Mineral chemistry of inclusions in basaltic sapphires from Southeast Asia

Abstract

Mineral inclusions in sapphires from Bo Phloi, Kanchanaburi, in particular, and some from Chanthaburi and Phrae in Thailand; Huai Sai in Laos; Pailin in Cambodia and Gia Nghia in Vietnam were selected for this study. Mineral inclusions discovered in Bo Phloi sapphires include alkali feldspar, nepheline, hercynitic spinel, zircon, manganiferous-ilmenite, silica-rich enstatite, pyrope-almandine garnet richer towards pyrope component, monazite, calcite, sapphirine, biotite-phlogopite and staurolite. These mineral inclusions can be categorized into two groups namely felsic alkaline suite and contact metamorphic suite, which appear to have originated from different environments. These inclusion suits has provided new evidences to propose a bimodal genetic model of Bo Phloi sapphire. Firstly, the felsic alkaline-related origin is based on the occurrence of felsic alkaline inclusion suite together with trace elements and REE’s geochemistry of sapphire-associated zircons indicating that majority of the sapphires appear to have crystallized in an environment related to high alkali felsic melt at lower crust. Secondly, the contact metamorphic-related origin is based on the finding of contact metamorphic inclusion suite, suggesting that some of these sapphires could also originate from the metasomatised crustal rocks and contaminated melt at contact zone of basaltic intrusion at upper mantle or lower crust. Furthermore, most mineral inclusions found in Southeast Asia sapphires also are similar to those inclusions observed in Bo Phloi sapphires, except ilmenite, monazite, calcite and biotite-phlogopite have not so far been found in Southeast Asia sapphires, while ferrocolumbite, pyrochlore and apatite inclusions appear in these sapphires. Therefore, the mineral inclusions found in Southeast Asia sapphires are also divided into two groups namely ‘syenitic melt-related’ and ‘contact metamorphic-related’ suites. It can be proposed that sapphires in Southeast Asia region also originated from the bimodal genetic model similar to those occur in Bo Phloi sapphires. U-Pb dating data of Bo Phloi zircon inclusion yield an age of 24±0.9 Ma including Th-U-Pb age of monazite inclusions at 25-30 and 42-45 Ma that are very close to the volcanism of Cenozoic basalts in Thailand. Therefore, the sapphire formation may have the same thermal history as partial melting in the upper mantle leading to early state of Cenozoic basaltic eruption of Thailand as well as Southeast Asia region