Granulite-facies Metamorphic Event Research Articles

Metamorphism and P-T Evolution of High-Pressure Granulites from the Fuping Complex, North China Craton

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M1), high-pressure granulite facies (M2), granulite facies (M3), and retrograde (M4) stages. The mineral assemblage of the prograde stage (M1) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M2) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M3 is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M1, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M2, 912~939 °C and 8.1~9.9 kbar for M3, 661~784 °C and 3.1~4.4 kbar for M4-1, and 637~638 °C, 1.1~1.3 kbar for M4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC.

Zircon geochronology and Hf–O isotopes of the Nulliak supracrustal assemblage (Saglek Block–Canada): Constraints on deposition age and setting, metamorphic age and environments of zircon crystallization

The Archean Saglek Block, North Atlantic craton, is one of the oldest crustal segments preserved on Earth. This area records two granulite facies metamorphic events: one in the late-Eoarchean at ca. 3.7 Ga and one in the Neoarchean, at ca. 2.75 Ga. Here, we report the results of combined U–Pb/Hf/O in-situ isotope analyses of detrital and metamorphic zircon domains from two meta-sedimentary samples, one psammitic, and one pelitic. Both samples are components of the oldest supracrustal assemblage of the Saglek Block: the Eoarchean Nulliak supracrustal assemblage. The isotopic data presented in this study: (i) confirm deposition age of the Nulliak supracrustal assemblage before ca. 3.65 Ga, (ii) indicate local provenance of the detrital zircon, (iii) suggest deposition of the sedimentary protoliths in a convergent setting, such as an arc, (iv) crystallization of the oldest zircon during the early Eoarchean in a protolith derived from a depleted mantle component, (v) involvement of material that experienced prolonged low temperature alteration during the late Eoarchean granulite facies metamorphism and (vi) selective metamorphic overprinting during the Neoarchean granulite facies metamorphism.The new zircon U–Pb/Hf/O isotope data presented here in conjunction with previously published analyses suggest that a depleted mantle domain existed in the early Archean, as reflected in the positive εHf signature of zircon from the Saglek Block. Yet, such positive Hf isotope deviations are not the norm, but rather the exception in the preserved early Earth zircon record. In addition, mildly elevated zircon δ18O values, i.e. those ≥ 5.9 ‰, are as common in the early Archean as they are in the late-Archean zircon record, implying that crustal reworking was an active and efficient process all through the Archean. Hence, no significant variation in the nature of the reworking and broader geodynamic processes occurred during this period of time.

Deciphering early Neoproterozoic and Cambrian high-grade metamorphic events in the Archean/Mesoproterozoic Rauer Group, East Antarctica

A U–Pb geochronological and rare earth element (REE) geochemical study of zircon, monazite and garnet was carried out on rocks of Mesoproterozoic and Archean crustal domains in the Rauer Group of East Antarctica. The zircon and monazite U–Pb age spectra define concordia intercepts mainly at ca 1200, 990–910, and 530–500 Ma, suggesting that the Mesoproterozoic crustal domain is a significant part of the Rayner Complex that also underwent early Neoproterozoic and Cambrian high-grade metamorphism. The age data, mineral inclusion assemblages in zircon, and REE features for zircon and garnet indicate that all the granulite facies mineral assemblages in this domain might have formed during early Neoproterozoic metamorphism. Some zircon and monazite grains or domains have experienced complete U–Pb isotopic resetting during Cambrian reworking, which did not result in new zircon and monazite growth. The Archean crustal domain consists mainly of Paleo–Mesoarchean orthogneisses interleaving with Neoproterozoic paragneisses that contain inherited metamorphic zircon domains with ages of ca 1330 and 970 Ma. The mineral assemblages in these gneisses formed during a single Cambrian granulite facies metamorphic event. Garnet-bearing and -free rocks cooled to solidus temperatures at ca 527 and 517 Ma, respectively, whereas the isotopic system of early-crystallized zircon was completely reset during the growth of new zircon. As such, all the zircon domains in the same sample could have the same concordant or weighted mean age. The 511 Ma monazite and 506 Ma zircon overgrowths in a paragneiss have REE contents in equilibrium with garnet, implying that later modification and isotopic resetting of zircon and monazite might have resulted in younger U–Th–Pb ages and, in this case, establishing the age–mineral assemblage relationship based on REE partition coefficients between zircon/monazite and garnet may be invalid. Overall, the available data support the notion that different crustal components of the Rauer Group were juxtaposed in the Cambrian as a consequence of the Gondwana assembly.

