Abstract of an invited paper at EUROPROBE Archaean-to-Proterozoic "Time-Slice" Symposium : Archaean and Proterozoic Plate Tectonics: Geological and Geophysical Records, St Petersburg, 1-4 November 2001
ANORTHOSITES: THE LOWER CRUSTAL CONNECTION
Jean-Clair Duchesne (University of Liège, Belgium)The consensus “hot spot” model for anorthosite petrogenesis, forged over the last decades and summarized by Ashwal (1993), implies a deep-seated magma chamber in which a mantle-derived magma ponds at the crust-mantle limit. Differentiation gives rise to a floated plagioclase cumulate at the roof of the chamber, from which, due to gravity instabilities, blobs of plagioclase mush start rising diapirically through the lower crust to emplace grossly at the limit between the upper and lower crusts, where they coalesce to form anorthosite plutons.
In the last decade, this model has been improved and partly modified to account for new field-, experimental and geochemical constraints.
A major step to confirm the polybaric character of the crystallization was to assess the PT conditions of stability of the typical high-alumina orthopyroxene megacrysts (with plagioclase exsolutions) which form subophitic aggregates with plagioclase megacrysts: this assemblage is stable at 10-13 kb in contrast with the pressure of final emplacement (<5 kb) (Fram and Longhi, 1992; Longhi et al., 1993). As for the diapirism mechanism, it was tested by finite element modeling taking into account the thermo-mechanical properties of anorthosite and mid- and lower crustal rocks (Barnichon et al., 1999). The model confirms that diapirism of an anorthosite mush through the lower crust can take place in a relatively short time (2-3 Myr), and gives rise to strain regimes and pluton shapes in agreement with field observations. The time interval constraint was shown to be an essential parameter: U-Pb ages revealed that the whole igneous evolution in the Rogaland anorthosites lasted less than 10 Myr (Schärer et al., 1996).
Experimental data on the basaltic systems in dry conditions have considerably modified the consensus view on the nature and source of the parent magma ponding in the deep-seated magma chamber (Longhi et al., 1999). The data show that the parent magma composition of anorthosite massifs (high-alumina basalt at Harp Lake, Nain; hypersthene monzodiorite or jotunite at Bjerkreim-Sokndal, Rogaland) are situated on a thermal maximum of the plagioclase + opx + cpx cotectic in the pressure range (10-13 kb) where the opx composition is typical of the high-alumina orthopyroxene megacrysts and the plagioclase composition is typical of that in the anorthositic rocks themselves. Consequently, the parent magmas cannot be generated by fractionation of a mantle-derived basaltic magma, but must result from the melting of a mafic rock (plagioclase + opx + cpx) at 40-50 km depths. The latter rock may be lower continental crust or foundered mafic plutons sunk out of the crust. These experimental data also put severe limitations to the production of intermediate magmas in “underplating” magma chambers by hybridization of contact anatectic acidic crustal melts with mantle-derived basic magma: the two melts simply cannot mix because of this thermal barrier. Re-Os isotope data brilliantly support a mafic lower crust origin, particularly in Rogaland where high Os isotopic ratios can only be accounted for by a mafic source, because there is no significantly older crust in SW Scandinavia (Stein et al., 1998; Schiellerup et al., 2000).
Detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their
emplacement at mid-crust levels (Emslie et al., 1994; Scoates and Chamberlain, 1997). In SW Scandinavia terrane boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material (the so-called Telemark Craton Tongue) underthrust to depths higher than 40-50 km (Andersson et al., 1996). In Poland the Suwalki anorthosite has emplaced in the Svecofennian platform along the EW trending Mazury lineament (Wiszniewska et al., 1999). In Southern Norway a lithospheric scale weakness zone (the Feda zone) may have channeled the emplacement of the Rogaland anorthosites (Duchesne et al., 1999). Finally, it has been suggested that crustal tongues (expression that we prefer to craton tongues) of lower mafic crust can reach sufficiently high temperatures to melt and produce the parent magma of massive anorthosites: this is the crustal tongue melting model (Duchesne et al., 1999). Concomitant with underthrusting, delamination along the weakness zone and asthenospheric uprise have to be invoked to explain the temperature rise. Such an hypothesis seems at least as good as the classical hot-spot hypothesis, which is unable to explain how several generations of anorthosites, separated by 100s Myr, can occur in the same province (Hamilton et al., 1998; Scoates and Chamberlain, 1997), unless the hot spot has been able to find its way back to the same location! As an alternative the crustal tongue melting model links the anorthosite production to a local geochemical property of the lower crust, which permits its melting each time heat is available.
In light of these new developments, the concept of anorogenic setting, typical of the consensus model of anorthosite formation, has also be nuanced. Emplacement along weakness zones can be favoured by relative movements between terranes either in post-collisional setting or, if a local source of extra-heat is provided, during tectonic rejuvenation (Duchesne et al., 1999).
In the classical model acidic rocks are produced by contact anatexis of crustal rocks surrounding the deep seated magma chamber. In our views this is no more the only possibility: jotunite magmas, parental to andesine anorthosite, can differentiate up to acidic melts. A well-defined liquid line of descent has been demonstrated in Rogaland by geochemical modeling and experimental studies (Vander Auwera et al., 1998; Wilmart et al., 1989). On the other hand troctolitic melts in the same pressure conditions (<4 kb) cannot reach acidic compositions, but will be forced to end their fractional crystallization at an olivine gabbro eutectic (Longhi et al., 1999).
Fe-Ti oxide mineral deposits, a major source of TiO2 pigment, are hosted in andesine anorthosites. We now begin to understand their genesis in light of the complex diapiric evolution of their host and fractional crystallization of their parent magma (Duchesne, 1999; Duchesne and Schiellerup, 2001). This provides new prospecting guides for deposits with low deleterious element contents (Cr, Mg, …) in ilmenite.
Finally, anorthosites have long been considered as a potential source of CO2 (see e.g. Touret, 1974). Carbon isotope studies in Rogaland (Wilmart et al., 1994) have confirmed that jotunite and related melts are indeed rich in carbon (up to 1500 pmm), but anorthosites themselves are C poor (< 50 ppm) due to their cumulate character. The carbon distribution observed at the contact between jotunitic intrusions and anorthosites or surrounding gneisses indicates that some CO2 has fluxed from the jotunitic intrusions over a distance of a few hundred metres. This suggests only minimal lateral propagation of the fluids by diffusion or infiltration.
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