O, SR, ND AND PB ISOTOPIC COMPOSITION OF THE KASUGA CROSS-CHAIN IN THE MARIANA ARC - A NEW PERSPECTIVE ON THE K-H RELATIONSHIP

The Kasuga system in the Mariana Arc consists of three submarine volca noes, Kasuga 1 (K1) along the magmatic front and Kasuga 2 (K2) and Kas uga 3 (K3) extending into the back arc. K1 and nearby Fukujin are domi nated by low- and medium-K tholeiitic andesites and basaltic andesites while K2 and K3 lavas are calc-alkaline and consist of primitive medi um- and high-K basalts and absarokites (Mg# = 66-80) and subordinate a ndesites and dacites. Construction of this complex on back-arc basin c rust that is no older than 45 Ma permits resolution of the sources and processes responsible for the increase in LIL contents in arc volcani c rocks with distance to the Benioff zone (the K-h relationship). New O, Sr, Nd and Pb isotopic data for fresh lavas from K2 and K3 and Pb i sotopic data for K1 are interpreted to better understand the significa nce of these enrichments. The range in deltaO-18 for four basalts (+5. 7 to + 6.0 parts per thousand) and three clinopyroxene separates (5.23 -5.57 parts per thousand) indicates an unmodified mantle source; delta O-18 for three andesites and dacites (6.0-6.2 parts per thousand) is c onsistent with their evolution by shallow fractionation of basalt. K2 and K3 Sr-87/Sr-86 and epsilon(Nd) data range from 0.70327 to 0.70410 and +2.9 to +6.1. These lavas do not show the elevated Sr-87/Sr-86 for a given epsilon(Nd) that is characteristic of lavas erupted along a m agmatic front; instead they suggest participation of OIB-like mantle. Pb isotopic data also show significant variations, with Pb-206/Pb-204 ranging from 18.77 to 19.13, DELTAPb-207 = + 5 to + 12, and DELTAPb-20 8 = +7 to +30. The trend of the Pb isotopic data and mantle-like Ce/Pb (10-23) and Ba/ La (10-27) preclude any significant role for subducte d crust or sediments in controlling these enrichments. Instead, the in verse variation of magma production rate and isotopic heterogeneity is interpreted to indicate diminished melting with distance from the mag matic front, such that the smaller volcanoes K2 and K3 are dominated b y the lowest temperature-melting fraction of a heterogeneous mantle. D iminished water flux results in preferential melting of OIB-type mantl e 'plums'-and attendant enrichments in resultant magmas-with depth to the Benioff zone.