The genetic relationship of the deformed peridotites and garnet megacrysts from kimberlites with asthenospheric melts
L.V. Solov'evaa, Yu.G. Lavrent'evb, K.N. Egorova, S.I. Kostrovitskiic, V.N. Korolyuk b , L.F. Suvorovac
a Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
c Institute of Geochemistry, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Kimberlite; deep-seated xenolith; deformed peridotites; megacrysts; geochemistry; asthenospheric liquids
Pages: 207-224
Abstract
New data on geochemistry of incompatible trace elements (Nb, Zr, Hf, Ti, Y, REE) in garnets (Ga) and clinopyroxenes (Cpx) from deformed high-temperature garnet lherzolites, harzburgites, and garnet megacrysts from the Udachnaya pipe are presented. According to textures and petrographic composition, the deformed Ga-bearing peridotites are subdivided into coarse- and fine-porphyroclastic. Rocks of the first type (Ga-lherzolites) are enriched in Ga and Cpx (up to 30% each); among them, there are xenoliths with deformed Ga, Cpx, and Ol megacrysts. Deformed fine-porphyroclastic peridotites (Ga-lherzolites and Ga-harzburgites) are depleted in Ga (≤10%) and Cpx (≤3-5%). Four Ga megacrysts are typical of a Cr-poor association (0.33-1.63% Cr2O3; 0.56-1.05% TiO2; Mg# = 0.67-0.86). Garnets from deformed high-temperature coarse-porphyroclastic Ga-lherzolites are similar in geochemistry to Ga megacrysts: Their chondrite-normalized REE curves rise smoothly from La to Yb, and their spidergrams show peaks of Nb, Zr + Hf, and Ti. The narrow marginal zones of Ga grains from Ga-lherzolites are enriched in LREE. The peaks of HFSE in the Ga curves are smoothed out. The element patterns of garnet from fine-porphyroclastic Ga-lherzolites and Ga-harzburgites show a sinusoidal distribution of REE and Zr + Hf and Ti minima on the spidergrams. The element patterns of Cpx from both types of rocks are nearly the same: The curves are convex in the field of LREE, drastically drop from Nd to Yb, and have minima of Nb, Zr + Hf, and Ti. The curves of melts (L) equilibrated with Ga (calculated from partition coefficients) show a drastic enrichment in LREE and peaks of Nb, Zr + Hf, and Ti for garnet from deformed coarse-porphyroclastic lherzolites and megacrysts. The L curves of fine-porphyroclastic lherzolites and harzburgites have distinct minima of Zr + Hf and more sharply slope from Nd to Yb. The L curves of Cpx have minima of Nb, Zr + Hf, and Ti and lie near the field of basaltoid kimberlites from the Udachnaya pipe. All these data are consistent with the model of percolative fractional crystallization. The matter of mantle plume arrived from the mantle transition zone might have been the source of asthenospheric liquids inducing specific magmatic metasomatism at the continental lithosphere-asthenosphere boundary. The source material was enriched in majorite and silicate-perovskite or in ancient oceanic crustal matter.
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