Amphibolite Facies Metamorphism Research Articles

Formation of Gold Mineralization under Amphibolite Facies Metamorphism: Ykan Deposit (Baikal–Patom Belt)

Formation of Gold Mineralization under Amphibolite Facies Metamorphism: Ykan Deposit (Baikal–Patom Belt)

Laser ablation (in situ) Lu-Hf geochronology of epidote group minerals

Epidote group minerals, including allanite, clinozoisite and epidote are common in a range of metamorphic, igneous and hydrothermal systems, and are stable across a wide range of pressure–temperature (P–T) conditions. These minerals can incorporate substantial amounts of rare earth elements (REEs) during their crystallisation, making them potential candidates for Lu–Hf geochronology to provide age constraints on various geological processes. Here we report on a first exploration into the feasibility of in situ Lu–Hf geochronology for epidote group minerals from various geological settings and compare the results with age constraints from other geochronometers. Magmatic allanite samples from pegmatites and monzogranites in the Greenland anorthosite complex, Coompana Province and Qingling Orogen provided dates consistent with magmatic events spanning from c. 2660 to 1171 Ma. In the Qingling pegmatites, a younger phase of hydrothermal allanite was dated at c. 215 Ma, consistent with the timing of regional REE mineralisation. Allanite from the Yambah Shear Zone, Strangways Metamorphic Complex, yielded Lu–Hf age of c. 430 Ma. It predates the garnet and apatite growth at c. 380 Ma, suggesting the Lu–Hf system can be preserved in allanite during prograde amphibolite-facies metamorphism. Additionally, Lu–Hf dates for hydrothermal clinozoisite and epidote are consistent with the timing of hydrothermal alteration and mineralisation in a range of settings, demonstrating the utility of the technique for mineral exploration. Despite the current lack of matrix-matched reference materials, the successful application of laser ablation Lu–Hf geochronology to epidote group minerals offers valuable geochronological insights into various geological processes that can be difficult to access through other geochronometers.

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.

Selective metasomatism of ultramafic cumulates within Archean supracrustal sequences

The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record.

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39−2.28 Ga magmatism and 1.93−1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452−439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

Multistage mineralizing episodes of the Proterozoic world-class Volta Grande gold deposit, Amazonian Craton, northern Brazil: Implications for the Bacajá Domain metallogenesis

The easternmost region of the Amazonian Craton in northern Brazil has been the focus of several mining exploration surveys, which led to the identification of the world-class Volta Grande gold deposit (∼6.0 Moz@1.02 g/t). The deposit is inserted in the Bacajá Domain (2.24–2.0 Ga), and part of the mineralization occurs in Proterozoic mylonite, gneiss, and associated metamafic rocks that underwent greenschist to amphibolite-facies metamorphism. Electrum (Au–Ag) occurs as grains in cm-sized quartz high-grade veins and veinlets (up to 34.1 g/t in some rock intervals) that follow the mylonitic foliation and are associated with pervasive carbonate alteration, synchronous with the dynamic metamorphism. Disseminated electrum grains, pyrite, chalcopyrite, arsenopyrite, tellurides, and galena occur associated with potassic, argillic, propylitic, and silicic alteration types mainly in fracture-controlled or pervasive styles. These geological features as a whole suggest an orogenic high-tonnage orogenic gold system. Isotropic core samples along a stratigraphic profile reveal late andesite, dacite, rhyodacite, and rhyolite lava flows and dikes and plutonic rocks such as quartz monzonite, granodiorite, monzodiorite, and subordinate microgranite crosscutting the older metamorphic rocks. These rocks reveal potassic, propylitic, argillic, silicic, and sericitic alteration types. Electrum grains, possibly related to boiling deposition, occur along cm-sized quartz, calcite, and adularia high-grade veins (up to 29.5 g/t in some rock intervals) with chalcopyrite, pyrite, arsenopyrite, tetradymite, and tellurobismuthite. Such mineralogy is compatible with a low-sulfidation epithermal system. Data integration points to at least two distinct mineralizing events leading to the Volta Grande gold deposit – orogenic gold and low-sulfidation epithermal – which explains such large-tonnage and high-grade ore accumulation. It configures a new exploration guide for the Bacajá Domain of the Amazonian Craton, attested by other worldwide regions with similar metamorphic, magmatic, and hydrothermal alteration characteristics.

