The widespread Neoproterozoic magmatism along the Yangtze block carries critical in-formation for understanding the Neoproterozoic evolution of the Yangtze block. In the northwestern margin of the Yangtze block,the Hannan (汉南) intrusive complex includes the Wudumen (五堵门),Erliba (二里坝) and Zushidian (祖师殿) granitoids. Using LA-ICP-MS U-Pb zircon dating method,the Wudumen and Erliba granitoids yielded magma crystallization ages of 785±4 and 778±3 Ma,respec-tively. Samples from these three granitoids show variable SiO2 contents ranging from 58.8% to 72.6%. They are characterized by enrichment of Al2O3 (14.97%-17.87%),Na2O (3.80%-5.33%) and Sr (504 ppm-741 ppm),and depletion of Y (<19 ppm) and HREE (e.g.,Yb<1.6 ppm),resulting in high Sr/Y (29-161) and (La/Yb)N (7.3-27.8) ratios. The geochemical features of the granitoids are comparable with those of adakite. The granitoids have zircon εHf(t) values of +3.65 to +10.05,whole-rock εNd(t) val-ues of -0.09 to +2.98 and whole-rock initial 87Sr/86Sr ratios of 0.703 4-0.703 9,indicating that their magma was derived from a juvenile crustal source. Together with geochemical and Hf-Sr-Nd isotopic compositions,it is suggested that the granitoids formed in island-arc setting and originated from partial melting of a subducted oceanic slab. The results support a model that the Yangtze block was sur-rounded by ocean and arc magmatism in its northern and northwestern margins in Neoproterozoic.
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.
In the Songpan-Garze fold belt of the northeastern Tibetan Plateau, an Indosinian lithospheric delamination model has been proposed, based on previous investigation of widespread granitoids. However, this model lacks comparable information from volcanism in the area. During the Indosinian delamination in the Songpan-Garze fold belt, whether partial melting of litho- spheric mantle taken place is debated. This paper reports U-Pb zircon LA-ICP-MS ages, geochemical and Sr-Nd-Hf isotopic compositions from the Aba and Wasai calc-alkaline volcanic rocks in the central Songpan-Garze fold belt. Obtained magma crystallization ages are 210±3 Ma for the Aba andesite and 205±1 Ma for the Wasai andesite. These are consistent with magma crystallization ages of the late Indosinian granitoids in the Songpan-Garze fold belt that formed in a post-collisional tectonic setting. The Aba and Wasai andesites have distinct geochemical singnatures. The former has higher Al2O3, K2O, Rb but lower Na2O, Ba and Sr contents, suggesting differences in their magmatic evolution. The Aba andesites have ISr values of 0.7070-0.7076 and εNd(t) values of -3.9 to -5.3, and the Wasai andesites have ISr values of 0.7075-0.7077 and εNd(t) values of -3.6 to -3.9. Zircons show εHf(t) values of -3.7 to 0.3 for the Aba andesites and -2.7 to 5.5 for the Wasai andesites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that fractional crystallization and crustal assimilation processes are not key roles for their magma evolution, implying that their chemical compositions are those of primary melts. We suggest that the magma of the Aba andesites originated predominantly from a crustal source, with a minor mantle-derived component. The source region of the magma was likely at the crust-mantle boundary. The magma of the Wasai andesites resulted from partial melting of lithospheric mantle, which was probably metasomatized by fluids so that it was amphibole bearing. The petrogenesis of the Aba and Wasai andesites provides an additional evidence for the l
CAI HongMing1,2, ZHANG HongFei1,2, XU WangChun1,2, SHI ZhangLiang1,2 & YUAN HongLin3 1 State Key Laboratory of Geological Processes and Mineral Resources, Wuhan 430074, China
