Granulite-facies Assemblages Research Articles

Exhumation of deeply subducted continental crust along contrasting pathways: Examples from the North Qaidam ultrahigh pressure metamorphic terrane, North Tibet

Deeply buried continental terranes now exposed in the core of mountain belts are characterized by dominant felsic gneisses enclosing eclogite and garnet peridotite. Some medium–low temperature eclogites can preserve peak mineral assemblages. In contrast, some eclogites and their country rocks have been turned to granulite-facies assemblages. The apparent discrepancy between the two types of eclogites may reflect different exhumation pathways and mechanisms. Based on detailed field investigation and geological mapping, two ultrahigh pressure (UHP) metamorphic units are recognized in the western North Qaidam Mountains. The Yuka-Luofengpo UHP metamorphic unit (YKU) is characterized by medium–low temperature eclogites and metapelites, whereas the Luliangshan UHP metamorphic unit (LLU) primarily consists of granulitized eclogites and pelitic granulites. The two units are separated by an early Paleozoic arc magmatic-metamorphic unit.The disparate metamorphic evolutions of the UHP metamorphic rocks from the YKU and LLU indicate that the two adjacent metamorphic units underwent different exhumation pathways. We combine our data with published geodynamic numerical modeling to infer that the YKU UHP metamorphic rocks detached from the subducted slab at different depths and exhumed within a cold continental subduction channel. In contrast, the granulitized eclogites and pelitic granulites of the LLU were vertically exhumed from the subduction channel through the hot mantle wedge into the overriding crustal plate by diapirism (relamination) and experienced a prolonged granulite facies overprint prior to exhumation to the shallow crust.

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

AbstractThe Entia Gneiss Complex represents the mid‐crustal core of the intracontinental Alice Springs Orogen. Located in the Harts Range, central Australia, it is characterized by the development of a domal structure including two sub‐domes separated by a steeply dipping median high‐strain zone. Dominantly, orthogneiss basement crops out through a structurally overlying cover sequence represented by the Harts Range Group and records evidence of hydration, recrystallization, and partial melting of precursor Paleoproterozoic granulite facies assemblages at amphibolite facies conditions. The structurally lowest parts of the Harts Range Group were metamorphosed to peak conditions of 10.5 kbar and ~880°C during rift‐related magmatism at 480–460 Ma, immediately prior to the onset of the Alice Springs Orogeny at 450–300 Ma. By contrast, the underlying Entia Gneiss Complex records widespread metamorphism and wet melting at upper amphibolite facies conditions. Phase equilibrium modelling and in situ LA–ICP–MS geochronology of rare, low‐variance kyanite–garnet‐bearing metapelites indicate that maximum P–T conditions of ~9 kbar and ~680°C were reached between c. 360–330 Ma, demonstrating a distinctive metamorphic and temporal evolution relative to the overlying Harts Range Group that can be linked to rheological stratification. Based on the integration of our results with existing monazite, zircon, and titanite geochronology, we interpret doming of the Entia Gneiss Complex to have involved compressive ascent of rheologically weakened crust below a region of extending upper crust near the termination of the Alice Springs Orogeny. In this way, the Harts Range Group represents a cooled, locally extending thrust sheet over ductile basement quasi‐concurrent with doming. Texturally‐late sillimanite combined with increasing Y content in monazite indicates high‐temperature, kyanite‐grade metamorphism was closely followed by decompression (3–4 kbar drop from peak conditions) and rapid cooling below 600°C during emplacement of the Entia Gneiss Complex at shallower crustal levels. The findings of this study highlight the feedback between hydration, retrogression, rheological weakening, and strain accommodation, thus allowing better evaluation of the thermomechanical history of gneiss dome formation within a narrow (<80 km wide) preconditioned intracontinental corridor.Highlights First modern P–T–t framework for gneiss dome formation in the Entia Gneiss Complex, central Australia. The onset of metamorphism, partial melting and ductile flow at c. 360 Ma was catalysed by hydration of granulite facies basement during rift inversion. Rapid exhumation of partially molten crust from depths of >20 km is marked by fluid‐mediated resetting of monazite down to c. 310 Ma. Rheological stratification enabled and accelerated exhumation of the highest‐grade corridor of a narrow intracontinental orogen.

