-
W
NORGES GEOTOGIS$ UNDERSO(EIS€
Oliprinl NGU BuLe1i.424, 65 74
D
P
WYN JAMES. JOHN G M]TCHELL,
R CI]ARD INESON & @YSIEIN NOFOGULEN
Geology and K/Ar chronology of the Malvika
scheelite skarns. Central Norwegian
Caledonides
Trondheim 1993
Geology and K/Ar chronology of the MAlvika scheelite skarns, Central Norwegian Caledonides
D
WYN JAMES, JOHN
G MTCIELL.
P. BICHARD NESON & OYSTEIN NORDGIJLEN
G.
F
M lchel. J
neson. p
a Nordguten. o. t993 ceoogy and KiAr chrono ogy or
rhe Mrrv ka sch€€ rro skarns,centratNoryeO an Cat€don des No. g6at. Lnde6 BDlt i24,6s-i4
James O.W,
Skarns prossnt on tho nodhern shore or rosentjord ov€Dnnr re 52 tabrc or rhe adiac€nt qne ss
(whrch rormod durin! peak regonar meramorphsm) ToA€rher wrh p€lroo.aphc evdenc€ ot iho
ate crystalzaton o, sch€€ne ths nd'cares thaj mnera zarion was posi moramorphic and ep
gonetc.KrArapparenragesotnn6mneraseparares(tourposr-scheeleamphboesrronsrarn.
lvo muscov i€s and rhree boties t'om inttusionn 6re concordanl ar rhe i. tever qivnq an tiver
ss variance woighi€d M6an (rvwM)age ot 402 1 2 Ma The crose agreeneil or r€surs rom
dttor€nl mn6r6s potnG to rapd posr orogenic uprn n rhe cenra Scandinavian caedond€s
Two oth€r poslsch66ire amphbore skarn sanp€s gave ndsrnouisriabe tal to) apparent ag€s
whose mean s 472
age woutd subsranra y consirain rhe iming ot regonar
matamolprrsh and mneiarzaton. However, ihe possb iry rh6r rhe ages are spurous due to the
pr€sonce oloxcess araon. cannot be excuded
D
wyn Janos & p. Rbhad tnesan Dep6 nent at Eanh Sdenc6s. UnrveEjty at Shened, Shet
hetd s3 7HF, u.K.
John G Mnchelt, schaat at phystcs, ,he unlvotsity, Newcas e upan rvne. NEl 7RU uK
Aysbn Nordguten Naryes geajaoBte LndeEorej\p pastbar\ 3006 Laae N.70a2 r@n.thenr.
Nway
lntroduction
Scheelite was flrstdiscovered within the Helge
lafd Nappe Compex (HNC) in 1969. lt was
ocated n skarns and quarlz ve ns, associated
with marble (Nissen 1969), near Mosloen (Fig.
1). Subsequently, scheelite was delecl€d in the
gold arsenopyrile deposits ol the Kolsvik area
(Skaarup 1974). A reconnaissance exploration
programme ior scheelite, in 1970, led to the
discovery oi several additional occurrences
on either s de of Tosenfjord. Briel descriptions
oi the geology and petrography of th€se prospects were provded by Skaarup (1974), who
considered them to have a syngonetic origin.
Syngenetc tungslen deposits are believed to
lorm by ihe accumulation on ihe sea{loor of
iungsten-rlch precipitales, which cryslallize as
a res!|l oi th€ mixing of tungsten bearing
solulions (€mitl€d frorn seaJloor smokers after percolaiing through rock wilh a high basic
gneous component) wth coder sea water
(Maucher 1965).
Four principa hypotheses may be postu ated
lor ihe lunqsten (scheelte) m neratzation at
Malvika:
a) Scheelite was syngenelic with lhe accumu-
lation ol the hosl lthologies.
b) Scheelite preclpitaled from tungslen-bear
ng hydrotherma iluds whch were mobi
ised by reqlonal metamorphism.
c) Scheelite precipitated fronr tungsien-bearng hydrothernr al ilu ds ofa magmatic origin,
or tungsten was l€ach€d from the country
rocks by f uids circu alinq around a granilic
d) Scheelile preciptaled aJter the cessation
o1 magmatic aciivty, from tungsten bearing
hydrothermal f uids.
