Metapelitic Assemblages Research Articles

Fluorine content of biotite in granulite-grade metapelitic assemblages and its implications for the Eastern Ghats granulites

Biotite inclusions showing high fluorine content (up to 3.3 wt.%) within porphyroblastic aluminous orthopyroxene (>10 wt.% Al 2 O 3 ) and cordierite suggest high temperature of melting for the prograde assemblages. Textural data indicate that biotite dehydration melting produces peak assemblages orthopyroxene + spinel + quartz + melt and orthopyroxene + sapphirine + quartz ± cordierite + melt in different microdomains depending on the bulk composition. A comparative study of the biotite composition with those of experimental data and present one indicates a minimum temperature of 950°C for the terminal stability of biotite-bearing assemblages. This also explains the appearance of observed peak assemblage sapphirine + orthopyroxene + quartz + cordierite + spinel directly from F-rich biotite as a consequence of topological changes in the KFMASH grids. Enrichment of fluorine in retrograde biotite can be explained by interaction of solid phases with the in situ melt fraction during subsequent retrogressive stage of the granulites. This is the first report of its kind from the northern part of the Eastern Ghats Belt, India.

Metamorphism of pelites in NKFMASH – a new petrogenetic grid with implications for the preservation of high‐pressure mineral assemblages during exhumation

AbstractA petrogenetic grid for metapelites in the system NKFMASH is presented. The P–T range is investigated in three sections: (1) The high‐ and ultrahigh‐pressure range is discussed in the system NFMASH because phengite is the only stable potassic phase. (2) The transition region is characterised by four NKFMASH‐invariant points that separate high‐pressure glaucophane‐bearing from medium‐pressure biotite‐bearing metapelites. (3) The medium‐pressure range contains the fifth NKFMASH‐invariant point. The univariant reactions of this point terminate the stability range of paragonite, which breaks down to form staurolite or kyanite and plagioclase during decompression and/or heating. As the growth of albitic plagioclase by decomposition of paragonite via continuous reactions may be conspicuous already before these staurolite‐ or kyanite‐producing reactions are reached, such albite porphyroblast schists are typical indicators of a former high‐pressure metamorphic history.Considering the preservation of high‐pressure metapelitic assemblages, those crossing the NKFMASH‐transition region during exhumation commonly dehydrate and preservation is unlikely. Three types of metapelites have a fairly good survival potential: (1) low‐temperature metapelites (up to c. 540 °C) with an exhumation path back into the chlorite + albite stability field, (2) assemblages with chloritoid + glaucophane, and (3) the relatively high‐temperature glaucophane + kyanite and jadeite + kyanite bearing parageneses, that are relatively dry at the onset of exhumation. A comparison with data from the literature shows that these rock types are the most abundant in nature.

Metapelitic assemblages in the Umm Zariq schists, central western Kid Belt, Sinai Peninsula, Egypt

The Kid Belt, one of the four metamorphic belts constituting the Pan-Afri- can basement of the Sinai Peninsula, underwent two main phases of ductile deforma- tion and regional metamorphism ranging in grade from the greenschist facies in the south to the upper amphibolite facies in the northeast. In the central western part of the Kid Belt, metapelites of the Umm Zariq Formation display the following mineral as- semblages (always + quartz + plagioclase An 20-38 + ilmenite): (i) garnet + biotite + muscovite ± andalusite, (ii) garnet + staurolite + biotite + muscovite ± cordierite ± andalusite, (iii) garnet + cordierite + biotite + muscovite ± andalusite ± (rare) sillimanite, testifying to the low pressure type of metamorphism under PT-conditions of the lower amphibolite facies. Conventional geothermobarometry yielded peak metamorphic tem- peratures around 570 ƒC and pressures of about 4kbar. Conformable results were ob- tained from a petrogenetic grid, calculated in the system KMnFMASH and applied to a sample with the low-variance assemblage (ii). The PT-area for the formation of the near-peak assemblage andalusite- staurolite- garnet- biotite extends from some 3 to 4kbar and 540 to 590 ƒC. Calculations on this mineral assemblage ( + plagioclase + muscovite + quartz) with the internally-consistent thermodynamic data set of Hol- land & Powell (1998) yield similar PT-conditions of 4.2 ± 0.9kbar and 591 ± 28ƒC

