塔东北库鲁克塔格中上寒武统白云岩岩石学、地球化学特征与成因探讨——与加拿大西部盆地惠而浦(Whirlpoolpoint)剖面对比

通过对比研究塔里木盆地东北缘库鲁克塔格隆起的乌里格孜塔格剖面与加拿大西部盆地寒武系白云岩的岩石学、地球化学及流体包裹体等特征,阐明了前者经历了三次主要白云岩(化)作用事件.①成岩早期-浅埋藏的粉细晶白云岩作用,Sr为42.75×10-6,Mn为315×10-6 ～506×10-6,Fe为0.2650％ ～0.403％;δ13 CpDB=一0.47‰ ～-0.28‰,δ18OPDB=-7.3‰ ～-7.43‰,87Sr/86Sr=0.71085;平均REE=18.84×10-6,δEu平均为0.66、δCe平均为0.81,估算的形成温度分别为47.6℃和55.12℃;②中(深)埋藏的细晶及中(粗)晶白云岩化作用;Sr为56.27×10-6,Mn为312×10-6,Fe为0.13％;δ13CPDB=-1.5‰,δ18OPDB=-8.5‰,87Sr/86Sr=0.7100; REE=18.08×10-6,δEu平均为0.72、δCe平均为0.77,盐水包裹体中的均一温度平均为108.34℃,盐度变化为4％～13.7％ NaCleqv;③沿裂隙或溶洞形成的粗-巨晶或鞍形的热液白云岩(石)交代充填:Sr为29.1×10-6 ～49.9×10-6,Mn为498×10-6～754×10-6,Fe为0.15％ ～ 1.14％;平均δ13 CPDB=-0.48‰,δ18OPDB=-8.82‰,87Sr/86Sr=0.70996;平均REE=15.08×10-6,δEu平均为0.70、δCe平均为0.78;鞍形白云石中的盐水包裹体的均一温度120～150℃,盐度为5％ ～ 12％ NaCleqv;溶洞中的粗-巨晶白云岩盐水包裹体的均一温度140～180℃,盐度为5％ ～ 18％ NaCleqv;与西加盆地惠而浦(Whirlpool point)剖面典型的热液白云岩相比较,乌里格孜塔格剖面中寒武系白云岩中缝洞中的粗-巨晶及鞍形白云石中的碳氧同位素、稀土总量、轻重稀土比均要高,但有序度、δEu、δCe、盐水包裹体形成温度及盐度相对低;因而推断它与西加盆地典型的岩浆期后热液来源不同,其流体来源于沿深部伸展走滑断裂-再循环地层热卤水,与海西晚期或燕山期-喜马拉雅期强烈挤压后弱伸展引起的二期或以上的压扭-走滑构造有关.

The petrographic and geochemical signatures and implication of origin of the Middle and Upper Cambrian dolostone in eastern margin Tarim: Comparative studies with the Whirlpool point of the Western Canada Sedimentary Basin

Comparative studies fault /fracture controlled the hydrothermal dolomitization,occurred pervasively in Cambrian at the Whirlpool point outcrops,the Western Canada Sedimentary Basin,with regard to petrographic,geochemical,isotopic and fluid inclusion demonstrated that three major dolomitization events in the Middle Cambrian Moheshan Group and Upper Cambrian Tereshakeitaga Group at Wuzilitaga outcrop,Northeast Tarim basin,have been recorded. ( 1) Near-micritic dolostone and fine-medium crystalline dolostone,formed at low temperatures during early stages of diagenesis,similar to the replaced host rock ( "matrix") in their succession and geochemical signatures; with negative value δ 13 C PDB ( - 0. 47‰ ～ - 0. 28‰) and δ 18 O PDB values ( - 7. 3‰ ～ - 7. 43‰) as well as the lower Sr contents ( 42. 75 × 10 - 6 ) ,rather higher content of Mn ( 312 × 10 - 6 ～ 506 × 10 - 6 ) and Fe ( 0. 2650% ～0. 403%) ; 87 Sr / 86 Sr = 0. 71085; REE = 18. 84 × 10 - 6 ,δEu =0. 66,δCe =0. 81,the estamted temperature 47. 6℃ for near-micritic dolostone and 55. 12℃ for fine-medium crystalline dolostone. ( 2) Fine-medium and medium-coarse crystalline dolostone supposed to formed during intermediate or deep burial condition,with depleted δ 13 C PDB ( -1. 5‰) and δ 18 O PDB ( -8. 5‰) as well as the lower Sr contents ( 56. 27 × 10 - 6 ) ,Mn ( 312 ×10 - 6 ) and Fe ( 0. 13%) ; with rather higher homogenization temperatures ( up to 108. 34℃ at average temperature) and estimates of low and medium salinities ( 4% ～ 13. 7% NaCleqv) . ( 3) Fractures and vugs-lining coarse or saddle dolomite precipitation,with the lowest Sr ( 29. 1 × 10 - 6 ～ 49. 9 × 10 - 6 ) ,the highest content Mn ( 498 ×10 - 6 ～ 754 × 10 - 6 ) and Fe ( 0. 15% ～ 1. 14%) ,δ 13 C PDB = - 0. 48‰ and depleted δ 18 O PDB = - 8. 82‰, 87 Sr / 86 Sr = 0. 70996; REE = 15. 08 × 10 - 6 ,δEu =0. 70,δCe =0. 78; formed under either relatively deeper burial conditions or possibly from hydrothermal fluids,which is supported by elevated homogenization temperatures ( 120 ～ 150℃ ) and estimates of slightly higher salinities ( 5% ～ 12% NaCleqv) for saddle dolomites; and highest homogenization temperatures ( 140 ～ 180℃ ) and estimates of medium to higher salinities ( 5% ～ 18% NaCleqv) for coarse and huge crystal dolomitse,in which completely occludes the fractures,breccias and vugs. Compare with the Cambrian hydrothermal dolomite ( HTD) occurred at the Whirlpool point outcrops,the Western Canada Sedimentary Basin,which has been related to the hydrothermal fluids,expelled from the Rocky Mountain fold-and-thrust belt during the Laramide orogeny. The coarse-huge crystal and saddle dolomite in fractures and vugs at Wuzilitaga outcrop have a feature of higher δ 13 C,δ 18 O,REE and HREE / HREE with a little lower contents of sequentiality ( degree of order) ,δEu and δCe,in addition to the presence of rather lower homogenization temperatures and salinity in fluid inclusions,suggested to intimately associated with a slightly hot basinal saline, presumably mostly derived from re-circulated of formation water,which have been funnelled upward along strike-slip faults and fractures,developed and facilitated during the Late Permian ( Late Hercynian) or from the Yansha to Himalaya ( Late Cretaceous to Quaternary) in Northeast Tarim.