Plagioclase Rims Research Articles

Mullite-corundum-spinel-cordierite-plagioclase xenoliths in the Skaergaard Marginal Border Group: multi-stage interaction between metasediments and basaltic magma

Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel+cordierite+plagioclase±corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about 1,000°C at a pressure of about 650 bars and at an oxygen fugacity about 0.2–0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum+mullite+sillimanite+ plagioclase+spinel+rutile±tridymite±cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An55 close to the basalt to An90 close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel+cordierite+plagioclase+quartz+K-feldspar +magnetite+ilmenite at about 800°C.

Mineral inventory of continuously erupting basaltic andesites at Arenal volcano, Costa Rica: implications for interpreting monotonous, crystal-rich, mafic arc stratigraphies

Except for the first 2 years since July 29, 1968, Arenal volcano has continuously erupted compositionally monotonous and phenocryst-rich (∼35%) basaltic andesites composed of plagioclase (plag), orthopyroxene (opx), clinopyroxene (cpx), spinel±olivine. Detailed textural and compositional analyses of phenocrysts, mineral inclusions, and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments. We suggest three distinct crystallization environments crystallized low Mg# (<78) silicate phases from andesitic magma but at different physical conditions, such as variable pressure of crystallization and water conditions. The dominant environment, i.e., the one which accounts for the majority of minerals and overprinted all other assemblages near rims of phenocrysts, cocrystallized clinopyroxene (Mg# ∼71–78), orthopyroxene (Mg# ∼71–78), titanomagnetite and plagioclase (An 60 to An 85). The second environment cocrystallized clinopyroxene (Mg# 71–78), olivine (<Fo 78), titanomagnetite, and very high An (∼90) plagioclase, while the third cocrystallized clinopyroxene (Mg# 71–78) with high (>7) Al/Ti and high (>4 wt.%) Al 2O 3, titanomagnetite with considerable Al 2O 3 (10–18 wt.%) and possibly olivine but appears to lack plagioclase. A fourth crystallization environment is characterized by clinopyroxene (e.g., Mg#=∼78–85; Cr 2O 3=0.15–0.7 wt.%), Al-, Cr-rich spinel, olivine (∼Fo 80), and in some circumstances high-An (>80) plagioclase. This assemblage seems to record mafic inputs into the Arenal system and crystallization at high to low pressures. Single crystals cannot be completely classified as xenocrysts, antecrysts (cognate crystals), or phenocrysts, because they often contain different parts each representing a different crystallization environment and thus belong to different categories. Bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions. On the other hand, they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources. The driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time may also go through compositional modification by fractionation and assimilation of country rocks. Thus, mafic magmas become basaltic andesite through mixing, fractionation and assimilation by the time they arrive at subvolcanic depths. We infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing, subvolcanic magma. Thus, the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite. Furthermore, we propose that this quasi-constant supply of small increments of magma is the fundamental cause for small-scale, decade-long continuous volcanic activity; that is, the current eruption of Arenal is flux-controlled by inputs of mantle magmas.

Dissolution and precipitation processes in deformed amphibolites: an example from the ductile shear zone of the Ryoke metamorphic belt, SW Japan

