Zircon dating of Neoproterozoic and Cambrian ophiolites in West Mongolia and implications for the timing of orogenic processes in the central part of the Central Asian Orogenic Belt

Abstract

We present new isotopic and trace element data to review the geochronological/geochemical/geological evolution of the central part of the Central Asian Orogenic Belt (CAOB), and find a fundamental geological problem in West Mongolia, which has traditionally been subdivided into northwestern early Paleozoic (formerly Caledonian) and southerly late Paleozoic (formerly Hercynian) belts by the Main Mongolian Lineament (MML). We resolve this problem with SHRIMP zircon dating of ophiolites and re-evaluation of much published literature. In Northwest Mongolia the Dariv–Khantaishir ophiolite marks the boundary between the Lake arc in the west and the Dzabkhan–Baydrag microcontinent in the east. Zircons from a microgabbro and four plagiogranites yielded weighted mean 206Pb/238U ages of 568±5Ma, 567±4Ma, 560±8Ma (Dariv), 573±8Ma and 566±7Ma (Khantaishir) that we interpret as reflecting the time of ophiolite formation (ca. 573–560Ma). Metamorphic zircons from an amphibolite on a thrust boundary between the Khantaishir ophiolite and the Dzabkhan–Baydrag microcontinent formed at 514±8Ma, which we interpret as the time of overthrusting. In South Mongolia the Gobi Altai ophiolite and the Trans-Altai Gurvan Sayhan–Zoolen forearc with an ophiolite basement were investigated. Zircons of a layered gabbro (lower ophiolite crust) and a leucogabbro (mid-upper crust) of the Gobi Altai ophiolite yielded crystallization ages of 523±5Ma and 518±6Ma. The age data constrain the formation time of ophiolite within ca. 523–518Ma. Zircons from four samples of the Gurvan Sayhan–Zoolen forearc, with similar hybrid adakite–boninite affinities, yielded 519±4Ma for an anorthosite, ≥512±4Ma for a hornblendite and 520±5 and 511±5Ma for two diorites. The ophiolite basement has an upper age limit of 494±6Ma, determined by dating a tonalite dike cutting the Zoolen ophiolite. Integrating available zircon ages as well as geochemical and geological data, we re-subdivide West Mongolia into: a latest Neoproterozoic-early Cambrian, arc–microcontinent collision zone north of the MML; a Cambrian Gobi Altai ophiolite–microcontinent collision zone and a Cambrian Trans–Altai forearc complex south of the MML. The central CAOB evolved in five phases: subduction initiation and arc formation (ca. 573 to>ca. 540Ma); arc–microcontinent collision (ca. 535–524Ma); a continuum of slab delamination, overthrusting, crustal thickening and surface uplift (ca. 519–482Ma) in Northwest Mongolia; initiation of new subduction zones in South Mongolia (ca. 523–511Ma); and continuing orogeny with local surface uplift. Overall, the current, documented timing of orogenic development in the central CAOB is largely consistent with a W/SW-Pacific style of evolution in terms of subduction initiation, short timescales of individual orogenies, and episodic subduction–collision during a continuing migration of subduction zones.