Metamorphic Recrystallization Research Articles

Fluid-metasomatized rocks with extremely low δ26Mg values in subducted oceanic lithosphere: Implications for mantle Mg isotope heterogeneity and the origin of low-δ26Mg magmas

The subduction of oceanic slabs with diverse lithologies greatly contributes to the geochemical heterogeneity of the mantle. Oceanic metasomatism leads to the formation of rocks with heterogeneous geochemical characteristics, which can be further modified by fluid–rock interactions during subduction zone processing. Investigations of exhumed high-pressure oceanic rocks not only shed light on the compositional variations that occur during oceanic metasomatism and subduction processes but also help in understanding mantle heterogeneity and magma genesis. Here, we present a comprehensive Mg–O–Sr–Nd isotope study on eclogite-facies Mg- and Ca-metasomatized rocks (Ti-clinohumite dikelets and metarodingites, respectively) and their host meta-serpentinites from the Voltri Massif (Ligurian Western Alps). Compared to their Fe–Ti gabbroic protoliths, the Ti-clinohumite (Ticl) dikelet and metarodingite are characterized by high MgO contents (∼32–38 wt%) and CaO contents (∼17–25 wt%), respectively. These Mg- and Ca-enriched rocks both display low δ26Mg values of −0.54 ‰ to −0.43 ‰ and −1.49 ‰ to −0.82 ‰, respectively, while the serpentinite hosts have mantle-like δ26Mg values of −0.26 ‰ to −0.20 ‰. The Ticl-chlorite rind rimming the metarodingite has a low δ26Mg value of −1.00 ‰. No carbonate minerals were found for metasomatized rocks that have low δ18O values ranging from +2.0 ‰ to +3.4 ‰, demonstrating that their low δ26Mg signature cannot be attributed to metasomatism by fluids involved with carbonate components. The increase in MgO content and decrease in δ26Mg in the Ticl dikelets are ascribed to fluid-induced mass transfer through low-δ26Mg fluids buffered by serpentinization of the host ultramafic rocks. The metarodingite has comparable MgO but extremely low δ26Mg values compared to those of the gabbroic protolith. By integrating reported Mg isotope data for low-grade rodingites, we propose that the low δ26Mg values of metarodingites are ascribed to isotopic exchange with serpentinizing fluids during progressive rodingitization reactions at the seafloor. However, the extremely low δ26Mg value of the Voltri metarodingite (as low as −1.49 ‰) may be attributed to further metasomatic modification in the subduction zone. High pressure metamorphic recrystallization of these metasomatized rocks play a limited role in their Mg isotope compositions. Therefore, our results show that the oceanic metasomatized metarodingite and Ticl-bearing dikes hosted in serpentinite display systematically low-δ26Mg features. Such isotopically light Mg compositions can be preserved during subduction to eclogite-facies conditions. Hence, subduction of these metasomatized rocks may lead to the transfer of a low-δ26Mg component into the deep mantle, which was overlooked thus far and may further contribute to the origin of low-δ26Mg magmas.

Fluid-Induced Metamorphism and Deformation at the Eastern Boundary of the Sveconorwegian Province

Abstract The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~ 30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~ 25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540°C to 600°C and 9 to 12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are <20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.

Microfabrics, In Situ Trace Element Compositions of Pyrite, and the Sulfur Isotope Chemistry of Sulfides from the Xitieshan Pb-Zn Deposit, Qinghai Province, Northwest China: Analysis and Implications