The aim of the present study was to estabish constraints on lhe r€latve and absoute
tmng of m neralization, and hence test the
vaidiiy of the above hypotheses. The K/Ar
radiometric ages presenled here may also
have incidenta va ue in clarifylng the tectonometamorph c evol!uon oi the Helgeland Nappe
Complex-
Regional Geology
The Helgeland Nappe Compex is the struclu
ra ly highest unii of the Uppermost Allochlhon
(Gee et a. 1985). The HNC is considered lo
consisl of at east two tectono stratigraphic
units (Nordgulen & Schouenborg 1990, Thors-
nes & Loseih 1991):
1. Mafc and ultramafic lithologies occur n the
weslern parl ot lhe HNC (Nordgulen &
Schouenborg 1990). These rocks have been
interpreted as th€ remnants of a disrnem
bered ophioiie (Husmo & Nordgulen 1988,
Thorsnes & Loselh 1991).
66 D wtn Janeset
FiS.
I
Georogicar map
at.
ol the Nelgeland Nappe
2.
A
HNC = Heqelaid Nappe Complex: FNC = Rdd'nsslriler Nappe
drer.r F! 2 Mod'red rrom Nordquren
compLex
Pluron: re.ranqe€ncioses
ComolerrVS=Vestanden,K=
supracrusial sequence o1 conunental
derivation has also been ldentiled.
lt
,ledium-
to coarse-grained and
megacrystic
is
qran tes and granodiorites typify the batholith
composed of strongly deformed, high grade
pe itic to semi-peiitic gneissos (partly mig
malitic) and calcareous metasedimentary
(Nordgulen & Llilchel1988, Nordgulen ei al.
1993). Radiometrcagedeterminations indicate
thal intrus ve activily was syn-orogenic, span
ning ihe period from the l,lidd e/Late Ordovi
cian to ihe Late silur an (Nordgulen et al. 1993).
Kolunq (1967) and Myrland (1972) indicated
that peak rnetamorphism reached a mandine-
A cac-alkaline intrusive complex, the Bindal
Batholilh, constiutes a substantial proportion
ol the HNC (Nordgulen & Schouenborg 1990).
Aeabgy and K/A.chrcnotagy 67
amph bo iie facies throughout much of the
HNC. The lormation of the dominant (S2)fola-
ton coincided with the peak ol meian,orphism
(Thorsnes & Loselh 1991). Laier deformation,
al events (D3 and D4) produced open, large
scale lods (Myrland 1972, Torldbakken &
Mickelson 1986, Gustavson 1988). These
events are responsible lor the present attitude
(c.60' dip to ENE) oi the rocks n the vicinity
of the study area (Thorsnes & Loseih 1991).
As 52 structures are prssent in inclusions
cut by the meqacrystic Andalshatten pluion
(daled
ai447!7
Ma; Nordgulen et at. 1993),
the loliation must have formed prior to or
during th€ Late Ordovician. There is therefore
evdence lrom lhe HNC, as trom the graniioid
terrane ol Smola-Hitra (Roberts 1980, Gauineb
& Aoberts 1989) and elsewhere in the Upper
Allochthon (Kullerud €t al. 1988) and Upper
most Allochthon (Claesson 1979), that a major
leciono magmatic €vent occurred during the
Ordovician (Nordgul€n & Schouenborg 1990).
Nevertheless, th€ final eastward thrusling ot
the HNC probablylook placeduring the Scandian continent-to-continent collision,
Geology and Petrography of the
Malvika skarns
Tungsten-bearing skarns occur in two zones
in the vicinity of Malvika, which is siluated on
the northern srde oi Tosenfjord (Figs. I & 2).
