Felsic Granulites Research Articles

Ultrahigh-temperature metamorphism revealed by felsic granulite xenoliths in southern Tibet

The Himalayan-Tibetan orogen experienced extensive thermal disturbance during the Indian-Asian collision, yet evidence for ultrahigh-temperature metamorphism (UHTM, >900 °C) remains scant. We report a UHTM event in the Lhasa Block, identified from felsic granulite xenoliths in Sangsang near the Tangra Yum Co Rift (TYR) of southern Tibet. These xenoliths exhibit a mineral assemblage of garnet + rutile + quartz + K-feldspar + plagioclase ± kyanite. Elemental mapping indicates garnet underwent high-temperature chemical homogenization. Employing Raman elastic geothermobarometry, Ti-in-quartz, and Zr-in-rutile geothermometers, we deduced pressure-temperature (P-T) conditions of 12−16 kbar and 870−975 °C, suggesting conditions were in the ultrahigh-temperature (UHT) range. The predicted P-T range is the most reliable estimation for Sangsang xenoliths to date, although the actual P-T conditions were likely higher due to viscous reset and chemical diffusion. An in situ garnet Lu-Hf date (ca. 26 ± 8 Ma) suggests that the UHT event occurred during the early Miocene, and in situ rutile U-Pb dates (ca. 12−11 Ma) indicate that the UHT conditions persisted until at least the timing of xenolith exhumation. Combined with the geological records in the Himalayan-Tibetan orogen, we suggest that the mantle upwelling contributed most to heating the Tibetan crust in the TYR and ultimately induced UHTM. Our study serves as an example of the UHTM formation mechanism within the active orogenic system.

Metasomatic Interaction of Ultramafic Mantle Xenoliths with their Felsic HP–UHT Granulite Host (Moldanubian Domain, Bohemian Massif in Lower Austria)

Abstract The St. Leonhard granulite massif in Lower Austria, dominantly formed by kyanite-bearing felsic granulite, encloses countless up to 5 cm sized mantle xenoliths of garnet clinopyroxenite and peridotite. The mineralogical, textural and chemical consequences of a mutual metasomatic interaction at the contact between these xenoliths and the host orthopyroxene-bearing felsic granulite are described. Movement of Mg, Al and, especially, Ca from the garnet clinopyroxenite to the granulite and migration of K and Na in the opposite direction, caused the breakdown of clinopyroxene and formation of orthopyroxene–plagioclase symplectite coronae at the expense of the garnet clinopyroxenite xenoliths. Around the peridotite xenoliths, monomineralic orthopyroxene coronae have developed due to the supply of Si from the host granulite. The P–T conditions of this interaction were established to 900°C to 1000°C and 1.0 to 1.2 GPa by thermodynamic modelling. The duration of coronae growth was constrained to 13 to 532 ka based on modelling of Fe–Mg interdiffusion underlying secondary compositional zoning of garnet from the garnet clinopyroxenite xenolith extending to the coronae. The most significant change in the host granulite was caused by the supply of Ca from the garnet clinopyroxenite xenolith, which led to the breakdown of the Al2SiO5 phase—probably kyanite—and stabilization of orthopyroxene. K-feldspar-poor haloes surrounding mantle xenoliths formed due to the depletion of K in the granulite adjacent to the garnet clinopyroxenite. The observed origin of felsic–intermediate orthopyroxene-bearing granulite by transformation of felsic kyanite-bearing granulite through the metasomatic interaction with mantle xenoliths implies that the deep crustal chemical exchange between mantle- and crust-derived lithologies may have an important consequences on composition, thermal structure and geodynamic evolution of orogenic lower crust especially in hot collisional orogens, such as the European Variscides.

