Pb Monazite Ages Research Articles

The “greywacke problem” explored in the Neoproterozoic of Saxo-Thuringia: new insights into sediment composition and metamorphic overprint

AbstractGreywackes make up a substantial part of the Cadomian basement of Saxo-Thuringia. Here, their classification as greywackes and the timing of metamorphic overprint are re-evaluated using a multi-method approach. Immature monotonous greywacke sequences from the Lausitz (Lausitz Block) and Leipzig groups (North Saxon Anticline), as well as from the eastern Thuringian Basin and parts of the Weesenstein Group (Elbe Zone) probably belong to a coherent unit, based on microscopic investigations supported by SEM Automated Mineralogy analyses and point counting data. However, due to the low matrix content (< 15%), the sedimentary rocks are more likely classified as lithic sandstones. The heterogeneous composition and in particular the highly mature character of the Clanzschwitz Group (North Saxon Anticline) and parts of the Weesenstein Group (Seidewitz Formation) suggest a younger, Late Cambrian to Early Ordovician sedimentation age. Typically, the metamorphic overprint of the “greywacke units” is very weak. Previous assumptions of Cadomian contact metamorphism triggered by Early Cambrian intrusions (ca. 540 Ma) could not be confirmed due to the local differences in the determined metamorphic ages. Late Cambrian to Early Ordovician (521–461 Ma) Th–U–Pb monazite ages are likely related to the tectonic transition from the collisional regime of the Cadomian orogeny to extensional processes in the course of the opening of the Rheic Ocean. Sporadic Late Ordovician (458–445 Ma) Th–U–Pb monazite and K–Ar fine-fraction ages were also obtained but the specific thermal trigger is still subject of debate. The Permo-Carboniferous metamorphic ages (314–286 Ma) indicate high-temperature metamorphism related to the post-Variscan extensional processes of Central Europe during this period. The youngest dated monazites are Jurassic in age and may have grown in association with the hydrothermal activity known from Central Europe at that time. Graphical abstract

Electron probe petrochronology of monazite- and garnet-bearing metamorphic rocks in the Saxothuringian allochthonous domains (Erzgebirge, Granulite and Münchberg massifs)

Abstract In the Saxothuringian Zone, a unique assemblage of high- to ultra-high-pressure and ultra-high-temperature metamorphic units is associated with medium- to low-pressure and temperature rocks. The units were studied in a campaign with garnet and monazite petrochronology of gneisses, micaschists and phyllites, and monazite dating in granites. P–T path segments of garnet crystallization were reconstructed by geothermobarometry and interpreted in terms of the monazite stability field, EPMA Th–U–Pb monazite ages and garnet Y + HREE zonations. One can recognize (1) Cambrian plutonism (512–503 Ma) with contact metamorphism in the Münchberg Massif. Subordinate monazite populations may indicate a (2) widespread but weak Silurian (444–418 Ma) thermal event. A (3) Devonian (389–360 Ma) high-pressure metamorphism prevails in the Münchberg and Frankenberg massifs. In the ultra-high-pressure and high-pressure units of the Erzgebirge the predominant (4) Carboniferous (336–327 Ma) monazites crystallized at the decompression paths. In the Saxonian Granulite Massif, prograde–retrograde P–T paths of cordierite-garnet gneisses can be related to monazite ages from 339 to 317 Ma. A (5) local hydrothermal overprint at 313–302 Ma coincides partly with post-tectonic (345–307 Ma) granite intrusions. Such diverse monazite age pattern and P–T time paths characterize the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny.

Relict Mesoarchean (2.99–2.94 Ga) metamorphism overprinted by late Neoarchean tectonothermal event(s) from the Sukma Group supracrustal rocks, Bastar craton, India: Evidence from new Lu-Hf and Sm-Nd garnet isochron and Th-U-total Pb monazite ages

