Explore the vast range of titles available.

The continental crust is the archive of the geological history of the Earth. Only 7% of the crust is older than 2.5 Ga, and yet significantly more crust was generated before 2.5 Ga than subsequently. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust. They yield marked peaks of ages of crystallization and of crust formation. The latter might reflect periods of high rates of crust generation, and as such be due to magmatism associated with deep-seated mantle plumes. Alternatively the peaks are artefacts of preservation, they mark the times of supercontinent formation, and magmas generated in some tectonic settings may be preferentially preserved. There is increasing evidence that depletion of the upper mantle was in response to early planetary differentiation events. Arguments in favour of large volumes of continental crust before the end of the Archaean, and the thickness of felsic and mafic crust, therefore rely on thermal models for the progressively cooling Earth. They are consistent with recent estimates that the rates of crust generation and destruction along modern subduction zones are strikingly similar. The implication is that the present volume of continental crust was established 2-3 Ga ago.

Apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40Ar/39Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions.

The Djourdé-Sinassi magmatic-migmatitic complex is part of the juvenile Neoproterozoic Garoua Domain located in Northern Cameroon. It is surrounded by the Zalbi, Poli and Rey-Bouba greenstone belts, limited to the east by the sinistral Tcholliré-Banyo Shear zone and to the south by the Poli Thrust System. The Djourdé-Sinassi magmatic-migmatitic complex is subdivided into a western magmatic domain made of diorite, tonalite, granodiorite, with numerous enclaves of mafic and ultramafic rocks, and an eastern domain dominated by metatexite migmatites enclosing ubiquitous mafic to ultramafic rafts. The plutonic rocks of the western domain display a gently dipping to vertical magmatic fabric with variable trends. Locally, trains of hornblendite enclaves with lobate boundaries and dismembered into a network of felsic veins points to injection of mafic magma that mingled with a host magmatic mush and/or partially molten rocks. The migmatites display a foliation delineated by the alternation of leucosome and mesosome layers, that are mainly subvertical and wrapped around the metric to kilometric size rafts of mafic and ultramafic rocks. A texturally continuous network of leucosome veins concordant to discordant relative to the syn-migmatitic foliation and the mineral assemblage attests for syntectonic melt segregation under granulite facies conditions (Hbl + Pl ± Bt ± Grt±Opx±Cpx ; peak: ≥800°C/8-10 kbar). The magmatic fabric of plutonic rocks and the syn-migmatitic foliation are locally transposed into subvertical shear zones with a steeply plunging mineral and stretching lineation. Kinematic criteria are identified in the subhorizontal plane perpendicular to the lineation, which indicates that deformation was not plane strain and occurred in 3D. N-S to NE-SW striking shear zones are sinistral whereas E-W to WNW-ESE trending ones are dextral. These shear zones first developed in the presence of melt but then under solid-state during progressive retrogression from amphibolite to greenschist facies (Ab+Qtz + Ep ± Op ± Chl+Ser+Ep). These data suggest that the Djourdé-Sinassi magmatic-migmatitic complex corresponds to the former root of the Central African Orogenic Belt originating from partial melting of a juvenile mafic crust. This root has been vertically extruded under a pure-shear dominated constrictional regime with vertical stretching in the presence of melt coeval with the injection of mantle-derived mafic magmas and was then progressively exhumed towards the surface. This deformation pattern is consistent with the position of the Sinassi complex surrounded by greenstone belts and on a larger scale, confined in between the West African - Congo cratons and Saharan metacraton.

The opening of the Arctic oceanic basins in the Mesozoic and Cenozoic proceeded in steps, with episodes of magmatism and sedimentation marking specific stages in this development. In addition to the stratigraphic record provided by sediments and fossils, the intrusive and extrusive rocks yield important information on this evolution. This study has determined the ages of mafic sills and a felsic tuff in Svalbard and Franz Josef Land using the isotope dilution thermal ionization mass spectrometry (ID-TIMS) U–Pb method on zircon, baddeleyite, titanite and rutile. The results indicate crystallization of the Diabasodden sill at 124.5 ± 0.2 Ma and the Linnévatn sill at 124.7 ± 0.3 Ma, the latter also containing slightly younger secondary titanite with an age of 123.9 ± 0.3 Ma. A bentonite in the Helvetiafjellet Formation, also on Svalbard, has an age of 123.3 ± 0.2 Ma. Zircon in mafic sills intersected by drill cores in Franz Josef Land indicate an age of 122.7 Ma for a thick sill on Severnaya Island and a single grain age of ≥122.2 ± 1.1 Ma for a thinner sill on Nagurskaya Island. These data emphasize the importance and relatively short-lived nature of the Cretaceous magmatic event in the region.

