2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Ba-Ti-rich micas from alkali Si-undersaturated rocks of the Calatrava volcanic field, Central Spain

Sharygin V.V.*, Schiazza M.**, Stoppa F.**

* V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia; ** Dipartimento di Scienze, Gabriele d'Annunzio University, Chieti, Italy

sharygin@igm.nsc.ru

The Miocene-Pliocene volcanism of the Calatrava field consists of over 200 monovolcanic centres which activity has been essentially explosive (Ancochea, 1999). It is represented by alkaline Si-undersaturated rocks (from tephritic nephelinite to olivine melilitite) and carbonatites (Ancochea, 1999, Cebriá, 1992; Lópes-Ruiz et al., 1993; Cebriá, Lópes-Ruiz, 1995; Bailey et al., 2005; Stoppa et al., 2011). These rocks contain abundant phenocrysts (up to 30 vol.%) of olivine and clinopyroxene. The groundmass of rocks consists of olivine, clinopyroxene, Ti-magnetite, fluorapatite as principal minerals and melilite, foids or feldspars. Depending on rock types the groundmass may bear melilite, leucite, nepheline, K-feldspar or plagioclase. Ba-Ti-mica, ilmenite, sodalite and perovskite occur rarely. The first data about the presence and chemistry of Ti-rich micas (without Ba determination) in groundmass of some Calatrava rocks (Morrón de Villamayor, Romani, Los Santos and Las Tontas) were previously given by Cebriá (1992).

Here we present new microprobe data for Ba-Ti-micas from some localities of the Calatrava volcanic field. Among all rock collection from Calatrava micas were found only in five samples from Morrón de Villamayor, Cuevas Negras, Asdrubal, La Vaquerizia and Colada de Ojailén. In all studied samples mica occurs as groundmass minerals and sometimes contains abundant inclusions of principal groundmass minerals. It should be noted that groundmass micas enriched in both Ba and Ti were previously described in alkali mafic rocks around the world (Mansker et al., 1979; Edgar, 1992; Zhang et al., 1993; Seifert, Kampf, 1994; Dunworth, Wilson, 1998; Greenwood, 1998; Kogarko et al., 2005; Sharygin, 2009; Sharygin, Kryvdik, 2010).

Microprobe data indicate strongly variable composition of micas in content of BaO (0.2-22.6 wt.%) and TiO₂ (7.7-13.0 wt.%) (Table 1). In general, most mica grains from particular rock do not show significant variations in chemical composition. The core-to-rim variations in some grains indicate the slight enrichment in BaO and depletion in K₂O. The reverse scheme also occurs. The compositions poorer in BaO (0.2-2.1 wt.%) and TiO₂ (7.7-9.1 wt.%) are typical of tephritic nephelinite from Colada de Ojailén whereas the high coupled enrichment in BaO and TiO₂ (up to 22.6 and 13.0 wt.%) is common of olivine melililite (La Vaquerizia) and olivine nephelinite (Asdrubal).

Table 1. Representative analyses (EMPA, wt.%) of Ba-Ti-micas from the Calatrava rocks.

Note: c, r - core and rim of grains; * - calculated from formula on the basis of 8 cations and 24 negative charges, Fe₂O₃ is estimated only as tetrahedral Fe³⁺ in formula. Ca-365 - olivine nephelinite, Asdrubal; Ca-373 - olivine melilitite, La Vaquerizia; Ca-344 - olivine leucitite, Morrón de Villamayor; Ca-389 - tephritic nephelinite, Villaneueva de San Carlos, Colada de Ojailén; Ca-362 - olivine nephelinite, Cuevas Negras.

To calculate formula for Ba-Ti-rich micas from Calatrava we used method on the basis of 8 cations and 24 negative charges, assuming an ideal variant when there are no vacancies in the octahedral and K sites. This method allows estimating maximal H₂O content (in the case when it did not analyze) and can not calculate octahedral Fe³⁺. Henderson and Foland (1996) previously indicated that cation-site deficiencies commonly reported for very Ba-Ti-rich micas in alkaline, basic igneous rocks may be artefacts of using 11-oxygen formulae rather than taking account of major replacement of excess O^2- for OH^- (oxymica substitution).

To identify the Calatrava Ba-Ti-micas we plotted their compositions on the classification diagram with ideal end-members (phlogopite - kinoshitalite - oxykinoshitalite - oxyphlogopite, Fig. 1) what has been available after the discovery of oxykinoshitalite and oxyphlogopite (Kogarko et al., 2005; Chukanov et al., 2010). According to this classification most micas from Colada de Ojailén and Morrón de Villamayor is titanian phlogopite; mineral from Cuevas Negras - barian oxyphlogopite, which is essentially higher in Ba than holotype (Chukanov et al., 2010); and micas from La Vaquerizia and Asdrubal - oxykinoshitalite, which is higher in Ba than holotype sample (Kogarko et al., 2005). No compositions are found in the kinoshitalite field. In general, a substitution K + Si + (Mg,Fe) + 2(F,OH) « Ba + Al + Ti + 2O is common of all the Calatrava micas.

The major and trace element compositions (Ancochea, 1999, Cebriá, 1992; Lópes-Ruiz et al., 1993; Cebriá, Lópes-Ruiz, 1995) show that all Calatrava rocks are approximately similar in the average contents of TiO₂ (0.7-3.8 wt.%) and Ba (445-1600 ppm). However, only mica from tephritic nephelinite and olivine leucitite indicates lower amounts of Ti and Ba. In the case of Ba it can be explained due to abundant presence of earlier feldspars in these rocks because these minerals (especially K-feldspar) are more preferable phases to accumulate Ba than mica.

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