Coupled petrological-geodynamical modeling of a compositionally heterogeneous mantle plume
Abstract Self-consistent geodynamic modeling that includes melting is challenging as the chemistry of the source rocks continuously changes as a result of melt extraction. Here, we describe a new method to study the interaction between physical and chemical processes in an uprising heterogeneous mantle plume by combining a geodynamic code with a thermodynamic modeling approach for magma generation and evolution. We pre-computed hundreds of phase diagrams, each of them for a different chemical system. After melt is extracted, the phase diagram with the closest bulk rock chemistry to the depleted source rock is updated locally. The petrological evolution of rocks is tracked via evolving chemi…
On the interpretation of retrograde reaction textures in granulite facies rocks
Retrograde reaction textures, such as partial pseudomorphs, coronae and symplectic coronae are a relatively common feature of granulite facies rocks and represent evidence of change of mineral modes or assemblages. Thus, such reaction textures represent one of the more potentially useful indicators of retrograde P-T evolutions. However, the interpretation of such textures requires the simultaneous consideration of the formation of the texture itself in terms of the spatial relationships in a rock, and of the changes in P-T conditions that drove the development of the reaction texture. Calculated phase diagrams provide a robust framework in which to consider reaction textures, especially via…
Thermobarometric constraints on pressure variations across the Plattengneiss shear zone of the Eastern Alps: implications for exhumation models during Eoalpine subduction
Forward and inverse mineral equilibria modelling of metapelitic rocks in the hangingwall and footwall of the Plattengneiss, a major shear zone in the Eastern Alps, is used to constrain their tectonometamorphic evolution and assess models for their exhumation. Forward (pseudosection) modelling of two metapelitic rocks suggests a steep clockwise P–T path with a near-isothermal decompression segment from a pressure peak at ~18–19 kbar and 670 °C to the metamorphic peak at 680–720 °C and 11–13 kbar. A subsequent decrease to 600–645 °C and 8–9 kbar is inferred from the late growth of staurolite in some samples. Conventional thermobarometric calculations (inverse modelling) on 18 samples with the…
Petrogenetic modelling of strongly residual metapelitic xenoliths within the southern Platreef, Bushveld Complex, South Africa
Xenoliths of quartz-absent Fe-rich aluminous metapelite are common within the platinum group element-rich mafic ⁄ ultramafic magmatic rocks of the Platreef. Relative to well-characterized protoliths, the xenoliths are strongly depleted in K2O and H2O, and have lost a substantial amount of melt (>50 vol.%). Mineral equilibria calculations in the NCKFMASHTO system yield results that are consistent with observations in natural samples. Lower-grade rocks that lack staurolite constrain peak pressures to � 2.5 kbar in the southern Platreef. Smaller xenoliths and the margins of larger xenoliths comprise micro-diatexite rich in coarse acicular corundum and spinel, which record evidence for the meta…
Emergence of blueschists on Earth linked to secular changes in oceanic crust composition
The oldest blueschists—metamorphic rocks formed during subduction—are of Neoproterozoic age1, and 0.7–0.8 billion years old. Yet, subduction of oceanic crust to mantle depths is thought to have occurred since the Hadean, over 4 billion years ago2. Blueschists typically form under cold geothermal gradients of less than 400 °C GPa−1, so their absence in the ancient rock record is typically attributed to hotter pre-Neoproterozoic mantle prohibiting such low-temperature metamorphism; however, modern analogues of Archaean subduction suggest that blueschist-facies metamorphic conditions are attainable at the slab surface3. Here we show that the absence of blueschists in the ancient geological rec…
Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach
The Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from ∼2.4 GPa to ∼4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India–Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure–temperature–time (P–T–t) path has been derived from thermomechanical simulations tailored for E…
The Proterozoic P–T–t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites
Integrated metamorphic and geochronological data place new constraints on the metamorphic evolution of a Neoproterozoic orogen in east Antarctica. Granulite-facies rocks from a 150 km stretch of the Kemp Land coast reflect peak conditions involving T � 870^9908 Ca t P� 7� 4^10 kbar, with pressure increasing westward towards an Archaean craton. Electron microprobe-derived (Th þ U)^Pb monazite ages from metapelitic assemblages indicate that the major mineral textures in these rocks developed during the c. 940 Ma Rayner Orogeny. Complex compositional zoning in monazite suggests high-T recrystallization over c. 25 Myr. Diversity in metapelitic reaction textures reflects silica and ferromagnesia…
Insights into the compositional evolution of crustal magmatic systems from coupled petrological-geodynamical models
Funding was provided by the VAMOS Research Center, University of Mainz (Germany) and by the ERC Consolidator Grant MAGMA (project #771143). The evolution of crustal magmatic systems is incompletely understood, as most studies are limited either by their temporal or spatial resolution. Exposed plutonic rocks represent the final stage of a long-term evolution punctuated by several magmatic events with different chemistry and generated under different mechanical conditions. Although the final state can be easily described, the nature of each magmatic pulse is more difficult to retrieve. This study presents a new method to investigate the compositional evolution of plutonic systems while consid…
Generation of Earth's early continents from a relatively cool Archean mantle
This research has been supported by DFG grant, SPP 1833 Building a Habitable Earth and MAGMA Consolidator Grant (ERC project #71143). Several lines of evidence suggest that the Archean (4.0 2.5 Ga) mantle was hotter than today's potential temperature (TP) of 1350 ° C. However, the magnitude of such difference is poorly constrained, with TP estimation spanning from 1500 ° C to 1600 ° C during the Meso‐Archean (3.2‐2.8 Ga). Such differences have major implications for the interpreted mechanisms of continental crust generation on the early Earth, as their efficacy is highly sensitive to the TP. Here, we integrate petrological modeling with thermomechanical simulations to understand the dynami…
U-Pb monazite ages from the Pakistan Himalaya record pre-Himalayan Ordovician orogeny and Permian continental breakup
The Greater Himalayan Sequence in India and Nepal records crustal thickening processes that took place during and following the onset of India-Asia collision (ca. 54–50 Ma). These resulted in late Eocene–early Miocene kyanite- and sillimanite-grade regional metamorphism, and Oligocene–Miocene crustal anatexis, which formed migmatites and leucogranites. We present new U-Pb monazite data for kyanite- and sillimanite-grade gneisses of the Neoproterozoic to Cambrian Tanawal Formation in the Pakistan Himalaya, which have metamorphic ages of 482.4 ± 7.9 Ma and 464.5 ± 4.0 Ma, respectively. These ages, together with along-strike equivalent rocks in the Lesser Himalaya of India and Nepal, help to d…
Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites
Calculated mineral equilibria are used to account for the formation of sapphirine-plagioclase, spinel-plagioclase and corundum-plagioclase symplectites replacing kyanite in quartz-plagioclase-garnet-kyanite granulite facies gneisses from the Southern Domain of the Athabasca granulite terrane, a segment of the Snowbird tectonic zone in northern Saskatchewan, Canada. Metamorphic conditions of >14 kbar and 800 °C are established for the high pressure, garnet-kyanite assemblage using constraints from P-T pseudosections and Zr-in-rutile thermometry. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near-isothermal decompression to <10 kbar. The che…
Quantitative phase petrology of cordierite-orthoamphibole gneisses and related rocks
Cordierite-orthoamphibole gneisses and rocks of similar composition commonly contain low-variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO-MgO-Al2O3-SiO2-H2O (FMASH), Na2O-CaO-K2O-FeO-MgO-Al2 O3- SiO2-H2O (NCKFMASH) and Na2O-CaO-K2O-FeO-MgO-Al2O3- SiO2-H2O-TiO2-Fe2 O3 (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite-orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calcu…
High-grade metamorphism and partial melting in Archean composite grey gneiss complexes
Much of the exposed Archaean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terrains were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a-x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. Calculated P–T pseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate…
Nonlithostatic pressure during subduction and collision and the formation of (ultra)high-pressure rocks
The mechanisms that result in the formation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks are controversial. The usual interpretation assumes that pressure is close to lithostatic, petrological pressure estimates can be transferred to depth, and (U)HP rocks have been exhumed from great depth. An alternative explanation is that pressure can be larger than lithostatic, particularly in continental collision zones, and (U)HP rocks could thus have formed at shallower depths. To better understand the mechanical feasibility of these hypotheses, we performed thermomechanical numerical simulations of a typical subduction and collision scenario. If the subducting crust is laterally homogen…
