Amphibolite Facies Conditions Research Articles

Middle Paleozoic metamorphic and sedimentary events in the central Korean Peninsula and their geological implications

The Middle Paleozoic tectonic evolution of the central Korean Peninsula (Gyeonggi Massif, Hongseong-Imjingang, and Okcheon zone of Okcheon Belts) remains controversial. Particularly, the occurrence of high-grade metamorphism and sedimentation need further examination. In this study, we conducted zircon U-Pb-rare earth element analyses from in the ultramafic-mafic complex in the central Korean Peninsula (Cheonan and Gapyeong areas) and the Paleozoic metasedimentary rocks in the Okcheon Zone of the Okcheon Belt to constrain the timing of maximum depositional and metamorphic ages. We also examined the metamorphic P-T-t path from garnet-bearing amphibolite in the central Korean Peninsula by pseudosection modeling and geothermobarometer. The results show that (i) some of the ultramafic-mafic complex and metasedimentary rocks in the central Korean Peninsula formed during the Middle Paleozoic (ca. 450–374 Ma); (ii) garnet-bearing amphibolite underwent successive metamorphism from amphibolite facies condition (7.5–8.0 kbar and 540–630 °C) at pre-peak stage to granulite facies condition (10.9–11.8 kbar and 740–820 °C) at peak stage, and then retrograded into amphibolite facies condition (5.7–7.7 kbar and 530–670 °C) along the clockwise P–T path during ca. 403–362 Ma. This and previous studies suggest that the central Korean Peninsula underwent subduction-related orogenic events during the Middle Paleozoic era, and it is well correlated to those of the orogenic events in the North Qinling belt.

The Permian Watershed tungsten deposit (northeast Queensland, Australia): fluid inclusion and stable isotope constraints

The Watershed scheelite deposit is located in an extinct fore-arc basin in the Mossman Orogen of North Queensland. This fore-arc region comprises multiply deformed, Ordovician–Silurian metasedimentary rocks of the Hodgkinson Formation, and it was intruded by Carboniferous–Permian granites of the Kennedy Igneous Association. At Watershed, the Hodgkinson Formation includes strongly deformed skarn-altered conglomerate, psammite and slate units, which record four deformation events that evolved from ductile (D1–3) to brittle–ductile (D4). Early, D1–2 scheelite mineralisation in Carboniferous monzonite and skarn-altered conglomerate formed during regional prograde metamorphism, which reached upper greenschist to lower amphibolite facies conditions. Permian, D4 scheelite mineralisation was deposited in transtensional, shear-related veins, vein haloes and skarn-altered conglomerate during retrograde, lower greenschist facies metamorphism. During D4, four stages of retrograde alteration (retrograde stages 1–4) affected the rocks. Fluid inclusion assemblages in retrograde stage 2, vein scheelite and quartz are characterised by a low-salinity H2O–NaCl ± CH4 fluid (XCH4 <0.01, 1.4–8.0 wt% NaCleq). The fluid inclusions show evidence for fluid mixing between a low (∼0 wt% NaCleq) and a medium (<8 wt% NaCleq) saline fluid. Scheelite mineralisation P–T conditions were determined at ∼300 °C and 1–1.8 kbar (i.e. a depth of 3.7–6.7 km), indicative of a high geothermal gradient (35–75 °C/km), which was likely caused by the heat from the Permian granites. The presence of pyrrhotite and arsenopyrite in D4 veins (retrograde stage 4), plus graphite and methane in the fluid inclusions in scheelite, indicates reduced mineralisation conditions. Oxygen isotope compositions (δ18OVSMOW) of retrograde stage 2 scheelite (+3.8 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰) indicate a fluid temperature of 306 ± 56 °C with δ18OVSMOW values between +4.7 and +8.3‰. Retrograde stage 3 muscovite δDVSMOW (−73.4 to −62.7‰) and δ18OVSMOW (+11.5 to +13.2‰) values were used to calculate the O–H isotopic compositions of the fluids in equilibrium with the minerals at various possible temperatures (250–300 °C). The results are consistent with a metamorphic origin for the mineralising fluid. Sulfur isotope compositions (δ34SCDT between −2.5 and +2.8‰) for vein-hosted, retrograde stage 4 sulfides indicate that sulfur could have come from seawater or seawater sulfate, which is consistent with the local geology, even though this range overlaps with magmatic sulfur isotope compositions. Metamorphic fluids probably originated from devolatilisation reactions in the Hodgkinson Formation during prograde metamorphism. Permian intrusions acted as heat source enhancing metamorphic fluid flow and metal transport.

