Assemblage Garnet Research Articles

Mineral equilibria constraints on open‐system melting in metamafic compositions

AbstractThe recent development of activity–composition relations for mineral and melt phases in high‐grade metamafic rocks allows mineral equilibria tools to be used to further aid our understanding of partial melting and the mineralogical consequences of melt segregation in these rocks. We show that bulk compositional data from natural amphibolites cover a wide compositional range, with particular variability in the content and ratios of Ca, Na and K indicating that low‐grade metasomatic alteration can substantially alter the igneous protolith chemistry and potentially affect the volume and composition of melt generated. Mineral equilibria calculations for five samples that span the compositional variability in our data set indicate that melting occurs primarily via the fluid‐absent breakdown of amphibole+quartz to produce a pressure‐sensitive peritectic assemblage of augite, orthopyroxene and/or garnet. The introduction of orthopyroxene at the onset of the amphibolite‐to‐granulite‐facies transition at lower pressure results in an increased rate of melt production until quartz is typically exhausted, and this is similarly seen for the introduction of garnet at higher pressure. Calculated melt compositions are dependent on the protolith composition, but initial solidus melting and biotite breakdown produce 1–3 mol.% of K‐rich granitic melts. As hornblende melting proceeds, 15–20 vol.% of either more granodioritic‐to‐tonalitic or granodioritic‐to‐trondhjemitic melt is produced. Once quartz is exhausted, intermediate to mafic melt compositions are produced at ultrahigh‐temperature conditions. Quartz‐rich lithologies with high Ca coupled to low Na and K are the most fertile under orogenic conditions, yielding up to 25 mol.% of sub‐alkalic granitic melt by 850°C. Such rocks did not experience significant subsolidus alteration. Altered compositions with low Ca and elevated Na and K are not as fertile, yielding less than 15 mol.% of alkalic granitic melt by 850°C. These melt volumes are enough to be segregated, and can make a contribution to granite magmatism and intracrustal differentiation that should not be overlooked.

Evolution of the Archean continental crust in the nucleus of the Yangtze block: Evidence from geochemistry of 3.0 Ga TTG gneisses in the Kongling high-grade metamorphic terrane, South China

Archean Tonalite-Trondhjemite-Granodiorite (TTG) rocks are scattered within the Kongling high-grade metamorphic terrane (KHMT) in the northern South China block. A comprehensive geochronological and geochemical study is carried out on the Taoyuan granitic gneisses, a newly recognized TTG suite in the northwestern KHMT. This suite has long been regarded as a Mesoproterozoic magmatic pluton, but U-Pb zircon ages of 2994 ± 22 Ma and 2970 ± 15 Ma are obtained by LA-ICP-MS method in this study. The Taoyuan gneiss suite is trondhjemitic in composition, and has high SiO2 (67.80–74.93 wt.%), Na2O (5.11–5.81 wt.%) contents with Na2O/K2O ratios greater than unity, and low Ni (2.56–7.61 ppm), Cr (1.26–7.67 ppm), Yb (0.32–0.82 ppm) and Y (4.48–11.5 ppm) contents. Plots show large variation in La/Yb and Sr/Y ratios and pronounced depletion in Nb, Ta and Ti in the primitive mantle-normalized spiderdiagram. The gneiss suite also displays two-stage Nd model ages close to its crystallization age with corresponding εNd(t) values of −2.5 to +3.5. It is thus suggested that the Taoyuan gneisses, in fact, is part of the Archean Kongling basement complex.Geochemical evidence implies that the TTG rocks may be derived from partial melting of subducted oceanic crust from a garnetiferous amphibolite source with residual assemblage of garnet + amphibole + plagioclase. Our study further indicates that the nucleus of the Yangtze block might experience a juvenile continental crustal growth during Mesoarchean. We also suggest that the Yangtze block may have its own crustal evolutionary history independent from the North China craton and the Tarim block before Paleoproterozoic.

Fluid-induced high-temperature metasomatism at Rundvågshetta in the Lützow-Holm Complex, East Antarctica: Implications for the role of brine during granulite formation

We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundvågshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex (LHC), East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite (K-feldspar + quartz + sillimanite + garnet + ilmenite) with foliation-parallel bands and/or layers of mafic granulite (plagioclase + orthopyroxene + garnet + ilmenite + quartz + biotite). The boundary between the two lithologies is defined by thin (about 1–20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet + plagioclase + quartz + ilmenite + biotite ± orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe + Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850–860 °C, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapelite–metabasite boundary (up to 40%), which is consistent with the field and thin section observations. The occurrence of brine (7.0–10.9 wt.% NaCleq for ice melting or 25.1–25.5 wt.% NaCleq for hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite (Cl = 0.22–0.60 wt.%) in the metasomatic rock compared to that in pelitic (0.15–0.24 wt.%) and mafic (0.06–0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation.

