| The volcanic rock series well developed in eastern Nanling Region is is a suite of dark gray andesite, light grey dacite, Porphyroclastic Lava intercalated with gray dacitic tuff, volcanic brecciated tuff, celadon fina grain feldspar quartz sandstone, sericite slates, silty slate, phyllites in the Nanjing basin, Southern Jiangxi areas. Because of the question of outcrop, it has been regarded to as a apart of the mesozoic volcanic rocks for a long-term, or being considered the Yutian Group in Early-Middle Jurassic, or being considered the Wuyi Group in Late Jurassic, or being considered the Heshui Formation and Yousheng Formation in early late cretaceous. The research about Qiaozishan section and Zhongzhai section in Nanjing basin shows that the volcanic rock series is conformably contact with the Huangzhudong Formation, underlies unconformably Middle Devonian Yunshan Formation, Establishment the Nanjing Formation, and placed at the top of the Longtouzhai Group. SHRIMP zircon U-Pb dating indicates that the 206Pb/238 U age range of fourteen analysis points of the andesite is 460 Ma~425 Ma with the weighted average of 442.1Ma±3.9 Ma and MSWD of 0.75, analysis points of the fifteen dacite is 450 Ma~425 Ma with the weighted average of 439.9Ma±3.7Ma and MSWD of 0.50. It indicate that Nanjing Formation belongs to the Late Ordovician-Early Silurian.The Andesite at Nanjing basin have porphyritic and massive structures, the phenocrysts are mainly plagioclase, biotite and hornblende. The content of Si O2 range of 57.9%~61.7%(the average is 59.47%),(Na2O+K2O) =3.94%~5.76%, the average is 4.90%, K2O/Na2O(=1.19~2.45, the average is 1.87) has a high ratio, plotted within the field of sub-alkaline series in the TAS diagram; The content of Mg O=2.68%~5.49%(the average is 4.37%), Mg#=41.7~54.9(the average is 48.6),(Fe2O3+Fe O)(=5.56%~8.72%, the average is 7.39%), plotting within the field of tholeiite series in the FAM diagram. Ti O2 content is relatively low(0.67%~0.79%, average is 0.76%), Al2O3 content id high(14.26%~16.08%, average is 14.86%), Ca O range of 3.94%~6.33%(the average is 4.93%). The total amount of REE(ΣREE =121~217×10-6) is relatively low, showing distinct rich LREE, negative Eu anomaly is obvious(δEu=0.60~0.76), and the shape of chondrite-normalized REE patterns inclines towards the right. The lithophile element(Rb、K) and REE has enrichment among trace element and depleted Nb, Ta, Zr, Hf and Ti. The low ratios of Nb/U, Nd/Th and Ti/Yb, and Pb anomaly, which show the earth’s crust were added. The(87Sr/86Sr)i value is 0.704265~0.704933,(143Nd/144Nd)i is 0.511881~0.511915, εNd(t) is-3.01~-3.67, and the singlestage Nd model ages(TDM2) vary from 1424 to 1478 Ma. In 87Sr/86Sr(i)-Si O2, εNd(t)-Si O2 and 87Sr/86Sr(i)-1/Sr diagrams present no linear relationship, show that the magma has less continental crust added during rising process. In(87Sr/86Sr)i-(143Nd/144Nd)i and εNd(t)-(87Sr/86Sr)i diagram, andesites at Nanjing basin are located near EMI enriched mantle, implying EMI mantle might have some contribution to the source district.Trace element and tectonic environment diagrams(Zr-Zr/Y, Th/Hf-Ta/Hf, Th/Zr-Nb/Zr and La/ Zr-Nb/Zr) show that andesites fall into the extensional tectonic environment. The andesite magama may be produced by the partial melting and fraetional crystallization from the juvenile lower crust which formed by melting between mantle derived magma and small amount of ancient lower crust for the Early Palaeozoic period. |
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