Locating the Indo-Antarctica suture – Correlating the Rengali, Rauer and Ruker terranes in Gondwana

The Rayner Complex of East Antarctica and the Eastern Ghats Province (EGP) of India are thought to have been contiguous in past supercontinents like Rodinia and Gondwana. These terranes have been correlated on the basis of similar granulite facies metamorphic imprints and isotopic age data that testify to Grenvillian (1100-900 Ma) and Pan-African (650-450 Ma) thermal signatures. The Grenvillian granulite facies metamorphic event is generally thought to represent collision between Antarctica and cratonic India, but the precise location of the Indo-Antarctic suture is disputed. The intensity of Pan-African age geological imprints is also variable in both continents, and their significance remains unclear. In this review, we correlate structural, metamorphic and geochronological data in both terranes and parts of their bounding cratons, and suggest that the Ruker Terrane and Rauer Group in Antarctica were continuous with the Rengali Province in India. Together with the established correlation between the EGP and the Rayner Complex, this implies that cratonic India along with the EGP-Rayner amalgam collided with the Archaean Ruker Terrane (part of the Crohn craton) at ~520 Ma along the southern Prince Charles Mountain in East Antarctica. This suture is distinct from the Grenvillian suture between EGP-Rayner and cratonic India.

Late Paleoproterozoic crustal thickening of the Jiao–Liao–Ji belt, North China Craton: Insights from ca. 1.95–1.88 Ga syn-collisional adakitic granites

The Jiao–Liao–Ji belt (JLJB) of the North China Craton is commonly regarded to have been involved in the global-scale collisional events that led to the amalgamation of the Columbia/Nuna supercontinent during the late Paleoproterozoic. Previous studies have revealed that the JLJB underwent a high-pressure granulite-facies metamorphic event with a clockwise P–T–t path during ca. 1.95–1.90 Ga. However, few studies have examined the contemporaneous magmatism and its genetic relationship to the high-pressure granulite-facies metamorphism. In addition, the petrogenesis of the ca. 1.95–1.90 Ga magmatism and its possible constraints on the crustal-thickening process are poorly understood. Our field investigations have discovered a series of late Paleoproterozoic granitoid plutons in Liaodong Peninsula, including the Fangjiawei, Qingchengzi, Songshugou, and Shizijie plutons. Systematic analyses of geochronology, petrology, and geochemistry were conducted to constrain the formation ages, petrogenesis, and tectonic environment of these granites. Zircon U–Pb dating reveals that the granites formed during ca. 1.95–1.88 Ga, coeval with the timing of the regional prograde or peak metamorphism and collision in the JLJB. They show geochemical characteristics of typical adakites, including high Sr (393–860 ppm) contents, low Y (1.03–3.30 ppm) and Yb (0.09–0.27 ppm) contents, and relatively high Sr/Y (125–832) and LaN/YbN (9.34–33.6) ratios. Low MgO, Cr, and Ni contents and the existence of ca. 2.5–2.0 Ga inherited zircons with ancient TDM2 ages indicate that the parental magmas were generated from partial melting of a thickened lower crust. The positive correlation between Dy/Yb and La/Yb ratios and the relatively low Nb/Ta (5.1–18.9, average of 13.6) ratios all suggest that garnets, rather than amphiboles or rutiles, were the dominant phase in the residual source, indicating a melting depth of 30–50 km. Our new results, combined with previous findings, support a rifting-and-collision model whereby the JLJB underwent a transformation from early intracontinental rifting to subsequent opening and closure of an incipient ocean, with the final collision between the Longgang Block and Nangrim Block occurring at ca. 1.95–1.88 Ga with substantial crustal thickening.

The Great Falls Tectonic Zone after the assembly of Laurentia: evidence for long-term tectonic stability from xenolith apatite

