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• Zircon inclusions in rubies have an U-Pb age of 36.2 Ma and phlogopite has Ar-Ar ages of 22.2–22.5 Ma. • The formation of the Yuanjiang ruby deposit was closely linked with the Cenozoic crustal tectonic evolution. • Ruby mineralization in the Ailao Shan-Red River shear zone was triggered by the local continental subduction-collision. • Exhumation of the deposits may have occurred during the later left-lateral shearing. The Yuanjiang marble-hosted ruby deposit in the middle Ailao Shan-Red River (ASRR) shear zone, is the only economic ruby deposit in China. This deposit is hosted in Paleoproterozoic sillimanite gneisses and the ores are found in metamorphosed platform carbonates. Rubies are unevenly distributed within calcite-rich marble layers and mainly associated with phlogopite, scapolite, graphite, pyrite. Rubies contain zircon, rutile, apatite, calcite inclusions. The genesis of the Yuanjiang ruby deposit and its genetic link to the ASRR shear zone remain poorly understood. In this contribution, its formation age has been indirectly constrained by the in-situ LA-ICP-MS U-Pb dating of zircon inclusions in rubies and 40Ar/39Ar dating of syngenetic phlogopite from the ruby-phlogopite bearing marble, which yielded the ages of 36.2 ± 1.1 Ma (MSWD = 1.4, 2σ, n = 4) and 22.2–22.5 Ma, respectively. The 36.2 Ma U-Pb age of metamorphic zircon inclusions falls within the range of local crustal thickening related subduction-collision and coupled with the spatial distribution in which ruby deposits were not restricted in the shear zone, we propose that ruby mineralization was triggered by the local continental subduction-collision rather than the large-scale left-lateral shearing. Due to the estimated closure temperature of the 40Ar-39Ar system is lower than the ore-forming temperature, the obtained phlogopite 40Ar/39Ar ages of 22.2–22.5 Ma represent the minimum ore-forming age of the Yuanjiang ruby deposit. These phlogopite ages reveal a slower cooling rate of ruby deposits within the shear zone (including the Yuanjiang and the Yen Bai ruby deposits) than outside of the shear zone (the Luc Yen deposit), which can be attributed to the additional heat from the left-lateral shearing movement. Therefore, we propose that the left-lateral shearing plays an important role in transporting marble-hosted ruby deposits in the Ailao Shan-Red River shear zone to the subsurface. [ABSTRACT FROM AUTHOR]

Structural and microstructural characteristics, deformation temperatures and flow vorticities of the northern Ailao Shan (ALS) high-grade metamorphic belt provide significant information regarding the nature and tectonic evolution of the Ailao Shan-Red River (ASRR) shear zone. Mineral deformation mechanisms, quartz lattice-preferred orientation (LPO) patterns and the opening angles of quartz c-axis fabrics of samples from the Gasa section indicate that the northern ALS high-grade metamorphic belt has experienced progressive shear deformation. The early stage shearing is characterized by a gradual decrease of deformation temperatures from >650 °C at the northeastern unit to ca. 300 °C at the southwestern unit, that results in the formation of migmatites, mylonitic gneisses, thin bedded mylonites, mylonitic schists and phyllonites from the NE to SW across the strike of the shear zone. The late stage low-temperature (300–400 °C) shearing is superimposed on the early deformation throughout the belt with the formation of discrete, small-scale shear zones, especially in the thin-banded mylonitic rocks along both margins. The kinematic vorticity values estimated by rotated rigid porphyroclast method and oblique grain-shaped/quartz c-axis-fabric method imply that the general shear-dominated flow (0.49–0.77) progressively changed to a simple shear-dominated flow (0.77–1) toward the late stage of ductile deformation. The two stages of shearing are consistent with early shortening-dominated and late extrusion-controlled regional tectonic processes. The transition between them occurred at ca. 27 Ma in the ALS high-grade metamorphic belt along the ASRR shear zone. The large amount of strike-slip displacement along the ASRR shear zone is predominantly attributed to accelerated flow along the shear zone during the late extrusion-controlled tectonic process. [ABSTRACT FROM AUTHOR]