Mesoarchean metasupracrustal successions can preserve evidence of tectonothermal reworking involving Paleoarchean felsic crust. Paleoarchean tonalite and trondhjemite of the Bastar craton, central India contain remnants of amphibolite-facies meta-supracrustal rocks of the Sukma Group, which likely represents the oldest Archean terrestrial sedimentary rocks. In order to constrain the time of metamorphic events combined garnet Lu-Hf and Sm-Nd geochronology and Th-U-total Pb dating of texturally preserved monazite from garnet-bearing metapsammite samples, representative of cover rocks overlying orthopyroxene-bearing tonalite basement, were conducted. Relics of an earlier garnet porphyroblast-forming metamorphic stage 1 (M1) developed at upper amphibolite-to granulite-facies conditions, are texturally distinct from the widespread metamorphic stage 2 (M2) marked by the garnet-breakdown to cordierite-anthophyllite symplectite. The M2-stage conditions, at ∼ 650–700 °C and ∼ 5.7 kbar, in the metasupracrustal remnants, are similar to those in garnet-amphibole-bearing quartzite and also as inferred from associated calc-silicate gneiss. Samarium-Nd chronology on whole-rock and porphyroblastic garnet fractions from three garnet-bearing quartzite samples yielded a weighted mean age of 2990 ± 10 Ma whereas Lu-Hf isotopic systematics on the same garnets from two of these quartzite samples yielded a weighted mean age of 2940 ± 31 Ma. Superposed Late Neoarchean M2 stage tectonothermal events include mylonitization of a ∼ 3.5 Ga basement tonaliteat 2525 ± 20 Ma and garnet formation within a Fe-rich quartzite at 2413 ± 21 Ma, as constrained from Sm-Nd isochron ages and similar to the in-situ weighted mean chemical age of 2367 ± 19 Ma of monazite included within M2-stage symplectitic cordierite around the older M1-stage garnet. The relict 2.99–2.94 Ga mid-crustal amphibolite-to granulite-facies M1 metamorphic assemblages stabilized within metasupracrustal rocks, formed in response to crustal thickening during Mesoarchean convergent tectonics, and were likely related to amalgamation of Paleoarchean crustal fragments.

Constraints on the timing of magmatism and rare-metal mineralization in the Fangzheng Rb deposit, Altai, NW China: Implications for the spatiotemporal controls on rare-metal mineralization

Amazonite granites are potential Rb resources given the abundance of Rb-bearing minerals they contain. The Altai Orogen, located in the southwestern portion of the Central Asian Orogenic Belt, is the world's most famous rare-metal metallogenic belt, containing numerous Li–Be–Nb–Ta–Cs deposits (e.g., the Koktokay No. 3 pegmatite deposit). The Fangzheng deposit, however, is the only known independent Rb deposit in this belt, with a proven Rb2O reserve of ∼12,940 tons at an average grade of 0.07%. The deposit is hosted by the Jiangjunshan pluton, which is composed largely of two-mica alkali feldspar granite and lesser amazonite granite on the peripheries, with amazonite pegmatite veins cutting the pluton and its wall-rocks. Although the pluton has been dated using various geochronological techniques, the magmatic and the rare-metal mineralization ages remain ambiguous given the complexity in zircon genesis and evolution, and their high Th and U contents. In this contribution, U–Pb geochronology of zircon, monazite, cassiterite, and columbite-group minerals, as well as their trace-element characteristics, are utilized to constrain the timing and evolution of magmatism and mineralization in the Fangzheng Rb deposit. Based on morphological and geochemical features, and cathodoluminescence images, zircon grains from different lithofacies are classified into three types — magmatic zircon, hydrothermally overprinted magmatic zircon, and hydrothermal zircon. The first two types of zircon grains are characterized by low Th/U ratios (0.22–4.00, with an average of 0.84) and ΣLREE contents (8.66–466, with an average of 90.6), indicative of a magmatic origin, whereas the third type is characterized by high Th/U ratios (0.22–7.26, with an average of 2.26) and ΣLREE contents (1154–4944 ppm, with an average of 2905 ppm), indicative of a hydrothermal origin. The magmatic and hydrothermally overprinted magmatic zircon grains from the two-mica alkali feldspar granite and amazonite granite yield U–Pb ages of 272.7±1.4 Ma and 272.5±1.7 Ma, respectively. Therefore, the earliest age of magmatism of the Jiangjunshan pluton is approximately 272 Ma (early–middle Permian). Based on the internal structure and geochemical characteristics of accessory minerals (monazite, cassiterite, and columbite-group minerals) selected from the ore bodies (amazonite granite and amazonite pegmatite), it is suggested that they crystallized during the magmatic–hydrothermal transition. The U–Pb ages of hydrothermal zircons (253.2±17.0 Ma), monazite (254.4±0.9 Ma), and columbite-group minerals (254.9±1.4 Ma) from the amazonite granite, as well as the U–Pb ages of monazite (253.3±0.7 Ma), cassiterite (250.4±1.4 Ma), and columbite-group minerals (254.5±0.8 Ma) from the amazonite pegmatite indicate that the age of rare-metal mineralization in the Fangzheng deposit is approximately 255–250 Ma (i.e., close to Permian/Triassic boundary). Combined with previous geochronological results and interpretations of tectonic evolution, it is evident that i) the middle Permian to Late Triassic is an important period of granitic magmatism and related rare-metal mineralization in the Chinese Altai, and ii) the rare-metal deposits formed in an intracontinental extensional setting after the tectonic transition from subduction to extension.

