摘要
白云岩成因机制的核心问题之一是白云石化流体的来源。本文选择塔北东部地区下奥陶统蓬莱坝组白云岩、云质灰岩作为研究对象,在沉积学和岩石学基础上对基质白云岩的稀土元素地球化学特征进行了分析研究。结果表明,白云岩、云质灰岩中Sc、Th、Hf、Zr等元素的含量远远低于这些元素在陆源沉积物中的含量,并且Th、Zr的含量与Y/Ho比值间不具有明显的正相关关系,La的正异常、较高的Y/Ho比值等特征均表明塔北东部下奥陶统蓬莱坝组碳酸盐岩形成的环境为未受陆源物质混染的浅海相沉积。而白云岩负Ce异常、正La异常、轻微富集的Gd正异常以及La正异常与Y异常的正相关性等典型稀土元素特征均反映了白云石化流体具有典型的海水来源特征;而基质白云岩与白云质灰岩之间的∑REE+Y,∑LREE和∑HREE含量差异表明,在白云石化过程中稀土元素发生了一定程度的流失,且HREE较之于LREE更易于发生流失,因此白云石化过程中成岩流体对HREE的移出量大于LREE的移出量是塔北东部下奥陶统白云岩具有相对富集的LREE及相对较低的Y/Ho比值的主要原因。结合岩石学、沉积学、微量元素以及稳定同位素特征的综合分析认为,塔北东部地区下奥陶统蓬莱坝组基质白云岩是海水白云石化作用的结果。个别白云岩样品的Eu正异常以及同层位构造裂隙方解石的δ^(18)O、^(87)Sr/^(86)Sr以及流体包裹体均表明下奥陶统蓬莱坝组受到后期热液流体的影响,但其对研究样品基质白云岩的影响程度比较弱。
The mechanism of dolomitization is still problematic in carbonate sedimentology, and it is also a hotspot in carbonate diagenesis. One of the keys in the processes of dolomitization is the origin of diagenetic fluids. Based on detail study of sedimentology and petrology, the authors analyzed BEE of the matrix dolomites and dolomitic carbonates of Penglaiba Formation of Lower Ordovician in eastera Tabei area. Lower concentrations of Se, Th, Hf, Zr, positive La anomaly, relatively higher ratio of Y/Ho show that carbonate sediments of Lower Ordovician in eastern Tabei area deposited in shallow marine environment that are free of terrigenous particles The characteristics of positive Ce anomaly, slightly positive Gd anomaly, positive La anomaly, positive Y anomaly, the positive correlation between positive La anomaly and Y anomaly indicate that matrix dolomites have resemble REE characteristics with sea-water and the diagenetic fluids of dolomitization probably originated from seawater. However, the different Sigma REE + Y, Sigma LREE and Sigma HREE between matrix dolomites and dolomitic limestones infer that REE was removed and HREE is more easily removed than LREE during dolomitization. Therefore, marine diagenetic fluids should be responsible for the relative enrichment of LREE and lower Y/Ho ratios, which are contrary to the REE characteristics of modern sea-water. The characteristics of petrology, sedimentology, trace elements and stable isotopes show that matrix dolomites of Lower Ordovician are products of marine dolomitization, and some samples which hove slightly negative Eu anomaly infers the existence of hydrothemal fluids, and the Lower Ordovician fracture filled calcites, which characterized by very negative delta(18) O, higher Sr-87/Sr-86 and some very high homogeneous temperature of fluid inclusions also indicate the relatively limited influence of hydrothermal fluids on the matrix dolomite.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第10期2405-2416,共12页
Acta Petrologica Sinica
基金
国家基础研究发展计划973项目(2006CB202304)资助.
