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The Granulite-Granite Linkage: Inferences Froll} S the Granulite-Granite Linkage: Inferences froll} s lDEPARTAMENTO DE PETROLOGIA Y GEOQUIMICA, FACULTAD DE CIENCIAS GEOLOGICAS, UNIVERSIDAD COMPLUTENSE, 28040 MADRID, SPAIN 2 DEPARTMENT OF GEOLOGY, BIRKBECK COLLEGE, MALIT STREET, LONDON WCIE 7HX, UK 3CNRS UMR 6524, D EPARTEMENT DES SCIENCES DE LA TERRE, UNIVERSITE BLAISE PASCAL, 5 RUE KESSLER, F-63038 CLERMONT-FERRAND, FRANCE 4DEPARTAMENTO DE GEOLOGIA, FACULTAD DE CIENCIAS DEL MAR, UNIVERSIDAD DE CADIZ, 1151O PUERTO REAL ''-'""JJHC,J. SPAIN if the xenoliths is lower than that outcropping mid-crustal A contribution the granites reduces the mantle contribution in models ifgranite nature if the lower continental crust in contrasts with the more mqficffJ7f!r:r-,Cf1LUflf con7iJo:ntum and estimaud in other European Herrynian areas) a non- Thermobarometric calculations based on min- crust in this Herrynian eral paragenesis indicate conditions around 850-950°(;, 7-11 th us the xenoliths lower con- this high-T high-P assemi'JLCl!.'!e IS a by Ket;vpftlUC coronas) trf111.'in(J'rt in the KEY WORDS: filsi;; lower continental crust; Sr-Nd .L.LO,'OYJ<,"-M& Iberian Belt alkaline magma. Felsic mela1!!J'leOlIS exhibit restitic mineral with up to 50% 37% sillimanite. Major and trace element mO,rJelLWP idea that the taU�-l1erc�vnuzn {J19JaLumlnollS INTRODUCTION The study of lower-crustal xenoliths has been a 8 Nd calculaud at an (J1)erage Herrynian age tool in understanding the process of anatexis involved in if300 Ma) are in the range O' 706-0' 712) and -1,4 to -8'2, the genesis of felsic (Downes & Duthou, These values match the isowpic if the Ruiz et Miller et 1992; Hanchar et outcrobtnnp late TIe Sr 1994; Pankhurst & Rapela, It is clear that granite sources in orogenic areas are not the outcropping An alkaline ultrabasic dyke swarm was intruded into metamorphic rocks but are located in deeper crustal the region in Mesozoic times (Villaseca & Nuez, levels. Lower crust is not easily accessible for study and These dykes have a N-S orientation and are cut can also be modified later orogenic events which by the tholeiitic Messejana-Plasencia dyke emplaced at could strongly its primary characteristics (Costa � 184 Ma (Schermerhorn et The anorogenic & However, in the case of the central Spain, alkaline is related to North Atlantic rifting uLLLHfJ�v'-' of lower-crustal lithologies have been exhumed Triassic times et The dykes, by alkaline magmas. this which are ultrabasic lamprophyres, has not been affected by �H'--·LLLLU'-'� contain a varied population of mainly felsic granulitic any or event since the xenoliths These granulite xeno­ orogeny, so the xenoliths are considered to represent the liths show very distinctive lower crust as it existed at the end of granite vH.LfJ�'LLv'�HL.vHL. and geochemical The Central System cropping Hercynian rrrr,-nllilT1r' 2 complex 000 km in surface located In this paper we present a r.Alrrr\rrr".r. internal of the Iberian Belt geochemical and isotopic of the granulitic huge batholith intruded into continental crust xenolith suite from the SCS that indicates a felsic restitic composed of metasedimentary pelitic schists of Pre­ character for the lower continental crust in this region. cambrian and lower Cambrian age, and metaigneous The role played by the late Hercynian mag- rocks related to early Palaeozoic orogenic matism in the of this restitic lower crust, and events (500 ± 20 Ma, Vialette et 1987; Valverde the differences between material at lower- and Vaquero et Orthogneisses are the most abund­ middle-crustal levels and granulitic ant rocks of the outcropping metamorphic series in the terranes, eastern region of the SCS (>80% of surface area in the Guadarrama region; Villaseca In the western of the SCS a more complex metasedimentary series PREVIOUS ESTIMATES OF LOWER­ is dominant, which is rich in metapsammites with local interlayered carbonate material (Ugidos et CRUS TAL COMPOSITION IN During the orogeny, this crust evolved from CENTRAL SPAIN: GEOPHYSICAL intermediate-P towards low-P, conditions (dated DATA at 335 Ma in the Guadarrama sector; Valverde Vaquero Deep seismic constrain the crustal struc- et reaching granulite-facies conditions in wide ture of central Spain. Banda et al. 1) showed that the zones of middle crust with consequent anatexis (e.g. the crust in this area is around 31 km thick and consists of anatectic granulitic terrane of Toledo; Barbero, four 2): Granitic plutonism took place in late Hp·rn�Tnll-::\n (Villaseca et (l) a SeCilITlentary cover up to 3 km thick, with P-wave consists mainly of with very velocities <3·5 scarce basic to intermediate plutonic rocks, and was (2) an upper emplaced at shallow crustal levels [see reviews by Mo- including a low-velocity from about 7 to reno-Ventas et al. Pinarelli & Rottura and 11 km (Vp = 5·6 km/s), with P-wave velocities in the Villaseca et al. The lack of sources range 5,6-6,1 km/s; in terms of isotopic composition in the vicinity of the (3) middle crust from 11 to 23 km depth and Vp � PTT"rl''An plutons has led to two models for the 6·4 of the SCS (1) between crustal (4) lower crust from 23 to 31 km depth and Vp = melts and mantle-derived magmas (Moreno-Ventas et 6,8-6,9 km/so Pinarelli & Rottura, (2) melting of intracrustal materials (Bea & l.".L'U� '�H'J- and magnetotelluric data from Carbo & Ca­ Villaseca et Although the outcropping me- pote (1985) corroborate the described of taigneous rocks (orthogneisses) more closely the crust in central Spain. More recently, deep seismic the isotope signatures of the SCS granites than the sounding undertaken the Iberian Lithosphere Hetero­ metasedimentary nevertheless they show clear geneity and Anisotropy Project showed the continental differences in Sr isotope composition with the crust of the SCS to be 34 km with the upper granites (Villaseca et No significant meta­ boundary of the lower crust located between 21 and morphic event has affected the SCS batholith after its 23 km depth DSS Group, The boundary IOIm,ltl()n; igneous textures are p reserved almost without between middle and lower crust is always well marked exception. by an increase in P-wave velocity up to 6,7-6,8 km/so Xenolith-rich localities Messejana-Plasencia dyke Lamprophyric dykes -' Syenitic dykes ". Sedimentary rocks � Metamorphic rocks o 10 20 km �=-�-----�...=.--=�� Granitoids Fig. I. Geological map of central Spain showing hmprophyric dykes and the location of the main outcrops with granulitic xenoliths. However, models derived fr-om ILl HA crustal data show more commonly as a thin layer above the Moho. Also, that there are no significant lateral inhomogeneities in some recent xenolith studies Hanchar et 1994) gross crustal structure, in marked contrast to the hetero­ indicate that the lower continental crust may contain a geneous Hercynian surface geology. larger supracIllstal component than previously thought. Data from P-wave coda in the SCS (Paulssen & Visser, Thus, in some regions, the lower-cIllstal composition 1993) also confirm a continental crust of around 30 km may be closer to tonalite rather thew cliorite as previously in thickness, similar to the average crustal thickness of estimated (Wedepohl, 1995). McLemlaJl & Taylor (1996) Phanerozoic fo ld belts of central Europe (Wedepohl, also recognized that estimates of the average composition 1995). The lowennost 8 km correspond to the gTanulitic of the continental crust may be shifted towards a more lower crust with P-wave velocities always in the range of acid composition. 6·5-6·9 km/so These values are more typical of felsic or pelitic compositions rather than maficgranulites (Vp fr om 6·9 to 7·5 km/s; Rudnick, 1992; Wedepohl, 1995). P­ wave velocities in the range 6·13-7·0 km/s are very TYPES OF GRANULITIC XENOLITHS common in garnet-bearing peraluminous granulites Although granulitic xenoliths occur in camptonitic dykes (Kern, 1990), these being one of the most abuncIant in several places in the SCS, two localities are most lithologies in the xenolith suite of the SCS (Villaseca & important because of the large number and variety of Nu ez, 1986). In Fig. 2, a sketch profile of the SCS xenoliths found in them. The first is a diatreme-like crust is compared with the average continental crust of outcrop in La Paramera (Nuez et al., 1981). It is composed Wedepohl (1995). of breccia, and has a surface area of around 20 000 m2• From seismological data there does not seem to be a The outcrop is elliptical in shape, with its long axis sigTlificant mafic granulitic or eclogitic layer in the lower striking N-S, the same direction as the dykes. In this crust of the SCS, which agTees with the lithologies of the locality, 25<Yo of the breccia is composed of granitic granulitic xenoliths studied in this work. Wedepohl (1995) and other wall-rock xenoliths and xenoCIysts, as well as stated that in the younger fo ld belts in Central Europe, granulite xenoliths. The second important locality is the malic granulites occur locally as voluminous bodies but dyke swarm of Peguerinos (Fig. 1) where the narrow Vp 6 7 8 6 7 8 "" Sediments Sediments '. _/ Metamorphic . -+ ;. ;. .,. .,.+ Fe/sic � ? . ' � �. rocks intrusives 10 ++++++ and metamorphic ... + ... + ;. + Fe/sic rocks ... -+ -+ ... ;- + + + -+ ;. 1- intrusives - ;. -+ + -+ + E 20 ++++++ ...::.::: 'I - .s::: Fe/sic Fe/sic - granulites granulites C. � 30 - C � . - MOHO -� � � � I � ,,////"1/ � � � ///////' ///,.// /.' Mafic //////// � � � / /."///// granulites / /,'///// � � � ,1/1,'" Mantle 40 HMOHO \ � . � � � � � � � � Mantle � � � � � � � � � � Standard profile Continental crust of the continental crust in Central Spain Fig. 2. Standard profile of the continental crust from Wcdcpohl (I compared with th e es timated continental crust in central Spain. Data ["om \'I'eclepohl are derivecl from intcgratecl 3000 km long European clata ["orn central Spain on the bases of geophys ical and petrological arguments.
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