Investigation of rock genesis and geodynamic significance based on Sr–Yb granite classification and geochemical characteristics: A case study from Guangxi, China

The intrusive bodies in the study area are categorized into two intrusive units, γπK2 and ηoπK2, based on rock structure characteristics and intrusive contact relationships. The primary lithologies are granite porphyry and quartz monzonite porphyry. Through LA-ICP-MS zircon U–Pb dating, the weighted mean ages of zircon ^206Pb/^238U for these two units were determined to be 84.09 ± 0.86 Ma and 86.10 ± 0.76 Ma, respectively. According to geochemical characteristics and Sr–Yb granite classification, the quartz monzonite porphyry is classified as Zhejiang-Fujian type granite, and the granite porphyry as Nanling type granite. The study explores the geodynamic significance of granites in the area based on Sr–Yb granite classification, identifying the lower crust metamorphic facies as amphibolite facies. The source rocks are primarily basic granulites and felsic granulites, reflecting a thermally active lower crust with poorly developed fluids. Additionally, Sr–Yb granite classification aids in determining the crustal thickness of the lower crust, which is crucial for understanding continental geological evolution. This paper discusses the optimal locations and conditions for mineralizing fluid precipitation, noting that tungsten-tin mineralization is associated with Nanling type granites and that the mineralization process generally occurs later than the diagenetic process under conditions of low pressure and low oxygen fugacity. Tungsten-tin prospecting should focus on areas where the surrounding rock temperature ranges from 300 to 500 °C, with particular attention to the contact zones of sedimentary rocks or near structural fault zones.

Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

The South Mountain Batholith (SMB) is a syn-tectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (∼4 kbar) andalusite–staurolite aureoles and are discordant to northeast-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage Pluton intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, and sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400–355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ∼40% of that perturbation.

Deformation-induced, retrograde transformation of kyanite to andalusite: An example of felsic granulite in the southern Bohemian Massif

Detailed microstructural, microfabric and chemical analyses of felsic granulite within and around a ductile shear zone at the Plešovice quarry in the southern Bohemian Massif revealed deformation-induced, retrograde transformation of kyanite to andalusite. Although andalusite is absent outside the shear zone, its fraction among Al2SiO5 polymorphs significantly increases up to ∼59% inside the shear zone, while the fraction of kyanite significantly decreases from ∼98% outside the shear zone down to ∼22% inside the shear zone. Three examples of andalusite and the relict kyanite pair suggest a common topotaxial relationship such that andalusite (010) is parallel to the precursory kyanite (100), while andalusite [100] and [001] are parallel to the precursory kyanite [010] and [001], respectively, indicating that the growth of andalusite was crystallographically controlled by the precursory kyanite.The felsic granulite in the ductile shear zone at the Plešovice quarry has been repeatedly subjected to brittle shearing accompanied by pseudotachylite formation and plastic shearing with mylonitization, suggesting development of the ductile shear zone at the brittle–plastic transition zone. Mylonitization of the felsic granulite was promoted by deformation-induced K-feldspar replacement by myrmekite at temperatures of 390–440 °C assuming a pressure of 250 MPa.

Ultrahigh-temperature metamorphism of the felsic granulites in the eastern North China Craton and their implications on the Neoarchean tectonic regime

The studies of the meta-pelitic and mafic rocks of the Eastern Hebei in the North China Craton have proved the existence of Neoarchean ultrahigh temperature (UHT) metamorphism. However, whether the metamorphosed felsic rocks of the area experienced the similar UHT metamorphism remains poorly constrained. Based on our new geological mapping and petrological observations, we identified felsic granulites in the Taipingzhai area of the eastern North China Craton. We combined thermodynamic modelling results with the temperature calculations of the ternary feldspar thermometer to retrieve the metamorphic history. Sample JD29–1 achieved the peak UHT condition at 9.3 kbar/1025 °C that was followed by a retrograde metamorphism at P < 8.0 kbar and T < 830 °C, whereas Sample JD11–2 recorded the peak P–T conditions of 8.0–9.5 kbar and 825–865 °C followed by a retrogression stage below 7.5 kbar/765 °C. Orthopyroxene crystals from both samples are subhedral to anhedral in crystal shape, and contain commonly high-TiO2 biotite inclusions and enriched-HREE and depleted LREE and MREE patterns, indicating a typical metamorphic origin. UPb zircon dating results reveal the protoliths of the felsic granulites were emplaced at 2537–2503 Ma and metamorphosed at 2503–2464 Ma. This study indicates that the UHT metamorphism was also recorded in the felsic granulites from the Eastern Block of the North China Craton. A Neoarchean mantle plume triggered sagduction model represents the most favorable tectonic regime that can well explain the existing metamorphic data from all the felsic, mafic and pelitic rock assemblages.

