Petrology and Geochemistry of Upper Mantle and Lower Crustal Xenoliths from Thailand

Rare spinel-lherzolite xenoliths from Thailand offer a unique opportunity to investigate the compositions and processes of the upper mantle beneath this SE-Asia region. These xenoliths occur in Cenozoic alkali basalts from two lithospheric fragments, namely the Shan-Thai (western side) and Indochina (eastern side), that were joined along the Nan-River suture. Major-element chemistry in whole rocks shows large variations (e.g., -33-49 wt MgO and 1-6 wt A}iO3), similar to those observed in eastern China spinel-lherzolite xenoliths. The Indochina xenoliths appear to be more fertile (i.e., lower in mg#), when compared to those of Shan-Thai. Xenoliths from both sides of the suture, however, yield comparable estimated temperatures (-890-1170°C), and often show evidence of interaction with the host basalts (i.e., spongy rims). Such metasomatic reaction profoundly depletes AIiO3 and Na2O in the clinopyroxene within the spongy rims, and in some cases, lead to the formation of K-Na rich, feldspathic glass.