Major Terrane Boundary Research Articles

Petrology and 40Ar–39Ar dating of shear zones in the Lanterman Range (northern Victoria Land, Antarctica): implications for metamorphic and temporal evolution at terrane boundaries

The Lanterman Fault Zone, a major terrane boundary in northern Victoria Land, displays a polyphase structural evolution. After west-over-east thrusting, it experienced sinistral strike-slip shearing. Sheared metabasites from the Wilson Terrane (inboard terrane) preserve a record of retrograde metamorphic evolution. Shearing took place under amphibolite-facies metamorphic conditions (roughly comparable to those reached during regional metamorphism) which later evolved to greenschist-facies conditions. In contrast, the Bowers Terrane (outboard terrane) preserves a prograde metamorphic evolution which developed from greenschist-facies to amphibolite-facies metamorphism during shearing, followed by greenschist-facies metamorphism during the late deformational stages. Laser step-heating 40Ar–39Ar analyses of syn-shear amphibolite-facies amphiboles yielded ages of 480–460 Ma, in agreement with a ∼480-Ma age obtained from a biotite aligned along the mylonitic foliation. These ages are younger than those (∼492 to ∼495 Ma) obtained from pre-shear amphibole relics linked to regional metamorphism of the Wilson Terrane. Results attribute the structural and metamorphic reworking during shearing to the late stages of the Cambrian-Ordovician Ross Orogeny and to the Middle-Late Ordovician probably in relation to the beginning of deformation in the Lachlan Orogen, thus precluding any appreciable impact of the Devonian-Carboniferous Borchgrevink event in the study area.

The Seridó Group of NE Brazil, a late Neoproterozoic pre- to syn-collisional basin in West Gondwana: insights from SHRIMP U–Pb detrital zircon ages and Sm–Nd crustal residence (TDM) ages

The Seridó Group is a deformed and metamorphosed metasedimentary sequence that overlies early Paleoproterozoic to Archean basement of the Rio Grande do Norte domain in the Borborema Province of NE Brazil. The age of the Seridó Group has been disputed over the past two decades, with preferred sedimentation ages being either Paleoproterozoic or Neoproterozoic. Most samples of the Seridó Formation, the upper part of the Seridó Group, have Sm–Nd T DM ages between 1200 and 1600 Ma. Most samples of the Jucurutu Formation, the lower part of the Seridó Group, have T DM ages ranging from 1500 to 1600 Ma; some basal units have T DM ages as old as 2600 Ma, reflecting proximal basement. Thus, based on Sm–Nd data, most, if not all, of the Seridó Group was deposited after 1600 Ma and upper parts must be younger than 1200 Ma. Cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca. 600 Ma Brasiliano deformation and metamorphism, so that SHRIMP and isotope dilution U–Pb ages must represent crystallization ages of the detrital zircons. Zircons from meta-arkose near the base of the Jucurutu Formation yield two groups of ages: ca. 2200 Ma and ca. 1800 Ma. In contrast, zircons from a metasedimentary gneiss higher in the Jucurutu Formation yield much younger ages, with clusters at ca. 1000 Ma and ca. 650 Ma. Zircons from metasedimentary and metatuffaceous units in the Seridó Formation also yield ages primarily between 1000 and 650 Ma, with clusters at 950–1000, 800, 750, and 650 Ma. Thus, most, if not all, of the Seridó Group must be younger than 650 Ma. Because these units were deformed and metamorphosed in the ca. 600 Ma Brasiliano fold belt during assembly of West Gondwana, deposition probably occurred ca. 610–650 Ma, soon after crystallization of the youngest population of zircons and before or during the onset of Brasiliano deformation. The Seridó Group was deposited upon Paleoproterozoic basement in a basin receiving detritus from a variety of sources. The Jucurutu Formation includes some basal volcanic rocks and initially received detritus from proximal 2.2–2.0 Ga (Transamazonian) to late Paleoproterozoic (1.8–1.7 Ga) basement. Provenance for the upper Jucurutu Formation and all of the Seridó Formation was dominated by more distal and younger sources ranging in age from 1000 to 650 Ma. We suggest that the Seridó basin may have developed as the result of late Neoproterozoic extension of a pre-existing continental basement, with formation of small marine basins that were largely floored by cratonic basement (subjacent oceanic crust has not yet been found). Immature sediment was initially derived from surrounding land; as the basin evolved much of the detritus probably came from highlands to the south (present coordinates). Alternatively, if the Patos shear zone is a major terrane boundary, the basin may have formed as an early collisional foredeep associated with south-dipping subduction. In any case, within 30 million years the region was compressed, deformed, and metamorphosed during final assembly of West Gondwana and formation of the Brasiliano–Pan African fold belts.