Four metamorphic massifs in southeastern Tibet – the Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) – have been suggested to constitute coherent parts of the Ailao Shan-Red River (ASRR) shear zone. This study details the cooling/exhumation histories of the crustal-scale shear zone by determining the cooling paths of the ALS massif and comparing these paths to those of the DCS massif. We show that the different parts of the ALS massif have experienced two stages of diachronous cooling histories. The two cooling stages of the ALS massif, i.e., an early rapid cooling of ca. 45–715 °C/Myr and a late slow cooling of 10–17 °C/Myr, differ significantly from the cooling of the DCS massif along the same shear zone, which featured an early slow cooling of 11–17 °C/Myr and a late rapid cooling of 30–140 °C/Myr. This study confirms that differing cooling/exhumation histories exist along the same crustal-scale shear zone. From the contrasting cooling/exhumation histories, we would argue that the ALS and DCS massifs are isolated units with different tectono-thermal histories during the Oligo-Miocene tectonic evolution of southeastern Tibet. [ABSTRACT FROM AUTHOR]

Highlights: [•] Epithermal-porphyry Cu–Au mineralization is related to the 40–32Ma high-K alkaline magmatism. [•] The porphyry Cu–Mo–Au and epithermal Au systems have magmatic S and Pb isotopes. [•] Paleotethyan subduction and Indian–Eurasian Plate interaction contributed to mineralization. [ABSTRACT FROM AUTHOR]

Abstract: The structural geology, timing of shearing, and tectonic implications of the ASRR shear zone, one of the most striking lineaments in Southeast Asia, have been the topics of extensive studies over the past few decades. The Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution. Our field structural analysis, detailed microstructural and fabric analysis, as well as the quartz, sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during Cenozoic tectonic evolution in the shear zone. Among the contrasting structural and microstructural associations in the shear zone, D2 structures, which were formed at the brittle to ductile transition during large-scale left-lateral shearing in the second deformation episode, predominate over the structural styles of the other two deformation episodes. Discrete micro-shear zones with intensive grain size reduction compose the characteristic structural style of D2 deformation. In addition, several types of folds (early shearing folds, , and late-shearing folds, ) were formed in the sheared rocks, including discrete to distributed mylonitic foliation, stretching lineation and shear fabrics (e.g., mica fish, domino structures, as well as sigma and delta fabrics). A sequence of microstructures from syn-kinematic magmatic flow, high-temperature solid-state deformation, to brittle–ductile shearing is well-preserved in the syn-kinematic leucocratic intrusions. Deformation structures from the first episode (D1) are characterized by F1 folds and distributed foliations (S1) in rocks due to pure shearing at high temperatures. They are preserved in weakly sheared (D2) rocks along the eastern margin of the ALS belt or in certain low-strain tectonic enclaves within the shear zone. Furthermore, semi-brittle deformation structures, such as hot striae and discrete retrogression zones, are attributed to normal-slip shearing in the third deformation episode (D3), which was probably locally active, along the eastern flank of the DCS range, for example. There are four quartz c-axis fabric patterns in the mylonitic rocks, including type A point maxima, type B Y point maxima with crossed girdles superimposition, type C quadrant maxima, as well as type D point and quadrant maxima combination. They are consistent with microscopic observations of microstructures of high-temperature pure shearing, low-temperature simple shearing and their superimposition. Integrated microstructural analysis and fabric thermometer studies provide information on both high temperature (up to 750°C) and dominant low-temperature (300–600°C) deformations of quartz grains in different rock types. Sillimanite and garnet fabrics, especially the latter, were primarily formed at the peak metamorphism during high-temperature pure shearing. The above structural, microstructural and fabric associations were generated in the tectonic framework of the Indian-Eurasian collision. The low-temperature microstructures and fabrics are attributed to left-lateral shearing along the ASRR shear zone from 27 to 21Ma during the southeastward extrusion of the Indochina block, which postdated high-temperature deformation at the peak metamorphism during the collision. [Copyright &y& Elsevier]

