Stress-controlled reaction pattern in the layered lower crust: Field evidence

High-K calc-alkalic plutons represent a significant proportion of the abundant magmatic bodies that intruded Borborema province (BP) of northeastern Brazil during the Neoproterozoic Brasiliano (Pan-African) orogeny. They consist of an association of mafic to intermediate (diorites to granodiorites) and felsic rocks (coarse-grained to porphyritic quartz monzonites to granites). Field and petrographic evidence indicates that the felsic and mafic rocks coexisted as contemporaneous melts, and major- and trace-element data favor magma mixing over fractional crystallization as the main petrogenetic process responsible for the petrographic and geochemical variability of these rocks. Major- and trace-element, oxygen-isotope, and radiogenic-isotope (Sr and Nd) data suggest that (1) the main source rocks of the granitoids are lower-crustal amphibolites having rare-earth-element (REE) and isotopie characteristics similar to the associated mafic rocks and (2) the source region of the diorites is the metasomatized subcontinental lithospheric mantle. These inferences imply that crustal growth occurred during the Brasiliano orogeny. Dewatering of the mantle and lower crust and addition of consolidated mafic rocks and I-type granitoids to the middle crust certainly strengthened the entire lithosphere, thus contributing to the final cratonization of the BP. Field evidence indicates that the BP high-K calc-alkalic plutons were emplaced in an intracontinental setting, implying that this magmatism was not subduction-zone related. Although the plutons are spatially associated with transcurrent shear zones, the scale of magmatism is too broad to be assigned to shear heating. 40Ar/39Ar data indicate that large areas of the BP underwent slow cooling, unlike orogenic belts where delamination or convective removal of the lithosphere occurred. Therefore, only large convective instabilities in the sublithospheric mantle may explain the thermal anomaly responsible for melting in the BP. It is proposed that a mantle plume impinging the base of the continental lithosphere under the BP may represent such a laterally extensive and long-lived heat source.

Genesis of magma for acid calc-alkaline volcano-plutonic formations

Decollements are conspicuous features of collision belts as a result of shortening and shearing of rocks of different competencies in response to intracontinental subduction and crustal stacking. When these decollements occur at upper levels, the classical thin skin tectonics of the foredeeps without important internal strain results; at mid crustal levels slaty cleavage and foliation development occurs. In the more internal parts of the chains, in the lower crust, near the crust mantle boundary, the more severe conditions (granulitic metamorphism) result in ductile deformation in the deepest parts of the mountain belts. Examples of middle and lower crust are well documented in Central China and the Variscan Belt of Europe from field evidence and deep seismic profiling. From these examples it appears more and more likely that most of the mid-crustal slate belts showing vertical slaty cleavage at the erosion surface overly flat decollements. At greater depth large decollements must occur as well in the lower continental crust and at crust-mantle boundary. Typical features of the deep seismic vertical profiles such as the strongly layered lower crust may represent the image of such deep decollements.

