1 CASSINIS2 (Boll. Vol. 27 Fasc ) Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 27, No. 3 (2008), pp , 6 figs., tab. Queste bozze, corrette e accompagnate dall allegato preventivo firmato e dal buono d ordine, debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, ROMA Permian to Triassic geodynamic and magmatic evolution of the Brescian Prealps (eastern Lombardy, Italy) G. CASSINIS (*), L. CORTESOGNO ( ), (**), L. GAGGERO (**), C.R. PEROTTI (*) & L. BUZZI (**) ABSTRACT Significant igneous events mark the geodynamic evolution of the Lombardy Southern Alps from Late Palaeozoic to Mesozoic times. During the Early Permian, a transtensional geodynamic regime developed along the southern Palaeoeurope, due to a dextral transform margin between Gondwana and Laurasia. The calc-alkaline, intermediate to acidic volcanics and alluvial to lacustrine deposits infilling fault-bounded pull-apart basins, are accompanied by widespread granitoids and minor gabbros from asthenospheric upwelling and crustal contributions. During the Late Permian to Anisian, an extensional regime linked to plate reorganization, and likewise to the opening of the Meliata back-arc basin to the east, gave rise to a second sedimentary cycle after a stratigraphic gap of about Ma. The siliciclastic and marine sedimentation, representative of a rapid shallow transgression from the east and lacking volcanic activity, was associated with a progressive peneplanation. The transition to epicontinental arc/back-arc conditions, due to the NW-wards subduction of Palaeotethys between Anisian to Carnian times, was defined by the extensive regression of the Bovegno «Carnieule», locally represented by evaporites and extra-formational clastics. In particular, the Ladinian-Carnian interval was characterized by a new magmatic cycle, with (base to top): () fine-grained rhyolitic tuffs («pietra verde» Auct.) interbedded with limestones; (2) large volumes of acidic high-k calc-alkaline to shoshonitic subvolcanics, rare intermediate dykes and abundant extrusives; (3) high-k calc-alkaline to shoshonitic subaerial volcaniclastic rocks. A marked geodynamic change probably related to late Carnian is suggested by transitional basaltic dykes (27±3 Ma) and lavas. This rifting event preludes the opening of the Neotethys Ocean. KEY WORDS: Brescian Prealps, Permian, Triassic, stratigraphic record, magmatic activity, geodynamic evolution. RIASSUNTO Evoluzione magmatica e geodinamica permo-triassica delle Prealpi bresciane (Lombardia orientale, Italia). (*) Department of Earth Sciences, University of Pavia, Via Ferrata, Pavia, Italy. (**) Department for the Study of the Territory and its Resources, University of Genoa, Corso Europa, Genova, Italy. L evoluzione geodinamica del Sudalpino Lombardo è caratterizzata, nel tardo Paleozoico e nel Trias, da significativi eventi magmatici. Durante il Permiano Inferiore, e cioè dopo le fasi deformative principali dell orogenesi varisica, si sviluppò un regime geodinamico transtensivo lungo il settore meridionale della Paleoeuropa, dovuto all esistenza di un margine trasforme destro tra il Gondwana e la Laurasia. La contemporanea messa in posto di vulcaniti intermedieacide calcalcaline e di sedimenti alluvio-lacustri che colmano bacini di pull-apart delimitati da faglie, è accompagnata dall intrusione di estesi granitoidi e di una minore quantità di gabbri legati ad una risalita astenosferica e contaminati da apporti crostali. Tra il Permiano Superiore e l Anisico, un regime estensionale connesso alla riorganizzazione di placche crostali, e similmente all apertura verso est del bacino di retro-arco di Meliata, originò un secondo ciclo sedimentario dopo una lacuna stratigrafica di circa Ma. La sedimentazione silicoclastica e marina, che testimonia una rapida trasgressione da est ed è priva di apporti vulcanici, fu associata con una progressiva peneplanazione del territorio. La transizione a condizioni epicontinentali di retroarco, dovuta alla subduzione verso NW della Paleotetide tra l Anisico e il Carnico, fu segnata all inizio dall ampia regressione della Carniola di Bovegno, rappresentata localmente da evaporiti e dal ritrovamento in essa di clasti extra-formazionali appartenenti alle sottostanti formazioni permo-triassiche. In particolare l intervallo Ladino-Carnico fu caratterizzato da un nuovo ciclo magmatico, che include dal basso verso l alto: () tufi riolitici a grana fine («pietra verde» Auct.) interstratificati con calcari; (2) notevoli volumi di rocce subvulcaniche acide da calcalcaline alte in K a shoshonitiche, rari dicchi a composizione intermedia ed abbondanti prodotti effusivi; (3) rocce vulcanoclastiche subaeree da calcalcaline alte in K a shoshonitiche. Nel Carnico probabilmente superiore si determinò un netto cambiamento geodinamico a seguito di una fase magmatica testimoniata da dicchi basaltici transizionali (27±3 Ma) e da lave. Questo evento magmatico innescò il «rifting» responsabile dell apertura dell Oceano Neotetideo. TERMINI CHIAVE: Prealpi bresciane, Permiano, Trias, stratigrafia, attività magmatica, evoluzione geodinamica. INTRODUCTION The Southern Alps is one of the major structural units of the Alps. It consists of a south-verging thrust-belt located to the south of a very important Alpine fault system, known as the «Insubric» or «Periadriatic» Lineament, dividing the Southern Alps from the north-verging Austroalpine, Pennidic and Helvetic units of the Alpine edifice (fig. ). The Brescian Alps, in eastern Lombardy, are a key area for modelling and interpretation of nature and history of Central Alps, because they consist of a complete succession from the Variscan crystalline basement up to Cenozoic deposits. In particular, this succession with products of Permian and Triassic magmatism appears highly qualified for a palaeogeographical and geodynamic reconstruction of the northern part of the Palaeotethys belt. In this paper we will focus on the Late Palaeozoic to Early Mesozoic succession south of the Alpine Adamello intrusion with the aim to evaluate the respective geodynamic and magmatic scenarios, based also on comparisons with the coeval European context. Therefore, the Permian-Triassic evolution of the Brescian Prealps can highlight better this important and much-discussed geological period, beginning from the last deformations of the Variscan orogeny up to the Late Triassic Neotethys rifting.
2 502 G. CASSINIS ET ALII Fig. - Major structural units of the Alps. The study area, located between the Periadriatic (Insubric) Lineament and the Po Plain, is evidenced by a bold square. Principali unità strutturali delle Alpi. L area studiata, che è localizzata tra il Lineamento Periadriatico (Insubrico) e la Pianura Padana, è messa in evidenza dal riquadro. PERMIAN SEDIMENTARY AND IGNEOUS EVOLUTION As in many parts of the Southern Alps the Variscan basement is unconformably overlain by Late Palaeozoic continental rocks, which can be generally subdivided into two major tectonosedimentary cycles (ITALIAN IGCP 203 GROUP, 986; fig. 2). In the Brescian Prealps the older cycle, Early Permian in age, is made up of calc-alkaline, acidic-to-intermediate volcanics and alluvial-to-lacustrine deposits, both infilling a fault-bounded subsiding basin isolated by metamorphic and igneous structural highs, and ranging in thickness up to 500 m and more. The Collio Basin, which is connected with the collapse of the Variscan orogen and with dextral transtensional tectonics (figs. 3 and 4), can be interpreted as a pull-apart basin (CASSINIS & PEROTTI, 994; CASSINIS et alii, 997). In fact, in this view, the debated area that experienced the Early Permian (and elsewhere also Late Carboniferous) wrench tectonics was part of a mid-european dextral megashear zone (ARTHAUD & MATTE, 977; ZIEGLER, 986, 988) (fig. 5). Detailed paleostructural investigations by CADEL (986) and CADEL et alii (996) on the nearby Orobic Basin, which revealed possible transpressional inversion of the former transtensional tectonics before deposition of the Upper Permian cycle, seem to confirm this interpretation. The history of the Collio Basin, from the lowermost volcanics («Lower rhyolitic ignimbrites») up to the uppermost «Auccia Volcanics» (represented by rhyolitic ignimbrites too), which stopped its evolution, is schematically and partially summarized in fig. 6. The basin section, however, was also interfingered with meter-thick Fig. 2 - Diagrammatic, simplified non-palinspastic cross-section (see trace on the inset map) through the Permian and the Lower Triassic of the central-eastern Southern Alps. Datum line: restored top of the Anisian succession, before the beginning of Middle Triassic tectonics (from CASSINIS et alii, 2002). TSU and TSU 2 are the first and the second of the «tectono-stratigraphic units» of the South-Alpine succession, respectively. Sezione schematica, semplificata e non palinspastica (di cui è data la traccia nella cartina inserita) attraverso il Permiano e il Triassico Inferiore del Sudalpino centro-orientale. Linea di riferimento: top ricostruito della successione anisica, prima dell inizio della tettonica mediotriassica (da CASSINIS et alii, 2002). TSU e TSU 2 sono rispettivamente la prima e la seconda delle «Unità tettono-stratigrafiche» relative alla successione sudalpina.
