A study of natural and experimental metasomatic assemblages in an ultramafic-quartzofeldspathic metasomatic system from the haast schist,South Island,New Zealand

The mineralogy of a metasomatic sequence formed between ultramafic and quartzofeldspathic protoliths from the Southern Alps of New Zealand consists of a forsterite-antigorite core surrounded concentrically by zones of antigorite-magnesite, magnesite-talc, talc, tremolite, chlorite and muscovite with discontinuous pods of albite associated with the muscovite zone. On the basis of trace element data the original ultramafite-schist contact is positioned between the present tremolite and chlorite zones.An experimental study of a metasomatic system was undertaken in an attempt to clarify diffusion relationships during the metasomatic event. To simulate the natural event, ultramafic and quartzofeldspathic natural starting materials were tightly packed in a gold tube with a graphite layer between to allow later identification of the original lithological interface. Run conditions were 450° C at 2 kb for 40 days. Phase dissolution and formation were analysed petrographically and component migration was examined with the electron-microprobe. The following hierarchical scheme of component migration, phase dissolution and phase formation is delineated: CO₂ migrates from the schist into the ultramafite forming first the antigorite-magnesite zone and then the magnesite-talc zone at higher values of CO₂. These zones are then partially overprinted by the formation of talc due to SiO₂ metasomatism. The SiO₂ is supplied from the schist by the dissolution of quartz and albite in the region adjacent to the ultramafite. The tremolite zone forms at the expense of the metasomatic talc zone upon the introduction of CaO from the schist into the ultramafite. Concurrent with tremolite formation, MgO migrates from the ultramafite into the schist to form the chlorite metasomatic zone. The growth of the chlorite zone causes dissolution of the pre-metasomatic micas and displaces K₂O from the chlorite zone further into the schist. Displaced K₂O and Na₂O are responsible for the formation of the muscovite zone and the albitite pods.     

Neoarchaean high-grade metamorphism in the Central Zone of the Limpopo Belt (South Africa): Combined petrological and geochronological evidence from the Bulai pluton

The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.     

Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India

In this study, we reconstruct the inverted metamorphic sequence in the western Arunachal Himalaya using combined structural and metamorphic analyses of rocks of the Lesser and Greater Himalayan Sequences. Four thrust-bounded stratigraphic units, which from the lower to higher structural heights are (a) the Gondwana rocks and relatively weakly deformed metasediments of the Bomdila Group, (b) the tectonically interleaved sequence of Bomdila gneiss and Bomdila Group, (c) the Dirang Formation and (d) the Se La Group are exposed along the transect, Jira–Rupa–Bomdila–Dirang–Se La Pass. The Main Central thrust, which coincides with intense strain localization and the first appearance of kyanite-grade partial melt is placed at the base of the Se La Group.Five metamorphic zones from garnet through kyanite, kyanite migmatite, kyanite-sillimanite migmatite to K-feldspar-kyanite-sillimanite migmatites are sequentially developed in the metamorphosed low-alumina pelites of Dirang and Se La Group, with increasing structural heights. Three phases of deformation, D₁–D₂–D₃ and two groups of planar structures, S₁ and S₂ are recognized, and S₂ is the most pervasive one. Mineral growths in all these zones are dominantly late-to post-D₂, excepting in some garnet-zone rocks, where syn-D₁ garnet growths are documented. Metamorphic isograds, which are aligned parallel to S₂ were subsequently folded during D₃. The deformation produced plane-non-cylindrical fold along NW–SE axis.In the garnet-zone, peak metamorphism is marked by garnet growth through the reaction biotite + plagioclase → garnet + muscovite. An even earlier phase of syn-D₁ garnet growth occurred in the chlorite stability field with or without epidote. In the kyanite-zone metapelites, kyanite appeared via the pressure-sensitive reaction, garnet + muscovite → kyanite + biotite + quartz. Staurolite was produced in the same rock by retrograde replacement of kyanite following the reaction, garnet + kyanite + H₂O → staurolite + quartz. These reactions depart from the classical kyanite- and staurolite-isograd reactions in low-alumina pelites, encountered in other segments of eastern Himalaya. In the metapelites, just above the kyanite-zone, melting begins in the kyanite field, through water-saturated and water-undersaturated melting of paragonite component in white mica. Leucosomes formed through these reactions are characteristically free of K-feldspar, with sodic plagioclase and quartz as the dominant constituents. With increasing structural height, the melting shifts to water-undersaturated melting of muscovite component of white mica, producing an early K-feldspar + kyanite and later K-feldspar + sillimanite assemblages and granitic leucosomes.Applications of conventional geothermobarometry and average P–T method reveal near isobaric (at P  8 kbar) increase in peak metamorphic temperatures from 550 °C in the garnet-zone to >700 °C for K-feldspar-kyanite-sillimanite-zone rocks. The findings of near isobaric metamorphic field gradient and by the reconstruction of the reaction history, reveal that the described inverted metamorphic sequence in the western Arunachal Himalaya, deviates from the classical Barrovian-type metamorphism. The tectonic implication of such a metamorphic evolution is discussed.     