Provenance of Detrital Rutiles from the Triassic–Jurassic Sandstones in Franz Josef Land (Barents Sea Region, Russian High Arctic): U-Pb Ages and Trace Element Geochemistry

Provenance study plays an important role in paleogeographic and tectonic reconstructions. Detrital zircons are commonly used to identify sediment provenance; however, a wide range of detrital zircon ages in clastic rock often represent a fingerprint of reworked older terrigenous successions rather than ages of magmatism and metamorphism in the provenance area. This study focuses on the provenance of detrital rutile grains in the Triassic–Jurassic sandstones from Franz Josef Land and shows the importance of multiproxy approaches for provenance studies. Trace element data demonstrate that most rutile grains were sourced from metapelitic rocks, with a subordinate population having a metamafic origin. The Zr-in-rutile thermometer and U-Pb geochronology suggest that detrital rutile grains were predominantly derived from rocks that underwent amphibolite facies metamorphism during the Paleozoic era, with a predominance of the Carboniferous–Permian ages. Therefore, we suggest that the provenance area for the studied sandstones on Franz Josef Land has a similar geological history to the Taimyr region and Severnaya Zemlya archipelago. We propose that this crustal domain extends across the Kara Sea and forms the basement to the north and east of FJL, representing a proximal provenance for the studied Mesozoic terrigenous rocks. This domain experienced both Middle–Late Ordovician and Carboniferous–Permian metamorphism. The comparison of U-Pb dating and the geochemistry of rutile, U-Th/He, and U-Pb dating of zircons showed that detrital rutiles are the powerful toll in provenance restoration and can give additional constrains when a provenance area locates within collisional-convergent settings.

Age and genesis of the Tongshan banded iron formation in the Zhongtiao region, North China Craton

Banded iron formations (BIFs) occur extensively within the late Neoarchean supracrustal successions of the North China Craton (NCC). The Zhongtiao region is located on the southern margin of the NCC and is a terrane that may host BIFs. The region is well known for exposed early Precambrian rocks, such as the Sushui Complex, which can be divided into three lithological units: Neoarchean tonalite-trondhjemite-granodiorite (TTG) rocks, Paleoproterozoic multi-phase granitoids, and supracrustal rocks (i.e., the Sushui Group). Despite previous studies focused on the granites within the Sushui Complex, limited information is available on the supracrustal rocks and the potential for associated BIFs that have undergone amphibolite-facies metamorphism. Here, for the first time, we conduct petrological, geochronological, and geochemical studies on a newly discovered BIF layer (i.e., the Tongshan BIF) and the associated metamorphosed volcanic and sedimentary rocks in the Sushui Group. This study constrains the formation age of the Tongshan BIF, its source characteristics, and the redox state of the depositional environment. SHRIMP zircon U–Pb analyses on biotite plagioclase gneiss (the footwall rock of the BIF) demonstrate that the Tongshan BIF was formed at ca. 2583.1 ± 9.8 Ma. The Tongshan BIF, occurrs in a volcano-sedimentary succession, and may have been deposited in a continental margin arc along the southern segment of the NCC and may best be considered as an Algoma-type BIF. Given a 2.58 Ga depositional age for the Tongshan BIF and the Sushui Group, and the more restricted depositonal nature of Algoma-type BIFs, this work highlights the potential for the identification of additional BIF layers in the Zhongtiao region.Two distinctive sedimentary facies are recognized in the Tongshan BIF based on mineral composition: an oxide facies BIF, composed of magnetite and quartz, and a silicate facies BIF, composed of magnetite, amphibole, and quartz. The Tongshan BIF displays distinct seawater-like, shale-normalized REY characteristics, including super-chondritic Y/Ho ratios, HREE enrichment relative to LREE, and positive La, Gd, and Y anomalies. Consistently positive Eu anomalies are also observed, indicating the influence of a high-T hydrothermal input. No true negative Ce anomalies occur in the Tongshan BIF, suggesting a dominantly anoxic water column contemporaneous to deposition. Further, the lack of true negative Ce anomalies extensively occurring in the Tongshan BIF is consistent with other BIFs observed globally, and confirms that ferruginous (anoxic and Fe-rich) conditions were maintained in deep seawater before to the GOE.

