Granulite Facies Metamorphism Research Articles

Tectonic processes and the evolution of the continental crust

The Earth is the only known planet where plate tectonics operates. We review features of Archaean and early Proterozoic geology that constrain tectonic environments and inform discussions of the onset of plate tectonics. There is the question of scale, and how the results of individual case studies are put into a wider global context. Global models may be difficult to test, and we seek to integrate evidence for plate tectonics being active with ancient records of subduction. We explore evidence for when the continental crust became rigid enough to facilitate plate tectonics, based on the occurrence of widespread dyke swarms and large sedimentary basins, relatively widespread granulite facies metamorphism, and evidence for crustal thickening. We argue that it remains difficult to constrain tectonic settings from contemporaneous metamorphic events without spatial control. Archaean cratons stabilised at different times in different areas from 3.1–2.5 Ga, juvenile continental crust changed from mafic to more intermediate compositions, there was a reduction in crustal growth at ca. 3 Ga, and increasing evidence for lateral movement of crustal fragments. These with the other changes at the end of the Archaean are taken to reflect the onset of plate tectonics as the dominant global regime.

Granulites of the Larba Block of the Dzhugdzhur–Stanovoi Superterrane: Reconstruction of the Formation Conditions

Abstract —We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur–Stanovoi superterrane. The Larba block is dominated by basic schists and garnet–biotite–orthopyroxene and garnet–biotite–cordierite–sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P–T parameters. The P–T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7–8 kbar, 800–850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4–5 kbar and T = 630–700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P–T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, TNd(DM) = 2.57–2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8–3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.

U-Pb geochronology of rutiles from Southern granulite Terrane, India: Implications for the cooling and exhumation of East Gondwanan terranes

The Precambrian Southern Granulite Terrane of south India figures prominently in the East African collisional orogen, linked to the assembly of the East Gondwana supercontinent along with Sri Lanka, Madagascar, Antarctica, and Africa. While the timing of this collision and associated magmatic-metamorphic processes are well constrained, the mechanisms governing the cooling and exhumation of these terranes remain uncertain. This study focuses on U-Pb rutile ages derived from granulites in the Southern Granulite Terrane and attempts to extract the cooling history of the terrane. Our new U-Pb rutile ages cluster between 422 to 458 Ma, postdating regional metamorphism and indicating an extended cooling period of ∼ 35 Ma whereas trace element signatures suggest their formation during granulite facies metamorphism with temperatures reaching up to ultrahigh-temperature conditions. Calculated cooling rates using these rutile U-Pb ages range between 2–6 °C/Ma similar to the cooling rate of ≤ 7 °C/Ma from other East Gondwanan terranes. Based on these results it is proposed that the coeval exhumation of these terranes was assisted by surface erosion after the final assembly of the East Gondwana supercontinent.

U-Pb geochronology and Hf isotopes of the Grenvillian Río Hondo gneisses, Puebla: redefining the western edge of Oaxaquia