HTLP metamorphism and fluid‐fluxed melting during multistage anatexis of continental crust (N Sardinia, Italy)

AbstractThe Variscan high‐grade metamorphic basement of northern Sardinia and southern Corsica record lower Carboniferous anatexis related to post‐collisional decompression of the orogen. Migmatites exposed in the Punta Bianca locality (Italy) consist of quartz + biotite + plagioclase + K‐feldspar orthogneisses, garnet and cordierite‐bearing diatexite and metatexites, derived from metasediments. Field evidence, petrographic observations, ELA‐ICP‐MS zircon and monazite dating and pseudosection modelling suggest that anatexis was apparently episodic involving two main stages of partial melting. Using pseudosection modelling, we infer that the first stage of partial melting is in the upper amphibolite facies (~0.45 GPa at ~740°C). Cordierite overgrowths replacing sillimanite, combined with the composition of plagioclase and K‐feldspar, suggest decompression followed cooling below the solidus at low pressures of ~0.3 GPa. The age of the first anatectic event is not precisely constrained because of extensive resetting of the isotopic systems during the second melting stage, yet few zircons preserve a lower Carboniferous age which is consistent with the regional dataset. This lower Carboniferous migmatitic fabric is offset by a network of pseudotachylyte‐bearing faults suggestive of cooling to greenschist facies conditions. Garnet/cordierite‐bearing diatexites incorporate fragments of pseudotachylite‐bearing orthogneiss and metatexites. Pseudosection modelling indicates nearly isobaric re‐heating up to ~750°C, followed by further cooling below the solidus. The inferred P–T path is consistent with decompression and cooling of the Variscan crust through post‐collisional extension and collapse of the thickened orogenic crust, followed by nearly isobaric re‐heating at low pressures (~0.3 GPa) yielding to a second melting stage under LP‐HT conditions. U/Th‐Pb monazite ages for diatexite migmatites indicate an upper bound of 310–316 Ma for the second melting stage, suggesting that the second melting stage is coincident with the regional phase of crustal shearing. The cause of the high geothermal gradient required for re‐heating during the second melting stage is unknown but likely requires some heat source that was probably related to dissipation of mechanical work within crustal‐scale shear zones. According to this interpretation, some upper Carboniferous peraluminous granite precursors of the Corsica–Sardinia Batholith could be the outcome rather than the cause of the late‐Variscan high‐T metamorphism.

Petrology and geochronology of andalusite- and sillimanite-bearing kyanite metapelites from the Gobi Altai Mountains: Evidence for prolonged convergent tectonics in the Central Asian Orogenic Belt

To further constrain the tectonic evolution of the convergent margin of the West Mongolian segment in the Paleo–Asian Ocean, we have investigated coarse-grained kyanite metapelites associated with quartz veins in the Gobi Altai Mountains, Mongolia, where the metamorphic belt is in a thrust contact with a Paleozoic accretionary complex. The metapelites contain coarse-grained granoblastic kyanite grains that are surrounded by sillimanite (Sil-II) with moat-like andalusite (And-II). The garnet grains in the garnet-bearing kyanite metapelites contain inclusions of andalusite (And-I) in their cores and sillimanite (Sil-I) in their mantles. Detailed textural characterization of the aluminosilicate phases indicates five distinct generations in the sequence andalusite (And-I), sillimanite (Sil-I), kyanite (Ky), sillimanite (Sil-II), and andalusite (And-II). Thermobarometric calculations and thermodynamic modeling suggest a hairpin-shaped P–T evolution with prograde conditions starting from ~530 °C at 3.5 kbar, reaching a peak of 600 °C at 6 kbar, and then a retrogression to ~550 °C at 4 kbar. U-Pb zircon and U–Th–Pb monazite ages indicate that the multistage aluminosilicate formation has occurred during c. 260–245 Ma. The detrital zircon age distribution is essentially similar to those in the accretionary sedimentary rocks from the entire Altai Range (a sharp peak at 550–490 Ma and a broad peak at 900–700 Ma), but is characterized by more abundant Proterozoic zircon grains and an older maximum deposition age (Early Ordovician) compared with Devonian pelitic gneisses in the Mongolian Altai. These petrological and geochronological results indicate the following development of the orogen: (1) Subduction of an oceanic plate beneath a microcontinental block as early as 550 Ma, evidenced by the youngest peak of detrital zircon ages. (2) Development of the accretionary wedge and volcanic front due to maturation of the arc until c. 360 Ma. The development of the volcanic front due to the magmatism has provided abundant Paleozoic detritus to the accretionary wedge and has stemmed supply of Proterozoic detritus from the inland at the same time. (3) Burial of the youngest accretionary wedge at c. 360–350 Ma, which is not observed in the current samples. Ridge subduction has also been suggested to have occurred in this period. (4) Arc magmatism in the upper (older) part of the accretionary wedge during the late Permian that resulted in prograde high-T and low-P metamorphism (formation of And-I). (5) Burial of the heated accretionary wedge to a depth of ~20 km due to subsequent collision (formation of Sil-I and Ky) at c. 260 Ma. (6) Rapid exhumation of the buried accretionary wedge to a shallow depth by thrusting (formation of Sil-II and And-II) at c. 245 Ma. In summary, the metapelites in the Altai Range clearly preserve the evolution of both the accretion and collision events as well as their transformation processes, which are likely to be a good natural example of the Wilson cycle for a period of ~300 Myr.

Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with 40Ar/39Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New 40Ar/39Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40Ar/39Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. 40Ar/39Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.

Monazite EPMA-CHIME dating of Sangmelima granulite and granitoid rocks in the Ntem Complex, Cameroon: Implications for Archean tectono-thermal evolution of NW Congo craton

Electron microprobe (EPMA) monazite Th–U – Pb ages have been determined from a suite of granulite and granitoids rocks of the Sangmelima granite-greenstone Belt (SGB) of the Ntem Complex in Cameroon or NW Congo Craton. The monazite-bearing rocks include charnockitic gneiss, migmatitic tonalite-trondhjemite-granodiorite (TTG) gneisses, granodiorite, monzogranite and a late pegmatite intrusion. The monazites of Sangmelima can be classified as monazite-Ce. Based on published ages and EPMA Th–U – Pb monazite age data from the SGB, the geodynamic evolution of the NW Congo Craton was refined through its tectonometamorphic and magmatic events. The major charnockitic and TTG magmatism event in Ntem Complex occurred between ~2950 and 2850 Ma. This Mesoarchean crystalline basement underwent two major ductile tectonothermal events M1 – D1 and M2 – D2. (1) The onset of M1 – D1 corresponds to the migmatization of TTG at 2843 Ma coeval with the emplacement of syn-kinematic granite dykes (monzogranites) at 2840–2825 Ma. The S1 regional penetrative foliation (D1) was progressively generated during the M1 metamorphic event, which led to the formation of associated hypersthene-garnet-bearing granulitic mineral assemblage that peaked at ~2830 Ma. (2) The M2 – D2 event between 2760 and 2740 Ma corresponds to the folding of S1 regional foliation by F2 folds and development of C2 shear zones synchronous with the anatexis event that generated the high-K granitoids at 2750 Ma. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D3 and protracted cooling of the Ntem Complex between ~2700 and 2550 Ma. Our new EPMA monazite U–Th – Pb ages from the SGB highlight similar magmatic and tectonothermal events of the adjacent northern Gabonese domain and the Chaillu Massif in Gabon part of the NW Congo craton (ca. 3.0–2.5 Ga).

Chemical composition and age of monazite-(Ce) in granitoids of the crystalline basement from the South Yamal

The relevance of the work is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to date by isotope research methods. Purpose of the work: study of the chemical composition of accessory monazite from granitoids of the crystalline basement of the South Yamal and determination of its age. Research methodology: quantitative analysis of the chemical composition of monazite was carried out using X-ray spectral electron probe microanalyzer CAMECA SX 100 (electron beam diameter from 1 μm, BSE, SE, Cat modes, determination of elements from beryllium to uranium). The spectra were obtained with the help of inclined wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. The age calculation was carried out according to the well-known methods of foreign authors in addition to some developments of the author. Results. The chemical composition of monazite makes it possible to classify it as a cerium variety, the content of radiogenic components varies greatly (in wt.%): ThO2 – 5.37–16.31, UO2 – 0.40–0.81, PbO – 0.08–0,19. There are significant concentrations of SiO2 (up to 3.5 wt.%), Y2 O3 (up to 1.8 wt.%) and CaO (up to 1.2 wt.%). It turns out that monazite implements hattonite (Th4+(U4+)+Si4+ → REE3++P5+) and cheralite (Th4+(U4+)+Ca2+(Sr2+,Ba2+,Pb2+) → 2REE3+) isomorphism types. The decent content of lead and high crystallinity of the substance makes it possible to use this mineral as a geochronometer mineral. Conclusions. New data on the chemical composition of monazite have been obtained, and the late Permian age of granitoids has been determined by microprobe dating. The values of the point U–Th–Pb ages of monazite together give a weighted average age of 256 ± 10 Ma (MSWD = 0.15) and an isochron of 254 ± 19 Ma (MSWD = 0.28), which almost ideally coincides with the results of isotopic U–Pb zircon dating from the same rock, 254 ± 3 Ma.

Tectonothermal evolution of a collisional orogen in the Khammam region, southeastern India: insights from structures, phase equilibria modeling and U–Th–(total) Pb monazite geochronology

The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet–kyanite metapelites of the KSB have been investigated to characterize P–T–t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P–T conditions of 7.9–8.1 kbar/790–810 °C and ~ 7.0 kbar/740–750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4–7.8 kbar/600–640 °C, followed by post-peak retrogression at 6.1–6.6 kbar/590–625 °C. U–Th–(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63–1.53 Ga and 1.48–1.38 Ga, while those in the metapelites were determined at 1.25–1.20 Ga and 1.18–1.10 Ga, respectively. These P–T–t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic–Late Mesoproterozoic. The younger ages (0.90–0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79–2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier–Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words)

Timing of high-grade metamorphism in the Kontum Massif, Vietnam: Constraints from zircon–monazite multi-geochronology and trace elements geochemistry of zircon–monazite–garnet

Metamorphic rocks in the Kontum Massif, including the Kannak, Ngoc Linh and Kham Duc Complexes are characterized by amphibolite- to granulite-facies metamorphism and their mono- versus poly-metamorphic evolution is a matter of debate. In this study, we constrained on the timing of high-grade metamorphism in this massif by combining laser ablation inductively coupled plasma mass spectrometry U–Pb zircon and electron microprobe chemical U–Th–Pb monazite ages with geochemical relation of trace elements between zircon–monazite–garnet. The results indicated that the analyzed pelitic and mafic granulites in the western Kannak Complex experienced the high temperature metamorphism at ca. 250 Ma with subsequent decompression. However, in the western Ngoc Linh Complex two high-grade metamorphic events were recorded in one of analyzed pelitic gneisses. The first high temperature metamorphism began at ca. 440–450 Ma with the attainment of the peak condition at ca. 430 Ma, overprinted by the second one at ca. 240–250 Ma with following decompression and exhumation stage at ca. 230 Ma. The other pelitic gneiss and mafic granulite detected only younger single event. Pelitic gneisses in the Kham Duc Complex recorded medium pressure metamorphism occurred at ca. 240 Ma without any traces of early metamorphic event.

