Geochronological and geochemical constraints on the metasomatised source for the Neoproterozoic (∼825 Ma) high-mg volcanic rocks from the Cangshuipu area (Hunan Province) along the Jiangnan domain and their tectonic implications

The amalgamation process of the Yangtze with Cathaysia Blocks has been one of the disputed issues that have attracted considerable attention due to its significance in understanding the assembly and breakup of the supercontinent Rodinia. However, it has been a matter of debate as to the tectonic nature (plume vs. subduction/collision vs. plate-rift) and timing of the amalgamation (1000 Ma vs. 880 Ma vs. 800 Ma) along the Jiangnan domain. To evaluate the previously proposed models and to constrain the final amalgamation timing of the Yangtze with Cathaysia Blocks, this paper presents a set of integrated geochemical and geochronological data for the high-mg volcanic sequence that is bounded between the Lengiiaxi and Banxi Groups. Two andesitic samples from the volcanic sequence (Cangshuipu) were dated at 824 +/- 7 Ma (CSP-35) and 822 +/- 28 Ma (CSP-8) using the SIMS and SHRIMP techniques, respectively. These samples have mg-numbers ranging from 71 to 25 with SiO2 of 57.51-73.80 wt%, classified as high-mg andesite and dacite. They show significant enrichment in LILEs and depletion in HFSEs with marked negative Nb-Ta, P and Ti anomalies. Their epsilon(Nd)(t) values range from -1.7 to -4.7 and initial Sr-87/Sr-86 values from 0.7051 to 0.7123. The generation of the Cangshuipu high-mg rocks might have been attributed to an interaction of the subducted sediment-derived fluid with the overlying refractory mantle. The contemporaneous dacitic rocks should be the product of the highly fractional crystallization from the high-mg andesitic magma. Integrated with the regional geology and available geochronological data, we propose that the Cangshuipu volcanic sequence might have formed in a collisional setting related to the Neoproterozoic orogenic process along the Jiangnan domain. The final amalgamation of the Yangtze with Cathaysia Blocks along the Jiangnan domain probably occurred between 815 Ma and 822 Ma (probably 820 Ma) rather than 1000 Ma, ca. 880 Ma or 800 Ma as previously proposed. Taking into account the subsequent development of the Nanhua rifting (probably after 815 Ma), it is inferred that a plate-rift model might be a more reasonable mechanism for the Neoproterozoic tectonic evolution along the Jiangnan domain. (C) 2012 Elsevier B.V. All rights reserved.