Pb Monazite Ages Research Articles

Geochronology in migmatites – a Sm–Nd, U–Pb and Rb–Sr study from the Proterozoic Damara belt (Namibia): implications for polyphase development of migmatites in high‐grade terranes

AbstractSm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high‐grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole‐rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre‐500 Ma Pan‐African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late‐stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high‐grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole‐rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late‐stage disturbance of the Rb–Sr isotopic system on the sub‐sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma−1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water‐undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high‐grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post‐metamorphic cooling in the Hercynian crustal cross‐section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma−1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.

Domainal variations of U–Pb monazite ages and Rb–Sr whole‐rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T >650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm3‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/Deff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.

TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK, SUPERIOR PROVINCE, QUEBEC, CANADA

Previously considered one of Earth’s largest granulite terranes, the Minto block, in northern Quebec, consists dominantly of granitic rocks with sparse supracrustal remnants that record low-pressure, high-temperature metamorphism to the amphibolite and granulite facies at ca. 2.7 Ga. Much of the region is underlain by plutonic rocks, including igneous charnockitic bodies, that provide little evidence of metamorphism. In contrast, pelitic assemblages in low-grade greenstone belts and their higher-grade shoulders define four metamorphic zones: 1) sub-staurolite-grade slates, found only in the Duquet belt, 2) staurolite – andalusite – sillimanite – garnet-zone schists, present in all belts, 3) marginal garnet – sillimanite ± kyanite migmatites, and 4) garnet – sillimanite ± cordierite and garnet – orthopyroxene granulites, as enclaves within plutonic units. A prograde low-pressure, hightemperature regional facies-series is defined by andalusite – sillimanite assemblages, thermobarometric analyses that indicate peak conditions ranging from 3 kbar, 560°C to 8.4 kbar, 840°C, and short segments of anticlockwise P–T trajectories preserved in two of the lowest-grade areas. Kyanite occurs in migmatites in the vicinity of a ca. 2.81 Ga shear zone bounding two distinct supracrustal assemblages and marks an earlier Barrovian event. All belts are polydeformed, with a common, broadly synmetamorphic, NNW-striking, steep foliation and down-dip lineation related to regional-scale F2 folds. Distributed late-metamorphic dip-slip shear appears responsible in some areas for cross-strike juxtaposition of low- and high-grade metamorphic zones. Older, generally poorly preserved D1 deformation, recognized in several belts, may represent accretionary structures. Variably developed younger deformation includes NNW dextral transcurrent brittle–ductile shear zones (S3), variably developed east-trending F4 folds, and WNW-striking pseudotachylite zones. U–Pb monazite ages of 2702 Ma date high-grade metamorphism of a pelitic granulite, crystallization of two diatexites, and staurolite-zone metamorphism in one schist. Together with similar zircon ages of crust-derived granites, these data point to a major collisional orogeny at this time. Older titanite (2748–2765 Ma) indicates areas that escaped >650°C metamorphism during the 2.7 Ga event. Younger monazite occurs in a migmatite-grade rock (2688, 2672 Ma) and staurolite-zone schists (2679–2642, 2648, 2637, and 2642–2628 Ma), similar to titanite ages in greenstones (2643–2550 Ma) and 40 Ar/ 39 Ar hornblende ages (2.64–2.56 Ga). These post-peak metamorphic ages in the greenstone belts based on monazite and titanite are attributed to hydrothermal growth from fluids channeled along permeable zones; they may reflect late reheating events observed throughout the Superior Province as leucogranitic magmatism, metamorphism in the deep crust, and mineral growth in shear zones.

