Geochemistry of the Meso- Neoproterozoic volcanic-intrusive rocks from Hunan Province and its petrogenic significances

The Meso-Neoproterozoic volcanic-intrusive rocks from Hunan in west section of Jiangnan orogen show very different geochemistry features. The Mesoproterozoic volcanic-intrusive rocks have narrow SiO₂ contents varying from 49.95% to 53.91%. They are mainly sub-alkaline basalts with low TiO₂ and Zr/Y ratios, high MgO, Ni, Cr, and Al₂O₃/TiO₂ ratios. They have different types of REE patterns, and display characteristics of arc volcanic rocks. The Neoproterozoic volcanic-intrusive rocks are alkaline and sub-alkaline basalts with broad SiO₂ varying 45.56% to 60.37%. They have high Zr/Y ratios and low MgO, Ni, Cr. Among them, the Neoproterozoic basic rocks are mostly alkaline basalts, with high TiO₂ contents and low Al₂O₃/TiO₂ ratios. They are obviously enriched in HFSE and show typical patterns of OIB, similar to the rift-basalts. Mesoproterozoic basalts from Nanqiao exhibit typical N-MORB features with high _εNd (t) values (6.86∼8.98 , and may represent the residual slab, which provides a new important evidence for Jiuling Island Arc in the middle section of Jiangnan orogen. Mesoproterozoic basalts from Yiyang show enrichment in Th and depletion in Nb and Ti with low _εNd (t) values (1.56 ∼1.76), similar to arc island volcanic rocks. On the other hand, they are enriched in MgO, Ni, and Cr with komatiitic basalts signatures, implying that they might be a product of interaction between the plume induced by subduction and the arc in the area. Early Neoproterozoic andesites from Yiyang are enriched in Th, La, Sm and Nd and depleted in Nb, Ti, Zr and Hf showing arc volcanic features. They suggest that they are the products of Grenville orogeny. Patterns normalized by primitive mantle of Neoproterozoic volcanic-intrusive rocks from Qianyang, Guzhang and Gaoqiao are similar to OIB, the _εNd (t) of basic rocks from Qianyang and Guzhang are 1.69∼1.05, indicating that they might be originated from asthenospheric mantle. The geochemistry of these Meso-Neoproterozoic volcanic-intrusive rocks give some implications for the evolvement of Rodinia from colliding to rifting in this area.