Garnet-bearing Mafic Granulites Research Articles

Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.

Seeing through metamorphic overprints in Archean granulites: Combined high-resolution thermometry and phase equilibrium modeling of the Lewisian Complex, Scotland

Abstract The Lewisian Complex in northwest Scotland presents a record of the transition from the Neo-Archean to the Paleoproterozoic. However, this record is complicated by a long and varied history after peak metamorphism that has erased and/or partially reset much of the early history of the rocks. Such overprinting is a common feature of Archean granulites and poses a substantial problem when trying to understand the tectonic processes that were active prior to the onset of modern plate tectonics. By combining careful petrography with phase diagram modeling and a range of exchange thermometers we obtain the peak and retrograde temperature history of the Lewisian Complex from a single, well-preserved, representative sample of garnet-bearing mafic granulite. We present the application of high-resolution electron probe microanalysis (HR-EPMA) to characterize sub-micrometer orthopyroxene exsolution lamellae in clinopyroxene. We discuss ways to mitigate issues associated with HR-EPMA including surface contamination, beam drift, standards, and the need to correct for secondary fluorescence effects. The resulting compositions from our HR-EPMA analyses provide an independent measure of the retrograde temperature conditions and can also be used to back-calculate the compositions of clinopyroxene in the peak assemblage. We obtain peak metamorphic conditions for the Lewisian of >11 kbar and >1025 °C, and constrain subsequent metamorphic overprints to 850 °C (Grt-Cpx), 590 °C (Opx-Cpx), and 460 °C (Mag-Ilm). These peak and retrograde temperatures span the range of those found in the literature. Whereas recent phase equilibrium studies assume equilibrium among all preserved high-T minerals, this study considers microstructural and mineral-chemical evidence for corona formation that reflects post-peak decompression with partial equilibration at ~850 °C, as recognized in some earlier studies.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt, NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare. In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun. Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet, orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 °C and 5.6–7.0 kbar, representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma, 413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with (La/Sm)N values of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high (87Sr/86Sr)i values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values (−4.3 to 2.5) and variable whole-rock εNd(t) values (−4.9 to 1.2) as well as high (87Sr/86Sr)i values (0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from −30.8 to −5.1, while the whole-rock samples yield consistent (87Sr/86Sr)i values (0.7301 to 0.7342) and negative εNd(t) values (−10.1 to −12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Metamorphic history and Neoarchean–Paleoproterozoic crustal growth of the central Trans-North China Orogen: Evidence from granulite- to amphibolite-facies rocks of the Hengshan complex

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional orogen exposed within the North China Craton (NCC). The orogen has undergone a complex tectonic evolution, but its role in the growth of continental crust is contentious. High-grade metamorphic rocks may carry key information required to clarify these issues. Here, using petrography, mineral chemistry, and zircon U–Pb–Hf isotopes, we investigate a suite of Neoarchean–Paleoproterozoic high-grade metamorphic rocks in the Hengshan Complex within the central TNCO. Garnet-bearing mafic granulites and amphibolites occur as lenses or dikes within tonalite–trondhjemite–granodiorite (TTG) gneisses. Three stages of metamorphic mineral assemblages are identified in a mafic granulite from the northern Hengshan Complex: (1) the prograde assemblage is preserved as small plagioclase and apatite inclusions within garnet porphyroblasts; (2) the peak mineral assemblage is recorded by a high-Ca garnet core coexisting with clinopyroxene, plagioclase, hornblende, biotite, and quartz; and (3) the retrograde assemblage is represented mainly by vermicular symplectites of hornblende + plagioclase and plagioclase coronae surrounding low-Ca garnet rims. Phase equilibria modeling and conventional geothermobarometry suggest that the mafic granulite records a clockwise P–T path, including decompression and cooling from the granulite-facies peak (12.5–14 kbar and >750 °C) to amphibolite-facies conditions during retrogression (7.3–8.3 kbar and 615–721 °C). In contrast, the amphibolite in the southern Hengshan Complex preserves only amphibolite-facies assemblages (hornblende + plagioclase + quartz), consistent with the metamorphic pressures of <7.1 kbar and temperatures below 720 °C.Zircon U--Pb dating of magmatic zircons shows that the protoliths of the TTG gneisses and amphibolite were emplaced at ca. 2.5 and 2.1 Ga, respectively. The metamorphic zircons reveal ages of 1.92–1.80 Ga for the mafic granulite (weighted mean of ca. 1.87 Ga) and 1.92–1.83 Ga for the amphibolites (weighted mean of ca. 1.88 Ga). These 1.88–1.87 Ga metamorphic ages are interpreted to represent the time when the rocks cooled to the solidus. By comparing Paleoproterozoic metamorphic ages across the TNCO, we infer that the subduction–collision–amalgamation between the Western and Eastern Blocks of the NCC lasted from 1.95 to 1.80 Ga at least, marking the final cratonization of the NCC. The ca. 2.5 Ga magmatic zircons within the TTG gneisses show positive εHf(t) values (+0.49 to +7.12) with model ages of 2.8–2.7 Ga, suggesting that the protoliths were derived from the partial melting of Neoarchean juvenile mafic crust. Zircon Hf model ages (TDM = 2.1–2.0 Ga) of the mafic granulite may represent the approximate time at which the mafic magma was generated in the mantle. Thus, the Hengshan Complex within the central TNCO records two episodes of crustal growth at 2.8–2.7 and 2.1–2.0 Ga, respectively. To further understand the processes involved in crustal evolution, we reconstructed a layered crust model for the Hengshan Complex, and the layered crust was strongly influenced by Paleoproterozoic thermal events.