An Exposed Hercynian Deep Crustal Section in the Sila Massif of Northern Calabria: Mineral Chemistry, Petrology and a P-T Path of Granulite-facies Metapelitic Migmatites and Metabasites

In the Sila massif of northern Calabria a continuous section is exposed through a segment of a Hercynian deep continental crust, which has been interpreted by previous workers as a stack of basement nappes (‘Monte Gariglione Complex’). The section consists essentially of metapelitic migmatites and subordinate metabasites and marbles, which were metamorphosed at medium-pressure–high-temperature granulite-facies conditions. A continuous metamorphic gradient through the exposed segment can be deduced from the systematic change in the compositions of ferromagnesian minerals in divariant metapelitic assemblages. This gradient is partly supported by conventional geothermobarometry and by applying the TWEEQU method. However, peak-metamorphic conditions are better defined by dehydration melting reactions, which reveal ∼4 kbar and 740°C at the top and ∼6 kbar and 770°C at the base of the section. Therefore the exposed rocks represent a former crustal depth range of ∼14–21 km at the thermal peak of metamorphism. The metamorphic evolution of the former deep crustal rocks has been reconstructed from reaction textures. The prograde P–T path took place in the stability field of sillimanite. The retrograde path is characterized by a stage of isothermal uplift after peak metamorphism to mid-crustal levels (10–15 km) followed by near-isobaric cooling to greenschist-facies conditions. The deep crustal section in northern Calabria resembles that of the Serre massif in southern Calabria, which represents the lowermost part of an exposed tilted cross-section through the Hercynian continental crust. Postkinematic mineral growth, reaction textures and isotopic age constraints indicate that the thermal conditions in the Calabrian crust during the Hercynian orogeny were mainly controlled by advective heat input through magmatic intrusions. As in southern Calabria, large granitic bodies were emplaced between the granulite-facies lower crust (‘Monte Gariglione Complex’) and the amphibolite- to greenschist-facies upper crust during regional metamorphism.

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.

Singularities in NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O)

A new petrogenetic grid is presented for the system NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O), in which Ca is incorporated in garnet, and CaAlNa−1Si−1 solid solution is considered for both the plagioclase and white‐mica structures. A compatibility diagram for plagioclase‐bearing metapelitic assemblages within the NCKFMASH system also has been derived. A dominant feature of the NCKFMASH grid is the singularities and associated singular reactions which occur along plagioclase+margarite‐ and plagioclase+paragonite‐bearing univariant equilibria. The singularities represent compositional coplanarities which occur in response to the CaAlNa−1Si−1 substitution occurring at different rates in plagioclase and white‐mica. This is controlled by a fundamental difference in the mixing within the two mineral structures. The singularities give rise to a number of intriguing phase diagram features, including azeotropes. From the results presented here, it is predicted that the occurrence of margarite and paragonite in pelitic rocks is controlled by equilibria related to the singularities. The presence of these white‐micas is strongly dependent upon bulk composition, and plagioclase‐bearing, margarite/paragonite‐free assemblages, typical of Barrovian‐type terranes, are predicted for bulk compositions of Mg/(Mg+Fe)≈0.4 and Ca/(Ca+Na)≈0.4 at for example 5.5 kbar.

Low-pressure crustal anatexis: the significance of spinel and cordierite from metapelitic assemblages at Nanga Parbat, northern Parkistan

Abstract Spinel-bearing domains in high-grade metapelitic rocks from Nanga Parbat represent zones of partial melting during biotite breakdown under vapour-undersaturated conditions. Spinel is essentially of the MgAl 2 O 4 -FeAl 2 O 4 solid solution, and is therefore not stabilized by the presence of trace elements such as zinc, but is restricted to a quartz-absent petrogenesis. A new petrogenetic grid for metapelites has been constructed to allow for both vapour-undersaturated and quartz-undersaturated conditions. This grid contains a quartz-absent invariant point in KFASH that predicts quartz-absent melting will occur in biotite-sillimanite assemblages at low pressures. A P–T pseudosection drawn for bulk compositions with intermediate Fe/Mg ratios shows that at low pressures biotite will break down initially in the presence of quartz, to produce cordierite and K-feldspar and melt. At higher temperatures the quartz-absent reaction will be crossed, and biotite will break down to produce spinel in addition to cordierite, K-feldspar and melt. The sequence of assemblages is biotite-cordierite, biotite-cordierite-spinel, and finally cordierite-spinel. This biotite-absent assemblage is not observed, indicating that the quartz-absent reaction did not go to completion. Limits can be placed on the pressures and temperatures at which the biotite breakdown reaction is crossed, from the coexistence of solid phases and granite melt over a range of water activities. For the Nanga Parbat assemblages, pressures and temperatures lay at about 720°C and 5 kbar at water activity, a H 2 O ≥ 0.6. These conditions are consistent with a rapidly exhuming terrane, as suggested by isotopic constraints and thermal modelling of the region.

