Zircon U-Pb and Lu-Hf isotopic systematics of the Daping plutonic rocks: Implications for the Neoproterozoic tectonic evolution of the northeastern margin of the Indochina block, Southwest China

摘要

The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U-Pb dating on the zircon cores and rims of a hornblende-gabbro yield ~(206)Pb/~(238)U weighted means ages of 873±9.1Ma and 769±7Ma, respectively, and that for cores, mantles and rims of a granodiorite yield ~(206)Pb/~(238)U weighted means ages of 981-987Ma, 829±10Ma and 761±11Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have ε_(Hf)~((t)) values ranging from -5.0 to -5.8 and +0.6 to +6.4. Corresponding single-stage model ages range from 1626 to 1662Ma and 1094 to 1311Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) ε_(Hf)~((t)) values of +3.3 to +12.3 with single-stage Hf model ages of 897 to 1235Ma; and (2) ε_(Hf)~((t)) values of -1.9 to -7.8 with single stage model ages of 1470-1667Ma. The zircon rims are characterized by positive ε_(Hf)~((t)) values (+5.4 to +8.2) with single-stage model ages ranging from 977 to 1108Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg~# (52.1-65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.