Ga Monazite Research Articles

Two Paleoproterozoic tectono‐thermal events in the Central Orogenic belt, North China craton, determined from EPMA Th–U–Pb monazite geochronology of the Zanhuang massif

AbstractTectono‐thermal records older than ca. 1.8 Ga in the North China craton have been poorly resolved given that they have been strongly overprinted by younger events, despite being key to understanding the tectonic evolution of the craton. We report results of monazite Th–U–Pb isochron geochronology via electron probe microanalysis on the Zanhuang massif in the North China craton, to determine its detailed Precambrian tectono‐thermal history. Results suggest that the ca. 2.1 Ga tectono‐thermal event can be divided into distinct ca. 2.17 Ga and ca. 2.10 Ga stages, reflecting metamorphism related to retroarc regional extension of the North China craton. Similarly, the previously established ca. 1.8 Ga metamorphic event can be divided into distinct ca. 1.88 Ga and ca. 1.79 Ma stages. Through a detailed summary and comparison, we interpret that the ca. 1.88 Ga monazite age reflects the timing of peak metamorphism, which was followed by ca. 1.79 Ga retrogression.

Fluid-fluxed partial melting of the Buncheon granitic gneiss in the Yeongnam Massif, Korea: Protracted (ca. 1.86–1.84 Ga) reworking of the Paleoproterozoic Korean arc

The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn-kinematic wet melting in the presence of ca. 0.9–1.4 wt% H2O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H2O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton.

Monazite record for the Paleoproterozoic Svecofennian orogeny, SE Finland: An over 150-Ma spread of monazite dates

The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP–HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750–800 °C and 5–6 kbar, followed by near-isothermal decompression down to 3–4 kbar. The analysed monazites yielded 207Pb/206Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the ≥1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89–1.86 Ga fits well with the high-grade metamorphic event at 1.89–1.88 Ga observed elsewhere in southern and central Finland. The 1.87–1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83–1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80–1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones.

Timing of Proterozoic magmatism in the Sunsas belt, Bolivian Precambrian Shield, SW Amazonian Craton

We present new U–Pb zircon and monazite ages from the Sunsas belt granitic magmatism in Bolivia, SW Amazonian Craton. The geochronological results revealed four major magmatic events recorded along the Sunsas belt domains. The older igneous event formed a granitic basement coeval to the Rio Apa Terrane (1.95 – 1.85 Ga) in the southern domain. The second magmatic episode is represented by 1.68 Ga granites associated to the Paraguá Terrane (1.69–1.66 Ga) in the northern domain. The 1.37–1.34 Ga granites related to San Ignacio orogeny represent the third and more pervasive magmatic event, recorded throughout the Sunsas belt. Moreover, magmatic ages of ~1.42 Ga revealed that the granitogenesis associated to the Santa Helena orogeny also affected the Sunsas belt, indicating that it was not restricted to the Jauru Terrane. Lastly, the 1.10–1.04 Ga youngest magmatism was developed during the Sunsas orogeny and represents the final magmatic evolution related to Rodinia assembly. Likewise, the 1.95–1.85 and 1.68 Ga inherited zircon cores obtained in the ~1.3 Ga and 1.0 Ga granite samples suggest strong partial melting of the Paleoproterozoic sources. The 1079 ± 14 Ma and 1018 ± 6 Ma monazite crystallization ages can be correlated to the collisional tectono-thermal event of the Sunsas orogeny, associated to reactions of medium- to high-grade metamorphism. Thus, the Sunsas belt was built by heterogeneous 1.95–1.85 Ga and 1.68 Ga crustal fragments that were reworked at 1.37–1.34 Ga and 1.10–1.04 Ga related to orogenic collages. Furthermore, the 1.01 Ga monazite age suggests that granites previously dated by zircon can bear evidence of a younger thermal history. Therefore, the geochronological evolution of the Sunsas belt may have been more complex than previously thought.

