Timing Of Granulite Facies Metamorphism Research Articles

P–T–t conditions of Early Palaeozoic low‐P high‐T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

AbstractHigh‐grade metamorphic rocks are widely exposed along the SE–NW‐ to E–W‐trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high‐grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse‐grained granulite mineral assemblages, defining a tight clockwise P–T path with near‐isobaric heating to a near ultrahigh‐temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse‐grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K‐feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post‐peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium‐pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine‐grained kyanite‐bearing mineral associations. Petrographic observations indicate that these fine‐grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post‐collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

Neoarchean–Paleoproterozoic HP-HT metamorphism in the Bhopalpatnam granulite belt, Bastar Craton (India): Insights from phase equilibria modelling and monazite geochronology

The Bhopalpatnam Granulite Belt (BGB), located within the southwestern part of the Bastar Craton (India), is composed of granite gneisses with numerous enclaves of high-grade rocks, including quartzofeldspathic gneisses, orthopyroxene-free mafic granulite, and garnetiferous two-pyroxene granulite. In addition, the marginal parts of the belt preserve patches of garnetiferous psammo-pelitic gneisses. New petrographic, metamorphic (conventional and phase equilibria modelling) and monazite geochronological investigations of these high-grade rocks constrain the conditions and timing of granulite-facies metamorphism and enable to demonstrate the tectonometamorphic history of the BGB. The garnet-bearing psammo-pelitic gneisses underwent medium pressure-high temperature (MP-HT) and retrograde metamorphism at P-T conditions of 9.4 ± 1.0 kbar/797 ± 39 °C and 5.1 ± 1.1 kbar/697 ± 69 °C, respectively. The quartzofeldspathic gneisses experienced high pressure-high temperature metamorphism (HP-HT; 12.0 ± 0.9 kbar/785 ± 48 °C), followed by retrogressive cooling at 10.6 ± 0.8 kbar/683 ± 42 °C. The orthopyroxene-free mafic granulites and two-pyroxene granulites witnessed HP-HT metamorphism at 11.0 ± 0.8 kbar/827 ± 45 °C, followed by MP-HT metamorphism at 8.4 ± 0.9 kbar/803 ± 66 °C and retrograde cooling at 5.7 ± 1.5 kbar/555 ± 80 °C. The U–Th–total Pb dating of monazites from the quartzofeldspathic gneiss yielded peak (HP-HT) metamorphic age at 2588–2461 Ma, MP-HT metamorphism at 2499–2308 Ma, and thermal events associated with granite magmatism at 2292–2111 Ma. Based on similar clockwise P–T history and Late Archean-Early Mesoproterozoic tectonothermal events, we suggest that the Bhopalpatnam and Karimnagar granulite belts evolved coherently until the Mesoproterozoic time and eventually rifted apart during the development of the Pranhita-Godavari rift basin between ∼ 1.6–1.4 Ga. The HP-HT and MP-HT granulite-facies metamorphic episodes identified in the BGB are correlated with the continent–continent collision between Bastar and eastern Dharwar cratons.

Relations between earthquake distributions, geological history, tectonics and rheology on the continents.

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400-600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

Palaeoproterozoic granulite-facies metamorphism in the eastern Alxa Block: New petrological and geochronological evidence from the Diebusige Complex

The Diebusige Complex, the highest-grade metamorphic complex in the Alxa Block, can provide significant insights into the origin, formation, evolution, and tectonic affinity of the Alxa Block. Petrographic observations show that the mineral assemblage of (garnet) magnetite quartzite from the Diebusige Complex is magnetite + quartz + hypersthene + plagioclase ± clinopyroxene ± garnet. The metamorphic temperature (T) is constrained to 800–860 °C by the two-pyroxene geothermometer, corresponding to granulite-facies metamorphism. Geochronological studies involving cathodoluminescence (CL) images, LA-ICP-MS U-Pb dating, and trace element analyses of zircons show that the time of granulite-facies metamorphism was approximately 1850 Ma. Combined with the corresponding published data, this paper suggests that the metamorphic basement rocks of the Alxa Block record Neoarchaean-early Palaeoproterozoic magmatic-metamorphic and late Palaeoproterozoic granulite-facies metamorphic events, thereby indicating the affinity between the Alxa Block and the Khondalite Belt (KB).