Caltepec fault zone: An Early Permian dextral transpressional boundary between the Proterozoic Oaxacan and Paleozoic Acatlán complexes, southern Mexico, and regional tectonic implications

The tectonic boundary between the Grenville‐age Oaxacan and Paleozoic Acatlán crystalline complexes in southern Mexico, named the Caltepec fault zone (CFZ), is characterized for the first time as a dextral transpressional, NNW trending and ENE dipping ductile fault zone of Early Permian age. The complexes are welded by a syntectonic magmatic epidote‐bearing granite along the entire length of the CFZ. From east to west, the 2–6 km wide CFZ consists of disrupted and retrograded banded gneisses of the Oaxacan complex, quartz‐feldspar mylonite, and the syntectonic magmatic epidote‐bearing Cozahuico granite (CZG) with huge xenoliths (up to several kilometers long and up to 600 m wide) of the Proterozoic gneisses, thrust westward over metasedimentary tectonites of the Acatlán complex. The CZG shows magmatic fabrics that represent a transition to solid‐state deformation characterized by subvertical foliation, subhorizontal NNE and SSE dipping mineral stretching lineation and dextral kinematics. The megaxenoliths underwent partial melting developing banded migmatites with layers of epidote‐bearing granitic neosome. The parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation, migmatization of xenolithic gneisses, and granite emplacement along the CFZ were coeval. The neosome yielded a U‐Pb zircon concordant age of 275.6 ± 1 Ma probably dating the peak of the tectonothermal event. We interpret the CFZ as a major terrane boundary accommodating transpressional interaction between the Acatlán and Oaxaquia blocks, which were amalgamated in an oblique convergent setting by Early Permian time, as the leading edge of Gondwana impinged onto the southern margin of Laurentia along the Marathon‐Ouachita suture to form Pangea.

Petrogenesis of High-K Arc Magmas: Evidence from Egmont Volcano, North Island, New Zealand