Mineral textural evolution and PT-path of relict eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland

The Nagssugtoqidian Orogen in South-East Greenland is a deeply eroded, Paleoproterozoic collision orogen. It consists of a variety of Archean and Paleoproterozoic rocks, most notably TTG gneiss, a variety of supracrustal rocks and basic dykes. This study aims at providing new insight into the geodynamic processes and subduction depth of this orogen by investigating the metamorphic evolution of garnet pyroxenite, retrogressed eclogite and amphibolite-facies rocks that are exposed within the Kuummiut Terrane of the Nagssugtoqidian Orogen. The garnet-pyroxenite has a dominant mineral assemblage of garnet, orthopyroxene, clinopyroxene and hornblende, while garnet-amphibolite and garnet-kyanite schist are made up of garnet, hornblende, plagioclase and quartz, and garnet, kyanite, biotite and quartz, respectively. Relicts of, and pseudomorphs after, eclogite-facies mineral assemblages are frequently found within basic metavolcanic rocks and Paleoproterozoic discordant basic dykes. In the retrogressed eclogite, the retrograde mineral reactions ceased prior to completion, resulting in the formation of two domains. A clinopyroxene domain consists of diopside-plagioclase symplectites, which are interpreted to have grown at the expense of omphacite. The symplectites are surrounded and partly replaced by hornblende and plagioclase. Omphacite (XJd 25–42) is preserved in a Na-rich sample, where it occurs in the core of large clinopyroxene and as inclusion in garnet and hornblende. In a garnet domain, garnet is variably replaced by an inner corona of plagioclase and an outer corona of amphibole +/− orthopyroxene and clinopyroxene. The degree of retrogression as well as the type of the retrograde assemblage in both domains appears to be dependent on fluid activity. Large garnet grains preserve Ca-rich cores, interpreted as prograde in origin, while Mg-rich garnet rims formed during eclogite-facies metamorphism and later re-equilibration. Pseudosection modelling combined with conventional geothermobarometry reveals a clockwise PT-evolution, involving eclogite-facies conditions of 17–19kbar and 740–810°C, followed by near-isothermal decompression to medium-pressure granulite-facies conditions (13.8–15.4kbar, 760–880°C) and subsequent decompression with minor cooling to high-pressure amphibolite-facies grades (8.8–10.9kbar, 660–840°C). These data show that rocks of the Kuummiut Terrane were exhumed from 70 to about 30km into the mid- and lower crust. The PT-path implies that exhumation initially was rapid and tectonically-controlled.

Reconstruction of multiple P-T-t stages from retrogressed mafic rocks: Subduction versus collision in the Southern Brasília orogen (SE Brazil)

The identification of markers of subduction zones in orogenic belts requires the estimation of paleo-geothermal gradients through pressure-temperature-time (P-T-t) estimates in mafic rocks that potentially derive from former oceanic units once. However, such markers are rare in supracrustal sequences specially in deeply eroded and weathered Precambrian orogens, and reconstructing their metamorphic history is challenging because they are commonly retrogressed and only preserve a few mineral relicts of high-pressure metamorphism. Metamorphosed mafic rocks from Pouso Alegre region of the Neoproterozoic Southern Brasília Orogen outcrop as rare lenses within continental gneisses. They have previously been classified as retrograde eclogites, based on the presence of garnet and the characteristic symplectitic texture replacing omphacite. These rocks were interpreted to mark the suture zone between the Paranapanema and São Francisco cratons. To test the possible record of eclogitic conditions in the Pouso Alegre mafic rocks, samples including the surrounding felsic rocks have been investigated using quantitative compositional mapping, forward thermodynamic modeling and in-situ dating of accessory minerals to refine their P-T-t history. In the metamorphosed mafic rocks, the peak pressure assemblage of garnet and omphacite (Jd20, reconstructed composition) formed at 690±35°C and 13.5±3.0kbar, whereas local retrogression into symplectite or corona occurred at 595±25°C and 4.8±1.5kbar. The two reactions were coupled and thus took place at the same time. A zircon U-Pb age of 603±7Ma was obtained for metamorphic rims and linked to the retrogression stage. Monazite and metamorphic zircon U-Th-Pb ages for the surrounding rocks are at ca. 630Ma and linked to peak pressure conditions similar to the one recorded by the mafic rocks. The low maximal pressure of 14kbar and the high geothermal gradient do not necessarily support subduction process-related metamorphism but, more likely, metamorphism related to continental collision.