Ediacaran to Jurassic geodynamic evolution of the Alborz Mountains, north Iran: geochronological data from the Gasht Metamorphic Complex

AbstractThe Alborz Mountains in north Iran underwent several tectono-metamorphic events during opening and closure of the Palaeotethys and Neotethys Oceans. These events are recorded by rare and discontinuously exposed metamorphic rocks, such as the HP-LT Asalem-Shanderman Complex and the Gasht Metamorphic Complex (GMC), that are considered to have been metamorphosed during the closure of the Palaeotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions, along with smaller volumes of granites. Different dating methods were applied to selected samples of the GMC basement to constrain the geological evolution of this part of the Alborz Mountains. A metagranite yielded two LA-ICP-MS U–Pb zircon ages of 638.4 ± 4.1 Ma and 590.3 ± 4.8 Ma that possibly date protolith crystallisation and later deformation and metamorphism, respectively, and a granite yielded a late Ediacaran 551 ± 2.5 Ma U–Pb zircon crystallisation age. A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for this unit. Derivation of the GMC from Turan Block basement is unlikely, as this has a different crustal makeup and is probably composed of Paleoproterozoic and early Neoproterozoic material. The zircon ages are similar to published ages from the Arabian-Nubian Shield, indicating that this part of the Alborz basement may have belonged to the northern margin of Gondwana in the Neoproterozoic before rifting and drifting away along with other Iranian blocks (the Cimmerian terranes) during opening of the Neotethys Ocean. Chemical Th-U-total Pb ages for metamorphic monazites from two metapelite samples yielded a very large range of spot ages, of which c. 80% falls between 200 and 250 Ma, that do not allow to distinguish between Eo-Cimmerian and Main Cimmerian events in the GMC. However, they may indicate that the amphibolite-facies peak metamorphism of the GMC occurred sometime in the Triassic, in any case much later than the Carboniferous metamorphism in the neighbouring Asalem-Shanderman Metamorphic Complex to the north. Peak-metamorphic amphibole from amphibolite, retrograde white mica and foliation-defining biotite from metapelites and magmatic white mica from granite yielded much younger 175.1 ± 0.5 to 177.0 ± 0.4 Ma 40Ar/39Ar plateau ages. The Toarcian 40Ar/39Ar ages for minerals with different nominal closure temperatures reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastward, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It was probably triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent.

Coesite-bearing garnet xenocrysts from diatexite in the Nordøyane domain, Western Gneiss Region, Norway: Implications for eclogite-melt interaction at ultra-high pressure

Ultra-high-pressure (UHP) metamorphic rocks in the Western Gneiss Region (WGR) of Norway formed in subducted Baltican crust during the Scandian phase of the Caledonian orogeny. In the Nordøyane UHP domain, eclogites locally preserve coesite and microdiamond. Along the north coasts of Haramsøya and Flemsøya, in situ orthopyroxene-eclogite bodies are surrounded by dioritic "diatexite selvages" containing abundant eclogitic enclaves and xenocrysts. In one location eclogite and diatexite are associated with a quartz diorite dyke that is highly contaminated with eclogitic material. The diatexite matrix and dyke have similar plagioclase + quartz + biotite assemblages with abundant embayed and fragmented garnet and clinopyroxene xenocrysts. Coesite is present in some garnet xenocrysts but has not been found in the adjacent eclogite bodies. Pressure-temperature (P-T) estimates from in situ eclogites range from 2.2-3.2 GPa and 810-840 °C, spanning the quartz-coesite transition, with P-T estimates from xenocrysts spanning a wider range. Diorites and retrogressed eclogites record amphibolite-facies conditions, ca. 1.0-1.5 GPa and 700-800 °C. The wide range and large uncertainties in P-T estimates reflect high-temperature re-equilibration and a relative lack of P-sensitive assemblages. Differences in texture, composition, and included assemblages indicate that the xenocrysts were not derived from the nearby eclogite bodies. By implication, their dioritic hosts were not locally derived but must have originated from a different, probably deeper, source. The results are integrated with a numerical model for WGR tectonic evolution to assess the sources of the melts and their possible effects on the exhumation of UHP rocks in western Norway.

