中国东部中生代火山岩中沸石岩的地质特征和成因

天然沸石是岩石圈上部分布最广的硅酸盐矿物之一,它普遍地见于各种火山岩和火山-沉积岩中。沸石不仅在岩浆期后热液阶段生成,而且在沉积-成岩作用阶段、表生作用(风化)阶段、低级变质阶段都可以生成。由于沸石对周围的物理化学条件和热、动力变质作用反应灵敏,因此,它是了解地质体形成时的地质环境及其变化的良好的指示矿物。本文概述了我国东部中生代火山岩中沸石岩的分布、时代、产出的地层岩性剖面、原岩类型、矿物组合及其生成顺序,并探讨了它的形成条件和成因类型,其中主要涉及具有工业意义的斜发沸石岩和丝光沸石岩。

GEOLOGICAL FEATURE AND GENESIS OF ZEOLITITE IN THE MESOZOIC VOLCANIC ROCKS IN EAST CHINA

Eastern region of China belongs to part of the Circum-Pacific orogenic belt.More than one hundred localities of zeolitites (mostly clinoptilolite rocks and or mordenite rocks) have been found in this region.The zeolitites occur in the acid volcanic rock and the tectonic basins (mainly faulting depression basin) of Mesozoic age.The stratigraphic times of zeolitite are Middle Jurassic-Upper Cretaceous,and a considerable amount of zeolitites are contained in the Upper Jurassic and Lower Cretaceous strata.The times of strata containing zeolitites tend to become younger from west to east.The lithologic profiles of strata occurring zeolitites may be classified into three types: (1) the strata consisting of eruptive-sedimentary facies series in which normal sedimentary bed (sandstone,conglomerate,shale) is interbeded with volcanic clastic bed (tuff,tuff breccia);(2) the strata consisting of eruptive-flooding facies series in which volcanie lava (rhyolite,spherulitic rhyolite,perlite,flow-breccia) is alternated with pyroclastic rocks (tuff,tuff breccia);(3) the strata consisting of flooding-erup-tive-sedimentary facies series.The primitive rocks altered to zeolitites are mainly perlite,flow-breccia,tuff,tuff breccia,etc.Natural zeolites occur as an alteration products of volcanic glass in the abovementioned rocks.The original textures remain well in the zeolitized rocks;.Mineral assemblages of zeolitites are possessed of similarity to a great extent.During alteration process of volcanic glass the minerals formed are clinoptilolite,mordenitp,montmorillonite,celadonite,cristobalite,chalcedonite,quartz,water-bearing iron oxides,and so on.A few occurrences of heulandite and analcime have been found.In addition,there exist residual minerals of primary rocks.In general,the formation succession of the altered minerals from volcanic glass is as follows:volcanic glass→montmotillonite celadonite→cristobaloe chalcedonite→mordenite,clinoptilolite→quartz water-bearing iron oxides Because of metastability the zeolites may further be altered to montmorillonite and clay minerals in a favourable condition.The spatial distribution of mineral assemblages of zeolitites display indistinct zonation.According to the formation condition of zeolites and the geological feature reflected by the zeolitites occurrence,we consider that the zeolitites in Mesozoic volcanic rocks occurring in eastern region of China have been formed by the reaction of acid volcanic glass with water medium in hydrologic open system.Here two genetic subtypes may be distinguished: (1) One is referred to the zeolitites,in freshwater lacustrine basin remote from volcanic passage,formed by the reaction of shardeshaped glass in pyroclastic rocks with the lacustrine water during diagenetic process.The zeolitites of this kind have commonly been found in the foregoing first strata,and the zeolite formed is normally clinoptilolite.The zeolitites of this genetic subtype occur stratiformly and psudostratiformly,with great quantity and homogeneous quality relatively.(2) The other involves the zeolitites,formed by the reaction of vitric volcanic lava (in a few cases,vitric pyroclastic rocks were alte mated with the lava) eiosed to volcanic passage after its formation with percolating groundwater or hot water originating in later stage of voleanism.This zeolitites are frequently related to the second strata.Mordenite is usually the characteristic of the zeolitites of this genetic subtype.The zeolitites occur lenticularly and psudost-ratiformly,mostly with small in quantity and varied in quality.Mineral species (clinoptilolite,mordenite,montmorillonite,celadonite,etc.) altered from volcanic glass depend chiefly on the formation temperature and component concentration in water medium.