Partial melting of oceanic sediments in subduction zones and its contribution to the petrogenesis of peraluminous granites in the Chinese Altai

Late Carboniferous magmatism in the Chinese Altai provides an important view of geodynamic processes active during crustal growth in the Central Asian Orogenic Belt (CAOB). In this study, five representative peraluminous granite plutons from the Chinese Altai were selected for systematic geochronological, geochemical and Sr-Nd-Hf isotopic analyses (Table 1). These granites were emplaced between 449 and 327 Ma in an active subduction zone, and have moderate to high SiO2 (66.54-76.13 wt%), moderate Na2O+K2O (6.27-7.66 wt%), and high Al2O3 contents (12.43-16.18 wt%). All granite samples in this study showed significant decoupling of the Nd and Hf isotope systems. Results show negative epsilon(Nd)(t) values (-3.3 to -0.9), and predominantly positive epsilon(Hf)(t) values (+0.24 to +8.01, n=57) except for a few negative epsilon(Hf)(t) values (-7.44 to -0.03, n=9), high Mg# values (28.69-53.33), high Nd/Hf ratios (4.26-43.57), and enrichment of large-ion lithophile elements (LILEs; e.g. Pb, Th, and U), suggesting that the granites were derived from the partial melting of oceanic sediments and the associated mantle wedge, with fractionation of plagioclase, K-feldspar and biotite. In situ zircon Hf isotopic analyses yield negative epsilon(Hf)(t) values from -30.6 to -13.7 for the zircon xenocrysts. The U-Pb ages and Hf isotopic ratios of these zircon xenocrysts were probably inherited from oceanic sediments. Zircon saturation temperatures suggest that these peraluminous granites were emplaced at 537-765 degrees C. We propose that: (1) the Nd isotopic system more faithfully reflects the source of peraluminous magmas in the Chinese Altai than the Hf isotopic system, and (2) the oceanic sediment recycling was an important process during continental growth in the CAOB.