Symplectitic Coronas Research Articles

Compositional zoning and evolution of symplectite coronas in jadeitite

Abstract Multistage fluid activities play an important role in the interaction between jadeitite and symplectite coronas; therefore, we studied the compositional zoning and evolution of representative Myanmar jadeitite. Under the influence of multistage Ca-, Na-, and Si-rich fluid activity, some minerals in Myanmar jadeitite formed symplectite coronas with concentric rings and multilayer metasomatic reaction rim structures. Additionally, the concentrations of Cr and Fe decrease from the core to the peripheral jadeite minerals, whereas the concentration of Si markedly increases. There is almost no Si or Ca in the chromite core, and the concentrations of Si and Ca increase sharply in rims composed of uvarovite. Due to Cr diffusion, the edge of the jadeite adjacent to kosmochlor is Cr-rich and Al-poor. The different element concentrations indicate that the uvarovite formed from the presence of a chromite and jadeite interaction, Si in the kosmochlor after metasomatism or an external Ca-rich fluid. One possible explanation for the formation of kosmochlor is the interaction between chromite and a Na- and Si-rich fluid. Also, Ca-rich fluid could have first interacted with chromite and formed uvarovite; subsequently, a Na-rich fluid could have entered and become saturated with kosmochlor, leading to the formation of kosmochlor surrounding the uvarovite.

Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons

High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 °C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 °C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819 ± 14, and igneous zircon cores of 3127 ± 14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090 ± 13 Ma (LA-ICP-MS) and 3125 ± 15 Ma (SHRIMP) with age cluster at 3120 ± 6 Ma. εHf(t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf(t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 m.y. younger (2794 ± 13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.

Ni-Fe-Cu-PGE ore deposition driven by metasomatic fluids and melt-rock reactions in the deep crust: The ultramafic pipe of Valmaggia, Ivrea-Verbano, Italy

The Valmaggia ultramafic pipe belongs to a group of Fe-Ni-Cu-PGE-rich, hydrous ultramafic intrusions emplaced within the Ivrea-Verbano Zone (Italy). According to the most recent geodynamic models these early Triassic ultramafic pipes represent a network of magma conduits derived from partial melting of continental lithospheric mantle metasomatized by fluids and melts related to the Eo-Varisican subduction. New extensive mapping and sampling of the Valmaggia pipe provides evidence of the close relationship between metasomatism and Fe-Ni-Cu-PGE mineralization. New geological data allow a 3D geometrical model of the pipe that intrudes the Main Gabbro domain. The pipe has a sub-vertical, tube-like main body with at least three lateral branches. Along its rims the pipe displays magmatic, soft contacts with an almost continuous meter-sized envelope of a peculiar marginal rock facies (“plagioclasite”) consisting of Na-bearing plagioclase, biotite and apatite and, locally, with pegmatoidal texture. The peridotite pipe consists of a primary anhydrous mineral assemblage comprising coarse-grained olivine, diopside, enstatite and Cr-rich spinel. These minerals are extensively overprinted by a metasomatic assemblage comprising Ti-rich pargasitic amphibole and phlogopite oikocrysts, enstatite, Al-rich spinel and PGE-bearing Fe-Ni-Cu sulfides, with accessory carbonates, Cl-F-rich apatite, zircon and monazite. Sulfide mineralization is preferentially located along pipe margins, and, subordinately, in the nearby plagioclasite contact rock. Sulfides are strictly associated with the metasomatic assemblage and occur as disseminated mm-sized droplets, network-like aggregates and cm-sized amoeboidal nodules rimmed by carbonates. Several features suggest that plagioclasite interacted with the ultramafic pipe, and infiltrated it, as a magma. Plagioclasite in contact with the pipe rock frequently displays a mottled texture, similar to that of gabbro, and where dark nodules, with ultramafic cores and pargasite- spinel-phlogopite-bearing symplectite coronas, are densely disseminated in white plagioclasite-rich groundmass. Metasomatic assemblages and symplectite textures, suggesting disequilibrium between pipe minerals and plagioclasite components, are widely developed also along the pipe margins and interstitial plagioclase pockets within the pipe. The metasomatic minerals provide calculated PT conditions ranging between 680° and 870°C and between 4 and 8kbar. Such conditions are compatible with the latest geodynamic model for the Ivrea Zone pipes proposing their emplacement in the lower crust. These PT constraints also likely apply to the conditions of ore deposition because of the strict relationships between the metasomatic assemblage and the sulfides. Carbonate crystals that are associated with sulfide-rich portions of the pipe are likely the product of CO2 vapor sticking to the sulfide melt surface resulting in their net buoyancy and ascent in the melt. The textural and chemical characteristics of the pipe mineral assemblage and sulfide mineralization suggest a polyphasic evolution dominated by metasomatism during interaction between magmas. At Valmaggia the alkali- and volatile-rich, metasomatizing magma may be analogous to slab-derived melts envisaged in the most recent geodynamic model for IVZ. Percolation of a metasomatic melt favours Valmaggia pipe-like geometries, including a tubular main body with lateral appendages.

Petrogenetic relations among titanium‐rich minerals in an anatectic high‐P mafic granulite

AbstractThe petrogenetic relations among Ti‐rich minerals in high‐grade metabasites is illuminated here through a detailed petrological investigation of an anatectic garnet–clinopyroxene granulite from the Grenville Province, Ontario, Canada containing rutile, titanite and ilmenite in distinct microtextural settings. Garnet porphyroblasts exhibit zoned Ti concentrations (up to 0.15 wt% TiO2 in their cores), as well as a variety of rutile inclusion types, including clusters of small, variably elongate grains and thin (≤1 μm) oriented needles. Calcite inclusions in garnet, commonly observed surrounding garnet cores containing quartz and clinozoisite, indicate the presence of evolving C–O–H fluids during garnet growth and suggest that the rutile clusters may have formed from subsequent Ti diffusion and rutile precipitation within existing fluid inclusions. Titanite forms large subhedral crystals and typically occurs where the primary garnet–clinopyroxene assemblage is in contact with leucosome containing megacrystic hornblende, silvialitic scapolite and calcic plagioclase. Many titanite crystals exhibit marginal subgrains that correspond with sharp changes in their major and trace element composition, likely related to a dissolution–precipitation or recrystallization process following primary crystallization. Clinopyroxene–ilmenite symplectite coronas surround titanite in most locations, likely forming from reaction with the hornblende‐plagioclase matrix (±fluids/melt). Integration of multi‐equilibria thermobarometry and Zr thermometry in rutile and titanite with phase equilibrium modelling allows definition of a clockwise P–T path evolving to peak pressures of ~1.5 GPa at ~750°C during garnet and rutile growth, followed by peak temperature conditions of ~1.2 GPa and ~820–880°C associated with melt‐present titanite growth, and finally cooling and decompression to regional amphibolite facies conditions (~1.0 GPa and ~750°C) associated with the formation of clinopyroxene–ilmenite symplectites surrounding titanite. P–T pseudosections calculated for the pristine (leucosome‐ and titanite ‐free) metabasite bulk composition reproduce much of the prograde phase relations, but predict rutile as the stable Ti‐rich mineral at the peak thermal conditions associated with melt‐present titanite growth. The P–M(CaO) and T–M(CaO) models show that bulk CaO concentrations have a significant effect on the stability ranges of titanite and rutile. Increased bulk CaO tends to stabilize titanite to higher pressure and temperature at the expense of rutile, with a ≥15% increase in CaO producing the observed titanite‐bearing assemblage at high‐P granulite facies conditions. Thus, the model results are consistent with the textural observations, which suggest that titanite stability is associated with a chemical exchange between the host metabasite and a Ca‐rich melt.

Metamorfismo de temperatura ultra-alta de granulitos com safirina de 2,0 Ga do Bloco Itabuna-Salvador-Curaçá, Bahia, Brasil