<p><strong>Keywords</strong>: Central Alps, magmatic system, thermodynamic modelling, crystallization, crustal contamination</p><p>Understanding which processes are active and quantifying their relative influence during the differentiation of intracontinental magmatic systems remains a major challenge, as these processes can either (1) involve magmas and their crystallization products (fractional crystallization, reactive melt flow...) and/or (2) crustal contamination through various vectors (bulk assimilation, reactive assimilation, host-rock partial melting…). Whereas the influence of some of these processes can be inferred from field evidence, it needs to be constrained and quantified. This question can be addressed in the Central Alps (N Italy, SE Switzerland), where a complete, crustal-scale post-Variscan (Permian) magmatic system has been documented from lower crustal (Braccia gabbro, Malenco unit) and mid-crustal intrusives (Sondalo gabbro, Campo unit) to upper crustal intrusives and extrusives (Bernina unit). We present preliminary results, combining field work to petrological and geochemical characterization and modelling.</p><p> </p><p>Petrological investigations on major element bulk-rock composition shows a complete differentiation trend from the less differentiated lower crust intrusive mafic rocks (Ol-gabbro, gabbro: 40-50 wt.% SiO₂, Mg# 45-75, 0.1-0.8 wt.% K₂O; and diorite: 45-60 wt.% SiO₂, Mg# 45-55, 0.15-0.5 wt.% K₂O), to upper crust felsic rocks (granite/rhyolite: 55-85 wt. % SiO₂, Mg# 5-50, 1-6 wt.% K₂O). By contrast, middle crust intrusive rocks encompass the full compositional range from Ol-gabbro and gabbro (45-50 wt.% SiO₂, Mg# 35-90, 0-3 wt.% K₂O), to alkali-rich diorite (50-60 wt.% SiO₂, Mg#: 40-55, 0.5-2 wt.% K₂O) and granite (50-85 wt.% SiO₂, Mg#: 5-50, 1-6 wt.% K₂O). To test the role of equilibrium and fractional crystallization, thermodynamic models were run using Rhyolite-MELTS software, and compared to experimental results in the 0-1 GPa pressure range from the literature. Some correlations between our samples compositions and the models (e.g., for CaO contents and Mg#) can be seen, but the latter fails at reproducing SiO<sub>2</sub> and K<sub>2</sub>O differentiation trends.</p><p> </p><p>Bulk-rock compositions indicate that magmas follow a composite differentiation trend between tholeiitic and calc-alkaline series, and the low abundance of olivine, even for the most primitive rocks indicates that before reaching the lower crust, magma was already fractionated during it ascent through the mantle. However, major differentiation does not seem to occur in the lower crust, being set fertile by previous tectono-metamorphic events. Instead, most of differentiation occurs in the fertile middle crust, since a wide major elements compositional range is observed. Both experimental and modelling results show that the observed diversity of composition cannot be attributed to fractional crystallization solely, notably by the high K₂O content at high Mg#. This suggest a potential role on crustal contamination; although evidence for contamination can be documented in the field (e.g., garnet, cordierite-bearing gabbro surrounding xenoliths), the extent of this contamination and its vectors remains to be constrained.</p>

Characterization of oxide assemblages of a suite of granulites from Eastern Ghats Belt, India: Implication to the evolution of C–O–H–F fluids during retrogression

Metamorphic Core Complex dynamics and structural development: Field evidences from the Liaodong Peninsula (China, East Asia)

Emplacement of mantle-derived magma (magmatic accretion) is the total thermal budget of the continental lower crust, often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism, and granulite facies metamorphism and crustal anatexis. The juxgenerate Yand heavy rare earth element (HREE)taposition of mafic cumulates and regionally distributed granulite depleted granitoids (Ellis, 1987). Accretion of mafic facies rocks has led some to consider the Ivrea zone (northern Italy, magma and migration of partial melt will internally Southern Alps) as an important exposure that demonstrates this stratify, chemically differentiate, and deplete the concausal relationship. However, regional PTt paths indicated by tinental lower crust in large ion lithophile elements metamorphic reaction textures and PT conditions inferred from (LILE). Magmatic accretion has been invoked to provide geothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism in rocks (Mafic Complex) at the Ivrea zone occurred during detectonic settings such as Phanerozoic extensional terranes compression from ambient pressures at the regional thermal maximum. (Lister et al., 1986; Gans, 1987; Fountain, 1989; Mareschal Field and petrographic observations, supported by PT estimates, & Bergantz, 1990; Jarchow et al., 1993) and magmatic indicate that regional retrograde decompression and emplacement of arcs (Hamilton, 1981; Kay & Kay, 1981; Bohlen & the upper parts of the Mafic Complex probably accompanied Lindsley, 1987; Hildreth & Moorbath, 1988). extension during the Late Carboniferous–Early Permian. A spatially However, as few exposed sections of lower continental restricted decompression-melting event accompanied final emcrust show contiguous mafic intrusions and regionally placement, depleting supracrustal rocks enclosed by an >2–3 km distributed granulite facies rocks, estimates of the extent aureole overlying the upper Mafic Complex by 20–30% granite of anatexis and metamorphism accompanying magmatic component. The upper Mafic Complex provided the thermal energy accretion have relied on numerical and analog simto reset mineral assemblages and locally overprint the regional ulations. These models yield disparate results depending prograde metamorphic zonation. The limited extent of the contact on whether heat transfer within the mafic intrusion aureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convective regional metamorphism and crustal anatexis. (Campbell & Turner, 1987; Huppert & Sparks, 1988) or conductive (Marsh, 1989; Bergantz & Dawes, 1994; Barboza & Bergantz, 1996). A comparison with field evidence is required to discriminate between the model

This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (>620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (<18 Ma).

