Quantifying partial melt fraction in the crust beneath the central andes and the Tibetan plateau

Phys. (.'hem. Earth (.,4), Vol. 26, No. 4-5, pp. 239-246, 2001 Pergamon

© 2001 Elsevier Science Ltd. All rights reserved 1464-1895/01/$ - see front matter

PII: S 1464-1895(01)00051-5

Quantifying Partial Melt Fraction in the Crust Beneath the Central Andes and the Tibetan Plateau F. R. Schilling I and G. M. Partzsch 2

~GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 2Mineralogisches Institut, Ruprecht-Karls-Universit~it, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany Received 14 June 2000; revised 22 September 2000; accepted 18 October 2000

Abstract. An interdisciplinary approach is used to quantify partial melt fractions and to infer the origin and distribution (melt structure) of melts located in the crust beneath the Central Andes and the Tibetan plateau. In these areas field observations of Low Velocity Zones (LVZ) and High Conductivity Zones (HCZ), which are commonly attributed to partial melting, are used to quantify melt fractions. Additional information is obtained from vp/vs ratios, seismic attenuation data, and heat flow density and gravity anomalies. These data accompanied by thermal modelling suggest that melts of mainly crustal origin are interconnected through dykes and veins. Experimental results and model calculations indicate that the minimum fraction of melt necessary to describe the LVZs and HCZs in the Central Andes and the Tibetan plateau is approximately 20 vol.%, and the melt has a non-ideal interconnectivity. © 2001 Elsevier Science Ltd. All rights reserved

I. Introduction The degree of partial melting and the interconnectivity of crustal melts are key parameters in the evolution of continental crusts (Brown et al. 1999). Therefore, processes of partial melting have been intensively studied since the 1970s, both experimentally and from field observations on relevant rocks (e.g., migmatites) (Meimert et al., 1973; BOsch et al., 1974; for a detailed discussion see Laporte et al. 1997). Migmatites are found in many orogens and are considered to indicate an ancient partially molten crust, with a degree of partial melting in the order of 20 to 30 vol%. (Nyman et al. 1995; Brown et al. 1999; Berger and Kalt, 2000). In some active orogens geophysical observations may be used to study the behaviour of a partially molten crust. A variety of different geophysical methods are being used to obtain information on the structure of the Earth's crust. Correspondence to: Frank Schilling, GFZ Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

In some orogens geophysical observations show the presence of High Conductivity Zones (HCZ), Low Velocity Zones (LVZ), and a rare high negative Bouguer anomalies of down to -450 mgal. The presence of these anomalies together with a strong attenuation of seismic waves l, high heat flow densities and a high ve/vs ratio2 point to the present existence of huge amount of melts in the crust beneath the Pyrenees, the Andes and the Tibetan plateau (Pham et al., 1986; Pons et al., 1995; Echternacht et al., 1997; Schilling et al., 1997, Chemilowski et al. 1999, Chen et a/.,1996; Nelson et al., 1996; Unsworth et al. 2000). Here we focus on two well studied regions - the Central Andes and the Tibetan plateau, which have been chosen for the high quality and variety of available geophysical observations. In the Central Andes, the existence of partial melts is well constrained by HCZs (Echternacht et al., 1997; Schilling et al., 1997), seismic observations (Schrnitz et al., 1997, Chemielowski et al., 1999) and thermal models (Amdt et al., 1997, Springer and FSrster, 1998). High V#Vsratios and low Q values in the same region strengthen the interpretation of abundant partial melts within the Andes. Within the Tibetan Plateau the existence of partial melts is concluded from a HCZ (Pham et al., 1986; Chen et al., 1996; Unsworth et al. 2000), which is linked to a strong seismic reflector (Nelson et al., 1996) and a LVZ (Kind et al., 1996) within an active thermal field (Wang et al., 1981). Knowledge of whether these melts partition from the host rock and subsequently ascend towards the surface or whether they remaining within the crust is a key to understanding the evolution of this orogen. The dynamic behaviour of the melts dependsnot only on the physical and chemical properties of the melt phase, but also on its three dimensional distribution, the melt structure. Laboratory studies in combination with model calculations can help to increase our understanding of the dynamic behaviour of partially molten crusts. I The absorptionof seismicenergyis usuallydescribedby Q, the quality factor. A high Q valuerepresentsa low attenuationof seismicwavesand a low absorption of seismic energy DecreasingQ values indicate an increasein the absorptionof seismicenergy. 2vp longitudinalwavevelocity,vs shearwavevelocity