Geology, geochronology and tectonic context of the Carreira Comprida Anorthosite in the evolution of the Goiás Massif, Porto Nacional Domain – Tocantins Province

The Carreira Comprida Anorthosite (CCA) is exposed in the Paleoproterozoic Porto Nacional High-Grade Metamorphic Terrene, in the north of the Goiás Massif, central Brazil, represented by an ancient pluton affected by late Neoproterozoic tectonics, with protoliths predominantly of anorthosite consisting of more than 85% labradorite (An60-65), with subordinate presence of clinopyroxene, quartz, hornblende and garnet. There is evidence of intrusive relationships in paragneisses country rocks of the Morro do Aquiles Formation, due to the effects of high-temperature contact metamorphism (∼750 °C) that reached the hornblende hornfels facies. Features of a magmatic nature are present as a cumulate texture formed by the accumulation of tabular plagioclase megacrystals by flotation associated with gravitational action due to the difference in magma density and magmatic flow, with clinopyroxene and interstitial (intercumulated) quartz, during the emplacement of the pluton and crystallization of magma. At the end of the Neoproterozoic, the body was affected by the tectonics of the transcontinental strike-slip system of the Transbrasiliano Lineament (Porto Nacional Transcurrent Shear Zone). U–Pb dating on igneous zircon crystals indicated an age of 2084 ± 4 Ma, which represents the age of emplacement of the Carreira Comprida Anorthosite in the Rhyacian, characterizing an event of gabbroic-anorthosite magmatism not known in the Goiano Massif. Lu–Hf isotopic data show variable εHf (2.08 Ga) values ranging from −1.3 to +4.5 and Hf-TDMC model ages with two crust formation episodes, one from the Siderian (2.46 - 2.38 Ga) and another in Neoarchean (2.74 - 2.59 Ga) which indicates that their parental magmas were derived from a depleted mantle source and range in composition from uncontaminated to variably contaminated by pre-existing older crust in the formation of the CCA. Considering that the ages of the Hf-TDMC model are similar to those of the orthogranulites from the Porto Nacional Complex, its indication as a possible source of the CCA is suggestive. Field relationships, primary structural features, petrographic and compositional data, geochronological and literature data suggest that the emplacement of the CCA is late-tectonic in relation to the evolution of the high-grade terrain of Porto Nacional, in the Rhyacian. After its crystallization, the CCA was affected by the tectonics of the dextral transcurrent system of the Transbrasiliano Lineament that implemented the Porto Nacional Shear Zone at the end of the Neoproterozoic, imposing strong deformation on the anorthosite and surrounding rocks, generating a variety of mylonites. These transformations generated a diversity of microstructures (stretch lineation, mylonitic foliation, tectonic banding, ribbon quartz, lentiform porphyroclasts, microshear, undulatory extinction, thinning of mechanical twinning, wedge twinning, deformation bands, mantle-core, subgrains, symplectites of garnet), which are suggestive of ductile tectono-metamorphic transformations that reached amphibolite facies conditions. Finally, the CCA is a rare example in Brazil of a massif-type anorthosite developed in the Paleoproterozoic (2.08 Ga). On the other hand, the Rhyacian granite bodies of the Ipueiras Suite (2.08 - 2.07 Ga) located very close to the anorthosite pluton are contemporary with the CCA, allowing them to be considered an association of the Anorthosite-Mangerite-Charnockite (AMC) type.

