Biotite Zone Research Articles

Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (KD≈2.5) and into chlorite relative to phengite (KD≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl3Si3O10(OH)2, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO2, CaAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low‐grade assemblages.

Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.

MINERAL EQUILIBRIA IN QUARTZ LEUCOAMPHIBOLITES (QUARTZ - GARNET - PLAGIOCLASE - HORNBLENDE CALC-SILICATES) FROM SOUTHEASTERN BRITISH COLUMBIA, CANADA

Pods and lenses of leucoamphibolite a few meters in maximum dimension contain quartz (Qtz) ‐ plagioclase (Pl) ‐ hornblende (Hbl) ‐ garnet (Grt) ‐ epidote (Ep), with or without biotite (Bt). They occur within Late Precambrian clastic metasedimentary rocks of the Esplanade Range of southeastern British Columbia, and range in metamorphic grade from upper biotite zone to kyanite (Ky) zone (defined in pelitic rocks). Mineral equilibria using the database of Berman provide estimates of metamorphic pressure (P), temperature (T), and fluid compositions. These estimates can be compared to those from coexisting pelitic rocks to provide a check on consistency. Grt‐Bt Mg‐Fe exchange geothermometry suggests temperatures ranging from 530 to 580°C at 7 kbar. The lack of kyanite in leucoamphibolites suggests a maximum P of about 7‐8 kbar. Pl‐Qtz‐Ep‐Grt assemblages without calcite suggest H2O-rich fluid compositions [generally X(H2O) > 0.8]. For a(H2O) = 1.0, Grt‐Ep‐Pl‐Qtz‐ H2O equilibria produce calculated temperatures near 565 °C at 7 kbar. Although there is considerable overlap, estimates of P and T from the leucoamphibolites do not agree very well with estimates from nearby pelites. On the basis of natural occurrences, ot her investigators have suggested a miscibility gap and near-constant activity of anorthite over the range An 40‐90. Electron-microprobe spot analyses in our study indicate that plagioclase compositions near ~An 40 and ~An90 occur within single grains and within single samples. Other samples have compositions concentrated near either An 40 or An90 or compositions with An≤40. These results are consistent with petrological studies reported by other researchers.

K–Ar age–chemistry–fabric relations of phengite from the Sanbagawa high‐pressure schists, Japan

AbstractThe Sanbagawa high‐pressure schists from central Shikoku in Southwest Japan have experienced high‐strain ductile deformation during exhumation and cooling. This study examines the effects of high‐strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine‐grained phengite crystals and are extremely fine‐grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation‐induced grain‐size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain‐size reduction of phengite is due to strain‐induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin‐section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite‐rich quartzose schist with strong grain‐size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain‐size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age.

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.

Metamorphism of the East Sector of the Southern Qinling Orogenic Belt and Its Geological Significance

Abstract The east sector of the southern Qinling belt is, lithologically, composed mainly of metapelites, ***qüartzites, marbles and small amount of metabasites and gneisses, whose protoliths are the Silurian, Devonian and less commonly the Sinian and Upper Palaeozoic. They have been subjected at least to two epochs of metamorphism. The early epoch belongs to progressive metamorphism which is centered on high amphibolite‐granulite fades in the Fuping area and changed outwards into low amphibolite facies (staurolite‐kyanite zone), epidote amphibolite facies (garnet zone) and greenschist facies (chlorite and biotite zones), the metamorphic age of which is about 220–260 Ma. This early‐epoch metamorphism belongs to different pressure types: the rocks from greenschist to low amphibolite facies belong to the typical medium‐pressure type which shows geothermal gradients of about 17–20 ***C/km and was probably produced by a crustal thickening process related to continental collision, and the high amphibolite‐granulite facies belongs to the low‐pressure type which shows geothermal gradients of about 25–38 ***C/km and was probably affected by some magmatic heats. Based on the basic characteristics of the P‐T paths of the different facies calculated from the garnet zonations, it can be deduced that the metamorphism of medium‐pressure facies series took place during an imbricated thickening process, rather than during the uplifting process after thickening. The late‐epoch metamorphism belongs to dynamic metamorphism of greenschist facies which is overprinted on the early‐epoch metamorphic rocks and is Yanshanian or Himalayan in age, probably related to intracontinental orogeny.

