Urs Schärer

The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina

The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts, C-S or C′-S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz 〈c〉 axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-lateral strain occurred under amphibolite-facies conditions (3–7 kb and 550–780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield UPb ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to ≈ 300°C between 22 and 17 Ma, before the end of left-lateral motion. The similarity of deformation kinematics, P-T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long “Midu gap” between the gneissic cores of the two ranges results from sinistral dismemberment of the shear zone by large-scale boudinage followed by uplift and dextral offset of parts of that zone along the Quaternary Red River Fault. Additional field evidence suggests that the Xuelong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear zone, which therefore reaches a length of nearly 1000 km. Surface balance restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, suggesting shear strains on the order of 30, compatible with a minimum offset of 300 km along the ASRR zone. Various geological markers have been sinistrally offset 500–1150 km by the shear zone. The seafloor-spreading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR zone, which yields a minimum left-lateral offset of 540 km on that zone. Comparison of Cretaceous magnetic poles for Indochina and South China suggests up to 1200 ± 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 ± 200 km on the ASRR zone, to which several tens of kilometers of post-Miocene right-lateral offset should probably be added. These results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. While being consistent with a two-stage extrusion model, they demonstrate that the great geological discontinuity that separates Indochina from China results from Cenozoic strike-slip strain rather than more ancient suturing. Furthermore, they suggest that this narrow zone acted like a continental transform plate boundary in the Oligo-Miocene, governing much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10–25% of the total shortening of the Asian continent. The geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike-slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mountain ranges.

The effect of initial230Th disequilibrium on young UPb ages: the Makalu case, Himalaya

Abstract Comparative U Pb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic206Pb which originate from an excess or deficit of230Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by U Th fractionation between the crystallising mineral and the magma. Therefore, the U Pb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess206Pb due to excess230Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of206Pb due to deficiency of230Th. The extent of this correction depends on the degree of Th U fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of206Pb were, with reference to the amount of radiogenic206Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the Th U partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic207Pb originating from the decay of initial excess231Pa, also enriched during crystal growth. The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for Th U fractionation in continental crust. The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected238U 206Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.

Magmatism and Metamorphism in the Lhasa Block (Tibet): A Geochronological Study

U-Pb systematics in small fractions of zircon, monazite, and sphene (5 to 20 grains each) have been analyzed in order to decipher the age and origin of magmatism in the Lhasa block and to date the tectono-metamorphic events. Additionally, we analyzed a few minerals for Rb-Sr. The results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust, the magma source regions of which consisted of Palaeozoic to Precambrian materials up to 2 b.y. in age. The following episodes can be distinguished: (1) an early Tertiary plutonism-volcanism in the southern Lhasa block (Yangbajing region), strongly related to intracontinental tectono-metamorphism, (2) a late Jurassic to early Cretaceous plutonism in the central and northern Lhasa block, and (3) a middle Jurassic low-grade metamorphism, affecting the basement at the northern margin 171 ± 6 m.y. ago. The early Tertiary magmatism and metamorphism in the south are probably a direct result of Tethys subduction (Indian plate) underneath the southern Tibetan Plateau (Eurasian plate). In contrast, the plutonism in the north-e.g., the emplacement of the 121 ± 2 m.y. old granite at Bange and 120-140 m.y. old granite near Anduo-appears to be a result of intra-block thrustings resulting from the collision of the Lhasa block and the Qintang block (Bangong-Nujiang suture). This collision probably occurred in middle Jurassic (Bajocian) times as indicated by the low-grade metamorphism at the northern margin of the Lhasa block. Moreover, the existence of a

UPb geochronology of Gangdese (Transhimalaya) plutonism in the Lhasa-Xigaze region, Tibet

531 ±_{14}^{13} m.y.

Intraplate tectonics in Asia: A precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China

old granite gneiss in the northernmost part of the Lhasa block confirms that early Palaeozoic plutonism, well known from the Himalayas, was very extensive, affecting large portions of the pre-Gondwanian continent (Pangaea).

The short duration and anorogenic character of anorthosite magmatism: U-Pb dating of the Rogaland complex, Norway

