Geochemistry of the Meso- to Neoproterozoic basic-acid rocks from Hunan Province, South China: implications for the evolution of the western Jiangnan orogen

The formation and evolution of the Jiangnan orogen at the southeastern margin of the Yangtze Block, South China, are an important and debatable topic. The Meso- to Neoproterozoic basic-acid rocks from Hunan Province record the history of the western Jiangnan orogen in the area. The Mesoproterozoic basalts and diabases from Nanqiao are the typical N-MORB, having very low K ₂O, low incompatible HFSE and REE, and depleted ε _Nd (T) values (+6.86 to +8.98). They may represent the fragments of an obducted oceanic crust or the relicts of the oceanic crust in a "forearc basin" along an ancient subduction zone, which provides new evidence for the existence of the Jiuling Old Island Arc. The Mesoproterozoic komatiitic basalts from Yiyang are high in Al ₂O ₃/TiO ₂, MgO, Ni and Cr, and are depleted in Nb and Ti. These plume-derived magmas are associated with island-arc tholeiites and exhibit the geochemical characteristics of the arc magma, suggesting that the Mesoproterozoic komatiitic basalts might be the products of the plume-arc interaction. The Neoproterozoic andesitic rocks from Baolinchong, with strong depletions of Nb, Ti and enrichment of LILEs, are of typical island-arc volcanic origin. The Neoproterozoic granites from northeastern Hunan are strongly peraluminous (SP) granites, with high ASI (>1.1) and high CaO/Na ₂O ratio (>0.3), suggesting that they might be derived from the partial melting of the psammitic sources in the Mesoproterozoic Lengjiaxi Group. The field geological, chronological, petrographical, geochemical, and isotopic features of these granites indicate that they are the products of post-collisional magmatism related to the breakoff of subducted slab, similar to the andesitic rocks from Baolinchong. The Neoproterozoic basic rocks from the western and central Hunan have the geochemical signatures of rifting basalts, with normalized patterns similar to ocean island basalts (OIB). Their occurrence in intraplate extensional environments is considered as indicators of the rifting of the area. Both the distribution style and field geological evidence of these Neoproterozoic basic rocks are not in line with a mantle plume model. Based on geochemical and petrological studies of the Meso- to Neoproterozoic basic-acid rocks, a preliminary model for the formation and evolution history of the western Jiangnan orogen in the area was put forward.