Mid-Neoproterozoic (ca. 845 Ma) metamorphism of the southwestern Yangtze Block and its tectonic implications

The evolutionary history of the Yangtze Block plays an increasingly important part in reconstructing Proterozoic paleogeography and associated supercontinent cycles. We document mid-Neoproterozoic amphibolite facies metamorphism related to crustal thickening and possible terrane accretion within an accretionary orogen along the southwestern margin of the Yangtze Block. Kyanite-staurolite bearing schists in the Dahongshan Group record a main deformation stage and a peak metamorphic assemblage Grt rim + Ky + St + Bt + Ms + Rt + Pl ± Ilm. Petrography and phase equilibrium calculations suggest a clockwise P-T path under medium-pressure facies condition. Monazite U-Th-Pb ages from the Dahongshan Group constrain the timing of peak metamorphism to 845 ± 6 Ma (n = 33, MSWD = 1.01), which overlaps the metamorphic climax in the northern segment of the southwestern Yangtze Block. Based on the comprehensive analysis of geochemical, metamorphic and sedimentary data from the Yangtze Block, we consider that the outboard domain occupied by the Dahongshan Group along with the Hekou, Huili, Julin (Yuanmou) groups constitutes a terrane(s) that was separated from the rest of the Yangtze Block by a marginal sea formed at the end of the Mesoproterozoic. Mid-Neoproterozoic re-accretion to the inner Yangtze Block during ongoing convergent plate margin activity resulted in the deformation and metamorphism of these rock units. Similarities in the Neoproterozoic evolution of the Yangtze Block, the Seychelles, and the western margin of Greater India suggest a protracted history of oceanic plate subduction periodically interrupted by coupling across the plate margin driving deformation and metamorphism.

EVIDENCES OF MINERAL MELTING IN THE ORES OF THE SVETLINSK GOLD DEPOSIT, SOUTH URALS, RUSSIA

For the large Svetlinsk gold deposit (South Urals) evidences of partial melting of minerals and possible participation of polymetallic melts in the concentration and redistribution of gold and other metals are given. Finding of bismuth and antimony minerals in ores, among which there are gold minerals new to the deposit (pampaloite, montbrayite and aurostibite), specific mineral intergrowths (polymineral Sb–Bi–Pb–Te–Ag–Au drop inclusions), enrichment of early sulphides with Low-Melting-point Chalcophile Elements (LMCE), high formation temperatures for ore assemblages (up to 400°C), as well as the occurring metamorphism of amphibolite facies indicate the possibility of the formation of such melts. Polymetallic melts at the deposit could be formed both by partial melting of early sulphides and directly from hydrothermal fluids. The signs of melting also include simplectites of calaverite and native gold in the marginal parts of the large montbrayite grain.

Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.

Evidence for a pre-Himalayan metamorphism in the High Himalayan Crystalline of the Miyar Valley (NW India)

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle.

