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Geosciences Journal

Vol. 13, No. 3, p. 317 329, September 2009DOI 10.1007/s12303-009-0030-4 The Association of Korean Geoscience Societiesand Springer 2009

Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeopro-

terozoic Lapland-Kola orogen (arctic European Russia): insights from

numerical modelling and multi-method in-situ micro-sampling geochronology

ABSTRACT: 4 0 Ar/3 9 Ar age spectra with progressively increasing

step ages are well known for metamorphic hornblende and have

been classically interpreted by partial loss of radiogenic argon bydiffusion processes during younger thermo-tectonic reworking.

Application of a number of numerical modelling tools based on

diffusion theory and that assume thermally activated loss of radio-

genic 4 0 Ar by solid-state volume diffusion suggests that staircase-

shaped age spectra of Neoarchaean tschermakitic hornblende from

the Lapland-Kola Orogenare due to argon losses of 4050% dur-ing reheating to 450 25 o C in Palaeoproterozoic time. However, in

hornblende samples that yielded staircase-type age spectra, biotite

occurs in the matrix, as well as intimately and abundantly inter-

grown with the amphibole along grain boundaries, cleavages, frac-

tures and other defects. Drilling of 1.5 mm diameter discs from

carefully selected hornblende grains in petrographic thin sections

permitted to minimise the effects of contaminant biotite inclusions

and/or compositional zoning of the amphibole.4 0

Ar/3 9

Ar laser probestep-heating of drilled biotite-free hornblende discs yielded flat age

spectra, suggesting absence of thermally activated radiogenic 4 0 Ar

loss. This would imply unrealistically contrasting temperature his-

tories for neighbouring grains. Apparent-loss age spectra, thus,

result from differential gas release of hornblende and an included,

earlier degassing minor contamination of much younger biotite,

which had apparently not been completely eliminated from the

amphibole separate, despite careful handpicking. This is confirmed

by the Ca/K ratio spectra a proxy for 3 7 ArCa/3 9 ArK of hornblende

that are flat for drilled biotite-free hornblende grains, but initially

low for hornblende separates. A drilled disc and a separate of horn-

blende from a biotite-free amphibolite did not yield apparent loss

spectra, but flat age and Ca/K ratio spectra, confirming the inter-

pretation of the role of biotite.

Key words: Archaean, argon loss spectra, geochronology, Kola pen-

insula, micro-sampling

1. INTRODUCTION

Time is a unique element in geosciences and the discov-

ery of radioactivity by Henri Becquerel in 1896 enabled

the development of radiogenic isotope geochemistry, open-

ing the way for measurements of the age of geological

materials and processes. This has enabled geoscientists to

place the evolution of the Earth and geological processes

within an absolute timeframe, which has fundamentally

changed thinking in the earth sciences.

Metamorphism is one of the prime geological processes.

Metamorphic rocks usually have undergone a complicated

tectono-metamorphic evolution. During this process they

have been affected by superimposed recrystallisation phases

under changing pressures and temperatures, as well as dif-

ferent fluid activities. Ages of such events in collision and

other tectonic zones have been recorded in micron-sized

domains of minerals, and must be interpreted in close rela-

tion to metamorphic mineral assemblages and the tectonic

fabrics of rocks. Such very small domains are hard to date

with the conventional geochronological techniques and

require the use of sophisticated micro-chronological meth-

ods. Used for the dating of Th- and U-bearing accessory

minerals are: SHRIMP (Sensitive High-Resolution Ion Micro-

Probe), different ion, electron and proton probe micro anal-

ysers applying the regression-based chemical ThUtotal

Pb isochron method (CHIME), and Laser Ablation Induc-

tively Coupled Plasma Mass Spectrometry (LA-ICPMS).

A number of40Ar/39Ar LASER (Light Amplification by Stim-

ulated Emission of Radiation) techniques have been used

for micro probe dating of small domains in K-bearing rock-

forming silicates. Instead of a laser probe, a microscope-

mounted drill can be used to obtain mm-scale discs of min-

erals from targeted sites in polished petrographic thin sec-

tions (Verschure, 1978). This micro-sampling techniquehas been applied to RbSr (Meffan-Main et al., 2004) and40Ar/39Ar laser step-heating (de Jong and Wijbrans, 2006)

dating.

In this study I use dating results that de Jong and Wijbrans

(2006) obtained on hornblende from the Lapland-Kola oro-

gen, which experienced a polyphase tectono-metamorphic

evolution of about 1 Ga from the late Neoarchaean to the

middle Palaeoproterozoic and is developed in the Kola

Peninsula, Arctic European Russia (Fig. 1, insert map). In

the present study I use numerical modelling of apparent

loss 40Ar/39Ar age spectra, i.e., spectra with progressively

rising apparent ages, they obtained on hornblende sepa-rates to shed light on the cause of apparent loss age spectra.

Koen de Jong* School of Earth and Environmental Sciences, College of Natural Science, Seoul National University, Seoul151-747, Republic of Korea

*Corresponding author: [email protected]


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318 Koen de Jong

Modelling results of such staircase-type age spectra are

compared with laser step-heating age spectra of drilled

hornblende discs without included biotite, and that were

essentially flat. A sharply discordant RbSr age for a horn-

blendeplagioclase pair is also discussed making use of the

insights gained from micro-sampled hornblende.

2. MICRO-CHRONOLOGY AND POLYPHASE TEC-

TONO-METAMORPHIC EVOLUTION

In geodynamic environments characterised by superim-

posed tectono-metamorphic events, rocks had to adapt to

changing physical conditions by recrystallisation during

P-T specific mineral reactions. Mineral zoning indicates

that chemical-potential gradients were not eliminated by

intracrystalline diffusion (Fisher, 1977). The occurrence of

zoned metamorphic minerals, or the presence of relics,

thus implies that disequilibrium conditions existed during a

succession in time of different metamorphic facies. Fabric-

forming minerals record the changing physical conditions

along P-T-d-t paths. This is also the case for polygeneticTh- and U-bearing accessory minerals like monazite, xeno-

time, euxenitepolycrase, allanite and zircon (Crowley and

Ghent, 1999; Cocherie et al., 2005; Dahl et al., 2005; Dumond

et al., 2008; Li et al., 2008; Suzuki and Kato, 2008; Bosse

et al., 2009). Fluid-assisted recrystallisation that affected

ionic bonds in minerals, consequently, played a paramount

role during exchange or loss of radiogenic daughter iso-

topes (Wijbrans and McDougall, 1986; Miller et al., 1991;

Villa 1998; Kerrich and Ludden 2000; de Jong et al., 2001;

de Jong, 2003).A major geochronological challenge is to obtaining age

estimates for the timing of specific tectono-metamorphic

phases or events in orogenic belts and tectonic zones. Because

the host rocks have deformed and metamorphosed in a

number of phases, leading to chemical and isotopic zoning

of minerals, radiometric ages of these events too have been

recorded in small domains of minerals. Secondary Ion Mass

Spectrometry (SIMS) has been specifically designed for

single-grain and in-situ 207Pb/206Pb and UPb age determi-

nations in accessory minerals with a high spatial resolution

of 1530 m using SHRIMP, Cameca IMS 1270 or Nano-

SIMS NS50 instruments (Takahata et al., 2008). Electronand proton probe microanalyses in polished petrographic

Fig. 1. Tectonic sketch map of the Kola Peninsula, modified after Timmerman (1996) and Daly et al. (2006). Palaeozoic nepheline syen-ite intrusives omitted for clarity. The location of Figure 2 with position the samples is outlined.


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Hornblende apparent partial loss age spectra 319

thin sections using CHIME (Suzuki and Adachi, 1991;

Montel et al., 1996) offer a ten times higher spatial reso-

lution (Cocherie et al., 2005; Dahl et al., 2005;Dumond et al.,

2008;Kusiak and Lekki, 2008; Suzuki and Kato, 2008).For correct interpretation, in-situ microchronological data

have to be linked to detailed (micro) structural and petro-

logic data. But it is often very difficult to link detailed U

ThPb age data of accessory minerals, which typically occur

as sub-mm grains, to the evolution of assemblages of meta-

morphic minerals or to fabric-forming main phase silicates.

By linking REE patterns in zircon (e.g., Kelly and Harley

2005; Rubatto and Hermann 2007) and monazite (e.g.,

Pyle and Spear 2001;Foster et al., 2002) to growth phases

of specifically garnet but also K-Feldspar (Mahan et al.,

2006), compositional changes and correlated age informa-

tion contained in the accessory minerals can be linked to

changes in the rock-forming mineral assemblage, and thus

to the pressure-temperature the evolution.In-situ dating in

conjunction with microstructural studies enabled incre-

mental growth of monazite to be related to phases of super-

imposed deformation (e.g., Dahl et al., 2005;Dumond et

al., 2008). An often overlooked aspect when interpreting

ages of robust accessory minerals is that although Pb dif-

fusion at high temperature is extremely slow - assuring

conservation of earlier recorded age information - fluid-rock

interaction under greenschist-facies conditions may lead to

profound chemical and isotopic alteration of monazite

(Crowley and Ghent, 2000; Li et al., 2008; Bosse et al.,

2009) and zircon (Gebauer and Grnenfelder, 1976). This

is not always recognised. A regional 40Ar/39Ar study in the

Tianshan (NW China) revealed that an erroneous assump-

tion of a Triassic UPb SHRIMP age on zircon for the

UHP metamorphism must, in all likelihood, be due to syn-

exhumation fluid-mediated recrystallisation; see de Jong et

al. (2009), Li et al. (2008) and Wang et al. (2009), for dis-

cussion. In contrast to accessory mineral dating, 40Ar/39Ar

dating can be applied to a wide range of K-bearing, com-

mon rock-forming minerals, like micas, feldspars and amphib-

oles. This is a big advantage as their growth can be

straightforwardly correlated to major phases of the tectono-

metamorphic evolution of rocks.