Sulfide Remobilization in the Metamorphosed Kayad Sedimentary Exhalative Zn-Pb Deposit, Western India: Evidence from Mode of Occurrence, Texture, Hydrothermal Alteration Features, and Trace Element Chemistry

Abstract The Kayad Zn-Pb deposit, situated within the Proterozoic Aravalli-Delhi fold belt in western India, is primarily characterized by sphalerite and galena along with pyrrhotite and chalcopyrite. The mineralization occurs as disseminated ores in quartzite, disseminated/laminated and massive ores in quartz-mica schist, and in pegmatite and quartz veins. The laminated ores conform to the regional schistosity and folding, whereas the massive Zn-Pb ores postdate the pervasive tectonic fabric, accumulating at the fold hinges. The massive ore is characterized by durchbewegung texture, discrete blebs of galena and chalcopyrite in a sphalerite matrix with low interfacial angles, and discrete intergrowths of sulfides and sulfosalts such as pyrargyrite, gudmundite, Ag-tetrahedrite, and breithauptite. Geochemical analyses of sulfides reveal microinclusions of sulfosalts comprising Ag, Sb, Cu, Tl, and As, which are regarded as low-melting chalcophile elements (LMCEs). Hydrothermal alteration is insignificant in the laminated and massive ores but prominent around Fe-Cu ± Zn-Pb and Zn-Pb ± Fe-Cu veins. The alteration assemblages in these veins evince a pervasive K + Na ± Fe alteration, later overprinted by a subsidiary Ca ± Na alteration. We interpret the laminated/disseminated ores to be of syndiagenetic sedimentary-exhalative (SedEx) origin formed within an euxinic basin. Conversely, the textural features, mineralogical composition, lack of associated hydrothermal alterations, and evident structural influence on the emplacement of the massive ores suggest they have been remobilized both via plastic flow and by sulfide partial melting. Temperature estimates of up to 650°C, derived from Ti-in-biotite geothermometry of the metamorphosed host rocks, indicate lower-middle amphibolite facies conditions during regional metamorphism. The initiation of melting at these temperatures was promoted by the desulfurization of pyrite to pyrrhotite in quartz-mica schist, aided by melting point depression due to the presence of LMCEs like Ag, Sb, and As.

Brittle initiation of dissolution–precipitation creep in plagioclase-rich rocks: insights from the Bergen arcs, Norway

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

Monazite and zircon U–(Th–)Pb dating reveals multiple episodes of HT metamorphism in the Cima Lunga unit (Central Alps): implications for the exhumation of high‐pressure rocks

High- to ultrahigh-pressure (HP–UHP) rocks recording high-temperature (HT) > 700 °C are well exposed in the Central Alps, making it an ideal region to study the timing of metamorphic stages and the mechanisms of deep-seated rocks exhumation. Here, we report an integrated dataset of petrological and U–(Th–)Pb dating of metapelites surrounding ultramafic lenses from the Cima Lunga unit. At the interface with ultramafics preserving (U)HP–HT assemblages (1.5–3.1 GPa, 650–850 °C), metapelites record higher P‒T values (1.3–2.7 GPa, 700–850 °C) and traces of partial melting, whereas the rest of the unit is dominated by amphibolite-facies conditions. U–Th–Pb dating on zircon and monazite from migmatites indicates that partial melting was episodic involving at least two stages at ~38 to 35 Ma and 33–30 Ma, respectively. While the 38–35 Ma stage matches the HP conditions (> 1.5 GPa) and it is recorded around only one lens with scarce volumes of melt, partial melting at 33–30 Ma is witnessed at lower pressure (~1 GPa) and more widely distributed around the lenses, as within the major shear zones. Far from the ultramafics, zircon from the amphibolite-facies metasedimentary rocks record inherited pre-Variscan ages, while monazite ages at ~22 Ma document mineral growth during the Barrovian cooling. Field and petro-chronological evidence highlight that multiple episodes of partial melting locally developed at the rheological interface promoted by the interplay of fluids extracted from the ultramafic lenses associated with shear heating. New evidence suggests that local variation of P‒T equilibria play a significant role during the exhumation history.Graphical

