Making and breaking an island arc: A new perspective from the Oligocene Kyushu‐Palau arc, Philippine Sea

Abstract

[1] The Kyushu-Palau Ridge (KPR) is a 2600 km long remnant island arc that is separated from the active Izu-Bonin-Mariana (IBM) arc by a series of spreading and rift basins. We present 40 Ar/ 39 Ar ages and geochemical data for the entire length of the Kyushu-Palau arc as well as for the conjugate arc which is stranded within the IBM fore arc. New 40 Ar/ 39 Ar ages indicate that the KPR was active between 25 and 48 Ma, but the majority of the exposed volcanism occurred in the final phase, between 25 and 28 Ma. Rifting of the Kyushu-Palau arc to form the Shikoku and Parece Vela basins occurred simultaneously along the length of the arc (circa 25 Ma), and at a similar distance from the trench. Unlike the IBM, the KPR has only limited systematic along-arc geochemical trends. Two geochemical components within the KPR indicate an origin in the suprasubduction mantle. First, EM-1-like lavas are identified in a restricted section of the arc, suggesting a localized heterogeneity. Second, EM-2-like arc volcanoes formed on juvenile West Philippine Basin crust, potentially reflecting ingress of mantle from the then active EM-2 province which lies in the west. Another geochemical heterogeneity is found at the KPR-Daito Ridge intersection where the arc developed on preexisting Cretaceous Daito Ridge crust. The geochemical characteristics at this intersection likely result from the involvement of sub-Daito Ridge lithospheric mantle. Subduction flux beneath the KPR generally matches post-45 Ma Eocene/Oligocene lavas in the IBM fore arc, involving fluids and melts derived from altered igneous crust.

Expedition 350 summary

“…This mature arc is labeled the "Kyushu-Palau arc" on Figure F2A and F2B. Eocene arc-front lava, dated at ~40-42 Ma by 40 Ar/ 39 Ar by Ishizuka et al (2011), has been drilled in what is now the Izu fore-arc basement. However, no Oligocene arc rocks (lava or intrusions) have been recovered in the Izu arc, although Oligocene turbidites with andesitic clasts rest upon Eocene lava in what is now the fore arc (Taylor, Fujioka, et al, 1990;Gill et al, 1994), indicating that Oligocene arc volcanism occurred somewhere west in the region.…”

“…However, 40 Ar/ 39 Ar dating on the lava from two different depths gave consistent and well-defined plateaus of 40.4 ± 0.8 Ma and 40.6 ± 0.3 Ma, or Eocene ages (Figure F7). Similarly, andesite lava "basement rocks" drilled in the modern fore-arc basin outboard of this, at ODP Site 793, are Eocene (41 Ma; Ishizuka et al, 2011). Farther outboard, in the outer arc high drilled at ODP Site 786, the basement consists of boninite lava (45.3-46.7 Ma) overlain by andesite lava (44.7 Ma; all by 40 Ar/ 39 Ar; Ishizuka et al, 2006a), all Eocene in age.…”

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“…This suggests that sinking of the downgoing plate was rapidly followed by a dramatic episode of asthenospheric upwelling and melting, sometimes enhanced by solute-bearing water fluxes released from the downgoing plate, over a zone that was thousands of kilometers long and as wide as 200 km (Reagan et al, 2010). As subduction proceeded, hydrous mantle melting overprinted decompression mantle melting, establishing the first mature arc in Eocene to Oligocene time (Taylor, 1992;Ishizuka et al, 2006aIshizuka et al, , 2006bIshizuka et al, , 2011. This mature arc is labeled the "Kyushu-Palau arc" on Figure F2A and F2B.…”

“…This mature arc is labeled the "Kyushu-Palau arc" on Figure F2A and F2B. Eocene arcfront lava, dated at ~40-42 Ma by 40 Ar/ 39 Ar by Ishizuka et al (2011), has been drilled in what is now the Izu fore-arc basement. However, no Oligocene arc rocks (lava or intrusions) have been recovered in the Izu arc, although Oligocene turbidites with andesitic clasts rest upon Eocene lava in what is now the fore arc (Taylor, Fujioka, et al, 1990;Gill et al, 1994), indicating that Oligocene arc volcanism occurred somewhere west in the region.…”

“…F7). Similarly, andesite lava "basement rocks" drilled in the modern fore-arc basin outboard of this, at ODP Site 793, are Eocene (41 Ma; Ishizuka et al, 2011). Farther outboard, in the outer arc high drilled at ODP Site 786, the basement consists of boninite lava overlain by andesite lava (44.7 Ma; all by 40 Ar/ 39 Ar; Ishizuka et al, 2006a), all Eocene in age.…”