Metamorphic Monazite Research Articles

Paleoproterozoic/Mesoproterozoic tectonism in the northern fringe of the Western Dharwar Craton (India): Its relevance to Gondwanaland and Columbia supercontinent reconstructions

AbstractA newly discovered suite of Paleoproterozoic/Mesoproterozoic amphibolite facies supracrustals and foliated granitoids between Pernem and Phonda along the northern Konkan coast of western India limits the northern boundary of the Archean Western Dharwar Craton (WDC). Top‐to‐the south transport of the suite is associated with progressively southward tightening of folds with down‐dip stretching lineation, obliteration of pre‐existing fabrics, and widespread stabilization of garnet (lacking in the WDC) along a mid‐crustal counterclockwise pressure‐temperature path. Th‐U‐total Pb ages in metamorphic monazites in the WDC supracrustals are 2.5 Ga; chemical ages of metamorphic monazites (2.3 Ga, 2.2 Ga, 1.9 Ga, and 1.7 Ga) in the WDC‐limiting Pernem‐Phonda tectonic zone decreases northwards. Based on the finding, we suggest that the southern boundary of the Central Indian Tectonic Zone (CITZ)—the accretion zone between the North and the South Indian Blocks—should be relocated ~ 550 km south from its perceived location near the Gulf of Cambay. In spite of the apparent geometric fit, we suggest the CITZ and the Trans North China Orogen may not be continuous as has been suggested in some Paleoproterozoic reconstructions of the Columbia supercontinent. The monazite ages in the footwall schists are similar to 2.5, 2.25, and 2.10 Ga Pb‐Pb ages in detrital zircon in the Sahantaha Group located north of the Paleoarchean Antongil‐Masora Block in Madagascar. We suggest that the Paleoproterozoic/Mesoproterozoic shear zone system extended westward into Madagascar north of the Antongil Block in East Gondwanaland, and this may clarify the unexplained source of zircon in the Sahantaha quartzites.

Constraints on the timing of late-Eburnean metamorphism, gold mineralisation and regional exhumation at Damang mine, Ghana

The Damang gold deposit in southwest Ghana is unique among known deposits in Ghana, comprising gold mineralisation in two distinct styles. Early gold, hosted in a stratigraphically controlled, auriferous quartz-pebble conglomerate horizon, is overprinted by later gold mineralisation contained in a sub-horizontal fault-fracture quartz vein array. A multi-system geochronological study is used to constrain the timing of igneous activity, regional metamorphism, gold mineralisation and the thermal history at Damang. U/Pb analysis of zircons from Birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178.0±9.3Ma and 2164.6±8.0Ma respectively, which is consistent with previous studies. The age of formation of staurolite-grade, amphibolite facies peak metamorphic mineral assemblages at 2005±26Ma is provided by U–Th–total Pb EPMA analysis of metamorphic monazite grains in the Tarkwa Phyllite. Measured 40Ar/39Ar biotite ages range between 1980±9Ma and 1898±11Ma. Argon diffusion modelling with the programme DIFFARG suggests that this age range could be achieved by a period of rapid cooling, at a rate of approximately 17°C/Ma, followed by a prolonged period of much slower cooling, at a rate of 0.15°C/Ma. The period of rapid cooling is interpreted to represent localised exhumation of the Damang host rocks during the latest stage of the Eburnean orogeny at the time of hydrothermal gold mineralisation. Given these age constraints, hydrothermal gold mineralisation is inferred to have occurred between approximately 2030Ma and 1980Ma. These ages constrain metamorphism, fluid flow and gold mineralisation at Damang and are the youngest currently recognised in the Birimian of SW Ghana.

LA–ICP–MS monazite U–Pb age and trace element constraints on the granulite-facies metamorphism in the Tongbai orogen, central China

