P–T–X relationships in the Precambrian Al–Mg-rich granulites from In Ouzzal, Hoggar, Algeria

The Al–Mg-rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al₂O₃–SiO₂ system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high-P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel-bearing assemblages; (2) a high-T stage characterized by the development of Sa–Qz-bearing assemblages; and (3) a later stage, in which garnet-bearing assemblages are replaced by more or less fine symplectites involving cordierite. During the course of early mineral reactions, the distribution coefficient, K_d, between the various ferromagnesian phases decreased significantly whereas Al₂O₃ in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high-P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low K_d value between orthopyroxene and garnet (K_d= 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al-solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale. Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re-equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C. We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation.     

Internally consistent calibrations for geothermobarometry of high-grade Mg-Al rich rocks in the system MgO-Al2O3-SiO2 and their application to sapphirine-spinel granulites of Eastern Ghats, India and Enderby Land, Antarctica

Sixty-three internally consistent geothermobarometers for mineral equilibria involving sapphirine (2:2:1 and 7:9:3), pyrope, cordierite, enstatite, Mg-tschermak orthopyroxene, quartz, spinel and sillimanite have been calibrated in the MAS system. The updated thermodynamic data of these minerals are consistent, within limits of error, with highP-T experiments on several mineral equilibria and calorimetric data. TheP-T conditions of the granulite facies metamorphism, spanning a range of 700 to more than 1000°C and 4 to more than 10 kbar, can be estimated simultaneously from these geothermobarometers andP-T-t trajectories can be deduced from the reaction coronas well preserved in these rocks because of the refractory nature of aluminous phases. The geothermobarometers have been applied to sapphirine-spinel granulites of Eastern Ghats and Enderby Land. TheP-T conditions of metamorphism (a-prograde/thermal peak and b-retrograde isothermal/isobaric decompression/cooling) estimated for these granulites are: (1) Eastern Ghats (Visakhapatnam): Paderu- (a) 900°C/8.3kbar, (b-1) 900°C/6.8kbar and (b-2) 740°C/5.4 kbar; Anantgiri- (a) prograde anticlockwise 930°C/6.2 kbar and (b) 870°C/6.8 kbar, 820°C/6.1 kbar; Anakapalle- (b) 845°C/8.5-6.2 kbar; and Araku- (b) 840°C/6.2 kbar to 795°C/5.9 kbar. Enderby Land (Napier complex): Spot height 945, Tula Mts.- (a) 970°C/9.1 ± 0.6 kbar, isobaric cooling (b) 885°C/ 7.75 kbar, isothermal decompression (b) 880°C/6.85 kbar; Mt. Hardy, Tula Mts.- (b) 885°C/6.75 kbar; Mt. Riiser-Larsen, Amundsen bay- (a) 1000°C/7.0 kbar prograde anticlockwise; Mt. Sones- (b) 920°C/ 6.8 kbar; Forefinger Point, SW Enderby Land- (b) 840°C/6.7 kbar, 810°C/6.5 kbar and 775°C/5.0 kbar. The estimatedP-T andP-T-t are mostly consistent with those inferred from the granulites of these areas.     

Evolution of Moldanubian rocks in Austria: review and synthesis

The Moldanubian zone in Austria comprises three major lithological units. Despite general agreement that nappe tectonics contributed to its current structure, the number and position of tectonic boundaries, or continental pieces that were involved in its evolution, as well as the age, extent and position of oceanic sutures are disputed. Recent models ascribe the Moldanubian tectonostratigraphic structure to its oblique, N- to NE-directed collision with Moravia only. The rocks of the Moldanubian Bunte series and Gföhl unit experienced a common, intensive overprint in the range 700–800 °C and 8–11 kbar. Textural evidence suggests that this overprint was attained during nearly isothermal decompression, so the rocks experienced higher pressures prior to this overprint. These conditions constrain a continent–continent collision environment that contributed to the formation of the Moldanubian granulites. The estimated metamorphic temperatures are close to T_max. During this Hercynian, high-T overprint, the minerals underwent extensive diffusion-controlled homogenization of elements. The early stages of retrogression of these units were characterized by isobaric cooling at c. 6 kbar in the range 650–500 °C that is related to the oblique collision of the Moldanubian and Moravian zones. Cooling to c. 400 °C is demonstrated by unstrained, diasporized corundum inclusions in garnet of common Moldanubian granulites. The available age data (including cooling ages) from metamorphic rocks show a very wide variation between 490 and 280 Ma that depends on sample characteristics and the dating method used. They demonstrate clearly, however, that the metamorphic overprint is Hercynian. The possibility that the large variation in ages reflects homogenization, resetting and closure of the isotopic systems attained at different, sample- and method-specific times is discussed. Age data varying between c. 370 and c. 346 Ma tentatively date different stages during the Hercynian, high-T decompression. The majority of zircon and monazite U/Pb ages as well as the hornblende and muscovite Ar/Ar cooling ages cluster between c. 345 and c. 326 Ma and date the effective closure conditions and the onset of rapid, nearly isobaric cooling. The continent–continent collision that formed the granulites pre-dates c. 370 Ma. The intra-Moldanubian nappe-stacking pre-dates thrusting of the Moldanubian zone over the Moravian zone. The range c. 340–335 Ma is the lower limit for completion of tectonic activity in the Moldanubian zone. The Moldanubian series are post-tectonically intruded by granitoids of the Southern Bohemian Pluton. Recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the Hercynian high-grade overprint. The Moldanubian Monotone series in Austria is separated from the other Moldanubian units by a conspicuous tectonic horizon. It also differs from them by its characteristic high-T , low-P overprint, which is best demonstrated by a widespread cordierite gneiss.     

Growth of plagioclase rims around metastable kyanite during decompression of high‐pressure felsic granulites (Bohemian Massif)

Samples of high‐pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high‐pressure) mineral assemblage of garnet – kyanite – plagioclase – K‐feldspar – quartz ± biotite show well‐developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K‐feldspar. Kyanite is regarded as a relict of the high‐pressure metamorphic assemblage that became metastable during transition to a low‐pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An_32–25 at the contact with kyanite to An_20–19 at the contact with the matrix or to the perthitic K‐feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim‐forming reaction to provide the Al₂O₃ component for the growth of plagioclase, whereas in other cases no Al₂O₃ from kyanite was necessary. In a majority of examples, the necessary Al₂O₃ was supplied with CaO and Na₂O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ～ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ～ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K‐feldspar grains.     

Phase equilibria modelling of kyanite‐bearing anatectic paragneisses from the central Grenville Province

Kyanite‐bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high‐P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K‐feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub‐aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P–T conditions of the metamorphic peak. These P–T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high‐T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P–T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P–T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P–T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P–T conditions of 12.5–13 kbar and 815–830 °C and flatter P–T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P–T evolution of high‐P anatectic quartzo‐feldspathic rocks.     

Low-pressure Granulites of the Lisov Massif, Southern Bohemia: Visean Metamorphism of Late Devonian Plutonic Arc Rocks

The Liov Granulite Massif differs from neighbouring granulitebodies in the Moldanubian Zone of southern Bohemia (Czech Republic)in including a higher proportion of intermediate–maficand orthopyroxene-bearing rocks, associated with spinel peridotitesbut lacking eclogites. In addition to dominantly felsic garnetgranulites, other major rock types include quartz dioritic two-pyroxenegranulites, tonalitic granulites and charnockites. Minor bodiesof high-pressure layered gabbroic garnet granulites and spinelperidotites represent tectonically incorporated foreign elements.The protoliths of the mafic–intermediate granulites (quartz-dioriticand tonalitic) crystallized 360–370 Ma ago, as indicatedby laser ablation inductively coupled plasma mass spectrometryU–Pb ages of abundant zircons with well-preserved magmaticzoning. Strongly metamorphically recrystallized zircons giveages of 330–340 Ma, similar to those of other Moldanubiangranulites. For the overwhelming majority of the Liov granulitespeak metamorphic conditions probably did not exceed 800–900°Cat 4–5 kbar; the equilibration temperature of the pyroxenegranulites was 670–770°C. This is in sharp contrastto conditions of adjacent contemporaneous Moldanubian granulites,which are characterized by a distinct HP–HT signature.The mafic–intermediate Liov granulites are thought tohave originated during Viséan metamorphic overprintingof metaluminous, medium-K calc-alkaline plutonic rocks thatformed the mid-crustal root of a Late Devonian magmatic arc.The protolith resembled contemporaneous calc-alkaline intrusionsin the European Variscan Belt. KEY WORDS: low-pressure granulites; geothermobarometry; laser-ablation ICP-MS zircon dating; whole-rock geochemistry; Sr–Nd isotopes; Moldanubian Zone     

The retrograde P–T–t path for low-pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low-P/high-T metamorphism

Several aspects of the petrogenesis of low-pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration-melting reactions to form migmatites under peak M2 P–T conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike-parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak-M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite-bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high-grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre-tectonic granites, is required.     