A number of Late Ediacaran post-collisional volcanic sequences are exposed in southern Sinai, which represents the extreme northern tip of the Arabian-Nubian Shield (ANS). To clarify the age and geochemical characteristics of such volcanism, two localities were selected for the present study: the Meknas and Iqna Shar’a volcanics. These undeformed and unmetamorphosed sequences include intermediate to felsic subaerial lava flows, tuffs and ignimbrites accompanied by deposition of immature clastic sediments. New SIMS U-Pb dating of zircons from two samples of the Meknas lava flows yielded ages of 593 ± 12 and 616 ± 1 Ma, while zircons from three samples of the Iqna Shar’a volcanics yielded ages of 600 ± 6, 616 ± 4, and 617 ± 6 Ma. Combined with field evidence, the zircon ages enable us to recognize two phases of post-collisional volcanic activity in southern Sinai, at 592-600 Ma and 616-617 Ma. Geochemically, the volcanic rocks of the two successions display large silica variations and are mostly medium- to high-K calc-alkaline rocks. The lower units of the earlier phase consist of andesite and dacite, whereas the upper units of the later phase are more evolved, rhyodacite to rhyolite. The evolved rhyolites of the second phase have characteristics that are transitional to alkaline A-type magmas, but this is attributed to extensive fractionation and does not require a change in the tectonic regime. Geochemically, the Meknas and Iqna Shar’a volcanics are enriched in most LILE and depleted in most HFSE. Moreover, they are generally enriched in LREE relative to HREE and characterized by moderate degrees of REE fractionation [(La/Yb)N = 7.0-12.8)]. They evolved from high-K calc-alkaline magmas that were generated in a post-collisional regime. Despite the temporal gap, it appears that all lavas in each locality are cogenetic and formed via fractional crystallization from a common parental melt. Although they erupted in a post-collisional setting, both volcanic suites display geochemical fingerprints of subduction influence, interpreted to reflect remelting of previously formed arc material ca. 750-650 Ma in age. These magmas were derived from the mafic lower crust, which likely melted due to lithospheric delamination. This is consistent with the Hf isotope ratios of their zircons, which consistently yield positive Hf(t) values (+3.2 ±1.5 and +4.3 ± 1.7, from Iqna Shar’a; +2.6 ± 2.3 and +5.3 ± 1.7 from Meknas). The 50-150 Ma time span between emplacement of this lower crust and its remelting was insufficient for its Hf isotope ratio to evolve to negative values considered representative of an ancient crustal source. Contamination by upper continental crust and fractional crystallization were responsible for the variation observed within the studied volcanic suites.

Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon

The Port Mouton pluton is unique among the Late Devonian peraluminous granitoid bodies in the Meguma Lithotectonic Zone of southwestern Nova Scotia in its lithological heterogeneity, extensive physical and chemical interaction with the country rocks, clear evidence for mingling and mixing with mafic magmas, and highly abundant pegmatites. New U–Pb age determinations on monazite establish an intrusion age of 373 ± 1 Ma, similar to the ages of other Meguma Lithotectonic Zone granitoid plutons and mafic intrusions. Field relations, petrology, and geochemistry define three stages of intrusion of the Port Mouton pluton: (i) early stage, discontinuously exposed around the outer margin of the pluton, dominated by coarse-grained tonalite-granodiorite, and with Rb/Sr < 0.55, Eu/Eu* > 0.40, and GdN/LuN < 2; (ii) middle stage, occupying the interior of the pluton, dominated by medium-grained granodiorite-monzogranite, and with Rb/Sr > 0.55, Eu/Eu* < 0.40, and GdN/LuN > 2; and (iii) late stage, consisting of abundant minor sheets throughout the pluton, dominated by fine-grained tonalite, granodiorite, and leucogranite that are similar to rocks of the early and middle stages. The Port Mouton pluton shows a wider range of 87Sr/86Sri (0.7036-0.7154), and a wider range and generally higher εNdi (–3.72 to +2.12), than other granitoid rocks in the Meguma Lithotectonic Zone, potentially reflecting a complex, partially equilibrated, interaction among mantle, lower crust, and upper crust. Field, petrological, and chemical evidence for the involvement of mantle-derived magmas and melting of upper crust permit modelling of the Port Mouton pluton granitoid compositions by three simultaneous mixing equations. These mixing model results suggest that the early stage granitoid rocks can form from simple three-component mixing relationships when the bulk distribution coefficients between residuum and melt for Sr and Nd range from 1.05 to 1.18, or two-component mixing combined with fractionation of material like the known felsic lower crust. The middle stage granitoid rocks only yield solutions involving two-component mixing and fractionation of material unlike the known felsic lower crust. We conclude that the Late Devonian mafic magmas played a major role in the formation of granitoid magmas in the Meguma Lithotectonic Zone by supplying heat and material to cause partial fusion of the Avalon lower crust.