Processes in granulite metamorphism
Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)
The metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consi…
Design and performance of the prototype Schwarzschild-Couder telescope camera
Journal of astronomical telescopes, instruments, and systems 8(01), 014007-1 (2022). doi:10.1117/1.JATIS.8.1.014007
High-grade metamorphism and partial melting of basic and intermediate rocks
Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) compositional system for a range of natural basic and interm…
Second asymptomatic carotid surgery trial (ACST-2): a randomised comparison of carotid artery stenting versus carotid endarterectomy.
Summary Background Among asymptomatic patients with severe carotid artery stenosis but no recent stroke or transient cerebral ischaemia, either carotid artery stenting (CAS) or carotid endarterectomy (CEA) can restore patency and reduce long-term stroke risks. However, from recent national registry data, each option causes about 1% procedural risk of disabling stroke or death. Comparison of their long-term protective effects requires large-scale randomised evidence. Methods ACST-2 is an international multicentre randomised trial of CAS versus CEA among asymptomatic patients with severe stenosis thought to require intervention, interpreted with all other relevant trials. Patients were eligib…
Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna
Abstract The mid to lower crustal metamorphic field gradient through amphibolite and granulite facies rocks in the Ivrea Zone offers the potential to study partial melting and melt loss in the crust. Metapelitic rocks in Val Strona di Omegna show a progressive evolution in migmatite structures from metatexites with rare isolated leucosome veins in the amphibolite facies rocks to stromatic migmatites and diatexites in granulite facies rocks. Little field or petrographic evidence for melting can be seen on crossing the position of the modelled wet solidus, consistent with the small amounts of melt predicted to occur by H2O-saturated melting. The first field evidence for partial melting, in th…
Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks
A set of thermodynamic models is presented that, for the first time, allows partial melting equilibria to be calculated for metabasic rocks. The models consist of new activity–composition relations combined with end-member thermodynamic properties from the Holland & Powell dataset, version 6. They allow for forward modelling in the system Na (Formula presented.) O–CaO–K (Formula presented.) O–FeO–MgO–Al (Formula presented.) O (Formula presented.) –SiO (Formula presented.) –H (Formula presented.) O–TiO (Formula presented.) –Fe (Formula presented.) O (Formula presented.). In particular, new activity–composition relations are presented for silicate melt of broadly trondhjemitic–tonalitic compo…
On the importance of minding one’s Ps and Ts: metamorphic processes and quantitative petrology
This Special Issue comprises a selection of the papers given at a two-day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powells 60th birthday. At this milestone, it is fitting to review Rogers career to date. He has published 200 scientific papers on topics that range from low- to high-grade metamorphism, from low- to ultrahigh-pressure (UHP) metamor- phism, and from thermodynamics to kinetics. Most of Rogers papers are multi-authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic proper…
Partial melting of metabasic rocks and the generation of tonalitic–trondhjemitic–granodioritic (TTG) crust in the Archaean: Constraints from phase equilibrium modelling
Abstract Rocks of tonalitic–trondhjemitic–granodioritic (TTG) composition preserved in Archaean terranes represent fragments of the Earth’s earliest-formed continental crust, and are thought to have formed via partial melting of hydrated metabasalt. The geodynamic environments in which such high-grade metamorphism and anatexis may have occurred in the early Earth is strongly debated. Constraining the pressure ( P ) and temperature ( T ) conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question. Phase equilibrium modelling has been undertaken for an enriched Archaean tholeiite bulk compo…
Metamorphism and melting of picritic crust in the early Earth
Abstract Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15 wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium …
False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data
Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and ⁄ or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P- T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P-T stability fields of assemblages. Mineral equilibria…
Field and petrographic evidence for partial melting of TTG gneisses from the central region of the mainland Lewisian complex, NW Scotland
The central region of the mainland Lewisian complex is dominated by granulite-facies tonalite–trondhjemite–granodiorite (TTG) gneisses that are highly depleted in some mobile trace elements (Cs, Rb, Th and U) relative to amphibolite-facies TTG gneisses elsewhere in the Lewisian complex and to the average composition of TTG gneisses worldwide. Over almost half a century of research there has been vigorous debate as to the origin of this depletion, in particular with respect to the role of partial melting and melt loss. Here we provide field and petrographic evidence that TTG gneisses across the central region partially melted during granulite-facies (Badcallian) metamorphism. Partial melting…
Plume — Lid interactions during the Archean and implications for the generation of early continental terranes
Abstract Many Archean terranes are interpreted to have a tectonic and metamorphic evolution that indicates intra-crustal reorganization driven by lithospheric-scale gravitational instabilities. These processes are associated with the production of a significant amount of felsic and mafic crust, and are widely regarded to be a consequence of plume-lithosphere interactions. The juvenile Archean felsic crust is made predominantly of rocks of the tonalite–trondhjemite–granodiorite (TTG) suite, which are the result of partial melting of hydrous metabasalts. The geodynamic processes that have assisted the production of juvenile felsic crust, are still not well understood. Here, we perform 2D and …
Garnet and spinel lherzolite assemblages in MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2: thermodynamic models and an experimental conflict
The recent publication of an updated thermodynamic dataset for petrological calculations provides an opportunity to illustrate the relationship between experimental data and the dataset, in the context of a new set of activity–composition models for several key minerals. These models represent orthopyroxene, clinopyroxene and garnet in the system CaO–MgO–Al2O3–SiO2 (CMAS), and are valid up to 50 kbar and at least 1800 °C; they are the first high-temperature models for these phases to be developed for the Holland & Powell dataset. The models are calibrated with reference to phase-relation data in the subsystems CaO–MgO–SiO2 (CMS) and MgO–Al2O3–SiO2 (MAS), and will themselves form the basis o…
Polymetamorphism in the mainland Lewisian complex, NW Scotland - phase equilibria and geochronological constraints from the Cnoc an t’Sidhean suite
The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet-bearing biotite-rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid-driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have c…
Using calculated chemical potential relationships to account for coronas around kyanite: an example from the Bohemian Massif
Corona textures around kyanite, involving for example zoned plagioclase separating kyanite from the matrix, reflect the instability of kyanite with the matrix on changing P-T conditions, commonly related to decompression. The chemical potential gradients set up between the kyanite and the matrix as a consequence of slow Al diffusion drive corona development, with the zoning of the plagioclase reflecting the gradients. Calculated mineral equilibria are used to account for corona textures involving plagioclase ± garnet around kyanite, and replacement of kyanite by plagioclase + spinel symplectite, in quartz + plagioclase + K-feldspar + garnet + kyanite granulite facies gneiss from the Blanský…
Partial melting of metagreywacke: a calculated mineral equilibria study
Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid-crustal depths (6–8 kbar), high-T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K-feldspar (±Fe–T…
Partial melting of metabasic rocks in Val Strona di Omegna, Ivrea Zone, northern Italy
Field and petrographic observations combined with major and trace element bulk rock geochemistry show that metabasic rocks within Val Strona di Omegna in the central Ivrea Zone partially melted during granulite facies regional metamorphism. A transition from granoblastic amphibolite facies metabasic rocks at the lowest metamorphic grades to metatexitic and diatexitic migmatites in the granulite facies records the effects of in situ fluid-absent partial melting. Coarse-grained euhedral clinopyroxene porphyroblasts within leucosomes are consistent with anatexis via incongruent fluid-absent melting reactions consuming hornblende, plagioclase and quartz to form clinopyroxene and melt. Field obs…
Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland
Abstract The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss…
Phase equilibrium constraints on conditions of granulite-facies metamorphism at Scourie, NW Scotland
Abstract: The metamorphic evolution of a metapyroxenite and metagabbro from Scourie, NW Scotland, is investigated using phase equilibria modelled in the NCFMASHTO (Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O) system. The calculated stability fields for the observed assemblages in each rock overlap and imply conditions of 8.5–11.5 kbar and 875–975 °C for the peak of granulite-facies (Badcallian) metamorphism. The lack of any evidence for the former presence of garnet in the metapyroxenite suggests that the rocks cannot have reached pressures much in excess of those recorded at the metamorphic peak. The growth of coronas of plagioclase, orthopyroxene and magnetite replacing garnet in t…
A year in the life of an aluminous metapelite xenolith—The role of heating rates, reaction overstep, H2O retention and melt loss
Abstract Xenoliths of aluminous metapelite within the Platreef magmatic rocks of the Bushveld Complex, South Africa, are mineralogically and texturally zoned, with coarse-grained margins rich in acicular corundum, spinel and feldspar and cores rich in finer-grained aluminosilicate and cordierite. Xenoliths exhibiting remarkably similar features occur within other intrusions, suggesting a common origin. Using a single 3 m wide xenolith as a case study, a model is proposed to explain their petrogenesis. Mineral equilibria calculations in the NCKFMASHTO system show that the thermal stability of the solid phases, in particular corundum, is highly sensitive to the quantity of H 2 O retained in t…
New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland
The research carried out for this study was part of YF's Master Thesis at the Institute of Geoscience, Johannes Gutenberg University, Mainz, which provided the funding for fieldwork and laboratory analyses. TJ acknowledges support from Open Fund GPMR210704 from the State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. In this study we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archaean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na2O-CaO‐K2O‐FeO‐MgO‐Al2O3-SiO2-H2O-T…
On equilibrium in non-hydrostatic metamorphic systems
Metamorphic geology has accumulated a huge body of observation on mineral assemblages that reveal strong patterns in occurrence, summarised for example in the idea of metamorphic facies. On the realisation that such patterns needed a simple explanation, there has been considerable a posteriori success from adopting the idea that equilibrium thermodynamics can be used on mineral assemblages to make sense of the patterns in terms of, for example, the pressure and temperature of formation of mineral assemblages. In doing so, a particularly simple implicit assumption is made, that mineral assemblages operate essentially hydrostatically. Structural geologists have studied the same rocks for diff…
Retrograde melt-residue interaction and the formation of near-anhydrous leucosomes in migmatites
Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P–T conditions change, gradients in chemical potential are established between the melt-rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H2O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with mel…
The effect of Mn on mineral stability in metapelites revisited: new a -x relations for manganese-bearing minerals
The a-x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a-x relations for metapelitic rocks in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn-bearing minerals were estimated using the micro-ϕ{symbol} approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-end-members were calibrated for orthopyroxene, cordierit…
The importance of iron speciation (Fe+2 /Fe+3 ) in determining mineral assemblages: an example from the high-grade aluminous metapelites of southeastern Madagascar
Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk-rock compositions when measured on a FeT basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock-forming oxides (i.e. Al2O3:FeT:MgO), but instead reflect variations in the pre-metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation – either Fe+2 or Fe+3. The relatively reduced rocks of the Amparihy…
Melt production, redistribution and accumulation in mid-crustal source rocks, with implications for crustal-scale melt transfer
