Multistage blocky calcite cementation in the Cretaceous carbonates of central Iraq: Insights from integrated petrology, geochemistry, and U−Pb dating Available to Purchase

Blocky calcite cement (BC) is ubiquitous and serves as a key archive for deciphering basin fluid circulation. However, the environmental conditions responsible for BC formation and reliable ways to distinguish its origin remain inadequately documented. To address this, the widespread BC in the Cretaceous Rumaila Formation of the Mesopotamia Foredeep, central Iraq, was investigated via integrated petrology, geochemistry and U−Pb dating techniques. Six distinct BC phases, occluded in various pore types, are distinguished. The first two BC stages, dated at ca. 93 Ma and 92 Ma, formed in the marine realm, originating from coeval normal oxic seawater and modified suboxic seawater. The subsequent four BC stages, dated between ca. 55 Ma and 21 Ma, formed in the burial realm where parent fluids—primarily formation water—were alternatively mixed with freshwater and low-temperature hydrothermal fluids. Early BC phases exhibit more pronounced recrystallization in open diagenesis systems, causing larger mean square weighted deviations (MSWDs >3) in U‒Pb data. The final BC phase yields two distinct U‒Pb ages, suggesting prolonged crystallization in minimally altered fluids within a progressively closed diagenetic system. These findings challenge the traditional view that BC typically forms in late diagenesis, revealing instead that multistage BC can span from synsedimentary to late diagenetic stages. Furthermore, the low Mg/Ca ratio and sulfate ion level of seawater during the Calcite Sea period may have facilitated early BC precipitation. The observed association between pore types and BC phases suggests that poorly connected, small-pore spaces are more conducive to early BC formation, providing a preliminary basis for identifying BC origins. Additionally, cathodoluminescence and trace element (rare earth elements and yttrium [REE+Y]) indices, combined with U‒Pb dating, enable the identification of multistage BC origins and the reconstruction of subtle temporal and spatial evolution of basin fluids. The fluid origins and pathways of burial BC are closely linked to the Zagros orogeny, providing new evidence that supports the latest revised collision time (ca. 56 Ma). This implies that tectonic-driven diagenesis extends far inboard from the orogenic front and plays an underestimated role in basin water-rock interactions.