AbstractThe granitic mylonite zone in the Cretaceous Ryoke metamorphic belt contains deformed amphibolites as thin layers. The amphibolite layers do not exhibit pinch‐and‐swell or boudinage structures, even when contained in a high‐strain granitic mylonite. This mode of occurrence suggests that they were deformed as much as the surrounding granite mylonite. In the highly deformed zone, strongly foliated amphibolites contain Ti‐rich brown amphibole porphyroclasts rimmed by Ti‐poor green amphibole, titanite and chlorite. These porphyroclasts are elongated, forming shear surfaces defined by preferential distribution of the chlorite and titanite. Porphyroclastic plagioclase in the strongly foliated amphibolites consists of two components: an anorthite‐rich core and an anorthite‐poor rim. Based on these observations, the mass‐balanced reaction occurring during deformation is defined as As the reaction products form a weak interconnected matrix, the strain rate of the amphibolites may be controlled by the rate of dissolution–precipitation through fluids. Weakly foliated amphibolites in the low‐strain zone exhibit cataclastic microstructures, whereas the strongly foliated amphibolites do not exhibit such features. These microstructural and chemical changes suggest that high‐strain amphibolites were initially deformed by cataclasis, followed by deformation through metamorphic reactions. During the metamorphism/deformation, old plagioclase grains with high Xan were not stable and dissolved, and new plagioclase grains with low Xan crystallized at the old plagioclase rim. Dissolution of old plagioclase and precipitation of new plagioclase occurred normal to and parallel to the foliation, respectively, reflecting incongruent pressure solution due to differential stress and changes in P–T–H2O conditions. The development of incongruent pressure solution is attributed to increased fluid flux in the strongly foliated amphibolites, as evidenced by the greater abundance of hydration‐reaction products in the strongly foliated amphibolites than in the weakly foliated ones.

Postcollisional granites in the South Tien Shan Variscan Collisional Belt, Kyrgyzstan

Two major types of the potassium-rich postcollisional granites of Permian age were studied in the South Tien Shan Variscan Collisional Belt. The first type, metaluminous granites of the Jangart complex, are located along the southeastern boundary of this belt with the Precambrian Tarim Block. The plagioclase rims on K-feldspar megacrysts (rapakivi texture), extremely high FeOt/(FeOt+MgO) ratio, indications of the low H2O and O2 fugacities, high concentrations of the incompatible elements especially light rare earth elements (LREE), Ba, Nb along with heightened contents of the compatible elements especially Ni, Cr and moderately initial 87Sr/86Sr ratio (Sr0=0.7075) make those granites similar to other rapakivi granites. The second type, peraluminous granites of the Inylchek complex, is located along the northwestern boundary of the belt with the Caledonian Kazakh Microcontinent. Those granites are rich in F, B, Li, Rb, Cs, Sn, Ta and heavy rare earth elements (HREE), but are poor in Ba, Sr, Ni and Cr and are characterized by the relatively high initial 87Sr/86Sr ratio (Sr0=0.7098). Li–mica granites are distributed widely among them. The granites of a transitional type (Uchkoshkon complex) occupy an intermediate geographic position between major granite types. The initial Sr isotopic composition of the transitional granites varies widely (Sr0 from 0.7080 to 0.7256). The variation in chemical composition of the coeval postcollisional granites across the Collisional belt, together with the variability of initial Sr isotopic ratios of the transitional granites, are thought to indicate basement heterogeneity. This heterogeneity, in turn, is probably related to thrusting of the Tarim Block under South Tien Shan during the collision.

The nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks, Montserrat

Recently erupted rocks from Montserrat, Lesser Antilles, were found to be coarsely porphyritic including plagioclase, magnesio–hornblende, orthopyroxene, titano–magnetite, and rare quartz phenocrysts. Zonations within plagioclase show three distinct periods within the evolution of the andesitic magmas. Initial steady state oscillatory overgrowths (An 49–54) represent an early period of growth within a confined andesitic magma chamber. Interaction with an intruding more mafic hotter magma resulted in thick zones of dissolution features that cut through these early growth layers. The dissolution zones and crystal fragments are overgrown by thin calcic (An 86) plagioclase rims. A plagioclase growth rate of 10 −7cm/s suggests that the calcic plagioclase rims were formed in approximately 10 h, consistent with the idea that mineral growth may increase magma viscosity inducing conduit plugging and shallow pressurization resulting in cycles of dome inflation, eruption, and explosive activity. Thermodynamic modeling of heating and mixing of the andesitic magma with a basaltic liquid produces plagioclase of composition An 79, not the observed composition of the late plagioclase (An 86). We conclude that the final plagioclase grew under non-equilibrium conditions as a result of the mixing event.

