Applications of Carbonate Geochemistry to Orogenic Systems: Examples from the Himalaya and the Hinterland of the North American Cordillera

Abstract

Carbonate minerals are amongst the most common materials on earth’s surface. They form in marine, lacustrine and terrestrial settings by inorganic and biogenic mechanisms. Therefore, they can be valuable archives of tectonic, climatic and biological process. In this dissertation, which is composed of four manuscripts, I explore the application of carbonate geochemistry to two of the largest orogenic systems on earth, the North American Cordillera and the Himalaya. In Appendix A, I investigate the alteration and preservation of the stable isotopic composition of carbonates in Himalayan foreland deposits. Assessing the preservation of primary isotopic signatures is critical for interpreting data derived from carbonates. Results show that soil carbonate nodules, tooth enamel, aragonitic fossil bivalve shells and possibly fossil tusk and eggshell all retain primary oxygen and carbon isotope composition during burial up to at least 4.5km, but their clumped isotope composition is altered at modest burial depths by exposure to temperatures of ∼100°C for <5Myrs. Carbonate cemented sandstones behave as open systems, allowing either initial cementation at variable depths or early cementation and later open-system alteration, while tooth enamel does not consistently produce realistic body temperatures regardless of sample age or species. The study highlights the susceptibility of carbonates to alteration and the challenge of reconstructing primary temperatures using clumped isotopes. In Appendix B I expand the dataset of Appendix A to include samples from all major carbonate units in the Himalaya, including the main carbonate bedrock assemblages and floodplain silts from foreland sediments. I also characterize the 87Sr/86Sr ratio of the main carbonate bedrock units. The study shows the alteration patterns of carbonates through lithification, metamorphism, exhumation and reburial. I also demonstrate the use of carbonate isotope geochemistry as a provenance tool and reconstruct a history of carbonate input to the foreland basin in western Nepal, showing a significant increase in the detrital carbonate bedrock component coincident with Lesser Himalayan duplexing. In addition, the study demonstrates that Himalayan carbonates have unusually high 87Sr/86Sr ratios such that their weathering may have played an important role in the increase in seawater 87Sr/86Sr ratios over the past ~40Myrs. In Appendix C I examine carbonate bedrock samples and calcite veins from the Monitor Range in the Great Basin, Nevada, USA. Carbonate stable isotopes, 87Sr86Sr ratios, and U-Pb dating demonstrate movement of faults in the range between 22-12Ma and precipitation of calcite from deeply circulating meteoric water. Older calcite veins from the range dated to 74-59Ma may be related to pluton emplacement at this time and coincide with silver and gold mineralization in the area. The oxygen isotope composition of water reconstructed from the veins suggests near modern elevation of the mountains for the past 22Myrs. Bedrock in the study area is altered to high temperatures and low oxygen isotope values, possibly resulting from a hydrothermal pulse. Appendix D expands on the findings from Appendix C, broadening the study area to five other mountain ranges in the Great Basin. The ages of calcite veins from these ranges are consistent with independent constraints on fault motion, including in ranges where Late Cretaceous and Eocene-Oligocene extension occurred. Veins from three of the ranges are likely related to pluton emplacement and hydrothermal flow events, in some cases associated with precious metal deposits. Paleoelevation reconstructions based on the oxygen isotopic composition of the veins are consistent with previous estimates of a >2.7km high plateau in the area since the Late Cretaceous. The study demonstrates that calcite veins can be used to trace episodes of hydrothermal flow, date phases of extension and reconstruct the regional evolution of paleoelvation.