Abstract
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 ( approximately 1-3 kbar), high f (sub O2) and crystallization temperature up to 727 degrees C, whereas it is 247-275 degrees C in the case of retrograde chlorite. Rutile and titanite in metagabbros are Fe-rich and replace Mn-bearing ilmenite precursors at high f (sub O2) . 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 degrees C, respectively. Geochemically, the Cryogenian metagabbros ( approximately 850-780 Ma) are tholeiitic to calc-alkaline in composition and interpreted as arc-related. Younger, fresh gabbros are calc-alkaline and post-collisional ( approximately 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). Copyright 2017 Springer-Verlag GmbH Germany and 2016 Springer-Verlag Berlin Heidelberg