U–Pb ages from felsic rocks of the External Ligurian sedimentary mélange (Northern Apennine, Italy): tracing the pre-Jurassic history of the hyperextended Adria continental margin

The External Ligurian Units are regarded as representative of the ocean continent transition between the Ligure‒Piemontese oceanic basin and the hyperextended Adria continental margin. The remnants of this transition are preserved as slide blocks embedded in a sedimentary mélange. This mélange sedimented during the Santonian to Campanian in the rear of the accretionary wedge that developed during the east-dipping subduction of the oceanic lithosphere below Adria. The main characteristic of this mélange is the occurrence of slide blocks of subcontinental mantle, lower continental crust, granitoids, gabbro, basalt and sedimentary rocks in a sedimentary matrix. In this paper, we provide petrographic observations coupled with U‒Pb zircon ages and δ 18 O zircon isotopes of the slide blocks of felsic granulites and granitoids. U‒Pb data indicate ages ranging from Neoproterozoic to Late Triassic. δ 18 O isotopes help to define the source of the magma of granitoids and felsic granulites, which was the continental crust. Overall, the collected data indicate that the slide blocks from sedimentary mélanges can be regarded as true archives that are able to provide useful constraints about the geodynamic history of the source area of these deposits. Supplementary material: The dataset related to the U–Pb and ∂ 18 O analysisis available at https://doi.org/10.6084/m9.figshare.c.6922535 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

A petrophysical study of the composition of Taiwan's middle and lower crust

A first-order approximation of the lithological make-up of an orogen's middle and lower crust can provide insights into its structure, as well as the tectono-metamorphic and geodynamic processes taking place there. In this study, we investigate the possible lithological and chemical composition of Taiwan's middle and lower crust by matching in situ physical properties measured by the TAIGER tomography data with isotropic wavespeeds, density, and major element composition for a variety of upper amphibolite and granulite facies rocks modelled at ambient pressure and temperature using the AbersHacker Macro. The modelling suggests that Taiwan's middle crust is possibly comprised of some combination of biotite-poor metapelite, garnet-poor felsic granulite, mafic granulite, amphibolite, and marble. The lower crust is likely comprised of mafic granulite, garnet-rich felsic granulite, biotite-free metapelite, and eclogite. Furthermore, the modelling shows that the modal abundance of garnet and/or sillimanite has a significant effect on physical properties, elevating seismic wavespeeds and density of felsic rocks to those of mafic rocks. The modelled wt% major oxide composition suggests that Taiwan's middle and lower crust have a more mafic chemical composition than that of global compilations of the continental crust. Nevertheless, this reflects the choices made when assigning rock types for the lithological mix used to calculate the wt% oxides, since increasing the percentage of garnet-rich metapelite and felsic granulite would result in a more felsic bulk composition.

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

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

Eoarchean to Paleoproterozoic Crust History in the Yakutian Diamond Province: Evidence from U–Th–Pb (LA-ICP-MS) and Lu–Hf Dating of Zircons from Crustal Xenoliths in Kimberlite Pipes

Abstract —Radiometric dating of zircon grains from crustal xenoliths in kimberlites of the Yakutian diamond province show that most of the Daldyn and Markha terranes were formed in Paleoarchean but preserve some fragments of Eaoachean and possibly even Hadean crust recycled in several tectonothermal events. The oldest zircons were crystallized about 3.2 Ga and recrystallized during later activity stages of 2.9, 2.8–2.7, and 1.9 Ga, whereby they gained radiogenic hafnium produced by 176Lu decay in the rock. The degrees crust rework and the tectonothermal stages varied across the Anabar tectonic province. The earliest events of 3.2 and 2.9 Ga left record in zircons from kimberlites in the Nakyn field, while the signatures of the 2.7 Ga activity are best pronounced in zircons from kimberlites sampled in the Upper Muna and Nakyn fields. On the other hand, zircons from lower crustal mafic granulite xenoliths in the Daldyn and Alakit-Markha kimberlites lack traces of the earliest crust history and only evidence of the last 1.9 Ga event, which remained mute in xenolith samples from the Upper Muna field. Zircons from felsic granulite and metadiorite xenoliths in the Udachnaya kimberlite, which represent middle and upper crust, show a peak at 2.6 Ga besides that of 1.9 Ga. The synthesized available data support several previous inferences: vertical and lateral heterogeneity of the crust in the Yakutian diamond province; absence of linkage between the crust recycling degree and major collisional zones of the Siberian craton; absence of the separate Markha terrane. Correlation of age peaks corresponding to thermal events in the crust history of the Anabar tectonic province with those of large igneous provinces allows a hypothesis that the revealed tectonothermal events may be related with the activity of superplumes.

