Volcanogenic Successions Research Articles

Chapter 4 Geological development and regional significance of an oceanic magmatic arc and its sedimentary cover: Permian Brook Street Terrane, South Island, New Zealand

Abstract The c. 450 km-long Brook Street Terrane (pre-Alpine Fault displacement) sheds light on processes of arc magmatism and related sedimentation. A very thick (up to 15 km) succession accumulated south of the Alpine Fault in the Takitimu Mountains during the Early Permian. Predominant arc-flank talus is intercalated with basic extrusive and intrusive igneous rocks. Volcaniclastic sediments mainly accumulated by mass-flow and turbidity current processes. The sediments were mostly derived from differentiated, arc-core, basaltic–andesitic rocks, contrasting with less evolved arc-flank flows and minor intrusions. Some igneous rocks are mildly enriched, supporting an extensional back-arc setting. After volcanism ended, Middle–Late Permian mixed carbonate–volcaniclastic gravity-flow deposits were derived from a non-exposed carbonate platform. Other volcanogenic successions in the south (Bluff, Riverton) represent smaller eruptive centres. In contrast, north of the Alpine Fault (e.g. Nelson), volcanism began with mostly felsic tuffaceous gravity-flow deposits, followed by extrusion/intrusion of clinopyroxene-rich, primitive magmas, related to arc rifting, and ended with an accumulation of a mixed basic–felsic volcaniclastic forearc apron. Taking account of regional comparisons, the Early Permian arc is interpreted as having formed adjacent to Gondwana (on accreted or trapped oceanic lithosphere), whereas the lithologies north of the Alpine Fault represent contrasting Late Permian continental arc magmatism.

A Keleti-Bakony triász időszaki vulkanogén képződményeinek petrográfiai vizsgálata és képződési körülményeik értelmezése

A Tethys közép-európai triász időszaki fejlődéstörténetének nyomozása az egymástól távolabbi területeken található nem vulkanogén üledékes kőzetek összehasonlító vizsgálatával régóta eredményesen zajlik. A térben és időben is sokkal változatosabb kifejlődésű vulkanogén képződmények ezirányú vizsgálatában rejlő lehetőségek még messze nincsenek kihasználva. Ennek leginkább az lehet az oka, hogy ezeknek a hazai kőzeteknek a részletes kőzettani leírása még nem történt meg. Ladin vulkanogén törmelékes képződmények a Dunántúli-középhegységen belül a Keleti-Bakony területéről ismertek. Az itt található középső–késő-triász vulkanogén kőzetek részletes petrográfiai vizsgálata lehetőséget teremtett arra, hogy kőzettani alapon is összevessük más hasonló korú, képződésük idején egymáshoz sokkal közelebb elhelyezkedett Dunántúli-középhegységben és Déli-Alpokban megtalálható vulkáni képződményekkel.A vizsgált keleti-bakonyi összletek az inotai Hideg-völgy útbevágásában lévő egykori felszíni feltárásban, valamint a Várpalota Vpt–3 és a Bakonykúti Bút–2 fúrások anyagában tanulmányozhatók. A vulkanogén összlet két részre tagolható: az alsó részben uralkodóan vulkanit törmelékszemcsékből álló, szenesedett növénymaradványt és elszórtan crinoidea váztöredékeket tartalmazó finomszemcsés üledékes kőzetet, fölül vulkanit- és mészkőkavicsokat nagy mennyiségben tartalmazó, Daonella vázelemekben gazdag durvaszemcsés rétegcsoportot találhatunk. A vulkáni törmelék szemcsék között döntően intermedier (andezit), kisebb mennyiségben bázisos (bazaltsalak, bazalt és mikrodolerit) és savanyú (riolit, aplit) vulkanitok és ezekhez kapcsolódó kristálytörmelékek jelennek meg.A vulkanogén összlet forrását olyan középső- és felső-triász vulkáni kőzetek (bázisos-intermedier lávakőzetek és freatomagmás kitörések termékei) szolgáltatták, melyeknek mai rokon képviselői a Dunántúli-középhegységben (legközelebb a balatonfői területen), de legnagyobb kiterjedésben a vele ezen időszakban szomszédos Déli-Alpokban fordulnak elő. A felső vulkanogén konglomerátum / kavicsos homokkő képződésére jó analógia a déli-alpi Wengeni Formáció és Marmoladai Konglomerátum keletkezése. E formációk nagy vastagságú törmelékes összlete szárazulatra került, döntően mészkő és bázisos-intermedier kőzetek lepusztulásával, majd tenger alatti tömegmozgásokkal üledékgyűjtő medencékbe történő felhalmozódásával keletkezett. Az alsó vulkanogén homokkő keletkezése ettől némiképp eltér. Bázisos-intermedier összetételű törmelékanyaga első lépésben sekélytengeri/szárazföldi környezetben freatomagmás kitörés eredményeként halmozódott fel, majd közvetlen ezután tenger alatti lejtő mentén történő áthalmozódás után ülepedett le véglegesen.

