Field and geochemical data bearing on the development of a mesozoic volcano-tectonic rift zone and back-arc basin in southernmost South America

A broad zone of dominantly subaerial silicic volcanism associated with regional extensional faulting developed in southern South America during the Middle Jurassic, contemporaneously with the initiation of plutonism along the present Pacific continental margin. Stratigraphic variations observed in cross sections through the silicic Jurassic volcanics along the Pacific margin of southernmost South America indicate that this region of the rift zone developed as volcanism continued during faulting, subsidence and marine innundation. A deep, fault-bounded submarine trough formed near the Pacific margin of the southern part of the volcano-tectonic rift zone during the Late Jurassic. Tholeiitic magma intruded within the trough formed the mafic portion of the floor of this down-faulted basin. During the Early Cretaceous this basin separated an active calc-alkaline volcanic arc, founded on a sliver of continental crust, from the then volcanically quiescent South American continent. Geochemical data suggest that the Jurassic silicic volcanics along the Pacific margin of the volcano-tectonic rift zone were derived by crustal anatexis. Mafic lavas and sills which occur within the silicic volcanics have geochemical affinities with both the tholeiitic basalts forming the ophiolitic lenses which are the remnants of the mafic part of the back-arc basin floor, and also the calc-alkaline rocks of the adjacent Patagonian batholith and their flanking lavas which represent the eroded late Mesozoic calc-alkaline volcanic arc. The source of these tholeiitic and calc-alkaline igneous rocks was partially melted upper mantle material. The igneous and tectonic processes responsible for the development of the volcano-tectonic rift zone and the subsequent back-arc basin are attributed to diapirism in the upper mantle beneath southern South America. The tectonic setting and sequence of igneous and tectonic events suggest that diapirism may have been initiated in response to subduction.     

Trace element geochemistry of Norwegian Lower Palaeozoic basic volcanics and its tectonic implications

The study of nearly 100 new trace element analyses of late Cambrian/early Ordovician greenstones from the Grong, Løkken, Støren and Stavenes areas of central and southern Norway indicates the presence of ocean floor type basalts in all four areas, and the presence of low-potassium tholeiites of island arc affinity in at least two of the areas. From this result an ocean floor-based Ordovician/early Silurian island arc complex and back-arc eugeosynclinal pile is recognised in the Trondheim region and its principal features described. This complex is considered to have been obducted during an early stage of the Middle Silurian orogeny upon an Eocambrian/Ordovician miogeosynclinal sedimentary succession which had developed on continental crust. During a later stage of the major folding and metamorphism of the juxtaposed sequences, further eastward thrusting took place such that total displacement of some allochthonous sheets may be in the order of several hundred kilometres.     

Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan

Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]_N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe³⁺? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]_N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO₂ contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin.     

Geochemistry of basalts from central and southern New Hebrides arc: implication for their source rock composition

The basaltic rocks from the central and southern islands of the New Hebrides-Aneityum, Tanna, Erromango, Efate, Emae, Tongoa and Epi, have geochemical features typical of island arc volcanics. They are enriched in LILE and depleted in Zr, Hf, Nb and Ta compared to N-type MORB. The rocks were derived from a similar upper mantle source as N-type MORB but with a higher degree of partial melting. In addition their source was enriched in LILE (K, Rb, Sr, Ba and LREE) probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab. The basalts from Futuna island which is located farther from the trench, display characteristics typical of calc-alkaline rocks. The Futuna basalts were generated from a different LILE-enriched upper mantle source. It seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere.     

Plate-plume-accretion tectonics in Proterozoic terrain of northeastern Rajasthan, India: Evidence from mafic volcanic rocks of north Delhi fold belt