Controls on the scales of equilibrium during granulite facies metamorphism

AbstractPhase equilibrium modelling is commonly employed to constrain the pressure–temperature (P–T) evolution of granulite facies rocks, from which their geodynamic setting may be inferred. However, defining a suitable equilibrium volume in such rocks is non‐trivial due to heterogeneities in protolith composition and open system behaviour, including melt loss or gain and deformation. Consequently, equilibrium volumes and the mineral assemblages they contain may vary both temporally and spatially within a single rock. Additionally, despite the extreme temperatures they attained, granulites commonly contain microtextures indicating gradients in chemical potential after the metamorphic peak. This study assesses the processes that control compositional heterogeneity between equilibrium volumes in a suprasolidus granulite and the consequences for the development of a range of microstructures. An approach combining phase equilibrium forward modelling and chemical potential diagrams is used to simulate the suprasolidus evolution of two adjacent equilibrium volumes in an Mg‐rich metapelite. Assuming equilibrium within the two compositional domains, evolving granulite facies assemblages vary along a high T/P (125°C/kbar) clockwise P–T path representative of high T/P metamorphic terranes. Many retrograde microtextures in metapelitic rocks can be reproduced by considering chemical potential (µ) gradients in µMgO, µFeO and µCaO between phases, assuming (a) the presence of melt on grain boundaries, (b) that Al2O3 is perfectly immobile and (c) that K2O and Na2O are perfectly mobile. Additionally, documented microtextures in granulites imply local SiO2‐undersaturation, despite the presence of matrix quartz in the rock. Preserving such chemical potential gradients requires that solid phases are chemically and physically isolated from melt. Efficient prograde melt drainage may lead to a loss in melt interconnectivity at high temperature (>900°C), meaning that equilibration of centimetre‐scale compositional domains is controlled by solid‐state diffusion. The presence of isolated pockets of viscous melt allows the formation of discontinuous intergranular reaction microtextures.

High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa

The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane.Some previous models have related the 3.2 Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2 Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.

Prolonged high-temperature, low-pressure metamorphism associated with ∼1.86 Ga Sancheong–Hadong anorthosite in the Yeongnam Massif, Korea: Paleoproterozoic hot orogenesis in the North China Craton

Elevated heat flow in association with mafic magmatism in an orogenic belt commonly leads to high-temperature, low-pressure (HTLP) metamorphism and the production of granulite-facies assemblages. We studied such a HTLP complex in the vicinity of the Sancheong–Hadong anorthosite-mangerite-charnockite-granite (AMCG) suite, Yeongnam Massif, Korea, in order to constrain the P–T conditions, timing, and duration of metamorphism. This complex primarily consists of massif-type anorthositic-gabbroic bodies emplaced at ∼1.87–1.86 Ga and a series of country rocks comprising orthopyroxene-bearing gneisses as well as anatectic granites and migmatites. Migmatitic gneisses were studied in detail because melt-related features are abundant and well preserved; for example, inclusion-rich peritectic phases such as cordierite or K-feldspar are characteristic for the prograde melt-forming stage, whereas biotite-quartz symplectites mantling garnet or orthopyroxene represent the cooling stage consuming melt. Pseudosection P–T analyses of migmatitic gneisses suggest peak metamorphic conditions of 810–840 °C and 5.9–6.2 kbar, followed by near-isobaric cooling and melt crystallization at ∼780 °C and ∼5.5 kbar. SHRIMP (sensitive high-resolution ion microprobe) U–Th–Pb ages of zircon and monazite from six migmatitic gneisses are in the range of 1870–1854 Ma. The oldest age, recorded only in high Y monazite, suggests that prograde metamorphism commenced at ∼1870 Ma. In contrast, melt crystallization had culminated by 1860–1855 Ma producing widespread leucosomes and anatectic granites. Our results suggest that high thermal gradient (∼40 °C km−1) attending the granulite-facies metamorphism is attributable to coeval, pulse-like emplacement of anorthositic-gabbroic magmas. Moreover, the HTLP metamorphism lasted over a period of ∼15 Ma, indicating a long-lived process corresponding to the late stage of Paleoproterozoic (∼1.95–1.85 Ga) hot orogenesis in the North China Craton.

