Generation Of Crust Research Articles

Geochemistry of gabbros and granitoids (M- and I-types) from the Nubian Shield of Egypt: Roots of Neoproterozoic intra-oceanic island arc

The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almost flat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. Similar to the trondhjemite, the tonalites show LILE enrichment and strong Nb depletions on the MORB-normalized spider diagrams. However, the tonalites have REE patterns which are enriched in LREE (La/Yb=1.71–5.54). The derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely. Therefore, high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage. The post-collision granitoids show considerable enrichment in LILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomalies relative to N-MORB. Their REE patterns are LREE-enriched (La/Yb=5–19), with negative Eu anomaly. These characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite and tonalites, and the granodiorite–monzogranite classify them as M-type and I-type granitoids, respectively. The partly tholeiitic intrusives of the Wadi Ranga area (South Eastern Desert) have lower K2O, Rb and LREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardly dipping subduction zone.The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean. The generation of high SiO2 (up to 74.5wt%), low K2O (0.56–1.78wt%) and slightly LREE-depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archaean.

Petrogenesis of Sierra Nevada plutons inferred from the Sr, Nd, and O isotopic signatures of mafic igneous complexes in Yosemite Valley, California

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher eNd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of 87Sr/86Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high 87Sr/86Sr (~0.7067) and low eNd (~−3.4) of bulk rocks, the homogeneity of 87Sr/86Sr in plagioclase, and the high δ18O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust.

Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model

The paradox of the Earth's continental crust is that although this reservoir is generally regarded as having differentiated from the mantle, it has an andesitic bulk composition that contrasts with the intrinsic basaltic composition of mantle-derived melts. Classical models for new crust generation from the mantle in two-stage processes fail to account for two fundamental facts: the absence of ultramafic residues in the lower crust and the hot temperature of batholith magma generation. Other models based on the arrival of already-fractionated silicic magmas to the crust have not received the necessary attention. Addition of new crust by relamination from below of subducted materials has been formulated as a process complementary to delamination of mafic residues. Here we show important support to relamination from below the lithosphere as an important mechanism for new crust generation in magmatic arcs of active continental margins and mature intraoceanic arcs. The new support is based on three independent lines: (1) thermo-mechanical modeling of subduction zones, (2) experimental phase relations and melt compositions of subducted materials and (3) geochemical relations between mafic granulites (lower crust) and batholiths (upper crust). The mineral assemblage and bulk geochemistry of lower crust rocks are compared with solid residues left after granite melt segregation. The implication is that an andesite magma precursor is responsible for the generation of new continental crust at active continental margins and mature oceanic arcs. According to our numerical and laboratory experiments, melting and eventual reaction with the mantle of subducted oceanic crust and sediments produce the andesite magmas. These ascend in the form of mantle wedge diapirs and are finally attached (relaminated) to the continental crust, where they crystallize partially and produce the separation of the solid fraction to form mafic granulites (lower crust) and granitic (sl) liquids to form the batholiths (upper crust).

Isotopic constraints on stratigraphy in the central and eastern Yilgarn Craton, Western Australia

Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu–Hf isotopic results, combined with existing Lu–Hf and Sm–Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.

Constraints and deception in the isotopic record; the crustal evolution of the west Musgrave Province, central Australia

