Nd-Sr-O isotopic evidence for source contamination and an unusual mantle component under Luzon Arc

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The Nd, Sr, and O isotopic compositions of 27 volcanic rocks from the northern Luzon Arc were determined. Their Nd and 87 Sr / 86 Sr displayed large correlated variations (-6.1 to +9.5, 0.70335 to 0.70622). Except for three abnormal samples, the 18 O (6.0 to 8.5%.) show an excellent correlation (r = 0.9) with the volatile contents. After correcting for the alteration effect using the correlation with volatile contents, the 18 O range from 5.2 to 6.4%. and are close to values shown by primary mantle-derived igneous rocks. The extreme deviation of the Sr and Nd isotopic ratios away from those of the depleted mantle coupled with a mantle oxygen isotopic ratio demonstrates unequivocally that the magma of Luzon Arc was contaminated not in the crust but in its source zone by an enriched mantle component. This enriched mantle component cannot be entirely ascribed to subducted sediment if these sediments have compositions similar to that of the nearby continental crust in Taiwan or to the Pleistocene sediment being subducted presently at the Manila Trench. Additional contribution from an enriched mantle component with isotopic composition similar to that of the EMI component proposed by and (1986) is required. Published Pb isotopic results (Sun, 1980) are consistent with the involvement of EMI. Such an EMI component has never been found before in an island arc setting. Ultramafic xenolith results ( , 1986a,b) suggest that this enriched mantle was formed in the mantle wedge via metasomatism by fluids possibly derived from the downgoing slab. However, the involvement of the lithosphere of the approaching Eurasia continent cannot be excluded.

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