Petrography and mineral chemistry of the Varena Iron Ore deposit, southeastern Lithuania: implications for the evolution of carbonate and silicate rocks and ore mineralization

The large Varena Iron Ore deposit of southeast Lithuania lies beneath 200–400 m thick sedimentary cover in the East European Craton. Several drilling projects have revealed that it contains ca. 70–200 million tons of iron ore. A metasomatic origin has been proposed by several investigators, with an igneous (mafic, ultramafic carbonatitic) origin also considered. Detailed micropetrography on a variety of ore-bearing and skarnitised samples through backscattered electron imaging, along with mineral chemical analysis and monazite dating by electron microprobe, provide support for a metasomatic origin. High-grade temperature (ca. 700–750° C) skarns containing forsterite, enstatite, spinel and diopside were formed preferentially in carbonate-rich rocks and were subsequently overprinted by amphibole-bearing (tremolite, actinolite, anthophyllite and hornblende) skarns. A low-temperature hydrous influx and changing redox conditions not only caused the serpentinization of the earlier skarns, but also deposited a considerable amount of iron ore in the form of magnetite. Iron was derived from various sources and was accompanied by sulphide, apatite, REE and other mineralizations. The ca. 1.78 Ga monazite grew during the regional metamorphism, while ca. 1.54 Ga is attributed to the last mineralization phases.

High-pressure eclogite facies metamorphism and decompression melting recorded in paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)

Thermobarometry and monazite U-Th-Pb geochronology of the phengite + chloritoid + biotite + garnet + kyanite bearing mica-quartz schist and of the nearby phengite + garnet bearing Córrego do Peixoto granite from Itaguara Sequence (IS) of the southern São Francisco Craton (SFC) have revealed that, at 2.05±0.05 Ga, metamorphic peak achieved ∼18.5 kbar and ∼626 °C. These pressure-temperature (P-T) conditions correspond to amphibole eclogite facies metamorphism that occurred around 60 km depth, consistent with geothermal gradient of about 10 °C/km, typical of subduction zones in continental collision settings. The continental collision-related tectonic exhumation would have triggered a decompression process of the high-pressure (HP) mica-quartz schist, which was partially melted to generate the Córrego do Peixoto granite at 2.05 Ga in P-T conditions inserted in the wet granite solidus field. The younger 1.93±0.02 Ga monazite age found in both mica-quartz schist and granite could be related to the orogenic collapse-related decompression late stage with amphibolite facies overprint. The mica-quartz schist has been modified by this late stage metamorphism, which partially overprinted primary mineral assemblages. Taking into account that ophiolites, represented in the IS by metamafic-ultramafic rocks, are typically associated to an accretionary wedge in continental collision settings, it is here suggested that mica-quartz schist is part of a probable Paleoproterozoic accretionary wedge formed above a paleo-subduction zone. It would represent a unique case of a very old accretion prism linked to a continental collision.

What the ca. 1.83 Ga gedrite-cordierite schists in the crystalline basement of Lithuania tell us about the late Palaeoproterozoic accretion of the East European Craton

ABSTRACTIn the western East European Craton (EEC), southern Lithuania, a suite of fine-grained, thinly bedded rocks of unusual composition has been shown to have originally comprised intermediate and felsic volcanic rocks. They extruded at ca. 1.83 Ga and were hydrothermally altered prior to metamorphism, which converted them into garnet-, gedrite-, anthophyllite-, staurolite- and cordierite-bearing schists. After the rocks have experienced a 630°C and 7 kbar metamorphism, they were uplifted to 15 km (5 kbar) probably at ca. 1.73 Ga. They were reheated to 640°C at ca. 1.50 Ga (monazite age). The monazite age of ca. 1.50 Ga is coeval with the emplacement of the neighboring 1.50 Ga Anorthosite-Mangerite-Charnockite-Granite (AMCG) Mazury complex. The ca. 1.83 Ga volcanic suites in Lithuania and northern Poland, together with the Oskarshamn-Jönköping belt (OJB) in south-central Sweden, may belong to the same chain of volcanic island arcs, and thus provide information on the evolution of the entire western EEC. The ca. 1.50 Ga metamorphic reworking and the replacement of the Mazury AMCG suite may have been triggered by the Danopolonian orogeny further west and, at a larger scale, accretion of the continental margin of Columbia.

