Assimilation-fractional Crystallization Processes Research Articles

Experimental re-melting of a continental crust: probing the deep storage zone of Campi Flegrei and Vesuvius magmas

Mantle magmas interact with surrounding rocks during their ascent and storage in the continental crust, leading to open system processes as wall rock partial melting. In this study, we have experimentally investigated the reactions between a leucosome depleted migmatite and a primitive K-basaltic of Campi Flegrei (Italy). Experiments were carried out at pressure of 0.8 GPa temperatures from 1250 °C to 1050 °C and constant temperature and thermal gradient conditions. The experimental products consist of biotite-free migmatite, glass and crystals of clinopyroxene, olivine, plagioclase and Cr-spinel with proportions that vary as a function of temperature. Open system isothermal experiments indicate that the chemistry of melts and phase relationships are controlled by the high Al2O3 content of leucosome depleted migmatite with the glass composition shifting from K-trachybasalt towards shoshonite as the temperature decreases from 1200 °C to 1125 °C. At temperatures \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\le$$\\end{document}1150°C, migmatite assimilation is not exclusively due to the assimilation fractional crystallization process because evidence of mingling and mixing is observed. T-gradient experiment shows melt composition ranging from shoshonite to phono-tephrite moving from the slightly crystalline zone (T = 1250 –1210 °C) at the bottom of the capsule towards the highly crystalline zone (T = 1160 –1140 °C). This SiO2-constant trend indicates that at temperature below the basalt solidus, the assimilation of leucosome depleted crust is represented almost exclusively by the biotite breakdown, leading to the increase in Al, Mg, Fe, Ti, and K activities in the system. The shoshonitic composition obtained in our experiments could represent the parental magma for both Campi Flegrei volcanic district and Vesuvius magmatic systems, indicating modification in a deep storage zone through mixing with the partial melts derived from restitic continental crust.

Petrological and geochemical constraints on the magmatic evolution at the Ampato-Sabancaya compound volcano (Peru)

In order to gain insights into continental arc magmatic processes, we have conducted a petrological and geochemical study of major and trace elements and Sr, Nd, and Pb isotopes of the Ampato-Sabancaya compound volcano, which belongs to the Andean Central Volcanic Zone (CVZ). Whole-rock compositions for Ampato and Sabancaya range from andesites to dacites (56.7–69.3 wt% SiO2) and both belong to a medium- to high-K calk-alkaline magmatic series. Ampato-Sabancaya samples are characterized by high contents of large-ion lithophile elements (LILE; e.g., K, Rb, Ba, Th), low concentrations of high field strength elements (HFSE; e.g., Nb, Zr) and heavy rare earth elements (HREE; e.g., Yb), with consequently high La/Yb and Sr/Y ratios. An increase in these ratios is usually interpreted as a result of magmatic differentiation in the presence of garnet in the deep crust. A detailed analysis reveals that the rocks of Ampato-Sabancaya display three different compositional groups. (1) The first, composed mainly of andesites (56.7–59.8 wt% SiO2), corresponds to lavas from the early stage of the Ampato Basal edifice, as well as pyroclastic deposits from the Ampato Upper edifice. (2) The second group corresponds to andesitic and dacitic compositions (60.0–67.3 wt% SiO2) from the Ampato Basal edifice (Moldepampa stage), the Ampato Upper edifice, and the Sabancaya edifice. (3) The third group corresponds to dacitic compositions (65.0–69.3 wt% SiO2) associated with the Corinta Plinian fallout and pyroclastic flow deposits from the Ampato Upper edifice. This last group of dacites, erupted during the Ampato Upper edifice stage, have drastically different compositions from the other groups with Sr/Y (<27) and Sm/Yb (<4.7) ratios lower than other lavas and lacking evidence of amphibole and/or garnet fractionation during their genesis. As a whole, Sr, Nd, Pd isotopic ratios suggest that mantle-derived magmas are significantly affected by assimilation processes during their evolution, due to the thick (65–70 km) continental crust beneath the CVZ in southern Peru. In summary, the magmatic evolution of group 1 and 2 can be explained by a two-step model in which primitive magmas evolved in the deep crust in the so-called melting-assimilation-storage-homogenization (MASH)-type reservoirs by assimilation-fractional crystallization (AFC) processes involving garnet and/or amphibole. Then, amphibole-dominated upper crustal AFC processes and magma mixing are responsible for the geochemical diversity of the main ASCV trend. In contrats, the group 3 dacites followed an upper crustal AFC process (without amphibole) from a different primitive magma, which did not suffer the high pressure, garnet-dominated AFC processes. This evolution highlights the complexities associated to magma genesis and differentiation at continental arcs contructed on a thick crust.

