PETROGRAPHY OF TRIASSIC IGNEOUS ROCKS FROM THE NORTH PAMIR

Several models have been developed for subduction zone processes along the Paleo-Tethys ocean prior to closure in the Triassic (Zhou and Graham, 1996; Change, 2000; Pullen et al., 2008). These models rely on igneous samples from the central and eastern Songpan-Ganzi/Hoh-Xil region of the Himalayan-Tibetan Orogen. To better understand the similarities and differences in the subduction zone processes of the northern Paleo-Tethys margin prior to closure, we investigate Triassic igneous rocks from the laterally equivalent Karakul-Mazar terrain of the northern Pamir. We build upon previously collected results which document the radiometric age and geochemistry of samples representative of magmatism in the Late Triassic. Six samples were investigated under plane-polarized light and investigated for igneous textures. Point counts were conducted to determine modal mineralogy for classification. Group 1 rocks include two monzogranites and a granodiorite. The samples are medium to course grained and include accessory minerals of garnet, orthopyroxene, amphibole, muscovite, and biotite. Reaction textures include myrmekite. Group 2 rocks include a granodiorite and a tonalite. Mineral phases are course grained and include orthopyroxene, amphibole, biotite, muscovite, and titanite as accessory phases. Group 3 consists of a single gabbro/norite sample. Pyroxene crystals are course grained and plagioclase crystals are medium to course grained. Accessory minerals include biotite and cordierite. In all samples, textures were evaluated to determine relative crystallization age of mineral phases. Finally we compare these results with results from whole-rock and trace-element geochemistry to evaluate characteristics of magmatism prior to ocean closure.