DIAGENETIC ALTERATION IN SAMPLES FROM THE HONGGULELENG FORMATION IN THE LATE DEVONIAN OF NORTHWESTERN CHINA

The Hongguleleng Formation records Late Devonian geology in northwestern China. Limestone, shale, and siltstone samples have been collected from the Boulongour Reservoir section. These rocks were deposited in a shallow water, highly fossiliferous, marine setting in an oceanic island arc complex. The Frasnian/Famennian (F/F) boundary of the Late Devonian is located 3.5 meters above the base of the limestone within the Hongguleleng Formation. As the first step in developing a global climate history of the Late Devonian using δ¹³C and δ¹⁸O, samples from the Hongguleleng Formation have been collected and analyzed for diagenetic alteration. Two procedures are being used as a prelude to isotopic analysis of the samples: cathodoluminescence of polished thin sections of limestone samples, and geochemical proxies. Limestone samples were analyzed under cathodoluminescence in order to determine which samples were diagenetically altered and no longer record the geochemical signals from the original environment of deposition. Using the extent of preservation of microstructure of fossils within the carbonates and CL responses, the degree of diagenesis was determined. Varying degrees of luminescence from brightly luminescent to nonluminescent are asymmetrically distributed throughout the Hongguleleng Formation, mirroring inspection in the field of faults at the base and nonpervasive altered intervals within the section. The F/F boundary is characterized by fossil fragments and matrix that are largely nonluminescent. The microstructure of fossils within these samples is intact. The first detailed analysis of the geochemical signals at the F/F boundary in this unique setting has been completed using major elements, trace elements, and rare earths. This geochemical data, in conjunction with cathodoluminescence, is being used to determine the degree of alteration within the formation. This preliminary survey of all samples aids in identification of altered brachiopods and carbonate matrix, which will be disregarded in future studies of δ¹³C and δ¹⁸O.