Prolonged anatexis of Paleoproterozoic metasedimentary basement: First evidence from the Yinchuan Basin and new constraints on the evolution of the Khondalite Belt, North China Craton

Discerning the history of burial, residence at depth and exhumation of high grade metamorphic rocks from an orogen is essential to understanding crustal differentiation and orogenesis. Such high grade rocks are inferred to exist beneath the Yinchuan Basin, which is overlain by a thick covering of younger sediments, and is located at the inferred southwest boundary of the Paleoproterozoic Khondalite Belt in the northwestern North China Craton. The metamorphic basement sampled from a recently completed deep borehole in the Yinchuan Basin is migmatitic and contains an assemblage of garnet, biotite, sillimanite, plagioclase, quartz, minor K-feldspar and local occurrences of muscovite. Phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system constrains the peak P–T conditions of anatexis to 6–10kbar at 760–810°C, followed by final crystallization at 4.5–5.5kbar and 690–710°C. Ti-in-zircon thermometry for the narrow metamorphic zircon rims provides a temperature range of 687–753°C, demonstrating formation during retrogression to the solidus. NanoSIMS U–Pb analyses on the metamorphic overgrowths of zircon grains yields an upper intercept age of 1895±36Ma. Chronological and chemical analyses of monazite indicate irregular bright gray domains with relatively high Th and low Y and HREE contents developed during prograde metamorphism (1962±8Ma). Conversely, dark domains with low Th and high Y and HREE overgrew during retrograde cooling (1892±14Ma) that was synchronous with the development of metamorphic zircon rims. LA-ICP-MS U–Pb analyses of detrital zircons reveal that the metamorphic basement beneath the Yinchuan Basin has a sedimentary protolith of Paleoproterozoic age. The results directly confirm the existence of Paleoproterozoic basement beneath the Yinchuan Basin, and the well-constrained P–T–t path supports the involvement of the basement (as part of the western Khondalite Belt) in burial and exhumation processes during the 1.96–1.89Ga period, during which anatexis occurred for ∼70Ma.

P–T–t path and tectonic significance of pelitic migmatites from the Lüliang Complex in Xiyupi area of Trans-North China Orogen, North China Craton

In order to reveal the metamorphic evolution of the Lüliang Complex, a combined study of petrography, phase equilibria modeling and zircon U–Pb dating is carried out on two mesosome of pelitic migmatites from the Jiehekou Group in the Lüliang Complex. Sample (15XYP-02) preserved the thermal peak assemblage of garnet, biotite, K-feldspar, sillimanite, quartz and ilmenite, without plagioclase. It recorded two near-isothermal decompressional indicators that rutile was moated by ilmenite and garnet was replaced by cordierite. Small plagioclase occurred in the rim of K-feldspar and garnet. Four stages (M1–M4) of metamorphic evolution have been identified based on the mineral assemblages and P–T pseudosection. It is interpreted to record pre-Tmax decompression from rutile-stability field (M1, represented by rutile relicts in matrix and biotite+sillimanite+quartz inclusions in garnet porphyroblasts) at ∼8.9kbar to the Tmax stage (M2, characterized by garnet rim with ilmenite and biotite+K-feldspar+sillimanite+quartz in matrix) of 815–838°C. The following suprasolidus post-Tmax cooling comprises M3stage (represented by appearance of cordierite in the assemblage of garnet+biotite+K-feldspar+sillimanite+ilmenite+quartz) of 780–838°C/5.0–6.3kbar and M4stage (defined by the final mineral assemblage of garnet+biotite+K-feldspar+plagioclase+sillimanite+cordierite+ilmenite+quartz) of ∼775°C/∼5.0kbar. Another sample (15XYP-21) preserved the thermal peak assemblage of garnet, biotite, plagioclase, K-feldspar, sillimanite, quartz. It also records pre-Tmax decompression from rutile-stability field at 9.0kbar to the Tmax stage of 796–821°C/6.4–7.1kbar, followed by suprasolidus post-Tmax cooling stage of 778–803°C/5.0–5.7kbar. The clockwise P–T paths are consistent with those of orogenic belt. Furthermore, the zircon U–Pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1.95Ga and 1.91Ga, respectively, in which, the former represents the early stage of cooling after Tmax, nearly close to the time of pressure peak metamorphism. Combined with those granulites in the Trans-North China Orogen, a common long-lived continent-continent collision processes was proposed, which may culminated at ∼1.95Ga and followed by a prolonged cooling process.

