Granulite-facies Event Research Articles

Suturing of the Archean Bastar craton with the Eastern Ghats Province to form the Greater Indian Landmass: Insights from geochemistry, U-Pb geochronology and phase equilibria modelling

The contact between the Archean Bastar craton (BC) and Proterozoic Eastern Ghats Province (EGP), central India, is marked by a suture zone termed Terrane Boundary Shear Zone (TBSZ). BC in this area is largely composed of hornblende-biotite granite with some mafic dykes. Rocks in the TBSZ include quartzofeldspathic (leptynite) gneiss, garnet-orthopyroxene-bearing granitoid, mafic granulites (Group A of cratonic affinity, and Group B of EGP affinity), Mg-Al granulite and an isolated exposure of orthopyroxene-bearing gneiss. Detailed geochemical analysis shows remarkable similarity between Hbl-Bt granite and Grt-Opx-bearing granitoid, with A-type affinity, and between mafic dykes and Group A mafic granulites. However, the Opx-bearing gneiss is geochemically distinct having I-type affinity, similar to TTG gneisses described from BC. Metamorphic phase equilibria analysis and trace element modelling shows that (i) melting of Opx-bearing gneiss would produce a ferroan granitic melt resembling the Hbl-Bt granite, (ii) metamorphism at appropriate P-T conditions would convert the granite to Grt-Opx-bearing granitoid and the mafic dyke to Group A mafic granulite. U-Pb geochronology of zircon constrains emplacement ages of the magmatic precursors of Opx-bearing gneiss and Grt-Opx-bearing granitoid as ca. 2.73 and ca. 2.5 Ga, respectively. These rocks were subjected to an early granulite facies metamorphism, followed by an amphibolite facies metamorphism, shearing and hydrous fluid flux. Geochronological data shows that the latter event took place at ca. 0.52 Ga, while the earlier granulite facies event can only be tentatively suggested to be of late Stenian/Tonian age. Collating all the evidence (including published geophysical and geochronological data), we suggest that the initial collision between BC and EGP took place during late Stenian/Tonian time as a consequence of formation of the Greater Indian Landmass, a part of Rodinia supercontinent. The TBSZ, probably initiated due to the late Stenian/Tonian collision, was reactivated and reworked tectonothermally at ca. 0.52 Ga, caused by far field stress effect of the Kuunga orogeny, which was strong enough to obliterate most of the imprints of the late Stenian/Tonian orogeny.

Origin of discordant U[sbnd]Pb dates in non-metamict zircons in intrusives deformed at granulite facies: Grain scale processes, and relevance to Cambrian orogeny, Eastern Ghats Belt, India

Interpreting the origin of UPb discordance in zircons in intrusives that experience high-T metamorphic-deformation events is challenging. We investigate in depth the origin of UPb dates obtained from garnet-bearing border zone granitoids and a sheared ferrodiorite dyke bordering the Balangir anorthosite pluton, eastern India. The crustal domain is known for discordant ages resulting in disputes regarding the age of intrusion and multiple granulite facies events. We shed light on the origin of the discordant dates utilizing CL imaging, electron backscatter diffraction analysis, trace element mapping in zircons, and Ti-in-zircon thermometry.Zircons in the foliated border zone granitoids (BZG) comprise (a) embayed cores with varying Y, U and Hf oscillatory zones, and (b) well-faceted chemically-homogeneous mantles crystallographically continuous with the cores, which truncate the oscillatory zones. In the intensely sheared ferrodiorite, cauliflower-shaped zircons possess profuse micropores and micro-fractures. Ti-in-zircon temperatures (700–950 °C) overlap in the texturally-distinct zircons, but are somewhat lower in the mantles/cauliflower-shaped zircons relative to the oscillatory-zoned cores in BZGs.LA-ICP-MS dates of zircons in four samples constitute a near-unique discordia line with the upper and lower intercepts at ca. 980 Ma and 495 Ma. Interestingly, dates from the mantles are highly variable with no correlation between age decrease and distance to the cores. The lack of concordant dates at the two intercepts, and the increase in discordance away from the intercepts preclude episodic zircon growth, and suggests a single-stage granulite facies event (ca. 495 Ma) that modified the ∼980 Ma zircon xenocrysts entrained from protoliths into the intrusives. We propose the age discordance was caused by melt-mediated high-T interface coupled dissolution-precipitation processes at ∼495 Ma. Variable inheritance of isotopic signatures of ∼980 Ma zircon xenocrysts in the younger zircons contributed to discordance in UPb systematics; the effect of variable inheritance of ∼980 Ma isotope signatures is less pronounced in the cauliflower-shaped zircons from the HFSE enriched ferrodiorite residual melts from high degree of plagioclase fractionation.Our results are consistent with a distinct Cambrian (∼495 Ma) orogeny involving emplacement of anorthosite pluton and related intrusives and high-T granulite-facies deformation-metamorphism in the ∼980 Ma Eastern Ghats Province, and are a direct record of Cambrian collision between the Eastern Ghats Province and the Archean cratonic nucleus.

