KFMASH System Research Articles

A new activity model for biotite and its application

A new activity model for biotite is formulated in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (KFMASHTO), which extends that for the KFMASH system by introducing a titanium-biotite and a ferric-biotite end-member (tbio: K(TiMg2)[(O)2(AlSi3)O10] and fbio: K(Fe3+Mg2)[(OH)2(Al2Si2)O10]), as well as a pyrophyllite end-member (pyp: Al2[(OH)2Si4O10]) that accounts for the presence of octahedral excess-Al in natural biotites. Phonon calculations applying density functional theory (DFT) using the software Castep yielded the standard entropies of tbio and fbio as Sotbio = 328.06 J/(mol·K) and Sofbio = 301.69 J/(mol·K), and their heat capacity functions. From experimental phase-equilibrium data, the enthalpy of formation value of tbio was constrained as ΔHf,tbioo\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {H}_{f,tbio}^{o}$$\\end{document} = −6124.68 ± 3.33 kJ/mol. Natural data were used to derive ΔHf,fbioo\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {H}_{f,fbio}^{o}$$\\end{document}= −5935.3 ± 6.6 kJ/mol. The single-defect DFT method was applied to parameterize important macroscopic mixing properties (macro-W’s) involving tbio and pyp end-members in the model (fbio was treated ideal). Castep-derived microscopic interaction energies (micro-w’s) are presented herein for KFMASH-biotite. The octahedral same-site (M1) Mg–Al mixing micro-w (wMgAl(M1)), the same-site tetrahedral Si-Al mixing parameter (wSiAl(T1)) and the related cross-site term are: wMgAl(M1) = 82.5 kJ/mol, wSiAl(T1) = 95.6 kJ/mol (two T1-sites) and wMgAlAlSi(M1T1)=\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${w}_{MgAlAlSi(M1T1)}=$$\\end{document} 175.1 kJ/mol. The linear combination of these micro-w’s gives a macroscopic Wphleas = 18.8 kJ/mol, that is not transferable to other mineral groups. Micro w’s for Mg-Fe mixing in biotite (wMgFe(M1), wMgFe(M2),wMgMgFeFe(M1M2)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${w}_{MgMgFeFe(M1M2)}$$\\end{document}), are all close to ideality. The biotite activity model of this study is thus a first example of next-generation activity models that use DFT- and thus physically based micro-w’s and reassembled macro-W’s for petrological calculations. Test calculations on 5 samples from low- to high-grade metamorphic environments covering metapelite to greywacke bulk-compositions using Perple_X suite of programs illustrate the performance of the new biotite activity model. Computed mineral-chemistries are in all cases in better agreement with measured compositions than resulting from published activity models of biotite.

A new activity model for Fe\u2013Mg\u2013Al biotites: II\u2014Applications in the K2O\u2013FeO\u2013MgO\u2013Al2O3\u2013SiO2\u2013H2O (KFMASH) system

The new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe–Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), using mainly the software Perple_X. These pseudosection calculations (termed ‘our calculation(s)’ in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261–286, 2014, 10.1111/jmg.12071), termed ‘W14 calculation’. Our calculations with the experimental bulk composition used by Zhou (Ti–Mg–Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 °C and 1.1 Kb with fO2 defined by the CH4–graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe–Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (AlVI) in these experiments. The experimental data of Ferry and Spear (—FS78, Contrib Mineral Petrol 66:113–117, 1978, 10.1007/BF00372150) on the Fe–Mg distribution between biotite and garnet (Grt) are well reproduced by our calculations. The computed composition of biotite (XFe) in equilibrium with garnet of Alm90Py10 composition and the resulting lnKD values as a function of temperature are in good agreement with the experimental brackets. An analogous W14 calculation on the same Fe-rich bulk composition predicts too high XFeBt in order of 0.1 mol fraction. The AlVI contents of biotite of about 0.3–0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218) in similar biotite–garnet exchange experiments performed with Alm80Py20 and Alm70Py30 garnets, are well reproduced by our, as well as by W14 calculations. The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained AlVI in the same order of ca. 0.3–0.4 apfu. A T–XFe (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of lnKD of the Mg/FeGrt/Bt exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high-T/low-P and a natural high-P metapelite. The pseudosections show that biotite according to our biotite model breaks down at lower temperatures and pressures than predicted from the W14 biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high-T/low-P metapelites. At higher pressures, biotite according to our calculations breaks down at lower pressures than computed with the W14 biotite model. Before biotite breaks down, however, its AlVI content based on our calculations could potentially be used for pseudosection barometry, similarly as the Si-in-phengite barometer. These trends need to be confirmed by a future extension of our model which incorporates Ti, Fe3+ and a di–tri-octahedral substitution.