Spatial distribution, P–T–t paths, and tectonic significance of high-pressure mafic granulites from the Daqingshan–Wulashan Complex in the Khondalite Belt, North China Craton

High-pressure (HP) mafic granulites in the Daqingshan–Wulashan Complex from the central segment of the Khondalite Belt of the North China Craton occur as irregular lenses or deformed dike swarms within dioritic–granitic gneisses and meta-sedimentary rocks. The HP mafic granulites contain three distinct mineral assemblages: a peak HP granulite-facies assemblage (M1) that consists of garnet+clinopyroxene+plagioclase+amphibole±quartz±Fe–Ti oxides; a post-peak decompression assemblage (M2) that is characterized by symplectites of orthopyroxene+clinopyroxene+plagioclase±amphibole±Fe–Ti oxides; a late cooling assemblage (M3) represented by symplectites of amphibole+plagioclase+Fe–Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrain the P–T conditions of the M1, M2, and M3 assemblages to P=1.25–1.40GPa and T=760–815°C, P=0.72–0.80GPa and T=825–860°C, and P=0.69–0.81GPa and T=645–730°C, respectively. In-situ U–Pb dating of zircons show that the protolith ages of the HP mafic granulites are 2100–2000Ma and the timing of peak HP granulite-facies metamorphism is 1960–1940Ma. The medium- to low-pressure amphibolite- to granulite-facies retrogression occurred mainly from 1920 to 1820Ma. Petrographic investigations, mineral compositions, metamorphic reaction histories, P–T calculations, and geochronological data define a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling, which suggests that the Daqingshan–Wulashan Complex underwent initial crustal thickening from 1960 to 1940Ma, followed by relatively rapid exhumation, cooling, and retrogression from 1920 to 1820Ma. This P–T–t path was likely the result of the Paleoproterozoic continental–continental (or arc) collision during the amalgamation of the North China Craton.

Discovery and geological significance of high-pressure mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, the Jiao–Liao–Ji Belt, the North China Craton

High-pressure (HP) mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, in the southwestern segment of the Jiao–Liao–Ji Belt of the North China Craton, occur as irregular lenses or deformed dike swarms within the Paleoproterozoic graphite-bearing paragneisses and marbles. Petrographic examination has revealed three distinct metamorphic mineral assemblages: a peak HP granulite-facies assemblage (M1) that consists of garnet+clinopyroxene+plagioclase+quartz±amphibole±Fe–Ti oxides, a post-peak decompression assemblage (M2) that is characterized by symplectites of orthopyroxene+clinopyroxene+plagioclase±amphibole±Fe–Ti oxides, and a late cooling assemblage (M3) represented by symplectites of amphibole+plagioclase+Fe–Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrained the P–T conditions of the M1, M2 and M3 assemblages to P=1.28–1.44GPa and T=757–805°C, P=0.50–0.80GPa and T=780–840°C, and P=0.55–0.73GPa and T=665–730°C, respectively. An integrated study involving laser Raman spectroscopy and scanning electron microscope analysis of mineral inclusions, cathodoluminescence imaging, and in-situ U–Pb dating of zircons showed that the protolith ages of the HP mafic granulites are mainly 2200–2000Ma and that the timing of the peak HP granulite-facies metamorphism ranges from 1950 to 1900Ma, as recorded by the cores of metamorphic zircons. The medium- to low-pressure amphibolite- to granulite-facies retrogression occurred mainly at 1900–1800Ma, as recorded by the rims of some zircon grains as well as zircon grains that contain inclusions of clinopyroxene+orthopyroxene+plagioclase+amphibole+sphene. Comprehensive petrographic, mineralogical, and geochronological investigations of the HP mafic granulites defined a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling, which further suggests that the Jiaobei Terrane had underwent initial crustal thickening during 1950–1900Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1900–1800Ma. This P–T–t path was probably generated by the Paleoproterozoic collisional orogenesis of the North China Craton.

The metamorphic evolution of Paleoproterozoic eclogites in Kuru-Vaara, northern Belomorian Province, Russia: Constraints from P-T pseudosections and zircon dating