Granulite-facies metamorphic rocks in the Tongbai orogen have great implications for the evolution of the whole Qinling–Tongbai–Dabie orogenic belt. However, the age and tectonic setting of the granulite-facies metamorphism have not been well constrained yet. In this contribution, BSE imaging, trace element analysis and U–Pb dating of monazite were undertaken for three felsic granulites, a gneiss and a leucosome from the Tongbai orogen. Most monazite grains from the granulite and gneiss samples exhibit no zoning, relatively low Th/U ratios, significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Their formation ages range from 423±2 to 430±3Ma with a weighted mean of 426±5, which is taken as the timing of the peak granulite-facies metamorphism. In the leucosome sample, the monazite shows a euhedral to subhedral shape, coupled with a core and rim structure. Besides being depleted in HREEs and Y, both of them have significantly negative Eu anomalies and variable U contents (1220–13323ppm) and Th/U ratios (7.2–136.7). This suggests that their formation was coeval with the crystallization of feldspar, metamorphic zircon, and garnet. The monazites yield a weighted mean age of 426±3Ma, suggesting nearly coeval partial melting during granulite-facies metamorphism. The relationship between metamorphic monazite and zircon within the same metamorphic sample is complex as revealed by metamorphic monazite and zircon U–Pb ages for the gneiss, granulite and leucosome samples. In the other two granulite samples, the monazite yields U–Pb ages of 423±2 and 425±3Ma, whereas only little overgrowth or recrystallization of metamorphic zircon. This indicates that monazite is more sensitive than zircon in recording of metamorphic age even at the granulite-facies conditions. Combined with previous metamorphic zircon U–Pb age results, the prograde, peak, and retrograde stages of the granulite-facies metamorphism in the Tongbai orogen can be constrained at ca. 440, 426, and 415Ma, respectively. Therefore, the granulite-facies metamorphism would have lasted for more than 20Myr. The prolonged granulite-facies metamorphism might result from the continuous northward subduction of the Paleotethyan oceanic crust beneath the North Qinling terrane.

Criteria to distinguish between regional and contact zone monazite – a case study from Proterozoic North Delhi Fold Belt (NDFB), India

North Delhi Fold Belt (NDFB) is a part of a NNE-SSW trending Proterozoic mobile belt hosting sulfide mineralization in Khetri Copper Belt (KCB) towards its western margin. It preserves assemblages of amphibolite facies Barrovian metamorphism locally overprinted by a thermal metamorphic event. Monazite (REEPO 4 ) is developed in both the assemblages. Detailed chemistry and chemical geochronology of monazites in regional and contact metamorphic assemblages depict distinct chemical traits and ages. Two stage growth of metamorphic assemblages at ∼1100 Ma and ∼920Ma is recorded. Contact zone monazite grew at ∼650Ma under hydrothermal conditions. It has low thorium and is cerium enriched in comparison to the regional metamorphic monazite. Low Th and high Ce/Th ratio in contact zone monazite is proposed as discriminating criteria between regional and contact metamorphic monazites.

Early Mesoproterozoic metamorphism in the Barossa Complex, South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ∼6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.

Large-scale metal zoning in a late-Precambrian skarn-type mineralization, Wadi Kid, SE Sinai, Egypt

A Precambrian skarn-type mineralization is recently discovered in the Wadi Kid area in southeast Sinai, Egypt. Two sulfide ore types define large scale metal zoning; Cu–Zn–Co in calc-silicate rocks and Zn–Pb–As–Ag in metapelites. The sulfides and host rocks underwent amphibolite facies metamorphism (2.1–4.2kbar and 500–620°C). Dating by the chemical Th–U-total Pb isochrone method yields an Th–Pb isochrone age of 660±25Ma for metamorphic monazite from metapelites. Overall structural and textural relationships of silicate and sulfide minerals favor syn-tectonic formation during granitoids emplacement in a continental margin setting. Large-scale metal zoning reflects variable distances from the causative pluton(s). The Wadi Kid area is highly prospective for Cu, Zn, Pb and Ag mineralization.

Paleoproterozoic metamorphic and deformation history of the Thompson Nickel Belt, Superior Boundary Zone, Canada, from in situ U–Pb analysis of monazite

