Mesozoic magmatism of Natuna Island, Indonesia: Implications for the subduction history of eastern Sundaland

Subduction beneath eastern Sundaland occurred throughout the Mesozoic forming a wide magmatic arc. This resulted in extensive magmatism, ophiolite formation, and development of a large accretionary complex that stretches north from Borneo, through Vietnam, and into China. However, much of this complex is currently submerged beneath the South China Sea, making the study of longitudinal variations in magmatic and subduction history difficult. This study focusses on the Mesozoic geology of Natuna, a remote island in the South China Sea, and uses fieldwork, whole-rock geochemistry, and zircon geochronology to determine the islands relationship to the subduction margin and how its magmatic history compares to examples from Borneo and Vietnam. Natuna records abundant subduction-related magmatism, with switches from extensional to compressional regimes driven by changes in the angle of subduction and rate of slab roll-back. Inherited zircons reveal the presence of Triassic–Early Jurassic crust beneath Natuna and indicate the formation of an Early Jurassic arc that has since eroded away. This is followed by Middle–Late Jurassic spreading in the fore-arc driven by slab-roll back and formation of the Pulau Tiga Ophiolite coeval with the deposition of cherts in the Bunguran Formation. A regional Late Jurassic–Early Cretaceous slab flattening then drove abundant I-type magmatism, however, the products of this are not observed on Natuna until the Late Cretaceous. Initial I/S-type granodiorite and porphyritic granite magmatism is recorded at 85–86 Ma and formed as mantle melts rose through and became partially contaminated by previously-formed arc crust. Following this, I/S-type granites record a period of crustal thickening and increased contamination from partially melted crust at 71–75 Ma. This crustal thickening marks the end of Palaeo-Pacific subduction beneath eastern Sundaland, compression in the fore-arc, ophiolite obduction, and formation of the Natuna Accretionary Complex along the Lupar Line.