Early Mesoproterozoic (1.72–1.68 Ga) convergence between Bastar and Eastern Dharwar cratons along the Bhopalpatnam orogen, southcentral India and implication for paleogeography of supercontinent Columbia

Temporal constraints on orogen initiation and duration are critical for the Paleoproterozoic reconstruction of cratons presently constituting India. Recent late Paleoproterozoic paleomagnetic pole data indicate proximity of Eastern Dharwar, Bastar and Singhbhum cratons and in view of global Mesoproterozoic-aged final Columbia assembly, reconstruction sequence of Indic cratons is critical to assess whether they constituted Columbia. The initiation time and duration of the Bhopalpatnam orogen, developed between the Eastern Dharwar and Bastar cratons, range from earliest Mesoproterozoic to Neoarchean. Therefore, in order to constrain the earliest stages of cratonic convergence and crustal thickening, the time of peak metamorphism was established on tectonites, sampled from > 300 km along newly classified longitudinal domains representative of the entire Bhopalpatnam orogen, by precise combined garnet Lu-Hf and Sm-Nd chronology of amphibolite-and granulite-facies rocks. Interpreting the Lu-Hf and Sm-Nd isochron weighted mean ages, an orogenic duration of 38 ± 19 Myr is inferred between the high-pressure amphibolite-facies through to granulite-facies event at 1721 ± 14 Ma and a post-peak cooling to ≤ 650 °C at 1683 ± 6 Ma, being the Sm-Nd garnet weighted mean age. Archean two-stage depleted mantle model ages (at 1.7 Ga) of tectonites and rotated dyke swarm orientations support the reactivation and reworking of the Paleo- to Neoarchean Bastar craton marginal rocks in response to the early Mesoproterozoic orogeny. Based on the clockwise P-T path involving an early high-pressure amphibolite-facies stage (Pmax ≥ 12 kbar at ∼ 750 °C) followed by a granulite-facies stage (Tmax ≤ 850 °C at lower pressures), we interpret the orogen to represent deeply exhumed thrust slices of the reworked Bastar craton leading edge underthrusted beneath the Eastern Dharwar craton. As paleomagnetic data indicates proximity or contiguity of Bastar and Eastern Dharwar cratons by 1.79 Ga, the Bhopalapatnam orogen could represent site of early Mesoproterozoic cratonic collision or locus of post amalgamation reactivation due to convergence.

Coronitic and Symplectitic Textures in Meta-Troctolite Reveal the Transition from Magmatism to Granulite-Facies Metamorphism in the Early Paleozoic Tongbai Orogen, Central China

AbstractNewly discovered meta-troctolite enclaves in Early Paleozoic Huanggang diorite within dominantly meta-volcaniclastic greenschist to amphibolite facies Erlangping unit in central China extends the known extent of granulite-facies metamorphism in the Tongbai Orogen. Evidence for this granulite facies event is preserved in coronitic and symplectitic textures developed in the meta-troctolites enclaves. The primary assemblage comprises olivine and plagioclase with minor orthopyroxene, clinopyroxene, spinel and amphibole. The late magmatic stage comprising an orthopyroxene (Opx1) corona around olivine and clinopyroxene corona around spinel formed by reaction of cumulus and intercumulus phases with trapped melt at around 1040–1060°C near the solidus. During troctolite cooling, metamorphic solid-state replacement of primary olivine and plagioclase produced orthopyroxene (Opx1)/clinopyroxene (Cpx1) + vermicular spinel (Sp1) symplectite, followed by an inclusion-free amphibole (Amp1) layer surrounded by amphibole + spinel symplectite (Amp2 + Sp2). Microstructural, pseudosection and mineral isopleth analysis coupled with prominent compositional zoning in the Opx1 corona and chrome-rich spinel (Cr-Sp) suggests that the symplectites formed by diffusion-controlled solid-state replacement at the interface of reactant minerals, probably during cooling to granulite-facies conditions of ~820–880°C and 6.7–7.0 kbar. U–Pb dates of rare metamorphic zircon, as indicated by growth zonation and identification of metamorphic minerals enclosed in grains by Raman probe, are ca. 450 Ma. The reaction textures record the transition from late magmatic to granulite-facies metamorphism with the heat supplied by magma upwelling at the base of Erlangping arc triggered by extension of Erlangping back-arc.

