Multistages of original sapphire formation related to basaltic magmatism in the Bo Phloi basaltic gem field, Kanchanaburi, Western Thailand: Evidence from trace elements and ages of zircons

Abstract

Trace element geochemistry of zircons in the Bo Phloi basaltic gem deposit, Kanchanaburi, Western Thailand indicates that majority of the sapphires and associated minerals appear to have crystallized in an environment related to highly alkaline felsic melt with subordinate collaboration of contact metamorphism at the lower crust. The U-Pb dating of a zircon inclusion in sapphire and zircons in two sapphire-bearing gravels yielded the average ages of 24 ± 0.9, 10.86 ± 0.14 and 4.4 ± 0.4 Ma, respectively. These dates are concordant with the timing of the Cenozoic basaltic volcanism in Thailand and Southeast Asia which span from late Oligocene to Pleistocene. These ages are, however, mostly pre-dated (one is close to) ~4–3 Ma ages of the Bo Phloi basalt. Therefore, the original sapphire formations appear to have the same thermal history when partial melting in the upper mantle had occurred extensively and led to early state of Cenozoic basaltic eruption in this region. These also indicate that the formation of sapphire from bi-modal origins (felsic melt and contact metamorphism) occurred episodically earlier during ~24–4 Ma as a result of the hybrid interplay of (lower) crustal and (upper) mantle materials. Later, the eruption of mantle-derived alkaline basalt carried the sapphire from both origins along with other xenoliths and xenocrysts to the surface during ~4–3 Ma.