Zircon U–Pb geochronology and clockwise P–T evolution of garnet-bearing migmatites from the Qinling complex in the Weiziping area of the Qinling Orogen, Central China: Implications for thermal relaxation after crustal thickening

Zircon U–Pb dating and phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system were carried out to garnet-bearing migmatites from the Qinling Complex in the Weiziping area of the Qinling Orogen, Central China, to constrain their metamorphic P–T path and elucidate the tectonic setting and geodynamic mechanism of Early to Middle Devonian high-temperature (HT) metamorphism in the North Qinling Belt. The mesosomes of the migmatites are composed of garnet-bearing biotite–hornblende–plagioclase and biotite–plagioclase gneisses, and the leucosomes can be divided into two types: wide leucosomes that were deformed, and narrow penetrative leucosomes. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb analyses yielded weighted mean 206Pb/238U ages of 398.8 ± 4.7, 394.6 ± 2.6, 399.5 ± 7.3, 372.5 ± 4.2, 370.8 ± 4.3, and 368.5 ± 4.2 Ma for metamorphic zircons within the sampled mesosomes, and a weighted mean age of 408.7 ± 5.7 Ma for igneous zircons within a deformed leucosome. Based on the published data and the results in this study, the timing of granulite-facies metamorphism is constrained to be ca. 409–395 Ma. The mesosome sample (garnet-bearing biotite–hornblende–plagioclase gneiss) has a peak mineral assemblage of garnet, biotite, plagioclase, hornblende, quartz, ilmenite, and melt, which equilibrated at P–T condition of 8.5–9.7 kbar at 745–820 °C as constrained by isopleths of XAn and XPy in a P–T pseudosection, suggesting that these gneisses underwent medium-pressure granulite-facies metamorphism. Conventional garnet–biotite–plagioclase–quartz (GBPQ) geothermobarometry yielded P–T conditions of 703 °C at 10 kbar and 685–748 °C at 7.1–8.9 kbar, which correspond to prograde and retrograde metamorphic stages, respectively. On the basis of these P–T results, the studied samples document a clockwise P–T path characterized by temperature increase and pressure decrease during the prograde stage, and temperature decrease and pressure decrease during the retrograde stage. Ti-in-zircon thermometry yielded a temperature range of 781–793 °C for metamorphic zircons from the mesosomes, further demonstrating that they underwent granulite-facies metamorphism at ca. 409–395 Ma. The prograde metamorphic stage characterized by temperature increase and pressure decrease suggests that thermal relaxation after crustal thickening has induced the partial melting and migmatization. Thus, the crustal thickening that may be caused by continental collision between the North China Block and the South China Block should occur no later than ca. 409 Ma.

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

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

A window into an ancient backarc? The magmatic and metamorphic history of the Fraser Zone, Western Australia

The Fraser Zone is a major lithotectonic domain of the Albany–Fraser Orogen, Western Australia, which records Proterozoic modification of the margin of the Archean Yilgarn Craton. The Fraser Zone is volumetrically dominated by gabbroic rocks and their metamorphosed equivalents. However, little is known of the pressure–temperature–time (P–T–t) history or the geodynamic setting of these mafic rocks. When considered within the context of existing P–T constraints from spatially-associated metapelitic rocks, modelled phase equilibria suggest that both the unmetamorphosed gabbros and the granulite facies metagabbroic rocks equilibrated at 950–900 °C and ∼7 kbar, interpreted to record the conditions of magmatic crystallisation and peak metamorphism, respectively. These data support the view that mafic magmatism was the thermal driver for high-T, low-P granulite facies metamorphism. The absence of garnet from the metagabbroic rocks, and the lack of evidence for its former presence (i.e., as inclusions), argues that, during metamorphism, the rocks never reached pressures above those they attained at the thermal peak. Coronæ of zircon around ilmenite in the magmatic rocks reflect a local supply of Zr as it exsolved from ilmenite, permitting earlier growth of zircon around ilmenite than elsewhere during melt crystallisation. U–Pb dating of coronal zircon (1315 ± 5 Ma) and a discrete magmatic zircon grain isolated from ilmenite (1296 ± 5 Ma) constrain the duration of magmatic crystallisation between ca. 10 and 30 Ma. Zircon in a metamorphosed gabbro constrains the timing of granulite facies metamorphism to 1293 ± 6 Ma, synchronous with final crystallisation of the mafic magmas. Based on the implied metamorphic evolution of these rocks and that of the surrounding supracrustal package, along with existing isotopic and geochemical data, we suggest the Fraser Zone probably formed in a backarc, or perhaps an intracontinental rift setting, and records successive emplacement of gabbroic rocks into a thick, sediment-filled basin. The older gabbroic rocks record hydration and reaction with the devolatilising and/or partially molten metasedimentary rocks into which they were emplaced. Subsequent granulite facies metamorphism of these hydrated rocks was driven by the heat provided by the intrusion of younger mafic magmas.

