| Rare spinel-lherzolite xenoliths occur in late Cenozoic alkali basalts in the SE-Asia region of Thailand. They offer a unique opportunity to understand the physico-chemical nature of the mantle within two contrasting lithospheric fragments, the Shan-Thai (western side) and Indochina (eastern side), which were joined by a Mesozoic Nan-River suture. Major-element chemistry in whole rocks varies considerably (e.g., ∼33–49 wt% MgO and 1–6 wt% Al2O3), with the Indochina xenoliths being more fertile than those of the Shan-Thai. Xenoliths from both sides of the suture, however, yield comparable estimated temperatures (∼890–1170°C). Trace-element compositions suggest variable extents of basaltic-melt extraction and metasomatism. Clinopyroxene trace-element modeling indicates that the majority of lherzolite xenoliths experienced small degrees (<6%) of partial melting, although up to 16% melting is necessary for a depleted sample (5% modal Cpx). In a unique xenolith, clinopyroxenes are significantly LREE-depleted (La/Lu = 0.01), possibly reflecting their derivation from garnet breakdown. In addition, the modeling of LREE-enriched samples, with pronounced HFSE (Ti, Zr, Hf) depletion, suggests metasomatic modification by carbonate-rich fluids/melts, unrelated to the host basalts. This metasomatic enrichment was more intense in xenoliths from the eastern microcontinent (up to 30%), compared to that occurred in samples of the western counterpart.; In the Chantaburi-Trat basaltic field of SE Thailand, granulite-facies xenoliths represent the only known occurrence of lower crustal material in this region. Important geochemical features of these granulite xenoliths include: (1) mafic compositions (43–49% SiO2), (2) high normative olivine, plagioclase, and diopside, and (3) low and generally flat REE abundances with positive Eu anomalies. These characteristics indicate cumulate origins, possibly from underplating basaltic magmas. Mineralogical and geochemical chemical evidence suggests that the original cumulate mineralogy (Ol, Pl, and Cpx) was transformed to granulite-facies assemblages (Ol, Pl, Cpx, and Gt) at ∼1100 to 1200°C and 15 to 18 kbar. Corundum inclusions were also produced at the upper limit of this P–T range.; The petrological and geochemical characteristics of spinel-lherzolite and granulite-facies xenoliths in the region of Thailand may reflect the combined effects of Mesozoic subduction, and asthenospheric upwelling and magmatism in the Cenozoic. |
