Crustal Residence Research Articles

THE U-PB AGE AND HAFNIUM ISOTOPE COMPOSITION OF ZIRCON FROM METAMORPHOZED ANDESITE OF THE CHORTOMLYK FORMATION AND RHYODACITE HYPABYSSAL INTRUSION OF THE SURA COMPLEX, CHORTOMLYK GREENSTONE BELT

Andesites and felsic volcanic rocks are observed at all stratigraphic levels of the Konka and Bilozerka groups, which comprise greenstone structures in the Middle Dnieper Domain. Their nature and age are still poorly known. The youngest felsic volcanic rocks of the Solone Formation of the Konka Group and comagmatic with the tonalite-trondhjemite-granodiorite (TTG) association of the Sura Complex hypabyssal intrusions were pre-viously dated by the U-Pb zircon SHRIMP method at ca. 3.1 Ga. The purpose of this study is to determine the U-Pb zircon age and geochemical features of i) metamorphosed andesites of the Chortomlyk Formation and ii) low-alkaline metarhyodacite hypabyssal intrusions that cut the rocks of the Sura Formation of the Konka Group. In the Chortomlyk Greenstone Belt, the thickness of volcanogenic rocks of the Chortomlyk Formation (dacite-andesite-tholeiite association) reaches 2000 m. The youngest felsic volcanic rocks of the Solone Formation and comagmatic hypabyssal intrusions are located within three large volcanic fields located near the Novomykolaivka* massif. Using the LA-ICP-MS method, U-Pb ages of two zircon populations from metamorphosed andesites of the Chortomlyk Formation were dated. Twenty-three crystals of transparent colourless zircon crystals yielded a concordant age of (3222 ± 6) Ma. The U-Pb age of the second population of large, brown, opaque zircon crystals is 3132–3073 Ma. Interpretation of the obtained ages is not straightforward and at least two options can be proposed: 1). The studied metaandesites are differentiated mafic magmas and the age of their formation is defined by the older zircon population, and the young population corresponds to the time of superimposed thermal processes during later intrusion of plagioclase granitoids of the Novomykolaivka massif; 2). The age of the metaandesite is defined by the younger population, while the older population is inherited from the protolith. We consider the second option as being far more likely. The first option contradicts the stratigraphic position of the dated rock. The studied metaandesite is low in potassium and belongs to the sodium series. Relative to TTG, they have higher Nb (16.2 ppm) and Y (25.9 ppm). Rare earth elements are weakly differentiated, (La/Yb)N = 3.91 with a strongly negative Eu anomaly, Eu/Eu* = 0.44. The U-Pb zircon age from the low-alkaline rhyodacite hypabyssal intrusion that cuts the Sura Formation of the Konka Group is (3085 ± 6) Ma. It has a highly differentiated REE pattern, (La/Yb)N = 16.2 and a positive Eu anomaly, Eu/Eu* = 1.21. The Nb (6.7 ppm) and Y (10.8 ppm) contents are low. They chemically resemble TTGs of the Sura Complex. Based on our data, the andesites of the Chortomlyk Formation of the Konka Group and the low-alkaline rhyodacite hypabyssal intrusions have the same U-Pb age, but different origins. The former were produced by the melting of older crustal rocks, and the latter were formed due to the partial melting of metabasites with garnet-bearing restite. Hafnium isotope composition in zircon from both samples reveals their juvenile nature, i.e., they crystallized from partial melts of rocks with short crustal residence times. Our isotope data agrees with the neodymium isotope composition of the felsic volcanic rocks of the Sura greenstone belt, which yielded εHf values of +1.8. These values are lower than the depleted mantle isotope composition at this time (3200–3000 Ma). *Novomykolaivka massif was formerly known as Chkalove massif.

