Age of the Irkut block of the Prisayan uplift of the Siberian platform basement: Dating minerals from metamorphic rocks

New geochronological (U-Pb, Pb-LS, Sm-Nd) studies were carried out for minerals from metamorphic rocks (aluminous plagiogneisses with sillimanite (kinzigites) and potassium shadow migmatites) to establish the sequence of metamorphic events in the Irkut block of the Prisayan marginal uplift of the Siberian platform basement. Obtained data permit the distinguishing of two main stages of regional metamorphism under the granulite and amphibolite facies conditions: 2480–2560 and 1860–1900 Ma. New age data in general are consistent with previously published zircon estimates of the Neoarchean and Paleoproterozoic ages of the granulite-facies metamorphism of the Irkut block. This gives grounds to consider the geochronological studies of garnet and monazite as promising tool for distinguishing age boundaries of metamorphic transformations in the areas of polycyclic evolution.     

Paleoarchaean tonalites in the Orekhovo-Pavlogradskaya palaeoproterozoic collsional zone (Ukrainian Shield)

The novel detailed geological and geochronological data for the tonalitie rocks and products of its structural and metamorphic overprint in the Orekhovo-Pavlogradskaya collision zone (Ukrainian Shield) are discussed. Magmatic crystallization of tonalites (3500 ± 13 Ma) followed by amphibolite (2863 ± 22 Ma) and granulite (2105 ± 40 Ma) facies metamorphic events, resulting in biotite-garnet gneisses formation. The last date also indicating the time of collision event. The earliest age of 3.64 Ga was found in the zonal zircon fragment within 3.5-Ga old magmatic zircon from tonalite.     

Dating Post-Metamorphic Cooling of the Eastern Granulites in the Mozambique Belt of Northern Tanzania Using the Garnet Sm-Nd Method

Granulites from the Usambara, Wami River and Uluguru areas in the northern part of the Mozambique Belt in Tanzania yield Sm-Nd garnet — whole rock ages of between 580 and 634 Ma with a mean of 609 ± 11 Ma (2σ). This mean age is only slightly younger than the previously published peak metamorphic age of 641 ± 2 Ma, suggesting that, contrary to some earlier arguments, garnet Sm-Nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes. Using published peak metamorphic temperatures of ∼810°C and cooling rates of 1–4°C/Ma, the mean age translates into garnet closure temperatures of 690 to 780°C.The similarity in garnet ages over widely separated areas, coupled with the previously established similarity in equilibrium PT conditions, indicate that isolated complexes that form the Eastern Granulites in the Tanzanian sector of the Mozambique Belt share the same thermal history and were formed under the same geodynamic setting.A few published garnet ages of between 525 and 545 Ma indicate a younger, less pervasive event of granulite facies metamorphism in the Belt. The bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of Gondwana took place in two stages. The ∼610 Ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of India, Madagascar, parts of eastern Antarctica, the Kalahari craton, the Congo craton and the Arabian-Nubian shield (forming the IMSLEK-ANS collage). On the other hand, the 525–545 Ma ages may mark cooling from a thermal event associated with the collision of Australo-Antarctica with the IMSLEK-ANS collage.     

胶北早前寒武纪变质基底多期岩浆-变质热事件:来自TTG片麻岩和花岗质片麻岩中锆石U-Pb定年的证据

本文采用LA-ICP-MS技术,对胶北地体TTG片麻岩和花岗质片麻岩中锆石进行系统原位U-Pb定年和稀土、微量元素的分析,发现研究区早前寒武变质结晶基底存在多期岩浆-变质热事件。4件TTG片麻岩和2件花岗质片麻岩锆石样品记录了2909±13Ma、2738±23Ma、2544±15~2564±12Ma和2095±12Ma 4组岩浆事件年龄,以及2504±16~2513±32Ma和1863±41Ma 2组变质事件年龄。结合以往TTG片麻岩和花岗质片麻岩的地球化学及Nd同位素研究发现,约2738Ma的TTG岩浆事件可能代表胶北地体地壳最主要的生长事件,而2544~2564Ma的岩浆事件则可能代表古老地壳重熔的最强烈岩浆事件,约2095Ma岩浆事件则反映了胶-辽-吉构造带内部在该时期与地壳拉张作用有关的岩浆活动。2504~2513Ma是研究区以及华北克拉通早前寒武基底最主要的一期变质热事件,可能与地幔柱(热点)岩浆的底侵作用有关,而TTG片麻岩记录的约1863Ma的变质年龄与研究区基性和泥质高压麻粒岩相岩石记录的麻粒岩相变质时代一致,暗示TTG片麻岩可能也经历了古元古代高压麻粒岩相变质作用,上述研究进一步表明胶北地体在古元古代的确存在一期陆-陆碰撞的重要造山事件。该项研究成果对于进一步深入探讨胶北乃至华北克拉通早前寒武纪变质基底的形成演化、岩浆-变质热事件序列及其构造背景具有重要的科学意义。     

Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites,South Marginal Zone,Limpopo Belt,South Africa

The metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta‐ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P–T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P–T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 ± 13 Ma. Peak metamorphic conditions of 852.5 ± 7.5 °C and 11.1 ± 1.3 kbar were attained at 2713 ± 8 Ma. The clockwise P–T path is characterized by heating in the sillimanite field along a P–T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near‐isothermal decompression at peak temperature and near‐isobaric cooling at ~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post‐date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of ~0.17 cm yr^?1. Peak metamorphism was followed by an isothermal decompression to 787.5 ± 32.5 °C and 6.7 ± 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640 °C prior to 2680 ± 6 Ma. This age for the end of the high‐grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 ± 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P–T path, the burial of the rocks to in excess of 30 km depth and the post‐peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and that the SMZ was metamorphosed as a consequence of continental collision along the northern margin of the Kaapvaal craton at c. 2700 Ma.     

大别山高级变质岩云母Rb—Sr等时线年龄测定及其地质意义

应用云母Rb Sr等时线法对大别山高级变质岩的后期改造历史进行了初步的探讨。采用重液变温技术将云母按比重的区别 ,分成若干组分 ,然后用常规的Rb Sr方法测定。应用这种技术对大别山黄土岭太古代麻粒岩相岩石中的黑云母和产于超高压变质带内的石马含石榴石片麻岩的白云母进行了Rb Sr等时线年龄测定。黄土岭太古代麻粒岩相片麻岩中的黑云母 全岩Rb Sr等时线年龄为 12 7± 9Ma ,与该区片麻岩中的角闪石的K Ar和Ar Ar年龄 ,及燕山期花岗岩的Rb Sr年龄一致 ,说明这一太古代下地壳岩石受到了燕山期深熔 岩浆事件的影响 ;大别山东南部石马含石榴石片麻岩 (榴辉岩相 )中的白云母Rb Sr等时线年龄为 191± 10Ma ,反映了印支 早燕山期区域性低级变质事件的时代。研究表明 ,大别山区的不同构造单元中产出的不同类型的高级变质岩自中生代以来有不同的后期演化历史。     

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.     

Single zircon evaporation ages: Evidence for the Mesoproterozoic crust in the southeastern Nigerian basement complex

The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the pres-ence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These "signatures" collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area.     

北秦岭造山带的早古生代多期变质作用

北秦岭造山带的秦岭岩群以高级变质岩石为特征,主要包括少量榴辉岩、高压麻粒岩和区域上广泛分布的麻粒岩-角闪岩相变质岩石。年代学研究显示秦岭岩群中不同岩石记录了多期变质作用。已有的定年资料给出北秦岭官坡地区的榴辉岩的年龄为500Ma左右,代表榴辉岩相的变质时代。结合岩相学资料,对两个高压麻粒岩样品的SHRIMP和LA-ICPMS U-Pb测定分别获得504±7Ma 和506±3Ma的年龄,应代表高压麻粒岩相变质时代。这表明高压麻粒岩和相邻的榴辉岩有相近的变质时代,但形成在造山带中不同的构热造环境中。西峡地区的角闪二辉麻粒岩的U-Pb定年给出两组早古生代年龄,一组为440±2Ma,可能代表了中低压麻粒岩相的变质时代,另一组为426±1Ma,应代表区域角闪岩相的变质时代。桐柏山北部的石榴二辉麻粒岩的U-Pb定年数据给出436±1Ma的年龄,为中压麻粒岩相的变质时代。这些资料表明北秦岭造山带经历了早奥陶世的俯冲和地壳增厚作用,并在晚志留世遭受了广泛的巴罗式区域变质作用。     

The Moslavačka Gora crystalline massif in Croatia: a Cretaceous heat dome within remnant Ordovician granitoid crust