Primitive high-Mg andesites from the Western Alps, Italy: Products of interaction of sediment diapir derived melts with mantle-wedge peridotite in a continental collision zone

Primitive (high-Cr, Ni), high-Mg andesites (HMA) at convergent plate margins are considered primary, mantle-derived melts, and are not part of the classical tholeiitic or so-called calc-alkaline igneous differentiation series. Their genesis is often considered the product of melting of lower crustal sequences or the subducting slab, though their genesis is far from being understood. Alternatively, they may represent differentiated rocks from boninitic precursors. Here we present isotope and geochemical data for a rare suite of high-Mg andesites that were preserved as decimetre-sized boulders in molasse-type sedimentary rocks of the Oligocene-Miocene Gonfolite Lombarda (Como Formation; Italy), as part of the post-orogenic alpine sedimentary succession. The HMA have moderate to high MgO (1.9–6.4 wt%), high Al2O3 (16.3–18.8 wt%), along with high Cr (30–306 ppm) and Ni (13–92 ppm), and moderate Yb (1.2–2.5 ppm) and Y (12–26 ppm) abundances, matching those of primitive HMA. These HMA are LILE- and LREE-enriched and most samples have negligible negative Eu anomalies. Primitive mantle-normalized compositions show depletions in Nb, Ta, P, Ti and Y. Most samples have unradiogenic Nd (εNd: −6.2 to −7.8) and radiogenic 87Sr/86Sr isotope compositions (0.708 to 0.710), elevated δ18O values (+7.6 ‰ to +9.5 ‰) and radiogenic Pb isotope compositions, with a pronounced variation in 207Pb/204Pb and 208Pb/204Pb at rather constant 206Pb/204Pb. Initial εHf isotope values range from −0.4 to −2.3, some of which deviate from the HfNd crust-mantle isotope array. One sample is unevolved (87Sr/86Sr: 0.708; εNd: −3.3, εHf: +1.8, δ18O: 8.5 ‰) whereas another sample is strongly evolved (87Sr/86Sr: 0.720; εNd: −9.3, εHf: −5.8, δ18O: 9.3 ‰); the latter probably representing a crustal melt. The remaining samples have intermediate compositions (87Sr/86Sr: 0.709–0.713; εNd: −6.2 to −8.5, εHf: −0.4 to −4.3, δ18O: 7.6–10.6 ‰) that either result from limited AFC processes or represent source heterogeneities. The sum of these features cannot be the result of a simple slab or mantle wedge melting process. To account for the observed geochemical signatures of the high-Mg andesites, a three-stage model is suggested that invokes melting of slab-derived sediment diapirs, followed by melt-rock-reaction in the mantle wedge and subsequent crustal assimilation-fractional crystallization (AFC). The diapirs that rise into the mantle wedge are composed of subducted wet, Alpine sediments. Hydrous dacitic (siliceous) partial melts derived from them react with the ambient mantle wedge and cause the formation of boninitic melts with elevated MgO, Ni and Cr; the archetype features of primitive high-Mg andesites. Subsequent interaction with the overlying arc crust is evidenced by crustal xenoliths, which may account for some isotope features of the rock suite. We consider this scenario most plausible because it is relatively simple, and explain most if not all geochemical features, based on modelling and experimental evidence.

Crust-scale reactive porous flow revealed by the brown amphibole in the IODP hole U1473A gabbros, Southwest Indian Ridge

Mid-ocean ridge magma reservoirs are mush-dominated systems, where reactive porous flow is a dominant post-cumulus process. However, the length scale of this process is poorly constrained, especially in ultraslow-spreading ridges. Here, we seek to address this question by presenting a comprehensive brown amphibole dataset of gabbros in IODP Hole U1473A drilled on the Atlantis Bank ocean core complex, Southwest Indian Ridge (SWIR). The results suggest a magmatic origin of the brown amphiboles. Intra-grain chemical variations and compositional zoning of these amphibole grains suggest they were crystallized in a closed system. Nevertheless, geochemical modelings indicate the remarkable Zr/Nd ratio variations in brown amphibole can only be explained by assimilation-fractional crystallization processes triggered by the reactive porous flow. Thus, the brown amphibole in olivine gabbros was crystallized from trapped melts residual after earlier melt-mush interaction. In this light, the reactive porous flow should have ceased prior to the brown amphibole precipitation. This, in turn, suggests the stratigraphic variations in amphibole trace elements reflect the final-stage reactive melt migration. The stratigraphic variations in amphibole trace elements reflect a major episode of reactive melt migration through an ∼800-m section upwardly. This implies that the porosity of the entire gabbroic section remained connected, providing a minimum estimate of the size of the melt-present zones in the lower oceanic crust during the spreading of the 57-60°E magmatic segment of SWIR. Our study reveals that large-scale reactive porous flow can occur in the magmatic segments of ultraslow-spreading ridges, and has important implications for the thermal and rheological evolution of the lower oceanic crust.

