Ensialic Arc Research Articles

Quaternary marginal basin volcanism in the Bransfield Strait as a modern analogue of the southern Chilean ophiolites

Abstract The Bransfield Strait is an actively-spreading, 65 km wide, Quaternary marginal basin at the northern tip of the Antarctic Peninsula. Across the Drake Passage from the Bransfield Strait are the Tortuga and Sarmiento ophiolites of southern Chile, which have been interpreted as uplifted, Jurassic-Cretaceous, marginal-basin curst that formed in a setting similar to the present Bransfield Strait. We present a comparison between the Bransfield Strait and the Chilean ophiolites, emphasizing new petrological and geochemical data for volcanic rocks dredged from two seamounts at the Bransfield Strait rift axis and published analyses of volcanic rocks from the marginal basin now preserved as the Chilean ophiolites. Volcanic rocks from the seamounts are about 100 000 years old, and major element abundances in these rocks are broadly similar to enriched mid-ocean ridge basalt (E-MORB). The rocks are slightly enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE), as in E-MORB, but they are moderately enriched in alkali, and alkali earth elements, as in arc basalts, and concentrations and ratios of diagnostic trace elements show island arc affinities. Geochemical discrimination diagrams suggest that Bransfield Strait lavas represent the initial stages of a petrogenetic transition linked to the tectonic transition from rifting in an ensialic arc to rifting in a marginal basin. Published rare earth element data from dykes and lavas in the Chilean ophiolites range from mildey depleted in LREE in the Tortuga ophiolite (Ce/Yb = 0.5 to 0.8, normalized to chondrites) to mildly enriched in LREE in the Sarmiento ophiolite (normalized Ce/Yb = 1.6 to 2.5). The latter are strikingly similar to data from the seamounts in the Bransfield Strait both in LREE enrichment (normalized Ce/Yb = 2 to 2.5) and total REE concentrations (20–45 times chondrite). We believe these chemical similarities, as well as other geological similarities pointed out by other authors, suggest that the Bransfield Strait marginal basin is at the same stage of development as was the marginal basin now preserved as the Sarmiento ophiolite.

Geochemical variations in Middle Ordovician volcanic rocks of the northern Miramichi Highlands and their tectonic significance

A detailed geochemical study of Middle Ordovician volcanic rocks, undertaken in the northern Miramichi Highlands of New Brunswick, shows that 10 basaltic suites can be distinguished. These suites are assigned to the Tetagouche and Fournier groups. The contact between these two groups is a major thrust zone, marked for over 70 km by a prominent blueschist zone. All the Tetagouche Group volcanic rocks have chemistries consistent with extrusion in a continental rift, but most Fournier Group basalts in the Miramichi Highlands have chemistries suggestive of an oceanic back-arc setting. The chemical signatures, stratigraphic variations, and structural data indicate that the northern Miramichi Highlands preserve a section across a telescoped Middle Ordovician back-arc basin that initially opened as a result of asthenospheric injection near the rear part of a Lower Ordovician ensialic arc.

Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland: Geochemical evidence of ensialic marginal basin volcanism

AbstractMetabentonites occur extensively in the Moffat Shale Group of the Southern Uplands of Scotland. At Dob's Linn 135 metabentonite beds, 1–50 cm thick, occur in Ashgill to Llandovery strata, representing an aggregate thickness of 6 m of compacted ash accumulated over approximately 25 Ma. The metabentonites are characterized by relatively high concentrations of trace elements, including Ba, Cs, Hf, Nb, Rb, Ta, Th, U, Y, Zr and REEs, which were inherited from evolved vitric ash. Immobile trace element data indicate that a spectrum of silicic ash compositions accumulated, ranging from subalkaline to mildly peralkaline. In the late Ordovician N. gracilis to G. persculptus biozones, subalkaline ash falls predominated, whereas peralkaline ash falls predominated in the Llandovery (Silurian) P. acuminatus to M. convolutes biozones, giving way to predominantly subalkaline ash falls during accumulation of the M. sedgwickii to R. maximus biozones. Changeovers in the dominant ash types are marked by increased proportions of ash. The magmas from which the ash types evolved were generated in an ensialic arc transitional to a back-arc setting, and involved attenuated sialic crust and mantle characterized by variable depletion in HFS elements. Lithological, petrological and REE characteristics suggest that the Moffat Shale Group is not exclusively pelagic in origin and probably accumulated in a back-arc basin bordering an ensialic arc terrane.