The Hf and Nd isotopic evolution of the Musgrave Province, central Australia, is used to constrain the timing of crust formation and lithospheric organisation of Proterozoic Australia. The dataset from this region challenges two widely held tenets of Hf and Nd isotope systematics, namely; that crust formation events can only be identified as periods when crystallisation ages correspond to model ages, and that linear Hf evolution arrays away from depleted mantle (along crustal Lu/Hf or Sm/Nd slopes) reflect reworking of the same source.Hf isotopes in Musgrave Province zircon crystals indicate two major crust formation events at c. 1900Ma and at 1600–1550Ma. Although no juvenile rocks or crystals are known from c. 1900Ma, radiogenic addition into the crust at this time is required to account for consistent Nd and Hf evolution patterns, which show no indication of an initially heterogeneous source. Oxygen isotopes in zircon grains confirm that much of the c. 1900Ma Hf isotopic signal is not compromised by mixtures. Furthermore, the correspondence between mantle extraction and the commencement of reworking of Archean material supports new crust generation at c. 1900Ma and a coupling between lower and upper crustal processes. The c. 1900Ma timing of juvenile addition is dissimilar to that in the Albany–Fraser and Arunta Orogens and may reflect continental arc development on the margin of a southern continent.The general Hf isotopic evolution trend of the Musgrave Province apparently reflects reworking from a dominant c. 1900Ma source with some additional unradiogenic and radiogenic input through time. However, in the 1220–1050Ma interval this apparent isotopic evolution contrasts with geological observations that indicate input of voluminous mantle-derived material. Intracontinental rifts and other regions with sustained very-high temperature crustal recycling processes generate magmatic provinces with extreme HFSE-enrichment. This can have a profound influence on isotopic evolution trends, suppressing typical juvenile addition patterns. Isotopic mixture modelling indicates that a significant volume of mantle derived material can be accommodated within HFSE enriched magmas without diverging isotopic signatures from apparent reworking trends. In the Musgrave Province, the crust had become so HFSE enriched during the prolonged Musgrave Orogeny (1220–1150Ma) that it was insensitive to mantle input, which is estimated to have been as much as 85% during this event.

The origin and response of zircon in eclogite to metamorphism during the multi-stage evolution of the Huwan Shear Zone, China: Insights from Lu–Hf and U–Pb isotopic and trace element geochemistry

The Huwan Shear Zone (HSZ) is an eclogite bearing transpressive wrench zone located along the Shangdan Suture that juxtaposes the Paleozoic Qinling and Mesozoic Hong'an–Dabie orogenic terrains. The region preserves a complex history that bridges the gap between adjacent orogenic terrains. Simultaneous in-situ trace element, U–Th–Pb and Lu–Hf‐isotope analysis of zircon grains from samples of the Xiongdian and Sujiahe eclogite identify a late Carboniferous to early Permian period of high pressure metamorphism, ca. 283 to 306Ma. Zircon grains are observed to respond to metamorphic overprint via a two stage process: (1) An initial prograde stage of fluid catalyzed interface coupled dissolution–reprecipitation, involving exsolution of a non-ideal solid solution thorite (ThSiO4) end member and loss of highly incompatible components (LREE and Pb), (2) A second stage of coupled zircon dissolution, coarsening, and new rim growth in equilibrium with garnet at high pressure conditions.We identify Proterozoic whole rock Sm–Nd and zircon grain Lu–Hf isotopic evidence which challenges the traditional interpretation that the Xiongdian and Sujiahe eclogite formed in response to early Paleozoic mantle melting and oceanic crust generation. We argue the Huwan Shear Zone contains no conclusive evidence of early/middle Paleozoic oceanic crust, but rather Proterozoic crustal components analogous to those found in the Northern Qinling Terrain and associated with formation of the Shangdan Suture. We present a simpler geodynamic model involving continuous convergence and accretion of terrains onto the southern margin of the North China Block during the Paleozoic Qinling and Mesozoic Dabie orogenies.

Small Mantle Fragments from the Renard Kimberlites, Quebec: Powerful Recorders of Mantle Lithosphere Formation and Modification Beneath the Eastern Superior Craton