Egmont Volcano (Mt Taranaki) is located 140 km west of the INTRODUCTION Taupo Volcanic Zone (TVZ), the principal locus of volcanic activity Egmont Volcano is a 2518 m high stratovolcano (Mt in the North Island of New Zealand, and is one of four closely Taranaki) resting on an extensive ring plain of volcanic associated Quaternary andesitic volcanoes in Taranaki province. debris in Taranaki province, western North Island, New Taranaki eruptives are enriched in K and other large ion lithophile Zealand. It is the youngest and largest of four Taranaki elements compared with their counterparts at Ruapehu in the southern volcanoes (Fig. 1), which together form a NW–SE trendTVZ, with the youngest Egmont andesites being the most K rich. ing lineament along which activity has migrated southEgmont andesites are invariably fractionated but isotopic information eastward with time (Neall et al., 1986). The Taranaki indicates that, unlike those at Ruapehu, they have not extensively volcanoes lie 140 km to the west of the Taupo Volcanic assimilated enriched crust. Ti/Zr, Ba/La, Ce/Pb, and K/Rb Zone (TVZ), the principal locus of subduction-related ratios indicate that a more depleted mantle wedge and compositionally magmatism in the North Island. different slab-derived fluids were involved in the generation of There are significant petrological contrasts between Taranaki primary magmas. Magmas parental to Egmont eruptives Egmont Volcano and the contemporaneous andesitic were relatively undersaturated, hydrous, high-Mg basalts generated stratovolcanoes of the southern TVZ, collectively referred by low degrees of partial melting in a depleted mantle wedge fluxed to as the Tongariro Volcanic Centre (TVC). For example, by deep slab fluids. Fractionation of these magmas at the base of the Egmont andesites are commonly hornblende bearing crust produced basaltic andesite and extensive ultramafic cumulates. whereas those of the TVC are generally clinopyroxene Plagioclase fractionation was suppressed by high aH2O. Rising and orthopyroxene types. Egmont eruptives are also geothermal gradients eventually resulted in partial anatexis of relatively potassic (Neall et al., 1986; Price et al., 1992; amphibolitic underplated crust, and interaction of basaltic andesites Stewart et al., 1996); they are high-K andesites according to the classification of Gill (1981). with these melts led to progressively more K-rich compositions. Both Taranaki and TVC are active volcanic systems associated with the Tonga–Kermadec–New Zealand convergent plate margin. On the basis of isotopic data, Price et al. (1992) suggested that both volcanic centres lie above mantle wedge with broadly similar chemical characteristics. The volcanic systems are, however, located on either side of a major terrane boundary and they

Selected Australian continental-scale lineaments: Observations and suggestions as to their origin

Australia is transected by a network of systematic continental-scale lineaments that are considered to be zones of concentrated, aligned tectonic activity which have apparent continuity over vast distances. The dominant directions form an orthogonal pair trending west-northwest and north-northeast: the King Leopold and Halls Creek Mobile Zones in northwestern Australia are an example of this. The consistent distribution of large-scale gravity and geological lineaments implies a common basement involvement for both, despite their expression at different levels within the crust. Continental-scale lineaments generally exhibit little to no offset where they intersect each other, suggesting an oscillatory history with little net offset. Geologic and chronological evidence demonstrate that many of the lineaments have been zones of reactivation since at least the Early Proterozoic (∼1880 Ma) and that they appear to cross major terrane boundaries. The lineaments have been sites of early tectonic activity that have undergone reactivation, resulting in repeated structural and depocentre control that is geographically associated with the continental-scale corridors. Alternative models for the origin of continental-scale lineaments include: a) a pre-existing lineament network maintained in an ancient basement underlying the entire continent; b) lateral propagation of crustal-scale structures; and c) alignment of genetically unrelated lineaments giving the appearance of continuity.

A comparison of the geochronology and geochemistry of plagioclase-dominated granitoids across a major terrane boundary in the SW Balitic Shield

To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identified and the timing of terrane accretion determined. New chemical and isotopic data confirm the terrane-separating character of a major shear zone, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Complex. These data relate to plagioclase-dominated granitoids to the east of the Mylonite Zone. Two zircon upper-intercept age determinations result in 1674 +24 −19 and 1688 +10 −10 Ma. Two separate crystals from the 1688-Ma rock give two single-zircon evaporation ages: one crystal gives 1646 ± 172 Ma (2σ) and the other 1654 ± 46 Ma (2s). Earlier investigations have pointed to vast amounts of similar looking granitoids to the west of the Mylonite Zone. However, these rocks are younger, approximately 1.60 Ga. The granitoids to the west of the Mylonite Zone are typical calc-alkaline rocks with a pronounced increase in the content of mafic minerals concurrent with a decrease in quartz. The trend of the eastern rocks is different, mainly involving changes in the quartz/total feldspar ratio. At the same SiO 2 content, the eastern rocks are lower in MgO and CaO and higher in (Na 2O + K 2O) and especially in Ba than their western counterparts. Also other elements like Ni show differences between the two suites. We conclude that these similar looking rocks belong to two different suites that are different in age and have different chemical signatures, probably caused by different source-rock chemistries. ϵ Nd1.68-values in the eastern suite range from + 2.0 to 2.5, suggesting that assimilation of large amounts of old continental rocks did not take place. Results from Sr-isotope determinations support this suggestion. The present results suggest, together with older data, that no granitoid rocks older than approximately 0.9 Ga are common to the two terranes separated by the Mylonite Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) orogeny.

