Pb Monazite 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

Paleozoic to Mesozoic paleotectonic reconstructions of Gondwana assembly: Insights from petrography, heavy mineral chemistry and detrital U–Th–total Pb monazite geochronology of sandstones in the Pranhita-Godavari Basin (SE India)

The provenance study of Paleo-Mesozoic sediments of the Pranhita-Godavari (P-G) Basin, located in the southeastern India is crucial for the reconstruction of the eastern Gondwanaland, including the India-Antarctica connection. This study examines petrography, heavy mineral chemistry and detrital monazite geochronology of the Permo-Carboniferous to Cretaceous, predominantly fluviatile sandstones in the P-G Basin to track the possible sediment sources in East Antarctica and Eastern India. The sandstones show a gradual shift in composition from feldspatho-quartzose to quartzose arenite from older to younger succession, accompanied by a change in source terranes in a changing setting from transitional continental to the cratonic interior. The heavy minerals of the sandstones are dominated by garnet, rutile, ilmenite, monazite and zircon in order of decreasing abundance. The mineral chemistry of garnet and rutile indicates the predominance of sources from amphibolite to granulite facies rocks with minor inputs from metapelitic sources. The increase in textural and mineralogical maturity of sandstones accompanied by a gradual increase in ilmenite alteration from the bottom to the top of the succession indicates the attainment of tectonic stability with time. U–Th–total Pb dating of detrital monazites yielded three distinct major clusters at (1) 2556–2125 Ma, (2) 969–715 Ma, and (3) 581–418 Ma. Based on the correlation of source composition and monazite geochronology and paleocurrent data, the source of sediments is traced to the orogens of the Eastern Ghats mobile belt, East Antarctica (including Rayner and Napier complex), Karimnagar granulite belt, Khammam schist belt, and Bhopalpatnam granulite belt, situated to the west, south, and southeast of the present outcrops of the P-G Gondwana succession. The younger age populations possibly represent the sediment input from Kuunga Orogen, which represents the youngest orogenic event (635–488 Ma) in the Gondwana assembly. The rocks of Kuunga orogeny, occurring to the south of the basin, was a minor contributor to the P-G Gondwana sediments of this basin. The new data supports that the Eastern Ghats Mobile Belt of India and East Antarctica were conjugate to each other till the Late Triassic Period, both of which contributed major sediments to the P-G Gondwana Basin.

Transtension or transpression? Tectono-metamorphic constraints on the formation of the Monte Grighini dome (Sardinia, Italy) and implications for the Southern European Variscan belt

This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature (~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt.Graphical abstract

Petrography, geochemistry and monazite geochronology of crustal xenoliths hosted by Afonso Cláudio Intrusive Complex, Araçuaí-West Congo orogen (southeast Brazil): Insights about contamination, magma sources and evolution of the post-collisional magmatism

The Afonso Cláudio Intrusive Complex (ACIC) is a typical post-collisional intrusion from Araçuaí-West Congo orogen (AWCO). The ACIC intruded allanite-bearing orthogneisses from AWCO pre-collisional tectonic stage (Rio Doce arc related rocks) and paragneisses from Nova Venécia Complex and it is composed of two lowered off-centered monzogabbroic and monzodioritic cores surrounded by quartz monzonite high hills, showing intense magma mingling and mixing between these domains. Xenoliths attributed to the enclosing gneisses have been commonly found widespread in the quartz monzonitic rocks, while are occasional in the monzogabbroic and monzodioritic domain. This paper presents the first detailed investigation of the xenoliths hosted by an AWCO post-collisional intrusion, focusing on petrography, major and trace element geochemistry and U–Th–Pb monazite dating of the main xenolith types. For comparison, major and trace element data investigation of the ACIC enclosing rocks has also been conducted. The correlated xenoliths and ACIC enclosing rocks show partially similar geochemical features, however both show different REE and multi-element normalized patterns compared to the ACIC main rocks. These results suggest that correlated xenoliths and enclosing rocks are cogenetic, however both could not represent the main sources of magmas that generated the ACIC. Geochemical modeling of igneous processes of previous ACIC dataset considering the investigated xenoliths and enclosing rocks as the main assimilant material suggests that mixing was responsible for both ACIC main rocks similar geochemical patterns and linear trends in the bimodal diagrams. This modeling also indicates that the evolution of the ACIC main rocks was ruled by different processes. Fractional crystallization with minor influence of assimilation of enclosing rocks controlled the magmatic evolution of monzogabbro and monzodiorite related to an enriched mantle source, while coupled assimilation and fractional crystallization ruled the evolution of quartz monzonite possibly associated to lower crust magmas mainly contaminated by enclosing rocks. U–Th–Pb monazite dating of two xenoliths showed that these rocks preserved similar main ages related to different stages of ACIC and AWCO post-collisional evolution. Both xenoliths showed main older ages of 478 and 477 Ma which are related to ACIC intrusion during AWCO post-collisional stage, while younger ages of 431 and 427 Ma are probably related to the intrusion of syenogranite dykes that crosscut the ACIC, representing an extension of AWCO post-collisional magmatism or a later unrelated thermal event. Due to the similarities between the typical AWCO post-collisional intrusions, the insights brought about the evolution of ACIC magmas could be considered to the typical AWCO post-collisional mantle and crustal magmas. This first investigation of xenoliths hosted by an AWCO post-collisional intrusion has showed that these strange rock pieces could bring new and important insights about the AWCO evolution, which shows an amazing and long-lasting magmatism with common occurrence of poorly investigated xenoliths.

