Formation Age and Evolution Time Span of the Koktokay No. 3 Pegmatite, Altai, NW China: Evidence from U-Pb Zircon and Ar-40-Ar-39 Muscovite Ages

摘要

The Koktokay No. 3 pegmatite is the largest Li-Be-Nb-Ta-Cs pegmatitic rare-metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones). However, the formation age and evolution time span have been controversial. Here, we present the results of LA-ICP-MS zircon U-Pb dating and muscovite Ar-40-Ar-39 dating. Four groups of zircon U-Pb ages (approximate to 210 Ma, approximate to 193-198 Ma, approximate to 186-187Ma and approximate to 172Ma) for Zones II, V, VI, VII, and VIII, and a weighed mean Pb-206/U-238 age of 965 +/- 11 Ma for Zone IV are identified. Also, Zones II, IV, and VI have muscovite Ar-40-Ar-39 plateau ages of 179.7 +/- 1.1Ma, 182.1 +/- 1.0Ma, and 181.8 +/- 1.1Ma, respectively. Considering previous U-Pb age studies (Zones I, V, and VII), the ages of emplacement, Li mineralization peak, hydrothermal stage of the No. 3 pegmatite are in ranges of 193-198Ma, 184-187Ma and 172-175Ma, with weighted mean Pb-206-U-238 ages of 194.8 +/- 2.3Ma, 186.6 +/- 1.3Ma and 173.1 +/- 3.9Ma, respectively. The No. 3 pegmatite formed in the early Jurassic. The results of xenocrysts suggest that there is another pegmatite forming event of around 210Ma in the mining district and the old zircon U-Pb ages imply that Neoproterozoic crustal rocks pertain to sources of the No. 3 pegmatite. Including the previous muscovite Ar-40-Ar-39 age studies (Zones I and V), a cooling age range of 177-182Ma is considered as the time of hydrothermal stage and end of formation. The evolution process of the No. 3 pegmatite lasted 16Ma. Therein, the magmatic stage continued for 9-11Myr and the magmatic-hydrothermal transition and hydrothermal stages were sustained at 5-7Ma. These time spans are long because of huge scale, cupola shape, large formation depth, and complex internal zoning patterns and formation processes. Considering some pegmatite dikes in the Chinese Altai, there is an early Jurassic pegmatite forming event.