Silicate‐perovskite and majorite signature komatiites from the Archean Abitibi Greenstone Belt: Implications for early mantle differentiation and stratification

Three komatiite‐tholeiite sequences from the Archean Abitibi Southern Volcanic Zone (SVZ), which are separated by major terrane boundaries, show systematic differences in composition, rare earth element (REE) patterns and high field strength element (HFSE) /REE fractionation. Komatiites from Tisdale Township are Al‐undepleted, with MgO = 12–24%, Al2O3/TiO2 = 13–17, CaO/Al2O3 = 1–1.3, and high Ni (722–1275 ppm) and Cr (1875–2820 ppm) contents. Mg‐tholeiites and Fe‐tholeiites are spatially associated with komatiites. Both komatiites and Mg‐tholeiites have flat REE patterns [(La/Yb)n = 0.6–1.2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1 (calculated using primitive mantle normalized values and adjacent REEs in the incompatibility sequence of elements in mid‐ocean ridge basalt; MORB), suggesting that they may have formed by high‐degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths (<300 km). Munro Township komatiites are Al‐undepleted, with MgO = 22–30%, Al2O3/TiO2 = 17–20, CaO/Al2O3 = 1.0–1.2, and high Ni (831–1590 ppm) and Cr (1250–2760 ppm) contents. They have light rare earth element (LREE) depleted patterns [(La/Yb)n = 0.2–0.4], and Nb/Nb*, Zr/Zr*, and Hf/Hf* > 1, consistent with having been derived from Mg‐perovskite enriched sources at depths >700 km below the transition zone, whereas spatially associated tholeiites have (La/Yb)n = 1–2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1, suggesting a shallow undepleted mantle source, distinct from the komatiites. Komatiites and tholeiites from Boston Township are characterized by MgO = 11–29%, Al2O3/TiO2 = 4.5–5.4, CaO/Al2O3 = l.4–2.5, high Ni (203–3420 ppm) and Cr (194–1965 ppm) contents; Al, Sc, Y, and heavy rare earth element (HREE) depletion, and pronounced negative normalized Zr and Hf anomalies (Zr/Zr* and Hf/Hf* < 1), in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300–600 km. The fractionated HFSEs and REEs of Munro ‐ and Boston‐area komatiites indicate that these Archean komatiites may have preserved some signatures of early mantle differentiation, which so far has only been inferred based on high‐pressure experimental studies and theoretical predictions. Further geochemical studies of Archean komatiites, especially precise multiple trace element analysis, combined with improved measurements of partition coefficients over a wide pressure range, will provide better understanding on early mantle differentiation, stratification, and the transition zone.

Determining the significance of high-grade shear zones by using temperature-time paths, with examples from the Grenville orogen

Ductile shear zones preserve essential information on processes that are active in orogenic roots, but the significance of these zones is often difficult to interpret. Structural, petrologic, and geochronologic data from shear zones yield elements of the history that are not necessarily synchronous. However, by combining these data with temperature-time ( T-t ) paths, insights are obtained into the nature of shear zones, the relation between bounding blocks, and orogenic evolution of the deep crust. This procedure is illustrated with two examples from the mid-Proterozoic Grenville orogen. T-t paths from ∼1160 to ∼900 Ma are based on U-Pb dating of metamorphic minerals—including garnet (closure temperature, T c , >800 °C), monazite ( T c ≅ 725 °C), sphene ( T c ≅ 600 °C), and rutile ( T c ≅ 400 °C)—and 40 Ar/ 39 Ar hornblende ages ( T c ≅ 480°C). Comparison of T-t paths from adjacent blocks allows predictions about the significance, kinematics, and timing of displacement of shear zones. In the Grenville orogen, T-t paths can distinguish between major terrane boundaries (e.g., the Carthage-Colton shear zone) and within-terrane shear zones (e.g., the Bancroft shear zone). Thus, these data can also be used to identify individual tectonic terranes in the deep crust. This integrated approach to analysis of shear zones provides constraints needed to determine the nature and rate of deep orogenic processes in areas that are complicated by high metamorphic grades.

