• journal_title：The Canadian Mineralogist
• Contributor：Milan Novák ; Petr Gadas
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.3.629
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：3
• firstpage：629
• section：Articles

A zoned lenticular body of anorthite- and grossular-bearing leucotonalitic pegmatite, ~3–4 m thick and ~20–30 m long, cuts serpentinized lherzolite at Ruda nad Moravou, Staré Město Unit, Czech Republic. Its zoned internal structure consists of the following (sub)units from the contact inward: contact unit (Amp > Di), a highly heterogeneous transitional unit dominated by a leucocratic subunit (Pl > Qtz >> Di ~ Ep > Grs) with lesser proportions of grossular (Grs >> Qtz > Di) and diopside (Pl > Qtz ≈ Di) subunits, a coarse-grained unit (Pl > Qtz >> Grs ≈ Ep) with a graphic intergrowth of grossular + quartz, a graphic-blocky unit (Pl > Qtz > Kfs >> Grs), and a quartz core. White to grayish plagioclase An_98–19 forms anhedral to euhedral grains, 0.1–2 mm to 10 cm in size. White to deep green K-feldspar occurs exclusively in the graphic-blocky unit. Colorless to pale brown grossular, varying from Grs_98–100Adr_0–1Prp_0–1 (grossular subunit, quartz core) to Grs_73–86 Adr_11–24Sps_1–2Prp_0–1Sch_0–4 (graphic intergrowths with Qtz), is present in all textural–paragenetic units in highly variable proportions. Subhedral to anhedral grains of pale to dark green diopside (X_Mg 97–82) ≤5 cm long occur almost exclusively in the contact unit and diopside subunit. Dark green grains of epidote-group minerals (epidote – clinozoisite – dissakisite – allanite), 0.1–20 mm in size, occur also in the outer units. Accessory minerals include common zircon as euhedral crystals of variable habits, ≤1 cm in size, locally associated with rare baddeleyite, U-rich zirconolite and gittinsite. Accessory titanite, fersmite, pyrochlore–betafite, apatite, monazite-(Ce), rutile, and biotite also were identified, along with several phases associated with subsolidus alteration: albite, clinozoisite, white mica, actinolite, Nb-rich titanite, Ba-rich feldspars, harmotome, chabazite-Ca, armenite, chlorite, prehnite, and pumpellyite-(Al). Plagioclase, diopside, and zircon show trends (decreasing Ca/Na, Mg/Fe, and Zr/Hf from outer to inner units) very similar to those expected of fractional crystallization. No simple compositional trends were recognized in grossular and epidote-group minerals except for decreasing ∑REE inward in the latter. Elevated contents of Mg and Cr in diopside and epidote-group minerals, limited exclusively to the contact unit and the diopside subunit, were controlled by contamination from the host serpentinite. Very low amounts of H₂O-, F-, P-bearing minerals and an absence of micas strongly suggest low activities of H₂O, F and P, whereas minerals containing CO₂, B, Li and Be are absent. These findings imply a pegmatite-forming melt poor in H₂O and other volatiles that crystallized sequentially inwardly from the contact with host rocks; crystallization likely proceeded during a single process. The mineral assemblages and chemical compositions of the minerals suggest a P > ~0.5–0.8 GPa and T > ~650–780°C of initial primary crystallization (liquidus conditions). The zonal internal structure, pegmatitic textures and fractionation trends in some minerals indicate rapid to moderate undercooling of the pegmatite-forming melt; low amounts of fluxes fit well with the boundary-layer model. The pegmatite from Ruda nad Moravou represents a new type of non-granitic pegmatite, leucotonalitic to quartz leucogabbroic in composition, significantly Ca-rich and with minor contamination by Mg.