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)
Abstract
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
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