Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts

Ashchepkov, I.V. and Kuligin, S.S. and Vladykin, N.V. and Downes, Hilary and Vavilov, M.A. and Nigmatulina, E.N. and Babushkina, S.A. and Tychkov, N.S. and Khmelnikova, O.S. (2016) Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts. Geoscience Frontiers 7 (4), pp. 639-662. ISSN 1674-9871.

Abstract

Mantle xenocrysts from Early Triassic kimberlite pipes from Kharamai, Ary Mastakh and Kuranakh fields in the Anabar shield of Siberia revealing similar compositional trends were studied to estimate the superplume influence on the SCLM. PT reconstructions using monomineral thermobarometry for 5 phases show division of the SCLM beneath the Kharamai field into 6 units: pyroxenitic Fe-rich (1-2 GPa) and Mg-rich (2-3 GPa) layers; middle with two levels of Gar -Sp pyroxenites at ∼3 and 4 -5 GPa; Ga- dunite –harzburgites ∼ 4.5-6.5 GPa subjected to Ilm-Px vein metasomatism, and a Mg-rich dunite lower part. In the Anabar shield (Ary Mastakh, Dyuken and Kuranakh fields) mantle lithosphere is composed of three large units divided into two parts: upper part with amphiboles and phlogopite; two levels of pyroxenites and eclogites at 3 and 4 GPa, and a lower part composed of refertilized dunites. Diagrams showing P- Fe#Gar clusters for garnets and omphacites illustrate the differences between SCLM of these localities. Differences of Triassic SCLM from Devonian SCLM are in simple layering; abundance of Na-Cr-amphiboles and metasomatism in the upper SCLM part, thick pyroxenite - eclogite layer and lower part depletion, heated from SCLM base to 5.0 GPa. Kharamai mantle clinopyroxenes represent three geochemical types: 1) harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; 2) lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; 3) eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and Sr, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe#Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian -Triassic superplume, but with no signs of delamination.

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