On the virtues and pitfalls of combined laser ablation Rb–Sr biotite and U–Pb monazite–zircon geochronology: an example from the isotopically disturbed Cape Woolamai Granite, SE Australia

Abstract Different mineral clocks in granite can provide age information reflecting various aspects of rock formation, including cooling or post-emplacement fluid–rock interaction. However, the dating tool chosen can yield inconclusive age information due to differences in closure temperatures and susceptibility to fluid alteration among chronometers. This has led to an inferred superiority of U–Pb in zircon over U–Pb in monazite or Rb–Sr in mica. Here, we investigate age systematics using Rb–Sr biotite grains, U–Pb in monazite and zircon in a Devonian granite from Australia. Single-grain laser ablation ICP-MS/MS biotite analyses are combined with zircon–monazite U–Pb ages and trace element systematics. Textural and trace element evidence combined with age systematics reveals a Rb–Sr closure age of c. 360–330 Ma relative to a putative 364 Ma emplacement age, suggesting hydrothermal alteration of the granite. Trace element systematics and magnetic susceptibility in biotite grains reflect their partial chemical reset and fluid overprint in the granite. However, similar systematics are also observed for zircon and monazite. Our multiple chronometer dating approach, studied with modern laser-ablation methods, highlights the need for detailed investigation of isotope and trace element systematics in single grains and that individual ages should be used cautiously when dating altered granitoids.

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.

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Monazite from lithium-bearing pegmatites of the Lipovskoye vein field, Middle Urals (composition and chemical dating)

The relevance of the research is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to study by isotope research methods. Purpose of the research is to study the chemical composition of monazite from lithium-bearing granite pegmatites of the Lipovskoye vein field and to determine their age. Research methodology. The quantitative analysis of the chemical composition of monazite was performed on a CAMECA SX 100 electron probe microanalyzer (IGG UB RAS, Ekaterinburg). Measurement conditions: accelerating voltage 15 kV, current strength 250 nA, electron beam diameter 2 μm. The pressure in the sample chamber is 2 ⋅ 10–4 Pa. The spectra were obtained on tilted 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. Standard samples: ThO2 , UO2 , Pb2 P2 O7 , diopside, synthetic rare-earth phosphates. The intensity measurement time at the peak for Th is 180 s, U is 100 s, and Pb is 500 s (240 s on one and simultaneously 260 s on another spectrometer), for Y and Si 20 s each, for the other elements 10 s; on the background – two times less. The detection limits for Th, U, and Pb in monazite are 290, 350, and 64 ppm, respectively. The oxygen content was determined under the assumption of the stoichiometry of the composition. Results. It has been established that monazite belongs to the cerium variety and is characterized by high contents of thorium (ThO2 up to 23.6 wt. %) and uranium (UO2 up to 2.5 wt. %). At the same time, cheralite-type isomorphism is realized in phosphate. In a closed Th–U–Pb-system (β = 0.92–0.97), according to the results of chemical dating (according to 20 analyses), monazite-(Ce) shows a weighted average age of 243 ± 7 Ma. When plotting the dependence (ThO2 + UO2 eq) – PbO, the points fall on one isochrone. Calculation of the age from the slope of the isochron gave a dating of 242 ± 17 Ma, MSWD = 0.21, probability = 1.00. Conclusion. It has been established that accessory monazite from lithium-bearing granitic pegmatites of the Lipovskoye vein field (Middle Urals) is of Triassic age. Apparently, this dating shows the time of the secondary transformation of lithium-bearing granitic pegmatites, which are often tectonized, and in some places even boudinaged.