New discovery of 3.84–3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Abstract Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838–3754 Ma trondhjemitic gneisses (4 samples), 3786–3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786–3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4–3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4–56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86–1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2–11.3) and lower Eu/Eu* values (0.62–0.79). The potassic granites have (La/Yb)N ratios of 20.7–55.2 and Eu/Eu* values of 0.75–1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3–3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1–3.5 Ga (mainly 4.1–3.8 Ga) and δ18O of 4.6‰–7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

O–Hf isotope ratios of Alvand S-type granite, western Iran, reveal crustal melting in an extensional regime

There are a few intrusive bodies in the central Sanandaj-Sirjan Zone (C-SaSZ), which is parallel to the Zagros suture zone, in western Iran. The Alvand batholith in the Hamedan area is one of these bodies and consists of rock types with large variation, in particular, two-mica garnet-bearing granite with an S-type signature. Zircon UPb dating gives crystallization ages of 170–163 Ma for the S-type granite. The 176Hf/177Hf ratios in the zircon grains vary from 0.282001 to 0.282780 (rim), with low 176Lu/177Hf ratios (0.002 to 0.018). The εHf(t) values (rim) are clustered into two main groups: a negative εHf(t) group (−6.9 to 0.0, n = 39), with some highly negative εHf(t) values (−25.8 to −21.9, n = 3), and slightly positive εHf(t) values (+0.3 to +2.9, n = 14) of inherited grains. The δ18O values for the zircon grains are positive and range from +10.4‰ to +11.8‰. The high δ18O (positive) and mostly negative εHf(t) values are more consistent with continental crustal sources for these rocks, and the Hf model ages (TDM2) show a wide range from 2.7 to 0.8 Ga, suggesting the heterogeneity of the crustal components. The long incubation time (2089 to 544 Ma) indicates a long history of crustal residence and crustal reworking. The incubation age of the zircon grains, >300 Ma, confirms the recycling of crustal material by partial melting. The simultaneous occurrence of S-type granites with mafic bodies, such as gabbro and diorite members with tholeiitic signatures, suggests that the injection of mafic magma increased the geothermal gradient from the bottom and sides along the major faults, which was the trajectory for the injection of hot mafic melts to shallow depths. These processes abnormally increased the geothermal gradients locally, and the partial melting of crustal and supracrustal materials produced S-type granites and migmatization among the hot blade-shaped mafic bodies. Therefore, the partial melting of crustal material in an extensional tectonic regime and/or in a thin continental crust occurred in a continental rift and/or in the Neo-Tethys passive margin. The present work concludes that the passive margin can be suggested as the tectonic setting for the generation of S-type granites, without the collisional regime that has been widely considered for the sources of this type of granite in the world.

Does a Shallow Magma Reservoir Exist in the Wudalianchi Volcanic Field? Constraints From Magnetotelluric Imaging

AbstractWe obtain new images of the 3D resistivity structure of the Wudalianchi volcanic field (WVF). No low‐resistivity structure is found beneath Laoheishan and Huoshaoshan volcanoes in the WVF, which challenges the hypothesis of one or more magma reservoirs in the upper crust of this region. We observe a low‐resistivity body beneath Weishan volcano; however, estimated magma emplacement rates conflict with the observed volume of that body. Due to the relatively low geothermal gradient and surface heat flow in the area, magma would cool rapidly and could not be sustained for a long time unless new magma was regularly introduced into the system, but our observations do not support this scenario. We suggest that the magma in the WVF mainly originates from the mantle or the base of the crust with little crustal residence, and that the low‐resistivity body in the upper crust is likely due to saline aqueous free fluids.

The Neoproterozoic Agudos Grandes granitic batholith, SE Brazil: Inferences on source areas from elemental and Sr-Nd-Pb isotope geochemistry