For a long time the Moslavačka Gora Massif in Croatia has been regarded as a major outcrop of Variscan crystalline basement of the South Tisia block. However, new geochronological data indicate that this massif consists of a Cretaceous S-type granite pluton intruding a Cretaceous low-pressure/high-temperature (LP/HT) metamorphic envelope. The age of the LP/HT metamorphism is estimated at ~90–100 Ma using the method of electron microprobe based monazite dating. The Central Granite was dated at 82 ± 1 Ma (LA-SF-ICP-MS zircon age). The metamorphic complex comprises mainly felsic anatexites and orthogneisses of granitic composition, some metapelites (paragneisses and mica schists) and amphibolites. Zircons from three different samples of metagranite were dated at 486 ± 6, 483 ± 6, and 491 ± 1 Ma, suggesting that most of the metamorphic complex represents an Early Ordovician granitic series. The Cretaceous regional metamorphism culminated in granulite facies conditions of ~750°C and 3–4 kbar. A retrograde metamorphic event at lower amphibolite facies conditions overprinted the metamorphic complex. This event is probably related to the intrusion of the Central Granite. The southeastern sector of the massif was additionally affected by post-granitic, predominantly NE oriented shearing at greenschist facies conditions. As yet there is no clear evidence for Variscan events in the Moslavačka Gora Massif. Mineral relics of a medium-pressure amphibolite facies metamorphism are preserved in amphibolites. They are older than the Cretaceous LP/HT regional metamorphism, but their age is presently unknown. Some indications for a Permian regional metamorphic event are provided by inherited zircons in the Central Granite that have been dated with a Permian age, and by Permian monazite relics in metapelites. The Cretaceous high heat flow regime recorded in the Moslavačka Gora Massif is unique in the subcrop of the Pannonian Basin and may be a local feature triggered by a mafic intrusion in the lower crust.     

Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge

The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton.     

Granulite complexes of the Dzhugdzhur-Stanovoi Fold Region and the Peristanovoi Belt: Age,formation conditions,and geodynamic settings of metamorphism

The available data on the age and formation conditions of the granulite complexes in the western Dzhugdzhur-Stanovoi Fold Region (Dambuki and Larba blocks) and the adjacent territory of the Peristanovoi Belt (Kurul’ta, Zverevsky, and Sutam blocks) are systematized. At least three Early Precambrian episodes of high-grade granulite-facies metamorphism dated at 2.85–2.83, 2.65–2.60, and 1.90–1.88 Ga are established in the geological history of the western Dzhugdzhur-Stanovoi Fold Region. Five granulite-facies metamorphic events are documented in the Peristanovoi Belt. The early granulite-facies metamorphism, migmatization, and emplacement of charnockite are related to the first event (2183 ± 1 Ma) in the Kurul’ta Block. The structural transformation and metamorphism of charnockite under conditions of granulite facies correspond to the second event (2708 ± 7 Ma). The enderbite belonging to the Dzhelui Complex (2627 ± 16) and charnockite of the Altual Complex (2614 ± 7 Ma) were emplaced during the third tectonic event, which was immediately followed by the emplacement of the Kalar anorthosite-charnockite complex (2623 ± 23 Ma). The first episode of Early Proterozoic granulite-facies metamorphism of the Sutam Sequence in the tectonic block of the same name was related to the fourth event, probably caused by collision of the Olekma-Aldan continental microplate and the passive margin of the Uchur continental microplate. Finally, granulite-facies metamorphism superimposed on rocks of the Kalar Complex in the Kurul’ta Block and high-pressure metamorphism in the Zverevsky and Sutam blocks (1935 ± 35 Ma) correspond to the fifth metamorphic event. The Late Archean metamorphic events are most likely related to the amalgamation and subsequent collision of the terranes which currently make up the granulite basement of the Dzhugdzhur-Stanovoi Fold Region with the Olekma-Aldan continental microplate. In the Early Proterozoic, the Aldan Shield and the Dzhugdzhur-Stanovoi Fold Region were separated by an oceanic basin. Its closure, and the collision of the Aldan and Stanovoi continental microplates, were accompanied by granulite-facies metamorphism and led to the formation of the Peristanovoi Belt, or Peristanovoi Suture Zone. This collision suture continued functioning in the Phanerozoic (from the Early Jurassic to the Early Cretaceous) with the formation of thick shear zones and greenschist retrograde metamorphism.     