The Xitieshan deposit, located in the central segment of the northern margin of the Qaidam Basin, is among the largest massive Pb-Zn sulfide deposits in China. This deposit, along with its ore-bearing rock series known as the Tanjianshan Group, underwent greenschist facies metamorphism due to subsequent orogeny. We investigated the in situ sulfur isotopes of sulfides with different occurrences to define the origin of ore-forming fluids. The δ34S values of sulfides from stratiform ores, massive ores in schist, stockwork ores in marble, schist and discolored altered rocks that constitute a typical double-mineralization structure range from −5.3‰ to +5.6‰ and from −1.7‰ to +32‰, respectively, indicating distinct biological and thermochemical reductions in seawater sulfates. These are similar to the sulfur isotopic characteristics of VSHMS deposits. Pyrite, whose LA-ICP-MS trace element compositions can provide significant information about metallogenic evolution and deposit genesis, is ubiquitous throughout the whole mineralization process. In these stratiform, massive and stockwork ores, three pyrite types were identified: colloform pyrite (Py0), fine-grained anhedral spongy pyrite (Py1) and coarse-grained euhedral pyrite (Py2). The contents of most metallogenic elements, such as Cu, Pb, Zn, Ag, Mo, Mn and Sn, decrease from Py0 to Py2 with the enhancement of metamorphic recrystallization. This suggests that the expelled elements appear as inclusions in primitive pyrite, contributing to the precipitation of new sulfide phases, such as sphalerite and galena. Orogenic metamorphism played an important role in controlling further Pb-Zn enrichment of the Xitieshan deposit. Moreover, there is another mineralization type, primarily occurring as sulfide veins in the undeformed Formation C siltstones of the Tanjianshan Group, which also crosscut early-formed sulfides, showing close-to-zero S isotopic compositions. In this mineralization type, pyrite (Py3) displays high Se/Tl (>10) and Co/Ni (>2.2) ratios, both indicating a minor superimposed post-orogenic magmatic–hydrothermal event.

Tracking a magmatic arc within a confined orogen: New evidence from the Araçuaí orogen (SE Brazil)

By comparison with igneous rock assemblages of well-exposed modern magmatic arcs and their typical calc-alkaline, magnesian, I-type signatures, many deeply eroded magmatic arcs have been characterized worldwide, unraveling ancient plate margins. Since the 1980's, different assemblages of Ediacaran to Cambrian magmatic rocks have been systematically mapped and characterized according to their distinctive petrographic, structural, lithochemical, geochronologic, and isotopic features, allowing to establish the pre-collisional, collisional, and post-collisional stages of the Araçuaí orogen (SE Brazil). Among them, the tonalitic-granodioritic batholiths and stocks with calc-alkaline, magnesian, I-type signatures, rich in dioritic to mafic enclaves, and related metavolcano-sedimentary successions compose a magmatic arc formed in the early Ediacaran, the Rio Doce magmatic arc. Extending for more than 1000 km along the Araçuaí-Ribeira Orogenic System (AROS), the Rio Doce arc shows a hybrid isotopic signature and crustal inheritance, compatible with modern magmatic arcs built on continental active margins. We present new field, petrographic, and lithochemical studies, coupled with U–Pb (LA-ICPMS) geochronology and isotopic (in-zircon Lu–Hf and whole-rock Sm–Nd) analysis, unraveling the least evolved isotopic signatures ever found for tonalitic-granodioritic plutons of the Rio Doce magmatic arc, and a comprehensive data comparison with modern magmatic arcs. The studied Santa Tereza and Jequitibá suites comprise tonalitic-granodioritic and Opx-bearing charnockitic rocks, with calcic-to-alkali-calcic-magnesian I-type signature, associated with gabbroic rocks. Zircon U–Pb data yielded Concordia ages of 612 ± 3 Ma (Santa Tereza) and 594 ± 3 Ma (Jequitibá) for magmatic crystallization, and 580 ± 5 Ma to 571 ± 3 Ma for metamorphic recrystallization, with associated εNd(612Ma) values from −0.06 to −7.05 (TDM model ages: 1.4–2.04 Ga), and εNd(594Ma) values from −6.67 to −8.8 (TDM model ages: 1.64–1.87 Ga). The zircon εHf(t) (+2.3 to −4.8) and Hf TDM model ages (1.33–1.65 Ga) assigned a moderately juvenile to evolved signature for the Santa Tereza suite, while the Jequitibá suite (εHf(t): −1.7 to −36.4; TDM model ages: 1.33–3.21 Ga) shows a more evolved signature. Altogether, data from the Rio Doce arc are similar to typical continental-margin magmatic arcs with the involvement of mantle melts and/or melting of juvenile crust, suggesting a westward slab retreat during ocean-floor subduction within the Araçuaí confined orogen.