Time calibration of a P-T path from a Variscan high-temperature low-pressure metamorphic complex (Bayerische Wald, Germany), and the detection of inherited monazite

A temperature–time path was constructed for high-temperature low-pressure (HT–LP) migmatites of the Bayerische Wald, internal zone of the Variscan belt, Germany. The migmatites are characterised by prograde biotite dehydration melting, peak metamorphic conditions of approximately 850 °C and 0.5–0.7 GPa and retrograde melt crystallisation at 800 °C. The time-calibration of the pressure–temperature path is based on U–Pb dating of single zircon and monazite grains and titanite separates, on 40Ar/39Ar ages obtained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and on 40Ar/39Ar spot fusion ages of biotite determined in situ from sample sections. Additionally, crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns, suggesting that peak temperatures of 850 °C cannot have prevailed much longer than 2.5 Ma. The temperature–time paths obtained for two areas approximately 30 km apart do not differ from each other considerably. U–Pb zircon ages reflect crystallisation from melt at 850–800 °C at 323 Ma (southeastern area) and 326 Ma (northwestern area). The U–Pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance. The preservation of inherited monazite and the presence of excess 206Pb resulting from the incorporation of excess 230Th in monazite formed during HT–LP metamorphism suggest that monazite ages in the migmatites of the Bayerische Wald reflect crystallisation from melt at 850–800 °C and persistence of older grains at these temperatures during a comparatively short thermal peak. The U–Pb ages of titanite (321 Ma) and 40Ar/39Ar ages of hornblende (322–316 Ma) and biotite (313–309 Ma) reflect cooling through the respective closure temperatures of approximately 700, 570–500 and 345–310 °C published in the literature. Most of the feldspars' ages (305–296 Ma) probably record cooling below 150–300 °C, while two grains most likely have higher closure temperatures. The temperature–time paths are characterised by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 °C/my at temperatures between 850–800 and 700 °C to 11–16 °C/my at temperatures down to 345–310 °C. Further cooling to feldspar closure for Ar was probably even slower. The lack of decompressional features, the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating, rapid uplift and extension due to lithospheric delamination as proposed elsewhere. Instead, the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material.

Timing and characterization of recurrent pre-Sveconorwegian metamorphism and deformation in the Varberg–Halmstad region of SW Sweden

The Ätran terrane of southwestern Sweden is a consequence of westward crustal growth away from an Archean core and records both Gothian (c. 1.75–1.55 Ga) and c. 1.0 Ga Sveconorwegian (Grenvillian) tectonism. In spite of widespread consensus on the first-order aspects of this westward growth model, there is a current debate as to whether the bulk of the deformation and migmatization in the Ätran terrane should be attributed to Gothian or Sveconorwegian events. U–Pb zircon, titanite, monazite and rutile ages from gneiss samples and three tectonostratigraphic groups of dykes in the Varberg–Halmstad region constrain the timing of major tectonothermal events. A felsic orthogneiss yields a protolith age of 1664±7 Ma. Mafic dykes of a first dyke group (Steninge dykes) intruded after the first recognized deformation (D1) and were subsequently metamorphosed at 1654±9 Ma. The Steninge dykes constrain D1 to the 1664–1654 Ma interval and D2 at 1654±9 Ma, requiring the D1/D2 gneissosity to have formed during Gothian orogenesis. Steninge dykes and migmatitic D1/D2 structures in the host gneisses are cross-cut by less deformed dykes of a second dyke group, one regional granitic suite dated at 1426+9/−4 Ma and a pegmatite swarm at 1399+7/−6 Ma. The second group of dykes therefore confirms a pre-1.43 Ga age for the main D1/D2 gneiss-forming event(s) consistent with previous models of regional Gothian deformation in this terrane. Group 2 magmatism also includes the Varberg Charnockite–Granite Association, from which a sample of charnockite is now dated at 1399+12/−10 Ma. In addition, metamorphism at 1438+12/−8 Ma resulted in new zircon growth in a mafic gneiss. Together, the Group 2 magmatic rocks and metamorphic zircons represent an inter-orogenic, 1.44–1.38 Ga thermo-magmatic episode in the Varberg–Halmstad region that has c. 1.4 Ga equivalents elsewhere in southern Sweden. Previous models of high heat flow and comprehensive metamorphism during a thermo-magmatic Hallandian event at c. 1.4 Ga is thus supported. Sveconorwegian deformation and recrystallization (D3–D4) occurred before 946+6/−4 Ma, as constrained by a post-kinematic third group of dykes. U–Pb ages for monazite (948±9 Ma), titanite (935±7 Ma and 932±5 Ma) and rutile (878±9 Ma) reflect Sveconorwegian cooling after peak metamorphism. During this stage, U–Pb ages and closure temperatures suggest slow cooling rates of 5–11°C/m.y. from 948 to 932 Ma and 2.5–5°C/m.y. from 932 to 878 Ma, attributed to late-stage erosion and isostatic uplift. The lack of regional penetrative migmitization during the Sveconorwegian granulite conditions is consistent with metamorphism of previously dehydrated rocks (during Gothian and Hallandian times) during eastward Sveconorwegian thrusting of western segments over the Ätran terrane.