Integrated garnet and zircon petrochronology reveals the timing and duration of orogenic events in the North China Craton

Radiometric data from accessory zircon in long-lived polymetamorphic granulite facies rocks frequently show a large spread, with no clear relationship to zircon textures. For example, the granulites and associated rocks of the North China Craton which have an apparently continuous spread of metamorphic zircon dates through the Palaeoproterozoic from c. 1.95–1.80 Ga. The data are commonly interpreted to reflect long-lived metamorphism or discrete metamorphic events, with intermediate data reflecting variable Pb loss or a mixture of multiple growth domains. Herein, we present an approach that integrates direct dating of the P–T indexed mineral garnet with accessory zircon petrochronology which allows diagnostic time constraints for polymetamorphic processes in high-temperature rocks. We present high-precision LuHf and SmNd dates for two garnet-bearing mafic granulites and the adjoining granite and quartzite from the Gushan area between the Khondalite Belt and the Trans-North China Orogen, North China Craton. Statistically robust LuHf dates of 1815 ± 4 Ma and 1808 ± 3 Ma and corresponding SmNd isochron dates of 1790 ± 6 Ma and 1800 ± 9 Ma were obtained from the two mafic granulites, overlapping the zircon UPb date range of c. 1959–1785 Ma. Samarium–Nd dates from the accompanying garnet-bearing granite and quartzite are indistinguishable at 1794 ± 6 Ma and 1780 ± 6 Ma. The corresponding bulk garnet LuHf isochrons are poorly constrained—two-point garnet–matrix dates range from 1945 ± 10 Ma to 1849 ± 10 Ma for the granite and from 1936 ± 10 Ma to 1840 ± 9 Ma for the quartzite. Upper intercept UPb dates of 1950 ± 24 Ma (granite) and 1936 ± 17 Ma (quartzite) are obtained from the metamorphic zircons in equilibrium with garnet. The close agreement of SmNd isochron-dates (c. 1.80 Ga), the overlapping clusters of zircon UPb dates (c. 1.96 Ga), the identical LuHf and UPb dates of c. 1.80 Ga within uncertainty in the two granulites, and the older LuHf dates and comparable zircon UPb dates (c. 1.96 Ga) obtained in the garnet-bearing granite and quartzite than in those of the mafic granulites, strongly indicate that these rocks experienced coeval metamorphism from c. 1.95–1.80 Ga. These data require a prolonged episodic tectonometamorphic event in the North China Craton, which is probably related to the amalgamation of the Columbia supercontinent.

Neoproterozoic crustal growth in southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation

The southern part of the Malawi Basement Complex is regarded as being part of the Mozambique Belt, which was formed by a series of collisions between Archean cratons and/or Proterozoic magmatic arcs during Neoproterozoic to Cambrian time. The region is particularly important because of its location near the junction between the East-African (ca. 600–550 Ma) and Kuunga (ca. 560–530 Ma) orogens, which means that complex magmatic or metamorphic episodes may have been recorded in high-grade metamorphic rocks in the region. Here we report on new petrological, geochemical, and zircon U–Pb age data on felsic to mafic orthogneisses as well as on a syn-tectonic syenite from the Lilongwe–Zomba–Blantyre area in southern Malawi, with a view to evaluating the timing and pressure–temperature (P–T) conditions of the collision events as well as the protolith formation. The geochemical data on the felsic orthogneisses suggested magmatic-arc signatures, whereas the mafic orthogneisses had both magmatic-arc and non-arc signatures. Metamorphic P–T estimates for the orthogneisses based on conventional geothermometry and phase-equilibria modeling indicated high-pressure granulite-facies conditions of 9.1–11.6 kbar and 840 °C–930 °C. Zircons obtained from charnockite, biotite gneiss, and garnet-bearing mafic granulite yielded magmatic ages of 1011–808 Ma, 1013 Ma, and 1076 Ma, respectively, suggesting the presence of Stenian–Tonian crustal growth related to arc magmatism. Ediacaran (573 Ma) intrusion of granitic rock was also inferred from magmatic zircons found within a granitoid gneiss. The timing of the high-grade metamorphism was constrained by the ages of the zircon rims and found to be 569–557 Ma, thus coinciding with the intrusion of syn-tectonic syenite at 562 Ma. The results of this study suggest that the southern part of Malawi is composed of remnants of the Stenian–Tonian magmatic arc, which were metamorphosed together with surrounding metasediments around 560 Ma. The Ediacaran high-grade metamorphism is consistent with the age of the Zambezi Belt located immediately southwest of the Malawi Basement Complex. Both the Lilongwe–Zomba–Blantyre area and the Zambezi Belt may correspond to a single suture zone formed by the latest Neoproterozoic collision between the Kalahari and Congo Cratons.

Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

This study presents high precision Zn stable isotope analyses for lower crustal rocks (9 granulites from Archean terrains and 30 lower crustal xenoliths) from the north margin of the North China Craton (NCC) to understand the behavior of Zn isotopes during deep crustal processes and the Zn isotopic composition of the lower continental crust (LCC). The lower crustal xenoliths range in composition from mafic to felsic with MgO contents of between 0.4 to 20.2 wt.%. The δ66Zn (the permil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) values of lower crustal xenoliths range from −0.17‰ to 0.38‰. The lack of clear correlation of δ66Zn with geochemical indicators (e.g., Al2O3, Cr, Ba/La, 87Sr/86Sr and 143Nd/144Nd) indicates that assimilation of Precambrian lower crust, fluid metasomatism, and accumulation of pyroxene and plagioclase had a limited effect on Zn isotopic compositions of these lower crustal xenoliths. The δ66Zn values (0.18‰ to 0.34‰) of garnet-bearing mafic granulites decrease with increasing FeO(T) and V contents, which are likely results of the accumulation of Fe-Ti oxides.The average δ66Zn of the lower continental crust is estimated to be 0.29 ± 0.02‰ (95% SE) using lower crustal xenoliths from the Neogene Hannuoba basalts. This δ66Zn value is similar to the estimated value (0.28 ± 0.04‰, 95% SE) obtained from the granulites from Archean terrains, suggesting that there is no significant difference in the Zn isotopic composition between the Archean and present-day lower continental crust. Combining the δ66Zn data of the lower crustal xenoliths and granulite terrains and different weighting methods, the Zn isotopic composition of the lower continental crust is estimated to be 0.28 ± 0.04‰ (95% SE).

A unique Paleoproterozoic HP–UHT metamorphic event recorded by the Bengbu mafic granulites in the southwestern Jiao–Liao–Ji Belt, North China Craton

The Wuhe Complex in the Bengbu area of the Jiao–Liao–Ji Belt, southeast North China Craton, contains garnet-bearing mafic granulites that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism. These granulites also experienced partial melting and occur as lenses within marbles. Petrographic observations and quantitative phase equilibria modeling reveal clockwise P–T paths, involving an inferred HP stage followed by decompressional, medium-pressure, granulite-facies metamorphism and subsequent cooling. The HP assemblage of garnet + clinopyroxene + plagioclase + K-feldspar ± amphibole ± quartz ± rutile indicates P–T conditions of 840–980 °C and 12–17 kbar. This was followed by post-peak, near-isothermal decompression with the development of orthopyroxene + clinopyroxene + plagioclase + K-feldspar + garnet + amphibole + ilmenite at 850–960 °C and 7–10 kbar, resulting in the development of orthopyroxene rims on resorbed garnet. Pyroxene and ternary feldspar thermometry yielded high temperatures of ~1150 °C and 1055–1087 °C at 10 kbar, respectively, which constrain the minimum crystallization temperatures of the igneous protoliths. The host and lamellae of the pyroxene and ternary feldspar are relict magmatic minerals/textures that survived metamorphism due to the silica-undersaturated bulk-rock conditions. Zr-in-rutile thermometry yielded temperatures of ~935 °C and 800 °C, with the former being consistent with the predicted peak metamorphic temperatures. Small amounts of melts (up to 5%) were generated during decompression of the Bengbu mafic granulites. The generated partial melts were mainly (quartz) monzonite at 900–920 °C, and the silica contents of the melts were controlled by the quartz stability field in P–T pseudosections. The partial melts were enriched in Na and strongly depleted in Fe–Mg at the peak pressure of ~14 kbar and 920 °C, and later evolved to Fe–Mg-rich and high-K compositions during decompression. The melt compositions in the studied rocks are similar when the pressures reached ~9 kbar. The modal proportion of amphibole increased as the melt H2O content decreased at lower pressures, indicating that the limited H2O remaining in the host rocks was consumed to produce amphibole. U–Pb geochronology of zircon containing inclusions of clinopyroxene, plagioclase, and apatite constrains the timing of metamorphism to 1930–1840 Ma, as is the case for HP granulites from Shandong, Liaoning, and southern Jilin in the central and northeastern Jiao–Liao–Ji Belt. The Wuhe HP–UHT mafic granulites were ultimately sourced from upwelling asthenosphere-derived magma at ~2.1 Ga, which intruded and crystallized at shallower depths. The igneous protoliths were then buried to middle–lower crustal levels and experienced HP–UHT granulite-facies metamorphism and partial melting at 1.95–1.90 Ga related to continental subduction and overthickening. The HP–UHT mafic granulites were rapidly exhumed at ~1.85 Ga and generated small volumes of (quartz) monzonite during decompression. The newly discovered Paleoproterozoic HP–UHT mafic granulites associated with partial melting suggest that the continent materials were deeply subducted to the lower crustal levels and that additional heating was not involved. The finding of the HP–UHT granulites, together with the widespread distributions of the granulite-facies metamorphic rocks and the determination of the clockwise P–T–t paths, reveal that the Paleoproterozoic Jiao–Liao–Ji orogenic belt extends at least 1000 km, starting from southern Jilin, passing through the southeastern Liaoning and Jiaobei terranes, and elongating to the Bengbu area in Anhui.