Ocean floor and Barrovian metamorphic events in the Mesozoic Peltetec ophiolitic belt (Ecuador)

ABSTRACT The Peltetec ophiolitic belt, exposed along the western margin of the Cordillera Real (Ecuador), comprises tectonic slivers of metamorphosed (greenschist facies) and variably deformed metaperidotites (serpentinites), gabbros (locally cumulate), greenschist and sub-greenschist rocks. New field data, deformation and petrographic observations, whole-rock major element, mineral composition and P-T determinations of the Peltetec metaophiolite allow determining the metamorphic-geodynamic events of a section of the active western margin of Gondwana during the Mesozoic. Two metamorphic events are proposed: (i) ocean-floor metamorphism during the formation of oceanic lithosphere under low-pressure amphibolite facies conditions (M0), followed by (ii) prograde metamorphism up to the greenschist facies along an intermediate (Barrovian) P/T gradient (M1; ~315°C and ~ 3.9 kbar, ca. 23°C/km). The first metamorphic event occurred under near-static stress conditions and rocks did not suffer significant deformation. During this event, pyroxenes were replaced by hornblende while plagioclase recrystallized. The second event produced strongly recrystallized and foliated greenschist facies rocks with a peak assemblage of actinolite + chlorite + albite + epidote + titanite ± phengite. We interpreted that this assemblage developed during the closure of a back-arc basin and the emplacement of the ophiolite onto the continental margin in the late Triassic or possibly by compressive events in the Early Cretaceous and/or Late Cretaceous. Later processes are related to the regional mostly strike-slip Peltetec fault/shear zone that produced displacement and mixing of blocks, as indicated by cataclastic structure present in most of the slivers, but without differentiable metamorphic recrystallization overprint.

Evolution of the neoarchean Kola alkaline granites, northeastern Fennoscandian Shield: insights from SHRIMP-II titanite and zircon U–Pb isotope and rare earth elements data

Abstract An U–Pb isotopic investigation combined with rare earth element data for titanites, zircons and coexisting accessories has been undertaken to gain insight into the formation of the Archean peralkaline granites of the northeastern Fennoscandian Shield and to test the stability of titanite during metamorphic and hydrothermal processes. The obtained set of isotope data shows that whereas more stable zircon retains a memory of the major episodes of granite evolution, the coexisting titanite provides additional information on crystallization, subsequent growth, cooling and alteration of the plutonic complexes. In addition to titanites formed at the magmatic stages of c. 2710 and c. 2650 Ma, the peralkaline granites contain titanite populations which have undergone major resetting at c. 1870 Ma during the burial metamorphism related to the Svecofennian orogeny and the 1760 Ma hydrothermal alteration near contemporaneous with regional metamorphism of c. 1780 Ma. The peralkaline granites which contain c. 2710 Ma titanites also include inherited titanite grains of an age of 2795 Ma. These data support the concept that the titanite can remain a closed system to Pb diffusion at temperatures of peralkaline granite melt (<800°C) as long as the crystals escape magmatic or metamorphic recrystallization.

Mineralogická charakteristika barytové mineralizace z lokality Ruda-hájenka (hornoměstský rudní revír, silezikum)

