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Abstract: This study presents Lu–Hf geochronology of zoned garnet in high‐P eclogites from the North Qilian orogenic belt. Selected samples have ~mm‐sized garnet grains that have been sampled with a micro‐drill and analysed for dating. The Lu–Hf dates of bulk garnet separates, micro‐drilled garnet cores and the remnant, rim‐enriched garnet were determined by two‐point isochrons, with cores being consistently older than the bulk‐ and rim‐enriched garnet. The bulk garnet separates of each sample define identical garnet–whole rock isochron date of c. 457 Ma. Consistent U–Pb zircon dates of 455 ± 8 Ma were obtained from the eclogite. The Lu–Hf dates of the drilled cores and rim‐rich separates suggest a minimum garnet growth interval of 468.9 ± 2.4 and 452.1 ± 1.6 Ma. Major and Lu element profiles in the majority of garnet grains show well‐preserved Rayleigh‐style fractionated bell‐shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P–T path from ~470–525°C and ~2.4–2.6 GPa. The exceptional high‐Mn garnet core in one sample indicates an early growth during epidote–blueschist facies metamorphism at <460°C and <0.8 GPa. Therefore, the Lu–Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu–Hf dates of rim‐rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet‐bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm‐sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469 Ma) and the ultimate closure (earlier than c. 452 Ma) of the fossil Qilian oceanic basin. [ABSTRACT FROM AUTHOR]

A U-Pb-He double-dating method is applied to detrital zircons with core-rim structure from the Ganges River in order to determine average short- and long-term exhumation rates for the Himalayas. Long-term rates are calculated from the U/Pb ages of metamorphic rims of the grains that formed during the Himalayan orogeny and their crystallization temperatures, which are calculated from the Ti-in-zircon thermometer. Short-term rates are calculated from (U-Th)/He ages of the grains with appropriate closure temperatures. The results show that short-term rates for the Himalayas, which range from 0.70 ± 0.09 to 2.67 ± 0.40 km/Myr and average 1.75 ± 0.59 (1 σ) km/Myr, are higher and more varied than the long-term rates, which range from 0.84 ± 0.16 to 1.85 ± 0.35 km/Myr and average 1.26 ± 0.25 (1 σ) km/Myr. The differences between the long-term and short-term rates can be attributed to continuous exhumation of the host rocks in different mechanisms in continental collision orogen. The U/Pb ages of 44.0 ± 3.7 to 18.3 ± 0.5 Ma for the zircon rims indicate a protracted episode of ∼25 Myr for regional metamorphism of the host rocks at deeper crust, whereas the (U-Th)/He ages of 42.2 ± 1.8 to 1.3 ± 0.2 Ma for the zircon grains represent a protracted period of ∼40 Myr for exposure of the host rocks to shallower crustal level. In particular, the oldest (U-Th)/He ages of the zircon grains are close to the oldest U/Pb ages for the rims, indicating that some parcels of the rocks that contain zircons were rapidly exhumed from deep to shallow levels in the stage of collisional orogeny. On the other hand, some parcels of the rocks may have been carried upwards by thrust faults in the post-collisional stage. The parcels could be carried upwards by the thrust faults that steepen as they near the surface, or by transient movement faults so that areas of rapid exhumation became areas of slow exhumation and visa versa on a time scale of a few Myr in order to maintain the continuous exhumation. In this regard, the Ganges River must be preferentially sampling areas that are currently undergoing above average rates of uplift. [ABSTRACT FROM AUTHOR]

Abstract: The geological evolution in the southwestern Chinese Tianshan orogen in NW China is poorly understood because of controversial geochronological results of the low-temperature eclogites. It is likely due to Sm–Nd and Rb–Sr isotopic disequilibrium in different mineral fractions, excess 40Ar in the high-pressure K-bearing minerals, and multi-stage growths of zircon in U–Pb system. Rutile, as a typical eclogite-facies mineral with high U/Pb ratio, is a feasible candidate to constrain the eclogite-facies metamorphic event. In this study, we measured U–Pb rutile age data using SIMS (Cameca IMS 1280) for low-temperature eclogites from southwestern Chinese Tianshan orogen, NW China. We demonstrate that SIMS is a powerful technique for in-situ U–Pb dating of rutile with U content as low as 1ppm. SIMS U–Pb rutile dating yielded an age of 318±7Ma, which may record the time when the rocks cooled down to the closure temperature of Pb in rutile (ca. 500°C), but should be very close to the time of eclogite-facies metamorphism in terms of common fast exhumation of coesite-bearing eclogite. This new proof strongly suggests a late Carboniferous collision between Kazakhstan–Yili block and Central Tianshan microcontinent. [ABSTRACT FROM AUTHOR]

Abstract: Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D1–D2, D3 and D4. The garnet-bearing assemblages crystallized in course of the deformation stage D1–D2 which led to the formation of the regional main foliation S2. In X Ca–X Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X Mg and variably decreasing X Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P–T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P–T trends. The P–T evolution started at ∼450°C/7 kbar, passed high-pressure conditions at 11–12kbar at variable temperatures (600–700°C) and involved a marked decompression toward 6–7kbar at high temperatures (700–750°C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613±33Ma and 586±15Ma. The EMP-monazite age populations between 613±33Ma enclosed in garnet and 605±12Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P–T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P–T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes. [Copyright &y& Elsevier]

Abstract: Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950±15Ma, and intruded by granite at 1056±11Ma. This complex has only weak evidence for a Pan-African metamorphism. Between the Maniamba Graben and the WSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062±13 and 946±11Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulite-facies metamorphism is dated at 953±8Ma and a nepheline syenite pluton is dated at 799±8Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555±11Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569±9 and 527±8Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549±13 and 486±27Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123±9 to 1042±9Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543±23 to 493±8Ma, and was intruded by voluminous post-collisional granitoid plutons between 511±12 and 508±3Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555Ma. The nappes include evidence for early Pan-African orogenic events older than 610Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555±11Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557±16Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton. [Copyright &y& Elsevier]