MINERAL ZONING IN THE BELL ISLAND PLUTON, SE ALASKA, WITH IMPLICATIONS FOR THE BEHAVIOR OF REE IN IGNEOUS ROCKS

Quantification of mineral zoning patterns can illuminate otherwise cryptic patterns of homogeneity or heterogeneity in plutons. Preliminary analysis of mineral zoning in the Bell Island Pluton, a large (>1300 km2) quartz diorite intrusion, suggest that amphibole records intra-plutonic variation in both bulk chemistry and pressure of crystallization. On the other hand, there is a surprising consistency in the crystallization history of allanite. Allanite is ubiquitous in the tonalite-quartz diorite Bell Island Pluton, with an average mode near 0.05 wt.%. Allanite generally occurs as cores in igneous epidote-clinozoisite and exhibits characteristic and consistent zoning patterns. REE-rich cores (all40-70%) grade outwards to epidote-clinozoisite with REE below microprobe detection limits. LREE (La, Ce, Pr) are highest in the allanite-rich cores of zoned crystals. Nd and Sm both initially increase as total REE 's decrease and are highest in intermediate zones. Y is generally low throughout, but tends to be highest in REE-rich epidote with less than 20% total allanite component. Approximately 80% of La and Ce , but less than 10% Y in the Bell Island Pluton is contained in allanite. Taking Y as a proxy for HREE (specifically Ho), fractionation of the observed amount of allanite from a the Bell Island quartz diorite would result in REE patterns evolving from strongly LREE-enriched to nearly flat. Perhaps more critical for interpreting REE behavior is that removal of allanite at various stages of crystallization will generate a suite of derivative, non-parallel REE patterns that is independent of and can be superimposed upon REE patterns related to major phase behavior.