The ammonium content of granites in the English Lake District
The ammonium content of granites in the English Lake District
复制标题
英国湖区花岗岩的铵含量
作者：
D. C. Cooper;A. Bradley
对湖区岩基所有暴露成分的花岗岩进行了铵分析，以寻找富含铵的沉积原岩的证据。结果表明，湖区花岗岩的铵含量与岩石所经历的蚀变类型和程度有关。来自岩基所有部分的新鲜的、几乎没有改变的花岗岩的铵含量很低(<；30ppm)，而高度蚀变的岩石含有高达250ppm的铵。最富铵的岩石是高度蚀变的斯基多花岗岩样品，其中含有可观的次生白云母、绢云母和粘土。铵的富集度与蚀变类型和赋矿岩性有关。在斯基多花岗岩光环的堇青石和石榴石角岩带内的粉质泥岩中，铵的强烈亏损(？lt；×0.3)为铵的来源提供了证据。鉴于一些沉积物的低铵含量，以及已知的伴随高级变质作用和相关脱水的损失，花岗岩的低原生铵含量不被认为是支持或反对沉积原岩的证据。
Abstract Granitic rocks from all the exposed components of the Lake District batholith have been analysed for ammonium to look for evidence of an ammonium-rich sedimentaryprotolith. The results indicate that the ammonium content of Lake District granites is related to the type and degree of alteration experienced by the rocks. Fresh, little-altered granites from all parts of the batholith have low ammonium contents (< 30 ppm), whereas highly altered rocks contain up to 250 ppm ammonium. The most ammonium-rich rocks are highly altered specimens of Skiddaw Granite containing appreciable secondary muscovite, sericite and clay. The degree of ammonium enrichment is related to the style of alteration and host-rock lithology. Evidence for the source of ammonium is provided by strong depletion (< × 0.3) of ammonium in silty mudstones within the cordierite andgarnet hornfels zones of the Skiddaw Granite aureole. In view of the low ammonium content of some sediments and the losses known to accompany high-grade metamorphism and associated dehydration, the low primary ammonium contents of the granites are not considered to be evidence either for or against a sedimentary protolith.