordinary chondrite metallography: Part 2. Formation of zoned and unzoned metal particles in relatively unshocked H, L, and LL chondrites

Abstract— We studied the metallography of Fe‐Ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of P‐free, Fe‐Ni alloy cooling experiments (described in Reisener and Goldstein 2003). Two types of Fe‐Ni metal particles were observed in the chondrites: zoned taenite + kamacite particles and zoneless plessite particles, which lack systematic Ni zoning and consist of tetrataenite in a kamacite matrix. Both types of metal particles formed during metamorphism in a parent body from homogeneous, P‐poor taenite grains. The phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles. Polycrystalline taenite particles transformed to zoned taenite + kamacite particles by kamacite nucleation at taenite/taenite grain boundaries during cooling. Monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process. The varying proportions of zoned taenite + kamacite particles and zoneless plessite particles in types 4–6 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism. Type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles. Type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles. The distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of Fe‐Ni alloy phase transformations during cooling. The distribution cannot be explained by hot accretion‐autometamorphism, post‐metamorphic brecciation, or shock processing.

Integrated tectonostratigraphic analysis of the Himalaya and implications for its tectonic reconstruction

The isotope geochronology of isochronously deposited Cambrian strata from different tectonostratigraphic zones of the Himalaya confirms new stratigraphic, sedimentological, and faunal evidence indicating that the Himalaya was a single continental margin prior to collision of India with Asia. Lesser, Greater, and Tethyan Himalaya represent proximal to distal parts of a passive continental margin that has been subsequently deformed during Cenozoic collision of India with Asia. Detrital zircon and neodymium isotopic data presented herein discount the prevailing myth that the Lesser Himalaya has a unique geochronologic and geochemical signature that is broadly applicable to modeling the uplift history of the Himalaya. The conclusion that all pre-Permian Lesser Himalaya strata lack young detrital zircons that are present in the Greater and Tethyan Himalaya underpins previous arguments that the Main Central Thrust forms a fundamental crustal boundary that separates the Indian craton from an accreted terrane to the north. The supposition that Himalayan lithotectonic zones differ in detrital zircon age populations has also been used to reconstruct the unroofing history of the Himalaya during foreland basin development in the Cenozoic. Our data conflict with the underlying assumptions implicit in these studies in that samples of similar depositional age from both the Lesser and Tethyan Himalaya contain detrital zircons with similar age spectra. Similarities between the Kathmandu Complex and the Tethyan Himalaya support stratigraphic continuity between the former and either age-equivalent Greater Himalayan protolith or the Tethyan. Assuming that the complex rooted along the Main Central Thrust, these strata would simply have escaped intense metamorphism during Cenozoic tectonism. Alternatively, the complex may represent a part of the Tethyan Himalaya that was emplaced during an early stage of movement along a south-directed thrust fault located near the present-day structural position of the South Tibetan Fault System.

Structure, microphysics, and surface area of the Arctic snowpack near Alert during the ALERT 2000 campaign

The seasonal snowpack at Alert (North coast of Ellesmere Island, 82°29.94′N, 62°20.55′W) was studied in February and April 2000, on land and on sea ice. The stratigraphy was studied, and the density and specific surface area (SSA) of each snow layer were measured. SSA was measured by CH 4 adsorption at 77 K using a volumetric method. On land, the snowpack was 10–50 cm thick and consisted of a depth hoar layer covered by one or more hard wind-packed layers with densities between 0.35 and 0.52. These were sometimes separated by soft layers of more or less faceted crystals. The surface was covered by recent precipitation and surface hoar. The stratigraphy on sea ice was more variable, with numerous hard wind-packed layers alternating with soft layers of depth hoar or faceted crystals. SSA values ranged from 125 cm 2/g for depth hoar to 1500 cm 2/g for diamond dust and dendritic snow. The total surface area of the snowpack was calculated from the thickness, density, and SSA of each layer, and ranged from 1160 to 3710 m 2 of snow surface area per m 2 of ground. These values were used to estimate the potential impact of the snowpack on atmospheric chemistry, by adsorption/desorption of trace gases. Using the example of acetone, whose adsorption behavior on ice is estimated, it is found that the snowpack may sequester most of the acetone of the (snow+boundary layer) system most of the year. The release during metamorphism of trace gases dissolved in snow is also discussed. We propose that the frequency and intensity of wind storms will strongly affect the release of trace gases, as this will determine whether intense metamorphism leading to depositional depth hoar can happen.