Combined U-Pb geochronology, and trace element analysis (Th, U, Y, Sr, rare earth elements: REE) by LA-Q-ICPMS and halogen chemistry (F, Cl, OH) by SEM-EDX are applied to apatite from a suite of amphibolite- and granulite-facies xenoliths from the Bearpaw Mountains (Montana, USA), to constrain cooling of Paleoproterozoic mid- to lower crust. Xenoliths hosting fluorapatite and hydroxyfluorapatite were emplaced at 54 to 50 Ma within K-rich, host lamprophyres (“minettes”). Granulite xenoliths yield apatite with highly dispersed U-Pb isotopic compositions. But discordia fit through their youngest populations yield Neoproterozoic-Cambrian (c. 650–500 Ma) lower intercepts. A single amphibolite xenolith from a shallower depth contains apatite with older single-grain apatite ages in the range of 1200–890 Ma. These lower intercept ages are much younger than previously reported zircon U--Pb ages from the same samples, which register a thermal maximum in the Palaeoproterozoic (c. 1700–1800 Ma). Trace element analysis of apatite confirms that most grains did not undergo retrogressive recrystallization. Additionally, there is a statistically significant link between apatite size and single grain ages in at least one sample. Our interpretation is that these apatite U-Pb ages represent slow cooling through the apatite Pb partial retention zone (c. 375–450 °C). Whether these data represent a distinct cooling event, or simple protracted cooling from a Palaeoproterozoic thermal peak, cannot be strictly constrained by these data. However, previous evidence from rutile U-Pb in xenoliths from nearby locations implies slow cooling from a Palaeoproterozoic peak. If a slow cooling scenario is assumed, by comparing apatite cooling ages to previous results from Ti-in-zircon thermometry on metamorphic zircon from the same xenoliths, a cooling rate of 0.14--0.33 °C/Myr can be estimated for these granulite xenoliths. Our apatite data thus contribute to a growing consensus on the geodynamic history of the northern Wyoming Craton and Medicine Hat Block, wherein lithospheric stability was maintained on a giga-year timescale from a granulite-facies metamorphic event associated with the assembly of Laurentia up until Laramide orogenesis.

Palaeoproterozoic granulite-facies metamorphism in the eastern Alxa Block: New petrological and geochronological evidence from the Diebusige Complex

The Diebusige Complex, the highest-grade metamorphic complex in the Alxa Block, can provide significant insights into the origin, formation, evolution, and tectonic affinity of the Alxa Block. Petrographic observations show that the mineral assemblage of (garnet) magnetite quartzite from the Diebusige Complex is magnetite + quartz + hypersthene + plagioclase ± clinopyroxene ± garnet. The metamorphic temperature (T) is constrained to 800–860 °C by the two-pyroxene geothermometer, corresponding to granulite-facies metamorphism. Geochronological studies involving cathodoluminescence (CL) images, LA-ICP-MS U-Pb dating, and trace element analyses of zircons show that the time of granulite-facies metamorphism was approximately 1850 Ma. Combined with the corresponding published data, this paper suggests that the metamorphic basement rocks of the Alxa Block record Neoarchaean-early Palaeoproterozoic magmatic-metamorphic and late Palaeoproterozoic granulite-facies metamorphic events, thereby indicating the affinity between the Alxa Block and the Khondalite Belt (KB).

New geochronological evidences of late Neoarchean and late Paleoproterozoic tectono-metamorphic events in the Miyun area, North China Craton

The Miyun area is located in the western margin of the Eastern Block of the North China Craton (NCC), and granulite- to amphibolite facies tectono-metamorphic events occurred in the Precambrian. In this contribution, high-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon from garnet-pyroxene amphibolite yielded the metamorphic age of 2507 ± 4 Ma. SIMS U-Pb dating of monazite from garnet-biotite gneiss yielded the metamorphic ages of 2496 ± 16 Ma and 1834 ± 21 Ma, respectively. Furthermore, 40Ar/39Ar dating of hornblende from amphibolite yielded plateau ages of 1817 ± 41–1795 ± 19 Ma, and the biotite yielded a plateau age of 1744 ± 12 Ma. Combined with previous studies and the closure temperatures of the dating minerals, it is inferred that the Miyun area experienced a late Neoarchean granulite-facies metamorphic event occurred at ~2.50 Ga, followed by a late Paleoproterozoic amphibolite-facies metamorphic event occurred at ~1.83–1.74 Ga. These two tectono-metamorphic events are ascribed to the Precambrian orogenic-related environments.

Anatexis and metamorphic history of Permian pelitic granulites from the southern Chinese Altai: Constraints from petrology, melt inclusions and phase equilibria modelling

