Contrary to their Tethyan counterparts, and despite substantial research, Upper Triassic shallow-water limestones from the Panthalassa Ocean remain poorly known. Their understanding is yet crucial to better constrain past depositional, ecologic, geographic and geodynamic conditions out of the Tethyan domain. The Taukha terrane, located in the Sikhote-Alin mountain range (Primorsky and Khabarovsk Krais, Far East Russia) is a key panthalassan terrane. Near Dalnegorsk City, North-East of Vladivostok, on the northern part of Taukha terrane, Upper Triassic marine carbonates abound. Based on field observations and facies analysis, eight major facies, corresponding to specific depositional environments within a carbonate platform, have been established. A conceptual depositional model corresponding to a mid-oceanic atoll-type system has been defined. A complete diagenetic study of Upper Triassic limestones from Panthalassa Ocean is performed, combining different approaches. Cathodoluminescence petrography coupled to LA-ICPMS (REE + trace elements) and SIMS (δ18O and δ13C) geochimical analysis have been used to precisely characterize calcite cements and accurately define the deposition and cementation conditions. The results evidences major events, from early marine diagenesis to accretion-related changes. A wide event of moldic dissolution, associated with formation of meteoric cements is dated thanks to in situ U-Pb ratios. Large calcitic cements in carbonate breccias linked to the dismantling of the system, posseses early marine affinity defined by REE and isotopic signals. The presented data and models are a new significant step to improve our knowledge of Upper Triassic environments in the Panthalassa Ocean, and more generally for the understanding of mid-oceanic limestone formation and their evolution through time.
Abstract Geothermometers are commonly used to reconstruct the diagenetic and thermal history of rocks. However, characterizing the timing, origin, and temperature of paleofluid flow remains challenging because it must be assessed indirectly through the analysis of microscopic cements that precipitate and fill intergranular spaces during fluid circulation. Here, we measure both the clumped isotope (Δ47) temperature and in situ U-Pb age of individual diagenetic calcite cements within a sedimentary section of the Paris Basin (France), whose thermal history has been previously inferred to be <60 °C. We show that cementation occurred during two stages associated with major events at the western European lithospheric scale: (1) the Bay of Biscay rifting (Late Jurassic–Early Cretaceous), and (2) north-south Pyrenean compression (Eocene) followed by east-west extension during the European Cenozoic rift system event (Oligocene). Related to both events, we report unexpectedly hot fluids, up to 110 °C, contrasting with the lower temperatures inferred from other geothermometers (e.g., fluid inclusions, clay minerals, apatite fission tracks, maturity of organic matter by Rock-Eval pyrolysis, or vitrinite reflectance). These high temperatures (>70 °C) have been measured for calcite cements containing single-phase aqueous fluid inclusions, challenging the commonly accepted assertion that the absence of nucleation of a vapor phase indicates crystallization at low temperature (∼<70 °C). We suggest that the kinetics of mineralization events prevented the recording of short-lived hot fluid flows by other geothermometers.
