Hydrothermal Fluids and Cold Meteoric Waters along Tectonic-Controlled Open Spaces in Upper Cretaceous Carbonate Rocks, NE-Iraq: Scanning Data from In Situ U-Pb Geochronology and Microthermometry

Abstract

The Upper Cretaceous carbonates along the Zagros thrust-fold belt “Harir-Safin anticlines” experienced extensive hot brine fluids that produced several phases of hydrothermal cements, including saddle dolomites. Detailed fluid inclusion microthermometry data show that saddle dolomites precipitated from hydrothermal (83–160 °C) and saline fluids (up to 25 eq. wt.% NaCl; i.e., seven times higher than the seawater salinity). The fluids interacted with brine/rocks during their circulation before invading the Upper Cretaceous carbonates. Two entrapment episodes (early and late) of FIs from the hydrothermal “HT” cements are recognized. The early episode is linked to fault-related fractures and was contemporaneous with the precipitation of the HT cements. The fluid inclusions leaked and were refilled during a later diagenetic phase. The late episode is consistent with low saline fluids (0.18 and 2.57 eq. wt.% NaCl) which had a meteoric origin. Utilizing the laser ablation U-Pb age dating method, two numerical absolute ages of ~70 Ma and 3.8 Ma are identified from calcrete levels in the Upper Cretaceous carbonates. These two ages obtained in the same level of calcrete indicate that this unit was twice exposed to subaerial conditions. The earlier exposure was associated with alveolar and other diagenetic features, such as dissolution, micritization, cementation, while the second calcrete level is associated with laminae, pisolitic, and microstromatolite features which formed during the regional uplifting of the area in Pliocene times. In conclusion, the hydrothermal-saddle dolomites were precipitated from high temperature saline fluids, while calcrete levels entrapped large monophase with very low salinity fluid inclusions, indicative for a low temperature precipitation from water with a meteoric origin.