3. 40Ar/39Ar LASER PROBE AND MICRO-SAMPLING

TECHNIQUES

The 40Ar/39Ar technique, like the KAr method from which

it is derived, is based on the natural radioactive decay of40K in 40Ca and 40Ar* (radiogenic argon). In contrast to the

KAr method, in order to be able to be dated with the 40Ar/39Ar technique samples have to be irradiated with fast neu-

trons in a nuclear reactor, which leads to the production of39Ar from the 39K isotope in the mineral. Because all target

isotopes are gases they can be directly measured with a noblegas mass spectrometer, after being extracted from the sam-

ple in a high-vacuum system. This is done in a number of

steps at increasingly higher temperature with furnace sys-

tems or a defocussed laser beam on mineral concentrates or

entire grains, or by spot dating using the laser as micro-probe. Other isotopes that are produced during irradiation

are 38Ar and 37Ar from respectively the 37Cl and 40Ca isotopes

in the samples. These can be used to obtain information on

the chemistry of the dated materials, in terms of e.g., Ca/K

and Cl/K ratios - proxies for 37ArCa/39ArK and

38ArCl/39ArK,

respectively - especially when combined with electron probe

microanalyses (EPMA). This geochemical information is

useful to identify the effect of mineral zoning, the presence

of exsolution features and/or included contaminant miner-

als or fluids on the age of the target mineral, as possible

chemically different phases degas at different temperatures.

In general degassing of a heterogeneous phase leads to

trends in 40Ar/39Ar age and Ca/K ratio spectra, either sym-

pathetically or antipathetically, as is well established for

hornblende (e.g., Berger, 1975; Berry and McDougall, 1986;

Onstott and Peacock, 1987; Kelley and Turner, 1991; Lee,

1993; Rex et al., 1993; Wartho, 1995a; Villa et al., 2000;

de Jong and Wijbrans, 2006).

The 40Ar/39Ar method is not only a powerful tool to

establish isotopic ages of events on mineral concentrates

using furnace systems; it also enables spot dating of

micrometer scale domains within minerals in thin sections

using state-of-the-art laser probe techniques. 40Ar/39Ar laser

microprobe spot fusion techniques have revealed age gra-

dients in mica (Maluski and Moni, 1988; Phillips and

Onstott, 1988; Scaillet et al., 1990; de Jong et al., 1992)

and hornblende (Kelly and Turner, 1991). Focussed ultra-

violet lasers ablate and strip thin layers from the surface of

crystals rather than melt the sample as other laser tech-

niques; data can be obtained spatially resolved to 10 m

(Kelley et al., 1994; Kelley and Wartho, 2000; Mulch et al.,

2002). A microscope-mounted drill can be used to micro-

sample minerals in polished petrographic thin sections,

while targeting parts of grains without zoning or included

phases. Such 1.5 mm diameter drilled discs can subse-

quently be analysed by step-heating with a defocused laser

(de Jong and Wijbrans, 2006).40Ar/39Ar dating applying var-ious laser techniques thus enables to relate age information

to the microstructures and petrology of rocks.

4. LAPLAND-KOLA OROGEN

The Lapland - Kola orogen (LKO) is a high-pressure

Palaeoproterozoic collisional belt located in the northern-

most part of the Fennoscandian (Baltic) Shield, one of the

best-known Precambrian regions on Earth. The orogenic

belt occurs to the North of the Karelian Craton, a classic

Neoarchaean granitegreenstone province, and stretches

for ca. 700 km from the Caledonian front in northern Nor-way to the southeast, where it is covered by the Palaeozoic


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320 Koen de Jong

sedimentary rocks of the Russian Platform (Fig. 1). Bou-

guer anomalies (Poudjom Djomani et al., 1999) indicate

that the belt continues farther to the southeast. The LKO

was formed by the amalgamation of terranes of predomi-nantly Neoarchaean age and juvenile Palaeoproterozoic

crust, including accreted island arc material (Timmerman

and Daly, 1995; Timmerman, 1996; Daly et al., 2001, 2006).

The orogenic belt formed part of a Palaeoproterozoic large-

scale tectonic system of Grenville or Himalayan dimen-

sions, which is traceable across the Atlantic Ocean to Green-

land and Labrador (Bridgwater et al., 1992).

