Petrogenesis and Magma Evolution of the Hornblende Gabbro from Northwest Elazığ, Eastern Türkiye: Constraints from Geochemistry, Sr-Nd Isotopes, and Mineral Chemistry

Abstract

The hornblende gabbro investigated in this study crops out in northwestern Elaz & imath;& gbreve;, eastern T & uuml;rkiye, within the Southeastern Anatolian Orogenic Belt (SAOB), where Late Cretaceous ophiolitic, volcanic, plutonic, and metamorphic units are widely exposed. This study examines the petrology, whole-rock geochemistry, Sr-Nd isotopic composition, mineral chemistry, and crystallisation conditions of these gabbroic bodies to constrain their petrogenesis and tectonomagmatic significance. Field observations show that the rock occurs as rounded to sub-rounded blocks with fresh inner cores and altered outer rims. Petrographic and XRD data indicate that the fresh gabbro mainly consists of plagioclase and amphibole, whereas the altered outer rims contain quartz and minor secondary phases. Whole-rock geochemical data classify the samples as low- to medium-K, tholeiitic, and predominantly metaluminous gabbro. Primitive mantle-normalised trace-element patterns display enrichment in large-ion lithophile elements and depletion in high-field-strength elements, whereas chondrite-normalised REE patterns show slight LREE enrichment, relatively flat HREE patterns, and weak Eu anomalies. Sr-Nd isotopic compositions are characterised by positive epsilon Nd(T) values (+4.4 to +5.3) and moderately radiogenic initial 87Sr/86Sr ratios (0.704792-0.705344), indicating a predominantly mantle-derived magma source affected by subduction-related modification, with limited crustal contribution. Mineral chemistry data show that amphiboles belong to the calcic amphibole group and plot in the magnesio-hornblende field. Amphibole thermobarometric calculations yielded temperatures of 873-991 degrees C and pressures of 1.49-3.26 kbar, corresponding to crystallisation depths of 5.1-15.3 km. Overall, the results indicate that the hornblende gabbro was derived from a mafic magma generated from a spinel lherzolite mantle source and crystallised in a subduction-related tectonomagmatic setting.