The peraluminous Aurumina Granite Suite in central Brazil: An example of mantle-continental crust interaction in a Paleoproterozoic cordilleran hinterland setting?

Peraluminous granites and tonalites/granodiorites of the Aurumina Suite crop out in the external zone of the Brasília Fold Belt on the western margin of the São Francisco Craton. Rocks of this suite intrude retrograde medium-grade schists and paragneisses of the Ticunzal Formation and display a magmatic muscovite±biotite±garnet assemblage and discrete graphite lamellae. The suite is divided into six members (Au1 through Au6), which grade from synkinematic to post-kinematic in character with ages spanning from 2.11 to 2.16Ga according to zircon U-Pb data. Compositional features of the Aurumina Suite and associated coeval metaluminous mafic to intermediate plutons suggest an origin by source hybridization processes characterized by reaction between metasedimentary rocks and basaltic melts that would have taken place at shallow depths (pressure<5kbar). This is supported by a narrow range of εNdT values between −4.2 and +0.4 for samples from both peraluminous and metaluminous units. Possible sources could correspond to old reworked crust that might also be related to the origin of ∼2.2Ga peraluminous magmatic rocks in the Almas-Dianópolis domain, although participation of unknown sources is also feasible. Similarities exist between the peraluminous magmatism of the Aurumina Suite and the Jurassic-Paleogene peraluminous granitic belt of the cordilleran interior of North America in terms of composition, rock association and tectonometamorphic context, which suggest a similar tectonic setting for the Aurumina Suite instead of the collisional setting suggested in previous works.

Eocene granulite-facies metamorphism prior to deformation of the Mianhuadi mafic complex in the Ailao Shan-Red River shear zone, Yunnan Province, SW China

The Mianhuadi mafic complex in the Ailao Shan-Red River (ASRR) shear zone, SW China, consists of granulite and mylonite, formed by granulite-facies peak metamorphism and retrograde amphibolite-facies metamorphism, with later local mylonitic deformation. Textures and mineral relics indicate that the protolith, represented by the mineral assemblage (M0) of olivine, clinopyroxene, plagioclase and Fe-Ti oxides, was emplaced as a mafic intrusion in the Late Permian as part of the Emeishan large igneous province. Two kinds of metamorphic mineral assemblages were identified: the peak assemblage (M1) of garnet+orthopyroxene+clinopyroxene+plagioclase+magnetite+ilmenite, and the retrograde assemblage (M2) composed of hornblende surrounding orthopyroxene and garnet, and hornblende+spinel symplectite around garnet. Pressure-temperature estimates yield 657–722°C and 7–9kbar, and 649–676°C and 5–6kbar for the M1 and M2 stages, respectively, roughly defining a clockwise P-T path with near-isothermal decompression following the peak metamorphism. These metamorphic conditions are quite unlike those (pressure up to 14kbar) of the Late Permian-Middle Triassic metamorphic event in the region. In addition, foliated biotite from mylonites has an 40Ar-39Ar plateau age of 34.3±0.4Ma, indicating that the ASRR shear zone was active at or before the Late Eocene. Combined with previous data, our new results indicate that the granulite-facies metamorphism of the Mianhuadi mafic complex occurred most likely during the Eocene, followed by rapid exhumation and ductile sinistral shearing in the Late Eocene. The formation of the granulite was likely related to the upwelling of the asthenosphere due to the delamination of overthickened lithospheric mantle after the India-Asia collision.

Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, NW China

Co-existing granulite and peridotite may represent relics of the paleo-suture zone and provides an optimal opportunity for better understanding of orogeny between two blocks. In this study, we carried out petrological and U-Pb zircon dating investigation on the HP mafic granulites associated with peridotite complex at Yushugou in Chinese South Tianshan. The studied samples include garnet-bearing high-pressure mafic granulites which can be subdivided into two types: Type I orthopyroxene-free and Type II orthopyroxene-bearing granulites and amphibolite. Type I granulite (Y21-2) has a mineral assemblage of garnet (33vol.%), clinopyroxene (32vol.%) and plagioclase (30vol.%); and Type II granulite (Y18-8) has a mineral assemblage of garnet (22vol.%), clinopyroxene (10vol.%), orthopyroxene (14vol.%), plagioclase (45vol.%) and quartz. Garnet in both granulites exhibits core-rim structure characterized by increasing grossular and decreasing pyrope from core to rim. Petrographic observations and phase equilibrium modeling using THERMOCALC in the NCFMASHTO system for the mafic granulites (Y21-2 and Y18-8) show three stages of metamorphism: Stage I (granulite facies) was recognized by the large porphyroblastic garnet core, with P-T conditions of 9.8–10.4Kbar and 860–900°C (Y21-2) and 9.9–10.6Kbar and 875–890°C (Y18-8), respectively; Stage II (HP granulite facies) has peak P-T conditions of 12.1Kbar at 755°C (Y21-2) and 13.8Kbar at 815°C (Y18-8) using mineral assemblages combining with garnet rim compositions with maximum grossular and minimum pyrope contents; Stage III (amphibolite facies) was characterized by the development of calcic amphibole in granulites with temperature of 446–563°C. Therefore, an anticlockwise P-T path characterized by simultaneous temperature-decreasing and pressure-increasing was inferred for the Yushugou HP mafic granulite. Studies of zircon morphology and inclusions, combined with zircon U-Pb dating and REE geochemistry indicate that their protolith’s ages of the mafic granulites were ∼430Ma, while the metamorphism could occur at three stages with ages of ∼390Ma, ∼340Ma and ∼320Ma, which may correspond to Stage I, II and III, respectively. We interpret the HP mafic granulites from the Yushugou granulite-peridotite complex to be formed by the cooling subduction of the lower crustal rocks from the hanging wall of central Tianshan block during the northward subduction of the south Tianshan paleo-ocean from Devonian to Carboniferous.

Origin and significance of diamonds and other exotic minerals in the Dingqing ophiolite peridotites, eastern Bangong-Nujiang suture zone, Tibet

We have recovered unusual mineral assemblages of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite from the peridotites of the Late Triassic Dingqing ophiolite along the Bangong-Nujiang suture zone in south-central Tibet. Diamond grains are 100–300 μm, pale yellow and reddish-orange to colorless, commonly anhedral in habit, and range in morphology from elongated to octahedral and subhedral crystals. They display a characteristic shift in the Raman spectra between 1325 cm -1 and 1333 cm -1 , mostly at 1331.51 cm -1 or 1326.96 cm -1 . Chromian spinels in the Dingqing harzburgites and dunites are high-Cr varieties with Cr# = 40.1–49.1 and Cr# = 65.3–69.6, respectively. Recovered diamonds, ultrahigh-pressure, and highly reduced minerals appear to have crystallized and then been encapsulated in chromian spinel grains over a wide range of depth in the diamond stability field near the mantle transition zone. These chromian spinels and their host peridotites were then brought up to shallow mantle levels by the convecting asthenosphere, and were subsequently trapped in a mantle wedge of an intraoceanic subduction zone. The peridotites underwent further partial melting processes and reactions with island arc tholeiitic and boninitic melts, significantly modifying their whole-rock and mineral compositions. The preexisting chromian spinel grains with the inclusions of diamonds and other exotic minerals coalesced to form podiform chromitites during this stage in a suprasubduction zone setting.

Petrological and geochemical evolution of the Central Gneissic Belt, Rengali Province, eastern India: Implications for the Neoarchean growth and orogenesis