Long-lived metamorphic P–T–t evolution of the Highland Complex, Sri Lanka: Insights from mafic granulites

Sri Lanka is composed of four major tectonic units known as the Wanni, Highland, Kadugannawa and Vijayan complexes, and preserves important records of the history of the Gondwana supercontinent. The Highland Complex records Neoproterozoic–Cambrian metamorphism associated with the assembly of the supercontinent Gondwana. We combine phase equilibria modelling with isotopic and trace element geochemistry to provide a time-integrated P–T evolution using representative garnet-bearing mafic granulites from the Highland Complex. New zircon U–Pb geochronology and garnet trace element data from one locality in the EHC, together with a dataset compiled from published results, indicate garnet and zircon existed in equilibrium during long-lived high temperature metamorphism for possibly longer than 100 Ma (ca. 660–520 Ma). Calculated pseudosections imply high-temperature to potentially ultrahigh temperature (>850 °C) conditions for peak metamorphism. The Highland Complex is characterised by a small decrease in pressure and temperature following peak conditions. This was followed by an accelerated rate of exhumation, leading to the partial replacement of garnet-bearing assemblages by lower pressure parageneses. Although the post-peak cooling/modest pressure decrease is a comparatively minor part of the P–T evolution, compiled zircon U–Pb data suggest it was the temporally dominant part of the thermobarometric history, comprising >100 Myr. The long-lived high post-peak temperatures require a stable in-situ heat source, which based on regional litho-tectonic correlations, was almost certainly elevated rates of crustal heat production. The minor cooling interval from P–T space is in fact is the temporally dominant part of the P–T evolution highlights that without time analysis, the P–T evolution does not convey the long-lived baric stability of the high-temperature history of the Highland Complex.

Poly-phase metamorphism of garnet-bearing mafic granulite from the Larsemann Hills, East Antarctica: P-T path, U-Pb ages and tectonic implications

A rare garnet-bearing mafic granulite boulder has recently been identified from the Larsemann Hills, East Antarctica, with the development of two-stage decompression textures. Detailed petrographic study and P-T estimates suggest three distinct metamorphic stages: (1) peak ultrahigh-temperature (UHT) stage (M1) represented by coarse-grained garnet-orthopyroxene-bearing porphyroblastic assemblage, with P-T conditions of ∼9.3 kbar and 900–950 °C; (2) post-peak decompression stage I (M2) represented by relatively coarse-grained orthopyroxene-plagioclase-ilmenite-bearing corona assemblage, with P-T conditions of 7.2–7.6 kbar and 810–830 °C; and (3) post-peak decompression stage II (M3) represented by fine-grained orthopyroxene-plagioclase-ilmenite-bearing symplectite assemblage on garnets and along garnet cracks, with P-T conditions of 6.2–6.9 kbar and 730–770 °C. Combined with P-T pseudosection calculation in the NCKFMASHTO system, these P-T data define a clockwise P-T path with post-peak two-stage decompression accompanied by important cooling. LA-ICP-MS zircon U-Pb age dating results show two weighted mean ages of 927 ± 13 Ma and 547 ± 30 Ma, suggesting that M1 UHT stage occurred in the early Neoproterozoic Rayner orogeny (990–900 Ma), whereas M2–M3 stages occurred during the early Palaeozoic Pan-African tectonic event (550–500 Ma). The peak P-T conditions, post-peak two-stage decompression P-T path and age data resemble those of the early Neoproterozoic garnet-bearing mafic granulite on Søstrene Island. It is inferred that the mafic granulite boulder may have been derived from the bedrock under inland ice sheet of the Larsemann Hills. Thus, the early Neoproterozoic metamorphism (M1) may reflect crustal thickening followed by post-orogenic extension likely associated with the assembly of Rodinia continent, while the overprinting of early Palaeozoic metamorphism (M2–M3) may reflect an extensive intra-continental reworking.

Metamorphic evolution and SHRIMP U-Pb geochronology of mafic granulites with double symplectites in the Fuping metamorphic complex, middle Palaeoproterozoic Trans-North China Orogen