The southern Up oJ tho 100 m€tr€'wid€ main
zone (Skaarup 1974) lies 100 m southwest
of Malvika and extends NNW for 700 m inand, to a height ol 400 metres- The second
(western) zone is exposed in a road-clt 200
m to the soulhwest, near Skjervikbugen. Thls
10 m wide zone extends for 70 m north
wards, wher€ it l€rminates against a promi
nent, ENE-WSW trending fault.
The skarns occur as heteroqeneols horizons not exceed ng 1.5 metres in width. They
are enclosed by leucocratic biotite gneiss or
melanocratic, weakly loliated, amph bole bioii
le gneiss. The lwo zones difler in the abundan
ce of marbe. The marble ln the eastern zone
Fg
2.
Ceoogca mapol ih€ area sutroundng lhe sch€€'te
tensively n the skarns oi the eastern zone,
whereas in thewestern zoneve ns and replace
menl assemblages are volumelricaly m nor.
Fig. 3 ilustrales the reaiionship between
skarn, marble and gne ss, in thewestern zone.
Each skarn horzon consists predorninanty
of two primary (.e. iirst-lormed) ithooges,
one consisting argely oi pyroxene and plag o
clase whereas the other is composed ma n y
of garnet and pyroxene. The former occurs as
a coniinuous horzon up lo 15 cm n wdth,
adlacent to gne ss. The garnet skarn, in contrasl, qenera ly occurs as disparate lenses (up
to 5cm wide) a ong the marble skarn contact
(Fig. 3). Fig.4 shows that garnet skarn probab
y formed by replacement ot marble. The gar
nel skarn was, n lurr, parlialy repaced by
an epidole + quartz (+ residual pyroxene)
occurs as widely scatlered inclusions, whereas
it s present as contnuous horizons enclosed
by <30 cm wide skarn horizons n tl'g wes
assemblage. This accounis for the erratic dlstribution of garnet skarn lenses, as in Fig. 3.
tern zone. Anastomosing vein networks and
assemblages formed by the replacement of
pyroxene) skarn is typica ly developed ai lhe
conlact between the garnet skarn and marble.
A zone of amphboe + plagiocase skarn
the first-lormed skarn assemblages occur ex-
A
<5mm wide selvage of wolastonlte (+
r6A D. \lynJanes et at.
Fig
3
Oarared skelch nap
ol a ryp'car skarn
hor zon or the skJerv'kbugen
lwesreiil zone
(.:smm wlde) is lound al lhe contact oi the
pyroxene + p aag oclase skarn with the gne ss.
pyroxene) is rarely deveoped in
ldocrase
place of the garnel skarn.
(l
A hydro!s assemblage cons sting mainly of
epidote + quartz + amphibole is extensively
deveoped al lhe expense ol the garn€t skarn.
Scheelite s locally a significant cornponent,
accounting for up to 20?" oi lhe assemblage.
To a lesser exl€nt hydrous mineralskarn (without scheelite) replaced the pyroxene + p agioc-
lase skarn. ln places it is spatially associated
with cross-cutting fractures or veins How-
ever. the reolacement skarn qeneraly occurs
more extensively, with preierental develop
ment along the contacts between the primary
skarn assernblages. Another type ol hydrous
mineral replacemenl is developed excusively
al the peripheres 01 quartz-calcite veins. lt
consists primarily of b ades of lerro-pargasitic
amphiboe and cubes of almandine-spessaF
tine-rich garnel (James 1991). variable proportions of pynhotite, calcite, quartz and epidote
occupy the interstices between amphibole and
Fs.4.
Camer skarn (G) rcpracinq marble (M) A = amphi-
boe wallrock skarn enclosnq
ciie voin lC) N = Hyd
rous r€placemeni ass€mba9e (alier !arn€r skarn)conssquanz
(wrh mmor srreelre).