The Thompson Nickel Belt is a 35km×200km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny, resulting in an elongate, nested metamorphic zonal pattern, parallel to the overall trend of the belt. The metamorphic pattern consists of an inner middle amphibolite-facies zone, flanked on both sides by upper amphibolite-facies zones, flanked in turn by granulite-facies zones. U–Pb data were collected by laser ablation–multiple collector–inductively coupled plasma–mass spectrometry from metamorphic monazite in polished thin sections from each of the metamorphic zones. In the middle amphibolite-facies and upper amphibolite-facies zones, monazite in metapelite is interpreted to have developed during the dominant, foliation-forming deformation event and yields ages of ca. 1776 and 1779Ma, respectively. Metapelite from the eastern granulite-facies zone contains ca. 1834Ma monazite grains intergrown with sillimanite that is pseudomorphous after andalusite and grew during the dominant foliation-forming event. Metawacke from the eastern granulite-facies zone contains ca. 1826Ma and 1792Ma monazite grains that are interpreted to be contemporaneous with the main foliation-forming event and were unstable in the presence of melt, and a third population of ca. 1748Ma monazite grains. In the western granulite-facies zone, metapelite contains ca. 1803Ma monazite grains intergrown with sillimanite that grew during the dominant foliation-forming event, and meta-iron formation contains ca. 1752Ma monazite grains characterized by oscillatory zoning and associated with melt pseudomorphs. A tectonic model is presented in which the initiation of collision between the Reindeer Zone and the Superior craton begins around ca. 1850–1840Ma. With continued convergence, rocks now present in the western and eastern granulite-facies zones were progressively buried and heated to middle amphibolite-facies conditions by ca. 1803 and 1792Ma, respectively. As the collision progressed, rocks now present in the upper amphibolite-facies and middle amphibolite-facies zones were buried and subjected to lower amphibolite-facies metamorphic conditions by ca. 1779 and 1776Ma, respectively. Progressive regional metamorphism may have been punctuated by local thermal events related to magmatism, such as at ca. 1830Ma in the eastern granulite-facies zone. Post-peak metamorphic cooling and differential uplift of crustal blocks related to transpressional deformation is recorded by ca. 1752 and 1748Ma monazite grains from the western and eastern granulite-facies zones, respectively.

Detrital monazite geochronology, Upper Jurassic–Lower Cretaceous of the Scotian Basin: significance for tracking first-cycle sources

Abstract Monazite geochronology was applied to an east–west transect of latest Jurassic and Lower Cretaceous deltaic sandstones of the Scotian Basin, to assess sediment sources and dispersal pathways. More than 200 detrital monazite grains yielded 694 electron microprobe age determinations with 1σ errors <±20%. Based on age, external morphology, zoning, inclusions and major element chemistry (rare earth element [REE], Th, Y), monazite grains represent more than 20 discrete sources. Similar proportions of euhedral and subhedral compared with irregular and rounded monazite grains in most age classes, together with comparison with detrital muscovite and zircon geochronology, suggest that most monazite is first cycle. Six types of REE distribution are recognized (A–F). Many igneous monazites show chemical zoning, contain sparse euhedral inclusions, and have REE distributions of types A and E. Many metamorphic monazites contain inclusions, commonly aligned, are generally rounded–subhedral to rounded, and have REE distributions of types B, C and D. Monazite geochronology shows important supply to the Scotian Basin from the Labrador rift shoulder as early as Tithonian; from Avalonian sources in the Tithonian; from Ordovician sources in northern New Brunswick, apparently via the Chaswood River; and from the inner continental shelf, particularly in the Hauterivian–Barremian.

Versatile Monazite: resolving geological records and solving challenges in materials science: Generalizations about monazite: Implications for geochronologic studies

With the advent of techniques that preclude mineral separation, ages from specific compositional domains in monazite [(Ce,La,Th)PO 4 ] have provided a wealth of information regarding the timing of the geologic evolution of numerous regions. However, confusion can arise when single grains show large differences in age that fail to correlate to chemistry or location within the monazite. Generalizations that lead to incorrect age interpretations include that monazite zoning in Y, Th, and/or the rare earth elements (REE) always identify: (1) distinct tectonic events; (2) environment of crystallization; and (3) provenance of detrital grains. Increasing Th contents in monazite do not always reflect: (1) increasing grade in metamorphic grains; (2) changes in silicate melt composition in igneous grains; (3) make the mineral more susceptible to alteration; nor (4) control the mineral’s uptake of REE. Metamorphic monazites from Himalayan garnet-bearing rocks with coexisting allanite show no relationship between Th content and REE. Instead, chondrite-normalized REE patterns of the allanite mirror those of the monazite, indicating the variations are related to the reactant that formed the mineral. Generalizations about Pb behavior in monazite remain problematic. Incorporation of Pb into monazite has thus far been precluded by experimental studies, yet common Pb has been measured in many studies of natural monazite. A clear understanding about controls of monazite composition and the role of the chemical and/or pressure-temperature ( P - T ) environment of the rocks in which it forms is required to correctly interpret the meaning of the mineral’s age(s).