Fluid–Melt–Rock Interaction during the Transition from Magmatism to HT-UHT-Granulite- and Amphibolite-Facies Metamorphism in the Ediacaran Adrar–Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid–melt–rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar–Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P–T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole–plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 ± 0.28 GPa and 975 ± 50°C, and ca. 0.82 ± 0.15 GPa and 894 ± 50°C, to amphibole-facies conditions at ca. 0.28 ± 0.28 GPa and 787 ± 45°C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th–U–Pb zircon ages of the igneous protoliths. An amphibole 40Ar–39Ar cooling age of 499 ± 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole 40Ar–39Ar closure temperatures of 520–555°C were obtained for an age range of 604–499 Ma and an average constant cooling rate of 4.2°C/My, suggesting that the lower-crustal blocks cooled down to the greenschist–amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O–CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

The onset of Neo-Tethys subduction in the Early Jurassic: evidence from the eclogites of the North Shahrekord Metamorphic Complex (Sanandaj-Sirjan Zone, W Iran)

Abstract Geodynamic models implying subduction of continental crust either consider this process happening during collision, when the continental margin of the lower plate attempts subduction, or in pre-collisional stages, when tectonic erosion of the upper plate or subduction of continental extensional allochthons drag continental crust in the subduction channel. In the Zagros orogen (W Iran), high-pressure rocks are known only from the Sanandaj-Sirjan Zone, NE of the Main Zagros Thrust. Here, eclogites of the North Shahrekord Metamorphic Complex suggest subduction of continental crust slices derived from the upper plate (Central Iran) during the onset of the Neo-Tethys subduction along the southern margin of Iran. Eclogites record a clockwise pressure-temperature-time path, with pre-eclogitic epidote-amphibolites-facies phase assemblages preserved in garnet cores, a high-pressure stage, and a subsequent retrogression at amphibolite-facies conditions. By means of forward thermodynamic modelling and 40Ar/39Ar geochronology, the peak metamorphism has been constrained at 1.9-2.1 GPa and 550-600 °C, in the 191-194 Ma time span. The following retrogression during exhumation lasted at least until 144 Ma. Our data suggest that the onset of the Neo-Tethys subduction traces back prior to 190 Ma, involving together with the Neo-Tethys oceanic lithosphere also slices of the upper plate continental crust scraped off by means of tectonic erosion processes.

Dominant slip systems of quartz under lower amphibolite-facies conditions identified from microstructures and CPOs in quartz phenocrysts

Identifying the dominant slip systems in naturally deformed quartz is crucial for understanding continental crust rheology. Although the basal <a> slip system is considered dominant in quartz under upper to middle crustal conditions, its activity is controversial because an oriented nucleation and growth model has been proposed for the c-axis distribution near the minimum strain axis. This study examined the crystallographic orientation and shape of quartz phenocrysts in a deformed granitic porphyry in the Ryoke belt, SW Japan, to clarify the dominant slip systems in naturally deformed quartz at temperature conditions of ∼400–500 °C utilizing optical and electron backscatter diffraction (EBSD) observations. Identified active slip systems include prism <a>, basal <a>, prism [c], and rhomb <a> through misorientation analyses. The aspect ratios of phenocrysts with dominant prism <a> and basal <a> slip systems are higher than prism [c] and rhomb <a> slip systems, indicating similar strengths between prism <a> and basal <a> slip systems, which are weaker than prism [c] and rhomb <a> slip systems under lower amphibolite-facies conditions. The c-axis of phenocrysts with dominant basal <a> distribute at pole figures peripheries, indicating basal <a> activation over the proposed oriented nucleation and growth model.