1rn9 mainy or epdole and
Bourd€r roadsid€. w€stern zone
ca
garnetcrystals.Coeval scheeliteisabsentfrom
this waLl-rock assemblage. Biotite is a minor
of the quartz-cacte veins and
schee ite is rarely present. The veins are placonstituent
Geolattr an.i K At.h.analagl 69
Fq 5 Pyr.xdnd srari p,o
P.liq rcll!adY3.r.ssihe
amphibo.lri.lre ltnes5
nar, exhibit prelerential orientation and are
restricted to the skarn.
Skaarup (1974) implred that the scheelite
bearing skarns were lormed by isochemrcal
HYDFOI]S SKABN
Stalc rv
transformation of tunqsten-rich marl protoilhs,
during regional metamorphism. However, Fig.
5 shows pyroxene + plaqioc ase skarn crosscutting the foiation of the adlacent gness al
a high angle. This indicates thai this type ol
skarn post-daled the 52 iabric in lhe gneissi
hence lhe skarn lormed alter the peak ol reg o
.-.
The characier ol the ore ai l,fdlv ka is a so
inconsistenl with Skaarup s (1974) conclu'
^
sions. lnvesligation of tungsten skarns worldwde have shown that schee ile compr ses al
leasl95% ol al skarn tungsten minerals (Kwak
1987) Ths is due lo the very hiqh negalive
free energy oi lormation of scheelite:
-52,320 Cai
CaO + WO, = caWO,
lvlole (Hsu & Gall 1973) ^G:il=
Only if the Ca actvty is low. or under super_
gene conditions, do other tungsten minerals
(mainly wolfram te) form. Hence, it is reason
able to assume that any lLrngslen present in
skarn and Ca rich precursors to skarn would
occur in sch€elite. The eariest generation of
scheelite in the Mevika skarns crystallized
ale in the primary paragenes s (minor schee
ile occurs as a late-stage infi I of the intersti
ces between the well-iormed prmary garnel
crystalsi see Fig. 6 for the compele paragenesis). This points to the ntroduction oJ iungsten
reranve
area
Ls
'nd'calire
or rhe
tnng
t^-
t-
or crysrarsalon wtr
aDF.r,
o 5koa '! on-ar
Am Spess cainer ot Sraqe
Mo k.c Epd.ie.onrposton
Ldnd
rli
r..pdoie c
>
c o\r
ozoisne
spans
V amph bo e,s re o parqr!1c tanalyses
repo,tcd n Jam-as lee1l
TA D Wyn Jahesel al
by eplgenetic hydrothermal fuids. Further
rnore, the most lntensive phase of scheelile
The dykes post-dale ihe adiacent primary
skarn, since xenoliths of skarn were present
within the dykes and an amphibole reaction
mineralization occurred later in lhe evolution
r€placem€nt of the
ol lhe skarn, during lhe
skarn was developed at the dyke-skarn con-
primary qarnet skarn by th€ hydrous mineral
skarn. The concenlration of scheelite present
in the primary skarn is far too small for the
scheelite in the roplacement skarn to have
tacl. Thr€e whole rock samples were obta ned
from fresh roadside exposures ol the twomica granodiorite pluton located 500 m north-
been derived solely by re-distribution ol prima-
Chip samples w€r€ puifi€d by crushing the
sample in a pesile and morlar and then pro
gressively removing the relatively magnotic
amphibole or biolile with an electromaqneuc
separalor, Purities in excess ot 99 percent
easi from the €astern zone.
ry skarn scheelite. Considering this together
wilh the evidence ofa post-reg ional metamorp
hic origin forthe primary pyroxene + plagiocase skarn, it is concluded that lt is highly improbable that t!ngsten mineralization in tho Malvi
ka area was syngenetic wilh the deposition
ol the hosl-lithologtes.
Whole-rock samples of the two-mica qrano-
diorite were crushed and sieved to obtain an
opiimal size fraclion of 90 lo 150 pm. Purities
greater rhan 99% were attained for biolile.