The Neoarchaean terranes of the LKO mostly have pro-

toliths formed during a major 2.682.96 Ga old juvenile

crust-forming event (Timmerman and Daly, 1995; Tim-

merman, 1996). Soon after its creation, the juvenile Neoar-

chaean crust was the site of major crustal stretching

between 2.352.50 Ga, indicated by the initiation of rift

basins, widespread intrusion of mafic dyke swarms, man-

tle-derived large layered basicultrabasic intrusions and

gabbroanorthosite complexes, locally accompanied by

anorogenic potassic granites (Fig. 1; Balashov et al., 1993;

Mitrofanov et al., 1995; Amelin et al., 1996; Sharkov and

Smolkin, 1997; Balagansky et al., 2001; Daly et al., 2006).

Heaman (1997) observed similar rocks in various Archaean

cratons in the North Atlantic region and suggested that

they may have been part of large igneous province related

to the break-up of a Neoarchaean supercontinent. The Neoar-

chaean terranes were peneplaned in the earliest Palaeopro-

terozoic (Zagorodny, 1982; Sturt et al., 1994; Bridgwater et

al., 2001) following rift inversion accompanied by uplift,

erosion and weathering (Daly et al., 2006). Anorogenic 2.3

2.1 Ga potassic granitic sheets, ca. 2.1 Ga doleritic dyke

intrusions, and alkaline K-Na basalts imply a second exten-

sional phase (Sharkov and Smolkin, 1997; Daly et al.,

2006). This finally resulted in the opening of oceanic

basins in the 2.101.97 Ga period, represented by the

Pechenga, Imandra - Varzuga and Polmak-Pasvik volca-

nosedimentary belts that are characterised by tholeiite pil-

low lavas and other basalt flows (Melezhik and Sturt, 1994;

Fig. 1). Part of these volcanosedimentary belts formed as

back-arc basins (Melezhik and Sturt, 1994; Brewer andDaly, 1997; Sharkov and Smolkin, 1997), suggesting sub-

duction of oceanic crust, which also formed 2.121.91 Ga

juvenile island arcs, like the granulite-facies Lapland and

Umba Granulite terranes and the amphibolite-facies Tersk

Terrane (Fig. 1; Huhma and Merilinen, 1991; Timmerman,

1996; Glebovitsky et al., 2001; Daly et al., 2001, 2006).

The observed ca. 2.1 Ga K-metasomatism and migmatisa-

tion in lower crustal granulite-facies xenoliths (Kelley and

Wartho, 2000; Kempton et al., 2001) may be related to

hydrous fluids released during dehydration of subducting

oceanic lithosphere. Subsequent collision ofNeoarchaean

terranes as well as underthrusting and metamorphism ofPalaeoproterozoic juvenile material is well constrained by

1.911.93 Ga UPb zircon ages, whereas 1.901.87 Ga Sm

Nd, UPb and 40Ar/39Ar ages constrain cooling and exhu-

mation of the collision belt (Bibikova et al., 1993; Mitro-

fanov et al., 1995; Tuisku and Huhma 1998; de Jong et al.,1999; Bridgwater et al., 2001; Glebovitsky et al., 2001;

Daly et al., 2001, 2006). Ca. 1.81.7 Ga post-orogenic granite

intrusions (Fig. 1), micaceous pegmatites, lamproites and

lamprophyre dykes occur throughout the Kola Peninsula

(Mitrofanov, 1996; Daly et al., 2006). The ca. 1.76 Ga-old

composite granite plutons of the Litsa-Araguba suite and

associated porphyritic granite dykes (Vetrin et al., 2002)

stitch terrane boundaries (Figs. 1 and 2). This final Palae-

oproterozoic anorogenic-type magmatism heralds the end

of the Precambrian tectonic evolution of the LKO. Mus-

covite and biotite, when not affected by the occurrence of

excess and/or inherited argon, have 1.751.70 Ga 40Ar/39Ar

ages on a regional scale, from the northern foreland of the

LKO to large tracks of the southern footwall (Belomorian

Terrane) and the Archaean foreland of the Karelian Craton

(de Jong et al., 1999). Flat-lying, predominantly clastic

Neoproterozoic (0.91.4 Ga) sediments occur locally along

the northern and southern limits of the Kola Peninsula

(Mitrofanov, 1996; Figs. 1 and 2) and constrain the final

exhumation of the LKO.

4.1. Murmansk Terrane: geology and age constraints

The Murmansk Terrane is one of the LKOs Neoar-

chaean terranes and separated from the other terranes by

the northwest-trending subvertical Murmansk Shear Zone

(Figs. 1 and 2). Equidimensional to low-aspect ratio mag-

matic bodies define its regional structure (Fig. 2), in contrast

to the other terranes of the LKO that have a clear structural

trend. The Murmansk Terrane predominantly comprises

amphibolite-facies, leuco- to mesocratic, tonalitictrondhjemitic

granodioritic orthogneisses (TTG gneisses) and intrusives,

some migmatite and only subordinate metasedimentary

material (Mitrofanov, 1996; Timmerman, 1996). The coarse-

grained TTG gneisses have a high-grade equilibrium micro-

structure. The often ill-defined foliation comprises a grain

shape fabric chiefly of oriented feldspar and quartz. A cen-timetre to decimetre-scale gneisses layering is defined by

variations in modal amounts of plagioclase (An 2530),

microcline, quartz, (blue) green amphibole or biotite. In some

TTG gneisses the layering is crosscut by veins of gneissic

tonalite, suggesting that deformation alternated with mag-

matism. Coarse-grained amphibolites occur as concordant

layers in the TTG gneisses or as decimetre-wide veins

cross-cutting the fabric of the host rocks. The tectonic lay-

ering is generally subvertically dipping; a lineation, when

present, is steeply plunging. The degree of preferred ori-

entation of the main constituents hornblende and plagio-

clase is variable. Because green hornblende may containrelics of clinopyroxene it is probably formed by retrograde