Magmatism and the geodynamics of basin formation

The magmatic response to extension of the continental lithosphere, with the resultant formation of sedimentary basins, varies widely. Quantitative models (e.g. McKenzie and Bickle, 1988), based upon an assumption of anhydrous partial melting in both lithosphere and asthenosphere, suggest that the temperature of the sub-lithospheric upper mantle, upwelling beneath the zone of extension, exerts a fundamental control on both the chemistry and volume of the erupted magmas. High volcanicity rifts are, in general, only likely to occur when the asthenospheric potential temperature is ca. 100–300°C greater than normal, associated with the activity of mantle plumes. In addition, such models also predict a strong correlation between the volume of magma produced and the amount of lithospheric stretching (β). More complex models (e.g. Gallagher and Hawkesworth, 1992) include partial melting of volatile-rich mantle source regions with lower than normal solidus temperatures. These may reside within mantle plumes or within enriched domains within the continental lithospheric mantle. Quantitative modelling of the magma generation process in the latter case is limited by our rather poor understanding of volatile-present partial melting of the range of potential source components. Geochemical and isotopic (Sr—Nd—Pb—O) studies of the igneous rocks emplaced within a basin are of fundamental importance in evaluating the relative roles of lithospheric versus asthenospheric source components in the petrogenesis of the magmas and the depth and degree of partial melting. In addition, these data may provide important constraints on the amount of crustal contamination which has occurred, particularly by partial melting of the lower crust. Knowledge of the latter is important in the development of models to explain crustal thinning. In many basins the bulk of the mantle-derived magmas may never actually reach the surface, but are underplating and intruding the lower crust instead. Identification of such a mafic underplate is critical if the amount of crustal thinning is to be evaluated correctly. The location of eruptive sites within individual basins is frequently determined by pre-existing basement structures. Within the extensive Mesozoic rift system of West and Central Africa, Pan-African lithospheric shear zones and deep basement faults have acted as foci for rift basin development, with the faults acting as magma pathways to the surface. In western and central Europe, Neogene—Recent extension has generated an extremely varied magmatic response which can be correlated with the Hercynian structural grain of the lithosphere. The Rhine graben, trending at a high angle to the Hercynian terrane boundaries is largely amagmatic. In contrast the Ohre rift of Czechoslovakia, which trends parallel to a major (Saxothuringian—Moldanubian) terrane boundary, is a classic example of a pure shear rift with major volcanic complexes located along the axial zone.

Emplacement and deformation history of the western margin of the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary

Cretaceous plutons of the western margin of the Idaho batholith were emplaced along and to the west of the major terrane boundary separating middle Proterozoic and Paleozoic continental rocks from mostly Mesozoic accreted oceanic-arc terranes of the Blue Mountain Province. This boundary is marked by a change in the lithology of pendants and inclusions within the batholith. Plutons form two newly named complexes of igneous and metamorphosed igneous rocks. The Hazard Creek Complex, emplaced west of the boundary between the oceanic arc and the continental margin, consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons. The Little Goose Creek Complex, which intruded the boundary between the oceanic arc and the continental margin, is primarily porphyritic granodiorite to granite orthogneiss. A preliminary U-Pb age of 111 Ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic-arc-continent boundary. The plutonic rocks were deformed both during and after emplacement in response to east-west compressive stresses. Cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary. The major deformation of the Hazard Creek Complex occurred during its emplacement. The dominant fabric in the Little Goose Creek Complex is due to subsolidus ductile deformation. The localization of two deformation events along the pre-existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long-lasting, crustal flaw.