Garnet-staurolite-mica schist from Rangli Rangliot,Eastern Himalaya: Constraints from phase equilibria and Thermobarometry

Rangli Rangliot is an integral part of lesser Himalaya. The area around Rangli Rangliot consists of garnetstaurolite-mica schist and it is characterized by mineral assemblage garnet-biotite-muscovite-staurolite-quartz± plagioclase. Different reaction textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The relative X_Mg in the minerals varies in the order: muscovite> biotite> staurolite> garnet, and the X_Mn decreases in the order: garnet>staurolite>biotite>muscovite. The P-T evolution of the garnet-staurolite-mica schist has been constrained through the use of internally consistent TWEEQU programme and Perple_X software in the KFMASH model system. The combination of these two approaches demonstrates that the garnet-staurolite-mica schist experienced peak pressure and temperature at 5.8 kbar and 590 °C. The proposed clockwise P-T path implies that rocks from the study area could have resulted from thickened continental crust undergoing decompression.     

Layered amphibolite sequence in NE Sardinia,Italy: remnant of a pre-Variscan mafic silicic layered intrusion?

A banded amphibolite sequence of alternating ultramafic, mafic (amphibolite) and silicic layers, tectonically enclosed within Variscan migmatites, outcrops at Monte Plebi (NE Sardinia) and shows similarities with leptyno-amphibolite complexes. The ultramafic layers consist of amphibole (75–98%), garnet (0–20%), opaque minerals (1–5%) and biotite (0–3%). The mafic rocks are made up of amphibole (65–80%), plagioclase (15–30%), quartz (0–15%), opaque minerals (2–3%) and biotite (0–2%). The silicic layers consist of plagioclase (60–75%), amphibole (15–30%) and quartz (10–15%). Alteration, metasomatic, metamorphic and hydrothermal processes did not significantly modify the original protolith chemistry, as proved by a lack of K₂O-enrichment, Rb-enrichment, CaO-depletion, MgO-depletion and by no shift in the rare earth element (REE) patterns. Field, geochemical and isotopic data suggest that ultramafic, mafic and silicic layers represent repeated sequences of cumulates, basic and acidic rocks similar to macrorhythmic units of mafic silicic layered intrusions. The ultramafic layers recall the evolved cumulates of Skaergaard and Pleasant Bay mafic silicic layered intrusions. Mafic layers resemble Thingmuli tholeiites and chilled Pleasant Bay mafic rocks. Silicic layers with Na₂O: 4–6 wt%, SiO₂: 67–71 wt% were likely oligoclase-rich adcumulates common in many mafic silicic layered intrusions. Some amphibolite showing a strong Ti-, P-depletion and REE-depletion are interpreted as early cumulates nearly devoid of ilmenite and phosphates. All Monte Plebi rocks have extremely low Nb, Ta, Zr, Hf content and high LILE/HFSE ratios, a feature inherited from the original mantle sources. The mafic and ultramafic layers show slight and strong LREE enrichment respectively. Most mafic layer samples plot in the field of continental tholeiites in the TiO₂–K₂O–P₂O₅ diagram and are completely different from N-MORB, E-MORB and T-MORB as regards REE patterns and Nd, Sr isotope ratios but show analogies with Siberian, Deccan and