P-T-fluid conditions of mineral equilibria in garnet-biotite crustal xenoliths from the Yubileinaya and Sytykanskaya kimberlite pipes, Yakutian kimberlite province.
Authors: Seliutina Natalia , Safonov Oleg , Yapaskurt Vasiliy , Varlamov Dmitry , Sharygin Igor , Konstantinov Konstantin 
Published: March 2021

&lt;p&gt;This study provides the results of research of the garnet-biotite crustal xenoliths from the Yubileinaya (372&amp;#177;4.8 Ma) and Sytykanskaya (363&amp;#177;13 Ma) kimberlite pipes of the Alakit-Markhinsky field (Siberian craton). Isotopic evidence on zircons from similar crustal xenoliths (Grt+Bt+Pl+Kfs+Qtz&amp;#177;Scp) showed Archean Hf model ages (TDM = 3.13-2.5 Ga) and thus indicated that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time (Shatsky et al., 2016).&lt;/p&gt;&lt;p&gt;The xenoliths are represented by the assemblage Grt+Bt+Pl+Kfs&amp;#177;Opx. Quartz is present only as rare inclusions in garnets. The rocks are coarse-grained, slightly foliated with garnets porphyroblasts of up to 5 cm in size. A spectacular feature of the rocks is an abundance of K-feldspar. Garnet grains are almost compositionally homogeneous, although they show a rimward decrease of the Mg and Ca contents indicating exchange reactions during cooling. Biotites are characterized by high F increasing from 1.5 wt.% in cores up to 2.2 wt.% in rims, as well as TiO&lt;sub&gt;2&lt;/sub&gt; up to 7.8 wt.%, which is typical for high-grade rocks. Orthopyroxene (up to 5.5 wt. % Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) relics are preserved both as inclusions in garnet and as individual grains in the rock matrix. Plagioclase occurs both as separate grains and as lamellae in potassium feldspar.&lt;/p&gt;&lt;p&gt;The bulk chemical compositions correspond to a metagraywacke. The REE spectra in these rocks are rather flat with slight enrichment in LREE. All the studied rocks are characterized by a distinct negative Eu anomaly (Eu/Eu* = 0.31-0.45).&lt;/p&gt;&lt;p&gt;Calculations using the PERPLEX software version 6.7.6 (Connolly, 2005) for Mg and Ca in Grt, Mg in Bt, and Ca in Pl indicated temperatures 630-730&amp;#176;C and pressures 5.8-7.2 kbar for the rocks. However, equilibria involving Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; in orthopyroxene corresponds to temperatures of 750-800&lt;sup&gt;o&lt;/sup&gt;&amp;#1057; at a similar pressure. It indicates that metamorphism of the garnet-biotite rocks reached higher temperatures, but they were actively modified later during cooling and insignificant decompression (by about 1 kbar). Calculations using the TWQ software version 2.3 (Berman, 2007) indicate consistent temperatures 610-680&amp;#176;C for the garnet-orthopyroxene and 640-690&lt;sup&gt;o&lt;/sup&gt;C for garnet-biotite Mg-Fe exchange equilibria. Calculations using the Grs+2Prp+Kfs+H&lt;sub&gt;2&lt;/sub&gt;O=Phl+3En+3An equilibrium demonstrated water activity below 0.1. Such low water activity could indicate an influence of highly concentrated alkaline Cl-F-bearing brines. This assumption is confirmed by extensive development of potassium feldspar, absence of quartz in the matrix, and elevated Cl contents of biotite, 0.1-0.3 wt. % at high #Mg (&gt;0.7) and F content.&lt;/p&gt;&lt;p&gt;&lt;em&gt;The study is supported by the Russian Science Foundation project 18-17-00206.&lt;/em&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;strong&gt;References &lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Berman, R. G. (2007). winTWQ (version 2.3): a software package for performing internally-consistent thermobarometric calculations.&amp;#160;Geological survey of Canada, open file,&amp;#160;5462, 41.&lt;/p&gt;&lt;p&gt;Connolly, T. M., &amp; Begg, C. E. (2005).&amp;#160;Database systems: a practical approach to design, implementation, and management. Pearson Education.&lt;/p&gt;&lt;p&gt;Shatsky, V. S., Malkovets, V. G., Belousova, E. A., ... &amp; O&amp;#8217;Reilly, S. Y. (2016). Tectonothermal evolution of the continental crust beneath the Yakutian diamondiferous province (Siberian craton): U&amp;#8211;Pb and Hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes.&amp;#160;Precambrian Research,&amp;#160;282, 1-20.&lt;/p&gt;