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ABSTRACT: Basement Rocks of Halmahera, Eastern Indonesia: Implications for the Early History of the Philippine Sea

Robert Hall, Gary Nichols, Paul Ballantyne

The oldest rocks known on Halmahera, eastern Indonesia, are petrologically and chemically similar to "supra-subduction ophiolites" and include boninitic volcanics resembling those dredged from the Marianas forearc. The age of the ophiolitic rocks is unknown; in east Halmahera they are overlain by Late Cretaceous and Eocene volcanics and associated sediments. Similar volcanics form the basement of western Halmahera. Plutonic rocks intruding the ophiolite and associated metamorphic rocks also yield Late Cretaceous to Eocene radiometric ages. The petrology and chemistry of the igneous rocks indicate an island arc origin. These rocks are locally overlain by shallow-water Eocene limestones and all are overlain unconformably by Neogene sediments. The Halmahera basement rocks ha e many structural, petrological, and stratigraphic similarities to submarine plateaus of the southern and northern Philippine Sea and basement terranes of the eastern Philippines. We suggest that these similarities indicate the existence of an extensive region of Late Cretaceous and Eocene volcanism built upon probable Mesozoic ophiolitic basement. The resultant thickened crust was later fragmented by spreading in the West Philippine Sea Central Basin and backarc spreading in the Eastern Philippine Sea. It is difficult to reconcile the present distribution of these crustal fragments with a linear arc, but equally difficult to propose a simple alternative. A proto-Philippine archipelago, with short-lived arcs separated by small oceanic basins, may be the closest modern analog. The develop ent of younger subduction zones has been influenced by the distribution of thickened crustal fragments as they have re-amalgamated since the Miocene.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990