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Restites, Eu Anomalies, and the Lower Continental Crust*

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Restites, Eu Anomalies, and the Lower Continental Crust* Gpahimica er C~~~himi~ Am Vol. 56, pp. 963-970 Copy&& @ 1992 Pe.r@monPress pk. Printedin U.S.A. Restites, Eu anomalies, and the lower continental crust* ROBERTAL. RUDNICK Research School of Earth Sciences, GPO 30x 4, Canberra, A.C.T. 2601, Australia (Received December 14, 1990; accepted in revisedform December 6, 1991) Abstract-Dehydration partial melting of the lower continental crust is the main process responsible for granite genesis in the post-Archean. If this melting occurs to undifferentiated mantle-derived rocks (i.e., basaltic protoliths), it should result in a granitic upper crust having a negative Eu anomaly and a mafic restitic lower crust having a positive Eu anomaly. However, most post-Archean high-pressure granulite facies terranes are not chemi~ly implement to granites because they themselves have bulk granitic compositions. If one restricts the discussion to m&c granulites from terranes, it is apparent that although their bulk composition is complementary to granite, most do not have positive Eu anomalies. Granite residues may, however, be present in granulite facies terranes if the parent rock giving rise to the granite had a negative Eu anomaly and a significant melt fraction were left behind with the residue. Such restitic granulites will have either no Eu anomaly or a negative one and “normal” LILE ratios due to the dominance of the incompatible element-rich melt. Granulite xenoliths do have the requisite compositions to balance the composition of the upper crust (i.e., mahc bulk composition and positive Eu anomalies); however, these samples are generally interpreted as melts or cumulates derived from underplated basaltic magmas rather than restites. Intracrustal differentiation may therefore proceed in several steps: intrusion and fractionation of basaltic magmas in the lower crust serves to ( 1) create matic and ultramafic cumulates (having positive Eu anomalies), (2 ) produce evolved magmas (having negative Eu anomalies) ( 3 ) melt the preexisting (evolved) crust to produce granites and evolved restitic granulites. The magmas produced by differentiation of the basalt may remain in the lower crust where they may be subsequently melted in a later underplating event or may mix with the crustal melts and intrude the upper crust. INTRODUCTION melts) do have the requisite features ofthe geochemical com- plement to the upper crust. They are generally mafic and TNE NEGATIVEEU anomaly documented in the upper con- have low abundances of K, Rb, Th, and U; many also possess tinental crust by TAVLOR and MCLENNAN ( 1985) on the positive Eu anomalies ( RUDNICK, 1992 )_ From geochemical basis of REE patterns in sedimentary rocks requires the pres- arguments, however, it can be demonstrated that most of ence of a positive Eu anomaly in the lower crust (if the bulk these m&c granulites form as a result of intrusion of mantle- crust has no Eu anomaly). They postulated that feldspar- derived basaltic melts near the Moho and, although cumulates bearing residues, left after granite extraction, would be the abound, very few appear to be restites ( ROGERSand HAWKES- likely hosts for this Eu signature in the lower crust. Since the WORTH, 1982; RUDNICKet al., 1986; RUDNICKand TAVLOR, negative Eu anomaly is a feature of post-Archean sediments 1987; D~WNES et al., 1990; KEMproN et al., 1990; Looc~ only, it follows that only crust that underwent ~fferentiation et al., 1990). The abundance of ma& compositions in gran- since the Archean would contain this positive Eu anomaly ulite xenoliths coupled with the observation that most xeno- signature (i.e., one might expect to observe it in post-Archean liths are older than the basaltic volcanism that carried them granulites). to the earth’s surface ( RUDNICK, 1992) suggests that basaltic However, the search for the restite that is chemically com- underplating has been an important process over a long time plementary to the upper granodioritic crust has been largely span (since the Proterozoic) and in a variety of tectonic set- unsuccessful. Taken from a simple mass-balance viewpoint, tings ( RUDNICK, 1990). the restitic lower crust should be characterized by a net pos- This paper addresses the following questions: Is post-Ar- itive Eu anomaly coupled with depletions in incompatible chean lower crust complementary to the upper crust through trace elements (i.e., IS, Rb, Cs, Th, U, and the LREE) relative a melt-restite relationship? If so, where is the cozening to the upper crust. However most granulites from surface positive Eu anomaly in the lower crust? Is basaltic under- outcrops do not have positive Eu anomalies and large num- plating important in post-Archean intracmstal differentiation? bers are not depleted in incompatible elements (RUDNICK To evaluate these questions I review the composition of et al., 1985). In fact, most granulite terranes, irrespective of granulites, working from a large, literature-derived chemical their age, are as geochemically evolved if not more evolved data base ( RUDNICKand PRESPER,1990), and compare these than the bulk upper continental crust (Fig. 1). data with models of restite composition derived from exper- In contrast, granulite facies xenoliths (samples of the lower imental studies and trace element partitioning behavior. De- crust carried rapidly to the surface by basaltic or kimberlitic spite the enormous ~rn~ition~ diversity amongst granu- lites, inferences may be made about the major processes re- * Presented at the symposium for S. R. Taylor, “Origin and Evo- sponsible for the differentiation of the crust and the interplay lution of Planetary Crusts,” held October 1-2, 1990, at the Research of partial melting, basaltic underplating, and granulite facies School of Earth Sciences, ANU. metamorphism on the generation of the lower crust. 963 R. L. Rudnick HZ0 as fluid inclusions in granulites ( TOURET and HARTEL, 1990). In addition, the heterogeneous oxygen isotopic values of granulites negates pervasive fluid circulation in many granulite terranes (VALLEY et al., 1983) and xenoliths ( KEMPTON and HARMON, 1992) and has been used to argue for fluid-absent metamorphism in some terranes (VALLEY, 8 1986). Recent experimental work has therefore sought to 5- model partial melting processes in the crust on the basis of dehydration reactions, where hydrous phases break down under increasing temperatures to form melt plus an anhy- 01 I drous mineral assemblage (POWELL, 1983 ) . 40 50 60 70 80 Fluid-absent melting of crustal rock produces some inter- %Si02 esting features ( VIELZEUFet al., 1990): FIG. 1. Average Si02 vs. MgO, which reflect degree of differentia- 1) Large amounts of melt are formed over narrow temper- tion, for individual granulite terranes shown relative to total crust ature intervals, coinciding with the breakdown of the ma- (T.C.) and upper crust (U.C.) compositions. Average compositions jor hydrous phase(s) , of terranes are calculated from the data compiled by RUDNICKand PRESPER( 1990) unless otherwise stated; crustal values are from 2) The metamorphic temperature is buffered by the incon- TAYLORand MCLENNAN( 1985). Terranesfor which the average gruent melting reaction, and composition is less evolved than the total crust composition are 3) The high temperature and moderate-to-low Hz0 contents numberedand include:( 1) Qianxi, (2) Ivreazone, ( 3) Anabar shield, of melts thus produced allow them to rise to shallow levels (4) Cabo Ortegal and (6) Furua complex. The remaining granulites are from: Arunta block, Australia (ALLEN,1979); Jequiecomplex, in the crust before they solidify. Brazil; Limpopo Belt, southern Africa; Minnesota River Valley, USA; It is these features that lead some petrologists to suggest that Uivak gneisses, Labrador; Kapuskasing structural zone, Ontario; fluid-absent melting is the main process by which the lower Scourian gneisses (SHERATONet al., 1973), Scotland;S. India;Napier complex, Antarctica; Vestfold Hills, Antarctica; Prydz Bay, Antarc- crust melts and consequently is the major means of crustal tica; Rayner complex, Antarctica; Musgrave ranges, Australia; Lap- differentiation (CLEMENSand VIELZEUF, 1987; VIELZEUFet land; Fenoscandia, Mexico; Adirondacks, USA; Fiordland, New al., 1990). Zealand; and Tromoy, Norway. The residual mineralogies produced by fluid-absent melting of a variety of rock types are listed in Table 1. The percent LOWER CRLJSTAL COMPOSITION melt produced corresponds to the complete consumption of the original hydrous phases, leaving an anhydrous residue. Based on our knowledge of the upper crust, which grows In each case this amount of melt is sufficient to allow sepa- mainly through intrusion of granites derived by partial melt- ration of the melt from its source region ( WICKHAM, 1987). ing in the deep crust, several chemical characteristics of the The residual mineralogies are similar to those observed in post-Archean lower crust can be inferred (TAYLOR and many granulites: residues produced by melting of gabbro MCLENNAN, 1985): (amphibolite) and tonalite correspond to mafic granulites 1) It should be depleted in the heat-producing elements (K, while residues produced by melting of graywacke and pelite Th, and U), as well as other large-ion lithophile elements correspond to various types of felsic and aluminous granulites, (LILE) such as Rb and Cs, relative to the upper crust. respectively. 2) It should be more mafic than the upper crust, having higher A1203, CaO, and MgO, and lower SiOZ, and Trace Element Signatures 3) It should contain, on average, a positive Eu anomaly The chemical composition of restite is a function of the complementary to that observed in the upper crust, as- proportions and character of the residual phases in addition suming that the bulk crust has no Eu anomaly. to the composition of the parent rock. In theory, the restites Such geochemical characteristics may be explained if the shown in Table 1 should all possess positive Eu anomalies lower crust is the residue left after partial melting to produce (due to the presence of residual feldspars) and be depleted in large-ion lithophile elements (LILE), showing high K/Rb the upper crust (TAYLOR and MCLENNAN, 1985).
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