The Crustal Tongue Melting Model and the Origin of Massive Anorthosites J

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The crustal tongue melting model and the origin of massive anorthosites J. C. Duchesne1*, J. P. LieÂgeois2,1, J. Vander Auwera1 and J. Longhi3 1L.A. GeÂologie, PeÂtrologie et GeÂochimie, UniversiteÂ de LieÁge, B-4000 Sart Tilman, Belgium, 2Department of Geology, Africa Museum, B-3080 Tervuren, Belgium, 3Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA

ABSTRACT Recent detailed field studies in several anorthosite complexes the Sveconorwegian Province terrane boundaries have been have shown that anorthosites are frequently associated with traced in deep seismic profiles to Moho offsets or to tongues of weakness zones in the crust which may have favoured their lower crustal material underthrust to depths higher than 40 km. In emplacement at mid-crust levels. Recent experimental data have Southern Norway, we suggest that a lithospheric-scale weakness shown that the parent magma compositions of various anorthosite zone (the Feda transition zone?) has channelled the Rogaland massifs lie on thermal highs in the relevant phase diagrams at 10± anorthosites through linear delamination, asthenospheric uprise 13kbar, indicating that these magmas cannot be derived by and melting of a mafic lower crustal tongue. fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40±50 km depths. In Terra Nova, 11, 100±105, 1999

extensively, particularly in connection Andersson et al. (1996), and major Introduction with the origin of related mangerites, lithospheric discontinuities in South Petrogenetic models for a number of charnockites and (rapakivi-) granites. Norway, to provide a constrained melt- igneous rocks such as anorogenic gran- Several magma types were invoked, ing mechanism for anorthosite genesis. ites, continental flood basalts or Pro- each with convincing arguments: high- terozoic AMCG [anorthosite, manger- alumina noritic basalt, e.g. in the Harp Petrological constraints on the ite, charnockite (rapakivi-) granites] Lake Complex (Emslie, 1978), trocto- nature of the anorthosite source suites postulate the development of a litic basalt (Nolan and Morse, 1986), rocks magma chamber at the base of the crust and jotunitic (Fe-Ti-P-rich hypersthene by underplating or intraplating. Such monzodioritic) liquid, well represented Experimental data on the liquidus equi- models are convenient to account for in Rogaland (Demaiffe et al., 1986; libria relevant to the anorthosite petro- Ahed the widespread geochemical and isoto- Duchesne, 1990; Vander Auwera and genesis in the range of 1 bar to 13 kbar Bhed pic evidence for interaction between Longhi, 1994). Granitic melts pro- have brought new constraints on the Ched magmas and crustal material. A mantle duced by crustal anatexis (e.g. Emslie nature of the source rocks of the par- Dhed origin for the ponding magma is gen- et al., 1994) and/or by differentiation ental magmas (Longhi et al., 1999). Ref marker erally accepted though its composition, from intermediate magmas (Duchesne Between 10 and 13 kbar, i.e. the pres- Fig marker varying from basaltic to picritic, is and Wilmart, 1997; Vander Auwera et sure conditions of a deep-seated mag- Table marker rather loosely constrained, as is the al., 1998) are voluminous. Contact ana- ma chamber where HAOM coexist Ref end melting process invoked. texis around the deep-seated magma with plagioclase, the phase diagram Ref start In the case of the AMCG plutonism, chamber and various amounts of con- (Fig. 1) shows a thermal barrier be- the discovery of high-Al orthopyroxene tamination between mantle-derived tween the eutectic liq + plag + l- megacrysts (HAOM) in most massif- magmas and crustal materials were in- pyx + sil + cpx and the reaction point type anorthosites (Emslie, 1975) was voked to explain the large variety of liq + plag + gar + opx + cpx (B in the decisive step in building up a poly- melts. A consensus model eventually Fig. 1). A jotunitic parent magma in baric model for the genesis of anortho- emerged (Ashwal, 1993), in which the Rogaland anorthosite complex, as sites: a basaltic magma ponds in a deep- `plumes' or `mantle swells' were gener- represented by the TjoÈ rn jotunite (TJ), seated magma chamber at the base of ating the basaltic magma underplating sits close to the liq + plag + opx + cpx the crust, where HAOM crystallize be- the crust. The possibility of a mafic cotectic, where the curve crosses the fore the magma products intrude to lower crustal source is rarely consid- plane of plag + opx + cpx composi- mid-crustal levels (Emslie, 1978). The ered, even though radiogenic isotopes tions and necessarily forms a local ther- depth of the deep-seated chamber was cannot differentiate easily between ju- mal maximum on the liquidus surface. constrained by experiments on the sta- venile lower crustal and mantle sources. Consequently, TJ cannot be derived by bility of 7±9% Al2O3 HAOM which However, recent petrological experi- fractionation of melt of olivine-domi- proved to be a liquidus mineral between mental work (Longhi et al., 1999) has nated mantle because such melts reach 11 and 13 kbar (Fram and Longhi, 1992). pointed to a crustal gabbronoritic pri- plagioclase saturation at the reaction The nature of the parent magma(s) of mary source for the parent magma of point (B) and then evolve down-tem- anorthosite suites has been discussed the anorthosite suite, thus questioning perature in the direction of the arrows the classical mantle origin. The `crustal away from TJ. TJ can only be produced *Correspondence: Tel.: + 32/43 66 2255; tongue melting' model proposed here by melting of a source rock whose Fax: + 32/43 66 2921; E-mail: integrates these data as well as deep- composition lies in the plag [email protected] seismic profiles recently interpreted by + opx + cpx plane, a gabbronoritic

