Stable isotope and rare earth element geochemistry of the Baluti carbonates (Upper Triassic), Northern Iraq

摘要

Stable isotope ratios of O-18/O-16 and C-13/C-12 and rare earth elements geochemistry of the Upper Triassic carbonates from the Baluti Formation in Kurdistan Region of Northern Iraq were studied in two areas, Sararu and Sarki. The aim of the study is to quantify the possible diagenetic processes that postdated deposition and the paleoenvironment of the Baluti Formation. The replacement products of the skeletal grains by selective dissolution and neomorphism probably by meteoric water preserved the original marine isotopic signatures possibly due to the closed system. The petrographic study revealed the existence of foraminifers, echinoderms, gastropods, crinoids, nodosaria and ostracods as major framework constituents. The carbonates have micritic matrix with microsparite and sparry calcite filling the pores and voids. The range and average values for twelve carbonate rocks of O-18 and C-13 in Sararu section were -5.3 parts per thousand to -3.16 parts per thousand (-4.12 parts per thousand) and -2.94 parts per thousand to -0.96 parts per thousand (-1.75 parts per thousand), respectively; while the corresponding values for the Sarki section were -3.69 parts per thousand to -0.39 parts per thousand (-2.08 parts per thousand) and -5.34 parts per thousand to -2.70 parts per thousand (-4.02 parts per thousand), respectively. The bivariate plot of O-18 and C-13 suggests that most of these carbonates are warm-water skeletons and have meteoric cement. The average sigma REE content and Eu-anomaly of the carbonates of Sararu sections were 44.26 ppm and 1.03, respectively, corresponding to 22.30 ppm and 0.93 for the Sarki section. The normalized patterns for the carbonate rocks exhibit: (1) non-seawater-like REE patterns, (2) positive Gd anomalies (average = 1.112 for Sararu and 1.114 for Sarki), (3) super chondritic Y/Ho ratio is 31.48 for Sararu and 31.73 for Sarki which are less than the value of seawater. The presence of sparry calcite cement, negative C-13 and O-18 isotope values, the positive Eu anomaly in the REE patterns (particularly for Sararu), eliminated Ce anomaly (Ce/Ce*: 0.916-1.167, average = 0.994 and 0.950-1.010, average = 0.964, respectively), and Er/Nd values propose that these carbonates have undergone meteoric diagenesis. The REE patterns suggest that the terrigenous materials of the Baluti were derived from felsic to intermediate rocks.