Neoproterozoic crustal growth of the Southern Yangtze Block : Geochemical and zircon U–Pb geochronological and Lu-Hf isotopic evidence of Neoproterozoic diorite from the Ailaoshan zone

Abstract

Abstract Neoproterozoic felsic igneous rocks associated with mafic-ultramafic bodies along the margins of the Yangtze Block, South China, can be used to constrain the continental crustal growth and secular evolution of the region. LA-ICPMS zircon U-Pb dating of the Adebo quartz diorite pluton in the Ailaoshan tectonic zone on the southern margin of the Yangtze Block gives the Neoproterozoic age of 800 ± 7 Ma and ɛHf(t) values in the range of -1.03 to +3.75 with two-stage model age of 1.3-1.6 Ga. The pluton is characterized by relatively low SiO2 (60.97-64.41 wt. %) and total alkalis (K2O + Na2O, 7.35-9.14 wt. %) and high Al2O3 content (16.98-18.21 wt. %) with mg-number of 36-39. REE-normalized patterns show enrichment in LREE with (La/Yb)cn of 11.36 to 19.77 and Europium negative anomalies with Eu/Eu* = 0.61- 0.74. The samples are characterized by negative Nb-Ta ((Nb/La)n = 0.18-0.35) and P, Ti, Sr anomalies and high Y concentrations (33.79-41.04 ppm) and low Sr/Y ratios (5.65-10.16). Their isotopic composition are similar to those of the Neoproterozoic mafic igneous rocks in the Ailaoshan zone and the southwestern Yangtze Block, indicating that the quartz diorite was produced by partial melting of mafic lower crust. The diorite also shows the similar geochemical characteristics with adakitic rocks from thickened lower crust or amphibolite and eclogite experimental melts. In combination with their arc-related geochemical signatures and synchronous developed adakitic rocks in the region, the Adebo quartz diorite pluton might be produced in a subduction-related tectonic setting during Neoproterozoic crustal growth along the margins of Yangtze Block.