In situ Río Hondo (Puebla, southern Mexico) gneiss samples, as well as clastic samples recovered from the nearby-outcropping latest Paleozoic Matzitzi Formation, a fluvial unit mainly sourced by Grenvillian rocks,  were historically interpreted as representative of the northernmost exposures of the Oaxacan Complex, the largest outcrop of ortho and metasedimentary units, made up of mostly 1.0–1.3 Ga protoliths, affected cfby local migmatization (ca. 1.1 Ga) and granulite facies metamorphism at ca. 0.98 Ga, constituting the most prominent outcrop of the Oaxaquia microcontinent.
 U-Pb geochronology and Lu-Hf isotopic determinations in zircon by LA-(MC)-ICP-MS were performed on both in situ and clast samples. In situ basement samples record a ca. 1.2 crystallization event, together with a younger one at ca. 1.02 Ga. both lacking inherited >1.3 Ga components. The clasts have an unimodal zircon U-Pb age distribution, recording a crystallization event at ca. 1.2–1.27 Ga. Scarce inherited zircon cores between ca. 1.4–1.6 Ga were found, with only a few samples with a broader age distribution, suggesting a detrital protolith with zircon cores as old as ca. 1.8 and 2.4 Ga. No zircon overgrowths or geochemical-petrographic evidences are indicative of granulite metamorphism. Furthermore, all the studied metaigneous samples show discordant zircon ages produced by Pb loss events barely constrained between the latest Paleozoic to the Mesozoic.
 Hf isotopes reveal that zircon crystals from clasts have a range of εHf (1.25 Ga) ≈+1 to +5 and yield Hf model ages from 1.7 to 1.9 Ga. On the other hand, the The zircon Hf isotopes of one analyzed basement sample reveal a higher range of εHf (1.25 Ga) ≈+7 to +9, and Hf model ages from 1.5 to 1.6 Ga.
 Both ≈1.2 Ga and 1.02 Ga events are consistent with magmatic ages previously documented elsewhere in Oaxaquia, interpreted as indicating portions of the NW Amazonia-Oaxaquia arc system with cratonic influence or to slices of Baltica thrust over Oaxaquia during the Grenville orogeny. However, the absence of granulite facies indicators, such as zircon metamorphic ages and/or granulite paragenesis (typically, in other Oaxaquia samples, orthopyroxene and garnet) are interpreted as prime evidence that the studied samples didn’t undergo such high grade of metamorphism. Río Hondo gneisses, as this sequence is informally named, must belong to a source that had the influence of an older continental crust and can be tentatively associated either with rocks recently described in the Sierra de Juárez, or those belonging to the central basement of the Maya block, currently exposed farther to the SE in Chiapas.

Neoarchean clockwise P-T trajectories with evolution sequence from HP to UHT metamorphism during decompression: Evidence from petrology, mineralogy, and phase equilibria modeling of mafic granulites in the Sittampundi Complex, southern India

The dispute of tectonic environments such as subduction-collision setting versus extension tectonic setting, is a common phenomenon in the Precambrian. Despite the ongoing controversy, the pressure–temperature (P–T) paths recorded by metamorphic rocks are often used as an indicator for distinguishing between subduction-collision and extension tectonic settings in the early Precambrian. It is widely acknowledged that high-pressure (HP) granulites may have formed in crustal-thickening-dominated collision/subduction environment. Conversely, ultra-high temperature (UHT) granulites may have formed in the elevated mantle heat flux environment. The Sittampundi Complex (SC), where HP granulites and UHT granulites coexist, plays a crucial role in unraveling the late-Neoarchean to early-Paleoproterozoic tectonic evolution of northern South Granulite-facies Terrane (NSGT) of India. In this study, we present new P–T paths from three HP mafic granulites overprinted by UHT granulite facies metamorphism in the SC and provide more data to unravel metamorphic evolution of NSGT. Phase equilibria modeling and traditional thermobarometry calculations illustrated that clockwise metamorphic pressure–temperature (P–T) paths were yielded, passing from 3.3 to 9.3 kbar/631–669 °C (prograde stage) through 11.9–18.5 kbar/828–986 °C (pressure-peak stage), and through 6.6–11.1 kbar/1019–1085 °C (temperature-peak stage) to 4.4–8.1 kbar/845–930 °C (retrograde stage). The HP mafic granulites exhibit clockwise P–T paths with a considerable heating process (ca. 150–180 °C) from the pressure-peak stage (HP-granulite sub-facies) to the temperature-peak stage (UHT-granulite sub-facies). Zircon U-Pb dating yields an age of 2513 ± 29 indicates that the metamorphism took place in the Neoarchean. The uncommon P–T paths from Neoarchean HP granulite in the NSGT suggest that during late Neoarchean, the NSGT had experienced the crustal-thickening dominated collision/subduction (assembly of micro-blocks of Dharwar Craton) and subsequently mantle diapirism process.

