陆相硅质岩成因类型及构造—气候指示意义

Genesis of Continental Siliceous Rocks and Their Tectonic-climatic Significance

硅质岩形成于特定的地球化学条件,具有重要的构造—气候—成岩指示意义。我国硅质岩的研究主要集中于海相地层,陆相硅质岩虽分布广泛但研究却很薄弱,成因解释由于借鉴海相燧石经验,以地球化学分析为首要手段,结论存在片面性和单一性,可能会引起古老大陆重要气候—构造—环境信息的遗漏缺失。国内外陆相硅质岩全面调研表明,陆相燧石主要存在三种成因类型,包括准同生燧石、埋藏成岩燧石和表生成岩硅结岩,各类燧石岩石矿物学特征不同,具有独特的古气候—古地理指示意义,因此精细的岩石—矿物学分析是辨别陆相燧石的首要手段。蒸发背景和pH变化是三类陆相燧石硅质溶解、沉降和交代的主要原因,且陆相燧石可以当作一类特殊蒸发岩。

Many siliceous rocks are located at key stratigraphic intervals and can provide critical information regard⁃ing the formation of sedimentary basins,tectonic orogens,and climatic settings.Previous studies have mainly fo⁃cused on the siliceous rocks that developed within marine strata in China.However,in continental strata of China,si⁃liceous rocks also have a wide distribution.Affected by the rich experience in studying marine siliceous rocks,most previous work of continental siliceous rocks was mainly based on geochemical analysis and has attributed continental siliceous rocks to biological and hydrothermal origins.Since the occurrences of continental siliceous rocks and their host rocks are much different from those hosted in marine deposits,the application of marine chert formation models to continental siliceous rocks will lead to the bias of some important conclusions on climate,tectonics,and environ⁃ments.We review the genesis of continental siliceous rocks,and three types of continental siliceous rocks are intro⁃duced:penecontemporaneous cherts,burial diagenetic cherts,and meteoric diagenetic silcretes.Based on silica pre⁃cursors and formation settings,three penecontemporaneous cherts are identified:Magadi-type cherts,Coorong-type cherts,and Bogoria-type cherts.Magadi-type cherts are commonly associated with saline,alkaline lakes and may be bedded lake-wide or localized in nodular deposits,forming from a sodium silicate precursor such as magadiite(Na⁃Si7Oi3[OH]3•3H2O).Coorong-type cherts are deposited by direct Opal-crystobalite precipitation due to seasonal water pH variation and are also associated with saline,alkaline lakes.Bogoria-type cherts form from a siliceous,probably gelatinous precursor that precipitated around submerged hot springs during high lake level.Burial diagenetic cherts are formed during burial by replacing other minerals,mostly carbonates.Most of the silicification processes encoun⁃tered in continental carbonates are interpreted as having occurred during the early burial diagenesis of lacustrine or palustrine sediments.Meteoric diagenetic silcretes or siliceous duricrusts are defined as the indurated products of sur⁃ficial and penesurficial silica accumulation and are formed by cementation and/or low-temperature replacements of all types of rocks and sediments.Silcretes are further divided into pedogenic and groundwater silcretes.When studying continental silica rocks,petrological and mineralogical analysis are the first and foremost means for determining type.Bedded(>1 mm)and laminated(<1 mm)cherts are mostly formed during penecontemporaneous periods and are transformed from hydrous precursors,but the possibility of the total replacement of bedded carbonated by silica can⁃not be excluded.Slumping,soft-sediment deformation,V-shaped shrinkage,and brecciation can further verify pene⁃contemporaneous cherts.Characteristics of Magadi-type cherts that differ from Coorong-type cherts are(1)a ground⁃mass mosaic of fine quartz crystals that vary in orientation from random to rectilinear;(2)finely disseminated inclu⁃sions of silicate clays,zeolites,and/or carbonates(predominantly calcite);(3)large crystal molds(probably after trona)concentrated near sample margins;and(4)inward-directed shrinkage cracks and/or more irregular internal voids filled with chalcedony,silicate clays,zeolites,and/or carbonates(predominantly calcite).Nodular cherts are mainly diagenetic products.Based on the mold shapes and mineral remains in cherts,the replacement precursors of silica can be identified and the paragenesis between silica and other minerals can be rebuilt.Cauliflower cherts are generally lacustrine silcretes.Pedogenetic and groundwater silcretes mainly appear in the form of cements,filling voids and replacing the matrix between skeleton particles.The origins of continental cherts are quite varied.If water pH increases higher than 9,many materials in lakes can release dissolved silica into the water,like detrital quartz,silicates,and tuff materials.A pH of 9 is a key point for silica precipitation,above which silica solubility can in⁃crease exponentially.Temperature variation is not an important condition for increasing silica concentrations in conti⁃nental settings,as water temperatures are generally lower than 100℃in continental settings and not sufficient to greatly enhance silica solubility.In this regard,continental cherts are mainly associated with evaporative alkaline wa⁃ters.Evaporation and pH variations are the main drivers inducing the dissolution,precipitation,and replacement of silica in continental conditions.In most cases,continental silica rocks can be regarded as a special evaporite.