The previously unknown remnants of old mining activity were recently found at the locality Ruda-hájenka in the SW part of the Horní Město Ore District (Northern Moravia, Czech Republic). Partly weathered fragments of baryte mineralization in association with acid metavolcanites of the Vrbno Group (Devonian, Silesicum) were found in the dump material. Baryte mineralization exhibits a fine-grained marble-like appearance and locally contains minor disseminated pyrite. A detailed mineralogical study also revealed the presence of trace amount of other hypogene sulphides (galena and sphalerite), whose chemical composition is similar to those reported from the Horní Město Ore District. In addition, baryte also contains individual grains or aggregates of quartz, K-feldspar, and muscovite, which are often corroded and replaced by baryte. The silicates contain negligible amount of Ba (up to 0.02 apfu), which is only slightly higher than those in the same mineral phases from the host metavolcanic rock. The isotope composition of sulphur in baryte and pyrite indicates at least two different sources of sulphur (marine sulphate and likely sulphur leached from wall rocks) and isotope disequilibrium between co-existing minerals. The texture and mineral composition of the studied mineralization resemble metamorphosed baryte mineralizations occurring in the wider area (including local stratiform baryte bodies), but the absence of elevated contents of Ba in feldspars and especially in mica contradicts with an idea of Variscan metamorphic re-crystallization of the studied mineralization together with its host rock environment under the greenschist facies conditions. Therefore, one cannot exclude the possibility that it can be a post-metamorphic vein mineralization, which experienced intense deformation and recrystallization at very low temperatures, associated with rejuvenation of fault-related tectonic activity. The supergene mineral phases are represented especially by limonite, less frequent are minerals of the hollandite group (hollandite, coronadite, cryptomelane), anglesite, cerussite, and a not very well identified Ce-mineral, most likely cerianite-(Ce). Supergene origin is also presupposed for rarely found acanthite.

Fine-grained petalite and spodumene dykes in the Stankuvatske Li-deposit, Ukrainian Shield: products of tectono–metamorphic recrystallisation

AbstractThe Palaeoproterozoic (~2.0−1.8 Ga) Stankuvatske Li deposit (Ukrainian Shield, Central Ukraine) represents an uncommon case of recrystallised, fine-grained petalite ± spodumene meta-pegmatite dykes with LCT affinity hosted in amphibolites and meta-ultrabasic rocks. The meta-pegmatite dykes show remnants of primary, pre-metamorphic zoning, with dominant magmatic albite, K-feldspar, quartz, Li-phases (petalite, spodumene, rarely triphylite and montebrasite), and accessory muscovite, fluorapatite, columbite-(Fe), tantalite-(Fe), cassiterite, Ta-rich rutile, zinco- and ferronigerite, gahnite, pyrite, sphalerite and zircon. The parental magma of the meta-pegmatites was peraluminous, and enriched in Li and P, though relatively poor in B and F during the late-magmatic stage. Metasomatic reactions between residual pegmatite magma and (ultra)basic country rocks resulted in the precipitation of holmquistite, triphylite, fluorapatite, tourmaline and Rb–Cs-rich biotite. Secondary generations of fine-grained petalite, spodumene, albite and K-feldspar were formed during post-magmatic stages, i.e. during hydrothermal–metasomatic alteration and/or subsequent tectono–metamorphic recrystallisation of the primary pegmatites. The initial subsolidus metasomatism of primary feldspars took place in alkaline conditions as a result of Na (partly K) for Li exchange.The presence of fibrolitic sillimanite and chrysoberyl, together with the scarcity of muscovite and (OH,F)-bearing minerals, point to metamorphic recrystallisation of the former Li-rich granitic pegmatites at relatively high-temperature and medium-pressure (~600±50°C; ~0.3−0.4 GPa) conditions.

Ore genetic implications of pyrite and sphalerite compositions fromRajpura‐Daribaand ZawarmalaPb‐Zndeposits, Northwestern India

Pb–Zn mineralization at Rajpura‐Dariba and Zawarmala, India, is hosted in the Palaeoproterozoic meta‐volcano‐sedimentary sequence of Aravalli‐Delhi Belt. We have studied different generations of pyrite (Py‐I, II and III) and sphalerite (Sph‐I, II and III), associated with ore mineralization and remobilization stages, from both the deposits. In Rajpura‐Dariba, high concentrations of the majority of trace elements in syngenetic Py‐I and Sph‐I suggest their formation from trace element enriched hydrothermal fluid in SEDEX environment. Enrichment of lattice bound elements in Py‐II (As, Co, Ni) and Sph‐II (Mn, Hg and Cd) during metamorphic recrystallization of early pyrite and sphalerite indicates their local derivation from host rocks. The Pb, Tl, Ag and Sb rich Py‐III and Sph‐III in discordant ore indicate their crystallization from low melting‐point chalcophile elements enriched sulfide melt. In Zawarmala, the trace element poor natures of diagenetic banded Py‐I and Sph‐I suggest that the initial ore‐forming fluid was depleted in trace elements. Enrichment of trace elements in Py‐II (As, Mn, Tl, Cu) and Sph‐II (Mn, Hg, As, Ga) in sphalerite‐pyrite vein ore signify their derivation from carbonate host rocks. The high abundance of trace elements in Py‐III and Sph‐III indicates their precipitation at lower temperatures from a trace element enriched hydrothermal fluid during the massive galena‐sphalerite ore formation. Pyrite and sphalerite from global SEDEX are trace elements enriched as compared to MVT deposits, where their compositions have remarkably correlation with host rocks. Our study highlights the influence of host rock in pyrite and sphalerite compositions and their ore genetic implications.

Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

AbstractWe present data on the pressure and temperature (P–T) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three‐dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak P–T conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous P–T constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of <5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate‐limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.

Tectonics and geothermal gradients from subduction to collision in the NW Variscan Iberian Massif

ABSTRACT The earliest tectonometamorphic record of tectonic slices incorporated to the base of an orogen holds the key to understand how an orogen is built. The tectonic pile of the NW Iberian section of the Variscan Orogen includes tectonic slices separated by crustal-scale thrusts. The earliest tectonometamorphic record in the uppermost parautochthon is calculated at 11–14 kbar and 450–500°C (P-T gradient about 13°C/km), suggesting a subduction-related metamorphic recrystallization at lower pressure than the overlying Lower Allochthon. Early conditions calculated in the autochthon (9–10 kbar and 425–450°C; 16°C/km) point to a relatively ‘cold’ collisional setting. Higher thermal gradients obtained from some sections of the autochthon (11–12 kbar and 700–725°C; 21°C/km) and the Lower Parautochthon (7.5 kbar and 550–700°C; 24–31°C/km), correspond to more advanced and ‘hot’ stages of collision. New U–Pb monazite geochronology indicates a 318–311 Ma age for the final formation of HT domes in the region. We propose the rapid decrease in P-T gradient (from <10 to 16°C/km) documents a fail to sustain further burial along a regular subduction zone. We consider the subsequent increase in the geothermal gradient (from 16 to 31°C/km) as the culmination of previous crustal accretion and the onset of crustal underthrusting and later processes in a collisional stage. We propose these switches in the early tectonometamorphic record of individual tectonic slices as potential markers to track the transition from subduction to collision in collisional orogens.

The effect of metamorphism on the aggregate properties of gabbroic rocks

Granitic rocks are durable materials sought after for the production of road and railroad aggregates. Granitic bedrock commonly, however, includes gabbroic components, which may enhance or decrease the aggregate performance. This study evaluates the variation in resistance to fragmentation (Los Angeles value, LA) and wear/abrasion (micro-Deval value, MDE) for the fraction 10/14 mm of gabbro in different metamorphic states. Samples were collected along a 150-km profile where metamorphic conditions grade from epidote–amphibolite to high-pressure granulite-facies, and the degree of metamorphic recrystallization varies with the amount of hydrous fluid. Rocks with no or incipient metamorphic recrystallization preserving their primary igneous fabric and interlocking texture meet the criteria for both asphalt base course and track ballast in railroad, with LA and MDE values below 25% and 14%, respectively. Mafic granulite and fine-grained amphibolite have LA values below 25% and can be used in unbound layers. Mafic granulites crystallize at high temperatures but commonly preserve a relict igneous texture due to limited hydration. Coarse-grained amphibolite and migmatitic amphibolite have the poorest performance. They recrystallized at hydrous conditions, leading to complete recrystallization, grain coarsening, and loss of interlocking igneous texture. This study shows that both temperature and infiltration of hydrous fluids significantly affect the technical properties. Even at high metamorphic temperatures, gabbroic rocks may yield aggregates of high technical performance. At hydrous conditions, however, recrystallization results in rock aggregates suitable for unbound layers only. The variation in metamorphic grade and hydration is easily assessed by the geologist in the field and by using standard petrographic microscopy.