Abstract: Exhumation of the high-grade metamorphic rocks from deep to shallow crustal levels in the Diancang Shan (DCS) massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust, through retrograde ductile–brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface. New 40Ar/39Ar ages of amphibole, muscovite, biotite and K-feldspar, mostly from mylonitic rocks, help constrain the thermal evolution of the massif during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Oligocene to Pliocene, e.g., 28–13Ma, 13–4Ma and 4–0Ma, respectively. The initiation of the unroofing history of the DCS massif resulted from ductile left-lateral shearing along the Ailao Shan-Red River shear zone (ASRR) since the late Oligocene. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif. The first stage of exhumation might have stopped due to the cessation of the left-lateral shearing at ca. 21Ma, although cooling continued until 13Ma ago to relatively low-temperature conditions. The second and third stages of cooling started from 13Ma and lasted until the recent active faulting. The diversity of cooling rates during the second stage from 13 to 4Ma suggests differences in exhumation rates at various localities. Ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage. The inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate. During the third stage, cooling and exhumation of the DCS massif since 4Ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16°C, the present day average surface temperature. Therefore, the diachronous cooling and exhumation continuously spanned to the surface. The tectonic exhumation of the DCS massif constitutes a part of the tectono-thermal evolution and diachronous exhumation of the metamorphic complexes in the fan-shaped area bounded broadly in SE Asia by the Red River fault zone and Sagaing fault zone since 36Ma. Extrusion-induced strike-slip shearing and rotation-associated normal faulting, played important roles in the diachronous cooling and exhumation of the DCS massif and other metamorphic complexes in SE Asia. [Copyright &y& Elsevier]

• Leucogranites in the ASRR shear zone experienced extensive fractional crystallization. • Sr-Nd-Hf isotope systematics indicate partial melting of amphibolite. • Leucogranite emplacement coincides with slip-strike movement of the ASRR at 27 Ma. We report for the first time the occurrence of leucogranite bodies that intruded the Ailaoshan metamorphic complex along the Ailao Shan-Red River (ASRR) shear zone. Leucogranites in Yuanyang, Southeast Yunnan province, are composed of quartz (~30%), alkali feldspar (~45%), plagioclase (~5%), muscovite (~13%) and biotite (~3%) and other minor silicate minerals (~3%, garnet, tourmaline). U–Pb dating of zircon from leucogranites yields a weighted mean age of 27.4 ± 0.5 Ma, suggesting a late Oligocene anatexis event. These leucogranites are strongly peraluminous with A/CNK values of 1.01–1.19 and have high SiO 2 (73.3–77.0 wt%) and K 2 O + Na 2 O (7.87–10.08 wt%), and low CaO (0.41–0.99 wt%), MgO (0.04–0.21 wt%), TiO 2 (0.01–0.06 wt%) and Fe 2 O 3 T (0.44–0.86 wt%) contents. They are geochemically similar to Himalayan leucogranites and are enriched in large-ion lithophile elements (e.g. Rb), and depleted in high-field-strength elements (e.g. Nb, Zr, and Hf). They have low Zr/Hf (12.3–22.5) and Nb/Ta (2.54–12.73) with negative Eu anomalies (Eu/Eu* = 0.05–0.45), indicative of extensive fractional crystallization of plagioclase and zircon. In addition, these rocks have low 87Sr/86Sr (t) (0.7072–0.7111), high ε Nd (t) (−1.53 to −3.61). They contain zircon grains with ε Hf (−5 to +1.8) and old two-stage Hf model ages of 1.24–1.02 Ga, implying that they were probably derived from partial melting of amphibolite (old crustal material) within the lower crust. In combination with previous studies of metamorphism of the ASRR shear zone, our study demonstrates that the leucogranites may have been generated in a highly dynamic setting involving both extrusion and extension that defined the beginning of left-lateral slip-strike movement of the ASRR shear zone at 27 Ma. [ABSTRACT FROM AUTHOR]