Thermal evolution of the lithosphere beneath the French Massif Central as deduced from geothermobarometry on mantle xenoliths

TECT~YSICS ELSEVIER

Tectonophysics275 (1997) 119-141

Thermal evolution of the lithosphere beneath the French Massif Central as deduced from geothermobarometry on mantle xenoliths Friederike Werling, Rainer Altherr* Mineralogisches lnstitut, UniversitiitHeidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany

Received 29 April 1996; accepted 17 July 1996

Abstract The chemical compositions of minerals in mantle xenoliths from Miocene-Holocene volcanic rocks have been used to model the thermal evolution of the lithospheric mantle beneath the French Massif Central. Reequilibration of mineral compositions after entrainment in melts and during ascent to the surface was generally weak as documented by Ca zoning patterns of olivine grains. About half of the 73 xenoliths studied are characterized by significant intra-grain compositional variations reflecting chemical disequilibrium and partial reequilibration under changing thermal conditions prior to their entrainment in melts. Different types of A1 and Ca zoning patterns of pyroxene grains are interpreted in terms of simple heating, simple cooling and two-stage thermal evolutions with early cooling followed by later heating. Diffusion modelling yields durations of several ka to less than a few Ma for these thermal events. Thermobarometric (P-T) data on 40 equilibrated xenoliths with more or less homogeneous mineral compositions plot along and above a 90 mW m -2 model steady-state geotherm with equilibration temperatures ranging from about 700 to 1280"C. Miocene and Pliocene-Holocene xenoliths define roughly similar P - T data arrays. Extrapolation of these arrays to the adiabatic upwelling curve of normal temperature asthenosphere results in an apparent lithospheric thickness of 70-80 km. These findings are in line with surface heat flow data and the results of seismic tomography. Keywords: lithosphere; lithospheric thickness; mantle xenoliths; spinel peridotite; geothermobarometry; French Massif

Central

1. Introduction Modern riffs are characterised by extensionrelated tectonic and magmatic activity, high surface heat flow and thermal uplift (Olsen and Morgan, 1995). Therefore, knowledge of the thermal evolution of the lithosphere is fundamental to the understanding of rifting processes. Relevant information may be obtained from the chemical compositions *Corresponding author. Tel.: +49 6221-54-8207. Fax: +49 6221-54-4805. E-mail: [email protected]

of minerals in mantle and crustal xenoliths hosted in rapidly erupted magmas (e.g., Wendlandt et al., 1995). Reequilibration of mineral compositions in xenoliths after entrainment in melt and during ascent to the surface is commonly weak. Therefore, most xenoliths provide important information on the thermal and dynamic evolution of their lithospheric source regions. Careful application of geothermobarometric techniques on chemically equilibrated xenoliths allows for estimations of equilibration temperatures and pressures. Given a sufficient number of xenoliths derived from various depths, these pres-

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0040- 195 1 (97)00018-8

120

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

sure-temperature (P-T) estimates can be used to reconstruct a geotherm (e.g., O'Reilly, 1989). Compositional zoning patterns in minerals from unequilibrated xenoliths in conjunction with diffusion models may yield important constraints on their P - T history and on changes in the state of their lithospheric source region (e.g., Henjes-Kunst and Altherr, 1992). This paper uses thermobarometric data on mantle xenoliths from the French Massif Central to model the thermal evolution of the lithosphere beneath the southern section of the European Cenozoic rift system (ECRIS). The Massif Central is particularly suited for such a study, since it is the largest area of Tertiary to Holocene volcanic activity in ECRIS and mantle xenoliths are widespread in space and time. Further interest results from the evidence for the ascent of a mantle plume beneath this area (Froidevaux et al., 1974; Coisy and Nicolas, 1978; Granet et al., 1995a,b).

2. Tectonic setting The French Massif Central is one of the largest outcrops of the Hercynian fold belt in Europe. Its crustal and lithospheric evolution is dominated by two geodynamic events: the Hercynian orogeny and the development of ECRIS in the Alpine foreland from Late Eocene to Recent (Ziegler, 1992; Prodehl et al., 1995). Within the Massif Central, the Limagne graben represents the most prominent graben structure (Fig. 1). Main subsidence occurred in the Late Eocene and Early Oligocene. Decreased and interrupted subsidence in the Late Oligocene and Early Miocene was possibly related to the thermal uplift of the Massif Central in the Miocene, 20-30 Ma after the initial stages of rifting (Bits et al., 1989). Refraction-seismic investigations (Zeyen et al., 1997) reveal a large-scale updoming of the Moho of about 2 km, compared to average western Europe. The mean depth to Moho is 28-29 km. Local crustal thinning of up to 20% is restricted to the grabens visible at the surface. In contrast, significant crustal thickening of 2-3 km is observed beneath the Miocene to Holocene volcanic field of Cantal (Fig. 1). Pn velocities generally range from 7.9 to 8.1 km s -1 . Only beneath the volcanic area of C6zallier (Fig. 1) lower velocities of about 7.7 km s -I are found (Zeyen et al., 1997). Seismic tomography

studies suggest that a mantle plume is impinging on the base of a thinned lithosphere beneath the French Massif Central (Granet et al., 1995a,b). Surface heat flow values range from 90 to 110 mW m -2 (Vasseur, 1982; Lucazeau et al., 1984). Tertiary to Holocene volcanic fields are located on the western flank (Petite Cha3ne de la Sioule, Cha3ne des Puys, Mont-Dore, C6zallier, Cantal, Aubrac) and in the southeastern continuation of the Limagne graben (Dev~s, Velay, Vivarais, Coirons) (Fig. 1). Within the Limagne graben proper, volcanic activity is restricted to isolated small volcanic edifices. Most volcanic fields (e.g., Aubrac, Coirons, Velay, Dev~s) show pronounced SSE-striking alignments of eruptive centres, consistent with the strike of the southern graben segments and the diffuse normal fault system in the southeastern continuation of the Limagne graben. The volcanic fields of the Cha3ne des Puys in the northern part and Escandorgue in the southern part of the Massif Central consist of a large number of cones aligned in a N-S direction (Fig. 1). Tertiary to Holocene volcanic activity in the Massif Central has been reviewed by Downes (1987a), Wilson and Downes (1991, 1992) and Wilson et al. (1995). According to K-At data (Briot and Harmon, 1989), minor pre-rift volcanism (65-38 Ma B.P.) occurred at the northeastern border of the Massif Central. About 20 Ma after the onset of rifting in the Late Eocene, a period of continuous basaltic volcanism started within the Limagne graben (Livradois, Forez: 19 Ma B.P.; Fig. 1). In the Late Miocene, 11-12 Ma B.P., volcanism commenced on the western flank of the Limagne graben (Cantal, Aubrac), in the southern area (Causses), and in the southeastern continuation of the Limagne graben (Dev~s, Velay, Coirons). Magmatic activity within the Limagne graben (Forez) lasted until the Late Miocene. The most intensive volcanic activity took place from 5 to 2 Ma B.P. During this period, volcanism continued in all areas except for Aubrac. Holocene volcanism is limited to the Cha3ne des Puys and Vivarais.

3. Sample localities and textures Mantle xenoliths are abundant in Tertiary to Holocene volcanoes of the Massif Central (Hutchison et al., 1975; Coisy, 1977; Brown et al., 1980; Downes, 1987b; Downes and Dupuy, 1987; FabriCs

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

121

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Fig. 1. Localities of mantle xenoliths in Tertiary to Holocene volcanics of the French M a s s i f Central. Solid circles represent spinel peridotites free of amphibole and/or phlogopite. Triangles denote amphibole or phlogopite-bearing spinel peridotites. A = Alcapi~s, Ac = Alleyrac, As = Alleyras, AV = Aurelle de Verlac, B = Burzet, CB = Cr~te de Blandine, CN = Ch~teau-Neuf, E = Eglazines, J = Josant, L = Longueroque, LA = Les Angles, LB = Le Buisson, LCh = Les Chanats, LCr = Le Chier, LCx = Le Cheix, LF = L e Fau, L J = Le Jouq, LJu = Labastide de Juvinas, L P = La Prade, Ma ---- Malnon, MB = M o n t Brianqon, Mb = Montboissier, MBo = Marais de Bor6e, M C = Mont Coupet, ML = Marais de Limagne, MP = M o n t Peylenc, MPx = Marais de Praclaux, N C = Nappe des Cussac, PB = Puy de Beaunit, PBe = Puy de Bessoles, P H = Puy de la Halle, PL = Pont Labeaume, PR = Puy du Roi, P V = Puy de Vergnes, R = Rochelambert, RP = Ray Pic, S = Santerre, Sa = Saucli~res, SJC ---- St. Jean le Centenier, SS = S o m m e t de la Sap~de, VP = La Vestide du Pal, Z = Zani~re.

122

F. Werling, R. Ahherr/Tectonophysics 275 (1997) 119-141

DunitOIe..~

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~/ Webstedte Opl(~Orthopyloxe~ile Clin¢l:yroxer. "/ ~Cpxe Fig. 2. Calculated mineral modes of representative Massif Central mantle xenoliths depicted in the IUGS classification scheme. Solid and open circles represent amphibole-bearing and amphibole-free spinel peridotites, respectively. The shaded field denotes literature data (Brown et al., 1980).

et al., 1987; Cabanes and Mercier, 1988; Wilson and Downes, 1991; Downes et al., 1996). Most abundant are spinel lherzolites and harzburgites. Pyroxenites are subordinate. Amphibole- and phlogopite-bearing xenoliths occur mainly in Holocene volcanic rocks of the Vivarais and in Quaternary rocks of the Dev~s and C6zallier areas. Garnet-bearing lherzolites and pyroxenites are extremely rare and have been reported from only three localities (Fig. 1): La Vestide du Pal in Vivarais (Berger, 1977), Eglazines in Causses (Berger and Brousse, 1976; Berger and Vannier, 1984) and Le Pouget near Montferrier (Babkine et al., 1968). Sample localities of xenoliths used in this study are shown in Fig. 1. The xenoliths are spinel peridotites and amphibole- and/or phlogopite-bearing spinel peridotites. Representative mineral modes calculated from bulk rock chemical compositions and averaged mineral compositions are shown in Fig. 2. In consistency with data from the literature (Brown et al., 1980) our samples range from lherzolites to depleted harzburgites. They cover the entire range of microstructures described for peridotite xenoliths from the French Massif Central. Using the nomenclature of Harte (1977), the xenoliths can be de-

scribed as coarse-grained, weakly porphyroclastic, and granuloblastic. Coarse-grained xenoliths usually have isogranular textures with no or only weak deformation features. A typical feature are medium-grained aggregates of orthopyroxene, spinel and minor clinopyroxene, interpreted as reaction products of garnet and olivine (Reid and Dawson, 1972; Henjes-Kunst and Altherr, 1992) or as having crystallised from magma pockets (Nicolas et al., 1987). Both interpretations assume the former stability of garnet and imply a decompression caused by vertical mass transfer in the lithosphere-asthenosphere transition zone. Characteristic of porphyroclastic microstructures is a bimodal grain size distribution. The significant reduction in grain size is caused by dynamic recrystallisation with subgrain rotation being the dominant process. Olivine and orthopyroxene porphyroclasts display moderate deformation features such as tabular subgrains and bent exsolution lamellae. Granuloblastic xenoliths are usually fine- to medium-grained with a few larger orthopyroxene porphyroclasts. Olivine and pyroxenes form mainly polygonal grains with straight boundaries and 120° triple junctions pointing to a static recrystallisation process. However, relicts of dynamic recrystallisation have also been observed. The different microstructural types in our xenolith collection seem to be randomly distributed in space and time. Deformed and undeformed xenoliths occur at the same volcanoes and individual xenoliths may be characterised by domains with different microstructures. This indicates a rather complex tectonic history with active deformation and subsequent annealing being variable in space and time even at a small scale. Our observations are at odds with those of Coisy (1977) and Coisy and Nicolas (1978) who found that xenolith textures indicating considerable deformation in the mantle are geographically limited to the centre of a core zone within the Massif Central and are temporally restricted to more recent xenolith occurrences.

4. Analytical techniques Minerals were analysed for major elements with (1) a CAMECA SX50 microprobe equipped with four wave-length dispersive spectrometers using an accel-

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

erating voltage of 15 kV and a beam current of 20 nA (University of Karlsruhe) and (2) a CAMECA Camebax microprobe equipped with a KEVEX energy-dispersive detector system using an accelerating voltage of 15 kV and a beam current of 10 nA (University of Mainz). Natural and synthetic oxides were used as standards for calibration. Correction procedures are according to the CAMECA PAP and ZAF correction programs for silicates and spinels, respectively. Consistency of the data sets was checked by interlaboratory comparison of data on reference minerals. In order to test the degree of equilibration between and within mineral grains, a large number of point analyses were performed with special emphasis on core and rim compositions. In samples showing significant intra-grain heterogeneity, element concentration profiles were run for several grains of each mineral phase. For spot analyses a beam diameter of less than 1 /zm was used. The cores of exsolved pyroxenes were analysed using a defocussed beam with a diameter of about 50 #m. Representative analyses of mineral cores and rims for selected samples are given in Table 1. Accurate determinations of Ca in olivine were carried out with the CAMECA SX50 microprobe at Karlsruhe using a special CAMECA analysis program for trace elements. In order to avoid effects of secondary fluorescence, olivine was mechanically separated from the pyroxenes and numerous grains were mounted for microprobe analysis. Ca was then analysed simultaneously with two spectrometers at a counting time of 100 s. As suggested by KShler and Brey (1990), an accelerating voltage of 20 kV and a beam current of 50 nA were applied. International mineral standards (Jarosewich et al., 1980) were used for calibration.

5. Mineral compositions Within individual xenoliths, olivine grains have uniform major element compositions with XMg (=Mg/(Mg+Fe)) ranging from 0.884 to 0.919 (Table 1). Ca contents are relatively constant in the inner parts of the grains (total range: 220--1180 ppm, Table 1), but show a strong outward increase in the outermost rims (up to 1760 ppm; Fig. 8). Orthopyroxenes have XMg between 0.887 and 0.918 (Table 1). The compositional intra-grain vari-

123

ation ranges from complete homogeneity to pronounced zoning in AI and Ca (Figs. 3 and 4). Clinopyroxenes are chromian diopsides with XMg and Xcr (• Cr/(fr -~- AI)) ranging from 0.881 to 0.934 and from 0.027 to 0.324, respectively (Table 1). Na20 (0.29-2.01 wt%) and TiO2 (0.00-0.91 wt%) are highly variable (Table 1). As for orthopyroxenes, the compositional intra-grain variation ranges from complete homogeneity to pronounced zoning in A1 and Ca (Fig. 3). Zoning in orthopyroxenes and clinopyroxenes occurs in approximately half of the xenoliths (36) and is not correlated with textures. The pyroxene zoning patterns are described in detail in Section 6.3. The compositions of spinels are highly variable. Xcr and XMg range from 0.082 to 0.586 and from 0.645 to 0.840, respectively (Table 1), with both parameters being negatively correlated. Furthermore, Xcr values of spinels are positively correlated with XMg of olivines and with Xcr of clinopyroxenes suggesting that all these parameters are controlled by bulk composition. In most xenoliths, chemical zoning in spinel grains is insignificant.

6. Temperatures and pressures 6.1. General considerations and selection of thermobarometers

Xenoliths that were equilibrated under ambient P - T conditions within the lithosphere and escaped

partial reequilibration after entrainment in melt generally bear homogeneous mineral phases and can serve to estimate geothermal gradients. In contrast, non-equilibrated xenoliths display compositionally zoned mineral phases that can be used to constrain the thermal evolution of the lithosphere. As stated in Section 5, homogeneity as well as varying degrees of compositional zoning are observed in the pyroxenes of xenoliths from the Massif Central. The distinction between equilibrated and non-equilibrated xenoliths is thus somewhat arbitrary. It was based on the coincidence of calculated core and rim temperatures. These were estimated using (1) the two-pyroxene thermometers of Bertrand and Mercier (1985) and Brey and K/Shier (1990) in a modified version as suggested by Brey and K6hler, 1990), (2) the Ca-in-orthopyroxene thermometer

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

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F. Werling, R. Altherr/Tectonophysics

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• • '0.03 ~

ca

- """ "

°°°°eo°e°oeeo°°°°~°

0.05 100

~

0.25

~

0.24

,.~ p..°o,~..oo

'4

'. 0.10

0.04 ,oA

~

~ 0.23

0.02

0.22

'

'

'

200

300

400

0.01 500

Distance [gm]

0.21

oOOO" q ~ " 30

Oo

AI ~ % 0 °

OO

0.70

¢° °

OOo~.~

oo

o~

' I O0

' 200

' 300

' 400

' 500

0.60 0 0.50 600

Distance [gm]

Fig. 4. Complex A1 and Ca zoning patterns of orthopyroxene (opx) and clinopyroxene (cpx) from non-equilibrated lherzolite samples 63/1, 40A/I and 62/1 indicating a two-stage thermal evolution. Early cooling is followed by later heating. See Section 6.3 for further explanation.

(Brey and K6hler, 1990), and (3) the Cr-A1 partitioning between orthopyroxene and spinel coexisting with olivine (Witt-Eickschen and Seck, 1991 ). Xenoliths that showed a difference between twopyroxene core and rim temperatures of less than 85°C (in most cases <40°C; Table 2, Fig. 5) were termed 'equilibrated'. For most of these xenoliths the differences between core and rim temperatures calculated with the Ca-in-orthopyroxene and orthopyroxenespinel thermometers were generally less than 13 I°C (in most cases <55°C; Table 2; Fig. 5). Deviations between Ca-in-orthopyroxene and orthopyroxenespinel core and rim temperatures tend to be larger

at the low-temperature end of the data arrays with core temperatures being higher than rim temperatures. This may be a consequence of incomplete re-equilibration after cooling. Ca zoning in olivine grains was not considered for this distinction as diffusion of Ca in olivine is rapid (Hainet al., 1996) and Ca contents of olivines thus easily adjusted during short-lived thermal events (see Section 6.4). Out of a total of 73 xenoliths investigated in more detail, 36 are equilibrated with respect to these criteria. These 'equilibrated' samples together with literature data on 4 additional samples (Berger and Brousse, 1976; Berger, 1977; Berger and

F. Werling, R. AItherr/Tectonophysics 275 (1997) 119-141 1300

1300

1200

1200

1100

1100

E10oo o

~"1000 P, E 900

-

80O

•

.•

i

,

700

•

j

i

,

800

.

,

t

t

i

900 1000 1100 1200 1300 Teore [°C] •

,

•

,

.

i

.

,

,-

1300

1100

1100

~" .o. 1000

~"1000 P. E 900

E

900 ~ "

ill,

,

,

i

,

i

•

,

, -

/ "

800

700

700

700

800

700

•

800

,

•

[

,

900 1000 1100 1200 1300 Tcom [°C]

,

•

,

,

J

,

i

,

i

IOpx-spl(WES9~

•

,

,-,

e~

800

6O0 6O0

•

I=Px(eM")l

600 600

1200

,=

,

800

1200

I-

-

700

7OO

1300

,

jill

E I,~ 900

600 600

•

131

T©om[°C]

900 1000 1100 1200 1300

6O0 600

."

700

800

i

i

i

I

900 1000 1100 1200 1300 Tcom [°C]

Fig. 5. Comparison between core and rim temperatures of 'equilibrated' xenoliths for various thermometers: 2px(BK90) = two-pyroxene thermometer of Brey and K6hler (1990); 2px(BM85) = two-pyroxene thermometer of Bertrand and Mercier (1985), in a modified version as suggested by Brey and K6hler (1990); Ca-in-opx (BK90) = Ca-in-onhopyroxene thermometer of Brey and K6hler (1990); Opx-spl (WES91) = thermometer based on Cr-A1 partitioning between orthopyroxene and spinel as formulated by Witt-Eickschen and Seck (1991). Solid and dashed lines correspond to 0, + 5 0 and - 5 0 ° C differences between core and rim temperatures,

Vannier, 1984) were used for estimating geothermal gradients. Pressures for garnet-free spinel peridotites were constrained by (1) the maximum pressure limit for plagioclase-bearing peridotites (0.7~).8 GPa at 800-1200°C for the NCFMAS system; O'Neill, 1981; Gasparik, 1987), and (2) the minimum pressure limit for the appearance of garnet calculated from spinel compositions according to Webb and Wood (1986). To well equilibrated xenoliths, the Ca-in-olivine barometer of K6hler and Brey (1990) was applied. The exchange of Ca between olivine and clinopyroxene, basically a function of pressure, is very sensitive

with respect to temperature. Mean values of twopyroxene rim temperatures (Brey and K6hler, 1990) in individual samples were used as input-temperatures to the Ca-in-olivine barometer equation as recommended by Brey and K6hler (1990). Considering the high diffusion velocity of Ca in olivine compared to that in pyroxenes, the reliability of this barometer is strongly dependent on the state of equilibrium in each particular sample. Disequilibrium effects may be responsible for unrealistically low pressure estimates obtained for some samples (e.g., 0.26 ± 0.34 GPa for 71/2; Table 2). For two garnet-spinel lherzolites (literature data), P - T estimates were obtained by combining the

F. Werling, R. Altherr / Tectonophysics 275 (1997) 119-141

132

II II

d q

d~ ~q

d

5 q

od ~q

o ~ o q q q q

o q

d q

d

©

. . . . vvv

~

. . . .

v v v v v v v v

M

~

.

.

V V V V V

vv

.

.

.

V~

.

.

.

.

.

.

.

V V V V V V V V

.

.

.

.

.

.

V V V V V V V

IIm~

-

q-Hqqq-Hq-H+l+l+lqqq

~ .

-

~

_

~

.

~ C ' - I ~ t

~

~= ~

~

~

,

~

,

~

~

,

~

r

~

~r~

~

0

"

,

o~

a a a q q q q q a a a a q

~~

_

_

~ -

~

o

_

_

_

~ q a q q q q ~ a a q q

g "'0

Q

'~ ~ ~ 8 A

q q q ~ a ~ a q ~ q q ~ q

~.

"-6

O

_

~

~

~

_

- ~ - ~

~

~

~

•~

o

II . -

O

V

O

-- ~

r~

ii ~

~

~o ~ O

.

o

~

O "0

=

~

~

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~

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q

a

~

q

a

q

q

q

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e oN'."

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-H-H-H-flq-t-Hq4-H-H+I+I-H-H ~ -

©

~ ~

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q

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,~,~,~,~,~,~,~,~,~

~~~~~~~~,~l

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._

F. Werling,R. Altherr/Tectonophysics 275 (1997) 119-141

Al-in-orthopyroxene barometer with the two-pyroxene thermometer of Brey and Krhler (1990). 6.2. P - T estimates on 'equilibrated' xenoliths and geothermal gradients

Temperature and pressure estimates for 'equilibrated' xenoliths are given as mean values with standard deviations (lcy) in Table 2. Equilibration temperatures (assumed pressure of 1.5 GPa) of 37 spinel peridotites from the Massif Central range from about 700 to 1200°C. Temperature differences between the four thermometers applied are <21 I°C for pyroxene cores and <100°C for rims (Fig. 6). For most of the samples, these differences are less than 50°C. Temperature estimates derived from the two-pyroxene thermometers (Brey and Krhler, 1990; Bertrand and Mercier, 1985, modified version according to Brey and Krhler, 1990) show a non-linear relationship. Ca-in-opx temperatures (Brey and Krhler, 1990) and opx-spl temperatures (Witt-Eickschen and Seck, 1991) tend to be higher than two-pyroxene temperatures (Brey and Krhler, 1990) in the lowtemperature region but lower or coincident in the high-temperature region of the data arrays. P - T estimates for 'equilibrated' spinel peridotites from Miocene volcanic fields are illustrated in Fig. 7A. Rim temperatures obtained from the twopyroxene thermometer of Brey and K0hler (1990) are used throughout as these are the input values for Ca-in-olivine barometry (Brey and Krhler, 1990). The observed P - T array suggests that the lithospheric mantle has been continuously sampled from a depth of about 70 km. The occurrence of hightemperature spinel-beating lherzolites (> 1100°C) is indicative for a high geothermal gradient. This is confirmed by pressure estimates on four samples derived from the Ca-in-olivine barometer of Krhler and Brey (1990) and by the P - T estimate on one garnet-spinel lherzolite from Causses. Extrapolation of the P - T data array to the 'adiabatic upwelling curve of normal temperature asthenosphere' (AAC; McKenzie and Bickle, 1989) results in an apparent lithospheric thickness of <80 km in the Miocene. Xenoliths from the Pliocene to Holocene volcanic fields yield P - T conditions that are broadly similar to those of xenoliths from Miocene volcanic rocks

133

(Fig. 7B). Taking into account a depth to Moho of 28-29 km as determined by refraction seismic studies corresponding to a pressure of about 0.8 GPa at the Moho, the pressure estimate obtained for xenolith 26A/1 from Devrs is unrealistically low (0.68 ± 0.10 GPa; Table 2). Extrapolation of the P - T data array to the AAC suggests an apparent lithospheric thickness of <80 km (or <60 km, if the P - T estimates obtained from literature data for one garnet lherzolite from Causses is neglected). 6.3. Thermal evolution of non-equilibrated xenoliths

Non-equilibrated xenoliths show significant intragrain compositional variations. Zoning patterns of large orthopyroxene and clinopyroxene grains are compatible with simple cooling, simple heating and two-stage thermal evolutions (i.e. early cooling overprinted by later heating). Simple cooling is mainly recorded by A1 zoning patterns with A1 decreasing from core to rim of both pyroxenes. Relatively fiat compositional patterns in some orthopyroxene cores at comparatively high AI and Ca levels (e.g., opx 81; Fig. 3) may reflect partial preservation of an earlier high-temperature state. Ca zoning patterns are often less pronounced than those of opx 81 (e.g., opx 1/1; Fig. 3), a feature that is most likely due to the higher diffusion velocity of Ca as compared to that of A1 (Sautter et al., 1988; Witt-Eickschen and Seck, 1991). This difference also explains why two-pyroxene temperatures are generally lower than opx-spl temperatures for pyroxene rim compositions (Table 3). Zoning patterns characteristic of simple heating are reflected by increasing AI abundances from core to rim in both pyroxenes (opx-cpx 27/2, opx-cpx 50A/l; Fig. 3). Concomitantly, Ca increases in orthopyroxene but decreases in clinopyroxene. The zoning patterns of both pyroxenes are bell-shaped. Since initial core compositions are not preserved, the original thermal conditions remain unknown. Due to the higher diffusivity of Ca relative to that of A1, two-pyroxene temperatures are higher than opx-spl temperatures (Table 3). Most of the non-equilibrated xenotiths are characterized by complex A1 and Ca zoning patterns in pyroxenes (Fig. 4) indicating a two-stage thermal evolution whereby cooling preceded heating. In

Werling, R. AItherr/Tectonophysics 275 (1997) 119-141

134

-H-H L

I

I

I

I

I

bl

1 1 1 1 1 1

VVVVVV

VV

V

+t-H-H +t-H-H

~,~,.

V

V

V

V

V

V

V

V

V

I

I

I

V

V

V

II ~z "~ .c

°°

'i

U..,

-H -H -H -H ~ -H 0 ~q

-~

¢,e3

m

0

H

n

~

--

re',

t "~

44 e-

=o

I

'43

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--

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~

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44+t

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rr;

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2

aaa ¢~ "~

~ . ~

,8~

~

E Werling, R. Ahherr/Tectonophysics 275 (1997) 119-141

135

1300 1200

oA

•

oo,,~

granuloblastic. •

po,p,,y~,,~c 1100

.",~'.-"

I I

,"/d"

I

""/.Y

o=_,

x IOOO[

1000 900

~" 8oo[ ,,~,,"

8OO 700 I

60O 60O

700

800

I

i

.

l

900 1000 1100 1200 1300

600

700

T2px (BKg0) [°C] 1300

•

,

•

i

.

i

•

,

.

800

.

.

.

.

.

.

.

.

.

900 1000 1100 1200 1300

T2px (BKS0) [ °c] ,

.

,

1300

.-.

.[~"

•

,

. . . . . . .

,

•

, .-.

,

1200 11001200 0" 1100 .o.

F, 1 o o o ..'"

.=..'" .~

ty'lD 900

900 ."

I"° 800

tx~

o

700

600

"'P'" """

o

~1000

600

D

800 700

•

I

700

,

i

800

,

i

l

I

,

I

,

I

,

900 1000 1100 1200 1300

T2px (BK90) [ °c]

600 "" . . . . . . . . . . ' 600 700 800 900 1000 1100 1200 1300

Tca.ln.opx IBKg01 [°C]

Fig. 6. Comparison of temperature estimates for 'equilibrated' xenoliths derived from various thermometers. 2px(BK90) = two-pyroxene thermometer of Brey and Krhler (1990); 2px(BM85) = two-pyroxene thermometer of Bertrand and Mercier (1985) in a modified version as suggested by Brey and Krhler (1990); Ca-in-opx (BK90) = Ca-in-orthopyroxene thermometer of Brey and Krhler (1990); Opx-spl (WES91) ---- thermometer based on Cr-AI partitioning between orthopyroxene and spinel as formulated by Witt-Eickschen and Seck (1991). Solid and dashed lines corresponding to 0, + 5 0 and - 5 0 ° C difference between core and rim temperatures are given for better orientation•

xenolith 63/1, the well defined A1 and Ca zoning patterns in cores and inner rims of both pyroxenes reflect cooling. The outermost rims of orthopyroxene grains, however, are characterized by increasing Ca abundances and some of the clinopyroxene grains also show slightly decreasing Ca abundances in their outermost rims (right-hand rim of cpx 63/1 in Fig. 4). As this thermal overprint is only seen in the outermost rims of pyroxene grains, this xenolith was probably heated just before entrainment in the host magma. In xenolith 40A/1 the inner cores of the pyroxene grains are homogeneous with constant AI and Ca

(Fig. 4). Towards the rims, AI decreases, while Ca stays constant. In the outermost rims A1 increases in both pyroxenes; Ca increases in orthopyroxene but decreases in clinopyroxene. These patterns suggest rehomogenization of Ca in the cores and inner rims during cooling, due to the higher diffusivity of Ca relative to A1, followed by heating. Indications of an older cooling event are erased in the zoning pattern of orthopyroxene 62/1 (Fig. 4). The increase of A1 and Ca from core to rim is compatible with a single heating event. The only evidence for an earlier cooling of this xenolith has been found in the complex A1 zoning pattern of

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

136

T e m p e r a t u r e [°C]

600 ,

500 0

700 ,

800 ,

900 1000 1100 1200 1300 140o , , , , , 0

~ 1.0

40

20r

00.

3.0 ~" [ • Ca-in-oVcpx ] El spl-0rt(Pmax)

i

4.0

100

,i

i

i

600 '

700 '

800 '

i

i

/ t 120

I

T e m p e r a t u r e (°C)

500 0 0.5 m _1.o

900 1000 1100 1200 1300 1400 ' ' ' ' I" 0

Pmaxfor pig Iherzolffe(NCFMAS)

~ [~!

"'~o~._~'~'q / / ~ f i ' ~ . ~ _

1

~90mWm'~

~1.5

2.0 == 2.5 ID. 3.0 3.5 4.0

20 40 0

60 " ~ I~, SE area W area / Ca-in-oVcpx • ~ | !~

Pres. . . . . . tirnates,....

spkgrt (Pmax)

0

AI-in-opx/grt

~1~

,

t

~ ,

~

~L[~it

80 .~.~" 100

/ ] PIIocene to I / I Recent xenollths I ,

,

120

,

Fig. 7. P - T estimates for 'equilibrated' xenoliths from Miocene (A) and Pliocene-Holocene (B) volcanic rocks. Temperatures were obtained by applying the two-pyroxene thermometer of Brey and K6hler (1990) to average rim compositions of pyroxene grains. For most spinel peridotite xenoliths minimum pressures (= P,nax for pig Iherzolite in the NCFMAS system: Gasparik. 1987; heavy line) and maximum pressures (spl-grt Pmu~: Webb and Wood, 1986) are given. For some xenoliths, equilibration pressures were estimated combining the Ca-in-olivine barometer (K6hler and Brey, 1990) with the two-pyroxene thermometer (Brey and K6hler, 1990). P - T data for two spinel peridotites and one garnet-spinel lherzolite from Eglazines, Causses (A) and one garnet-spinel lherzolite from La Vestide du Pal, Vivarais (B) were obtained by applying the two-pyroxene thermometer and the Al-in-opx barometer of Brey and K6hler (1990) to mineral analyses reported by Berger and Brousse (1976), Berger and Vannier (1984) and Berger (1977). These data are marked with Lit. Data point ['or sample 26A/1 in (B) is marked, since this sample is mentioned in the text (at the end of Section 6.2). AAC is adiabatic upwelling curve for normal temperature asthenosphere (McKenzie and Bickle, 1989). Steady-state conductive geotherms (Chapman, 1986) are given for comparison only.

clinopyroxene. The abrupt increase of Al within about 50 /~m from the edge is characteristic of a younger heating event which is also suggested in the slight decrease of Ca towards the rim. These

observations suggest AI diffusivity to be slightly higher in orthopyroxene than in clinopyroxene. The varying degrees of reequilibration during the later heating event recorded by the xenoliths can be explained by variable time intervals between thermal overprint and entrainment in rising magmas. Pyroxene zonation types are virtually not related to the spatial distribution of eruptive centres, to the eruption ages of xenoliths or to pyroxene core temperatures. This suggests that the observed thermal phenomena may be due to local rather than regional processes. Furthermore, in clinopyroxene grains with complex zoning, features induced by heating are restricted to the outermost rims (Fig. 4) suggesting a relatively short duration of the heating event. It is thus plausible that local heating was caused by intruding magmas followed by cooling. This hypothesis can be tested by model calculations on the observed A1 zoning patterns in clinopyroxenes. The durations of heating and cooling events can be roughly estimated provided the diffusion coefficient of A1 in clinopyroxene (DAIcpx) is known. Using the 27Al(p,~g)28Si nuclear reaction Sautter et el. (1988) obtained a value of 0.37 x l0 -I~ cm 2 s t at 1180°C for DAIcpx. By modelling AI diffusion profiles in clinopyroxenes of an eclogite xenolith Sautter and Harte (1990) estimated values ranging from 1.1 x 10 -]6 cm 2 s -1 to 5 x 10 2o cm 2 s ] at temperatures of about 1200°C. While their maximum value is well consistent with the experimentally determined value of Sautter et al. (1988), their minimum value is thought to considerably underestimate A1 diffusivity in clinopyroxene, as mentioned by the authors themselves. For our calculations based on ( 1) radial diffusion in a sphere and (2) linear diffusion in a semi-infinite medium (Crank, 1975) we used both values as extremes. Xenolith 27/2 from the Pliocene-Quaternary volcanic field of Dev~s contains clinopyroxenes with purely heating-related zoning patterns (Fig. 3). Depending on the diffusion model and coefficient used heating durations between 1,92 ka and 12.8 Ma are obtained (Table 4). Clinopyroxene 50All (Miocene eruption age; Causses) characterized by an exceptionally broad diffusion zoning (Fig. 3) yields durations between about 35 ka and 77 Ma. Taking into account that the apparently broad zoning of this clinopyroxene may well be due to geometric

F. Werling, R, Altherr/Tectonophysics 275 (1997) 119-141

137

Table 4 Estimated diffusion times for Ca zoning patterns in olivine and A1 zoning patterns in clinopyroxene based on models for radial diffusion in a sphere and linear diffusion in a semi-infinite medium (Crank, 1975) Sample

a

Dt/a 2

(~m)

xt/2 (/~m)

t for radial diffusion with DAI cox =

t for linear diffusion with DAI cpx =

1.1 x l O - 1 6 c m 2 s

5.0x lO-20cm2s-I

1.1 x 10 -16 cm 2 s - I

5.0 x 10 -20 cm 2 s - I

1.17 Ma 0.80 Ma 2.47 M a -

1.15 0.65 1.80 1.40

2.54 1.43 3.96 3.07

4.22 Ma -

5.83 ka 34.80 ka

I

All zoning patterns in clinopyroxene (simple cooling) 1/1 (g) 21/2 (g) 80/2 (g) 81 (c)

175 205 255 -

0.006 0.003 0.006 -

20 15 25 22

0.53 ka 0.36 ka 1.12 ka -

ka ka ka ka

Ma Ma Ma Ma

All zoning pattern in clinopyroxene (simple heating) 27/2 (c) 50A/1 (g)

365 -

0.005 -

45 110

1.92 ka -

12.8 M a 76.7 M a

Ca zoning patterns in olivine (heating) Sample

a

Dt/a 2

(#m) 32/1 49/1 52/2 68/1 71/2

(c) (c) (c) (c) (c)

1215 1150 530 1570 882

xl/2 (#m)

<0.0005 0.0005 0.0075 <0.0005 0.0010

20 10 50 15 20

t for radial diffusion with Dca °1 =

l for linear diffusion with Dca °l =

1.7 x 10 -12 cm 2 s I

7.2 x 10 -14 cm 2 s - I

1.7 × 10 -~2 cm -~ s I

7.2 x 10 -14 cm 2 s - t

<50 45 143 <84 53

<3.3 2.9 9.3 <5.4 3.4

27 7 170 15 27

1.8 0.4 I 1.0 1.0 1.8

days days days days days

a a a a a

days days days days days

a a a a a

Letters in parentheses after sample numbers refer to textural type: (c) = coarse-grained, (g) = granuloblastic, a = radius of sphere; r = distance from centre of sphere; D = diffusion coefficient; t = duration of diffusion event; xl/2 = half width of diffusion profile. For radial diffusion in a sphere values for Dt/a 2 were obtained by comparing normalized concentrations (e - c~)/(c0 - cl ) as a function of r/a with model curves for various values of Dt/a 2 (Crank, 1975), It was assumed that the initial concentration ct equals the measured concentration in the cores of the mineral grains and that the constant surface concentration co equals the measured concentration at the outermost rims. Zoning patterns in some clinopyroxene grains (50A/l, 81) do not show constant core compositions and thus do not allow using the model for spherical diffusion. Linear diffusion was modelled by using the relationship x v 2 = (Dt) ~/2 (Crank, 1975).

effects (section not including the grain centre) and that a diffusion coefficient of about 1 0 - 1 6 c m 2 s - I is much more likely than a value of 5 × 10-2o cm 2 s -l , we conclude that heating in the non-equilibrated xenoliths was indeed caused by local thermal pulses related to the intrusion and stagnation of magmas during Neogene igneous activity. Clinopyroxenes of some xenoliths show simple cooling-related A1 zoning patterns with constant core compositions (e.g., 21/2 and 80/2; Fig. 3) that yield diffusion periods between about 0.36 ka and 4 Ma (Table 4). The short-lived cooling processes documented by the AI zoning patterns of these clinopyroxenes are therefore not related to thermal relaxation of the lithosphere after the Hercynean orogeny. Most likely, cooling of these peridotites occurred after their source regions were heated and metamorphosed by intruding magmas during the evolution of ECRIS.

6.4. Interpretation of Ca zoning patterns in olivine Ca zoning patterns of olivine grains (Fig. 8) show the effect of heating during transport by the host magma. For both, 'equilibrated' and non-equilibrated xenoliths (see Section 6.1), Ca abundances in olivine are approximately constant in the inner parts of the grains, but show a strong outward increase in the outermost rims, The width of the zones with elevated Ca abundances varies from 30 to 150 # m for different grains and xenoliths. Considering the high diffusivity of Ca in olivine (Hain et al., 1996), these zoning patterns probably reflect rapid partial reequilibration of Ca due to heating and/or decompression. Such conditions are thought to be realised in an ascending host magma. When applied to the outermost rim compositions of olivine grains, the Ca-in-olivine thermome-

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141

138 2000-

r-149/1 ( C a u s s e s ) I I, ' 1 T = 1288°C (1.5 GPa) 1600T = 1180°C (0.1 GPa) 1400 P (core) = 1.85 GPa (1183°C) O.O.1200. I T (core) = 1154°C (1.5 GPa) ~ 1000. 1600 1

800.

....

6OO

i ....

t ....

500

0 1200 -

i

1000

,

,

-

1500

1000

•

i

.

.

2000

T = 1140°C (1.5 GPa) T = 1043°C (0.1 GPa)

600

A

P (core) = 0.69 GPa (960°C)

,= 600

'~anaaAz~aaA~xAtu~xt~zx~AA'~a~l~

0

400 200 0

400

800

1200

1600

2000

2400

',7oL

l n n n ~ ~'~,, T=1184°C (1.5 GPa) _,vj~ T=1084oC (0.1 GPa) 900~

0

8 600-J 500 1 0

0

0

7

P (core) =1.02 GPa (1066°C ', .

0

.

.

0 .

.

.

~

.

.

~ P = 0.76 GPa (1066°(3) i i i i i i i i i 200 400 600 800 1000 1200 1400 1600 1800

Distance [jam] Fig. 8. Representative Ca zoning patterns of olivine grains. Samples 4/2 and 49/1 are classified as 'equilibrated' (compare P-T data in Table 2), sample 32/1 is non-equilibrated (Table 3). Pressure estimates for olivine cores are based on the Ca-in-olivine barometer of KShier and Brey (1990). Temperatures for olivine rims are calculated using the Ca-in-olivine thermometer according to K6hler and Brey (1990).

GPa for xenolith 32/1, Fig. 8). This may be due to complete readjustment of Ca abundances in these olivine grains resulting from heating before xenolith entrainment in melt. The observed Ca diffusion profiles in olivine (Fig. 8) allow for an estimation of diffusion times after entrainment in host magma if values for the tracer diffusion coefficient of Ca in olivine (Dca °l) are known. Hain et al. (1996) carried out 42Ca tracer diffusion experiments in San Carlos olivine (Fogo) along three crystallographic axes at 1000-1500°C/1 atm/fo~ = 10- ' ° . Using their Arrhenius equations yields values between about 7.2 x 10-j4 cm 2 s -1 and 3.6 x 10 -]3 cm 2 s -~ for temperatures between 1200 and 1250°C. Jurewicz and Watson (1988) obtained a value of 1.698 × 10-12 cm 2 s i at 1220°C/1 atm/fo2 = 10-s. Thus, values of 1.7 x 10-j2 cm: s -l and 7.2 x 10 -{4 cm 2 s -I can be considered as maximum and minimum values for Dc~ °j at near-liquidus temperatures of basaltic melts. The duration of the observed Ca diffusion into olivine grains was calculated using two different models (Crank, 1975): (1) radial diffusion in a sphere and (2) linear diffusion in a semi-infinite medium. Resulting time spans range from 7 days to 11 years (Table 4) corroborating the assumption that the Ca diffusion into olivine grains was indeed caused by heating and decompression after entrainment in the host magma. 7. Conclusions

