True Oceanic Crust Research Articles

Structural - tectonic update of the central Vietnam margin based on analysis of the merged gravity - magnetic and regional seismic data

The central Vietnam margin is part of the western continental margin of the Vietnamese East Sea - the transition zone from the Indochina continental block to the true oceanic crust. In this paper, the authors present new results from merging and interpreting gravity and magnetic data from different sources in combination with 2D seismic and Ocean Drilling Program well data for the central Vietnam margin.The results illustrate the reliability of the merged gravity - magnetic data in regional structural analysis and tectonic division. Five structural layers are vertically identified and characterised by density and thickness. Spatially, six tectonic zones are defined with different Bourguer and magnetic anomaly background values: the boundaries between these zones are clearly delineated by sudden changes of these values. In addition, development signs and extent of the Tuy Hoa Shear Zone on basement rocks are also better resolved compared to the earlier seismic data.

Preserved ancient oceanic lithosphere within the Buem structural unit at the eastern margin of the West African Craton

This work presents new major and trace element geochemical data and the first SmNd and LuHf isotopic data for the mafic suite of the Buem structural unit (BSU) in Ghana in order to infer its origin and geodynamic setting, relationship to the Pharusian suture zone and significance in the Dahomeyide belt. The BSU, which occurs about 50 km to the west of the Pharusian suture zone, comprises well-preserved, weakly metamorphosed, mafic volcanic and mafic-ultramafic plutonic rocks. Two lava types can be distinguished: Type I and Type II lavas that are spatially separated and geochemically distinct. The Type I lavas are subalkaline and show trace element patterns akin to mid-ocean ridge basalt (MORB). Their εNd800Ma values of +5.3 to +6.2 and εHf800Ma values of +11.1 to +17.2 are indications of juvenile magma derived from a depleted mantle source in the spinel stability field. The Type II lavas have alkaline signatures with trace element patterns that resemble ocean island basalts. They formed by smaller degrees of disequilibrium partial melting at depth in which garnet likely played a role as a residual phase having εHf800Ma and εNd800Ma values of +3.2 to +5.1 and + 4.1 to +6.3 respectively. The gabbros show similar trace elements concentration and isotopic ratios as the BSU lavas, an indication of a similar mantle source and environment of formation. Because the geochemical features of the BSU mafic suites are comparable to the MORB-type high-pressure rocks of the Pharusian suture zone, both represent the remnant of the Pharusian oceanic crust, formed during the breakup of Rodinia. The BSU corresponds to the true oceanic crust of the West Gondwana Orogen continental margin-type ophiolite, sharing similar lithological and geochemical features to other continental margin-type ophiolites in the world.

Architecture of a magma poor passive margin – Insights from the Somali margin

The architecture of magma-poor continental margins, from break-up to the establishment of an oceanic spreading centre has received significant recent interest. A general consensus as to their characteristics exists and although it has been applied to margins globally is still dominated by the Iberian-Newfoundland “type” margin. Additionally, many studies rely upon sparsely distributed 2D seismic reflection profiles, or single seismic sections. Questions remain as to the validity of the model across other magma-poor margins, in particular the role of magmatic additions during the rifting phase and their relationship to the creation of true oceanic crust at the onset of sea-floor spreading.Our data comprise a 2D seismic reflection survey from the Obbia Basin, northern Somalia passive margin. The data are extremely well imaged to over 13 s two-way time, providing insights into the crustal structure. Here, we present 4 representative sections from this larger survey, which combined with gravity modelling, describe the character and crustal architecture along the margin from the proximal rifted continental crust to the oceanic domain.We find that the Somali margin is a poorly understood example of a magma-poor margin exhibiting Iberian-type architecture. The margin comprises a stretched proximal domain that passes seaward into a strongly thinned necking zone across which the lower crust is completely thinned. We identify a high amplitude reflection (HAR) that corresponds to the decoupling level, and into which crustal faults sole out. Seaward, the HAR merges with the top of the exhumed mantle directly underlying post-rift sediments. In contrast to other magma-poor margins, the Somali margin has a narrow necking zone and a wide zone of exhumed mantle (in excess of 100 km). On most sections true oceanic crust is never encountered, with only the distal portion of one section exhibiting oceanic crustal character. We place the continent-ocean transition zone in excess of 200 km east of the shoreline. In the north, the basin is affected by transform faults; where this occurs we note a presence of volcanics within the crust and post-rift sediments. Gravity modelling supports the addition of magmatic material in these regions. This addition of volcanic material, in contrast to that proposed at transform faults in other passive margins, is spatially restricted to the transform faults, emplaced post-kinematically, and is not associated with oceanic crust formation. The results of this study not only provide new insights into the architecture on non-typical non-magmatic margins but also reveal the importance of incorporating these findings into plate-reconstructions.

