从岩石学及微区同位素探讨四川盆地灯影组皮壳-葡萄状白云石成因

Applications of petrography and isotope analysis of micro-drill samples to the study of genesis of grape-like dolomite of the Dengying Formation in the Sichuan Basin

四川盆地灯影组中的"皮壳、栉壳及葡萄状"白云石是一种非常特殊结构白云石。对四川盆地灯影组葡萄-皮壳状及基岩岩石学、碳氧与锶同位素等研究表明:皮壳-葡萄状白云石早期由内纤状(AC)、外为束状-放射状海底纤柱状(RFC)的文石及犬牙状高镁方解石或粒状方解石构成。不同尺度"球状-哑铃"或菱形的泥微晶白云石沿C轴呈台阶式相向生长,反映了海水潜流带为主的生长模式。微区同位素揭示了环带中的粉细晶白云石要比黑、白相间纤状白云石中的δ^(13)C,δ^(18)O负偏明显;黑色纤状又较白色纤状白云石δ^(13)C,δ^(18)O正偏。环带至少有3种变化情形:(1)δ^(18)O(PDB),δ^(13)C(PDB)负偏,87Sr/86Sr较高,较宽的明、暗阴极条带,指示了周期性弱氧化与弱还原环境的变化海水并叠加了大气淡水作用;(2)有序度变化大,从环带核部至边缘,δ^(13)C(PDB)缓慢下降,δ^(18)O(PDB)缓慢升高,87Sr/86Sr接近同时代的海水平均值,核部中等橙红向外为不发光-暗淡发光,反映了核部弱还原向外变为弱氧化、海水并有持续大气淡水的补给;(3)有序度较低、从环带核部至边缘,δ^(13)C(PDB)缓慢上升,δ^(18)O(PDB)先升后降,边缘稍为偏负,中等橙红色、间隔有相对暗淡阴极发光,推测为停滞浓缩海水为主、后有弱氧化大气淡水补给。据此推断,皮壳-葡萄状白云石是在新元古南华纪冰期的特殊海水背景下,经历了过饱和海水中微生物中细菌参于下成核、准稳态含镁矿物沉淀(AC-RFC)、大气淡水下水岩作用粒状方解石(RFC-PC)和浓缩海水中的准同生云化作用过程。

The shell-comb and grape-like dolomite of the Dengying Formation in the Sichuan Basin has an unusual textures. The petrographic data,carbon,oxygen and strontium isotope analysis on matrix and grape-like dolomites microdrillsamples have been used to investigate the origin of shell comb and grape-like dolomite. The grape-like dolomite is characterized by length-fast fascicular-optic fibrous aragonite cement,and bladed radial slow high-Mg calcites as well as granular calcites. The micritic dolomite in opposite direction growth,which consists of different size of either spherical and dumbbell forms or rhombic shape,has formed in pattern rhythm layers of banding or laminated along the terrace of C-axis of crystal. The zonation,which consists of microcrystalline,fine crystalline dolomite,has generally a more distinctive negative values of δ^13C（ PDB） and δ^18O（ PDB） than that of the black and white fibrous dolomite of grape-like dolomite. The black or dull fibrous dolomite has a positive δ^13C（ PDB） and δ^18O（ PDB）,compared with that of white fibrous dolomite.It may mainly indicate a marine origin for shell comb and grape-like dolomite. At least three different fluid environment oscillation to generate the banding of shell comb grape-like dolomite：（1） One of them is characteristic of lower negative ofδ^13C（ PDB） and δ^18O（ PDB） and much higher ^87Sr/^86 Sr compared to that of matrix dolomite or seawater. The wide medium bright orange luminescent bands with interval of dull bright zones is supposed to form in seawater overprinted with fresh water,indicate an periodic environment changes between weak oxidation and weak reduction;（2） The second has a sharp variation of sequences,with slow decrease of δ^13C（ PDB） and increase of δ^18O（ PDB） from its centre to rim of zonation. Normal seawater value of87Sr/86 Sr,medium bright orange luminescent in the centre to dull or non-luminescent in outer zones indicates it generated in a seawater with continuous recharge of fresh water,and weak reduction in the center shifted to weak oxidation in the outer of zone;（3） The third one does not have very clear zonation,and negative18O（ PDB） in the rim with a slow increase of δ^13C（ PDB） and increasing and later decreasing of δ^18O（ PDB）. It also shows a cathodoluminescence feature similar to that of matrix dolomite,which has been interpreted as a weak reduction and later oxidation environment. The environment first developed in condense seawater and was ventilated by fresh water in later diagenesis periods. In conclusion,the shell comb and grape-like dolomite has been developed in an unusual Neoproterzoic seawater characterized by very high Mg/Ca. It is coeval with the precipitation of algal laminated stromatolite and thrombolite,in forms of the microbial mat,mound,biofilms and reef. The environment model can most likely account for rapid growth of isopachous,fibrous aragonite and bladed high-Mg calcites cement,and growth of later granular calcites induced or promoted by bacterial microbial modulation,and para-contemporaneous dolomitization in condense seawater. The fluid is oversaturated seawater at the beginning,and later superimposed by a cyclic subaerial exposure of fresh water.