New 40Ar/39Ar ages and revised 40K/40Ar* data from nephelinitic–phonolitic volcanic successions of the Trindade Island (South Atlantic Ocean)

The Trindade Island is located in the South Atlantic Ocean, 1170km from the Brazilian coast and represents the eastern end of the submarine E–W Vitória–Trindade Chain. This is interpreted as the Trindade plume track beneath the South American plate during the Cenozoic. Almeida (1961) recognized five volcanogenic successions at Trindade in decreasing age: the Trindade Complex (TC) and the Desejado (DF), Morro Vermelho (MV), Valado (VF) and Paredão (PF) formations, composed of nephelinitic to phonolitic effusive-pyroclastic deposits, dykes and necks. Here, we present new 40Ar/39Ar ages and re-evaluation of available 40K/40Ar* data that, coupled with previous petrological information, allowed us to reconstruct the volcanic history of the island (i.e., 3.9–0.25Ma) distinguishing near synchronous volcanic episodes and solving several stratigraphic uncertainties reported in the literature.The geochronological dataset show that the nephelinitic–phonolitic volcanism at Trindade was discontinuous throught time, being marked by periods of high volcanic activity and periods of quiescence, which suggests variable melt production and eruption rates. The peak of the volcanic activity occurred between 3.9 and 2.5Ma (i.e., Lower Pliocene to Lower Pleistocene) that is represented by the TC, which correspond to the largest volume of volcanic deposits preserved in the island. The volcanic activity slows down progressively from 2.5Ma to cease at ca. 1.6Ma, period that is represented by the DF. This volcanism was followed by a quiescent period that lasted until the ephemeral nephelinitic volcanism of the MV (no age), VA (no age) and PF (ca. 0.25Ma). Thus, the volcanic activity in Trindade was ceased completely at ca. 0.25Ma, event registered in the uppermost volcanic deposits of the PF and that represents the last volcanic activity in the Brazilian territory.

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy, and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K2O/Na2O, K2O/TiO2 and intermediate CaO/Al2O3 that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K2O/Na2O, K2O/TiO2 and CaO/Al2O3 that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K2O/TiO2 similar to those from VF, and intermediate CaO/Al2O3 as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO2–eclogites) whereas the less important EM component to recycled marine or continental sediments.

The Moldanubian Thrust Zone — A terrane boundary in the Central European Variscides refined based on lithostratigraphy and U–Pb zircon geochronology