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

Abstract The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago

U-Pb geochronology of rutile: deciphering the cooling history of the Oaxacan Complex granulites, southern Mexico

Rutile (TiO2) is a heavy mineral, commonly found as accessory in many lithologies, such as basic igneous rocks, high-grade metamorphic units, as well as a detritus in sedimentary clastic rocks. Its chemical composition is sensitive to the crystallization environment, allowing a characterization of either metabasic or metasedimentary protoliths in metamorphic rocks. Thanks to the capability to accept U in its crystalline network, at least in metasedimentary, high-grade protoliths, rutile can be dated by U-Pb geochronology. Furthermore, its closure temperature of ca. 600 °C for the U-Pb system makes rutile a suitable chronometer, complementary to zircon, to unravel provenance and exhumation paths in both sedimentary siliciclastic cover and basement units. Besides, the Zr-in thermometer allows for a very precise calculation of the rutile crystallization temperature.
 In the example case presented here, focused on granulite facies units of the Grenvillian Oaxacan Complex (OC), rutile crystallisation took place in the range 808–873 °C. Data for different localities indicate that cooling and exhumation after the Zapotecan granulite facies event (ca. 990 Ma) was heterogeneous among the different tectonic slices that constitute the OC. Cooling occurred in the central sector (Nochixtlán-Oaxaca) right after the granulite peak, with fast cooling rates of ca. 40 °C/Ma. To the north and south, the cooling to ca. 600 °C was much slower, with calculated cooling rates of ca. 3 °C/Ma for the northern OC outcrops in Coatepec (Puebla) to ca. 6 °C/Ma south of Ejutla (Oaxaca). This can be related to a combination of factors, such as an early collapse of some sectors of the orogen, a change of conditions in the subducing plate, or more in general, to a sudden change in the geodynamic conditions during the Zapotecan orogeny and Amazonia-Baltica amalgamation.
 This application example to some metasedimentary lithologies belonging to the OC demonstrates how the exhumation after the Zapotecan granulite facies event (ca. 990 Ma) was heterogeneous among the different tectonic slices that compose the OC, having occurred in the central sector (Nochixtlán-Oaxaca) right after the granulite peak, with fast cooling rates of ca. 40 ºC/M.y., whereas to the North and South the cooling to ca. 600 ºC was much slower, with calculated cooling rates of ca. 3 ºC/M.y. (north, OC outcrops in Coatepec, Puebla) to ca. 5.5 ºC/M.y. south of Ejutla (Oaxaca). This can be related to a combination of factors, such as an early collapse of some sectors of the orogen, change of conditions in the subjecting plate, or more in general, to a sudden change in the geodynamic conditions during the early stages of the Rodinia amalgamation.
 This example sharply illustrates the advantage of employing microanalytical techniques, able to resolve restricted crystal-domain chemical variations, to obtain accurate and precise temperature and age values. Furthermore, it is paramount to combine several mineral species with different closure temperatures, and collected in well-defined, recognized tectonic slices, to understand their behavior and construct meaningful cooling curves through geologic time, capable to better characterize and interpret their tectonic evolution.

Petrochronology of the Terre Adélie Craton (East Antarctica) evidences a long-lasting Proterozoic (1.7–1.5 Ga) tectono-metamorphic evolution — Insights for the connections with the Gawler Craton and Laurentia

