U-Pb Zircon Ages,Geochemical and Sr-Nd-Hf Isotopic Compositions of Granitoids in Western Songpan-Garze Fold Belt:Petrogenesis and Implication for Tectonic Evolution

Granitoids are widespread in the Songpan-Garze (松潘-甘孜) fold belt,western China. These granitoids provide insight into regional tectono-magmatic events,basement nature and tectonic evolution. However,previous studies mainly focused on the eastern Songpan-Garze fold belt. In this ar-ticle,five granitoid intrusions from the western Songpan-Garze fold belt have been studied. These in-trusions are composed of quartz-diorite and granodiorite. Using LA-ICP-MS zircon dating method,the obtained magma crystallization ages are 219±2 Ma for the quartz-diorite and 216±5 Ma for the grano-diorite. The ages,combined with regional geological analyses,show that they formed in a post-collisional tectonic setting. The quartz-diorite and granodiorite display co-linear variation in their chemical compositions. REE compositions for both the quartz-diorite and granodiorite show strongly fractionated patterns with (La/Yb)N=5.02-18.34 and Eu/Eu*=0.44-0.89. The quartz-diorites have initial 87Sr/86Sr ratios (ISr) of 0.709 29-0.711 97 and εNd(t) values of -8.6 to -6.1 and the granodiorites have ISr values of 0.705 49-0.709 97 and εNd(t) values of -8.3 to -4.3. Zircon Hf isotopic data show εHf(t) values of -3.8 to +1.6 for the quartz-diorites and -1.2 to +3.0 for the granodiorites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that the quartz-diorites and granodiorites have similar petrogenesis. We suggest that the magmas for the quartz-diorites and granodiorites were derived from partial melting of lower crustal mafic source,resulting from amphibole dehydration melting reaction. The probing of the magma source reveals that the western Songpan-Garze fold belt contains an unexposed continental basement,which is similar to the eastern Songpan-Garze fold belt. Geodynamically,it is proposed that a lithospheric delamination model can account for the magma generation for the quartz-diorites and granodiorites in the western Songpan-Garze fold belt.