A-type granite and adakitic magmatism association in Songpan-Garze fold belt,eastern Tibetan Plateau:Implication for lithospheric delamination

摘要

The Songpan-Garze fold belt covers a huge triangular area (> 200,000 km~2),confined by the South China (Yangtze),North China and Tibetan Plateau continental blocks.In the Songpan-Garze fold belt,Triassic adakitic granitoids have been identified.However,whether there are Triassic A-type granites is unclear.Here,we report our first finding of an A-type granite (Nianbaoyeche),which occurs in the central part of the Songpan-Garze fold belt.The Nianbaoyeche granite (~ 820 km2) is characterized by arfvedsonite in its mineral assemblage.Using both LA-ICPMS and TIMS U-Pb zircon dating methods,we obtain a magma crystallization age of 211 ±1 Ma,which is slightly younger than Triassic adakitic granitoids (216-221 Ma) in the Songpan-Garze fold belt.The Nianbaoyeche granite is enriched in Si,K,Na,Rb,REE,HFSE (Nb,Ta,Zr,Hf),with elevated FeO~(tot)(FeO~(tot)+MgO) and Ga/Al ratios,but is depleted in Al,Mg,Ca,Ba and Sr.The REE compositions show moderately fractionated patterns with (La/Yb)_N=2.67-7.54 and Eu*/Eu=0.09-0.34.These geochemical characteristics indicate that the Nianbaoyeche granite has an A-type affinity.Geochemical data and U-Pb zircon age,combined with regional studies,show that the Nianbaoyeche granite formed in a post-collisional tectonic setting.Sr-Nd isotopic data for the granite exhibit I_(Sr)=0.7090-0.7123 and e[so;pm_(Nd)(t)=-2.72 to-4.26 with T_(DM)=1.15-1.51 Ga,suggesting that the magma has a dominantly crustal source,though a minor contribution from the mantle cannot be ruled out.Melting to produce an A-type granite may have resulted from Triassic lithospheric delamination after Triassic crustal thickening of the Songpan-Garze fold belt due to convergence between the Yangtze,North China and North Tibet continental blocks.The lithospheric delamination model also helps to explain the Triassic adakitic magma generation in the Songpan-Garze belt.We conclude that association of A-type granite and adakitic granitoids in post-collisional environment could be a useful indicator of lithospheric delamination.