Monazite as a monitor of melting, garnet growth and feldspar recrystallization in continental lower crust

AbstractMonazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are consistent with the garnet‐rich rocks representing the residual products of ultrahigh temperature melting of biotite‐bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64–2.61 Ga Y‐rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium‐poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid‐absent melting of biotite + plagioclase + quartz ± sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61–2.55 Ga Y‐depleted, Th‐rich monazite domains crystallized in the presence of garnet + ternary feldspar ± orthopyroxene + peraluminous melt. The c. 2.58–2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high‐pressure (HP) grossular‐rich garnet at the expense of plagioclase during crustal thickening, culminating at &gt;950 °C. Re‐heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn‐kinematic growth of (La + Ce)‐enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0–1.2 GPa and 600–700 °C. Monazite grains in this study are marked by positive Eu‐anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu‐anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu‐enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.

Growth, preservation of Paleoproterozoic-shear-zone-hosted monazite, north of the Western Dharwar Craton (India), and implications for Gondwanaland assembly

We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).

Proterozoic evolution of the Mojave crustal province as preserved in the Ivanpah Mountains, southeastern California

In the western U.S., numerous exposures of Precambrian crustal rocks were exhumed from mid-crustal depths during Cenozoic tectonic events and provide a view into the deep crust. This natural laboratory provides an excellent opportunity to study orogenic and continent-forming processes at deeper levels. The Ivanpah Mountains of southeastern California contain exposures of Paleoproterozoic migmatites and banded gneisses that belong to the Mojave crustal province. New, detailed U–Pb geochronology of monazite and zircon from ortho- and paragneisses reveal the timing of the formation of these rocks and multiple periods of metamorphism and magmatism. Paragneisses have detrital zircon populations of dominantly 1.80–2.15Ga (dominant) and 2.4–2.8Ga, although the youngest detrital zircon grain is 1.754±24Ga. Values of δ18O(zircon) of the two detrital age populations are 7.0±2.2‰ and 6.3±1.4‰ (2SD), respectively, consistent with original, igneous values. Based on the 1.76Ga age of metamorphic zircon rims, the metasediments were intruded by a calc-alkaline magmatic suite including gabbro, tonalite, and porphyritic granite at 1.76Ga, although the contacts are obscured by pervasive metamorphism and deformation.U–Pb ages of metamorphic overgrowths on detrital zircons from the paragneisses indicate multiple events at ∼1.76, 1.74, 1.70 and 1.67Ga which documents a periodicity of tectonism through time. The oxygen isotope ratios of the metamorphic zircon overgrowths from the paragneisses remained relatively constant through time within each sample, consistent with growth during in situ partial melting. However, oxygen isotope ratios of zircons from 1.74Ga leucocratic material are lower in δ18O, and therefore could not have been generated by partial melting of the adjacent paragneiss. Monazite from both igneous and metasedimentary rocks preserves two periods of growth at 1.75 and 1.67Ga. In situ, U–Pb dating of monazite reveals that 1.67Ga monazite inclusions are found in major rock-forming minerals, and therefore we interpret this as the timing of fabric formation and migmitization, which is younger than previously believed and is interpreted to be the dominant metamorphic pulse in the protracted middle crustal tectonism. Thermobarometry indicates peak metamorphic conditions of ∼3.5kb and ∼740°C, which is consistent with partial melting and the metamorphic mineral assemblage in these rocks (sillimanite+K-feldspar, muscovite absent). This paper shows (1) multiple pulses identified within 100Ma orogeny, (2) zircon components and their overgrowths demonstrate mixing of Archean, 1.8 and younger reservoirs, (3) low pressure granulite facies metamorphism occurred at 1.67Ga and may have been related to continental extension of a tectonically over-thickened crust.