One billion years of tectonism at the Paleoproterozoic interface of North and South Australia

The Mount Woods Domain, in the northeastern Gawler Craton, occupies a tectonically important location in Proterozoic Australia, yet there is very little published U–Pb geochronology data from this region to underpin tectonic models. New LA-ICP-MS U–Pb monazite and detrital zircon geochronology reveal Archean to Paleoproterozoic basement in the central Mount Woods Domain, comprising metasedimentary rocks and garnet-bearing granite with protolith ages of c. 2550–2400 Ma and metasedimentary rocks deposited after c. 1855 Ma. The southern Mount Woods Domain contains younger metasedimentary sequences deposited after 1750 Ma. Metamorphic monazite and zircon geochronology combined with phase equilibria modelling show the rocks of the central Mount Woods Domain were metamorphosed to granulite facies between 1700 and 1670 Ma, reaching pressure and temperature conditions of 4.8–5.3 kbar and 800–840 °C. Monazite geochronology from samples located along major shear zones and in the westernmost Mount Woods Domain record amphibolite facies metamorphism and reworking at 1570–1550 Ma, with a further phase of shear zone activity along the northern margin of the Mount Woods Domain at c. 1480 Ma. Laser ablation inductively coupled plasma triple quadrupole mass spectrometry (LA-ICP-QQQ-MS) Rb–Sr biotite ages from across the Mount Woods Domain range between 1480 and 1390 Ma. The protracted geological history in the Mount Woods Domain from c. 2500–1400 Ma provides a piercing point linking different regions of Proterozoic Australia and western Laurentia during the tenure of the Nuna supercontinent.

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.

The tectonic evolution of Thelon tectonic zone, Canada: a new model based on petrological modeling linked with Lu–Hf garnet and U–Pb accessory mineral geochronology

Petrological modeling and geochronology (Lu–Hf garnet, U–Pb monazite, zircon, and titanite) for seven mid-amphibolite to granulite-facies metamorphic rocks in the Thelon tectonic zone (Ttz) provide new insights into the evolution of this orogen. A Grt-Sp-Sil diatexite records 2.01–2.00 Ga garnet and monazite growth during >830 °C contact metamorphism associated with voluminous convergent margin plutonism. The most widespread, 1.96–1.90 Ga metamorphism is associated with clockwise pressure ( P)–temperature ( T) paths, indicating it was driven by crustal thickening. Earlier (1.96–1.92 Ga) and lower temperature (630 °C–730 °C; 6–8 kbar) metamorphism in the eastern Slave craton contrast with ∼860 °C (7.5 kbar), 1.91–1.90 Ga diatexites in the central, Paleoproterozoic plutonic rock-dominated, core of the Ttz. These differences are interpreted to reflect lower vs. upper plate settings, respectively. A contribution of mantle heat is suggested by the counter-clockwise P–T path and 885 °C (5.7 kbar) conditions recorded by diatexite associated with c. 1.9 Ga peraluminous leucogranite that dominates a corridor along the Ttz–Rae craton boundary. A tectonic model is proposed wherein the Ttz evolved at an accretionary margin after rifting of Ttz microcontinent (mTtz) off the Rae craton at c. 2.2–2.1 Ga. Voluminous plutonism at 2.01–1.98 Ga formed via east-dipping subduction under mTtz. Following 1.97 Ga collision of the Slave craton and mTtz, west-dipping subduction generated 1.96–1.95 Ga plutonism in the composite mTtz-Slave upper plate. Collision of Rae craton at 1.95–1.94 Ga led to crustal thickening, widespread 1.91–1.90 Ga high-grade metamorphism and extensive crustal melting in the central Ttz.