Constraints on the Palaeoproterozoic tectono‐metamorphic evolution of the Lewisian Gneiss Complex, NW Scotland: Implications for Nuna assembly

AbstractDespite extensive investigation, the tectono‐thermal evolution of the Archean crust in the Lewisian Gneiss Complex in NW Scotland (LGC) is debated. Most U–Pb zircon geochronological and metamorphic studies have focused on rocks from the central region of the mainland LGC, where granulite facies assemblages associated with the oldest (Badcallian) tectono‐metamorphic event at c. 2.75 Ga are overprinted by younger amphibolite facies assemblages related to the Inverian (c. 2.5 Ga) and subsequent Laxfordian (c. 1.9–1.65 Ga) tectono‐thermal events. In the southern and northern regions of the mainland LGC, deformation and metamorphism associated with the Laxfordian event are pervasive, although the timing and conditions are poorly constrained. Here, we present new field, petrographic and structural data, U–Pb zircon and titanite geochronology and phase equilibrium modelling of amphibolite samples from the northern and southern regions. Our field observations show that in both regions, pre‐Laxfordian structures are significantly reworked by steep NW‐striking fabrics that are themselves pervasively overprinted by co‐axial deformation and amphibolite facies metamorphism related to the Laxfordian event. In situ U–Pb titanite geochronology yields Laxfordian ages of 1853 ± 20 Ma in the southern region (P = 6–8 kbar and T = 640–690°C) and 1750 ± 20 Ma and 1776 ± 10 Ma in the northern region (P = 6–7.5 kbar and T = 740–760°C). While U–Pb dating of zircon rims from felsic gneisses in the central region shows a dominant Inverian metamorphic overprint at c. 2500 Ma, zircon rims in felsic gneisses from the northern and southern regions commonly yield Laxfordian dates as young as c. 1800 Ma. Combined, the results support the idea that, during the Palaeoproterozoic, the central region of the LGC acted as low‐strain domain, in which intense deformation and metamorphism were restricted to crustal‐scale shear zones. By contrast, in the southern and northern regions, early (c. 1.85 Ga) and late (c. 1.75 Ga) Laxfordian deformation and fluid‐mediated metamorphism were much more pervasive and at higher P–T conditions than previously proposed. The diachronous Laxfordian evolution of the southern and northern regions indicate that they reflect early and late snapshots of collisional to transpressional tectonics in the mainland LGC. The long‐lasting Laxfordian evolution documents the collision of the Rae and North Atlantic cratons during the Palaeoproterozoic amalgamation of the supercontinent Nuna, with implications for the palaeogeographic configuration of NW Scotland during Palaeoproterozoic Nuna.

Geochronology and geochemistry of early Paleozoic magmatism in the Qilian orogen: Constraints on closure of the Proto-Tethys Ocean

The Proto-Tethys Ocean was a Neoproterozoic to early Paleozoic ocean that opened upon the breakup of Rodinia and closed by Paleozoic oceanic and continental subduction. The Qilian orogenic belt is the northernmost orogen of the Proto-Tethys domain and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The processes responsible for extensive early Paleozoic magmatism in the Qilian orogen are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of early Paleozoic mafic and felsic rocks in the Qilian orogen. Gabbroic dikes from the Central Qilian block have LA-ICP-MS zircon U-Pb ages of 499–497 Ma with a wide range of εHf(t) values of −9.94 to + 4.28 and TDM1 model ages of 1557–929 Ma, and dioritic dikes have a LA-ICP-MS zircon U-Pb age of 489 ± 3 Ma, positive εHf(t) values of +7.61 to +10.53, TDM1 model ages of 684–790 Ma, and an amphibole 40Ar/39Ar age of 486.3 ± 4.3 Ma. Both sets of dikes were derived from partial melting of a heterogeneous spinel-facies lherzolite lithospheric mantle that was modified by fluids and melts from a subducted slab. The protolith of amphibolites from the Hualong block in the South Qilian belt was a mafic intrusion of 454 ± 2 Ma with negative εHf(t) values of −9.69 to −7.79 and TDM1 model ages of 1390–1478 Ma. Its trace element signature indicates that sediment-derived melts were involved in the magma formation. An amphibole 40Ar/39Ar age of 418.7 ± 3.8 Ma from an amphibolite records the timing of amphibolite facies metamorphism. Felsic volcanic tuffs and granitic intrusions from the Central Qilian block have zircon U-Pb ages of 507–440 Ma with a broad range of zircon εHf(t) values of −9.60 to + 11.87 and TDM2 model ages of 2037–584 Ma. They have geochemical characteristics similar to I-type and S-type granites that were sourced from partial melting of juvenile and ancient continental crust with minor input of mantle materials. The formation of the felsic rocks indicates the transition from an arc-related setting to a post-collisional setting. Combining our results with existing data, we suggest that the Central Qilian block was affected by both southward subduction of the North Qilian ocean and the northward subduction of the South Qilian ocean, resulting in the formation of an Andean-type continental arc along the northern margin of the Central Qilian block and a subduction-accretion system along the southern margin of the Hualong block at ca. 530–440 Ma. Collision of the Central Qilian, Hualong, Quanji and Qaidam blocks at ca. 440–418 Ma marks the final closure of the Proto-Tethys Ocean.

