Petrogenesis and tectonic significance of Late Jurassic-Early Cretaceous volcanic-intrusive complex in the Tianhuashan basin, South China

The Late Mesozoic geological framework of South China is characterized by voluminous volcanic rocks and their intrusive counterparts. However, the ages, petrogenesis, and tectonic implications of the Late Mesozoic magmatism in South China, especially those in the Qinhang zone, are poorly constrained. The Tianhuashan basin is one of the important volcanic basins along the southern margin of the Qinhang zone, and is mainly composed of the Daguding and Ehuling Formations and their intrusive counterparts. Laser ablation-inductively coupled plasma-mass spectrometry (IA-ICPMS) U-Pb zircon data presented here in conjunction with our previously published data suggest that there are two distinct pulses of magmatism within the Tianhuashan basin. The granite porphyry was emplaced at similar to 158-157 Ma, whereas large-scale volcanic-intrusive rocks were formed at similar to 144-137 Ma, rather than in the Late Jurassic as previously regarded. The Tianhuashan volcanic-intrusive rocks have similar whole-rock element geochemical and Sr-Nd and zircon Hf isotopic compositions, suggesting that they were probably derived from similar source regions. These rocks exhibit clearly negative and variable bulk rock epsilon(Nd)(t) values in a range of -12.07 to -8.69 and low zircon epsilon(Hf)(t) values from -12.25 to +2.31, with Paleo- and Mesoproterozoic two-stage model ages for both Nd and Hf isotopes (1639-1915 Ma and 1051-1970 Ma, respectively), suggesting their derivation dominantly through partial melting of ancient crustal materials with involvement of a mantle-derived component in variable proportions and followed by differential assimilation of upper crust during magma ascent. An extensional setting associated with the roll-back of the Paleo-Pacific slab, is favored to explain the petrogenesis of the Tianhuashan volcanic-intrusive complex. The epsilon(Hf)(t) of zircon increases from Late Jurassic granite porphyry to Early Cretaceous magmatic rocks, suggesting progressive crust-mantle interaction in generating the episodic intermediate-felsic magma eruption during late Mesozoic. Our results suggest that the continental extension along the Qinhang zone was weak during Late Jurassic and large-scale lithospheric extension did not take place until Early Cretaceous with ongoing development of slab roll-back. (C) 2013 Elsevier B.V. All rights reserved.