Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

Granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high‐variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene–sillimanite–quartz‐bearing assemblages in granulite facies rocks reflects the need for bulk rock X_Mg of greater than approximately 0.60–0.65, with pressures and temperatures exceeding c. 8 kbar and 850 °C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P–T paths in ultrahigh‐temperature granulite facies terranes. However, a potentially wide range of P–T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P–T vector. Therefore, the interpretation of P–T paths in high‐grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P–T path, and in turn on tectonic models derived from these paths.     

Near-isothermal decompression within a clockwise P–T evolution recorded in migmatitic mafic granulites from Clavering Ø, NE Greenland: implications for the evolution of the Caledonides

The high grade rocks (metapelites and metabasites) of Clavering Ø represent the easternmost exposures of granulites in the Palaeozoic Caledonian Orogen of East Greenland. Mafic granulites which occur as sheet‐like bodies and lenses within metapelitic migmatites and orthogneiss complexes have experienced migmatisation and mineral equilibria which define a clockwise P–T path incorporating a near‐isothermal decompression segment. Textures demonstrate the existence of early garnet‐clinopyroxene‐melt assemblages which equilibrated at >8–11 kbar and 850–915 °C. Subsequently, decompression melting led to formation of orthopyroxene‐plagioclase‐melt assemblages at conditions below >8–11 kbar. Continued syn‐deformational decompression is indicated by a combination of both static and syn‐deformational recrystallization textures which generated finer grained orthopyroxene‐plagioclase assemblages. P–T constraints indicate these assemblages equilibrated at c. 5.0–6.5 kbar at 850–915 °C. These data are consistent with the rocks undergoing a stage of rapid tectonic‐induced exhumation involving some 3.0–4.5 kbar (c.10–12 km) uplift as part of a clockwise P–T path in a collisional setting.     

The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis,melt productivity,melt loss and the preservation of cordierite

Cordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C^?1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.     

The metamorphic evolution of metapelitic granulites from Radok Lake, northern Prince Charles Mountains, east Antarctica; evidence for an anticlockwise P–T path

Mineral textures in metapelitic granulites from the northern Prince Charles Mountains, coupled with thermodynamic modelling in the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) model system, point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde cooling (i.e. an anticlockwise P–T path). Textural evidence for the increasing temperature part of the path is given by the breakdown of garnet and biotite to form orthopyroxene and cordierite in sillimanite‐absent rocks, and through the break‐down of biotite and sillimanite to form spinel, cordierite and garnet in more aluminous assemblages. This is equated to the advective addition of heat from the regional emplacement of granitic and charnockitic magmas dated at c. 980 Ma. A subsequent increase in pressure, inferred from the break‐down of spinel and quartz to sillimanite, cordierite and garnet in aluminous rocks, is attributed to crustal thickening related to upright folding dated at 940–910 Ma. The terrane attained peak metamorphic temperatures of c. 880 °C at pressures of c. 6.0–6.5 kbar during this event. Subsequent cooling is inferred from the localised breakdown of cordierite and garnet to form biotite and sillimanite that developed in the latter stages of the same event. The textural observations described are interpreted via the application of P–T and P–T–X pseudosections. The latter show that most rock compositions preserve only fragments of the overall P–T path; a result of different rock compositions undergoing mineral assemblage changes, or changes in mineral modal abundance, on different sections of the P–T path. The results also suggest that partial melting during granulite facies metamorphism, coupled with melt loss and dehydration, initiated a switch from pervasive ductile, to discrete ductile/brittle deformation, during retrograde cooling.     

Role of H2O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane,southern West Greenland

The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M_1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M_1A to ～850 °C and 7.5 kbar at fluid‐saturated conditions. Following M_1A, the rocks retained their M_1A H₂O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ～700 °C and 6.5–7 kbar, prior to reworking during M_1B. These low H₂O contents allowed for the formation of garnet overgrowths and coronas during M_1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths.     

A multi-stage pressure—temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats,India?

Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (X_Mg>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.     

