Compositional evolution (Li, Be) in cordierite-group minerals from metamorphic and magmatic rocks of the Bory Granulite Massif, Moldanubian Zone, Czech Republic and its implications for origin of related granitic pegmatites

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-02-10 DOI:10.1016/j.chemgeo.2025.122668

S. Hreus , J. Kocáb , M. Novák , M. Vašinová Galiová , P. Gadas

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Abstract

Samples of cordierite-group minerals (CGM) from various rocks in the Bory Granulite Massif (BGM), Moldanubian Zone - granulite (1), leucosome to pegmatitic leucosome in migmatites (2), small zoned granitic pegmatites (2) and large, complexly zoned granitic pegmatites (11) were examined by the means of EPMA and LA-ICP-MS. CGM are morphologically and compositionally highly variable from subhedral grains, ∼0.5–5 cm in size, of cordierite (XMg₅₇) from granulite, subhedral grains, < ∼5 cm, of cordierite-sekaninaite (XMg₅₆_–₄₄) from leucosome to pegmatitic leucosome, cordierite (XMg₆₀) from graphic unit and subhedral grains to large conic subhedral crystals, up to 0.7 m long, of sekaninaite (XMg₂₈_–₃) mainly from albite unit of complexly zoned pegmatites. Concentrations of Be and chiefly Li in CGM vary significantly - granulite (Be = 84–89 ppm, Li = 204–242 ppm), leucosome to pegmatitic leucosome (Be = 184–317 ppm, Li = 204–338 ppm), complexly zoned pegmatites (outer and blocky units - Be =150–200, Li = 577–904 ppm; albite unit - Be ≤51 ppm, Li = 572–3662 ppm; ≤0.79 wt% Li₂O). The following substitution mechanisms were revealed: homovalent R²⁺ substitutions - major Mg = Fe in all types of CGM, minor Mg + Fe = Mn in sekaninaite with low XMg, moderate to minor heterovalent substitution NaLi = □ R²⁺and insignificant substitution NaAl = □ Si in sekaninaite from complexly zoned pegmatites. Textural evolution from small bodies of leucosome to large complexly zoned pegmatites with Li-rich sekaninaite, and to Li-rich lepidolite pegmatites in BGM somewhat resembles evolution of simple anatectic pegmatites to albite-spodumene pegmatites from the Austroalpine Unit Pegmatite Province (Eastern European Alps); however, pegmatites from BGM with Li-bearing sekaninaite and lepidolite + petalite crystallized at lower P ∼ 2 kbar. Variations in concentrations of Be and Li in the individual paragenetic types of CGM and chiefly high Li and very low Be in sekaninaite manifest that pegmatitic melts underwent several steps of magmatic fractionation evident chiefly in complexly zoned pegmatites.

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捷克Moldanubian地区Bory麻粒岩地块变质岩和岩浆岩中堇青石组矿物的组成演化（Li, Be）及其对相关花岗伟晶岩成因的指示意义

采用EPMA和LA-ICP-MS对Bory麻粒岩地块（BGM）、Moldanubian带麻粒岩(1)、混染岩中的白粒体-伟晶体(2)、小带状花岗伟晶岩(2)和大、复杂带状花岗伟晶岩（11）中不同岩石的堇青石群矿物（CGM）样品进行了研究。CGM在形态和成分上有很大的变化，从大小约0.5 - 5cm的堇青石（xm57）到麻粒岩、半面体颗粒，<；~ 5 cm的堇青石-绢英石（XMg56-44）从白色小体到伟晶质白色小体，堇青石（XMg60）从图形单元和亚面体颗粒到大的锥形亚面体晶体，长达0.7 m，绢英石（XMg28-3）主要来自复杂分带伟晶岩的钠长石单元。CGM中Be（主要是Li）的浓度变化显著——麻粒岩（Be = 84-89 ppm, Li = 204-242 ppm）、白色小体到伟晶岩白色小体（Be = 184-317 ppm, Li = 204-338 ppm）、复杂带状伟晶岩(外部和块状单元——Be = 150-200, Li = 577-904 ppm；钠长石单位- Be≤51 ppm， Li = 572-3662 ppm；≤0.79 wt% Li2O)。结果表明：在所有类型的CGM中，均存在同价R2+取代——主要的Mg = Fe，在低XMg的绢铁铁矿中存在次要的Mg + Fe = Mn，在复杂带状花岗岩中，绢铁铁矿中存在中等至次要的异价取代NaLi =□R2+和不显著的取代NaAl =□Si。BGM从白色小体到含富锂绢云母的大型复杂分带伟晶岩，再到富锂的锂云母伟晶岩，其结构演化过程与奥阿尔卑斯单元伟晶岩省（东欧阿尔卑斯）的单纯净长石伟晶岩到钠辉石伟晶岩的演化过程有些相似；然而，BGM的伟晶岩与含锂的绢云母和锂云母+花瓣岩在较低的P ~ 2 kbar下结晶。不同共生型CGM中Be和Li含量的变化，以及以高Li和极低Be为主的绢绢岩表明，伟晶岩熔体经历了多次岩浆分馏，这主要体现在复杂分带伟晶岩中。

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.