The Terre Adélie Craton displays superimposed strain fields related to the Neoarchean (2.6–2.4 Ga, M1) and Paleo-Mesoproterozoic (1.7–1.5 Ga, M2) metamorphic events. M1 is a regional granulite facies event, constrained by P-T modelling at ~0.8–1.0 GPa – 800–850 °C, followed by a decompressional retrogression in the upper amphibolite facies at ~0.6 GPa – 750 °C. M2 Stage 1 P-T peak is constrained at 0.6–0.7 GPa – 670–700 °C, followed by a steep P-T path down to 0.3 GPa – 550 °C. Retrogression after M2 PT peak occurred in a context of dextral shearing along the Mertz Shear Zone along with thrust motions within the eastern Terre Adélie Craton. In this paper, we present a series of 63 new 40Ar/39Ar ages of biotite and amphibole pairs in mafic rocks from a complete traverse of the Terre Adélie Craton. 40Ar/39Ar dating constrains M2 amphibolite facies metamorphism at a regional scale between 1700 and 1650 Ma, during stage 1 peak metamorphism. During retrogression, lower amphibolite facies recrystallization mainly occurred along vertical shear zones and mafic dykes between 1650 and 1600 Ma (Stage 2), followed by amphibolite to greenschist facies metamorphism until after 1500 Ma (Stage 3). At the scale of the Mawson continent, this event is related to the growth of an active margin above an oblique subduction zone. The supra-subduction model best explains opening of Dumont D'Urville and Hunter basins at 1.71 Ga followed by their rapid closure and metamorphism at 1.70 Ga. In this context, episodic shear zone reactivation and magmatic dyke emplacement led to a partial reequilibration of the 40Ar/39Ar system until <1500 Ma. This latter phase of mafic magmatism largely coincides with a hot spot event at the scale of the Gawler Craton and western Laurentia paleocontinent.

Paleoproterozoic amphibolite facies retrogression and exhumation of Archean metapelitic granulites in the Southern Marginal Zone of the Limpopo Belt, South Africa

Granulite-facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis. The timing of retrogression is key to understanding the geodynamic circumstances of such events. This study has investigated amphibolite-facies retrogression of melt-depleted granulite-facies metapelites from the South Marginal Zone (SMZ) of the Limpopo belt. The granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at ~2.7 Ga, under conditions of ~11 kbar and 875 °C. Peak metamorphic garnet is replaced by cordierite + orthopyroxene symplectites, with cordierite subsequently partially replaced by fine-grained intergrowths of amphibolite-facies assemblages, including rutile. Zr-in-rutile thermometry yielded a temperature range of 731–846 °C for the granulite-facies rutiles and 629–690 °C for rutile in the retrograde assemblage; all rutile generations yielded a youngest U-Pb age of ~2.1 Ga. Some granulite-facies rutiles record older dates (2.55─2.25 Ga) which are non-systematic with regard to crystal size and represent non-reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite-facies metamorphism between ~2.7 and 2.2 Ga. The youngest ~2.1 Ga rutile U-Pb age represents the time of final exhumation-related cooling of the rocks following a discrete Paleoproterozoic amphibolite-facies retrograde event, Coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 °C. The lack of age-size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite-facies retrograde event which is inferred to be not much older. U-Pb dating of zircon from the same rocks confirms that the metapelites underwent granulite-facies metamorphism at 2695 ± 35 Ma. Thus, the metapelites of the SMZ record both an Archean granulite-facies event and a polymetamorphic Paleoproterozoic amphibolite-facies history that culminated in a mid-amphibolite facies peak inducing partial retrogression of the granulites. The Paleoproterozoic amphibolite-facies event is tentatively correlated with mantle melting documented between ~2.3 and 2.1 Ga to produce mafic and intermediate volcanic rocks in the Pretoria Group on the Kaapvaal Craton.

Looking beneath the Stawell and Bendigo zones in Victoria, Australia: a view through the granite window

Information, mainly from the granitic and silicic volcanic rocks in the Stawell, Bendigo and Melbourne structural zones in the state of Victoria, shows that the sources of both the S- and I-type rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with the sources of similar granitic rocks in the Melbourne Zone, consistent with the absence of the mainly Proterozoic Selwyn Block beneath most of the SBZ. Below a mid-crustal décollement in the SBZ, the crust is evidently highly variable and possibly includes thinned Proterozoic crust. There is geochronological evidence for ca 400 and ca 370 Ma granulite-grade metamorphic events here, and, after this double bout of metamorphism, and depletion in the silicic melt component, the constituents of the entire deep crust of the SBZ would have densities similar to those of overlying, much lower-grade Cambrian metabasaltic to boninitic rocks. Thus, granitic magmas may have formed here by partial melting of a variety of rock types, probably with back-arc affinities, with ages that may extend back to the Proterozoic. Therefore, the basement of the SBZ is unlikely to consist solely of thick ocean-floor rocks, as in some current interpretations.KEY POINTSThe sources of the Devonian granitic rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with those of the Melbourne Zone granites.Two Devonian granulite-facies events left the melt-depleted deep SBZ crust with densities similar to those of overlying Cambrian metabasaltic rocks.The SBZ Devonian granitic magmas probably formed by partial melting of heterogeneous Proterozoic to Cambrian arc-related crust, below the mid-crustal décollement.

