Early History of Island Arcs - Evidence from the Mariana Trench

Among the many problems concerning the initiation and early history of island arcs is the question - what underlies arc plutonic and volcanic series ? Conventional wisdom is that arcs are built on older ocean crust. Their deep roots should include high-Mg eruptions, crystal cumulates complementry to higher-level differentiated lavas, plutonic rocks compositionally equivalent to lavas, and depleted mantle residue complementary to extracted arc magmas. If built on older ocean crust, the "roots" should overlie seafloor sediments, pillows, dikes, etc. (i.e., ophiolites). Probable exposures of deep roots of arcs are rare; exposures such as Tonsina Complex, Alaska and Zambales Range, Luzon are notable for a lack of evidence for subjacent ocean crust. The Zambales shows unmistakeable evidence for continuity from depleted harzburgite/dunite to a thick layered cumulate series of wehrlite,dunite, pyroxenite; these are overlain by norite, gabbro and arc tholeiite basalt. Much of the cumulate series and norite-gabbro has blasto-mylonite texture. These petrologic features suggest upwelling of parental peridotite mantle into a realm of oceanic lithosphere extension. The nascent arc has replaced former ocean crust rather than being built on it. The extensional regime, owing to subduction and seaward trench roll-back, has driven opening of back arc basins, caused forearc rifting, and provided a rift setting for initiation of arc magmatism. Rocks dredged from the Mariana Trench (14 to 18 N) from depths of 9000 to 3000 m, display evidence for near-solidus penetrative deformation that developed higher amphibolite facies mylonites and blasto-mylonites. Rocks include depleted peridotite, pyroxenite, wehrlite, cumulate amphibolite, norite/gabbro, tonalite. High-T metamorphism is shown by equilibrium assemblages of labradorite-bytownite, AL-hornblende, ortho- and clino- pyroxene. Highly deformed homogeneous calcic plagioclase has cross-cutting veins of xenoblastic plagioclase and rare veins of hornblende. Epidote or other An-substitutes are absent except for minor veinlets associated with retrograde talc, tremolite, chlorite. The high-T penetrative deformation and recrystallization are evidence for near-solidus deformation of newly formed arc crust and upper mantle as a consequence of shallow -level crust/mantle extension. Close association of these arc-series rocks with Eocene boninites makes it likely that they all formed at the Palau-Kyushu Ridge and have progressively migrated eastward tracking roll-back of the Mariana Trench.