Weakly paramagnelic muscovit€ proved d iii
cult to separate from compound grains which
consisled of non-maqnetic quartz or feldspar
in combination with magnetic biotite or ilmenite. Th s resulled in a purity ol95% for musco'
ln orderto consvain the liming of mineraliza-
tion, and lhereby tesi the afore-mentioned
hypotheses, a K/Ar geoch ronological investiga-
tion was underlaken.
K/Ar isotopic age determinations
The amphiboles (Table l)were chipped direct
ly lrom tresh roadside exposures of the vein
wall rock skarn. Biolile chip samples R11 and
R12 w€re obtained from pegmaliiic lwo-m ca
Potass um analyses were carried out using
a Corning 450 flam6 photometer with a lithium
internal standard. Argon isolopic analyses
granitic dykes located in ihe western zone.
Tabe r. KiAr dererm narions ot
Tosboln ror sampe oca res)
bo
€. muscovire and
amprr bote s€parares trom
lhe
MA v ka area (s6e map sheer 1825 I
Aimosprr€ric-'
conran
Iw. dr.a gtanoa)atte
ML\Lae e
Twa mrca qtanadhnte Biatite
Iwa hica gtanitic dykes Biotite
Wat\ack skan Anphibate
R2
F9
R6
R4
R7
R3
N
40070.72435s
40050 724330
40070 724355
44070-724355
40070-7243ss
40070 72435s
40070 724355
40074124355
(1
72010
(1s14r
'o don p€ !on' 16 O'sl
O20l x rO-2
019)x
0 75510 020)1
t
r0.2
10,
\2.202 !0 O24) 10 2
(1 610 i O O2O)x rO ?
(211510 023)x 10 2
0.99010020)x102
(2 29010 020)x 10,
'Moa^ or ihree anaysas. 'Mean or tuo analys€s
'"qq(6 or r^o J€ ordod d., o
constants alr6. sl€qer a JAger 0977)
0
lo
rod
Age
naion(%) lMal
142
7.5
63
45
72
ro)
Geolagy and K AtchranatagY
were periormed by the isotope dilltion method
on a modified Kratos MS10 mass spectro
meter (Wilkinson et aL. 1986).
Nine of the fourteen appareni ages (Table
1) are concordant at th€ 16 level. This group
comprises the two muscovites (R10 and R17),
the three biotites (Rl1, R12 and R17) and
tour oi the amphiboles (R4, R6, Rg and R14)
The lnverse variance welghled Mean (IVWM)
of the nine agos is 402 :t 2 Ma At the 20
conJidence level, twelve oi the Jourteen ages
are concordant with an IVWM ol 399 a 2
Ma (excluding only amphiboles R3 and R8)
Such concordancy in apparent age among
difterent minerals, and among samples oi the
same mineral(with vary ng chemistries), indica-
les that the age obtained has not been pertur
bed by factors such as the incorporaiion ot
nit al argon into ihe crystal lattices or the ioss
or gain of varable argon/polassium caused
by weather ng or melasomatism (Dalrymple &
Lanphere 1969). The age obtained may thus
be regarded as representatve ol an evenl in
rhe tectono-therm al h istory oJ ihe host lthologi
6s whch led to seltng (or loial reseiting) ol
the K/Ar isotoprc systern n the mcas and
ThF c 400 Ma age dcco ds w t'l olher K A'
ages of mlcas irom the Centra Scandrnavan
Caledorrdes
lu'
(1985) eporlFd
KA
age<
lrorr Ihe Wesrerr Grerss Fegio'l ia thF rdrga
4!0 to 3/0 Ma and a whoe ro.\ cample o
kentallente from the Bindal area (c.30 km
souihwesl ol Melvika)qave an age ol3991l0
Ma (Nordgulen & Milchell 1988). Ages within
the range 420-380 lvla have been nterpreted
a5 li'lng Jpl l ard coolng al ll'e Fnd ol thc
Caledo'rrdn O oqe'ry req SLJn el dl '967r
rlerce, I1e ages obldned lom ll^a anerdls
we have studied are regarded as having an
eq'i vale'rr \rglrl'Lan.e lor lhe Helge ard \ap
'
The cdncordancy ol ihe ages rs hrghly signts
ficant, qlven that muscovile has a somewhal
hlgher blocking temperature than biotite and
fiai
amphibole has a signilicanily higher block
ng lemperaru'F tnan mLs.ovrlF rDd vmpla 8
Lanphere 1969). This implies that cooing
through ihe respeclive thresholds for argon
retenlion n each of these minerals must have
occJled wnhn a li1lF scdle ol les\ Ihan 5
tula. h ll'e .o"rlert oi a reg onal Telan olphrl
le"arn lhs rnpres a process ol rapid Jplrft
Thrs tnfe'ercF aLco'ds will^ ll^e vrews o{ Tucl_
er et al. (1987), who obtained U/Pb ages from
71
the area to the souihwest of Trondheimstjord
that pointed to a ... very shod perod of buri
al and upilt in the Western Gneiss Begion.