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hydration. Also Na-plagioclase and minor quartz are probably

formed by retrogression. Hornblende is generally compo-

sitionally zoned with rims that are richer in actinolite. Gar-

net occurs occasionally in plagioclase. Hornblende in both

gneisses and amphibolites is locally replaced by


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322 Koen de Jong

clase RbSr age of biotite-free amphibolite MT-27 is com-

parable to the 40Ar/39Ar age of hornblende, whereas in

biotite-bearing amphibolite MT-11 it is comparable to the40Ar/39Ar age of biotite.

5. MODELLING OF AGE SPECTRA

Generally, isotopic ages in metamorphic rocks are inter-

preted within the framework of Dodsons 1973 diffusion

theory, which states that minerals in geological systems

start to accumulate isotopes formed by radioactive decay,

like radiogenic argon (40Ar*) and strontium (87Sr), in their

crystalline lattices if the temperature is below a critical

value. Above this so-called blocking (Jger, 1967) or clo-sure (Dodson, 1973) temperature 40Ar* is regarded to be

lost from minerals by volume diffusion. Age zoning of

grains, revealed by 40Ar/39Ar laser probe spot dating, show40Ar* gradients (Phillips and Onstott, 1989; Lee et al.,

1990; Scaillet et al., 1990; Kelley and Turner, 1991; de Jong

et al., 1992; Hames and Cheney, 1997). Already before the

advent of laser probe spot dating, furnace step-heating of

mineral separates that yielded 40Ar/39Ar age spectra with

progressively rising apparent ages had been interpreted as

caused by partial argon loss by diffusion processes from

so-called lower retentive lattice sites during younger thermo-

tectonic reworking or slow cooling (Turner, 1969). Thisprocess would result in younger apparent ages during the

early stages of degassing experiments in the laboratory.

Staircase-type hornblende age spectra, like those obtained

by de Jong and Wijbrans (2006) for hornblende separate

MT-10 (Fig. 3a), have been described widely from meta-

morphic terranes (e.g., Berger, 1975; Dallmeyer, 1975; Har-

rison and McDougall, 1980; Onstott and Peacock, 1987;

Wartho, 1995a) and have been classically interpreted as

pointing to partial resetting by thermally activated 40Ar* loss

by solid-state volume diffusion during younger thermo-tec-

tonic reworking. Spectra with very old apparent ages for

the first increments, similar to those for hornblende sepa-

rate MT-11 (Fig. 3b), have been explained by limited uptake

of excess argon that was superimposed upon partial loss of

argon (Harrison and McDougall, 1980; Berry and McDou-gall, 1986; Wijbrans and McDougall, 1987). In contrast,

thermally undisturbed minerals will, according to the dif-

fusion theory, show flat spectra, not unlike the spectra of

the hornblende drilled grain and separate of MT-27 (Fig.

3c). In order to test these assumptions and to gain insight

into the reasons behind the finding of flat age patterns for

drilled hornblende grains from samples that yielded dis-

turbed age spectra for the separate (MT-11; Fig. 3b), spec-

tra were modelled using the Double Pulse Program of the

University of Toronto (McMaster, 1987) and the MacAr-

gon programme (Lister and Baldwin, 1996). Such model

calculations are based on diffusion theory and comparableto Turners 1969 model and assume thermally activated

Fig. 3. 4 0 Ar/3 9 Ar age spectra (lower panels) and Ca/K ratio spectra (upper panels) of samples MT-10 (a), MT-11 (b) and MT-27 (c) fromthe Murmansk Terrane, acquired by step-heating of separates of 1020 milligram of hornblende (ac) and 59 milligram of biotite (a,b) with a resistance-heated furnace, and with a defocused laser probe on ca. 1.5 mm diameter drilled hornblende discs (b, c). Degassingtemperatures indicated in o C; those for biotite in italics. Data and analytical procedure are given in de Jong and Wijbrans (2006). RbSr ages of hornblende-plagioclase pairs (Cliff et al., 1997) in amphibolites MT-11 (b) and MT-27 (c) and their error are indicated by theobliquely ruled bar.