proto-Atlantic rift tholeiites. Comparisons with Thingmuli, Skaergaard and Kiglapait rocks and with experimental data suggest that the Monte Plebi intrusion was an open-to-oxygen system with fO₂ FMQ. Mafic and ultramafic samples yielded _Nd(460)=+0.79 /+3.06 and ⁸⁷Sr/⁸⁶Sr=0.702934–0.703426, and four silicic samples _Nd(460)=–0.53/–1.13; ⁸⁷Sr/⁸⁶Sr=0.703239–0.703653. Significant differences in Nd isotope ratios between mafic and silicic rocks prove that both groups evolved separately in deeper magma chambers, from different mantle sources, with negligible interaction with crustal material, and were later repeatedly injected within a shallower magma chamber. The spectrum of Sr and Nd isotope data is consistent with a slightly enriched mantle metasomatized during an event earlier than 460 Ma. The metasomatising component was represented by alkali-Th-rich fluids of crustal origin rather than by sedimentary materials, able to modify alkali and Sr–Nd isotope systematics. Monte Plebi layered amphibolites might represent the first example of a strongly metamorphosed fragment of an early Paleozoic mafic silicic layered intrusion emplaced in a thinning continental crust and then tectonically dismembered by Variscan orogeny.     

Inferred phase relations in part of the system Au–Ag–Te: an integrated analytical study of gold ore from the Golden Mile, Kalgoorlie, Australia

Summary Integrated X-ray powder diffraction, scanning electron microscopy, electron probe, and transmission electron microscopy studies have identified the rare contact assemblage calaverite–sylvanite–hessite in a sample of gold ore from the Golden Mile deposit, Kalgoorlie, Australia. The presence of coexisting calaverite–hessite at Kalgoorlie is a non-equilibrium assemblage whereby the stable hessite-bearing assemblage is hessite–sylvanite, which formed from the breakdown of the -phase or -phase below 120°C, stützite+-phase, or sylvanite+stützite+-phase, as predicted by Cabri (1965).     

Magmatic inclusions in rhyolites,contaminated basalts,and compositional zonation beneath the Coso volcanic field,California

Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO₂) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO₂, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO₂-rich mafic lavas erupted near the Sierra Nevada fault zone.Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO₂). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted.The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller T. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.     

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D₂ shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D₃ shearing and intense quartz veining. The development of a S₂–S₃ crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D₂ shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D₃ is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S₃. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ¹⁸O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D₂ deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D₃ deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

Rb−Sr- und K−Ar-Isotopen-Alter an Pegmatiten aus Kor- und Saualpe,SE-Ostalpen, Österreich