3 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION 503 Fig. 3 - Schematic geological map of the investigated central sector of the Southern Alps including the main Permian continental basins and neighbouring areas (modified after CASSINIS et alii, 997). Carta geologica schematica dell area sudalpina indagata, con indicati i principali bacini continentali permiani e le regioni adiacenti (da CASSI- NIS et alii, 997, modificato). volcaniclastic mass-flow deposits of intermediate composition, including sparse andesite clasts, of which the source was located to the east beyond the Caffaro Valley, near the Brescia-Trento border; these asymmetric bodies, named «Dasdana beds», were interpreted as the result of the explosion of a sublacustrine rhyodacitic cryptodome disrupted and reworked in the Collio paleolake, successively topped by fallout products (BREITKREUZ et alii, 200a, b); they become rapidly thinner westwards and only some of them can be followed through the basin and used as marker beds. Extensive geochemical and petrographical investigations of all these volcanics were carried out in recent decades (PEYRONEL-PAGLIANI, 965; PEYRONEL-PAGLIANI Fig. 4 - Interpretative schematic model of the Early Permian tectonic setting of the central South-Alpine region examined, before the deposition of the Verrucano Lombardo-Val Gardena Sandstone red beds. The N direction during Permian times is also shown (from CASSINIS & PEROTTI, 994). Modello schematico interpretativo dell assetto tettonico relativo al Permiano inferiore dell area sudalpina considerata, prima della deposizione dei red beds del Verrucano Lombardo e dell Arenaria di Val Gardena, Nel grafico è indicata l orientazione del N durante il Permiano (da CASSINIS & PEROTTI, 994).
4 504 G. CASSINIS ET ALII Fig. 5 - Tectonic late-variscan framework of Europe according to ARTHAUD & MATTE (977), modified and simplified from CASSINIS & PEROTTI (994). Ricostruzione della tettonica tardo-varisica in Europa in accordo con ARTHAUD & MATTE (977), modificato e semplificato da CASSINIS & PEROTTI (994). & CLERICI-RISARI, 973; CASSINIS et alii, 975; ORIGONI- GIOBBI et alii, 979; BARGOSSI et alii, 993; CORTESOGNO & GAGGERO, 999; BREITKREUZ et alii, 200a, b). As a whole, a subalkaline, calc-alkaline affinity arises for the volcanites of the Collio Basin (e.g. in the SiO 2 vs. Zr/TiO 2 diagram; fig. 7), with the scarce intermediate volcanics showing a particularly high Zr/TiO 2 ratio. The REE patterns are relatively homogeneous, with significant fractionation of LREE and an almost flat HREE profile (fig. 8). The negative Eu anomaly is weak in all samples, especially for rhyodacites and rhyolites, compared with other Permian equivalents. The hydrothermalized shales at the top of the lower Collio Formation (Pian delle Baste member) underlying the «Dasdana I beds» (i.e. the first volcaniclastic marker cropping out within the Collio Formation; BREITKREUZ et alii, 200a), if normalized to the Average European Shale (HASKIN & HASKIN, 966), show weak enrichment in HREE (fig. 8). Gold in trace amount was also detected. In the average crust-normalized spiderdiagram (WEAVER & TARNEY, 984; fig. 9), the volcanites of the Collio Basin have a weak positive anomaly for U N and sometimes Th N, with pronounced enrichment in a reworked andesite clast. Tb N shows a positive anomaly and Y N, Yb N are enriched; a slight positive Ti N anomaly is restricted to andesites, although all other samples display small negative peaks. The negative Ba N occurs in andesites and dacites, whereas no anomaly occurs in either ignimbrite. Sr N shows a pronounced negative anomaly for almost all lithologies, even if a Sr loss can be sometimes associated with spilitization, as evidenced by parallel low Ca abundances. The lava at the basalt/basaltandesite boundary has a different pattern with a weak Sr N positive anomaly. The interpretation of the geodynamic significance of the igneous activity related to the lower cycle is quite controversial. The Permian volcanic and intrusive rocks are characterized by a calc-alkaline trend (D AMICO & MOT- TANA, 976; ROTTURA et alii, 998); this led some authors to suggest that magmatic products originated in an Andean-type continental margin (VISONÀ, 982; STILLE & BULETTI, 987; DI BATTISTINI et alii, 990), due to the Palaeotethys northwards-subducting slab (STAMPFLI, 996; VAVASSIS et alii, 2000; ZIEGLER & STAMPFLI, 200). These products, which are widespread in southwestern Fig. 6 - Palaeotectonic cross-section (see trace A-A on the inset map) through the Permian of the western Collio Basin, in the Brescian Prealps (from CASSINIS & PEROTTI, 997). Sezione paleotettonica (v. traccia nell inserto) lungo il settore occidentale del Bacino di Collio, nelle Prealpi bresciane (da CASSINIS & PEROTTI, 997).
