Mineral chemistry as a tool for understanding the petrogenesis of Cryogenian (arc-related)–Ediacaran (post-collisional) gabbros in the western Arabian Shield of Saudi Arabia
Metagabbros and gabbros in the Ablah-Shuwas belt (western Saudi Arabia) represent part of significant mafic magmatism in the Neoproterozoic Arabian Shield. The metagabbros are Cryogenian, occasionally stratified and bear calcic amphiboles (hornblende, magnesio-hornblende and actinolite) typical of calc-alkaline complexes. These amphiboles suggest low pressure (~1–3 kbar), high $$f_{{{\text{O}}_{2} }}$$fO2 and crystallization temperature up to 727 °C, whereas it is 247–275 °C in the case of retrograde chlorite. Rutile and titanite in metagabbros are Fe-rich and replace Mn-bearing ilmenite precursors at high $$f_{{{\text{O}}_{2} }}$$fO2. On the other hand, younger gabbros are fresh, layered and comprised of olivine gabbro and olivine–hornblende gabbro with an uppermost layer of anorthositic gabbro. The fresh gabbros are biotite-bearing. They are characterized by secondary magnetite–orthopyroxene symplectitic intergrowth at the outer peripheries of olivine. The symplectite forms by deuteric alteration from residual pore fluids moving along olivine grain boundaries in the sub-solidus state. In fresh gabbros, ortho- and clinopyroxenes indicate crystallization at 1300–900 and 800–600 °C, respectively. Geochemically, the Cryogenian metagabbros (~850–780 Ma) are tholeiitic to calc-alkaline in composition and interpreted as arc-related. Younger, fresh gabbros are calc-alkaline and post-collisional (~620–590 Ma, i.e., Ediacaran), forming during the late stages of arc amalgamation in the southern Arabian Shield. The calc-alkaline metagabbros are related to a lithospheric mantle source previously modified by subduction. Younger, fresh gabbros were probably produced by partial melting of an enriched mantle source (e.g., garnet lherzolite).