Zusammenfassung An sieben Pegmatiten aus der Kor- und Saualpe wurden insgesamt 12 Rb–Sr-Gesamtgesteins-Analysen durchgeführt, an Muskoviten aus den Pegmatiten sind 16 Rb–Sr-Analysen und 15 K–Ar-Bestimmungen gemacht worden.Alle Pegmatite haben zwei Hellglimmergenerationen, die deutlich unterscheidbar sind.Mit der Rb–Sr-Methode ergeben die großen Muskovite (bis 20 cm ) in acht Fällen praealpidische Alterswerte: 240 bis 265 M, J. Mit der gleichen Methode wird an den Muskoviten der zweiten Generation (bis 3 cm ) alpidisches Alter bestimmt: 72 bis 122 M. J.Die Rb–Sr-Gesamtgesteins-Analysen deuten auf ein geöffnetes System für Rb–Sr in alpidischer Zeit, vergleichbar mit den Paragneisen (Morauf, 1980b).Die K–Ar-Bestimmungen an den Muskoviten beider Generationen ergeben alpidische Alterswerte; 71–116 M. J. Es ist ein Einfluß der Korngröße auf den Alterswert feststellbar sowie Unterschiede zwischen Kern und Rand bei den Muskoviten der ersten Generation.Prae-alpidische Mineral-Alter sind im Bereiche der Kor-und Saualpe nur noch in großen Pegmatit-Muskoviten nachweisbar, da durch eine altalpidische druckbetonte Metamorphose (vor 80 M. J.) nicht nur kleine Glimmer verjüngt/gebildet wurden, sondern auch die Gesamtgesteine zum teilweise offenen System für Rb und Sr wurden. Die prägende Deformation muß nach den vorliegenden Daten ebenfalls altalpidisch sein.

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Rb–Sr and K–Ar isotopic ages of pegmatites from Koralpe and Saualpe, South-Eastern Alps, Austria

Summary 12 Rb–Sr-whole rock analyses on seven pegmatites of the Kor- and Saualpe were performed, as well as 16 Rb–Sr-determinations and 15 K–Ar measurements on the white micas of these pegmatites. All the pegmatites show clearly two generations of mica growth.The white mica from the first generation with books up to 20 cm yielded eight pre-Alpidic Rb–Sr-ages between 240 and 265 my. The second generation of white micas ( up to 3 cm) give Alpidic ages of 72–122 my.The K–Ar-determinations on white micas of both generations yielded only Alpidic ages within the range of 71–116 my. Both grainsize as well as position in the large grains (core-rim) influence the age value: Smaller grains and rims of first generation muscovite giving younger ages, than the larger grains as well as cores of the first generation.The Rb–Sr-whole-rock-analyses point to an open system for Rb and Sr during the early Alpidic metamorphism, similar to the paragneisses (Morauf, 1980b). Considering all the data of Kor- and Saualpe (Morauf, 1980 a, b and this paper) it can be shown that 1) pre-Alpidic mineral ages only survived in large pegmatitic muscovites; 2) During the influence of the intensive old-Alpidic metamorphism small pre-Alpidic white micas where not only rejuvenated, but new micas grew and kyanite recrystallized, while the whole rock was partially open for the Rb–Sr-system.According to the data presented in this paper the main deformation of the area has to be of Alpidic age.

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Mit 3 Abbildungen     

A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

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

A low-pressure metamorphic zonation ranging from biotite tomigmatite zones occurs in the Sierra Albarrana area (VariscanBelt of southwestern Iberian Peninsula) in uppermost Precambrianto Lower Palaeozoic metasedimentary rocks. The principal deformationin this area is related to a major ductile shear zone whosecentral part is localized immediately to the southwest of theSierra Albarrana Quartzites. The metamorphism is synchronouswith respect to this deformation. The metamorphic zones aresymmetrically distributed with respect to the Sierra AlbarranaQuartzites. Pressure–temperature (P–T) conditionsare 3.5–4 kbar and range from 400°C (biotite zone)to 500°C (staurolite–garnet zone) up to 650–700°C(migmatite zone). We have not detected pressure variations alongthe different metamorphic zones. Relic kyanite is observed inthe form of inclusions in andalusite within veins in the lower-gradepart of the staurolite–andalusite zone. The low-pressuremetamorphism of the Sierra Albarrana area arises from a two-stagehistory including moderate crustal thickening followed by subsequentlocalization of deformation in a transcurrent shear zone duringpeak P–T conditions. Channelized fluid flow within themajor ductile shear zone may have contributed to the heat budgetof the low-pressure metamorphism. KEY WORDS: fluid flow; Iberian Massif; low-pressure metamorphism; shear zone; Sierra Albarrana area     