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

Multi-scale isotopic heterogeneity reveals a complex magmatic evolution: An example from the wallundry suite granitoids of the lachlan fold belt, Australia

Open-system magmatic processes are expected to impart various sorts of isotopic heterogeneity upon the igneous rocks they produce. The range of processes under the “open-system” umbrella (e.g., simple two-component mixing, magma mingling, assimilation with fractional crystallization) cannot usually be uniquely identified using data from a single isotope system. The use of bulk-rock, mineral separate and in situ techniques and multiple isotope systems allows the characterization of isotopic variability at different sampling scales, illuminating details of the petrogenesis of a magmatic system. This approach has been applied to granitoids of the Wallundry Suite in the Lachlan Fold Belt, Australia. The Wallundry Suite exhibits variations in mineral assemblage, mineral composition and trends in bulk-rock major and trace element compositions consistent with the involvement of liquid-crystal sorting processes such as fractional crystallization. In situ paired O-Hf isotope data from zircon in six samples show an array indicating the isotopic evolution of the melt phase. Similarly, bulk-rock Sr-Nd-Hf isotope arrays support open-system magma evolution. These data combined with the petrographic observations and major and trace element geochemical variations suggest some form of assimilation-fractional crystallization process in the petrogenesis of the Wallundry Suite. Added complexity is revealed by two observations: 1) the isotopic variations are only weakly coupled to the lithology and major element compositions of the samples; and 2) there are distinguishable differences between the Hf isotope compositions of bulk-rock samples and those of the magmatic zircons they host. To varying degrees the rocks consistently show negative ΔεHfbulk-zrc values (i.e., the bulk-rock compositions have less radiogenic Hf isotope values than their coexisting zircons). The preservation of distinctly low Nd and Hf isotope ratios in an Fe-Ti oxide mineral separate suggests that the bulk-rock vs. zircon discrepancy is caused by the presence of unmelted components derived from a contaminant of continental origin (i.e., a rock with low Sm/Nd and Lu/Hf and thus unradiogenic Nd and Hf). Evidently, a complex interplay of assimilation, crystallization and melt segregation is required to account for the data. This investigation demonstrates that such complexity can, nevertheless, be disentangled through comparison of complementary isotope data at multiple sampling scales.

Contrasting magmatic evolutions of the Three Sister Volcanoes reflect increased heat flow, crustal melting and silicic magmatism in the Central Oregon Cascade Arc

The Three Sisters stretch, one of the most magmatically active segments of the Oregon Cascade Arc, includes the Three Sisters volcanoes proper (North, Middle and South) as well as the shields and cinder cones of the Mount Bachelor Chain and McKenzie Pass. The Three Sisters record a magmatic transition from fundamentally basaltic andesitic activity in older North Sister products to more diverse activity at Middle Sister, and unusually silicic activity in a Cascade volcano, including rhyolite lava, at South Sister.North Sister magmas reflect magmatic evolution of basaltic andesite through crystal fractionation, mostly of olivine, and incorporation of crustal melts. Middle Sister, although mostly composed of basaltic andesite and andesite, produced dacitic magma through assimilation fractional crystallization processes involving incorporation of silicic melts. South Sister developed a subjacent magma chamber under the southern half of the volcano, where andesitic magmas evolved, dominantly through fractional crystallization, to dacite and rhyolite. A prominent compositional gap at South Sister between dacite and rhyolite and depletion of middle and heavy rare earth elements in rhyolite suggest that amphibole was prominently involved in fractionation processes involving separation of silicic magma from a gabbroic or amphibolitic crystal mush. Silicic magma eventually formed a stagnant cap in the system, where it evolved through small scale fractional crystallization. Rhyolite magma was finally heated to elevated temperatures prior to eruption, presumably through influx of less evolved magma into the base of the stagnant silicic cap and was erupted in late rhyolite lava of Rock Mesa and the Devil's Hill Chain.South Sister lies along strike of the Northwest Rift of Newberry Volcano, which forms the northwestern terminus of the High Lava Plains trend of south-central Oregon. The increased magmatic activity of the Three Sisters stretch and unusual abundance of silicic products, including rhyolite, of South Sister may reflect the arrival and intersection of the High Plains magmatic anomaly with the Cascade Arc.