Constraints on the Devonian–Carboniferous closure of the Rheic Ocean from a multi-method geochronology study of the Staré Město Belt in the Sudetes (Poland and the Czech Republic)

This paper attempts to clarify the complex nature of how and when the Rheic Ocean closed in what is now Central Europe and, with respect to the various terranes that were involved, offer a regional chronology for the associated structural, metamorphic and igneous processes that accompanied and followed this closure. The Variscan orogen in Europe originated from the multiple collisions of Gondwana-derived terranes (the Armorican Terrane Assemblage) with Laurussia: however, many important structural details on the timing of these collision-related events remain obscure. In the Sudetes, the Staré Město Belt represents a WNW-dipping part of the Rheic suture that developed from the continental collision of the eastern terranes of the Armorican Terrane Assemblage (now in the Bohemian Massif) with the Brunovistulian Terrane (a part of Laurussia/Old Red Continent). In this study, the results of monazite Th–U–total Pb, garnet Lu–Hf and zircon U–Pb geochronology were integrated into a newly established D1–D3 tectonometamorphic sequence. A Th–U–total Pb age of ~368Ma from a monazite that grew concurrently with D2 metasedimentary garnet, as well as Lu–Hf ages of ~361Ma and ~355Ma obtained from D2 metasedimentary garnets, implies that the regional contractional deformation and progressive metamorphism of D2 took place mainly during the Famennian (Late Devonian) and extended into the Visean (Middle Mississippian of the Early Carboniferous). The ion probe U–Pb zircon ages of ~355Ma and ~359Ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during D1–D2. Metamorphic monazites dated at ~340Ma provide a time for the subsequent D3 dextral transpressional regime. The closure of the Staré Město Belt segment of the Rheic Ocean probably resulted from a head-on hard collision between the westerly subducting Brunovistulian promontory of Laurussia and the eastern members of the Armorican Terrane Assemblage. Thus, the Rheic Ocean closed during the Late Devonian at ~370–360Ma and preceded the collision of the Armorican Terrane Assemblage with East Avalonia at the western margin of what is now the Bohemian Massif. Following ocean closure, the Rheic slab may have broken off, resulting in the suture zone becoming dominated by lateral “tectonic escape” movements of the colliding terranes at ~340–330Ma (Visean). Syntectonic D3 intrusions of granodiorite/tonalite magma acted as a hot lubricant and stitched the suture zone together.

A geochronological and petrological study of anatectic paragneiss and associated granite dykes from the Day Nui Con Voi metamorphic core complex, North Vietnam: constraints on the timing of metamorphism within the Red River shear zone

AbstractThe Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.

Combined thermobarometry and geochronology of peraluminous metapelites from the Karakoram metamorphic complex, North Pakistan; New insight into the tectonothermal evolution of the Baltoro and Hunza Valley regions

AbstractPetrographic analysis of peraluminous metapelites from two separate regions of the Karakoram metamorphic complex, North Pakistan, has produced new insights into the P–T–t evolution of the deep crust along the south Asian margin before and after the India‐Asia collision. Average P–T estimates and pseudosection construction in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (MnNCKFMASHTO) system using THERMOCALC have provided prograde and peak metamorphic conditions and U–Pb geochronology of metamorphic monazite has provided age constraints. Two new events in the tectonothermal evolution of the Hunza Valley have been documented; an andalusite‐grade contact metamorphic event at 105.5 ± 0.8 Ma, at unknown P–T conditions, associated with the widespread subduction‐related granite magmatism before the India‐Asia collision, and a kyanite‐grade overprint of sillimanite‐grade rocks with peak P–T conditions of ∼7.8 kbar, 645 °C at 28.2 ± 0.8 Ma associated with the ongoing India‐Asia collision. A kyanite‐grade event observed in the Baltoro region with similar peak P–T conditions (∼7.4–8.0 kbar, ∼640–660 °C) is interpreted to have occurred sometime after 21.8 ± 0.6 Ma, however, previous studies have suggested that this event commenced in the Baltoro as early as c. 28 Ma. A calculated prograde P–T path for this kyanite‐grade event in the Baltoro indicates that garnet first nucleated on an initially high geothermal gradient (∼30 °C km−1) and grew during a significant increase in pressure of ∼2.6 kbar over a temperature increase of ∼100 °C. This event is thought to represent evidence for conductive heating of the middle crust during early stages of intrusion and lateral migration of the Baltoro batholith, with thermal conditions comparable with tectonic models of magmatic over‐accretion.