5 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION 505 SiO 2 wt% Andesite Sub-alkaline basalt Rhyodacite Dacite Tr-and Rhyolite Trachyte Phonolite Alkbas Basanite Trachybasanite Nephelinite Comendite Pantellerite Zr/TiO Figure 7 Fig. 7 - Zr/TiO 2 vs. SiO 2 diagram (WINCHESTER & FLOYD, 977) for volcanic and subvolcanic rocks of the Collio Basin (Southalpine domain). Symbols: star: andesite dyke; filled circle: andesite flow; empty circle: andesite clasts; square: subintrusive dacite and breccia; filled cross: basal ignimbrite; triangle: rhyolite domes; asterisk: Dasdana I Beds; empty cross: Auccia ignimbrite. Diagramma Zr/TiO 2 vs. SiO 2 (WINCHESTER & FLOYD, 977) per rocce vulcaniche ed ipoabissali del Bacino di Collio (dominio Sudalpino). Simboli: stella: dicco andesitico; cerchio pieno: colata andesitica; cerchio vuoto: clasti andesitici; quadrato: dacite subintrusiva e breccia; croce piena: ignimbrite basale; triangolo: duomi riolitici; asterisco: Strati di Dasdana I; croce vuota: ignimbrite di Auccia. Europe and northern Africa (such as in Atlas, Pyrenees, Massif Central, Sardinia, Briançonnais, Ligurian and Southern Alps), as well as in central European areas (i.e. along the Variscan orogen) are generally present within or at the margins of interspaced continental basins, alternating with fluvio-lacustrine deposits. Intrusive bodies of intermediate and acidic compositions (such as, in the Southern Alps, the Biella, Baveno, Sabion, Bressanone and Cima d Asta granitoids) also occurred. Moreover, the presence of minor angular unconformities, strong erosional processes and abrupt changes in the volcanic and sedimentary sequences, confirm the stepwise tectonic development of the basin structures. However, none of the palaeostructural data shows evidence of active subduction processes in the Southern Alps domain or in other related areas of Variscan Europe (VAI & COCOZZA, 986). On the contrary, all evidence seems to indicate the existence of a transtensional regime. The contradiction between the overall geotectonic setting and the calc-alkaline orogenic affinity of the magmatic products may be explained by chemical modification of mantle sources by previous subduction processes (MASSARI, 988; DAL PIAZ & MARTIN, 998; ROTTURA et alii, 998). However, this model contrasts with the evidence that the remnants of the Cambro-Ordovician and Devonian-Dinantian lithospheric extension represent a local oceanization (BODINIER et alii, 988; BRIAND et alii, 988; VON RAUMER et alii, 990; MÉNOT & PAQUETTE, 993). Furthermore, in the Provence-Corsica-Sardinia domain and Alpine areas, as well as in Pyrenees, no evidence exists for Palaeozoic calc-alkaline andesitic magmatism associated with active margin systems. The closest in time eclogitic event developed between Ordovician and Dinantian times (radiometric ages: ~ Ma; GEBAUER et alii, 988; PAQUETTE et alii, 989; BOUCHAR- DON et alii, 989) under 750 T 850 C and.0 P GPa conditions (LIÉGEOIS & DUCHESNE, 98; PAQUETTE et alii, 989; STENGER et alii, 989; BOUCHARDON et alii, 989; CORTESOGNO et alii, 993, 998), hardly supports a direct genetic link between subduction and arc-related magmatism. The younger megasequence of the Brescian Prealps, dominated by fluvial red beds (Verrucano Lombardo, max. 500 m thick), began long after, during Mid?-Late Permian times, with a stratigraphic gap which has been tentatively estimated at about Ma in Val Trompia, but probably extending into Middle Permian (Guadalupian) times. According to MASSARI et alii (994) and MAS- SARI & NERI (997), this upper cycle also includes the Werfen/Servino formations (Induan-Olenekian p.p.) and the overlying units, up to the Anisian (Bitinian) tectonics. According to CASSINIS et alii (2007), the P-T boundary in central and eastern Lombardy coincides with a gap, estimated at about 3-4 Ma, between the Verrucano Lombardo and the Servino Formation. This second tectonosedimentary cycle was probably induced by a widespread intra-permian kinematic plate reorganization which, in the Southern Alps, was accompanied by extinction of the volcanic activity and switching of depocentres. It also shares affinities with other deposits in a number of European countries, such as Spain, France, Italy and Bulgaria. In the Brescian Prealps, crustal thinning and thermal contraction, due to the new tectonic regime and the lack of volcanism, governed the subsidence. This change probably seems compatible with an extensional phase, which led to a progressive erosion and flattening of the previous irregular topography and, successively, to a rapid shallow Tethys transgression, generally from the east, of the Lower Triassic Werfen and Servino deposits (fig. ). In this view, the tectonic activity could be related to the southern opening of Neotethys, along the northern Gondwana margin, and to the formation of the Meliata-Maliak and Svanetia back-arc basins in Eastern Europe, north of Palaeotethys. From the above, this structural reorganization is identifiable with the «Mid-Permian Episode» of DEROIN & BONIN (2003), which was characterized by specific tectonic, magmatic, thermal and basinal features. In particular, according to these authors, the large-scale strike-slip regime evolved from dextral shear in the Late Carboniferous and Early Permian to sinistral shear in the Mid- and Late Permian. Compressional movements or tilting were also recognized during this period (e.g. CASSINIS, 964; PROST & BECQ-GIRAUDON, 989; CADEL et alii, 996; CASSINIS &PEROTTI, 997; CASSINIS et alii, 995, 997; SCIUNNACH, 200). In our opinion, the «Mid-Permian Episode» was followed by a long-wavelength extensional regime in the Southern Alps. TRIASSIC SEDIMENTARY AND IGNEOUS EVOLUTION In the Brescia region, the Late Permian sedimentary cycle ended, between the Early and Middle Triassic, with a widespread regression (Bovegno Carnieule) locally characterized by evaporites and extra-formational clastics derived from subaerial areas (fig. ).
6 506 G. CASSINIS ET ALII ANDESITE DYKE AND FLOWS SUBINTRUSIVE DACITE AND BRECCIA La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ANDESITE CLASTS RHYOLITE DOMES La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu BASAL IGNIMBRITE AUCCIA IGNIMBRITE La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu DASDANA I BEDS HYDROTHERMALIZED SHALE La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 0. Fig. 8 - REE patterns (normalized to NAKAMURA, 974) for the main Collio Basin lithologies. The shale of the Collio Formation is normalized to the Average European Shale (HASKIN & HASKIN, 966). Symbols as in fig. 7. Configurazioni delle REE (normalizzate a NAKAMURA, 974) per le principali litologie del Bacino di Collio. L argillite della Figure Formazione del Collio 8 è normalizzata all Argillite Europea Media (HASKIN & HASKIN, 966). Simboli come in fig. 7. During the early p.p.-middle Anisian (Angolo Limestone), there was a lowering of the region related to block-faulting and the onset of a reliable transcurrent climax, which has also been recorded eastwards in the Dolomites and Cadore (DOGLIONI, 984, 987). Carbonate platforms occurred locally in this disconnected submarine environment but their disappear, in the late Anisian, by the broadening of a clear basin sedimentation (Prezzo Limestone; fig. ). The basin setting and the slightly younger complex facies scenario of the Triassic stratigraphic record were probably linked with the appearance and evolution of new tectono-magmatic activity, which mainly developed during Ladinian and Carnian times in the form of dykes, sills, laccoliths and lava flows. Mineralogical, petrographical and chemical data, as well as sedimentological investigations of these volcanic and sub-volcanic bodies, show variable changes and sometimes contrasting features that are difficult to interpret, from calc-alkaline to alkaline seriality and from acid to basic composition. Figs. and 2 show the most indicative samples for these Ladinian and Carnian igneous bodies in the Brescia province. LADINIAN From a general viewpoint, the Triassic igneous activity of the investigated area began in the early Ladinian, associated with the Buchenstein Formation.
7 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION ANDESITE DYKE ANDESITE FLOWS Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb ANDESITE CLASTS Rb Ba Th U K NbSUBINTRUSIVE Ta La Ce Sr Nd P Hf DACITE Zr Sm Ti Tb Y TmYb AND BRECCIA Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb BASAL IGNIMBRITE Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb RHYOLITE DOMES Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb DASDANA I BEDS Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb AUCCIA IGNIMBRITE Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb Rb Ba Th U K Nb Ta La Ce Sr Nd P Hf Zr Sm Ti Tb Y TmYb 0.0 Figure 9 Fig. 9 - Rock/Average crust normalized spiderdiagrams (WEAVER & TARNEY, 984) for the main Collio Basin lithologies. Symbols as in fig. 7. Diagrammi spider normalizzati del rapporto roccia/crosta media (WEAVER & TARNEY, 984) per le principali litologie del Bacino di Collio. Simboli come in fig. 7.
