Revista Brasileira de Geociências 17(4):366-371 , dezembro de 1987

FROM OROGENIC TO ANOROGENIC MAGMATISM : A PETROLOGICAL MODEL FOR THE TRANSITION CALC-ALKALlNE - ALKALlNE COMPLEXES

BERNARD BONIN*

ABSTRACT In many orogenic belts , the internal parts display a closely related in space and time association of calc-alkaline and alkaline igneous centres. The magmatic massifs are emplaced, during late-stage orogenic processes and post-orogenic tectonic episodes , in pull-apart molassic basins associated with large shear fault zones. When considering the nature of the igneous products in these structures, one can recognize members of orogenic suites (ca/c-a/ka/ine sensu lato) as well as of anorogenic suites (namely a/kaline and tholeiitic). Strike-slip movements and associated distensive fault zones control the locat ion and emplacement of magmatic produ cts but do not influence the nature of primary liquids. Therefore, the typical sequence: a. calc-alkaline batholith emplacement (uplift and unroofing), b. late calc-alkaline volcanic activity , c. early alkaline volcanic plateaus, and d. subsequent plutonic-volcanic alkaline ring-eomplexes has its petrogenetic explanation to be searched in deeper leveis than at the present erosion levei where it is observed.

RESUMO Em muitos cinturões orogênícos as partes internas exibem uma associação estreita no espaço e no tempo , de centros cálcio-alcalinos e alcalinos. Os maciços magmáticos são colocados, durante os estágios tardios dos processos orog ênicos e episódios tectônicos p ós-orogênicos, em bacias molássicas de pull-apart vinculadas a grandes zonas de falhamento. Quand o se considera a naturez a dos produtos ígneos nessas estruturas, podem-se reconhecer membros das suítes orogênicas (cálcio­ -alcalina sensu lato ) bem como das suítes anorogênicas (denominadas alcalina e toleítica). Movimentos transco rrentes e zonas distensivas de falhamento associado controlam a locação e a colocação dos pro­ dutos magmático s, mas não influenciam a natureza dos líquidos primários. Assim sendo, para se explicar petrogeneticamente a seqüência típica: a. colocação do batólito cálcio-alcalino (ascensão e intrusão), b. atividade vulcânica-alcalina tardia, c. platô s vulcânicos alcalinos precoces, e d. subseqüen­ tes complexos alcalino anelares plutono-vulcânicos, é necessário pesquisa em níveis mais profundo s que os atuais níveis descobertos pela erosão o permitem.

LATE- TO POST-OROGENIC MAGMATIC CENTRES Strait of the D'Entrecasteaux Islands, southeast Papua. DURING PHANEROZOIC TIMES There is abundant Lavas in this area are entirely comendite but basaltic and evidence that distinctive igneous rock association occur intermediate rock ty pes are foun d as lava blocks in in specific tectonic environrnents (for a review, see Pitcher pyroclastic deposits and as inclusions in comendite flows.The 1987). One such is the pera1kaline felsic rock association porphyritic comendites are Holocene in ageand show a close The tectonic settings of th is petrographically and geochem­ spatial association with Quaternary andesitic volcanoes. ically distinctive group of igneous rocks have been outlined Holocene pantellerite lavas and associated pyroclastics by MacDonald (1975): areas of uplift and rift formation, comprise the whole of the isolated volcano of Mayor Island more rarely of extensional tectonics, in continental as well in the Bay of Plenty on the northeastern side of North as in oceanic environments. The common factor of these Island, New Zealand. Intermediate rock types occur as lava various tectonic settings is that they are characterized by blocks and have been interpre ted as a part of the magmatic conditions of crustal -tension. In this context, the close association. spatial and tempo ral association of peralkaline felsic rocks The occurrence of obsidian in the Kaeo area ofNorthland with calc-alkalíne belts (now conceptually representing near the northern tip of North Island has been documente d converging plate boundaries and compressional tectonic since 1909 but the vent from which pantelleritic obsidian regimes) seems at a first glance to be anomalous, as stressed originated has not been identified. Obsidian blocks are by Srnith et ai. (1977). This "anomaly" is in fact a role for overlying Plio-Pleistocene basalts. the last stages of several orogenies having worked at the Srnith et ai. (1977) 'outline the general petrographical earth surface. and geochernical features of the peralkaline rhyolite associa­ tion: peralkaline rhyolites are either comendites (less Pe ralkaline rhyolites associated with andesitic peralkaline and Fe-rich, more alurninous) or pantellerites belts SOUTHWEST PACIFIC AREA Probably, the (more peralkaline and Fe-rich, less alurninous); the charac­ most recent occurrences of peralkaline volcanic rocks teristically low Ca, Ba, Sr and high contents of elements associated with andesitic volcanoes are located in three such as Th, U, and Zr support the hypothesis that peralkaline areas of the southwest Pacific (Smith et ai. 1977), namely rhyolites originate by fractional crystallization from a in the D'Entrecasteaux Islands, southeastern Papua-New transitional basalt parental magma. Guinea, and on Mayor Island and in the Kaeo area of Because it lies in the zone of interaction between the Northland, New Zealand. Indo-Australían and Pacific plates, the tectonic sett ing of Peralkaline silicic volcanic rocks, unique among the southeastern Papua has in general been interpreted in terms volcanic products of west Melanesia, occur in the Dawson of island are processes. In fact, Smith et aI. (1977) point

* Laboratoire de P étrographie-Volcanologie. U.A. 728 du C.N.R.S., Université Paris-Sud, F-9l 4DS ORSAY CEDEX, France Revista Brasileira de Geociências, Volume 17, 1987 367 out that this is an oversimplified view of a complex plate Present ly, no satisfactory explanation in terms of plate boundary which has undergone dramatic polarity shifts boundary tectonics of the elose andesite-comendite associa­ within the past 50 Ma. They prefer the following interpreta­ tion has been proposed, because of the great complexity tion of the area: the late Cenozoic andesitic volcan ism, of rnicroplates implicated in the genesis of the Western although not elosely linked to recent subduction, is a con­ Mediterranean Basin (e.g. Dewey et aI. 1974). sequence of earlier subduction and a Quaternary spreading centre lies in the Woodlark Basin immediately to the east of the D'Entrecasteaux Islands . Thus, a late Tertiary Late- to post-Hercynian magmatic activity in Permo-Triassic compressional regime in southeast Papua is at present being time Rifts and pull-apart basins related to large shear replaced from the east by a tensional regime associated with fault zones are tectonically emplaced in the Hercynian sea-floor spreading in the Woodlark Basin. Srnith et aI. orogen (Arthaud & Matte 1977). They are not directly ( 977) suggest that peralkaline rhyolite products are early associated to ocean-type extensional regime but to stríke-slip manifestations of extensional tectonics and indicate a movements induced by the ending Variscan orogenic remarkably elose response of the petrogenetic process to process and essentially by the complete modification of the a change in tectonic regime. stress field (Ziegler 1983). During the whole Carboniferous The same geodynamic explanation is proposed from times, the compressional regime had an essentially north­ Mayor Island and Kaeo pantellerites. The peralkaline silicic -south direction and , at the Carboníferous-Permían bounda­ volcanism is interp reted as a consequence of back-arc ry, significant modifications of the compressional axis have spreading, especially where spreading centres impinge on resulted in a large dextral E-W shear movements (Fig . 1). andesit ic ares,