A área de pesquisa se situa no Cráton do São Francisco, no sul da Bahia, Brasil. Nessa região, no segmento crustal Bloco Itabuna-Salvador-Curaçá (BISC), ocorrem rochas supracrustais associadas a gnaisses tonalíticos/trondhjemíticos a monzoníticos e rochas máficas subordinadas, de idades arqueana e paleoproterozoica, todos fortemente deformados e reequilibrados na fácies granulito em 2,07 a 2,08 Ga. As rochas supracrustais incluem granulitos com safirina constituídos por quartzo, ortopiroxênio, silimanita, granada, mesopertita, plagioclásio, biotita e cordierita. Com base nos arranjos microestruturais entre os minerais constituintes dos granulitos com safirina, a paragênese de pico metamórfico é Grt1 + Opx1 + Bt1 + Qtz + Sil1 + Spr1 + Mp. Microestruturas de reações entre minerais, como coroas de simplectitos ao redor de porfiroblastos, permitiram identificar as seguintes paragêneses retrógradas, interpretadas como formadas durante a ascensão tectônica dessas rochas: Grt1 ± Qtz = Opx2 + Sil2, Grt1 = Opx2 + Spr2 (+ Sil2); Opx1 + Sil1 + Qtz = Crd e Grt1 + Qtz = Opx3 + Crd. Por sua vez, simplectitos de Bt2 + Qtz formados pela reação Opx1-2 + Mp + H2O = Bt2 + Qtz estão relacionados à evolução retrógrada durante resfriamento. Cálculos termobarométricos realizados com aplicação de termobarômetros clássicos e método de multiequilíbrios (THERMOCALC, por exemplo) indicaram P = 7-11 kbar e T ≥ 900°C para os granulitos com safirina. Um dos mecanismos associados à elevada temperatura está relacionado à adição de material radioativo com a presença de magmas basálticos durante a formação do BISC. A cristalização fracionada desses magmas produziu intrusões tonalíticas que datam de aproximadamente de 2,15 a 2,13 Ga, próximo do pico do metamorfismo de 2,07 a 2,08 Ga.

Metamorphic P–T–t paths of the Zanhuang metamorphic complex: Implications for the Paleoproterozoic evolution of the Trans-North China Orogen

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N–S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M1–M3). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase+quartz±biotite±apatite±magnetite±rutile±ilmenite define the prograde metamorphic stage (M1). The assemblage of the peak metamorphic stage (M2) consists of biotite+plagioclase+quartz±kyanite±hornblende±magnetite±ilmenite in equilibrium with the lowest-XMn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M3) and the development of symplectitic coronas of biotite+plagioclase+quartz±hornblende±magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P–T conditions of 4.5–5.9kbar and 551–596°C for stage M1, 9.6–12.3kbar and 770–830°C for stage M2, and ∼4.7kbar and ∼635°C for stage M3. The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P–T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U–Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to ∼2507Ma, and the second age dates to ∼1839Ma. The first age group (∼2507Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (∼1839Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO).

Forward modeling corona growth in a partially eclogitized leucogabbro, Bourbon Island, North-East Greenland

Coronitic textures are common in partially eclogitized igneous bodies, such as gabbros, leucogabbros, and anorthosites, east of the Germania Land Deformation Zone in North-East Greenland. Coronas formed by prograde metamorphic processes that transformed the gabbroic bodies to eclogite facies, and record frozen stages of the prograde metamorphic evolution of these rocks. A metaleucogabbro-norite body on Bourbon Island in Jøkelbugt is characterized by three concentric areas: a coronitic core, a mottled inner rim with areas of completely eclogitized material surrounded by a matrix of coronitic metaleucogabbro, and an outer rim of strongly foliated and completely retrogressed amphibolite. The Bourbon body preserves four stages of the prograde metamorphic history: Stage I, Stage II, Stage III, and Eclogite Stage. Stage I coronas are found only in the core of the body, which is the least reacted part of the leucogabbro-norite and the closest to the protolith, and is characterized by the corona sequence Pl rim/Grt + Kfs + Amp/Grt + Amp/Cpx rim. The typical corona sequence for Stage II is Pl rim/Grt + Pl + Zo/Cpx rim/Amp rim. Stage III samples show a Pl rim + Ky + Scp/Grt + Pl + Qtz/Qtz + Pl sequence, with the relict clinopyroxene being replaced in part by microcrystalline aggregates of Cpx + Amp + Pl. The Eclogite Stage shows relict Pl completely replaced by Grt, and the relict Cpx completely replaced by aggregates of Omp + Pl + Kfs + Amp. We tested open-system grain boundary diffusion (OSGBD) theories to model the prograde Stage I symplectitic coronas. The observed ratio of the thickness of the different layers is Pl rim:Grt + Kfs + Amp:Grt + Amp:Cpx rim equal to 3:1.3:0.95:0.5. These ratios are very close to the modeled ones of 2.7:1.1:1:0.5. Furthermore, subtle textural changes within the Grt + Kfs + Amp corona were also reproduced by the model. The model gave us insight into the conditions of the metamorphic system in which the coronas formed. The sequence Pl rim/Grt + Kfs + Amp/Grt + Amp/Cpx rim formed by diffusion driven reactions in an open system involving gain of Fe, K, and Na, and loss of Ca and Mg at the original clinopyroxene–plagioclase boundary. Relative mobilities of the different components within the corona layers were L MgMg > L AlAl > L SiSi > L CaCa > L KK > L FeFe > L NaNa. Fluid circulation was active to some degree during the transformation to eclogite. The differences between core, inner rim, and the two domains within the inner rim of the metaleucogabbro-norite can be explained by different degrees of fluid circulation in different portions of the rock. The presence of phases containing Cl and P, such as scapolite, in completely eclogitized samples supports the presence of fluid circulation in the system. Another possible explanation for the mottled appearance of the inner rim is protolith heterogeneity.

