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