EVIDENCE FROM MURIAH, INDONESIA, FOR THE INTERPLAY OF SUPRA-SUBDUCTION ZONE AND INTRAPLATE PROCESSES IN THE GENESIS OF POTASSIC ALKALINE MAGMAS

The high-K alkaline volcano Muriah is situated in central Java and has erupted two lava series, a younger highly potassic series (HK) and an older potassic series (K). The HK series has higher K2O contents for a given MgO content; greater silica undersaturation; and higher concentrations of LILE (Rb, Sr, Ba, and K), LREE (La and Ce), and HFSE (Nb, Zr, Ti, and P), than the K series. The HK series lavas have incompatible trace element patterns similar in many respects to ocean island basalts. The K series has slightly higher Sr-87/Sr-86 (0.70453-0.70498) and delta-O-18 (+6.2 to +8.4 parts per thousand) and lower Nd-143/Nd-144 (0.512530-0.5126588) than the HK series (for which Sr-87/Sr-86 = 0.70426-0.70451, delta-O-18 = +6.52 to +7.0 parts per thousand, and Nd-143/Nd-144 = 0.512623-0.512679), and higher LILE/HFSE and LREE/ HFSE ratios. DELTA-7/4 and DELTA-8/4 are high and do not show any systematic change from the K to the HK series. The proposed model for the Muriah lavas involves three source components: (1) the asthenosphere of the mantle wedge of the Sunda arc, which has Indian Ocean MORB characteristics; (2) a metasomatic layer situated at the base of the lithosphere, which has characteristics similar to enriched mantle (i.e., EMII), (3) subducted pelagic sediments from the Indian Ocean. Trace element and isotope data indicate that the characteristics of the K series are produced by mixing of two endmember magmas: an undersaturated magma derived wholly from within-plate sources and a calc-alkaline magma derived from the subduction-modified asthenospheric mantle. The calc-alkaline magma is believed to be contaminated by the arc crust before mixing. Low-pressure fractionation took place in the K series after mixing. Initial lithospheric extension in the Bawean trough (in which Muriah is located), may be responsible for decompressive melting of the metasomatic layer and thus the production of the HK series lavas. The magmas erupted from Muriah show a transition from intraplate to subduction zone processes in their genesis.