Compositional zonation in garnets in peridotite xenoliths

Garnets in 42 peridotite xenoliths, most from southern Africa, have been analyzed by electron probe to seek correlations between compositional zonation and rock history. Xenoliths have been placed into the following 6 groups, based primarily upon zonation in garnet: I (12 rocks)-zonation dominated by enrichment of Ti and other incompatible elements in garnet rims; II (10 rocks)-garnet nearly homogeneous; III (8 rocks)-rims depleted in Cr, with little or no related zonation of Ti; IV (3 rocks)-slight Ti zonation sympathetic to that of Cr; V (3 rocks)-garnet rims depleted or enriched in Cr, and chromite included in garnet; VI (6 rocks)-garnets with other characteristics. Element partitioning between olivine, pyroxene, and garnet rims generally is consistent with the assumption of equilibrium before eruption. Although one analyzed rock contains olivine and pyroxene that may have non-equilibrated oxygen isotopes, no corresponding departures from chemical equilibrium were noted. Causes of zoning include melt infiltration and changes in temperature and pressure. Zonation was caused or heavily influenced by melt infiltration in garnets of Group I. In Groups III, IV, and V, most compositional gradients in garnets are attributed to changes in temperature, pressure, or both, and gradients of Cr are characteristic. There are no simple relationships among wt% Cr₂O₃ in garnet, calculated temperature, and the presence of compositional gradients. Rather, garnets nearly homogeneous in Cr are present in rocks with calculated equilibration temperatures that span the range 800–1500 °C. Although the most prominent Cr gradients are found in relatively Cr-rich garnets of rocks for which calculated temperatures are below 1050 °C, gradients are well-defined in a Group IV rock with T1300 °C. The variety of Cr gradients in garnets erupted from a range of temperatures indicates that the zonations record diverse histories. Petrologic histories have been investigated by simulated cooling of model rock compositions in the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃. Proportions and compositions of pyroxene and garnet were calculated as functions of P and T. The most common pattern of zonation in Groups III and IV, a decrease of less than 1 wt% Cr₂O₃ core-to-rim, can be simulated by cooling of less than 200 °C or pressure decreases of less than 1 GPa. The preservation of growth zonation in garnets with calculated temperatures near 1300 °C implies that these garnets grew within a geologically short time before eruption, probably in response to fast cooling after crystallization of a small intrusion nearby. Progress in interpreting garnet zonations in part will depend upon determinations of diffusion rates for Cr. Zonation formed by diffusion within garnet cannot always be distinguished from that formed by growth, but Ca–Cr correlations unlike those typical of peridotite suite garnets may document diffusion.     

Corundum formation by metasomatic reactions in Archean metapelite,SW Greenland:Exploration vectors for ruby deposits within high-grade greenstone belts

Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO₂ in combination with addition of Al₂O₃, MgO, K₂O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO₂. The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO₂ allowing corundum to stabilize in the former rocks. Furthermore, addition of Al₂O₃ via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface.     

Pan-African high pressure granulites from SE-Kenya: Petrological and geothermobarometric evidence for a polycyclic evolution in the Mozambique belt