This study focuses on the evolution of the Central Gneissic Belt of the Archean Rengali Province which evolved as a craton-margin orogenic belt. The Central Gneissic Belt is constituted of charnockite gneiss, migmatitic hornblende gneiss and granite gneiss often showing gradational contacts. While mafic granulite occurs as enclave within the charnockite gneiss, amphibolite and calc-silicate granofels enclaves are present within the granite gneiss. Mafic granulite shows peak metamorphic assemblage of garnet+clinopyroxene+plagioclase+quartz±orthopyroxene which was stabilized at 10.6±0.5kbar and 860±20°C. Charnockite gneiss with the peak assemblage of orthopyroxene+quartz+plagioclase+K-feldspar was metamorphosed at 792±48°C and 7.6±0.4kbar. Amphibolite and migmatitic hornblende gneiss contain hornblende along with plagioclase and garnet and these rocks were metamorphosed at 800±20°C, 8.5±0.2kbar and 695°C, 8kbar respectively. Later meta-dolerite dikes exhibit relic igneous textures which are slightly modified by greenschist-facies metamorphism. Charnockite gneiss, migmatitic hornblende gneiss and granite gneiss show similar trace and REE characteristics (moderate fractionation in terms of La and Yb, LREE enrichment and flat HREE pattern) implying the same protolith composition for these rock groups. Based on the field, petrographic and geochemical data, we propose that the protoliths for the charnockite gneiss, the migmatitic hornblende gneiss and the granite gneiss crystallized as fractionated magma in within-plate syncollisional setting during the ca. 2860–2780Ma orogeny at the Rengali Province.

Heavy mineral studies of Gondwana sandstones of Eastern Arunachal Himalaya and implications for provenance

A thin linear belt of Permian Lower Gondwana rocks occur in the Eastern Himalayas from Arunachal Pradesh to Sikkim. The Lower Gondwana Group of rocks consists of shale, siltstone, sandstone, carbonaceous shale and coaly matter and is sandwiched between Miri Group and Siwalik Group of rocks. Heavy mineral composition of sandstones is extensively used in the provenance studies as they are the surviving remnants of the rather abundant but unstable mafic components of the source rock. The sandstones of Lower Gondwana Group bear the heavy mineral assemblage of chlorite, biotite, zircon, ilmenite, epidote, garnet, amphibole, chloritoid, brown tourmaline, magnetite, staurolite, rutile, and opaques which is indicative of a provenance of metamorphic rocks with input of igneous rocks. Provenance sensitive mineral index ratios were also calculated to see the variation in the ratio of two or more stable minerals with the same characteristics.

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.

The implications of overstepping for metamorphic assemblage diagrams (MADs)

Metamorphic assemblage diagrams (MADs, also known as pseudosections) are based on the assumption of chemical equilibrium throughout. Nucleation of a porphyroblast, however, requires a driving force that is achieved through overstepping of the isograd reaction. By making an assumption about the degree of overstepping required to nucleate garnet, aluminosilicate, staurolite, and cordierite, a modified MAD can be constructed that offers insights into metamorphic parageneses beyond those provided by an equilibrium phase diagram.Assuming a representative value of 300J/mol oxygen of affinity for the nucleation of a porphyroblast, modified MADs have been constructed for a typical pelitic bulk rock composition. The resulting diagrams show a displacement of the garnet isograd to higher temperature (T), a shrinking of the stability field for garnet+chlorite, an expansion of the field for aluminosilicate, and a shrinking of the stability field for cordierite. Furthermore, the size of the stability field for staurolite+garnet depends on which reaction produces aluminosilicate: if aluminosilicate nucleates from an assemblage of garnet+staurolite the field of staurolite+garnet is greatly expanded; if aluminosilicate nucleates from an assemblage of chlorite+muscovite, the field shrinks dramatically and may be eliminated entirely. If garnet is a reactive phase, then kyanite is predicted to nucleate prior to or nearly simultaneously with staurolite in typical Barrovian metamorphic trajectories. Finally, certain equilibrium assemblages present in only small regions of P–T space may not be realized unless sufficient driving force for nucleation or progressive reaction can occur over limited changes in P and T. The key consideration is the amount of affinity required to drive the reaction at the P–T conditions of interest.

Metamorphic evolution of Palaeoproterozoic anatectic migmatites in the eastern part of the Aravalli–Delhi Fold Belt, India: constraints from thermodynamic modelling and monazite dating