Petrology and P–T path of the Fuping mafic granulites: implications for tectonic evolution of the central zone of the North China craton

The Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton. The dominant rock types in the Fuping Complex are high‐grade tonalitic–trondhjemitic–granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks. The petrological evidence from the mafic granulites indicates three stages of metamorphic evolution. The M₁ stage is represented by garnet porphyroblasts and matrix plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende. Orthopyroxene+plagioclase symplectites and clinopyroxene+plagioclase±orthopyroxene coronas formed in response to decompression during M₂ following the peak metamorphism at M₁. Hornblende+plagioclase symplectites formed as a result of further isobaric cooling and retrograde metamorphism during M₃. The P–T estimates using TWQ thermobarometry are: 900–950 °C and 8.0–8.5 kbar for the peak assemblage (M₁), based on the core compositions of garnet, matrix pyroxene and plagioclase; 700–800 °C and 6.0–7.0 kbar for the pyroxene+plagioclase symplectites or coronas (M₂); and 550–650 °C and 5.3–6.3 kbar for the hornblende+plagioclase symplectites (M₃), based on garnet rim and corresponding symplectic mineral compositions. These P–T estimates define a clockwise P–T path involving near‐isothermal decompression for the Fuping Complex, similar to the P–T path estimated for the metapelitic gneisses. The inferred P–T path suggests that the Fuping Complex underwent initial crustal thickening, subsequent exhumation, and finally cooling and retrogression. This tectonothermal path is similar to P–T paths inferred for the Wutai and Hengshan Complexes and other tectonic units in the central zone of the North China craton, but different from anti‐clockwise P–T paths estimated for the basement rocks in the eastern and western zones of the craton. Based on lithological, structural, metamorphic and geochronological data, the eastern and western zones of the craton are considered to represent two different Archean to Paleoproterozoic continental blocks that amalgamated along the central zone at the end of Paleoproterozoic. The P–T paths of the Fuping Complex and other tectonic units in the central zone record the collision between the eastern and western zones that led to the final assembly of the North China craton at c. 1800 Ma.     

Isobaric cooling and anti-clockwise P–T paths in the Variscan Odenwald Crystalline Complex, Germany

Mineral assemblages in Al₂O₃‐rich, SiO₂‐ and K₂O‐poor metapelitic rocks from the western Odenwald Crystalline Complex (Variscan Mid‐German Crystalline Rise, southern Germany) include corundum, spinel, cordierite, sillimanite, garnet and staurolite. Quartz is absent from almost all samples. Therefore, the applicability of conventional geothermobarometry is very limited or even impossible. Detailed petrographic investigation on selected samples permits inference of the sequence of appearance and disappearance of several mineral assemblages. The recognition of such partial re‐equilibration stages and their associated mineral assemblages, together with mineral stabilities predicted from KFMASH pseudosections, enables the determination of the pressure‐temperature (P–T) trajectories experienced by these rocks during the Variscan metamorphism. The rocks were metamorphosed under low‐P/high‐T conditions and underwent an anti‐clockwise P–T evolution. A pressure increase from about 2 kbar to 4 ± 0.5 kbar was accompanied by heating. Peak metamorphic conditions were reached at pressures of 4 ± 0.5 kbar and temperatures of at least 640 °C, probably even higher. The retrograde evolution is characterised by near‐isobaric cooling from ≥ 640 °C to approximately 550 °C. The rocks underwent the anti‐clockwise evolution in a subduction‐related magmatic arc setting. The close spatial association of the low‐P/high‐T rocks with recently discovered metabasic eclogites in the eastern part of the Odenwald Crystalline Complex may indicate a fossil paired metamorphic belt in the Central European Variscides.     

Multistage progressive evolution of rare osumilite‐bearing assemblages preserved in ultrahigh‐temperature granulites from In Ouzzal (Hoggar,Algeria)

Osumilite is reported in Palaeoproterozoic Al–Mg‐rich granulites from the Khanfous area (Tekhamalt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite + sapphirine + biotite + orthopyroxene + sillimanite and osumilite + orthopyroxene + sillimanite + quartz ± biotite (±K‐feldspar) in silica‐deficient and silica‐saturated granulites respectively. Osumilite coexists with F‐rich biotite (X_F ≈ 0.6). The observed microstructures, the mass balance of metamorphic reactions and P–T pseudosections modelled for bulk‐rock and reaction‐microdomain compositions indicate a clockwise P–T metamorphic evolution at ultrahigh temperatures, without substantial post‐peak deformation. The peak P–T conditions recorded by the osumilite‐bearing assemblages are 8.5–9.0 kbar and 930–980 °C. During retrogression, osumilite was partially or totally replaced by fine‐grained pseudomorphs of cordierite + orthopyroxene + K‐feldspar + quartz at ~7 kbar and ~850 °C. This study confirms that osumilite can occur only in Mg‐rich metamorphic rocks that experienced ultrahigh‐temperature metamorphism under anhydrous conditions. In the presence of a hydrous fluid, it is replaced, even at high temperatures, by cordierite‐bearing assemblages. This important feature explains the rarity of osumilite in granulite facies rocks and its common replacement by cordierite + orthopyroxene + K‐feldspar + quartz pseudomorphs. The peak conditions suggest that a delamination of the lithospheric mantle underneath the In Ouzzal crust brought the asthenosphere close to the Mohorovi?i? discontinuity.     

