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 Javaand has erupted two lava series, a younger highly potassic series(HK) and an older potassic series (K). The HK series has higherK2O 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 patternssimilar in many respects to ocean island basalts. The K serieshas slightly higher ⁸⁷Sr/⁸⁶Sr (O70453 GenBank] -O70498) and ¹⁸O (+6?2to + 8?4%o) and lower ¹⁴³Nd/¹⁴⁴Nd (0?512530–0?5126588)than the HK series (for which 87Sr/86Sr = 0?70426–0?70451,<518O = +6?52 to +7–0%o, and 1*3Nd/1*4Nd= 0?512623–0?512679),and higher LILE/HFSE and LREE/ HFSE ratios. A7/4 and A8/4 arehigh and do not show any systematic change from the K to theHK series. The proposed model for the Muriah lavas involvesthree source components: (1) the astheno-sphere of the mantlewedge 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.,EMU); (3) subducted pelagic sediments from the Indian Ocean. Trace element and isotope data indicate that the characteristicsof the K series are produced by mixing of two endmember magmas:an undersaturated magma derived wholly from within-plate sourcesand a calc-alkaline magma derived from the subduction-modifiedasthenospheric mantle. The calc-alkaline magma is believed tobe contaminated by the arc crust before mixing. Low-pressurefractionation took place in the K series after mixing. Initiallithospheric extension in the Bawean trough (in which Muriahis located), may be responsible for decompressive melting ofthe metasomatic layer and thus the production of the HK serieslavas. The magmas erupted from Muriah show a transition fromintraplate to subduction zone processes in their genesis.