A new report of the early Palaeozoic hornblendite in South China and its tectonic significance

Outcrops of early Palaeozoic mafic rocks are limited in the South China Block (SCB), which become a barrier to unlock the related tectonic evolutionary history. We present a new report of early Palaeozoic Pengxi hornblendite at Xiajiang County, Jiangxi Province, in this paper. The Pengxi hornblendite has a concordant zircon U-Pb age of 433 +/- 2 Ma (40 grains) which could represent its formation time. It is characterized by high MgO content (16.8-19.6 wt.%) and Mg-# (78-80), compatible elements concentration (Ni = 280-506 ppm, Cr = 1,580-1,780 ppm), and evident depletions of Nb, Ta, and Ti which are similar to the arc-related magmatism. Combining with the enriched Nd (epsilon(Nd)(t) = -5.3 to -7.8) isotopic feature, we suggest that the hornblendite was derived from partial melting of metasomatized subcontinental lithospheric mantle (SCLM). The lack of other field geological evidences in the study and adjacent areas, such as volcanic rocks, pyroclastic sedimentary rocks, and turbidites, did not support the contemporaneous subduction model. The study area is located near the boundary of the Yangtze and Cathaysia blocks which was a Neoproterozoic subduction belt characterized by ophiolites, volcanic rocks, and turbidites. Mafic rocks formed at different periods (e.g., 800, 500, and 430 Ma) in the SCB have similar enriched Nd isotopic compositions and La/Nb ratios which suggest that the Silurian hornblendite with arc-like geochemical features may not be the result of contemporaneous subduction but likely inherited from the Neoproterozoic subduction. During Silurian post-orogenic collapse event, the partial melting of enriched SCLM generated the Pengxi hornblendite. This work can provide new data to the further discussion of the controversies in the tectonic background of the SCB.