I-type, K-rich calc-alkaline hornblende-biotite granitoids (HKCA) suites are typical of both pre- and post-collisional tectonic settings, and their petrogenesis is challenging, since they are dominated by rocks with intermediate composition (e.g., 60–68 wt% SiO2; 10–20 vol % mafic mineral) that are neither typical products of crust nor of mantle partial melting. These granitoids are particularly abundant as the main components of Ediacaran batholiths intruding the Neoproterozoic Ribeira Fold Belt in SE Brazil, e.g. constituting ∼70% of the exposition of the Agudos Grandes Batholith, where they were emplaced at 620-610 Ma, as indicated by U–Pb zircon dating from our previous work and new data presented here. Two types of coeval granitoids comprise an expanded suite of mafic to felsic titanite and allanite-rich biotite granitoids (Itapevi-type) and a single occurrence of biotite-muscovite leucogranite (Turvo-type). Elemental and isotope whole-rock data indicate that different sources were involved in the genesis of their parent magmas, with crust with old residence predominating for the HKCA and Itapevi types, as indicated by very negative (−16 to −20) εNd(t) and low initial 206Pb/204Pb (16.4–16.7), and younger metasedimentary rocks as the source of the Turvo leucogranite (εNd(t) (∼-14); initial 87Sr/86Sr = 0.717). This “syn-orogenic” magmatism was immediately followed, as demonstrated by field and geochronological data, by ∼605-600 Ma circumscribed plutons dominated by more fractionated (±muscovite)-biotite granites that derive from melts generated in the middle crust from sources that are slightly younger than those of the HKCA granitoids, as indicated by slightly less negative (εNd(t) (similar to those of the Turvo leucogranite) and higher 206Pb/204Pb (17.1–17.3). After a ∼15 Myr lull, A-type plutons were emplaced in a post-orogenic setting, probably from mantle and crust sources. The syn-to late-orogenic (620-600 Ma) magmatism of the Agudos Grandes was produced by a thermal anomaly, probably associated with a continental arc. A combination of elemental and isotope geochemistry and inherited zircon data suggests that crustal melting was probably induced by ascension of mantle-derived basic magmas, and initiated in hot zones at the lower portions of a previously thickened crust, formed by old Paleoproterozoic to Archean material (the basement of the Apiaí-São Roque Domain of the Ribeira Belt?), and progressed to the middle portions of the crust, where it provoked partial melting of a different crustal domain with younger age and crustal residence (Embu Complex orthogneisses and metasediments).

Combined bulk-rock Lu-Hf and Sm-Nd isotopic study of Archean granitoids and mafic rocks from Sangmelima terranes (Ntem Complex, south Cameroon): Geodynamic implications

Hafnium and neodymium isotopes provide robust tool tracers for tracking the mechanisms of crustal generation and differentiation, from mantle extraction to the late geological processes undergone by the studied rocks. Though these two isotope systems commonly behave similarly and define a positive correlation between 143Nd/144Nd and 176Hf/177Hf referred to as the “terrestrial array”, they may differ in terrains having a complex geological history, such as Archean cratons. This study presents the first combined whole-rock Hf and Nd isotopic analyses of Archean mafic and felsic igneous rocks from the Sangmelima terranes in the Ntem Complex (located in the Northwest of Congo Craton (NW CC) in southern Cameroon). It mainly consists of Mesoarchean charnockites and tonalite-trondhjemite-granodiorite (TTG) suites, greenstone belts, mafic enclaves or dykes, and potassic granitoids. Mesoarchean charnockite and TTG suites have subchondritic initial 176Hf/177Hf and 143Nd/144Nd ratios with a mean initial (suffix (i)) ε176Hf(i) and ɛ143Nd(i) of –3.30 ± 1.40 and –3.85 ± 0.65 (2SD) respectively, implying the involvement of Eo- to Paleoarchean components in their petrogenesis. The bulk rock initial Hf and Nd isotope compositions of the gabbro and gabbronorites are suprachondritic (ε176Hf(i) = +3.4 to + 8.2; ε143Nd(i) = +0.9 to + 1.6), indicate that they were derived from a depleted mantle source. Coupled with pre-existing data from the literature, these new Hf-Nd isotopic data indicate that two episodes of mantle-derived mafic magmatism in the Ntem Complex occurred during the Mesoarchean period (i.e., at ∼ 3.1 and 2.86 billion years (Ga)). Neoarchean dolerite dykes show broadly chondritic to supra-chondritic ε176Hf(i) and ε143Nd(i) values of –0.28 to + 4.53 and TCHUR ages of 2.74 – 2.76 Ga, suggesting the derivation of doleritic magma from a depleted mantle source with either none or limited crustal residence time. Considering other Archean domains of the NW CC, there were multi-stage magmatism events and crustal growth events occurred at ∼3.75 – 3.31 Ga, ∼3.26 – 3.0 Ga, ∼2.92 – 2.85 Ga, and ∼2.75 – 2.72 Ga from Eoarchean to Neoarchean in the NW Congo Craton.