ter (K6hler and Brey, 1990) yields temperatures of 1140-1290°C at an assumed pressure of 1.5 GPa (Fig. 8). For each xenolith these temperature values are significantly higher than two-pyroxene or spl-opx-ol rim temperatures. Ca-in-olivine rim temperatures calculated for 0.1 GPa range from 1043 to 1180°C (Fig. 8) and are thus equal to or lower than assumed liquidus temperatures of alkali-basaltic and basanitic melts. When applied to Ca abundances of olivine cores, the combination of the Ca-in-olivine barometer (K6hler and Brey, 1990) with the two-pyroxene thermometer of Brey and K6hler (1990) yields acceptable pressures for the 'equilibrated' xenoliths (Table 2 and Fig. 7) but results in unrealistically low pressures for unequilibrated xenoliths (e.g., 0.69

In the French Massif Central, rift-related basanitic to alkali-basaltic magmas have been erupting since the Early Miocene. Xenoliths entrained in these magmas provide important information on the thermal evolution of their lithospheric source regions. Most of these xenoliths are characterized by compositionally heterogeneous mineral grains indicating chemical disequilibrium. Apart from partial reequilibration en-route to the surface (Ca zoning patterns of olivine grains) caused by heating in the host magmas and by decompression (lasting several days to months), most of the intra-grain compositional variations reflect thermal processes before xenolith entrainment in melt. A1 and Ca zoning patterns of coexisting pyroxenes in xenoliths can be subdivided into three main types indicating different thermal evolutions of

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-14l

their host rocks: simple cooling, simple heating and a two-stage history with early cooling followed by later heating. Diffusion times necessary to produce the observed A1 zoning patterns range from several hundred years to several Ma. Since the occurrence of the various pyroxene zoning types is unrelated to the spatial and temporal distribution of eruptive centres and to pyroxene core temperatures, they are interpreted to reflect local rather than regional thermal processes. Most probably, the source regions of the xenoliths were (repeatedly) heated and metamorphosed by intruding magmas. About half of the peridotitic xenoliths are characterized by more or less homogeneous mineral compositions and m a y thus be regarded as 'equilibrated' under the prevailing pressure-temperature conditions. Application o f geothermobarometric techniques on these xenoliths yields equilibration temperatures ranging from about 700 to 1280°C and constrains xenolith extraction to a depth range from the M o h o at 2 8 - 2 9 k m to about 70 km. Two P - T arrays defined by Miocene and by Pliocene to Holocene xenoliths intersect the adiabatic upwelling curve of normal temperature asthenosphere (AAC; McKenzie and Bickle, 1989) at depths of less than 80 km. Apart from local thermal perturbations, there is no evidence for a significant change of the geothermal gradient in the last 20 Ma. The results obtained from geothermobaromety on mantle xenoliths are in agreement with surface heat flow data (Vasseur, 1982; Lucazeau et al., 1984) and models derived from teleseismic tomography (Granet et al., 1995a,b; Sobolev et al., 1996, 1997).

Acknowledgements This study was financially supported by the Deutsche Forschungsgemeinschaft (grant to R.A.) within the frame o f the Collaborative Research Center 108 (SFB 108) at the University o f Karlsruhe. Technical assistance by H. Frohna-Binder, H.-E Meyer, B. Schulz-Dobrick, E Ullmer, K. Wacker ( t ) and E. Weiher-Schr6der is gratefully acknowledged. We thank U. Achauer, K. Fuchs, V. Garasic, O. Novak, C. Prodehl, J.R.R. Ritter, S.V. Sobolev, G. Stoll, E. Werling and H. Zeyen for fruitful discussions. Critical reviews by A. Kalt, J. Fabribs, G. Franz and G. Brey as well as incisive criticism by S.Y. O ' R e i l l y

139

helped to substantially improve the manuscript. This is SFB 108 Publication No. 581 and Mineralogical Institute of Heidelberg publication No. 18.

References Babkine, J., Conqurrr, E and Vilminot, J.C., 1968. Drcouverte d'une enclave d'ari~gite h grenat dans l'absarokite du Pouget (Hrrault). C. R. Acad. Sci. Paris, 267, SEt. D: 1266-1269. Berger, E., 1977. Sur la presence d'une lherzolite h grenats en enclave dans le basalte alcalin de la Vestide du Pal (Ard~he): conditions d'rquilibre, implications prtrogEnrtiques et grotectoniques. C. R. Acad. Sci. Paris, 284, Srr. D: 709-712. Berger, E. and Brousse, R., 1976. Une lherzolite h grenats du pipe d'Eglazines (Loz~re, France) stabilisre a 1400°C et 30 kbar. C. R. Acad. Sci. Paris, 282, Srr. D: 1477-1480. Berger, E. and Vannier, M., 1984. Petrology of megacrysts, mafic and ultramafic xenoliths from the pipe of l~glazines, Causses, France. In: J. Kornprobst (Editor), Kimberlite Conference, Vol. 1. Elsevier, Amsterdam, pp. 155-168. Bertrand, P. and Mercier, J.-C.C., 1985. The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system? Earth Planet. Sci. Lett., 76: 109-122. Bl~s, J.L., Bonijoly, D., Castaing, C. and Gros, Y., 1989. Successive post-Variscan stress fields in the European plate (French Massif Central and its borders): comparison with geodynamic data. Tectonophysics, 169:79-111. Brey, G.P. and K6hler, T.P., 1990. Geothermobarometry in four phase lherzolites, II. New thermometers, and practical assessment of existing thermometers. J. Petrol., 31: 1353-1378. Briot, D. and Harmon, R.S., 1989. Relationships between isotopes and geological settings in volcanics from the Massif Central. Terra Abstr., 1: 343. Brown, G.M., Pinsent, R.H. and Coisy, E, 1980. The petrology of spinel-peridotite xenoliths from the Massif Central, France. Am. J. Sci., 280A: 471-498. Cabanes, N. and Mercier, J.-C.C., 1988. Chimie des phases min6rales et conditions d'6quilibre des enclaves de lherzolite spinelle de Montferrier (H6rault, France). Bull. Min6ral., 111 : 65-77. Chapman, D.S., 1986. Thermal gradients in the continental crust. In: J.B. Dawson, D.A. Carswell, J. Hall and K.H. Wedepohl (Editors), The Nature of the Lower Continental Crust. Geol. Soc. London, Spec. Pub1., 24: 63-70. Coisy, P., 1977. Structure et chimisme des peridotites en enclaves dans les basaltes du Massif Central, modules grodynamiques du manteau suprrieur. Thesis, Nantes. Coisy, P. and Nicolas, A., 1978. Regional structure and geodynamics of the upper mantle beneath the Massif Central. Nature, 274: 429-432. Crank, J., 1975. The Mathematics of Diffusion. Oxford University Press, London. Downes, H., 1987a. Tertiary and Quaternary volcanism in the Massif Central, France. In: J.G. Fitton, J.G. and B.G.J. Upton (Editors), Alkaline Igneous Rocks. Geol. Soc., London, Spec. Publ., 30: 517-530.

140

E Werling, R. Altherr/Tectonophysics 275 (1997) l lg-141