Skarn classification and element mobility in the Yeshan Iron Deposit, Eastern China: Insight from lithogeochemistry

Skarns form by significant fluid-mediated exchange of mass and heat between igneous rocks and their surrounding wall rocks into which they intruded. Quantification of the mass exchange associated with skarn alteration requires knowledge of the skarn protoliths, which are often masked by metamorphic recrystallization and intense calc-silicate metasomatism. To overcome this challenge in characterizing the Yeshan skarn Fe deposit in Eastern China, a cross-section through the complete rock sequence, from the carbonate wall rock to the pluton, was systematically sampled, and analyzed for bulk-rock major and trace elements. Underpinned by the skarn zonation model, ln(SiO2/Al2O3), ln(SiO2/TiO2), and REE + Y values in the skarns were used to distinguish the various skarn protoliths. The effectiveness of the ln(SiO2/Al2O3) and ln(SiO2/TiO2) is supported by the variable mobility of Si, Al, and Ti during magma-derived fluid infiltration into the carbonate wall rocks at Yeshan. The effectiveness of REE + Y is based on their significant concentration differences in the carbonate wall rocks and igneous rocks at Yeshan. These geochemical indexes may be applicable to the characterization of protoliths and mass transfer in skarn deposits where igneous rocks intruded carbonate wall rocks.

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

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.

Sľudy a chlority ako indikátory podmienok metamorfózy karbonátových hornín gelnickej skupiny južného gemerika (Slovenská republika)

Investigated metacarbonates are located in the Early Paleozoic rocks of Southern Gemericum unit, Slovakia. Metacarbonates are part of Vlachovo Formation (Tichá Voda, Stará Voda) and Bystrý potok Formation (Betliar, Čučma, Malá Hekerová, Smolník - Mária Terézia, Holec). Detailed studies indicated metamorphic pressure-temperature (P-T) conditions of 3 - 7 kbar at 330 - 370 °C for the Gelnica Group. The calculated P-T conditions were verified using chlorite geothermometer and phengite geobarometer. Mineral association of metacarbonates consists of Si rich (3.10 - 3.35 apfu) phengitic micas, Fe-Mg chlorites (clinochlore and chamosite), quartz, K-feldspars and accessory fluorapatite. Micas and Mg-Fe chlorites originated from metamorphic recrystallization of clay material with incorporations of basic pyroclastic material during the regional metamorphosis of pelitic protolith, which sedimented continuously with carbonates. Metacarbonates belong to the upper parts of Gelnica Group, which were metamorphosed under the greenschist facies conditions.

Tonian and Cryogenian 40Ar/39Ar hornblende and muscovite ages for the São Gabriel Terrane, Dom Feliciano Belt, southern Brazil

Geochronological 40Ar/39Ar investigations in the São Gabriel Terrane, Dom Feliciano Belt, Brazil, associated with previous structural geology and geochronology studies, allow for a better understanding of Neoproterozoic events in this region. The Sanga da Cachoeira Complex and metasedimentary rocks of Arroio Marmeleiro Complex (U–Pb provenance age &gt; 1.5 Ga) show 40Ar/39Ar spectra with no successful plateau age, attributed to the alteration or excess 40Ar. Tonian (ca. 729 ± 9 Ma) 40Ar/39Ar hornblende age of metadiorites of the Cambaí Complex (U–Pb protolith age ca. 750 Ma) is interpreted to register the age of an early structure in this unit, where hornblende is a mineral of the composite banding fabric formed by remnants of original bedding, locally preserved, and added by metamorphic recrystallization. A cooling rate of at least 5°C/Ma is suggested for the Cambaí Complex, from its crystallization to its metamorphic event. The Cryogenian (&lt;690 ± 8 Ma) 40Ar/39Ar hornblende and muscovite ages belonging to the Imbicuí Complex (protolith age ca. 869 Ma) whose ages are related to the late structure formed by a short and intense activity of the Cerro Branco Shear Zone, which experienced a rapid cooling of 12°C/Ma. The ages from the rocks belonging to such a diverse tectonic setting, including continental and intraoceanic island arc, oceanic crust low‐K amphibolites and meta‐harzburgites, and metasediments, presenting similar nature and fabric orientation, indicate that major tectonic‐magmatic–metamorphic activity was responsible for the accretion of these lithotectonic units that gave rise to the São Gabriel Terrane in southern Brazil.