Gradients in fluid composition across metacarbonate layers of the Wepawaug Schist, Connecticut, USA

Metamorphic index mineral zones, pressure–temperature (P–T) conditions, and CO2–H2O fluid compositions were determined for metacarbonate layers within the Wepawaug Schist, Connecticut, USA. Peak metamorphic conditions were attained in the Acadian orogeny and increase from ~420 °C and ~6.5 kb in the low-grade greenschist facies to ~610 °C and ~9.5 kb in the amphibolite facies. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with progressive increases in metamorphic intensity. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering syn-metamorphic quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. It has been postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. In this model, the infiltrating fluids drove prograde CO2 loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence. The model predicts that significant gradients in the mole fraction of CO2 ( $$X_{CO_2 } $$ ) should have been present during metamorphism, but new estimates of fluid composition indicate that differences in $$X_{CO_2 } $$ preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection–dispersion-reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO2 loss by diffusion and mechanical dispersion. These gradients, although typically too small to be measured by field-based techniques, would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger gradients in fluid composition may have existed across some layers during metamorphism, but large gradients favor rapid reaction and would, therefore, seldom be preserved in the rock record. Most of the H2O needed to drive prograde CO2 loss probably came from regional dehydration of surrounding metapelitic schists, although H2O-rich diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that structurally underlie the Wepawaug Schist.

Emplacement age for the afic-ultramafic plutons in the northern Dabie Mts. (Hubei): Zircon U-Pb, Sm-Nd and 40Ar/39Ar dating

The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9±0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende40Ar/39Ar ages of 116.1±1.1 Ma and 106.6±0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500°C. The hornblende40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15–25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.

Quartz and clinoenstatite exsolutions in clinopyroxene of garnet-pyroxenolite from the North Dabie Mountains, eastern China

The exsolution lamellae of quartz and clinoenstatite are idenfied in diopside of garnet-pyroxenolite from the North Dabie Mountain by transmission electron microscopy, which is interpreted that the lamellae are originally exsolved from a former ultra-high-pressure clinopyroxene due to decreasing of pressure. Study of petrography shows that there is compositional zoning in the diopside itself. It is implied that the garnet-pyroxenolite had undergone intensive high-temperature granulite facies and high-amphibolitic facies retrogressive metamorphism, while the peridotite (the garnet-pyroxenolite’s host rock) emplaced the crust.

Development of high thermal gradients by coeval transpression and magmatism during the Variscan orogeny: insights from the Cap de Creus (Eastern Pyrenees)

The Cap de Creus massif in the Eastern Pyrenees is a fragment of the Variscan mid-crust. Geological mapping and meso- and micro-scale analysis of the relationships between fold structures, metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes: deformation, metamorphism and magmatism. Low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of ∼5km. Zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid. Sillimanite schists and small migmatite pods are distributed around the small intrusions of basic–intermediate composition. The inferred maximum thermal gradient is >80°C/km. This thermal pattern is similar to patterns observed in other Variscan massifs in the Pyrenees, where high metamorphic gradients are interpreted in different ways. In the NE Cap de Creus, the type and distribution of prograde metamorphism was caused, or at least enhanced, by the intrusion of diorites and granitoids. Alternatively, deformation developed at prograde conditions and synchronously with magmatism, displays a highly heterogeneous distribution, producing a sub-vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity. This zone of high strain, developed in a broad transpressive regime, could have favoured the injection of magmatic sheets from deep to mid-crustal levels. Progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones.

