شيمي كاني هاي اصلي گابروهاي آلكالن مجموعه افيوليتي جنوب درياي خزر، شمال ايران (شرق گيلان)

Mineral Chemistry of Alkali Gabbros from the Southern Caspian Sea Ophiolite: Northern Iran (Eastern Gilan)

In the southern Caspian Sea ophiolite complex, in the structrural zone of Gorgan-Rasht, some gabbroic bodies have penetrated the southern Caspian Sea ophiolite rocks that have upper cretaceous age. These ophiolite rocks have tholeiitic nature but these gabbros fall in alkaline rocks field (Si02: 45. 5-48. 5; A1203: 14. 8-18. 45; MgO: 3. 2-4. 27; Fe203(t):10. 45-13. 3; CaO: 8. 2-9. 41; Ti02:2. 79-3. 82; (Na20 +K20):5. 2-6 and Ti/V:83-103). In spite of the relationship and accompaniment of alkaline gabbros andophiolitic rocks complex, it seems after formation of tholeiitic units, alkaline gabbrospenetrated into that but since any effect of alkaline gabbros is not found in younger Cenozoic units, it is suggested that these rocks are not very younger of their hostedrocks. The mineral chemistry of alkali gabbros of the Southern Caspian Sea ophiolite(SCO) is consistent with crystal-liquid fractionation of primitive basalts at lowpressures. The presence of low magnesian clinopyroxene, presence of zoning in clinopyroxene together with the presence of plagioclase (with low An%), indicate low pressure crystal fractionation of primitive basaltic melts. Mineralogical and geochemical data suggest that the alkali gabbros are formed and enriched MORB source.