Uncertainties in break-up markers along the Iberia–Newfoundland margins illustrated by new seismic data

Abstract. Plate tectonic modellers often rely on the identification of “break-up” markers to reconstruct the early stages of continental separation. Along the Iberian-Newfoundland margin, so-called break-up markers include interpretations of old magnetic anomalies from the M series, as well as the “J anomaly”. These have been used as the basis for plate tectonic reconstructions are based on the concept that these anomalies pinpoint the location of first oceanic lithosphere. However, uncertainties in the location and interpretation of break-up markers, as well as the difficulty in dating them precisely, has led to plate models that differ in both the timing and relative palaeo-positions of Iberia and Newfoundland during separation. We use newly available seismic data from the Southern Newfoundland Basin (SNB) to assess the suitability of commonly used break-up markers along the Newfoundland margin for plate kinematic reconstructions. Our data show that basement associated with the younger M-series magnetic anomalies is comprised of exhumed mantle and magmatic additions and most likely represents transitional domains and not true oceanic lithosphere. Because rifting propagated northward, we argue that M-series anomaly identifications further north, although in a region not imaged by our seismic, are also unlikely to be diagnostic of true oceanic crust beneath the SNB. Similarly, our data also allow us to show that the high amplitude of the J Anomaly is associated with a zone of exhumed mantle punctuated by significant volcanic additions and at times characterized by interbedded volcanics and sediments. Magmatic activity in the SNB at a time coinciding with M4 (128 Ma) and the presence of SDR packages onlapping onto a basement fault suggest that, at this time, plate divergence was still being accommodated by tectonic faulting. We illustrate the differences in the relative positions of Iberia and Newfoundland across published plate reconstructions and discuss how these are a direct consequence of the uncertainties introduced into the modelling procedure by the use of extended continental margin data (dubious magnetic anomaly identifications, break-up unconformity interpretations). We conclude that a different approach is needed for constraining plate kinematics of the Iberian plate pre-M0 times.

Breaking up continents at magma-poor rifted margins: a seismic v. outcrop perspective

The break-up of continents at magma-poor rifted margins is a complex yet little understood process accounting for intricate interactions between tectonic and magmatic processes. Whereas high-resolution seismic data may resolve the first-order architecture of ocean–continent transitions (OCTs), direct access to rocks remains limited to deep-sea drilling. In this study, we combine seismic observations from the East Antarctica margin (Geoscience Australia Survey 228) with field observations from the Bracco–Levanto area (Northern Apennines, Italy), representing modern and fossil examples of ultra-distal magma-poor rifted margins respectively. The combination of detailed structural mapping and petrological studies from fossil examples with architectural features observed in seismic sections from present-day OCTs enables to bridge the different observation scales. Field evidence indicates that the magmatic budget is not zero, as magma is present throughout the exhumation process. Extensional detachment faults that exhumed serpentinized mantle and deep-seated gabbro intrusions are truncated by later high-angle normal faults, which most probably acted as feeders for the emplacement of massive syn-extensional basalts. These observations suggest a polyphase tectonic and magmatic evolution of the ultra-distal margin prior to the formation of the first true oceanic crust, which can be studied in detail only by combining seismic and outcrop observations.