Geochemistry and Isotopic Characteristics of Apatite and Zircon From Late Jurassic Granites in the Jiaobei Terrane, East China: Implications for Petrogenesis and Geodynamic Setting

The North China Craton (NCC) was stable for more than 2.0 Gyr before a Jurassic–Cretaceous large-scale lithospheric thinning event, but the geodynamic setting during the early phases (Late Jurassic) of NCC reworking remains controversial. We present new petrological and whole-rock geochemical data, zircon and apatite geochemistry, U–Pb ages, O isotopic data, and Sr–Nd isotopic data for two phases of Late Jurassic granite (Linglong and Luanjiahe) from the Jiaobei terrane, southeastern NCC. LA-ICP-MS zircon U-Pb dating suggests that the Linglong granite formed about 6 Myr earlier than Luanjiahe granite (158 Ma vs 152 Ma), after the inception of the paleo-Pacific plate subduction. High zircon U/Yb ratios, high δ18O values [7.89 ± 0.10‰ to 7.67 ± 0.14‰ (2σ)], and inherited zircon age spectra, as well as high apatite F/Cl ratios and Sr–Nd isotopic compositions, suggest that the Linglong and Luanjiahe granites formed by partial melting of ancient thickened lower continental crust of the NCC and Yangtze Craton. Magma evolution modelling based on Rb and Rb/Nb data suggests a similar decoupled assimilation-fractional crystallization process for the generation of Linglong and Luanjiahe granite but with different assimilation degrees. The water contents of parental magma evaluated by using whole-rock Ba, Sr and apatite F, Cl data indicate that the Linglong granite was formed in a relatively water-rich environment than Luanjiahe. This is consistent with the presence of amphibole and minor negative Eu anomalies in the Linglong granite, as water input can promote amphibole fractionation and suppresses plagioclase crystallization. Considering the similar magma sources but distinct water contents of the granites, and the oblique Paleo-Pacific plate subduction setting in the Late Jurassic, the fluids were likely released from the ocean plate beneath a stacked thickened crust. Since the earliest mafic dikes (OIB-type) in the NCC are coeval with the Luanjiahe granite, we suggest that the lower water contents of the Luanjiahe granite were associated with roll-back that resulted in an increasing distance from slab to continental crust. Such a tectonic transition from subduction compression (158 Ma) to initial extension (152 Ma) in the Late Jurassic perhaps possibly marks the beginning of the reworking of the NCC.

Petrogenesis of isotopically enriched Quaternary magma with adakitic affinity associated with subduction of old lithosphere beneath central Myanmar

We present a model for the petrogenesis of magma with adakitic affinity in an old subduction zone, which does not involve slab melting and is constrained by new geochronological and geochemical data for Mt. Popa, the largest of three Quaternary volcanoes in central Myanmar (Popa, Monywa and Singu). The edifice is composed of Popa Plateau (0.8–0.6 Ma) with high-K rocks and a stratovolcano (< 0.33 Ma) predominantly composed of medium-K rocks with adakitic affinity (Mg# 45–63, Sr/Y > 40). The distinct K contents indicate that the adakitic magmas cannot be derived from Popa high-K rocks, but they share trace-element signatures and Sr–Nd isotope ratios with medium-K basalts from Monywa volcano. Our estimation of water contents in Popa magma reveals that primary magma for medium-K basalts was generated by partial melting of wedge mantle with normal potential temperature (TP 1330–1340 °C) under wet conditions (H2O 0.25–0.54 wt%). Its melting was probably induced by asthenospheric upwelling that is recognized by tomographic images. Mafic adakitic magma (Mg# ~ 63, Sr/Y ~ 64) was derived from the medium-K basaltic magma in fractional crystallization of a garnet-bearing assemblage at high pressure, and felsic adakitic rocks (Mg# ~ 45, Sr/Y ~ 50) were produced by assimilation-fractional crystallization processes at mid-crustal depths.

Construction and Destruction of Bontău Composite Volcano in the Extensional Setting of Zărand Basin during Miocene (Apuseni Mts., Romania)

The Eastern part of the Miocene Zărand extensional basin witnessed the generation and evolution of the largest composite volcano in Apuseni Mts., named recently Bontău. The volcano is filling the basin at the junction between the South and North Apuseni Mountains. The Bontău Volcano is known to be active roughly between ~14–10. In spite of heavily forested and poorly exposed volcanic deposits, it was possible to identify its complex evolution. The volcano suggests an original oval-shaped edifice base currently showing a north-oriented horseshoe-shaped debris avalanche eroded crater. The early effusive volcanic activity was contemporaneous with the emplacement of individual and/or clustered volcanic lava Domes. Late-stage summit dome generation was followed by several volcanic collapses all around the volcanic edifice producing large volcanic debris avalanche deposits (DADs), accompanied by numerous debris flows all around the volcano periphery. Thick pumice pyroclastic flow deposits found below DADs at the periphery may suggest that the slope failures were proceeded by a Plinian eruption. The debris avalanche crater is the last event in the volcano evolution exposing several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes, most probably including the former central vent area of the volcano. The volcano proximal effusive and explosive deposits display a change in the composition of the erupting magma (increased SiO2 from 53.4% to 60.6%) that resulted in an increase of viscosity and the construction of the summit lava domes. Such domes are however only found as various size blocks in DADs. The volcanism connects with the two steps of geotectonic evolution of the Zărand Basin: The initial construction period during regional extension started ~16 Ma up to 12.3–12.1 when the Bontău volcano and surrounding domes were generated. The second period, younger than 12 Ma, corresponds to NW-SE compressional tectonics developed only in the Bontău volcano with summit dome generation and, finally, assists volcano destruction and DADs generation. Newly performed geochemical and Sr and Nd isotopic data studies attest to a calc-alkaline character and suggest an evolution via assimilation-fractional crystallization processes of a dominant MORB-like mantle source magma. Also, they confirm the amphibole (±pyroxene) andesites to be the most evolved lithology. The stepwise changes in fracture propagation in the Zărand extensional setting along with a change to more hydrated and fractionated magma favored in ~4 Myrs of the evolution of the Bontău volcano lead to multiple pulses of the longest-lived magma chamber in the whole Miocene volcanism of the Apuseni Mts.

