Late Ordovician to early Devonian adakites and Nb-enriched basalts in the Liuyuan area, Beishan, NW China: Implications for early Paleozoic slab-melting and crustal growth in the southern Altaids

摘要

We report newly-defined Nb-enriched basalts, adakites and dacites from the Beishan, NW China of the southern Altaids based on field, geochemical, isotopic and geochronology studies. Two phases of adakites (adakite-I and adakite-II) have been defined, which are calc-alkaline, and characterized by high Na _2O/K _2O ratios (1.49-1.71 and 2.32-3.64) and Sr contents (494-1213ppm and 325-494ppm), negligible to positive Eu anomalies, strong depletion of HREE (e.g., Yb=0.48-0.93ppm and 0.50-0.99ppm) and Y (6.87-9.80ppm and 6.02-10.30ppm), and enriched in Rb, Sr, Ba, K and depleted Nb and Ti. They are characterized by relatively low ε _(Nd)(t) values (-0.8 to -0.9 and +0.6 to +3.8) and relatively constant high (~(87)Sr/ ~(86)Sr) _i ratios (0.70635-0.70636 and 0.70583-0.70651). The zircons of adakite-I have relatively low ε _(Hf)(t)(-0.8 to +2.7). The Nb-enriched basalts are sodium-rich (N _2O/K _2O=1.31-4.44), with higher TiO _2, P _2O _5, Zr and Nb contents and (Nb/Th) PM, (Nb/La) PM and Nb/U ratios than typical arc basalts. They are relatively enriched in Rb, Ba, U, Pb and K, depleted in Nb, and minor negative to positive Ba, Zr, Sr and Ti. They have low positive ε _(Nd)(t) (+0.9 to +2.3) and relatively high (~(87)Sr/ ~(86)Sr) _i (0.70556-0.70691) ratios. The dacites are typical arc magmas, with moderately enriched LILE, distinctly negative Eu, Nb, Sr and Ti anomalies. They have positive ε _(Nd)(t) (+2.2) and relatively high (~(87)Sr/ ~(86)Sr) i (0.70786). We argue that the Liuyuan adakites were most probably related to the melting of young/hot subducted crust of the Paleo-Asian Ocean, which included tectonically-subducted radiogenic crustal material and/or inheritance from highly radiogenic oceanic crust (e.g. OIB). The Nb-enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and/or further changed by components other than adakites (e.g., minor slab-derived fluids). Based on own zircon SIMS U-Pb dating of these key rock types, we further propose that from the late Ordovician to early Devonian, large volumes of magma consisting of late Ordovician Nb-enriched basalts (451Ma) and dacites (442Ma), late Silurian adakite-I (424Ma), early Devonian adakite-II (374Ma) and I-S-A-type granites (436Ma-380Ma), developed in the southern Altaids. Together with other geochronological data from the literature, we conclude that subducted oceanic slab-melting was frequent from 470Ma to 370Ma. Our results suggest that frequent hot (and/or young) oceanic crustal subduction and slab-melting were important mechanisms in the accretionary growth of the Southern Altaids.