Metamorphic P–T paths of metapelitic granulites from the Larsemann Hills, East Antarctica

Through detailed textural observations, a peak M1 assemblage garnet+orthopyroxene+cordierite+K-feldspar has been identified in a structurally early Al-rich metapelitic granulite lens from the Larsemann Hills, East Antarctica. The M1 assemblage has been overprinted by M2 cordierite corona and M3 orthopyroxene+cordierite symplectite on garnet grains. Quantitative modeling for the peak M1 assemblage via the THERMOCALC program in the KFMASH system suggests that it was formed by the crossing of the univariant reaction garnet+biotite=cordierite+orthopyroxene+K-feldspar+melt under P–T conditions of 6–8kbar and 840–880°C, followed by post-peak near isobaric cooling. However, the average P–T calculations for the boron-bearing pelitic granulite indicate that peak M1 conditions reached ~9.0kbar and ~900°C, and the overprinting M2 assemblage formed under P–T conditions of ~7.0kbar and 800–850°C, reflecting a post-peak near isothermal decompression. P–T estimates show that M3 conditions reached 4–5kbar and 700–750°C. These imply that the M1 metamorphic evolution of the region displays contrasting P–T paths, while M2 to M3 evolution indicates a decompression-cooling process. The available chronological data support that the M1 metamorphic evolution occurred during the late Proterozoic (1000–900Ma) Grenvillian high-grade compression tectonic event (D1), and was accompanied by strong magmatism, showing a close affinity to the northern Prince Charles Mountains and the Rayner complex. However, the overprinted M2 to M3 metamorphic evolution formed during the early Palaeozoic (~530Ma) Pan-African high-grade tectonic events (D2–D3), and was associated with an important intracontinental reworking. This study presents an example for interpreting a complex polymetamorphic history.

Stability of chloritoid + biotite-bearing assemblages in some metapelites from the Palaeoproterozoic Singhbhum Shear Zone, eastern India and their implications

Abstract The arcuate Singhbhum Shear Zone (SSZ) of the East Indian Shield area, fringing the Archaean Singhbhum Craton, exposes a mélange-like ensemble of polymetamorphosed and highly tectonized siliciclastic sediments, mafic–ultramafic extrusive, tourmaline-rich rocks, magnetite–apatite rocks, granites and Cu–Fe–U sulphide ores of Meso- to Palaeoproterozoic age (1.5–1.77 Ga). Metapelites from two localities in the SSZ developed the enigmatic assemblage chloritoid–biotite–garnet–chlorite, which are the first reported from India and the eighth in world occurrence. Textural studies and algebraic analyses of the phase compositions in the KFMASH system indicate the operation of the reaction, chloritoid+biotite→garnet+chlorite+H 2 O and this reaction has a negative slope in pressure–temperature ( P – T ) space. Quantitative geothermobarometry and pseudosections in the NCMnKFMASHO system indicate that this reaction occurred during prograde metamorphism that culminated at 6.3±1 kbar and 490±40 °C. The stability of the chloritoid+biotite is also sensitive to the MnO content of the bulk rock, and to the chemical potential of H 2 O and oxygen ( μ H 2 O and μ O 2 , respectively), of the ambient fluid phase. Thrusting of continental crust in a collisional setting is invoked to explain the peak metamorphic temperatures and the clockwise P – T trajectory is construed from the petrological study of the chloritoid–biotite schist.

