Showing papers on "Granulite published in 2022"

OUP accepted manuscript

Abstract: Abstract In collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.

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

Abstract: 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 eHf(t) values of 0.1–4.9 and whole-rock eNd(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 eHf(t) values (−4.3 to 2.5) and variable whole-rock eNd(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 eHf(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 eNd(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.

Critical metal enrichment in crustal melts: The role of metamorphic mica

Abstract: Metals such as Li, Be, V, Co, Nb, In, Cs, Sn, Ta, and W are considered resources that are critical for modern economies. They can be significantly enriched in granites and pegmatites, but the mechanisms of enrichment remain poorly understood. Many metal-enriched granitic magmas form through mica dehydration reactions during high-grade metamorphism. The preferential incorporation of these metals into micas provides a mechanism for concentration and mobilization during crustal melting. Comprehensive data sets of these elements and their partitioning in metamorphic micas across different metamorphic grades are currently lacking. We present the first extensive in situ laser ablation–inductively coupled plasma–mass spectrometry element data set collected from metasediment-hosted muscovite and biotite from three different metamorphic cross sections traversing sub-greenschist- (~400 °C) to granulite-facies conditions (>900 °C). Within the same sample, Li, V, Co, Cs, and Ta concentrations are higher in biotite, whereas Be, In, Sn, and W concentrations are higher in muscovite. Subsolidus micas record only nonsystematic compositional variations between samples. Suprasolidus biotites show systematic depletion in Li, Be, Sn, and Cs and enrichment in V and Co with increasing temperature in the highest-grade (muscovite-absent) samples. Indium and W reach peak concentrations in biotite at 750 °C and 850 °C, respectively. Muscovites record systematic enrichment in In and W and depletion in Be, Sn, and Cs with increasing metamorphic grade. These distinctive trends appear to be independent of tectonic setting (i.e., continental collision and crustal thinning). Our data set highlights the importance of higher-temperature melting (>750 °C), in particular, biotite breakdown reactions, for the release of Li, Be, Sn, Cs, and W into crustal melts.

Eburnean/Trans-Amazonian orogeny in the Nyong complex of southwestern Cameroon: Meta-basite geochemistry and metamorphic petrology

Abstract: New and published geochemical, petrologic, and geo-thermobarometric data of meta-basites were combined to constrain the geodynamic evolution of the Paleoproterozoic Nyong complex (NyC) in southwestern Cameroon, a part of the Central African tectonic belt. Their geochemical signatures, including (La/Yb)N = 0.53–56.26, slightly positive Eu anomalies (0.21–1.04), abundance of Pb and U, negative Nb–Ta anomalies, and REE depletion, suggest various degrees of partial melting of mostly spinel-bearing mantle sources. In addition, the meta-basite protoliths can be classified as IAB-, E-MORB-, N-MORB-, and exceptionally OIB-compositional types with minor crustal contamination. Petrologically, the meta-basites are characterized by coronitic textures, marked by microblasts and porphyroblasts of garnet surrounded by plagioclase or by both plagioclase and clinopyroxene, indicating metamorphic recrystallization under decompression. Our new geo-thermobarometric data outline various segments of an overall clockwise pressure‒temperature path. In the different meta-basite samples, the pressure peak at eclogite-facies conditions (∼625 °C, 11.7 kbar) was followed by nearly isobaric heating to amphibolite-facies conditions (600–690 °C, 11.5 kbar) or decompression and heating to amphibolite- (∼740 °C, 10.8 kbar) and granulite-facies (∼800 °C, 10.0 kbar) conditions. The retrograde path evolved from granulite- (850–750 °C, 11.3–7.6 kbar) and amphibolite- (790–550 °C; 12.4–3.0 kbar) to epidote-amphibolite- (∼600‒490 °C, 8.0–7.6 kbar) and greenschist-facies (∼400 °C, 4.2 kbar) conditions. Combining the new with published, in general high to middle temperature-middle pressure data from various rock types, with published high temperature-high pressure data from eclogites highlights distinct burial-exhumation paths for the NyC rocks: these paths document a clockwise pressure-temperature evolution, typical for a collisional setting associated with the Rhyacian-Orosirian Eburnean/Trans-Amazonian orogeny in Cameroon. The eclogite data outline a deep burial–isothermal decompression evolution, typical for a subduction setting predating the collisional setting.

