D. J. Waters

The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet

This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (>620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (<18 Ma).

Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 ± 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720–770 °C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ∼30–80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya.

Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya

New monazite U-Pb geochronological data from the Everest region suggest that ∼20–25 m.y. elapsed between the initial India-Asia collision and kyanite-sillimanite–grade metamorphism. Our results indicate a two-phase metamorphic history, with peak Barrovian metamorphism at 32.2 ± 0.4 Ma and a later high-temperature, low-pressure event (620 °C, 4 kbar) at 22.7 ± 0.2 Ma. Emplacement and crystallization of the Everest granite subsequently occurred at 20.5–21.3 Ma. The monazite crystallization ages that differ by 10 m.y. are recorded in two structurally adjacent rocks of different lithology, which have the same postcollisional pressure-temperature history. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armored within silicate minerals. During the low-pressure metamorphic event, the armoring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite-producing reactants, and spots within monazite that are rich in Ca, Fe, Al, and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence.

Metamorphism, melting, and extension: Age constraints from the High Himalayan Slab of southeast Zanskar and northwest Lahaul

We present evidence for two distinct stages of Tertiary metamorphism (M1 and M2) in the High Himalayan Slab of southeast Zanskar and northwest Lahaul, as well as evidence for an older, pre‐Himalayan metamorphism (pre‐M1). The M1 was a regional Barrovian‐type event related to crustal shortening and thickening of the Indian plate margin, while M2 was associated with crustal melting and the emplacement of the Gumburanjon leucogranite into the Zanskar Shear Zone at the top of the slab. U‐Pb dating of metamorphic and magmatic accessory phases constrains the timing of M1 between 30 and 37 Ma and the crystallization and emplacement age of the Gumburanjon leucogranite at 21–22 Ma. Inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least Lower Paleozoic in age and that they experienced a major pre‐M1 thermal perturbation at ca. 450–500 Ma. Whether this was associated with a regional Barrovian‐type metamorphism or whether it was a thermal event related to the intrusion of large Cambro‐Ordovician granites remains uncertain. The 40Ar/39Ar cooling ages of muscovites from metamorphic schists range from \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Pressure, temperature and time constraints on Himalayan metamorphism from eastern Kashmir and western Zanskar

22.0\pm 0.6

Attenuation and excision of a crustal section during extensional exhumation: the Carratraca Massif, Betic Cordillera, southern Spain

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Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains

20.4\pm 0.6