AbstractGarnetiferous pelitic to psammopelitic migmatites widespread across the central and eastern part of the Aravalli–Delhi Fold Belt in NW India record two distinct orogenies, e.g. the Aravalli Orogeny (1.7–1.6 Ga) and the Delhi Orogeny (1.0 Ga). In this study, we integrate field geological studies with textural and mineral–chemical analyses,P–Tpseudosection modelling andin situmonazite dating in anatectic migmatites in the Aravalli Supergroup occurring along the Deoli–Shahpura segment. The study reveals formation of peak assemblages of garnet + sillimanite + biotite + K-feldspar + melt and garnet + muscovite + K-feldspar + melt in two anatectic migmatite samples.P–Tpseudosection modelling suggests that anatexis in the gneisses occurred at ~8 kbar and 700–800°C along a tight-loop clockwiseP–Tpath. Monazite ages from the migmatites indicate that the anatexis occurred at ~1.73–1.74 Ga. This age is similar to the Palaeoproterozoic anatexis (at 7–8 kbar) and charnockite emplacement in the Sandmata and the Mangalwar complexes, the subsolidus amphibolite-facies metamorphism in the Rajpura–Dariba and Pur–Banera supracrustal belts, and the A-type granite magmatism in the North Delhi Fold Belt. We propose that the Palaeoproterozoic migmatites in central and eastern Rajasthan are part of the one crustal unit that underwent anatexis during an accretion event along the NE–SW-trending Aravalli orogenic belt.

Paleozoic intrusive rocks from the Dunhuang tectonic belt, NW China: Constraints on the tectonic evolution of the southernmost Central Asian Orogenic Belt

The Dunhuang tectonic belt (DTB) is of great importance for understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt (CAOB). In this study, the temporal-spatial distribution, petrogenesis and tectonic setting of the Paleozoic representative intrusive rocks from the DTB were systematically investigated to discuss crustal evolution history and tectonic regime of the DTB during Paleozoic.Our results reveal that the Paleozoic magmatism within the DTB can be broadly divided into two distinct episodes of early Paleozoic and late Paleozoic. The early Paleozoic intrusive rocks, represented by a suite metaluminous-slight peraluminous and medium- to high-K calc-alkaline I-type granitoids crystallized at Silurian (ca. 430–410Ma), are predominantly distributed along the northern part of the DTB. They were probably produced with mineral assemblage of eclogite or garnet+amphibole+rutile in the residue, and were derived from magma mixing source of depleted mantle materials with various proportions of Archean-Mesoproterozoic continental crust. The late Paleozoic intrusive rocks can be further subdivided into two stages of late Devonian stage (ca. 370–360Ma) and middle Carboniferous stage (ca. 335–315Ma). The former stage is predominated by metaluminous to slight peraluminous and low-K tholeiite to high-K calc-alkaline I-type granitic rocks distributed in the central part of the DTB. They were also generated with mineral assemblage of amphibolite- to eclogite-facies in the residue, and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust. The later stage is represented by adakite and alkali-rich granite exposed in the southern part of the DTB. The alkali-rich granites studied in this paper were possibly produced with mineral assemblage of granulite-facies in the residue and were generated by partial melting of thickened lower continental crust. Zircon Hf isotopes and field distribution of those Paleozoic intrusive rocks reveal that both the Silurian and the late Devonian magmatic activities predominantly represent crustal growth processes in the DTB, accompanied by different degrees of reworking of pre-existing continental crust. However, the middle Carboniferous (ca. 335–315Ma) magmatic activity reflects a crustal reworking process. The Silurian and late Devonian intrusive rocks were most likely formed in the arc-related subduction zones, whereas, the middle Carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension, representing the final stage of Paleozoic orogeny in the DTB.These Paleozoic magmatic rocks further suggest that the DTB has reactivated from a stable block to an orogen and undergone two episodes (the early Paleozoic and the late Paleozoic) of orogeny during Paleozoic. It represents a Paleozoic accretionary orogen of the southernmost margin of the CAOB between the Tarim Craton and North China Craton, and tectonically extends northward to the Beishan orogen and westward to the eastern South Tianshan Belt.

Paleoproterozoic multistage metamorphic events in Jining metapelitic rocks from the Khondalite Belt in the North China Craton: Evidence from petrology, phase equilibria modelling and U–Pb geochronology