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40Ar* loss by solid-state volume diffusion, zero argon con-

centration at grain surfaces and uniform grain size. Diffu-

sion theory stipulates that a thermal overprinting will result

in diffusion of

40

Ar* from the less retentive sites within thecrystal. This results in lower apparent ages during the early

stages of degassing in the laboratory. Despite the acicular

habit of hornblende a spherical diffusion geometry is gen-

erally assumed for the mineral (Kelley and Turner, 1991;

Lister and Baldwin, 1996). This agrees with the quite equi-

dimensional grains that were used for step-heating and the

extensive array of parting planes at high angles to the c-

axis of the mineral in thin section.

The Double Pulse Program allows for loss and/or gain of40Ar* during two superimposed events (t1 and t2) that are

separated in time. The input parameters include the number

of steps, their size and apparent ages, as well as the ages

of the main event and two later partial resetting phases t1

and t2, each with the percentage of inflicted 40Ar* loss and/

or gain. Based on these input parameters the program opti-

mises the fit by an iterative process. The output model

curve is modified so that individual apparent ages of steps

are modelled. The modelling suggests that hornblende in

separates MT-10 and MT-11 suffered argon losses of 40

50% in one or two pulses, with less than 5% of excess Ar

incorporation for the latter sample.

The MacArgon programme enables a number of differ-

ent thermal scenarios to be modeled. In our case, the most

successful models have a thermal history that is compara-

ble to the orogenic evolution of the LKO that is fairly well

constrained. The main input parameters were: a fast Neoar-

chaean cooling, followed by a peneplanation from 2350

Ma on (section 4), which was followed by underthrusting

to mid crustal levels during the 1.951.90 Ga Lapland-

Kola orogeny that is associated with a substantial reheating

(Fig. 4). The latter event is constrained by growth of stau-

rolite and cordierite in chlorite-muscovite schists, which

represent severely retrogressed Archaean rocks of the Cen-

tral Kola Terrane (Timmerman, 1996). The age spectra of

hornblende separates of MT-10 and MT-11 were success-

fully modelled by applying a Palaeoproterozoic reheating

to 450 25 oC, which is not necessarily followed byreheating to 400 oC at 1750 Ma, associated with the post-

tectonic magmatism of the Litsa-Araguba suite (Figs. 5a

and b). Stronger or less reheating could not fit the observed

age spectra of the hornblende separates. Applying a slow

cooling model approach resulted in a good fit as well. The

successful thermo-tectonic scenarios resulted in biotite

model ages as young as observed, as the mineral is above

the closure temperature for a long time. Modelling of the

flat age spectra of MT-27 suggest a thermally undisturbed

Neoarchaean isotope system and rapid cooling. These results

would imply that neighbouring samples experienced sharply

contrasting thermal histories. A key sample is hornblendeMT-11 that yielded an apparent partial loss spectrum of

progressively increasing apparent ages for the separate, not

shown by the age spectrum of drilled grain that shows

slightly decreasing apparent ages to ca. 2.64 Ga instead (Fig.

3b). This would imply that neighbouring grains within a

single sample would have experienced sharply contrasting

thermal histories. Consequently, modelling gives conflict-

ing and unrealistic results. This implies that phenomena

other that the thermal evolution of the samples alone, lie

behind the finding of apparent thermal loss age spectra for

some separates and flat spectra for the biotite-free drilled

grains.

6. Ca/K RATIO SPECTRA

Step-heating of hornblende MT-27 yielded fairly con-

stant Ca/K ratios of 1212.5 for the separate and 1314 for

the single grain during the entire 39Ar release (Fig. 3c); val-

ues that agree with Ca/K ratio of 1314 obtained by EPMA

(de Jong and Wijbrans, 2006). In contrast, separates MT-10

and MT-11 have progressively increasing Ca/K ratio spec-

tra (Figs. 3a and b). But, the drilled hornblende grain from

the latter sample has a fairly constant Ca/K ratio of about

12 that is comparable to the ratios of 1011.5 of the sep-

arate in the 9801270 oC range (Fig. 3b). Ca/K ratios of66.5 for the main degassing of hornblende separate MT-

Fig. 4. Three synoptic thermal scenario of the evolution of theMurmansk Terrane applied to model the age spectra that were ableto constrain the age spectra of hornblende separates of MT-10 andMT-11. The three models differ only in Palaeoproterozoic reheat-ing. The thin solid black line reheating to 475 o C; thick solid lightgrey line reheating to 425 o C at 1.951.90 Ga, both followed byreheating to 400 o C at 1750 Ma, associated with the post-tectonicmagmatism; thin dashed dark grey line, reheating to 475 o C at1.951.90 Ga, without late magmatism-related reheating.


