Petrology of Volcanoclastic Rocks from Baiyinchang and Palaeo-Marine Volcanism
白银厂的火山碎悄岩岩石学与古海相火山作用

Marine volcanic rocks are widespread over North Qilian Mountains in Northwest China, in which felsic volcanoclastic ones are mainly concentrated at two Neoproterozoic-Cambrian continental rift zones namely Qingshuigou-Bailiugou and Heishishan-Baiyinchang volcanic domes respectively. Felsic volcanoclastic rocks of various sorts are especially abundantly occurred at Baiyinchang dome, such as crystal tuffs, very fine-grained tuffs, agglomerates, breccia and patchy rocks, clastoporphyritic lavas of quartz keratophyre, as well as lava with pillow and ropy structures of keratophyre. In this paper, we deal mainly with crystal tuff, patchy rocks and pillow lava. As crystal tuffs have cyclothem structures with crystal size varying from bigger than 2mm to smaller than 1mm, and crystal contents varying from 30% to 60% and are geochemically similar to those of lava of the same origin, it is considered that the crystal tuffs are the products of subaqueous explosive eruptions which would produce billowing eruption columns in which vitric ash would be removed and thus crystals be concentrated. The chemical differences such as the loss of SiO 2 and alkaline and the increase of Al 2O 3, MgO, FeO*, might be attributed to the alterations of submarine hydrothermal circulating mineralization during or after volcanism. Patchy rocks are named for their patch structures, which made the rocks bearing brecciated appearances. Two types of patches are recognized here. The first one is termed tuffaceous patch which has silky lightness and is geochemically similar to that of tuffs (for example in REE patterns and hygromagmatophile element spider-diagrams). The other one is called chloritic patch which is composed mainly of chlorite(40%~60%), quartz, sericite, calcite and varying amount of pyrite. It is considered that patches of this type are altered matters of felsic volcanoclastic rocks related to the circulating mineralization during the periods of inactive volcanism. Furthermore, chloritic patches have similar patterns in REE and trace element spider-diagrams. Felsic pillow and ropy lava are first recognized here in Baiyinchang and rarely described in China, especially for Palaeozoic marine volcanic rocks. Researches on fluid inclusions in quartz of rocks and ores from Baiyingchang show that water depths may be greater than 1500m which is bellow the pressure compensate level of keratophyritic lava. Thus keratophyritic eruptions may become lava flows like those of basaltic lava on seafloor or rhyolitic ones on land to form pillow and ropy lava.