Mineral compositional and textural evidence for magma mingling in the Saraykent volcanics

Upper Cretaceous–Paleocene calc-alkaline dacitic and rhyodacitic lava flows and domes are exposed in the Saraykent (Yozgat) region within Central Anatolian Crystalline Complex (CACC). Saraykent volcanics are generally plagioclase+hornblende+biotite+titanomagnetite±quartz±augite–phyric. Subrounded, vesicular magmatic enclaves, ranging from a few millimeters to a few centimeters in size, are present in the Saraykent dacites. The enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites. Almost all phases in Saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling. Plagioclase phenocrysts display sieved and normal type populations, a wide range in rim compositions, and oscillatory zoning. Maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol% increase in An. Some plagioclase core compositions are unusually Ca-rich. They range from An 27 to An 75 for the Saraykent dacites and from An 30 to An 66 for the Saraykent rhyodacites. Hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma. Both hornblende and biotite crystals show reverse zoning in terms of decreasing Mg/Mg+Fe. Quartz phenocrysts are corroded and embayed. Presence of normally and reversely zoned augites in the same sample, and high-Ti high-Al augites more comparable with basaltic compositions, are other lines of evidence for disequilibrium. The enclave hornblendes are both edenitic and pargasitic, the latter being more typical of mafic compositions. The enclave biotite crystals have higher Mg/Mg+Fe 2+ ratios than the host dacite biotite crystals and display normal, reverse and oscillatory zoning. Plagioclase core compositions vary between An 44 and An 55 in the enclaves. Although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression, this cannot explain the observed compositional change and normal type of plagioclase in the same sample. Similarly, a cognate (i.e. fractionation) origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite. However, other criteria such as oscillatory zoning and unusually calcic cores of plagioclases; heterogeneity of plagioclase rims; resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the Saraykent volcanics.

Mantled feldspar from the Noring granite, Peninsular Malaysia: petrography, chemistry and petrogenesis

Mantle feldspar found in the Noring granite, have been investigated in terms of their petrography and geochemistry. Petrographically the mantle feldspar consists of whitish plagioclase rim which mantle the pinkish K-feldspar core. The K-feldspar core consists of a single crystal or several crystals usually showing simple twinning. Plagioclase rim is generally composed of numerous euhedral to subhedral plagioclases with size usually less than 1 mm. In places the plagioclase rim consists of a single crystal about 2 mm thick. :irormation of the mantled feldspar can be explained in two stages, (1) magma mixing and (2) late stage replacement. The plagioclase rim probably precipitated from a different magma type to the Noring magma probably of andesitic composition. The K-feldspar that has crystallised in the Noring magma act as substrates for the growth of plagioclase, producing the mantled texture. The aggregates of individuals crystals that have floated together in the magma attached to the K-feldspar as in the synneusis model of Vance (1969). Once this nucleation takes place there is relatively the straight forward process of crystal growth yielding the mantled texture (Hibbard, 1981). Small irregular outline plagioclase inclusions occur in the K-feldspar core being most abundant near the contact with plagioclase rim suggesting that the K-feldspar core replacing and enclosing relicts of plagioclase rim at the very late stage.

Trace element distribution within olivine-bearing gabbros from the Northern Apennine ophiolites (Italy): evidence for post-cumulus crystallization in MOR-type gabbroic rocks

The trace element distribution in three selected olivine-bearing gabbros from the Northern Apennine ophiolites has been determined. These rocks consist of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyroxene. Fe-Ti-oxides, titanian pargasite, orthopyroxene and apatite occur as interstitial accessory minerals. Plagioclase, clinopyroxene and accessory minerals were analysed for rare earth (REE) and selected trace elements by secondary ion mass spectrometry. Both plagioclase and clinopyroxene are compositionally zoned. The plagioclase rims have slightly lower anorthite component and higher light REE (LREE), Ba and K than the cores. Likewise, the clinopyroxene rims show a slight Mg and Cr decrease, and a marked increase in Zr, REE and Y relative to the core. The rims of plagioclase and clinopyroxene, Fe-Ti-oxides, apatite and titanian pargasite most likely formed through post-cumulus fractional crystallization of interstitial liquid. It is argued that such interstitial liquid had an exotic component, probably related to the infiltration of highly evolved, slightly LREE enriched liquid in the cumulate pile. On the basis of mass balance calculations, we show that Fe-Ti-oxides play an important role in the Ti budget of the whole rock, as does apatite for LREE.