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324 Koen de Jong

10 (Fig. 3a; 9701085 oC range) are comparable to values

of 6.17.7 obtained by EPMA (de Jong and Wijbrans,

2006). This shows that the age information contained in

the spectra of drilled grains and separates for the degassingabove 970 oC refers essentially to hornblende. However,

the low Ca/K ratios of hornblende separates MT-10 and

MT-11 for gas release below 970 oC (Figs. 3a and b) point

to degassing of included K-rich phases, like the observed

tiny biotite crystals that occur intimately intergrown in the

amphibole. Also the main degassing of biotite from the

matrix of these samples took place at temperatures below

1000 oC (Figs. 3a and b). In contrast, hornblende grain MT-

11 from which biotite inclusions could be avoided by well

targeted drilling yielded spectra with fairly constant appar-

ent ages and Ca/K ratios for (Fig. 3b). This shows that such

typical partial-loss age spectra stem from the differential

degassing of hornblende and small amounts of included

biotite, and are thus the result of a binary mixing (Berger,

1975; Rex et al., 1993; Ahn and Cho, 1998; de Jong and

Wijbrans, 2006). This interpretation implies that the low

retentive hornblende sites in numerical models may in fact

be an artifact related to inclusions of earlier degassing

biotite in natural hornblende.

This interpretation similarly questions the classic inter-

pretation of flat age spectra and age plateaux as simply

reflecting a homogeneous distribution of Ar in the crystal

lattices of minerals not been affected by thermally induced

Ar loss. There is a lot of evidence that Ar release during

step-heating of hydrous minerals like amphiboles under

vacuum occurs as a consequence of chemical and struc-

tural changes within the crystals, rather than by volume dif-

fusion (Gaber et al., 1988; Lee et al., 1991; Wartho et al.,

1991; Wartho, 1995b). A major consequence of this is that39ArK and

40Ar* may be released simultaneously from cores

and rims of crystals, leading to homogenisation of age gra-

dients (Lee et al., 1990; Kelley and Turner, 1991; Lee, 1993).

7. GEOLOGICAL SIGNIFICANCE

The transformation of hornblende into biotite indicates

hydration and an increased activity of K-ions. Biotite growthis confined to grain boundaries, cleavage planes and lattice

imperfections in hornblende implying localised ingress of

an aqueous fluid. This can be understood from the obser-

vations that grain and sub grain boundaries are disorgan-

ised and rich in vacancies due to mismatch of mineral

lattices (Lomer and Nye 1952), and that grain-boundary

diffusion is generally faster than intracrystalline diffusion

(Fisher, 1977). Consequently, K would be expected to be

mobile on grain boundaries and lattice imperfections.

Under hydrous conditions these areas will hence be coated

by a fluid film and may become the site of concentrated

fluid ingress. As biotite-bearing quartz-feldspar gneissessurrounding the amphibolites are richer in K, the localised

hydration implies an open system on at least the scale of

several dm to metres. The first increment of MT-10 has an

apparent age that is comparable to the youngest biotites

from the matrix of gneisses and amphibolites of the Mur-mansk Terrane, like MT-11 (1800 6 Ma). The integrated

age of 1988 8 Ma of matrix biotite MT-10 is likely to be

due to excess and/or inherited argon as this date is far older

than the regionally occurring 1.75 1.70 Ga 40Ar/39Ar min-

imum mica ages (de Jong et al., 1999). These data suggest

that fluid ingress leading to biotite growth occurred during

the middle Palaeoproterozoic.

The apparent age of 2645 Ma of the final degassing of

hornblende separate MT-11, which is the least affected by

included younger biotite, is concordant with the apparent

age of 2650 Ma for the flat part of spectrum of biotite-free

drilled hornblende grain from this sample (Fig. 3b). These

ages are concordant with the weighted mean ages of ca.

2.64 Ga for both hornblende step heating experiments on

MT-27 (Fig. 3c). The apparent age of the final degassing of

hornblende separate MT-10 that is least affected by included

younger biotite is 2548 Ma (Fig. 3a), suggesting that horn-

blende in the tonalitic gneiss is younger than in amphib-

olites MT-11 and MT-27 that occur as bands in tonalitic

gneisses. Because amphibolite MT-11 forms a band in tonal-

itic gneiss MT-10, differences in thermal history cannot be

the reason of the observed age difference between the

hornblendes in both samples. In slowly cooled rocks, age

gradients occur at the scale of hornblende grains (Kelley

and Turner, 1991). Consequently, neighbouring grains with

different diameters can in principle yield different ages, as

Dodsons (1973) diffusion theory predicts. The typical grain

size of hornblende crystals measured in cross section per-

pendicular to c-axes in thin section is much smaller for MT-10

than MT-11, viz. 2500 m, respectively. These

two samples thus display an age-grain size relationship in

line with diffusion theory, hornblende MT-11 with the larg-

est grain size being the oldest. However, as hornblende in

MT-27 (>3500 m in cross section) is still larger but not

older than in MT-11, it is unlikely that grain size is the gov-

erning factor behind the observed difference in age between

hornblende in the two amphibolites and the tonaliticgneiss. As the amphibolites form sub-concordant, but bou-

dinaged layers in the gneisses the younger age of horn-

blende MT-10 might be due to a longer lasting tectono-

metamorphic recrystallisation of the gneisses. However,

because the last three steps for hornblende MT-10 yielded

progressively older ages and higher Ca/K ratios than the

previous steps, pointing to degassing of a more Ca-rich con-

taminant phase, like clinopyroxene (de Jong and Wijbrans,

2006), a full understand of the meaning of the 2548 Ma

age is difficult.