8 508 G. CASSINIS ET ALII Fig. - Schematic and simplified Upper Permian-Upper Triassic stratigraphic succession of the Brescian Prealps. The black bodies indicate Triassic igneous rocks cropping out, as dykes, sills, laccoliths, cryptodomes and subaerial extrusive products, in this South-Alpine sector. Vertical distances are time-related. Successione stratigrafica schematica e semplificata, relativa all intervallo Permiano Superiore-Triassico Superiore, delle Prealpi bresciane. I corpi neri indicano rocce ignee triassiche affioranti, sotto forma di dicchi, filoni-strato, laccoliti, criptodomi e prodotti eruttivi subaerei, in questo settore del Sudalpino. Le distanze verticali sono proporzionate ai tempi. In contrast with South-alpine sectors to the east of Lake Garda, the Anisian seems to lack igneous evidence. However, some mm-sized reworked clastics found locally in the upper Anisian Prezzo Limestone could indicate the embryonal presence of very fine volcanic products. The Buchenstein consists of well-bedded ( to 30 cm thick or more), compact, sometimes pseudo-nodular, gray and blackish limestones, with thin intercalations of graygreenish pelite (fig. ). Siliceous nodules are typically present. Mm- to cm-thick, pale green, generally graded and resedimented volcaniclastics («Pietre verdi» Auct.) also occur. The voluminous Ladinian magmatism of Mount Guglielmo is characterized by coarse volcanic breccias, with brecciated lavas and tuffs at the top. The dm-sized volcanic clasts are angular and include dacite lavas (with plagioclase, biotite and quartz), carbonatic rocks and rare fine-grained diorites; within the tuffs, mainly formed by plagioclase, biotite and quartz phenoclasts, mm-fragments of dacite lavas occur. The Ladinian brecciated lavas can be generally interpreted as relatively homogeneous andesites with plagioclase, clinopyroxene and biotite phenocrysts. The texture of the lavic clasts seems consistent with igneous crystallization under extrusive or subvolcanic conditions. Lithic clasts of sideromelane, metamorphic basement (phyllades, micaschists) and, locally, of carbonate rocks occur sporadically. The geochemical analyses of brecciated lavas, clasts of lava included in the pyroclastic rocks, and a sample of fine-grained pyroclastics indicate a trachybasalt to dacite composition, with relatively high Al 2 O 3 contents (~4-9 wt%) and low values of Fe 2 O 3 total (3.5-7 wt%). The abundance of major elements agrees with a calc-alkaline nature. The sub-alkaline characterization is confirmed by trace element correlation (Zr/TiO 2 vs. Nb/Y; WIN- CHESTER & FLOYD, 977; fig. ). The REE pattern is positively fractionated (La N /Sm N = ), whereas the heavy HREE show a nearly flat trend (Gd N /Yb N =.3-.9; fig. 2). Generally, this Ladinian activity seems connected, in the Brescian Prealps, with a polyphase but substantially unitary explosive event. In conclusion, at that time pyroclastic deposits were widely dominant, in association with abundant breccias and tuffs, and without a regular horizontal distribution. The Buchenstein and related igneous products were locally capped by the basin deposits of the Wengen Formation and, later, by the limestones of the well-developed carbonate platform of the Esino Formation, which waned at the beginning of the early Carnian (fig. ).
9 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION 509 The first unit generally consists of ill-defined black marls and marly limestones, with subordinate graygreen, sometimes graded and laminated volcaniclastic turbidites. In contrast, the Esino Formation is represented by massive gray-whitish limestones and dolostones, locally oncolitic and stromatolitic, rich in algae, gastropods and bivalves. Rhyolite Comendite pantellerite Phonolite LADINIAN-CARNIAN SUB-VOLCANIC ACTIVITY However, north of Bovegno, between the Trompia and Camonica valleys, several sub-volcanic rock bodies occur. This so-called «Montecampione group» (CORAZ- ZATO et alii, 200) consists of sills (always apophyses of major bodies) and small laccoliths. While the former subvolcanic structures were generally emplaced at the different stratigraphic levels within the whole Servino Formation, the latter, essentially represented by the Monte Muffetto, Corne di Regoia and the punched laccolith of Dosso Sparviero, were preferentially injected near the contact between the Verrucano Lombardo and the Servino Formation. The igneous bodies show a minimum volume of about km 3 and could represent a shallow (presumably of about 300 m) magmatic reservoir feeding a volcanic edifice (CORAZZATO et alii, 200; ARMIENTI et alii, 2003; CORAZZATO & GROPPELLI, 2004). The sub-volcanic rocks of the «Montecampione group» have a porphyritic texture, with phenocrysts of quartz, pink feldspar, pyroxene, amphibole, and biotite and diffuse mafic microcrystalline enclaves. According to ARMIENTI et alii (2003), they can be classified following the terminology of volcanic rocks because of the presence of abundant devitrified groundmass and a vacuolar texture. They belong in alkaline series as trachyandesites to trachytes, although the sodic or potassic affinity is impossible to determine for the high fluid circulation. CASSINIS & ZEZZA (982) identified the laccolithic masses of Monte Muffetto, Dosso Sparviero and Corno Mura as mainly dacitic in composition and the sills of Beccheria di Bassinale as high-k andesites. Later, ARMIENTI et alii (2003) related the geochemical features of this association to high-k to shoshonitic series, thus suggesting an origin in a back-arc basin environment. Moreover, this igneous area results affected by numerous faults, as the Val Rossello tectonic line, westwards of the Early Permian Collio Basin, which possibly represented a magmatic intrusion line. The radiometric dating (Rb-Sr, whole rock and biotite) of two sub-volcanic units (respectively M. Muffetto: 23±5 Ma and Dosso Sparviero: 226±4 Ma; CASSI- NIS & ZEZZA, 982), the detailed petrographical and geochemical analyses, as well as a tentative stratigraphical reconstruction allow to ascribe these igneous products to undefined Middle-Late Triassic times. Therefore, their emplacement should be, at least in part, coeval with the Ladinian-Carnian volcanism spanning from the Buchenstein up to the Val Sabbia Sandstone, but preceding to the later described, final late Carnian basaltic episode of the Brescian Prealps. Moreover, the Ladinian-Carnian sub-volcanic rocks of the «Montecampione group» show close petrographical, geochemical and geometrical affinities with the magmatic bodies occurring near Collio in Upper Val Trompia, similarly intruding the Verrucano Lombardo and the Servino Formation CARNIAN Rhyodacite dacite Andesite Alkalibasalt Subalkaline basalt Trachyandesite Trachyte Basanite Nephelinite Nb/Y 0. Fig. - Zr/TiO 2 vs. Nb/Y diagram (WINCHESTER & FLOYD, 977) for Ladinian and Carnian volcanic products in the Brescia Figure region. Symbols: diamond: Irma trachyte dyke; large square: Pezzoro Val Cavallina basalts; small square: data from DE CAPITANI & MORONI (982); filled triangle: Brozzo; inverse filled triangle: Lodrino and Coren; filled circle: Cesovo; inverse empty triangle: Carnian andesites, including Croce di Marone (light grey); empty triangle: Ladinian trachy- and K-andesite. Diagramma Zr/TiO 2 vs. Nb/Y (WINCHESTER & FLOYD, 977) per i prodotti vulcanici ladinici e carnici in provincia di Brescia. Simboli: losanga: dicco trachitico di Irma; quadrato grande: basalti di Pezzoro Val Cavallina; quadrato piccolo: dati di DE CAPITANI & MORONI (982); triangolo pieno: Brozzo; triangolo pieno inverso: Lodrino e Coren; cerchio pieno: Cesovo; triangolo vuoto inverso: andesiti carniche, incluso Croce di Marone (grigio chiaro); triangolo vuoto: trachi- e K-andesiti ladiniche. The deposits of the shallow-marine Esino platform, which ended at sites with thin calcareous beds (the socalled «Plattenkalk» Auct., probably basal Carnian in age), are generally overlain by the continental volcanics La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 2 Fig. 2 - REE patterns (normalized to NAKAMURA, 974) for the Triassic basalts of the Brescia region. Symbols as in fig.. Configurazioni delle REE (normalizzate a NAKAMURA, 974) per i basalti triassici della area sudalpina considerata. Simboli come in fig..
