High-pressure Granulite Research Articles

High-pressure granulite from Jixian, Eastern Hebei, the North China Craton: implications for Neoarchean to early Paleoproterozoic collision tectonics

Abstract Precambrian high-pressure (HP) granulites provide important information for reconstructing ancient continental nuclei. Here we report granulites in Eastern Hebei, North China Craton (NCC). They experienced three metamorphic events related to Neoarchean–early Paleoproterozoic orogenesis. The garnet–clinopyroxene granulite defines the M 1 event at P–T conditions of 11–13 kbar and 780–830°C, while the two-pyroxene granulite was produced during the M 2 event at 7–9 kbar and 850–950°C. Both the garnet–clinopyroxene and two-pyroxene granulites experienced amphibolite retrogression during the M 3 event at 5–7 kbar and 710–730°C. Geochemical compositions of these granulites exhibit affinity to island-arc andesites. Zircon U–Pb dating shows that their magmatic precursors were erupted at c. 2538 Ma, and have experienced two-stage growth of zircon rims at c. 2458 and c. 2285 Ma, respectively. The c. 2458 Ma age may represent orogenic events during the amalgamation of micro-continental blocks of the eastern NCC, and the c. 2285 Ma age may be interpreted as the effect of late Paleoproterozoic magmatism. We suggest that the Neoarchean andesitic protoliths of the granulites were metamorphosed at HP granulite-facies conditions during collision of micro-continental blocks, and then exhumed to shallow levels. These early Paleoproterozoic HP granulites recorded the amalgamation of micro-continental blocks to reach the cratonization of the eastern NCC.

Constraining the age of high-pressure metamorphism of paragneisses from the Eastern Himalayan Syntaxis using zircon petrochronology and phase equilibria

Abstract The metamorphic evolution of the Eastern Himalayan Syntaxis (EHS) is of primary importance in understanding the tectonic framework of the India–Asia collision. However, the age of formation and exhumation of the high-pressure granulites from the EHS remains controversial. In this paper, we report the age of metamorphism of high-pressure (HP) granulite-facies paragneisses from the Namche Barwa Complex (NBC). These paragneisses consist of variable proportions of garnet, biotite, plagioclase, K-feldspar, quartz, rutile and ilmenite. Using phase equilibrium modelling of garnet growth combined with partitioning of rare earth elements (REEs) between zircon and garnet, we link multistage zircon growth to garnet growth to date the prograde stage to the interval 32.6–25.2 Ma, the pressure peak to c. 25–24 Ma and the retrograde stage to the interval 23.5–13.1 Ma. Taking into account published ages from the literature, for the NBC we propose that the prograde to peak metamorphism persisted until c. 25–24 Ma. This was followed by exhumation and melt crystallization during the interval 24–8 Ma, and cooling through the amphibolite facies after c. 8 Ma.

Phase equilibrium modelling and SHRIMP zircon U–Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications

The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M1–M4). The M1–M3 stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M1) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M2 is defined by the ilmenite that have partially replaced rutile; and the M3 is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M4). Combined with the phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system, the MP pelitic granulites record a clockwise P–T path, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.

2.7 Ga high-pressure granulites of the Teton Range: Record of Neoarchean continent collision and exhumation

2.7 Ga high-pressure granulites of the Teton Range: Record of Neoarchean continent collision and exhumation

P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: Insights from phase equilibria and geochronology