Abstract   The northern part of the Aravalli mountain belt of northwestern Indian shield is broadly composed of three Proterozoic volcano-sedimentary domains, i.e. the Bayana, the Alwar and the Khetri basins, comprising collectively the north Delhi fold belt. Major, trace and rare earth element concentrations of mafic volcanic rocks of the three basins exhibit considerable diversity. Bayana and Alwar volcanics are typical tholeiites showing close similarity with low Ti–continental flood basalts (CFB) with the difference that the former shows enriched and the latter flat incompatible trace element and rare earth element (REE) patterns. However, the Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. It appears that the melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field possibly in the presence of mantle plume. During ascent to the surface the Bayana tholeiites suffered crustal contamination but the Alwar tholeiites erupted unaffected. Geochemically, the Khetri volcanics are arc-like basalts which were generated in a segment of mantle overlying a Proterozoic subduction zone. It is suggested that at about 1800 Ma the continental lithosphere in northeastern Rajasthan stretched, attenuated and fractured in response to a rising plume. The produced rifts have undergone variable degrees of crustal extension. The extension and attenuation of the crust facilitated shallowing of the asthenosphere which suffered variable degree of melting to produce tholeiitic melts – different batches of which underwent different degrees of lithospheric contamination depending upon the thickness of the crust in different rifted basins. The occurrence of subduction-related basaltic rocks of Khetri Belt suggests that a basin on the western margin of the craton developed into a mature oceanic basin.     

Element transport by dehydration of subducted sediments: Implication for arc and ocean island magmatism

High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.     

Petrology of the Rumble seamounts,southern Kermadec Ridge,southwest Pacific

The Rumble seamounts are major Quaternary submarine volcanoes which lie at the southern end of the Tonga-Kermadec island arc. Rocks dredged from three of the volcanoes are olivine and pyroxene phyric basalts and highly por-phyritic plagioclase-rich basaltic andesites. Plagioclase phenocrysts display patterns of iron enrichment similar to those observed in rocks from other parts of the arc. Systematic major and trace element variations indicate that the specimens constitute a closely related suite of low-K arc-type rocks showing characteristic depletion in some large ion lithophile elements. They are closely comparable to basaltic rocks of the Kermadec Islands and their genesis may be linked to the currently active subduction system beneath the arc.     

The Karoo igneous province — A problem area for inferring tectonic setting from basalt geochemistry

Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin.When data for the “southern” Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of “within-plate” basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a “subduction component” has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities.In contrast, the data for “northern” Karoo basalts largely plot in the “within-plate” field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks.     

Magma generation and crustal accretion as evidenced by supra-subduction ophiolites of the Albanide–Hellenide Subpelagonian zone

Abstract Ophiolites of the Mirdita–Subpelagonian zone form a nearly continuous belt in the Albanide–Hellenide orogen, including mid‐ocean ridge basalt (MORB) associations in the western Mirdita sector and supra‐subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities in the eastern part of the belt (i.e. eastern Mirdita, Pindos, Vourinos). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Aspropotamos sequence (Pindos). These basalts alternate with pure MORB and are cut by boninitic dykes. The distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10–20%) of lherzolitic sources generated pure MORB, leaving clinopyroxene‐poor lherzolite as a residuum. Approximately 10% water‐assisted partial melting of this latter source, in an SSZ setting, may in turn generate basalts with MORB/IAT intermediate characteristics, whereas IAT basalts and boninites may have been derived from 10–20% and 30% partial melting, respectively, of the same source variably enriched by subduction‐derived fluids. In addition, boninites may also have been derived by comparatively lower degrees of hydrated partial melting of more refractory harzburgitic sources. A generalized petrologic model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusives (from ultramafic cumulates to gabbronorites and plagiogranites), as well as sheeted dykes and volcanics (from basalts to rhyodacites) forming the bulk crustal section of the SSZ ophiolites, may be accounted for by shallow fractional crystallization from low‐Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intraoceanic arcs. The most appropriate tectono‐magmatic model for the generation of the SSZ Tethyan ophiolites implies low velocity plate‐convergence of the intraoceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll‐back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/IAT intermediate basalts and boninitic magmas.     

Zircon SHRIMP U-Pb ages and trace element geochemistry of the Kuhai gabbro and the Dur’ngoi diorite in the southern east Kunlun tectonic belt,Qinghai, Western China and their geological implications

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A’nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur’ngoi diorite in the Kuhai-A’nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur’ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur’ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A’nyêmaqên suggests that the southern margin of the “Qilian-Qaidam-Kunlun” archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.

     

Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.     