Two different Pan-African tectono-metamorphic events are recognised in the Taita Hill Tsavo East National Park/Galana river area, SE-Kenya (Mozambique belt) based on petrographic and geothermobarometric evidence. Structurally, this area can be subdivided into four units: (1) the easternmost part of the basement along the Galana river is characterized by subhorizontal slightly to the west and east dipping foliation planes. Migmatic paragneisses with intercalated marbles, calcsilicates and metapelites and bands of amphibolites are the dominant rock type. (2) The western part of the Galana river within the Tsavo East National Park is a ca. 25 km wide shear zone with subvertical foliation planes. The eastern part shows similar rocks as observed in unit 1, while towards west, metasedimentary units become rare and the main rock types are tonalitic gneisses with intercalated amphibolites. (3) A 10 km wide zone (Sagala Hills zone) between the strike slip zone (unit 2) and the Taita Hills (unit 4) is developed. This zone is characterized by elongated and folded felsic migmatic amphibole and garnet bearing orthogneiss bodies with intercalated bands of mafic rocks. (4) The Taita Hills are a slightly to the N dipping nappe stack. The main rock type in the Taita Hills are amphibole–biotite–plagioclase–quartz ± garnet ± clinopyroxene ± scapolite bearing migmatic gneisses with mafic bands. In the southern part, metapelites, marbles and some amphibolites are common.Although the geological structures are different in units 1 and 2, the calculated PT conditions are similar with peak PT of 760–820 °C and 7.5–9.5 kbar. Temperatures in unit 3 (Sagalla Hills zone) and unit 4 (Taita Hills) are slightly higher ca. 760–840 °C, but pressure is significantly higher, ranging from 10 to 12 kbar. Sillimanite growth around kyanite, garnet zonation pattern, mineral reaction textures, and PT calculations constrain a “clock-wise” PT-path with near isobaric cooling following the peak of metamorphism. The different PT conditions, tectonic setting, and a different age of metamorphism are evidence that units 1 and 2 (Galana river) belong to a different tectono-metamorphic event than unit 3 (Sagala Hills zone) and 4 (Taita Hills). The major shear zone (unit 2) marks a tectonic suture dividing the two different tectono-metamorphic domains. It is also likely that it played an important role during exhumation of the granulite facies rocks from units 3 and 4.     

Late Paleozoic retrograded eclogites from within the northern margin of the North China Craton: Evidence for subduction of the Paleo-Asian ocean

Retrograded eclogites from the central part of the northern margin of the North China Craton, Hebei Province, China occur as separate tectonic lenses or boundins within garnet–biotite–plagioclase gneisses of the Paleoproterozoic Hongqiyingzi Complex characterized by amphibolite facies paragneisses. The petrographic features and mineralogical compositions represent three main metamorphic stages: (1) the peak eclogite facies stage (P > 1.40–1.50 GPa, T = 680–730 °C), (2) the granulite facies stage and (3) the amphibolite facies stage (P = 0.67–0.81 GPa, T = 530–610 °C) formed during decompression. The major and trace element and Sm–Nd isotopic data suggest that most of the retrograded eclogite samples had protoliths of tholeiitic oceanic crust with geochemical characteristics of mid-ocean ridge basalt (MORB) or island arc tholeiite (IAT) environment, and were contaminated by crustal components during subsequent subduction. Zircon SHRIMP isotopic dating of two different textural varieties of retrograded eclogite defines a weighted mean age of 325 Ma, which is interpreted as the peak metamorphic age of the eclogites and reflects the occurrence of eclogite facies metamorphism related to subduction of Paleo-Asian Oceanic crust beneath the North China Craton during the Late Paleozoic. Finally, we show that the retrograded eclogite from Hebei Province is not related to the Baimashi retrograded eclogite at the northern foot of the Heng Mountains, approximately, 300 km to the southwest.     

Geochemistry and argon thermochronology of the Variscan Sila Batholith,southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral ⁴⁰Ar/³⁹Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO₂ content in the granitic rocks ranges from 57 to 77 wt%; cumulate gabbro enclaves have SiO₂ as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to 300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (50–2100), as well as low values of Nb/U (2–28) and La/Th (1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock ¹⁸O ranges from +8.2 to +11.7; the mafic cumulate enclaves have the lowest ¹⁸O values and the two-mica granites have the highest values. ¹⁸O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7). The initial Pb isotopic range of the granitic rocks (²⁰⁶Pb/²⁰⁴Pb 18.17–18.45, ²⁰⁷Pb/²⁰⁴Pb 15.58–15.77, ²⁰⁸Pb/²⁰⁴Pb 38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.     