Modification of Archean lower crust of the North China Craton by magma underplating during the Mesozoic: Evidence from zircon U-Pb-Hf-O isotopes of granulite xenoliths

Previous studies of felsic granulite and pyroxenite xenoliths have confirmed the existence of the Archean lower crust beneath the Fuxin region which experienced magma underplating in the Phanerozoic. U-Pb-Hf-O isotopes of zircons in felsic granulite xenoliths from the Fuxin late Cretaceous basalts in the North China Craton were analyzed to evaluate the role of magma underplating in the modification of the Archean lower crust during the Phanerozoic. The felsic granulites recorded a uniform upper intercept age of 2.5 Ga and a spread of concordant Mesozoic zircons from 177 to 105 Ma, with a peak at 118 Ma. Most Precambrian zircons show discordant U-Pb ages and obvious core-rim structure. The Mesozoic zircons from the granulite xenoliths can be divided into two groups: zircons with an age of >106 and those with an age of ≤106 Ma. Zircons with ages older than 106 Ma are characterized by an unzoned or core-rim structure and enriched Hf-O isotopic compositions, whereas zircons with ages of ≤106 Ma mainly exhibit oscillatory zoning and have depleted Hf-O isotopic compositions. Some >106 Ma zircons with strongly negative εHf(t) values of −40.4 to −24.9 are considered to be age resets of ancient zircons. The relatively higher Hf-O isotopic values for the rest of the >106 Ma zircons indicate that various proportions of underplated magmas with high εHf(t) and δ18O values contributed to their formation. The underplated magmas may have originated from the lithospheric mantle, metasomatized by melts/fluids from subducted Paleo-Asian and Paleo-Pacific oceanic crusts. The ≤106 Ma zircons probably crystallized in a melt from the depleted mantle. These observations suggest that long-lived magma underplating occurred beneath the Fuxin region during the Mesozoic, and the derivation of underplated magmas changed from an enriched lithospheric mantle to a depleted asthenospheric mantle at ∼106 Ma. Combined with our previous studies, we conclude that magma underplating not only provided heat for remelting, but also added exotic material in the modification of ancient lower crust beneath the Fuxin region during the Mesozoic. Furthermore, continuous modifications of cratonic lower crust by magma underplating weaken its refractory and made it more susceptible to chemical erosion and partial melting, which played an important role in the lower crust destruction of the craton.

Metamorphic P–T–t evolution deciphered from episodic monazite growth in granulites of the Chencai Complex and implications for the Early Paleozoic Orogeny, West Cathaysia terrane, South China

Abstract The Early Paleozoic Orogeny in eastern South China has been highly controversial. It has been alternatively interpreted to have formed in an intra-plate setting driven by far-field tectonic forces or at plate boundaries involving subduction–collision. The West Cathaysia terrane in the core of the orogen is characterized by extensive magmatism, intense deformation and especially high-grade metamorphism. Identifying early Paleozoic high-pressure (HP) metamorphism and establishing a complete P–T–t path from the high-grade metamorphic rocks could help us understand the tectono-thermal evolution process and nature of the Early Paleozoic Orogeny. Here, we present results from a felsic granulite from the Chencai Complex in the northeastern West Cathaysia terrane. Petrographic evidence, mineral compositions and phase equilibria modelling indicate that the granulite underwent a pre-peak HP stage with P–T conditions of 13.3–14.7 kbar/696–718°C and low geothermal gradients of 13–14°C km −1 , and a peak high-temperature stage with P–T conditions of 9.7–11.0 kbar/785–820°C. A clockwise P–T path involving pre-peak decompressional heating, post-peak near-isothermal decompression and near-isobaric cooling processes was constrained for the HP felsic granulite. In situ monazite U–Pb geochronology combined with previous results date these metamorphic processes at c. 440, c. 425 and c. 400 Ma, respectively. Our new metamorphic and geochronological data from the HP felsic granulite support the case that the Early Paleozoic Orogeny was a typical collisional one.

Multiple Episodes of Zircon Growth during Anatectic Metamorphism of Metasedimentary Rocks in Collisional Orogens: Constraints from Felsic Granulites in the Bohemian Massif

Multiple Episodes of Zircon Growth during Anatectic Metamorphism of Metasedimentary Rocks in Collisional Orogens: Constraints from Felsic Granulites in the Bohemian Massif

Timescales of ultra-high temperature metamorphism and crustal differentiation: Zircon petrochronology from granulite xenoliths of the Variscan French Massif Central

Lower crustal ultra-high temperature (UHT) metamorphism and the resulting production and transfer of granitic magmas represent key processes of intracrustal differentiation. The timescales of these phenomena are debated, due to the complexity of the granulite zircon U-Pb record and because direct links between granulites and granites are difficult to establish. To address these issues, we present the results of zircon petrochronology (coupled U-Pb dating, trace element and Lu-Hf isotopic analyses) for lower crustal felsic and mafic granulite xenoliths from the Variscan eastern French Massif Central and compare them with data from well-characterized mid-/upper crustal migmatites and granites. The felsic and mafic granulites represent pre-Variscan meta-sedimentary and meta-igneous mafic rocks, respectively; which experienced Variscan UHT peak metamorphism at 940–970°C and 8 ± 2 kbar. Zircons from the felsic granulites show U-Pb dates spreading over ∼50 Myr between ∼315 and ∼265 Ma, correlated to Ti-in-zircon temperatures decreasing from 940–970°C to 800°C and REE contents consistent with growth in equilibrium with garnet at (U)HT conditions. This is best explained by continuous crystallization of zircon upon cooling from the thermal peak owing to decreasing Zr solubility of residual melt, despite significant prograde melt loss. Zircons from the mafic granulite only record the last stage of this time-temperature evolution (295–265 Ma; 800–900°C) due to later zircon saturation. Upper crustal granite emplacement started at 340 Ma and culminated at the age of the lower crustal thermal peak of 313 ± 3 Ma (defined by the highest-temperature zircons from felsic granulites), reflecting melt extraction along the ∼27 Myr prograde path of the lower crust. In turn, the crystallization ages of mid-crustal migmatites (315–300 Ma) and the lower crustal granulites (315–265 Ma) are consistent with slow cooling in the presence of melt. These results provide a direct assessment of the timescales of melt production and residence at the crustal scale; and validate the granulite-granite connection in the framework of the melt loss theory in migmatitic systems.

Petrogenesis of protoliths and U–Th–Pb SHRIMP ages in zircons of the Cerro Negro Paleoproterozoic mylonitic igneous suites, Tandilia belt: Adakitics local fingerprints in the Rio de la Plata craton

A dataset of 23 geochemical analyses of mylonitized igneous rocks from the Cerro Negro hill, at the northwestern-most tip of the Azul Megashear Zone of the Tandilia Belt (TB), was evaluated to characterize protoliths: mafic granulites, felsic granulites, garnet-bearing hornblende gneissic and garnet-bearing granite. The evaluation of major and trace elements points to rocks derived from an arc-related suite of metaluminous to peraluminous magmas. Felsic granulites and garnet-bearing granite displayed REE patterns with Eu/Eu* >1.5 and K/Rb ratios around 250, suggesting accumulation of plagioclase feldspars during differentiation of mafic underplated magmas. An exceptional adakite-like signature is reported based on high Sr/Y (20–100) for low Yb (<15 ppm) and high La/YbN (20–70) ratios for low YbN (<4) in a felsic granulite and a garnet-bearing granite.A garnet-bearing hornblende gneissic mylonite and a xenolith of garnet-bearing striped gneiss, were selected for U–Th–Pb SHRIMP in zircon. Crystallization ages of 2092 ± 2 Ma and 2163 ± 5 Ma were determined respectively. A lower intercept at 610 ± 40 Ma obtained in the discordia diagram of the garnet-bearing stripped gneiss unravels a Brasiliano overprint in TB.We propose a petrogenetic model of two stages of incremental melting of a subducting slab based on adakitic signatures seen in some of the studied rocks. LILE-depleted mafic granulites derived from an initial stage of dehydration of the slab with constant K/Rb ratio (217–346). Subsequent partial-melting of the remaining LILE-depleted eclogite led to adakitic magmas as source of the garnet-bearing granite (2163 ± 5 Ma) and felsic granulite protolith. Adakitic source shows increasing K/Rb (189–354) ratio with decreasing Rb. Finally, a calc-alkaline magmatism produced the younger granite (2092 ± 2 Ma) which intruded preexisting rocks prior to their high-grade mylonitization. This model is consistent with the exceptional conditions hypothesized for adakites petrogenesis in the context of the Archean-Paleoproterozoic transition.