Proterozoic granulite formation driven by mafic magmatism: An example from the Fraser Range Metamorphics, Western Australia

Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293Ma. Peak metamorphic conditions at c. 1290Ma were c. 850°C at pressures of 7–9kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290Ma, essentially coincident with the emplacement of mafic rocks at 1292Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400°C) occurred at 1260Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234±17Ma.

Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan

We dated and geochemically characterized peridotite-bearing mélanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan mélange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10km long; protolith age≥ca. 445–466Ma). Metamorphic zircons of a mafic granulite (εNd(t)=5.0) yielded a weighted mean 206Pb/238U age of 341±8Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (εNd(t)=−4.3) have a mean age of 332±13Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341–332Ma) of mantle diapirism through continental crust. A dolerite dike (εNd(t)=2.3) emplaced into metaperidote has a crystallization age of 335±5Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (εNd(t)=−3.6) intrudes the mélange matrix and has an emplacement age of 324±5Ma, thus providing an upper time limit for a tectonic movement that led to mélange formation. The older time limit of deformation (ca. 362–352Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (εNd(t)=−3.3) and a dacite (εNd(t)=−5.6) have eruption ages of 433±4Ma and 435±3Ma that date formation of the mélange matrix. A microgabbro dike (εNd(t)=10.0) cutting metasediments (i.e. mélange matrix) has an emplacement age of 279±3Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the mélange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (εNd(t)=−7.0 and −6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341±2Ma and 329±4Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (εNd(t)=8.7) has an emplacement age of 297±3Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan–Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan.

Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite ( P = 14–18 kbar, T ≈ 800 °C) yield U–Pb ages of 24.0 ± 0.3 Ma and 18.8 ± 0.3 Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490 ± 3 Ma and 24.2 ± 0.4 Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0 ± 0.4 Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼ 24 Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17 Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼ 0.6 to ∼ 2.0 Ga with peak at 0.8–1.2 Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼ 600 Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼ 24 Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene.

On the difficulty of assigning crustal residence, magmatic protolith and metamorphic ages to Lewisian granulites: constraints from combined in situ U–Pb and Lu–Hf isotopes

Abstract Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c . 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c . 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c . 3.05–3.2 Ga. Supplementary material: Analytical methods and data tables are available at: http://www.geolsoc.org.uk/SUP18395

Formation of planar deformation features (PDFs) in zircon during coseismic faulting and an evaluation of potential effects on U–Pb systematics

Zircons from pseudotachylyte and cataclasite veins in the Svarthumlevatnet metagabbro, SW Norway, are faulted, brecciated and display multiple sets of planar deformation features (PDFs) that we assign to coseismic deformation in the middle crust during Caledonian continent collision. The zircons give a U–Pb upper intercept age of 1507 ± 4 Ma, interpreted as a minimum age of the gabbro, and a lower intercept age of 968 ± 31 Ma, likely indicating the time of granulite-facies metamorphism, but they do not record the Caledonian coseismic faulting even when the fault planes are healed by thin veins of low-U zircon. The work indicates that PDFs are not exclusive to meteorite impacts, but may also develop during seismic faulting.

Timing of magmatism and metamorphism in the Gruinard Bay area of the Lewisian Gneiss Complex: comparison with the Assynt Terrane and implications for terrane accretion—reply

U-Pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the Gruinard Bay, Badcall Bay and Lochinver areas of the mainland Lewisian Gneiss Complex of north-west Scotland. Zircons from the gneisses contain complex zoning patterns making them amenable to SHRIMP ion microprobe analysis, with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation. From Gruinard Bay, a tonalitic granulite yielded a protolith age of 2,825±8 Ma for oscillatory-zoned zircon cores, with recrystallised low-U rims preserving an age of 2,733±12 Ma. An inherited 2,905±15 Ma age component was recognised in a sample of trondhjemitic gneiss, the protolith of which is estimated to have been emplaced at 2,858±11 Ma. Low-U rims yielding an age of 2,729±20 Ma are also present, which together with the rim age from the granulite are considered to date granulite facies metamorphism in the Gruinard region. For comparison with the Gruinard Bay gneisses, a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at Badcall Bay in the Assynt terrane. Age data do not show definitive relationships with zoning patterns in the zircons, and show a spread along concordia typical of previous ion microprobe age data from the Scourie gneisses. Both samples do, however, show a significant isotopic disturbance at 2,480 Ma, the most recent estimate for the Badcallian metamorphism. Protolith ages for components of the Assynt terrane range between ca. 2,900 and 3,030 Ma, and appear to be older than the 2,860–2,825 Ma gneiss protoliths from the Gruinard Bay area. The timing of granulite facies metamorphism also appears to be different between the two areas, suggesting that the two granulite regions of the mainland may not be related as previously thought. Instead, they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the Palaeoproterozoic. The boundary between the two terranes is thought to lie to the south of Lochinver, corresponding with the Strathan Line, because of the observed 2,480 Ma age signature to the north and 2,730 Ma signature to the south. By defining a new mainland terrane, the Gruinard Terrane, the number of mainland blocks, which includes the Assynt (central), Rhiconich (northern) and Southern terranes, has been expanded.