THE MANTLE RIDGE OF THE GALICIA PASSIVE MARGIN: GEOCHEMICAL EVIDENCES FOR THE DENUDATION OF A PIECE OF THE LITHOSPHERIC MANTLE DURING OCEANISATION

The northern part of the Iberian continental passive margin is characterised by the presence of a ridge made of mantellic and magmatic rocks. This ridge, roughly parallel to the margin and continuous over several hundred kilometres, was emplaced at the end of the major rifting event that preceded North-Atlantic ocean opening. Petrological and geochemical studies on mantle rocks, whose results are presented here, are essential to characterise mantle affinity and processes that have affected this part of the mantle during or before continental break-up. In this work we would like to answer some questions like: Does this mantle correspond to the upwelling of the upper asthenospheric mantle at the end of the rifting event or rather correspond to the denudation of lithospheric mantle that was beneath the American-Iberian continent before oceanisation? Does the mantellic and magmatic ridge represent the first North-Atlantic oceanic lithosphere? The studied peridotites were sampled by submersible on the northwest part of the Galicia bank during the Galinaute II campaign (Boillot et al., 1995). As all samples are weathered, we have only performed analyses on fresh minerals. All peridotites have common mantle assemblage (Ol+Opx+Cpx+spinel) but according to microscopic observations and geochemical results we have defined three different types of peridotites. The first type is characterised by aluminous spinel (Cr* 0.2) and rare Cpx which is poorer in Na and Ti, richer in Cr, and have rather flat REE patterns, but, strikingly, lower middle and heavy REE contents compared to type 1 Cpx. Type 2 Cpx has also the lowest observed eNd values (-2.9 and +4.4. The third type of peridotite is rather different: it is characterised by a strong high-temperature strain, by the occurrence of scattered pargasitic amphiboles and dioritic dykelets made of pargasite and plagioclase, and by the presence of plagioclase rim around spinel. Dioritic dykelets are contemporaneous with the rifting high-temperature strain of the host mantle (Feraud et al., 1988). Amphibole in these dykelets shows rather homogeneous geochemical features: +5.6 and +5.8 eNd values and REE contents about one hundred times chondrite, with middle REE maxima and LaN/SmN ratios < 1. Scattered amphibole in peridotites shows the same kind of REE patterns but variable REE contents ranging from 5 to 80 the chondrite composition. From all these results, the first point to note is that the Galicia mantle ridge has been locally influenced by fluid circulation (type 3 peridotites), which was channelled inside active simple shear zones during rifting; moreover, the fluid associated with strain have probably favoured partial recrystallisation of the mantle from spinel to plagioclase facies. Type 1 and 2 peridotites probably give us more information on mantle affinity before oceanic opening, the most important fact being their contrasted eNd values. In this geodynamical context of passive rifting we can’t explain the lowest eNd values by metasomatism of the asthenospheric mantle reservoir by enriched fluids during the rifting or oceanisation. Moreover, the eNd values calculated at rifting time are always so different that we can’t explain the present isotopic values by different degrees of partial melting and so, different Sm/Nd time integrated evolution of an initially homogeneous asthenospheric mantle. Therefore, geochemical features of the Galicia ridge samples indicate that the mantle was already heterogeneous before rifting and probably represents a part of a sub-continental lithospheric mantle. To conclude, we can say that the Galicia mantle ridge is neither an asthenospheric diapir mantle emplaced during the rifting event, nor the mantellic part of the first North-Atlantic oceanic lithosphere, but likely is a piece of the lithospheric mantle that was beneath the American-Iberian continent before North-Atlantic opening. The Galicia mantle shows geochemical heterogeneities similar to what is known in many orogenic lherzolite massifs (Beni-Bousera, Ronda, Pyrenees, etc.) but these heterogeneities have probably been amplified during the rifting processes.