The rare occurrence of Precambrian eclogite has been used to argue when plate tectonics initiated. Well-preserved kyanite-bearing eclogites were collected in the Kuru-Vaara quarry, northern Belomorian Province, Fennoscandian Shield. Textural observations and phase equilibria modeling were applied to constrain their P-T evolution. The eclogite is characterized by the peak pressure assemblage garnet+omphacite+kyanite+amphibole+quartz. The garnet has inclusions of zoisite, omphacite and kyanite. This omphacite with high jadeite (up to 33mol%) and 5% Ca-Eskola content bears oriented rods of quartz, amphibole, and rutile. Omphacite was replaced by diopside+plagioclase±orthopyroxene symplectites and garnet surrounded by plagioclase coronas. Kyanite was mantled by the assemblage sapphirine+spinel+plagioclase±scapolite. The symplectites of diopside+plagioclase±orthopyroxene are further replaced by amphibole. P-T estimates of peak eclogite facies conditions are c. 18–20kbar and 720–820°C. The granulite facies assemblages formed at c. 11–13kbar and 870–915°C. Zircon U-Pb dating reveals the protolith ages of 2802–2929Ma for the eclogites. The c. 1896Ma zircon ages correspond to the time of prograde metamorphism in eclogite facies. The age of 1885±7Ma for a corundum-bearing mica pegmatite vein that intruded into the eclogites reflects the infiltration of fluid at amphibolite facies conditions. Thus, the Kuru-Vaara eclogite was metamorphosed during the Paleoproterozoic, not the Archean. The P-T path of Kuru-Vaara eclogites was characterized by eclogite facies metamorphism overprinted by granulite facies and retrograde amphibolite facies metamorphism within c. 10Ma. This process reflects the fast exhumation of the eclogites. The increase in temperature from eclogite to granulite facies in the HT-UHT domain suggests a hot Precambrian tectonic environment.

Comment on “Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications”. Tectonophysics, 662, 219–242 (2015) by Ishwar-Kumar et al.

Determining the possible tectonic regimes active during the Neoproterozoic is crucial for the knowledge of the evolution of the super-continent Gondwana. In Madagascar, that occupies a key position in Gondwana, there is an on-going debate regarding the location of possible suture zones and the implications in terms of paleo-geography. Recognizing high-pressure to ultra-high pressure conditions in mafic rocks is commonly viewed as a strong argument for paleo-subduction zones. Ishwar-Kumar et al. (2015) report unusual high pressure conditions (24kbar) in Neoproterozoic to Cambrian rocks from North-Central Madagascar (Andriamena Complex). They propose a geodynamic model in which exhumation of the high pressure terranes from up to ~80km to ~40km occurred via vertical extrusion during the collision of various crustal blocks after subduction and closure of an oceanic domain during the formation of Gondwana in the late Neoproterozoic to Cambrian. We question this model and in particular the (ultra-)high pressure conditions reported, because their estimation is based on a misinterpretation of the petrography and inaccurate thermodynamic modeling for the crucial metabasite sample. The authors suggest that garnet-quartz coronas around orthopyroxene and ilmenite coexist with clinopyroxene. The postulated garnet-clinopyroxene-quartz assemblage is interpreted to document an eclogite facies overprint. However, the presence of abundant plagioclase in the sample and the lack of high jadeite content in clinopyroxene clearly refute the postulated eclogite facies conditions. According to the presented photographs clinopyroxene is part of the rock matrix. We therefore suggest that the sample represents a common two-pyroxene granulite, formed at mid- to low-pressure granulite facies conditions of >700°C and <6kbar, consistent with PT data of former studies for the Andriamena Complex. Garnet-quartz-bearing coronas produced at the expense of the granulite-facies assemblage could have been produced during isobaric cooling. Although a subduction zone may have been active during this period of time in Madagascar, this interpretation is not justified by the data presented by Ishwar-Kumar et al. (2015).

Kiosk domain, Central Gneiss Belt, Grenville Province, Ontario: A Labradorian palimpsest preserved in the ductile deep crust

