| The volcanic rock series under the unconformable surface of Cretaceous red clasticrocks at Shengyuan basin in Xiajiang-Guangfeng volcanic rock belt, Jiangxi, are knownas Wuyi Group, and can be further divided into three Formations, i.e. Shuangfengling,E’huling and Shixi Formation. And andesites develop in the top of ShuangfenglingFormation. SHRIMP zircon U-Pb ages show: the206Pb/238U weighted average age of fouranalyzed points of zircon in andesites is448.7±2.2Ma, the206Pb/238U weighted averageage of five analyzed points is220.2±5.4Ma, and the206Pb/238U weighted average age ofseven analyzed points is129.8±2.3Ma. And the first two ages represent the ages ofcaptured zircons or residual zircons, and the latter stands for the forming age of andesites.The206Pb/238U weighted average age of eleven analyzed points of rhyolites in mid-upperShuangfengling Formation is134.3±1.6Ma, generally consistent with the youngest zircon206Pb/238U age of andesites within the error range, and corresponds with the stratasequence, further confirming the reliability that129.8±2.3Ma is the forming age ofandesites.Andesites at Shengyuan basin have porphyritic and massive structures, thephenocrysts are mainly plagioclase, biotite and clinopyroxene, and K-feldspar shell canbe occasionally seen in plagioclase phenocrysts. SiO2content of andesites is57.1%~59.3%(averaging in58.33%), content of (Na2O+K2O)(=5.54~6.94%, averaging5.94%), is high, plotting within the field of sub-alkaline series in the TAS diagram;(Fe2O3+FeO)(=6.96%~8.09%, averaging7.56%), content of MgO (=1.86%~2.5%,averaging2.23%) is low, Fe2O3/FeO=0.45~1.18(averaging0.76), plotting within thefield of tholeiite series in the FAM diagram; K2O(=3.14%~3.58%，averaging3.30%) has a high content, K2O/Na2O(=1.07~1.34, averaging1.26) is high, plotting within the fieldof high-K calc-alkaline series in the SiO2-K2O diagram, and belonging to the shoshoniteseries field in SiO2-K2O/Na2O diagram; Al2O3content id high(17.0%%~19.7%,averaging17.99%), TiO2content is relatively high (0.96%~1.17%, averaging1.06%).The total amount of REE (ΣREE=213~287×10-6)is relatively high, showing distinct richLREE ((La/Sm)N=3.93~4.37), and the values of Eu (δEu=0.90~1.17) and Ce(δCe=0.70~0.94) are not depleted; The lithophile elements (Rb, Sr, Ba, Th) enrichsignificantly, and high field strength elements as Sr and Nd are depleted enormously.(87Sr/86Sr)iis0.711266~0.712299, εNd value is-3.51~-5.35,206Pb/204Pb is18.12~18.35,207Pb/204Pb is15.59~15.90, and208Pb/204Pb is38.60~38.60.87Sr/86Sr(t)-SiO2and87Sr/86Sr(t)-1/Sr diagrams show positive relation, and in εNd(t)-SiO2å'ŒÎµNd(t)-1/Nddiagrams present no linear relationship, indicating the insignificance of continental crustcontamination during the process of magma uprising and the mixture in mantle area isthe result of the join of crustal materials. In143Nd/144Nd-206Pb/204Pb diagram, andesitesat Shengyuan basin are located near EMII enriched mantle, while87Sr/86Sr-206Pb/204Pband143Nd/144Nd-87Sr/86Sr diagrams show the trend of andesites is the same with EMIIenriched mantle, implying EMII mantle might have some contribution to the sourcedistrict. In the Ba/La-Ce/Yb, Cs/Rb-Ce/Yb, Rb/Ba-La/Yb,(La/Sm)N-(Ba/La)N,Zr/Ba-Ce/Y, Th/Yb-Ta/Yb and Th/La-Rb/La diagrams, andesites have distinctcharacteristics of mantle fluid alteration, indicating in early Early Cretaceous or evenearlier period, the fluids produced during the subduction of the ancient Pacific plate tosoutheast China alter the lithospheric-mantle, making the latter evolving into EMIIenriched mantle.In the Zr-Zr/Y, MgO-FeO-Al2O3and Th/Hf-Ta/Hf diagrams, andesites fall into theextensional tectonic environment.To sum up, andesites at Shengyuan basin are formed under the extensional tectonicsetting in early Early Cretaceous by partial melting of EMII enriched mantle and the joinof some crustal materials during its uprising process. |
PDF Full Text Request