Metapelitic rocks of the Jining Complex (sillimanite-cordierite-garnet (Sil–Crd–Grt) gneisses, sillimanite-garnet (Sil–Grt) gneisses and quartzofeldspathic rocks) are exposed in the eastern segment of the Khondalite Belt (KB) in the North China Craton (NCC). The Sil–Crd–Grt gneisses have preserved polyphase mineral assemblages and microstructural evidence of anatexis, resulting from biotite dehydration melting. Petrological observations revealed that the Sil–Crd–Grt gneisses contain three metamorphic assemblages: a peak assemblage of garnet porphyroblast and matrix biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, a post-peak near-isothermal decompressional assemblage of garnet+cordierite+biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, and a decompressional cooling assemblage of garnet+biotite+cordierite+K-feldspar+plagioclase+quartz+ilmenite+magnetite. A clockwise P–T path was defined involving the inferred peak stage followed by post-peak near-isothermal decompression and decompressional cooling stages, with P–T conditions of 790–825°C and 9–10kbar, 810–890°C and 6.0–6.5kbar, and 780–810°C and 4.0–5.5kbar, respectively. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS) U–Pb analyses of the Sil–Crd–Grt gneisses and Sil–Grt gneisses for the detrital and metamorphic zircons yielded a protolith age of ∼2.0Ga and the late Paleoproterozoic metamorphic age of 1895–1885Ma. The results reveal that the metapelitic rocks of the Jining Complex underwent continent–continent subduction or collision in the peak metamorphic stage, followed by a post-collisional exhumation event in the post-peak decompressional stage, and a subsequent decompressional cooling stage between the Yinshan and Ordos blocks to form the Paleoproterozoic KB.

The metamorphic evolution of Paleoproterozoic eclogites in Kuru-Vaara, northern Belomorian Province, Russia: Constraints from P-T pseudosections and zircon dating

The rare occurrence of Precambrian eclogite has been used to argue when plate tectonics initiated. Well-preserved kyanite-bearing eclogites were collected in the Kuru-Vaara quarry, northern Belomorian Province, Fennoscandian Shield. Textural observations and phase equilibria modeling were applied to constrain their P-T evolution. The eclogite is characterized by the peak pressure assemblage garnet+omphacite+kyanite+amphibole+quartz. The garnet has inclusions of zoisite, omphacite and kyanite. This omphacite with high jadeite (up to 33mol%) and 5% Ca-Eskola content bears oriented rods of quartz, amphibole, and rutile. Omphacite was replaced by diopside+plagioclase±orthopyroxene symplectites and garnet surrounded by plagioclase coronas. Kyanite was mantled by the assemblage sapphirine+spinel+plagioclase±scapolite. The symplectites of diopside+plagioclase±orthopyroxene are further replaced by amphibole. P-T estimates of peak eclogite facies conditions are c. 18–20kbar and 720–820°C. The granulite facies assemblages formed at c. 11–13kbar and 870–915°C. Zircon U-Pb dating reveals the protolith ages of 2802–2929Ma for the eclogites. The c. 1896Ma zircon ages correspond to the time of prograde metamorphism in eclogite facies. The age of 1885±7Ma for a corundum-bearing mica pegmatite vein that intruded into the eclogites reflects the infiltration of fluid at amphibolite facies conditions. Thus, the Kuru-Vaara eclogite was metamorphosed during the Paleoproterozoic, not the Archean. The P-T path of Kuru-Vaara eclogites was characterized by eclogite facies metamorphism overprinted by granulite facies and retrograde amphibolite facies metamorphism within c. 10Ma. This process reflects the fast exhumation of the eclogites. The increase in temperature from eclogite to granulite facies in the HT-UHT domain suggests a hot Precambrian tectonic environment.

Petrography and geochemistry of the Pedra Dourada Granulite, southeastern Minas Gerais, Brazil

The Pedra Dourada Granulite (PDG) occurs at the southeastern portion of the Aracuai Belt, north of the town of Ponte Nova. It comprises bodies up to 45 km2 inserted into amphibolite-facies gneisses of the Mantiqueira Complex. Motivated by the discrepancy of metamorphic grade with surrounding rocks, this paper presents results of the petrographic and geochemical investigation of the PDG. The unit is comprised of meta-igneous and metasedimentary rocks. Meta-igneous rocks dominate and include felsic granulites (biotite ± garnet-bearing and orthopyroxene-bearing) and subordinate mafic granulites. Metasedimentary rocks are aluminous granulites with Al-rich mineral assemblages (garnet, sillimanite, spinel). Geochemical data show that most of felsic protoliths are peraluminous rocks including granites, granodiorites and diorites of calc-alkaline character, chemically similar to granitoids of convergent tectonic settings. Mafic protoliths are metaluminous rocks comprised of gabbros and subordinate diorites of tholeiitic affinity, compositionally similar to plate margin basalts. Aluminous protoliths may be peraluminous pelitic rocks and wackes, analogous to sediments from convergent environments. The mineral assemblages indicate that these rocks were metamorphosed under medium-P granulite-facies conditions. Coronitic garnet textures suggest a near-isobaric cooling (IBC-path) after metamorphic peak.