The zircon age populations of metavolcano-sedimentary successions in the Orlica–Śnieżnik Dome (OSD), Staré Město Belt (SMB) and Velké Vrbno Dome, the Sudetes (Poland and Czech Republic), have been used to refine the location of the Moldanubian Thrust Zone (MTZ), which is a boundary between the Saxothuringian Terrane of Gondwana descent and the Brunovistulian Terrane being a promontory of Laurussia. In the northern continuation of the MTZ, a set of multiply activated, regional-scale thrusts developed and brought into contact rocks of different ages and geological histories. Metarhyolites in the Orlica–Śnieżnik Dome and the Staré Město Belt have similar geochemistry and U–Pb isotopic zircon records, which is taken in favour of their coeval formation and common Saxothuringian affinity. Felsic metavolcanic rocks from the OSD and from the thrust-bounded upper and lower units of the SMB yielded protolith ages of 500±3Ma and 493±4Ma to 498±5Ma, respectively, which indicates that metavolcano-sedimentary successions in the OSD and SMB were deposited in Late Cambrian times. Structurally below these rocks, there are the highly sheared Brousek quartzites with detrital zircons that yielded a maximum depositional age of ~530Ma. The mylonitic quartzites accommodate deformation induced by tectonic transport on the East Nýznerov Thrust, which is interpreted as the easternmost margin of Saxothuringia. This fault separates Palaeozoic rocks of the Saxothuringian Terrane from Neoproterozoic bimodal volcanogenic succession in the Velké Vrbno Dome of Brunovistulia, dated at ~558Ma. The late-Variscan thermal events in the Early Carboniferous left imprints in the form of U-rich rims around the zircons of the metavolcanic rocks from the Staré Město Belt and only a very minor overprint in the zircons from the Velké Vrbno Dome and Orlica–Śnieżnik Dome.

Eocene sedimentary facies in a volcanogenic succession on King George Island, South Shetland Islands: a record of pre-ice sheet terrestrial environments in West Antarctica

About 34 Ma ago there was a radical change of climate that led to the formation of Antarctic ice sheet. King George Island, located in the South Shetland Islands volcanic arc (northern Antarctic Peninsula region), is one of a few places in West Antarctica which shows a geological record of sedimentary environments preceding development of the ice sheet. The Eocene sedimentary facies occur in the dominantly volcanogenic succession of King George Island. They have been recognized in the Arctowski Cove and Point Thomas formations (Ezcurra Inlet Group) and in the Mount Wawel Formation (Point Hennequin Group) in Admiralty Bay, and in the Mazurek Point Formation (Chopin Ridge Group) and Lions Cove Formation (Polonia Glacier group) in King George Bay. They record a cooling trend in terrestrial environments that began at termination of the Early/Middle Eocene Climatic Optimum, and was followed by a significant deterioration of climate during Late Eocene and earliest Oligocene, directly preceding glacial conditions in the northern Antarctic Peninsula region. The ongoing research confirms the existence of three preglacial climatic stages (PGS-1 &ndash; PGS-3) during Eocene &ndash; earliest Oligocene, from humid, warm to moderate climate (PGS-1), through cool and dry climate (PGS-2), up to cold and humid conditions (PGS-3). Studies were carried out on usually fine-grained volcanoclastic sediments, containing Podocarpaceae&ndash; Araucaria &ndash; Nothofagus plant fossils assembly. Beds of reworked pyroclastic material alternate with lava flows or volcanic agglomerates, as well as ex situ blocks of Eocene volcanogenic sediments on a moraine. Calculated geochemical indices of weathering (CIA, PIA and CIW) confirm moderate to high chemical weathering under warm and humid climate conditions at the beginning and deterioration of conditions in the end of Eocene.

Stratigraphic relationships of Carboniferous volcanogenic successions in the Clifton–Carroll block and Werrie Syncline, northern Tamworth Belt, southern New England Orogen