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

Abstract: 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.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

Abstract: The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

Abstract: The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Tectonothermal transition from continental collision to post‐collision: Insights from eclogites overprinted in the ultrahigh‐temperature granulite facies (Yadong region, central Himalaya)

Abstract: Eclogites and granulites are fossil records of orogenies and provide crucial evidence of geodynamic processes. Incomplete knowledge on both hampers the establishment of detailed tectonothermal models for the central Himalaya. Utilizing detailed petrography, pseudosection modelling, mineral thermometers, and zircon petrochronology, the granulitized eclogites discovered firstly at Yadong region at the southern tip of the Yadong–Gulu Rift record five metamorphic stages. The epidote–amphibole eclogite facies metamorphism (M1) is preserved in the garnet core including the relics of omphacite. The peak eclogite facies metamorphism (M2) is represented by the garnet rim and melt‐related polymineralic inclusions. From M1 to M2, the eclogites experienced a prograde path from 1.7 GPa/620°C to 2.1 GPa/750–770°C. The clockwise pressure–temperature path, the occurrence as coherent layers in gneiss, and zircon U–Pb dating imply that the Indian crust subducted to a maximum depth of ~60 km in the central Himalaya, close to the Moho of southern Lhasa block at c. 17 Ma. Afterwards, the exhumation started with decompressional heating. In the high‐pressure granulite facies metamorphism (M3), almost all the omphacite broke down to the symplectites of diopside and plagioclase. The subsequent two‐pyroxene granulite facies metamorphism (M4) is characterized by the intergrowth of clinopyroxenes and orthopyroxenes, high‐Ti amphibole, and antiperthite. The exhumed eclogites eventually reached ultrahigh‐temperature conditions of ~0.8 GPa/950–1,000°C in M4 and were near completely transformed into mafic granulites. At the final stage, these granulitized eclogites cooled in the middle crust to amphibolite facies (M5) of 0.6 GPa/700–750°C. Given the spatio‐temporal consistencies between the granulitized eclogites, post‐collisional magmatism, and north‐trending rifts, the heat source of the ultrahigh‐temperature metamorphism was attributed to the upwelling of asthenospheric mantle due to slab tear of subducted Indian lithosphere. The granulitized eclogites from Yadong evolved from the high‐pressure eclogite to the ultrahigh‐temperature granulite facies, which recorded the transitions from collisional convergence to post‐collisional extension during their exhumation.

Abiotic passive nitrogen and methane enrichment during exhumation of subducted rocks: Primary multiphase fluid inclusions in high‐pressure rocks from the Cabo Ortegal Complex, NW Spain

Abstract: Primary multiphase fluid inclusions (MFI) were studied in one eclogite and two granulites from the Cabo Ortegal Complex (COC, NW Spain) by means of Raman imaging, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM‐EDS) and Focused Ion Beam ‐ Scanning Electron Microscopy (FIB)‐SEM. Complementary, secondary MFI in pyroxenites from COC were also investigated. MFI hosted in eclogite and granulites occur along growth zones or in 3D clusters in garnet porphyroblasts suggesting a primary origin at high‐pressure (HP) metamorphic conditions. The mineral assemblage of MFI is mainly composed of Fe‐Mg‐Ca‐carbonates and phyllosilicates ± graphite ± quartz ± corundum ± pyrite ± apatite ± rutile and a fluid phase composed of nitrogen ± methane ± carbon‐dioxide. The mineral proportions vary among the lithologies. Dominant carbonates and hydrous silicates are interpreted as step‐daughter minerals (crystals formed in the MFI after entrapment as a result of fluid–host interaction), whereas apatite, quartz and rutile are considered in part as accidentally trapped minerals since they also occur as crystal inclusions together with MFI in each rock type. Quartz and corundum occur together in MFI in ultramafic granulite and are regarded as step‐daughter minerals in this lithology. These observations suggest that the MFI are products of post‐entrapment reactions of a homogeneous COHN fluid system with the host mineral. Thermodynamic calculations in the CaFMAS‐COHN system confirmed that bulk composition of the MFI in eclogite is similar to the host garnet+COHN composition except for a potential lost of H2O. Carbonation and hydration reaction between the host (i.e. garnet or pyroxene) and the fluid inclusion results in the consumption of all CO2 and part of the H2O from the fluid phase producing Ca‐Fe‐Mg‐carbonates and hydrous step‐daughter minerals, mostly pyrophyllite and chlorite. Nitrogen content of the originally trapped COHN fluid in eclogite was estimated to have a maximum value of 10 mol% at peak HP conditions and 30–40 mol% at retrograde conditions that is within the range of the observed MFI in the residual fluid (13–68 mol%). Pseudosection modelling confirmed the stability of the phase assemblage in the MFI in a specific low‐pressure, low‐temperature stability field (between 300°C and 400°C at pressures < 1 GPa), caused by H2O‐ and CO2‐consuming reactions possibly in a single step. Our findings indicate that such processes in the exhuming HP units may play a role in global nitrogen and carbon cycling as well as potentially contributing to nitrogen and methane supply to subsurface–surface environments during devolatilization in the forearc regions of convergent plate margins.