This occurred at about 395 Ma (the liminq ol
coo ing through the c.500'C threshold forisotopic diflusion of lead in titan te), vinually conternporaneous wlth coolng in the MelvLka area.
Fu(her evdence of rapid uplit is provided
by Johansson et al. (1990) irom their study
of the metamorphic evolution of the Roan area
of Central Vestranden, aso in the Weslern
tor six hornGness Region.
blende separates glve pateau ages around
400 Ma (ndicaling the timing ofcooling through
about 500'C).'Ar-'Ar ages of six muscovte
separates indicale subsequent coo ing ihro!gh
the r c.350'C blocking temperature at 390 395
Ma. This rapd cooling is alleged to have been
accompanied by minimal uplift (Johansson el
al. 1990). However, in a regional melamorphic
environment. it is difficull to envisage by what
process rapid coolnq could occur other than
by rapid upllft to high crustal levels. Fb-Sr
dating by Piasecki & cliff (1988) on muscovite
books frorn shear-zone pegmatites from the
Blugn area of Cenlral Vesiranden gave ages
of 389 a 6 and 386 I 6 Ma. Small, mairx'
size muscovite and botlte lrom the same
mylonte zones, on lhe other hand, yieded
ages oi 378 I 5 and 365 1 5 Mai these
younger ages were interpreted by Piasecki &
cliff 1o reLale to post-shoaring uplit and cooling.
ln lhe present study, among the eght amphiboles daled one gave an age siqnificanlly older than thal oJ the main group at the 1o
confidence lev€l (415 I 7 Ma). Two other
samples gave indisting! ishab e (at 1d) appa
renl ages whose mean s 472 Ma, whlch s
distinguishably olderlhan the remaining amphi
lf the c.472 N4a aqe accurately records a
tectono-lhermal (or crystallizat on) event then
the timing of amphibolite faces metamorphism
in the HNC would also be consirained. The
high-grade metamorphic event must have preceded lhe crystallization ol the undeiormed,
unmetamorphosed, amphibole walL-rock skarn
Arnphibollte tacies metamorphism and the
deve opment of the 52labric, (which co ncided
wilh the peak of melamorphism (Gustavson
1988, Tlorsnes & Loselh 1991) woLld rhFretore have to be related to tectonism of Early
Ordovician age, or earlier.
T2 D WynJameset al.
ll the c.472 Ma aqe is valid then this would
also constrain the timing and m€chanlsm ot
minera ization. As scheelte pr€-dated the foF
mation oi the amphibole wall-rocks, the main
mineralization ev€nt could not be associated
with lhe adjacent two-mica granodioriie plu
ton. This ls because the pluion is known to
be yolnger lhan 430 [,4a since it cuts the
adjacenl monzodiorite (Fig.2), whlch has glven
a U/Pb age of 430 1 7 Ma (Nordguten et al.