Public domain satellite gravity inversion offshore Somalia combining layered-Earth and voxel based modelling

At the end of 2016, Spectrum Geo released two long offset seismic reflection profiles across the Somalian rifted margin, in the Juba Lamu and Obbia basins respectively – part of two larger 2D surveys acquired in 2014 and 2015/16 (Stanca et al., 2016; Figure 1). In frontier areas, potential field data interpretation, in particular public domain satellite-derived gravity, can play a key role in the early stages of exploration by identifying basement structure, sediment thickness, and the continental-oceanic crust transition (COT) or continental-oceanic boundary (COB), and hence indirectly contribute to the understanding of thermal history and the hydrocarbon system. This is particularly relevant over the Somalia margin, where geophysical surveys have been extremely sparse until very recent years. A high-gradient in the gravity is often used as a marker of the COB along passive margins (see Pawlowski (2008) for a review), expressing laterally contrasting physical properties between the continental and oceanic crusts. However, at magma-poor rifted margins, such as the Somalia margin, the lithosphere undergoes a progressive thinning/stretching process and the transition between unaltered continental crust and oceanic crust becomes gradual (Manatschal, 2004). Crustal hyper-extension, serpentinization, embrittlement, and exhumation of mantle peridotites can precede accretion of true oceanic crust, which in turn can blur the simple transitional model between a dense and highly magnetized oceanic crust and a weakly magnetized and lighter continental crust. Along a magma-poor margin, analysis of potential field data can distinguish the different rifting domains and quantify crustal thinning (Stanton et al., 2016; Cowie et al., 2015). In offshore Kenya and Somalia, the image is further complicated by thick sedimentary cover.

Petrogenetic study of Mesoproterozoic volcanic rocks of North Delhi fold belt, NW Indian shield: implications for mantle conditions during Proterozoic

Mesoproterozoic North Delhi fold belt of NW Indian shield comprises three volcano-sedimentary basins viz. Bayana, Alwar and Khetri aligned parallel to each other from east to west. Each basin contains excellent exposures of mafic volcanic rocks. Major, trace and rare earth element abundances of volcanic rocks of the three basins are significantly diverse. Bayana and Alwar volcanics are tholeiites bearing close similarity with low Ti-continental flood basalts. However, Bayana volcanics are characteristically enriched in incompatible trace elements and REEs while Alwar volcanics display least enriched incompatible trace element abundances and flat REE patterns. The Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. REE based source modeling suggests that Bayana suite was formed from the melts derived from 1 % to 10 % (avg. 4 %) of the partial melting of a spinel lherzolite source giving a residual mineralogy of 56 % Olv, 25 % Opx and 19 % Cpx. Whereas Alwar suite evolved through 12 %–20 % (avg. 15 %) partial melting of the same source with a residual mineralogy 61 % Olv, 25 % Opx and 14 % Cpx. Khetri volcanics are exposed at two localities Kolihan and Madhan–Kudhan. The Kolihan volcanics were derived from 1 % to 6 % (avg. 4 %) partial melting with residual mineralogy 56 % Olv, 25 % Opx and 19 % Cpx whereas the magma of Madhan Kudhan volcanic suite was generated by 15%–30 % partial melting of the same source leaving behind 64 % Olv, 25 % Opx and 11 % Cpx as residual mineralogy. This source modeling proves that melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field under the influence of mantle plume. During the course of ascent, Bayana melts were crustally contaminated but Alwar melts remained unaffected. There was two tier magma production in Khetri region, one from the partial melting of the mantle wedge overlying the subducted oceanic plate which formed Kolihan suite and two the melting of the subducted plate itself generating Madhan–Kudhan volcanics. It is interpreted that during Mesoproterozoic (1,800 Ma), the continental lithosphere of NW Indian shield suffered stretching, attenuation and fracturing in response to a rising plume. Consequently, differential crustal extension coupled with variable attenuation brought the asthenosphere to shallower setting which led to the production of tholeiitic melts. These melts enroute to the surface suffered variable lithospheric contamination depending upon the thickness of traversed crust. The Khetri basin attained maturity which resulted in the generation of true oceanic crust and its subsequent destruction through subduction. The spatial existence of three suites of mafic volcanics of diverse chemical signatures is best example of subduction–plume interaction. It is therefore, proposed that the Mesoproterozoic crust of NW Indian shield has evolved through the operation of a complete Wilson cycle at about 1,832 Ma, the age of mafic volcanics of Khetri basin.

