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Cadernos Lab. Xeolóxico de Laxe ISSN: 0213-4497 Coruña. 2003. Vol. 28, pp. 213-229

Amphibolite vs. banded amphibolite: a case study in the São Martinho-Arronches area, Tomar Cordoba Shear Zone, NE Ossa Morena Zone, Portugal Anfibolita versus anfibolita bandeada: el caso de São Martinho-zona de Arronches, Zona de Cizalla de Tomar Cordoba, Zona NE de Ossa Morena, Portugal

DE OLIVEIRA, D. P. S.1; WIECHOWSKI, A.2; ROBB, L. J.3 & INVERNO, C. M. C.4

Abstract Amphibolites and banded amphibolites form an integral part of the Série Negra rocks that outcrop within the Tomar Cordoba Shear Zone that forms the boundary between the Ossa Morena Zone and the Central Iberian Zone. The terms amphiboli- te and “ ” (here changed to banded amphibolite) have been regularly used by exploration geologists when referring to the amphibolitic-type lithologies, with or without a marked schistosity, that outcrop in the Ossa Morena Zone, e.g. in the vicinity of the São Martinho gold prospect. In this study the term amphibolite is used for homogeneous, massive-texture rocks at the mesoscopic scale whereas ban- ded amphibolite refers to rocks that exhibit a distinct banding at the outcrop scale. Microscopically both rock types show a schistosity, which is better defined in the banded amphibolites. These rock types have a rather similar mineralogy. Geochemically, in terms of their major element composition, there is little differen- ce between them. However, each exhibits a distinct mineral chemistry: the amphi- boles in the amphibolites are -magnesiohornblende while those in the ban- ded amphibolites are mostly magnesiohornblende and minor tschermakite. Likewise, the in the amphibolites plot in the - and labra- dorite-bytownite fields while those in the banded amphibolites are largely constrai- ned to the fields with a few outliers in the andesine and bytownite fields. Cadernos Lab. Xeolóxico de Laxe Coruña. 2003. Vol. 28, pp. 213-229

Tentative geochemical signatures for protolith nature and tectonic setting indicate a pro- bable basic protolith for both amphibolites and banded amphibolites although the tex- tures preserved in the banded amphibolites may also indicate a sedimentary-tuffaceous origin. Key words: Amphibolite, banded amphibolite, Série Negra, Tomar Cordoba Shear Zone, NE Ossa Morena Zone. (1) Economic Geology Research Institute-Hugh Allsopp Laboratory (EGRI-HAL), University of the Witwatersrand, Private Bag 3, WITS 2050, Rep. South Africa. Present address: Instituto Geológico e Mineiro, Apartado 7586, 2721-866 Alfragide (Lisbon), Portugal (2) Department of Mineralogy and Ore Deposit Research, RWTH Aachen, Wüllnerstr. 2, D-52062 Aachen, Germany (3) Economic Geology Research Institute-Hugh Allsopp Laboratory (EGRI-HAL). University of the Witwatersrand, Private Bag 3, WITS 2050, Rep. South Africa (4) Instituto Geológico e Mineiro, Apartado 7586, 2721-866 Alfragide (Lisbon), Portugal CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 215