A continental back-arc setting for the namaqua belt: Evidence from the kakamas domain

Abstract: A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from ∼1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at ∼1350 Ma occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at ∼1290–1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at ~1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between ∼2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 1350–1240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at ∼1210 Ma (peak conditions: 4.5–6 kbar and 770–850 °C), associated with voluminous, predominantly S-type granitoid orthogneisses between ∼1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until ∼1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a ‘hot orogen’ developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts. Ductile D2 deformation resulted from “bottom-driven” tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at ∼1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between ∼1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080–1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at ∼990 Ma.

Garnet, zircon, and monazite age and REE signatures in (ultra)high‐temperature and high‐pressure rocks: Examples from the Caledonides and the Pamir

Abstract: Rare earth element (REE) signatures of high‐U/Pb and high‐Th/Pb accessory minerals are typically used to link their ages to specific petrological processes (‘petrochronology’)—most notably the growth or breakdown of garnet. Although this approach is powerful, gaps in our understandings of REE systematics in high‐grade rocks exist, particularly regarding the degree to which these minerals chemically equilibrate at extreme conditions. To investigate this, we performed comparative chronology and REE analysis of garnet, zircon, and monazite in (1) a fluid‐rich, ultrahigh‐pressure (UHP) migmatite from the Western Gneiss Region, Norway, and (2) dry felsic granulite xenoliths from the Pamir, Tajikistan. Zircon and monazite from the hydrous migmatite provided ages of 450–370 Ma and a range of Gd/Yb values across this time span. A Lu‐Hf garnet age of 422 ± 2 Ma coincides with the age at which zircon and monazite exhibit the highest Gd/Yb values, as well as the largest range therein. The degree of dispersion in these values is substantial, especially for monazite. Zircon and monazite in the dry xenoliths provide age clusters between 50 and 11 Ma, recording pulsed growth and recrystallization. The Lu/Hf garnet ages for these samples are 41–38 Ma. The accessory minerals of that age are texturally associated with garnet, yet have the lowest, not the highest, Gd/Yb values. In both cases, there is evidence that co‐genetic garnet and accessory minerals achieved REE equilibrium during growth. However, the age and compositional record of chronometric accessory minerals that were co‐genetic with garnet are distinctly different between the two case examples. In the hydrous migmatite, supply‐limited garnet growth in the presence of fluid phase resulted in strongly zoned garnet and a correspondingly large range in Gd/Yb values among co‐genetic zircon and monazite. The range in Gd/Yb values match among these phases and collectively capture the strong fluctuations in the REE composition of matrix micro‐domains caused by garnet growth. In this case, it is the range in Gd/Yb values, rather than a specific composition that is diagnostic for garnet growth. In the felsic granulites, garnet and accessory minerals growth occurred in a fluid‐limited regime, in which short fluid pulses triggered reactions that had been likely significantly overstepped. The seemingly contradictory pattern of increasing Gd/Yb after garnet growth in these samples is the result of garnet having high Gd/Yb and continued zircon growth forcing further HREE depletion of the matrix with time. The situation that high, rather than low, Gd/Yb values in zircon and monazite indicate equilibrium with garnet may be common in dry granulites and other anhydrous high‐temperature rocks. Our findings provide an improved framework for the reliable identification of the accessory minerals that equilibrated with garnet, even when the REE‐age record of these minerals is complex, dispersed, and seemingly contradictory. In addition, the combined zircon‐, monazite‐, and garnet‐age and REE‐record refines the regional tectonic interpretation of our samples.

Supplemental Material: Early Paleozoic Cascadia-type active-margin evolution of the Dunhuang block (NW China): Geochemical and geochronological constraints

Abstract: Table S1: U-Pb LA-ICP-MS data for zircons from granulite and amphibolite. Table S2: U-Pb LA-ICP-MS data for zircons from metapelite and metapsammite. Table S3: Zircon trace-element compositions from granulite and amphibolite. Table S4: Hf isotopic compositions for zircons from granulite and amphibolite.

Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks

Abstract: Abstract Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P – T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.

Thorium zoning in monazite: A case study from the Ivrea–Verbano zone, NW Italy

Abstract: Metamorphism and partial melting of the lower crust is commonly assumed to cause depletion in heat producing elements (HPEs; K, U, Th). In the deep crust, volumetrically subordinate metasedimentary layers, which are source to crustal granites (sensu lato), host the majority of Th ± U, primarily within the REE + Th + U + Y phosphate mineral monazite. We examine the spatial and temporal distribution of Th within monazite grains in metasedimentary rocks from the lower crustal section of the Ivrea–Verbano Zone (Italy), using textural, compositional and geochronological data. We link this to outcrop and regional scale trends described by in‐field gamma‐ray spectrometry data (in‐field GRS) for the purpose of understanding how Th distribution is controlled by progressive metamorphism and partial melting. In‐field GRS data shows that the whole rock budget of Th does not change between granulite facies rocks and their unmelted equivalents but is significantly lower in rocks that have undergone more significant melt loss at ultra‐high temperature (UHT) conditions. Concurrently, the bulk Th budget of monazite increases with metamorphic grade to granulite facies conditions and is greatly reduced in UHT samples. Monazite geochronology returns dates mostly in the range 240–320 Ma with two main peaks at circa 290 and 270 Ma. Textural and chemical constraints indicate that these dates record the timing of pre‐peak to peak metamorphic conditions. Amphibolite facies monazite compositional zones are absent from granulite facies monazite, in contrast to examples from lower‐pressure terranes. This is consistent with the expanded stability of allanite relative to monazite with increasing pressure having an important role in determining the internal structure, composition and extent of inheritance of monazite in going from amphibolite facies to granulite facies rocks. We propose high‐pressure granulites should preserve less monazite inherited from amphibolite facies conditions than low‐pressure granulites. Monazite is preserved at all metamorphic grades and presents a mineralogical mechanism for retaining Th in residual deep crust during partial melting and after melt loss.

Zircon age depth-profiling sheds light on the early Caledonian evolution of the Seve Nappe Complex in west-central Jämtland

Abstract: The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths. The earliest of the deep subduction events has been dated to ca. 500–480 Ma. Evidence of this event has been reported from the northern exposures of the SNC. Farther south in the central and southern segments of the SNC, (ultra)high-pressure rocks have yielded younger ages in the range of ca. 470–440 Ma. This study provides the first record of the early Caledonian event in the southern SNC. The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tväråklumparna microdiamond-bearing gneiss. These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6 ± 3.8 Ma. A second, chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3 ± 3.6 Ma, which corroborates previous geochronological evidence for granulite facies metamorphism at this time. Based on these results, we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician, but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism. • First record of the early Caledonian event in the southern SNC in central Jämtland. • –U-Pb zircon dating of Tväråklumparna microdiamond-bearing gneiss. • Two chemically-distinct zircon overgrowths record two high grade metamorphic events. • Zircon depth-profiling applied to polymetamorphic rock.

New Insight From the First Application of Ti-in-Quartz (TitaniQ) Thermometry Mapping in the Eastern Khondalite Belt, North China Craton

Abstract: The thermal regime of the lower crust is a critical factor that controls crustal anatexis, high-grade metamorphism, and granite formation, which finally results in crustal differentiation. However, the large-scale thermal regime in the Precambrian continental crust is generally not well established. In this study, we first applied Ti-in-quartz (TitaniQ) thermometer to map the temperature variation in the lower crust within an area of ∼10,000 km2 in the Paleoproterozoic eastern Khondalite Belt, North China Craton. The studied rocks are aluminous gneisses/granulites, which contain abundant quartz that generally coexists with rutile. The results show that matrix-type quartz with substantial rutile exsolution generally contains the maximum Ti concentration, which is <300 ppm higher than that of inclusion-type quartz. This result suggests that two quartz types probably formed at the prograde and near-peak to early cooling metamorphic stages, respectively. Therefore, the temperature mapping result based on the maximum Ti concentrations of the matrix-type quartz can better represent the thermal regime than inclusion-type quartz. Our regime shows that the hottest Paleoproterozoic lower crust is underneath the Liangcheng-Heling’er-Zhuozi area, where ultrahigh-temperature (UHT) metamorphism is closely associated with abundant charnockite. The hottest region may represent the root of an ancient large hot orogeny. Our study provides a new insight into the formation of UHT metamorphism.