Petrochemical study of regional/contact metamorphism in metaclastic strata of the central White-Inyo Range, eastern California

Sedimentary rocks of the White-Inyo Range have been metamorphosed by episodically emplaced Jurassic–Cretaceous calc-alkaline arc plutons, reflecting stages in the thermotectonic history of this evolving continental margin. A north-northwest–trending anticlinorium is manifested in strata ranging in age from latest Precambrian to middle Paleozoic. Most middle and late Mesozoic (80–180 Ma) intrusive igneous bodies crosscut or deflect the regional fold pattern and incipient axial-plane cleavage in the country rocks—hence granitoid emplacement was largely postkinematic and, accordingly, isogradic surfaces transect structural levels. Retrograde andalusite ± cordierite-bearing metapelitic assemblages and diopside + grossular-bearing calc-silicate wall-rock skarns at plutonic contacts reflect high-temperature thermal maxima in the inner aureoles. Bulk-rock chemistries, mineral parageneses, and compositions of coexisting phases for several noncalcareous, weakly foliated sequences, the uppermost Proterozoic Wyman meta-argillites, the Lower Cambrian Deep Spring and Andrews Mountain metaquartzites, and the Montenegro phyllites, demonstrate that the overall metamorphic zonation is due to advective heat transport by the postkinematic granitoids. The higher grade parageneses do not represent a synchronous thermal event, but instead constitute a composite of local recrystallization episodes attending calc-alkaline pluton emplacement over an ≈100 m.y. interval, overprinting a pervasive, weak, chloritic metamorphism. The latter may have been produced during late Paleozoic and/or early Mesozoic folding. Bulk-rock X-ray fluorescence analyses demonstrate that Wyman meta-argillites are rich in CaO, Al 2 O 3 , Cr, Th, and Sr, and low in SiO 2 , TiO 2 , P 2 O 5 , Na 2 O, and K 2 O, indicating the former presence of carbonate, abundant clays, and minor detrital feldspars and iron oxides. Deep Spring and Andrews Mountain metaquartzite analyses exhibit high concentrations of titania, iron, silica, alkalies, Zr, Nb, and phosphate, but are poor in Al 2 O 3 , CaO, Cr, Th, and Sr, reflecting original protoliths enriched in alkali feldspars, quartz, and magnetite. Montenegro phyllites have compositions intermediate between the other two investigated metasedimentary units; however, they are relatively low in CaO and Sr, and high in Al 2 O 3 , suggesting noncalcareous precursor sediments containing abundant clay minerals, quartz, and alkali feldspars. Metaquartzites and phyllites are more oxidized than the sparsely carbonaceous meta-argillites. Except for a gradual decrease in volatile contents, the analyzed rocks do not exhibit systematic chemical variation with increasing metamorphic grade. Contrasts in phase compositions and proportions reflect protolith chemistries as well as inferred conditions of recrystallization. The areal disposition of visually estimated neoblastic biotite is mapped as isopleths transecting the three investigated sequences. Titanium contents of biotite and white mica, and An contents of plagioclase increase toward the larger, more mafic Barcroft, Cottonwood- Beer Creek, and Joshua Flat plutons, whereas the cation proportions of Si in white micas decrease. Taking into account the ferric iron contents of white micas, Fe 2+ Mg −1 fractionation becomes less pronounced adjacent to these thermal highs. Metamorphic grade west of the range crest is greenschist facies chlorite zone (≈300 °), but rises abruptly on the extreme north adjacent to the Barcroft granodiorite, and gradually increases eastward toward the Beer Creek–Cottonwood and Joshua Flat plutons. An ill-defined metamorphic culmination is directly north of the Papoose Flat granite. Along contacts with plutons, hornfelsic wall rocks approached temperatures of 500–600 °C. On the basis of rare preservation of cordierite and/or andalusite, phase chemistry, and mineralogic thermobarometry, pressures attending regional and thermal recrystallization were about 3 ± 1 kbar. The activity of H 2 O was moderate (2 ± 1 kbar) during recrystallization of carbonate-poor rocks. Compositions of coexisting phases exhibit systematic, sympathetic variations, compatible with a close approach to local chemical equilibrium.