SARDINIA The same picture can be outlined for the comendites of San Pietro Island , Sardinia, where this rock ty pe has been defined. The context is somewhat different as it implies thickened continental blocks. In this island, two magmatic suites have been emplaced at nearly the same time . The fírst ensemble comprises hype rsthene basalts, andesites and plagíoclase-bear íng rhyolites and it is markedly calc-alkal íne, The second ensemble is compositionally more restricted, with alkaline metaluminous rhyolites, comendites and pantellerites . Geochronological K/Ar data indicate that comendites, metalurninous alkaline rhyolites and calc-alkaline and esites are synchronous, yielding identical ages of about 15 Ma (Arana et aI. 1974). Mineralogical and geochernical studies evidence the role of the fractional crystallization in the differentiation of rhyolitic liquids (Laridhi 1981). Andesites and plagíoclase­ -bearing rhyolites have originated from the same mantle source yielding a Th/Ta rat io of 5-8. Alkaline rhyolites display distinctly different trace element contents and rati os, for example Th/Ta ratios of 3.5 for the metaluminous Figure 1 - Sketch map ofEurope during Stephanian-Autu­ rhyolit es, 4.0 for the comendites and 3. 2 for the nian times: Diagonal lines: inactive fold belt; cross-hatched pantellerites, indicating a distinctly different mantle source. area : active fold belt; squares: alkaline igneous centres; In her review, Laridhi (1981) shows that San Pietro cireles: calc-alkaline igneous centres. BBF- Bay ofBiscay comendites can be distinguished from comendites emplaced Fault zone; GSF - Gibraltar Strait Fault zone; TL ­ in an extensional regime by higher H20 contents (5-6 wt%), TornquistLine lower halogen contents (respectively 0.32% F and 0.39% CI) and a F/Cl ratio of 0.83. San Pietro Island is one piece of a large calc-alkaline During this period, a widespread volcanic activity is at volcanic belt emplaced in grabens in Sardinia Island work in the Hercynian orogen as well as in the Laurasian (Coulon 1977). Two volcanic cycies have been recognized cratonic foreland. This magmatic activity is not so related 50 far: to waning Variscan orogenic processes as to pre-Alpine • a subductíon-related calc-alkaline volcanism, Oligocene tectonic processes, prelude to subsequent Tethys oceanic to Middle Miocene in age (from 29 to 13 Ma). Alkaline basin opening. However , the nature of the magmatic 14-15 Ma old rhyolites (metalurninous rhyolites, products are strongly differing according to previous comendites and pantellerites), are spatially and temporally structural zones (Fig. I). In the cratonic foreland as well related to the culmination of this volcanic cyele. Note tha t as in the Hercynian orogen boundaries, the magmatism is the locus typicus for the pantellerite rock type is located in markedly alkaline since the beginning of Permian (e.g. the neighbour Pantelleria Island , but the peralkaline silicic Oslo Graben, Scottish Lowlands...). On the contrary, in volcanic products are a little younger (8 Ma). the fold belt itself, volurninous calc-alkaline volcanic • a Plío-Quaternary (3.5 Ma to present) alkaline volcanism, activity is interpreted by Mossakovsky (1970) as a magmatic represented by basalts, trachytes and phonolites. consequence of an active continental margin betweeri

L 368 Revista Brasileira de Geociências. Volume 17, 1987

Laurasia continental plate and Palaeotethys oceanic plate. Arthaud & Matte (1977) and Ziegler (1983) have questioned this interpretation at least in the western European part of the Hercynian orogen and have preferred a model of continent-continent transcurrent fault boundary. Geological, petrological and geochernical data indicate no significant change in magmatic characteristics from the Upper Carboniferous (Stephanian, ca. 300 Ma) to the Lower Permian (Autunian, ca. 280 Ma). The geological history of the western Mediterranean area of the Hercynian orogen is marked by the emplacement of huge calc-aIkaline batholiths, more or less associated with peralurninous granitoids, during the Carboniferous, followed by a rapid uplift, erosion and unroofing (Bonin et alo 1987b). During the Early and Lower Permian, volcanic episodes, yielding the same calc-aIkaline magmatic character, produced lava flows and pyroc1astic deposits, lying unconformably on peneplaned Carboniferous formations (VelIutini 1977). The :- con stant magmatic characters, evidenced by petrological . and geochemical studies, must be compared to the dramatic change in the tectonic context, postulated by tectonic and structural reconstructions. Moreover, the Middle Permian (Fig . 