Late Carboniferous volcanogenic successions in the Clifton–Carroll block, west of the Werrie Syncline, represent the southernmost extremity of the Willuri Formation, a unit which extends northwards into the Rocky Creek region in northern New South Wales. Two ignimbrites, the Iventure and Tagggarts Mountain Ignimbrite Members, previously named in the Currabubula Formation, are present in the Willuri Formation in the Clifton–Carroll block. The Taggarts Mountain Member extends farther northwards through both Gunnan and Tulcumba Ridges. The western limb of the Werrie Syncline, which is separated from the Clifton–Carroll block by the Kelvin Thrust, contains an Upper Devonian to Upper Carboniferous succession. The largely non-marine, Lower Carboniferous Merlewood Formation is typified by andesite flows, two of which are major, within an area between the Donnellys Gap Fault in the north and Piallaway in the south. The Iventure and Taggarts Mountain Ignimbrite Members in the overlying Upper Carboniferous Currabubula Formation provide a correlation with the Willuri Formation in the Clifton–Carroll block to the west, and the more widespread Taggarts Mountain Ignimbrite Member with the Willuri Formation in Gunnan and Tulcumba Ridges to the north.

The Cretaceous Songliao Basin: Volcanogenic Succession, Sedimentary Sequence and Tectonic Evolution, NE China

Abstract: The Songliao basin (SB) is a superposed basin with two different kinds of basin fills. The lower one is characterized by a fault‐bounded volcanogenic succession comprising of intercalated volcanic, pyroclastic and epiclastic rocks. The volcanic rocks, dating from 110 Ma to 130 Ma, are of geochemically active continental margin type. Fast northward migration of the SB block occurred during the major episodes of the volcanism inferred from their paleomagnetic information. The upper one of the basin fill is dominated by non‐marine sag‐style sedimentary sequence of siliciclastics and minor carbonates. The basin center shifted westwards from the early to late Cretaceous revealed by the GGT seismic velocity structure suggesting dynamic change in the basin evolution. Thus, a superposed basin model is proposed. Evolution of the SB involves three periods including (1) Alptian and pre‐Aptian: a retroarc basin and range system of Andes type related to Mongolia‐Okhotsk collisional belt (MOCB); (2) Albian to Companian: a sag‐like strike‐slip basin under transtension related to oblique subduction of the Pacific plate along the eastern margin of the Eurasian plate; (3) since Maastrichtian: a tectonic inverse basin under compression related to normal subduction of the Pacific plate under the Eurasian plate, characterized by overthrust, westward migration of the depocenter and eastward uplifting of the basin margin.

Redefinição do Grupo Bom Jardim (Neoproterozóico Iii) em sua área-tipo: litoestratigrafia, evolução paleoambiental e contexto tectônico

The Camaqua Group is composed of sedimentary and volcanogenic successions that crop out in the south-central portion of the Rio Grande do Sul State (southern Brazil). This supergroup is partially covered by Parana Basin units and has been divided into sub-basins by the uplift of basement highs. This post-depositional tectonic segmentation, and the lithologic similarity among sedimentary successions of different stratigraphic levels makes it difficult to establish a formal lithostratigraphic column of the Bom Jardim Group without detailed field work. This paper presents a review of the lithostratigraphy of the Bom Jardim Work in its type-area, constituted by siliclastic, volcanic (basalts, andesites and rhyolites) and volcaniclastic rocks grouped in three units, with transitional contacts among each other. Cerro da Angelica Formation: rhythmic intercalation of sandstones, siltstones and claystones, as well as pebbly sandstones and minor volcaniclastic rocks (peperites), formed in sub-lacustrine fans and overlying deltaic systems. Hilario Formation: volcanic rocks of basic, intermediate and acid composition (basalts, latite-basalts, latites, andesites and rhyolites) placed in subaquatic environments, and minor pyroclastic rocks (lapilli-tuffs and coarse-grained lithic and vitric tuffs) placed by gravity flows or settled in water. Picada das Gracas Formation: rhythmic layers of fine-grained sandstones, siltstones and claystones formed in pro-delta environments, overlain by a thick succession of conglomerates and pebbly sandstones of proximal fan-deltas. Fluvial and fluvial-dominated deposits occur at the top of the unit.