Origin and composition of fluid responsible for metasomatic processes in shear zones of the Bolshie Keivy tectonic nappe, baltic shield: Carbon isotope composition of graphite

The formation of the observed structure of the Keivy terrain is related to the nappe formation during the Paleoproterozoic collisional processes at the southwestern boundary of the Kola fragment of the Archean Craton. The Bolshie Keivy tectonic nappe with a length of >160 km is the upper part of the tec� tonostratigraphic section of the Keivy terrain (1-3) and composed of plates and lenses of blastomylonitic, predominantly highaluminum metamorphic and metasomatic rocks of the socalled Keivy Series (4). It contacts by shear zones with rocks of the lower Leb� yazhinskii nappe mainly composed of biotite and gar� net-biotite gneisses, and, to a lesser extent, biotite- amphibole gneiss and amphibolite gneisses and amphibolites (interpreted as metavolcanites) with numerous bodies of alkaline metasomatites. Plates and lenses of rocks in the considered nappe are sepa� rated from each other by shear zones and often zones of silicification with variable thicknesses. There is a clear connection between repeated metasomatism and narrow fluidtransporting shear zones, elongated for many tens of kilometers. Among the main varieties of metasomatic rocks in the Bolshie Keivy nappe are kyaniterich rocks with variable con� tents of quartz, plagioclase, staurolite, biotite and muscovite, containing kyanite-quartz veins, as well as the areas of plagioclasites and plagioclaserich crystal� line schists interpreted as protolith relics. They are mainly concentrated in the shear zones of the lower and upper parts of nappe, at the contact with metavol� canites of the Lebyazhinskii nappe. These metasoma� tites were the objects of isotope-geochemical investi� gations aimed at solution of the problem of the origin and composition of fluid responsible for metasomatic processes in the shear zones of the Bolshie Keivy tec� tonic nappe. Based on the analysis of parageneses and thermo� dynamic calculations, we determined the physico� chemical conditions of the formation of metasoma� tites and host metamorphic rocks. The calculated val� ues of the water activity in dense hydrous-saline fluid vary from 0.4-0.6 in aluminiferous gneiss to 0.6-0.8 in metasomatites. Petrogenetic diagrams of the stabil� ity of parageneses in P-T coordinates were plotted for the most typical rocks of the KFMASH system for dif� ferent water activities. As a result, the general P-T trend (Fig. 1) was obtained for the Bolshie Keivy nappe reflecting the evolution of metamorphic and metaso� matic petrogenesis in the anticlockwise direction, first by the line of isobaric cooling (6.3-7.2 kbar and 640- 490°C) and, then, isothermal decompression (up to 5-4 kbar and ~500°C). Analysis of the P-T evolution is limited by the area of stability of staurolite paragene� ses. The more lowtemperature alterations in the con� sidered zones were not reflected in considered estima� tions. The age of metasomatites (1762-1720 Ma) is determined by local methods (SHRIMP II) (2). The area of isothermal decompression is confirmed by the results of fluid inclusion study (Table 1). More detailed analysis of these results will be made in a spe� cial paper. In this study we mention only two peculiar� ities of the fluid. The first of them implies that in the period of fluid capture and the formation of primary and refilled (primary-secondary) inclusions, as well as during the later processes (secondary inclusions), fluid was heterogeneous consisting of at least phases: pure CO 2 and solutions of calcium and sodium chlorides with the maximal salinity of >40 wt %. The second fea� ture, the most important for us, implies that in addi� tion to the abovementioned phases, most of the stud� ied samples contained inclusions of methane and

Geochemistry, provenance, and metamorphic evolution of Gabal Samra Neoproterozoic metapelites, Sinai, Egypt