2) is a criticaI period as the magmatic activity becomes in alI areas definitely aIkaline (Bonin 1982, 1986) whereas the geodynamic Figure 2 - Sketch map afMediterranean regians during context, always marked by large shear faulting, is of a L ate Permian and Triassic periads: 1. principal shear zanes; complete reversal in the sense of movement of the same 2.Permian alkaline pravinces;3. Corsican alkaline pravince; transcurrent faults (Bonin 1982, 1986, Bonin et ai. 1987a, 4. Matterharn transitianal pravince; 5. Triassic alkaline 1987b). From the Middle Permian to the Upper Triassic provinces ; 6. Triassic tholeiitic provinces; 7. lines of (275 a 200 Ma) (Fig. 2), early volcanic lava plateaus are continental fragmentation and subsequent oceanic intruded by numerous aIkaline ring-complexes, constituting opening;and 8. transcurrent faults with indication oftheir the Western-Mediterranean aIkaline province (Bonin 1985). senseofmovement The magmatic rock types are dorninantly Krich ignimbrites (Estérel, Corsica, Central Alps) and subsolvus biotite granites (Corsíca, Baveno pink granite in Italy, Alps, Morocco...). CALC·ALKALlNE- A LKALlNE T RAN SIT ION ANO Minor but significant occurrences of monzonites (Monz oni­ CRUSTAL ACCRET ION OURING UPPER PROT ERO· -Predazzo massif in Italy, Corsica), peraIkaline granites ZOIC TIMES A worldwide orogenesis has occurred in (Evisa area in Corsica) and basic-íntermedíate rock types the Upper Proterozoic, its name changes as a function of (Corsica, Alps) substantiate the aIkaline character of the the location of studied areas: Pan-African in African and magmatic association. neighbo uring Arabia, Brazilian in South America, Cadornian This alkaline province predates the Tethys oce anic in West Europe... but its modern characters, in terms basin opening by a very short time interval: the latest of global plate tectonics, are particularly noteworthy aIkaline products are 200 Ma old , the earliest tholeiitic (Black 1984). differentiates are 180 Ma old. Geochemical and isot opic studies of the alkaline province (Bonin 1982, 1986, Bonin Cadomian and Cambrian granitoids in Armoricain Massif, et ai. 1987a) reveal that, if minor (less than 10%) crustal France The northern region of the Massif Armoricain, contamination by wall-rock diffusion at the magma located in French Brittany and Normandy, displays the chamber leveI and leaching by geothermal waters at oldest geologic formations recorded in this part of the the ring-complex leveI could have occurred, the parent Caledo no-Hercynían orogen. Lower Proterozoic gneissic magma is mantle, as evidenced by the lowest Sr isotopic basement has yielded ages of ca. 2,000 Ma, comparable to ratio (0.703) recorded for an albitic peraIkaline granite Eburnean described in Africa (Cogne & Wright 1980). No (Evisa complex, Bonin et ai. 1978). Neither a genetic magmatic activity has been recorded during the 1,700-1 ,000 derivation from calc-alkaline magmas nor a significant Ma, indicating that Massif Armoricain is closely related to crustal contribution by partial melting to produce aIkaline the south-atlantic domain (Eburnean and Trans-Amazonian magmas are substantiated by geochemical data (Bonin cratonic areas). et ai. 1987a). Upper Proterozoic formations are welI preserve d and Thus, in Perrnian times, a calc-alkaline aerial volcanism not strongl y affected by subsequent Caledono - Hercynian has been folIowed, less than 10 Ma after, by aIkaline aerial orogenies. At the Upper Proterozoic - Cambrian boundary, volcanic and hypabyssal plutonic activities. The exceptional intense magmati c activity have been related to south­ Baveno complex (Caíroni et ai. 1985) displays rnixed and east wards subduction processes (Auvray 1979). A volcanic nearly synchronous calc-alkalíne and aIkaline products . ensemb le, dat ed at 650 Ma and composed of a calc-alkaline Moreover, for the 280-270 Ma period, synchronous activity spillite-keratophyre suite, is intruded by large batholithic of discrete igneous centres displaying contrasting magmatic ma ssifs, dated at 615 Ma (Graviou 1984). Diorites, associations cannot be prec1uded. granodiorites and granites constitute a composite and linea r Revista Brasileira de Geociências. Volume 17, 1987 369

plutonic belt from the Tregor, in Brittany, to the Channel In the northern prolongation of the Adrar des Iforas , Islands and the Cap de la Hague, in Normandy. Trace the Trans-Saharan mobile belt displays similar features in element data (Graviou 1984) indicate that calc-alkaline Ahaggar (Boissonnas 1973). The "Taourirt" granites granitoids have originated from a partia I melting of constitute a graniti c belt showing calc-alkaline affinities lherzolite upper mantle in the presence of water percolat ing and forcefui intrusions for the earliest phases, and alkaline from the subducting hydrated oceanic plate ; no significant affinities and ring-complexes for the larest. Two plutonic crustal contribution is evidenced . massifs are made up of mixed and nearly synchronous Subsequent aIkaline volcanic-plutonic fonnations are calc-alkaline and aIkaline rock types : Tioueiine and described, as small stocks made up of subsolvus biotite Ti-n-Erit. In the Tioueiine complex , ring-dykes composed granites, dyke swarms of microgranite and ignimbritic lava of alkaline syenites and hypersolvus granites are emplaced piles (Auvray 1979). Alkaline granites have been dated at into alkaline hyperso1vus-subsolvus granites and a central 552 ± 8 Ma and ignimbrites at 547 ± 57 Ma, therefore monzogranite showing some calc-alkaline affinities. The substantiating their synchronous emplacement time. Sr Ti-n-Erit is a1so made up of a central monzo granite and of isotopic initial ratios range from 0.706 to 0.710, suggesting peripheral alkaline hypersolvus granites and quartz-syenítes. that crustal contribution was only minor. Age group ing Field observations indicate ambiguous relations, as in around 550 Ma is also found for some calc-alkaline bodies Tioue iine, calc-alkalíne granite postdates alkaline granitoids in the Channel Is1ands. whereas, in Ti-n-Erit, the reverse is true. A 560 Ma age has Note , however, that, at nearly the same periods , a determined by Rb/Sr isochron method. In the nearby markedly orogenic but located in a more continental craton, the aIkaline complexes of In-Zíze and Gara environment magmatism has taken place in the Mancellia Adjemamaye yield a little younger ages. region (Jonin 1981). Calc-alkaline cordierite-bearing granodio rites yield ages of 615-580 Ma and subsequent A PETROGENETIC MODEL FOR THE MAGMATIC 1eucogranites are a little younger: 525 Ma. Crusta1 contribu­ TRANSITION CALC-AL KALl NE-ALKALlNE During tion in the generation of1eucogranitic magmas is evidenced the late orogenic stages as well as during the first tensional by the high Sr isotopic initial ratio (0 .716). Synchronous stages, shear fault zones contro1 the site of emplacement ignimbritic volcanic emissions are widespread throughout of magmatic provinces. Pressure release caused by the the Mancellia region (Boyer 1974). . fractu ring can enhance and/or produce a partial melting in deep zones in the thickened orogenic lithosphere and in underlying asthenosphere. However, the nature of the Crostal accretion at the Upper Proterozoic in Pan-African source rocks und ergoing partial melting plays a significant domains The Pan-african orogeny iswidespread through­ role in the natur e of primary magmas (Fig. 3). out in Africa: it consists in many cratonic domains affected The late- to post-orogenic high-K calc-alkaline magmatic by thennal rejuvenation effects and associated anatectic provinces canno t be considered as directly subduction­ magmatism and in the Trans-Saharan belt which has been -related , as during their time of differentiation and em­ proved to be a fine example of crustal accretion (B1ack placement , only regional uplift after continent-continent et a/o 1979). collisional events is recorded. Presently , no satisfactory The Adrar des Iforas (Mali) batholith disp1ays, as in a experimental data have been obtained on the production similar context in Saudi Arabia, a very fast transition from of calc-alkaline liquids with thermodynamic conditions calc-alkaline subduction-related and late-orogenic plutonic postulated by post-collisional uplift proce sses. Liegeois & magmatism toward alkaline intra-plate magmatism (Liegeois Black (1984) have demon strated that the source rocks for & Black 1984). Oceanic closure around 600 Ma led to a the calc-alkaline liquids producing the huge batholith of collision between the passive continental margin of the the Adrar des Iforas (Mali) at the end of the Pan-African West African craton and the active continental margin orogeny are likely to lie in a lithos pheric subducted unit to the East (Adrar des Iforas) displaying is1and are and (Fig. 38). cordilleran volcanoc1astic assemblages bordering a defonned Calc-alkaline magmas (for a review, see Sekine and continental mass intruded by a huge composite batholith. Wyllie 1982) are produced from : Four families of magmatic events have been fixed: 1. cale­ • melting of the subducted hydrated oceanic crust, -alkaline pre-tectonic (before 600 Ma); 2. calc-alkaline • melting of mantle peridotite flushed by aqueous fluids late-tecto nic (600-590 Ma); 3. calc-alkaline post-tectonic rising from the subducted oceanic slab, (590-540 Ma) ; and 4. alkaline. post-tectonic (580-540 Ma). • hybridization with hydrous siliceous melts generated in The low Sr isotopic initial ratios (0.7035-0.7060) for all the. oceanic crust and then reacting with the overlying the plutons indicate mantle origin with a possible contribu­ peridotite, followed by subsequent partial melting of wet , tion from lower crust but without contamination by old hybrid olivine-free pyroxenites. supracrustal rocks. Sekine & Wyllie (1982) have presented experimental The petrogenetic model , based on geochemical dat a evidences of the third petrogenetic mode!. In this hypothe­ obtained in Adrar des Iforas (Mali), proposes: a lithospheric sis, calc-alkaline magmas can be produced as long as hydrous (upper mantle with some contributions from subducted sileceous melts, resembling rhyoda cites in composition can oceanic crust and possibly from lower crust ) source be­ "be extracted from the dehydrating subducted oceanic crust. coming deeper with time in the same region for the cale­ At depths of 100-150 km postulated for the melting zone -alkaline families and an asthenospheric source for the in the subducted slab, water solubility in silicate liquids alkaline family, with a possible contribution from the can reach 25 wt% in Sekine & Wyllie's experi ments. As it depleted lithospheric source for the earliest of the alkaline is likely that the oceanic crust contains less than 25% H20 , complexes. In this model, two contrasting sources are it can be conc1uded that each batch of magma results in required successively to account with the magmatic families. drying its source. Therefore, calc-alkaline magmas have 370 Revista Brasileira de Geociências, Volume 17, 1987

lOW_K CAlC_ALKAlINE merely a mantle source but must receive a significant crustal contribution. During ocean-continent convergence process (Fig. 3A), the production of calc-alkalíne magmas is made easy by a continuous influx of water inducing the production of hydrous siliceous melts in the subducted slab, and the subsequent hybridization of the overlying mantle . When a contínent-contínent collision occurs after the oceanic closure (Fig. 3B), the water influx decreases and then disappears as the subducted oceanic slab becomes more and more dehydrated and, therefore, unable to A SUBDUCTION produce significant amounts of water-rich siliceous melts. (100 Ma) Then, neither hybridization process can occur in the overlying mantle nor calc-alkalíne magma be extracted. CALC_ ALKALI NE In the Pan-African orogeny as well as in Phanerozoic times, it appears that, 30-50 Ma after the end ofthe subduc­ tion process, no calc-alkaline magmatic response is possible. An important conclusion is that any calc-alkaline magmatic activity is related to a 30-50 Ma old subduction process, even if a synchronous subduction is well established (as in the case of the Andine batholith, Pitcher 1986). A complete reversal of thermodynamic conditions can be outlined for the production of alkaline liquids (Fig. 3C). "Liegeois & Black (1984) have evidenced a new LIL-enriched but water-poor asthenospheric source for the alkaline volcanic plateaus and plutonic ring-complexes of the ALKALlNE Adrar des Iforas (Mali). The same result has been obtained for the Permo-Triassic Western Mediterranean province (Bonin et ai. 1987a, 1987b). However, a crustal contamina­ ~~..~ tion during the ascent and the differentiation of alkaline liquids cannot be precluded: íf primary liquids are only ------.:..:""-:..:.

ascent and differentiation in the crust , complex fluid-liquid­ • Alkaline magmatic events are synchronous with tensional -rock interactions control the evolution of residual liquids tectonics and can overlap with the latest calc-alkaline and the typical K-enrichment. intrus ions. • If water is unavailable at the asthenospheric levei, alkaline • It takes about 500 Ma after an orogeny to dissipate liquids are produced from a LIL-enriched mantle . When significantly Hp in the crust and to observe a dramatic staying in a magma chamber emplaced in the crust , primary change in the nature of the felsic end-members (granites be­ liquids are evolving under the control of crustal waters: fore, nepheline syenites with associated carbonatites after). in a newly formed continental crust, meteoric, cognate and • After this criticai 500 Ma períod, the crust becomes interstitial waters are present in great amounts, so that truly cratonic as far as magmatism is concerned. evolved liquids are granitic in composition; on the contrary, This model is valid for all orogenic cycles related to plate in old catazonal dehydrated crusts , insufficient water tectonics, in the Phanerozoic as well as in the Upper supplies together with abundant CO2 fluids lead to Proterozoic. Similar features can be outlined in even older feldspathoidic evolved liquids . mobile belts : intruding the Trans-Amazonian forrnations in • It can exist a criticai time when calc-alkaline magmas, Brazil, numerous complexes, often showing ring-structures, issued from waning subduction-related melting zones, are displaying alkaline and peralkaline granites with associated emplaced synchronously with alkaline magmas, originating basic and intermediate rock-types and postdating 2,000 Ma from a fresh Lll-rich mantle source. old h ígk-K calc-alkaline batholiths, have been recorded. A time control is evidenced by space and time relation­ Any attempt of geodynamic reconstructions of the ships of magma series: Proterozoic event s must take into account the systematic • Subduction-related calc-alkaline magmatism can survive association of calc-alkaline magmatic formations shortly to the end of the subduction processes as long as 30·50 Ma. followed by alkaline volcaníc-plutonic products.

REFERENCES

ARANA, v.; BARBERI, F.; SANTACROC E, R. - 1974 - Some COULON, C. - 1977 - Le volcanisme calco-alcafin c énoz oique de data on the comendite area (San Pietro and San Ant ioco Sardaigne (Itafie): pétrographie, géochimie et gen ése des laves islands , Sardinia). Bull. Volcanol., 38 :715-736 . and ésitiques et de s ignimb rites. Signification géodynamique. ARTHAUD, F. & MATTE , Ph. - 1977 - Late Paleozoic strike-slip Marseille, 350 p. (Thêse Doct. Etat , Univ. Aix-Marseille IIl ). faulting in southern Europe and northern África: Result of a DEWEY, J. F.; PITMAN IIl, W.C.; RYAN, W.B.F.; BONNIN, J.­ right-Iateral shear zone between the Applachians and th e UraIs. 1974 - Plate tectonics and the evolution of the Alpine systern. Geol. Soe. Amer. Bull., 88 :1305 -1320 . Geol. So e. Amerc. Bull., 84 :3137-3180. AUVRAY, B. - 1979 - Gen êse et évolution de la croú te continen­ GIRET, A.- 1983 - Le plutonisme océanique intraplaqu e, ex ­ tale dans le Nord du Massif Armoricain. Renn es, 681 p, (Th êse emple de l'archipel Kerguelen, Terres Australes et Antarctiques Doct. Etat, Univ. Renne s). Françaises. Paris, 290 p. (Thêse Doct. Etat , Univ, P. et M. Curie). BLACK, R. - 1984 - The Pan-African event in th e geological GR AVIOU, P' - 1984 - Pétrogen êse des magmas calco-alcalins: f frame work of Africa . Pangea, 2:8-16. exemple de s grani torde s cadomiens de la région trégorroise BLACK, R.; CABY, R.; MOUSSINE-POUCHKINE, A.; BAYER, R.; (Massif Armoricain). Rennes, 236 p. (Th êse Doct. 39 cycle, BERTRAND, J.M.L. ; BOULLlER, A.M.;FABRE, 1.; LES· Univ. Renne s). QUER , A. - 1979 - Eviden ce for Precambrian plat e tectonics JONI N, M. - 1981 - Un batholite fini-précambrien: le batholite in West-Africa. Nature, 278 :223·227. mancellien (Massif Armo ricain, France). Êt ude pétrographique BOIS'SONNAS, J. - 197 3 - Les granites à stru ctures concentriques et géochimique. Brest, 320 p.(Th êse Doct. Etat, U.B.O.). et quelques autr es granites tardifs de la chaine pan-africaine en LARIDHl, N. - 1981 - Contribution à l' étude des laves hyperalca­ Ahaggar (Sahara Central, Algérie}. Paris, 66 2 p. (Th êse Doct. fines de I'Íle San Pietro (Sardaigne) . Orsay, 178 p. (Thése Doct . Etat, Univ. Paris Vl) . 39 cycle, Univ. Paris-Sud). BONIN, B. - 198 2 - Les granites des complex es annulaires. Orl éans, Ll EGEOIS, J.P. & BLACK, R. - 1984 - Pétrographie et géochro­ B.R.G .M., 183 p.(Manuels et Méth odes no. 4). nologie Rb-Sr de la transition calco-alcaline-alcaline fini­ BONIN, B. - 1985 - Alkaline magmatic provin ces in Corsica and in -Panafricaine dan s l' Adrar des Iforas (Mali): accrét ion crusta le w. Africa : role of continental d íslocation s, relationship with au Précambrien sup érieur. In: KLERKX , J. & MICHOT, J. eds. oceanic openings. In: COLL. ON AFR . GEOL. , 13, St. Andrews, Géologie Africaine, Tervuren, p. 115-145. 1985. Abstracts... C.I.F.E.G. Occas. Publ. , 3, p. 268-269. MacDONALD, R. - 1975 - Tectonic settings and magma associa­ BONIN, B. - 1986 - R ing complex granites and anorogenic mago tions. Bull. Volcano l., 38:575-593. matism. Oxford, North Oxford Academic Publishers Ltd. , MASSAKOVSKY , A.A. - 1970 - Upper Paleozoic volcanic belt of 188 p. (Studies in Geology). Euro pe and Asia. Geotectonics, 4:24 7-253 . BONIN, B.; GRELOU-GRSINI, C.; VIAL ETTE, Y. - 1978 - Age, PITCHER , W.S. - 1986 - Granites and yet more granites forty origin and evolution of the anorogenic complex of Evisa years on. Geol. Rdsch.• 76:51-79. . (Corsica): a K-Li·Rb·Sr study, Contrib. Min eral. PetroI., 65 : SEKINE, T. & WYLLl E, P.J .- 1982 - The systern granite-perido ­ 425-4 32. tite-H20 at 30 kbar, with appli cations to hybridization in BONIN, B.; PLATEVO ET, B.; VIAL ETTE , Y. - 198 7a, in press ­ subductio n zone magmatism. Contrib, Mineral. Petrol., 81 : Th e geodynamic significance of aIkaline magmatism in Corsica 190-202 . and West Africa. In: BOWDEN, P. & KINNAIRD, J.A. eds, SMITH, I.E.M.; CHAPPELL , B.W.; WARD, G.K.;FREEMAN, R.S. African Geology Reviews, Wiley and Sons, London. - 1977 - PeraIkaline rhyolites associated with andesitic ares BONIN, B.; VISONA , D.; KEKELlA , M.A. - 198 7b , in press ­ of th e southwest PacificoE.P.S.L. . 37 :230·236. Hercynian Plutonism. In : F.SASSI ed. I.G.C.P. 5 "Thematic VELLUTINI, P.J.- 1977 - Le magmatisme permien de la Corse du Volume", UNESCO. Nord-Ouest. Son ext ension en Méditerranée occi dentale. BOYER, C. - 1974 - Volcanisme s acides paléozo iqu es dan s le Marseille, 257 p. (Th êse Doct. Etat, Univ. Aix-Marseille 1Il). Massif Armoricain. Orsay, 435 p.(Thêse Doct, Eta t, Univ. ZIEGLER, P.A. - 1983 - Caledonian and Hercynian crustal consol­ Paris-Sud). idation of the European Alpine foreland . Terra Cognita, 4: CAIRONI,V.F.; BORIANI, A.C.; GIOBBI-GRIGONI, E.; ODDONE 5 1-58. M. - 1985 - The Late-H ercyn ian granites of Lago Maggiore (Italy) and their geological setting. Terra Cognita, 6 :37. COGNE,J. & WRIGHT. A.E. - 1980 - L'orogê ne cadomien. In: MANUSCRITO 438 INT. GEOL. CONGR ., 26, Paris, 1980. Colloqu e " Géologie de Recebido em 11 de março de 1987 l'Eu rop e" , p. 29·55. Revisão aceita em 24 de abril de 1987