Evidence for pre-Cretaceous history and partial Neogene (19–9 Ma) reequilibration in the Karakorum (NW Himalayan Syntaxis) from 40Ar–39Ar amphibole dating

Description

Amphiboles and a biotite from amphibolites and amphibolitic gneisses of the Karakorum Metamorphic Complex (KMC, NW Himalaya), were dated using the 40Ar/39Ar method. Isotope correlations were compared to electron-microprobe analyses. Both AlIV and AlVI contents of amphibole increase from the south near the Shyok Suture Zone to the Dassu-Askole Area in the north, with an increase in metamorphic grade (from anchizone to upper amphibolite facies). In the lower metamorphic zone, discordant age spectra were obtained. They are related to epidote-amphibolite overgrowth of amphibole rims on older (Precambrian to Mesozoic) magmatic cores. In the Shyok Suture Zone Ar/Ar ages of 38–62 Ma are contemporaneous with the emplacement of the Ladakh Batholith granodiorites, while middle Cretaceous magmatic Ar/Ar ages (w120 Ma) are locally preserved in tschermakite relics. In the Southern Karakorum basement, a minimum Ar/Ar age of 651 Ma was preserved, while an age of 208.5G2 Ma was preserved in Panjal Trap-like diorites. At the margin of the domes zone, more recent metamorphism resulted in mineral growth at 20–5 Ma. In the domes zone, amphibole Ar/Ar ages of 17 Ma (Askole) and bimodal age spectra of 15–17 and 9–13 Ma (Dassu) extend towards the east the zone of Neogene amphibolite to granulite facies metamorphism known in the Nanga Parbat Syntaxis.

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