The Hengshan complex has drawn attention for the occurrence of typical high-pressure mafic granulites, representing the Paleoproterozoic collisional orogeny in the North China Craton. Mafic granulites occur as boudins of various scales in tonalite–trondhjemite–granodiorite (TTG) gneisses. They are composed of garnet, clinopyroxene, plagioclase, orthopyroxene, hornblende, biotite, quartz, rutile and ilmenite. Garnet porphyroblasts are usually surrounded by plagioclase coronas, and early jadeite-rich clinopyroxene is totally decomposed to the symplectite of clinopyroxene and plagioclase. Two high-pressure granulites and one two-pyroxene granulite were studied by the pseudosection approach, and clockwise P–T paths with four metamorphic stages were recognized. The pre-peak prograde stage was identified from garnet core to mantle compositions, with increasing P–T from 630 to 710 °C at 11–13 kbar. The peak pressure stage hasn’t been well preserved but its mineral assemblages inferred from petrography to be garnet + jadeite-rich clinopyroxene + quartz + rutile ± plagioclase, with P–T condition of 760–820 °C at ∼15 kbar. The post-peak decompression with slight heating is characterized by symplectite, plagioclase coronas, and the formation of orthopyroxene. Locally, two-pyroxene granulite assemblages develop, defining P–T condition of 6–8 kbar and 840–860 °C for the temperature peak (Tmax). The post-Tmax cooling is represented by the occurrence of amphibolite facies assemblages especially in boudin margins with P–T condition of 6–8 kbar and 720–760 °C, suggesting an isobaric cooling process with fluid infiltration. U–Pb dating of metamorphic zircons record ages mainly ranging from 1897 to 1830 Ma with weighted mean ages of 1862 ± 11 Ma for the high-pressure granulite and 1837 ± 7 Ma for the two-pyroxene granulite. These metamorphic ages were interpreted to record the post-peak cooling stage especially the time close to solidus, which is also supported by the result of Ti-in-zircon thermometer. A summary of P–T–t evolution for metamorphic terrains corresponding to different crust levels in Hengshan–Wutai area indicates that the main episode of crust thickening-dominated collisional orogeny is ∼1.95 Ga or earlier and finished at ∼1.92 Ga. This was followed by post-orogenic cooling-uplifting or the overprinting of another thermal-tectonic event during the period of 1.92–1.80 Ga.

Petrologic and zircon U–Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta–igneous rocks, occurring as lenses within the meta–sedimentary rocks of the Badu Complex. However, metamorphic state of these meta–sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P–T loop. Results from integrated U–Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100–50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to granulite facies metamorphism of these supracrustal rocks occurred in the Early–Middle Triassic. Based on the presence of HP granulite facies pelitic rocks, it is inferred that significant underthrusting was involved during the Indosinian Orogeny which introduced these supracrustal rocks to lower crustal levels.

Permo-Triassic high-pressure metamorphism in the central western Korean Peninsula, and its link to Paleo-Tethyan Ocean closure: Key issues revisited

Permo-Triassic high-pressure (HP) mafic granulites, together with the Bibong retrogressed eclogite, preserved along the central western Korean Peninsula provide important insights into the Late Permian to Triassic collisional orogeny in northeast Asia. The metamorphic pressure–temperature–time (P–T–t) paths of these rocks, however, remain poorly constrained and even overestimated, owing to outdated geothermobarometers and inaccurate isopleth techniques. Here we evaluate the metamorphic P–T conditions of Triassic HP mafic granulites including those in Baekdong, Sinri and Daepan and the Bibong Triassic retrogressed eclogite in the Hongseong area, and the Permo-Triassic Samgot mafic granulite in the Imjingang Belt of the central western Korean Peninsula through the application of modern phase equilibria techniques. The Baekdong and Samgot mafic granulites and the Bibong retrogressed eclogite yield a range of 12.0–16.0 kbar and 800–900 °C, representing HP granulite facies conditions. The Sinri and Daepan granulites from the Hongseong area show relatively lower grade metamorphic conditions between HP granulite and normal granulite facies, and are characterized by sub-isothermal decompression during exhumation. The similarities in the metamorphic ages and the post-collisional igneous activity from the central western Korean Peninsula indicate that the Triassic ages represent the retrograde stage of the metamorphic P–T paths. In contrast, the Late Permian metamorphic ages, which are older than protolith ages of the post-collisional igneous rocks, correspond to the possible prograde stage of metamorphism. The P–T–t paths presented in this paper, together with the metamorphic ages and post-orogenic igneous events reported from these areas suggest trace of the subduction, accretion and exhumation history, and indicate a tectonic linkage among the northeast Asian continents during the Paleo-Tethyan Ocean closure.