Contrasting accreted terranes in the southern Appalachian Orogen, basement beneath the Atlantic and Gulf Coastal Plains, and West African orogens

A variety of lithostratigraphic units may be outlined within the pre-Mesozoic crystalline basement of the Atlantic and Gulf Coastal Plains. These include: (1) a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern Alabama and southeastern Mississippi (Wiggins Uplift). Hornblende and biotite within higher-grade units record 40Ar/ 39Ar post-metamorphic cooling ages of ∼310−300 Ma; (2) a suite of contrasting igneous rocks (granite, basalt, and agglomerate) and serpentinite which occurs along the Brunswick-Altamaha Magnetic Anomaly in southwestern Alabama; and (3) an extensive, apparently coherent tectonic element (the Suwannee Terrane) comprised of undeformed granite (in which biotite records 40Ar/ 39Ar post-magmatic cooling ages of ∼530−525 Ma), low-grade felsic metavolcanic rocks, a suite of high-grade metamorphic lithologies (St. Lucie Metamorphic Complex in which hornblende yields 40Ar/ 39Ar post-metamorphic cooling ages of ∼515−510 Ma), and a succession of Lower Ordovician-Middle Devonian sedimentary rocks (characterized by Gondwanan fauna). The West African Orogens (Mauritanides, Bassarides, and Rokelides) record a locally complex, polyphase tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African Craton at ∼700 Ma. This led to development of a rift-facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences. Rifting was limited, and a western ensialic arc began to develop by ∼680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ∼650 Ma (Pan-African I orogenesis). Following widespread deposition of late Paleozoic glacial and flyschoid sediments, a second tectonothermal event occurred between ∼550 and 500 Ma (Pan-African II orogenesis). The West African Orogens were marked by general tectonic quiescence throughout most of the early and middle Paleozoic. Late Paleozoic (∼300 Ma) collision of Gondwana and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record an associated penetrative tectonothermal overprint. Characteristics of pre-Mesozoic basement units which comprise the Suwannee Terrane in the Florida subsurface suggest they represent extensions of the Bassaride-Rokelide Orogen. There is no apparent record of Paleozoic tectonothermal activity in the Suwannee Terrane. This contrasts markedly with penetrative late Paleozoic reworking of basement units in the Wiggins Uplift. This suggests that a major dextral transcurrent fault was active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various basement units of the Coastal Plain are not correlative with any of the non-Laurentian terranes exposed in the southern Appalachian Orogen (e.g., Carolina terrane of the eastern Piedmont) which accreted in the Ordovician-Devonian to exterior positions along the eastern margin of the North American craton. These exotic Appalachian terranes were transported into their present structural positions during late Paleozoic collision of Gondwana and Laurentia. They are separated from Gondwanan elements of the Suwannee Terrane by a suture approximately marked by the Brunswick-Altamaha Magnetic Anomaly.

Pétrologie des roches volcaniques de sillon de roches vertes archéennes de Matagami–Chibougamau à l'ouest de Chapais (Abitibi est, Québec), 2. Le groupe hautement potassique d'Opémisca

In the western part of the Chapais syncline (Abitibi East, Quebec), the Opemisca Group unconformably overlies the Roy Group at a low angle. It consists of a thick turbidite sequence covered by an interdigitated sequence of lavas and alluvial cone sediments. The subaerial lavas include two sequences evolving from porphyric metabasalts to metatrachyandesites and porphyric metatrachytes (lower sequence) or to K-rich aphanitic meta-andesites (upper sequence). These lavas, with calc-alkaline to shoshonitic affinity, have high K2O, Ba, Sr, and Th contents and show highly enriched LREE spectra.The behaviour of major elements, trace elements, and lanthanides suggests an origin from partial melting of a mantle source consisting of a garnet lherzolite enriched in K, Sr, Rb, Ba, and Th by volatile elements and also by crustal contamination and by fractional crystallization mechanisms. The evolution of the lavas of the lower sequence, progressively deficient in Y, seems to have been controlled by fractional crystallization of a plagioclase, clinopyroxene, and olivine mixture; this was followed for the metatrachyandesites and metatrachytes by high H2O activity of a feldspar, amphibole, titanomagnetite, and apatite mixture. The evolution of lavas enriched in Y in the upper sequence seems to have been controlled by weak H2O activity of the anhydrous plagioclase, clinopyroxene, and olivine assemblage.The petrographic and geochemical data suggest an emplacement similar to that occurring on the active continental margins of the central Andes and implies the existence of a late-stage ensialic arc. [Translated by the journal]