Garnet-bearing mafic granulites occur as lenses within TTG gneiss or metapelite in the Fuping Complex, middle Palaeoproterozoic Trans-North China Orogen. They are composed mainly of garnet, clinopyroxene, orthopyroxene, hornblende, plagioclase, quartz and accessary zircon and titanite. Four stages of metamorphic mineral assemblages have been found in these mafic granulites: (a) the prograde assemblages (M1) are inclusions preserved in the garnet porphyroblasts, including clinopyroxene, hornblende, plagioclase and quartz; (b) the metamorphic peak assemblages (M2) consist of garnet porphyroblasts plus matrix minerals clinopyroxene + orthopyroxene + hornblende + plagioclase + quartz + zircon + titanite; (c) the first retrograde assemblages (M3) are represented by clinopyroxene + orthopyroxene + plagioclase + quartz symplectites rimming the garnets; and (4) the final retrograde assemblages (M4) are hornblende + plagioclase + quartz symplectites rimming or adjacent to the garnets. The retrieved metamorphic P-T paths pass from 530–600 °C/6.5–7.0 kbar (M1) through 680–820 °C/7.2–11.7 kbar (M2) to 730–760 °C/3.2–3.9 kbar (M3) and finally to 600–640 °C/3.2–3.7 kbar (M4). All the P-T paths of the mafic granulites are clockwise, two of these even contain near isothermal decompression (ITD) and subsequent near isobaric cooling (IBC) segments. SHRIMP U-Pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of ∼1849 ± 9Ma, which is consistent with the metamorphic time of surrounding TTG gneisses. Therefore, the Fuping Complex records the collision event between the Eastern and Western Blocks of the North China Craton that led to the final assembly of this craton during the late Paleoproterozoic.

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.

Granulite-facies metamorphism at ca. 570-580 Ma in the Porangatu Granulite Complex, central Brazil: implications for the evolution of the Transbrasiliano Lineament

The Porangatu Granulite Complex is exposed in the central part of the Neoproterozoic Tocantins province in central Brazil, along the boundary between the Brasilia Belt to the east and the Araguaia Belt to the west. This is part of the transcontinental Transbrasiliano-Kandi shear system. The complex includes garnet-rich enderbite and charnockite, high-grade gneisses as well as lenses of garnet-bearing mafic granulite or amphibolites, and in situ anatectic charnockite, elongated in the NNE-SSW direction along the Talisma Shear Zone (TSZ). These rocks represent suites of ortho-derived rocks of calc-alkaline affinity and small contributions of tholeiitic basalts and aluminous paragneisses. The structural framework records thrust components probably related to the early stages of an oblique collision during the evolution of Neoproterozoic Brasiliano orogens, and can be understood as involving a collisional system of two crustal blocks, initially with thrust components which in its final stage evolved to a transcurrent system with dextral movement. This led to intense imbrication, generation of mylonitic foliation, stretching lineation, tectonic banding and rotation of structures and minerals. The heterogeneous and progressive ductile deformation was accompanied by metamorphic re-equilibrium in late Neoproterozoic time. Granulite facies conditions reached a metamorphic maximum at temperature and pressure above 850°C and 10 kbar, in an almost anhydrous environment, with or without anatexis. Zircon U-Pb SHRIMP analyses for two selected rock samples indicated the combined age of 580 ± 7 Ma for a charnockite and 548 ± 48 Ma for a mafic granulite from which the charnockite is throught to have been derived. The mafic granulite contains zircon grains of ca. 2.1 Ga, indicating Paleoproterozoic igneous protoliths involved in Neoproterozoic high-grade metamorphism. In addition, older inherited zircon grains of ca. 3.1 and 2.0 Ga ( 207 Pb/ 206 Pb ages) in charnockite also confirm the existence of older Archaean and Paleoproterozoic material in this region, possibly derived from the Goias Massif. A 0.88 Ga inherited zircon grain is suggestive of derivation from the Goias Magmatic Arc. This Neoproterozoic age for the high-grade metamorphism is substantially younger than those reported for other granulites in the Brasilia Belt (ca. 0.65 Ga), suggesting that the Porangatu Granulite Complex is more probably associated with the evolution of the younger Araguaia Belt. The new field, structural, petrographic and geochronological data suggest that the Porangatu Granulite Complex was involved in a high-temperature ductile strike-slip shear zone juxtaposing terrains of different ages (Archaean, Paleoproterozoic, Neoproterozoic), crustal nature and level (lower and middle continental crust), strongly reworked during the final stages of the Brasiliano orogeny, and represents the exposed roots of the Tocantins orogen.

Metamorphic evolution and geochronology of the Dunhuang orogenic belt in the Hongliuxia area, northwestern China

