显生宙的海绿石主要形成于具低沉积速率的外陆架至上斜坡深水环境,而前寒武纪海绿石则多见于具高沉积速率的浅水环境。为揭示导致这种明显差别的主要环境控制因素,本文对华北中元古代铁岭组叠层石礁内发育的海绿石进行了包括野外观察、岩相学和XRD、SEM、EDS、ICP-MS分析在内的综合研究。据海绿石的分布位置和基质孔隙度特征,本文将铁岭组自生海绿石分为3种类型:类型Ⅰ产于孔隙最发育的叠层石柱和微晶基质交接带,类型Ⅱ位于较少孔隙的基质中,类型Ⅲ常见于致密的叠层石柱体内。这三种类型的海绿石都具有高且稳定的K_2O含量(平均〉8%),指示孔隙水具有高K^+离子浓度。从Ⅲ型至Ⅰ型海绿石,Fe_2O_3含量逐渐升高,而Al_2O_3含量逐渐降低,表明基质孔隙度对海绿石演化具有控制作用,海绿石的成熟度演化需要Fe的持续补充。由于叠层石碳酸盐岩明显缺乏陆源碎屑输入,因此Fe可能主要来自海水,要求存在很浅的Fe氧化还原界面,这也得到了碳酸盐岩中Ce异常证据的支持。我们认为多孔的基质、高K^+浓度以及次氧化海水条件是促进中元古代浅水海绿石沉淀和演化的重要环境因素。
The origin of glaucony in the Phanerozoic sediments has been fairly understood,which generally results from marine authigenesis with slow sedimentary rate,especially in outer shelf environments.In contrast,the origin of glaucony in the Precambrian sediments is still poorly constrained,particularly that why it was commonly deposited in much shallower environments with relatively high sedimentary rate is not fully understood.To address this issue,we carried out an integrated study,including field,petrography,XRD,SEM,EDS,ICP-MS investigations on the glaucony and on its host carbonate in stromatolitic reef of the Mesoproterozoic Tieling Formation.Three types of autochthonous glaucony were recognized based on their distribution and substrate porosity,including type 1(in the most porous belt between stromatolitic column and micritic matrix),type 2(in less porous matrix)and type 3(in dense stromatolitic column)glaucony.They were all characterized by constant and high K_2O contents(av.〉8%),indicating initial high K+concentration in pore water,and an evolution from type 3 to type 1 glaucony by addition of total Fe_2O_3 and simultaneous release of Al_2O_3.These features likely suggest that substrate permeability has controlled the glaucony evolution,and glauconization requires persistent supplement of Fe.Since the stromatolitic carbonates are basically devoid of detrital inputs,Fe may have been largely sourced from seawater.In this case extremely shallow Fe-redox chemocline was required,which is also supported independently by Ce anomalies in the carbonates.Thus,we proposed that porous substrate,high K^+ concentration and suboxic conditions in the Mesoproterozoic shallow seawaters have favored the precipitation and evolution of glaucony.