10 5 G. CASSINIS ET ALII and/or fluvio-deltaic red to subordinate green sandstones, known as the «Arenarie di Val Sabbia» (fig. ), which prograde towards the NW. These latter deposits are arranged in cm- to dm-thick, often amalgamated beds, which pass subordinately to ill-stratified wine-red pelites and, locally, to caliche and/or dolomitic concretions. Fining-upwards cycles and turbiditic flows are also common. The Val Sabbia Sandstone can be petrographically classified as volcanic litharenites, with prevalent andesite clasts and rare quartz, and secondly as feldspathic litharenites, essentially characterized by altered plagioclases. The age of the reddish Val Sabbia Sandstone, which passes laterally NW-wards into the fossiliferous Gorno Formation, is generally ascribed to early and middle Carnian times (ASSERETO & CASATI, 965; ALLASINAZ, 966). This continental scenario is clearly connected with the erosion of the products of both a coeval (presumably early Carnian) igneous rocks and earlier volcanic edifices located southwards (probably along the so-called «Southern Mobile Belt» of BRUSCA et alii, 982), testified by the presence of Ladinian andesite clastics. The best examples of the Carnian magmatism in the Brescian Prealps occur in the Irma Valley to the east, and between the Croce di Marone-Val di Colonno to the west. In the former area, the igneous deposits consist of porphyric rhyolites and rhyodacites, with phenocrysts of plagioclase, K-feldspar, biotite and minor quartz, which were probably emplaced as subaerial flows and subvolcanic bodies. A conspicuous dyke of whitish, aphyric, micro-cryptocrystalline rhyolite, quartzose-feldspathicmicaceous in composition, which cuts the underlying lower-middle Anisian Angolo Limestone, is also locally associated between the Val Squassai and Vizale. In contrast, at the Croce di Marone (south of Mount Guglielmo) the Carnian volcanism is characterized by graded and compacted pyroclastic products, which are intercalated with porphyric trachyandesite and even trachytic lavas, often with glomeroporphyric aggregates and phenocrysts of sanidine and biotite±plagioclase, or minor phenocrysts of plagioclase, biotite±clinopyroxene and rare hornblende. The bulk rock composition of the Triassic volcanism in the Brescia region (fig. 3) records some data for these Carnian products, also based on previous work (CASSINIS & ZEZZA, 982). Generally, the Carnian volcanic complex of the investigated area can be interpreted as marginal to a southern igneous centre of mixed activity, with prevalent subaerial explosive phases. A high- to medium-k sub-alkaline affinity, to shoshonite, is considered the most typical feature. The Gorno Formation, sedimented in a lagoonal environment, occurs above the «Plattenkalk» formation and laterally to the Val Sabbia Sandstone in the proximity of Lake Iseo (fig. ). It consists of dark-gray fine limestones and calcarenites, rich in bivalves, alternating with blackish marly limestones and pelites, in cm- and dm-thick regular or lenticular beds. As already stated, the age pertains to the early and middle Carnian, and perhaps to the beginning of the late Carnian. Upwards the Gorno Formation is followed by the mixed terrigenous-carbonate-evaporitic S. Giovanni Bianco Formation (fig. ). This unit can be locally subdivided into two parts, separated by an unconformity related with a relative fall in sea-level, which marks the transition to more quartz-rich compositions (GARZANTI et alii, 995). The lower part is represented by red alluvial clastic beds and mudrocks (apparently similar to those of the underlying «Arenarie di Val Sabbia»), which interfinger with the top of the Gorno Fm. and with greenish medium- to fine-grained siliciclastics alternating with yellow dolostones. Otherwise, the upper part mainly consists of dolomitized siltstones, fine-grained sandstones, carbonate and gypsum deposits of semi-arid and possibly arid conditions. Based on correlation with fossiliferous outcrops of central Lombardy, the S. Giovanni Bianco Formation is generally assigned to the late Carnian. The overlying tectonically-controlled, intraformational limestone breccias of the Castro Sebino Formation and the subsequent lower member of the Dolomia Principale carbonate platform (fig. ), of which the onset have been related to latest Carnian times (JADOUL et alii, 992a, 994), may be considered the earliest episode connected to the rifting and spreading of the Piedmont- Ligurian Ocean in the Jurassic (JADOUL et alii, 992a, b; BERTOTTI et alii, 993). LATE CARNIAN IGNEOUS ACTIVITY As noted above, the youngest Triassic magmatism is represented by a swarm of vein-type basic bodies of various dimensions (up to m thick and more), spanning from the Variscan crystalline basement up to the S. Giovanni Bianco Formation. In general, in Val Cavallina (Pezzoro) the local dolerite intrudes discordantly the phyllades and micaschists of the Variscan metamorphic basement; at Brozzo these igneous bodies are sub-parallel to the Prezzo calcareous nodular beds, near the boundary with the mid-anisian «Brachiopod Horizon» on the top of the Angolo Limestone; in the Lodrino Valley these dykes cut the Buchenstein, whereas in other sectors they intrude the overlying Wengen and Val Sabbia formations. In the field, these basic rocks show a melanocratic to mesocratic composition, with a weakly porphyric texture (plagioclase, olivine, biotite) and a doleritic mesostasis, at least in the larger intrusions. Chilled margins are generally common. The groundmass is intersertal to ophitic, and characterized by (a) euhedral zoned plagioclase and olivine, (b) euhedral to skeletal ilmenite, (c) subhedral to poikilitic augite (2-3 mm) with an aegirine rim, and (d) interstitial brown to green hornblende. Accessory minerals are apatite and zircon. Secondary alteration processes, under sub-greenschist or hydrothermal conditions, are particularly widespread in the thin dykes. These basic bodies vary from picrobasalts to phonotephrites and the continuum in composition supports a co-magmatic origin and evolution for fractionation. A sub-alkaline basaltic character is typical. The REE pattern (fig. 2) shows high concentrations for the LREE (La N /Sm N =.8-2.2) and a flat trend for the HREE (Gd N /Yb N =.3-.6). The relatively more primitive compositions in the gross Pezzoro body are characterized by high-mgo (>9 wt%), high-al 2 O 3 concentrations (>5 wt%) and a high alkaliindex (MIDDLEMOST, 975; fig. 4A). The low content of P 2 O 5 (generally <0.3 wt%) and the relative enrichment in Ti and Zr but not in Y are compatible with continental intraplate tholeiites (PEARCE & CANN, 973; FLOYD & WINCHESTER, 975; WINCHESTER & FLOYD, 977; PEARCE & NORRY, 979; PEARCE, 982), as shown in the 2Nb-Zr/4-Y diagram (MESCHEDE, 986; fig. 4B).