Sensitive high‐resolution ion microprobe U‐Pb ages of the Latest Oligocene‐Early Miocene rift‐related Hidaka high‐temperature metamorphism in Hokkaido, northern Japan

Abstract In order to define the timing of granulite facies metamorphism, sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206Pb/ 238U ages (n = 31) with low Th/U ratios, mostly <0.1). The internal structure of zircons and their SHRIMP U‐Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene‐Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene‐Early Miocene Hidaka high‐temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back‐arc rifting of the Kuril and Japan basins.

Single zircon ages of migmatitic gneisses and granulites in the Obudu Plateau: Timing of granulite-facies metamorphism in southeastern Nigeria

A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded 207Pb/ 206Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from collisional processes during continental amalgamation to form the Gondwana supercontinent. The sources of the sediments, which were deposited at ≈605 Ma and metamorphosed at 574 Ma, comprise older igneous and metamorphic protoliths (including inherited xenocrystic zircons up to 2.5 Ga in age). The Palaeoproterozoic zircons seem to have survived Pan-African melting.

Late Pan-African magmatism in northeastern China: SHRIMP U–Pb zircon evidence from granitoids in the Jiamusi Massif

We present the first evidence of late Pan-African magmatism in the Jiamusi Massif of northeastern China. Recent SHRIMP U–Pb zircon studies of the Mashan Complex have identified a major ∼500 Ma granulite facies metamorphic event in the area, but no older igneous rocks have previously been dated. Two granitoid samples have been analysed using SHRIMP U–Pb zircon techniques: a deformed granitoid from near Sandaogou contains zircons with well-defined oscillatory zoning that define a weighted mean 206 Pb / 238 U age of 523±8 Ma, taken to be the age of the granite protolith; and a weakly-deformed porphyritic granite from near Jiamusi, which also contains zircons with oscillatory zoning of igneous origin, has a weighted mean 206 Pb / 238 U Ma crystallisation age of 515±8 Ma. In addition, a deformed pegmatite from Liu Mao, which contains oscillatory-zoned igneous zircons, was also analysed. These zircons are rich in uranium and the data are discordant, so their interpretation is not equivocal. If the four oldest 207 Pb / 206 Pb ratios are taken to represent the minimum age of zircon crystallisation, they define an age of 501±18 Ma. Several zircon crystals from both granite samples also reveal evidence of recrystallisation in cathodoluminescence (CL) images and this has been dated at 505±4 and 497±5 Ma in the Sandaogou and Jiamusi samples, respectively. This is the timing of granulite facies metamorphism in the southern area of the Mashan Complex and these ages are only slightly older than the ∼490 Ma granulite facies metamorphism identified in southern Siberia, possibly indicating that they had a common evolution. Alternatively, the presence of plutonic igneous rocks with ages between ∼515 and ∼525 Ma in the Jiamusi Massif is comparable to several other late Pan-African terranes. In particular, the timing of granite emplacement at ∼515 Ma and high-grade metamorphism at ∼500 Ma is identical to events recorded in the Delamerian Orogen of South Australia. The recent discovery of ∼500 Ma metamorphic zircons in the Halls Reward metamorphic belt in Queensland, NE Australia, may indicate a continuation of late Pan-African events across Australia, supporting the view that the Jiamusi Massif lay along the northern margin of Australia at this time, together with the Tarim, North China and South China blocks.

Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.