Mesoarchaean to Neoproterozoic (3.2–0.8 Ga) crustal growth and reworking in the Aravalli Craton, northwestern India: Insights from the Pur-Banera supracrustal belt

The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D1–D3, each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga.

U–Th-total Pb ages of monazite from the Eckergneiss (Harz Mountains, Germany): evidence for Namurian to Westfalian granulite facies metamorphism at the margin of Laurussia

Four samples of garnet-bearing metapsammitic gneisses and one sample of a kinzigitic sillimanite-garnet-cordierite gneiss from the Eckergneiss Complex (Harz Mountains, Germany) were studied in detail for U–Th-total Pb dating with the electron microprobe. Monazite in all samples occurs in the matrix and show similar chemical compositional variation. A single stage monazite growth during the highest stages of metamorphism was deduced and chemical ages of all samples are between 313 ± 4 and 328 ± 6 Ma. These ages are interpreted as the time of granulite facies metamorphism in the Eckergneiss Complex. This metamorphism post-dates the formation of the main foliation, presumably developed under amphibolite facies conditions, as indicated by overprinting static grain growth and partial melt microstructures. It pre-dates the significantly younger intrusion of the Harzburg Gabbronorite and the Brocken Granite that are of post-Variscan age. Our results, in combination with P–T conditions for the peak of granulite facies metamorphism, show that the Eckergneiss Complex must have resided at a depth of at least 15 km during the Namurian to Westfalian. Later exhumation of the Eckergneiss Complex and its tectonic emplacement within the low-grade metamorphic Rhenohercynian rock units of the NW-Harz most likely occurred along the Acker-Bruchberg Thrust Zone, which marks the transition between the allochthonous and autochthonous domains of the Harz Mountains. At Early Permian time, this imbricated sequence was intruded and sealed by the post-Variscan intrusions.

Recognition of Pan-African-aged metamorphism in the Fisher Terrane, central Prince Charles Mountains, East Antarctica

The Precambrian tectonic architecture of the East Antarctic Shield is subject to considerable uncertainty because of limited outcrop. The Fisher Terrane, located in the central region of the Prince Charles Mountains, evolved during the Mesoproterozoic as a volcanic arc system and provides key data about the evolution of this region. Our study provides evidence that the Fisher Terrane formed in proximity to a Proterozoic plate boundary and was subsequently metamorphosed during the late Neoproterozoic to early Cambrian during the final stages of Gondwana amalgamation. U–Pb detrital zircon geochronology reveals that metasedimentary rocks from within a metasedimentary–metavolcanic package cropping out at Fisher Massif were deposited after c. 1300 Ma, and contain detritus derived from the Rayner–Eastern Ghats Terrane. This suggests that the Fisher Terrane was not an isolated oceanic arc but rather formed on the same tectonic plate as the Rayner Complex. Metapelitic schists from within the same metasedimentary package yield metamorphic U–Pb monazite ages of c. 512–509 Ma, corresponding to a regionally recognized Pan-African-aged event. This event has not been previously identified in the Fisher Terrane, and demonstrates that Pan-African-aged metamorphism affected all parts of the Prince Charles Mountains. Calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2.5–4.0 kbar and 550–615°C, corresponding to apparent thermal gradients of 146–220°C kbar −1 . Such conditions plausibly relate to metamorphism taking place in an extensional setting. Supplementary material: Methods and summarized mineral chemistry data, sample locations, U–Pb zircon and monazite data, and representative electron microprobe mineral analyses are available at https://doi.org/10.6084/m9.figshare.c.4392710

U-Pb zircon and U-Th-total Pb monazite ages from the Phulbani domain of the Eastern Ghats Belt, India: Time constraints on high-grade metamorphism and magmatism in the lower crust

