Petrology of volcanoclastic rocks from Baiyinchang and Palaeo-marine volcanism.

Marine volcanic rocks are widespread over North Qilian Mountains in Northwe st China, in which felsic volcanoclastic ones are mainly concentrated at tw o Neoproterozoic-Cambrian continental rift zones namely Qingshuigou-Bailiug ou and Heishishan-Baiyinchang volcanic domes respectively. Felsic volcanocl astic rocks of various sorts are especially abundantly occurred at Baiyinch ang dome, such as crystal tuffs, very fine-grained tuffs, agglomerates, bre ccia and patchy rocks, clastoporphyritic lavas of quartz keratophyre, as we ll as lava with pillow and ropy structures of keratophyre. In this paper, w e deal mainly with crystal tuff, patchy rocks and pillow lava. As crystal t uffs have cyclothem structures with crystal size varying from bigger than 2 mm to smaller than 1mm, and crystal contents varying from 30% to 60% and ar e geochemically similar to those of lava of the same origin, it is consider ed that the crystal tuffs are the products of subaqueous explosive eruption s 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 SiO2 and alkaline and the increase of Al2O3, MgO, FeO*, mig ht be attributed to the alterations of submarine hydrothermal circulating m ineralization during or after volcanism. Patchy rocks are named for their p atch structures, which made the rocks bearing brecciated appearances. Two t ypes of patches are recognized here. The first one is termed tuffaceous pat ch 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 chlor ite(40%similar to 60%), quartz, sericite, calcite and varying amount of pyr ite. It is considered that patches of this type are altered matters of fels ic volcanoclastic rocks related to the circulating mineralization during th e periods of inactive volcanism. Furthermore, chloritic patches have simila r patterns in REE and trace element spider-diagrams. Felsic pillow and ropy lava are first recognized here in Baiyinchang and rarely described in Chin a t especially for Palaeozoic marine volcanic rocks. Researches on fluid in clusions in quartz of rocks and ores from Baiyingchang show that water dept hs 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.