Garnet-bearing mafic granulites and amphibolites from the Hongliuxia area of the southern Dunhuang orogenic belt, northwestern China, commonly occur as lenses or boudinages enclosed within metapelite or marble, which represent the block-in-matrix feature typical of orogenic mélange. Three to four generations of metamorphic mineral assemblages are preserved in these rocks. In the high-pressure amphibolites, prograde mineral assemblages (M1) occur as inclusions (hornblende+plagioclase+quartz±chlorite±epidote±ilmenite) preserved within garnet porphyroblasts, and formed at 550–590°C and 7.7–9.2kbar based on geothermobarometry. The metamorphic peak mineral assemblages (M2) are composed of garnet+hornblende+plagioclase+quartz+clinopyroxene, as well as titanite+zircon+rutile+apatite as accessory minerals in the matrix, and are estimated to have formed at 640–720°C and 14.1–16.0kbar. The first retrograde assemblages (M3) are characterized by “white-eye socket” symplectites (hornblende+plagioclase+quartz±biotite±epidote±magnetite) rimming garnet porphyroblasts, which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under P–T conditions of 610–630°C and 5.6–11.8kbar. The second retrograde assemblages (M4) are intergrowths of actinolite and worm-like quartz produced by the breakdown of the matrix hornblendes, and formed under P–T conditions of ∼490°C and ∼2.8kbar. For the high-pressure mafic granulites, the prograde assemblages (M1) are represented by plagioclase+quartz preserved within the garnet porphyroblasts. The metamorphic peak assemblages (M2) are garnet+matrix minerals (clinopyroxene+plagioclase+quartz+hornblende+rutile+zircon) and were estimated to have formed at ∼680°C and ∼15.4kbar. The retrograde assemblages (M3) are characterized by fine-grained patches of hornblende+plagioclase+quartz rimming the garnet porphyroblasts, as well as hornblende rimming clinopyroxene in the matrix, and were inferred to have formed at ∼620°C and ∼4.2kbar. For the metapelitic gneiss, the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals (biotite+plagioclase+quartz+ilmenite+zircon), which were estimated to have formed at ∼630°C and ∼8.9kbar. The mafic granulites and amphibolites record fairly similar clockwise P–T paths that include nearly isothermal decompression processes, which suggest that they experienced subduction and subsequent rapid tectonic exhumation. SIMS and LA-ICP-MS U–Pb dating of zircons and 40Ar/39Ar dating of hornblende suggest that the metamorphism occurred at ∼430–390Ma. Field occurrences, different protolith ages of the mafic granulites and amphibolites, and the considerable gap in peak P–T conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a mélange accumulation process during the Paleozoic caused by the Silurian–Devonian orogeny, which is possibly associated with the closure of the Liuyuan ocean, a branch of the Paleo-Asian ocean near the southern Central Asian Orogenic Belt.

Metamorphic evolution and SIMS U-Pb geochronology of the Qingshigou area, Dunhuang block, NW China: Tectonic implications of the southernmost Central Asian orogenic belt

Garnet-bearing mafic granulite and amphibolite exposed as lenses, boudins, or interlayers within metasediments in the Qingshigou area, Dunhuang block, southernmost Central Asian orogenic belt, record important information for understanding the tectono-metamorphic evolution of subduction and collision zones in the southern Central Asian orogenic belt during the mid-Paleozoic. Three stages of metamorphic assemblages (M1, M2, M3) are recognized in the high- and medium-pressure mafic granulite and amphibolite. In the high-pressure mafic granulite, the prograde assemblage (M1) is represented by inclusion minerals (hornblende + plagioclase + quartz) preserved in garnet porphyroblasts; the metamorphic peak assemblage (M2) is characterized by garnet porphyroblasts and matrix minerals (garnet + clinopyroxene + plagioclase + quartz ± zircon ± titanite); and the retrograde assemblage (M3) is marked by coronitic symplectite (hornblende + plagioclase + quartz ± magnetite) rimming the garnet porphyroblasts. In the medium-pressure mafic granulite, the prograde assemblage (M1) of hornblende + plagioclase + quartz is included in the garnet porphyroblasts; the peak assemblage (M2) consists of garnet + orthopyroxene + clinopyroxene + plagioclase + quartz ± zircon ± titanite (M2) in the matrix; and the retrograde assemblage (M3) of hornblende + orthopyroxene + plagioclase + quartz (M3) surrounds the garnet porphyroblasts. In the amphibolite, the prograde assemblage (hornblende + plagioclase + quartz + ilmenite) is preserved as inclusions in garnet (M1); the peak assemblage (M2) is composed of garnet + hornblende + plagioclase + quartz ± zircon ± titanite; and the retrograde assemblage (M3), consisting of hornblende + biotite + plagioclase + quartz + epidote + magnetite, rings the garnet porphyroblasts. Geothermobarometric calculations suggest that the metamorphic pressure-temperature paths pass from 568 °C and 8.8 kbar through 607 °C and 10.6 kbar and 861 °C and 16.9 kbar and finally to 598 °C and 4.4 kbar for the high-pressure mafic granulite; from 756 °C and 9.0 kbar through 750–874 °C and 9.3–11.6 kbar to 675 °C and 4.7 kbar for the medium-pressure mafic granulite; and from 686 °C and 7.6 kbar through 715–766 °C and 10.6–11.2 kbar to 671 °C and 5.6 kbar for the amphibolite, and the paths show clockwise pressure-temperature loops typical of an orogenic process. The metamorphic peak of the high-pressure mafic granulite lies in the eclogite facies, which is indicative of a subduction zone environment. High-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon indicates that the metamorphism occurred in the Early Silurian (ca. 430 Ma) and lasted for at least 65 m.y. This study reveals a possible southward subduction history of a branch of the Paleo–Asian Ocean, the Liuyuan Ocean, from the Silurian to Late Devonian, which may be an important event in the accretionary history of the Central Asian orogenic belt.

