Dolomitization of felsic volcaniclastic rocks in continental strata: A study from the Lower Cretaceous of the A'nan Sag in Er'lian Basin, China

Dolomitization of fine-grained volcaniclastic rocks is common in the Lower Cretaceous of the A'nan Sag in the Er'lian Basin of China. Analysis of core samples shows that the organic-rich volcaniclastic rocks are mainly composed of reworked felsic volcanic materials and terrigenous clay minerals. The fine-grained volcaniclastic rocks can be divided into four types: volcaniclastic rocks without carbonatization, volcaniclastic rocks with ferroan dolomites, dolomitized and calcified volcaniclastic rocks, and calcified volcaniclastic rocks. The parent rocks of the volcaniclastic rocks have high silicon and potassium contents and low iron and magnesium contents, and are probably felsic magma of the talc-alkaline series. The average values of delta C-13(PDB) of the carbonate minerals are about 3.13 parts per thousand; the average values of delta O-18(PDB) are about -16.74 parts per thousand. The compositions of C and O isotopes are probably influenced by bacterial methanogenesis. Iron, magnesium, and calcium are probably derived from illitization of terrigenous smectite. A model for dolomitization of felsic volcaniclastic rock is proposed, including three stages: 1) mixed sedimentation and bacterial methanogenesis (< 75 degrees C); 2) transformation of clay minerals (> 70 degrees C) and dolomitization (75 to 97 degrees C); and 3) dissolution. Late dissolution of authigenic carbonate minerals, creating abundant secondary pores, is significant for hydrocarbon accumulation. (C) 2017 Elsevier B.V. All rights reserved.