From Triassic metamorphism to Early Cretaceous anatexis in the Dabie orogen, central China: Constraints from in-situ U-Pb age and Hf and O isotopes of zircon from migmatites

The Dabie orogen underwent a ‘complete’ orogenic cycle from Triassic collisional orogeny to Early Cretaceous tectonic collapse. Zircons of the migmatite in the North Dabie terrane recorded the processes from Triassic granulite-facies metamorphism to Early Cretaceous anatexis. Three types of zircon are recognized in the migmatite: the Neoproterozoic inherited magmatic zircon, the Triassic metamorphic zircon, and the Late Jurassic–Early Cretaceous anatectic zircon. The Neoproterozoic inherited magmatic zircons suggest the protolith of the migmatite has a tectonic affinity with the Yangtze Craton (YC). The Triassic metamorphic zircon domains have variable εHf(t) values from −4.1 to 3.6 and low δ18O compositions ranging from −4.36‰ to −0.30‰. The anatectic zircon domains have clearly negative εHf(t) values of −19.8 to 0.8 (mostly less than −10.0) and relatively high δ18O values from −4.24‰ to 3.49‰. Based on the chronological and isotopic data of migmatite and the results of previous studies, the Triassic granulite-facies metamorphism was closely related to the asthenosphere upwelling after the slab break-off during the orogenic stage, and the Early Cretaceous anatexis respond to widely heat event during the collapse stage of orogen.

Crustal evolution of the northern Nyika Subdomain of the Palaeoproterozoic Ubendian belt in Malawi

The Ubendian belt extends from eastern Congo through SW Tanzania and NW Zambia across northern Malawi and into Mozambique. It results from the Palaeoproterozoic accretion of a number of crustal blocks followed by the collision of the Tanzanian and Congo cratons. The Ubendian belt has been classically divided into eight “terranes”, (termed “subdomains” in this work), based on rock assemblage, magmatic and metamorphic history. This paper is concerned with the geological evolution of the southernmost of these, the Nyika Subdomain.We present an extensive dataset to decipher the geological history of the northern part of the Nyika Subdomain, including field observation, LA-ICP-MS U/Pb geochronology on zircon and monazite and a metamorphic petrological study of the extensive characteristic migmatitic cordierite-bearing gneisses. The U/Pb zircon data records four magmatic pulses. The first is represented by xenocrysts dated between 2200 and 2150 Ma. The second, major pulse is dated at ca. 2030 Ma, followed by minor magmatic activity at ca. 1980 Ma. This event is coeval with regional granulite facies metamorphism (ca. 780 °C and 0.6 Gpa), constrained in age U/Pb monazite and xenotime dated at 1977 ± 14 Ma and 1984 ± 13 Ma, respectively and also probably dates the collision between the Congo and Tanzania cratons. A fourth event represented by the intrusion of the Nyika Suite granites at 1944 ± 7 Ma terminates the Palaeoproterozoic history. There follows a long time interval with no recorded orogenic history before the tectonic activity during the Neoproterozoic Pan African orogeny. The regional significance of the evolution of the Nyika Subdomain with respect to the adjacent Ufipa Subdomain and the “Bangweulu Block” to the west are discussed within the framework of the Ubendian orogeny and the wider Palaeoproterozoic history of the region.

The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)

In this contribution, we report new P-T (pressure–temperature) and U-Pb zircon geochronological data from the Lima Duarte Nappe and basement rocks from the Mantiqueira Complex, Central Ribeira Orogen, southeastern Brazil, highlighting their relevance to the understanding of the West Gondwana assembly. The lower metasedimentary sequence of the Lima Duarte Nappe records a passive margin environment, with a majority of Paleoproterozoic detrital zircon grains, with probable main source rocks located in the São Francisco paleocontinent. The upper foreland sequence of the Lima Duarte Nappe has a prevalence of Tonian-Cryogenian detrital zircon grains, absent in the lower passive margin sequence and source rocks were most likely in a magmatic arc developed during the convergence between the São Francisco and the Paranapanema paleoplates. Zircon rims from the Lima Duarte passive margin and foreland sequences, and zircon grains from an amphibolite of the Lima Duarte Nappe record granulite facies metamorphism at ca. 560–590 Ma (Th/U ratio < 0.1). Similar ages are recorded by CL-dark zircon rims and overgrowths in orthogneisses of the Mantiqueira Complex and are interpreted to reflect basement reworking. Metapelitic metatexites from the foreland sequence record granulite facies peak metamorphism at ∼ 780–800 °C and 8–9.5 kbar in the sillimanite-rutile stability field, constrained by phase equilibrium modeling. Rutile inclusions in garnet record minimum peak temperatures of ∼ 750 °C. The retrograde mineral assemblage in the matrix is marked by the disappearance of rutile and the presence of ilmenite and mineral compositions re-equilibrated at upper amphibolite facies. Late Tonian (ca. 850–750 Ma) metamafic rocks within the Mantiqueira and Juiz de Fora complexes might record an extensional intraplate mafic magmatism episode that culminated with the establishment of the passive margin at the south of the São Francisco paleocontinent. The Lima Duarte metasedimentary sequence along with the Canastra and Carrancas groups (western and southern portions of the Brasília Orogen, respectively) represent a Tonian proximal passive margin environment, developed at the margin of the São Francisco paleocontinent. The Lima Duarte Nappe metasedimentary units and the orthogneisses of the Mantiqueira Complex record Ediacaran syn-collisional metamorphism related to the Araçuaí-Ribeira Orogenic System.

Zimbabwe's coloured gemstone endowments - A regional geological overview

Zimbabwe hosts a varied array of coloured gemstones. With the exception of emerald deposits and several world-class pegmatites, few of the gemstone occurrences have received detailed attention from the scientific or mineral exploration communities. In the present contribution we summarize the status of knowledge of the gemstone deposits and occurrences in Zimbabwe, paying particular attention to the geological settings in which they were formed. Synthesis of this regional geological approach reveals that there may be significant exploration potential for further gemstone occurrences, particularly in the extensive pegmatite fields and in Al-enriched orogenic belts that have undergone greenschist to granulite facies metamorphism. Further socio-economic and developmental initiatives will aid in optimizing the value generation from this important sub-sector.

The Potential Role of Sulfur during Granulite-facies Metamorphism, Oxidation, and Geochemical Transformation of the Granitoid Lower Crust

The Potential Role of Sulfur during Granulite-facies Metamorphism, Oxidation, and Geochemical Transformation of the Granitoid Lower Crust

Metamorphic evolution of a metabasite dyke from Ogbagi Akoko area, Southwestern Nigeria

Petrography, mineral chemistry and whole rock analyses were conducted on a metabasite dyke hosted by pelitic gneiss (metapelite) in Ogbagi Akoko area with the aim of determining its metamorphic evolution. The dyke is about 40 cm wide and traceable for some meters along strike. It has a discordant contact relationship that is pencil sharp with the host pelitic gneiss and lack any internal fabric. The mineral assemblage includes: garnet +biotite + plagioclase + quartz + orthopyroxene + K-feldspar + ilmenite, with apatite occurring as an accessory mineral. Plagioclase has anorthite content of about 76 (An76) suggesting bytownite (An76 Ab24 Or0) while garnets are poikiloblastic almandine-pyrope rich with XFe of 0.73-0.75. Orthopyroxene has a composition of Wo0 En50 Fs50 suggesting hypersthene that is depleted in Ca. The mineral assemblage, presence of bytownite, absence of amphibole and lack of internal fabric indicate that the rock has undergone metamorphism at a high temperature and moderate pressure without deformation at granulite facies metamorphism. The absence of deformation structure in the dyke is an indication that the intrusion occurred after the cessation of the regional metamorphism but prevalent high temperature that recrystallized the rock. The whole rock composition and some elemental ratios of the dyke indicate that the metamorphism had little or no effect on the bulk chemistry of the rock.

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.

Reaction-diffusion modelling of petrological mixing mechanisms in the evolution of continental crusts

Petrological assimilation is a key process in the evolution of high-grade metamorphic terrains in the continental crusts. This study examines the mechanisms of such macroscopic assimilation between felsic (F) and mafic (M) constituents as two petrologically interacting continuum phases, as observed in the Chotanagpur Granite Gneissic Complex (CGGC), India, which underwent amphibolite to granulite facies metamorphism (∼775 to 900°C and 7 to 11 kb) between from the Paleoproterozoic to the late Mesoproterozoic. From field investigation we could recognized four interface patterns: planar, wavy, fingering and incoherent, which are generated at the interface between the F and M units. We have adopted the Turing type reaction-diffusion (RD) approach, which is a well established theoretical model to interpret any complex auto-regulatory pattern in natural and physical sciences, to understand the physics of the self-organizing interface geometries observed across the CGGC. The RD model findings suggest that these patterns are constrained by a combination of: diffusion coefficients (DF, DM) of F and M, a linear or non-linear reaction term (R) that describes phase interactions and a pinning field (W) that introduces microscale heterogeneity. For linear interactions, F − M undergo homogeneous mixing and show planar/wavy interfaces, when DF = DM and W = 0. The mixing turns heterogeneous as DF ≠ DM and W &amp;gt; 0, resulting in phase boundary migration with a fingering pattern. Non-linear reaction coupling enhances heterogeneous mixing and produces incoherent phase boundaries where F-phases host relics of M-phases, following a power-law size distribution. Striking similarities of interface patterns and fractal dimensions estimated from model and CGGC validate the proposed mechanism of macroscopic petrological assimilation. We argue that RD model provides a new insight into the genesis of hybrid rocks in metamorphic terrains.

Metamorphosed Plagiogranite Veins In Salma Eclogites, Belomorian Eclogite Province

The Salma subduction eclogites of the Meso-Neoarchean Belomorian Eclogite Province have compositions of N-MORB oceanic crust protoliths and contain felsic veins of plagiogranite compositions. Felsic veins consist of metamorphic minerals sush as garnet, quartz and kyanite, the earliest high-pressure inclusion of phengite and zoisite in garnet. Veins have geochemistry characteristics that are typical for island arc granites or adakites. They are compositionally comparable with eclogite-bearing tonalite-trondhjemite-granodiorite (TTG) gneisses, modern adakite, and Archaean andesite association of the Vedlozero–Segozero greenstone belt. The origin of these felsic veins can be represented as a product of melting of fluid-modified oceanic mafic slab at PT-conditions of eclogite metamorphism.Geothermometry and pseudosection calculations of metamorphic conditions in felsic veins, for eclogitic mineral assemblage of pyropic garnet, kyanite, high-silica phengite and quartz gave high pressures conditions (ca. 16–22 kbar) at relatively low temperatures (ca. 600–650 °C); for secondary decompression plagioclase ± biotite coronas between quartz, kyanite and garnet gave 14–11 kbar at 650–700 °C. This estimate corresponds “warm” subduction, which is most consistent with the parameters of the modern subduction of the base of the Cascadia plate . Mineral assemblages in eclogites, that included the felsic veins, give 15–11 kbar at 750–700 °C. Thermodynamic modelling indicates that the eclogites hosting the felsic veins did not reach partial melting conditions under the estimated PT conditions: exposure temperatures to the mafic rocks must have been significantly higher. A probable explanation for high-pressure parageneses in felsic veins may be the formation of plagiogranites as a result of the melting of the mafic slab under higher temperature and pressure conditions than were measured in this work. Thus, the high-pressure paragenesis of garnet, phengite, zoisite, and kyanite reflects the path of rock exhumation through eclogitic conditions.Zircon from the felsic veins gave several Archean and Paleoproterozoic ages ∼ 2.87, 2.78, ∼2.70 and, ∼1.9 Ga. The oldest age gave the fluid-modified oceanic zircon (∼2.9 Ga) from Fe-Ti eclogites directly near the veins. The oldest igneous zircons in the veins are represented by a population with the age of ∼ 2.87 Ga and correspond to the igneous age of some TTG gneisses, which do not contradict a hypothesis of the plagiogranite formation during the melting of the mafic slab in the subduction zone. A Neoarchean zircon population of ∼ 2.78 Ga is often present in eclogitized mafic rocks and surrounding TTG gneisses which can be associated both with the formation of TTG gneiss protoliths in the subduction zone (?) and with the recrystallization of older zircon as a result of fluid or thermal impact. The ɛHf of the older population (∼2.87 Ga) is + 3 to + 4, indicating a juvenile origin; the ∼ 2.78 Ga population has similar Hf-isotope compositions, suggesting that their ages reflect resetting (non-zero Pb loss) of the older zircons. The Sm-Nd model TDM ages of Grt-Ky veins have Meso-Neoarchean values (3.0–2.75 Ga) with positive εNd = 1.8–3.8 that may indicate a juvenile nature of sources of the plagiogranite melt. The eclogitized rocks of the Belomorian province generally underwent the granulite facies overprint at ∼ 2.74–2.64 Ga ago that is coincides with the age of granulite facies metamorphism of regional and global rank. The Paleoproterozoic evolution of the Salma eclogites and the East European Craton as a whole: at 2.5–2.4 Ga and 2.0–1.9 Ga with formation of the Lapland–Mid-Russia–South Baltia intracontinental and Svecofennian accretionary orogens was terminated in Paleoproterozoic time at ∼ 1.87–1.7 Ga. These age values correspond to closure of Pb-Pb and Sm-Nd isotopic systems of rock-forming metamorphic minerals of the felsic veins.

