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Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
Ophiolitic rocks form the basement of the eastern part of the island of Halmahera in eastern Indonesia. The ophiolite is tectonically dismembered but all the elements of a complete ophiolite are present with the uncertain exception of a sheeted dyke complex. The ophiolite was formed in a supra-subduction zone setting before the Late Cretaceous and is interpreted to represent the forearc of a Mesozoic arc whose remnants are now found within and close to the margins of the Philippine Sea Plate. The plutonic rocks of the ophiolite include periotites interpreted as a ‘mantle sequence’ and cumulates interpreted as the deeper parts of an ophiolitic crustal sequence. The mantle sequence includes subordinate lherzolites of relatively enriched chemistry which represent upper mantle previously depleted by the extraction of MORB. A high degree of melting of the lherzolites under hydrous conditions left a mantle residue of depleted harzburgites, which are more refractory than the harzburgites of Troodos and Oman. Abundant cumulates which crystallized from the magma include peridotites and gabbroic rocks rich in orthopyroxene. The chemistry and mineralogy of the cumulates indicate open-system crystallization from a high Si, Mg, low Ti, LREE-depleted magma. Highly calcic plagioclase characteristic of island-arc plutons and found in the cumulate sequence of the Troodos ophiolite is not found in the Halmahera cumulates. The Halmahera ophiolite is interpreted as the onland analogue of the present-day Mariana forearc. Rocks dredged from the Mariana Trench, which like Halmahera is also situated at the margin of the Philippine Sea Plate, are remarkably similar to those of the Halmahera ophiolite.
