白银厂火山碎屑岩岩石学与古海相火山作用

Petrology of Volcanoclastic Rocks from Baiyinchang and Palaeo-Marine Volcanism

应用古火山地质学和岩石地球化学对白银厂中酸性火山穹隆内的凝灰岩、晶屑凝灰岩、中酸性枕状、绳状熔岩和具有特殊构造的补丁岩等火山碎屑岩进行了较为系统的研究 ,认为火山晶屑凝灰岩为酸性岩浆经爆发作用形成的火山碎屑流经细火山灰与富晶体碎屑部分分离后较快速度沉降并堆积成岩 ,产于火山喷口附近。海底成矿热液蚀变作用使其 Si O2 、Fe O、Mg O、CO2 等化学成分发生变化。凝灰质千枚岩则是细火山灰在海水中经缓慢的沉降后形成于远离火山口的火山斜坡上的火山 -沉积变质岩。根据“补丁”的成分可将补丁岩分为两种类型 :绿泥石质和火山凝灰质。其中绿泥石质补丁原为成矿热液在火山作用间歇期于喷流口附近因成矿蚀变作用所形成的薄厚不一的层状体或不规则体 ;凝灰质补丁的原始物质则是同时在火山口附近沉积的火山物质与海底沉积物之混合物 ,二者均被后期火山作用破碎并被其产物重新胶结而形成补丁岩。枕状角斑岩和“冻鱼状”角斑岩乃是同质岩浆在水深大于压力补偿深度条件下溢流相喷发产物 ,它的确定标志着对白银厂火山岩穹中火山作用认识上的进一步提高 ,也为建立白银矿田古火山机构、深入研究成矿作用提供了必要的基础。

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.