Geochemistry And Tectonic Evolution Of Alikaline Rocks In The Southern Margin Of The North China Craton

Alkaline rocks are widely distributed at the southern margin of North China Craton. These alkaline rocks intrusion in different times, with different origin. This paper chooses the Mesoproterozoic Longwangzhuang A-type granite, the Neoproterozoic Shuangshan hornblende syenite and Mesozoic Zhangshiying hornblende syenite and Taishanmiao A-type granite as the research object. Through the study of geochemical and zircon U-Pb and Hf isotope to explore its origin, tectonic implications. In order to investigate the evolution history of the southern margin of the North China Craton.The Mesoproterozoic Longwangzhuang granite has high silicon(SiO2=70.7 72.68%), high alkali (Na2O=2.45-3.54%, K2O=4.56-6.35%). In the K2O-SiO2 diagram the samples are ploted in high-k calc alkaline and alkaline shoshonite series. The fractionation of LREE and HREE is high with LREE/HREE=9.6-17.6, has obvious negative Eu anomalies. Enrich in Th, U, Rb, K large ion lithophile elements with depletion in P, Ti Ta, Zr high field strength elements. The characteristics of the Longwangzhaung granite has the typical of A type granite, such as high SiO2 (>70%), FeOt/MgO, (K+Na)/Al and 10000*Ga/Al ratio. The most important is its intruded in the environment of high temperature, the whole rock zirconium calculation results show that the crystallization temperature at 1030-1098℃. The Longwangzhaung A type granite is belongs to A1-type granites indicateing that the tectonic environment is within the plate. The A type granite is the product of partial melting of the tonalite crust, the residual phase is plagioclase. In the era of 1.6Ga. The Longwangzhaung A type granite intruded in the era of 1.6Ga which may be the response of the decomposition of the Columbia supercontinent.The zircon LA-ICP-MS U-Pb dating results of the Neoproterozoic Shuangshan hornblende syenite is 806+11 Ma. The Shuangshan hornblende syenite with low SiO2 content (54.1-58.3wt.%) indicating the intermediate rock; high alkali (Na2O=5.13-10.13%, K2O=2.52-8.83wt.%). In the TAS diagram all samples are ploted in the nepheline syenite. Enrichment in Rb, Th, U and other large ion lithophile elements (LILE). Ba has obvious depleted and weak depleted of K. Be different from the general rock of island volcano,the Shuangshan hornblende syenite has no Nb, Ta, Zr, Hf and HFSE (HFSE) depletion. Enriched in Nb and Ta indicates the original magma of Shuangshan are contributed from the primitive mantle components. Combined with Sr-Nd-Pb isotopic of the published from othes, we consider that the magma of Shuangshan is formed by the mixing of the crust magma and mantle magma, followed by crystallization differentiation. The Shuangshan hornblende syenite formed in the Neoproterozoic probably is the response of Rodinia supercontinent in the southern margin of North China.Here we report an integrated study of zircon U-Pb dating, Lu-Hf isotopesic data, and whole-rock chemical compositions on two plutons of the Zhangshiying (ZSY) and the Taishanmiao (TSM) two plutons in the southern margin of the North China Craton (NCC), in order to unravel their petrogenesis and geological evolution. Zircon U-Pb dating for the ZSY syenites and the TSM granites yield a weighted mean 206Pb/238U age of 122.8 ± 1.5 Ma and 112.1 ± 3.2 Ma, respectively. The ZSY syenite has high SiO2 (65.59-68.1wt.%), and Al2O3 (15.39-15.64 wt.%) contents with slight variation. It is characteristized by high Sr(468-666 ppm), Sr/Y(35-45) (La/Yb)N(20.1-22.1) ratios, yet low Y(13.1-16.6 ppm), Yb(1.465-1.778 ppm), and negligible Eu anomalies with depletion in Nb, Ta, P, Ti and Y. Which suggests that the syenite is close affinity with adakitic rocks. Zircons from this pluton give εHf(t) values from -17.6 to -5.7 with an average of -15.2. Their corresponding two stage model ages (TDM2) ages (1.29-1.91 Ga) are much younger than the basement rocks of the south margin of the NCC, indicatinge that the ZSY syenite was generated by partial melting of thickened lower crust with less involvement of juvenile crust as a result of underplating of the mafic magmas. In contrast, the TSM pluton has characters of A-type granite, such as, high SiO2 (69.7 to 71.87wt.%), total alkalis (Na2O+K2O= 8.5-9.4 wt.%), and HFSE(Zr, Nb, Ga, Y), but low CaO (0.27-1.25 wt.%), Ba, Sr and Eu. Their εHf(t) values rangs from -12.4 to -1.6 with an average of -7.6 and their TDM2 rangs from 1.10 to 1.63 Ga with an average of 1.38 Ga. zircon saturation temperatures range from 830℃ to 900℃ in these sampls. We propose that the TSM granites results from partial melting of tonalitic crustal sources with a plagioclase-rich residual at high temperatures and low pressure. Juvenile crust input, most likely underplated mafic magmas, is registed by the higher εHf(t) values than the precursor of TSM granites at the southern margin of NCC, which lends abundant heat.for their re-melting. Our results suggested that the thickened LCC is occurred prior to the intrusionve of the ZSY adakitic rockes at(～123 Ma). When the TSM granites are intrusive at ～112 Ma, implying an extensional environment, may constrained the (begin or end) time of lithospheric thinning and decratonization of NCC.The southern margin of the North China Craton has experienced at least three extensional tectonic cycles. During Mesoproterozoic the eastern part of North China continental block and the western continental block collision events corresponds to the Columbia super continent. During the Neoproterozoic the Shuangshan pluton is the signs of extension of super continent of the Rodinia. During the Mesozoic the North China and the South China plate collision and subsequent transition in the tectonic system in the southern margin of the North China. The Zhangshiying hornblende syenite was formed, and then the incursion of the Taishanmiao A-type granite represents the thin of lithosphere at the southern margin of the North China Craton...