Neoproterozoic subduction along the Ailaoshan zone, South China: Geochronological and geochemical evidence from amphibolite

Abstract

Lenses of amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U–Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping amphibolite, has mg-number of 71–76 and (La/Yb)cn ratios of 7.2–7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3–18.4ppm), resembling Nb-enriched basalts. The ɛNd(t) values for Group 1 range from −3.45 to −2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U–Pb zircon age of 803±7Ma, whereas two samples for Group 2 give U–Pb zircon ages of 813±11Ma and 814±12Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these amphibolites are inferred to constitute part of an early Neoproterozoic (∼815–800Ma) arc-back-arc basin system. The Neoproterozoic amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block.