1993). Th€ c.472 lua age would also imply
that scheelite mineralizalion predates known
igneous activity by about 25 Ma (ignoring the
Rb/Sr Cambrian dates obtained by Nissen
(1986) which have recently b€en supplanted
by more reliable, younger U/Pb ages (Nordgul€n et al. 1993). Furthermore, ihe eartiest scheelite mineralization ai l\,4efuika co!ld not rhen
be related lo th€ Kolsvik qold mineralization,
since the lalter
is iinked to Early
Devonian
britUe d€forrnation (P. lhlen, pers. comm. 1991).
These inferences are dependant on ih6 as-
sumption that the c.472 [4a apparenl age s
Conclusions
Geological ovidence shows that pyroxene +
plagioclase skarn, at Malvka, post-dated the
32 fabrc in the gneiss, indicating that
the
skarn formed after the peak of regional meta
morphism. This and the petrographic evidence
of scheelite crystallizing late in th€ skarn paragenesis demonslrale thattungsl€n was deposited lrom epigenetic hydrolhema fluids. This
refutes Skaarup s (1974) suggestion that the
lungsten was deposited conlemporaneously
with ihe accumuiation ol the hosl litholog es
K/Ar apparent ages of biotite, muscovte and
three arnphibole separales deline the cooling
history of the Helgeland Nappe Complex at
the end oi the Caledonian orogeny. The indistinguishabilty (at 1o).oi apparent ages from
these minerals mplies that rapid uplifi was
respons ble lor the cooling, a conclusion wh ch
is consistent with the findings ol two geochro
noog cal investigations of the Western Gneiss
Begion (Tucker et al. 1987, Johannson et al.
r990).
geologically significant. However, pr€vious
K/Ar investigations 01 the Uppermost Alloch-
a
thon have encountered problems with excess
EVENT
argon (Wilson 1972, Wilson & Nicholson 1973).
Wilson (1972) was of the opinion that the ex
cess argon in the sulitielma reqion was in
herited as a result of argon outgassing fron
the 1700 1800 [4a basement during regionat
metamorphism. Given the proximily of the
Suliijelma region it is a possibilily that radioge,
nic argon, released from basemont rocks, was
present in fluids circulating through the Malv-
408
438
a
ka area at the time oJ cooling through the
argon blocklnq tomperaiure ol amphibole (at
a time later than 472 [,4a). However, since the
total radiogenic argon and potassium contents
of the two samples wirh rhe oldest appar€nt
ages differ by aboul 15% it would be an improbable coincidence that excess argon resulted
in their indisunguishable apparent ages. The
difterence (c.55 [ra) between ihe two oldest
--
!
Y
505
-F
Mai w Mne.,Ai r,n
concordant ages and the next oldest age is
also inconsisl€nl with the randomizing €ffecl
of excess argon on apparent ages,
ln vi€w of the conlradictory evidence descri-
bed above, further radiometrlc ages woutd
n€ed to be obtained, utilising olher isoiopic
methods (e.g (Ar - rAr or Sm/Nd - see Better
al. 1989), b€fore the two oldest KlAr amphibole
ages obtained can be confidently regarded
as havlng chronological signilicance.
F9 7 Chronorogicar cont€xr ot scho6h6 mmemisaion
The upper back reclang 6 show the wVM (wrh 1a etror
ba4 ot the rwo ordesi (amphboe war.rock skah) K/Ar
ages. A and B indicare rh€ rm nq or..vslarzalion ot rhe
monzodio' e puron no rrea5lolMl!rdtseeFq 2land
Andarshsien pruron resp6d vety 1u Pb on zr.on. Nordgu
a n o,ess) aoe or 2 m.a Granodor Le
es
'nd'.
lrre possble lime span or intusion or rhe cross-cutinc
granodior16lhat €s adiacenrlo ihe Li Pb daled moizod orte
ren €r
Ceotaqy and K/Ar chranolagy 73
Two oldest amphibole samples gave indist
inguishable (at 1o) apparent ages whose mean
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Manusctipt received July 1992: revised typescript accepted February 1993.
J
geor Soc