Differentiating marine vs hydrothermal processes in Devonian carbonate mounds using rare earth elements (Kess Kess mounds, Anti-Atlas, Morocco)

作者：Franchi Fulvio^*; Hofmann Axel; Cavalazzi Barbara; Wilson Allan; Barbieri Roberto

来源：Chemical Geology, 2015, 409: 69-86.

DOI：10.1016/j.chemgeo.2015.05.006

• Kess Kess
• Carbonate mounds
• Hydrothermalism
• Carbonate geochemistry
• Rare earth elements
• Palaeo-redox

Petrological and geochemical analyses were carried out on Early Devonian Kess Kess mound limestones of the Seheb el Rhassel Group exposed in the Hamar Laghdad Ridge (Tafilalt Platform, Eastern Anti-Atlas, Morocco) in order to evaluate marine vs hydrothermal processes for the origin of the mounds. Hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter-mound facies as cavities, veins, and dykes.
Shale-normalized rare earth element (REE) patterns for limestones display pronounced light REE depletion compared to middle REE (average Pr-SN/Dy-SN = 0.46) and heavy REE (average Pr-SN/Yb-SN = 0.55), a super-chondritic Y/Ho ratio (up to 87) and positive La anomaly consistent with precipitation from normal marine seawater. Within our data set the Ce anomaly, as well as U and other trace element concentrations vary with facies providing evidence for variable redox conditions. The presence of positive Ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition. Samples from mound facies and quartz veinlets in the upper part of the group show REE patterns consistent with precipitation from hydrothermal fluids.
Early Devonian hypothetical seawater REE patterns were calculated from samples showing normal marine REE patterns (i.e. inter-mound facies) and from samples with weaker marine signature (i.e. fossiliferous mound limestones and plumbing system deposits). Hypothetical patterns are slightly enriched in Sigma REE compared to modern open ocean seawater.
This study reveals that REE analysis is a powerful tool for understanding polygenetic carbonate systems. It sheds light into the genesis of the Kess Kess mounds by reconstructing fluid pathways and palaeo-redox conditions.

• 出版日期2015-8-20