Hydrothermal Dolomitization and Subsequent Retrograde Dissolution in Qixia Formation, West Sichuan: A Case Study of Incomplete and Halfway-Back Dolomitization
详细信息
- 责任者：Huang Sijing (State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation ; Chengdu University of Technology ; Chengdu 610059 ; China) ; Pan Xiaoqiang
- 刊名：Journal of Chengdu University of Technology
- 出版年：2013
- 卷期：40(3)
- 关键词：白云石化 ; dolomitization ; 四川 ; Sichuan Province
- 页码：p.288-300
- 图表：illus., 43 refs.
- 分类号：0803
- issnNo：ISSN1671-9727
- isbnNo：CN51-1634/N
摘要
The study of hydrothermal dolomite is significant theoretically and practically because of its particular mechanism.The Middle Permian Qixia dolomites in West Sichuan are essentially the fabric-destructive crystalline dolomites,including the non-planar anhedral and saddle dolomites that are related to the elevated precipitating temperatures.113 fluid inclusions in different dolomite phases have an average homogenization temperature of 136℃,varying from 67℃(planar subhedral dolomite) to 243℃(non-planar saddle dolomite).Most of the temperatures cover the range between 80℃ and 180℃.The analysis of burial history indicates that the geotherm could not provide such a high temperature range.The related thermal event forming the Emeishan basalt developed in West Sichuan at 259 Ma B.P.(late middle Permian) and had the paleo-heat flow of 60 to 80 mW/m2(higher than 100 mW/m2 for few drills),suggesting that the Qixia Formation in West Sichuan was influenced by the magmatism related to thermal effects during its shallow burial stage.Therefore,the hydrothermal process was the dominant dolomitizing mechanism in Qixia Formation of West Sichuan.The elevated temperature was caused by the extraneous thermal events,and overcame the kinetic obstacle for dolomite precipitations,especially under the conditions of low Mg/Ca ratios.The Qixia Formation in West Sichuan is mineralogically lack of carbonate rocks with dolomite content of more than 90%.And the dolomite content is essentially between 40% and 60% in rock samples,suggesting that the process of dolomitization was incomplete.The insufficiency of completely dolomitized bulks is the result of short duration of the related thermal event forming the Emeishan basalt(6 Ma,between 257±3 Ma B.P.and 263±5 Ma B.P.) and of the shortage of magnesium ions in strata.Based on the textural observation,the dissolutions and fragmentations are commonly found in Qixia dolomites in West Sichuan,especially in the saddle dolomite.Most of the dissolved pores in dolomites are filled by sparry calcite of lower homogenization temperatures(mainly between 70℃ to 80℃),suggesting that the saddle dolomites,after its precipitation,have experienced the temperature retrogression of 70℃ at least(and 150℃ at most).The fragmentation and dissolution of dolomites and the re-precipitation of calcites(i.e.dedolomitization) were caused by temperature retrogression and Mg-depletion.Therefore,the dolomitization in Qixia Formation of West Sichuan is a halfway-back process and produces the limy dolomites or dolomitic limestones.