Carboniferous–Triassic tectonic and thermal evolution of the middle crust section of the Dervio–Olgiasca Zone (Southern Alps)

AbstractA well‐preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio–Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre‐Alpine (rift‐related) history. We reconstruct the P–T–t–d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U–Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono‐thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite‐facies Carboniferous metamorphic basement (7.6–10 kbar and 610–660°C at 318–312 Ma) was affected by pervasive extensional deformation (5.1–7.6 kbar and 580–660°C) in the Middle‐ to Late‐Permian (257.5 ± 3.8 Ma). Pegmatite intruded at 249.8 ± 1.1 Ma in an extensional phase that re‐equilibrated rocks of the basement at 3.5–4.5 kbar and 560–600°C. During the Middle‐ to Late‐Triassic (241–235 Ma), the basement experienced static thermal recrystallization (T = 689 ± 41°C and ~5.0 kbar). This Late‐Anisian to Early‐Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large‐scale extensional detachments developed in the upper crust (e.g., the Lugano‐Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post‐Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting.

Late Mesoproterozoic to early Neoproterozoic evolution of the Meghalaya gneissic complex, NE India: significance in Rodinia assembly

ABSTRACT The Meghalaya Gneissic Complex (MGC) of northeast India is a little-studied high-grade metamorphic terrain, located close to the Late Palaeoproterozoic to early Neoproterozoic orogenic front. We include mapping, petrography, mineral chemistry, geochemistry, phase equilibria modelling, and U-Th-total Pb monazite geochronology to investigate the evolutionary history of deformed and metamorphosed ortho- and para-gneisses from the central MGC. The metapelite and metamafic rocks of the MGC are folded to E-W upright, antiforms-synforms; and intruded by charnockites during progressive deformations. Reconstructing the metamorphic history of pelite (garnet-cordierite granulite) and two pyroxene granulite, using a TiMnNCKFMASH and TiNCKFMASHO system, reveals a clockwise P–T path where the peak metamorphism was associated with mid crustal burial heating to granulite facies (~825°C and 5.5 to 6.5 kbar) and subsequent cooling with a minor decompression (~ 816°C and 5.3 kbar). Chemically zoned monazites were grown episodically during 1035 ± 13 Ma (n = 21), 976 ± 14 Ma (n = 16), and 494 ± 14 Ma (n = 26). The granulite metamorphism here initiated at 1035 ± 13 Ma and reached thermal peak at 976 ± 14 Ma. This contribution emphasizes the importance of late Mesoproterozoic to early Neoproterozoic metamorphic evolution of the MGC related to the collision of North and South Indian cratonic blocks. The studied P-T path differs from the previously reported Meso- to early Neoproterozoic paths of Central and Western India and close to Western Australia. This finding further explains the change in the early Neoproterozoic collisional pattern of the Central Indian Suture in extreme east (near MGC) and its significance in reconstructing the western Rodinia.

Rhyacian evolution of high-grade metamorphic rocks of the Porto Nacional Granulite Complex, based on geocronological data U-Pb-Hf in zircon and U- Pb in monazite

The Porto Nacional Complex in the northern portion of the Goiás Massif, in north-central Brazil, represents a Paleoproterozoic/Archean microcontinent reworked in the Neoproterozoic during the Brasiliano Cycle. This NE-SW-trending belt of high-grade metamorphic rocks consists, predominantly, of orthogranulites in addition to aluminous paragneisses that reached high-grade metamorphic conditions in the granulite facies (∼860 °C and 9 kbar). The zircon U–Pb geochronological results obtained on orthogranulite (enderbite) crystal cores yielded an age of 2.16 Ga, however, the crystal borders present lower values (2.09 Ga). These results represent, respectively, the age of igneous crystals of tonalitic protoliths, and high-grade metamorphism. The zircon crystals of another enderbite sample yielded lower values of about 2.09 Ga, reinforcing the interpretation of metamorphic zircons. The U–Pb geochronological results between 2.09 Ga and 2.10 Ga obtained on monazite from the aluminous paragneisses, coincide with the values obtained for enderbite metamorphic zircon crystals, allowing us to define the high-grade metamorphism age and the formation of igneous protoliths in the Rhyacian. This small age difference demonstrates the metamorphism role contemporary to the magmatism of the tonalitic protoliths of the Porto Nacional Complex, related to the Late-Transamazonian orogeny. The Hf-TDMC model ages suggest two crust formation episodes that gave rise to the following rocks: one from the Siderian (2.40–2.48 Ga) and another from the Neo-Mesoarchean (2.52–3.01 Ga). The positive petrogenetic parameters ƐHf(t) (+3.9 to +5.2) evidence a mantle source derivation for the Siderian material, while the negative and positive petrogenetic parameters ƐHf(t) (−4.6 to +3.3) indicate a mixture between crustal and juvenile material for the Neo-Mesoarchean material. These results allow us to disregard the proposed correlation between this high-grade metamorphic terrain with the Porangatu-Talismã Granulitic Belt, which occurs to the south of this region, whose thermo-tectonic evolution is related to the late Neoproterozoic. On the other hand, there are records of several other Rhyacian similar granulitic terrains, such as the granulites of the Bacajá and Tartarugal Grande in the Amazonian Craton, the Itabuna-Salvador-Curaçá Granulitic Belt in the São Francisco Craton, as well as in the Bakhuis Belt, in Suriname, and Limpopo Belt in the West African Craton.

