TRACE-ELEMENT COMPOSITION OF GARNET AS A TOOL TO DISTINGUISH PEGMATITE FAMILIES

Garnet is a common accessory mineral in both, NYF and LCT families of granitic pegmatites. The composition of garnet from granitic pegmatites falls within the almandine-spessartine ((Fe Mn)₃Al₂Si₃O₁₂) solid-solution series. Major-element compositions of garnet from LCT pegmatites (Alm_0.00-73.02Spss_19.87-95.20) overlap those of garnet from NYF pegmatites (Alm_5.21-86.78Spss_2.26-90.86). Here we examine the trace-element composition of garnet to determine geochemical differences between families and to aid in the classification of granitic pegmatites, including those of doubtful chemical affiliation. Trace-element compositions of garnet from 28 pegmatites, representing NYF and LCT families, were obtained by LA-ICP-MS. Particular elements (Li, Na, P, Sc, Ti, Zn, Ga, Ge, Y, Zr, Sn, Gd, Tb, Dy, Ho, Er, Tm, Yb, Hf, and Ta) were chosen for detailed statistical analysis based on common substitutions within the garnet structure as well as having <10% of all observations below the detection limit. These were examined using principal component analysis (PCA) and linear discriminant analysis (LDA) following a log-ratio transformation. Results of the PCA suggest that the elements Gd, Tb, Dy, Y, Ho, Er, Tm, and Yb are indicative of NYF garnet, whereas Li, P, Ga, Ge, Zn, and Ti are typical of LCT garnet. Garnet from the Spruce Pine District in North Carolina exhibits elemental enrichment closely matching that of NYF pegmatites. However, garnet from the LCT pegmatites of the Pala District in California show affinity towards Zr, Hf and Ta, while those from Mt. Apatite (Maine), Greenlaw (Maine), and Batchellerville (New York) show a “mixed” geochemical signature. Successful classification results using LDA varied based on the training set and a priori classification scheme. A larger dataset would increase the validity of this method but preliminary results suggest that LDA could be used to distinguish between pegmatite families using trace element contents in garnet.