中国东部中生代陆相火山岩型铁矿成矿背景和火山岩浆性质

本文系笔者在探讨火山岩型铁矿成矿地质背景过程中,对中国东部中生代陆相火山作用某些基本特征及与铁矿成矿作用关系的一点认识。笔者在另一文曾较详细论述了中国东部中生代火山岩各岩带、亚带、岩区和岩省火山作用及其产物的差异性,给出了各地火山岩平均化学成分,分出了碱性玄武岩-粗安岩-粗面岩(或响岩)、玄武岩-安山岩-英安岩-流纹岩和流纹岩-英安岩-(安山岩-玄武岩)三类火山岩组合及幔源型或幔壳混染型和壳源型两类不同成因火山岩;并指出玄武岩浆多形成于拉张构造环境,流纹岩浆往往发育于挤压构造环境,可把由某组分(上地幔和地壳)局部熔融形成的出熔岩浆之组分看作压力的函数(当然还有温度的影响);笔者还从中国东部中生代构造格局和地壳应力场特征探讨了中国东部不同地区中生代火山岩的成因。

Ore-Forming Background And Characte Ristics Of Magmas Of The Mesozoic Volcanic Iron Deposits In East China

This paper only discusses two problems concerning ore-forming background of the volcanic type iron deposits in the Mesozoic continental volcanic rocks of East China: 1) the types of the volcanic basins and their relation to the nature of magmas, and 2) the nature of magmas and their relation to the iron-forming process. There are about 200 relatively large volcanic basins in East China. These volcanic basins may be grouped into downwarping-faulted and downwaxping-faulted basins according to their forming tectonism, or divided into inherited and overlapping (including hemi-overlapping or hemi-inherited) basins based on the relationship between the Mesozoic volcanic sequences and the pre-Mesozoic basement. Generally speaking, the inherited basins, examplified by Ningwu and Luzong basins, are (early stage) downwarping- (late stage) faulted basins, characterized by intermediate to intermediate-basic volcanic rocks, which were formed in the intense stress field and are rather unique in East China. Most basins in East China are overlapping basins. The most typical ones are developed on the Precambrian rises, and are characterized mainly by intermediate-acid to acid volcanic rocks. The hemi-inherited basins are more complicated: some basins in Yanshan and western Shandong were formed in the tensile stress field and mainly developed intermediate volcanlic rocks, some basins in Daxinganling and Southeast China were formed in the compressive stress field and chiefly developed intermediate-acid to acid volcanic rocks. The magmas in Ningwu and Luzong basins, which are well-known for the presence of large-sized iron deposits, are intermediate-basic magma with more alkali and less silica and were crystallized and differentiated under relatively low oxygen fugacity. The author firmly believes that the low oxygen fugacity was an important factor for the differentiation of high FeO residual liquid during fractionation crystalluation and that the oxygen fugacity may be determined by Fe2O3/FeO value of volcanic rocks and composition of pyroxene and some other minerals. Compared with the intermediate-basic magma in Ningwu and Luzong basins, the andesitic volcanic magmas in other areas were not favorable for the formation of volcanic type iron deposits because of their more silica, less alkali (especially sodium) and higher oxygen fugacity. Besides, the volcanic iron deposits in the intermediate-acid to acid rocks are only discovered in Lishan, Guangdong and Shibatai, Hebei and were formed at the andesite stage in the process of evolution of the magmas, and at this stage the magmas had more alkali and lower oxygen fugacity. Therefore,it can be said that Ningwu and Luzong basins enjoy the exceptional advantages in the ore-forming background for the Mesozoic volcanic iron deposits in East China.