The thermal aureole of the Lochnagar Complex: mineral reactions and implications from thermal modelling

Synopsis The Caledonian Lochnagar Complex in the NE Highlands of Scotland comprises a central granite and earlier marginal quartz diorites. Contact metamorphism of the Dalradian country rocks has caused a thermal overprint on upper amphibolite facies regional metamorphic assemblages. Distinctive assemblages in metapelites and amphibolites can be mapped across the aureole, showing that the thermal effects are greatest where the marginal quartz diorites occur. Mineral chemistry in the metapelites can be related to contact metamorphic reactions and to a change in bulk rock chemistry due to partial melting of the metasediments in the high-grade parts of the aureole. Amphibolites within the aureole show no evidence of partial melting; their mineral chemistry is controlled by contact metamorphic reactions. The form of the aureole is a function of the different phases of intrusion, and mathematical modelling has been applied to distinguish between different intrusive scenarios. It is likely that two phases of contact metamorphism, related to early quartz diorite and later granite intrusion, were necessary to produce the highest grades of metamorphism. Mismatch between the modelled aureole shape and that observed in some areas is probably due to enhanced local heat flow and the lateral extension of the intrusion below the present erosion level.

Metamorphic evolution of the assemblage tremolite + talc + calcite + dolomite + quartz within a sample of siliceous dolomite from the southern Damara Orogen (Namibia)

Magnesian calcite temperatures of calcites formed during metamorphic reactions and fluorine distribution between OH-bearing minerals have been measured for the assemblage {ie180-1} within a sample of siliceous dolomite from the southern Damara Orogen. Two groups of temperatures were found: A group with higher temperatures in the range 583 to 545°C for reaction sites where tremolite was formed, and a second group with lower temperatures from 522 to 476°C for reaction sites where talc has grown. From special reaction textures it is evident that the reaction sites of the higher temperature group related with temolite is older, whereas talc has grown after the formation of tremolite with decreasing temperature. The transition from the tremolite stability field to the talc stability field occurred at a temperature between the two temperature groups. Each measured magnesian calcite temperature can be assigned to a microscopic fluid vein which connects single reaction sites either with tremolite or with talc. Single microveins developed at different times, because the prevailing temperature during the formation of individual fluid veins and the connected reaction sites are different. The mineral associations and the reaction textures of the sample were formed within a period of time necessary for cooling the rock at least from about 580 to 480°C. The textural relations after the successive reaction processes appear to be the result of a single univariant reaction. Distribution of fluorine between tremolite-phlogopite pairs and tremolite-talc pairs show that the fluorine content of the tremolite may be inherited in the latter case. A plausible P-T path is outlined explaining the evolution of assemblages in dolomites and metapelites of the same area which contain staurolite and kyanite. The first appearance of staurolite and kyanite in the field is situated on the lower grade side of the zone containing assemblage (I). The conversation of the present field occurrence of assemblage (I) is due to the fading of fluid infiltration into the dolomites.

Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

The northern part of the Saih Hatat window, Oman, shows high-pressure metamorphic rocks derived from shelf sediments and pre-Premian continental basement, and is atypical of sub-ophiolite metamorphism elsewhere. The high-pressure rocks are divided into structural units originally bounded by foreland-propagating thrusts formed during ophiolite obduction, although now many contacts are backthrusts, normal faults or extensional shear zones. Metamorphic breaks exist across many unit boundaries. The deepest unit (As Sifah) has eclogite-facies assemblages in metabasites and metapelites which record evolution of P – T conditions along a clockwise path culminating al 23±2.5kbar, 540±75°C, contrasting markedly with overlying units (5–10 kbar, 200–500°C), although separated by <l0km on the ground. The dominant penetrative structures in the eclogites predate exhumation, but broad zones in the enveloping and overlying schists show a later, greenschist-facies extensional fabric. Phengites from eclogite-facies schist show discordant 40 Ar/ 39 Ar apparent ages. Our tectonic model relates all the high- P units to a single convergent event in the Late Cretaceous. The As Sifah eclogites were exhumed in two stages: (i) tectonic emplacement against other units at c . 20–25 km depth, and (ii) exhumation of the entire high- P zone by culmination collapse after obduction.

