Almandine-spessartine garnets, from the Oxford County pegmatites and the Palermo No. 1 pegmatite, record significant compositional variations according to the degree of evolution of their hosting rock. Garnets from the most fractionated pegmatites (Mt. Mica, Berry-Havey, and Emmons) show the highest Mn, Nb, Ta, Zr, and Hf values, followed by those from the intermediate grade pegmatites (Palermo No. 1) and, finally, garnets from the barren pegmatites show the lowest values (Perham and Stop-35). Iron, Ca, and Mg contents follow an inverse order, with the highest contents in the latter pegmatites. Major element zoning shows increasing Mn values from core to rim in most garnet samples, while trace element zoning is not systematic except for some crystals which show a core to rim depletion for most of these elements. Chondrite normalized HREE (Heavy Rare Earth Elements) spectra show positive slopes for garnets from barren pegmatites, both positive and negative slopes for those associated with the intermediate pegmatite, and negative or flat slopes in garnets from the highly fractionated pegmatites. Ion exchange mechanisms, including Fe²⁺₋₁Mn²⁺₁, (Fe²⁺, Mn²⁺)₋₁Si₋₁Li₁P₁; and, (Y, Ho³⁺)₂(vac)₁(Fe²⁺, Mn²⁺)₋₃, could explain most of the compositional variations observed in these garnets. These compositional variations are the reflection of the composition of the pegmatitic magma (barren pegmatites originate from a more ferromagnesian magma than fractionated pegmatites); and of the coexisting mineral phases competing with garnets to host certain chemical elements, such as biotite, schorl, plagioclase, apatite, Fe-Mn phosphates, Nb-Ta oxides, zircon, xenotime, and monazite.

中文翻译：

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石榴石作为伟晶岩演化的指标：牛津伟晶岩领域伟晶岩案例研究（美国缅因州）

来自牛津郡伟晶岩和巴勒莫 1 号伟晶岩的铝镁锰石榴石根据其母岩的演化程度记录了显着的成分变化。来自分馏最多的伟晶岩（Mt. Mica、Berry-Havey 和 Emmons）的石榴石显示出最高的 Mn、Nb、Ta、Zr 和 Hf 值，其次是来自中级伟晶岩（巴勒莫 1 号）的石榴石，最后，来自贫瘠伟晶岩的石榴石显示出最低值（Perham 和 Stop-35）。铁、钙和镁的含量顺序相反，后者的伟晶岩含量最高。在大多数石榴石样品中，主要元素分带显示从核心到边缘的 Mn 值增加，而微量元素分带不是系统的，除了一些晶体显示大多数这些元素的核心到边缘耗尽。球粒陨石归一化 HREE（重稀土元素）光谱显示来自贫瘠伟晶岩的石榴石的正斜率，与中间伟晶岩相关的石榴石的正斜率和负斜率，以及来自高度分馏伟晶岩的石榴石的负斜率或平坦斜率。离子交换机制，包括 Fe²⁺ _-1 Mn ²⁺ ₁ , (Fe ²⁺ , Mn ²⁺ ) _-1 Si _-1 Li ₁ P ₁；和， (Y, Ho ³⁺ ) ₂ (vac) ₁ (Fe ²⁺ , Mn ²⁺ ) _-3，可以解释在这些石榴石中观察到的大部分成分变化。这些成分变化是伟晶岩岩浆成分的反映（贫瘠伟晶岩起源于比分馏伟晶岩更具铁镁质岩浆）；以及与石榴石竞争以容纳某些化学元素的共存矿物相，例如黑云母、斜长石、斜长石、磷灰石、Fe-Mn 磷酸盐、Nb-Ta 氧化物、锆石、磷钇矿和独居石。