The different responses of trace elements to equilibrium and disequilibrium melting: Implications for crustal differentiation and granite compositions

Equilibrium and disequilibrium meltings are two common mechanisms in the geochemical differentiation of the continental crust. However, responses of trace elements to the two melting processes are still not fully understood in granite petrogenesis. To address this, we conducted trace element modelling of open system melting using phase equilibrium calculations combined with solubility equations for accessory minerals and mineral–melt partition coefficients based on three melt-reintegrated felsic granulite compositions. The results indicate systematic differences between the compositions of disequilibrium (Type I) and equilibrium (Type II) melts involving water-present melting reaction, fluid-absent melting reaction involving biotite, and fluid-absent melting reaction in the absence of biotite. Type I melts generally have higher Rb, Ba, Co, Ni, V, and Cr contents, and Rb/Sr and Nb/Ta ratios, and lower Sr, P, Th, U, and light rare earth element contents than Type II melts. Significantly, disequilibrium melting involving biotite can produce melts with high Nb contents and super-chondritic Nb/Ta ratios, which may be a reservoir for the missing Nb on Earth. Compared with Type II melts, the trace element contents and ratios of Type I melts exhibit a larger range, more complex trends, and are more sensitive to changes in melting reaction types and melt loss. This suggests that, in addition to petrogenetic processes such as crystal entrainment, fractional crystallization and magma mixing, the disequilibrium melting would also contribute to the compositional diversity of granites. The modelling results also show that the trace element signatures of granites that are commonly used to determine their petrogenetic processes and tectonic setting may lead to erroneous conclusions if the granitic melts were produced by disequilibrium melting.

Variation in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: A Consequence of Various Mantle Sources or Crustal Contamination

The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. Mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concetrations of TiO2 and immobile trace elements (REE, Zr, Nb, and most positive εNd(Т) va-lues. The first type of mafic granulites show elevated (La/Sm)n and enrichment in Th and LREE relative Nb which is typical of basalts of subduction origin or crustal contaminated basalts. The absence of negative correlation between (La/Sm)n and εNd(Т) and a clear positive correlation of TiO2 with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental mantle which metasomatized by melts formed from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO2 content, positive εNd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Ma-fic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)n, depletion of Nb relative to Th and LREE, and negative εNd(Т) values. Post magmatic influence of granitoids lead to the enrichment of mafic granulites in biotite and apatite, an increased in concentrations of K2O, P2O5, a signi-ficant enrichment of Zr, Nb, Th, LREE, and negative εNd(Т) values. The difference between mafic granulites of the first and second types is not resulted from crustal contamination, but is due to the melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted for the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements, presumably by felsic melts generated from the rocks of subducting plate, immediately preceded mafic magmatism.

Geochemical variation in biotite from the Devonian South Mountain Batholith, Nova Scotia: Constraints on emplacement pressure, temperature, magma redox state and the development of a magmatic vapor phase (MVP)

