Calcite U-Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia

Abstract. The North Pamir, part of the western syntax of the India-Asia collision zone, preserves remnants of a poorly investigated Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U-Pb geochronology using laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS). Calcite dating yielded Mississippian ages, mostly overlapping each other within errors. REE + Y data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U-Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors and provide a test of the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.