APPLICATION OF HIGH SPATIAL RESOLUTION MAJOR AND TRACE ELEMENT ANALYSES OF ACCESSORY MINERALS IN METAMORPHIC TECTONITES BY ELECTRON MICROPROBE

In-situ microanalyses of accessory minerals have been drawing renewed attention from researchers, using techniques such as SIMS, LA-ICP-MS and EPMA, to solve various geologic problems. However, due to the large beam size (~10-60 μm) of SIMS and LA-ICP-MS, some micron-sized crystals with narrow zones or thin rims (< 5 μm) cannot be accurately analyzed. These narrow domains commonly provide key constraints on the petro-tectonic history. The recently installed CAMECA SXFive field emission (FE) EPMA at University of Florida allows high-precision major and trace element analyses at high spatial resolution (e.g., 1-5 μm), making it possible to accurately analyze the narrow domains. We have applied newly developed techniques to garnet-cordierite migmatites from the central Damara Belt in Namibia. EPMA X-ray mapping of monazite reveals seven generations of growth. Monazites in garnet are typically tiny (10-20 μm), showing four narrow zones with decreasing Yttrium (Y) concentrations (G1-G4), whereas monazites in biotite (and some quartz) commonly reveal four zones with increasing Y contents (G4-G7), in which the high-Y rim (G7) is only 2-5 μm wide. Some monazite grains in cordierite or associated with apatite preserve all seven generations (G1-G7) within one grain. The Y-zoning patterns either indicate monazite growth with coexistence of garnet formation (G1-G4, prograde metamorphism) or during garnet breakdown (G4-G7, retrograde metamorphism). In addition, geothermometers like zircon and quartz show varying occurrences as inclusions or assemblages with cordierite, biotite, apatite, and monazite. The CAMECA SXFive FE EPMA has been used to date these narrow monazite domains based on the U, Th, Pb concentrations, and to estimate the metamorphic temperatures based on the Ti content in zircon and quartz. A detection limit of 95-120 ppm, 45-90 ppm, 50-55 ppm, and 6 ppm for Th, U, Pb, and Ti respectively have been achieved. Moacyr monazite, NIST 610 and natural homogenous zircon crystal (N-Zrn-3G) were chosen as secondary age or elemental standards to monitor the EPMA data. In this study, we will show how detailed mineral growth history and regional metamorphic (T-t) evolution can be unraveled by using EPMA to analyze the micro-scale textural and geochemical information from accessory phases in the metamorphic tectonites.