High-grade metamorphism flying under the radar of accessory minerals

Uranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event.

Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Geochronological evidence for Archaean and Palaeoproterozoic polymetamorphism in the Central Zone of the Limpopo Belt, South Africa

The Central Zone of Limpopo Belt is a polymetamorphic and complexly deformed Precambrian terrane whose tectonic evolution is still uncertain because details on the timing of its evolution are not well established. We report zircon ages for newly discovered rocks in the Central Zone which document three distinct high-grade tectono-metamorphic events at 3.22, 2.62 and 2.02 Ga. The youngest of these has been well established, but the older two events were so far largely based only on rare zircon overgrowth rims. Most of our samples were collected from a large enclave in the 2.61 Ga Bulai pluton and constitute a strongly deformed, brecciated high-grade assemblage of supracrustal rocks, meta-anorthosite and granitoid gneisses, some of which are most likely equivalents of the pre-3.33 Ga Beit Bridge Complex. Magmatic zircon ages of 3.34–3.33 Ga for anorthositic and tonalitic gneisses in the enclave breccia support this correlation. Ca. 3.22 Ga metamorphic zircons, possibly reflecting the D1/M1 event, were only found in one felsic granulite sample, and their regional significance remains uncertain. Abundant metamorphic zircons grew at ca. 2.62 Ga under granulite-facies conditions (D2/M2), and some of these experienced variable degrees of recrystallization and/or Pb-loss during the third granulite-facies event at ca. 2.0 Ga (D3/M3). Rocks outside the Bulai enclave contain abundant newly-formed ca. 2.02 Ga metamorphic zircons but only rarely preserve evidence for the ca. 2.62 Ga event since these late Archaean zircons show strong recrystallization and/or Pb-loss at ca. 2.0 Ga. We suggest that the Bulai pluton “protected” the rocks inside the enclave from severe overprinting at ca. 2.0 Ga because these rocks remained dry, whereas the assemblages outside the Bulai pluton experienced fluid-assisted metamorphic overprinting at ca. 2.0 Ga.Our new data sugggest that the tectonic evolution of the Central Zone was more complex than previously thought. We favour a scenario where collision involving the Limpopo Belt occurred in the Neoarchaean, whereas the high-grade Palaeoproterozoic event was most likely intracontinental and records far-field effects of plate movements elsewhere. The tectonic setting controlling the early Archaean D1/M1 event remains unknown, largely because of insufficient data and extensive overprinting during later times.

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7km long and 0.8km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8GPa/460°C and reached pressures up to 2.5GPa at 550°C. The subsequent granulite facies overprint (1.6–1.8GPa/800–880°C) affected the rocks only partially; by replacement of omphacite into diopside+plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite facies mineral assemblage. The results of this study are compared with other granulite massifs in the Moldanubian Zone. In addition, a possible scenario for the Variscan eclogite and subsequent granulite facies metamorphism in the Bohemian Massif is discussed.

Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon

The Carlos Chagas batholith (CCB) is a very large (~14,000km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U–Pb ages (from 825 to 490Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny. Magmatic zircons (ca. 578–588Ma) are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825–600Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U–Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a two-stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the two metamorphic events in the batholith reached granulite facies conditions (>800°C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570–550Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535–500Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after the intrusion of the batholith. This introduced the batholith to a depth in excess of 30km and produced widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts from the CCB that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies event.