The P–T–X conditions of staurolite-garnet metabasites, NW Iran: Implications for metamorphism during Arabia-Eurasia collision

The staurolite-bearing Avajiq metabasites (AMB) crop out near the NW border of Iran with Turkey and contain the peak assemblage garnet + Ca-amphibole + staurolite + biotite + chlorite1 + ilmenite. The AMB record peak pressure–temperature (P-T) conditions of >1 GPa at ∼650 °C; the moderately high pressure is consistent with the presence of staurolite in metabasalt. Garnets exhibit a weak prograde zoning, with relatively homogenous cores and a slight increase in Mg and decrease in Ca, Fe, and Mn toward the rim, consistent with garnet growth during a clockwise P-T path. Texturally late margarite + biotite + muscovite correspond to retrograde P-T conditions of ∼0.5 GPa at ∼500 °C. This stage was followed by further retrogression under greenschist facies conditions recorded by late pumpellyite + paragonite ± calcite. Based on the MORB-like composition of the AMB, it has been unclear whether the metabasites are associated with Neo-Tethyan ophiolites in the area or formed within a continental setting. However, the petrologic association of metabasites with micaschist, gneiss and marble and upper amphibolite facies conditions of metamorphism indicate that the basaltic rocks likely metamorphosed during Arabia-Eurasia continental collision, following consumption of the intervening Neo-Tethys Ocean and emplacement of the ophiolitic rocks in the region.

Petrogenesis of Mesozoic pegmatites in the Dahongliutan Li-mineralized belt (Western Kunlun, NW China)

The genesis of pegmatite and the associated Li-mineralization mechanism remain debated. The intimate temporal-spatial relationship between the pegmatites in the Dahongliutan Li-mineralized belt (incl. #509 Daoban and Akshay Li-poor pegmatites, and Akshay Li-rich pegmatite) and the Dahongliutan granite pluton allows us to discuss the origin of pegmatite and the Li-mineralization mechanism by conducting detailed geochemical and geochronological investigations on the two rock units in the Western Kunlun orogen (NW China). The Li-poor pegmatites in the Dahongliutan Li-mineralized belt exhibit similar or much lower Li, Rb, and Cs compared to the adjacent granite, and their εNd(t) values (-8.7–-10.0) are significantly lower than those of the granite (-7.5–-8.1) as well. However, the Akshay Li-rich pegmatite shows extremely high Li, Rb, Nb, Ta, and initial Sr isotopic ratios (0.7223–0.7227, obtained from apatite), in contrast to the Dahongliutan granite (ISr = 0.7108–0.7157). The whole-rock δ7Li values of the Dahongliutan granite, pegmatites, and the wall-rock Bayanharshan Group metapelite are + 0.76–+5.67 ‰, +0.8–+14.4 ‰, and −4.2–+2.8 ‰, respectively. Zircon and cassiterite U-Pb dating yielded 206Pb/238U ages of 216 ± 2 Ma and 195 ± 2 Ma for the #509 Daoban Li-poor pegmatite and Akshay Li-rich pegmatite, respectively. Modeling of trace elements and Li isotopes suggests that the pegmatites in the Dahongliutan Li-mineralized belt did not originate from fractional crystallization of granitic magma. Instead, they may have formed through direct anatexis under amphibolite-facies conditions. Abundant Li-rich claystone, occasionally accompanied by evaporite, may exist in the magma source of the Li-rich pegmatite, which is the key factor controlling pegmatitic Li mineralization.

PT conditions of deformation of the Gol-e-Gohar shear Zone, SW Iran: Insights from analysis of quartz c-axis fabrics, recrystallization mechanisms and syndeformational fluid inclusions