The Kiosk domain is a thrust sheet within a multi-component Grenvillian thrust stack, the polycyclic (evidence for pre-Grenvillian orogeny present) Bonfield-Kiosk–Algonquin stack. The stack is overlain to east and west by thrust sheets of predominantly monocyclic gneiss and uplifted along a ramp of Archean parautochthonous rocks. Central to the thrust stack, the Kiosk domain contains a homoclinal southeast-inclined core locally preserving granulite facies assemblages. Predominantly amphibolite facies shear zones, bounding and within the stack, and southeast trending folds overprint the margins of the core fabric. Finally, the Kiosk basal shear zone is locally reworked by extension. Monzonitic orthogneiss in Kiosk domain contain zircon with cores indicating protolith ages of ca. 1640Ma, and dark mantles yield an age of migmatization (and possibly of granulite facies) of ca. 1450Ma. However, metadiabase dykes containing granulite assemblages cut the ca. 1450Ma leucosome and share a southeast trending LS fabric with the country rock of the homoclinal core suggesting the possibility of an early Grenvillian, granulite event. Dating the amphibolite facies overprint is generally challenging, thin zircon rims yielding ages with large errors suggest an Ottawan (ca. 1090–1030Ma) age for stacking, consistent with a 1078±28Ma age for a shear zone which cuts the internal fabric of the stack. The Kiosk domain thus preserves successively younger Grenvillian events from core to rim interpreted to represent early thickening (homoclinal core LS fabric), retrogression-related softening (pegmatites), nappe flow (marginal shears and folds), and post convergence extension (localised shearing).The record of Paleo- early Mesoproterozoic igneous-metamorphic history preserved in the Kiosk core is complemented by detrital zircon data from a quartzite member of a metasedimentary assemblage. The detrital zircon display an age concentration spanning ca. 1500–2000Ma with a Penokean (ca. 1850Ma) maximum. Pre-Labradorian components of the age spectrum are similar to published spectra from the Penokean Upper Animikie sediments from the Lake Superior region and may represent reworking of these during early Pinwarian deposition on the Labradorian basement. We speculate that the Kiosk metasediments, metamorphosed at high grade with their hosts at ca. 1459Ma represent a pre-or early-Pinwarian back-arc (age of youngest detrital grain, ca. 1500Ma) in Labradorian crust inverted in the Pinwarian Orogeny and consistent with A-type plutonism of this age present in the Central Gneiss Belt and interpreted to have been emplaced in extended back-arc crust. The Kiosk domain is thus established as a Labradorian member of the peri-Laurentian Great Paleoproterozoic Accretionary Orogen (Condie, 2013), having received detritus from other members of the accretionary orogen as well as Laurentia.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

Olivine‐bearing symplectites in fractured garnet from eclogite, Moldanubian Zone (Bohemian Massif) – a short‐lived, granulite facies event

AbstractRecent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.

Silica-undersaturated spinel granulites in the Daqingshan complex of the Khondalite Belt, North China Craton: Petrology and quantitative P–T–X constraints

The Da’nangou silica-undersaturated spinel-bearing granulites in the Daqingshan Complex of the Khondalite Belt, North China Craton, occurred within quartzo-feldspathic rocks or interlayered with garnet–cordierite–biotite gneiss. These granulites preserve a distinct mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite with reaction textures that are useful for deciphering the metamorphic evolution of the Khondalite Belt. Quantitative pressure, temperature, and bulk composition constraints on the development and preservation of a characteristic granulite facies mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite are assessed with reference to calculated phase diagrams. Combined studies of mineral assemblages, phase relations of spinel granulites in P–T–X pseudosections calculated for the NCKFMASH system, and the origin of the protoliths of these rocks reveal that spinel-bearing assemblages formed mainly in high-variance fields. Quantitative P–T–X pseudosections indicate that the spinel-bearing granulite facies assemblages are restricted only to conditions of extremely low Si with iron enrichment and high Al. A clockwise P–T evolution is inferred from the P–T pseudosection and the observed microtextures of the spinel granulites involving near-isothermal decompression and decompressional cooling stages, which is comparable to the metapelitic rocks of the Daqingshan Complex. The metamorphic history of the spinel-bearing granulites can be subdivided into the following successive stages. The peak assemblage of biotite+garnet+K-feldspar+plagioclase+sillimanite+liquid was stable at ∼830°C and ∼10.5kbar. During the post-peak decompressional stage at ∼870°C and ∼8kbar, partial melting introduced by the biotite dehydration events occurred, followed by melt extraction taking SiO2 and alkali elements, the restitic silica-undersaturated Al–Fe-rich composition remains. The coarse-grained spinel-bearing assemblage was therefore produced by the following biotite dehydration-melting reactions: biotite+garnet+sillimanite→spinel+K-feldspar+liquid and biotite+sillimanite±quartz→spinel+liquid. Symplectic spinel rimming the garnet was produced by the garnet breakdown reaction garnet+sillimanite+liquid→spinel+K-feldspar+plagioclase as the decompression and cooling. During the cooling stage after decompression at ∼800°C and ∼6.5kbar, garnet was further broken down by consumption of melt that had not been segregated, to give rise to the biotite+plagioclase symplectites related to the reactions garnet+spinel+liquid→biotite+plagioclase and garnet+liquid→biotite+plagioclase±quartz. Such silica-undersaturated spinel-bearing granulites were likely to occur in the exhumation and cooling processes after continental subduction–collision, as indicated by the P–T path.