Cheralite from Visakhapatnam Area in the Eastern Ghats Granulite Belt, India

Cheralite is reported for the first time from a folded sillimanite schist layer in the garnetiferous quartzo-feldspathic gneisses from the Visakhapatnam area in the Eastern Ghats granulite belt. Cheralite of scarce occurrence was recorded from charnockites of Araku area in the catchment of Gosthani river basin, which was incidentally related to the concentration in the beach placers as cheralite component in Visakhapatnam - Bhimunipatnam Coastal region. The monazite data in the Eastern Ghats from quartzo-feldspathic gneiss, sapphirine/cordierite granulites generally shown ThO 2 range between 6-10% while hypersthene granulites have 9-19% with an exception of 2% in allanite bearing hypersthene granulites. Cheralite from spinel bearing sillimanite schist layer contains 14-22% ThO 2 and shows occasional enrichment towards the grain margins. Plagioclase rims noticed around ilmenite in the spinel bearing sillimanite schist are indicative of partial melt in the granulites. The pegmatite formation along the sheared contacts of garnetiferous quartzo-feldspathic gneisses and spinel bearing sillimanite schists/gneisses is related to metamorphic segregation phenomena in the quartzo-feldspathic gneisses. The enrichment of Th, U and Si is due to the element mobilities during high grade metamorphism. Based on the occurrence of cheralite in the major source rocks, garnetiferous quartzo-feldspathic gneisses, garnetiferous sillimanite gneisses and hypersthene granulites, it is believed that these rocks have substantially contributed to the placer concentrations in the coastal sediments.

Preeruption conditions and timing of dacite‐andesite magma mixing in the 2.2 ka eruption at Mount Rainier

Analytical, field, and experimental evidence demonstrate that the Mount Rainier tephra layer C (2.2 ka) preserves a magma mixing event between an andesitic magma (whole rock SiO2 content of 57–60 wt %) and a dacitic magma (whole rock SiO2 content of 65±1 wt %). The end‐member andesite (a mix of an injected and chamber andesite) and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions. Many pumices, however, contain mixtures of the end‐members. The end‐member dacite contains a microlite‐free matrix glass with 74–77 wt % SiO2, orthopyroxene rims of Mg57–64, clinopyroxene rims of Mg66–74, and plagioclase rim anorthite contents of An45–65. The temperature and oxygen fugacity, from Fe‐Ti oxide compositions, are 930±10°C and 0.5–0.75 log units above NNO. The mixed andesite contains Mg73–84 orthopyroxene rims, Mg73–78 clinopyroxene rims, An78–84 plagioclase rims, and Mg67–74 amphibole rims. The temperature from Fe‐Ti oxides, hornblendeplagioclase, and two‐pyroxene geothermometry is 1060±15°C, and the oxygen fugacity is approximately one log unit above NNO for the injected andesite. The chamber andesite is estimated to be a magma with a ∼64–65 wt % SiO2 melt at 980°C and a NNO oxygen fugacity. We conclude that the andesitic and dacitic magmas are from separate magma storage regions (at &gt;7 km and ∼2.4 km) due to differences in the bimodal whole rock, matrix glass, and phenocryst compositions and the presence or absence of stable hornblende. The time involved from the mixing event through the eruption is limited to a period of 4–5 days based on Fe‐Ti oxide reequilibration, phenocryst growth rates, and hornblende breakdown. The eruption sequence is interpreted as having been initiated by an injection of the 1060±15°C andesitic magma into the ∼980°C (&gt;7 km) andesite storage region. The mixed andesitic magma then intersected a shallow, ∼2.4 km, dacitic storage system on its way toward the surface. The eruption became more dacitic over time, and the final products some show evidence of partial reequilibration between the andesite and dacite.