11 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION SiO 2 wt% SiO 2 wt% Fig. 3 - Harker correlations of the Triassic volcanism in the Brescia region. They include data for the Ladinian products and the analyses of CAS- SINIS & ZEZZA (982; dots, empty triangles and filled diamonds). Other symbols as in fig.. Correlazioni di Harker per il vulcanismo triassico dell area di Brescia, comprensivo dei dati composizionali dei prodotti ladinici e delle analisi di CASSI- NIS & ZEZZA (982; punti, triangoli vuoti e rombi pieni). Altri simboli come in fig
12 52 G. CASSINIS ET ALII TABLE Sr and Nd isotope compositions of Ladinian-Carnian basalts and andesites from the southern Alpine domain. Composizioni isotopiche Sr e Nd di basalti e andesiti ladino-carnici del Sudalpino. These Carnian basalts have initial 87 Sr/ 86 Sr in the range (tab. ), close to lithospheric values, and define an isochron in the Rb-Sr system (not reported), therefore resulting a suite of cogenetic rocks. They are characterized by higher values of radiogenic 87 Sr than OIB and, in 43 Nd/ 44 Nd vs. 87 Sr/ 86 r plot (fig. 5A), they are close to the MORB-OIB mantle array. Nd isotopic compositions are very homogeneous (initial 43 Nd/ 44 Nd: ). The isotopic data suggest a likely lithospheric mantle source characterized by depleted remnants. Discriminant trace element ratios of Carnian andesites (e.g. Zr/Nb, La/Nb, Ba/Nb, Rb/Nb, K/Nb, Ba/La) are close to the crustal values and, together with high 87 Sr/ 86 Sr, may result from (I) magma contamination with crust-derived materials, (II) selective Sr enrichment in the mantle source due to fluid circulation inducing metasomatism and (III) enrichment processes due to secondary alteration. In the genesis of Carnian andesites, the isotopic data suggest the interaction between mantle-derived magmas and crustal materials. Large ion lithophile element concentrations are relatively dispersed however, total Sr contents are homogeneous, as well as the Eu anomaly (Eu/Eu* 0.79) and the Sr isotopic composition (initial 87 Sr/ 86 Sr: ). The Mt. Guglielmo andesite has Nd isotopic composition (tab. ; fig. 5A) significantly depleted compared with the MORB-OIB mantle array and it is close to the volcanic rocks from central active volcanic zones of the Andes. The whole features suggest analogies with an arc setting. On the basis of these evidences, and starting from potential mantle sources for the andesite liquids, a model of concurrent crustal assimilation and fractional crystallization has been calculated (fig. 5B). The selected potential contaminants were: (I) a Carboniferous S-type peraluminous granitoid of the Moldanubian zone, inferred to derive by dehydration melting of a metasedimentary protholith (LIEW & HOFMANN, 988) with production of peraluminous felsic melts in equilibrium with a garnet-bearing residuum (ROTTURA et alii, 998), used to provide the isotopic signature of the deep pre-variscan crustal metasedimentary source; (II) an I-type metaluminous granitoid of the Saxothuringian zone originated in the partial melting of a lower crustal source (WHITE & CHAPPEL, 983) or of high-grade metamorphic basement with a mantle component contribution (LIEW & HOF- MANN, 988). The potential mantle sources for Carnian andesites were focussed on a MORB-type parental magma, reported for mafic rocks in Corsica, Southalpine and Alpine domains (VOSHAGE et alii, 990; COCHERIE et alii, 994; MILLER & THÖNI, 995), and on a picrobasaltic composition, assumed by KAGAMI et alii (99) as potential parental liquid of the Tertiary calc-alkaline Adamello batholith (Southern Alps) and assumed as isotopic image of the subcontinental lithopheric mantle (SLM). Experimental works (ULMER, 987) confirm that picrobasaltic liquids are issued from partial melting of a garnet lherzolite at about P = 30 kbar and T = 400 C (KAGAMI et alii, 99). Therefore N-MORB, E-MORB and SLM isotopic compositions were used to model assimilation-fractional crystallization paths, calculated according to DE PAOLO equation (98). The rate of crustal contamination relative to fractional crystallization was assumed r = 0.6, likely matching the extent of such deep crustal processes (e.g. ROTTURA et alii, 998). The fractionating assemblage for MORB-type parental liquids (paths a, b, d and e, fig. 5B) consists of wt% olivine, 30 wt% orthopyroxene, 30 wt% clinopyroxene and 30 wt% plagioclase as in VOSHAGE et alii (990). The fractionating assemblage for SLM represented by picrobasaltic parental liquids (paths c and f, fig. 5B; KAGAMI et alii, 99) consists of 40 wt% olivine, wt% orthopyroxene, 40 wt% clinopyroxene and wt% plagioclase. The Sr and Nd isotopic composition observed in the Carnian andesite was reproduced starting from a subcontinental, lithospheric-mantle-type parental liquid by addition of about 40 wt% of S-type peraluminous granitoid component to the picrobasaltic composition. MORB-type mantle sources and the addition of an I-type component to all considered mantle sources yielded unsatisfactory results. The 40 Ar/ 39 Ar radiometric dating for the interstitial igneous amphiboles of the dyke intruded in the metamorphic basement of Val Cavallina provided an average plateau age of 22.5 Ma and an isochron of about 27.8 Ma (fig. 6). This dating and the cogenetic characteristics of all the respective manifestations are in good agreement with the attribution of this basic igneous «event», related by GARZANTI et alii (995) in the Brescia region, from Lake Iseo to Val Nozza (Sabbia Valley), to late Carnian times. GENERAL OVERVIEW AND CONCLUSIONS The Permian to Triassic geodynamic evolution of the Brescian Prealps is marked by significant igneous and sedimentary events and, in synthesis, can be subdivided into four principal stages.
13 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION 53 LOWER PERMIAN This period is characterized by the presence of an active dextral transform margin between Laurasia and Gondwana that locally induced a transtensional to strikeslip tectonic regime and the collapse of the Variscan orogen. The stratigraphic succession is made up of continental alluvial-to-lacustrine deposits and calc-alkaline, acidic-to-intermediate volcanics that filled narrow faultbounded basins. A possible Palaeotethys crust subduction northwards beneath the south-eastern margin of the European continent, that could have influenced the Late Carboniferous to Permian calc-alkaline igneous intrusions widespread in the Southern Alps and South European Variscan terrains (STAMPFLI, 996; ZIEGLER & STAMPFLI, 200), cannot be excluded. However, petrogenetic model for the Early Permian magmatism integrating the tectonic, petrologic and palaeogeographic setting during the Pangaea built-up (CORTESOGNO et alii, 998) should take into account a thick crustal pile created through an early Ordovician- Dinantian underthrusting of the thinned Cambro-Ordovician crust and the subsequent stacking of crustal nappes (e.g. CARMIGNANI et alii, 994). The onset of a dextral trasform margin between Laurasia and Gondwana induced the dissecting of the Variscan orogen, associated with consequent thermal uplift and metamorphic re-equilibration in the crust, dehydration reactions and crustal melting. At this moment, released fluids could cause localized metasomatism of the underlying mantle. The subsequent transtensional collapse of the orogenic belt and the consequent adiabatic decompression, recorded as eclogite-granulite transition at the lowest levels of the crust pile, could trigger melting processes in the lithospheric mantle and at the lower crust-mantle. Records of decompression, such as the ascent of liquids from the mantle to the middle crust (as gabbros and basic dykes), associated with crustal melting, pre- to syn-kinematic granite emplacement in the host basement, are clear in Sardinia and Ligurian basements. In this environment, the conditions for