海绿石的成因、指相作用及其年龄意义

Genesis,diagnostic role and age significance of glauconites

海绿石以独特的绿色自生色和球粒形状与围岩形成明显反差,关于其成因、演化、沉积和地层学意义目前存在3种普遍被认可并被采用的观点:海绿石是典型的海相沉积自生矿物,原地海绿石是"慢速、弱还原、较深水环境"的典型指相矿物之一;通常是海侵相的产物,含海绿石的地层在浅海沉积中常被作为海侵时期"凝缩段"及其相关沉积的识别标志之一;是沉积年代学中K_Ar、40Ar/39Ar年龄理想的测定对象。在采用海绿石作为典型指相矿物的过程中,应注意海绿石可以形成于多种沉积环境中,只有原地海绿石才能作为海侵时期"凝缩段"及其相关沉积的识别标志;海绿石年龄往往呈"年轻化"或"老化",没有火成岩定年准确,只有成熟、富钾的海绿石才是最好的定年对象。

With its unique green color and pellet-like shape,glauconite contrasts remarkably with the surrounding rocks.Currently there are three generally adopted concepts：glauconite is a kind of typical marine sedimentary mineral,special in-situ glauconite is one of the diagnostic minerals indicative of slow depositional rate,weak reduction and deep-water environment,glauconites are often deposited during the transgressive period,and therefore the glauconite-bearing formations in shallow sediments are often regarded as condensed section in the transgressive period.Glauconites are ideal measurement objects of K-At and 40Ar/39Ar ages.Their genesis,evolution,and sedimentary as well as stratigraphic significance have been widely used.It should be noted that glauconites can be formed in a variety of sedimentary environments,only in-situ glauconites can be used to identify the condensed section,but its age may be younger or older than the real age.