Geochronology, petrogenesis, and tectonic setting of amphibolitic rocks from the Tutak metamorphic Complex, Sanandaj-Sirjan Zone, Iran

Amphibolitic rocks of the Tutak Metamorphic Complex (TMC), in the southern part of the Sanandaj Sirjan Zone (SaSZ), are associated with meta-dolerites, felsic intrusions, mica-schists, and marbles as bodies and interlayers. Rutile grains from an amphibolite (ES-18-1) yield a U-Pb age of 56.08 ± 1.13 Ma (Late Paleocene-Early Eocene) while Zr-in-rutile thermometry indicates peak temperatures ∼ 616 to 674 °C. These data are inferred to reflect the timing of medium-pressure/medium-temperature (MP/MT) amphibolite facies metamorphism in Late Paleocene-Early Eocene rather than upper Cretaceous time in the TMC complex. Geochemically, the amphibolites are from an Nb-enriched basalt-andesite to basalt protolith with sub-alkaline to alkaline affinity. The amphibolites have moderate 87Sr/86Sr(t) ratios (0.702451 to 0.712720), 143Nd/144Nd(t) ratios (0.512122 to 0.512209), positive ƐNd(535 Ma) (3.38 to 5.08) values and two-stage Nd model ages (TDM) between 0.84 and 1.3 Ga. The geochemical trends and (La/Yb)cn> 3, Nb/La> 0.6, Sm/Nd< 0.3, Zr/Y (4.42 to 7.47, avg. 5.72) ratios with moderate 87Sr/86Sr(535) ratios and positive ƐNd(535 Ma) values indicate that the mafic protoliths of the amphibolites are probably derived from the low partial melting of subcontinental lithospheric mantle in a spinel lherzolite field, with enriched mantle (E-MORB/OIB-like) nature. Subsequently, they experienced minor crustal contamination in the early stages of the development of intra-continental-type back-arc basin (BABB). Lack of subduction signals in the amphibolites with E-MORB/OIB-like features may result from the roll-back of subducting Proto-Tethys oceanic slab during the Cambrian similar to other peri-Gondwana terranes.

Current state of knowledge of the gold mineralization at Imonga-Saramabila, Maniema (DR Congo): a petrographic and mineralogical study of the mineralized vein system