The origin and evolution of diamondiferous lithospheric mantle sampled by the Neoproterozoic Renard kimberlites (eastern Superior Province, Quebec) is constrained based on mantle-derived microxenoliths and xenocrysts. The dataset illustrates the wealth of knowledge that can be gleaned from small samples (1·2 mg–2·2 g) through an integration of multiple, mainly single mineral based approaches. Our samples document the presence of an ∼200 km thick lithosphere with a ‘cold’ (38 mW m−2 surface heat flow) model geotherm at the time of kimberlite emplacement (c. 632 Ma), resulting in a large diamond window from 130 to 200 km (42–60 kbar). On the basis of the mantle xenolith and xenocryst record and excluding megacrysts, the lithospheric mantle beneath Renard was dominated by peridotite (91%), composed of lherzolite (72% of samples), harzburgite (24%) and wehrlite (5%), with minor eclogitic (3%) and websteritic (6%) portions. Comparatively abundant harzburgite probably establishes the principal diamond source, but elevated Na contents in eclogitic garnet suggest the additional presence of diamond-stable eclogites. A number of events have modified the lithospheric mantle underlying the eastern Superior Province, including the following: (1) evolving ‘kimberlitic’ melts pervasively re-fertilized the originally strongly depleted lithospheric mantle with respect to highly and moderately incompatible trace elements; (2) less pervasive fluid style metasomatism is indicated by selective re-enrichment of highly incompatible elements that occurred within a depth range of 125–170 km. In situ Pb isotope data obtained for clinopyroxenes suggest a model age of ∼2·7 Ga for the protolith(s) of the cratonic lithospheric mantle beneath Renard. This age coincides with a major phase of continental crust generation within the Superior Province and throughout the Laurentia supercontinent (e.g. Greenland).

Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago

Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

U–Pb and Hf isotope study on detrital zircons from the Paranoá Group, Brasília Belt Brazil: Constraints on depositional age at Mesoproterozoic – Neoproterozoic transition and tectono-magmatic events in the São Francisco craton

The eastern portion of the Brasilia belt, part of the Neoproterozoic Tocantins Province in central Brazil, is mainly constituted by metasedimentary rocks that partially represent the evolution of a passive margin developed during the Meso-Neoproterozoic in the western margin of the São Francisco paleocontinent. New in situ U–Pb and Lu–Hf isotope analyses on detrital zircons by LA-MC-ICP-MS were obtained for clastic sedimentary rocks of the Paranoá Group, considered to be the oldest unit of the this sedimentary sequence. The U–Pb data define a maximum depositional age of 1.54Ga and a minimum depositional age of ∼1.04Ga, the latter obtained on diagenetic xenotime overgrowth on zircon grains. Combined U–Pb and Lu–Hf data permitted to reconstruct the tectonomagmatic evolution of the potential source areas identified in the São Francisco craton. Two zircon populations at 1.54 and 1.78Ga, with ɛHf(t) values from +5.1 to −5.5, are possibly related with the Mesoproterozoic extensional magmatism registered in the Arai and Espinhaço rifts, located in the Brasilia belt and in the São Francisco craton respectively. The most important Paleoproterozoic zircon population, at 2.0–2.25Ga, shows positive ɛHf(t) (0 to +8) and Paleoproterozoic TDM values indicating a continuous long-period of magmatism with generation of juvenile crust during the Paleoproterozoic. Two minor zircon populations, at 2.1 and 2.15Ga, showing negative ɛHf(t) values, 0 to −10.1, and TDM values between 2.65 and 3.22Ga may represent arc related magmatism, with reworking of Archean crust, during the evolution of an active margin of the São Francisco craton. A third minor zircons population, at 2.05Ga, with strongly negative ɛHf(t) values, between −8.6 and −17.7, and TDM values between 2.93 and 3.46Ga, may represent a collisional magmatism that characterized the final stage of an active margin evolution with reworking of Early Archean crust. Several minor zircon populations of Archean ages (2.7, 2.85, 3.1 and 3.42Ga), derived from the Archean nuclei in the interior of the São Francisco craton, suggest the presence of different magmatic pulses that repeatedly reworked a juvenile Paleoarchean crust. The oldest event of generation of juvenile crust is registered by zircons with U–Pb age of 3.42Ga, positive ɛHf(t) values of 3.0 and TDM of 3.54Ga.

