Burial Metamorphism of the Ordos Basin in Northern Shaanxi

Abstract Burial metamorphism has been found in the Ordos basin of northern Shaanxi. On the basis of a rather intensive study of burial metamorphism of sandstone, it has been shown that the evolution from diagenesis to metamorphism involves four stages: cementation of clay minerals, regrowth of pressolved quartz and feldspar, cementation of carbonates and formation of laumontite. On that basis it has been put forward that the laumontite is formed by burial metamorphism of clay and carbonate minerals. According to the thermodynamic data of minerals, the conditions under which laumontite is formed are T<250°C and Xco2<0.17. High‐resolution SEM and TEM studies of clay minerals in mudstone show that there occur a mixed layer assemblage of bertherine and illite/chlorite and trapsformation from bertherine to chlorite. On that basis coupled by the X‐ray diffraction analysis the author suggests the following transformation of clay minerals during burial metamorphism: the earliest smectite‐kaolinite assemblage changes into the bertherine‐illite mixture with increasing depth, then into the illite/chlorite mixed layer assemblage and finally into dispersed individual illite and chlorite. The reaction of the transformation is:smectite+kaolinite+K+ = illite+chlorite+quartzAccording to the study of the oxygen isotope thermometry of the coexisting illite‐quartz pair, the temperature of the above transformation is lower than 180°C.