Considerable heating during clockwise decompression of a long-lived Paleoproterozoic hot orogen: Evidence from the Xuanhua Complex, North China Craton

Granulites comprise the root of collisional orogens, and decoding their evolution may provide key information on orogenic processes from collisional thickening to post-collision extensional thinning. However, the records of collisional orogeny in most cases have been obscured or even erased by later high-/ultrahigh-temperature (HT/UHT) overprinting. Here, we report new petrological, geochemical and geochronological data for granulite-facies rocks in the Xuanhua Complex of the northern Trans-North China Orogen (TNCO). We reveal the complex metamorphic features and tectonic evolution of a long-lived Paleoproterozoic hot orogen as well as their link with Phanerozoic orogens. The rocks are composed mainly of garnet + clinopyroxene + orthopyroxene ± amphibole + plagioclase + quartz + rutile + ilmenite with symplectitic intergrowths of clinopyroxene + orthopyroxene ± amphibole + plagioclase ± quartz. Pseudosection modeling and thermobarometric calculations suggest that the high-pressure (HP) mafic granulites record clockwise pressure–temperature (P-T) paths, from the prograde stage (M1) to the pressure-peak stage (M2; 13.5–15.0 kbar and 850–875 °C), followed by considerable heating during decompression to the temperature-peak stage (M3a; 8.0–9.6 kbar and 910–950 °C) and final cooling to the M3b stage. Magmatic zircon U-Pb dating reveals c. 2.1–2.0 Ga protolith ages for the granulites. Metamorphic zircon and titanite U-Pb results, and trace elements in zircons indicate that the age of c. 1.92 Ga is possibly close to the lower limit of HP granulite-facies pressure-peak, and c. 1.84–1.81 Ga ages are related to the timing of the final cooling stage. Hence, by comparison with existing geological data, the synthesized metamorphic P-T-t data of the Xuanhua granulites indicate two stages of tectonic evolution, including: (1) crustal thickening by continental collision between the Western and Eastern Blocks at c. 1.95–1.92 Ga, resulting in an early HP granulite-facies metamorphism at relatively low thermal gradients of 16–19 °C/km; and (2) the collision-thickened orogenic lithosphere was thinned controlled by continental extension and gravitational instability, which might provide additional heat in the interval c. 1.90–1.84 Ga, resulting in a later UHT overprinting at reduced pressures and higher gradients of 30–36 °C/km. In addition, the post-collisional extension accompanying exhumation decompression shows a long-lived timescale of > 80 Myr, corresponding roughly to a slow exhumation rate of < 0.3 mm/yr. To sum up, these features of long-lived tectono-thermal evolution (tectonic transition from crustal thickening to lithospheric thinning, and prolonged orogeny and exhumation), slow exhumation rates, and the occurrence of UHT overprinting, are becoming more commonly identified in Precambrian hot orogens like the TNCO.

Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M1), and the peak stage (M2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M3a) and final cooling stages (M3b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende ± biotite ± clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P–T paths, including isothermal decompression and cooling from the peak stage (M2: 10.4–10.8 kbar/830–860 °C and 9.6–11.2 kbar/850–930 °C, respectively) to the retrograde stages (M3a and M3b: 7.5–8.0 kbar/840–870 °C and 7.8–8.2 kbar/650–700 °C, respectively; M3: 7.2–8.6 kbar/670–800 °C). Zircon U–Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5–2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51–2.45 Ga and c. 1.90–1.85 Ga. The c. 2.51–2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95–1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive εHf(t) value of 7.64 and 2.8–2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8–2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8–2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC.To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P–T–t paths with near-isothermal decompression segments and low geothermal gradients (c. 21–26 °C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95–1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.

High-P metamorphism in the Mesoproterozoic: Petrochronological insights from the Grenville Province

In the Grenville Province, high-P rocks that are discontinuously exposed along the margin of the allochthonous belt preserve the record of deep crustal processes and are an essential puzzle piece to understanding Mesoproterozoic geodynamics. The Manicouagan Imbricate zone (MIZ) is one of the three high-P domains from the Grenville Province. It is located in the Central Grenville between the Parautochthonous Belt to the North and the orogenic hinterland to the South, that were metamorphosed during the 1005–980 Ma Rigolet and 1080–1020 Ma Ottawan orogenic phases, respectively. In the western MIZ, the Lelukuau Terrane (LT) mostly consists of Labradorian-age (∼1650 Ma) mafic suites with a fringe of aluminous rocks at its southern edge. Metamafic samples from the Western and Eastern parts of the MIZ display a peak assemblage of garnet, clinopyroxene, plagioclase, rare pargasite or edenite, and quartz ± kyanite. Pseudosection modelling suggests high-P granulite peak conditions at ca. 14 to 16 kbar and 800–900 °C, with the scarcity of hydrous phases and quartz explaining the lack of evidence for partial melting. Zircon cores from the Western LT sample show a maximum magmatic age of ca 1.6 Ga. Lu–Hf and Sm–Nd dating on garnet from this sample yield ages of 1020 ± 7 Ma and 1005 ± 13 Ma, respectively, overlapping within error and inferred to represent peak metamorphic conditions followed by fast cooling. In the Eastern LT sample, garnet Lu–Hf dating yields two ages that are consistent with a petrographically preserved two stage growth, at 1033 ± 6 Ma and 1013 ± 6 Ma, while the Sm–Nd age indicates cooling at 1003 ± 8 Ma. The recorded high-P granulite facies conditions highlight a late Ottawan to Rigolet-age localized crustal thickening at the margin of the hinterland during the propagation of the orogen to the NW, with a possible younging of the high-P granulite-facies metamorphism from the Eastern to Western LT. These new results indicate that the high-P belt in the Central Grenville does not represent the exhumed base of an Ottawan age orogenic plateau, as previously proposed, and that no tectonic hiatus exists between the two orogenic phases, as generally thought. Finally, this publication highlights the diversity and diachronicity of the high-P domains in the Grenville Province.