Neoproterozoic arc accretion along the ‘eastern suture’ in Sri Lanka during Gondwana assembly

Sri Lanka is composed of four major tectonic units: the Wanni, Highland, Kadugannawa and Vijayan complexes, with important records of Neoproterozoic tectonic events including subduction-related vertical and lateral continental growth associated with the final assembly of the Gondwana supercontinent. Here we investigate the boundary between the Highland (HC) and Vijayan (VC) complexes, which we define as the ‘eastern suture’ in Sri Lanka. We report granitoid gneisses, diorite, clinopyroxenite, garnet-bearing mafic granulites and serpentinized ultramafics with bands of chert and quartzite. Our petrologic and geochemical data suggest subduction-related magmatism, marking the boundary between the HC and VC as an active continental margin. We report zircon U–Pb ages and Lu–Hf isotopic data which show that the protoliths of granitoid gneisses including hornblende biotite gneiss, granodioritic gneiss and meta-granite were emplaced at ca. 924–966Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic–Cambrian. The zircons from garnet-bearing mafic granulite and clinopyroxenite show magmatic core ages of 617±27Ma and 772±6Ma respectively, with minor to significant lead-loss at about 550–580Ma. Our data also identify Ordovician ultramafic magmatism with CO2 influx and mantle metasomatism at 485±6Ma, with the formation of magnesite deposits in the serpentinite. The zircon Hf isotopic data from the Vijayan gneisses and the mafic–ultramafic intrusions show depleted mantle signature, with only minor input of older continental material. The data obtained in this study, and those from recent studies suggest that the Vijayan and Wanni Complexes are similar aged Neoproterozoic magmatic arcs, and that the Vijayan–Highland boundary marks an active convergent margin similar to the western boundary between the Wanni and Highland Complexes. The Cambrian high-grade metamorphism is extensively recorded in all rock types. The Neoproterozoic records from Sri Lanka are consistent with double-sided subduction and accretion of arcs on both sides through the closure of the HC segment of the Mozambique Ocean, with the HC representing a Cambrian Himalaya in Sri Lanka constructed during the collisional event associated with the final assembly of Gondwana. Comparable rock suites with similar ages and tectonic setting in East Antarctica and southern India support arc assembly and closure of intervening oceans along multiple subduction systems.

Lithological units at the boundary zone between the Jining and Huai'an Complexes (central-northern margin of the North China Craton): A Paleoproterozoic tectonic mélange?

In the central-northern margin of the North China Craton, the tectonic boundary between the Jining and Huai'an Complexes is well exposed in the Gushan area. In this boundary zone, exotic blocks of orthopyroxene-bearing monzonitic gneiss, garnet-bearing mafic granulite, marble, and calc-silicate were identified in the meta-psammitic and meta-pelitic matrix. The likely nature of the rock units is a tectonic mélange, which is a key to understand the tectonic evolution of the craton.In this paper, geochronological and geochemical studies have been carried out on representative lithologies from the Gushan area to discuss their petrogenesis and geodynamic settings. The opx-bearing monzonitic gneisses (U–Pb zircon emplacement age of 2.17Ga) are high-K calc-alkaline to shoshonitic I-type rocks with trace-element magmatic arc signature. The trace elements, whole rock Sm–Nd and zircon Lu–Hf isotopes suggest that they were possibly derived from partial melting of Neoarchean lower crust (garnet amphibolite) in a continental arc. The protoliths of the garnet-bearing mafic granulites are inferred to have formed between 2.37 Ga and 1.95Ga, possibly at 2.2Ga. They show N-MORB geochemical affinity and were most likely produced by partial melting of spinel lherzolite in a mid-ocean ridge setting. Detrital zircons from one metapsammite sample (garnet-bearing quartzofeldspathic gneiss) yield nearly concordant 207Pb/206Pb ages ranging from 2.1Ga to 2.0Ga. In situ U–Pb and Hf isotopic data of zircons from the metapsammite sample support that the metasedimentary protoliths were probably derived from a mid-Paleoproterozoic continental arc. Three metamorphic age groups (1.95–1.93Ga, 1.86–1.83Ga, and 1.80Ga) are revealed by the opx-bearing monzonitic gneisses, garnet-bearing mafic granulites, garnet-bearing quartzofeldspathic gneiss, and sillimanite–garnet–K-feldspar gneiss. The predominant group (1.95–1.93Ga) is considered to record the age of near-peak granulite facies metamorphism. The 1.86–1.83Ga group may result from decompression or the thermal imprint of the collision between the Eastern and Western Blocks. The 1.80Ga group is consistent with the age (1.81Ga) of the ductile deformation of the garnet–K-feldspar mylonite, and is interpreted as the timing of a relatively rapid tectonic extension. Thus, the Paleoproterozoic Gushan mélange formed and evolved in a subduction–collision–exhumation process, and finally situated in a likely suture zone between the Eastern and Western blocks or sub-cratons.