1. Introduction between amphibolites and banded amphi- bolites and makes tentative observations The terms amphibolite versus “amphibole regarding the protolith nature of these schist” (here modified to banded amphibo- rocks from samples collected from outcrop lite) have been regularly used by gold and trenches in the area (figure 1) to inves- exploration companies operating in the tigate whether the differences observed at Iberian Ossa Morena Zone (e.g. Rio Tinto the mesoscopic scale are also evident at the Zinc, Portuglobal-Explorações Mineiras, microscopic scale, in view of the economic Lda., Auspex Minerals Ltd.) when refe- gold potential of these rocks in the area. rring to the amphibolitic-type lithologies that outcrop in the gold prospect areas, as is the case of São Martinho, east of Alter 2. GEOLOGICAL AND STRUCTU- do Chão. These amphibolitic-type litholo- RAL SETTING gies (s.l.) are locally hosts to (significant) The study area is located within a poly- gold mineralisation on their own or at the gon that has at its extreme limits the transition between them and bioti- towns of Alter do Chão and Arronches in te of the Série Negra (de OLIVEI- the W and SE respectively (figures 1 and RA, 2001). 2). The Série Negra (Black Series) is a pac- In São Martinho, amphibolites and kage consisting of metasedimentary (meta- banded amphibolites at times outcrop- arenites and metapelites), basic igneous ping adjacent to each other, form part of (amphibolites and banded amphibolites) the Série Negra rocks that outcrop within and felsic volcanic (metarhyolites) rocks the Tomar Cordoba Shear Zone (TCSZ) in (e.g. OLIVEIRA et al., 1991; de OLIVEI- the NE Ossa Morena Zone. The amphibo- RA, 2001). The Série Negra occurs juxta- lites and banded amphibolites outcrop posed on both the north and south limbs of south of the Blastomylonitic Belt where an asymmetric flower structure that con- the Série Negra rocks have reached amphi- tains, from north to south, low-grade bolite facies as opposed to metamorphic rocks ( facies) to those outcropping north of the intermediate-grade metamorphic rocks Blastomylonitic Belt that have only rea- (amphibolite facies) separated by a central ched greenschist facies metamorphic corridor of high-grade metamorphic rocks grade. In light of the potential economic (the Blastomylonitic Belt; figure 2) chris- importance of these rocks and the distinc- tened initially as the TCSZ. The TCSZ tion between the terms amphibolite and is, however, conventionally understood banded amphibolite (“amphibole schist”) today to be the zone located immediately that have crept into company reports on southwards of the Central Iberian Zone the area and other metallogenic studies, up to and including the Barreiros tecto- this paper, that does not intend to be a nised granitoids (e.g. OLIVEIRA et al., paper on fine metamorphic petrology, exa- 1991; figure 2). mines and records differences in texture, The TCSZ is a geologically complex bulk geochemistry and mineral chemistry and diverse zone of intense deformation 216 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Figure 1. Polygon of the São Martinho-Arronches area with sample locations. and metamorphism contemporaneous Morenos-Caia Subdomain, Assumar with a large sinistral displacement, which Subdomain and the Alter do Chão-Elvas may be due to a large intracontinental Subdomain. Each of these subdomains is a sinistral fault active during the Variscan thrust fault-bounded package of rocks and Orogeny (BERTHÉ et al., 1979) with the thrust faults and vergence are arranged sinistral displacements of 100 (BURG et in a flower structure more or less centered al., 1981) to 300 km (ABALOS & in the Blastomylonitic Belt (figure 1). EGUÍLUZ, 1992). This displacement The Blastomylonitic Belt (approxima- caused mylonitisation and retrograde tely the core of this flower structure) is metamorphism (under greenschist to made up primarily of Palaeoproterozoic amphibolite facies) to all previous structu- age [2237 and 1700 Ma, by de OLIVEI- res and mineral assemblages (QUESADA RA et al., (2002) and ORDOÑES-CASA- & MUNHÁ, 1990). PEREIRA & SILVA DO (1998), respectively] migmatitic (2001) considered the Tomar Cordoba of the Campo Maior Formation. Shear Zone a major Eohercynian-Hercynian Within the TCSZ outcrops the sinistral transcurrent fault overprinting a Neoproterozoic Série Negra rocks (maxi- Cadomian arc localised at a convergent mum age for the final stages of sedimenta- margin of Gondwana. tion, ca. 565 Ma; SCHÄFFER et al., The Portuguese sector of the TCSZ 1993); the name derived as a result of the comprises a series of polymetamorphic overwhelming majority of dark-coloured structural-tectonic subdomains defined by rocks that make up this series. The Série PEREIRA in 1995 and 1999. These sub- Negra outcrops scarcely both north and domains are, from north to south, Urra- south of the Blastomylonitic Belt. Mosteiros-Ouguela Subdomain, Degolados- Stratigraphically the Série Negra is made Campo Maior Subdomain, Arronches- CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 217