An investigation into the correlation between microstructural characteristics and deformation temperatures of quartz in the middle crust is of utmost importance in yielding substantial knowledge of the nature of deformation zones. Mylonitic rocks from the Gol-e-Gohar Shear Zone were investigated to infer innovative insights into the ductile deformation pressure–temperature (PT) conditions of the Sanandaj–Sirjan metamorphic rocks within the Zagros, southwest Iran. This approach used a multi-method strategy including quartz c-axis fabrics, recrystallization mechanisms and fluid inclusions (Fls) data. Geothermobarometric analysis based on these data highlight deformational PT conditions ranging from ∼400 to ∼500 °C and P < 5 kbar that correspond to ductile deformation with greenschist to lower-amphibolite conditions at depth of ∼ 12–19 km. Measured opening angles of quartz c-axis fabric and dominance of basal < a > slip system indicate crystal plastic deformation occurred in T > 400 °C. Microstructures highlighting the occurrence of Grain Boundary Migration (GBM) confirm temperature of 500–600 °C. The deduced P–T–salinity Fls history of deformation-related fluid inclusions is highlighted by Fls (B-type) with H2O-CO2–rich metamorphic fluids. These fluids were trapped during ductile deformation in temperatures ranging from ∼ 355 to ∼ 535 °C and pressure of P < 5 kbar. The petrofabric analyses show that the mylonitic rock in the Gol-e-Gohar shear zone have undergone metamorphism at the amphibolite facies condition, and then followed by a retrograde greenschist facies metamorphism. These results emphasize the utility of microstructural and fluid inclusion techniques to constrain the history of mylonitic rocks exhumation in the deformation zones.

GEOCHEMISTRY AND PETROGRAPHY OF METAMORPHIC SOLE AMPHIBOLITES FROM THE SLATINA QUARRY, MT. ZRINSKA GORA, CROATIA

The investigated Mt. Zrinska Gora amphibolites are a part of the metamorphic sole of the Banovina ophiolite complex. Mineral composition and other petrographic characteristics of amphibolites and adjacent peridotite were investigated by polarised microscopy and chemical analyses of rocks were obtained by a combination of inductively coupled plasma mass and emission spectrometry. The main minerals in these garnet-bearing amphibolites are amphibole and plagioclase with accessory garnet, sphene, spinel, opaque mineral ± clinopyroxene ± quartz ± actinolite ± zoisite-clinozoisite ± prehnite ± pumpelyite and ± clay minerals. Kelyphitic corona is developed around garnet. The amphibolites have nematoblastic, nematogranoblastic, porphyroblastic and porphyroclastic textures and homogenous to foliated structures. The presence of clinopyroxene in some of the investigated amphibolites points to their possible formation under P‒T conditions of upper amphibolite facies. The greenschist facies retrograde metamorphism and subsequent surface weathering of amphibolites is also evident. The chemical composition of rocks indicates that protoliths of most amphibolites were tholeiitic basalts, initially formed in island arcs and the adjoining back-arc basins. During the intraoceanic subduction in Jurassic Neotethys Ocean basalts/basaltic tuffs of island arc tholeiite affinity and back-arc basin basalts, positioned on the top of the down-going oceanic slab, were welded to the base of the hot over-riding mantle wedge, metamorphosed to amphibolite rocks and obducted together with ophiolites on the Adria continental margine. The new described amphibolite type having peridotite protolith characteristics originated as the consequence of the hydration of the bottom part of the overlying mantle wedge.

P–T Paths of Cooling and Metamorphism under Conditions of Low-Grade Amphibolite Facies in the Xenoliths of Granulites in the Siberian Craton

P–T Paths of Cooling and Metamorphism under Conditions of Low-Grade Amphibolite Facies in the Xenoliths of Granulites in the Siberian Craton