Geology and controls on mineralisation in the Eastern Goldfields region, Yilgarn Craton, Western Australia

The Yilgarn Craton in Western Australia contains evidence of the oldest crust on Earth. Greenstone successions developed after c. 3000 Ma show a complex history of juvenile crust generation and crustal reworking. There are at least three periods of greenstone-related magmatism in the Yilgarn Craton. The earliest recognised greenstone development consists of volcanic and sedimentary successions deposited between c. 3000-2900 Ma. A mantle plume at c. 2800 Ma produced large mafic-ultramafic igneous complexes and probably initiated rifting on the eastern side of the craton and incipient rifting in the NW. A second major plume, at c. 2700 Ma, was focussed along the rupture created by the c. 2800 Ma event and may have been associated with the re-accretion of lithospheric blocks created by the earlier event. Komatiites generated by the c. 2700 Ma plume contain world-class Ni deposits, and structures developed subsequent to the peak of plume activity host world-class Au deposits. Recent studies of Ni and Au deposits in the Eastern Goldfields Superterrane have shown how features ranging in scale from the lithosphere to regional structural and stratigraphic controls to local volcanological and sedimentological variations can affect the size and distribution of deposits. This understanding is now being applied in exploration targeting.

Magmatic evolution of the South Shetland Islands, Antarctica, and implications for continental crust formation

Lavas from the South Shetland Islands volcanic arc (northern Antarctic Peninsula) have been investigated in order to determine the age, petrogenesis and compositional evolution of a long-lived volcanic arc constructed on 32-km-thick crust, a thickness comparable with average continental crust. New 40Ar–39Ar ages for the volcanism range between 135 and 47 Ma and, together with published younger ages, confirm a broad geographical trend of decreasing ages for the volcanism from southwest to northeast. The migration pattern breaks down in Palaeogene time, with Eocene magmatism present on both Livingston and King George islands, which may be due to a change in both subduction direction and velocity after c. 60 Ma. The lavas range from tholeiitic to calc-alkaline, but there is no systematic change with age or geographic location. The compositions of lavas from the north-eastern islands indicate magma generation in a depleted mantle wedge with relatively low Sr and high Nd isotopic compositions and low U/Nb, Th/Nd and Ba/Nb ratios that was metasomatized by hydrous fluids from subducted basaltic oceanic crust. Lavas from the south-western islands show an additional sedimentary influence most likely due to fluid release from subducted sediments into the mantle wedge. Although magmatic activity in the South Shetland arc extended over c. 100 m.y., there is no evolution towards more enriched or evolved magmas with time. Few South Shetland arc lavas are sufficiently enriched with incompatible elements to provide a potential protolith for the generation of average continental crust. We conclude that even long-established subduction zones with magmatic systems founded on relatively thick crust do not necessarily form continental crustal building blocks. They probably represent only the juvenile stages of continental crust formation, and additional re-working, for example during subsequent arc-continental margin collision, is required before they can evolve into average continental crust.

A critical assessment of Neoarchean “plume only” geodynamics: Evidence from the Superior Province

Criticisms of Archean plate tectonic models rarely offer detailed descriptions of alternative paradigms. The catalytic delamination-driven model for the coupled genesis of Archean crust and sub-continental lithospheric mantle described by Bédard (2006) is an exception in that a relatively comprehensive, but essentially 2-D, model is described. Although this novel model is frequently cited as an example of the diversity in geodynamic thought for the Archean, it is rarely assessed with reference to events in a specific craton, such as the Superior Province or a particular greenstone belt, such as the Abitibi. When this type of assessment is undertaken it becomes clear that the apparent simplicity of the model, one of its supposed virtues, constitutes a critical weakness. Although Bédard (2006) developed the catalytic model to overcome the perceived “melt productivity deficit” of Archean plate tectonic models, the catalytic process at the heart of the model is likely to be inefficient and provides insignificant improvements to crustal generation rates associated with subduction and “non-catalytic” plume tectonics. The model implies a near global craton in the Mesoarchean to Neoarchean but fails to address that fate of this craton during and after “crustal rigidification”. Seismic reflectors observed beneath Archean cratons are characterized as failed delamination features but their global abundance suggests that syn- to post-tectonic TTG suites could not have been generated as a result of crustal restite delamination. The brief descriptions of suggested mechanisms for the generation of minor rock types, such as sanukitoids, mask the complexity inherent in the overall model. When considered in terms of the evolution of the Superior Province as a whole, it is evident that many of these suggestions are not only implausible, they are inconsistent with the timing and distribution of geological criteria used to mark the onset of “crustal rigidification”.

