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.

Highlights

  • Hydrothermal fluids and meteoric waters are characterized by a significant heterogeneity in geochemical composition

  • Visual petrography of fluid inclusions and microthermometry, stable isotopes, and U-Pb dating, two main entrapment episodes of aqueous FIs were identified from their optical characteristics at room temperatures; 1

  • An early entrapment episode of fluid inclusions in HT dolomite cements exhibited a range of homogenization temperatures with (i) a first population of Th values between 83 ◦C and 120 ◦C and (ii) a second population with Th values between 130 ◦C and 160 ◦C

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Summary

Introduction

Hydrothermal fluids and meteoric waters are characterized by a significant heterogeneity in geochemical composition. To gain better insights into the origin and evolution of these diagenetic fluids, techniques such as conventional petrography, cathodoluminescence, and stable and radiogenic isotopes have been applied [1,2,3,4,5,6]. The researchers used these methods to understand the complex diagenetic settings in hydrothermal dolomitization. One of the major questions is to determine the number of pulses and the injection age of these HT fluids, in order to unravel the possible driving mechanisms for their upward flow [13]

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