240

E R. Schilling and G. M. Partzsch. QuantifyingPartml Melt Fraction 7l°W

(a)

70*W

69*W

68"W

67"W

66~W

85 = W

64*

W

~oo: 2~ ~ :

2. Geophysical and Geological Observations in the Central Andes and the Tibetan Plateau 2.1 Central Andes

22~

23~ :

24oi

25~ =

(b)

The Andes are located at the convergence zone between the oceanic Nazca plate and the South American plate. This Cordilleran-type mountain belt is the largest active subduction-controlled orogen on Earth with a length o f = 7500 km. In the central part, between 16 ° to 25°S, the Andes show extreme dimensions with a 800 km wide mountain belt and elevations o f more than 6 0 0 0 m (Fig. la).

(a)

w

Preandean Depression

WesternCo~illera

E

020 40-

0.0005 S/m

608O 0 001 S/rn

100 km

1 O0 k m

I

I

(b) S

Fig. !: (a) Locationmap of the Central Andes, showingthe morphostructural umts after Reutter et al (1988). The triangles indicate the present volcanic arc with elevations up to > 6000 m. CC - Costal Cordillera, LV - Longitudinal Valley, PC - Precorddlera, PD - Preandine Depression, WC - Western Cordillera, AP - Altlplano, PU - Puna, EC Eastern Cordillera. SA - Subandean Ranges. APVC- Altiplano-Puna Vulcanic Complex, AF, WF - schematic run of the Atacama Fault and West Fissure, respectively. (b) Major tectonic features of the Tibetan Plateau Mare BoundaryThrust (MBT), Indus-TsangpoSuture (IST) (slmphfied after Rodgers and Schwartz, 1997).

Many laboratory studies and models are based on the concept o f dihedral angels (Beere, 1975a; 1975b) derived for an idealized wetting behaviour. This concept is still widely used even though it only approximates the isotropic surface energies o f minerals and melts (Holness, 1995). Actual studies, however, show strong deviations from an ideal wetting behaviour by three-dimensional imaging o f migmatites (e.g., Brown et al., 1999), in laboratory studies o f partially molten rocks (e.g., Lupulescu and Watson; 1999, Partzsch et al., 2000), or as a result o f nonhydrostatic stress conditions (Jin et al., 1994). In this contribution we combine geophysical observations, field data, laboratory studies, and modelling to (i) quantify partial melt fractions, (ii) discuss the origin o f melts, and (iii) deduce the melt structure. The study is focused on two well-examined regions in the Central Andes and beneath the Tibetan Plateau.

Lhozag

Zang-Bo

Lhasa

Yang Ba Jan

N

2O ~'n

100 km

!

I °° .4. 4 0 k i n

Fig. 2. Slmphfled electrical conductwity models (2-D inversion of magneto-telluric measurements) in {a) the Central Andes (Echternachtet al., 1997) and {b) the Tibetan Plateau (Pham et al., 1986).

The Central Andes are characterized by several geophysical anomalies. A crustal thickness o f up to 70 km below the Altiplano and the Western Cordillera has been deduced from seismic and gravimetric measurements (James, 1971; Ocola and Meyer 1972; Wigger et al., 1994). The Moho-discontinuity is clearly resolved in seismic tomography beneath the Western Cordillera (Haberland, 1998; Rietbrock, 1999). A distinct low velocity zone (LVZ) and a zone o f high seismic attenuation are observed beneath the magmatic arc (Wigger et al., 1994; Haberland, 1998). The Bouguer anomaly reaches -450regal in the magmatic arc and even the residual field amounts to approximately -40 mgal (G6tze et al., 1994). High heat flow values o f >100 mW/m 2 have been measured in the Western Cordillera (Henry and Pollack, 1988; Giese, 1994; Hamza and Mufloz, 1996; Springer and FSrster, 1998). Magneto-telluric and geomagnetic deep soundings in northern Chile have revealed a pronounced HCZ (Schwarz et al., 1994, Echternacht et aL, 1997) beneath the Western Cordillera, which constitutes the present magmatic arc o f the South American continental margin with active volcanism (Fig. 2).

E R. Schillingand G. M. Partzsch:QuantifyingPartialMelt Fraction The recent seismic studies of the eighties and nineties conducted between 20 ° and 26°S (mainly within the framework of the Berlin-Potsdam Special Research Centre "Deformation Processes within the Andes") reveal a more detailed structure within the Andean crust. Seismic discontinuities and broad low-velocity zones (e.g., Wigger et al., 1994) were deduced from active seismic experiments, whereas areas with high ve/vs ratios and low Q values (e.g., Haberlandt, 1998; Rietbrock, 1999) were observed in 3D topographic inversions from passive seismic experiments. The thickening of the Andean crust has been explained by different mechanisms such as magmatic addition (e.g., James, 1971; Thorpe et aL, 1981; Pet-ford and Atherton, 1996), tectonic shortening (Reutter et aL, 1988; Sheffels, 1990; Scheuber et al., 1994) and thermal uplift in combination with shortening (Isacks, 1988). A combination of different mechanisms seems to he necessary to explain the present crustal thickness (Scheuber et al., 1994; Giese et aL, 1999).

241

3. Experimental Studies

A granulite sample from the Coastal Cordillera from the Bolfm Complex is used to study the three dimensional development of the melt distribution and the electrical conductivity during partial melting. The sample corresponds to intermediate crustal levels and is related to the Jurassic volcanic activity. Lukassen (1992) gives a detailed sample description. The electrical conductivity measurements and melt distribution experiments were performed in a high temperature resistance furnace. The oxygen fugacity was controlled (Waft and Weill, 1975) with H2/N2/CO2 gas mixtures and monitored with a zirconia sensor (Deines et aL, 1974) The electrical conductivity measured with a two electrode arrangement using iron saturated platinum electrodes (Partzsch, 1998). Temperature [~CI 1.0 ~50 1100

2.2. Tibetan Plateau

0.5

The Tibetan Plateau is the largest topographical mass on Earth, with an unusually high average elevation of approximately 5 km above the sea level. Its uplift is associated with the India-Asia continent-continent collision, penetrating from India into Asia (Molnar and Tapponnier, 1975; Molnar 1988). This collision results in a crustal thickness of up to 70km (Pham et al., 1986). The lithosphere of the Tibetan Plateau was assembled by the accretion of continental and island arc terranes into southern Asia during the closure of the Tethys followed by a continent-continent collision (Dewey et aL, 1988)(Fig. lb). This collision has formed complex geological structures in the Tibetan plateau, which were examined during the INDEPTH project by different geophysical methods (e.g., Brown et al., 1996; Kind et al, 1996, Nelson et al.; 1996, Chen et al., 1996). Several geophysical anomalies quite similar to those observed in the Central Andes are detected beneath the Tibetan plateau. Seismic studies indicate that the crustal thickness exceeds 70 km. The crust shows LVZs and a strong reflector of seismic waves within the upper crust (Nelson et al., 1996; Brown et aL, 1996; Kind et al., 1996; Makovski et al. 1996). A HCZ with conductivities up to 0.33 S/m (Fig. 2) were detected that are closely related to the LVZs (Pham et al., 1986; Chert et al, 1996; Unsworth et al., 2000). The Tibetan Plateau is characterized by strong geothermal activity with more than 600 hydrothermal areas, which are concentrated in the so-called "Himalayan Geothermal Belt (HGB)" (Wang et al., 1981). The geothermal activity is closely related to recent magrnatic activity.

0.0

~' -0.5 c -1.0

~

-1.5

1050

11~00

9~0

900

850

measured

oa,,umo, an

':./

"%~_.

I

]

ideal interconnected network of melt I

o

~ •

id rock

-2.0

-2.5 -3.0 0.70

. 0.75

.

.

. 0.80

.

.

.

.

0.85

0.90

Temperature in [IO001K] Fig. 3: Arrhenius diagram of the experimental determined electrical conductivity (e) of a granulite from the Boifin Complex (Chile). Conductivitieswere modelledfor an idealnetworkof melt (0) and meltin insulatedpockets(r'l), usingthe measuredmelt fractions(Fig. 6). Details of the modellingare presentedin the text.

The measurements of the electrical conductivity were conducted at rising temperatures. Impedance spectra were taken automatically every 6 K in the frequency range between 1 and 10 6 I-Iz with a computer controlled impedance spectrometer (> 40 individual frequencies for each temperature and sample). For details of the evaluation procedure see Partzsch et al. (2000). The experimental results are shown in Fig. 3. The maximum error is represented by the size of the symbols. For temperatures below 1000°C the error is much smaller than the height of the symbols. Melting experiments were performed under the same experimental conditions using the same equipment. The phase content and texture of the quenched samples were examined in thin sections and the chemical composition of the phases (minerals and melts) was determined with an electron microprobe. The modal phase content was evaluated with the point counter method. The detailed

242

E R. Schilhng and G. M. Partzsch: QuantifyingPartial Melt Fraction

10

Interconnected ~

~,

1

R~

~ "~

~"~ .~ ~ _~

0

4. Discussion 4.1. Electrical Conductivity

~.~

y

. . . . . . . . . . . . . . 10 20

30 40 50 60 Xmelt [%]

70 80

90 100

Fig. 4: Calculated electrical conductivity of a partially molten rock as a function of the melt portion (Eq. 2). An interconnected melt along grain boundaries is assumed for modelling (bold solid line - HashmShtrikman upper bound). A high conducting(10 S/m) and an intermediateconducting (5 S/m) melt are used m the calculatlon. The conductivityof the solid rock is set to 0.01 S/m, comparableto the observed conductivity of the solid rock in Fig. 3. Between 14 and 27 vol.% melt are necessaryto obtain the 1 S/m observedbeneath the Western Cordilleraand 6 to 12 vol.% of melt to obtain the 0.33 S/m observedbeneath the Tibetan Plateau (lower hmit of the necessarymelt fraction). If the melt is concentrated in insulated melt pockets ~ 90 vol.% melt is required to describe the observedconductwity tthin hne - upper hmit of the necessarymelt fract=on).

description of the experiments is given in Partzsch

et aL

(2000) and Schilling et al. (1997). The observed melt distribution at normal pressure is in good agreement (Partzsch et al., 2000) with observed melt distributions in high-pressure experiments on comparable samples (e.g., Laporte et al. 1997). As the melt distributions at normal pressure are similar to those in high-pressure experiments, the same mixing models can be applied. With increasing P H e o the solidus temperature decreases. Therefore, compared to the presented laboratory experiments, melting will start at lower temperatures at crustal conditions, At low temperatures (up to about 1010°C) a linear increase of the conductivity is observed (Fig. 3). The activation energy E,4 for the solid rock is calculated from the slope of the curve according to f r-- "~ o ' = troeXpt--~TAl

A strong HCZ is observed under both the magmatic arc in the Central Andes (Schwarz et al., 1994; Echternacht et al.;

1997) andtheTibetanplateau(Phametal.,1986;Chenet al., 1996). The top of the HCZ in the Central Andes with a

,n ,~lm,,.,-"'Meltinsulated ~ ~ in pockets ~ 5 ~ ' "

"I

0.01

~

tm~/ Tibetan olateau (0.33 S , . . . i s

~', 0.1

=

~

/

,--, t~

~lt

(1)

where k is the Boltzmann constant, T the temperature in Kelvin, and 60 is a constant. Below 1030°C the calculated EA is equal to 1.4 _+0.1 eV (Fig. 3). Between 1030 and 1070°C a change in the slope is observed. This steeper increase in electrical conductivity with increasing temperature is due to partial melting (Presnall et al., 1972; Rai and Manghnani, 1978; Schilling et al., 1997; Partzsch et al., 2000). The low temperature behaviour (straight line Fig. 3) has been used as ~ot,a in the model calculations later in this paper(Eq. 3, Fig. 6).

conductivity of ~ I S/m is detected at a depth of ~ 20 kin. This is similar to the depth where a HCZ is observed within the Tibetan plateau (20-25km depth). The overall conductance in the Andes is approximately 30 kS, the highest value known in the world, requiting the HCZ to be

at least 30 km thick (Echternacht et al., 1997). The 6 kS overall conductance of the Tibetan plateau implies a thickness of the HCZ of ~ 20 km (Pham et aL, 1986; Nelson et al. 1996). The electrical conductivity of two-phase composites (e.g., melt and rock) can be modelled by the HashinShtrickman upper and lower bound (HS ÷ and HS')(Hashin and Shtrikman, 1962). The average DC conductivity cr in the HS ÷ and HS- model is described by: o-HS ÷ =

~

3xl coo-

L

3o-,+ (1- x~)tgo-)

o-H 1+

1 (2)

O-HS_=o-,II 3 (1--X') COO/with coo---o-,-o-, Where xt is the volume fraction of the melt, o'i and trlt are the conductivity of the melt and solid phase, respectively. For HS + phase I is interconnected into a 3D network (ideal interconnected network of melt), while in HS" melt is insulated in pockets. The as modelled conductivities are shown in Fig. 4. If the observed electrical conductivity (Fig. 2) is modelled by a highly conductive silicate melt (5 and 10 S/m, Satherly and Smedley, 1995; Partzsch et al. 2000), at least 14 vol.% and 6 vol.% of interconnected melt are necessary to explain the observed behaviour in the Central Andes and Tibet, respectively (Fig. 4). These values are a minimum estimation for the partial melt portion within these crusts for an ideal interconnected network of melt. Due to perturbation problems and the likelihood of some melt occurring in melt pockets, the real amount of melt will probably be even higher (Baby, 1997). 4.2. Seismic Velocity: The low seismic velocity zones (LVZ) and high ve/vs ratios observed in active and passive seismic experiments beneath the Western Cordillera (Central Andes) and Tibet can also be explained by partial melting (Schmitz et al., 1997; Kind et al., 1996). If partially molten rocks are used to model the elastic behaviour in the Central Andes, approximately the same amount of melts (15-20 vol%) as in the electrical conductivity estimations are necessary to

E R. Schillingand G. M. Partzsch:QuantifyingPartial Melt Fraction explain the observed behaviour (Schmitz et al, 1997, Schilling, 1998). The electrical conductivity is a much more sensitive parameter in the evaluation of melt fractions, mainly due to the huge difference in the electrical conductivity of melt (~ 10 S/m) and the solid rock (~, 0.01 S/m) compared to the relatively small variation in the elastic properties (vp (melt) 3 km/s; vp (rock) -~ 7 km/s). However, the interrelation of different petrophysical properties is a key to get a more detailed picture about the quantity and distribution of melt, whereas the use of only one petrophysical property may be ambiguous. The interrelation of different properties gives us the possibility to test different assumptions and to strengthen the conclusions.

4.3. Heat Flow: A maximum heat flow density of more than 140 mW/m 2 with an average around 90 mW/m 2 is observed in the Central Andes (Springer and FOrster, 1998; Springer, 1999) and in Tibet (Wang et al., 1981) compared to an average heat flow density of ~ 65 mW/m 2 on Earth. The highobserved heat flow density indicates partial melting in a thickened crust, even if a large amount of convective heat transport and a high radiogenic heat production (1-2 laW/m 3) are taken into account (Schilling, 1998).

3

mWlm" !.5 10 ",g

K

20 25

P-

30

I.........

35 ............ I 0 250 500 750 1000 1250 1500 Temperature[*C] Fig. 5: A modelledgeothermfor a surfaceheat flowdensityof 90 mW/m2, assuming a temperature dependentthermal conductivityand an internal heat productionof 1.3 ~tW/m3 (for details see Arndt et al. 1997). The hydrous solidi of differentcrustal rocks(0) muscovitegranite, @ tonalite, ® gabbro)are plottedas thin solid lines accordingto Wyllie(1977).

243

In a 1D conductive heat-transfer-model (Arndt et al., 1997) the temperature at about 20 km depth reaches the wet solidus of typical crustal rocks (Fig. 5), for a heat flow density of 90 mW/m 2 as observed in the Andes and the Tibetan plateau. The frictional heat generation during a collision may also contribute to heating of the crust, but the internal heat production of crustal materials alone will be sufficient for partial melting (Fig. 5). Therefore, in both orogenic systems the presence of partial melts can be related to the extreme thickness of the crusts.

4.4. Melt Distribution The electrical conductivity is very sensitive to the melt distribution. A highly inter-connected network of melt has a high conductivity, whereas melt in melt pockets contributes only little to the overall conductance. Measured electrical conductivities and observed melt fractions are therefore used to quantify the melt distribution. The Hashin Shtrikman upper and lower bound (Eq. 2) was used to model the electrical conductivity as a function of temperature (Fig. 3) using the melt fractions from the melting experiments. For the first model a complete interconnected network of melt at grain surfaces is assumed (ideal interconnected network) whereas in the second model the melt is assumed to reside in insulated pockets. If, for example, most of the melt is situated in insulated pockets the volume fraction of the melt might be high without a strong influence on the overall conductance (Fig. 4). The same amount of an ideal interconnected network of melt however will lead to a much stronger conductivity increase (Fig. 4). The deviation of the measured electrical conductivity from the calculated conductivities gives information about the melt distribution (Fig. 6). The ratio of interconnected melt to melt in insulated pockets is deduced from a comparison between measured and modelled (HS ÷ and HS-) conductivities. The conductivity of a partially molten rock is mainly the result of the interconnected melt phase (Schmeling, 1986; Schilling et al., 1997). Therefore, the melt fraction located in melt pockets mainly reduces the amount of interconnected melt and the observed conductivity can be related to the interconnected melt fraction. The interconnected melt fraction X,,te,co,~,ed, is approximated as the weighted arithmetic mean of the conductivity of melt in melt pockets o r (HS-) and melt interconnected at grainboundaries o~B (HS +) and used to model the observed conductivity Omeo~u~a. The calculations were performed by extrapolating the electrical conductivity of the solid rock from the subsolidus behaviour (solid line in Fig. 3). The conductivity of the melt phase used in the calculations is 10 S/m at 1300°C and is corrected for lower temperatures using an Arrheniuns equation with activation energy of 1 eV.

244

E R. Schilling and G. M. Partzsch: Quantifying Partial Melt Fraction

50

100

~.

10000

90

ascending of m a g m a

o ;>. 80 70 E 60 I 50 o 40 0 30 E 20 .E 10 x 0 1000

i

0

1050

• 1100

40

-~

30 ~_.

~o')

100.

20~

~

10.

10

E

1,

~

o.1

0 1150

1000.

m Error bar 0.1

Temperature [*C]

0.3

1

3

10

Diameter of the melt pipe [kin] Fig. 6: The observed melt portion and the interconnected melt fraction as a function of temperature The interconnected melt fraction is modelled according to the observed conductivity (for details see text).

Fig. 7: The minimum ascending velocity (meters/year) for a basaltic or andesitic magma ascending through a 70 km thick crust (Springer et al., 1998). At lower velocities the melt would crystallize during the ascent. The error bar of the modelling is indicated by a !

o'rr~s.~ = X,nte~o..~.a,d O'G.B + ( 1 -- X, nte~..~ed ) O'p

If we take the laboratory experiments (e.g., above, Jin et al., 1994) and field observations (e.g., Brown et al., 1999)

Xmtereonnected -- (Tmeasured -- ~ P O'GB -- O'p

(3)

ooB and (rp are modelled by the Hashin Shtrikman upper and lower bound, respectively, using the observed melt frations. As only flee parameter in Eq. 3 remains the melt fraction X,,te,~o,~e,ed. The as deduced melt fractions as a function of the temperature are plotted in Fig. 6 together with the observed melt fractions. Up to a mek fraction of~. 15 vol.% (Fig. 5) most of the melt remains in pockets and therefore contributes only little to the overall conductance (Figs. 2 & 4). The resistance at melt fractions < 15 vol.% is dominated by the high resistivity of the solid grains. At higher melt fractions the intercounectivity of the melt increases rapidly through penetration of the grain boundaries (Partzsch et al., 2000) leading to a higher overall conductance (Fig. 2). The deduced ~. 20 vol% melt (Fig. 6) necessary to form an nearly complete interconnected network of melt is in good agreement with melt structure observations in laboratory experiments on partially molten crustal rocks (e.g., Laporte et al. 1997) and field observations (e.g., Brown et al., 1999). These results show that most of the melt is interconnected at about 20 vol.%, but that there are still some "dead ends" of melt. With increasing volume of the investigated body these "dead ends" might play an increasing role due to perturbation problems. Theoretical perturbation approaches indicate that ~ 50% of melt are necessary to get an interconnected network of melt in huge bodies (Bahr, 1997). Laboratory experiments under non-hydrostatic conditions however show, that shear stress result in melt migration, which would significantly increase the interconnectivity of melt (Jin et al., 1994). These experimental results are in good agreement with field observations indicating that in the crustal environment melt tends to form an interconnected network already at lower melt fractions of about 20 vol.%.

into account, a melt fraction of at least 20 vol.% seems to be required to form an interconnected network of melt over huge areas, which is capable of increasing the conductivity to the observed values of 1 S/m (Figs. 2 & 6). While the interconnectivity of melt is the most critical parameter in modelling melt fractions, this also seems to be the most reasonable lower limit of melt fractions to increase significantly the electrical conductivity. Therefore at least 20 vol.% of melts are assumed to describe the observed HCZs in the Central Andes and within the Tibetan plateau. The low Q values (Haberland, 1998; Rietbrock, 1999) strengthen the assumption of a high melt fraction. If the melt were to penetrate all surfaces, no shear wave should be observable. Currently observable Vs waves in the area of the HCZ indicate that a rigid (solid) network exists. We thus conclude that the melt is interconnected through dykes and veins.

4.5. Origin of Melts The combination of observations and thermal modelling can be used to reveal the origin of the partial melts within the Central Andes. Two dimensional temperature models of the Andes indicate that an advective heat transfer should be taken into account (Springer et al., 1998; Springer, 1999) to model the observed heat flow density. Magma ascent from the mantle wedge at ca. 0.3 km/Ma would be in agreement with temperature models (Springer et al., 1998) and the observed magmatic activity. However, due to the high solidus temperature of basaltic/andesitic melts a slowly ascending magma would crystallize within the crust (Springer et aL, 1998). A three-dimensional advection model was used to model the minimum ascending velocity, which is necessary for basaltic melts to reach the surface through a 70 km thick crust (Fig. 7). A temperature dependent thermal

E R. Schilling and G. M. Partzsch: Quantifying Partial Melt Fraction conductivity according to A m d t et a t (1997) and a solidus temperature o f the melt o f 1000-1200°C is assumed (error bar). A cylindrical geometry for the ascending melt body is chosen. Other g e o m e t r i e s , such as a dyke would lead to a slightly smaller minimum ascending velocity (less than 50% difference). Nevertheless, very high ascending velocities are necessary for reasonable diameters o f the dykes/pipes. In a pipe with a diameter o f 100 m the minimum ascending velocity is in the order o f ~ 3 km/a. I f the viscosity o f the melt is added to the calculations, an even higher minimum ascending velocity would result (Petford, 1996). I f we assume that the observed geophysical anomalies are the result o f an andesitic or basaltic melt composition within the crust, a higher surface heat flow density would be expected due to the high solidus temperature o f basic magmas. We might also assume that this assumed high amount o f basic melts is caused by an anomalous high ascending o f basaltic or andesitic melt during the last decade(s). This would imply that over the last decade the geophysical signature should have changed dramatically. All magneto-telluric observations and seismic studies c a r d e d out over the last 20 years in similar areas however do not show any short-term variations. From temperature modelling, observed heat flow densities and observed magmatic activity during the last 10 Ma, it is not reasonable that the huge amount o f melt deduced within the Andean crust can be explained by basaltic or andesitic melts. We thus conclude, that probably most o f the partial melt in the Andes has a crustal origin and composition (granite-like) and temperatures far below 1000 °C.

Many helpful discussions with J. Amdt, H. Brasse, P. Giese, M. Springer and M. Unsworth are gratefully acknowledged. The constructive comments o f the reviewers P. Glover and N. Bagdassarov are great fully acknowledged. We thank S. Sinogeikin and A. Whittington for critical reading o f the manuscript. This contribution was made possible due to the financial support by the D F G (SFB 267 and Heisenberg grant).

Acknowledgements.

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