121 Petrology, Igneous and Experimental

Tuesday, 29 October 2002
8:00 AM - 12:00 PM

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

Paper number: 121-8
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

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.