Transmissionselektronenmikroskopische Untersuchung von durchst�ubtem Plagioklas in einem kalk-alkalischen Andesit vom Oze-Hiuchigatake Vulkan, Zentral-Japan

A detailed TEM study of dusty plagioclase revealed for the first time that the dust materials are mainly composed of glass, alkali feldspar, quartz and other fine-grained minerals. Detailed textures and the chemistry of glass inclusions are also described. b-domains with smoothly curving boundaries are found in the dusty zone and the clear rim. The plagioclase in the dusty zone inverts from C\(\bar 1\) into I\(\bar 1\) by (1) annealing in a stable C\(\bar 1\) field and then quenching on eruption and, (2) compositional change from less calcic to more calcic during partial dissolution or (3) annealing in a metastable C\(\bar 1\) field. The calcic clear plagioclase rim crystallizes metastably as C\(\bar 1\) and then inverts into I\(\bar 1\) on eruption. The glass inclusions are more enriched in SiO2 (> 80 wt.%) than residual glasses in the groundmass. It is estimated that alkali feldspar, quartz and other fine-grained minerals were crystallized from the melts entrapped in the dusty zone during eruption. It is concluded that the dusty plagioclase was formed by the dissolution of less calcic plagioclase in basic magma during magma mixing.

Pan-African structures and metamorphism in the Makuti group, north-west Zimbabwe

The Makuti Group is composed of arkosic gneisses, amphibolites, marbles, calc-silicate rocks and minor pelitic schists and forms part of the Pan-African supracrustal sequence within the Zambezi belt in north-west Zimbabwe. The group was affected by two generations of folds (D z1 and D Z2). The D Z1 folds are isoclinal and plunge gently to the NW and SE. D z2 folds are upright and refold D z1 folds on an almost coincident axis giving rise to a type III fold interference pattern. Metamorphism within the Makuti Group is syn-D Z1 and pre-D Z2. Temperatures calculated from garnet-biotite (grt-bt) geothermometers increase to the south from 538±49°C in upper greenschist/low amphibolite facies, to 595±46°C for mid-amphibolite facies and 718±30°C for the upper amphibolite grade. Upper amphibolite grt-bt temperatures are supported by amphibole-plagioclase and garnet-hornblende (grt-hbl) temperatures. Compositional zoning is reflected by high anorthite content in plagioclase rims (An 24–30) relative to the cores (An 10–16) and by high X gro+sps cores and low X pyp+alm cores relative to the rims in garnets. Temperatures calculated using garnet and plagioclase rim compositions are 50°C higher than core temperatures. Thus growth zoning in plagioclase and garnet preserve a portion of the prograde P-T path. The metamorphic and structural data concur with the southward thrusting of the Zambezi belt.

Isotopic constraints on the production rates, crystallisation histories and residence times of pre-caldera silicic magmas, Long Valley, California