Hydrothermal mica deposits in altered meta-ultrabasites from north-central Sri Lanka

Extensive vein-type mica deposits at Elahera in north-central Sri Lanka are associated with meta-ultrabasite (MUB) boudins in impure marbles. Mineralization occurs as discontinuous, lenticular bodies concordant with the impure marbles at the contacts with MUB. Impure marbles show typical granulite facies assemblages overprinted by retrograde minerals. The original assemblages of MUBs have been texturally and mineralogically replaced completely, but the alteration did not progress into impure marble country rocks. The alteration products are mainly phlogopite with minor tremolite, calcite and quartz. The micas have likely been formed as a result of the breakdown of diopside in the MUBs. Rock textures and chemical data of whole rocks and minerals suggest that the alteration is due to infiltration of external hydrothermal fluid through the contacts of MUB boudinage layer and impure marbles. The calcium, magnesium and CO2 required for formation of alteration minerals may have been acquired from the host rock, whereas silica, K, Cl and Al were externally derived and transported with the fluid. The most likely source of hydrothermal fluid is exsolved fluid from nearby granitic pegmatites, based on field relations showing preferred contents of mica near pegmatite contacts.

Polymetamorphic evolution of the granulite-facies Paleoproterozoic basement of the Kabul Block, Afghanistan

The Kabul Block is an elongate crustal fragment which cuts across the Afghan Central Blocks, adjoining the Indian and Eurasian continents. Bounded by major strike slip faults and ophiolitic material thrust onto either side, the block contains a strongly metamorphosed basement consisting of some of the only quantifiably Proterozoic rocks south of the Herat-Panjshir Suture Zone. The basement rocks crop-out extensively in the vicinity of Kabul City and consist predominantly of migmatites, gneisses, schists and small amounts of higher-grade granulite-facies rocks. Granulite-facies assemblages were identified in felsic and mafic siliceous rocks as well as impure carbonates. Granulite-facies conditions are recorded by the presence of orthopyroxene overgrowing biotite in felsic rocks; by orthopyroxene overgrowing amphibole in mafic rocks and by the presence of olivine and clinohumite in the marbles. The granulite-facies assemblages are overprinted by a younger amphibolite-facies event that is characterized by the growth of garnet at the expense of the granulite-facies phases. Pressure-temperature (P-T) conditions for the granulite-facies event of around 850 °C and up to 7 kbar were calculated through conventional thermobarometry and phase equilibria modeling. The younger, amphibolite-facies event shows moderately higher pressures of up to 8.5 kbar at around 600 °C. This metamorphism likely corresponds to the dominant metamorphic event within the basement of the Kabul Block. The results of this work are combined with the litho-stratigraphic relations and recent geochronological dating to analyze envisaged Paleoproterozoic and Neoproterozoic metamorphic events in the Kabul Block.

Dehydration melting and the relationship between granites and granulites

For more than half a century, thought about granite genesis and crustal evolution has been guided by the concept of partial melting in the lower crust. In this model, granitic magmas produced at depth are lost to shallow levels, leaving behind a more mafic, volatile poor residue that is depleted in incompatible components (H2O, alkalis, and heat-producing elements). Although granite extraction must be the dominant process by which crust is modified over time, the preferred model of granite genesis triggered by metamorphic dehydration reactions (dehydration melting) does not adequately explain important aspects of granite formation. The temperatures required for voluminous granite production by dehydration melting need heat and mass input to the crust from mantle-derived mafic magmas. In addition, prediction of the H2O contents of granitic liquids by extrapolation from low-pressure experiments to deep-crustal pressures (P) and temperatures (T) implies that the H2O resident in hydrous minerals is insufficient to account for large granite volumes, such as anorogenic granite batholiths in continental interiors. To test this, we conducted new experiments on the H2O contents of simple granitic liquids at 10kbar and 800–950°C. We confirm previous extrapolations from lower P and T indicating that a minimum of 3–4wt% H2O is present at the studied P and T in a granitic liquid in equilibrium with quartz and feldspars. For large-scale melting, this is much more than could have been supplied by the H2O resident in biotite and amphibole by dehydration melting at these conditions, unless lower-crustal temperatures were higher than generally inferred. Another problem with the dehydration-melting model is that the crystal chemistry of the large-ion lithophile elements (LILE) does not favor their partitioning into granitic liquids; rather, U, Th, Rb and the rare earth elements (REE) would more likely be concentrated in the postulated mafic residues. Finally, observations of migmatite complexes reveal many features that can not be satisfied by a simple dehydration-melting model.We suggest that the volatile components CO2 and Cl are important agents in deep-crustal metamorphism and anatexis. They induce crystallization and outgassing of basalt magmas at lower-crustal levels, where the combination of latent heat and liberated H2O may contribute to granite production, leading to larger melt fractions than for simple dehydration-melting models. Since the Cl and CO2 are very insoluble in granite liquids, granite generation leads naturally to production or separation of a coexisting metamorphic fluid with low H2O activity. Such a fluid could coexist with granulite-facies assemblages and yet be capable of dehydration, alkali exchange and LILE extraction to explain many chemical processes of deep-crustal metamorphism not readily explainable by dehydration melting.