Timing and rate of isothermal decompression in Pan-African granulites from Rundvågshetta, East Antarctica

Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near‐isothermal decompression at granulite facies temperatures in rocks from the Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite‐bearing assemblages, and secondary cordierite–sapphirine‐bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post‐peak metamorphic history is characterized by near‐isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post‐dated pervasive ductile deformation. K/Ar and ⁴⁰Ar/³⁹Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high‐temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and ⁴⁰Ar/³⁹Ar closure ages indicates that near‐isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one‐dimensional models for exhumation‐controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr^?1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re‐equilibration.     

Silica-undersaturated sapphirine, spinel and kornerupine granulite facies rocks, NE Strangways Range, Central Australia

Small pods of silica-undersaturated Al-rich and Mg-rich granulite facies rocks containing sapphirine, pleonastic spinel, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene-rich aluminous granofels and metapelitic gneisses containing sapphirine, spinel or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper. Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900–950 °C and 8–9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the sequential crossing of the following reactions in the system FMAS: cordierite + spinel + corundum = sapphirine + sillimanite, cordierite + spinel = orthopyroxene + sapphirine + sillimanite, sapphirine + spinel + sillimanite = orthopyroxene + corundum, sapphirine + sillimanite = cordierite + orthopyroxene + corundum. Phase stability relationships in FMAS and MASH indicate an anticlockwise P–T path terminated by isobaric cooling. Such a path is exemplified by early low-P mineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase in aH₂O and aB₂O₃ with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1 P–T path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.     

Separating metamorphic events in the Fosdick migmatite–granite complex,West Antarctica

The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M₁) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M₁ metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M₁ conditions. The Cretaceous (M₂) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M₁ garnet constrains M₂ conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M₂ event, which would facilitate overprinting of M₁ compositions during the M₂ prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M₁ overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.     

Upper‐amphibolite facies partial melting of paragneisses from the Epupa Complex,NW Namibia,and relations to Mesoproterozoic anorthosite magmatism

The evolution of the mineral assemblages and P–T conditions during partial melting of upper‐amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P–T–X phase diagrams in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper‐amphibolite facies low‐ and medium‐pressure metapelites, which occur in many high‐grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite–sillimanite–quartz assemblages were replaced via melt‐producing reactions by cordierite‐bearing assemblages. In a Southern Zone, they were replaced via melt‐producing reactions by garnet‐bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt–Sil–Crd–Bt peak assemblages. Peak‐metamorphic conditions of 700–750 °C at 5.5–6.7 kbar in the Southern Zone and of ～750 °C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back‐reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite–sillimanite and/or biotite–muscovite intergrowths. Upper‐amphibolite facies metamorphism and partial melting (c. 1340–1320 Ma) in the rocks of the Southern Zone of the Orue Unit, which underwent probably near‐isobaric heating–cooling paths, are attributed to contact metamorphism induced by the coeval (c. 1385–1319 Ma) emplacement of the Kunene Intrusive Complex, a huge massif‐type anorthosite body. The lower‐pressure metapelites of the Northern Zone are interpreted to record contact metamorphism at an upper crustal level.     

Evolution of high-Mg–Al granulites from Sunkarametta, Eastern Ghats, India: evidence for a lower crustal heating–cooling trajectory

A suite of high-Mg–Al granulites from Sunkarametta, Eastern Ghats Belt, India, shows contrasting prograde assemblages of extremely aluminous orthopyroxene+cordierite+sapphirine and similarly aluminous orthopyroxene+Ti-rich spinel in closely associated domains. Textural and compositional characteristics indicate that both were derived from prograde dehydration–melting of biotite–plagioclase–quartz-bearing protoliths. The former assemblage was stabilized at relatively more magnesian bulk composition. Geothermobarometric data and petrogenetic grid considerations place 'peak' metamorphic conditions at c. 950 °C and 9 kbar. Subsequent to peak metamorphism, the rocks cooled to c . 700–750 °C, with slight lowering of pressure, and the retrograde reactions also involved melt–solid interaction. The inferred P – T  trajectory is one of heating–cooling at lower crustal (25–30 km) depths.     

Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implications

Interpretations based on quantitative phase diagrams in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).     

P–T evolution of the Precambrian Metamorphic Complex,NW Iran: a study of metapelitic rocks

The Mahneshan Metamorphic Complex (MMC) is one of the Precambrian terrains exposed in the northwest of Iran. The MMC underwent two main phases of deformation (D₁ and D₂) and at least two metamorphic events (M₁ and M₂). Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite‐facies conditions. The dominant metamorphic mineral assemblage in metapelitic rocks (M₁) is muscovite, biotite I, Garnet I, staurolite, Andalusite I and sillimanite. Peak metamorphism took place at 600–620°C and ∼7 kbar, corresponding to a depth of ca. 24 km. This was followed by decompression during exhumation of the crustal rocks up to the surface. The decrease of temperature and pressure during exhumation produced retrograde metamorphic assemblages (M₂). Secondary phases such as garnet II biotite II, Andalusite II constrain the temperature and pressure of M₂ retrograde metamorphism to 520–560°C and 2.5–3.5 kbar, respectively. The geothermal gradient obtained for the peak of metamorphism is 33°C km⁻¹, which indicates that peak metamorphism was of Barrovian type and occurred under medium‐pressure conditions. The MMC followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal relaxation following tectonic thickening. The bulk chemistry of the MMC metapelites shows that their protoliths were deposited at an active continental margin. Together with the presence of palaeo‐suture zones and ophiolitic rocks around the high‐grade metamorphic rocks of the MMC, these features suggest that the Iranian Precambrian basement formed by an island‐arc type cratonization. Copyright © 2010 John Wiley & Sons, Ltd.     

Palaeoproterozoic high-T, low-P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

A sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al₂SiO₅ polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.     

Gedrite–garnet–sillimanite-bearing granulites from Amessmessa area, south In Ouzzal, Hoggar, Algeria

Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. This P–T evolution is consistent with low to medium a_H2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north of In Ouzzal.     

High to ultrahigh temperature contact metamorphism and dry partial melting of the Tasiuyak paragneiss,Northern Labrador

Contact aureoles of the anorthositic to granitic plutons of the Mesoproterozoic Nain Plutonic Suite (NPS), Labrador, are particularly well developed in the Palaeoproterozoic granulite facies, metasedimentary, Tasiuyak gneiss. Granulite facies regional metamorphism (M_R), c. 1860 Ma, led to biotite dehydration melting of the paragneiss and melt migration, leaving behind biotite‐poor, garnet–sillimanite‐bearing quartzofeldspathic rocks. Subsequently, Tasiuyak gneiss within a c. 1320 Ma contact aureole of the NPS was statically subjected to lower pressure, but higher temperature conditions (M_C), leading to a second partial melting event, and the generation of complex mineral assemblages and microstructures, which were controlled to a large extent by the textures of the M_R assemblage. This control is clearly seen in scanning electron microscopic images of thin sections and is further supported by phase equilibria modelling. Samples collected within the contact aureole near Anaktalik Brook, west of Nain, Labrador, mainly consist of spinel–cordierite and orthopyroxene–cordierite (or plagioclase) pseudomorphs after M_R sillimanite and garnet, respectively, within a quartzofeldspathic matrix. In addition, some samples contain fine‐grained intergrowths of K‐feldspar–quartz–cordierite–orthopyroxene inferred to be pseudomorphs after osumulite. Microstructural evidence of the former melt includes (i) coarse‐grained K‐feldspar–quartz–cordierite–orthopyroxene domains that locally cut the rock fabric and are inferred to represent neosome; (ii) very fine‐ to medium‐grained cordierite–quartz intergrowths interpreted to have formed by a reaction involving dissolution of biotite and feldspar in melt; and (iii) fine‐scale interstitial pools or micro‐cracks filled by feldspar interpreted to have crystallized from melt. Ultrahigh temperature (UHT) conditions during contact metamorphism are supported by (i) solidus temperatures >900 °C estimated for all samples, coupled with extensive textural evidence for contact‐related partial melting; (ii) the inferred (former) presence of osumilite; and (iii) titanium‐in‐quartz thermometry indicating temperatures within error of 900 °C. The UHT environment in which these unusual textures and minerals were developed was likely a consequence of the superposition of more than one contact metamorphic event upon the already relatively anhydrous Tasiuyak gneiss.     

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.