Pre-caldera high-silica rhyolites of Glass Mountain, California erupted episodically from 2.1 Ma until the catastrophic eruption of the Bishop Tuff at 0.74 Ma. The lavas are extremely evolved, with Rb/Sr ratios between 128 to 3640, the latter being the highest recorded from a volcanic rock. Glass separates from pre-1.2 Ma lavas define two geographically controlled RbSr isochrons. Lavas adjacent to the current caldera rim define an isochron age of 2.047 ± 0.013 Ma with an initial ratio of 0.7063 ± 2, and lavas more distant from the caldera define an isochron of 1.894 ± 0.013 Ma with the same initial ratio. The isochrons are consistent with the magmas forming within 26 ka, which implies a minimum magma production rate of 0.75 × 10 −3 km 3/yr over this period. New 40Ar 39Ar ages on sanidine and biotite have established that lavas defining each isochron were erupted over a long time interval, the isochron ages being up to 360 ka older than the youngest eruption age. RbSr isotope data are reported for minerals from three lavas with eruption ages of 1.990 ± 0.012, 1.866 ± 0.014 and 1.686 ± 0.011 Ma. Petrographically early apatite inclusions in biotite and biotite inclusions in feldspar and quartz have glass-mineral RbSr ages that are indistinguishable from the relevant regional isochron. Sr diffusion in feldspar is slow at the magmatic temperatures inferred for Glass Mountain rhyolites (∼ 700°C) such that over 0.5 Ma the cores of large feldspars ( > 1 mm) will retain > 99.9% of their original Sr. The cores of sanidine and plagioclase yield glass-mineral ages that are up to 300 ka older than eruption ages. Feldspar rim ages for two samples are indistinguishable from eruption ages. The rims of sanidines and plagioclases from the third sample are 110 and 280 ka older than the eruption age and 180 and 20 ka younger than the cores. These mineral age data probably reflect the combination of extended periods of mineral growth and partial isotopic exchange with the host liquid during protracted residence in a magma reservoir. However, the Ar and Sr isotopic data for biotite phenocrysts are consistent with the presence of a significant component that is recycled from earlier magmatic pulses. Due to the extreme Rb/Sr ratios of the rocks and minerals it is possible to very precisely resolve the time difference between the formation of different phases, assuming that they crystallised from the same host magmas (e.g., T plagioclase − T FeTi oxide = 6.8 ± 0.1 ka) and the maximum time taken to form a phase (e.g., T plagioclase core − T plagioclase rim = 32.3 ± 0.2 ka). The timescale for mineral growth is shorter in the chemically more evolved and crystal-poor lavas, consistent with these magmas having resided at higher levels in the magma chamber with shorter residence times and being more liable to extrusion. By using mineral inclusion relationships, average mineral growth rates are estimated to be between 7 × 10 −13 and 8 × 10 −14 cm/s. These values are significantly lower than those measured in basaltic systems and probably reflect a combination of the slow cooling rate of the Glass Mountain magma chamber(s) and the highly polymerised nature of high-silica magmas.

Reactions and textures in wollastonite-scapolite granulites and their significance for pressure-temperature-fluid histories of high-grade terranes

Reaction textures in wollastonite-scapolite calc-silicates provide important evidence for pressure-temperature-fluid histories in granulites. Pressure-temperature, T-aCO2 and P-aCO2 diagrams, calculated for appropriate mineral compositions using the internally consistent dataset of Holland and Powell (1990), are presented and used to provide a framework for interpretation of the reaction textures. Garnet rims replacing wollastonite and scapolite in calc-silicate gneisses is a common texture in many granulite terranes. This texture can plausibly be developed in post-peak P-T histories dominated by cooling (e.g., near-isobaric cooling) and does not generally imply the infiltration of hydrous fluids. Examples from the Arunta Complex, Australia, and the Northern Prince Charles Mountains, Antarctica, are consistent with cooling without fluid influx, at 7–8 kbar and aCO2 of 0.3–0.5. Contrasting textures involving the replacement of garnet by wollastonite+scapolite or wollastonite+plagioclase symplectites, and the growth of wollastonite+plagioclase rims or coronas on scapolite-quartz boundaries, can be interpreted as essentially closed-system features and modelled through modal analysis of reaction products. In the case of the Rauer Group, Antarctica, near-isothermal decompression from ca. 8 to 6 kbar at 850–800°C and aCO2 in the range 0.35–0.45 is indicated by these textures, based on calculated P-T-aCO2 grids in the CaOAl2O3SiO2CO2 (CASV) and more complex systems. Calculated reactions producing wollastonite+scapolite from garnet-bearing assemblages do not involve decarbonation, but may consume CO2 with increasing temperature in the absence of calcite. Hence, these calc-silicates may act as CO2 “sinks” on the prograde segments of clockwise P-T paths. Conversely, scapolite-wollastonite granulites will act as sources of post-peak CO2-rich fluids liberated upon cooling and potentially trapped elsewhere as post-peak carbonic fluid inclusions.