Abstract Ascent of granitic melt initiates under suprasolidus conditions in the mid- to lower crust before continuing through subsolidus rocks to higher crustal levels. Whereas migration of melt in suprasolidus rocks can occur in pervasive net-like structures and involve relatively small melt volumes, ascent through the subsolidus crust requires more focused, dyke-like structures and larger volumes to prevent freezing. Migmatites in the Aus granulite terrain, southern Namibia, preserve evidence that large-scale melt redistribution and accumulation occurred in the near-source region under suprasolidus conditions. Melt that was mainly produced in metapelitic rocks utilised pervasive small-sca…
Archaean Intracrustal Differentiation from Partial Melting of Metagabbro--Field and Geochemical Evidence from the Central Region of the Lewisian Complex, NW Scotland
The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic^ultramafic bodies that exhibit macroscopic features diagnostic of anatexis. In situ partial melting of metagabbroic rocks produced patches and sheets of coarse-grained plagioclase-rich leucosome containing euhedral peritectic clinopyroxene.These leucosomes connect with larger, laterally continuous tonalite or trondhjemite sheets that record segregat…
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 Na O-CaO-K O-FeO-MgO-Al O - SiO -H O-TiO -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 p…
Phase Relations, Reaction Sequences and Petrochronology
At the core of petrochronology is the relationship between geochronology and the petrological evolution of major mineral assemblages. The focus of this chapter is on outlining some of the available strategies to link inferred reaction sequences and microstructures in metamorphic rocks to the ages obtained from geochronology of accessory minerals and datable major minerals. Reaction sequences and mineral assemblages in metamorphic rocks are primarily a function of pressure ( P ), temperature ( T ) and bulk composition ( X ). Several of the major rock-forming minerals are particularly sensitive to changes in P–T (e.g., garnet, staurolite, biotite, plagioclase), but their direct geochronology …
Clockwise, low- metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny
Abstract The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5 kbar and 825 ° C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼ 550–600 ° C at below 4 kbar, whereas the replacement of sillimanite by garnet–cordierite and sub…
On parameterizing thermodynamic descriptions of minerals for petrological calculations
A new regularization approach, termed micro-ϕ, is outlined for parameterizing activity–composition (a–x) relations and other aspects of the thermodynamic descriptions of minerals for petrological calculations. In the context of the symmetric formalism, a formulation of a–x relations that is easily generalizable to multi-component minerals, parameterization with micro-ϕ extends from where there are good data available to constrain, for example, interaction energies, to where there are little or no data. This involves decomposing the interaction energies, which are macroscopic between end-members, into their microscopic components involving interactions between elements on sites. Micro-ϕ invo…
Is the Crucible Reproducible? Reconciling Melting Experiments with Thermodynamic Calculations
Experimental studies and thermodynamic modelling have advanced our understanding of partial melting in the crust and have provided a frame-work for the interpretation of migmatites, residual granulites and granites. Each approach has advantages and pitfalls, and each is more appropriate than the other for investigating particular aspects of the melting process. A comparison of these two approaches may be useful because, together, they potentially give more information. A comparison of a small number of experiments with model calculations using equivalent bulk compositions shows important consistencies between the results, especially regarding the overall topologies of key melting equilibria…
New mineral activity-composition relations for thermodynamic calculations in metapelitic systems
New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (2011, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the fir…
Origin, age, and significance of deep-seated granulite-facies migmatites in the Barrow zones of Scotland, Cairn Leuchan, Glen Muick area
Funding for this work was provided by the Johannes-Gutenberg University of Mainz. Petrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones.Phase diagrams constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system have constrained the P–T conditions of peak metamorphism in the Glen Muick region of the upper‐sillimanite zone (Sill+Kfs) to have been at least ~840 ◦C at ~9 kbar (high‐pressure granulite facies). These conditions are approximately ~120 ◦C and ~3 kbar higher than those reco…