Petrogenesis of Cenozoic Basaltic Rocks from Jiangsu Province,China:Evidence from Geochemical Constraints

Cenozoic （Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt. We present major, trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area. On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province, we suggest that these basalts may belong to the same magmatic system. The alkali basalts found in Jiangsu province have higher ~FeO, MgO, CaO, Na20, TiO2 and P205 and incompatible elements, but lower A1203 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine, pyroxene and minor plagioclase. In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/~FeO ratios. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB. Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma. The 143Nd/144Nd vs. S7Sr/S6Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EM1 component in the study area. According to Shaw＇s equation, the basalts from Jiangsu province may be formed by 1%-5% partial melting of a depleted asthenospheric mantle source. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen （1982）, we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.

马里亚纳群岛和约翰逊岛近海区锰结核的地球化学

1 前言 最近几年,人们对在海洋环境中沉积的锰结核作为潜在矿床已有相当兴趣。以前的研究表明,在不同的海洋环境中形成的结核,其成分是相当不同的。 已提出各种各样的假说来解释海洋环境中锰结核及其相关沉积物的成因。深海锰结核的放射性化学年代测定是其中用来解决关于结核的起源及浓度史难题的方法之一。 尽管关于细菌性Mn^(2+)氧化作用的动态和物理学的研究指出了明显的微生物成分,但自然环境中微生物Mn^(2+)一非生物Mn^(2+)氧化作用的相对重要性尚未清楚地描述(Ghiorse,1988)。 本文目的旨在讨论马里亚纳群岛和约翰逊岛近海区的锰结核的矿物学和化学特征,以便弄清楚它们的起源。 马里亚纳群岛附近的锰结核采自153°20′E及23°N附近海山的约1200m的水深处(图 1),而约翰逊岛附近的结核采自169°32′W和16°26′N海底的约2400m的水深处(图1)。两个取样位置均位于太平洋板块上,靠近火山岛或海山。