Metapelites are exposed at Wadi Ba’ba, east of Abu Zenima city; represent the northwestern extension of the Fieran-Solaf Metamorphic Complex, Sinai Peninsula, Egypt. The metapelites are characterized by qtz + pl (An 24–28) + bt + grt ± crd ± sil mineral assemblage, indicating upper amphibolite facies with peak metamorphic conditions of 700 °C and pressures of 7 kbar, as determined by conventional geothermobarometeric methods. This resulted in incipient migmatization, forms patches of leucosomes and melanosomes. Geochemical investigation indicates that the precursor sediments of the metapelites had been deposited as immature Fe-rich shales from source materials of dominantly intermediate composition. Source area exhibited weak to moderate chemical weathering in a tectonically active continental marginal basin within a continental-arc system. A strong shallow-dipping foliation, characterizing the metapelites, was folded around an open antiform with sub-horizontal south plunging hinge. Phase equilibria calculations in the KFMASH system indicate that the peak metamorphic conditions formed at 730–750 °C and 6.8–7.9 kbar. This was followed by a retrogression formed at 770–785 °C and 3.9–4.5 kbar. Hence, this implies an isothermal decompression and rapid exhumation of the metapelites from depth (25–29 km) in the lower crustal level at peak conditions, continuous to include shallow to middle crustal level (14–17 km), at overprint retrograde conditions. Subsequent isobaric cooling took place at 720–750 °C and 3.6–4.5 kbar. The resulting isothermal decompression followed by isobaric cooling clockwise P–T path of the metapelites is more likely, in which the high-temperatures attained maximum conditions during isothermal decompression were enhanced by heat flux, due to the presence of an active magmatic arc that formed on top of subducting young lithosphere. This is supported by a moderate geothermal gradient of 27–43 °C/km and dating compatibility of the Sinai granitoids and the metamorphic complexes. The P–T path segment records the tectonothermal histories of crustal thickening as a result of the East and West Gondwana collision at the metamorphic peak. This was subsequent by extensional and crustal thinning with syn-metamorphic magmatic intrusions, during P–T path retrogression, which resulted in the final assembly of the Arabian–Nubian Shield during Neoproterozoic.

P– T modeling of kyanite and sillimanite paramorphs growth after andalusite in late Paleozoic Pyeongan Supergroup, South Korea: Implication for metamorphism during the Mesozoic tectonic evolution

Metasedimentary rocks of the Pyeongan Supergroup in the northeastern flank of the Taebaeksan Basin, South Korea, commonly contain two Al 2SiO 5 polymorphs (And + Ky and And + Sil), or more rarely, three coexisting Al 2SiO 5 polymorphs. Lower-grade rocks preserve chloritoid + andalusite ± kyanite in muscovite–chlorite phyllite, and higher-grade rocks contain andalusite + sillimanite (± kyanite) + staurolite ± garnet in mica schists. Kyanite and sillimanite occur as partial paramorphs after andalusite. Textural relations and relative metamorphic P– T conditions of the Al 2SiO 5-bearing mineral assemblages suggest a crystallization sequence of andalusite → kyanite → sillimanite, with the growth of staurolite and garnet in the kyanite and sillimanite stability fields. Metamorphic P– T conditions determined from geothermobarometry, compositional isopleths of chloritoid and garnet, and petrogenetic grids in the MnO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O (MnKFMASH), MnKFMASH–T(TiO 2)–O(Fe 2O 3), and Mn–N(Na 2O)–C(CaO)–KFMASH systems could be divided into two regimes temporally: a clockwise path at lower temperature (LT)/medium pressure (MP) conditions, and higher temperature (HT)/low pressure (LP) conditions. The peak metamorphic P– T conditions for regional LT/MP metamorphism are ca. 5.0–6.0 kbar at 550–580 °C, and those for HT/LP metamorphism are ca. 3.0–4.8 kbar and 590–610 °C. Kyanite paramorphs after andalusite may result from a LT/MP clockwise P– T path associated with a rapid contractional deformation during the early to middle Mesozoic, and then sillimanite paramorphs after andalusite may have been formed by magmatic heating due to the later intrusion of leucocratic granite.