Petrological Investigations and Zircon U‐Pb Dating of High Pressure Felsic Granulites from the Yushugou Complex, South Tianshan, China

AbstractAs a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite‐peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure (HP) granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative felsic granulite samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism (Stage I) with P‐T conditions of 9.8–10.4 kbar at 895–920°C was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism (Stage II) shows P‐T conditions of 13.2–13.5 kbar at 845–860°C, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anticlockwise P‐T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA‐ICP‐MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are ∼430 Ma, with two age groups of ∼390 Ma and 340–350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of ∼390 Ma, which may be related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism (340–350 Ma) may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.

The metamorphic evolution of Salma-type eclogite in Russia: Constraints from zircon/titanite dating and phase equilibria modeling

Combined geochronology with phase equilibria modeling, the P-T-t evolution of metamorphic rocks can be deciphered. Four different representative samples, including retrogressed eclogite, garnetite, paragneiss and tonalite–trondhjemite–granodiorite (TTG) gneiss have been selected for detailed petrography and U/Pb dating from the Belomorian Eclogite Province in Russia. Zircon cores yield Archean ages for the protoliths of these samples. Zircon rims of the TTG gneiss have recorded Neoarchean anataxis at ca 2.73 Ga, whereas 1.91–1.92 Ga zircon rims of the garnetite and the paragneiss have dated high-pressure granulite facies metamorphism. Titanite in the retrogressed eclogite has recorded amphibolite facies retrogression at ca 1.86 Ga. Phase equilibria modelings for retrogressed eclogite, garnetite, and paragneiss define P-T conditions of 17–18 kbar/below 790 °C for the peak eclogite facies metamorphism, 11–12 kbar/870–900 °C for the high-pressure granulite facies metamorphism, and 8–10 kbar/650–710 °C for titanite’s stability which corresponds to the amphibolite facies metamorphism. A clockwise P-T path is inferred for the Salma-type eclogite, which is characterized by thermal relaxation from the peak eclogite facies to the high-pressure granulite facies, and nearly isobaric cooling from the high-pressure granulite facies to the amphibolite facies.

High-pressure granulites in the Fuping Complex of the central North China Craton: Metamorphic P–T–t evolution and tectonic implications

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite–Tonalite–Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ilmenite + quartz within garnet core; an inferred peak assemblage composed of garnet ± hornblende + plagioclase + clinopyroxene + rutile/ilmenite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene ± orthopyroxene + plagioclase, coronae of plagioclase ± clinopyroxene ± hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P–T conditions of 12–13 kbar and 760–800 °C (Pmax) and a post-peak history under P–T conditions of ∼9.0 kbar and 805–835 °C (Tmax), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the Pmax to the Tmax is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz ± diopside = hornblende + plagioclase + liquid and garnet + quartz + hornblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the Tmax is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68–6.88 ppm and crystallization temperatures of 602–712 °C. SIMS zircon U–Pb dating on these zircons yields 207Pb/206Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage of metamorphism. The P–T–t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton.

Metamorphic P–T paths and zircon u–pb ages of Paleoproterozoic metabasic dykes in eastern Hebei and northern Liaoning: Implications for the tectonic evolution of the North China Craton