Determining the extent and nature of Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake Superior region, north–central USA

Twenty-one hornblende and mica 40Ar/ 39Ar dates from central and northwest Wisconsin, USA, provide important information on the timing, spatial extent, and intensity of Mazatzal-age metamorphism and deformation which overprinted the Paleoproterozoic (1870–1820 Ma) Penokean orogenic belt in the southern Lake Superior region. 1760–1750 Ma mica plateau ages from bedrock beneath undeformed 1750–1630 Ma quartzites are interpreted as the time of lower temperature (∼300–350°C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma (1576–1614 Ma). The complete absence of 1760–1750 Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300–350°C during Mazatzal-related deformation and metamorphism. The ∼1600 Ma mica ages are interpreted to date the cooling phase of this metamorphism. Two anomalously young biotite dates are interpreted to indicate partial resetting associated with Mesoproterozoic rifting at 1100 Ma. Ten hornblende 40Ar/ 39Ar dates obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 2503±18 Ma and two ages of 1853 and 1830 Ma are interpreted as remnant evidence of Archean and Penokean age amphibolite metamorphic events, respectively. The majority of hornblende ages are younger than the Penokean orogeny, scattering between 1796 and 1638 Ma. Microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages. This suggests that shearing and recrystallization did not play an important role in the retention or loss of argon. The 1638 Ma hornblende age is concordant with the Mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid-related resetting associated with that event. Six other post-Penokean ages scatter over a 70 million year interval (1796–1723 Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature (350–500°C) resetting of hornblende argon systematics at ∼1650–1630 Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved 350–500°C metamorphism and penetrative deformation.

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)

Meta‐peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re‐examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre‐Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian‐type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta‐peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730–760 °C were calculated for the upper‐amphibolite facies meta‐peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre‐M2 histories: whereas the cover‐associated horizons have been affected by low‐grade serpentinization prior to metamorphism, the basement‐ associated meta‐peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement‐associated meta‐peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca‐loss in the second group. The cover‐associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement‐associated meta‐peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.

Contrasting Oligocene and Miocene thermal histories from the hanging wall and footwall of the South Tibetan detachment in the central Himalaya from 40Ar/39Ar thermochronology, Marsyandi Valley, central Nepal

In the Marsyandi valley of central Nepal, a major strand of the South Tibetan detachment system, the 18–22 Ma Chame detachment, places epidote‐amphibolite to amphibolite facies calc‐silicate rocks and marbles of the Tibetan sedimentary sequence on amphibolite facies pelitic gneisses and calc‐silicate rocks of the Greater Himalayan sequence. Although the resulting metamorphic discontinuity is minor and sometimes cryptic, 40Ar/39Ar thermochronologic results from the area reveal that the hanging wall and footwall of the detachment had distinctive thermal histories. Hanging wall phlogopites and biotites yield cooling ages of 27.1 – 29.9 Ma, compared with footwall biotite ages of 14.1 – 16.6 Ma. U‐Pb monazite thermochronology demonstrates that the Greater Himalayan sequence experienced peak amphibolite facies conditions at approximately 22 Ma, but the 40Ar/39Ar results require hanging wall metamorphism to be an Oligocene (or older) phenomenon. These events are interpreted as representing the “Neohimalayan” and “Eohimalayan” metamorphic phases proposed by previous workers in the central Himalaya. Some of the dated hanging wall phlogopites grew synchronously with development of SW vergent macroscopic folds in the Tibetan sedimentary sequence, implying that Eohimalayan metamorphism was associated with an important phase of crustal shortening in this sector of the Himalaya. Despite the intensity of Neohimalayan metamorphism below the Chame detachment, evidence for Eohimalayan metamorphism and igneous activity is preserved in the footwall rocks of the Marsyandi drainage. Inherited approximately 35 Ma monazites of either metamorphic or igneous origin have been found in the upper Greater Himalayan sequence in this area, and one hornblende separate from the uppermost footwall yields a 40Ar/39Ar age of 30.0 ± 3.0 Ma. This hornblende date and a similar result from the same structural level in the nearby Kali Gandaki valley, if robust, suggest that the duration of the Neohimalayan event was extremely short, probably less than 1 million years and that peak temperatures in the upper part of the Greater Himalayan sequence were not substantially greater than about 900 K.