Monazite and xenotime petrogenesis in the contact aureole of the Makhavinekh Lake Pluton, northern Labrador

High-temperature (700–900°C) metamorphism in the contact aureole of the Makhavinekh Lake Pluton (MLP), northern Labrador, led to the growth of monazite and xenotime during progressive replacement of regional garnet-bearing assemblages (M1) by lower-pressure symplectitic coronas of orthopyroxene + cordierite (M2). In the inner aureole (<500 m from the contact), where M1 garnet is strongly resorbed, high-Y+HREE monazite (XY+HREE 0.14–0.18) occurs as small isolated grains and as discontinuous rims on partially resorbed pre-M2 monazites that were liberated from garnet. Xenotime also occurs as small isolated grains within M2 coronas. Ion-microprobe dating of thin, high-Y rims indicates that new monazite growth occurred during M2. Monazite–xenotime miscibility-gap temperatures are consistent with Al-solubility-in-orthopyroxene thermometry estimates, indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite. M2 monazite records, therefore, the temperature and timing of M2 metamorphism. Two net-transfer reactions, modelled using singular value decomposition in the system P-Y-HREE-LREE, are proposed to account for the growth of M2 phosphates: (1) 38 Grt1 + 1 Mnz1 = 1.13 Mnz2 and (2) 737 Grt1 + 1 Ap = 1 Mnz2 + 3.4 Xno2. Reaction (1) conserves P and gave rise to locally coronitic high-Y overgrowths on partially resorbed pre-M2 monazite, whereas reaction (2) accounts for the growth of small new monazite and xenotime grains. Both reactions were highly localized within individual M2 coronas due to slow intergranular diffusion accompanying fluid-undersaturated metamorphism in the MLP aureole. Similar monazite-forming reactions are expected in other polymetamorphosed granulites.

Impact‐induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites

AbstractMid‐crustal Archean pelitic granulites in the Vredefort Dome experienced a static, low‐P granulite facies overprint associated with the formation of the dome by meteorite impact at 2.02 Ga. Heating and exhumation were virtually instantaneous, with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave. Maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 °C and locally even exceeded 1350 °C. This led to comprehensive melting of the precursor Archean granulite assemblages (Grt + Bt + Qtz + Pl + Ksp ± Crd ± Opx ± Sil) followed by peritectic crystallization of aluminous alkali feldspar+Crd + Spl ± Crn ± Sil parageneses and the segregation of small, evolved, biotite leucogranite bodies. However, at a distance of c. 6 km from the centre pre‐impact rock features are largely preserved, although partial replacement of garnet by symplectitic coronas of Crd + Opx ± Spl ± Pl and biotite by orthopyroxene indicate that peak temperatures approached 775 ± 50 °C. Thin interstitial moats of K‐feldspar are closely associated with the orthopyroxene coronas; they are interpreted as the remnants of low‐proportion partial melts generated by biotite breakdown. Both the textures and mineral compositional data support reduced equilibration volumes in these rocks, which reflect rapid isobaric cooling following shock heating and exhumation. The high temperatures and strong lateral thermal gradient are consistent with the modelled impact‐induced isotherm pattern for a 200–300 km diameter impact crater.

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P– T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P– T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.