Phase relations in andalusite‐sillimanite type Fe‐rich metapelites: Tono contact metamorphic aureole, northeast Japan

ABSTRACTMineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2‐3 kbar. The bulk rock compositions of metapelites are aluminous with A′ values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low‐grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5 polymorphs is limited in typical pelitic rocks.The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K‐feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite‐sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid‐bearing assemblages from Bt‐Chl‐Cld, And‐Bt‐Cld, to And‐Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low‐temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P‐T model depicting the relative stability of chloritoid and staurolite at low‐ and medium‐pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.

Geology and tectonomagmatic evolution of the eastern Himalaya along the Chomolungma-Makalu transect

Abstract The Lesser Himalayan units, the base of the Tethyan sedimentary sequence and the intervening High Himalayan Crystallines have been studied along a transect in the eastern Nepal Himalaya and southern Tibet. The lowest Lesser Himalayan tectonic unit exposed along the transect is the parautochthonous Tumlingtar Unit, a thick sequence of phyllite and quartzite of presumed Late Proterozoic age. Metamorphic grade is low (chlorite and biotite zones) and deformation much weaker than in the overlying unit, the Lesser Himalayan Crystallines, a unit including large bodies of granitic orthogneiss of probable Early Palaeozoic age. Metamorphic grade of the Lesser Himalayan Crystallines is higher than in the Tumlingtar Unit, being characterized by garnet-biotite-muscovite-kyanite-staurolite assemblages in metapelites and biotite-muscovite-kyanite in blastomylonitic orthogneiss. A belt of metasedimentary and metagranitic thrust sheets separate the Lesser Himalayan Crystallines from the overlying High Himalayan Crystallines (HHC). Its position corresponds to that of the Main Central Thrust (MCT) as defined in central Nepal. The HHC are a gneiss sequence of 6–7 km visible thickness in which metasediments of upper amphibolite facies (Barun Gneiss) are associated with large tabular bodies of granitic orthogneiss that occur especially in the middle part of the sequence and which display evidence of water-deficient partial melting. The age of the granite protolith from which the orthogneiss derives is Early Paleozoic, possibly Cambrian. The top of the HHC consists of biotite paragneiss and micaschist (Black Gneiss) hosting networks and lens-shaped bodies of tourmaline leucogranite which may attain a thickness of 1-2 km (Makalu, Baruntse) and evolve into cylindrical diapirs with gently north-plunging long axes (Nuptse). At the regional scale the top of the HHC coincides with the muscovite-in isograd and appears as a major intracrustal detachment zone which brought the HHC into contact with low-grade metasediments of the Tethyan Zone (North Col Fm) during the Miocene uplift of the Himalayan nappe pile, with attendant muscovite dehydration melting of the metasediments occurring at the top of the HHC. The top of the HHC, the overlying North Col Fm. and the leucogranite sills hosted in both, are affected by a ductile extensional tectonics which appears broadly synchronous with granite emplacement and is characterized by extension lineations trending 20° to 40°. Extensional structures occur as far south as Makalu, whereas further south ductile deformations have the same orientation but are compressional. In the area south of Makalu the ductile deformation involves some of the Miocene leucogranite bodies, suggesting that Miocene compression in the lower and middle HHC may have been synchronous with ductile extension in the upper HHC. Relations between metamorphism and deformation are inconsistent with a wholly post-metamorphic thrusting of the HHC over the Lesser Himalayan units and rather suggest that the Miocene exhumation of the HHC may be better explained by changes in the internal dynamics of the orogenic wedge produced by the Early Himalayan collision between northward-drifting India and Eurasia.