Nature of the contact zone between the North Cascades crystalline core and the Methow Sequence In the Ross Lake Area, Washington: Implications for Cordilleran tectonics

Previous analyses of the accretionary history of NW Washington‐SW British Columbia have suggested the possibility of large‐scale, dextral‐slip faulting events to explain the present day regional outcrop pattern. The contact zone between the crystalline Skagit Complex and the volcano‐sedimentary Methow sequence has been considered to be a major fault, named the Ross Lake fault by Misch (1966), and has been considered to be a possible large‐scale strike‐slip fault by later workers. Detailed mapping of this contact zone in the Ross Lake area shows that the brittle faults and mylonites thought by Misch to express dextral shearing along the contact are not continuous, throughgoing structures, and that the contact between Skagit orthogneisses and Methow sequence strata is primarily intrusive. Furthermore, the mylonites show both left‐ and right‐lateral shear. The Skagit‐Methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions. Based on these observations and regional synthesis of timing relations, deformation in the contact zone is interpreted to express regional ENE‐WSW shortening, not major dextral slip, in the early Tertiary. Since no terrane‐bounding fault exists, the Cretaceous‐Tertiary history of the Ross Lake area can be interpreted as follows: (1) Jurassic(?)‐Early Cretaceous forearc/rift basin sedimentation (deposition of the lower Methow sequence); (2) 110–95 Ma, imbrication of the lower Methow sequence and its basement of oceanic crustal rocks by east‐vergent thrusting, with concomitant development of a west‐derived foredeep in the upper Methow sequence; (3) 95–85 Ma, arc magmatism, with genesis of the Skagit migmatite from Methow and older basement protoliths; and (4) Paleocene‐Early Eocene folding, faulting, uplift, and exposure of a relatively intact 25+ km crustal section, termed the Skagit‐Methow crustal section by Kriens and Wernicke (1987).

Deep Seismic profiling in the Nosop Basin, Botswana: cratons, mobile belts and sedimentary basins

Deep Seismic reflection data from the Kalahari of western Botswana has been interpreted in terms of the tectonics of southern Africa. The 600 km of 12 fold data exhibit good quality reflections in the range 5–15 s TWT. The data outline the Nosop Basin, a depositional centre with greater than 10 km of Palaeozoic and younger sediments. Five prominent reflecting horizons are traced in the basement underlying the sediments. On the basis of the Seismic data, the Nosop is now interpreted as a single basin. The previous multi-basin interpretation of the aeromagnetic data -esults from features within the magnetic basement. The uppermost two basement units are interpreted as Proterozoic sediments that rest on Archean crust. The Seismic data show that the crust appears to thicken to the southeast as the Kaapvaal Craton is approached. At the same time, the sedimentary cover progressively thins suggesting that the western edge of the Kaapvaal is an extended margin. Interpreted deep crustal shear zones indicate a major terrane boundary separating the Proterozoic from the Archean. Depths to basement inferred from aeromagnetic data are consistent with the depth to the Seismic boundary associated with the top of the Archean. Some of the aeromagnetic expression may be due to Karoo volcanic intrusives in the basement. A thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby Ghanzi-Chobe belt and allows the mid-crustal layering to be dated as late Proterozoic.

Deep, high-amplitude reflections from a major shear zone above the subducting Juan de Fuca plate

Seismic reflection data show that a regionally extensive band of landward-dipping reflections exists above the subducting Juan de Fuca plate at the western Canadian continental margin. The reflections truncate at depth a major terrane boundary mapped near the surface, and must have a structural origin. Many reflections are very strong, having reflection coefficients locally as high as 10%-20%.These amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts, if the pores are very thin; however, seismic data from the accreted sedimentary melange farther seaward, from which much of the deep reflective zone is derived, suggest that significant porosity contrasts do not occur alone. Because higher reflection amplitudes are found in regions of inferred high strain, both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks, possibly originating close to the subduction decollement.