Submarine fan facies in the Torlesse terrane, New Zealand

Abstract The Torlesse terrane is an assemblage of late Palaeozoic and Mesozoic greywackes allochthonous to other coeval rocks in New Zealand. The sedimentology of these greywackes indicates submarine fan depositional processes; two exceptions are Middle Triassic (Kaihikuan) non-marine to shelf deposits and one occurrence of Jurassic non-marine beds. The source terrane is inferred to be an ensialic arc, and deposition occurred in a variety of basinal settings in a region of subduction. The internal tectonic complexity of the greywacke strata and the rather simple map pattern of the biostratigraphic zones reflect tectonic kneading and a systematic outbuilding of an accretionary prism.

Cretaceous to Eocene volcanic arc activity in the Andes of northern Ecuador

During the Cretaceous to Eocene the W. part of N. Ecuador received the tholeiitic and calk-alkaline magmatic products of one volcanic arc, which built up on a thin or dense basement, at least partly of oceanic origin. Accumulation of this arc was preceded by early Cretaceous volcanicity and flexuring in the E. and was brought to a close by late Senonian to Eocene orogenic movements. The partly ensimatic arc of N. Ecuador contrasts markedly with the totally ensialic arc of S. Ecuador and Peru, and suggests an important change in basement type near 2°30'S (the latitude of Guayaquil). Oceanic basement in N. Ecuador was probably trapped in the Lower Cretaceous by initiation of subduction at a trench in approximately the same relative offshore position as at present.

Provenance of Ordovician deep-water sandstones, Tourelle Formation, Quebec, and implications for initiation of the Taconic orogeny

The Lower Ordovician Tourelle Formation crops out on the Gaspé Peninsula in the allochthonous external domain of the Quebec Appalachians. Coarse sandstone beds dominate this submarine fan deposit, and contain immature detritus (volcanic fragments, radiolarian chert, amphiboles, pyroxenes, chromite) shed from initial uplifts within the Taconic orogenic belt. Pre-Tourelle sediments were eroded directly from a Lower Paleozoic Grenville-type craton and its fringing carbonate bank. Recycling of these sediments provided the bulk of the detritus found in the Tourelle sandstones.Volcanic rock fragments comprise 4% of framework grains. They are intermediate to acid in average composition, contain primary biotite, and record prehnite–pumpellyite grade metamorphism in the source area. This volcanic source also contributed fresh angular plagioclase (albite) to the Tourelle sand fraction. The volcanic source was probably an ensialic arc associated with the closing of the Iapetus Ocean, and is tentatively identified with the Cambrian to Middle Ordovician Tetagouche volcanics of northern New Brunswick.In addition to rounded, recycled grains of zircon, tourmaline, garnet, and sphene, the Tourelle heavy mineral suite contains prismatic hornblende, hypersthene, and clinopyroxene. These minerals may have been derived from arc lavas, their intrusive equivalents, or from an obducted ophiolite sheet. Ophiolite obduction and erosion is preferred because the [Formula: see text] heavy mineral population contains 18% chromite, or 8.2 × 10−4 g Cr/cm3 sandstone. This is 1/15 the chromite concentration of peridotite, an essential component of any ophiolite suite.Provenance criteria for these syn-orogenic sandstones require a plate tectonic model involving southeastward subduction beneath a block of continental crust, and emplacement of ophiolite. Previous models for the Quebec Appalachians are unable to account for detrital composition of Tourelle sands and must undergo major modification.This paper reveals that sedimentary petrography is essential to satisfactory unravelling of orogenic history. Neglect of sediment composition may invalidate tectonic reconstructions based solely on stratigraphic and structural considerations.