Localized granulite and eclogite facies metamorphism at Flatraket and Kråkeneset, Western Gneiss Region: U–Pb data and tectonic implications

Abstract The Flatraket Complex, in the ultra-high-pressure (UHP) domains of the Western Gneiss Region (WGR) of Norway, preserves granulite facies assemblages, which were locally overprinted by eclogite and amphibolite facies metamorphism. Zircon and monazite indicate magmatic crystallization of the rocks at 1680–1640 Ma and constrain the timing of the granulite facies overprint at 1100 Ma. This age is older than previously reported ages of 1000–950 Ma for regional metamorphism reaching anatexis and locally granulite facies in the WGR. The granulites at Flatraket may have developed as a consequence of local metasomatism, perhaps linked to metasomatism occurring at the same time in the nearby Sandvik peridotite. Granitic rocks from neighbouring Kråkeneset indicate magmatic emplacement at ≥1650 Ma, during the event that formed the Flatraket Complex and the bulk of the WGR. A gabbro body at Kråkeneset is dated at 1255±8 Ma by baddeleyite, which was not affected by the granulite event, implying that the rock remained impermeable to fluids, reacting instead to some degree during the Caledonian UHP event.

High-grade metamorphism of ironstones in the Mesoarchaean of southwest Swaziland

A sequence of high-grade rocks collectively referred to as Mkhondo Valley Metamorphic Suite is exposed around the Nhlangano Gneiss Dome in southwest Swaziland and also less extensively further north. Meta-ironstones are part of this sequence and have been studied specifically with two main objectives in mind, firstly, to test the hypothesis of the high-grade metamorphic sequence correlating with lower-grade Pongola-age rocks nearby, and secondly, to assess the potential of these meta-ironstones for retrieving P-T conditions of metamorphism. The studied ironstones contain substantial Al due to terrigenous clastic input and also include Mn-rich varieties. Three compositional groups of meta-ironstones can be distinguished: garnet-orthopyroxene-bearing ones, garnet-bearing, orthopyroxene-free ones, and a garnet-poor to garnet-free type. A comparison of bulk rock compositions between these high-grade meta-ironstones and low-grade Mn-poor to Mn-rich ironstones from the Mozaan Group supports a stratigraphic correlation. Pseudosection modelling demonstrates that aluminous meta-ironstones provide useful P-T constraints at high peak temperatures as they do not tend to produce significant amounts of melt, if any. The presence of Al is critical for introducing garnet to amphibolite and granulite-facies assemblages, which also allows to apply conventional thermobarometers. The combination of pseudosection modelling and conventional thermobarometry yielded peak metamorphic conditions of 850–890 °C at 7 to 8 kbar. These data are compatible with the ones obtained from associated metapelitic gneisses, except that the latter indicate decompression to lower pressure granulite-facies conditions. The petrological, structural and geochronological data from southwest Swaziland confirm major orogenic activity in the eastern Kaapvaal Craton during the Neo-Archaean.

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 occurred largely by fluid-absent incongruent reactions consuming plagioclase, quartz, hornblende and biotite to produce melt and peritectic clino- and orthopyroxene. The preservation of dry, granulite-facies assemblages requires loss of melt, consistent with the presence of an interconnected network of leucosomes and larger felsic sheets that probably record segregation and transfer of melt to higher crustal levels. Regardless of whether or not partial melting and melt loss can explain fully the unusual geochemical signature of the central region TTG gneisses, these fundamental processes did occur. Supplementary material: Figure S1, showing additional field photographs of boulders from Poll Eòrna, is available at www.geolsoc.org.uk/SUP18580 .

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.