Petrologic modeling of chloritoid–glaucophane schists from the NW Iberian Massif

Two P– T pseudosections computed with Thermocalc and Perplex software for high-pressure pelitic Ceán Schists from the NW Iberian Massif show that the paragenesis chloritoid + glaucophane, preserved as relict micro-inclusions in garnet porphyroblasts, is stable above 17–18 kbar, indicating a first stage of subduction with 65–70 km of burial. The subsequent growth of biotite and albite porphyroblasts, according to their stability fields in a MnNCKFMASH P– T phase diagram, indicates strong decompression accompanied by slight heating to reach a metamorphic peak at ∼ 500 °C. This mineral association, described in many subduction-related terranes around the world as one of the high-pressure indicators for metapelites, has not been reported previously in the NW Iberian Massif. The schists contains an initial high-pressure assemblage formed by chloritoid + garnet (Alm 0.58 Prp0 0.03 Grs 0.38 Sps 0.09) ± glaucophane + phengite (3.5–3.4 Si p.f.u.) + paragonite + chlorite + epidote + rutile + ilmenite + quartz, preserved as micro-inclusions in garnet, chloritoid and albite porphyroblasts defining an S 1 internal fabric. The matrix foliation (S 2) additionally contains a high-pressure association formed by garnet (Alm 0.68 Prp 0.04 Grs 0.25 Sps 0.03) + phengite (3.4–3.3 Si p.f.u.) + paragonite + winchite + barroisite + hornblende + chloritoid + chlorite + epidote + rutile + ilmenite + albite + quartz ± biotite. An initial pseudosection calculated in the KFMASH system with Thermocalc 3.26 gives pressure estimates through Si-content in phengite barometry that are in agreement with conventional thermobarometry and Thermocalc average P– T calculations, but is unable to describe the full complexity of the mineral assemblages of the schists. For this reason, a more complete P– T pseudosection in the model system MnNCKFMASH was calculated with Perplex 07. This provides a reliable succession of mineral assemblages that are consistent with the petrographic observations and allows a mineralogical sequence to be set for each metamorphic event in the P– T evolution of the high- P pelitic Ceán Schists.

Tectonic significance of structural successions preserved within low-strain pods: Implications for thin- to thick-skinned tectonics vs. multiple near-orthogonal folding events in the Palaeo-Mesoproterozoic Mount Isa Inlier (NE Australia)

Event-wise structural successions and their regional correlation across the orogenic belts are always been problematic, resulting in construction of conflicting tectonic models. However, in recent years, it has been demonstrated that complex deformation and metamorphic overprinting relationships and their correlation across large regions can be resolved through careful quantitative analysis of fabrics preserved within low-strain pods at different scales. In this study, integrated macro, meso and microstructural analyses of the southern Selwyn Range of the Palaeo-Mesoproterozoic Mount Isa Inlier, NE Australia, have revealed that the tectonic evolution involved four dominant fabric-generating events. These differ from those reported previously based on interpretation of the Mount Isa deep seismic section. D 1 is preserved in low-strain zones as outcrop-scale recumbent folds (S 1 subparallel S 0) with E–W trending fold axes. D 2 is characterized by upright to inclined N–S trending folds with a pervasive regional foliation (S 2), subparallel to S 0/S 1. D 3 was very heterogeneously developed in the region and appears to be controlled by rheological contrast. Three structural domains are recognized. The Eastern Domain (ED) and Central Domain (CD) are characterized by shallow dipping S 3 foliation. S 3 foliation is rare in the Western Domain (WD). Deformation during D 4 was intense in the WD and produced complex overprinting patterns. All these observations from macro to microscopic scale suggest that the regional S 2 initially formed as a steep foliation and was deflected to shallow to moderate dips due to the effect of subvertical shortening (D 3). These observations are difficult to reconcile with previously suggested thin- to thick-skinned nappe-style tectonics in the region. Metamorphic field gradient is higher (sillimanite zone) in the ED, which represents deeper crustal levels than the rocks exposed in the CD and WD separated by pre-existing faults. Andalusite porphyroblasts in the WD preserve steeply dipping S 2, whereas staurolite porphyroblasts preserve gently dipping S 3, suggesting staurolite grew after andalusite. This interpretation is consistent with the appearance of mineral phases in the KFMASH system along a prograde P– T path (M 2) for D 2 and D 3. An earlier higher pressure P– T path (M 1) is proposed for D 1 based on the metamorphic mineral assemblages in the MnNCKFMASH system.