The Cretaceous volcanic succession around the Songliao Basin, NE China: relationship between volcanism and sedimentation

AbstractWith volume ratio of 8:1:1.5 amongst acidic, intermediate and basaltic rocks, the Cretaceous volcanics around the Songliao Basin are a series of high‐K or medium‐K, peraluminous or metaluminous, calc‐alkaline rocks, lacking typical basalts and peralkaline members of typical rift‐related types. Their eruption ages range between 133 and 127 Ma, 124 and 122 Ma and 117 and 113 Ma respectively. They are high in total (Rare earth element) REE contents (96.1–326 ppm), enriched in LREE and depleted in HREE (LREE/HREE = 4.6–13.8), with negative Eu and Ce anomalies (Eu/Eu* = 0.04–0.88; Ce/Ce* = 0.60–0.97). They have enriched large‐ion lithophile elements (e.g. K, Ba, Th) and depleted high field strength elements (e.g. Nb, Ti and Y), suggesting a subduction‐related tectonic setting. The volcanic activities migrated from south to north, forming a successively northward‐stepping volcanic series and showing a feature significantly different from the overlying sedimentary sequence striking northeast. Thus, an overlap basin model was proposed. Accompanied by opening of the basin, the volcanogenic succession was formed at the block‐faulting stage (131–113 Ma) owing to the closure of the Mongolia–Okhotsk ocean in the Jurassic and early Cretaceous, while the overlying sedimentary sequence was unconformably deposited at the spreading stage (Albian–Maastrichtian) owing to the oblique subduction of the Pacific plate under the Eurasian plate. The volcanic succession constitutes the lower unit of basin filling and is the forerunner of further basin spreading. Copyright © 2002 John Wiley &amp; Sons, Ltd.

40 Ar/ 39 Ar and K/Ar dating on the volcanic rocks in the Songliao basin, NE China: constraints on stratigraphy and basin dynamics

Precise 40Ar/39Ar and K/Ar ages were determined on sanidine and whole-rock separates of volcanic rocks in the Songliao basin (SB). The volcanism is characterized by a series of continual and episodic eruptions in the late Jurassic (147–157 Ma) and early Cretaceous (113–136 Ma). A fairly accurate chronostratigraphic column was first established based on those data. The volcanic activities happened synchronously with closure of the Mongolia–Okhotsk ocean north to the SB. Structures of the volcanogenic successions are S-dipping or SE-dipping fault blocks coupled with N-dipping or NW-dipping major detachment systems. Distribution of the volcanics, revealed by high-resolution seismic reflections, is neither restricted within nor in agreement with the framework of the overlying sedimentary sequence striking NNE. This new evidence fits the overlap model, proposed in the present paper, that formation of the volcanic successions in the SB is related to the subduction of the Mongolia–Okhotsk plate under the Mongolia–North China block during the Jurassic and early Cretaceous, whereas the overlying sedimentary sequence unconformably on the volcanics is tectonically controlled by the oblique subduction of the Pacific plate under the Eurasian plate since late Aptian. In addition, the coal-bearing epiclastic sediments intercalating the volcanics were deposited at the subduction–quiescence episodes when extensional collapse occurred.

Two cases of continental collisions and related tectonics during the Proterozoic period in India — insights from gravity modelling constrained by seismic and magnetotelluric studies