Figure 2. Excerpt from the 1:500000 geological map of Portugal showing the simplified geology of the NE Ossa Morena and SE Central Iberian Zones (adapted after OLIVEIRA et al., 1992). The sha- ded area corresponds to the location of the study area that is located south of the Blastomylonitic Belt in amphibolite facies-grade metamorphic rocks (adapted after de OLIVEIRA, 2001). 218 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003) up of the (lower) Morenos and (upper) ESE of trig. beacon Travesso (figure 1; Mosteiros Formations (OLIVEIRA, et al., locations of samples DP42 and 43). Here 1991). The Morenos Formation is made the amphibolitic rock outcrops are domi- up of micaceous schists that are locally nantly massive (with local banding) in garnet-bearing, limestones and calc-silica- character with a dark colour and locally te assemblages, meta-arkoses, meta-areni- with ferruginous staining when slightly tes (quartzites) and micaceous and silice- weathered. They outcrop adjacent to ous schists, amphibolites and pyroclastic quartz-biotite schists, and black quartzi- rocks (OLIVEIRA et al., 1991). The tes of the Série Negra and are intruded by Mosteiros Formation consists of black aplitic veins, rhyolites and several apo- schists/slates, , black cherts theses of the Barreiros tectonised grani- (quartzites?), limestones and amphibolites toid (figure 2.10 in de OLIVEIRA, (OLIVEIRA et al., 1991). North of the 2001). Further to the E, near Assumar, Blastomylonitic Belt and unconformably the distinct banding of the banded overlying the Mosteiros Formation occurs amphibolites is more evident although the Urra Formation made up of a lower outcrops are limited to small occurrences porphyry unit and an upper /grey- in road cuttings or stream beds. wacke unit (OLIVEIRA et al., 1991) Two phases of Variscan deformation are usually of a pale green colour (de Oliveira, identified in this sector (GONÇALVES et 1998). At the TCSZ borders, a (Lower) al., 1972a, b). The first phase of deforma- Cambrian sequence of platform sediments tion (D1) develops folds with WSW ver- is preserved, which unconformably over- ging axial planar schistosity (S1) with a lies the Neoproterozoic Série Negra meta- tendency of the axial planes becoming sedimentary rocks (see figure 1) and con- more horizontal as one moves southwes- sists of micaceous schists, amphibolites, twards. The second phase of deformation metamorphosed carbonate rocks and peli- (D2) generated folds with subvertical axial tic schists (OLIVEIRA et al., 1991; planes or strongly dipping to the NE and PEREIRA, 1995). striking NW-SE. D2 deformation genera- The amphibolites of the Série Negra tes refolding of the D1 structures and in the vicinity of São Martinho are develops crenulation cleavage (S2) mostly hidden below a Palaeogenic clast- [PEREIRA, 1995]. supported elluvium deposit that covers The TCSZ is intruded by several pre- most of the area. Hence, in this area stu- Hercynian [e.g. the Late Cambrian dies have mostly to resort to borehole (weighted mean 207Pb/206Pb age of 508 ± samples. However, on the edges of this 8.1 Ma) Barreiros tectonised granitoids, deposit further to the SSW of trig. bea- de OLIVEIRA et al., 2002; the (ultra)basic con São Martinho (figure 1; location of to felsic intrsusions of Alter do Chão- sample DP91), small (< 2m-wide) out- Cabeço de Vide,Campo Maior and Elvas), crops of amphibolite occur. The best syn-Hercynian (e.g. the as yet to be dated exposure of amphibolites and banded elongated granitoid bodies E of Alter do amphibolites is in the railway cutting Chão; figure 1) and late- to post- CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 219