Late-Orogenic Juvenile Magmatism of the Mesoproterozoic Namaqua Metamorphic Province, South Africa, and Relationships to Granulite-Facies REE-Th and Iron Oxide Mineralizations

Abstract The Bushmanland Subprovince of the Mesoproterozoic Namaqua-Natal orogenic belt in southern Africa hosts numerous occurrences of monazite-magnetite-(biotite-apatite-sulfide)-bearing veins and granitoid dykes, including the Steenkampskraal vein system, which is one of the highest-grade REE-Th ore deposits in the world. Here, we provide whole-rock geochemical data along with zircon and monazite U-Pb dates and monazite Sm-Nd isotope analyses of these REE-Th-rich veins and granitoid dykes. The U-Pb geochronology indicates that the monazite-rich veins and granitoid dykes formed between 1050 and 1010 Ma, contemporaneously with late-Namaqua granulite-facies metamorphism. They are also coeval with the Koperberg Suite mafic magmas at 1060–1020 Ma and the late stage of a major event of A-type granitoid magmatism that occurred from 1100 to 1030 Ma (i.e. the Spektakel Suite). Similar to the mafic intrusive rocks from the Koperberg Suite, monazite-rich veins and granitoid dykes, located in the southern part of the Bushmanland Subprovince, have more radiogenic Nd isotopic compositions (ɛNd(t) ∼ −1 to zero) than equivalent dykes and veins to the north (ɛNd(t) ∼ −12 to −6). Mafic rocks of the Koperberg Suite reach Th and La concentrations of &amp;gt;400 ppm that significantly exceed those of other rock types from the region, except for the monazite-rich veins and granitoid dykes, which suggests a genetic link between these rocks. Within veins and granitoid dykes, monazite, biotite and magnetite are commonly anhedral and occur interstitially between the felsic minerals; they are, thus, late crystallizing phases. The whole-rock REE-Th concentrations of the granitoid dykes increase with Fe-Mg contents. Therefore, their incompatible element enrichment is not linked to assimilation-fractional crystallization processes. The Nd isotopic signature as well as Fe-Mg- and REE-Th-rich character of the Koperberg Suite and monazite-rich granitoid dykes might reflect partial melting of lithospheric mantle domains, metasomatized during previous Namaqua subduction events, and the mixing of mantle-derived melts with REE-Th-rich metamorphic fluids during their ascent through the crust. We propose that the monazite-magnetite vein mineralizations represent Fe-P-rich immiscible liquids that exsolved from mantle-derived magmas with compositions similar to the most mafic and monazite-rich granitoid dykes. Within this petrogenetic model, conjugate silicate-rich immiscible liquids formed the more felsic granitoid dykes characterized by lower modal abundances of biotite, magnetite and monazite. Although they do not reach similarly high REE-Th concentrations, other A-type granitoids from the region, represented by the Spektakel Suite, also share geochemical affinities with mafic igneous rocks from the Koperberg Suite; they may have originated by melting of underplated equivalents of these late-orogenic mafic rocks.

Crustal architecture studies in the Iranian Cadomian arc: Insights into source, timing and metallogeny

