Petrological constraints upon the provenance and genesis of the East Halmahera ophiolite

Abstract

This petrological and geochemical study of the ophiolitic rocks of the island of Halmahera (eastern Indonesia) has resulted in the first detailed interpretation of their tectonomagmatic provenance and suggested modern analogues around the western Pacific margin. Rocks of ophiolitic affinity are common in the eastern part of Halmahera, but structural dismemberment means that an intact ophiolite stratigraphy is not preserved. However, samples representative of each level of a “complete” ophiolite (with the possible exception of sheeted dykes) have been collected. A “mantle sequence” dominated by depleted harzburgite (spinel cr # = 62, olivine Fo 90.4, bulk (Al 2O 3 + CaO) = 1.2 wt%) suggests it is a mantle residue which has undergone a high degree of partial melt extraction. Subordinate lherzolite of relatively enriched chemistry (spinel cr # = 17, olivine Fo 90.4, bulk (Al 2O 3 + CaO) + 4.2 wt%) is interpreted as locally “fertile” upper mantle material. Cumulate rocks are well represented, particularly by olivine-free gabbronorite in which orthopyroxene and clinopyroxene occur in approximately equal modal proportions, and contain clinopyroxene with low TiO 2 (av. 0.29 wt%). Both pyroxenes appear before plagioclase in the crystallisation sequence, and therefore the Halmahera cumulate rocks are distinct from gabbroic rocks formed at mid-oceanic spreading ridges. The cumulus mineralogy is generally comparable with cumulates of the Papuan and Marum ophiolites of New Guinea and with cumulates dredged from the Mariana Trench; it is consistent with open-system crystalisation from a relatively high-Si, high-Mg, low-Ti magma derived from a high degree of partial melting of a lherzolitic mantle source. This correlates with the evidence from the harzburgites and suggests that the ophiolitic rocks were formed in a supra-subduction zone environment. The plutonic rocks are interpreted as resulting from approximately 20% melting of depleted oceanic upper mantle, triggered by hydrous input on the initiation of subduction. However, highly calcic plagioclase characteristic of island arc plutons occurs in neither the Halmahera nor the Mariana Trench cumulate rocks. Although volcanic rocks are not abundant in eastern Halmahera, trace element analysis has revealed the presence of a number of distinct and non-cogenetic suites, of boninitic affinity, of island arc affinity and one of oceanic island/seamount origin. The first of these is correlated with the Halmahera cumulate rocks on the basis of strongly depleted incompatible element chemistry. Arc rocks are identified by pronounced negative Nb anomalies, whereas the alkalic rocks are enriched in TiO 2 and Nb and interpreted as seamount material accreted with the ophiolitic rocks during subduction. Similarly polygenetic volcanics have been reported from the Mariana Trench region, which is proposed as a modern analogue for the ophiolite terrane of eastern Halmahera. The plutonic rocks represent the deeper levels of the ophiolite preserved in the forearc region which were subject to intrusion by a number of later magmatic pulses.