Yang and Rivers
 Geological Materials Research
 v.4, n.1
The origin of Mn and Y annuli in garnet and the thermal dependence of P in garnet and Y in apatite in calc-pelite and pelite, Gagnon terrane, western Labrador

Panseok Yang and Toby Rivers

Department of Earth Sciences, Memorial University of Newfoundland St. John’s, NF, Canada A1B 3X5 <pyang@mun.ca, trivers@sparky2.esd.mun.ca>
(Received June 25, 2001; Published March 11, 2002)


X-ray compositional maps and zoning profiles for major and trace elements have been measured in garnet, apatite and epidote-group minerals from calc-pelitic and pelitic schists from western Labrador, by electron microprobe and by laser ablation ICP-MS. High Y abundance in apatite adjacent to resorbed garnet and a decreasing modal abundance of apatite with garnet growth indicate that apatite participates in major rock-forming mineral reactions. Phosphorus concentration in garnet porphyroblasts coexisting with apatite decreases smoothly from core to rim and depends moderately on metamorphic grade. Apatite coexisting with xenotime shows high-Y cores and the concentration of Y in apatite increases with metamorphic grade.

Many of the analyzed garnet porphyroblasts exhibit Y annuli which provide information about different physicochemical processes operating during garnet growth. We describe criteria to fingerprint three of these processes in the analyzed garnets. (i) Yttrium annuli produced by garnet resorption and regrowth are characterized by an asymmetric shape with a steeper slope on the inner side of the annulus and by a decoupled variation between garnet-compatible and garnet-incompatible elements at the resorption margin. (ii) Y annuli produced by breakdown of Y- and REE-rich trace phases such as xenotime, monazite, apatite and epidote-group minerals are indicated by REE peaks in LREE (monazite, allanite), MREE (epidote), or HREE (xenotime, zircon). (iii) Y annuli associated with diffusion-controlled growth occur in garnet with oscillatory textural zoning defined by alternating inclusion-rich and inclusion-poor zones, with increases in concentrations of garnet-compatible elements such as Y and HREE correlating with inclusion-free zones. Mn annuli in some garnets were produced by sporadic local breakdown of Mn-rich epidote and in several cases indicate that thin-section scale equilibrium during annulus formation was not achieved, raising questions concerning the use of Mn concentration as a time marker in garnet growth modeling.

Keywords: garnet, apatite, Y and Mn annuli, phosphorus zoning, REE peaks

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Copyright © 2002 by the Mineralogical Society of America