Abstract Sixty four size fractions of zircon, monazite, xenotime and titanite were dated by the UPb method to temporally and spatially constrain left-lateral movements in the Red River shear zone. Between the depths of 15 and 20 km, the shearing led to the formation of the high-grade metamorphic gneisses that form the bulk of the ∼ 4200 m high Diancang Shan and ∼ 3100 m high Ailao Shan mountain ranges. The omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust, and deformation patterns are very homogeneous over the entire length of the belt. North of the gneisses, subvolcanic bodies intrude unmetamorphosed cover rocks—in particular Eocene red beds—along the projected trend of the Red River shear zone. For a leucogranitic layer in the Diancang Shan gneisses, an age of 24.2 ± 0.2 Ma (2σ) is obtained for monazite and 22.4 ± 0.2 Ma is obtained for xenotime, whereas zircon formed through two growth stages at 24–23 Ma. These ages record solidification of the anatectic melts over a period of at least 1.8 ± 0.4 Ma, during which left-lateral shear was continuously active. In the Ailao Shan gneisses, zircon and titanite ages are 26.3 ± 0.3 and 26.1 ± 0.2 Ma for two monzonitic bodies, and 24.1 ± 0.2 and 22.4 ± 0.2 Ma for two pegmatitic layers that occur within the monzonitic intrusions. These results show that zircon, monazite, titanite and probably also xenotime are closed systems for U and Pb under melting conditions ( > 650°C). Therefore, the ages date mineral formation in the melt, rather than cooling below certain closure temperatures. A significantly older age of 35.0 ± 0.1 Ma is obtained for a monzonitic subvolcanic intrusion north of the Diancang Shan gneisses. Because these intrusions follow the trend of the shear zone, they most likely represent the surface expression of early activity along that zone, indicating that strike-slip movement had already occurred in latest Eocene time. The total set of UPb ages shows that magmatic and metamorphic activity occurred from 35 to 20 Ma, strengthening the idea that the Red River shear zone absorbed most of the intraplate movements required to compensate the opening of the South China Sea between 32 and 16 Ma. The existence of monzonitic-syenitic intrusions along the Red River zone implies that melting affected source rocks other than the Diancang Shan and Ailao Shan gneiss protoliths. Potential sources to produce such rocks lie in the deep crust, possibly including melts from the mantle. Consequently, the Red River shear zone must be a deep-rooted structure, along which melts migrated to the surface. Moreover, the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike-slip activity.

Evolution of the SE-Asian continent from U-Pb and Hf isotopes in single grains of zircon and baddeleyite from large rivers

Abstract A series of different plutons from the Lhasa-Xigaze segment of the Gangdese (Transhimalaya) belt has been studied by high-resolution U Pb analyses of zircon (using zircon fractions of 5–100 grains each, selected upon specific grain characteristics). For two diorites, located east of Xigaze (Dazhuka), the zircons yield concordant ages of 93.4 ± 1.0 and 94.2 ± 1.0 m.y., respectively. Also concordant ages of 41.1 ± 0.4 and 41.7 ± 0.4 m.y. have been obtained for two granodiorites, collected southwest of Lhasa (Qushui). The precision on the ages of two granites from the Xigaze and Lhasa area, is limited by two factors: the presence of inherited radiogenic lead and the occurrence of subsequent lead loss. However, some concordant zircons, detected in both granites, define approximate ages of about 67 and 53 m.y., respectively. The inherited lead components show that melting of Precambrian material was involved in magma genesis. The U Pb ages substantiate a magmatic activity lasting from mid-Cretceous (Cenomanian) to Eocene (Lutetian) time. Such a period of plutonism at the southern margin of Eurasia, as well as the occurrence of magma generation from continental crust, suggest that the Gangdese range results from the subduction of Tethys oceanic lithosphere (Indian plate) underneath Eurasia (Eurasian plate). If this model is true, the collision of India with Eurasia (along the Lhasa-Xigaze sector) postdates the emplacement of the 41 m.y. old Gangdese granodiorites, i.e. the collision occurred after Lutetian time.

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

Abstract The age and origin of five leucogranites from the High and Tethys Himalaya, and two country-rock gneisses were investigated by U Pb dating of zircon fractions and single grains, and fractions of monazite. Additionally, Th U concentrations in whole rock powders and isotopic compositions of Pb in leached K-feldspars were determined. Monazites yield ages of 16.8 ± 0.6 m.y. for the Nialam migmatite-granite, 15.1 ± 0.5 m.y. for the Lhagoi Kangri granite, 14.3 ± 0.6 m.y. for a granite from Mt. Everest, and 9.8 ± 0.7 m.y. and 9.2 ± 0.9 m.y. for two varieties of the Maja granite. These data, together with monazite ages of 21.9 ± 0.2 and 24.0 ± 0.4 m.y., determined earlier on the Makalu granite [1], substantiate a period of intracontinental granite emplacements from 24 to 9 m.y. ago, i.e. from uppermost Oligocene to late Miocene times. Such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of India with Eurasia. Furthermore, the individual ages, together with structural relationships between granites and country rocks suggest that granite formation and tectono-metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m.y. Inherited lead components, present in all granite zircons point to large proportions of Precambrian material in the magma source regions, up to 2200 m.y. old. Th U systematics between monazite and country rocks indicate that U has been leached from most of the granites after crystallisation of monazite. Zircon dating of the Kangmar granite gneiss, which occurs in a window through the Tethys Himalayan sediments, shows that this pluton, transformed to a gneiss during the Alpine orogeny, crystallised in lowermost Palaeozoic times 562 ± 4 m.y. ago.