The Great Lakes area in Central Africa forms a large metallogenic province that hosts important deposits of gold mineralization. We present a petrographic, mineralogical and geochemical study of unique borehole samples from the Imonga-Saramabila gold deposit, a historical mine site located in the Maniema Province (DR Congo) in the Karagwe-Ankole belt (KAB) in the Great Lakes area and one of the only places in the Maniema province with accessible boreholes allowing to study the mineralization. The samples are metasedimentary rocks belonging to the Mesoproterozoic Kivu Supergroup, with bedding-parallel meta-igneous rocks. These rocks have undergone upper greenschist to lower amphibolite facies metamorphism, based on the presence of andalusite and chiastolite porphyroblasts, and are affected by hydrothermal alteration. The porphyroblasts formed during peak metamorphism and posterior to a first vein generation. Three additional vein generations were identified at Imonga based on crosscutting relationships, with the second and third events overprinting the porphyroblasts by intense chloritization, and associated with sulfide mineralization. The fourth vein generation is again barren. The first veining event formed pre-folding and the three subsequent generations postdate folding, as concluded based on the relationship of the veins with the cleavage. Only one important folding event is proposed based on the development of only one cleavage. Gold occurs as free gold or is included in pyrite in the second (and maybe third) vein generation. Based on the paragenesis, structural characteristics, and the link between veining and metamorphic minerals, the gold mineralization at Imonga is interpreted to be linked to the early Neoproterozoic (~980 Ma) compressional deformation event, associated with the amalgamation of the Rodinia supercontinent.

Protracted localization of metamorphism and deformation in a heterogeneous lower-crustal shear zone

Deformation of the continental crust is influenced by the pre-existing structural framework, fluid availability, strain rate, and pressure-temperature conditions. We investigate the evolution of a large (hundreds-of-meters wide) heterogeneous shear zone and associated brittle and ductile deformation structures (Lofoten, Norway) using structural analysis, mineral-chemical and microstructural observations, and U-Pb dating. The shear zone developed through a sequence of metamorphic stages: (1) migmatization and granulite-facies metamorphism, (2) eclogite-facies metamorphism, and (3) amphibolite-facies metamorphism. Stage (1) was related to magmatic activity at lower-crustal conditions prior to 1.7 Ga. Stage (2) likely occurred during the early phases of the collision between Baltica and Laurentia (Caledonian collision). Stage (3) was related to shortening during the main phase of the Caledonian collision (∼430–400 Ma) and was accompanied by hydration of the shear zone. We demonstrate how mechanical heterogeneities influence the deformation style from the centimeter to meter scale. Zones with a pre-existing fabric deformed by ductile shearing and folding, whereas homogeneous, dry rocks fractured. Fractures provided precursors for small-scale shear zones. These contrasting deformation styles occurred repeatedly. Consequently, pre-existing structures define the deformation style and serve as conduits for the channelization of fluids over extended periods of time.

Late orogenic high-temperature overprint in (ultra)high-pressure ([U]HP) metamorphic rocks of the North Qinling Orogen, Central China: Insights into the geodynamics of the exhumation of (U)HP metamorphic rocks

Exhumed (ultra)high-pressure ([U]HP) metamorphic rocks are markers of paleo-subduction zones and can provide crucial implications for the geodynamic evolution of subduction-exhumation. A variety of (U)HP metamorphic rocks from many collisional orogens have commonly documented a low-pressure high-temperature overprint, which appears to indicate a pervasive geodynamic process behind (U)HP metamorphic rock exhumation. A good case study of garnet amphibolite and associated garnet-biotite gneiss with an unambiguous low-pressure high-temperature overprint in the Xixia-Xiaguan area of the North Qinling Orogen, central China, provides a significant opportunity to uncover this geodynamic process. A detailed study of petrography, mineral chemistry, and phase equilibria modeling suggests that the investigated garnet amphibolite and garnet-biotite gneiss have jointly experienced multistage exhumation processes with near-isothermal decompression from the (U)HP eclogite facies (26.8−29.0 kbar/740−775 °C) to the medium-pressure (MP) amphibolite facies (8.7−10.0 kbar/740−775 °C), subsequent remarkable near-isobaric heating to the low-pressure (LP) granulite facies (6.8−8.2 kbar/832−835 °C), followed by final decompression and cooling to the LP amphibolite facies (5.8−6.8 kbar/740−757 °C). U-Pb age dating of zircons from the garnet amphibolite and garnet-biotite gneiss yielded metamorphic ages of ca. 500 Ma, 450 Ma, and 400 Ma, which we consider to represent (U)HP eclogite-facies, LP granulite-facies, and late LP amphibolite-facies metamorphism, respectively. According to the regional geology and the metamorphic history we have determined, this multistage exhumation process of two-step exhumation stages delimited by a phase of LP near-isobaric heating was associated with slab rollback. Accordingly, we propose a tectonic model of exhumation driven by slab rollback for the geodynamic evolution of (U)HP metamorphic rock exhumation in the North Qinling Orogen. Additionally, a combination of previous studies and our new results suggests that exhumation driven by slab rollback appears to be a fundamental mechanism for the (U)HP metamorphic rock exhumation within collisional orogens.

Paleozoic Tectonothermal Evolution in the West Qinling Orogen, Central China: Petrological and Chronological Evidence from Garnet Amphibolites

The Qinling Complex is located in the core of the northern Qinling Orogen and plays a key role in understanding the tectonic evolution of the Qinling Orogen, but its metamorphic evolution remains controversial. The combined investigation of petrographic observation, zircon U-Pb dating, and phase equilibria modeling for garnet amphibolites from the Tianshui area in the West Qinling Orogen is reported in this study. The results show that the garnet amphibolites record a clockwise P-T path characterized by a pre-TMax decompression heating stage, a temperature peak at P-T conditions of 0.84–0.99 GPa and 869–886 °C, followed by a decompression cooling stage. Zircon U-Pb dating yields four age populations of ~479 ± 4 Ma, ~451 ± 8 Ma, ~411 ± 4 Ma, and ~377 ± 6 Ma. The 479–450 Ma reflects the timing of the pre-TMax high–medium pressure upper amphibolite-facies metamorphism. The metamorphism at peak temperature condition occurred at c.411 Ma and was followed by decompression cooling to c.377 Ma. The Ordovician high–medium pressure metamorphism is related to the continental collision, which is slightly later than the HP–UHP eclogite-facies metamorphism in the East Qinling Orogen. The HT granulite-facies metamorphism at peak temperature condition took place at reduced pressures, suggesting thinning of the collision-thickened orogenic crust. Therefore, the northern West Qinling Orogen experienced a tectonothermal evolution from initial crust thickening to thinning during the Paleozoic collisional orogeny.

Late Mesoproterozoic to early Neoproterozoic metamorphic history of the North Wulan terrane, NW China: New constraints on the evolution of Rodinia

Late Mesoproterozoic to early Neoproterozoic metamorphic and magmatic events related to the assembly of Rodinia are identified in microcontinents (such as North Wulan terrane, Qaidam block, Central Qilian terrane, Quanji Massif, and Hualong terrane) from the northeastern Tibet Plateau. However, the peak P–T conditions, P–T paths, and timing of the metamorphic events are still controversial. In this contribution, we integrate petrography, phase equilibrium modelling, mineral chemistry and zircon geochronology to develop a detailed metamorphic history of felsic paragneiss and garnet amphibolite from the North Wulan terrane. Zircon U–Pb geochronology reveals two stages of metamorphism at 1.2–1.0 Ga and ∼0.9 Ga. The earlier metamorphic event caused granulite-facies metamorphism with peak P–T conditions of 800–850 °C at 8–10 kbar. The second metamorphic event (∼0.9 Ga) reached amphibolite-facies conditions at 700–750 °C and 6–7 kbar. Granulite-facies metamorphism probably took place in an arc-related setting, whereas amphibolite-facies metamorphism was likely associated with tectonic extrusion during the collision between the North Wulan terrane and unknown continental fragments. The North Wulan terrane and surrounding continental blocks/fragments in the northeastern Tibet Plateau share a similar evolution that includes long-lived oceanic subduction from 1.2 to 1.0 Ga followed by continental collision at ∼0.9 Ga at the periphery of Rodinia. Our results improve the understanding of the tectonic processes responsible for microcontinent evolution in the northeastern Tibet Plateau during the assembly of Rodina, and provide important constraints on the reconstruction of Rodinia.