Nagoya University
NagoyaUniversity

Whatminor

phasesreveal

the timing of major

aboutphase growth

DUNKLEM
Daniel J.
NTPR, .Ikepan

Accessory minerals

occur

in all rocktypes as significant

hostsfora widevarietyoftrace

Although the main body of research into the fbrmationof high-temperature
metamorphic
lithologies
isbased on the chemical interaction
ofmajormineralphases,there isin the role of accessory phases in petrogenesis.
This change in fbcus isgrowing interest
largelydue to advancesin microbeam
techniques that allow fbr quantitative
analysis andcompositional
imaging of sub-100 micron mineral grainswithin thin sections, wuereas
elements,

radioisotope-bearing

hostsamples

minerals

and analyzed

such as monazite

and zircon are conventionallyto determinethe timing ofpetrogenesis,

separatedfromrecent studies by electronion (SHRIMP)and laser-ablation(CHIME-EMP),(LAICPMS)analysis of these minerals in
petrographiccontext reveal that,likemajor metamorphic
phases,accessory minerals havecomplexhistories
of growth that can be tied to specific processesduringthe progradeand
retrograde stages of a metamorphic

cpisode.

In the firstexample,
monazitein a single thin-sectionofmetapelitefrom the granulite-grade
Higo Terrane of central Kyushu reveals a wealth of associations with metamorphic
anddeformationaltexturesin the major phases,Monazite occurring with enhedral growth zones

inhigh-Ca,
low-Mg cores of garnet,and as clusters thatrepresent pseudomorphs ofallanite, represents growth beforeand
duringpartialmelting
at peak metamorphic
enclosedtemperatures, Flattenedgrainsand trailsof low Th

biotiteto definean S2

monazite,aligned withsillimanite andrepresent
planar fbliation,
growth during retrograde
metamorphismand defbrmation.
Moderate Th ! low Y domains on resorbed monazite
grain
margins
recordlate-stage
retrogression,
with hydrous fluids
releasedprobabiy in associationby the crystallization of anatectic melt. CHIME ages of the monaz{te growth stages cannot beresolved;
CHIME monazite and zircon and SHRIMP zircon ages from a variety of Higolithologies
restrict the timing of major
mineral growth to 110-130Ma. Resultsfrom otherdatingmethods
whichsuggestthat peak metamorphism
occurred at c.250Ma
appear to be
influencedby isotopic
inheritance.

axial

grainsfrom metapelite samples of the granulite-gradeLutzowHolm Complex of east Antarcticashow
a complex
rangeof textures and ages. wnereas
previousSHRIMP ages from zircon grain separates concentrate around 530-550Ma, and wereinterpreted
as timing peak metamorphism,
newanalysesyield a spread of age estimates from510 to 61OMa. In-situ
analysesof zircon
inclusions
in garnetporphyroblasts
reveal a stage ofIn the second

example,zircon-27NII-Electronic Library Service

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Vrich zircon growthat c.600Ma,
with flat
HREE-MREE
indicatingprogradegrowth
profilesin the presenceof garnet.Zircon preservedin a garnetmegacryst grown in association withfelsicpegmatitethat fi11sD2 boudin necks in a metapelitic layershow multiple stages ofmetamorphic
withoutwardcores, c,570Ma
rims
growth,with c,600Ma flatH-MREE (garnet-equilibrated)
steep to flat
H-MREE growth zoning, and resorbtion priorto incorporationinand crystallization of the felsic
pegmatiteat c,540Ma,

the garnetmegacryst

100Ma spread of zircon

Although the

be interpreted
as recording
separate metamorphic
events,associations
between zircon, garnetand felsicmelt suggests thatages couldthe texturaland chemical

event,
growth was progressivethrough a prolongedmetamorphic
proportionof zircon growth at 530-550Ma occurred during retrograde
zirconandthat a large

ratherthan peakmetamorphism.

In both examples,
withoutInagedata cannot be usedcarefu1 exarnination

metamorphic

ofthe

terraneswithgeochronologythrough micrebeam

to constrain

the timing of major

phase growth
between minor and major phases.
histories,petrographically-constrainedparageneticrelationships
complicatedtechniques isessential.

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