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 (M1and 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 M1was a regional Barrovian‐type event related to crustal shortening and thickening of the Indian plate margin, while M2was 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 M1between 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‐M1thermal 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. The40Ar/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 $22.0\pm 0.6$ \end{document} to \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 $20.4\pm 0.6$ \end{document} Ma, suggesting that cooling of the High Himalayan Slab below the nominal closure temperature for Ar loss in muscovite (∼400°–350°C) occurred during the Early Miocene. Muscovite and biotite40Ar/39Ar cooling ages fro