Mesoarchean Epithermal Gold Mineralization Preserved at Upper Amphibolite-Facies Grade, Qussuk, Southern West Greenland

The Qussuk gold occurrence, located in southern West Greenland (North Atlantic Craton), is a flattened, transposed, and metamorphosed, 20 km long Au ± Cu system interpreted as an epithermal mineralization system of Mesoarchean age. The Qussuk mineralization is hosted in the 3.075 Ga volcanosedimentary rocks of a relict andesitic arc. The volcanic arc component is embedded in 3.06 to 3.0 Ga plutonic, predominantly tonalitic rocks and was gradually deformed and metamorphosed as the original arc matured into a microcontinent. Metamorphism culminated with a sillimanite-grade amphibolites-facies thermal event and mobilization of granite at around 2.995 to 2.975 Ga. The premetamorphic hydrothermal alteration associated with the mineralization includes acid leaching revealed by quartz-alumina rocks with very high rare earth element (REE) contents and Ga-Al fractionation, and subsequent reintroduction of LIL elements including K, Th, and U together with the mineralizing agents. Premetamorphic carbonate alteration is widespread but unrelated to the mineralized system. Complexly zoned zircon with igneous-hydrothermal-metamorphic histories and metamorphic monazite bracket the mineralizing event to 3.075 to 3.02 Ga (most likely >3.06 Ga) in age. The zoned zircons have volcanic/igneous, 3.075 Ga oscillatory zoned cores with igneous-type REE distributions, <3.07 Ga hydrothermal spongy growth phases, diverse REE distributions and abundant inclusions of quartz and biotite, and homogeneous metamorphic, 2.995 Ga rims. Geochronological, petrographical, and geochemical evidence demonstrates that the gold mineralization at Qussuk was formed prior to peak deformation and metamorphism. The Qussuk gold occurrence illustrates that epithermal gold deposits existed at the Archean, and that they can be preserved up to upper amphibolite metamorphic conditions, further highlighting the prospectivity of ancient high-grade cratons and volcanic belts worldwide for primary gold deposits.

The age and rate of displacement along the Main Central Thrust in the western Bhutan Himalaya

In western Bhutan, the Main Central Thrust (MCT) is broadly folded, creating multiple exposures of the fault surface over a ~70km across-strike distance. This unusual map pattern presents a unique opportunity to map the MCT and document both the magnitude and age of displacement. In situ Th-Pb (SIMS and LA-ICP-MS) geochronology of metamorphic monazite from the immediate hanging wall of the MCT indicates that prograde monazite growth in Greater Himalayan (GH) rocks continued until 20.8±1.1Ma, whereas crystallization of in situ melts, characterized by high Y monazite overgrowths, occurred during cooling from ca. 15–10Ma. Prograde monazite growth at 15Ma in Lesser Himalaya (LH) rocks in the immediate footwall requires that LH footwall strata began to be buried at this time, and the MCT had reached its southernmost, exposed extent. By combining prograde monazite ages in the immediate hanging wall and footwall, the duration of MCT displacement is bracketed between 20.8±1.1 and 15.0±2.4Ma. Immediately north of our study area, a published estimate of shearing along the outer-South Tibetan detachment (STD) argues for displacement between 20 and 15Ma, coeval with the age range for MCT displacement that we document in this study. However retrograde monazite grains as young as 10Ma suggest that GH rocks were cooling until ~10Ma, 5Myr later than motion on the outer-STD immediately to the north. This cooling was either the result of continued displacement on the MCT, or growth of a duplex that passively folded the MCT. Using a sequential reconstruction, we estimate a total displacement of ~230km, which is the sum of displacements on the MCT and the structurally-lower Paro Thrust, over a duration of 5.8±2.6Myr. This indicates a horizontal shortening rate of 4.0+3.2/−1.3cm/yr, which exceeds present rates estimated from geodetic measurements across the Himalaya, and MCT displacement rates (c. 2cm/yr) inferred from petrologic and thermal models in central Nepal but is indistinguishable from plate convergence rates calculated for eastern Bhutan between 23 and 20Ma (3.3±0.7cm/yr). Our study highlights that displacement on the MCT alone achieved plate velocity rates in western Bhutan, and that the age and rate of MCT displacement varied significantly across the Himalaya.