A new insight into metasomatism through mass-density-solid volume variation maps

In order to present a new petrological approach to quantify mass transfer redistribution, especially regarding chemical zonation, published data of a centimeter sized aluminium-rich clinopyroxene from the Bergen Arcs in Norway is used. The sample comes from the island of Holsnøy where the dry granulite-facies protolith underwent hydration reaction under amphibolite-facies conditions. This aluminium-rich clinopyroxene developed kinks during deformation along which fluid has been introduced. It reacted in two different ways: to garnet plus a less aluminous pyroxene along kinks and to chlorite along cleavage planes. Compositional maps from electron probe microanalyzer (EPMA) were coupled to the mass balance equation of Gresens (Gresens, 1967) to quantify for each pixel present in a studied surface area, element gains and losses, density changes and solid volume variations. This new method allows to estimate the solid volume variation associated with the reaction by assuming minimal mass transfer, or by considering an immobile element i.e. aluminium immobile or by preserving the mass during the reaction. Results demonstrate that for all three assumptions made, the actual mass behaviour does not change significantly for major elements as opposed to the solid volume variation and the actual sum of mass transfer. In the high strain domain (kink) the element losses and gains in replacing the aluminium-rich clinopyroxene by garnet, are balanced by the opposite gains and losses associated with the less aluminous clinopyroxene. This implies that the local fluid chemical composition required for the formation of one mineral is equal to that in equilibrium with the second one after the reaction. The same observation can be made regarding solid volume variation that is balanced between garnet and the less aluminous clinopyroxene. Considering now the specific reaction in the kink and outside the kink (chlorite), mass transfer and redox conditions (Fe3+ and Fe2+) tend to behave in the same way supposing a temporal relationship between these two parageneses.

Sm-Nd age determination of eclogites from Sredna Gora Mountains – HP metamorphic event or final retrogression

The eclogites from Sredna Gora Mts. occur as small isolated bodies enclosed in amphibolites, two-mica schists and migmatized gneisses. They have tholeiitic composition and characteristics of MOR basalts. Three equilibrium mineral associations are distinguished in eclogites: (1) high-pressure (HP) – Grt+CPx(Omp)+Zo+Rt; (2) middle-pressure-high temperature (MP/HT) – Opx+CPx+Pl and middle pressure – middle temperature (MP/MT) presented by Amp+Pl+Qzt+Ep+Ilm+Ttn. The P-T conditions of HP stage reached values of 1.6–2.02 GPa and 650–690 °С. After the HP stage and CPx-Pl symplectite formation, the eclogites underwent a short-lived granulite facies overprint under temperatures similar to those of the HP event. The P-T conditions of the retrograde amphibolite facies overprint are in the range 620–650 °C and 0.6–0.8 GPa. The obtained age of 319±28 Ma by Sm-Nd garnet+whole rock method is interpreted as age of the final retrograde metamorphism under P-T conditions of high temperature amphibolite facies.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

THE Fe-Cu DISCONNECT: UNRAVELING A COMPOSITE IRON OXIDE COPPER-GOLD DEPOSIT IN THE OLYMPIC Fe-Cu-Au PROVINCE, GAWLER CRATON

Abstract The northern Olympic Cu-Au province, Gawler craton, Australia, includes a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralization such as the 8.37 million tonne Cairn Hill deposit. Cairn Hill has long been considered a deep, magnetite end member of the iron oxide copper-gold (IOCG) family that is largely represented in the southern Olympic province by the 1590 Ma hematite-dominated Olympic Dam, Carrapeteena, and Prominent Hill deposits. In contrast to the southern district, the deposits in the northern Olympic Cu-Au province are hosted in rocks that experienced multiple phases of high-temperature metamorphism and deformation. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are crosscut by ca. 1515 Ma granitic dikes. A second high-temperature event is recorded by U-Pb monazite geochronology at ca. 1490 Ma and involved deformation and metamorphism along the Cairn Hill shear zone at conditions of 4.6 to 5.3 kbar and 740° to 770°C. The 1490 Ma event reworked the iron lodes and 1515 Ma granitic dikes. However, Cu mineralization at Cairn Hill occurs in brittle fractures and quartz-biotite veins, overprinting the 1490 Ma deformation and metamorphism. Despite a spatial association between magnetite and Cu, the long thermal history that affected magnetite mineralization and the clear petrographic links between magnetite and high-temperature granulite facies minerals contrast with the late, low-temperature hydrothermal Cu mineralization and indicate the two are not paragenetically related. Therefore, the spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralization to create the Fe-Cu deposit observed today. Although this fits within the broad IOCG deposit family, exploration strategies for Cairn Hill-style composite deposits should be distinct from IOCG deposits with cogenetic Fe and Cu.