The structure, evolution and symmetry of the magma-poor rifted margins of the North and Central Atlantic: A synthesis

Magma-poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting, the serpentinization of the mantle beneath crust thinned to less than 8 ± 2 km, and the unroofing of a broad zone of mantle within the continent–ocean transition, accompanied by the development of detachment and other large-offset faults. These observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes. Although the paucity of magmatism may be explained by depth-dependent extension of the lithosphere, and by pre-depleted sub-crustal lithosphere, rifting above initially cool sub-lithospheric mantle also may explain the subsidence deficit observed at some margins: synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere. As it extends, thins and cools, ductile creeping layers in the mid- and deep crust become progressively more brittle, resulting in increased coupling between the upper and lower crust, and eventually the embrittlement of the entire crust, faults cutting from the surface across the Moho, bringing water into the mantle and causing its serpentinization. Increased coupling and the development of serpentine detachments predict the development of late-stage asymmetry once the entire crust is brittle. Such detachments are imaged on some margins and inferred on others; analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric. Similar analyses show that depth-dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning. Instead this “extension discrepancy” may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting, related to the changes in the rheological character and strength of the lithosphere as it is thinned. Complex polyphase faulting continues after complete crustal separation, resulting in the exhumation of broad expanses of peridotitic basement, the top of which is everywhere marked by an exhumed slip surface, similar to the corrugated surface observed at mid-ocean ridges. The similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the COT and true oceanic crust difficult and possibly moot.

Architecture of volcanic rifted margins: new insights from the Exmouth – Gascoyne margin, Western Australia

Australia's western continental margin formed during Gondwana breakup between Australia – Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic crust accreted to the continental fragments. In this context, the margin segment from the outer Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of volcanic margin formation. However, many recent analyses based on selective interpretation of geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture and the age of breakup on this margin. These differences profoundly impact on models for both the global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in this margin, based on the integrated interpretation of two deep crustal transects with existing seismic-reflection, refraction, gravity, magnetic and geochemical data. Interpretations are constrained by data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic-margin transition zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning, between upper and lower crust, provide support for models of depth-dependent thinning previously proposed for this margin.

Fossil wood and Mid-Eastern Europe terrestrial palaeobiogeography during the Jurassic–Early Cretaceous interval

Palaeobiogeography plays an important role in the evolution of continental plants. This has been demonstrated mainly for modern biota and for past biota on a very large scale only. During the Jurassic–Early Cretaceous Mid-Eastern Europe was an archipelago, thus a particularly suitable area for a more detailed study. We investigated the area's plant palaeobiogeography, using fossil wood, with information from both a literature survey and investigation of new samples. There is a clear north–south differentiation of wood floras. The northern part of the archipelago, which was connected by a shallow sea, has a homogenous flora. A small terrane in the south, separated by true oceanic crust, seems to have had a peculiar flora, lacking widely distributed elements but displaying an endemic taxon with Gondwanan affinities. Compared to Western Europe, Mid-Eastern Europe has a Jurassic–Early Cretaceous wood flora with similar diversity, except for the Late Jurassic, when it was limited to a single taxon, the widespread Agathoxylon Hartig. The wood flora of northern Gondwana is less diverse across the time interval under consideration, except for the Late Jurassic again. Taphonomic bias cannot be ruled out, but this low diversity during the Late Jurassic suggests stressful climatic conditions for Mid-Eastern Europe.