U–Pb ages of metamorphic monazite and detrital zircon from the Northampton Complex: evidence of two orogenic cycles in Western Australia

The Pinjarra Orogen, stretching along the western coast of Western Australia, is one of the least understood orogens on the continent. Exposure is limited to fault-bound basement blocks within the Phanerozoic Perth Basin, which have been interpreted as either remnants of a Mesoproterozoic orogen or as allochthonous blocks within a Neoproterozoic orogen. Thus even the timing of orogenesis is controversial. However, since the orogen is in a critical position for reconstructions of both Rodinia and Gondwana, a better understanding is highly desirable. New U–Pb ages of detrital zircon cores, metamorphic zircon rims and metamorphic monazites from the Northampton Complex, the northernmost and largest of the exposed basement blocks, provide better insights into the sedimentary provenance and metamorphic history of the metasediments exposed in this part of the orogen. Both the monazite ages and the youngest metamorphic zircon rims constrain metamorphism in the Northampton Complex to 1090–1020Ma. Additionally, a large number of older metamorphic zircon rims pre-date this event and were inherited from the Albany-Fraser Orogen in southwestern Australia, which is also identified as the main source region for the detrital zircon cores. Additional sources of detrital zircons are the Mawson Continent (the South Australian Craton and parts of East Antarctica) and possibly the North Australian Craton, as well as synsedimentary volcanic rocks. However, the combination of ages from the cores and inherited metamorphic rims shows that zircons from the Mawson Continent/North Australian Craton were not directly transported to the Northampton Complex but arrived there after being incorporated into the Albany-Fraser Orogen and then eroded from there. The dataset records therefore two orogenic cycles: (1) erosion of the Mawson Continent/North Australian Craton, deposition along the craton margins, collision between the Mawson Continent and North-West Australian Craton and metamorphism in the Albany-Fraser Orogen and (2) erosion of the Albany-Fraser Orogen, deposition along Australia's western margin and metamorphism during the Pinjarra Orogeny. This highlights the care that must be taken in detrital zircon studies, since many of the detrital cores record a second-order provenance. It also explains how zircons from the Mawson Continent could be transported across the Albany-Fraser Orogen that should normally have been a barrier to northwest-directed sediment transport. Finally, in light of these new and previously published data, we support the interpretation that the Pinjarra Orogen is a Mesoproterozoic orogen formed along Australia's western margin.

Reactive monazite and robust zircon growth in diatexites and leucogranites from a hot, slowly cooled orogen: implications for the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield, Sweden

Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80 Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83–1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.

Tracing the provenance and recrystallization processes of the Earth's oldest detritus at Mt. Narryer and Jack Hills, Western Australia: An in situ Sm–Nd isotopic study of monazite

Mount Narryer and Jack Hills metasedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia, contain zircons with ages up to 4.4 Ga, the oldest known crustal materials on Earth, and monazites up to 3.6 Ga. In this study, we have investigated 147Sm– 143Nd systematics of detrital and metamorphic monazites from these metasedimentary rocks using laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). All detrital monazites have negative initial εNd(t) values, indicating that their parental magmas formed by remelting of older crustal materials. A comparison between the initial εNd(t) values of the detrital monazites and granitoids in the Narryer Gneiss Complex indicates that the Mt. Narryer and Jack Hills sediments were partly derived from the most isotopically enriched surrounding granitoids with ages of ca. 3.6 and 3.3 Ga. The metamorphic monazites generally have lower initial εNd(t) values when compared to the detrital monazites. However, the detrital and metamorphic monazites show similar distributions of εNd(t) at the ages of sediment deposition (3.28 Ga for Mt. Narryer and 3.05 Ga for Jack Hills). In addition, multiple analyses on single monazite grains having core-rim structures reveal that the cores and the recrystallized rims had identical Nd isotopic compositions at the time of recrystallization. These findings indicate that older monazites are source of light rare earth elements for younger metamorphic monazite formation and, therefore, that monazite can inherit its primary Sm–Nd isotopic signature during the recrystallization processes. We calculated the Nd model ages for all analyzed monazites to estimate crustal residence time of their source materials. We find that no igneous monazites older than 4.0 Ga were recrystallized to form the monazites. This implies that the lack of Hadean monazites is not due to recrystallization of ancient monazites during later metamorphism, but due to high-Ca compositions of the parental magmas of Hadean detritus, which prevent growth of magmatic monazite.

