Alteration and burial dolomitization of fine-grained, intermediate volcaniclastic rocks under saline-alkaline conditions: Bayindulan Sag in the Er'Lian Basin, China

摘要

A novel type of burial dolomitization related to alteration of intermediate volcaniclastic rocks is discussed based on mineralogical, petrological and geochemical analyses. Lacustrine, fine-grained ( < 0.1 mm), volcaniclastic rocks from the Lower Cretaceous Bayindulan Sag in the Erlian Basin, China, contain diagenetic minerals produced from reactions between volcanic materials (volcaniclastics and ash fall) and diagenetic fluids derived from a saline alkaline lake. Mineral products include: high concentrations of diagenetic ankerite, dolomite, ferroan calcite, analcime with high ratio of Si to Al, Wyoming montmorillonite, illite-smectite mixed layer, and amorphous Si-rich material. These rocks can be divided into two types based on the occurrence and composition of authigenic carbonate minerals: non-carbonatized volcaniclastic rocks (Non-Carb-VR) and carbonatized volcaniclastic rocks (Carb-VR). Iron, magnesium, calcium, and sodium are probably derived from alteration of intermediate and calc-alkaline volcanic materials and from illitization of Wyoming montmorillonite. Petrographic textures indicate that ferroan calcite and non-ferroan dolomite predated ankerite (78-103 degrees C). Geochemistry analysis (TiO2 versus Al2O3, Zr/TiO2 versus Nb/Y, and Zr versus Nb) indicates that the precursor rocks were dacitic or rhyodacitic in composition. The compositions of heavy C (0.2 parts per thousand-8.4 parts per thousand, V-PDB) and light O ( -17.5 parts per thousand to - 7.0 parts per thousand, V-PDB) isotopes are probably influenced by methanogenic fermentation. The process of burial dolomitization can be divided into two stages: (1) mixed sedimentation and bacterial methanogenesis; and (2) alteration of volcanic materials and transformation of clay minerals. This study documents a new process for dolomite formation through the alteration of volcanic materials.