Discussion on Hercynian tectonometamorphic evolution of the Bosost dome (French–Spanish central Pyrenees)

R. L. Gibson writes: Pouget invokes a diapiric model for the development of structural–metamorphic domes in the Hercynian terrane of the Pyrenees. His model is based on conclusions drawn from the Bosost dome, namely that the core of this dome rose relative to the margins during regional metamorphism (deduced from P–T paths) and that the syn-metamorphic regional foliation formed in a dome-shaped attitude (from structural studies). Some comment is required, however, both on the methodology Pouget uses to construct the P–T paths presented in his paper and on his extension of the model to other domes in the Pyrenees. Pouget uses P–T estimates obtained from 4 garnet-bearing metapelite samples to construct P–T paths for different parts of the Bosost dome (fig. 4). Two P–T estimates were obtained from each sample using garnet core and rim compositions together with appropriate matrix phases and a straight-line vector was drawn linking the plotted results. Pouget maintains that this vector represents part of the P–T path followed by the sample during metamorphism. Such reasoning is, however, seriously flawed in that no guarantee exists that this straight line represents the true evolutionary path for the assemblage (see Spear et al. 1990 ). Pouget does not provide supplementary evidence, either from the analysis of continuous garnet zoning profiles or from the analysis of the metapelitic assemblages in terms of the KFMASH reaction grid (Harte & Hudson 1979) to corroborate his ‘shortest-line’ assumption. Based on this assumption, Pouget discerns two types

Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships

In the southeastern Reynolds Range, central Australia, a low‐P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high‐grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz‐bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.

Proterozoic tectonic reworking in the Rudall complex, Western Australia

Metasedimentary gneisses and schists in the northern part of the Rudall Complex are divided into the Yandagooge and Tjingkulatjatjarra Formations. The Yandagooge Formation occurs as inclusions in, and is intimately interlayered with, a composite orthogneiss, which is the dominant rock type of the Rudall Complex. The Tjingkulatjatjarra Formation is not disrupted by the orthogneiss, and is interpreted as being younger than both the orthogneiss and Yandagooge Formation, possibly forming a cover sequence to these rocks. The Yandagooge Formation and orthogneiss record a tectonic event, D1, that pre‐dates the Tjingkulatjatjarra Formation, but all rock units were intensely deformed and metamorphosed during the Watrara Orogeny, which comprises events D2–3 and began at ∼ 1330 Ma. The recognition of marker horizons in the metasedimentary rocks enables regional structural analysis, and the identification of mesoscopic to macroscopic recumbent folds produced during D2, which were redeformed by upright D3 folding. Metapelitic assemblages produced during D2 include kyanite + staurolite + biotite and garnet + staurolite + biotite, and suggest that the Watrara Orogeny was a consequence of the burial of the Rudall Complex to depths of ∼ 20 km. These Barrovian‐style assemblages are unusual for the mid‐Proterozoic orogenic belts of Australia, and the Watrara Orogeny could be equivalent to the Capricorn Orogeny, which was consequent to the juxtaposition of the Pilbara and Yilgarn Cratons. Unconformably overlying low‐grade Late Proterozoic cover rocks, together with the basement rocks, were mildly affected by a regional folding and thrusting event during the Paterson Orogeny at ∼ 700 Ma.

Phase equilibria and P-T-X(Fe-Mg) relations involving Gl-St-Cd-Als-Bi-Mu-Oz-Kfs-H 2 O in medium-high grade metapelites (KFASH, KMASH and KFMASH sistems)

In different areas of the Hercynian in the Iberian Peninsula some reactions are repeatedly observed in pelites, these reactions are: Staurolite + muscovite + quartz = Biotite+ Al 2 SiO 5 + H 2 O Garnet + muscovite = Al 2 SiO 5 + biotite+ quartz Biotite + Al 2 SiO 5 + quartz = Cordierite + K feldspar+Hp In order to examinate the P-T stability fields of these, and other similar reactions, aH the univariant equilibria in multisystems with Gt-Cd-St-Bi-Mu-Als-Qz-HP, Gt-St-Bi-Mu-Fk-AIs-QzHp and Cd-St-Bi-Mu-Fk-AIs-Qz-H 2 O in Kp-FeO-AI 2 O r SiO 2 -H 2 O (KFASH) system have been calculated, and their corresponding P-T grids have been constructed. The expansion of these reactions into divariant surfaces through the P-T-X (Fe-Mg) space was made by studying the assemblage Gt-Cd-St-Bi-Mu-Fk-AIs-Qz-HP in K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (KFMASH) with a Fe/Fe+Mg relationship Gt > St > Bi > Cd such as observed in most of natural pelites. A resultant grid was obtained by combining those obtained in the aboye systerns. This grid has been P-T located for P H 2 0 = P 19 near QFM buffer, and excess muscovite and quartz conditions Reaction slopes in this grid were calculated within different P-T surroundings from thermodynamic data as weH as by considering the existing experimental data. In addition to the stability fields of reactions the P-T-X Fe-Mg theoric relations for three univariant and thirteen divariant reactions have been obtained. The grid confirms the imposibility of staurolite-K feldspar and Garnet-Cordierite-Muscovite coexistence, as weH as the extension of the stability fields for Garnet-Staurolite, Cordierite-Staurolite and Garnet-Cordierite assemblages in muscovite-poor metapelites.