Age of crustal melting, emplacement and exhumation history of the Shivling leucogranite, Garhwal Himalaya

We report a U–Pb monazite age of 23.0±0.2 Ma for the Shivling leucogranite, a tourmaline+muscovite±biotite leucogranite at the top of the High Himalayan slab in the Garhwal Himalaya, north India. The Shivling–Bhagirathi leucogranite is a viscous near-minimum melt, emplaced as a foliation parallel laccolith via a dyke network not far from its source region. Prograde heating occurred soon after the India–Asia collision at c. 50 Ma up to melting at 23 Ma and high temperatures (>550 °C) were maintained for at least 15 Ma after garnet growth. The leucogranite was emplaced at mid-crustal depths along the footwall of the Jhala fault, a large-scale low-angle normal fault, part of the South Tibetan Detachment system, above kyanite and sillimanite grade gneisses. The geometry of the leucogranite laccolith shows biaxial extension and boudinage both perpendicular (north-northeast–south-southwest) and parallel to the strike (west-northwest–east-southeast) of the mountain range. Unroofing occurred by underthrusting beneath the High Himalayan slab along the Main Central Thrust zone, progressively ‘jacking up’ the leucogranites, removal of material above by low-angle normal faulting, and erosion. Very rapid cooling at rates of 200–350 °C/Ma between 23–21 Ma immediately followed crystallization, as tectonic unroofing and erosion removed 24–28 km of overburden during this time. K–Ar muscovite ages are 22±1.0 Ma and fission track ages of zircons from >5000 m on the North Ridge of Shivling are 14.2±2.1 and 8.8±1.2 Ma and apatites are 3.5±0.79 and 2.61±0.23 Ma. Slow steady state cooling at rates of 20–30 °C/Ma from 20–1 Ma shows that maximum erosion rates and unroofing of the leucogranite occurred during the early Miocene. This timing coincides with initiation of low-angle, north-dipping normal faulting along the South Tibetan Detachment system.

P– T– t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica

Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P– T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P– T– t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd. The low-pressure granulite-facies to upper-amphibolite-facies stage is dated by six nearly concordant U–Pb monazite ages of 484–494 Ma from three migmatite samples and correlates to the late Pan-African Ross Orogeny in Cambro-Ordovician times. The age of the medium-pressure granulite-facies assemblages is not constrained by geochronological data. Either they form relics of the Precambrian Antarctic Craton, or they represent an early metamorphic stage of the Ross Orogeny. Rb–Sr and K–Ar dating on biotites yielded 470, 468, 470 Ma and 473, 469, 470 Ma (±5 Ma each), indicating the time of cooling to 450–300°C. This is confirmed by 40Ar– 39Ar plateau ages of 476±2, 472±3 and 470±2 Ma for these biotites. A late tectonic pegmatite yielded a concordant U–Pb monazite age of 489±3 Ma, while slightly discordant U–Pb data of two zircon fractions are explained by recent minor lead loss of ca 490 Ma old zircons. Cooling to ca 500–350°C is dated to 472±2 Ma by concordant 40Ar– 39Ar plateau ages of two muscovite fractions. The cooling history of the basement from high-grade conditions to the blocking temperature of Rb–Sr and K–Ar in micas took place within ca 15–20 Ma. Cooling rates of 18–25°C Ma −1 can be derived, if continuous cooling is assumed. U–Pb data points of zircons as well as Sm–Nd whole rock model ages between 1.8 and 1.9 Ga indicate that at least part of the migmatites derive from Early Proterozoic crustal protoliths. Comparing the new P– T– t data from the northern Wilson Terrane with those from the southern Wilson Terrane, a common tectono-metamorphic history becomes evident for a 600 km long sector of the Ross Orogenic belt at the Pacific end of the Transantarctic Mountains, at least since the granulite-facies metamorphic event.

