LESSONS FOR ANCIENT ARC STUDIES FROM THE INTRA-OCEANIC IZU-BONIN-MARIANA ARC SYSTEM IN THE WESTERN PACIFIC

Continental crustal growth has mostly occurred at convergent margins for as long as Plate Tectonics (sensu lato) has operated, and this is revealed by magmatic growth at intra-oceanic arc systems (IOAS). IOAS now comprise ~40% of Earth's ~55,000 km of convergent margins, and this proportion may have been greater in Precambrian time. The IBM arc system is a representative modern IOAS to which possible ancient arcs can be usefully compared. IBM crust is 3000 km long, 200 km wide (from trench to magmatic arc), and 25 km thick, tapering to almost nothing at the trench. Ancient arcs likely had similar original dimensions but subsequently have been shortened, compressed in width and thickened, at the same time magmatically reworked and metamorphosed. Several points about IOAS from the IBM example guide interpretation of ancient arc-like terranes: 1) Because IOAS are removed from continents, they lack accretionary prisms. Instead, low sediment flux to the trench results in tectonic erosion of forearc crust. 2) IBM forearc crust was generated during a short (5-10 Ma) “infant arc” episode of forearc spreading, including low-K tholeiite and rhyodacite ± boninite; this is the origin of many forearcs and ophiolites. IBM forearc mantle lithosphere is underlain by harzburgite, and is an in situ SSZ ophiolite. 3) IBM forearc is 100-150 km wide with low heatflow and abundant serpentinized harzburgites; serpentinite diapirism and mudflows may occur. Blueschist may be exhumed during episodes of slab break-off or when subduction channel return flow occurs. Transient episodes of high heatflow in the forearc occur rarely when very young seafloor is subducted. 4) Following arc infancy, IBM magmatism retreated to the magmatic front, ~100-150 km from the trench. IBM arc lavas are mostly fractionated low-K tholeiitic and medium-K calc-alkaline basalts but include subordinate primitive basalts, andesites, and rhyodacite rocks, as well as some high-K/shoshonitic lavas. These magmas reflect hydrous melting of the mantle wedge; adakitic rocks formed by melting of subducted young (>20 Ma) oceanic crust can also occur. 5) Rear-arc volcanism occurs at a few arc cross-chains. 6) Back-arc basins are a ubiquitous part of IOAS and are dominated by hybrid arc/MORB basalts where seafloor spreading occurs and arc-like bimodal lavas where rifting occurs.