Kyanite-garnet-bearing Cambrian rocks and Grenville granulites from the Ayer's Cliff, Quebec, Canada, lamprophyre dike suite: Deep crustal fragments from the northern Appalachians

Granulite facies metamorphic xenoliths have been found in several Monteregian-age lamprophyric dikes at Ayer9s Cliff, Quebec, Canada. From geothermobarometric calculations,it is shown that middle to lower crustal metapelitic assemblages formed at 700-800 °C and 8-10 kbar, whereas mafic assemblages formed near 1200 ±100 °C and 14 ±1 kbar. The metapelitic assemblages are interpreted as Cambrian metasediments similar to those found farther south in Vermont. The mafic assemblages are interpreted as Grenville basement. The occurrence of these lithologies at Ayer9s Cliff and in similar dikes in Vermont confirms the seismic interpretation of Grenville basement beneath a large part of the western Appalachians in southern Quebec and northern New England.

A case of obduction-related high-pressure, low-temperature metamorphism in upper crustal nappes, Arabian continental margin, Oman: P-T paths and kinematic interpretation

In the southeastern Oman Mountains, the Saih Hatat tectonic window reveals Arabian continental material beneath the oceanic (sensu. lato) nappes i.e., the Hawasina sedimentary units and the huge Sumail ophiolite. The northeastern (internal) half of the window suffered an intense deformation associated with a high-pressure, low-temperature ( HP- LT) metamorphism, approximately dated at 80-75 Ma according to the available stratigraphie data. After the deformation, due to the absence of collision, the obduction-inherited structural and metamorphic pattern was virtually unmodified. A metamorphic map is established based on mineral assemblages in metapelites and metabasites. Low-grade assemblages occur in the highest units (Muscat nappes, Quryat unit). Characteristic minerals are Fe Mg carpholite (± lawsonite) associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites. High-grade rocks outcrop in the deepest and easternmost units (As Sifah antiform) and are typified by an epidote-glaucophane-bearing eclogite assemblage. Intermediate units (Hulw) show assemblages of intermediate grade (chloritoid schists and glaucophane-bearing metabasites). Higher grade assemblages such as talc-chloritoid were not found. Inverted gradients are observed locally in the Muscat nappes (pyrophyllitebearing units above kaolinite-quartz-bearing units) and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures. A complex P- T evolution is fairly well documented by the mineral assemblages. Prograde evolution in the subophiolitic (continental rocks) began with a rather “hot” gradient of about 30°–35°C/km (with chloritoid growth in the higher units). Thereafter pressure increased with the temperature remaining constant or even decreasing (chloritoid replaced by kaolinite + oxides and carpholite growth) in synkinematic conditions. The peak P- T conditions are close to 8 kbar, 270°C for the Muscat nappes and 11 kbar, 400°C for the As Sifah unit. The corresponding surface-related gradient is low (10°–15°C/km). During the retrograde evolution, pressure and temperature generally decreased concomitantly, but in some of the upper units temperature remained constant or increased slightly for some time, at which point sudoite growth from carpholite took place. The metamorphic pattern and mineral evolution are interpreted in the framework of the Late Cretaceous obduction. The already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin (with coeval basaltic volcanism). Pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite, the maximum thickness of which was about 25 km in the As Sifah area. In this area, the overburden was also attributable to the accumulation of continental material (up to 5 km thick) by means of a complex set of synmetamorphic, often conjugated folds and thrusts. The basal peridotites of the obducted slab were probably not hotter than 300° C during its thrusting upon the continental margin. The temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas. Late thrusting in the same southerly direction controlled the retrograde evolution, together with erosional and gravity-driven tectonic unloading.

Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism

Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively‐exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less‐well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.