We present zircon U-Pb (SHRIMP) and monazite U-Th-total Pb (EPMA) ages from a suite of high-grade rocks of the Phulbani domain of the Eastern Ghats Belt, India. These ages are integrated with the metamorphic and magmatic histories of the Phulbani domain to put precise time constraints on the tectonothermal evolution of the northern part of the Eastern Ghats Belt. Zircon from the coarse-grained charnockite rock shows crystallization age of ca. 970 Ma. Aluminous granulite contains spinel and quartz-bearing mineral assemblage indicating ultrahigh temperature metamorphism (>900 °C at 8–10 kbar) which possibly occurred during ca. 987 Ma as revealed from monazite included in porphyroblastic garnet. Monazite in the aluminous granulite and the migmatitic felsic gneiss grew dominantly at 966 ± 4 Ma and 968 ± 4 Ma ages respectively, which are interpreted as the cooling ages subsequent to peak metamorphism. Oscillatory-zoned zircon grains of the felsic augen gneiss yield ca. 1173 Ma age which is interpreted as the crystallization age of the granitic protolith. This ca. 1173 Ma age granite possibly composed a part of the Proterozoic basement of the Eastern Ghats Province. A monazite age of 781 ± 15 Ma from the aluminous granulite may indicate the timing of a localized shear-induced thermal process in the Phulbani domain. The presently studied rock suite thus recorded three distinct events (ca. 1173 Ma, ca. 1000–900 Ma and ca. 781 Ma) of magmatism, metamorphism and deformation of Eastern Ghats Belt. Metamorphic and magmatic histories of the Phulbani domain are similar to those of the adjacent Visakhapatnam domain and the Rayner Province of East Antarctica during the time frame ca. 1000–900 Ma.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5Ga) Betul Belt and the Neoproterozoic (∼1.0Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MSU-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2Ga and 0.95Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0Ga and 0.95Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

Proto-Andean evolution of the Eastern Cordillera of Peru

The Eastern Cordillera of Peru represents one of the longest (>1200km) Paleozoic metamorphic and magmatic belts exposed along the western Andean margin of South America. In this study, we examine the tectonothermal evolution of a key segment of the metasedimentary basement of the Eastern Cordillera of Peru (the Huaytapallana Complex) and demonstrate that it has experienced a hitherto undocumented high-grade orogenic event at 260Ma (latest Middle Permian) based on U–Pb and Th–Pb monazite age data from paragneisses and U–Pb dating of zircon rims from leucosomes. These ages are interpreted as recording crystallization and are consistent with 255Ma rutile growth in lower-grade units. U–Pb apatite data (c. 260–230Ma) in all units are consistent with slow cooling from this 260Ma metamorphic peak. U–Pb zircon geochronology of pre-tectonic plutons yield ages ranging from c. 302Ma to c. 260Ma. These geochronological data are augmented by new U–Pb apatite age data from other segments along the Eastern Cordillera of Peru. A regional synthesis of existing geochronological constraints from the Eastern Cordillera of Peru demonstrates that the margin has experienced a polycyclic orogenic history. Deformation and magmatism occurred at c. 480Ma and c. 435Ma during the Famatinian orogenic cycle, was followed by a Late Silurian to Early Carboniferous (c. 420–350Ma) magmatic and metamorphic gap, and terminated with Gondwanide magmatism and metamorphism at c. 315Ma and c. 260Ma. These Famatinian and Gondwanide orogenic phases can be correlated into the Proto-Andean margin of Argentina and Chile and are thus of regional extent. The evolution of the Proto-Andean margin is thus best explained by changes in tectonic plate reorganization in a long-lived Paleozoic accretionary orogen which was undergoing phases of advance and retreat, resulting in magmatic pulses and orogenic phases which can be correlated along the length of the plate boundary.

New P–T and U–Pb constraints on Alpine Schist metamorphism in south Westland, New Zealand

Metamorphic mineral compositions of a staurolite-bearing greyschist from the middle reaches of the Moeraki River valley in south Westland reveal peak equilibration at c. 558 ± 50 °C and c. 6.1 ± 1.2 kbar. Two c. 83 Ma U–Pb monazite age populations from the cores of monazite-apatite-allanite-epidote corona structures in mylonitised schists from near Fox Glacier confirm that Alpine Schist metamorphism occurred during the Late Cretaceous. The published spread in Late Cretaceous metamorphic ages indicates that metamorphism was diachronous or was a protracted event. Further dating is required to pin down the cryptic transition into the Jurassic–Early Cretaceous metamorphosed Otago Schist, but the Alpine Schist must extend at least 11 km east of the Alpine Fault in south Westland and overprint the suture between the Pounamu and Rakaia terranes. The P–T–t results imply that the Late Cretaceous crust represented by portions of the Alpine Schist was probably of similar thickness to that beneath the Southern Alps today, but with dehydration and partial melting occurring near the base. The crust under Westland and Otago may be dry and therefore strong.