Evolução tectônica do arco magmático Juruena entre os rios Aripuanã e Juruena: noroeste de Mato Grosso, Brasil

The area of study is located in the southwest of the Amazon Craton, northwest of Mato Grosso State and it comprises a region between Aripuanã and Juruena rivers. This paper proposes a geotectonic evolutionary model of a single continental magmatic arc for this Paleoproterozoic segment; its orogenesis starts around 1,820 Ma with subduction, oceanic plate consumption, crust generation followed by continental blocks oblique collision processes after approximately 160 Ma, resulting in the formation of the Juruena Magmatic Arc. The initial phase litotypes are represented by oceanic crust remnants from Bacaerí-Mogno Complex and sub-volcanic to volcanic rocks from Paranaíta Suite and Colíder Group respectively, which configures a marginal ridge. Southwestern, this belt is in contact with the Juruena Complex, a portion of the arc deformed in a ductile regime. This segment is composed by Vitória Suite, São Pedro and São Romão Granites and Vespor Mafic Suite. Volcanic rocks scattered in the complex, apparently filling retro-arc basins, are included in the Roosevelt Group. Post-collisional Type A granitogenesis, related to extensional tectonic, is represented by Serra da Providência Suite. The U-Pb LA-ICPMS geochronology results from two Paranaíta Suite samples, one Vitória Suite and one Vespor Mafic samples are 1,797 ±14, 1,769 ±17, 1,783 ±14 and 1,773 ±15 Ma, respectively. Ten rock samples were also examined by Sm-Nd method, resulting in T DM ages between 2.25 and 1.88 Ga and εND(t) values between -1,9 and +1,92. The isotopic results are compatible with the expected for the Juruena Magmatic Arc.

High Heat Producing Volcano-Plutonic Rocks of the Siner Area, Malani Igneous Suite, Western Rajasthan, India

he volcano-plutonic rocks of the Siner area of the Malani Igneous Suite (MIS) are characterized by high content of radioactive elements (U, Th, K) and are classified as high heat producing felsic volcano-plutonic rocks of A-type granitoid. Microgranite shows highly comparable heat production (26.07 HP) and total heat generation value (62.06 HGU in average) as compared to other granite (HP = 12.73; HGU = 26.57), rhyolite (HP = 4.98; HGU = 11.85) and trachyte (HP = 5.00; HGU = 11.91). The volcano-plutonic rocks of the present show higher average value of total HGU than the average value of 3.8 HGU for the continental crust, which suggests a possible linear relationship among the crustal heat generation of the MIS.

Development and evolution of subaerial halite crust morphologies in a coastal sabkha setting

The halite crusts of the coastal Abu Dhabi Sabkha were monitored and described in order to assess their temporal evolution and their potential for preservation and recognition in ancient sabkha sequences. The position of polygon ridges is controlled by subtle differences in the sabkha surface topography – typically in the form of low lying mud ridges and exposed halite. Where this topography remains undisturbed, then successive generations of halite polygons were observed to form with borders in the same location. As each generation of halite crust effectively ‘rebuilds’ the controlling mud ridge, then this process can be seen as self perpetuating until disturbed by a major erosional event. In marginal marine settings, such as the Abu Dhabi Sabkha, the preservation potential of the halite crust is negligible. While associated sedimentary structures have been employed to infer the presence of an earlier halite crust in other geographic settings, these all require a net influx of sediment to the depositional system. The absence of such a sediment supply in the coastal-fringing Abu Dhabi Sabkha is inferred to be a major constraint to the preservation and recognition of halite crusts and their associated features into the sedimentary record.

Tectonic subsidence of the Lomonosov Ridge

The Cenozoic sedimentary record revealed by the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44–18 Ma. According to conventional thermal kinematic models, the microcontinent should have subsided to >1 km depth owing to rifting and subsequent separation from the Barents–Kara Sea margin at 56 Ma. We propose an alternative model incorporating a simple pressure-temperature ( P - T ) relation for mantle density. Using this model, we can explain the missing stratigraphic section by post-breakup uplift and erosion. The pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central Eurasia Basin could have been restricted and confined by non-volcanic thinning of the mantle lithosphere at an early stage (ca. 56–40 Ma). In response to a rise in temperature, the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite. The consequence of lithosphere heating and related mineral phase transitions would be post-breakup uplift followed by rapid subsidence to the deep-water environment observed on the Lomonosov Ridge today.

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K 2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization. The low abundances of trace elements, together with low K 2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO 2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.

Chronological and genetic relationships between intrusive rocks of the Berdyaush pluton, South Urals, in light of new U-Pb and Sm-Nd isotopic data

Geological observations in combination with previously published and new isotopic data allowed us to reconstruct the history of geological events that eventually gave rise to the formation of the Berdyaush pluton situated on the western slope of the South Urals: (1) emplacement of gabbro into Lower Riphean sedimentary rocks (1410–1390 Ma); an enriched mantle source of gabbro arising in the Archean or Paleoproterozoic; (2) formation of granitic melt in the lower crust; Archean TTG association subsequently enriched in K and correlative elements as a result of interaction with enriched mantle-derived magmas and related fluids was a magma source; mixing of mantle and crustal magmas in the course of their synchronous ascent with formation of hybrid intrusive rocks; injections of mafic and hybrid melts into incompletely solidified granite; fragmentation of such injections with the formation of melanocratic nodules; emplacement of basic dikes into the cooled granite—all these events took place 1410–1360 Ma ago; (3) discrete episodes of partial melting of enriched mantle source with waning intensity; formation of minor volumes of melt, which solidified under auto- and paraautochthonous conditions as local domains highly enriched in incompatible elements (1360–1270 Ma); (4) partial melting of those domains resulting in the formation of minor nepheline syenite intrusions (915–800 Ma), containing relict zircon grains dated at >1270 Ma; (5) injections of mantle-derived alkaline melt contaminated with crustal granitic material as microsyenite and syenite porphyry dikes (700–500 Ma ?). Thus, the Berdyaush pluton is a projection of a local domain of mantle and crustal magma generation, which periodically resumed its activity over almost a billion years.

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

Amphibole fractionation in the deep roots of subduction-related magmatic arcs is a fundamental process for the generation of the continental crust. Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes. The Chelan Complex in the western U.S. forms the lowest level of a 40-km thick exposed crustal section of the North Cascades and is composed of olivine websterite, pyroxenite, hornblendite, and dominantly by hornblende gabbro and tonalite. Magmatic breccias, comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes. Phase equilibria, textural observations and mineral chemistry yield emplacement pressures of ∼1.0 GPa followed by isobaric cooling to 700°C. The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole. Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3% of olivine websterite, 12% of pyroxene hornblendite, 33% of pyroxene hornblendite, 19% of gabbros, 15% of diorite and 2% tonalite. Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process. Crustal assimilation during fractionation is thus not required to generate peraluminous tonalite.

Initial burst of oceanic crust accretion in the Red Sea due to edge-driven mantle convection

The 500 m.y. cycle whereby continents assemble in a single supercontinent and then fragment and disperse again involves the rupturing of a continent and the birth of a new ocean, with the formation of passive plate margins. This process is well displayed today in the Red Sea, where Arabia is separating from Africa. We carried out geophysical surveys and bottom rock sampling in the two Red Sea northernmost axial segments of initial oceanic crust accretion, Thetis and Nereus. Areal variations of crustal thickness, magnetic intensity, and degree of melting of the subaxial upwelling mantle reveal an initial burst of active oceanic crust generation and rapid seafloor spreading below each cell, occurring as soon as the lid of continental lithosphere breaks. This initial pulse may be caused by edge-driven subrift mantle convection, triggered by a strong horizontal thermal gradient between the cold continental lithosphere and the hot ascending asthenosphere. The thermal gradient weakens as the oceanic rift widens; therefore the initial active pulse fades into steady, more passive crustal accretion, with slower spreading and along axis rift propagation.