Abstract Here we report the composition of biotite from the peraluminous Devonian South Mountain Batholith (SMB) of southwestern Nova Scotia (Canada), the largest intrusive body within the Appalachian orogen. The batholith was emplaced in two phases: an early (379–375 Ma) granodiorite-monzogranite suite (Stage 1) and a later (375–372 Ma) more-evolved monzogranite-leucogranite suite (Stage 2). Biotite analyses (major and minor elements) were obtained on 55 unmineralized samples representing 11 plutons. Regardless of the stage of pluton emplacement, biotite is commonly interstitial to alkali feldspar, quartz and plagioclase, indicating similar timing of biotite saturation. This suggests that biotite chemistry records conditions at similar extents of magma evolution for the chosen suite of samples. Biotite compositions are Fe-rich, with Fe/(Fe+Mg) ranging from 0.6 to 0.98, and Al-rich, with IVAl ranging from 2.2 to 2.9 atoms per formula unit (apfu; 22 oxygen basis), the latter reflecting the coexistence of other Al-rich phases, such as muscovite, garnet, aluminosilicates, and cordierite. Biotite anion sites are dominated by OH (&amp;gt;3 apfu), followed by F (~0.3 apfu) and Cl (≤0.02 apfu), with a general trend of decreasing OH, increasing F and a marked decrease in Cl, with increasing differentiation. Pressure (P) is estimated from the Al content of biotite to be between 280–430 MPa, consistent with a range of 240 to &amp;lt;470 MPa derived from phase equilibria and fluid inclusion microthermometry combined with mineral thermobarometry. Temperature (T) calculated from the Ti content of biotite ranges from 603–722 °C. Comparison of P-T estimates with water-saturated granite phase relations suggest minimum water contents of 6–7 wt% for the SMB magmas. The redox state of the SMB was estimated by comparing biotite Fe#-Ti relations with compositions calculated using the MELTS thermodynamic model, as experiments have shown that biotite Fe# increases with decreasing fO2 at a given extent of crystallization. Results of MELTS modeling for the most primitive magmas of the SMB sample suite indicate that the observed biotite Fe#-Ti variation is consistent with crystallization at FMQ to FMQ-1, with more oxidizing conditions suggested for the most strongly differentiated samples. To constrain the origin of the biotite anion site variation, a quantitative model using biotite-melt exchange coefficients (KD) derived from existing experimental data was used to track the change in biotite OH-F-Cl abundances as a function of crystallization, with or without an extant magmatic vapor phase (MVP). The model reproduces the relative OH, F, and Cl abundances in biotite, and suggests that SMB crystallization occurred in the presence of a MVP. The relatively reduced redox state of the SMB, similar to other peraluminous granitoid occurrences worldwide, aligns with other measures of fO2 for the SMB, including the occurrence of primary ilmenite. The observed correspondence between the estimated fO2 and that imposed by graphite-gas equilibrium suggests a role for reduced carbon in the generation and evolution of the SMB. This is consistent with evidence for SMB interaction with graphite-bearing felsic granulites of the underthrust Avalon terrane, and assimilation of carbonaceous and sulfidic metasediments during pluton ascent and emplacement. Reducing conditions and development of a MVP have implications for granophile element concentration processes in the SMB magmatic system. Low fO2 during crystallization affects the mineral/melt partitioning and solubility of the redox-sensitive elements Sn, W, U, and Mo, serving to suppress early SnO2 precipitation, and cause both an increase in W/Mo and an overall buildup of all four elements in evolving SMB liquids. Available experimental data indicate that reducing conditions also shifts DMVP/melt to favor partitioning into the melt phase. Therefore, early vapor exsolution under reducing conditions also lessens the extraction efficiency of these redox-sensitive elements to the MVP, further underscoring the role of extensive crystallization as an important metal enrichment process.

High-pressure/ultrahigh-temperature mafic and felsic granulites associated with magnetite deposits from the Jiaobei terrane, Jiao-Liao-Ji Belt, North China Craton

