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[Display omitted] • Eocene lamprophyre magmatism is found in South Kalimantan, Borneo. • The lamprophyre has a Priabonian 40Ar/39Ar age of 37.45 ± 0.01 Ma, indistinguishable from the sediments into which it intrudes. • Present day South Kalimantan lithosphere is 50–75% thinner than what is expected for lamprophyre petrogenesis indicating substantial lithospheric thinning since the Priabonian. • Magmatic rocks with well characterised petrogenetic pathways, such as lamprophyres, can be used as paleo-lithospheric thickness indicators. The petrogenetic history of well characterised magmatic rocks may contain invaluable information of paleo-lithospheric thickness and structure that can inform on tectonic evolution. Petrographically distinctive rocks, such as lamprophyres, provide temporal and spatial constraints to build, assess, and verify tectonic models. An alkaline lamprophyre dyke that intrudes the rift-related sediments of the Eocene Tanjung Formation of the Senakin Peninsula in South Kalimantan, Indonesia has been investigated using petrology, whole rock geochemistry and Ar-Ar geochronology. The Senakin lamprophyre dyke is mostly geochemically homogenous but petrographically stratified with vesicle free monchiquite alkaline lamprophyre mineralogy at the base and an amygdale bearing kaersutite phenocryst free upper zone. The major and trace element chemistry is consistent with an intra-plate Ocean Island Basalt like affinity. 40Ar/39Ar dating of hornblende and groundmass aliquots indicates an emplacement age of 37.45 ± 0.10 Ma (2σ), which is consistent with the Late Eocene age of the Tanjung Formation derived from microfossil analysis. The petrology and geochemistry of the Senakin lamprophyre dyke is consistent with magma genesis at > 2GPa beneath a thick lithosphere followed by fractional crystallisation at Moho depths equivalent to 0.5–0.8 GPa. This indicates a much thicker southeastern Kalimantan paleo-lithosphere. Lamprophyre magmatism was initiated by lithospheric extension associated with the opening of the Makassar Strait and also regional thermal upwelling. This extension continued to thin the lithosphere to its current thickness equivalent to 0.5 GPa. [ABSTRACT FROM AUTHOR]

Syn-sedimentary mafic volcanism has been identified within a rift setting (Eocene Tanjung Formation) in the Senakin Peninsula, southeast Kalimantan. Fine-grained, dark-grey basalt lava occurs and has prominent vertically oriented columnar jointing. Petrographically, the basalt is composed of small euhedral pyroxene, olivine, and lath-shaped plagioclase phenocrysts within a very fine-grained dark coloured groundmass. A volcaniclastic unit also occurs and in outcrop has sharp contacts with underlying and overlying sedimentary mudstone. The unit is composed of cm-scale clasts of fine-grained to glassy textured basalt with vesicles of varying size and abundance. Euhedral pyroxene phenocrysts are observed within the clasts, although some with overprinting alteration. Palagonite alteration on the margins of some clasts is noted and is indicative of mafic composition volcanic material that has come into contact with sea water. Presence of bivalve and coral fragments in sandstone and mudstone underlying the volcaniclastic unit indicates emplacement into a marine environment. Core description from 33 locations over an 18 km transect length show that both the basalt and volcaniclastic sediments are extensive throughout the east Senakin area. Lithological relationships and compositional similarities between the basalt and volcaniclastic sediment suggest they are related and were contemporaneous with sedimentation within the Tanjung Formation. It is proposed that the basalt unit is designated the Tanah Rata Basalt Member of the Tanjung Formation. If a wider distribution occurs for the volcaniclastic unit it is proposed that it is termed the Gumbil Volcaniclastic Member of the Tanjung Formation. [ABSTRACT FROM AUTHOR]