Metabasic dykes are described from Qianxi (JD1462) and Qinglong (JD15158) in the eastern Hebei and Qingyuan (15Q18) in the northern Liaoning that clearly crosscut the foliation of Neoarchean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and supracrustal rocks in the Longgang Block. Petrographic observations and phase modeling using THERMOCALC suggest that these metabasic dykes have experienced metamorphism of high-pressure (HP) granulite facies and amphibolite facies characteristic of clockwise P–T paths. Their peak P–T conditions were constrained to be 11.5 ± 0.6 kbar/815 ± 62 °C for sample JD1462, 8–9 kbar/570–585 °C for JD15158 and 10.7 ± 0.6 kbar/780 ± 70 °C for 15Q18, corresponding to apparent geothermal gradients of 18–19 °C/km. Zircon U–Pb dating yields metamorphic ages of ∼1.82 Ga for samples JD15158 and JD1462, interpreted to represent the peak stage of amphibolite facies and the post-peak decompression stage of HP granulite assemblage. Zircons in sample 15Q18 yield four age groups of ∼1.84 Ga, 1.94 Ga, 2.12 Ga and 2.49 Ga, interpreted to represent the ages, respectively, of the post-peak decompression stage of HP granulite assemblage, a mixed result from zircons of ∼1.84 Ga and ∼2.12 Ga, the emplacement of basic dykes, and the captured zircon grain from country TTGs. Thus, the metamorphic evolution of metabasic dykes may correlate to a crustal thickening event occurred close to 1.84–1.82 Ga, which is separated from the collision event at ∼1.95 Ga prevailed in the North China Craton. The metabasic dykes show weakly right-declined REE patterns and are depleted in Nb, Ta and Ti, inferred to represent a within-plate extensional setting at ∼2.12 Ga, consistent with the magmatic events occurred in the Trans-North China Orogen and Jiao-Liao-Ji Belt. As a consequence, the Archean Longgang Block was largely influenced by the Paleoproterozoic magmatic and metamorphic events.

Dumortierite- and corundum-bearing quartz–feldspar–mica rocks of the Belomorian eclogite province: An example of melting of phengite + quartz

In the Salma eclogite of the Belomorian eclogite province, a dumortierite–phengite–corundum–bearing quartz–feldspar rock has been studied: its primary HP mineral paragenesis included garnet, phengite, and quartz. The phengite–quartz rocks were formed during dehydration and/or melting of boroncontaining rocks when they were dipped in the Meso- Neoarchaean subduction zone to a depth of not less than 70 km. As a result of the subsequent superimposed high-temperature metamorphic events under PT conditions of high-pressure granulite facies, the phengite in quartz underwent incongruent dehydration melting with formation of complex polymineral pseudomorphs, consisting of feldspars, biotite, newly formed muscovite, kyanite, corundum, and dumortierite. New estimates of the metamorphic temperature (850–900°C according to the melting reactions of phengite and the dumortierite field of stability; about 1000°C by the reintegrated composition of feldspar–mesoperthite) that affected the HP parageneses of Salma eclogitized rocks are at least 50–100°C (or even more) higher than them estimated earlier.

Partial melting during exhumation of Paleozoic retrograde eclogite in North Qaidam, western China

Ultrahigh pressure (UHP) rocks are often overprinted by high pressure (HP) granulite-facies metamorphism during exhumation. Here we investigate retrograde eclogite from the Lvliangshan area in North Qaidam which is associated with abundant felsic veins. The eclogite adjacent to the vein shows conversion to garnet amphibolite. Petrographic, mineralogical, geochemical, geochronological data suggest that the eclogite has been experienced peak eclogite facies metamorphism at c. 440Ma, followed by granulite facies retrogression at c. 420Ma, and amphibolite facies overprinting at <420Ma. The retrograde eclogite has microstructural records for partial melting with elongated plagioclase grains occurring as veinlets along the garnet-ilmenite grain boundaries and interspersed in clinopyroxene. Moreover, highly cuspate, wedge–shaped and triangular plagioclase grains are distributed along the garnet boundaries, and banded felsic veins (Pl+Qz) occur along the boundaries of amphibole in garnet amphibolite. Whole-rock geochemistry suggests that the felsic veins have 438.1–1106ppmSr and 2.87–12.18ppm Y with Sr/Y ratios from 81.98–166.2, exhibit pronounced positive Eu anomaly, and depletion of HREE, resembling those of adakitic rocks. Felsic veins contain zircon grains with an U-Pb age of ∼422Ma, similar to the age of HP granulite–facies metamorphism (420Ma) of retrograde eclogite. Combined with the identical Hf isotope compositions between the granulite–facies zircon grains from the host retrograde eclogite and those from the felsic vein, we infer closed system partial melting during HP granulite facies retrogression of the retrograde eclogite. The tonalitic (high Sr/Y ratio and enrichment of LREE) chemical feature, low Rb/Sr and high Ba/Rb ratios of the felsic veins suggest that zoisite and sodic amphibolite dehydration-melting together account for the formation of felsic veins during exhumation from the eclogite–facies to granulite–facies stages.

