The Types, Metallogeny and Conditions to form Ultrabasite Complexes of Lesser Caucasian and Karadag - Talyshian fold zones

دسته پترولوژی
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری پانزدهمین گردهمائی علوم زمین
تاريخ برگزاری ۲۷ بهمن ۱۳۷۵

Two types of Ultrabasite complexes differed by chemism, metallogeny and by form of manifestation in the Earth's crustal conditions were established as a result of researches carried out during last decade.

One of them is suitable to weakly - differenciated ophiolitic complexes (Mesozoic, Lesser Caucasus), composed of lherzolite and harzburgite which is differed not much from medi-mantle peridotite by content of lithophilic elements. The gabbroides average content is close to basalt of effusive series, related to specific oceanic tholeiite.

Another type is completely differenciated and suitable to alkaline complexes (Late - Eocene - Oligocene, Karadag and Talysh), consisted of micaceous pyroxenites and peridotites, gabbro­ pyroxenites, alkaline gabbro and gabbro - syenites. The latter and rocks of effusive series ­ trachybasalts, leucititic basanites, trachyandesites and leucititic phonolites have numeral like petro-geochemical properties, typical of alkaline olivine basalts.

The trends of both complexes differentiation to be compared allow to find out a rupture among rock of hyperbasite - gabbro ophiolitic complex in MgO/ Al۲۰۳ ratio, ranged from ۳۰-۵۰.
But, differenciating is observed for (س FeO + Ti۰۲) / Mg ratio, presented by leykogabbro
formation, enriched some elements – Fe, K, Na ,Ti and melanogabbro - being more Mg, lesser Fe, which shows concealed Fe - Mg fractionation. This smoother transition is recognized for alkaline basite - ultrabasite complex in MgO/ Al۲۰۳ ratio ۲.۰-۳.۰ for pyroxenites, peridotites, and ~ ۰.۳-۰.۴ for alkaline gabbro and trachybasalts. In the process of differenciation to average differences one observes increasing of alkaline parts (at K/Na> ۱) and decreasing of ferrum and titanium.
Alkaline peridotites are differed from ophiolitic ultrabasites by increased content of ferrum, alumina as well as of significant lithophilic elements - Ta, La, U, Th, Zr, Hf, Ti, P. The differentiated nature of hyperbasites having alkaline gradient determines a wide range of attribuated mineralization Tr - Ti - Ta - Nb. Non differentiated nature of tholeiitic ophiolitic hyperbasites recognizes, mainly, mantle profile of metallogenic specialization, characterized by narrow range - chromite and platinoids.
Petro-geochemical features of complexes data allow to consider ultrabasites of ophiolitic
complexes as suitable to mantle conditions to generate. For the first stage Fe-Mg fractionation is predominated (to form dunite - harzburgite). Lately, while forming gabbroids, Fe-Ti fractionion has been determined as well as Al and Ca relative accumulation.
Alkaline basite - ultrabassite complex is linked with selective melting of upper mantle parts.
The latter is composed of hyperbasites, containing phlogopites, which is important. The mentioned hyperbasites, being as a result of metasomatic proceeding of initial matter, performed by mantle
Ophiolitic - peridotite - gabbroid complex and effusive radiolarite one have been formed in structures of oceanic crust and are suitable to early stage of development of folded zones.
Alkaline peridotite - gabbroid complex and volcanogenic alkaline - basaltic one are suitable to rifts back arcs, which have been formed in more consolidated margin structures of data system on late stage of its development.
Difference of contents between basite - hyperbasite complexes used to determine by more deep - seated nature of initial ultrabasic melts within folded belt of old age to compare to increasing difference of contents initial mantle substrates in dependence ontime, as well as relative enrichment of lithophilic elements with large ion - radius.

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