Multiple mafic magmatic and high-grade metamorphic events revealed by zircons from meta-mafic rocks in the Daqingshan–Wulashan Complex of the Khondalite Belt, North China Craton

Meta-mafic rocks in the Daqingshan–Wulashan Complex, in the central segment of the Paleoproterozoic Khondalite Belt of the North China Craton, are composed predominantly of garnet-bearing mafic granulites, mafic granulites, and amphibolites. They occur mainly as irregular lenses and as a set of deformed dyke/sill swarms within dioritic–granitic orthogneisses and Al-rich paragneisses. An integrated study involving laser Raman and scanning electron microscope analyses of mineral inclusions from zircons, cathodoluminescence imaging, and LA–ICP–MS U–Pb dating of zircons, shows that magmatic zircons in these meta-mafic rocks record four age groups of 2500–2450, 2300–2100, 1970–1930, and ca. 1850Ma, indicating multiple mafic magmatic events occurred within the Daqingshan–Wulashan Complex. In addition, four groups of metamorphic zircons with ages of ca. 2450, ca. 1950, ca. 1900, and ca. 1850Ma are present within these meta-mafic rocks. Zircons of the first (oldest) group contain medium- to low-pressure (MLP) granulite facies mineral inclusions of orthopyroxene+clinopyoxene+amphibole+plagioclase+quartz, and yield ages of ca. 2450Ma, representing the timing of the early MLP granulite facies metamorphic event within the Daqingshan–Wulashan reworked basement complex. In contrast, zircons of the second group are free of mineral inclusions, and yield ages of ca. 1950Ma, which is considered to be the timing of the regional high-grade metamorphic and structural event probably caused by the collision of the Yinshan and Ordos Blocks. Zircons of the third group contain MLP granulite facies mineral inclusions of orthopyroxene+clinopyoxene+amphibole+plagioclase+quartz, similar to the zircons of the first group, but yield younger ages of ca. 1900Ma, interpreted as the timing of the regional MLP granulite facies metamorphism that most likely occurred during the early stage of post-orogenic exhumation of the Daqingshan–Wulashan Complex. Zircons of the fourth group contain MLP amphibolite facies mineral inclusions of clinopyoxene+amphibole+plagioclase+quartz, and yield the youngest ages of ca. 1850Ma, which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post-orogenic exhumation of the Daqingshan–Wulashan Complex. Combined with previous geological data of the Khondalite Belt, it is speculated that the multiple mafic magmatism and contemporaneous high-grade metamorphism of the Daqingshan–Wulashan Complex from the Khondalite Belt were most likely generated by an orogenic process involving early subduction–collision and late exhumation in an active continental margin arc.

Partial melting of the Archaean Thrym Complex of southeastern Greenland

High-grade Archaean rocks of the remote Thrym Complex of southeastern Greenland expose a lower crustal section that formed at a depth of >30km at temperatures reaching 1000°C and pressures approaching 1.5GPa. The complex is part of the North Atlantic Craton, which is bound to the north by the Trans Hudson–Nagssugtoqidian–Ammassalik Orogen and to the south by the Makkovik–Ketilidian Orogen. The rocks in the complex include orthogneiss, migmatitic orthogneiss with banded or nebulitic fabrics, migmatite with a breccia-like structure (agmatite), and narrow bands of lensoidal mafic and ultramafic rocks (granulites) with associated widespread garnet-bearing mafic granulite, and a minor proportion of garnet–cordierite–sillimanite–biotite gneiss. The latter high-grade rocks form part of the substrate for the emplacement of the protoliths for orthogneiss that was synchronous with the ca. 2790–2700Ma Skjoldungen Orogeny.At least two suites have been recognised in the Thrym Complex based on geochemistry. The oldest is a tholeiitic suite of mafic to ultramafic granulites, and the other is a calc-alkaline suite that includes felsic to intermediate orthogneiss. Most of the orthogneiss has a granodioritic (oldest) to quartz monzodioritic (younger) composition with rare monzogranitic, tonalitic, monzonitic and quartz dioritic protoliths. Historically, the orthogneiss units have been called TTGs, although monzogranite and tonalite are rare, and no trondhjemite is observed. In addition, they could also be called adakitic on geochemical grounds. Instead of using this terminology, which has genetic connotations, the orthogneiss is referred to as predominantly granodioritic to quartz-monzodioritic.Assuming that modern tectonics operated during the Neoarchaean, at least some of the mafic and ultramafic granulites and orthogneiss exposed in the Thrym Complex represent the root zone of a magmatic-arc that might have been relaminated around 2760Ma during the collisional Skjoldungen Orogeny.

Geochemical modelling of the tonalitic and trondhjemitic granulites from the Itabuna-Salvador-Curaçá Block, Bahia, Brazil

The studied tonalitic and trondhjemitic granulites are located in the SSE granulitic domain of the Sao Francisco craton, Bahia, Brazil, where they represent most of the southern part of the Archean and Paleoproterozoic Itabuna-Salvador-Curaca Block (ISCB). Chemically, the tonalitic and trondhjemitic granulites belong to a low-K calc-alkaline suite; their REE patterns are steep with strong LREE/HREE fractionation and no significant Eu anomaly. Garnet-bearing mafic granulites that occur as enclaves in the tonalitic and trondhjemitic granulites were derived from basalts and/or gabbros of tholeiitic affinity. Geochemical modelling showed that the tonalitic and trondhjemitic granulites were produced by moderate fractional crystallization of an assemblage of hornblende and plagioclase, with subordinate amounts of magnetite, apatite, allanite and zircon. The garnet-bearing mafic granulites would be the source of the magmas that generated these rocks. Partial melting left a residue made up of plagioclase, garnet, orthopyroxene and hornblende.