The Geochemical Characteristics And Geological Significances Of The Basic Dike Swarms And Syenite Porphyry Veins In The Ziyang,South Shanxi

A large number of alkali dike swarms are exposed in the Early Paleozoic strata in arcuate structure of Daba mountains of south Qinling zone of Qinling orogenic belt, ziyang area, they intruded into bedding and distributed near NW–SE with the same trend of the regional strata. The rocks of alkali dike swarms include gabbro, diabase, gabbro–diabase and syenite porphyry. The SHRIMP baddeleyite U–Pb dating of gabbro of Li Jia Gou area manifest that the basic rocks crystallization age were 415.2±6.7Ma, meanwhile,we obtained 432.9±6.2Ma intrusion age of the syenite porphyry veins by the SHRIMP zircon U–Pb dating,combining with the previous’ dating results and stratigraphic age information of this area, we get a conclusion that the basic rock and syenite porphyry were formed in the Early Silurian. The basic dike swarms and syenite porphyry veins constitude a suit of typical bimodal dike swarms, indicating an extension event of the crust occurred during the end of Early–Paleozoic.The Mg# of the basic rock range form 32.29 to 55.55 and the syenite porphyry range form 28.22 to41.47, which demonstrated that the basic rock and syenite porphyry were product of primitive magma which had undergone relatively high evolution degree. The main element geochemical characters of the basic rock include: relative low silicon(SiO2=40.64%~52.27%), high Ti(TiO2=2.51%~4.35%), enrich Na(Na2O=2.10%~4.86%) and high alkalinity rate(AR=1.25~2.04). Harker diagrams show that the clinopyroxene and magnetite were the main fractional crystalline mineral during evolution of magma. The basic rocks is similar to the OIB basalt REE distribution patterns, and no negative Eu anomalies which suggest that crystalline of plagioclase as the main mineral facies did not occurred during evolution of magma. The basic rocks also characterized by raising a peak of Ba, Nb and Ta, as well as weak in the valley of Rb, K, Ti with uplift right–inclined in primitive mantle–normalized element distribution patterns.The syenite porphyry belong to metaluminous with SiO2 contents of 59.62%–64.30%, alkali contents of10.13%~12.11%, and Aluminum Saturation Indexes of 0.88–1.8. Its ∑REE contents range from120.86×10-6 to 421.52×10-6, with(La/Yb)Nvalues ranging from 17.50 to 34.36 and LREE/HREE ranging from 9.13 to 17.45, weak positive Eu anomalies(δEu=0.94–1.15, average in 1.05), and the feature of relative LREE enrichment and HREE depletion in the right–declined REE distribution patterns. Comparing to the primitive mantle values, it is characterized with the enrichment of LILEs(such as Cs, Ba), and HFSEs(especially Nb, Ta, Zr, and Hf).The basic rock and the syenite porphyry both showed high radiogenic Sr and Pb, low Nd isotopiccomposition characteristics. Combineing with Sr, Pb, Nd isotope discrimination diagram, we draw a conclusion that the basic rock and syenite porphyry were derived from enrichment mantle source region with similar OIB, have properties of the mixture components with EMⅡand HIMU. They were not affected by contamination of continental crust during magmatic intrusion, according to related diagrams(such as SiO2 vs 87Sr/86 Sr, SiO2 vs Ta/Nb, 1/87 Sr vs 87Sr/86Sr) and trace element ratios(such as Nb/U, La/Nb,La/Nb, Zr/Nb, Rb/Nb). The good correlation between Nb/Ba vs 87Sr/86 Sr suggest that ancient enriched lithospheric mantle had effected on the magma source. The special geochemical nature of source region should be related to its source area matter mantle metasomatism. The characteristics of the basic rock and syenite porphyry rock`s major element, trace element and isotopic indicate that the basic rock magma originated in mixed magma with main garnet–lherzolite and a small amount spinel–lherzolite low partial melting under high pressure environment, the porphyry magma originated in the enrichment of the mantle magmatic independently and had a certain genetic relationships with the basic rock magma source.The characteristics of major element and trace element tectonic environment discrimination diagram indicated that the basic rock and the syenite porphyry were formed in a tectonic rift environment in the continental, basic rock samples all drop–in intracontinental rift alkaline basalt and mantle plume basalt mixed area in Ta/Hf vs Th/Hf and Th/Nb vs Nb/Zr diagrams, indicating that the set of bimodal dike swarms have the cause of mantle plume. Mantle plume activity may be the main driving force for tectonic evolution in south Qinling region.