Sub-arc Mantle Heterogeneity of the Northern Luzon Volcanic Arc: Mineral and Whole Rock Compositional Variability in Mantle Xenoliths from Lutao Island

Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo(92.5-88.9)), orthopyroxene (Mg# = 94.6-89.2), and clinopyroxene (Wo(49.1-38.1)En(57.0-45.4)Fs(3.0-11.0)) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# < 45) and low-Al (Cr# > 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95-2.07 wt %; Mg# = 88.5-90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (Sr-87/Sr-86(i) = 0.704400-0.707908; epsilon(Nd)(t) = 0.0 - +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 degrees C with the majority of temperature estimates >1000 degrees C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded Delta FMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1-2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya <1000 degrees C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (<6 Ma) between the Luzon arc and the Eurasian margin.