Zircon and monazite U-Pb ages of the Mashan Complex of the Jiamusi Block of NE China: a link to Gondwana?

ABSTRACT The tectonic affinities of the microcontinents in the eastern Central Asian Orogenic Belt of NE China, particularly the Jiamusi Block, have been hotly debated in the last decade. The Mashan Complex, which is the oldest exposed basement of the Jiamusi Block and consists of khondalitic rocks (felsic granulite, marble, and graphite schist) and orthogneisses, is the key geological unit to look into the tectonic affinity of the Jiamusi Block. In this study, we carried out LA-ICP-MS U-Pb dating on zircon and monazite minerals from the granitic gneisses in the Mashan Complex, to constrain the affinity of the Jiamusi Block. Magmatic zircons from two granitic gneisses yielded weighted mean 206Pb/238U ages of 517 ± 7 Ma and 527 ± 4 Ma, respectively, which are interpreted as the protolith ages. These age data verify a late Pan-African magmatic event in the Jiamusi Block. Metamorphic zircons from one granitic gneiss gave a weighted mean 206Pb/238U age of 510 ± 3 Ma, which constrained the metamorphic time of the Mashan Complex. This metamorphic age is further supported by monazite dating results – the monazites from three granitic gneiss samples yielded weighted mean 206Pb/238U ages of 489 ± 3 Ma, 498 ± 3 Ma, and 500 ± 3 Ma, respectively, which represented the metamorphic recrystallization ages of these monazites. These data consistently indicate a regional metamorphic event immediately after the late Pan-African magmatism in the Jiamusi Block. Our new data, combined with previously published late Pan-African geochronological data from the Jiamusi Block, support that the Jiamusi Block has an affinity to the Gondwana.

Variation in technical properties of granitic rocks with metamorphic conditions

Granitic rocks constitute a global raw material key asset for aggregate production. Their technical performance is, however, highly variable and knowledge on the aggregate functionality of granites in different geological settings is incomplete. This study investigates systematic variations in resistance to fragmentation (Los Angeles value) and wear (Micro-Deval value) for granitic rocks along a 150 km long east–west trending metamorphic gradient across the Eastern Segment of the Sveconorwegian Province in Scandinavia. In essence, the metamorphic gradient represents a transition from pristine granite to stromatic migmatitic orthogneiss and granulite. Along this profile, the aggregate functionality is governed by the behaviour of quartz and feldspar during metamorphic recrystallisation. Two critical metamorphic variables were identified: temperature and hydrous fluids. Recrystallisation at comparably low metamorphic temperatures, ≤ 600 °C, result in irregular grain shapes and fine-grained quartz aggregates that in turn result in low Los Angeles and Micro-Deval values, i.e., high-quality materials for road construction. At higher metamorphic temperatures, > 600 °C, grain coarsening and smoothening of grain boundaries result in significantly higher Los Angeles and Micro-Deval values. At high temperatures, high availability of hydrous fluids promoted extensive partial melting and post-deformational crystallisation which resulted in remarkably poor aggregate performance, with Los Angeles and Micro-Deval values consistently above 40% and 12%, respectively. In contrast, deformation at high temperature but low availability of hydrous fluids resulted in inequigranular textures with high proportions of very fine-sized crystals, interlocking textures, and aggregates with Los Angeles and Micro-Deval values down to 19% and 5%. The consistent variation in metamorphic microtextures correlate with macro-fabrics at the outcrop scale, allowing assessment of aggregate functionality already in the field. For migmatitic gneisses, the proportion and strain state of leucosome are the most critical parameters.