The Aravalli Delhi Mobile Belt (ADMB) in NW India includes the Proterozoic Aravalli and Delhi rifts and fold belts. A seismic profile and gravity modelling across the ADMB shows several dipping reflections and suggests a thick crust (45–46 km) and a high density (3.04 g cm −3) dome-shaped body in the lower crust. This high density body in the lower crust may represent underplated lower crust caused by extension during evolution of the Proterozoic rifts. Part of the gravity high observed over the ADMB is attributed to the upthrust blocks of high density lower crustal granulite rocks. The gravity low along the eastern margin of the ADMB is modelled as being due to a westerly dipping (45°) low density body in the crust, in accordance with seismic reflectors which may represent remanents of the subducted crustal rocks. The Hindoli group of rocks above this body is characterized by high conductivity, indicating a high fluid content and a fractured nature which may be related to the collisional tectonism of this region. A sharp gravity high along the western margin of the ADMB is attributed to the basic rocks of an ophiolite melange. The gravity low and high west of the ADMB are attributed to plutonic activity and bimodal calc-alkaline volcanics in linear basins and are related to arc and back-arc rift magmatism, respectively. The ADMB, therefore, represents a Proterozoic collision zone between the Bundelkhand craton towards the east and the Rajasthan craton towards the west with a plausible subduction towards the west along the eastern margin of the ADMB. The thrusting is mainly confined to the obducted block towards the west, although some thrusting is inferred even in the subducted block. The central Indian Suture (CIS) is located at the southern margin of the Satpura Mobile Belt (SMB) in central India and separates high grade metasediments and granulite towards the north from low grade Proterozoic volcanogenic successions towards the south. A seismic profile and gravity modelling across the CIS show a thick crust with a thicker crust north of the CIS compared to the south and a contact in the lower crust whose projection on the surface coincides with the CIS. The CIS at the surface coincides with a low density (2.65 g cm −3) and conductive body that is interpreted to be fluid-filled and fractured in nature. The high density (2.81 g cm −3) body modelled in the upper crust north of the CIS coinciding approximately with a highly conductive body and dome-shaped seismic reflectors may represent an upthrust block from the lower crust as high density granulite rocks are exposed in this region. Short wavelength gravity highs and lows south of the CIS represent exposed metavolcanics and granite intrusions which represent tholeiitic series rocks to calc-alkaline series rocks associated with island arcs and back arc magmatism. Therefore, the large wavelength gravity low south of the CIS is modelled as a low density body below the Moho, which conforms with seismic reflectors and may represent crustal rocks subducted towards the south. Thrusting in this case is indicated in the subducted block towards the north of the CIS. The bipolar gravity anomaly across the CIS, therefore, indicates a collision zone between the Bundelkhand craton towards the north and Bhandara–Deccan cratons towards the south.

Neoproterozoic and Cambrian successions deposited on the East European platform and Cadomian basement area in Poland

A short review of the Neoproterozoic and Cambrian sedimentary and volcanogenic successions in Poland is concerned with depositional and tectonic developments. The successions accumulated in shallow shelf and deeper slope environments. They overlie Palaeoproterozoic crystalline basement of the Baltic Craton and Gondwana-derived Neoproterozoic Cadomian basement. This discrepancy in the nature of the basement is viewed as indicating distinct terranes. During the course of Palaeozoic time, the discrete terranes were assembled along the Teisseyre-Tornquist Lineament and the Trans-European Suture Zone adjacent to the Baltica palaeocontinent.

Geochemistry of the late Proterozoic Kapp Hansteen igneous rocks of Nordaustlandet, Svalbard

Proterozoic igneous rocks occur in three areas in Nordaustlandet, Svalbard, and are found in the upper part of the Lower Hecla Hoek succession, the Botniahalveya Supergroup. The rocks have been called porphyrites in Botniahalveya. metadiabases in Prins Oscars Land and quartz porphyries in both areas as well as in the Sabinebukta area. All rocks have been metamorphosed under the greenschist facies conditions. The porphyrites are calc-alkaline acid andesites and dacites of medium to high K20 type, possibly showing a transition to tholeiitic series. The quartz porphyries are calc-alkaline rhyolites of high K20 type. The metadiabases are subdivided into two: the basic dykes of low K20 type and relatively high Fe tholeiite series, while the main bodies are acid andesites of medium to high K2O and low Fe tholeiite series. The basic dykes fall in the oceanic rock field of the Ti02-K20-P205 diagram, and are most likely belonging to the island arc type vo!canism. The metadiabases of main bodies and the porphyrites, and possibly the quartz porphyries, are chemically continuous. The medium to high K2O contents, and their Ti02-K20- P205 ratios suggest that these three rock groups are non-oceanic and resemble the rock associations of the areas having thick continental crust. This conclusion agrees with the reported high initial SrH7Mra tios and the existence of a distinct unconformity at the basc of this volcanogenic succession.