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Qtz (b) {Pl,Ilm} (a) oxygen units Di

{Wo,Ilm} 13 kb oxygen units aug plag + ilm + ... 13 kb ± 1 bar aug + ilm sil aug ol aug

TJ gar B lpyr Opx lpyx lpyr plag sil 1 bar Ol Opx sil sil Qtz ol plag lpyr TJ

Pl Ol ol gar

Fig. 1 Liquidus equilibria at 13 kbar projection into the system Ol (olivine)±Pl (plagioclase)±Qtz (quartz)±Wo (wollastonite). (a) projection from the wo (wollastonite), or (orthose) and ilm (ilmenite) components; (b) projection of plagioclase-saturated liquids from the pl (plagioclase), ilm and or components. Light solid lines show augite-saturated and plagioclase-saturated liquid boundaries at 1 bar calculated for TJ-like projection. Heavy lines show 13 kbar liquid boundaries. Arrows indicate direction of lowering temperature. Double arrows show peritectic lines. TJ (ellipse) is the composition of the jotunitic parent magma in the Rogaland province. Diamond is HAOM composition. Dashed lines connect coexisting phases. Abbreviations: lpyx, low-Ca pyroxene; sil, silica phase; plag, plagioclase; ol, olivine; opx, orthopyroxene; aug, clinopyroxene; gar, garnet (simplified from fig. 3 of Longhi et al., 1999). rock, which cannot be a mantle rock. through the two reaction points and Nain anorthosite province straddles Longhi et al. (1999) have also demon- further (Fig. 1). Contamination by the the limit between the Nain and Church- strated that some high-alumina basaltic crust of a basaltic magma in a deep- ill (Rae) Provinces (Wiebe, 1992; Em- liquids, such as the parent magma of seated magma chamber is thus possible, slie et al., 1994); the Lac St Jean and the Harp Lake Complex, show the especially at the radiogenic isotope le- Havre±St. Pierre anorthosite com- same character as TJ despite higher vel, but fractionation does not lead to plexes are associated with long linea- Mg contents, and that more troctolitic silica-rich liquids. ments (Higgins and van Breemen, 1992; melts, parental to various massifs such van Breemen and Higgins, 1993); in the as Kiglapait or Laramie, plot on the Laramie anorthosite complex two gen- Emplacement mechanism and liq + opx + cpx + plag cotectic at high erations of anorthosites (separated by structural setting pressure, and thus are also consistent an interval of some 0.3 Gyr) are linked with derivation from a mafic source. The diapiric uprise through the lower to a terrane boundary (Scoates and These experiments have constrained crust of a plagioclase crystal mush was Chamberlain, 1997); the Suwalki an- the composition of the source rocks. initially proposed by Martignole and orthosite has emplaced in the Svecofen- Mafic granulites±a major component Schrijver (1970). It has been success- nian platform along an EW lineament of the lower crust±have adequate aver- fully simulated using finite-element (Wiszniewska et al., 1999). age compositions, although they vary modelling (Barnichon et al., 1999). In Southern Norway and Sweden, greatly between different tectonic pro- The model accounts for the shape of several major crustal lineaments have vinces (Rudnick and Fountain, 1995). the diapir and its associated rim-syn- been recognized in the field and in The best candidates are layered intru- cline; the computed strain is consistent geophysical profiles: from east to west, sions of basaltic kindred such as the with field measurements, and the em- the Dalsland Boundary Thrust (DBT, Stillwater Complex which have higher placement time (2±3 Myr) with geo- Fig. 1), the Kristiansand±Porsgrunn mg-numbers than average lower crust. chronological data. The simulation shear zone (KPSZ, Fig. 1), which sepa- Because of the thermal barrier at considers an isotropic lower crust and rates the Bamble and Telemark Pro- high pressure, mixing with silica-rich the absence of a regional stress-field, vinces, and the Mandal±Ustaoset Line (crustal) material has no effect on the conditions approximating those of an- (MUL, Fig. 1) between Telemark and major element chemistry of liquids si- orogenic/within-plate settings. Rogaland±Vest Agder. Geophysical tuated on the olivine side of the barrier. Several anorthosite provinces, how- data indicate a Moho offset south-east Whatever the amount of mixing, these ever, coincide with structural weak- of the Rogaland anorthosite complex liquids are inexorably brought back to nesses in the lithosphere, which appear (in ILP-11 profile; Andersson et al., the ol + opx cotectic by decreasing to have controlled and favoured the 1996). This offset is a major litho- temperature and forced to follow it emplacement of diapirs. Indeed, the spheric structure and could correspond

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The crustal tongue melting model . J. C. Duchesne et al. Terra Nova, Vol 11, Nos 2/3, 100±105 ...... to the transition zone of Falkum (1998) Ga, Bingen and van Breemen, 1998a); Link with major lithospheric `separating the south Rogaland an- the climax of their metamorphism has structures: the crustal tongue orthosite Province to the west and the been dated by U±Pb on monazite at melting model Agder migmatitic terrane to the east' 1.02±1.00 Ga and a second phase at and marked by the N±S elongated (100 0.930±0.925 Ga (Bingen and van Bree- Deep seismic profiles in the Skagerrak km 6 2±5 km) Feda augen gneiss dis- men, 1998b) is contemporaneous with have been interpreted as reflecting Sve- playing ubiquitous local shear zones Rogaland AMCG emplacement at conorwegian structures by Andersson (Falkum, 1998). Despite quite impor- 0.930±0.920 Ga (SchaÈ rer et al., 1996). et al. (1996) (Fig. 3). At depth the Tele- tant late deformation linked to intru- Further east a number of postcolli- mark±Bamble terrane boundary, i.e. sion of late plutons, this transition zone sional granites such as the Bohus and the KPSZ, becomes a surface along is characterized by general N±S-strik- FlaÊ granites (Fig. 2) have been related which underthrusting has taken place. ing and E-dipping foliation planes, to major terrane boundaries (Anders- A slab of lower crust±the Telemark with common strong subhorizontal son et al., 1996). Although more accu- Craton Tongue±decoupled from the N±S-orientated mineral lineations par- rate age determinations are needed to upper and mid crusts, can be identified allel to the fold axis (Falkum, 1998). constrain the timing of this outburst at depths between 30 and 50 km in the This Feda weakness zone could be si- of late plutonism, it appears that a downward prolongation of the KPSZ. milar to the southern part of the Man- major plate-scale structure channels Further east another terrane boundary, dal±Utsaoset Line, which coincides granite emplacement. We suggest that along which the Bohus and FlaÊ granites with an augen gneiss and is straddled large discontinuities, such as terrane are elongated, corresponds at depth by numerous late granites (Sigmond, boundaries or the Feda transition with a Moho offset. Andersson et al. 1985) (Fig. 2). Both Feda and Mandal zone, also control the anorthosite (1996) have suggested that the Bohus, augen gneisses emplaced during the emplacement during the same postcol- FlaÊ and other postcollisional granites in Sveconorwegian orogeny (1.05±1.03 lisional event. South Norway were produced by ana-

Fig. 2 The Sveconorwegian Province with major terrane boundaries: MUL, Mandal-Utsaoset line; KPSZ, Kristiansand-Porsgrunn shear zone; DBT, Dalsland boundary thrust; MZ, Mylonite zone; PZ, Protogine zone. The seaward extension of the Bohus granite and the Moho offset on deep seismic profile ILP-11 after Andersson et al., 1996. Location of the profile across the Skagerrak shown on Fig. 3 is indicated by a heavy line.

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NW SE crustal mafic material to the mantle as Bamble Skagerrak graben Bohus granite a variant of the crustal foundering process (Arndt and Goldstein, 1989).

10 Depth km Telemark Concluding remarks 30 Moho The crustal tongue melting model ap- TCT 50 pears as an alternative to the plume 50 km model and provides a plausible mechanism to account for the crustal ori- Fig. 3 Structural interpretation of the deep seismic profile under the Skagerrak and gin of anorthosite parent magmas. It onshore extension. See Fig. 2 for location of the profile (after fig. 5 of Andersson et emphasizes the role of deep lithospheric al., 1996). tectonic structures in producing a variety of magmas from granitic to high- texis of underthrust mid-crustal mate- posed model accounts for the produc- alumina basaltic and in controlling rial due to tectonic burial and conse- tion of a large variety of magmatic their emplacement. quent temperature increase. material which can intrude over a wider When produced as a direct conse- We propose that a modified version time interval than the anorthosites quence of collision, anorthosites can be of Andersson et al.'s model also ac- themselves. The preservation of the classified as post-collisional because (i) counts for the formation of anortho- Telemark craton tongue might possibly the magmatism postdated only by some sites. Crustal tongues of lower mafic be due to a more refractory composi- tens of Myr the collision climax, and (ii) crust reach sufficiently high pressure by tion or to a thicker lithospheric mantle, the emplacement is controlled by struc- underthrusting to account for the sta- which buffered the temperature rise tures directly resulting from the colli- bility pressure of HAOM (11±13 kbar and prevented its melting (Black and sion; when linked to a reactivation of a corresponding to 40±50 km). After a LieÂ geois, 1993). When the tongue major structure, they belong to the with- thermal relaxation time of several tens melts, only a Moho offset is left. in-plate/anorogenic magmatism but of Myr (Platt and England, 1994) and Anorthosite formation can thus be with the same geochemical signature. linear lithospheric delamination along directly linked to reactivation of large The thermal and mechanical viabili- the shear zone allowing an astheno- lithospheric discontinuities during the ty of the model now remains to be spheric uprise (Black and LieÂ geois, post-collisional period, as defined by tested. We believe that the petrological 1993), as is the case, e.g. under the San LieÂ geois (1998). In a number of cases, and structural parameters are now suf- Andreas shear zone where the astheno- however, the weakness zone is much ficiently compelling to make these sphere is close to Moho (Lachenbruch older than the anorthosite emplace- further investigations worthwhile. et al., 1985), the temperature rises to ment, which suggests its reactivation. produce jotunitic melts at c. 12008C In Laramie, two generations of an- (Vander Auwera and Longhi, 1994) orthosites were produced along the Acknowledgements and high-alumina basalts at c. 13008C same limit: the first generation (1.76 The Belgian Fund for Joint Research (Fram and Longhi, 1992). By melting, Ga) is late-orogenic (post-collisional) supported part of the project. Partial support the tongue becomes a magma chamber and the second (1.43 Ga) anorogenic also came from a NASA grant (NAG5± (Fig. 4) which evolves in a similar way (Scoates and Chamberlain, 1997). In 4558) to J. Longhi. Lamont-Doherty Earth as in the classical model, but with the the Nain province, the suture seems to Observatory Contribution no. 5975. J. Mar- variant that magmas and crystal be some 0.4±0.5 Gyr older than the tignole and B. Bingen are greatly thanked for mushes are channelled by crustal struc- anorthosites produced 1.3 Ga (Emslie their constructive reviews. tures instead of rising vertically. Acidic et al., 1994) and another generation of rocks can be generated during the anorthosites at & 2.1 Ga has been iden- References whole process both by differentiation tified along the boundary (Hamilton et of the mafic magma and/or by anatexis al., 1998). In AõÈ r (Niger, Sahara), the Andersson, M., Lie, J.E. and Husebye, E.S., 1996. Tectonic setting of post-orogenic of the lower crust. Initial melting of a emplacement of the & 0.41 Ga subvol- granites within SW Fennoscandia based mafic lower crustal tongue should pro- canic anorthosite (similar to massif- on deep seismic and gravity data. Terra duce relatively small volumes of grani- type) and associated alkaline ring-com- Nova, 8, 558±566. tic melt, which subsequently may in- plexes (Moreau et al., 1994) is linked to Arndt, N.T. and Goldstein, S.L., 1989. An trude to higher levels in the crust than a reactivation of a & 0.6 Ga shear zone open boundary between lower continental the anorthosites or may be overtaken separating two contrasted terranes crust and mantle: its role in crust and passed by larger anorthositic dia- (LieÂ geois et al., 1994). Reactivation of formation and crustal recycling. pirs with greater buoyant velocities. a crustal underthrusting can trigger a Tectonophysics, 161, 201±212. Granulitic residues of anatexis become new plutonism episode if another frag- Ashwal, L.D., 1993. Anorthosites. Springer, Berlin. gradually involved in the melting pro- ment of the crust is forced into the Barnichon, J.-D., Havenith, H., Hoffer, B., cess and would give rise to some A-type mantle or if an additional heat source Charlier, R., Jongmans, D. and or rapakivi-granites (Clemens et al., develops as, e.g. through linear delami- Duchesne, J.C., 1999. The deformation of 1986), although asthenosphere could nation along the shear zone. the Egersund-Ogna massif, South also produce alkaline magmas (LieÂ - The crustal underthrusting is also an Norway: Finite Element modelling of geois et al., 1998). In short, the pro- elegant mechanism to transfer lower diapirism. Tectonophysics, 303, 109±130.

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Fig. 4 (left) The crustal tongue melting model. (a) collision stacking of terranes produces underthrust lower crust tongues; (b) granitic liquids produced by anatexis of mid-crustal material intruded at higher levels along the terrane bound- CLM CLM aries; due to linear delamination along the weakness zone, the rise in temperature melts a tongue of suitable composition some tens of Myr later, and a deep-seated asthenosphere (a) magma chamber develops in which plagioclase floats to accumulate at the roof; a mafic cumulate sinks; (c) anorthosite diapirs rise through the crust, channelled by the weakness zone, and coalesce higher at mid-crust level to constitute a province of anorthosite massifs; granitic melts are also produced and follow the same path; (d) the mafic cumulate, left behind, CLM becomes indistinguishable from the man- CLM tle; a Moho offset is the only scar of the former magma chamber. Abbreviation: CLM, continental lithospheric mantle. asthenosphere (b)

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