ABSTRACT: Magmatism in Nascent Arcs: Evidence from the Eocene Marianas

Robert J. Stern, Sherman H. Bloomer

Modern arc systems have well-defined volcanic fronts that separate an amagmatic frontal arc and fore-arc from the magmatic arc. The position of the magmatic front is determined by the thermal state of the mantle above the subduction zone, with the volcanic front 100-150 km above the subduction zone and, depending on the dip of the subduction zone, 100-200 km or more away from the trench. The problem addressed here is whether or not this spacing exists during the initiation of an arc following the development of a new subduction zone. We take as our example the initiation of the Mariana arc during the mid-Eocene.

The entire eastern margin of the present Philippine Sea was converted from a ridge-ridge transform into a convergent margin following the major tectonic reorganization about 45 Ma ago. The first melts generated in the new subduction zone were boninites and tholeiites. Boninites do not form later in the history of the arc because these manifest the transient interaction of a near-ridge geotherm and hydrous fluids; instead, magmas generated in mature arcs are tholeiitic and calc-alkaline, generated by melting at greater depth in the mantle. Mid- to late Eocene igneous activity in the Marianas encompassed at least the entire area occupied by the modern arc, from the trench to the active arc, giving a minimum width of 200 km; subsequent tectonic erosion and back-arc rifting removed substa tial volumes of the initial arc. This is more than an order of magnitude wider than the magmatic arc of the post-Eocene Marianas and, assuming a crustal thickness of 6 km was generated, suggests magma production rates of about 160 km³km^-1Ma^-1, about 5× greater than the average arc accretion rate. We conclude that igneous activity associated with subduction zone initiation leads to much more extensive melting of the mantle than in mature subduction zones, and suggest that a very large fraction of the total magmatic growth experienced by arcs may happen during the initial stages.

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