Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo, Indonesia)

The Meratus ophiolitic series (SE Borneo) present a specific assemblage tha t have recorded (1) a continental extensional episode mostly seen within th e peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic acti vity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. T he ophiolitic series comprise ultramafic rocks with minor metavolcanic rock s. The ultramafic rocks include dominant Iherzolites and pyroxenites with r ather scarce harzburgites acid dunites. Spinel peridotite, mineral chemistr y data and bulk rock Rare Earth Element (REE) abundances show that most roc ks underwent a low degree of partial melting. However, a few samples displa y significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase f acies, as seen from phase chemistry data. These peridotites are locally cro sscut by dikelets containing high-temperature K- and Cr-rich amphiboles. La vas closely associated with the Meratus peridotites have REE compositions r anging from the ones typical of enriched MORE (E-MORB) to normal MORE (N-MO RB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fr actional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enrich ed subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back- are basin basalts (BABB) now associated with E-MORB and N-MORB have also be en found in the metamorphic soles of the peridotites. These rocks would hav e formed in a back-are basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation) . The ophiolitic series was later crosscut by calc-alkaline melts (Manunggu l Formation). The Meratus ophiolitic series hence displays a dual origin. T hey witness 1) a continental episode mostly seen within the peridotites, an d 2) later subduction-related events marked by the emplacement of calc-alka line magmas. (C) Elsevier, Paris.