Lithospheric transition from the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic

A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt—the Variscan Iberian Massif—to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa–Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results show that the Variscan Iberian crust has a roughly constant thickness of ∼30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (∼6 km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of ∼200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required to fit the observed geoid, gravity and elevation over the Ossa Morena and South Portuguese zones. Such mass deficit can be interpreted either as a lithospheric thinning of 20–25 km or as an anomalous density reduction of ∼25 kg m −3 affecting the lower lithospheric levels. Whereas the first hypothesis is consistent with a possible thermal anomaly related to recent geodynamics affecting the nearby Betic–Rif arc, the second is consistent with mantle depletion related to ancient magmatic episodes that occurred during the Hercynian orogeny.

Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin crust within the ocean–continent transition (OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic crust. This crust is usually found to be underlain by upper mantle material of 7.2–7.4 km/s velocity at shallow depths (1–2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges.

New Data Regarding Hotly Debated Topics Concerning UHP Metamorphism of the Dabie-Sulu Belt, East-Central China

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic mélange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are of two types— metasedimentary “in situ” and igneous “foreign.” The paragneisses contain UHP garnets + white micas, and are uniformly older (235 ± 5 Ma) than the orthogneisses (210 ± 5 Ma), which are devoid of UHP mineralogic indicators. (3) Fluids were active under UHP conditions and allowed the formation of UHP hydrous phases such as phengite and zoisite. However, the aqueous fluids may have been restricted to certain channels/pathways during exhumation. External fluids were absent until ascent of the UHP rocks to middle-crustal levels. (4) The Yangtze and Sino-Korean continental blocks collided during 230 to 240 Ma, when supracontinental rocks experienced UHP metamorphism. The HP metamorphic event dated as >400 Ma might record a subduction of oceanic crust during the Paleozoic. (5) An ancient mantle wedge is revealed by geochemical characteristics of Mesozoic magmatic rocks developed on the southern margin of the Sino-Korean craton, the hanging wall of the UHP-rock-bearing unit. Seismic tomography images reveal that the Yangtze block extends beneath the Dabie-Sulu orogenic belt. This indicates that both oceanic and continental crust had a northward subduction polarity. (6) Taking petrologic and geochronological data into account, we prefer a multistage exhumation model. The UHP rocks were exhumed rapidly during the first stage (230 to 200 Ma), perhaps reflecting a corner-flow mechanism. Then, buoyancy and mantle upwelling brought the UHP rocks up to middle-crustal levels during the second stage (200 to 170 Ma). Extension and thermal uplift, as well as erosion, eventually exposed the UHP rocks to the surface in the third stage (170 to 120 Ma).

Puncoviscana folded belt in northwestern Argentina: testimony of Late Proterozoic Rodinia fragmentation and pre-Gondwana collisional episodes

Stratigraphic correlations and tectonic analysis suggest that the Puncoviscana fold belt of northwestern Argentina was an intracontinental basin with bimodal igneous suites that formed in connection with the breakup of the Rodinia supercontinent (at ∼800 Ma). Several lines of evidences point to an initial lithosphere rupture, possibly induced by a rising mantle plume. The earliest synrift igneous products are represented by ultra-potassic dykes and alkaline lava flows of high LREE/HREE and low Zr/Nb–Y/Nb ratios. The dyke emplacements and the initiation of rifting were probably synchronous. They pass laterally and upwards (middle part of the Puncoviscana succession) into basalts of alkaline transitional character (OIB-like source). The distinctive chemical feature of these lavas are very similar to the source of oceanic island basalts; thus, they are thought to represent a magmatism associated with the rift and rift-drift transition stage. During this stage of rifting probably true oceanic crust was formed. The upper part of the Puncoviscana sequence, Late Precambrian/Lower Cambrian in age, comprises a thick and monotonous sequence of pillow lavas, massive basaltic flows and minor volcanic breccias and hyaloclastites. These lavas exhibit MORB trace element characteristics with high FeOt and TiO2, low K2O and P2O5, flat light REE spectra, little or no depletion in Nb and Ta. This volcanism consists of the major and latest effusive episode from the Puncoviscana basin which was slightly modified by subduction processes. The geodynamical model proposed for the generation of these volcanic rocks could have been developed in two stages. In the first stage the volcanic event is compatible with a progressive opening of a continental rift leading to formation of a mature oceanic basin. In contrast, the second stage shows the effects of a completed Wilson cycle including a primitive volcanic arc which continued until the accreted Cuyania-Arequipa-Belen-Antofalla (CABA) terrane against the proto-Gondwana western borderland of the Amazonian shield (∼535 Ma).