Garnet-bearing mafic and felsic granulites associated with magnetite deposits that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism occur in the Yangjiazhai area of the Jiaobei terrane within the Jiao-Liao-Ji Belt, North China Craton. In mafic granulite, the early prograde assemblage (M1) is preserved as inclusions in the core of garnet. Matrix minerals and the garnet mantle define the peak metamorphic assemblage (M2). Near-isothermal decompression (M3) is recorded by the garnet rim and symplectitic minerals. The final cooling process is defined by the late growth of amphibole (M4). Abundant magnetite occurring in some mafic granulite samples may be inherited from the protolith. The felsic granulite records the peak metamorphic stage (M2) defined by the matrix minerals and the garnet mantle, and the retrograde stage defined by the symplectite and the garnet rim (M3). P-T estimations using pseudosection techniques and two-feldspar thermometry yield ∼8–9.8 kbar/710–828 °C for the prograde stage M1, ∼14.5–15 kbar/1040–1073 °C for the peak P-T stage M2, 9.1–10 kbar/965–1015 °C for the post-peak stage M3 and 6.6–9.4 kbar/820–853 °C for the cooling stage M4 as recorded by the mafic granulite. P-T conditions of ∼14–16.5 kbar/985–1077 °C for the peak P-T stage M2 and ∼7.8–9 kbar/889–902 °C for the post-peak stage M3 were derived from the felsic granulite. Thus a clockwise trajectory can be defined by constraining peak-metamorphic high-pressure/ultrahigh-temperature (HP-UHT) conditions. Zircon U-Pb dating and REE characteristics show a contrasting information. Oscillatory zoned zircon domains yield an age of ∼2150 Ma with a steep HREE characteristic, which is interpreted to indicate a magmatic origin of the mafic granulite. The peak HP-UHT stage obtained from metamorphic zircon domains in felsic and mafic granulites is constrained by an age of ∼1950–1900 Ma, which possibly corresponds to a collisional event shortly after subduction. The decompression occurred at ∼1880 Ma following the subsequent cooling at ∼1820 Ma indicating a rapid exhumation of the HP-UHT granulite followed by slow cooling.

Latest Cambrian stage of evolution of Precambrian continental crust in the Aktyuz high-pressure Complex (Chu-Kendyktas terrane; North Tien Shan): New evidence from the SW part of the Central Asian Orogenic Belt

Metamorphic crustal formations of the Aktyuz block (SE part of the Chu-Kendyktas terrane; SW segment of the Central Asian Orogenic Belt) include garnet-bearing orthogneisses and gneissic granites of the Aktyuz Complex, garnet-bearing ortho- and paragneisses of the Kemin Complex and paragneisses with schists of the Kokdzhon Complex. The gneisses of the Aktyuz and Kemin Complexes associated with intensively altered eclogites, are referred to the retrogressed felsic granulites, which likely experienced high-pressure re-equilibration and dehydration melting under eclogite facies conditions. The eclogite-bearing garnet-mica gneisses of the Aktyuz Complex contain zircons with magmatic cores, overgrown by the rims with the low Th/U ratios of 0.005–0.05. The obtained age clusters of ca. 844 Ma and ca. 490 Ma likely characterize two stages of the rocks’ evolution in the late Neoproterozoic (emplacement of the gneisses’ protoliths) and in the latest Cambrian (high-pressure metamorphism of the gneisses’ protoliths). The garnet-epidote gneissic granites of the Aktyuz Complex and garnet-bearing chloritized orthogneisses of the Kemin Complex yielded late Neoproterozoic (Tonian) protoliths’ crystallization ages of 820–805 Ma, but these rocks do not show any evidence of the later re-equilibration and apparently avoided high-pressure metamorphism. Thus, the protoliths of the late Neoproterozoic orthogneisses represented by anorogenic granitoids, comprised Precambrian basement of the Aktyuz block in the Chu-Kendyktas terrane, and some part of the felsic rocks was involved into Early Palaeozoic subduction processes. Detrital zircons from the metasedimentary formations of the Kokdzhon and Kemin Complexes of the Aktyuz block display the main age peaks at 600, 800, 1000 Ma and weaker peaks at ∼1.5 and 2.5 Ga. The protoliths of the rocks were terrigenous lithologies, which are believed to have been formed after eroded felsic complexes of mostly Ediacaran, late Neoproterozoic, Mesoproterozoic and Palaeoproterozoic-to-Neoarchean ages, and accumulated during the Cambrian. The rocks likely made up sedimentary cover of the Chu-Kendyktas terrane and constituted the sand-siltstone-shale series. The presence of varisized rims of 495–471 Ma in the detrital zircons of the metasedimentary formations of the Kokdzhon and Kemin Complexes is consistent with the near-peak-to-retrograde stages of the latest Cambrian-Middle Ordovician metamorphic evolution of the rocks. The age estimates obtained for the crustal complexes of the Aktyuz block correlate well with those of the similar complexes known from the adjacent Issyk-Kul (North Tien Shan) terrane (Makbal Complex) and Zheltau terrane (Southern Kazakhstan; Koyandy Complex) in the SW part of the Central Asian Orogenic Belt.