Multistage metamorphism of the Huangtuling granulite, Northern Dabie Orogen, eastern China: implications for the tectonometamorphic evolution of subducted lower continental crust

Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Palaeoproterozoic metamorphic processes, Triassic continental subduction‐collision and Cretaceous collapse of the Dabie Orogen. Six stages of metamorphism are resolved, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high‐pressure/high‐temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630–700 °C); (IV) medium‐pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low‐pressure/high‐temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent at c. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to Triassic subduction/collision between the Yangtze and Sino‐Korean Cratons. The dry lower crustal granulite persisted metastably during the Triassic subduction/collision because of the lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabie Orogen, possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high‐pressure–ultrahigh‐pressure metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction‐collision and orogenic collapse.     

青藏高原拉萨地体北部的前寒武纪变质作用及构造意义

青藏高原南部拉萨地体中分布的角闪岩相至麻粒岩相变质岩一直被认为是前寒武纪变质基底,但并没有获得可靠的年代学证据。本文运用原位锆石U-Pb定年方法,在拉萨地体北部那果地区的变质岩中获得了约720Ma的变质年龄,从而证明拉萨地体北部在新古元代经历了角闪岩相变质作用和近同期的岩浆作用。基于这一成果和在拉萨地体中、南部高级变质岩中陆续获得的中、新生代变质年龄,对拉萨地体变质作用的时、空变化及其成因进行了初步探讨。     

Tectono-metamorphic evolution of a high- to medium-grade ductile deformed metagabbro/metadiorite from the Arenosa Creek Shear Zone,Western Sierras Pampeanas,Argentina

Textural and compositional changes affecting a quartzose metagabbro/metadiorite in a ductile shear zone which is part of the Early Palaeozoic (Famatinian) orogenic belt of the Western Sierras Pampeanas, Argentina, allow reconstruction of its tectono-metamorphic evolution as well as the metamorphic conditions achieved. On the basis of paragenetic associations and microfabrics, three overprinted deformation events are differentiated: a) a relict ductile event developed within the granulite facies, at temperatures exceeding 800 °C and pressures ≤5–5.5 kbar; b) a two-stage mylonitic deformation event under high-to medium-grade metamorphic conditions, at temperatures 500–700 °C and pressures between 6 and 7 kbar and c) a brittle deformation event developed at low greenschist facies below 400 °C. Evidence of the first event is preserved in the protolith although the subsequent deformation obliterated it partially. Slight modal and chemical changes were detected between the protolith and its mylonitic products. Deformation ages of the mylonites that vary from 441.9 ± 1.9 Ma to 438.7 ± 1.9 Ma are internally consistent and compatible with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied in time and space.     

First SHRIMP U–Pb zircon dating of granulites from the Kontum massif (Vietnam) and tectonothermal implications

The Kontum massif in Central Vietnam represents the largest continuous exposure of crystalline basement of the Indochina craton. The central Kontum massif is chiefly made of orthopyroxene granulites (enderbite, charnockite) and associated rocks of the Kannack complex. Mineral assemblages and geothermobarometric studies have shown that the Kannack complex has severely metamorphosed under granulite facies corresponding to P–T conditions of 800–850°C and 8±1 kbars. Twenty-three SHRIMP II U–Pb analyses of eighteen zircon grains separated from a granulite sample of the Kannack complex yield ca 254 Ma, and one analysis gives ca 1400 Ma concordant age for a zoned zircon core. This result shows that granulites of the Kannack complex in the Kontum massif have formed from a high-grade granulite facies tectonothermal event of Indosinian age (Triassic). The cooling history and subsequent exhumation of the Kannack complex during Indosinian times ranged from ∼850°C at ca 254 Ma to ∼300°C at 242 Ma, with an average cooling rate of ∼45°C/Ma.     

Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis,south Tibet: petrology,zircon geochronology and implications for the subduction of Neo‐Tethys and the Indian continent beneath Asia

The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus‐Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium‐P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36–33 Ma. However, the High Himalayan rocks experienced only a single high‐P granulite facies metamorphic event at 37–32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium‐P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo‐Tethyan ocean, and that the Oligocene (37–32 Ma) high‐P (1.8–1.4 GPa) rocks of the High Himalayan and coeval medium‐P (0.8–1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Age and geochemical constraints for partial melting of granulites in Estonia

Summary The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.     

An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolds Range,central Australia*

ABSTRACT The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly >2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ～3°C Ma^–1. The last regional high-grade metamorphism in the Reynolds Range occurred at ～1.6 Ga, not ～1.78 Ga as previously thought. The high-grade event at ～1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.