∼1.6 Ga ultrahigh‐temperature granulite metamorphism in the Central Indian Tectonic Zone: insights from metamorphic reaction history, geothermobarometry and monazite chemical ages

AbstractIn this study, we present precise pressure–temperature (P–T) and age constraints of ultrahigh‐temperature (UHT) metamorphism of the Bhandara–Balaghat granulite (BBG) domain at the southern margin of the Central Indian Tectonic Zone (CITZ). Supracrustal and metaigneous granulites of this domain, which lie as detached pods and lenses of various sizes within felsic gneiss–migmatite association record protracted high‐T crustal anatexis events, broadly synchronous with and/or punctuated with felsic and mafic plutonism. Magnesian metagreywacke protolith of the supracrustal suite records extensive biotite melting and subsequent melt extractions at deep crustal, UHT metamorphic conditions (T ≥ ∼900°C at P∼8 kbar), producing restitic mineral assemblages of garnet + rutile, garnet + cordierite and garnet + aluminous orthopyroxene. The diversity of the mineral assemblages is related to the domainal‐scale variation of the bulk rock composition. In situ chemical age dating of five monazite grains, which occur in the different textural settings of the garnet + cordierite + orthopyroxene + rutile‐bearing granulite reveals two age domains: (1) Pervasive ∼1.6 Ga domain, which is recorded in monazites occurring as inclusions in garnet and in the leucosome matrix is correlated with the timing of the UHT metamorphism. (2) ∼1.47 Ga domain reflects a fluid‐mediated recrystallization event leading to dissolution and re‐precipitation of older monazite. The ∼1.6 Ga monazite chemical ages provide robust constraints on the timing of the earliest stage of tectonothermal processes in the CITZ (defined here as the ‘Central Indian Orogeny’). The significance of the ∼1.6 Ga hot orogenesis in interorogen correlation is discussed. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay belt, Rae Province, Nunavut

Petrology and in situ SHRIMP monazite geochronology provide new constraints on the polymetamorphic and polydeformational history of the Committee Bay belt, Rae Province, Nunavut, Canada. Rare ca. 2.58 Ga monazite inclusions in garnet porphyroblasts in the eastern part of the belt are interpreted to reflect regional contact metamorphism (M 1) associated with voluminous 2.61–2.58 Ga plutonism. Monazite also occurs as ca. 2.35 Ga inclusions in andalusite in the central supracrustal belt, but is absent from the east. These data complement previous studies to the west, and suggest that the Committee Bay belt represents a ca. 2.35 Ga tilted crustal section that was tectonically thickened ± imbricated during the Arrowsmith orogeny (D 1–M 2). Penetrative reworking of the belt occurred during a diachronous D 2–M 3 tectonometamorphic event which involved northwest-vergent thickening to depths of ∼5 kbar across most of the belt. Although D 2 folds and fabrics across the belt are geometrically correlative, monazite is ∼20 Myr older in the western migmatite domain (ca. 1.86 Ga) than in the western supracrustal belt (ca. 1.84 Ga). This temporal difference is attributed to burial of more radiogenic lithologies in the migmatite domain. Tectonic thickening is attributed to ca. 1.87 Ga collision of Meta Incognita microcontinent with the southeastern flank of the Rae Province. The eastern supracrustal belt exposes similar amphibolite-facies (M 3) northeast-striking, northwest-vergent D 2 structures that are dated at 1.815 Ga. This younger reworking may reflect the influence of a major northward projecting promontory of the Superior Province which continued to affect the softened hinterland of the Rae Province during terminal collision at ca. 1.82 Ga. The central Committee Bay belt records a younger, ca. 1.79 Ga structural overprinting (D 3–M 4) which involved dextral shearing and differential burial to temperatures above monazite growth in a gentle synformal structure formed during the final stages of amalgamation of Laurentia.

Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera

Zircon and monazite from granitic sheets and dikes in the Monashee complex, Canadian Cordillera, were investigated to determine whether igneous crystallization occurred at 1� 9G a or 50 Ma with 1� 9 Ga inherited zircon and monazite. Four of the five samples are weakly deformed to undeformed, despite occurring in a gneiss dome at the structurally deepest exposed level of the orogen that elsewhere was strongly deformed and partly melted at 50 Ma. Based on U^(Th)^Pb ages from zircon and monazite, field relationships, and mineral composition and zoning, we conclude that the granitic rocks crystallized at 1� 9 Ga and were metamorphosed at 50 Ma. All dated zircon is 1� 9 Ga (except for 2� 3^2� 0G a inherited cores) and 1� 9 Ga monazite makes up 490% of the population in four samples. The remainder of the monazite is 50 Ma and all monazite in one sample is 50 Ma. Composition and zoning of 1� 9 Ga zircon and monazite are uniform within samples, yet differ between samples, indicating growth from 1� 9 Ga magmas that are unique to each sample. This relationship is unlikely if the grains are inherited because the host rocks are heterogeneous 2� 3^2� 1 Ga gneisses. The 1� 9 Ga zircon and monazite have zoning that is consistent with growth from magmas, whereas the 50 Ma monazite has variable composition and zoning that suggest growth from diverse metamorphic fluids. The results demonstrate that part of the Monashee complex was last strongly deformed and partly melted at 1� 9 Ga, and thus largely escaped Cordilleran tectonism.

High-resolution dating of granite petrogenesis and deformation in a lower crustal shear zone: Athabasca granulite terrane, western Canadian Shield

The Athabasca granulite terrane represents > 20,000 km 2 of continental lower crust exposed in the hanging wall of the ca. 1.85 Ga thrust–sense Legs Lake shear zone in the western Canadian Shield. The terrane and the thrust zone are cut by the Grease River shear zone. The structure hosts a syn-tectonic granite dike with a high-precision ID-TIMS zircon 207Pb/ 206Pb crystallization age of 1922.6 ± 0.4 Ma (MSWD = 3.1, 2 σ). Detailed textural analysis and in situ electron probe microanalysis (EPMA) of monazite in the dike indicate three discrete episodes of strain accumulation, each corresponding to syn-kinematic growth of monazite. The results provide evidence for a previously unrecognized > 100 m.y. episodic record of sub-horizontal NW–SE shortening and dextral shear strain along the Grease River shear zone. High-Ca domains in ca. 1.92–1.90 Ga monazite are linked to protracted high- T strain (ca. 650–700 °C) at lower crustal depths (ca. 1.0 GPa) and correlated with indentation of the Slave craton along the Great Slave Lake shear zone. Syn-kinematic overgrowths of low-Ca monazite + apatite are interpreted as products of deformation-enhanced dissolution and re-precipitation of monazite at 1846 ± 9 Ma (MSWD = 1.1, 2 σ). The third generation of syn-kinematic monazite yield ca. 1.8 Ga ages. The dataset constrains, for the first time, coeval movement along the strike–slip Grease River shear zone and the thrust–sense Legs Lake shear zone during ca. 1.85 Ga dextral transpression and uplift of continental lower crust to middle crustal levels (ca. 0.5 GPa). Circa 1.8 Ga syn-kinematic monazite domains in dynamically recrystallized feldspars support brittle–ductile reactivation along the Grease River shear zone at shallower crustal levels, coincident with intra-continental strike–slip strain during culmination of the Trans-Hudson orogeny.

Significance of ~ 1.5 Ga zircon and monazite ages from charnockites in southern Lithuania and NE Poland

The presence of 1.52–1.50 Ga charnockites from the anorthosite–mangerite–charnockite–granite (AMCG) Mazury complex in southern Lithuania and NE Poland, in the western East European Craton (EEC) is revealed by secondary ion mass-spectrometry (SIMS) and EPMA geochronology. Early 1.85–1.82 Ga charnockites are related to major orogeny in the region whereas the newly studied charnockites intrude the already consolidated crust. The 1.52–1.50 Ga charnockite magmatism (SIMS data on zircon) was followed by high-grade metamorphism (EPMA data on monazite), which strongly affected the surrounding rocks. The 1.85–1.81 Ga zircon cores in Lazdijai and 1.81 Ga monazite domains in the Lanowicze charnockites represent the protolith age of a volcanic island arc. The 1.52–1.50 Ga charnockite magmatism and metamorphism are likely related to the distal, Danopolonian, orogeny further to the west, at the margin of Baltica. The c.1.52–1.50 Ga AMCG magmatism and metamorphism in the western EEC as well as the paired accretionary-rapakivi suites in Amazonia, may be the inboard manifestations of the same early Mesoproterozoic orogeny associated with the juxtaposition of Amazonia and Baltica during the amalgamation of the supercontinent Columbia.

Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

In situ U-Pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the Witwatersrand basin, South Africa, reveals two episodes of tectonothermal activity. A major event between 2.06 and 2.03 Ga is recorded in the Wit-watersrand and Transvaal Supergroups in the northwestern and central basin, and broadly coincides with the ca. 2.06 Ga Bushveld event. In the central and southern basin, a previously unrecognized event has been dated between 2.14 and 2.12 Ga. The widespread geographic and stratigraphic occurrence of Bushveld-aged monazite and xenotime, including both auriferous reefs and unmineralized strata, indicates that metamorphism and fluid flow associated with magmatism was pervasive, affecting most of the succession (>10 km thick) in the central and northern parts of the basin. The metamorphic phosphate dates, which are younger away from the complex, indicate a lag of 20–30 m.y. between emplacement and phosphate growth in the central basin (∼100 km south), suggesting that heat related to magmatism was transferred southward at an average rate of 3–5 mm yr −1 . The absence of 2.06–2.03 Ga phosphates in the Welkom goldfield at the southern end of the basin implies that Bushveld-related heating and fluid flow did not affect this part of the basin. The intergrowth of ca. 2.045 Ga monazite with gold in quartz-pebble conglomerate from the West Rand goldfield indicates that fluid flow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.

A SHRIMP U–Pb and LA-ICP-MS trace element study of the petrogenesis of garnet–cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt, South Africa

The Central Zone of the Limpopo Belt (South Africa) underwent high-grade metamorphism at ∼2.7–2.5 and ∼2.03 Ga. Quartz-rich, garnet-, cordierite-, biotite- and orthoamphibole-bearing, feldspar-free gneisses from the western Central Zone reached granulite-facies conditions (∼800 °C at ∼8–10 kbar) followed by decompression. Garnet from one such sample shows significant zonation in trace elements but little zonation in major elements. Zoning patterns suggest that the early prograde breakdown of REE-rich accessory phases contributed to the garnet trace element budget. Monazite from the sample yields a SHRIMP weighted mean 207Pb– 206Pb age of 2028 ± 3 Ma, indistinguishable from a SHRIMP zircon age of 2022 ± 11 Ma previously measured on metamorphic overgrowths on ∼2.69 Ga igneous zircon cores. New zircon and monazite formed before, or at, the metamorphic peak, and occur as inclusions in garnet. Monazite appears to have formed through the breakdown of early allanite ± xenotime ± apatite. Trace element zoning patterns in garnet and the age of accessory phases are most consistent with a single tectonometamorphic event at ∼2.03 Ga. The plagioclase and K-feldspar-free composition of the garnet–cordierite–orthoamphibole gneisses requires open system processes such as intense hydrothermal alteration of protoliths or advanced chemical weathering. In the studied sample, the ∼2.69 Ga igneous zircons show a prominent negative Eu anomaly, suggesting equilibrium with plagioclase, or plagioclase fractionation in the precursor magma. In contrast, the other minerals either show small negative (∼2.03 Ga monazite), no (∼2.02 Ga zircon and garnet) or positive Eu anomalies (orthoamphibole). This suggests that the unusual bulk compositions of these rocks were set in after ∼2.69 Ga but before the peak of the ∼2.03 Ga event, most probably while the protoliths resided at shallow or surficial crustal levels.

Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study

A combined geochronological, geochemical, and Nd isotopic study of felsic high-pressure granulites from the Snowbird Tectonic Zone, northern Saskatchewan, Canada, has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry (ID-TIMS) techniques. The terrane investigated is a 400 km 2 domain of garnet–kyanite–K–feldspar-bearing quartzofeldspathic gneisses. Monazite in these granulites preserves a complex growth history from 2.6 to 1.9 Ga, with well-armored, high Y and Th grains included in garnet yielding the oldest U–Pb dates at 2.62 to 2.59 Ga. In contrast, matrix grains and inclusions in garnet rims that are not well-armored are depleted in Y and Th, and display more complicated U–Pb systematics with multiple age domains ranging from 2.5 to 2.0 Ga. 1.9 Ga monazite occurs exclusively as matrix grains. Zircon is typically younger (2.58 to 2.55 Ga) than the oldest monazite. Sm–Nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of Archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial ɛ Nd. In contrast, grains that contain a Paleoproterozoic component show more positive initial ɛ Nd, most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of HREE's. Our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith. The ca. 2.55 Ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed. Very high-pressure (> 1.5 GPa) metamorphism during the Paleoproterozoic at 1.9 Ga produced kyanite from garnet breakdown, and resulted in limited growth of new monazite and zircon. In the case of monazite, this is likely due to the armoring and sequestration of early-formed monazite such that it could not participate in metamorphic reactions during the high-pressure event, as well as the depletion of the REE's due to melt loss following the early melting event.

Evolution of the Amphibolite‐Granulite Facies Transition Exposed by the Vredefort Impact Structure, Kaapvaal Craton, South Africa

Geological mapping is integrated with high‐precision U‐Pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite‐granulite facies boundary exposed in the Vredefort impact structure of the central Kaapvaal Craton. Our study has revealed that the preimpact configuration of the amphibolite‐granulite facies boundary is best preserved in panels between impact‐related brittle faults and that past concepts of the boundary must be revised. A regional lithologic transition exists between amphibolite facies Outer Granite Gneiss (3.084–3.09 Ga) and a heterogeneous assemblage of granulite facies gneisses (ca. 3.1–3.3 Ga). Tabular deformed quartz syenite bodies, dated at $$3.084\pm .001$$ Ga, parallel the metamorphic transition and crosscut both amphibolite and granulite facies rocks. Although two episodes of late Archean deformation modified the boundary, it has retained its primary intrusive continuity since 3.08 Ga. Subsequent activity includes minor dolerite dikes, 2.2 Ga monazite growth, 2.02 Ga meteoritic impact, and episodic hydrothermal zircon growth events from 615 to 380 Ma. Our results suggest that the Vredefort amphibolite‐granulite facies transition can shed valuable insight into Archean crustal evolution and the nature of metamorphic boundaries in cratonic crust. The nature and paleodepth of the Vredefort metamorphic transition are compatible with the seismic characteristics of the unexposed crust of the central Kaapvaal Craton. The absence of major tectonic activity post–3.08 Ga in the Vredefort crystalline basement is consistent with the presence of a lithospheric mantle root that stabilized the central cratonic crust against subsequent disruption. The lithologic, metamorphic, and structural complexity of the transition zone between amphibolite and granulite facies rocks in the Vredefort crystalline basement may be typical of this metamorphic boundary in intracratonic crust.

Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories

High-resolution X-ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural, metamorphic, and tectonic analysis. X-ray maps commonly show complex Th, U, and Pb zoning that reflects monazite growth and overgrowth events. Age maps constructed from the X-ray maps simplify the zoning and highlight age domains. Microprobe dating offers a rapid, in situ method for estimating ages of mapped domains. Application of these techniques has placed new constraints on the tectonic history of three areas. In western Canada, age mapping has revealed multiphase monazite, with older cores and younger rims, included in syntectonic garnet. Microprobe ages show that tectonism occurred ca. 1.9 Ga, 700 m.y. later than mylonitization in the adjacent Snowbird tectonic zone. In New Mexico, age mapping and dating show that the dominant fabric and triple-point metamorphism occurred during a 1.4 Ga reactivation, not during the 1.7 Ga Yavapai-Mazatzal orogeny. In Norway, monazite inclusions in garnet constrain high-pressure metamorphism to ca. 405 Ma, and older cores indicate a previously unrecognized component of ca. 1.0 Ga monazite. In all three areas, microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts.