Crystallization conditions of the alkaline-basic dike from the Yllymakh Massif,Central Aldan: Evidence from melt inclusion data in minerals |
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Authors: | L I Panina A T Nikolaeva E Yu Rokosova |
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Institution: | 1.Institute of Geology and Mineralogy, Siberian Branch,Russian Academy of Sciences,Novosibirsk,Russia |
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Abstract: | Alkaline-basic dike from the Yllymakh Massif (Central Aldan) has been studied. Its partially crystallized matrix contains
corroded phenocrysts of olivine and hypidiomorphic phenocrysts of clinopyroxene and pseudo-, epileucite. It was found that
phenocrysts of clinopyroxene contain abundant primary inclusions, Ti-magnetite and apatite bear only single inclusions, whereas
olivine is enriched in secondary inclusions, which are confined to the cleavage of host mineral (along second and third pinacoids)
and its cracks. The homogenization temperatures of the primary inclusions in clinopyroxene and secondary inclusions in olivine
are approximately equal and lie within 1260–1240°C. The compositions of melt inclusions in olivine and clinopyroxene are also
similar and corresponded to the malignite-pseudoleucite phonolite-monzonite pulaskites, which are developed at the Yllymakh
Massif. Unheated inclusions in apatite and Ti-magnetite compositionally approach monzonites and nepheline syenites—tinguaites,
respectively. It was concluded that the alkaline basaltoid magma was presumably parental magma for the entire rock complex
of the Yllymakh Massif. Its crystallization and differentiation presumably provided all observed rock variety from ultrabasics
(early derivatives located at depth) and malignites (later derivatives) to leucite phonolites, monzonites, and alkaline pulaskites,
which were obtained during subsequent stages of the melt evolution. The parental magma, and especially its derivatives, were
enriched in BaO (0.8–0.1 wt %), Cl (0.1–0.3 wt %) and trace elements (primarily, LREE and MREE), which are several times higher
than mantle values. At the same time, ion microprobe (SIMS) study showed that derivative melts were dry: contained only 0.01–1.13
wt % H2O. The trend of melts conserved in the minerals and the massif rocks corresponds to the evolution of alkalinebasaltoid magma
with increase in Si, Al, alkalis and decrease in Mg, Ca, and Fe, i.e. the Bowen trend. The considered alkaline-basic dike
was presumably formed from the derivative of leucite-phonolite melt, which during emplacement captured olivine xenocrysts
from previously fractionated ultrabasic rocks. The parental magma was presumably derived by high-degree melting of garnet-spinel-facies
depleted mantle at some influence of crustal material. |
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