genesis of intermediate calc-alkaline liquids, with prevalent mantle and/or lower crust signature, and of acid, crustal melts (GREEN, 982) could occur. The progressive evolution of a transform margin, with the formation of major wrench and connected normal faults, favoured the ascent to the surface of magmas residing in the middle crust, and possible consequent hybridization processes. UPPER PERMIAN-ANISIAN The Upper Permian-Anisian p.p. succession, which sealed the Variscan orogeny, marked the onset of widespread erosion and cessation of volcanic activity. In our opinion, this sedimentary cycle was preceded by an important transpressional inversion of the former transtensional tectonics, which led to the most significant intra-permian gap estimated in the Brescian Alps at about 20 Ma and to extensive terrestrial conditions. Most likely, this tectonic event can be generally related to the «Mid- Permian Episode» of DEROIN & BONIN (2003) and with compressional and/or transpressional events (e.g. CASSI- NIS, 964; PROST & BECQ-GIRAUDON, 989; CADEL et alii, 996; CASSINIS & PEROTTI, 997). According to French geologists, it also represents the beginning of the long Zr/ Tholeiitic basalts 4 Al 2 O 3 wt% A C 2Nb High Alumina or Calc-alkaline basalts 2 Fig. 4 - (A) Alkali-index (MIDDLEMOST, 975; A.I.) vs. Al 2 O 3 and (B) 2Nb-Zr/4-Y diagram (MESCHEDE, 986) for the Triassic basalts of the Southalpine domain: A: intra-plate alkali basalts; A2: intra-plate alkali basalts and intra-plate tholeiites; B: E-type MORB; C: intraplate tholeiites and volcanic-arc basalts; D: N-type MORB and volcanic-arc basalts. (A) Indice degli alcali (MIDDLEMOST, 975; A.I.) in relazione ad Al 2 O 3. (B) Diagramma 2Nb-Zr/4-Y (MESCHEDE, 986) per i basalti triassici del dominio Sudalpino: A: basalti alcalini intraplacca; A2: basalti alcalini e tholeiitici intraplacca; B: E-type MORB; C: basalti tholeiitici intraplacca e di arco vulcanico; D: basalti N-MORB e di arco vulcanico. Alpine sedimentary cycle. Recently, the debated transition from Pangea «B» to Pangea «A» configurations, i.e. from the end of the Variscan orogeny to the beginning of the Alpine Era, has also found a valid supporter in MUTTONI et alii (2003). The corresponding Verrucano Lombardo red beds, which transgressed outside of the Collio Basin and onto surrounding metamorphic highs, can be generally inter- B D A B Y
14 54 G. CASSINIS ET ALII SLM E-MORB BSE MORB OIB N-MORB d 0.8 e 0.8 f NVZ SVZ 0.8 I-type metaluminous granitoid OCEANIC ISLAND ARCS CVZ Sr/ 86 Sr initial Sr/ 86 Sr initial 0.4 c b a A B S-type peraluminous granitoid Fig. 5 - (A) Initial (28 Ma) 87 Sr/ 86 Sr vs. 43 Nd/ 44 Nd for volcanic products of the Southalpine area. MORB, OIB, oceanic island arc volcanic rocks, volcanic rocks from the northern (NVZ), central (CVZ) and southern (SVZ) active volcanic zones of the Andes (WIL- SON, 99) are reported for comparison. Filled circle: Mt. Guglielmo andesite; square and triangle: basalts from Val Cavallina and Brozzo. (B) Initial (28 Ma) 44 Nd/ 43 Nd vs. 87 Sr/ 86 Sr for Mt. Guglielmo andesite. The model mixing fractional crystallization vectors from the potential mantle and crustal sources of the Permian magmatism are reported. BSE: Bulk Silicate Earth from FAURE (986). N-MORB from HART et alii (999). E-MORB type magma from COCHERIE et alii (994). SLM: subcontinental lithospheric mantle from KAGAMI et alii (99). Carboniferous S-type peraluminous granitoid of the Moldanubian zone and I-type metaluminous granitoid of the Saxothuringian zone from LIEW & HOFMANN (988). All compositions recalculated for t = 28 Ma. All paths calculated according to DE PAOLO equation (98), assuming a rate of crustal contamination relative to fractional crystallization r = 0.6. The mineral K D values used for calculation of the bulk partition coefficients for Sr and Nd were taken from FUJIMAKI et alii (984) and DOSTAL et alii (983). Numbers on the paths 0.8, 0.6 and 0.4 represent proportions of residual magma. (A) Correlazione tra rapporto iniziale (28 Ma) di 87 Sr/ 86 Sr in relazione a 43 Nd/ 44 Nd per i prodotti vulcanici del Sudalpino. I campi delle rocce vulcaniche MORB (basalti di dorsale medio oceanica), OIB (basalti di isole oceaniche), archi di isole oceaniche e vulcaniti dei segmenti volcanici attivi settentrionale (NVZ), centrale (CVZ) e meridionale (SVZ) delle Ande (WILSON, 99) sono riportati per confronto. Cerchio pieno: andesite di Monte Guglielmo; quadrato e triangolo: basalti della Val Cavallina e Brozzo. (B) Correlazione tra rapporto iniziale (28 Ma) di 87 Sr/ 86 Sr e 43 Nd/ 44 Nd per l andesite di Monte Guglielmo. Sono riportati i vettori modello di mixing e cristallizzazione frazionata dalle sorgenti potenziali di mantello e della crosta per il magmatismo permiano. BSE: Bulk Silicate Earth da FAURE (986). N-MORB da HART et alii (999). Magma E-MORB tipo da COCHERIE et alii (994). SLM: mantello litosferico sottocontinentale da KAGAMI et alii (99). Granitoide peralluminoso tipo S della zona Moldanubiana e granitoide metalluminoso tipo I della zona Saxoturingiana da LIEW & HOFMANN (988). Tutte le composizioni sono state ricalcolate a t = 28 Ma. Le traiettorie sono state calcolate secondo l equazione di DE PAOLO (98), considerando un tasso di contaminazione crostale relativo ad una cristallizzazione frazionata r = 0.6. I valori di coefficiente di distribuzione K D per il calcolo dei coefficienti di ripartizione per Sr e Nd da FUJIMAKI et alii (984) e DOSTAL et alii (983). I valori 0.8, 0.6 e 0.4 rappresentano le frazioni di liquido residuale. preted as fluvial deposits in a region affected by a longwavelength extensional regime, conjoined with crustal thinning and thermal contraction. This change led to progressive erosion and flattening of the previous irregular topography, and to a rapid shallow Tethys transgression, from the east, of the Lower Triassic deposits. Generally the Verrucano sediments, compared with the products of the lower cycle, appear more widely distributed although less thick. In a general view, these new Permian tectonics could be related to an important plate reorganization which led to the opening of Neotethys along the northern Gondwana margin, and of Meliata-Maliak and Svanetia backarc basins in Eastern Europe, both developed to the north of Palaeotethys. After a not-yet-defined hiatus (presumably from the Lopingian up to the lower Griesbachian) at the Permian- Triassic boundary (CASSINIS et alii, 2007), follows the transitional/shallow-marine Servino. In Brescia province the thickness of this unit (20-50 m), compared with that of the lateral Werfen Formation (up to a maximum of about 400 m) towards the eastern Southern Alps, is compatible with more moderate subsidence activity, probably linked to synsedimentary faults (Giudicarie Line?) and/or to structural and palaeogeographical local conditions. At the end of the Early Triassic and during the early Anisian the Bovegno Carnieule marks a regressional phase, which led to the establishment of a sabkha environment and, locally, to deposition of clastics eroded from underlying units (such as the Servino and Verrucano Lombardo formations). Moreover, according to MASSARI et alii (994), this event concludes the Upper Permian cyclic evolution and is followed by a very different stratigraphic succession, with alternating sedimentary and igneous rocks. Subsidence generated a lowering of the investigated region during the early p.p. to middle Anisian (Angolo Limestone), probably due to block-faulting and a transcurrent climax, also shown eastwards (Dolomites and Cadore) by DOGLIONI (984, 987). Several small carbonate platforms (Camorelli and Mt. Guglielmo) coexisted in this heterogeneous marine palaeotopography. Later, basin sedimentation of the upper Anisian Prezzo Limestone covered the platforms. LADINIAN-LOWER/MIDDLE CARNIAN This time-interval was characterized by the appearance and evolution of new tectono-magmatic activity, which developed in the Brescian Prealps in the form of dykes, sills, laccoliths, and lava flows. The general subsidence of the region was governed by strike-slip to extensional tec-
