THE LATE TRIASSIC CARBONATITE TUFFS IN CENTRAL YANGTZE BLOCK, SOUTH CHINA: MANTLE SOURCE, CRUSTAL CONTAMINATION AND TECTONIC IMPLICATIONS

Carbonatites can derive from crystal fractionation, liquid immiscibility or melting of carbonate-metasomatized mantle, but the compositions of magmas, processes experienced during emplacement and tectonic setting are still debated. The Indosinian orogeny, as a part of amalgamation of microcontinents through Triassic in Southeast Asia, involves a Late Triassic Carbonatite tuff layer in central Yangtze Block, South China. These carbonatite tuffs are composed of calciocarbonatite and magnesiocarbonatite (10.64-32.48% SiO₂ and 25.38-42.38% CaO). They have similar REE pattern with light REE fractionation and strongly negative Ti and Eu anomalies (δ_Eu=0.48-0.83). However, they have low ε_Nd(t) values of -9.32 to -3.53 and relative high (⁸⁷Sr /⁸⁶Sr)_i ratios of (0.705961-0.708771), which distinguish them from most carbonatites around the world. U–Pb age data for 121 zircons from these tuffs range in age from Archean to the Late Triassic, showing a youngest and highest age population of 212 ± 1 Ma and subordinate peaks around 800 Ma, 1850 Ma and 2450 Ma. Additionally, the zircons grains of tuffs yielded initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280910 to 0.282813 and ε_Hf(t) values from −23.1 to 16.8. The REE patterns, HFSE geochemistry, NTT negative anomalies, Sr-Nd isotopes characteristics, major zircon age populations and Hf isotopic compositions of carbonatite tuffs suggest that the magma are derived by melting of a carbonate-metasomatized mantle with crustal contamination during emplacement. Consequently, we suggest a model of syncollisional rifting perpendicular to the Indosinian Orogeny to explain the origin of the tuffs and tectonic evolution of the Late Triassic Yangtze Block, South China.