Ballantyne, P. D.; (1990) The petrology of the ophiolitic rocks of the Halmahera region, Indonesia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis contains the first detailed petrological and geochemical study of the ophiolitic rocks of the island of Halmahera in eastern Indonesia. Rocks of ophiolitic affinity occur extensively across the eastern half of Halmahera, although an intact ophiolite stratigraphy cannot be seen, presumably because of a combination of poor exposure and structural dismemberment. However, samples representative of each level of a "complete" ophiolite (with the exception of sheeted dykes) have been collected. "Mantle sequence" rocks are represented predominantly by depleted harzburgites, suggesting a mantle residue which has undergone a high degree of partial melt extraction. Subordinate Iherzolites of relatively enriched chemistry are interpreted as local zones of "fertile" upper mantle material. Cumulate rocks are well represented, particularly by olivine-free gabbronorites in which orthopyroxene and clinopyroxene occur in approximately equal modal proportions and the clinopyroxene has low TiO2. Ortho- and clinopyroxene occur before plagioclase in the crystallization sequence. The cumulus mineralogy is consistent with open-system crystallisation from a high Si, Mg, low Ti magma derived from a high degree of partial melting of a Iherzolitic mantle source region. This correlates with the evidence from the harzburgites and suggests that the ophiolitic rocks were formed in a supra-subduction zone environment. Although volcanic rocks are not abundant in eastern Halmahera, geochemical analysis reveals the presence of at least four, distinct and non-cogenetic, groups; one of boninitic affinity, two of island arc and one of oceanic island/seamount origin. The first of these groups is correlated with the cumulate rocks on the basis of strongly depleted rare earth and trace element chemistry. Similarly polygenetic volcanics have been reported from the Mariana forearc region, which is proposed as a modem analogue for the ophiolite terrain of eastern Halmahera. The plutonic rocks are interpreted as resulting from the partial melting of depleted oceanic upper mantle on the initiation of subduction; with development of the subduction zone, these rocks were preserved in the forearc region and were subject to intrusion by a number of later magmatic pulses.

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