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
Qian Yixiong, Feng Jufang, He Zhiliang, Zhang Keyin, Jin Ting, Dong Shaofeng, You Donghua, Zhang Yongdong
2017, 38(4):
665-676.
doi:10.11743/ogg20170404
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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 microdrill-samples 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 δ13C(PDB) and δ18O(PDB) than that of the black and white fibrous dolomite of grape-like dolomite.The black or dull fibrous dolomite has a positive δ13C(PDB) and δ18O(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 of δ13C(PDB)and δ18O(PDB) and much higher 87Sr/86Sr 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 δ13C(PDB) and increase of δ18O(PDB) from its centre to rim of zonation.Normal seawater value of 87Sr/86Sr,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 negative 18O(PDB) in the rim with a slow increase of δ13C(PDB) and increasing and later decreasing of δ18O(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.
