Preliminary 40Ar/39Ar thermochronologic results from two transects across the Altyn Tagh (AT) range allow new interpretations for the genesis of the range and provide constraints for its subsequent displacement. Recent reports of Middle to Upper Ordovician pillow lavas in the NE portion of the range as well as the first reports of high-pressure metamorphic rocks within the range are interpreted as the remnants of a lower Paleozoic ocean subducted within the AT. Muscovite and biotite ages from schist, gneiss, and granodiorite document a significant metamorphic and intrusive event throughout the range at 435¿20 Ma. This fabric is crosscut by undeformed, postorogenic microgranite plutons dated at 383¿7 Ma by muscovite. These data suggest that an ocean basin closed after the Early Silurian and prior to the Middle Devonian, forming the herein named Lapeiquan suture. The lack of similarly aged plutons to the north and the suggestion of a synchronous arc to the south implies that subduction was south dipping (present geography). If the Altyn Tagh contains a middle Paleozoic suture, lateral continuations should be identifiable. Possibilities to the west include the Kudi suture in the western Kunlun Shan. Comparisons of lithologies and radiometric data reveal significant similarities between the Kudi suture and the Altyn Tagh, suggesting that the AT originated as part of the same middle Paleozoic orogenic belt. Reconstruction of the Paleozoic structure of northern Tibet is complicated by Cenozoic deformation, particularly along the Altyn Tagh fault. Although this could be used as a measure of the finite post-Devonian deformation, a more thorough understanding of Paleozoic geodynamics is a prerequisite to unraveling subsequent displacements. ¿ 1999 American Geophysical Union