Geochemical variation with time in the Cenezoic volcanic rocks of southwest Hokkaido, Japan

Abundances of major and trace elements were determined for the Tertiary volcanic rocks from SW Hokkaido. The Late Miocene to Pliocene volcanic rocks of this region show geochemical features similar to those of the Quaternary rocks, that is, K/Si, Th/Si and LREE/HREE ratios increasing across the arc, east to west, from the Pacific to the Japan Sea side. In contrast, the Early Miocene volcanic rocks, which are geographically restricted to the Japan Sea coast, are distinct from all later volcanics and show “within-plate” characteristics — in particular, high concentrations of HFS elements. The Quaternary basalts have low Hf/Yb ratios and Hf contents, whereas the Early Miocene basalts are high in Hf/Yb and Hf, similar to Hawaiian alkali basalts. The compositional variation with time may result from the progressive depletion of incompatible HFS elements in the mantle source. Th/Yb ratios increase from Early Miocene to Quaternary, possibly reflecting increase in the LIL element contribution to the mantle source during that time.     

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.     

An unconventional proterozoic paleomagnetic polar wander path for North America and its advantages

Statistically-contoured paleomagnetic poles for the Proterozoic of North America trace a sub-equatorial polar wander path that differs from more-conventional APW tracks. This path is modified by two types of hairpins, those that close in the northern hemisphere and are closely synchronous with the Hudsonian and the Grenvillian events, and those that close in the southern hemisphere and do not appear to have tectonic significance. The north-closing hairpins correspond to major shearing through the continent. The most plausible displacement of North America during the Proterozoic is an anti-clockwise rotation along a small circle centered close to the equator. A major reorganization of the APW path in the late Proterozoic probably corresponds to the beginning of subduction processes.     

Sea-floor spreading: The central anomaly magnetization high

Indications of a narrow region of high magnetization within the central magnetic anomaly on some mid-ocean ridges are found on near-bottom and sea surface magnetic profiles. This zone, which probably represents the most recent extrusions onto the ocean floor, is similar to the narrow region of high magnetization found on the Mid-Atlantic Ridge at 45°N with a suite of dredge samples. This narrow region is probably the result of the initial high magnetization of pillow basalts when they are extruded onto the ocean floor and the subsequent rapid oxidation of the outer variolitic zone of the pillows. The large-amplitude, short-wavelength (<15 km) magnetic anomaly found within the central anomaly over both slow- and fast-spreading ridges is produced by this narrow magnetization high. This magnetic anomaly can be used to locate the region of most recent extrusions on most ridges. The absence of this short-wavelength anomaly on some ridges may reflect the episodicity with which basalts are extruded onto the ocean floor.     

Lithophile elements in Huronian low-Ti continental tholeiites from Canada, and evolution of the Precambrian mantle

Huronian basalts from central Ontario, Canada, dated at about 2450 Ma and associated with an early rifting episode, are classified as siliceous, low-TiNb tholeiites. They display strong enrichment in large-ion lithophile (LILE) and light rare earth (LREE) elements compared to modern oceanic lavas. The tectonic setting and geochemistry resemble Mesozoic rift-related low-Ti flood basalts, including the Ferrar Group of Antarctica, and the Parana and equivalent Etendeka volcanics of south Brazil and Namibia, respectively. High LILE/LREE ratios are also similar to subduction-related island arc tholeiites, and it is suggested that enrichment of the Huronian lithospheric mantle source occurred through ancient subduction of crustal material, probably during formation and consolidation of the Archean continental crust.Melting models suggest that Huronian subcontinental mantle source compositions, derived from least contaminated, aphyric, mafic end-members, had already undergone a complex evolution, including withdrawal of Archean basalts and hydrous enrichment in incompatible components. Despite several subsequent melting episodes and a second, probably magmatic, enrichment event, however, many aspects of the Huronian source signature were preserved, and appeared in later basaltic products of this mantle mass. Keweenawan volcanics, for example, dated at about 1100 Ma, preserve low P, Zr, Ti and HREE abundances.     

Oceanic versus island arc origin of ophiolites

The silica content of basaltic rocks is an unreliable variable with which to distinguish ultramafic-mafic complexes developed at ocean ridges from those potentially formed beneath volcanic island arcs. Data from Appalachian ophiolites supports the view that silica metasomatism is responsible for the high silica content of supposed calc-alkaline basaltic rocks found in ophiolites such as Troodos, and that the high-silica (70 wt.%) leucocratic rocks associated with ophiolites are of tholeiitic rather than calc-alkaline parentage. The use of titanium as a discriminant of tectonic environment is also suspect because the titanium content of basalts associated with Appalachian ophiolites as well as those recently recovered from the Atlantic ocean floor ranges from values even lower than those typical of island arc tholeiites to values typical of abyssal tholeiites. However, the internal stratigraphy of ophiolites in both the Appalachian and Tethyan systems can only be explained on the basis of the postulate that ophiolites originate at oceanic spreading centres rather than beneath island arcs.     

