| The Wuyi-Yunkai orogen (called an early Paleozoic orogenic belt in South China) is an early Paleozoic intraplate orogen in the southeast China, which is a major orogenic belt in the East Asia. It formed at a similar time as the classic Caledonian orogeny in Europe and reflected an important tectonic event.Wuyi-Yunkai orogen often evolve syn-orogen and post-orogen, one salient feature of the orogen is that despite the wide occurrence of syn-to late-orogenic granites and high-grade rocks in the orogen, syn-to late-orogenic volcanic rocks and mafic- uiltramafic rocks have been poorly discovered. Ultramafic rocks are derived from mantle and provided important information of the mantle, thus the studies of the ultramafic rocks help us to understand the nature of the mantle, the crust-mantle interaction, evolutionary process and the dynamics during orogenic event. Their chronology provided by ultramafic have an important time constraint for recovering the structural evolution process of the orogen.Gannan ulramafic intrusions are located in the northern brach of the early Paleozoic Wuyi-Yunkai orogen, which is exposed in Songshutang and Wushitou region. Songshutang ultramafic rock type mainly consists of serpentite (protolith is peridotite), wehrlite, olivine websterite and hornblendite. Wushitou ultramafic rocks consist of wehrlite. The zircon U-Pb dating of the Songshutang serpentite, olivine websterite, wehrlite and Wushitou wehrlite gave consistent magmatic ages of 437.8±2.1 Ma,437.3±2.2 Ma,437.6±2.2 Ma and 437.5±2.3 Ma, respectively. Such ages are synchronous with the widespread, commonly late-orogenic felsic magmatism dated mostly at the 440 Ma-415 Ma range and are concordance of Chayuan basic-inter mediate volcanic rock magmatic ages (435 Ma), which represent one tectonic thermal event in Caledonian in Wuyi-Yunkai orogenic belt and constraint the time of the collapse during the extension of post-orogen.The Songshutang and Wushitou ultramafic rocks have low SiO2 (38.87%～43.97% for Songshutang; 33.25%～46.08% for Wushitou) and TiO2 (0.24%～0.47% for Songshutang; 0.07%～0.12% for Wushitou) contents, but high MgO contents (16.37%-25.85%; 26.18%～ 30.12%), suggesting that they are tholeiitic series. They have low rare earth element (REE) contents and show slightly LREE enrichments with negative anomalies of Eu. They enriched in large ion lithospheric element (LILE:Rb, Ba, Th and U) and Cr and Ni, but depleted in high field strengthen element (HFSE:Nb, Ta and Ti). They have high (87Sr/86Sr)i but low εNd(t) and εHf(t) (Songshutang ultramafic rocks:(87Sr/86Sr)i=0.70435～0.70753,εNd(t)=-8.30～-9.20,εHf(t)=-2.56--10.93; Wushitou ultramafic rocks:(87Sr/86Sr)i=0.71336～0.71463,εNd(0=-5.42,εHf(t)=-5.18～-8.17). Their model ages are older than their formation ages, show feature of enriched-lithospheric mantle.A comparative study on Sr-Nd isotopic composition indicate that the Yangtze could exsit anciet enriched-lithospheric mantle. Combination with the trace element features and Sr-Nd and Hf of zircon isotopic composition, we suggest that the magma could be derived from the lithospheric mantle metasomatized by subduction fluids. The Songshutang and Wushitou ultramafic rocks display features of high concentrations of MgO, Cr, Ni, and low SiO2, diagram plotting Nb/U vs SiO2/MgO and Nb/La vs MgO are no correlation indicated that they were suffered negligible amount of contamination by crust during the ascent of the magma The good correlations between MgO and indicate the magama experienced fractional crystallization of the clinopyroxene and olivine during the formation of the magma.Combined above petrographical and geochemical studies with the orogenic root delamination model, we believe that the ultramafic rocks could be products of the partial melting of the metasomatized lithospheric mantle, which were heated by the asthenosphere upwelling resulting from the lithospheric delamination during the post-orogen in the Wuyi-Yunkai orogenic belt. The asthenosphere upwelling, basaltic underplating and regional decompression after the collapse could explain the widespread occurrence of the post-orogen granite along the orogenic... |
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