Pelitic granulites from the southern Chinese Altai record a prominent Permian high–temperature reworking event. The lack of solid constraints on anatexis and metamorphic P–T path of these rocks has hindered our understanding of the thermo–tectonic history of Chinese Altai orogen. Here, we present a detailed field and petrological study of the pelitic granulites cropping out at Wuqiagou area, Fuyun county. They occur as lenses or interlayers hosted within biotite–plagioclase gneisses, with a rutile–bearing assemblage composed of garnet ± orthopyroxene + cordierite + biotite ± K–feldspar + plagioclase + quartz ± spinel ± sillimanite + ilmenite. These pelitic granulites preserve diagnostic features of anatexis, such as small patches of leucosomes at macroscopic scale, melt inclusions in garnet porphyroblasts and highly cuspate quartz at microscopic scale. Melt inclusions (up to ~40 μm) are now crystallized into a cryptocrystalline aggregate of feldspars, quartz and biotite (i.e. they are nanogranitoids). Phase equilibria modelling in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) 10–compositional model system suggests a possible clockwise P–T path involving a post–peak isobaric cooling (IBC) process from 800 to 850 °C to near solidus conditions (700–720 °C) at 5.0–6.0 kbar. SIMS zircon UPb geochronology indicates that the granulite–facies metamorphism took place at 284.1 ± 2.6 Ma, which is coeval with spatially associated mantle–derived mafic intrusions. Therefore, we propose that the Permian granulite–facies metamorphic event occurred in a post–orogenic extensional tectonic setting, which was associated with the input of external heat, related to underplating and intruding of mantle–derived magmas possibly as a result of the Tarim mantle plume activity.

Low–pressure granulite–facies metamorphism in the Southern Chinese Altay orogenic belt, NW China: P–T estimates, U–Pb ages and tectonic implications

The Chinese Altay orogenic belt formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), accompanying with remarkable metamorphism. Its tectono–metamorphic evolution history is still a controversial topic. In this contribution, we investigate the petrography, metamorphic peak P–T conditions and zircon geochronology of the newly discovered garnet–absent metapelitic and felsic gneisses at Wuqiagou area in the southern Chinese Altay. They are characterized by an assemblage of orthopyroxene + biotite + plagioclase + quartz + Fe–Ti oxides ± cordierite ± K–feldspar, in which most K–feldspar grains have transformed into perthites with thin albite lamellae. The Wuqiagou garnet–absent gneisses experienced pervasive partial melting, as testified by microstructures such as mineral pseudomorphs after melt films or pockets. Phase diagram modelling constrains the peak P–T conditions to 3.5–5.5 kbar and 800–900 °C, with possible geothermal gradients of 45–75 °C/km, indicative of a prominent low–pressure granulite–facies metamorphic event in the southern Chinese Altay. SIMS zircon U–Pb dating results show a weighted mean age of 255.8 ± 1.8 Ma. This age is interpreted to represent the timing of this low–pressure granulite facies metamorphism, which is highly coeval with the timing of their hosted mafic granulite lenses (∼255 Ma) and nearby mantle–derived mafic intrusions (∼257 Ma). Based on available petrological, geochemical and chronological data, we propose that the Permian low–pressure granulite–facies metamorphic event in the southern Chinese Altay was likely associated with the intrusions of deep–derived mafic magma at a relatively shallow crustal level (12–18 km) in a post–orogenic extensional setting, with a possible link with the Tarim mantle plume activity.

Low–pressure metamorphism of mafic granulites in the Chinese Altay orogen, NW China: P–T path, U–Pb ages and tectonic implications

Mafic granulites occur as massive lenses within host paragneisses at Wuqiagou area in the southern Chinese Altay orogen. They comprise a suite of fine–grained and coarse–grained mafic granulites, both characterized by an assemblage of orthopyroxene, clinopyroxene, hornblende, biotite, plagioclase, quartz, Fe–Ti oxides, minor K–feldspar, and accessary apatite and zircon. The peak– and post–peak metamorphism evolution is respectively represented by orthopyroxene–clinopyroxene–bearing assemblages, and the clinopyroxene–plagioclase–bearing or hornblende–biotite–plagioclase–bearing associations. Thermobarometries and pseudosection modelling constrain the peak P–T conditions at ~5.5 kbar and 770–820 °C, with gradients of 40–45 °C/km for the this stage, and at ~3.0 kbar and 630–670 °C for the retrograde evolution, indicative of a possible P–T path with a post–peak cooling and slight decompression process. LA–ICP–MS and SHRIMP zircon U–Pb age dating results of two mafic granulite samples show weighted mean ages of 275 ± 1.6 Ma and 271 ± 3.2 Ma, respectively. These ages were interpreted to represent the timing of the low–pressure granulite–facies metamorphic event in this region, suggesting relatively high thermal conditions in the late Paleozoic. This metamorphism was highly coeval with the deep–derived mafic intrusions and Tarim mantle plume activity (~275 Ma). Accordingly, we concluded that the Permian regional thermal anomaly in this region might be closely related to the Tarim mantle plume activity.