Variscan syncollisional extension in the Iberian Massif: structural, metamorphic and geochronological evidence from the Somosierra sector of the Sierra de Guadarrama (Central Iberian Zone, Spain)

The rocks affected by the Variscan orogeny in the Sierra de Guadarrama (Central Iberian Zone) can be separated into two domains: a high-grade Western Domain and a medium- to low-grade Eastern Domain. These domains are separated by the Berzosa–Riaza shear zone. A new integrated analysis of structural, metamorphic and U–Pb monazite age data, indicates that the Berzosa–Riaza shear zone is a major D2 extensional structure associated with oblique, crustal-scale extensional shearing and important decompression. Low-P/high-T metamorphism associated with the D2 event overprints earlier Barrovian metamorphic assemblages formed during the Variscan D1 compressional event. The kilometre-scale Berzosa–Riaza shear zone exhibits a S–C mylonitic front at its base and a crenulation front at its top, with successive fabric development under different P–T conditions indicating crustal thinning. Textural relationships and U–Pb dating of monazite indicate that the main D2 extensional event took place between 337±2 and 326±3 Ma (Early–Mid Carboniferous). A late-D2 extensional episode overprinted the major D2 ductile shear zone along narrow, low-grade, oblique-normal detachment zones. Kinematic indicators in both D2 and late-D2 fabrics indicate a displacement of the upper structural levels to the southeast. Field relationships, microstructures and qualitatively derived P–T–t–d paths suggest that the Early–Mid Carboniferous syncollisional extension in the Sierra de Guadarrama was associated with the major extensional collapse of the Variscan hinterland (Central Iberian Zone) and was contemporaneous with the earliest thrusting in the foreland.

High-precision U–Pb monazite geochronology of the c. 806 Ma Grampian Shear Zone and the implications for the evolution of the Central Highlands of Scotland

Two pegmatites and mylonitic rocks from the Grampian Shear Zone yield U–Pb monazite ages of 806 ± 3 Ma, 808 –9 +11 Ma, and 804 –12 +13 , respectively. These confirm that shearing and pegmatite crystallization was essentially synchronous. Lower intercepts of c. 440 Ma indicate disturbance during Ordovician reworking. These new data question the concept of a single latest Proterozoic–early Palaeozoic orogeny in the Grampian terrane and the significance of the Great Glen Fault.

U–Pb zircon, monazite, and sphene ages for granitic orthogneiss of the Barkerville terrane, east-central British Columbia

Granitic orthogneiss forms an important component of the Barkerville terrane of southeastern British Columbia. Rb–Sr whole-rock ages for the orthogneisses are ambiguous and range from Late Proterozoic to mid-Paleozoic, with large associated errors. U–Pb dating of zircon, monazite, and sphene has been employed in an attempt to establish precise crystallization ages for two of the orthogneiss bodies. U–Pb systematics for zircons from both bodies show the combined effects of inheritance of zircon cores and postcrystallization Pb loss. This complexity precludes a precise estimate of the age of emplacement of the granitic protoliths of the gneiss. The data do, however, constrain possible emplacement ages for the bodies to between 335 and 375 Ma (Late Devonian – mid-Mississippian).A U–Pb age of 174 ± 4 Ma for metamorphic sphene from one of the orthogneiss bodies is interpreted as dating the end of the second phase of deformation in the area. Two nearly concordant U–Pb ages of 114 and 117 Ma for monazite from the second body remain problematical. These data suggest either that the monazite grew during a relatively young shearing and (or) metamorphic event that locally affected the Barkerville terrane or that the closure temperatue for the U–Pb system in monazite is lower than had previously been inferred, or both.