Decoding polyphase migmatites using geochronology and phase equilibria modelling

AbstractIn this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–300 Ma (from cores of polychronic grains, dominantly from deeper structural levels in the central and western sectors of the complex) and 120–96 Ma (from rims of polychronic grains, dominantly from the central and western sectors of the complex, and from monochronic grains, mostly from shallower structural levels in the eastern sector of the complex). For five paragneisses and two orthogneisses, Lu–Hf garnet ages range from 116 to 111 Ma, c. 12–17 Ma older than published Sm–Nd garnet ages of 102–99 Ma from three of the same samples. Garnet grains in the analysed samples generally have Lu‐enriched rims relative to Lu‐depleted cores. By contrast, for three of the same samples, individual garnet grains have flat Sm concentrations consistent with high‐T diffusive resetting. Lutetium enrichment of garnet rims is interpreted to record the breakdown of a Lu‐rich accessory mineral during the final stage of garnet growth immediately prior to the metamorphic peak, and/or the preferential retention of Lu in garnet during breakdown to cordierite in the presence of melt concomitant with the initial stages of exhumation. Therefore, garnet is interpreted to be part of the Cretaceous mineral paragenesis and the Lu–Hf garnet ages are interpreted to record the timing of close‐to‐peak metamorphism for this event. For the Devonian–Carboniferous event, phase equilibria modelling of the metasedimentary protoliths to the paragneiss and a diatexite migmatite restrict the peak P–T conditions to 720–800 °C at 0.45–1.0 GPa. For the Cretaceous event, using both forward and inverse phase equilibria modelling of residual paragneiss and orthogneiss compositions, the P–T conditions after decompression are estimated to have been 850–880 °C at 0.65–0.80 GPa. These P–T conditions occurred between c. 106 and c. 96 Ma, determined from Y‐enriched rims on monazite that record the timing of garnet and biotite breakdown to cordierite in the presence of melt. The effects of this younger metamorphic event are dominant throughout the Fosdick complex.

Timing of formation and exhumation of the Montagne Noire double dome, French Massif Central

New U–Pb monazite, zircon, and xenotime ages date the Late Carboniferous crystallization of the anatectic Vialais granite in the Montagne Noire Axial Zone and the high-temperature deformation and metamorphism of the augen gneiss (Ordovician granite protolith) from which the granite was likely derived. The U–Pb monazite ages obtained from the augen gneiss (308±3Ma), late kinematic Vialais granite (303±4Ma), and post-kinematic leucogranite (298±2Ma), bracket the high-temperature deformation and metamorphism at ~310–300Ma, clearly postdating regional contraction and nappe emplacement (>320Ma). The planar–linear and locally linear fabrics in the augen gneiss outline a regional-scale double dome structure (Caroux and Espinouse sub-domes) containing smaller (km-scale) upright folds. Compared to the Caroux sub-dome, the larger, migmatitic Espinouse sub-dome contains abundant leucosome, leucogranite bodies and late-kinematic intrusions (Vialais granite). The Vialais granite displays a weak magmatic foliation that defines the main layering of the Espinouse migmatite. Ellipsoidal quartz–sillimanite nodules concentrated above the roof of the granite recorded an increment of strain (vertical shortening, E–NE elongation) during granite emplacement and crystallization, consistent with the extension event that exhumed the domes. These new geochronological and structural data suggest that the Montagne Noire double dome formed in a pull-apart structure within a dextral strike–slip system. Upright folding in the sub-domes has been traditionally assigned to a regional contraction event; alternatively, we propose that local contraction is associated with the convergence of low-viscosity crust beneath the upper crust pull-apart. Dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low-viscosity orogenic crust converging and rising to fill the gap created by upper crust extension. This interpretation solves the long-standing problem of apparent coeval contraction and extension in the Montagne Noire double dome and establishes migmatite dome emplacement in strike–slip corridors as a style of late Variscan tectonics.