The role of slab melting in the petrogenesis of hi

Authors: S König S Schuth C Münker C Qopoto

Publish Date: 2006/10/12

Volume: 153, Issue: 1, Pages: 85-

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Abstract

The petrogenesis of highMg andesites HMA in subduction zones involves shallow melting of refractory mantle sources or alternatively the interaction of ascending slabderived melts with mantle peridotite To unravel the petrogenesis of HMA we report major trace element and Sr–Nd–Hf–Pb isotope data for a newly found occurrence of HMA in the New Georgia group Solomon Islands SWPacific Volcanism in the Solomon Islands was initiated by subduction of the Pacific plate beneath the Indian–Australian plate until a reversal of subduction polarity occurred ca 10 Ma ago Currently the Indian–Australian plate is subducted northeastwards along the San Cristobál trench forming the younger and still active southwestern Solomon island arc However a fossil slab of Pacific crust is still present beneath the arc The edifice of the active volcano Simbo is located directly in the San Cristobál trench on top of the subducting Indian–Australian plate Simbo Island lies on top of a strikeslip fault of the adjacent Woodlark spreading centre that is subducted beneath the Pacific plate Geochemical and petrological compositions of volcanic rocks from Simbo are in marked contrast to those of volcanic rocks from islands north of the trench mostly arc basalts Simbotype rocks are opxbearing HMA displaying 60–62 wt SiO2 but rather primitive Mg–Ni–Cr characteristics with 4–6 wt MgO up to 65 ppm Ni up to 264 ppm Cr and Mg from 67 to 75 The compositions of the Simbo andesites are explained by a binary mixture of silicic and basaltic melts Relict olivine phenocrysts with Fo88–90 and reactionrims of opx also support a mixing model The basaltic endmember is similar to backarc basalts from the Woodlark Ridge A slab melt affinity of the silicic mixing component is indicated by GdN/YbN of up to 22 that is higher if compared to MORB and other arc basalts from the Solomon Islands 87Sr/86Sr ɛNd and ɛHf values in the analysed rocks range from 07035 to 07040 +64 to +79 and +12 to +144 respectively These values reveal the presence of the Indian–Australian mantle domain beneath Simbo ie the Indian–Australian plate and also beneath all other volcanic islands of the New Georgia group which are located north of the San Cristobál trench 206Pb/204Pb 207Pb/204Pb and 208Pb/204Pb values 1843–1852 1549–1555 and 1813–1834 respectively confirm the presence of slab melts from the subducted Pacific plate beneath southern Simbo where the highest GdN/YbN ratios are reported A spatial shift towards an Indian–Australian slab signature is observed when approaching the active San Cristobál trench on northern Simbo reflecting the decreasing influence of slab melts from the old subducted Pacific plateThis research was supported by the DFG German Research Foundation project MU 1406/2–3 Heidi Baier and Jasper Berndt are thanked for lab support We thank Radegund Hoffbauer from Universität Bonn for XRF analyses and Dieter GarbeSchönberg from Universität Kiel for quadrupole ICPMS analyses Andrew Mason and Stanley Basi provided support in organizing the field campaign in the Solomon Islands Journal reviews by two anonymous reviewers helped to improve the manuscript

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