Geochemical properties of the variolitic andesite from Susong in the Dabieorogenic belt, China

The variolitic andesite from the Susong County in the Dabie Mountains impli es that it was erupted in water. The mineralogy of the varioles is primaril y radiate plagioclase (albite sind oligoclase), with little pyroxene, hornb lende and quartz (derived from alteration). The pyroxene, hornblende and qu artz are in the interstices between plagiocalse. The matrix consists of gla ss, hornblende, chlorite, epidote and zoisite. It is clearly subjected an e xtensive alteration. The andesite has an uncommon chemical composition. The SiO2 content is about 56.8%, TiO2 = 0.9%, MgO = 6.4%, Fe2O3 (tot) = 6.7%si milar to 7.6%, 100Mg/(Mg + Fe) = 64.1 similar to 66.2. Mg-# is significantl y high. The andesite has high abundances of large-lithophile trace elements (e.g. K, Ba. Sr, LREE), e.g. La/Nb = 5.56 similar to 6.07, low abundances of high-strength-field elements (HFSE e.g. Ta, Nb, P, Ti), particularly Ta and Nb strongly depleted. These are consistent with the characteristics of subduction-related magmas. In the spider diagram of trace elements, from Ce to right hand, the abundances of elements decrease quickly, showing a char acter of the continental margins. There has a strong punishment of light-ra re-earth elements, with a significant diffraction of REEs (the mean value o f (La/Yb)(N) is 32.84). No Eu anomaly, but there are anomaly high (La/Yb)(N ) = 28.63 similar to 36.74, (La/Y)(N) = 70.33 similar to 82.4. The elements Y and Yb are depleted greatly, Y<20 <mu>g/g, Y-N = 2.74 similar to 2.84, Y b-N = 2.18 similar to 2.35. From the La-(La/Sm) diagram, the andesite is de rived from partial melting. But the epsilone value of Nd is - 18.7 similar to -19.2, so that the material source may be the mantle materials affected by the crustal materials. The Nd model age is 1.9 Ga indicating that the va riolitic basaltic andesite was resulted from the mantle wedge of North Chin a block, which had the Nd model age of 2.5Ga, when the Yangze block which h ad the Nd model age of 1.7Ga subducted beneath it. So the variolitic andesi te has characteristics of the island-are volconic rocks oil a continental b asement in the vicinity of the destructive continental margin.