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The Crustal Tongue Melting Model and the Origin of Massive Anorthosites J Paper 232 Disc 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, Universite 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 100 *C 1999 Blackwell Science Ltd Paper 232 Disc Terra Nova, Vol 11, Nos 2/3, 100±105 The crustal tongue melting model . J. C. Duchesne et al. ............................................................................................................................................................................................................................................. 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.
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