Textures and geochemistry of the Saramta peridotites (Siberian craton): Melting and refertilization during early evolution of the continental lithospheric mantle M.A. Gornova ⇑ , V.A. Belyaev, O.Yu. Belozerova Institute of Geochemistry after A.P. Vinogradov SB RAS, 664033, P.O. Box 309, Irkutsk, Russian Federation article info Article history: Available online 15 November 2012 Keywords: Mantle partial melting Cratonic peridotite Trace element Refertilization abstract The Saramta peridotite massif is located within the Sharyzhalgai complex, SW margin of the Siberian cra- ton. The Saramta massif was formed in the Archean and then juxtaposed with granulites of crystalline basement of the Siberian craton. The Saramta harzburgites are highly refractory in terms of lack of resid- ual clinopyroxene, olivine Mg-number (up to 0.937), and spinel Cr-number (0.5), suggesting high degree of partial melting. Detailed study of their microstructures shows that they have extensively reacted with a SiO 2 -rich melt, leading to the crystallization of orthopyroxene, clinopyroxene, amphibole and spinel at the expense of olivine. The major element compositions of the least reacted harzburgites are similar to the residues of refractory peridotites produced by the fractional melting (initial melting pres- sures >3 GPa and melt fractions 40%). Moreover, non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti, but highly enriched in LREE. A two-stage history is proposed for the Saramta peridotite: (1) primitive mantle underwent depletion in the garnet stability field followed by melting in the spinel sta- bility field; (2) refractory harzburgites underwent refertilization by SiO 2 -rich melt in supra-subduction zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result of such a melt–rock reaction. The Saramta peridotites are similar to low-T coarse-grained peridotites of subcraton- ic mantle. Processes of their formation, as reflected by textures and composition of minerals of the Sara- mta peridotites, are characteristic of the early stages of subcratonic mantle formation. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Studies of peridotite xenoliths from kimberlite pipes and orogenic peridotite massifs provide direct information about sub- continental lithospheric mantle. Peridotite xenoliths from kimber- lites are subdivided into two types: coarse-grained and deformed. Coarse-grained peridotites are characterized by low equilibrium temperatures and pressures (<1100 °C, 2–6 GPa), and represent the upper part of cratonic lithospheric mantle formed mainly in the Archean (Pearson et al., 2003). This type of peridotites demon- strates high Mg-number of olivine (0.915–0.94) and high modal orthopyroxene up to 55 vol.%. High Mg-number indicates their ori- gin as a residue after >30% melting and high content of orthopyrox- ene suggests polybaric melting at 3–4 GPa (Walter, 1998). Melting reaction of peridotite in this pressure interval is as follows: clino- pyroxene + garnet + olivine ? orthopyroxene + liquid, and can re- sult in formation of residue, containing not more than 30 vol.% orthopyroxene (Kelemen et al., 1998; Kinzler and Grove, 1998; Walter, 1998). For higher orthopyroxene modes (>30%) in coarse- grained peridotites, several explanations were suggested: (1) metamorphic differentiation (Boyd, 1989), (2) mixing of low- orthopyroxene residual peridotites with high-pressure igneous cumulates containing high proportion of orthopyroxene (Herzberg, 1993), and (3) reaction of the rock with SiO 2 -rich melt (Kelemen et al., 1998). Recent studies revealed evidence supporting wide occurrence of lithospheric mantle interacted with percolating melts (Kelemen et al., 1997; Dijkstra et al., 2003; Seyler et al., 2007; Brunelli et al., 2006). This process leads to dissolution of some minerals and crystallization of the others. The above transformations can be found in textures of rocks and composition of newly formed minerals. The melt-peridotite interaction takes place both beneath mid-ocean ridges and in supra-subduction zones. Beneath mid- ocean ridges, deep-formed melts become orthopyroxene-undersat- urated during rise up to the surface, and react with the mantle per- idotites, precipitating olivine at expense of orthopyroxene. This process results in formation of reactive dunites, which act as trans- port channels of melts. These reactive dunites were recognized in the mantle section of the Oman ophiolite, which is classic ophiolite example (Kelemen et al., 1995). In the Oman, clinopyroxene and spinel from reactive dunites are in equilibrium with N-MORB (Nor- mal Mid-Ocean Ridge Basalt) melt, while surrounding harzburgites are not in equilibrium with such a melt (Kelemen et al., 1995). The last low-pressure fractions of melt from depleted peridotite can 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.10.004 ⇑ Corresponding author. Tel.: +7 3952 429564; fax: +7 3952 427050. E-mail address: magornova@yandex.ru (M.A. Gornova). Journal of Asian Earth Sciences 62 (2013) 4–17 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes