PETROLOGY OF ORDOVICIAN COLLISION-RELATED LAYERED PERIDOTITE-GABBRO INTRUSIONS (exemplified by the Mazhalyk intrusion, Southeastern Tuva)
E.V. Borodina, V.V. Egorova, and A.E. Izokh
Institute of Geology, Siberian Branch of RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090
Keywords: Layered intrusions, peridotite-gabbro complexes, fractional crystallization, parental magma, komatiites, rare-earth elements, mantle source
Pages: 1025-1042
Abstract
The Mazhalyk intrusion, a reference of the Mazhalyk peridotite-gabbro complex of Southeastern Tuva, has a rhythmically layered internal structure. Its age (484.2  2.3 Ma) corresponds to the Ordovician accretion-collisional step of evolution of the Earth's crust in Central Asia. The intrusion contains rocks of marginal facies and layered series, including ultramafic, subultramafic, mafic, and anorthosite groups of rocks. The upsection succession of cumulate associations is as follows: Ol+Sp=>Ol+Sp+Cpx=>Ol+Sp+Cpx+Pl=>Ol+Sp+Cpx+Pl+Opx=>Ol+Sp+Cpx+Pl+Opx+Amf=>Cpx+Pl+Opx=>Cpx+Pl+Opx+Amf. Variations in mineral compositions are: Fo 83.1 to Fo 71.5 for olivine; An 90.14 to An 81.72 for plagioclase; En 53.19-43.04 Fs 16.1-6.71 Wo 48.05-32.16 for clinopyroxene, with mg# = 100 Mg/(Mg+Fe) ranging from 88.6 to 76.3; En 82.17-71.68 Fs 27.85-16.03 Wo 2.01-0.18 for orthopyroxene, with mg# ranging from 84.0 to 73.1, and amphibole with mg# = 81.63 to 62.99. Petrochemistry of the Mazhalyk rocks is consistent with fractional crystallization of a picritic parental magma. The low content of alkalies (under 1-2 wt.%) in rocks and accumulation of total iron in later differentiates suggest that the parental magma of the Mazhalyk intrusion is similar to a tholeiite magma. All the Mazhalyk rocks, irrespective of their position in section, are characterized by a weakly differentiated trend of REE distribution. The content of REE in them is less than 10 chondrite units and exceeds the REE content in the primitive mantle and in the N-MORB source by a factor of 2-4. The geochemistry of the Mazhalyk intrusion is distinguished by a Eu maximum expressed in all rocks and depletion in HREE, which suggests that garnet is present in the mantle source.Mineralogical, petrographic, and geochemical features of the Ordovician collisional peridotite-gabbro intrusions (Mazhalyk intrusion) are similar to those of island-arc intrusions of Vendian-Cambrian age (Central intrusion). This similarity suggests that the parental magmas of both collisional and island-arc intrusions were formed by partial melting of depleted subduction-related mantle source, corresponding in composition to garnet lherzolite. The Mazhalyk intrusion is distinguished from island-arc intrusions by higher mg# of parental magma, which is due to higher degree of its melting under the effect of the mantle plume. |