GARNET MICROSTRUCTURES FROM AN ULTRA-DEEP (>185 KM) OROGENIC PERIDOTITE

Relic majoritic (or super-silicic) garnets were recently discovered within orogenic garnet peridotites at the island of Otroy, Western Gneiss Region, W. Norway (Van Roermund and Drury, 1998; Van Roermund et al., 1999). However within the Otroy garnet peridotites relic majoritic garnet microstructures are rare and the petrogenetic relationship( s) between relic majoritic- and “normal” garnet is unknown. For this reason we have made a detailed SEM study of microstructures and solid-phase inclusions present exclusively within all garnets from the Otroy garnet peridotites. MAJORITIC GARNET MICROSTRUCTURES AND CHEMISTRY The super-silicic garnet microstructure is characterised by the following microstructural elements: a) Majoritic garnet nodules consist of polycrystalline garnets ranging in size from 2 to 8 mm. Garnet-garnet grain boundaries are straight or gently curved, triple point junctions are common, interstitial orthopyroxenes decorate some garnet-grain boundaries. b) In the larger garnet grain cores two pyroxene exsolution needles occur 5-10 mm thick and oriented // grt. The relative vol% between exsolved cpx and opx is 1: 9. c) Two-pyroxene-bearing garnet-cores are surrounded by 2 mm thick precipitation-free rims. Backscattered scanning electron microscopy was used to estimate the amount of majorite component present in garnet nodules before exsolution. Large interstitial orthopyroxenes at garnet grain boundaries comprise up to 3.6 vol.%. In the garnet grain cores the maximum pyroxene content is 1 vol.%. Including both types of pyroxene-exsolution the maximum amount of exsolved pyroxene is 3.6-4.0 vol.%. This corresponds to pressure estimates around 6-6.5 GPa and a minimum depth of origin for the Otroy garnet peridotites in the range of 185-200 km. Microprobe analyses of exsolved pyroxenes and garnethost reveal homogeneous mineral compositions, except for Al2O3 wt% in opx. The most dominant Al2O3 wt% value in opx is 0.7-0.8. Estimated PT conditions for mineral-chemical equilibration, using standard geothermobarometric methods, indicate upper mantle conditions around 805 ± 40°C and 3.2 ± 0.2 GPa. These PT estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory extends backwards to twice as great a depth-range than can be obtained by thermobarometric methods. In addition to the exsolved two pyroxenes we have found native nickel-particles as solid inclusions in relic majoritic garnets. This is indicative for relatively reduced oxygen fugacity conditions. The nickel particles are surrounded by complex symplectitic coronas involving Ni-Al oxide, Orthopyroxene, SiO2 and Ni-bearing garnet. NON-MAJORITIC GARNET MICROSTRUCTURES AND CHEMISTRIES The following solid-phases occur as inclusions in “normal” pyrope-rich garnets: a) Ni-Fe-Cu sulfides. Two types occur: 1) co-linear trails of rounded particles less than 1 mm in size and interpreted to represent precipitation along healed cracks. 2) Coarsened irregular-shaped inclusions up to 50 mm in size. The latter are often present in clusters. b) (Fe-) Ti (oxide??) needles (oriented // grt). c) (Fe-) Ti -oxides randomly precipitated or decorating dislocations. d) Isolated spinel needles and precipitates. With respect to the garnet host two types occur: oriented and randomly distributed crystals. e) Recrystallised garnet 2 - spinel - cpx/amphibole - orthopyroxene (± Ni-Fe-Cu sulfides) mineral assemblages. In addition to the various solid-garnet-inclusions garnetdeformation microstructures are present including naturally decorated dislocations. In thin-section garnet is often heavily fractured, and a penetrative crack-spacing down to the micron-scale is not uncommon. More widely-spaced or more heterogeneously fractured garnets exhibit the following microstructural domains: i) in between the cracks garnet is geometrically strained and deformation-induced microstructures such as undulous extinction and cross-hatched deformation bands are visible. This is recognised by the local non-isotropic nature of garnet. Within some of the strained domains dislocations are identified due to the fact that micron-sized particles have been precipitated along the dislocation cores. TEM and attached analytical facilities demonstrate that the solid inclusions are titanium-oxides, most likely rutile. ii) In between the cracks garnet is fully isotropic and strainfree. Within such domains randomly oriented solid inclusions are recognised one order of magnitude larger than the precipitates that decorate dislocations in strained garnet domains. All solid garnet inclusions and deformation microstructures predate the well-known two-pyroxene-spinel corona’s that surround garnet and form due to the instability of the mineral assemblage garnet-olivine. All garnet microstructures are consistent with a polycyclic exhumation model involving diapiric upwelling of deep-mantle peridotites to lithospheric depths followed by isobaric-cooling down to normal continental lithosphere conditions. Crustal incorporation occurs during renewedsubduction of continental crust during continental collision subsequently followed by final exhumation.