The Jalal Abad magmatic rocks, situated at the southern edge of the Saghand-Bafgh-Zarand district, include a thick pile of Cadomian extrusive and pyroclastic units intruded by younger granitoid stocks. New zircon U–Pb ages show eruptions at ∼552 Ma, followed by emplacement of granodiorite at ∼537 Ma. The Jalal Abad magmatic rocks have typical high-K and shoshonitic signatures, and are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSE). Zircon ɛHf(t) from the Jalal Abad magmatic rocks ranges from +3.9 to −3.9 for volcanic rocks and −1.2 to +8.1 for granodiorite. Zircon δ18O values for the Jalal Abad are variable from +5.1 to +8.8‰, progressively higher than those of mantle-derived melts. The whole-rock ɛNd(t) values range between −7.7 to −7.4 for granodiorite, −4.6 to −3 for volcanic rocks and −6.2 to −8.2 for ignimbrites/tuff. The whole-rock Nd and zircon Hf crustal model ages (TDMC) for the Jalal Abad magmatic rocks range between 0.8 and 2.3 Ga. All of the Jalal Abad magmatic rocks have quite similar trace element patterns, and slightly different whole-rock Nd and zircon Hf isotopic composition, indicating the involvement of the thick continental crust during the formation of these rocks. Modeling of zircon Hf–O data, bulk-rock trace elements, and Sr–Nd isotopes suggest the magmas were generated by interaction of mantle–derived melts with thick continental crust through assimilation/fractional crystallization (AFC) processes. However, crustal architecture studies in the Iranian Cadomian arcs show that AFC processes were more important during the Mesoarchean–Early Neoproterozoic (3000–1000 Ma), whereas juvenile magmas became increasingly important to the Cadomian (600–500 Ma) magmatism. Early Cambrian intrusive magmas seemingly intruded sedimentary sequences in the study region and provided magmatic constituents and a heat source for hydrothermal processes and mineralization.

Petrogenesis of Early Cretaceous volcanic rocks of the northeastern North China Craton: Constraints from elemental and Sr–Nd–Pb isotope geochemistry

This study examines the geochemical and whole-rock Sr–Nd–Pb isotopic compositions of Early Cretaceous volcanic rocks of the southern Jilin Province (Tonghua and Huanren areas) and Liaodong Peninsula (Xiuyan, Dandong, and Guiyunhua areas), northeastern North China Craton (NCC). Two groups of volcanic rocks are recognized in the northeastern NCC. The geochemistry and isotopic compositions of the intermediate-acid volcanic rocks indicate that a systematic decrease in whole-rock εNd(t) is accompanied by an increase of Zr/Nb. Modeling results suggest that those intermediate-acid volcanic rocks experienced the assimilation-fractional crystallization (AFC) process with different lower continental crust (LCC) components, indicating the heterogeneity of LCC in the northeastern NCC. As for basic volcanic rocks, they are characterized by “weakly enriched” isotopic signatures, i.e., moderately radiogenic Sr (0.7062–0.7082), unradiogenic Nd (−5.29 to −1.14), and highly radiogenic Pb compositions. The narrow scope of Sr–Nd–Pb isotopic compositions of whole-rock indicates insignificant crustal assimilation. Instead, the high Th/Yb ratios were largely inherited from source enrichment through the input of subducted sediment. Furthermore, element-isotopic modeling suggests that the “weakly enriched” mantle-derived rocks have resulted from a depleted mantle source metasomatized via the sediment-derived melt. Our results provide powerful petrological and geochemical evidence on the Early Cretaceous subduction of the Paleo-Pacific plate beneath the northeastern NCC and suggest the metasomatic process by subducted sediment-derived melt may play an important role in depleted mantle enrichment.

Tin Enrichment in Magmatic-Hydrothermal Environments Associated with Cassiterite Mineralization at Ardlethan, Eastern Australia: Insights from Rb-Sr and Sm-Nd Isotope Compositions in Tourmaline

Abstract Primary cassiterite mineralization is often associated with highly evolved granites, but the magmatic and hydrothermal processes that produce these deposits are often difficult to decipher. In this study, we employed the chemical and Sr-Nd isotope compositions of tourmaline to monitor processes of Sn enrichment in the magmatic and hydrothermal stages of the Ardlethan granite (Australia) and its associated Sn deposits. Initial 87Sr/86Sr (0.710–0.717) and ɛNd (–5.0 to −1.0) values of late magmatic tourmalines indicate derivation of the Ardlethan granite via an assimilation-fractional crystallization (AFC) process in which incorporation of Ordovician sediment into an I-type granitic parental magma produced an enrichment of Sn at least 30 times over that of the assumed mafic-dominated igneous source of the granite. The rare earth element and Sn concentrations of tourmaline in the greisen deposits together with δ18O of coprecipitated quartz indicate that exsolution of a late-stage, Cl-rich fluid from the Ardlethan granite led to cassiterite mineralization in these deposits. In contrast the Fe/(Fe + Mg) and initial εNd (–9.2 to −12.9) compositions of tourmaline that coprecipitated with cassiterite in the large breccia pipes adjacent to the Ardlethan granite suggest that granite-derived fluids scavenged Sn by chemical leaching of an older S-type granite that hosts the pipes. This study shows that tourmaline can act as a robust monitor of key geologic processes in complex and dynamic magmatic-hydrothermal Sn systems and that its 87Sr/86Sr and ɛNd isotope compositions are especially useful for constraining the nature of magmatic and hydrothermal sources that contributed to these deposits.