Oxygen isotope geochemistry of the Omeo Metamorphic Complex, Victoria: Implications for metamorphic fluid flow, mineralisation and anatexis

Metamorphosed turbidites from the Omeo Metamorphic Complex show only minor changes in δ18O values with increasing metamorphic grade from 13.4 ± 1.7% in the chlorite and biotite zones to 12.3 ± 1.0% in the sillimanite + K‐feldspar zone. Rocks within 5 km of the S‐type granite at Hume Dam have δ18O values of 6.8–8.1% that probably reflect interaction with heated meteoric‐igneous fluids. Interaction with igneous fluids has also occurred close to other I‐ and S‐type granites in this region. However, pervasive metamorphic fluid‐rock interaction in this terrain did not occur, which limits the region's potential for hydrothermal mineralisation. Anatexis at high grades was probably via dehydration‐melting reactions that consumed muscovite and biotite, which is consistent with there being little fluid present during metamorphism. Small (kilometre scale or less) S‐type granites in the sillimanite + K‐feldspar zone have δ18O values similar to those of the surrounding metasediments and probably formed by melting of those rocks. By contrast, larger (tens of kilometres scale) Ca‐rich, peraluminous, S‐type granites have lower δ18O values than the surrounding metasediments, and may represent melts of underlying middle to lower crust.

Geology, structure and metamorphism of the Lesser Himalayan metasedimentary sequence in Pokhara region, western Nepal

The Lesser Himalaya in the Pokhara region, western Nepal, comprises low grade metamorphic rocks of the Nawakot Complex in the south and mylonitic rocks of the Main Central Thrust Zone in the north. It is delimited in the north by the Main Central Thrust (MCT), which brings the kyanite gneisses of the Higher Himalayan Crystallines over the Lesser Himalayan metasediments. Cross-cutting pegmatite veins injected prior and posterior to the main deformation event are reported for the first time from the MCT Zone. The Phalebas Thrust, Lumle Thrust and the MCT are the major structural discordances, which divide the area into various tectonic zones with different style and intensity of deformation. At least three deformation events are recognised in each tectonic zone. Sense of shearing is always top to the south in the MCT Zone.
 The Higher Himalayan Crystallines and the Lesser Himalayan metasediments show marked differences both in history and grade of metamorphism. The gneisses of the Higher Himalayan Crystallines exhibit polyphase metamorphism. The older high grade (kyanite-grade) metamorphism formed pyrope-rich cores in garnets and plagioclase with An>20%. The younger retrograde metamorphism caused to recrystallise phengitic muscovite and to form retrograde chemical zoning in garnets. The Lesser Himalaya shows syn- to post-tectonic, garnet-grade, inverted prograde metarnrophism. The grade of metamorphism increases from chlorite zone in the south to the biotite zone below the Lumle Thrust and the garnet zone in the MCT Zone. The garnets show prograde zoning pattern, muscovite gradually becomes poor in celadonite component, biotite becomes rich in Ti-content, and Ca-amphiboles become rich in Na and K from south towards the MCT. Both the Lesser Himalayan metasediments and the Higher Himalayan Crystallines have experienced the late-stage weak regression due to uplift and erosion.

Rb-Sr, Sm-Nd and K-Ar systematics of metamorphosed pillowed basalts and associated Besshi-type deposits in the Sanbagawa Belt, Japan