Pitfalls and new approaches in granulite chronometry: An example from the Limpopo Belt, Zimbabwe

Timing and duration of metamorphism are particularly difficult to decipher in granulites because the metamorphic temperature commonly experienced by such rocks exceeds the closure temperature of all geochronometers other than the U–Th–Pb systems in a few minerals. Age constraints on granulite metamorphism therefore often rely on data from a single geochronometer and consistency tests are rarely possible. This case study was conducted on a single hand specimen of an exceptional late Archaean granulite from the Northern Marginal Zone of the Limpopo Belt, Zimbabwe. This rock not only contains zircon, monazite, and xenotime as accessory phases but is also depleted in Pb to such an extent that dating of orthopyroxene was possible. In situ 207/206∗Pb SIMS zircon, monazite, and xenotime ages, conventional U–Pb zircon and orthopyroxene ages, and Pb–Pb step leaching orthopyroxene ages were obtained. The in situ 207/206∗Pb SIMS ages of monazite and xenotime inclusions in cordierite are well within an independent age bracket as are ages of an interstitial monazite grain. Interstitial xenotime, however, retains a more than 55 Ma younger age than the cordierite shielded grain. These ages are incompatible with a monazite/xenotime U–Pb closure temperature of 725±25°C but suggest that grains which were shielded from processes leading to additional Pb loss are closed to significant Pb volume diffusion at temperatures ≥800°C. Zircon ages reflect partial protolith inheritance, total resetting during peak temperature conditions, and disturbance during minor hydrothermal overprint and are therefore not useful, on their own, to constrain the timing of granulite facies metamorphism. U–Pb and Pb–Pb step leach systematics of orthopyroxene yielded ages identical to the in situ phosphate ages, well within the independent age bracket. However, the step leach experiment showed that these ages merely date monazite and zircon inclusions rather than the host orthopyroxene itself. Simple model calculations are used to demonstrate that even minute contamination of low μ minerals with high μ inclusions will have significant effects on the obtained mixed age and may lead to erroneous conclusions regarding the U–Pb closure temperature of e.g. pyroxene and garnet. These pitfalls in granulite geochronology will usually result in an overall apparent age dispersion and time scales derived from field studies should therefore be viewed as upper limits.

SmNd isotopes and ree geochemistry of madras granulites, india: an introductory statement

Abstract Major and trace elements including REE as well as SmNd isotope analyses of eight granulite samples from the Madras area are presented. The data provide evidence for the origin of some of the granulites, but in other cases demonstrate metasomatic changes in major and trace element contents, including LREE. The data furthermore demonstrate that the analysed granulites are not cogenetic. However, when plotted in a SmNd isochron diagram, the samples yield a linear array which corresponds to an age T = 2555 ± 140 Ma and ϵNd(T) = +1 ± 1.4. This age of 2555 ± 140 Ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith. The ϵNd(T) = 1 value may be indicative of a near chondritic mantle source, or alternatively of mixing between a depleted mantle and a crustal component.

Uranium-lead and lead-lead investigations of minerals from the Broken Hill lodes and mine sequence rocks

A regression analysis of combined rock and lode apatite data for the 207 Pb/ 206 Pb vs. 204 Pb/ 206 Pb plot gives an age of 1,565 + or - 20 m.y. (2sigma ) which is similar, within the experimental error limits, to the minimum U-Pb ages for monazite and Pb-Pb age for sphene from the mine sequence rocks of about 1,595 m.y. The possible differences in apparent age of the monazite-sphene and apatite may reflect the different blocking temperatures of the U-Pb systems in the two minerals. Cooling ages indicate that the Broken Hill Block remained at temperatures in excess of 600 degrees C for over 60 to 70 m.y. and would have allowed extensive concentration and/or recrystallization of the sulfide orebodies. On a concordia plot, the U-Pb data for zircons from two samples of upper granite gneiss and two of Potosi gneiss lie on an array with an apparent age of 1,663 (super +9) (sub -8) m.y. (2sigma ) and with a mean square of weighted deviates of 24.6. This age is interpreted as the time of granulite facies metamorphism. The least magnetic zircon fractions of samples close to mineralization contain from 2.2 to 16 ppm of common Pb. The abundant common lead in the zircons indicates that the sulfides were already in existence by 1,660 m.y., even if only in a disseminated form. An alternative explanation of the isotopic data is that there have been two major metamorphic events at Broken Hill during the middle Proterozoic: the granulite facies at approximately 1,660 m.y. and another at approximately 1,595 m.y. Thirty-five analyses of 21 sphalerites and pyrrhotites show uniform lead isotope ratios throughout the orebody because of the presence of galena inclusions.--Modified journal abstract.