Linking magmatic processes and magma chemistry during the post-glacial to recent explosive eruptions of Ubinas volcano (southern Peru)

Understanding the links between the magma differentiation processes, the magma plumbing system and the magma composition at arc volcanoes is of paramount importance for volcanic hazard assessment. In this work we focus on the post-glacial, Holocene, historical, and recent eruption products of Ubinas volcano (Peru), which display an overall decrease in silica content from the older, plinian (VEI 3–5), rhyolitic eruptions (69–71 wt% SiO2) to the historical and recent (2006–2009, 2013–2017), vulcanian (VEI 1–2) basaltic andesitic eruptions (55–57 wt% SiO2). Based on a comprehensive study of the major and trace elements and the Sr-Nd-Pb isotopes, we conclude that this temporal pattern reflects the evolution of the Ubinas magmas in the middle-to-upper crust by a coupled Assimilation-Fractional Crystallization (AFC) process involving a cumulate composed of plagioclase, amphibole, clinopyroxene, orthopyroxene and Fe–Ti oxides, with minor amounts of olivine and biotite at the mafic and felsic end-members, respectively. Upper crustal assimilation is limited to 5–8 vol%, but the overall radiogenic Sr-Nd-Pb signature of the Ubinas magmas requires a larger crustal component, which must therefore occur at middle to lower crustal depths. The petrology of the Ubinas magmas also points to an overall increase in P-T conditions: the large Holocene dacitic and rhyolitic eruptions record temperatures ranging from 800 to 850 °C and pressures in the range of 200–400 MPa, whereas the historical and recent (2006–2009, 2013–2017) basaltic andesitic eruptions provide higher temperatures and pressures (1000 °C, >300–400 MPa). Overall, the thermo-barometry, phase equilibrium and geochemical constraints allow us to propose the existence of a middle-to-upper crust magma column composed of a highly crystalline magma mush containing batches of liquid magma, which seems to be continually recharged from deeper levels. On the basis of the petrological nature of the historical basaltic andesitic eruptions (1667 CE, 2006–2009, 2013–2017), we postulate that during the last centuries, Ubinas experienced a recharge-dominated process, with no evidence for a rejuvenation of the silica-rich reservoir that fed the large Holocene dacitic to rhyolitic eruptions. This study highlights the importance of detailed petrological studies of Holocene sequences at explosive arc volcanoes in order to constrain the magmatic processes and conditions that control large explosive eruptions.

Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

U-Pb zircon geochronology and geochemical constraints on the Ediacaran continental arc and post-collision Granites of Wadi Hawashiya, North Eastern Desert, Egypt: Insights into the ~600 Ma crust-forming Event in the northernmost part of Arabian-Nubian Shield

The Wadi Hawashiya, North Eastern Desert of Egypt, is a mountainous granitic terrain exposing two Ediacaran granitoid suites, namely the Hawashiya Older Granites (HOG) and the Hawashiya Younger Granites (HYG). The HOG comprise normal to high-K, calc-alkaline, metaluminous granitoids (mainly quartz diorites), while the HYG are strongly high-K, calc-alkaline, slighty peraluminous syeno- and monzogranites. Zr is strikingly high (av. 335 ppm) in the HOG and is notably depleted (av. 108 ppm) in the more evolved HYG. The HYG have very low Nb contents almost matching those of the HOG (Nb ~ 6 ppm), which is typical of I-type granites generated at arc-type settings. Although the REE patterns of the HOG and HYG are comparably LREE-enriched with moderately steep slopes, the patterns of the HYG have significant deep negative Eu anomalies of variable magnitudes, whereas those of the HOG usually exhibit very shallow negative and occasional positive Eu anomalies. The assessed apatite crystallization temperatures for both granitoid suites are higher than those estimated from the zircon and monazite thermometers indicating that apatite crystallized before zircon and monazite from the melt. HOG evolved by assimilation-fractional crystallization processes from mantle-derived magmas, which subsequently interacted with crustal sources during ascent and residence in higher crustal levels. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments. The HYG were emplaced (586–601 Ma) after the collision between the juvenile crust of the Arabian-Nubian Shield (ANS) and the older pre-Neoproterozoic continental blocks of Saharan Metacraton. The emplacement of HOG (622-611 Ma) had occured earlier (ca. 10 Ma) in a normal mature continental arc. The geochemical signatures and age data suggest that the terminal collision between East and West Gondwanaland in the northern tip of the ANS occurred before ~620 Ma, most likely at ~630 Ma, and transitioned to an extensional setting at ~600 Ma.