Samples from metamorphosed pillowed basalts and related Besshi-type deposits occurring in the Sanbagawa belt of the Shikoku Island, southwest Japan, have been analyzed for 87Sr/86Sr, 143Nd/144Nd and 40 K/40Ar. This is to investigate the tectonic settings in which the original submarine volcanism and associated Besshi-type mineralization occurred, as well as the age of metamorphism. Eight whole-rock samples of the pillow lavas metamorphosed in pumpellyite-actinolite facies conditions yield a Rb-Sr isochron age of 107 ± 15 Ma with an initial ratio of 0.70401 ± 0.00006, while they do not define a Sm-Nd isochron. We interpret the results as the metamorphic age, an interpretation consistent with the previously reported Rb-Sr whole-rock age for the Sanbagawa pelitic schists. The overall ranges of the initial epsilon values at T = 107 Ma are: ɛNd (T ) = +7.8 to +4.3; ɛSr(T ) = +2.2 to −7.0, suggesting that the most likely source for the pillowed basalts is depleted oceanic mantle, a conclusion supported by the previous Pb isotope studies. The K-Ar ages determined for twelve mineral separates from the Besshi-type deposits range from about 60 to 112 Ma, with a mean age of about 80 Ma, in agreement with the previous K-Ar and Ar-Ar data for the Sanbagawa pelitic and basic schists. The youngest age, 60 Ma, was obtained for sericite from the Hinooku deposit metamorphosed in pumpellyite- actinolite facies conditions, while the oldest one for hornblende from the spotted amphibolite in the immediate vicinity of the Shiragayama deposit metamorphosed in albite-biotite grade. The oldest age, 112 Ma, is interpreted to date the peak metamorphism, consistent with the Rb-Sr data, though a possibility of excess Ar cannot always be ruled out. In view of the closure temperatures of muscovite (350 °C) in the biotite zone, it is suggested that our K-Ar age data (<about 80 Ma) represent the age of the retrograde metamorphism or subsequent uplift. Datable microfossils found in the Sanbagawa belt of Shikoku suggest that the submarine basaltic volcanism and related Besshi-type mineralization occurred in an oceanic basin away from the trench region in Late Triassic (conodont) to Late Jurassic (radiolarian) times.

Mineralogical Transformation of Bioweathered Granitic Biotite, Studied by HRTEM: Evidence for a New Pathway in Lichen Activity

AbstractThe question of whether clay minerals can be biogenically transformed as a result of lichen activity at the lichen-rock interface remains unresolved. We applied several microscopical and analytical techniques—scanning electron microscopy-back-scattered electron (SEM-BSE), energy dispersive spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM)—in an attempt to address this issue. Unaffected granitic biotite and bioweathered material from the granitic biotite and Parmelia conspersa lichen thalli interface were examined using HRTEM after ultrathin sectioning. The n-alkylammonium treatment of ultrathin sections was carried out in order to study the biogenous mineralogical transformation of the biotite. Microsamples proceeding from unaffected biotite zones demonstrated homogenous 10-Å d(001)-value biotite phase. HRTEM images of lattice fringes of samples taken from the lichen-biotite contact zone reveal large areas of both unexpanded (10-A) and randomly and R = 3 distributed expanded (from 14- to 30-Å) layers of phyllosilicates identified as interstratified biotite-vermiculite. Results of artificial biotite weathering (replacement of K by Ca ion) also revealed the biotite-vermiculite phase formation, indicating that K release in biotite is one of the mechanisms responsible for interstratified mineral phase formation. Two parallel processes, physical exfoliation of biotite and inter-layer ionic exchange of K and subsequent vermiculite formation, are the mechanisms for biotite bioweathering induced by lichens.

Pressure‐temperature path of Sanbagawa prograde metamorphism deduced from grossular zoning of garnet

The zonal structure of prograde garnet in pelitic schists from the medium‐grade garnet zone and the higher‐grade albite‐biotite zone was examined to investigate the evolution of prograde P–T paths of the Sanbagawa metamorphism. The garnet studied shows a bell‐shaped chemical zoning of the spessartine component, which decreases in abundance from the core towards the rim. Almandine and pyrope contents and XMg [=Mg/(Mg+Fe2+)] increase monotonously outwards. The general scheme of the zonal structure for grossular content [XGrs=Ca/(Fe2++Mn+Mg+Ca)] can be summarized as: (1) XGrs increases outwards (inner segment) and reaches a maximum at an intermediate position between the crystal core and the rim, then decreases towards the outermost rim (outer segment) (2) the inner segment of garnet in the garnet zone samples tends to have a higher XGrs/XSps values for a given XSps than those in the albite–biotite zone samples (3) average XSps at the maximum XGrs position in the albite–biotite zone samples ranges from 0.02 to 0.12 and is lower than that in the garnet zone samples (0.13–0.32) (4) the maximum XGrs in the albite–biotite zone samples (0.34–0.39 on average) tends to be higher than that in the garnet zone samples (0.26–0.36), and (5) differences of XGrs between the maximum and rim in the albite–biotite zone samples are between 0.10 and 0.14 and higher than those in the garnet zone samples (&lt; 0.11). These facts imply that albite–biotite zone materials (a) were recrystallized under lower dP/dT conditions at an early stage of the prograde metamorphism (b) began their exhumation under higher P–T conditions and (c) have been continuously heated during exhumation for a longer duration than the garnet zone materials. The systematic changes of prograde P–T paths can be interpreted as documenting the evolution of the Sanbagawa subduction zone.

Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand

The Alpine Schist immediately west of the Main Divide of the Southern Alps is a west‐dipping duplex system consisting of an imbricated stack of rock slabs, each c. 250–1000 m thick. The imbricated stack has low grade, little deformed pumpellyite‐actinolite facies semischists at the base, overlain by progressively higher grade and more deformed schists. The structurally highest slab mapped consists of multiply‐deformed biotite zone schist. The duplex lies on the hanging wall of the northwest‐dipping Main Divide Fault Zone, which separates semischist from structurally underlying greywacke. Rock slabs are internally disrupted by faults subparallel to layering, and consist of lozenge‐shaped blocks of 10–100 m. Fault zones separating rock slabs consist of <50 cm thick zones of weakly lithified cataclasite and soft gouge. Slickensides on fault surfaces plunge northwest. The duplex formed during late Cenozoic rise of the Southern Alps. Four generations of postmetamorphic veins cut rock slabs of the duplex system. The earliest veins contain quartz, chlorite, calcite, and sulphides and lie subparallel to foliation in the higher grade slabs only. A distinctive set of open‐space‐filling fissure veins in a lower greenschist facies slab contains euhedral quartz, adularia, bladed calcite, and chlorite. The whole duplex system is cut by steeply dipping shears and fractures containing quartz and ankerite infilling and breccia cementation. Late stage iron oxy‐hydroxide‐coated fractures, generally steeply dipping, cut all rock slabs. Pyrite and, locally, arsenopyrite accompanies quartz and ankerite veins in extensional sites of steeply dipping late stage fractures, and gold contents up to 850 ppb occur. This gold mineralisation occurred within c. 1 km of the surface and had epithermal affinities. Hydrothermal fluids mineralising the various fractures range in temperature between 350°C and near‐ambient temperatures. The quartz‐adularia‐calcite veins formed at 0.5–2.5 km depth before duplex formation, and later veins formed at shallower levels after duplex formation. Vein carbonates have δ 18O(smow) between 11.4 and 26.1%o, and δ13C(pdb) between ‐0.6 and ‐11.4%o. Calculated isotopic ratios for the mineralising fluids show little evidence for meteoric origin. The fluid may be meteoric water which has undergone isotopic exchange with host rocks, metamorphic water, or a mixture of these, and mineralisation occurred during rise of the fluids from depth.

Contrasting structural styles during polyphase granitoid intrusion, South Victoria Land, Antarctica

Four granitoid plutons intruded in the Walcott Bay area, South Victoria Land, during the Paleozoic Ross Orogeny, display markedly different structural styles. Bonney Pluton is a variably megacrystic hornblende‐biotite granite batholith which outcrops over an area of 1000 km2. The southern end of the body is located in the Walcott Bay area, and it is the oldest of the four granitoid bodies (530–520 Ma). The pluton synkinematically intruded Koettlitz Group metasediments under mid‐upper amphibolite facies conditions (700°C, 5 ± 1 bars). Bonney Pluton fabrics are highly variable, with weak magmatic fabrics in the pluton centre grading to well‐developed, solid‐state deformation fabrics in high strain zones at the southwestern margins. Armitage Pluton is a non‐megacrystic hornblende‐biotite body, which intruded central parts of Bonney Pluton while it was incompletely crystallised. The body forms randomly oriented dikes and stocks and is chemically distinct from Bonney Pluton. A weak magmatic fabric is developed through the body, subparallel to magmatic lineations in central parts of Bonney Pluton. Chancellor Orthogneiss is a strongly deformed biotite granite, which discordantly intrudes metasediments as randomly oriented dikes and stocks. Solid‐state deformation fabrics overprint magmatic fabrics throughout the body, indicating post‐emplacement ductile deformation. This body was emplaced into host rock close to or at the brittle‐ductile transition, at least biotite zone conditions. Hidden Granite is an undeformed biotite granite which discordantly intrudes metasediments and older granitoid bodies as randomly oriented dikes and stocks. Mafic dikes coevally intrude the granite, infilling fractures with consistent northeast—southwest orientations, at a late stage of granite crystallisation. Hidden Granite was emplaced into a brittle host rock, above the brittle‐ductile transition. The changing structural styles of the four plutons reflect intrusion during progressive uplift. The nearly parallel nature of the elongation direction of post‐emplacement ductile fabrics in Chancellor Orthogneiss, and the extension direction indicated by brittle structures in Hidden Granite, suggests a geometric relationship exists between these fabrics and structures. The northwest—southeast elongation direction also remains relatively constant during post‐emplacement deformation of the plutons.

紀伊半島北西部三波川変成帯の曹長石―黒雲母帯

In the Sanbagawa metamorphic belt in the northwestern part of the Kii Peninsula, occurrences of biotite and its altered products are newly recognized. Pelitic schists in the area within the previously accepted garnet zone occasionally contain biotite and/or a mineral, which is optically almost identical to biotite. The latter mineral has variable K2O contents less than 6 wt%, and has similar stoichiometry to biofite except low-K content. EPMA and XRD analyses suggest that the mineral is weathered biotite, which contains interlayered vermiculite. Taking the localities of such altered biotite-bearing pelitic schists into consideration, the biotite zone is newly established as a narrow region along the east-west trending axis of the Dainichi synform. The Fe-Mg partition coefficients for garnet-chlorite pairs in the pelitic schists of biotite zone indicate that the biotite zone in the Kii Peninsula corresponds to the lower-grade part of the albite-biotite zone in central Shikoku.

Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif)

A low-pressure metamorphic zonation ranging from biotite to migorogenic belts, including the Variscan Belt of Southwest matite zones occurs in the Sierra Albarrana area (Variscan Belt of Europe. De Yoreo et al. (1991) presented a review of some southwestern Iberian Peninsula) in uppermost Precambrian to Lower possible tectonic settings of low-pressure metamorphism. Palaeozoic metasedimentary rocks. The principal deformation in Those workers considered low-pressure metamorphism this area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500°C is localized immediately to the southwest of the Sierra Albarrana and pressure does not exceed that of the aluminium Quartzites. The metamorphism is synchronous with respect to this silicate (Als) triple point. The following four tectonic deformation. The metamorphic zones are symmetrically distributed situations were envisaged for the low-pressure metawith respect to the Sierra Albarrana Quartzites. Pressure–temperature morphism (De Yoreo et al., 1991): (1) magmatic arcs; (P–T) conditions are ~3·5–4 kbar and range from ~400°C (2) regions of crustal extension; (3) continent–continent (biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C collision zones where thermal relaxation occurs after (migmatite zone). We have not detected pressure variations along crustal thickening; (4) regions with significant fluxes of the different metamorphic zones. Relic kyanite is observed in the aqueous fluids, especially those located above subduction form of inclusions in andalusite within veins in the lower-grade part zones. of the staurolite–andalusite zone. The low-pressure metamorphism of This paper is concerned with the description of a highthe Sierra Albarrana area arises from a two-stage history including temperature–low-pressure facies series terrane (the Sierra moderate crustal thickening followed by subsequent localization of Albarrana area, Variscan Belt, southwestern Iberian Masdeformation in a transcurrent shear zone during peak P–T conditions. sif ) (Figs 1 and 2). In this area, a metamorphic zonation Channelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developed have contributed to the heat budget of the low-pressure metamorphism. at low pressure (Garrote, 1976; Gonzalez del Tanago & Peinado, 1990). As will be shown here, the metamorphic zonation is not related to post-thickening thermal relaxation but rather exhibits a roughly symmetric pattern

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

Post-metamorphic quartz veins which occur over hundreds of square kilometres in the biotite zone of the Dalradian metamorphic belt consist of three principal types: anhedral quartz with pyrite, anhedral quartz with hematite, and prismatic quartz with hematite or rutile. The oxide minerals in anhedral veins have formed by oxidation of pre-existing sulphides, and gold was mobilized during this oxidation. Anhedral quartz veins formed from an aqueous fluid with up to 5 wt% dissolved salts and 16 wt% CO2 at about 300 °C. Texturally later prismatic quartz crystals formed from a compositionally similar fluid which was undergoing phase separation at the H2O-CO2 solvus at 160–200 °C and 500 to 1200 bars fluid pressure. Oxygen isotope ratios for quartz from the veins range from 12.0 to 15.3‰, with hematite-bearing veins generally isotopically heavier than pyrite-bearing veins. Calculated fluid oxygen isotope ratios range from + 8‰ for pyrite-bearing veins to -2‰ for late prismatic crystals. The mineralizing fluid contained a substantial component of meteoric water whose isotopic and chemical composition evolved with progressive water-rock interaction. Evolution of meteoric fluid composition involved migration of oxidation and oxygen isotope fronts in the down-flow direction as head-driven water passed through structurally controlled fractures in the schist pile. A gold solubility trough occurs for the observed fluid in the oxidation frontal zone. Gold remobilization and reprecipitation occurred progressively as the oxidation front migrated through the schist pile.

Retrogressive fluids and vein formation during uplift of the Priestley metamorphic complex, north Victoria Land, Antarctica

The poly-deformed Priestley schist (Wilson Terrane) of north Victoria Land, Antarctica ranges in metamorphic grade from lower greenschist facies to upper amphibolite facies. All grades of schist have been affected by structurally controlled retrogressive H2O-CO2 fluids with 45–70 mole % CO2. The fluids have deposited quartz-carbonate veins with pyrite and chlorite or biotite in late stage structures. Veins typically constitute &lt; 1% of the rock mass, but in one greenschist facies area &gt; 10% of the rock is vein. Veins in higher grade schists have been boudinaged after formation, and many have been annealed. Primary fluid inclusions are preserved in veins in biotite zone schists in two localities. At one locality, entrapment of immiscible fluids (water with c. 8 and 45 mole % CO2) occurred during vein formation, at about 280–300°C and 700 ± 200 bars fluid pressure. The aqueous fluid is slightly saline (4 wt % NaCl equivalent). At the other primary fluid inclusion locality, veins were formed from a single phase fluid (c. 70 mole % CO2) at 200–350°C and 1600 ± 500 bars fluid pressure. Both these vein systems are inferred to have formed between 2 and 8 km depth, near the brittle-ductile transition. Retrogressive fluid mobility and vein formation occurred throughout schist in the Priestly metamorphic complex during uplift in the latter part of the Ross Orogeny (c. 490 Ma), following near-isobaric cooling at metamorphic depths.

Conditions and timing of metamorphism in the southern Abitibi greenstone belt, Quebec

Regional metamorphism, ranging in grade from the subgreenschist-facies to the greenschist–amphibolite-facies transition, affects all Archean supracrustal rocks (&gt;2677 Ma) in the Rouyn–Noranda area. Contact metamorphic minerals associated with the posttectonic Preissac–Lacorne batholith (2643 Ma) show no evidence of a regional retrograde event. Accordingly, the age of regional metamorphism can be bracketed between 2677 and 2643 Ma. Three reaction isograds were mapped in subgreenschist-facies metabasites, dividing the low-grade rocks into three metamorphic zones: the pumpellyite–actinolite zone, the prehnite–pumpellyite zone, and the prehnite–epidote zone. In addition, the pumpellyite–actinolite–epidote–quartz bathograd, corresponding to a pressure of approximately 200 MPa, occurs on both sides of the Porcupine–Destor fault. Low-pressure regional metamorphism is also indicated both by the occurrence of an actinolite–oligoclase zone, and the persistence of pre-regional-metamorphic andalusite. The coincidence of andalusite and the actinolite-oligoclase zone indicates that pressure was &lt;330 MPa at the greenschist-amphibolite transition. The geothermal gradient during metamorphism was approximately 30 °C/km. Regionally, isograds dip shallowly to the north and trend subparallel to lithological and structural trends. Metamorphic minerals in metabasites define tectonic fabrics only near major fault zones and in zones of CO2 metasomatism. In biotite zone metasedimentary rocks the schistosity is defined by mica and amphibole. These textures indicate that metamorphism and fabric development were coeval. However, the actinolite–epidote isograd cuts the Porcupine–Destor fault, indicating that regional metamorphism postdates movement along this fault. The strong fabrics associated with the Porcupine–Destor and Larder Lake–Cadillac faults must have developed through a process dominated by flattening strain.

Crack geometries and deformation by the crack-seal mechanism in the Sambagawa metamorphic belt

Geometries of cracks sealed by quartz formed in the retrograde metamorphism of the Sambagawa high-pressure type metamorphic belt were measured. The relation obtained between width and length of cracks displays a power law having power exponents ranging from 0.64 to 0.98 for a wide range of lengths. The crack aspect ratios increase from 0.008 in the chlorite zone to 0.03 in the biotite zone. The average grain size of infilling quartz is proportional to the crack width, suggesting that the crack width is governed by the growth rate of infilling quartz. The frequency distribution of spacings of neighbouring cracks shows a power-law relation. Combining these data, deformation by the crack-seal mechanism is inferred to be controlled by the crack-spacing power index (fractal dimension) and growth rate of infilling quartz. The boundary between deformation by the crack-seal mechanism and the dislocation-creep regime is delineated as a function of differential stress, temperature and fractal dimension of the crack distribution pattern.

Evolution of the Western European continental crust: implications from Nd and Pb isotopes in Iberian sediments

This paper presents Smz.sbnd;Nd and geochemical data on fine-grained sediments of the Iberian Massif, Spain, together with new and published Uz.sbnd;Pb results of selected sedimentary rocks. Profiles from two major zones (Cantabrian and Central Iberian Zone) have been investigated. Both profiles exhibit almost complete stratigraphic successions from uppermost Precambrian to Lower Carboniferous. Paleozoic shallow-depth platform sediments are essentially unmetamorphosed; the Precambrian syn-orogenic succession underwent a metamorphic overprint up to biotite zone. 147 Sm 144 Nd ratios range from 0.0988 to 0.1457 and Nd contents from 6 to 112 ppm. ϵ Nd[today]-values vary from −16.5 to −7.6, Pb contents from 1.7 to 97 ppm. Samples with high ϵ Nt[ T] ( > −3) are found to be low in μ 2[330Ma] ( < 9.75). Mean Nd crustal residence ages ( T DM) are 1.92 Ga (Cantabrian Zone) and 1.96 Ga (Central Iberian Zone). As juvenile Cadomian crust is present in the Iberian Massif, the oldest components have to be significantly older than 2 Ga. Within the European Variscides comparably old ( T DM) were so far only described from the Armorican Massif, France. An active margin of the Gondwana continent is the most probable provenance of the uppermost Precambrian strata as indicated by their Cadomian age, syn-orogenic facies and juvenile components. Precambrian as well as samples from the Precambrian-Cambrian boundary gave ϵ Nt[ T]-values > −4.8, significantly higher than average Paleozoic rocks. Contributions of juvenile Cadomian crust to the syn-orogenic sediments must be in the order of 30–50% to fit the Nd and Pb data. There is no evidence for any addition of mafic material according to major or trace elements, or to Sm Nd ratios. Lower Cambrian samples are already below −7.3 in ϵ Nd[ T]. Thus, compared to the total pre-existing crust, contribution of juvenile crust was very small. ϵ Nd[ T]-values of almost all samples ranging in stratigraphic age from 515 to 410 Ma are strikingly negative ( −12.9 to −10.1) and thus indicate the absence of newly differentiated continental crust in the provenance (s). Only the Tremadocian of the southern Central Iberian Zone gave high ϵ Nt[ T]-values ( −4.0 to + 1.0). Again there is no evidence for addition of mafic material to the strata. As Precambrian and Tremadocian ϵ Nt[ T] peaks are separated by the lowermost Cambrian of ϵ Nt[ T] > -7, juvenile crust may have been added related to the pre-Ordovician unconformity (Sardic phase). During the Middle Paleozoic (410–330 Ma), ϵ Nt[ T]-values increase from — − 12 to ∼ − 7, indicating a continuous contribution of juvenile continental crust of ∼20%. As major terranes of the Iberian Massif were not amalgamated until Late Paleozoic, it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones. Nd isotope data of this study compared with published results of Brittany, France, suggest that the Medio-North ArmoricanCentro-Iberian domain persisted from uppermost Precambrian to the Variscan.