کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6436319 | 1637563 | 2015 | 15 صفحه PDF | دانلود رایگان |
- δ7 Li (â°) were measured in white and dark mica at pegmatite field scale
- δ7 Li (â°) are interpreted with rare-element content from each sample and compared to literature data
- We suggest a possible inheritance of δ7 Li (â°) from a common crustal source to granite- and pegmatite-forming melts
- The role of δ7 Li (â°) as isotopic tracer of LCT-type deposits is discussed
Isotopic compositions in the Monts d'Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from â 3.6 to + 3.4â°. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = â 1.5â°). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs and Ta of the micas are consistent with pegmatite differentiation, δ7Li (â°) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from an intermediate unit (â 1.7â°) to an internal pegmatitic unit (+ 3.4â°). The nature of this δ7Li variation suggests that there was extensive fractional crystallization during the pegmatite's consolidation. The independence of δ7Li (â°) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (â°) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite-petalite subtypes) observed throughout the Monts d'Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tends to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica and garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallization at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems.
Journal: Chemical Geology - Volume 411, 14 September 2015, Pages 97-111