The Neoproterozoic rocks of the Bir Madi area, south eastern desert, comprise a Metagabbro-Diorite Complex(GDC) and a Tonalite-Granodiorite Suite (TGrS). The GDC has a weak tonalitic to strong calc-alkaline character and ismade up of olivine gabbro, hornblende gabbro, diorite and monzodiorite. The olivine gabbro is characterized by abundanceof augite and labradorite with pseudomorphic serpentine. The hornblende gabbro is mainly composed of hornblende,labradorite, andesine and minor amounts of quartz with or without augite. The diorite consists essentially ofandesine, hornblende, biotite and quartz. The GDC is compositionally broad, with a wide range of SiO2 (46-57 %) andpronounced enrichment in the LILE (Ba and Sr) relative to the HFSE (Nb, Y and Zr). The GDC rocks exhibit petrologicaland geochemical characteristics of arc-related mafic magmas, derived possibly from partial melting of a mantle wedgeabove an early Pan-African subduction zone of the Neoproterozoic Shield. The tonalite and granodiorite have a calcalkalineaffinity and show the geochemical signatures of I-type granitoids. The TGrS contains amphibolite enclaves andfoliated gabbroic xenoliths. Based on the field evidence and geochemical data, the GDC and TGrS are not related to asingle magma type through fractional crystallization. The presence of microgranular amphibolite enclaves in the tonaliticrocks suggest against their generation by partial melting of a mantle-derived basaltic source. The tonalitic magmaoriginated from partial melting of an amphibolitic lower crust by anatexis process at a volcanic arc regime duringconstruction of the Arabian-Nubian Shield. Fractional crystallization of K-feldspar and biotite gave more developedgranodiorite variety from the tonalitic magma. The gabbroic xenoliths are similar in the chemical composition to theinvestigated metagabbros. They are incompletely digested segments from the adjacent metagabbro rocks incorporatedinto the granitic magma through an assimilation process.

The role of assimilation at the roof of the axial magma chamber (AMC) at fast spreading mid‐ocean ridges is investigated using field, geophysical, and geochemical data. Field observations from ophiolites indicate that roof assimilation is a common process, but provide little constraint on the amount of material assimilated. Arguments based on geophysical data that include the subsidence of the layer 2A/2B boundary across the width of the AMC, and the depth of large axial summit troughs, require at least ∼50–100 m of assimilation. However, significant variations in the depth of the AMC on the East Pacific Rise (EPR), over short along‐axis distances, suggest up to ∼500 m fluctuations in the depth of the AMC may be common on short timescales. Geochemical modeling of the over‐enrichment of chlorine in EPR basalts suggests that ∼20% of the oceanic crust may go through a cycle of crystallization, alteration, then assimilation. This is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation. Assimilation most likely occurs on a number of different timescales ranging from decadal, due to perturbations in the thermal regime related to diking, to longer‐period fluctuations controlled by magma supply from the mantle. Large‐scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation, and for the interpretation of the geochemistry of the oceanic crust.

Geological Evolution of the Karakoram Terrane since Neoproterozoic

The Ediacaran Ferani and Rutig volcano-sedimentary successions of the northernmost Arabian-Nubian Shield (ANS): New insights from zircon U–Pb geochronology, geochemistry and O–Nd isotope ratios