The discrepancy between the RbSr age of 1881 23

Ma for the hornblende-plagioclase pair of MT-11 and theNeoarchaean integrated 40Ar/39Ar age has been explained


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by substantial excess argon incorporation by amphibole

(Cliff et al., 1997). However, both the drilled hornblende

grain and the mineral separate of sample MT-11 are only

marginally affected by excess argon uptake. This is revealedby excessively old ages for the first steps and by the

slightly older total gas age of the drilled grain with respect

to the grain separate (Table 1). However, the 2650 Ma appar-

ent ages of the final parts of the spectra are concordant to

weighted mean ages of ca.2.64 Ga of hornblende MT-27

(Figs. 3b and c). Excess argon is often incorporated het-

erogeneously, leading to important age differences between

neighbouring samples and even between grains in a single

sample, or parts of grains (de Jong et al., 2001), making the

interpretation by Cliff et al. (1997) unlikely. The horn-

blende-plagioclase RbSr age of 1881 23 Ma in the biotite-

bearing amphibolite is comparable to the 1988 8 Ma inte-

grated 40Ar/39Ar age of biotite (Fig. 3b) in this rock. The

ingress of an aqueous fluid enriched in K implied by the

localised growth of biotite points to recrystallisation of the

amphibolite, suggesting that plagioclase in this rock might

have been affected by recrystallisation and isotope exchange

too.

Cliff et al. (1997) calculated the two-point RbSr min-

eral-isochron for the hornblende-plagioclase pair on the

assumption that they grew and recrystallised in equilibrium

with a fluid phase in the rock, which also is the initial Sr

reservoir. Such fluid no longer being present, the whole-

rock or a Rb-poor and Sr-rich mineral is taken as a proxy

for the initial 87Sr/86Sr of the fluid, like plagioclase (Free-

man et al., 1997) or apatite (Meffan-Main et al., 2004).

Implicit in this calculation is that the analysed minerals,

viz. hornblende and plagioclase, sampled the same Sr res-

ervoir during (re)crystallisation. If this is not the case the

result is a geologically meaningless mixed age. Especially

the different abilities of the minerals used for the calcula-

tion to interact with circulating (late-stage) fluids may have

a strong influence on the age result (Andriessen, 1991).

The RbSr age of a mineral will depend on the relative

rates of loss or gain of Rb, Sr and 87Sr by a mineral during

cooling (Jenkin et al., 2001). Assuming that plagioclase in

MT-11 was above its closure temperature the mineral could(continue to) exchange strontium and attain equilibrium

with the (Palaeoproterozoic) metamorphic fluid. Biotite

growth in hornblende MT-11 and the implied K-influx

show open system behaviour, at least on the scale of sev-

eral dm to metres. As the gneisses surrounding the amphib-

olite are Neoarchaean their recrystallisation may have

released 87Sr too that was added to the fluid that penetrated

the amphibolite. If a given plagioclase was in equilibrium

with a fluid that has a raised 87Sr/86Sr ratio and above its

closure temperature for Sr exchange, 87Sr could in principle

have diffused inward along a concentration gradient (Jen-

kin et al., 1995, 2001), and so altering the isotopic signa-ture of the feldspar. Such a process would render the

assumption that plagioclase is a proxy for the initial 87Sr/86Sr

ratio invalid. Significant Sr isotope exchange could result

in a much younger age for the plagioclase-hornblende pair,

not Neoarchaean, but an age comparable to the ca.

1900Ma RbSr age of MT-11. Pettke and Diamond (1997)

described a very rare and extreme case where circulating

fluids carried radiogenic strontium leached from a base-

ment, thereby making the whole-rock point meaningless.

In contrast to hornblendes MT-10 and MT-11, biotite is

absent from the matrix of amphibolite MT-27 and from the

hornblende itself. Hornblende MT-27 seems thus not affected

by Palaeoproterozoic fluid infiltration. MT-27 yielded a

RbSr age of 2680 19 Ma for a hornblende-plagioclase pair,

which is concordant to the 2657 10 Ma integrated 40Ar/39Ar hornblende age and only marginally elevated above

the ca. 2640 Ma apparent ages of the main increments ofboth step-heating experiments on this sample. The use of

Fig. 5. Modelled age spectra of MT-10 (a) and MT-11 (b) on thebasis of the thermal scenario of Figure 4, using the MacArgonsoftware.


10/13


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Manuscript received June 4, 2009

Manuscript accepted September 9, 2009

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