Downes, H., 1987b. Relationship between geochemistry and textural type in spinel lherzolites, Massif Central and Languedoc, France. In: P.H. Nixon (Editor), Mantle Xenoliths. J. Wiley, New York, pp. 563-574. Downes, H, and Dupuy, C., 1987. Textural, isotopic and REE variations in spinel peridotite xenoliths. Massif Central, France. Earth Planet. Sci. Lett., 82:121 +135. Downes, H., Vaselli, O. and Zangana, N., 1996. Contrasting styles of interaction between asthenospheric melts and shallow mantle lithosphere - - is there a geodynamic control'? J. Conf. Abstr., 1(1): 141. Fabribs, J., Figferoa, O. and Lorand, J.-P., 1987. Petrology and thermal history of highly deformed mantle xenoliths from the Montferrier basanites, Languedoc, Southern France: a comparison with ultramafic complexes from the North Pyrenean Zone. J. Petrol., 28: 887-919. Froidevaux, C., Brousse, R. and Bellon, H., 1974. Hot spot in France? Nature, 248: 749-751. Gasparik, T., 1987. Orthopyroxene thermobarometry in simple and complex systems. Contrib. Mineral. Petrol., 96: 357-370. Granet, M., Stoll, G., Dorel, J., Achauer, U., Poupinet, G. and Fuchs, K+, 1995a. The Massif Central (France). New constraints on the geodynamical evolution from teleseismic tomography. Geophys. J. Int., 121: 33-48. Granet, M., Wilson, M. and Achauer, U., 1995b. Imaging a mantle plume beneath the Massif Central (France). Earth Planet. Sci. Lett., 136: 281-296. Hain, A., Stahl, S.+ Specht, W., Laqua, W. and Palme H., 1996. Tracer diffusion of 42Ca in olivine. Terra Nova, Abstr. Suppl. 1, 8: 25-26. Harte, B., 1977. Rock nomenclature with particular relation to deformation and recrystallisation textures in olivine-bearing xenoliths. J. Geol., 85: 279-288. Henjes-Kunst, E and Altherr, R.. 1992. Metamorphic petrology of xenoliths from Kenya and northern Tanzania and implications for geotherms and lithospheric structures. J. Petrol., 33: 1125-1156. Hutchison, R., Chambers, A.L.+ Paul, D.K. and Harris, EG., 1975. Chemical variation among French ultramafic xenoliths - - evidence for a heterogenous upper mantle. Mineral. Mag., 40: 153-170. Jarosewich, E., Nelen, J.A. and Norberg, J.A., 1980. Reference samples for electron microprobe analysis. Geostandards Newsl., 4: 43-47. Jurewicz, A.J.G. and Watson, E.B., 1988. Cations in olivine, Part 2. Diffusion in olivine xenocrysts, with applications to petrology and mineral physics. Contrib. Mineral. Petrol.. 99: 186-201. K6hler, T.R and Brey, G.E, 1990. Calcium exchange between olivine and clinopyroxene calibrated as a geothermometer for natural peridotites from 2 to 60 kb with applications. Geochim. Cosmochim. Acta, 54: 2375-2388. Lucazeau, F., Vasseur, G. and Bayer, R., 1984. Interpretation of heat flow data in the French Massif Central. Tectonophysics, 103:99-119. McKenzie, D. and Bickle, M.J., 1989. The volume and corn-

position of melt generated by extension of the lithosphere. J. Petrol., 29: 625-679. Nicolas, A., Lucazeau, F. and Bayer, R., 1987. Peridotite xenoliths in Massif Central basalts, France: textural and geophysical evidence for asthenospheric diapirism. In: EH. Nixon (Editor), Mantle Xenoliths, J. Wiley, New York, pp. 563-574. Olsen, K.H. and Morgan, E, 1995. Introduction: progress in understanding continental rifts. In: K.H. Olsen (Editor), Continental Rifts: Evolution, Structure, Tectonics. Developments in Geotectonics, 25. Elsevier, Amsterdam, pp. 3-26. O'Neill, H.St.C., 1981. The transition between spinel lherzolite and garnet lherzolite, and its use as a geobarometer. Contrib. Mineral. Petrol., 77:185-194. O'Reilly, S.Y., 1989. Nature of the East Australian Lithosphere. In: R.W. Johnson (Editor), Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press, pp. 290-297. Prodehl, C.. Mueller, St. and Haak+ V., 1995. The European Cenozoic rift system. In: K.H. Olsen (Editor), Continental Rifts: Evolution, Structure, Tectonics. Developments in Geotectonics, 25. Elsevier, Amsterdam, pp. 133-212. Reid, A.M. and Dawson, J.B., 1972. Olivine-garnet reaction in peridotites from Tanzania. Lithos, 5:115-124. Sautter, V. and Harte, B., 1990. Diffusion gradients in an eclogite xenolith from the Roberts Victor kimberlite pipe: (2) kinetics and implications for petrogenesis. Contrib. Mineral. Petrol., 105: 637-649. Sautter, V., Jaoul, O. and Abel, F., 1988. Aluminum diffusion in diopside using the 27Al(p,F)2SSi nuclear reaction: preliminary results. Earth. Planet. Sci. Lett., 23:109-114. Sobolev, S.V., Zeyen, H., Stoll, G., Werling, E, Altherr, R. and Fuchs, K., 1996. Upper mantle temperatures from teleseismic tomography of French Massif Central including effects of composition, mineral reactions, anharmonicity, anelasticity and partial melt. Earth Planet. Sci. Lett., 139: 147-163. Sobolev, S.V., Zeyen, H., Granet, M., Stoll, G., Achauer, U., Bauer, C., Werling, F., Altherr, R. and Fuchs, K., 1997. Upper mantle temperatures and lithosphere-asthenosphere system beneath the French Massif Central constrained by seismic, gravity, petrologic and thermal observations. In: K. Fuchs, R. Altherr, B. Mtiller and C. Prodehl (Editors), Stress and Stress Release in the Lithosphere - - Structure and Dynamic Processes in the Rifts of Western Europe. Tectonophysics, 275: 143-164 (this volume). Vasseur, G., 1982. Synth6se des r6sultats du flux geothermique en France. Ann. Geophys., 38: 189-201. Webb, S.A.C. and Wood, B.J., 1986. Spinel-pyroxene-garnet relationships and their dependence on Cr/AI ratio. Contrib. Mineral. Petrol., 92: 471480. Wendlandt, R.F.. Altherr, R., Neumann, E.-R. and Baldridge, W.S., 1995. Petrology, geochemistry, isotopes. In: K.H. Olsen (Editor), Continental Rifts: Evolution, Structure, Tectonics. Developments in Geotectonics, 25, Elsevier, Amsterdam, pp. 47-60. Wilson, M. and Downes, H,, 1991. Tertiary-Quaternary extension-related alkaline magmatism in Western and Central Europe. J. Petrol., 32:811-849.

E Werling, R. Altherr/Tectonophysics 275 (1997) 119-141 Wilson, M. and Downes, H., 1992. Mafic alkaline magmatism associated with the European Cenozoic rift system. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume I. Case History Studies on Rifts: Europe and Asia. Tectonophysics, 208: 173-182. Wilson, M., Downes, H. and Cebria, J.-M., 1995. Contrasting fractionation trends in coexisting continental alkaline magma series; Cantal, Massif Central. France. J. Petrol., 36: 17291753. Witt-Eickschen, G. and Seck, H.-A., 1991. Solubility of Ca and AI in orthopyroxene from spinel peridotite: an improved version of an empirical geothermometer. Contrib. Mineral.

141

Petrol., 106: 431-439. Zeyen, H., Novak, O., Landes, M., Prodehl, C., Driad, L. and Him, A., 1997. Refraction-seismic investigations of the Northern Massif Central (France). In: K. Fuchs, R. Altherr, B. Miiller and C. Prodehl (Editors), Stress and Stress Release in the Lithosphere - - Structure and Dynamic Processes in the Rifts of Western Europe. Tectonophysics, 275: 99-t 17 (this volume). Ziegler, P.A., 1992. European Cenozoic rift system. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume I. Case History Studies on Rifts: Europe and Asia. Tectonophysics, 208: 91-111.