Geochronology and metamorphic P - T - X evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution

Abstract Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860±50 °C and 7–8 kbar (M 1 ) were followed by a decrease of pressure to 4.3±1 kbar and of temperature to around 700 °C, associated with the development of migmatites (M 2 ). After cooling, a third thermal phase at c . 650 °C and 3–4 kbar (M 3 ) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151±8 Ma, the granulite-facies and migmatitic metamorphisms (M 1 –M 2 ) at 2062±39 Ma and the medium-grade metamorphic assemblage (M 3 ) at 614±11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan-African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution.

First Report of the Spinel + Quartz Assemblage from Kodaikanal in the Madurai Block, Southern India: Implications for Ultrahigh-Temperature Metamorphism

We report the equilibrium assemblage of spinel (X Mg = Mg/(Fe+Mg) = 0.40-0.43, ZnO~0.58 wt%, Cr2O3 = 0.39-0.44 wt%) and quartz enclosed within the core of poikiloblastic garnet in pelitic gneiss from the Kodaikanal region, Madurai Block, southern India. The petrogenetic grid in the KFMASH system suggests that such Zn- and Cr-poor spinel associated with quartz is an indicator of ultrahigh-temperature (UHT) metamorphism, which is confirmed by calculated peak P-T conditions using a garnet-orthopyroxene assemblage in charnockite (860-990°C and 7.5-8.5 kbar) and antiperthitic feldspar in a leucocratic layer of pelitic migmatite (T ~1000°C). Detailed observation of retrograde textures as well as mineralogical and thermobarometric data suggest that the area underwent near isobaric cooling and then decompression after the UHT event. Comparison of available P-T trajectories of the Madurai Block and adjacent areas suggests that the Kodaikanal region underwent ~1000°C UHT metamorphism similar to other domains in the Madurai Block, although the peak pressure condition is slightly lower than other areas, possibly indicating shallower crustal levels. Results of this study confirm that the Madurai Block is an example of a regional UHT terrane associated with the Pan-African orogeny attending final amalgamation of the Gondwana supercontinent.

Phase Equilibria in the Process of Anatexis in High-grade Metapelites

Anatexis is a widespread phenomenon in high-grade metamorphic rocks, which has drawn considerable attention of geologists. Due to the advancements in the study of phase equilibrium since the beginning of the 1990s, the formation and fractionation of melts and relevant retrograde reactions can be quantitatively studied using the software THERMOCALC and the pseudosection method. There are three main melting regimes in metasedimentary rocks: melting at water-saturated solidus, muscovite dehydration melting, and biotite dehydration melting reactions. Calculation of phase equilibria in KFMASH and NCKFMASH systems indicates that the phase relations of ferromagnesian minerals are controlled by the KFMASH system. But the H 2O-saturated solidus in the KFMASH system is 50–60°C, higher than the actual minimum solidus in metapelites. Thus, the phase relations at temperature above the solidus of metapelite can be better simulated in the NCKFMASH and more complicated systems. Recent researches indicate that the preservation of granulite facies requires the melt loss. The formation of migmatite includes partial melting, partial segregation and extraction of melt, and partial back and retrograde reactions.

Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application

Specially designed experiments were carried out in the temperature range 950-1100 °C at 800 MPa to grow coexisting sapphirine and spinel-bearing metapelitic assemblages in the KFMASH system. Equilibrium phase assemblages contain these two phases with stoichiometric compositions. Compositional data obtained from the two coexisting phases indicate a strong dependence of the Mg-Fe distribution coefficient on temperature. An empirical thermometric expression has been formulated from the experimentally constrained relationship, lnKD = -6277/T(°K) + 6.4202, where KD = (XMg/XFe)Spr.(XFe/XMg)Spl. Application of this thermometer to natural mineral pairs indicate that earlier temperature estimates based on sapphirine-spinel need to be revised upward by 120-160 °C.

Modelling phase–assemblage diagrams for magnesian metapelites in the system K2O–FeO–MgO–Al2O3–SiO2–H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X A as well as P and T. The predicted mineral associations for the low- and high-X A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.

Implicaations of micro-compositions of garnet and biotite from high-grade meta-pelites

Abstract Based on detailed studies on the compositional zoning of garnet and biotite in pelitic rocks from the Jingshan group of granulite facies in north Jiaodong, P-T pseudosections with isopleths of Fe/(Fe+Mg) in garnet and biotite were calculated in the KF-MASH system for two representative rocks of sillimanite-garne-cordierite-biotite gneiss (Vbi/Vg>1) and sillimanite-garnet gneiss (Vbi/Vg 1 can nearly preserve their peak compositions, and biotites in garnet-rich rocks with Vbi/Vg<0.2 ...

Metamorphic evolution and reworking of the Sleaford Complex metapelites in the southern Eyre Peninsula, South Australia

The partial melting processes of Al‐rich and cordierite‐absent garnet‐orthopyroxene‐bearing leucosome assemblages in the reworked Archaean Sleaford Complex in southern Eyre Peninsula, South Australia are examined. The Kimban Orogeny produced a pervasive development of garnet‐K‐feldspar‐dominated leucosome assemblages. The textural relationships suggest that the peak assemblage garnet‐orthopyroxene‐K‐feldspar involved a melt phase and formed through the divariant partial melting reaction garnet + biotite = orthopyroxene + K‐feldspar + liquid in the KFMASH system, during a prograde KD1 heating event. A cordierite‐bearing metapelite also preserves the peak KD1 assemblage of garnet‐cordierite‐orthopyroxene‐K‐feldspar, and the textural relationships suggest that peak assemblages formed through the crossing of the univariant partial melting reaction garnet + biotite = orthopyroxene + cordierite + K‐feldspar + liquid in the KFMASH system, during the KD1 prograde heating stage. It is suggested that peak metamorphic P‐T conditions reached 850–900°C and 0.9–1.0 GPa, and that the preservation of peak assemblages means that some melt was lost near or during the metamorphic peak. The pervasive interstitial K‐feldspar grains nucleated synchronously with decompression, and the second generation of small euhedral garnets are inferred to have formed through decompression‐generated partial melting during a KD2 high‐strain shear event. The P‐T conditions of 700–750°C and 0.5–0.6 GPa were attained during the KD2 high‐grade shear event. The inferred clockwise P‐T path from KD1 to KD2 is compatible with a collisional tectonic model during the Kimban Orogeny (ca 1730–1700 Ma), whereas syn‐KD2 decompression‐cooling was a result of rapid tectonic exhumation of the terrain.

Calculated phase equilibria for low‐ and medium‐pressure metapelites in the KFMASH and KMnFMASH systems