Hercynian rocks (e.g. the Nisa and Santa 4. DEFINITION WITHIN THE Eulália granite batholiths). STUDY AREA Within the study area the distinction 3. PREAMBLE is made between amphibolite and banded In order to follow our approach, a brief amphibolite. This distinction stems from review of field and microscopic features of the the more pervasive and penetrative schis- tosity (parallel to the schistosity in enclo- amphibolitic rocks follows. Amphibolites sing quartz-biotite schists) of the banded are rather massive metamorphic rocks amphibolites vs. the more massive, homo- composed chiefly of and pla- geneous character of the amphibolites at gioclase (with An 17) and are diagnostic ≥ the outcrop scale. The banded amphiboli- of the amphibolite facies of metamor- tes show better-defined segregation of phism. They are amongst the most com- quartz and crystals into discre- mon rocks formed by regional metamor- te bands. Figure 3 exemplifies these featu- phism of moderate-to- high-grade res. However, these two lithologies often (WILLIAMS et al., 1982). Amphibolite grade into each other and no sharp con- facies metamorphism applies over a tem- tacts between banded amphibolites and perature range of approximately 500-650 amphibolites are seen. Banded amphiboli- ºC and pressures of 3-10 kb (figure 2.1 in tes extend for up to tens of metres within SHELLEY, 1995). either amphibolites or quartz biotite Amphibolite protoliths are commonly schists. Though both banded amphiboli- , dolerite and . Banded tes and amphibolites host gold mineralisa- amphibolites can be formed from a variety tion, it is more common in the former. of rocks mostly from (ultra)basic to inter- mediate igneous rocks but also from 5. MINERALOGY AND TEXTURES impure calcareous and dolomitic sedimen- tary rocks (HYNDMAN, 1985). The mineralogy of the amphibolites is In thin section amphibolites show a generally similar in samples collected from outcrops and from trenches and preferred orientation of the hornblende boreholes. The only real difference betwe- prisms (WILLIAMS et al., 1982) defining en samples is the amount of alteration a schistosity, a lineation or both (SPRY, these have suffered as a result of the 1969; SHELLEY, 1995), better defined in various prograde and retrograde metamor- the banded amphibolites, where it is also phic events. well expressed mesoscopically. Banding in In both the amphibolites and banded the banded amphibolites may be produced amphibolites the grain size is extremely by segregation of constituents during heterogeneous and can range from 10 µm recrystallisation or, alternatively, may be to 250 µm in diameter. This value takes inherited from bedding or layering in into account not only the amphibole size sedimentary or igneous rocks, respectively but also that of quartz, plagioclase and (BATES & JACKSON, 1987). (when present). 220 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Figure 3. Photomicrographs highlighting the textural differences between banded amphibolite (A) and amphibolite (B). The former has a strongly developed schistosity with clear separation betwe- en alternating quartz/-rich bands (light) and amphibole-rich bands (dark) while the latter has a distinctly massive character. (Adapted after de OLIVEIRA, 2001).

Mineralogically the banded amphiboli- ber of samples also falling in the andesine tes are made up of amphibole, plagioclase and bytownite fields (figure 4B). plus minor quartz with titanite and apati- The amphibolites are more massive te with very few opaques set in a nemato- and consist of strongly coloured amphibo- blastic texture. No biotite was observed in le with mostly untwinned xenoblastic pla- the thin sections but discrete bands of bio- gioclase, epidote plus minor quartz, tita- tite from intervening schists have been nite, , and rare rutile. observed in borehole core. Microchemical Garnet is common in higher-grade amphi- analyses of the and feldspars bolites (SHELLEY, 1995), but these have (average analyses shown in appendix 1 and only been found in one sample near appendix 2 respectively) allow these mine- Assumar in a hornblende-garnet schist rals to be classified, respectively, as predo- (sample DP69, figure 1; de OLIVEIRA, minantly magnesiohornblende (with a 2001,) similar to an amphibolite in weak shift towards the actinolite and mineralogy but interpreted as a retrogres- tschermakite fields) (figure 4A) and predo- sed by PEREIRA (1999). minantly labradorite with a reduced num- Microprobe analyses of the CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 221 and feldspars in the studied amphibolites 6.2. Protolith signatures through allow these minerals to be classified as geochemistry actinolite-magnesiohornblende, while the feldspars range from oligoclase to Since the contacts between the amphi- bytownite clustering into a oligoclase- bolites and banded amphibolites are gra- andesine group and a different labradori- dational, the geochemical analyses refer te-bytownite group (figures 4A and B), mostly to amphibolites and only in three both amphibole ands plagioclase dia- cases where clear meso- and microscopic grams being distinct from the banded textural relationships were observed, ban- amphibolites case. ded amphibolites were analysed. All sam- From this purely mineralogical point ples were free of analysis-contaminating of view it would seem that both amphibo- features, e.g. quartz veins. lites and banded amphibolites in this area Geochemically these rocks range from could be derived from basic protoliths. intermediate to basic in terms of their However, other alternative protoliths for SiO2 content (table 1). Several plots, the latter would also have to be suggested, shown in figure 5, highlight some of the since the presence in the banded amphi- relationships obtained for these rocks in bolites of discrete biotite bands may indi- terms of their possible protoliths and tec- cate a possible tuffaceous protolith tonic setting. (WILLIAMS et al., 1982) and in other Using volcanic rock nomenclature cases, thin, variable bands rich in quartz, diagrams, all samples plot in the basic (carbonates) and biotite suggest a sedi- fields of the diagrams shown by figures mentary origin (SHELLEY, 1995). 5A and B. The diagram in figure 5A shows a spread of the amphibolite sam- ples from the alkaline basalt to subalkali- 6. GEOCHEMISTRY ne basalt fields while the samples of the 6.1. Methodology banded amphibolites plot on the border between the /basalt and subalka- The samples were analysed by X-ray line basalt fields. The diagram in figure Fluoresecence (XRF) at the Dept. of 5B mirrors the spread of amphibolite Geology, University of the Witwatersrand samples across the basalt and basalt-ande- in Johannesburg using a Philips PW 1400 site fields whereas the banded amphiboli- with a rhodium X-ray tube at 50kV and tes all plot within the basalt field. This 50mA. Strict quality control for major approach corroborates, in a way, the pre- elements was done using the method laid vious considerations about a dominant out by NORRISH & HUTTON (1969) basic protolith for these rocks. while quality control for the trace ele- In order to evaluate the possible magma ments was carried out using the method type the amphibolites may have been deri- laid out by FEATHER & WILLIS (1976). ved from, the set of samples has been plot- Absolute percentage errors and the stan- ted on the FeO(t)-(Na2O+K2O)-MgO dard deviation are set out in table 1. (AFM) diagram (figure 5C). The spread of 222 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Figure 4. A- Mineral classification diagram for the calcic amphiboles in amphibolite vs. banded amphibolite. Although both sets of amphiboles plot on the same field of the LEAKE et al. (1997) diagram, the amphiboles in the banded amphibolite, mostly classified as magnesiohornblende, while the amphiboles in the amphibolites cluster loosely across the actinolite and magnesiohorn- blende fields. B- Classification diagram for the feldspars in the amphibolites and banded amphi- bolites. The distribution of the types of feldspars that occur in the two lithologies is different. All diagrams plotted from data whose averages are shown in appendices 1 and 2. CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 223 the amphibolite samples across both the suggesting a within-plate versus a more tholeiitic and calc-alkaline fields of the dia- MORB tectonic setting for amphibolites gram is observed. The banded amphibolite and banded amphibolites, respectively. samples group tightly within the tholeiitic However, the large scatter and lack of dis- field (figure 5C). The diagram in figure 5D crete grouping in any of the designated shows a plot of the samples with respect to fields makes this diagram hard to interpret their possible tectonic setting, tentatively and is not conclusive.

Table 1. Major and selected minor element geochemistry used for generating figures 5A, B, C and D. Fe2O3 was converted to FeO(t) and all major element values recalculated to 100% for the pur- poses of plotting the AFM diagram. Major element values expressed in wt% and minor element values in ppm. Absolute errors for analytical methods are shown: average of 5 calibrations for major elements and 3 calibrations for trace elements. 224 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Figure 5. Geochemical relationships (after de OLIVEIRA, 2001) obtained for banded amphibolites and amphibolites. Figures A and B deal with the nomenclature of the various rocks (A - after WIN- CHESTER & FLOYD, 1977; B - after COX et al., 1979). The discrimination of tholeiitic from calc- alkaline series is shown in the AFM diagram (C - after KUNO, 1968). The Zr/Zr-Y diagram for dis- criminating between within-plate, MORB and arc is shown in D (after PEARCE AND NORRY, 1979).

7. DISCUSSION Geochemical data are inconclusive with respect to establishing a tectonic set- Amphibolite and banded amphibolite ting perhaps due to an insufficient num- constitute a suite of rocks that are clearly ber of samples collected. The observed distinguishable in the field on a mesoscopic geochemistry and mineralogy indicate a scale. However, the above mentioned probable basic protolith for both amphi- approaches and results, as well as the analy- bolites and banded amphibolites, but the tical data show that these rocks have a high protolith could be more variable for the degree of variability that is dependant on latter since they may also exist textures the mineralogy of each individual sample. akin those found in rocks of sedimentary- CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 225 tuffaceous origin as exemplified by the author and supervised by the second and presence, in the banded amphibolites, of fourth authors. We would like to ackno- both discrete biotite layers and also the wledge the contribution of Prof. F. M. abundance of distinct amphibole-rich Meyer of the Institut für Mineralogie und layers juxtaposed by plagioclase and Lagerstättenlehre (Aachen, Germany) in quartz layers. Therefore, perhaps, this supplying microprobe facilities and R. suite of rocks should be generically treated Solá of the IGM who helped with the dia- as amphibolite (s.l.) from a geochemical grams. The senior author benefited from point of view but from a textural and a Praxis XXI PhD bursary (BD/15877/98) mineralogical point of view these amphi- awarded by the Fundação para a Ciência e bolite and banded amphibolite should be a Tecnologia. Messrs. A. Gouveia and A. treated as individual entities as they have Verde are thanked for polished thin sec- been and continue to be by exploration tion preparation. Mr. J. L. Pinto is than- geologists working in the area and in all ked for help with sample collection and the Ossa Morena Zone. field work.

ACKNOWLEDGEMENTS This work represents a small part of a much larger Ph.D. undertaken at the University of the Witwatersrand Recibido: 07-XI-02 (Republic of South Africa) by the senior Aceptado: 20-I-03 226 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

Appendix 1. Average microanalytical data for amphiboles. Structural formula calculated on an anhydrous basis to cations per 23O after RICHARD & CLARKE (1990); Fe2+ and Fe3+ calculated after ROBINSON et al. (1981). [Probe: JEOL JXA.8900R; 25 nA beam current; 15 kV accelerating voltage; 1 µm beam diameter; 10 and 5s counting times (peak and background respectively)]. CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 227

Appendix 2. Average microanalytical data for feldspar. (Probe data as per appendix 1). 228 de Oliveira et al. CAD. LAB. XEOL. LAXE 28 (2003)

REFERENCES GONÇALVES, F.; ASSUNÇÃO, C. T. & COELHO, A. V. P. (1972a). Notícia explicativa da folha 33- ABALOS, B. & EGUILUZ, L. (1992). Evolución C (Campo Maior). Serviços Geológicos de geodinámica de la zona de cisalla dúctil de Portugal. Badajoz-Córdoba durante el Proterozoico GONÇALVES, F.; PELEJA, A. F. & JARDIM, J. J. Superior-Câmbrico Inferior. In: Paleozoico infe- (1972b). Geological map of Portugal, Sheet 32-B rior de Ibero-América, Gutiérrez-Marco, J.C.; (Portalegre), Scale 1:50000. Serviços Geológicos Saavedra, J. & Rábano, I. (eds.), Universidad de de Portugal. Extremadura, pp.: 577-591. HYNDMAN, D. W. (1985). Petrology of igneous and BATES, R. L. & JACKSON, J. A (eds.) (1987). metamorphic rocks, 2nd. Ed. Mcgraw-hill Book , 3rd Ed., American Company, 786 pp. Geological Institute, Alexandra, Virginia, KUNO, H.; (1968). Differentiation of basalt mag- Thompson Shore, Inc., 788 pp. mas. In: Hess, H.H. and Poldervaart, A. (eds.), BERTHÉ, D.; CHOUKROUNE, P & JEGOUZO, Basalts: The Poldervaart Teatrise on Rocks of P. (1979). Orthogneiss, and non coa- Basaltic Composition, Vol. 2. Interscience, New xial deformation granites: the example of the York, pp.: 623-688. south Armorican shear zone. Journal of LEAKE, B. E.; WOOLEY, A. R.; ARPS, C. E. S.; Structural Geology, 1: 31-42. BIRCH, W. D.; GILBERT, M. C.; GRICE, J. BURG, J. P.; IGLESIAS, M.; LAURENT, P. H.; D.; HAWTHORNE, F. C.; KATO, A.; MATTE, P. & RIBEIRO, A. (1981). Variscan KISCH, H. J.; KRIVOVICHEV, V. G.; LIN- intracontinental deformation: The Coimbra- HOUT, K.; LAIRD, J.; MANDARINO, J. A.; Córdoba Shear Zone (SW Iberian Peninsula). MARESH, W. V.; NICKEL, E. H.; ROCK, N. Tectonophysics, 78: 161-177. M. S.; SCHUMACHER, J. C.; SMITH, D. C.; COX, K. G.; BELL, J. D. & PANKHURST, R. J. STEPHENSON, N. C. N.; UNGARETTI, L.; (1979). The interpretation of igneous rocks. George, WITAKER, E. J. W. & YOUZHI, G. (1997). Allen and Unwin, London, 450 pp. Nomenclature of amphiboles: Report of the de OLIVEIRA, D. P. S. (1998). The rare earth-bea- subcommittee on amphiboles of the internatio- ring Llandeilian quartzites in the Vale de nal mineralogical association, commission on Cavalos-Portalegre area, Central Iberian Zone, new minerals and mineral names. The Canadian Portugal - Their better understanding through Mineralogist, 35: 219-246. geological mapping and rock geochemistry. NORRISH, K. & HUTTON, J. T. (1969). An accu- Comunicações, Actas do V Congresso Nacional de rate X-ray spectrographic method for the analy- Geologia, 84 (1): B142-B145. sis of a wide range of geological samples. de OLIVEIRA, D. P. S. (2001). The nature and ori- Geochimica et Cosmochimica Acta, 33 (4): 431-453. gin of gold mineralisation in the Tomar Cordoba OLIVEIRA, J. T.; OLIVEIRA, V. & PIÇARRA, J. Shear Zone, Ossa Morena Zone, Eastern Portugal. M. (1991). Traços gerais da evolução tectono- Ph.D. Thesis (Unpubl.), University of the estratigráfica da Zona de Ossa Morena. Witwatersrand, Republic of South Africa, Comunicações Serviços Geológicos Portugal, 77: 3-26. 352 pp. OLIVEIRA, J. T.; PEREIRA, E.; RAMALHO, M.; de OLIVEIRA, D. P. S.; POUJOL, M. & ROBB, L. ANTUNES, M. T. & MONTEIRO, J. H. J. (2002). U-Pb geochronology for the Barreiros (1992). Carta Geológica de Portugal 1:500000. tectonised granitoids and Arronches migmati- Serviços Geológicos de Portugal. tic gneisses: Tomar Cordoba Shear Zone, east ORDOÑES-CASADO, B. (1998). Geochronological central Portugal. Rev. Soc. Geol. España, 15 (1- studies of the Pre-Mesozoic basement of the Iberian 2): 105-112. Massif: The Ossa Morena Zone and the allocthonous FEATHER, C. E. & WILLIS, J. P. (1976). A simple complexes within the Central Iberian Zone. PhD method for background and correction thesis, Swiss Federal Institute of Technology of spectral peaks in trace element determina- Zürich, ETH Nº 12.940, 233 pp. tion by X-Ray fluoresecence spectrometry. X- PEARCE, J. A. & NORRY, M. J. (1979). Ray Spectrometry, 5: 44-48. Petrogenic implications of Ti, Zr, Y and Nb CAD. LAB. XEOL. LAXE 28 (2003) Amphibolite vs. banded amphibolite 229

variations in volcanic rocks. Contributions to DOOLAN, B. L. (1981). Phase relations of Mineralogy and Petrology, 69: 33-47. metamorphic amphiboles: Natural occurrence PEREIRA, M. F. C. de C. (1995). Estudo tectónico da and theory. Mineralogical Society of America- megaestrutura de Crato-Arronches-Campo Maior: A Reviews in Mineralogy, 9B: 1-228. Faixa Blastomilonítica e limite setentrional da Zona SCHÄFER, H. J.; GEBAUER, D.; NAGLER, T. F. de Ossa Morena com o autóctone Centro Ibérico & EGUILUZ, L. (1993). Conventional and ion- (Nordeste Alentejano). MSc thesis (unpubl.), microprobe U-Pb dating of detrital zircons of Faculty of Science; University of Lisbon, 108 pp. the Tentudía Group (Série Engra, SW Spain): PEREIRA, M. F. C. de C. (1999). Caracterização da implications for zircon systematics, stratigraphy, estrutura dos domínios setentrionais da Zona de Ossa tectonics and the Precambrian/Cambrian boun- Morena e seu limite com a Zona Centro Ibérica, no dary. Contributions to Mineralogy and Petrology, nordeste alentejano. Ph.D. thesis (unpubl.), 113: 289-299. University of Évora, 114 pp. SHELLEY, D. (1995). Igneous and metamorphic rocks PEREIRA, M. F. & SILVA, J. B. (2001). A new under the microscope. Classification, textures, micros- model for the Hercynian Orogen of tructures and mineral preferred orientations. Gondwanan France and Iberia: discussion. Chapman & Hall, 445 pp. Journal of Structural Geology, 23: 835-838. SPRY, A. (1969). Metamorphic Textures. Pergamon QUESADA, C. & MUNHÁ, J. (1990). Press, 360 pp. Metamorphism. In: Pre-Mesozoic Geology of WILLIAMS, H.; TURNER, F. J. & GILBERT, C. Iberia, Dallmeyer, R.D. & Martinez Garcia, E. M. (1982). Petrography. An introduction to the (eds.), Springer Verlag Berlin Heidelberg, pp.: study of rocks in thin sections (2nd Ed.). W.H. 314-319. RICHARD, L. R. & CLARKE, D. B. (1990). Freeman and Company, S. Francisco, 626 pp. AMPHIBOL: A program for calculating struc- WINCHESTER, J. A. & FLOYD, P. A. (1977). tural formulae and for classifying and plotting Geochemical discrimination of different analyses of amphiboles. American Mineralogist, magma series and their differentiation products 75: 421-423. using immobile elements. Chemical Geology, 20: ROBINSON, P.; SPEAR, F. S.; SCHUMACHER, 325-343. J. C.; LAIRD, J.; KLEIN, C.; EVANS, B. W. &