High pressure amphibolite-granulite facies metamorphism in the Paleoproterozoic Rudall Complex, central Western Australia

The Rudall Complex forms the basement of the Paleoproterozoic to Neoproterozoic Paterson Orogen, which was deformed during at least three major orogenies. The complex comprises a sequence of metamorphosed clastic and chemical sediments, and basalts, deposited in a marginal basin environment, prior to extensive felsic intrusion. Deformation of the sequence during the first “Yapungku” orogeny between 2000 and 1760 Ma, included extensive thrust stacking from ENE to WSW. The thermal peak of the associated high-pressure metamorphism was late- to post-tectonic with respect to this deformation. Thermobarometry on amphibolites and mafic granulites that contain various combinations of the minerals hornblende, plagioclase, quartz, garnet, othopyroxene and clinopyroxene, indicates that temperatures peaked at around 800°C, while pressures reached 1200 MPa. The late timing of peak temperatures together with the presence, in some amphibolites, of amphibole-plagioclase symplectitic coronas around garnet, indicates a steeply decompressive clockwise P-T-t path. Peak metamorphic pressures indicate that the crust was locally thickened by at least 40 km, during an event that showed similarities to Phanerozoic collisional tectonics. It is suggested that this deformational and metamorphic event records the collision between the Pilbara Craton and a continent to the northeast. The Paleoproterozoic evolution of Australia has previously been attributed to ensialic processes characterised by low-pressure metamorphism and anti-clockwise P-T-t paths. The metamorphic history of the Rudall Complex is not consistent with this view, and the documented high-pressure metamorphism is broadly synchronous with a medium-pressure metamorphic event in central Australia (the Strangways Orogeny), and possibly also in northwestern Australia (the Capricorn Orogeny). Collisional plate-tectonic processes played a significant role in the Paleoproterozoic evolution of northern Australia.

Contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the Grenville Province of western Labrador

Contrasting coronas around olivines, pyroxenes, and ilmenites occur in metagabbros in two adjacent terranes in the Grenville Province of western Labrador. In the parautochthonous Gagnon terrane, coronas around olivines display evidence of evolution from double coronas of orthopyroxene- and fluorine-rich pargasitic amphibole, through triple coronas of orthopyroxene – garnet (clinopyroxene) – pargasitic amphibole, to double coronas of orthopyroxene – garnet (clinopyroxene) (parentheses enclose phase occurring as inclusions in previous phase). In the allochthonous Lac Joseph terrane, coronas around olivines consist of orthopyroxene – [clinopyroxene–spinel] or orthopyroxene–[pargasitic amphibole – spinel] (brackets enclose phases occurring in symplectic intergrowths). Possible reactions that explain the development of these assemblages are investigated, and it is concluded that the system was closed at the level of the corona. The silica required for the olivine–plagioclase reaction in Gagnon terrane was generated by the FMQ reaction during cooling and controlled the rate of reaction along olivine–plagioclase contacts. Alumina was relatively immobile, resulting in the formation of corundum and spinel inclusions in plagioclases in Gagnon and Lac Joseph terranes, respectively.Evolution of the corona gabbros in Gagnon terrane took place in two stages during isobaric cooling after emplacement, whereas formation of the symplectite coronas around olivines in Lac Joseph terrane is compatible with a single-stage reaction during isobaric cooling following emplacement. It is qualitatively estimated that the garnet-bearing coronas in Gagnon terrane formed at higher pressures than the spinel-bearing coronas in Lac Joseph terrane.Greenschist- and amphibolite-facies assemblages in noncorona gabbros in Gagnon terrane occur in locations corresponding to the regional greenschist- and amphibolite-facies metamorphic zones and are accordingly interpreted as being Grenvillian features. There is no apparent Grenvillian imprint in the gabbros in Lac Joseph terrane.

Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss, North Cascades, Washington

In upper amphibolite-facies Skagit Gneiss, certain rocks, usually carrying hornblende, display post-kinematic breakdown of almandine-rich garnets to symplectite consisting of plagioclase plus biotite and/or hornblende. Other almandine-rich garnets, widespread in hornblende-free schists and gneisses, remained stable to the end of the metamorphic cycle. Analysed garnets with symplectite coronas have 14 to 30 mol.% grandite. A set of garnets free of such coronas have 6 to 9% grandite and contain relatively more pyrope.