Two phases of granulite facies metamorphism during the Neoarchean and Paleoproterozoic in the East Hebei, North China Craton: Records from mafic granulites

Archean cratons are commonly reworked during the late Paleoproterozoic orogeny, which may result in multi-phase granulite facies metamorphism in many domains. The East Hebei terrane of the North China Craton is developed with two phases of granulite facies metamorphism in the Neoarchean and Paleoproterozoic. The Neoarchean granulite facies metamorphism is recognized mainly in the rafts or slivers of supracrustal rocks from the Taipingzhai ovate-structural domain, characteristic of anticlockwise P-T paths. The mafic granulites at this domain contain two-pyroxene granulite assemblages which are commonly overprinted by a late phase of granulite facies marked by tiny-grained vermicular orthopyroxene-bearing assemblages. They only record the Neoarchean metamorphic ages at 2.53–2.52Ga and 2.51–2.48Ga as indicated by zircon U-Pb data, para-simultaneously with the emplacement of TTG rocks. In contrast, the Paleoproterozoic granulite facies metamorphism can be well recognized in the Saheqiao linear-structural belt, which is characterized by high-pressure granulite assemblages in mafic rocks with clockwise P-T paths. In this belt, the Archean two-pyroxene granulite assemblages are locally preserved, showing “red-eye socket” overprinting textures consisting of garnet coronas or fine-grained garnet and clinopyroxene bearing assemblages surrounding the early-stage pyroxenes. Zircon U-Pb data indicate that mafic granulites from the Saheqiao belt record not only the Neoarchean metamorphic ages of ∼2.50Ga but also some Paleoproterozoic ages of 1.97–1.83Ga. The first application of Lu-Hf garnet-whole rock isochron method to garnet-bearing mafic granulites from the Saheqiao belt yields ages of 1766.6±7.0 and 1787±22Ma, confirming that the overprinting of garnet-bearing assemblages occurred in the late Paleoproterozoic. Thus, the Saheqiao linear-structural belt may represent a strongly reworked domain during the Paleoproterozoic orogeny, rather than an Archean high-grade greenstone belt as previously considered.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5Ga) Betul Belt and the Neoproterozoic (∼1.0Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MSU-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2Ga and 0.95Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0Ga and 0.95Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

Isotopic evidence for Neoarchean continuity across the Snowbird Tectonic Zone, western Churchill Province, Canada

The Snowbird Tectonic Zone (STZ), the ca. 2800km-long boundary between Rae and Hearne domains of the western Canadian Shield, remains of uncertain tectonic significance, but is currently interpreted as a ca. 1.9Ga suture. The eastern Rae domain is well studied and underlain by a >20,000km2 region of high pressure granulites, containing ca. 2.60Ga arc rocks, and granulite to amphibolite-grade structures, which formed at ca 2.57Ga (D1/M1) and 1.90Ga (D2/M2). The 1.85Ga Legs Lake shear zone juxtaposed the high-P granulites of the eastern Rae domain structurally above lower amphibolite- to greenschist-grade supracrustal gneisses of the Hearne domain. Zircon U-Pb and Hf isotopic analyses were acquired from igneous and metasedimentary rocks across the STZ and Rae-Hearne boundary to assess the validity of the suture model, with specific focus given to the >5km wide Legs Lake shear zone. Results indicate that both 2.60–2.55Ga magmatism and deformation continue unimpeded across the STZ and Rae-Hearne boundary. Furthermore, Hf isotopic data from igneous zircon from 2.60Ga plutonic rocks across the STZ display little variation in εHf indicating that no major crustal-break corresponds to the Rae-Hearne boundary, or any of the shear zones that parallel the STZ on the ground. Older plutonic rocks show a greater variability in εHf, which may indicate a disparate origin. Grade, intensity, and timing of Paleoproterozoic features vary along the transect and display little continuity across the Rae-Hearne domain boundary. In conjunction with existing field and isotopic data, we interpret that the Rae and Hearne domains were together by 2.60Ga, and that the STZ was host to repeated Paleoproterozoic reactivation driven by the latter stages of Taltson-Thelon orogenesis, final indentation of the Slave craton into the Rae domain, and accretionary processes on the southern periphery of the Hearne domain from 1.93 to 1.84Ga. Furthermore, assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them.