On causal links between flood basalts and continental breakup

Temporal coincidence between continental flood basalts and breakup has been noted for almost three decades. Eight major continental flood basalts have been produced over the last 300 Ma. The most recent, the Ethiopian traps, erupted in about 1 Myr at 30 Ma. Rifting in the Red Sea and Gulf of Aden, and possibly East African rift started at about the same time. A second trap-like episode occurred around 2 Ma and formation of true oceanic crust is due in the next few Myr. We find similar relationships for the 60 Ma Greenland traps and opening of the North Atlantic, 65 Ma Deccan traps and opening of the NW Indian Ocean, 132 Ma Parana traps and South Atlantic, 184 Ma Karoo traps and SW Indian Ocean, and 200 Ma Central Atlantic Margin flood basalts and opening of the Central Atlantic Ocean. The 250 Ma Siberian and 258 Ma Emeishan traps seem to correlate with major, if aborted, phases of rifting. Rifting asymmetry, apparent triple junctions and rift propagation (towards the flood basalt area) are common features that may, together with the relative timings of flood basalt, seaward dipping reflector and oceanic crust production, depend on a number of plume- and lithosphere- related factors. We propose a mixed scenario of `active/passive' rifting to account for these observations. In all cases, an active component (a plume and resulting flood basalt) is a pre-requisite for the breakup of a major oceanic basin. But rifting must be allowed by plate-boundary forces and is influenced by pre-existing heterogeneities in lithospheric structure. The best example is the Atlantic Ocean, whose large-scale geometry with three large basins was imposed by the impact points of three mantle plumes.

Geochemical evidence for progressive, rift-related early Palaeozoic volcanism in the western Sudetes

The volcanigenic rocks of the Kaczawa Mts (western Sudetes, Poland), in the eastern Variscides, show changing geological and geochemical evolution during early Palaeozoic time. The lower part of the succession (Cambrian (?)–Ordovician) has three components. 1. Shallow marine to subaerial metabasalts, associated with limestones and volcaniclastics. The lavas are dominantly of a transitional tholeiitic–alkaline type and their trace element patterns typically represent a rift-related environment. They pass laterally (and upwards ?) into more depleted basalts resembling enriched MORB, with Nd-isotopic characteristics indicating contamination by continental crust. 2. Interlayered rhyodacitic lavas and volcaniclastics which show negative ɛ Nd values, suggesting formation of the original magma by crustal melting. 3. An overlying alkaline bimodal suite of lavas and volcaniclastic rocks, as well as alkaline metabasites of shallow-intrusive character. The geochemistry of the latter resembles oceanic island volcanics, but they may well have been emplaced in the same evolving rift environment. The upper part of the Kaczawa succession, Ordovician-Silurian (?) in age, comprises a thick and monotonous sequence of deep-marine pillowed and massive metabasalts, associated with black shales and cherts. These lavas exhibit MORB trace element characteristics, with minor evidence of crustal contamination. During this stage of rifting, true oceanic crust probably formed. It is thus suggested that the studied part of the Kaczawa succession developed in a progressively evolving rift, initially within an ensialic environment, and finally reaching the stage of a basin underlain by oceanic-type crust. Together with similar Cambrian-Ordovician volcanic-sedimentary associations, widely distributed in western Europe, from Portugal, through France and Germany, they represent a record of the Early Palaeozoic rifting in the northern periphery of Gondwana.

Proterozoic c-type eclogites hosting unusual TiFe±Cr±Cu mineralization in northeastern Brazil

Abstract Mafic to ultramafic rocks occur at Bodoco, about 640 km west of Recife, cofolded with Early Proterozoic gneisses and migmatites, close to the tectonic contact with Middle to Late Proterozoic metasedimentary rocks of the Salgueiro-Cachoeirinha Group. The jadeite content of clinopyroxenes, the Fe/Mg partitioning between clinopyroxene and garnet, the Si activity in phengite, and the Na content of amphiboles in retrograde symplectites in gabbroic members, suggest a progressive metamorphic evolution up to C-type eclogites at 13 kbar/525° C, followed by a retrograde path of medium P/T ratio, to the greenschist facies. The final event transformed parts of these mafic rocks into banded amphibole gneisses and migmatites. The metagabbroic members contain up to 12% rutile. Metapyroxenitic members bear two types of massive ores: (a) cumulatic CrTi spinel bands and; (b) network ilmenomagnetite. The CrTi spinel is unusual in that it has a composition between ulvospinel, magnetite, and chromite and has a zonation of CrMg enrichment from core to rim which may be explained by peripheral Ti loss during high-pressure metamorphism. Several Ti occurrences in the Floresta area, 400 km west of Recife, including the 20 Mt Riacho da Posse deposit show striking similarities with the Bodoco occurrences: (1) Massive ores with high Ti/Fe ratios related to ultramafic rocks (olivine cumulates); (2) gabbroic intercalations composed of only garnet and amphibole, with the same mineral compositions recorded in Bodoco, lacking plagioclase but rich in rutile; and (3) migmatitic-gneissic wall rocks in the vicinity of the contact with younger metasedimentary rocks. A similar tectono-stratigraphic setting, a few km from the contact with the Late Proterozoic Macururo Group, and high pressure metamorphic host rocks (mafic granulites), has been described in the literature for the Serrote da Lage deposit of massive titanomagnetite with a 60 Mt interstitial copper mineralization. The only other eclogite occurrence recorded in Brazil, at Pouso Alegre, state of Minas Gerais, has an identical geological setting. Chemistry of the metagabbroic rocks of Bodoco and Floresta indicate an oceanic, tholeiitic-picritic character while the rare earth element patterns of the ultramafic rocks suggest a “modified ocean-floor peridotite” nature. Positive Co/Ni and very high Cu/Pd ratios in the sulphides also suggest an oceanic origin. All these features, with exception of the high Ti/Fe ratio, support an ophiolitic character of the mafic rocks. A detailed geochronological study may help to determine whether the mafic-ultramafic rocks represent relics of a true oceanic crust of the same age as the Salgueiro-Cachoeirinha Group, obducted onto older continental crust on an active Late Proterozoic continental margin, or if the proximity to the contact between basement and metasedimentary rocks is coincidental.

Petrochemistry of the Yellowknife volcanic suite at Yellowknife, N.W.T.

The petrochemistry of the Yellowknife volcanic belt at Yellowknife (Kam, Duck and Banting formations) shows the suite to be more chemically complex than previously thought. The Kam Formation (65% basalt, 27% basaltic andesite, 17% andesite, and 2% dactite) has a strong tholeiitic trend, involving pyroxene fractionation, which shows iron enrichment to FeO/(FeO + MgO) = 0.75. After altered samples are removed from consideration, it can be shown that the Kam is chemically analogous to modern ocean-floor or ocean-rise tholeiites. The andesitic members of the Kam were partially melted from a hornblendic region, presumably former oceanic crust, possibly with a slight sialic component. The Duck Formation (11% basalt, 21% basaltic andesite, 59% andesite, and 9% dacite) shows considerable evidence of alkali mobility, with varied carbonate content. It is thought to have been largely derived as partial melts from former oceanic crust via hornblende fractionation. The Banting Formation (14% basalt, 7% basaltic andesite, 38% andesite, and 41% dacite) was also derived from an ocean-crust parent, with hornblende fractionation forming a calc-alkaline suite. There is also consistent major- and trace-element evidence that the relationship between Kam parental magmas and Duck–Banting parental magmas involved magnetite as well as hornblende. A petrogenetic model, starting from garnet or spinel lherzolite, with extensive fractionation and (or) partial melting of a preexisting oceanic crust, which may be either true oceanic crust or the lower part of a thin continental crust, is proposed. The whole suite appears to have been formed in one series of related events, developing a rifted ensialic environment at a late stage in the evolution of the Slave craton. It is shown that major- and trace-element discrimination diagrams yield highly varied tectonic conclusions about this sequence, and it is concluded that in general the diagrams are inapplicable to Archean volcanic sequences.