AbstractPetrogenetic grids in the KFMASH and KMnFMASH model systems calculated with the software thermocalc 3.1 are presented for the P–T range 0.5–12 kbar and 450–900 °C, for assemblages involving garnet, muscovite, chloritoid, biotite, chlorite, staurolite, cordierite, spinel, orthopyroxene, K‐feldspar, Al2SiO5 phases, quartz, water and melt. Based on calculated compatibility diagrams and P–T and T–MMn [Mn/(Mg + Fe + Mn)] pseudosections for different metapelitic bulk compositions, the principal conclusions are that the addition of Mn to the KFMASH system: (i) enhances the stability of garnet, and, to a lesser extent, aluminosilicates; (ii) reduces the stability of staurolite, cordierite and, to a lesser extent, chlorite; and (iii) extends the medium pressure stability of muscovite and the low‐P stability field of K‐feldspar. The influence of Mn on individual mineral stabilities is strongly related to rock composition, in particular, to the relative contents of Al2O3 and K2O. For metapelites of a range of compositions and MMn values, P–T pseudosections in the KFMASH system, in most cases, do not adequately predict the mineral assemblages observed in natural assemblages under medium and low‐pressure conditions. In contrast, the P–T pseudosections in the KMnFMASH system generally provide more satisfactory results, suggesting that MnO is one of the non‐KFMASH components that should not be neglected in documenting the phase equilibria of medium‐ and low‐P metapelites.

Silica depleted melting of pelites. Petrogenetic grid and application to the Susqueda Aureole, Spain

AbstractMetapelitic rocks in the low pressure contact metamorphic aureole around the Susqueda igneous complex, Spain show a number of features that make them an ideal testing ground for the modelling of silica‐undersaturated melting. Rocks in the aureole experienced localized depletion in silica by the segregation of quartz veins during a pre‐anatectic, regional cordierite‐andalusite grade metamorphic event. These rocks were then intruded by gabbroic to dioritic rocks of the Susqueda igneous complex that formed a migmatitic contact metamorphic aureole in the country rocks. This migmatisation event caused quartz‐saturated hornfels and restite formation in rocks that had experienced no quartz vein segregation in the previous regional metamorphic event, but silica‐undersaturated melting in those rocks that were previously depleted in silica. Silica‐undersaturated melting is investigated using a new petrogenetic P–T projection and equilibrium pseudosections calculated in the KFMASH and NCKFMASH systems, respectively. The grid considers quartz absent equilibria and a range of phases that form typically in silica‐undersaturated bulk compositions, for example corundum. It is shown that the quartz‐rich precursors in the Susqueda contact aureole produced about 10% melt during contact metamorphism. However, most of this melt was extracted leaving behind rocks with restitic bulk compositions and minor leucosome segregation. It is suggested that the melt mixed with the host igneous rocks causing an apparent magmatic zoning from diorite in the centre of the complex to tonalite at the margins. In contrast, the quartz‐poor precursors (from which the quartz veins segregated) melted in the silica‐undersaturated field producing a range of assemblages including peritectic corundum and spinel. Melting of the silica‐undersaturated rocks produced only negligible melt and no subsequent melt loss.

Liquid compositions from low‐pressure experimental melting of pelitic rock from Morton Pass, Wyoming, USA

AbstractLow‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.

Garnet-chloritoid-kyanite metapelites from the Raspas Complex (SW Ecuador): a key eclogite-facies assemblage

The Raspas Complex (Ecuador) contains one of the few eclogitic bodies in the northern Andes. It consists of meta- peridotites, eclogites, and metapelites. The latter display three assemblages: (i) garnet + chloritoid + kyanite, (ii) garnet + chlori- toid and (iii) garnet + chlorite, in all cases with quartz and muscovite in addition. The growth of these assemblages was coeval with the main ductile deformation, and was followed by minor reequilibration (chlorite growth in garnet + chloritoid samples and chloritoid + quartz aggregates replacing garnet and kyanite in garnet + chloritoid + kyanite samples). Detailed microprobe anal- yses show increasing magnesian compositions for garnet (from core to rim) and chloritoid (inclusions within garnet compared to matrix grains) in kyanite-bearing samples. The above data are interpreted in the framework of the KFMASH system. Reaction progress along the divariant reaction Cld = Grt + Ky explains the change in chemistry of coexisting phases. The divariant Grt-Cld- Ky assemblage has a narrow stability field, and the P-T conditions are estimated at about 20 kbar, 550-600°C. Decompression, recorded by chloritoid-quartz pseudomorphs of garnet, probably occurred as temperature decreased.