Geochronology and geochemistry of Neoproterozoic Hamamid metavolcanics hosting largest volcanogenic massive sulfide deposits in Eastern Desert of Egypt: Implications for petrogenesis and tectonic evolution

The Gebel Abu Hamamid area forms the middle part of Shadli Metavolcanics Belt of the South Eastern Desert, which hosts the largest volcanogenic massive sulfide deposits in Egypt. The host-rock succession belongs to the Younger Hamamid Metavolcanics (YHM) group, consisting of mafic and felsic lavas with variable volcaniclastics. Although several studies have attempted to clarify the crustal evolution of the Neoproterozoic basement rocks of the South Eastern Desert, researchers have yet to develop a deep understanding of the petrogenesis, tectonic setting, and ages of these host rocks. In the current study, we newly report whole-rock geochemistry, mineral chemistry, and in-situ U–Pb zircon dating from basaltic to rhyolitic rocks of the YHM group to describe their magmatic evolution. Results indicate that the mafic rocks are characterized by tholeiitic affinity and arc-like geochemical signatures with significantly enriched large-ion lithophile elements and depleted high field strength elements. The parental magma was likely generated by partial melting (~10–15%) of a spinel-lherzolite source mantle that was metasomatized by fluids from a former subduction event, followed by magma ascent under low pressure and low oxygen fugacity conditions. The felsic lavas (~695 Ma) are characterized by subduction-related geochemical characteristics with significant enrichments in Zr, Hf, and Sm. They formed from lithosphere-derived magma enriched by melts from the ancient subducted slab, and later experienced an assimilation-fractional crystallization (AFC) process. The YHM group was formed under extensional geodynamic conditions (back-arc basin setting). The zircon grains display hydrothermal overgrowth at the rims, most likely due to regional metamorphic and hydrothermal changes. This characteristic is likely the effect of hydrothermal fluids associated with the Jurassic-Cretaceous tectonothermal event in the South Eastern Desert.

New Constraints on the Main Mineralization Event Inferred from the Latest Discoveries in the Bor Metallogenetic Zone (BMZ, East Serbia)

This study aims at better constraining the link between magmatism and metallogeny in the south-easternmost sector of the Bor Metallogenetic Zone (BMZ), where the world-class copper and gold deposit of Čukaru Peki was recently discovered. The obtained U/Pb zircon ages confirm the earlier knowledge that the major Cu–Au porphyry and epithermal mineralization in the BMZ is genetically related to the first volcanic phase (‘Timok andesite’; 85–90 Ma). However, the data also suggest that during this phase, two subgroups of andesite porphyry were formed; they are named volcanic phase 1A (V1A) and volcanic phase 1B (V1B). The V1A andesite (89–90 Ma) is plagioclase-hornblende phyric, holocrystalline and ubiquitously hydrothermally altered and/or mineralized, whereas the V1B (85–86 Ma) is hornblende-plagioclase phyric, holo- to hypocrystalline, fresh, and non-mineralized. According to our simplified model, the contrasting productivity of the V1A and V1B is explained by fluctuations during AFC (assimilation-fractional crystallization) processes of water-rich parental magma, which have controlled the order of crystallization of hornblende and plagioclase in the V1A and V1B andesite.

Across-arc geochemical variations in the Paleogene magmatic belt of Iran

Across-arc geochemical variations between igneous rocks from the magmatic front (MF) to rear-arc (RA) are commonly observed for convergent margins, but the processes that are responsible for these variations are unclear. To address these questions, we studied a well-exposed Cenozoic continental arc, the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran and its rear-arc counterpart to the north.North Iran RA magmatism is bimodal, characterized by mafic to felsic igneous rocks of calc-alkaline to shoshonitic affinity. The new zircon U -Pb ages show that plutonic rocks are ~ 42–38 Ma whereas volcanic rocks are older at ~52 Ma. 87Sr/86Sr(t) and εNd(t) values of UDMB RA igneous rocks range from 0.70483 and 0.70783 and –2.5 and + 3.6, respectively. Zircons from plutonic rocks show εHf(t) values ranging from +1.8 to +9.8, higher than volcanic rocks with εHf (t) between +2.6 and –9.2. Isotopic data agrees with inferences from incompatible trace element ratios (e.g., Th/Yb, Zr/Nb and La/Yb) that mafic plutonic rocks originated from an enriched mantle source, whereas felsic volcanic rocks show more interaction with continental crust. High magmatic fluxes did not occur at the same time in the MF and RA; UDMB MF flared-up at ~ 54–37 Ma and 20–5 Ma, overlapping RA magmatic flare-ups at 45–40 Ma and 30–25 Ma. The available major and trace element data from the UDMB show that most magmatic rocks experienced strong fractional crystallization and assimilation-fractional crystallization processes, making it difficult to identify across-arc geochemical variations. Moreover, the bulk rock εNd(t) and zircon εHf(t) values are highest at the MF, but the across-arc isotopic variability seems to be mainly controlled by crustal influence. High-flux magmatism in the UDMB was related to protracted heating of the crust as a result of subduction of Neotethys oceanic lithosphere. This weakened the continental lithosphere, leading to strong extension in Paleogene time accompanied by decompression melting and magmatic flare-ups.