Tropicana translated: a foreland thrust system imbricate fan setting for c. 2520 Ma orogenic gold mineralization at the northern margin of the Albany–Fraser Orogen, Western Australia

Abstract Neoarchean rocks of the Tropicana Zone, including granites with subduction-zone affinities, formed in a terrane adjacent to, or on the margin of, the Yilgarn Craton at the commencement of a long-lived, amphibolite to granulite facies event – the 2722–2554 Ma Atlantis Event. Early stages of this event overlap with extensive komatiite emplacement within the Eastern Goldfields Superterrane (Yilgarn Craton), suggestive of a plume-related rift environment, which was followed by 2660–2630 Ma greenschist facies, orogenic gold mineralization. This indicates differences in the tectonic evolution of the Tropicana Zone compared with within the craton, although isotopic data show similarities in crustal sources. At c. 2520 Ma, the Tropicana Zone was retrogressed to greenschist facies as it was thrust onto the Yamarna Terrane (Yilgarn Craton), forming a northwesterly directed fold-and-thrust belt above the flat-lying Plumridge Detachment. This fold-and-thrust belt is host to the c. 2520 Ma, Tropicana gold deposit. The Plumridge Detachment may extend north to the Yamarna greenstone belt, linking to the Yamarna Shear Zone – the boundary between the Burtville and Yamarna Terranes. The fertility of the Tropicana Zone is related to its Neoarchean geodynamic setting within a continental arc environment, implying that deformed margins of Archean cratons may be prospective for Neoarchean Au deposits.

Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia

Rare remnants of a Mesozoic subduction high pressure (HP) accretionary complex are exposed on Diego de Almagro Island in Chilean Patagonia. We herein focus on the Lazaro unit, a coherent slice of oceanic crust exposed on this island that has been first affected by high temperature (HT) metamorphism followed by a lower temperature deformation event (LT). Its Pressure-Temperature-time (P-T-t) evolution is reconstructed using field and petrographic observations, phase relations, thermobarometry and geochronology. Remnants of a primary amphibolite to HP granulite-facies event in mafic rocks comprising garnet (with ilmenite exsolutions), diopside, trondhjemitic melt, pargasite, plagioclase±epidote are reported for the first time in neosomes, indicating peak P-T conditions of 1.1–1.3GPa and c. 750°C. This peak T paragenesis has been thoroughly overprinted by a phengite-chlorite-actinolite assemblage during isobaric cooling down to c. 450°C. U-Pb dating of zircon metamorphic rims from a metasedimentary rock yielded a homogeneous age population of 162±2Ma for the HT event. Sm-Nd dating of two peritectic garnet-bearing samples yield ages of 163±2Ma and 163±18Ma for the HT event. Multi-mineral Rb-Sr dating of a metasedimentary rock overprinted by LT deformation suggests retrograde shearing between 120 and 80Ma. Our results show that the HT event in the Lazaro unit took place at around 160–165Ma, shortly before the onset of Patagonian Batholith emplacement. Partial melting of subducted oceanic crust reported in the Lazaro unit is related to the early stages of hot subduction along the Gondwana western margin. The Lazaro unit remained at c. 40km depth along the subduction interface for >80Ma, recording the deformation and long-term cooling of the subduction channel environment until the upper Cretaceous.

The timing of metamorphism in the Odenwald–Spessart basement, Mid-German Crystalline Zone

New in situ electron microprobe monazite and white mica 40Ar/39Ar step heating ages support the proposition that the Odenwald–Spessart basement, Mid-German Crystalline Zone, consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition. The monazite ages constrain tectonothermal events at 430 ± 43 Ma, 349 ± 14 Ma, 331 ± 16 Ma and 317 ± 12 Ma/316 ± 4 Ma, and the 40Ar/39Ar analyses provide white mica ages of 322 ± 3 Ma and 324 ± 3 Ma. Granulite-facies metamorphism occurred in the western Odenwald at c. 430 and 349 Ma, and amphibolite-facies metamorphism affected the eastern Odenwald and the central Spessart basements between c. 324 and 316 Ma. We interpret these data to indicate that the Otzberg–Michelbach Fault Zone, which separates the eastern Odenwald–Spessart basement from the Western Odenwald basement, is part of the Rheic Suture, which marks the position of a major Variscan plate boundary separating Gondwana- and Avalonia-derived crustal terranes. The age of the Carboniferous granulite-facies event in the western Odenwald overlaps with the minimum age of eclogite-facies metamorphism in the adjacent eastern Odenwald. The granulite- and eclogite-facies rocks experienced contrasting pressure–temperature paths but occur in close spatial proximity, being separated by the Rheic Suture. As high-pressure and high-temperature metamorphisms are of similar age, we interpret the Odenwald–Spessart basement as a paired metamorphic belt and propose that the adjacent high-pressure and high-temperature rocks were metamorphosed in the same subduction zone system. Juxtaposition of these rocks occurred during the final stages of the Variscan orogeny along the Rheic Suture.