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.

Neoproterozoic reworking of a Mesoproterozoic magmatic arc from the north-eastern part of the Central Indian Tectonic Zone: Implication for the growth and disintegration of the Indian shield in the Proterozoic supercontinental cycles

Thermotectonic evolution of the rocks of the Central Indian Tectonic Zone (CITZ) is crucial to understand the growth of the Indian shield and its response to the supercontinental cycles of the Earth. In this study we present field, petrology, geochemistry, in-situ U-Pb zircon and Th-U-total Pb monazite dates from a suite of felsic orthogneisses from the least studied Makrohar Granulite Belt (MKGB) of the CITZ. Petrology and geochemical data identify two rock groups: charnockitic orthogneiss (COG) and garnet orthogneiss (GOG). Geochemical fingerprints suggest that the magmatic protoliths of the gneisses are S-type granitoids, formed in a continental arc setting. In situ U-Pb zircon dates yield the time of arc magmatism at ca. 1400–1350 Ma. This is the first report of mid-Mesoproterozoic arc magmatism in the CITZ. These arc granites were subsequently metamorphosed at granulite facies condition that culminated at ∼800–850 °C, 7–7.5 kbar. A prominent gneissic (migmatitic) fabric was formed with garnet ± orthopyroxene + plagioclase + K-feldspar + quartz as peak metamorphic assemblages. After reaching the recorded P-T maxima, the studied rocks followed a decompression and cooling path to 4 kbar and 500 °C. The inferred clockwise P-T path is consistent with reworking of the Mesoproterozoic arc granites in a continent–continent collisional setting. In-situ Th-U-total Pb monazite and in-situ U-Pb ages of zircon overgrowth constrain the age of deformation and metamorphism in the span of ca. 974–913 Ma. In combination with the published information, the results of this study are consistent with the views that (a) the Indian shield participated as a coherent block in the Columbia supercontinent since ca. 1450 Ma (or before), (b) the ca. 1400–1350 Ma arc magmatism is presumably linked to collapse or closure of a basin (either north or south of the MKGB) in the CITZ in response to the prolonged accretionary phase of the Columbia supercontinent and (c) during the assembly of the Rodinia supercontinent, localized dismembered fragments of the Columbia got re-amalgamated during late Mesoproterozoic to early Neoproterozoic. Archean cratons got fused along the Proterozoic mobile belts (such as Central Indian Tectonic Zone, Eastern Ghats Mobile Belt). Different units within these mobile belts also experienced local compression during this time frame of the Grenvillian orogeny in the Indian subcontinent.

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

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

Pressure–temperature–time constraints on metamorphism in the southeastern Taltson Domain, Saskatchewan, Canada

The southeastern Taltson Domain is located on the southern margin of the Rae Craton in northern Saskatchewan, Canada, and encompasses the region south of the Athabasca Basin and west of the Snowbird Tectonic Zone. There has been relatively little metamorphic geochronology and no quantitative pressure–temperature (P–T) constraints from the southeastern Taltson Domain, which hinders attempts to link its metamorphic history to surrounding regions. New in situ LA-ICP-MS U–Pb monazite and zircon ages from rare metasedimentary rocks from the southeastern Taltson Domain reveal a polymetamorphic history. A cordierite–sillimanite–spinel-bearing migmatite from Fournier Lake records high thermal gradient metamorphism at 1988 ± 21 Ma, reaching P–T conditions of 4–6.4 kbar and > 900 °C. The cordierite–sillimanite–spinel assemblage overprints an earlier biotite–sillimanite-bearing assemblage that formed at 2075 ± 31 Ma. An orthopyroxene–garnet gneiss from within the Snowbird Tectonic Zone has a dominant monazite population at c. 1.90 Ga, but contains rare monazite ages between c. 2.19–2.01 Ga that support a phase of metamorphism prior to 2.0 Ga. The main metamorphic event to have affected the southeastern Taltson Doman occurred at c. 1.94–1.90 Ga. Phase equilibria modelling suggests peak P–T conditions were 8.3–11.2 kbar and 800–930 °C. Monazite with high HREE contents yields weighted mean ages of 1926–1918 Ma, interpreted to reflect prograde metamorphism prior to significant garnet growth. Zircon from an extremely garnet and zircon rich sample (now silicified and chloritized as a result of younger alteration) formed at c. 1918 Ma. REE compositions of the zircon indicate that it formed in equilibrium with garnet at temperatures of ∼ 830 °C, suggesting peak metamorphism occurred around c. 1.92 Ga. HREE-poor monazite with weighted mean ages of c. 1910–1895 Ma formed after the majority of garnet growth and either reflect the timing of melt crystallisation or recrystallisation during retrogression and fluid flow along major shear zones in the region. The P–T conditions and c. 1.93–1.90 Ga ages from the southeastern Taltson Domain are very similar to other rocks in the Dodge–Snowbird Domain of the Rae Craton, and demonstrate that the footprint of mid-P granulite facies metamorphism extends south and west into the southeastern Taltson Domain.

Late Neoproterozoic extended continental margin development recorded by the Seve Nappe Complex of the northern Scandinavian Caledonides

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record the opening and closure of the Iapetus Ocean, culminating with the collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises relics of the outermost Baltoscandian passive margin that were subducted to mantle depths during ocean closure. Subduction of these rocks overprinted much of the Neoproterozoic record for Iapetus Ocean formation, and as a result, much of the work conducted in the SNC has focused solely on the Caledonian orogenic history.In this study, we combined petrological and geochronological work to expand the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the understudied Váivančohkka-Salmmečohkat region of the northern Scandinavian Caledonides. The work focused on rocks that belong to the upper gneiss unit, which constitutes part of SNC in the region. The unit comprises migmatitic paragneisses and garnet-mica schist containing metamafic bodies. U-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age of metamorphism in 602 ± 5 Ma and 599 ± 3 Ma, respectively. A similar U-Pb age of 604 ± 8 Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian ages were identified. Likewise, no evidence for high or ultra-high pressure conditions was found, neither in the gneisses/schists nor in the metamafic rocks. Petrographic observations and calculated metamorphic P-T conditions indicate that rocks belonging to upper gneiss unit underwent upper-amphibolite facies metamorphism in a melt stability field; 8.0–10.5 kbar at 750–790 °C.Since the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism, the studied part of the SNC offers an extraordinary insight into the Late Neoproterozoic history of the hot, extended Baltoscandian margin.

Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation

AbstractClassic tectonic models for the Caledonian orogeny in Scotland involve Ordovician collision of Laurentia–Midland Valley arc (Grampian orogeny), followed by middle Silurian collision of Laurentia–Baltica (Scandian orogeny) and 500–700 km of sinistral displacement along the Great Glen fault separating the Northern Highlands (Moine Supergroup) from the Grampian Highlands (Dalradian Supergroup). A review of the timing of magmatic and metamorphic rocks across Scotland allows a simpler explanation that fits with a classic Himalayan-style continent–island arc–continent collision. Late Cambrian – Early Ordovician NW-directed ophiolite obduction (Highland Border complex) coincided with the ending of stable continental shelf sedimentation along the eastern margin of Laurentia. Following collision between Laurentia and the Midland Valley arc–microcontinent in Early Ordovician time, crustal thickening and shortening led to almost continuous regional metamorphism from c. 470 to 420 Ma, rather than two discrete ‘orogenies’ (Grampian, Scandian). U–Pb monazite and garnet growth ages indicating prograde metamorphism, and S-type granites related to melting of crustal protoliths are coeval in the Grampian and Northern Highlands terranes. There is no evidence that the Great Glen fault was a terrane boundary, and strike-slip shearing post-dated emplacement of Silurian – Early Devonian granites. Late orogenic alkaline granites (c. 430–405 Ma) in both Moine and Dalradian terranes are not associated with subduction. They are instead closely related to regional alkaline appinite–lamprophyric magmatism resulting from simultaneous melting of lower crust and enriched lithospheric mantle. Caledonian deformation and metamorphism in northern Scotland, with continuous SE-directed subduction, show geometry and time scales that are comparable to the Cenozoic India–Kohistan arc–Asia collisional Himalayan orogeny.