A Geochemical Study of Magmatism across a Major Terrane Boundary: Sr and Nd Isotopes in Proterozoic Granitoids of the Southern Laramie Range, Wyoming

Proterozoic granitoids in the Laramie Range of southeastern Wyoming crop out on both sides of an inferred suture between the Archean Wyoming craton and Proterozoic terranes to the south. These rocks offer an unusual opportunity to test the various hypotheses for the geometry of the suture. No difference in major or trace element composition is observed across the suture, nor is there a large difference in isotopic composition. Nd and Sr isotopic data indicate that only a small proportion of Archean material has contributed to this granitoid suite, even in two samples that intrude Archean gneiss. The isotopic data therefore require a dominantly Proterozoic lower crust both beneath the Proterozoic rocks and beneath the margin of the Archean block. This observation is inconsistent with models in which the suture between the Archean and Proterozoic blocks dips to the south. This inconsistency can be reconciled by magmatic underplating or mechanical underthrusting of Proterozoic crust beneath the Archean block during terrane accretion or post-accretion strike-slip faulting.

Discussion on trace element geochemical correlation in the reworked Proterozoic Dalradian metavolcanic suites of the western Ox Mountains and NW Mayo Inliers, Ireland

C. S. Jones & P. T. Leat write: In a recent paper Winchester et al. 1987 presented geochemical data for Caledonized metavolcanic rocks from the Western Ox Mountains and NW Mayo Inlier, Ireland. They found geochemical differences between two metabasalt units that occur within a thick metasedimentary sequence of semi-pelite and psammite, and are separated by a syn-metamorphic dislocation, (the Glennawoo Slide; Taylor 1969 ). Winchester et al. demonstrated that the metabasalts south of the slide have trace element compositions similar to mid ocean ridge basalt (MORB), whereas metabasalts north of the slide, have compositions similar to continental tholeiites and tholeiites of the ocean island basalt (OIB) association. They concluded from these data that the two groups of basalts were erupted in different crustal settings and proposed that the Glennawoo Slide is a major terrane boundary which subsequently brought the two units together. If accepted, this conclusion would have major implications for the tectonic history of the Irish Caledonides. The psammitic nature of the metasedimentary rocks closely associated with both groups of metabasalts indicates that neither was erupted in a truly oceanic setting. We argue here that both groups of metabasalts could have been erupted in the same continental margin environment, and we summarize structural, stratigraphic and metamorphic evidence which precludes the identification of the Glennawoo Slide as a terrane boundary as defined by Jones et al. 1983 . Geochemical evidence. A recent trend in basalt geochemistry has been the increasing awareness that degree of partial melting of mantle has a major

Menai Strait fault system: An early Caledonian terrane boundary in north Wales

A steep fault system runs parallel to the Menai Strait, which separates Anglesey from the mainland of north Wales. The Menai Strait fault system is interpreted as a major terrane boundary along which terrane docking took place during the late Precambrian or earliest Cambrian. The evidence is based on (1) marked geologic dissimilarities exhibited by the pre-late Early Cambrian basement to these terranes and (2) the existence of early, major ductile shear zones within the fault system. Rocks as varied as blueschists, low-grade melange, and shallow-level granitic complexes were juxtaposed along these steep shear zones prior to the eruption of ash-flow tuffs and deposition of upper Lower Cambrian marine sediments in the Welsh basin. Since the amalgamation of the Anglesey area with the Avalonian terrane (then a part of Gondwana), frequent Phanerozoic reactivation of the resulting composite terrane boundary has been restricted to mostly dip-slip fault movements.