Recognition and significance of c. 800 Ma upper amphibolite to granulite facies metamorphism in metasedimentary rocks from the NW margin of the Yangtze Block

The evolutionary history of the NW Yangtze Block is important in interpreting its location and relationship with the Rodinia supercontinent. Although a Neoproterozoic arc-related tectonothermal event is recognized in the Micangshan area of the NW Yangtze Block, its timing and P–T conditions are poorly constrained. We address this issue by focusing on the garnet–biotite gneiss that represents the main lithology of the Huodiya Group on the NW margin of the Yangtze Block. This gneiss is predominantly composed of garnet, biotite, feldspar and quartz. The peak mineral assemblage in the matrix consists of garnet + biotite + quartz + plagioclase + K-feldspar. The retrograde stage is characterized by embayed rims of the garnet and its associated biotite in the matrix. The P–T conditions in the peak and retrograde stages are constrained to 7–8 kbar and c. 710°C and 5–6 kbar and 650–675°C, respectively, and suggest that 4–5 vol.% melt was produced during an upper amphibolite–granulite facies metamorphic event. The first report of monazite U–Pb dating in the Huodiya Group of the Micangshan area yielded a weighted mean 206Pb/238U age of 802 ± 5 Ma. The studied samples contained detrital igneous zircons with 206Pb/238U dates >800 Ma, whereas the metamorphic zircons yielded a weighted mean 206Pb/238U age of 797 ± 9 Ma. Based on the compressional tectonic environment and previous studies, we propose that the Yangtze Block was probably located on the periphery of the Rodinia supercontinent. Supplementary material: The analytical methods are summarized in Supplementary Text File 1. Representative mineral composition and the results of U–Pb isotopic analyses of monazites and zircons are presented in supplementary table 1–7. The results of trace element concentrations in garnets are presented in Supplementary table 8. https://doi.org/10.6084/m9.figshare.c.4698356

Zircon U–Pb ages and Hf isotopes of the Huai’an gneisses from the Tianzhen-Xinpingbu area: Implications for the tectonic evolution of the Trans-North China Orogen

One of the major advancements in understanding the formation and evolution of the North China Craton was the recognition of a Himalayan-type collisional belt in the central part of the craton, called the Trans-North China Orogen (TNCO), along which two micro-continental blocks, named the Eastern Block and Western Block, joined together to form a coherent craton. However, controversy still remains about the pre-collisional tectonic setting and architectures of the TNCO. In this contribution, field-based zircon U–Pb and Lu-Hf isotopic investigations were carried out on the Huai’an gneisses from the Tianzhen-Xinpingbu area in the northern part of the TNCO. The Huai’an gneisses consist predominantly of tonalitic-trondhjemitic-granodioritic (TTG) gneisses with minor gabbroic, dioritic and granitic gneisses. Our zircon U-Pb results show that the plutonic protoliths of the Huai’an gneisses were emplaced at 2.51–2.40 Ga and experienced a granulite-facies metamorphic event at 1.86–1.80 Ga. The magmatic zircons of these samples yield εHf(t) values from +1.9 to +7.0 and TDMC ages of 2862–2619 Ma. These positive εHf(t) values, associated with the lack of old inherited zircons, suggest the derivation of their precursor magmas from a juvenile crust without contamination of old continental crust. The TDMC model ages indicate that 2.8–2.6 Ga is the best estimation of the mantle extraction time for the source rocks in the Huai’an gneisses, revealing a major period of juvenile crustal growth. Thus it can be inferred that the partial melting of subducted oceanic crust slab produced the magmas of the Huai’an gneisses, which subsequently interacted with overlying mantle wedge during the ascent, suggesting an intra-oceanic arc environment for the Huai’an gneiss complex. Taken together, the development of the Huai’an gneiss complex was involved in a subduction-related magmatic event in an intra-oceanic arc at ~2.5 Ga, followed by collision between the Eastern and Western blocks at ~1.85 Ga.

Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

Zircon U-Pb age constraints for Tonian-early Cryogenian deposition of metasedimentary rocks from the Schirmacher Oasis, East Antarctica: Implications for correlations across the Mozambique Ocean

Schirmacher Oasis, central Dronning Maud Land, East Antarctica, a dispersed fragment of the 660-610 Ma East African Orogen (EAO), exposes platformal-facies high-grade metasedimentary rocks. Here we constrain the depositional age of these sedimentary rocks to test models relating to timing of the closure of the Mozambique Ocean, the southern segment of the EAO, since Tonian time. Zircon, a proxy for detritus, was dated by LA-MC-ICPMS (206Pb/238U ages) that includes both magmatic and (older) metamorphic populations, in addition to those formed during the granulite-facies metamorphic events (660-600 Ma and 580-555 Ma) having an age range between 1240 and 500 Ma. In conjunction with field evidence, their depositional time is constrained to the Neoproterozoic (Tonian to early Cryogenian, i.e. 1000 >Tdep >800 Ma and 930 >Tdep>800 Ma) with their provenance likely dominated by metamorphic (orogenic) over magmatic rocks.These metacarbonate and psammitic rocks are correlated with similar rocks from the Montepuetz Complex, Northeastern Mozambique, metacarbonate rocks from the Sør Rondane Mountains and with the Linta Group, Vohibory Domain, Southeastern Madagascar. The coeval deposition of platformal sedimentary protoliths, interpreted as occurrence of multiple Mozambique Oceanic strands, were incorporated into the continental arc crust during collision between the Congo and Azania cratonic blocks forming the EAO.

Pressure-temperature evolution during two granulite-facies metamorphic events (2.62 and 2.02 Ga) in rocks from the Central Zone of the Limpopo Belt, South Africa

The Central Zone of the Limpopo Belt in southern Africa is a classical example of a polymetamorphic terrane that experienced three metamorphic events at 3.22 Ga (M1), 2.66–2.61 Ga (M2) and 2.02 (M3). However, the geodynamic significance of these events is controversial as their P-T evolution was poorly constrained.We present new petrological and geochronological data for the M2 and M3 events. The Neoarchean M2 event is well-preserved in samples from a newly discovered enclave of supracrustal and magmatic rocks in the 2.612 Ga Bulai pluton that shielded the enclave against a Paleoproterozoic M3 overprint, which is dominant in rocks exposed outside the pluton. P-T pseudosection modelling for migmatitic Grt-Crd metapelites from the enclave and the rock sequence outside the pluton indicates an opposite metamorphic evolution, a counterclockwise P-T path for M2 but a clockwise path for M3.Spinel-quartz assemblages in the enclave metapelites record early heating to UHT conditions (ca. 930 °C, 5.5 kbar) at intermediate crustal levels. The peak-pressure M2-assemblage garnet-cordierite-sillimanite-K-feldspar-plagioclase-quartz ± rutile formed at ca. 860 °C and 7.5 kbar during a moderate pressure increase. Cordierite coronas around garnet record subsequent decompression. We interpret the counterclockwise P-T evolution during M2 to result from convergence of the Zimbabwe and Kaapvaal Cratons, which has led to subduction of oceanic lithosphere beneath the Zimbabwe Craton. Subsequent slab retreat has caused asthenosphere upwelling, thereby inducing delamination of the lithospheric mantle. Intrusion of hot asthenospheric material beneath the crust induced early UHT metamorphism at mid-crustal levels. The subsequent moderate pressure increase was related to the ensuing collision of the cratons. Post-peak decompression records incorporation of the enclave into the Bulai pluton melt and uplift in the ascending granitoid melt.The peak M3-assemblage garnet-cordierite-sillimanite-K-feldspar-rutile-plagioclase-quartz formed in metapelites occurring outside the pluton during near-UHT conditions (ca. 880 °C, 7.5 kbar). Cordierite coronas around garnet record subsequent decompression. New growth of garnet and sillimanite at the expense of cordierite documents late near-isobaric cooling to <700 °C. The resulting clockwise P-T path for M3 granulite-facies metamorphism is interpreted to record an intracontinental Paleoproterozoic transpressional orogeny within the consolidated Kaapvaal-Zimbabwe Craton, which was induced by far-field tectonic processes.

Petrology, geochemistry and zirconology of impure calcite marbles from the Precambrian metamorphic basement at the southeastern margin of the North China Craton

Impure calcite marbles from the Precambrian metamorphic basement of the Wuhe Complex, southeastern margin of the North China Craton, provide an exceptional opportunity to understand the depositional processes during the Late Archean and the subsequent Palaeoproterozoic metamorphic evolution of one of the oldest cratons in the world. The studied marbles are characterized by the assemblage calcite+clinopyroxene+plagioclase+K-feldspar+quartz+rutile±biotite±white mica. Based on petrography and geochemistry, the marbles can be broadly divided into two main types. The first type (type 1) is rich in REE with a negative Eu anomaly, whereas the second type (type 2) is relatively poor in REE with a positive Eu anomaly. Notably, all marbles exhibit remarkably uniform REE patterns with moderate LREE/HREE fractionation, suggesting a close genetic relationship.Cathodoluminescence imaging, trace elements and mineral inclusions reveal that most zircons from two dated samples display distinct core-rim structures. Zircon cores show typical igneous features with oscillatory growth zoning and high Th/U ratios (mostly in the range 0.3–0.7) and give ages of 2.53−2.48Ga, thus dating the maximum age of deposition of the protolith. Zircon rims overgrew during granulite-facies metamorphism, as evidenced by calcite+clinopyroxene+rutile+plagioclase+quartz inclusions, by Ti-in-zircon temperatures in the range 660–743°C and by the low Th/U (mostly <0.1) and Lu/Hf (<0.001) ratios. Zircon rims from two dated samples yield ages of 1839±7Ma and 1848±23Ma, respectively, suggesting a Palaeoproterozoic age for the granulite-facies metamorphic event. These ages are consistent with those found in other Precambrian basement rocks and lower-crustal xenoliths in the region, and are critical for the understanding of the tectonic history of the Wuhe Complex.Positive Eu anomalies and high Sr and Ba contents in type 2 marbles are ascribed to syn-depositional felsic hydrothermal activity which occurred at 2.53−2.48Ga. Our results, together with other published data and the inferred tectonic setting, suggest that the marbles' protolith is an impure limestone, rich in detrital silicates of igneous origin, deposited in a back-arc basin within an active continental margin during the late Archean and affected by synchronous high-T hydrothermalism at the southeastern margin of the North China Craton.

A relic slice of archean–early Paleoproterozoic basement of Jiaobei Terrane identified within the Sulu UHP belt: Evidence from protolith and metamorphic ages from meta-mafic rocks, TTG–granitic gneisses, and metasedimentary rocks in the Haiyangsuo region

A great quantity of metamorphic rocks (Haiyangsuo Complex), including high-pressure (HP) mafic granulites with ‘red-eye-circle’ textures, retrogressive products of Grt-, Qtz-, Ep-, and Cpx-bearing amphibolites, TTG–granitic gneisses, and metasedimentary rocks, is distributed extensively within the Haiyangsuo region, northeastern Sulu ultrahigh-pressure (UHP) belt. The Haiyangsuo Complex crops out over an area of at least 70km2, rather than the 15-km2 area mentioned in previous studies. Based on integrated studies of petrography, mineral phase transformations, mineral inclusions, cathodoluminescence (CL) images of zircons, and zircon U–Pb LA–ICP–MS and amphibole Ar–Ar age dates, together with previous P–T condition estimates, abundant Neoarchean to Early Paleoproterozoic igneous protoliths, including ca. 2.6–2.5Ga meta-mafic rocks and ca. 2.7Ga, ca. 2.55–2.5Ga, and ca. 2.4Ga TTG–granitic gneisses, were identified in the Haiyangsuo region, whereas the Late Paleoproterozoic (∼1.7Ga) and Neoproterozoic (ca. 800–650 Ma) protolith ages common in the Sulu HP–UHP rocks have not been observed. The metasedimentary rocks in the region studied have a deposition age of ca. 2.1–1.9Ga, showing protolith ages similar to those of the voluminous Khondalite series of the Jingshan and Fenzishan groups from the Jiaobei Terrane, southwestern Paleoproterozoic Jiao–Liao–Ji orogenic belt.Two discrete episodic HP granulite–facies metamorphic events were identified for the first time in the meta-mafic rocks of the region and their country rocks. The first episodic HP granulite-facies metamorphism (M1) is characterized by the relict coarse-grained mineral assemblage of garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)±quartz (Qtz), which formed under P–T conditions of 740–850°C and 1.0–1.26GPa. In contrast, the second HP granulite-facies metamorphism (M21) is characterized by ‘red-eye-circle’ textures, with a typical fine-grained mineral assemblage of Grt+Cpx+Pl±Qtz surrounding the relict coarse-grained HP granulite-facies minerals of M1. It formed under P–T conditions of 765–845°C and 1.14–1.75GPa. Both of the HP granulite-facies metamorphic events were overprinted and replaced by a late amphibolite-facies retrogression (M22), which is typified by a fine-grained mineral assemblage of Amp+Pl+Grt and formed under P–T conditions of 580–660°C and 0.6–0.8GPa. Abundant metamorphic zircon U–Pb dating revealed that the two discrete episodic HP granulite-facies metamorphic events in the Haiyangsuo Complex occurred and recorded in metamorphic zircon domains with HP granulite-facies mineral inclusions at Late Paleoproterozoic (ca. 1.87–1.86Ga) and Middle Triassic (ca. 245–225Ma), respectively, and that each was overprinted by a subsequent amphibolite-facies retrogression at∼1.84Ga and 220–214Ma, respectively. Various metamorphic rocks (Haiyangsuo Complex) in the Haiyangsuo region have rock association, petrographical, geochronological, and P–T evolution affinities to the Paleoproterozoic Jiaobei Terrane, SW Jiao–Liao–Ji Belt, but are distinguished from the Sulu UHP rocks. Hence, the Haiyangsuo Complex is exotic to the Sulu UHP belt and can be regarded as an inseparable part of the Jiaobei Terrane during the Paleoproterozoic. The unique Middle Triassic HP granulite-facies metamorphic event first identified within the Haiyangsuo Complex coincides with that (245–225Ma) of the adjacent Jiaodong Terrane, NE Sulu UHP belt, indicating that the Haiyangsuo Complex was also involved in Triassic collision–subduction and experienced HP granulite-facies metamorphism at a lower crustal level. The ultimate formation of the Haiyangsuo Complex along the southeastern margin of the Jiaobei Terrane, SW Jiao−Liao−Ji orogenic belt, was genetically related to the collision between the North China and South China cratons. Thus, the juxtaposition of the Haiyangsuo Complex to the Sulu UHP metamorphic belt took place in the Middle–Late Triassic rather than in the Jurassic.

Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen (southeastern Brazil): Evidence from the polymetamorphic Carlos Chagas batholith

From the earliest (ca. 630Ma) pre-collisional plutons to the latest (ca. 480Ma) post-collisional intrusions, the Araçuaí orogen (SE Brazil) records an outstanding succession of granite production events in space and time. The Carlos Chagas batholith (CCB) is the largest (~14,000km2) granitic body ascribed to the collisional plutonism (G2 supersuite) in the back-arc region of the Araçuaí orogen, to the east of the Rio Doce magmatic arc. A wide range of monazite and zircon ages (>725Ma to ca. 490Ma) have been found in CCB granites, recording a rich history of crustal recycling and inheritance, magmatic crystallization and anatexis. The CCB includes a dominant granite richer in garnet than in biotite, in which three mineral assemblages can be identified: 1) Qz+Pl+Kfs+Bt+Grt+Ilm±Rt; 2) Qz+Pl+Kfs+Bt+Grt+Ilm+Sil; and 3) Qz+Pl+Kfs+Bt+Grt+Ilm+Sil+Spl. Rocks which contain mineral assemblage 2 and 3 all contain two generations of garnet. Textural evidence for the presence of former melt, recognized in all studied CCB samples, includes: silicate melt inclusions in poikiloblastic garnet, pseudomorphed thin films of melt surrounding both generations of garnet, pseudomorphed melt pools adjacent to garnet and biotite, and plagioclase and quartz with cuspate-lobate shapes occurring among matrix grains. Both generations of garnet crystals (Grt1 and Grt2) are unzoned in terms of major element concentration, contain small rounded inclusions of Ti-rich biotite and, in addition, the Grt2 crystals also contain inclusions of remnant sillimanite needles. Microstructural evidence, in combination with mineral chemistry, indicates that the garnet crystals grew during two distinct metamorphic-anatectic events, as the peritectic products of fluid-absent melting reactions which consumed biotite, quartz and plagioclase, in the case of Grt1, and which consumed biotite, quartz, plagioclase and sillimanite in the case of Grt2. P–T pseudosections calculated via Theriak-Domino, in combination with in situ U–Pb monazite and zircon dating, provide new constraints on the thermal evolution of the back-arc region of the Araçuaí orogen. Data from assemblage 1 suggests P–T conditions for the first granulite-facies metamorphic event (M1) at 790–820°C and 9.5–10.5kbar, while the assemblage 2 records P–T conditions for a second granulite-facies metamorphism (M2) of around 770°C and 6.6kbar. Monazite and zircon within garnets from the different assemblages give age peaks at 570–550Ma (M1) and 535–515Ma (M2), recording two anatectic events in the CCB during a single orogenic cycle. The PT conditions for these metamorphic events can be related to: i) M1, striking crustal thickening, probably involving thrusting of the magmatic arc onto the back-arc region; and ii) M2, decompression related to the gravitational collapse of the Araçuaí orogen.

The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences

The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goias state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite-facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in eNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeiropolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.