Paragonite stability at 700°C in the presence of H<Subscript>2</Subscript>O–NaCl fluids: constraints on H<Subscript>2</Subscript>O activity and implications for high pressure metamorphism

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H₂O at 700°C, 1.5–2.5 GPa, in the presence of H₂O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H₂O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H₂O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H₂O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30–2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293–301, 1979). Lowering X(H₂O) decreases the pressure of paragonite breakdown to 2.10–2.20 GPa at X(H₂O)=0.90 and 1.85–1.90 GPa at X(H₂O)=0.75. The experiments at X(H₂O) = 0.62 yielded the assemblage albite + corundum at 1.60 GPa, and jadeite + kyanite at 1.70 GPa. This constrains the position of the isothermal paragonite–jadeite–kyanite–albite–corundum–H₂O invariant point in the system Na₂O–Al₂O₃–SiO₂–H₂O to be at 1.6–1.7 GPa and X(H₂O)~0.65±0.05. The data indicate that H₂O activity, a(H₂O), is 0.75–0.86, 0.55–0.58, and <0.42 at X(H₂O)=0.90, 0.75, and 0.62, respectively. These values approach X(H₂O)², and agree well with the a(H₂O) model of Aranovich and Newton (Contrib Miner Petrol 125:200–212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H₂O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7–2.0 GPa, 550–650°C. The absence of paragonite requires a fluid with low a(H₂O) of 0.3–0.6, which could be due to the presence of saline brines.     

Field and geochemical characteristics of the Mesoarchean ( 3075 Ma) Ivisaartoq greenstone belt, southern West Greenland: Evidence for seafloor hydrothermal alteration in supra-subduction oceanic crust

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075 Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside) + quartz + plagioclase ± hornblende ± scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside + garnet + amphibole + plagioclase + quartz ± vesuvianite ± scapolite ± epidote ± titanite ± calcite ± scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial ε_Nd values of + 1.3 to + 5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents Mesoarchean supra-subduction zone oceanic crust.     

Geochemical and Sr-Nd-Pb isotopic characteristics of Late Cenozoic leucite lamproites from the East European Alpine belt (Macedonia and Yugoslavia)

In the East European Alpine belt, leucite-sanidine-phlogopite-olivine-bearing volcanic rocks of Late Cenozoic age occur at eight localities within the Vardar suture zone and at one locality in the Southern Carpathian fold-and-thrust belt. Most of these volcanics are characterized by high Mg# (66.6–78.6), high abundances of Ni (117–373 ppm) and Cr (144–445 ppm) as well as high primary K₂O contents (5.63–7.01 %) and K₂O/Na₂O values (1.93–4.91). Rocks with more differentiated compositions are rare. A lamproite affinity of these rocks is apparent from their relatively low contents of Al₂O₃ (9.9–14.3 wt%) and CaO (6.2–8.3 wt%) in combination with high abundances of Rb (85–967 ppm), Ba (1,027–4,189 ppm), Th (18.9–76.5 ppm), Pb (19–54 ppm), Sr (774–1,712 ppm) and F (0.16–0.52 wt%), and the general lack of plagioclase. Although eruption of the magmas took place in post-collisional extensional settings, significant depletions of Nb and Ta relative to Th and La, low TiO₂ contents (0.92–2.17 %), low ratios of Rb/Cs, K/Rb and Ce/Pb as well as high ratios of Ba/La and Ba/Th suggest close genetic relationships to subduction zone processes. Whereas Sr and Nd isotope ratios show relatively large variations (⁸⁷Sr/⁸⁶Sr = 0.7078–0.7105, ¹⁴³Nd/¹⁴⁴Nd = 0.51242–0.51215), Pb isotope ratios display a very restricted range with ²⁰⁶Pb/²⁰⁴Pb = 18.68–18.88 and variable but generally high 7/4 (11–18) and 8/4 (65–95) values. The observed petrographic, geochemical and isotopic characteristics are best explained by a genetic model involving preferential melting of phlogopite-rich veins in an originally depleted lithospheric mantle source, whereby the metasomatic enrichment of the mantle source is tentatively related to the addition of components from subducted sediments during consumption of Tethyan oceanic lithosphere.Editorial responsibility: J. Hoefs