Metamorphic history of the central Himalaya, Annapurna region, Nepal, and implications for tectonic models

Pressure-temperature-time (P-T-t) conditions of metamorphism have been determined in the Annapurna region of central Nepal that place new constraints on the structural and tectonic evolution of the Himalayan orogenic wedge. Peak P-T conditions increase structurally upward: ∼525 °C and 8 kbar in the Lesser Himalayan sequence, 650 °C and 12 kbar at the base of the Greater Himalayan sequence across the Main Central thrust, 750 °C and 12 kbar in the middle of the Greater Himalayan sequence, and 775 °C and 13 kbar near the top of the Greater Himalayan sequence. Metamorphic monazite ages in the Greater Himalayan sequence also increase structurally upward: 16–21 Ma for subsolidus growth at the base of the Greater Himalayan sequence to ∼25 Ma for peak-T metamorphism and anatexis near the top of the Greater Himalayan sequence. These ages are several million years older than at equivalent structural levels at Langtang, ∼200 km to the east. The P-T-t data recommend reinterpretation of the Bhanuwa fault within the Greater Himalayan sequence as a thrust, and the presence of a different thrust structurally above the Bhanuwa thrust, here named the Sinuwa thrust. The new data are consistent with progressive stacking of tectonic slices, with calculated overthrust rates that are consistent with some (but not all) models that presume ∼2 cm/yr convergence across the Himalaya since 25 Ma. Despite differences in absolute ages, similarities among the chemical systematics of monazite, peak P-T conditions, and overthrust rates calculated for Annapurna when compared to Langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers, although the timing and duration of movement on discrete thrust surfaces may differ.

A short‐duration pulse of ductile normal shear on the outer South Tibetan detachment in Bhutan: Alternating channel flow and critical taper mechanics of the eastern Himalaya

In easternmost Bhutan the South Tibetan detachment (STD) is a ductile shear zone that juxtaposes the Radi (or Sakteng) klippe of the Tethyan Sedimentary Series from underlying high‐grade Greater Himalayan rocks. In situ LA‐ICPMS U‐Th‐Pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the STD within the shear zone at the base of the klippe, constrains north vergent normal shear to between 25 and 20 Ma. Coeval thrusting on the Main Central Thrust during this time supports a phase of channel flow–viscous wedge model activity, lasting only ∼3 Ma. Geochronologic data from the eastern Himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the Late Oligocene through to the Late Miocene, switching from channel flow–viscous wedge behavior to critical taper–frictional wedge behavior, each phase lasting approximately only 2 to 5 Ma. The tectonic evolution of the eastern Himalaya is comparable to central and western Himalayan tectonics during the Early Miocene, but during the Middle Miocene metamorphism and magmatism in the eastern Himalaya migrated toward the orogenic hinterland, a process not widely documented elsewhere in the Himalaya, thus highlighting the need for an orogenic model in three spatial dimensions.

Discovery of Ordovician–Silurian metamorphic monazite in garnet metapelites of the Alpine External Aiguilles Rouges Massif

The pre-Mesozoic, mainly Variscan metamorphic basement of the Col de Berard area (Aiguilles Rouges Massif, External domain) consists of paragneisses and micaschists together with various orthogneisses and metabasites. Monazite in metapelites was analysed by the electron microprobe (EMPA-CHIME) age dating method. The monazites in garnet micaschists are dominantly of Variscan age (330–300 Ma). Garnet in these rocks displays well developed growth zonations in Fe–Mg–Ca–Mn and crystallized at maximal temperatures of 670°C/7 kbar to the west and 600°C/7–8 kbar to the east. In consequence the monazite is interpreted to date a slightly pressure-dominated Variscan amphibolite-facies evolution. In mylonitic garnet gneisses, large metamorphic monazite grains of Ordovician–Silurian (~440 Ma) age but also small monazite grains of Variscan (~300 Ma) age were discovered. Garnets in the mylonitic garnet gneisses display high-temperature homogenized Mg-rich profiles in their cores and crystallized near to ~800°C/6 kbar. The Ordovician–Silurian-age monazites can be assigned to a pre-Variscan high-temperature event recorded by the homogenised garnets. These monazite age data confirm Ordovician–Silurian and Devonian–Carboniferous metamorphic cycles which were already reported from other Alpine domains and further regions in the internal Variscides.