کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
6441184 1638719 2011 16 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Mineral stability in peralkaline silicic rocks: Information from trachytes of the Menengai volcano, Kenya
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
Mineral stability in peralkaline silicic rocks: Information from trachytes of the Menengai volcano, Kenya
چکیده انگلیسی

Electron microprobe analyses are presented for phenocrysts and matrix glass in peralkaline, silica-oversaturated trachytes from the Menengai volcano, Kenya Rift Valley. The dominant phenocryst assemblage is alkali feldspar-hedenbergite-fayalite-titanomagnetite-apatite. Aenigmatite, amphibole and quartz occur rarely in more peralkaline rocks. QUILF calculations indicate that the trachytic magmas crystallised at temperatures of 854-870 °C, at relatively low oxidation states (ΔFMQ − 1.6 to − 1.7) and silica activity (aSiO2 (Qtz) of 0.60-0.66). The new analyses are used, along with published data, to outline the distribution of the main phases in the compositional spectrum of peralkaline quartz trachytes and rhyolites. There is uncertainty about the nature, or even existence, of a low-temperature zone in the alkali feldspar primary phase region, the equivalent of the thermal valley in the haplogranite system. Quartz phenocrysts may appear early or late in the crystallisation sequence, even in rocks of similar bulk composition, its appearance perhaps being a function of the F content of the melts. Whereas hedenbergite and fayalite show fairly systematic compositional trends with increasing host rock peralkalinity, amphibole compositions are variable, for reasons not yet understood. Aenigmatite crystallisation is at least partly controlled by oxygen fugacity and silica activity. With rare exceptions, ilmenite and titanomagnetite are incompatible phases but the factors controlling their relative stabilities are not clear. It appears that peralkaline trachyte-rhyolite sequences evolve along many crystallisation paths, the paths perhaps being strongly influenced by pH2O, pF2, melt F/Cl ratios and perhaps total pressure.

Research highlights► Phenocryst and glass compositions in trachytes, Menengai volcano. ► Mineral-melt relationships in peralkaline silicic rocks. ► P-T-X controls on mineral stability with increasing peralkalinity. ► Nature of low-temperature zone in alkali feldspar primary phase region.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Lithos - Volume 125, Issues 1–2, July 2011, Pages 553-568
نویسندگان
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