15 PERMIAN TO TRIASSIC GEODYNAMIC AND MAGMATIC EVOLUTION 55 tonics, probably linked to arc/back-arc conditions of an eastern subduction zone, bounding the Meliata Basin. The bulk-rock chemistry of the related volcanic and subvolcanic products exhibits an acidic to basic calc-alkaline composition, which contrasts with an extensional regime and generally active subsidence during the Middle Triassic. Based on investigations essentially carried out in the eastern Southern Alps, many authors suggest a number of geodynamic hypotheses to reconcile the contradiction between the calc-alkaline nature of the volcanism and the apparent extensional to strike-slip tectonics. They may be summarized as follow: (a) volcanism connected with aborted rifting (FERRARA & INNOCENTI, 974; BECHSTÄDT et alii, 978); (b) volcanism linked to subduction processes, with shallowing of the continental lower crust in the lithospheric mantle (CASTELLARIN et alii, 980; CASTELLARIN & ROSSI, 980); (c) volcanism connected to late Variscan evolution of an ensialic back-arc marginal basin (MARINELLI et alii, 980), as also indicated by ARMIENTI et alii (2003) for the subvolcanic bodies of Mt. Muffetto (between the Camonica and Trompia valleys) which point to a back-arc setting associated with the great depths achieved by a subducting slab. CRISCI et alii (984) suggested a further hypothesis, i.e. that this Triassic magmatism derived from partial melting of the upper mantle, strongly modified during the previous Variscan orogenesis and contaminated by crustal material, and later intruded during the first «rifting stages» of the early Middle Triassic. However, research on Triassic and Late Carboniferous shoshonitic rocks of the Dolomites and other extra- Southalpine areas suggests that the upwelling of these magmas could have been controlled by transcurrent tectonics (SLOMAN, 989; SCHALTEGGER & CORFU, 992; BONADIMAN et alii, 994). As already stated, DOGLIONI (984, 987) pointed this out in some studies on the Middle Triassic tectonics of the Dolomites, which would have been accompanied by important transpressional/transtensional events, along with block-faulting and other tectonic structures. The possibility that both the Triassic and Permian volcanic episodes in the Southern Alps could have been essentially triggered by transcurrent tectonics arouses great geological interest. However, as BRACK & RIEBER (993) suitably pointed out, the wide geodynamic context in the South-Alpine domain, as well as in the nearby areas, is as yet largely unknown. In our opinion, partially according to MARINELLI et alii (980), GARZANTI et alii (995) and ARMIENTI et alii (2003) on the basis of available data, a better explanation of the geodynamic regime of the South-Alpine realm during the Ladinian to early/middle Carnian consists of an arc/back-arc setting, characterized by transtensional to locally transpressional conditions. The related subduction zone is probably identifiable at the eastern margin of Paleoeurope, where the Palaeotethys subducted beneath the Laurasia Plate towards the NNW (ZIEGLER & STAMPFLI, 200). The Brescian sector of the Southern Alps could represent the western continental lateral termination of the Meliata oceanic back-arc basin. UPPER CARNIAN Apparent Age (Ma) TFA = ± 0.73 Ma WMPA = ± 0.62 Ma ± ± ± ± 0.9 0,0 0,2 0,4 0,6 0,8,0 Cumulative 39 Ar Fraction Fig Ar/ 39 Ar release temperatures. Single step ages climb from 40 to ca 2 Ma. The flat middle part of the spectrum yields a weighted mean plateau age of 22.5 Ma corresponding to an isochron age of 27.8 Ma. Spettro delle temperature di rilascio di 40 Ar/ 39 Ar. I singoli intervalli di rilascio variano da 40 a circa 2 Ma. La parte mediana piatta corrisponde ad una età media di plateau di 22.5 Ma, corrispondente ad una età isocrona di 27.8 Ma. In presumed late Carnian times, transitional basaltic dykes (27±3 Ma; 40 Ar/ 39 Ar dating) and lavas, emplaced below or at the base of the San Giovanni Bianco Formation and inducing marked changes in the thickness of the stratigraphic succession that evolves from continental to evaporitic and marine environments, indicate a substantial change in the geodynamic pattern of the region. The uppermost intraplate tholeiites can be related to extensional rifting conditions as the associated rhyolitic bodies, which at least for the high Nb/Y exhibit an alkaline composition. These basic bodies are generally connected with partial melting of a modified subcrustal mantle (WILKINSON & BINNS, 977; PRESTVIK & GALES, 985), or to evolution of MgO-enriched melts (COX, 980) and eventually modified by crustal contamination (PATCHETT, 980). The occurrence of continental tholeiites suggests the presence of thermal anomalies localized in a lithospheric, generally depleted mantle, below a crust not yet thinned, and in the framework of incipient regional subsidence. In conclusion, the distribution of intraplate tholeiites in regions affected by continental breakup and marine ingression indicates that, in the Brescian Prealps, the Triassic basaltic dykes represent the onset, in relatively marginal areas, of Jurassic Neotethyan rifting. This event introduced into the investigated region a new major rifted extensional carbonate sequence. The uppermost Carnian- Norian «Dolomia Principale» and its local basal breccias sealed once for all the previous Palaeotethys signature. APPENDIX: ANALYTICAL METHODS Whole rock major and trace element abundances for the volcanic rocks were carried out by XRF techniques at the X-RAL Laboratories Canada. Losses on ignition (LOI) were determined by the gravimetric method. Rare Earth elements were analyzed by ICP-MS at the X-RAL Laboratories, Canada.
16 56 G. CASSINIS ET ALII 40 Ar/ 39 Ar age determinations were carried out on amphibole phenocrysts separate from the Val Cavallina dyke. The separated fractions were analyzed by the 40 Ar/ 39 Ar incremental method at the Actlab Laboratories (Canada). The sample wrapped in Al foil was loaded in evacuated and sealed quartz vial with K and Ca salts. The samples were irradiated in nuclear reactor for 48 hours. The neutron gradient did not exceed 0.5% on sample size. The Ar isotope composition was measured in a Micromass 5400 static mass spectrometer. 200ºC blank of 40 Ar did not exceed n*- STP. Sr and Nd were analyzed at the Geochemistry Laboratory of Trieste University. Samples for isotopic analysis were dissolved in teflon vials using a mixture of HF-HNO 3 and HCl purified reagents. Sr and Nd were collected after ion exchange and reversed-phase chromatography, respectively; the total blank for Sr was less that 20 pg. The Sr and Nd isotopic composition were obtained using a VG 54E mass spectrometer and «Analyst» software (LUDWIG, 994) for data acquisition and reduction. The 87 Sr/ 86 Sr and 43 Nd/ 44 Nd were corrected for fractionation to 86 Sr/ 88 Sr =0.94 and 46 Nd/ 44 Nd=0.729, respectively, and the measured ratios were corrected for instrumental bias to NBS 987 and La Jolla standard values of and The reported errors represent a 95% confidence level. ACKNOWLEDGEMENTS This paper is in memory of our friend Luciano Cortesogno. It has benefited from the careful reviews of Giampaolo De Vecchi (Padova) and Michele Mattioli (Urbino). The authors are also grateful to Paola Pittau and Claude Spinosa, as organizers of the meeting «Sardinia, Italy, and the Development of the Western Mediterranean Basin» held in Boise (May 3-5, 2004) during the GSA «Rocky Mountain (56 th Annual) and Cordilleran (0 th ) Joint Meeting», where this paper was preliminarily presented, for having permitted its publication. 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