A reappraisal of polymetamorphism in the Eastern Ghats belt — A view from north of the Godavari rift

Evidence collated from different parts of the Eastern Ghats belt north of the Godavari rift (barring the “Western Charnockite Zone” ) indicates that this sector evolved through a series of compressive structures (F₁ to F₃), with prolific migmatization in quartzofeldspathic and metapelitic gneisses synchronous with F₁ shortening, as was the syn-F₁ emplacement of profuse megacrystic K-feldspar-bearing granitoid bodies. Thereafter, melt productivity of the rocks (synchronous withF ₂– F₃ folding) sharply decreased. Mineral parageneses stable in the S₁, S₂ and S₃ fabrics indicate persistence of granulite facies conditions. P-T estimates on orthopyroxene + garnet + plagioclase + quartz assemblages anchored to recrystallized mosaic that overgrow all penetrative fabric elements in mafic granulites, granitoids and quartzofeldspathic gneisses are in the range of 900‡-950‡C and P≅ 8–9 kbar. This estimate is comparable to those retrieved from sapphirine-bearing paragenesis in Mg-Al metapelites that appear to be diachronous in relation to the fabric elements, and arguably disrupt the granoblastic mosaic. These facets in the northern sector of the orogenic belt are compatible with either a single cycle of tectonic events (i.e., F₁, F₂ and F₃ in continuum), or temporally-separate thermo-tectonic events, with the peak of earlier metamorphism (pre- to syn-F₁) at lower temperature (in the granulite facies) in comparison to the record of high post-F₃-T_max values. It is suggested on the basis of the above evidence that the late Proterozoic/Pan-African granulites in the Eastern Ghats belt north of the Godavari rift, are unlikely to be reworked equivalents of any older granulitic crust, such as the ∼1.6 Ga granulites south of the rift. Instead, the temporally disparate sectors may represent different crustal segments with unconnected pre-amalgamation tectonic history. However, if the ∼ 1.6 Ga granulites of the Western Charnockite Zone continue northwards across the rift, as suggested by recent isotope data, there are serious doubts as to the validity of a north-south division within the Eastern Ghats belt.     

Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central Australia

Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800–875 °C and >9–10 kbar) and subsequent decompression in a kilometre‐scale, E‐W striking zone of noncoaxial, high‐grade (c. 700–735 °C and 5.8–6.4 kbar) deformation. However, new SHRIMP U‐Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta‐volcaniclastics) yield ²⁰⁶Pb/²³⁸U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield ²⁰⁶Pb/²³⁸U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400–300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475–460 Ma & 400–300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300–1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed.     

Stable Isotope Investigations on the Granulite Facies Terrains of Brazil: A Review

Stable isotope data from three areas in the granulite facies terrains of Brazil are assembled and discussed. All the three areas (Jequié, Guaxupé and São José do Rio Pardo) are from the São Francisco Craton. The carbon isotope composition of the fluid inclusion CO₂ in the Archean granulite terrain of Jequié indicate the participation of two distinct sources, an upper mantle source and an internal source of Archean organic matter. The isotope data may be interpreted in terms of the granulite genesis due to the intrusion of magma in the lower crust. The singularly uniform carbon isotope data for CO₂ fluid inclusion of plutonic granulites is a clear indication of the magmatic addition of CO₂-rich volatiles from deeper crustal sources. In the São José do Rio Pardo area sulfur and carbon isotope data of scapolites from the granulites imply the derivation of volatiles from internal sources. The whole rock oxygen isotope data of the amphibolite and granulite facies gneisses from Guaxupé indicate a small scale variation of d¹⁸O values, compatible with the chemical data, suggesting the preservation of pre-metamorphic oxygen isotope composition. The isotope data of the granulites from São Francisco Craton indicate non-pervasive fluid flow during metamorphism.