Late Mesozoic low‐P/high‐T metamorphism preceding emplacement of Cretaceous granitic rocks in the Gosaisho‐Takanuki district, Abukuma metamorphic belt

Abstract The central part of Abukuma metamorphic belt consists of two geologic units, the Gosaisho Group and the Takanuki Group. Although the deformation styles differ between the Gosaisho and the Takanuki Groups, their rock facies show a gradual transition. In both Groups early regional low‐pressure (over 3 kb) metamorphism has been overprinted by contact metamorphism. Evidence for the P/T condition of the regional metamorphism is recorded in cores of armored minerals. Metamorphic zones have been defined on mineral rim assemblages of meta‐mafite, meta‐pelite and meta‐calc‐siliceous schist and on the degree of graphitization of meta‐pelite. The mineral‐core chemistry of plagioclase, Ca‐amphibole and garnet changes with increasing metamorphic grade, and indicates that the regional metamorphism of the Gosaisho Group took place in a high pressure region of the andalusite stability field. The Takanuki metamorphic rocks are structurally overlain by the Gosaisho Group and have undergone regional metamorphism whose conditions have passed near the triple point of Al‐silicates and kyanite has crystallized. The contact aureoles in both groups are developed around middle Cretaceous granitic intrusions. Rims of plagioclase, Ca‐amphibole and garnet overgrew on the mineral‐cores during the contact metamorphism. The regional metamorphism began after the sedimentation of Jurassic chert and was succeeded by the contact metamorphism in the middle Cretaceous.

High‐pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino‐Korean and Yangtze cratons in central and eastern China. These eclogites usually have high‐P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high‐P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO2 phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO2 phase, and are divided into prograde eclogites and mantle‐derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle‐derived eclogites have high MgO/(FeO + Fe2O3) and Cr2O3 compositions in bulk rock and minerals, and consist mainly of pyrope‐rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular‐rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile.Pargasitic and taramitic amphiboles, calcic plagioclase (An68), epidote, zoisite, K‐feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near‐isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P &gt; 26 kbar and T= 500–750°C in the western part to P &gt; 28 kbar and T= 810–880°C in the eastern part of the high‐P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near‐isothermal decompression during the retrograde stage. The conditions at which symplectites re‐equilibrated tend to increase from west (P &lt; 10 kbar and T &lt; 580°C) to east (P &gt; 9 kbar and T &gt; 680°C). Equilibrium temperatures of Type II mantle‐derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.

Retrograded eclogites in the Olekma granite-greenstone region, Aldan Shield, Siberia

Relict eclogite facies assemblages occur in basic material which forms boudins and isoclinally folded bands in tonalitetrondhjemite gneisses in the northeastern part of the Archaean Olekma granite-greenstone region, Aldan Shield, Siberia. The basic material is interpreted as the remains of dykes which were deformed and metamorphosed during a major orogenic event. The central parts of thicker (3–7 m) basic boudins contain relict garnet and clinopyroxene with symplectic intergrowths of sodic plagioclase. Garnets are surrounded by kelyphitic rims of plagioclase and hornblende, and clinopyroxenes are partially replaced by hornblende. In the margins of the boudins (which have similar bulk composition to the central parts) garnet and clinopyroxene are absent, and the mineral assemblage is hornblende and plagioclase, i.e. an amphibolite facies paragenesis. The petrographic observations combined with mineral chemistry suggest that during the early stages of metamorphism the (anhydrous) basic rocks had garnet +jadeite-rich clinopyroxene assemblages. Subsequently, during decompression and continuing deformation, sodic plagioclase exsolved from the clinopyroxene, and garnet and clinopyroxene were variably recrystallised to give rise to the hornblende + plagioclase amphibolite facies assemblages. The eclogites must have equilibrated at 10–15 kbar, and geothermometry of the amphibolite facies assemblages suggests conditions of 650–700°C and 5.5–7 kbar. During decompression and retrogression of the eclogites, local partial melting of the host gneisses took place, particularly between boudins of mafic material.

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.

Plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the Sierra Nevada Batholith, California

Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.