Geologic and geochronologic constraints on the evolution of the Red Sea-Gulf of Aden and Afar depression

New K-Ar age determinations show that the development of Southern Afar since 14 Ma ago involved five stages of tectonism and volcanism at 14-11, 11-10, 9-7, 5-4 and post-1.6 Ma ago; while the major phases of rifting for the Red Sea-Gulf of Aden have been shown by geological and geophysical data to be 25, 14, 10 and 4 Ma ago. It is suggested that since 14 Ma ago, identical periods of volcanism preceded by rifting have affected both areas.Continental breakup in the Red Sea region was initiated along large transcurrent faults followed by extension manifested by normal faulting, block tilting in the brittle crustal region and repeated dyke injection. Two large scale transcurrent faults were identified, the Marda (NW-SE) and the Ambo (ENE-WSW) which, it is suggested, controlled the trends of rifting in the Red Sea and Gulf of Aden, respectively. Structural and geological evidence indicate that these faults have had a relatively long history, since they were active in pre-Jurassic times. Because at least the Ambo fault cuts across the N-S trends of the late Proterozoic basement, its orientation must be primarily related to the Phanerozoic rift tectonics.This model avoids overlap of the Arabian plate with the Danakil and Aisha blocks because it involves only limited true oceanic crust. The new age data indicate that the initiation of the Aden and Red Sea rifts was not accompanied by active volcanism on land until the Oligocene-Miocene (30-5 Ma ago). Following that time, volcano-tectonic rifting continued without substantial hiati.

A transect of the early Proterozoic Cape Smith foldbelt, New Quebec

A transect of the Cape Smith foldbelt near Asbestos Hill consists of eight blocks separated by north-dipping, high-angle reverse faults. The most southerly block (Block 1) contains a succession of quartz-rich, continent-derived clastic sediments resting unconformably on Archean gneisses. Mafic volcanic rocks occur in the next block to the north (Block 2) and become progressively more important in the two subsequent blocks (Blocks 2a and 3), along with fining of the clastic sediments. These blocks are interpreted as lateral equivalents, representing the collapsed southern edge of a basin. A similar succession is preserved in the northernmost two blocks (Blocks 6 and 7), and may be part of the northern edge of the basin. The mafic rocks of Blocks 2, 2a, 6 and 7 are Ti-rich tholeiites with all the chemical features of contemporary continental tholeiites, and are associated with minor amounts of rhyolite. They are thought to reflect an episode of continental rifting. Thick high-Mg (komatiitic) layered sills are present in many of the blocks, and the two central blocks (Blocks 4 and 5) consist almost exclusively of komatiitic basalt, cogenetic low-Ti tholeiitic basalt and associated intrusives. The basalts occur in orderly sequences changing progressively upwards from komatiitic to tholeiitic, and have built up a succession over 4 km thick. The more northerly block (Block 5) is on average much less magnesian than the one further south (Block 4). The tholeiitic rocks from these two blocks are similar to modern MORB in both major and trace-element characteristics. The less magnesian block may represent early Proterozoic oceanic crust, with the more magnesian block formed at an earlier stage in the spreading before a steady state had been achieved. Although the foldbelt very probably represents a continental rift zone, and quite possibly one in which true oceanic crust was generated, the general absence of sediment in the “oceanic” blocks, the absence of subduction-related volcanics and the similarities of the two margins suggest that it does not reflect a major suture zone. It was probably an ocean-basin of limited width, destroyed by north-dipping subduction.