| A great deal of alkali basic dyke swarms, spatially distributed in the trend of near NW-SE, are exposed in the Early-Paleozoic strata in south Qinling zone of Qinling orogenic belt, Ziyang— Langao area. Isotopic dating research on these basic dykes shows that they were formed in late Early-Paleozoic (471.4Ma413.1Ma). The dyke swarms can be divided into pyroxenite, diabase, gabbro-diabase. They, together with the snychronogenic trachyte and basalt distributed in the area, constitute a suite of bimodal volcanic rock association, indicating that an extension event of the crust occurred in this area during the end of Early- Paleozoic. The low Mg# of the rocks indicates that the basic dykes were products of magma which had undergone relatively high evolution degree. All these rocks are enriched in LREE with right inclined REE patterns. They, except for trachytes, have slight or none Eu anomalies, implying that crystallization of plagioclase as the main mineral facies did not occur during early evolution period of magma. The basic dykes are also characterized by relatively LILE and obvious Nb and Ta enrichment, as well as depletion of Sr, K and P in a trace element, similar to that of the OIB basalts. Most rocks are of high-Ti (TiO2%>4%, Ti/Y>500) characteristics which are as the same as the high-Ti basalts in Emeishan area. The dykes in the study area are characterized by high Ti (TiO2=1.61%7.03%, av. 3.89%), Zr (83×10-6388×10-6, av. 229 ×10-6) and Nb (16.95 ×10-696.05×10-6, av. 54.4×10-6), which are similar to the basic rocks (with average Ti 2.25%, average Zr 285 ×10-6 and average Nb 58 ×10-6) formed in the continental rift in East China during Cenozoic. In addition, except for the alkalic basic rocks, tholeiitic basalts are absent in the south Qinling area. The above evidences demonstrate that the dyke swarms were formed in the early phase of continental rift. Besides, all the rocks are of Th/Ta=0.731.16, Ta/Hf=0.471.11 which is accordant with those basalts from the mantle plume(Th/Ta<1.6, Ta/Hf>0.3). Thus the dyke swarms may be derived from enriched mantle source related to the mantle plume.ISr of all the rocks ranges from 0.702319 to 0.705549, ε Nd (t) ranges from 2.387302 to 5.088255. All rocks are plotted in the areas surrounded by the EM I, HIMU and BSE in a diagram of Sr-Nd, implying that their magmatic sources was related to the three-end members ofmantle. Pb isotope composition shows ^Pb/^Pb = 18.44-20.73, 207Pb/204Pb=15.596-15.751, 208Pb/204Pb=38.6641.22, with the same Pb isotope composition as C component (206Pb/204Pb=19.2-19.8, 2O7Pb/204Pb=15.55-15.65, 208Pb/204Pb=38.8-39). This further provides the evidences that the dyke swarms were trigged by the mantle plume magmatism. The high Nb/U, and negative correlation of 1/Sr to "Sr/^Sr display that they had not been contaminated with the crust during magmatic intrusion. The good correlation between Nb/Ba and ^Sr/^Sr implies that the lithosphere mantle had important contribution to the magma source. Trachyte has as the same isotope feature as the basalts, indicating that they were derived from the same source, and resulted from products during different evolution phases of the same magma source, which is similar to that of the basalts in Emeishan.Important activities of the mantle plume occurred in late early-Paleozoic in south Qingling have been documented in this paper, which could be an important part for deep dynamics in late early-Paleozoic in Qinling orogenic belt. Extension of the crust happened in late Paleozoic because of the activities of the mantle plume which might have important impact on formation of MianLue oceanic basin.
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