龙门山地区凉水洞剖面中泥盆统观雾山组白云岩成因

Genesis of Middle Devonian Guanwushan Formation dolomite in Ganxi cross section in Longmenshan,Sichuan,China

以野外剖面实测与岩石学特征分析为基础,结合主元素、痕量元素、稀土元素和碳氧同位素等地球化学测试结果,对四川盆地龙门山地区中泥盆统观雾山组白云岩类型及其成因进行系统分析。该区主要发育残余结构白云岩和晶粒白云岩;晶粒白云岩进一步分为粉-细晶白云岩、中晶白云岩和粗晶白云岩,其形成受控于埋藏过程。残余结构白云岩和粉-细晶白云岩,由他形-半自形白云石晶体组成,具有较低的U/Th值和V/(V+Ni)值,Fe和Mn含量较高,Sr和Na含量较低,δLa正异常,δCe负异常,δEu无明显异常,δ13C值变化不大,包裹体均一温度较低,δ18O值明显偏负,形成于弱还原性浅埋藏环境下早成岩阶段A期;中-粗晶白云岩由他形白云石晶粒组成,具有较高的U/Th值和V/(V+Ni)值,Fe和Mn含量比细晶白云岩要高,Sr和Na含量也较高,δLa正异常,δCe负异常,δEu无明显异常,包裹体均一温度较高,δ13C值变化不大,δ18O值明显偏负,形成于还原性较强的深埋藏高温环境,应为晚成岩阶段A期成岩阶段。

The dolomite genesis of the Middle Devonian Guanwushan Formation in Longmenshan area,Sichuan Basin is studied based on field profile measurement,rock structure analysis,combined with analysis of major elements,trace elements,rare earth elements and carbon & oxygen isotopes.Guanwushan Formation in the area has two types of dolomites,relict texture dolomite and granular dolomite.Granular dolomite is controlled and formed during the burial process,and can be further subdivided into crystal powder-fine crystal dolomite,medium crystal dolomite and coarse crystal dolomite.Geochemical analysis shows that the residual structure dolomite and fine-grained dolomite which are composed of euhedral and subhedral dolomite crystals,are characteristic of lower U/Th and V/(V+Ni)value,higher Fe and Mn content,lower Sr and Na content,positive anomaly of δLa,negative anomaly of δCe,no obvious abnormal of δEu,minor variation of δ13C,obviously negative value of δ18O,with lower homogenization temperatures of fluid inclusions.It is considered that the Guanwushan Formation dolomites in Longmenshan area formed in weakly reducing shallow burial environment at early diagenetic stage A.Medium-coarse crystalline dolomite is mainly composed of subhedral dolomite.Geochemical analysis reveals that the dolomite has high U/Th and V/(V+Ni)value.The content of Sr and Na is high and the content of Fe and Mn is higher than that of finegrained dolomite.The medium-coarse grained dolomite possesses positive anomaly of δLa,negative anomaly of δCe,minor abnormality of δEu,minor variation of δ13C value,obvious negative value of δ18O,with high homogenization temperatures of fluid inclusions.This type of dolomite is considered to be formed in the deep burial high temperature environment with strong reduction at late diagenetic stage A.