羌塘盆地半岛湖地区上侏罗—下白垩统礁灰岩地球化学特征和成岩流体性质

GEOCHEMICAL CHARACTERISTICS AND DIAGENETIC FLUID FROM REEF FACIES CARBONATE ROCKS OF UPPER JURASSIC-LOWER CRETACEOUS IN BANDAOHU AREA,QIANGTANG BASIN,TIBET

半岛湖地区生物礁以发育灰岩为主,包括以造礁生物为主的骨架岩和非礁碳酸盐岩,针对生物礁相碳酸盐岩的显微组构分析和阴极发光分析,测得不同点礁各类礁相碳酸盐岩的常微量元素和碳、氧同位素组成,分别为:35件样品Mn,Sr,Fe元素平均质量分数为129×10-6,270×10-6和1 320×10-6,具有较低的Mn含量、较高的Sr含量、Fe含量,表明在成岩过程中海源流体发挥了主导作用,同时遭受了不同程度的大气淡水成岩流体的影响;碳同位素值变化不大且都为正值,变化区间为1.4‰~3.5‰,平均值为2.61‰;氧同位素值变化区间为-7.4‰^-2.7‰,平均值为-5.25‰,古海水盐度指数Z值变化较小,为126.6~132.7,平均值为130.5。结果表明,成岩流体盐度较高,溶蚀孔洞较为发育的颗粒灰岩和礁灰岩具有相对更低的δ18 O值,晚侏罗—早白垩世时期,羌塘盆地北部大量有机碳被高速埋藏是导致半岛湖地区礁相碳酸盐岩碳同位素值普遍偏高的首要原因。

The organic reef in Bandaohu area is dominated by limestone,including bio-dominated reef and non-reef skeleton rock carbonates. Microfabrics and cathodeluminescence of reef carbonate rocks in study area are studied,and their trace element concentrations, carbon and oxygen isotope compositions measured. It shows that the average quality score of Mn, Sr,Fe of 35 various types of carbonate rocks are 129 × 10^-6 , 270 × 10^-6 and 1 320 × 10^-6 , respectively, with lower Mn concentration,higher concentration of Sr and Fe concentration,indicating that during diagenesis, the seawater fluid played a leading role, while suffering the effect of different levels of meteoric water fluid. Carbon isotope has minor variation with a positive value,and with a variation interval of 1.4%0- 3.5%0. Oxygen isotope value ranges from -7.4%0 to -2.7%0, with an average of -5.25%0. Salinity index value of Z of ancient seawater is small,ranging from 132.7 to 126.6 ,with an average of 130. 5. It reveals that diagenetic fluid has higher salinity,and the reef limestone and grain limestone with well developed dissolution cavities have relatively lower δ^18O values. In the Late Jurassic-Early Cretaceous period, large amounts of organic carbon buried at high speed in the northern part of Qiangtang Basin results in the high in isotope values in reef facies carbonate rocks in Bandaohu area.