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In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode.Zircon intergrown with or included within M2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2σ, n = 20). The youngest observed results are found in zircon outside M2 minerals, some overgrown by M3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

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[1] Ongoing extension in Tibet may have begun in the middle to late Miocene, but there are few robust estimates of the rates, timing, or magnitude of Neogene deformation within the Tibetan plateau. We present a comprehensive study of the seismically active South Lunggar rift in southwestern Tibet incorporating mapping, U-Pb geochronology and zircon (U-Th)/He thermochronology. The South Lunggar rift is the southern continuation of the North Lunggar rift and comprises a~50 km N-S central horst bound by two major normal faults, the west-dipping South Lunggar detachment and the east-dipping Palung Co fault. The SLD dips at the rangefront~20°W and exhumes a well-developed mylonite zone in its footwall displaying fabrics indicative of normal-sense shear. The range is composed of felsic orthogneiss, mafic amphibolite, and leucogranite intrusions dated at 16 and 63 Ma. Zircon (U-Th)/He cooling ages are Oligocene through late Pliocene, with the youngest ages observed in the footwall of the SLD. We tested~25,000 unique thermokinematic forward models in Pecube against the structural and (U-Th)/He data to fully bracket the allowable ranges in fault initiations, accelerations, and slip rates. We find that normal faulting in the SLR began in the middle Miocene with horizontal extension rates of~1 mm a À1 , and in the north accelerated at 8 Ma to 2.5-3.0 mm a À1 as faulting commenced on the SLD. Cumulative horizontal extension across the SLR ranges from <10 km in the south to 19-21 km in the north.

To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P-T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern PanAfrican belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5-11 kbar and 810±40°C during peak metamorphism at 650-620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet-clinopyroxenequartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P-T evolution is characterized by late-stage kyanite in metapelites and garnet-clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P-T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70-100 Ma prior to continental collision. The P-T -t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.

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SHRIMP U-Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite-norite intrusive complex, both from the ultrahigh temperature (UHT; >900°C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2r, n ¼ 8) and 1052 ± 5 Ma (1r, n ¼ 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma. In contrast, samples from the UHT zone exhibit four major age groups: (1) magmatic cores yielding ages over 1500 Ma (2) magmatic cores giving ages of 1034 ± 13 Ma (2r, n ¼ 4) and 1056 ± 10 Ma (1r, n ¼ 1) (3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1r) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and (4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2r, n ¼ 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2r, n ¼ 4). This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950°C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th ⁄ U-Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.