Source and Generation Parameters of the Granitoid Melts of the Archean Charnockite–Enderbite Complex in Karelia, with Reference to the Pon’goma-Navolok Massif

—The paper presents authors’ original detailed data on rocks of the Archean Pon’goma-Navolok charnockite−enderbite complex in northern Karelia. The rocks practically have not been modified and are preserved within a rigid block among Paleoproterozoic zones of ductile deformations and metamorphism. The geochemistry of the rocks and their isotope−geochemical features indicate that the protolith from which the enderbite melts of the main phase of the massif were derived may have been amphibolites. The enderbite melts were derived from these amphibolites under the effect of K2O-, Na2O-, and SiO2-bearing fluids; and the enderbites were subsequently charnockitized with the involvement of fluids enriched in K2O and SiO2. Physicochemical modeling indicates that the enderbite melt was derived from the amphibolite protolith at a depth of about 45 km (P = 14.8 kbar, T = 1030−1080°C) under the effect of saline H2O−CO2 fluid. Comparison of the P−T parameters of the granulite-facies metamorphism of the metabasites and the parameters under which the enderbite melts were derived indicates that Archean granulite-facies metamorphism in the Belomorian belt in northern Karelia was of contact but not regional nature and was induced by the high-temperature field of an emplaced enderbite massif. The orthogneisses hosting the Pan’goma-Navolok massif inherit geochemical features of the unsheared, ungneissose, and unmetamorphosed enderbites. This means that enderbites analogous to those of the Pan’goma-Navolok massif may have served as the protolith of some of the orthogneisses, and that enderbites may have been spread more widely in the Archean than the currently preserved single enderbite massifs.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

Pseudotachylytes in felsic lower-crustal rocks of the Calabrian Serre massif: A record of deep- or shallow-crustal earthquakes?

Pseudotachylytes (quenched frictional melts produced on a fault by seismic slip) in dry rocks exhumed from the mid-lower crust are potential indicators of earthquakes that either nucleated at, or propagated to, depths below the main shallow brittle-ductile transition zone. Establishing whether these pseudotachylytes effectively record deep-crustal earthquakes, or shallow-level earthquakes overprinting the mid-lower-crustal rocks during the exhumation path, may represent a major challenge. This challenge is mainly related to the fact that the mineral assemblage of a pseudotachylyte develops out of equilibrium during the coseismic thermal transient leading to melting and melt quenching. Here we investigate pseudotachylytes within peraluminous, sillimanite-garnet-rich, migmatitic paragneiss of the Serre Massif in Calabria (Southern Italy). These exhumed lower-crustal rocks experienced granulite-facies metamorphism (∼700–800 °C; ∼600–800 MPa), partial melting and dehydration during the late Variscan Orogeny (ca. 320–280 Ma). The crosscutting pseudotachylytes contain hercynite and sillimanite microlites, globular-shaped poikilitic cordierite and plagioclase, and rare cauliflower- to subhedral-shaped garnet. The pseudotachylytes are pristine, not affected by ductile deformation, recrystallisation or extensive alteration by fluid after their formation. A Rb-Sr isochron age of 51.4 ± 5.1 Ma is obtained for the pervasively kinked biotite in the host rock immediately adjacent to the pseudotachylyte and associated with earthquake damage, while an age of 105.3 ± 4.1 Ma is obtained for the undeformed host-rock biotite. This indicates that the granulites were cooler than the closing Rb-Sr temperature of biotite (ca. 300–400 °C) in the Cretaceous and that the studied pseudotachylytes formed by shallow seismic faulting. Therefore, sillimanite, hercynite, garnet, plagioclase, and cordierite all formed during quenching of the frictional melt well above the ambient temperature. Modelling of cordierite growth during melt quenching indicates that cordierite should have started to crystallise at T > 900 °C to achieve the grain size (up to 10 μm in diameter) observed in the pseudotachylyte. Modelling and microstructural observations allow the crystallisation sequence of microlites during melt cooling to be established. These microlites include cauliflower garnet which, in this case, did not develop in a deep-seated faulting context as commonly reported.