Inert gas abundances in basalts of the Lesser Antilles island arcs: possible implications for their volcanic evolution

Measurements of noble gas element abundances in igneous rock samples from the Lesser Antilles island arc (LAIA) show an enrichment of Ne, Kr and Xe relative to Ar, when compared to the atmospheric inert gas composition. The element concentrations vary as a function of time and a cyclic variation appears to occur in the pre-Pliocene stage of arc development. The pattern of elemental abundances closely resembles that of ocean floor basalts, suggesting that, (1) the origin of the inert gases in these samples is related to the subduction of oceanic lithosphere responsible for the formation of the LAIA, and (2) the subduction process does not modify the noble gas patterns of the materials being subducted.There is no evidence of a change in concentration or composition of noble gases occurring at ca. 9 Ma, the time when it has been suggested the LAIA shifted to form the western, younger, volcanic Caribbees. However, a major hiatus in the noble gas evolution is recognised at ca. 16 Ma.     

Ophiolitic breccias associated with allochthonous oceanic crustal rocks in the East Ligurian Apennines,Italy — a comparison with observations from rifted oceanic ridges

In the East Ligurian segment of the North Apennines, eugeosynclinal sequences which contain ophiolitic rocks have been tectonically emplaced onto approximately coeval miogeosynclinal sediments. These allochthonous sequences represent the floor of a Mesozoic ocean which closed during the early Tertiary. The ophiolitic rocks consist of serpentinite, gabbro, pillowed and massive basalts, and breccias derived from these lithologies. They are overlain with depositional contacts by Upper Jurassic-Cretaceous pelagic cherts, limestones, and a shale/limestone sequence.The ophiolitic breccias attain thicknesses up to 100 m and strike lengths up to a few kilometres, and consist largely of unorganized accumulations of sand- to block-sized clasts. Compositions at specific horizons may range from oligomict breccias containing gabbro, basalt, or serpentinite fragments, to polymict breccias consisting of any mixture of these lithologies. Most of the breccias probably represent slow talus accumulations at the base of major submarine fault scarps which have exposed gabbro and serpentinite to submarine erosion. Direct exposure of gabbro and serpentinite on the ocean floor is also indicated by the occurrence of stratigraphically intact contacts between these lithologies and overlying pelagic sediments (generally cherts). The distribution and thickness of the breccias and volcanics, and the distribution of the gabbro and serpentinite, can vary greatly within distances of a few kilometres, thus producing complex heterogeneous sequences consisting of laterally impersistent lithological units.Recent observations and deep drilling of the Mid-Atlantic Ridge and other rifted ridges have revealed occurrences of significant thicknesses of basaltic, serpentinitic, and gabbroic breccias upon and within the volcanic layer of the oceanic crust, as well as the direct submarine exposure of plutonic rocks. It is therefore likely that the East Ligurian sequences represent parts of rifted ridge-generated crust. If so, then the complexity of the East Ligurian sequences suggests that the upper part of rifted ridge-generated crust may in places possess large variations in its stratigraphy over small (<10 km²) areas.Smooth, non-rifted (fast-spreading) ridges, which have very reduced topography and lack major fault scarps, should form ophiolitic complexes deficient in breccias containing fragments of plutonic igneous rocks. Most large ophiolitic complexes do not contain plutonic rock-bearing breccias, and were therefore probably formed at smooth ridges. The apparently preferential preservation of this type of ophiolitic complex, as opposed to the rifted ridge-type crust in East Liguria, may be related to the less pervasive and less intense fracturing of smooth ridges. This resulted in greater “cohesion” and lateral continuity of smooth ridge-generated crust during later tectonic emplacement into allochthonous positions in orogenic belts.     

Nd-isotope systematics of ∼2.7 Ga adakites, magnesian andesites, and arc basalts, Superior Province: evidence for shallow crustal recycling at Archean subduction zones

An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ?_Nd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ?_Nd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ?_Nd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.