Partial melting of granodiorite: insights from ediacaran granulite-facies metamorphism in Southern Brazil

AbstractIn the Southern Ribeira Belt (Brazil), the Itatins Complex, composed of ortho- and para-derived migmatites record granulite facies metamorphism. Our work focuses on the ortho-derived migmatite, which displays features indicative of partial melting ranging from in-situ melting (cm-thick lenses and veins) to accumulations of melt as leucosomes and patches of diatexite. The unit comprises a biotite felsic granulite (metagranodiorite) with a residual neosome and a leucocratic neosome, generally referred to as leucosome. Phase equilibria modelling using a modified bulk composition from the geochemical standard JG-1 (granodiorite) was carried out to evaluate whether a protolith of granodiorite bulk composition could produce the mineral assemblages observed in the residual neosome and the compositions of melt and residue produced by the partial melting. Isopleth thermobarometry of plagioclase, biotite and orthopyroxene points to P-T conditions of 0.7–1.0 GPa and 845–872 ºC. The modified granodiorite bulk composition generated just 10 mol % melt at these conditions, far lower than the amount estimated from outcrops (around 32 %). A T-XH2O pseudosection shows that increasing H2O content enhances the fertility of the bulk composition and it can become capable of producing melt volumes similar to that observed in outcrop. The melt compositions determined for partial melting of the modified granodiorite bulk composition in open- and closed-systems using THERMOCALC are granitic, and peraluminous. The melts have SiO2 contents between 52.02 mol % and 77.27 mol. % and show XFe values of 0.68 and 0.86. In K2O vs. Na2O+CaO, K2O vs. XFe and K2O-(FeO+MgO)-(Na2O+CaO) space natural samples (5 from the residual neosome and 2 from leucosome) from the Itatins Complex plot in the direction of the trends for melt and residue compositions obtained from modelling of the modified granodiorite bulk composition. The differences between modeled compositions and real rocks are interpreted to result from the interaction of melt with its residual counterpart. Chondrite normalized REE patterns from the residual neosome shows considerably greater depletion of HREE than geochemical standard JG-1 (granodiorite), and the two analyzed leucosomes show lower HREE contents. Zircon rims have REE patterns interpreted to indicate metamorphic recrystallization. U-Pb zircon Concordia ages indicate that the protolith formed during the Paleoproterozoic (2137 Ma) but that metamorphic recrystallization is Neoproterozoic (between 632 and 601 Ma). The metamorphic age pre-dates the intense granitic magmatism that affected the Southern Ribeira Belt between 600 and 570 Ma.

Geology and LA-ICP-MS U–Pb geochronology of the Nova Brasilândia belt, SW Amazonian Craton: New ages, re-evaluation of existing geochronological data, and implications for the evolution of the Sunsás orogen

New geological and LA-ICP-MS U–Pb geochronological data, combined with literature data, are used to characterize more precisely the timing of tectonic events in the Nova Brasilândia belt in the southwestern Amazonian Craton, Rondônia, Brazil. The U–Pb ages of zircon crystals were obtained in the rocks of the Rio Branco domain in the southern part of the belt. They reveal a period of sedimentation between 1241 ± 7.5 and 1137 ± 9 Ma and high-grade metamorphism between 1137 ± 9 and 1127.6 ± 2.3 Ma. Mafic-felsic magmatism of the Rio Branco Suite was dated between 1119.7 ± 2.7 and 1106.2 ± 2.8 Ma. U–Pb ages from specific groups of recrystallized zircon (cores and rims) extracted from high-grade calc-silicate gneiss (1019 ± 15 Ma), metagabbro (1016.7 ± 3.9 Ma), and anatectic granite (1011.3 ± 9.6 Ma) may represent episodes of metamorphic recrystallization and the youngest migmatization. The LA-ICP-MS U–Pb zircon ages establish the timing of two orogenic phases: an accretionary phase between approximately 1137 and 1106 Ma and overprinted metamorphism during the final collisional phase between approximately 1096 and 1011 Ma. The Nova Brasilândia orogeny recorded on the southwest margin of the Amazonian Craton can be described as an accretionary-collisional type that developed between approximately 1137 and 1010 Ma.