Magma mixing at arc volcanoes is common, but the manner in which mixing or mafic recharge may trigger volcanic eruptions is unclear. We test ideas of eruption triggering for the 1103 ± 13 years B.P. Chaos Crags eruption at the Lassen Volcanic Center, Northern California. We do so by applying mineral-melt and two-mineral equilibria from mafic enclaves and host lavas from six eruptive units of the Chaos Crags eruption to calculate crystallization conditions. Understanding that Chaos Crags are a type location for magma mixing, we estimate these P-T conditions by employing some apparently new methods to reconstruct pre-eruptive liquid compositions-which can be independently verified using various mineral-melt equilibria. We find that crystallization of \"host\" rhyodacite magmas occurs within the upper crust (at pressures of 0-1.7 kbar) over an approximate 300 °C interval (temperatures ranging from 669-975 °C) and that mafic magmas (which occur as enclaves within the host felsic samples) crystallized over an approximate 250 °C temperature interval (ranging from 757-1090 °C), also within the upper crust, though extending to middle-crust depths (0-3.9 kbar). Notably, both host lavas and mafic enclaves contain crystals that are inherited from their opposing end-member, and both magma types contain plagioclase crystals that appear to have equilibrated with the resulting intermediate composition magmas; these intermediate composition plagioclase crystals indicate that some amount of time passed between both the recharge of magma into a felsic reservoir and the mixing event that caused an exchange of crystals before eruption. We propose that mafic recharge-though it may have been the ultimate triggering event-did not immediately precede any of the eruptive events at Chaos Crags. The most mafic (least mixed) enclaves in our collection are nearly aphyric, indicating that they were likely the first melts to enter the system, and quenched upon intrusion into a cold, upper-crust felsic magma. Many high-T olivine grains in enclaves also coexist with clinopyroxene, plagioclase, and amphibole crystals that crystallized from only slightly more evolved liquids, at temperatures that are low enough (e.g., 800-900 °C) to have possibly quenched earlier-formed, high-T Ol crystals, perhaps negating the use of Ol diffusion profiles as a record of mixing-to-eruption timescales (at Chaos Crags, at least, they would only provide minimum times, which could be orders of magnitude less than actual times). And more crystalline enclaves record more mixing and more cooling. It thus appears that recharge is required to reinvigorate an otherwise dormant Chaos Crags system, as described by Klemetti and Clynne (2014), but ∼250 °C of cooling and crystallization, as recorded by many enclaves, provides the immediate cause of eruption-through increased magma overpressure by the exsolution of a fluid phase and increased buoyancy.

Cathodoluminescence (CL) and trace element analyses were performed for fluorapatite from the gabbro and Fe-Ti oxide ores in the upper zone of the Hongge Fe-Ti oxide-bearing, mafic-ultramafic layered intrusion in SW China. The fluorapatite is closely associated with Fe-Ti oxides and interstitial to plagioclase and clinopyroxene. The fluorapatite grains in one thin section vary from ∼10 to 800 µm in width and ∼50 to 1200 µm in length. Coarse-grained fluorapatite crystals (>200 in width) in the same thin section show both simple and complex CL images. The coarse-grained fluorapatite crystals with simple CL images show discontinuous, thin dark rims along grain boundaries, whereas those with complex images show clearly bright veinlets across the grains. On the other hand, fine-grained fluorapatite crystals (<200 µm in width) show complex CL images and can be divided into four types, i.e., concentric, chaotic, banded, and overall dark. The concentric type shows distinctly bright core surrounded by dark mantle that is irregularly zoned, whereas the chaotic type shows disordered bright and dark sectors in the interior with a thin dark rim. The banded type shows unevenly distributed bright and dark bands. The overall dark type shows a relatively dark and uneven image. Fluorapatite grains contain 1.84-2.74 wt% F, 0.07-0.19 wt% Cl, and 0.86-1.63 wt% OH. Coarse-grained fluorapatite grains have total rare earth elements (REE) concentrations ranging from 2278 to 3008 ppm and Sr/Y of 9 to 13. Fine-grained fluorapatite grains have relatively high REE (2242-4687 ppm) and low Sr/Y of 6 to 14 in the bright cores, sectors, and bands and relatively low REE (1881-2728 ppm) and high Sr/Y of 9 to 15 in the dark mantles, sectors, and rims under CL imaging. On the thin section scale, the bright sections under CL imaging for fine-grained fluorapatite have much higher REE contents than those for similar bright CL images for coarse-grained fluorapatite. The highly variable REE concentrations among fluorapatite grains and the sections within a single fluorapatite are attributed to a prolonged crystallization process and overprint by fluid metasomatism. The coarse-grained fluorapatite may have crystallized earlier than fine-grained fluorapatite. Then variable degrees of hydrothermal metasomatism released REE from the fine-grained fluorapatite so that diverse CL images developed in the crystals. This study reveals that magmatic apatite from a layered intrusion can be intensively modified by later-stage fluid-induced metasomatism in both trace element composition and CL image texture. Reconstruction of primary melt compositions using apatite from layered intrusions should therefore be treated with caution.

Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O.

The Precambrian Southern Granulite Terrane (SGT) of south India is well-known for the preservation of high- to ultrahigh-temperature (HT-UHT) granulites, prominently exposed in its central part forming a linear belt referred to as the Kambam UHT belt. This belt also hosts widespread occurrences of mafic granulites that are observed in close spatial association with the HT-UHT granulites. This study presents detailed petrology, geochemistry and geochronology of representative mafic granulites from the area to understand their petrogenesis and tectonic setting. The results demonstrate that mafic granulites are low- to medium-K tholeiites, with continental arc affinity, formed by the partial melting of a subduction-modified enriched mantle source. The composition of the parent mantle source is modelled with a spinel/garnet lherzolite contribution ratio between 100/0 and 70/30, suggesting the mixing of spinel and garnet bearing melts during asthenosphere upwelling. Zircon U-Pb geochronology of mafic granulites constrains their emplacement between 612 Ma and 625 Ma, that subsequently underwent metamorphism between 581 Ma and 531 Ma. This overlaps with the timing of HT-UHT metamorphism in the Kambam UHT belt bracketed between 593 and 532 Ma. Zircon Hf isotopic studies reveal parent magma generation from reworked melting sources involving Archean and Proterozoic components. These results propose an alternative heat source for the formation of HT-UHT granulites in the Kambam UHT belt which can be designated as a major terrane boundary within the SGT.

The Central Iberian Zone (CIZ) hosts several mafic suites showing variable geochemical affinities and signatures evolving through time. To provide a better understanding of magmatic processes, possible sources and the temporal evolution of the mafic magmatism in the CIZ, this work presents new geochemical and geochronological data on mafic intrusions hosted by three structures: the Guadalmez syncline (GS), the Almadén syncline (AS) and the Codosera syncline (CS). According to the U-Pb dating on apatite, the alkaline dolerites from the GS and AS were emplaced during at least two events, the first one at the end of the Silurian, and the second one during the upper Devonian to Carboniferous. The mafic rocks from the CS present a transitional affinity and were emplaced during the Carboniferous, either as a single major event around 348±33Ma or as a series of discrete events between 359 ± 22 Ma and 332 ± 17 Ma. They are therefore associated with the first stages of the Variscan orogeny. The mafic suites of the AS and GS show no evidence of crustal contamination, whereas the rocks from the CS may have experienced slight crustal contamination. All studied rocks show an enrichment in LREE, high Gd/Yb>2.9 and Ti/Y>554 ratios; likely indicating the presence of residual garnet in the mantle source. Some of the mafic rocks in this study are contemporaneous with a mafic magmatic event already reported in the Armorican Massif. This allows us to propose the existence of a widespread mafic magmatic event that took place between the Upper Devonian to the Mississippian in both the Central Iberian Zone and in the Armorican Massif at around 360 Ma.

Commingling between contemporaneous mafic and felsic magmas is now widely recognized in a broad range of intrusions and intrusive complexes. These interactions are important features for two main of reasons: (1) the rapidly chilled margins of mafic magma against silicic magma commonly preserve the compositions of mafic liquids, and (2) because the mafic magma solidifies rapidly, the resulting (final) configurations of mafic and felsic magmas can provide insights into physical processes and changing viscosity contrasts and rheologies of magmas and felsic crystal mush during crystallization of the mafic magma. Mingling of contrasted magmas was first recognized in the 1950s. Wider recognition of interactions between mafic and silicic liquids led to concepts of \"net-veining\" in the 1960s, \"intramagmatic flows\" (chilled basaltic layers separated by felsic cumulates) in the 1970s, and in the 1990s to \"mafic-silicic layered intrusions\" (MASLI), which could be as much as a few kilometers thick and more than 100 km2 in area. It was quickly appreciated that these MASLI preserved stratigraphic records of mafic replenishments into silicic magma chambers floored by felsic crystal mush. Volcanic studies had anticipated the occurrence of this last type of intrusion on the grounds that extensive ponding of basaltic magmas beneath silicic chambers was seen to be essential to keep large silicic systems like Yellowstone active for millions of years. This paper looks at the history of changing perceptions and interpretations of magma mingling and whether or not \"sill complexes\" are distinct from mafic-silicic layered intrusions. The stratigraphy of mafic-silicic layered intrusions records changing magmatic compositions, events, and processes in a temporal framework comparable to that provided by coeval volcanic rocks. As a result, careful study of MASLI has great potential for linking plutonic and volcanic processes and events.