WHAT ROCK IS IT? CHARACTERIZATION OF MAGMATITES FROM THE MAGNET COVE IGNEOUS COMPLEX, HOT SPRING CO., ARKANSAS

Alkali magmatites from the 94 Ma old Magnet Cove Igneous Complex (MCIC) include aphanites and phanerites of insufficient mineralogical and chemical characterization. Main reasons are the small grain size of the minerals and the presence of a hydrothermal alteration mineralogy. Both make the identification of minerals difficult and hamper the collection of quantitative, petrographic data. This means that genetic interpretations of the MCIC are problematic.

Our recent investigations demonstrate that these difficulties can be overcome by a substantial improvement of the investigation methodology.

(1) Conventional light optical polarization microscopy is used to examine polished thin sections first to assess the recognizable mineralogical composition qualitatively under transmitted and reflected light.

(2) XRD/Rietveld results give identification and quantities of minerals. They have to be refined by using the microscopic findings. This allows plotting on the A-P-F diagrams for aphanites and phanerites, and the derivation of the rock names.

(3) Comprehensive whole-rock chemical analysis results are used for the rock identification based on TAS-diagram plotting and further characterization.

Only the combination of individual results such derived allow recognition of rock type, quantitative mineralogy and petrochemistry that are the basis for any genetic interpretations. These techniques are also seen as the most practical and necessary approach to obtain the essential data needed for proper characterization of any rock.