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.

U-Pb systematics and trace element characteristics in titanite from a high-pressure mafic granulite

Simultaneous acquisition of U-Pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic, high pressure granulite using LA-ICP-MS split-stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high-temperature, melt-present environments. Although the titanite studied here have a comparatively low initial Pb (Pb0) component (Pb0/Pb*), the Pb0 is highly radiogenic relative to model crustal values, indicating inheritance from U-bearing accessory minerals consumed in the melt/titanite-forming reactions. Additionally, titanite crystals typically exhibit core-rim decreases in Pb0/Pb*, as defined by 204Pb/206Pb, calculated 206Pb0/206PbT, and uncorrected 206Pb/238U spot date profiles. Near the margins this is clearly dominated by local U-enrichment, but in the uniformly low-U interiors outwardly decreasing Pb0/Pb* appears to reflect decreasing Pb0 concentrations during growth. The positive correlation among Pb0 and Sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation, Given the high crystallization temperatures (>800°C) and likely slow cooling rates (~5°C), our data support slow Pb diffusivity in titanite, even at high temperature conditions, as has been proposed in a number of recent studies.Along the outer ~50–100μm, U, Th, Zr, and REE concentrations are variably elevated relative to the crystal interiors, with profiles taking one of two forms: 1) sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge, or 2) gradually increasing to highest concentrations nearest the crystal edge. High-contrast BSE imaging shows that the former profiles are associated with ~1–2μm wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is (or formerly was) in contact with matrix feldspar (i.e. crystallized melt), and are inferred to represent trace-element-enriched dissolution-precipitation reaction fronts. The latter profiles are associated with diffuse, locally wispy, brightness gradients adjacent to resorbed crystal boundaries, subgrain boundaries, or thicker bright bands formed in contact with matrix hornblende crystals, and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro-porosity pathways at some point in the recrystallization process.

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.

Ultrahigh temperature (UHT) mafic granulites in the East Hebei, North China Craton: Constraints from a comparison between temperatures derived from REE-based thermometers and major element-based thermometers

The extrapolation of the newly developed REE-based thermometers of Liang et al. (2013) and Sun and Liang (2015) to the Neoarchean mafic granulites in the East Hebei of North China Craton shows that the REE-based thermometers yield temperatures that are 150–350°C for clinopyroxene–orthopyroxene pair and 100–150°C for garnet–clinopyroxene pair higher than those from the major element-based Fe-Mg thermometers, implying that the REE-based thermometers are more reliable to recover the peak metamorphic temperature of granulites. Detailed petrographic investigations suggest that three types of mafic granulites from east to west in the East Hebei can be recognized, as two-pyroxene granulites in the Taipingzhai zone, garnet two-pyroxene granulites in the Saheqiao zone and garnet clinopyroxene granulites in the Malanyu zone. P-T estimation using major element- and REE-based thermobarometers yield UHT conditions of 1025–1060°C/0.9GPa and 950–1070°C/1.0GPa for the clinopyroxene-orthopyroxene pair in granulites from the Taipingzhai and Saheqiao zones, and high-pressure granulite conditions of 800–875°C/1.0–1.4GPa for the garnet-clinopyroxene pair in granulites from the Malanyu zone. All the granulites show the P-T paths characterized by cooling from their peak conditions to late amphibolite-facies of 720–780°C. The peak P-T conditions of the two-pyroxene and garnet-bearing granulites indicate a paired type of metamorphism with contrasting geothermal gradients of ~40°C/km and ~20°C/km, being particular in the Neoarchean high-grade terranes.