Garnet growth and diffusion zoning during the late Paleoproterozoic metamorphism, North Altyn area, SE Tarim Craton, NW China: Implications for a long-lived hot orogen

Metamorphic rocks encode invaluable information on the burial and exhumation history of rocks that is critical to our understanding of orogens. In this work, zircon U−Pb dating, pseudosection, and garnet zoning modeling were performed on samples from the North Altyn area, SE Tarim Craton, NW China. Zircon U−Pb geochronological data indicate that the peak metamorphism occurred at ca. 2.0 Ga. Pseudosection isopleth thermobarometry using garnet growth zoning of the sample from the northern part of the North Altyn area (Kalatashtagh) reveals a decompression and heating path to a peak temperature of 735 °C and 0.74 GPa under the equilibrium crystallization model. The preservation of growth zoning under high temperature requires relatively rapid cooling. In contrast, garnet from the southern part of the North Altyn area (Aktashtagh) preserves diffusion zoning. Metamorphic peak occurs at 670−735 °C and 0.67−0.73 GPa, which suggests a high dT/dP-type metamorphism. Diffusion zoning modeling suggests an extremely slow cooling rate of 1−5 °C/m.y., which indicates prolonged (∼100 m.y.) crustal residence time at high temperature. This requires a long-term heat supply, most likely from radiogenic heat from buried sediments or heat conducted from the asthenosphere. The high dT/dP and slow cooling rate, combined with extensive crustal anatexis from previous studies, indicate that the late Paleoproterozoic North Altyn Orogen is a long-lived hot orogen rather than a subduction−accretionary complex. We propose a tectonic model that involves a ca. 2.1−2.0 Ga arc−back-arc system in the SE Tarim Craton. The back-arc basin was subsequently closed by northward underthrusting due to arc−arc/continental collision at ca. 2.0−1.95 Ga during the integration of the Tarim Craton into the Columbia/Nuna supercontinent.

Evidence for contemporaneous deposition of the Duitschland and Rooihoogte formations (Transvaal Supergroup): Implications for tempo and mode of Earth’s Great Oxidation

The stratigraphic correlation of the ca. 2353 ± 18 Ma to 2316 ± 7 Ma Duitschland and Rooihoogte formations, Transvaal Supergroup, South Africa, becomes critically important when viewed in the light of the Great Oxidation Event (GOE), since both formations record the transition from mass-independent (MIF-S) to mass-dependent fractionation of sulfur isotopes (MDF-S) indicative of the transition to an oxygenated atmosphere. At present, discrepancies exist as to whether the Duitschland and Rooihoogte formations are two distinct formations, formed in an older and younger depositional basin, respectively, or alternatively were deposited contemporaneously in the same basin at different positions to shorelines of the basin. A decoupled depositional history would indicate an oscillating oxygenation trajectory of the planet, whereas a contemporaneous deposition of the two formations would point towards a sudden increase of atmospheric oxygen. To investigate the controversy of a contemporaneous versus a consecutive depositional relationship, this study presents a sedimentological investigation of 11 stratigraphic profiles intersecting the Duitschland to lower Timeball Hill and Rooihoogte to lower Timeball Hill formations. The 11 profiles are correlated between the Transvaal Supergroup in the Transvaal area of South Africa and the adjoining Kanye area in Botswana and show that the Duitschland and Rooihoogte formations are laterally correlative. The sedimentological observations are reinforced by similar major and trace element data as well as Sr–Nd isotope compositions measured in four scientific drill cores, intersecting both the Duitschland and the Rooihoogte formations. Consequently, the documented disappearance of MIF-S in these two formations should be regarded as a single-step atmospheric change, thereby removing the best supporting evidence for a protracted and dynamic oxygenation trajectory. Furthermore, bulk sediment TDM(Nd) of the Duitschland and Rooihoogte formations consistently display Archean crustal residence ages in the interval between 2.9 Ga and 3.3 Ga for most samples. Geochemical discrimination functions and zircon distribution patterns suggest a collisional tectonic setting and measured bulk sediment Nd isotope data combined with literature zircon Hf isotope data indicate a potential collision between a Rae-type craton and the Kaapvaal craton around 2.5 Ga.

The compositional diversity and temporal evolution of an active andesitic arc stratovolcano: Tongariro, Taupō Volcanic Zone, New Zealand

New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka.

Arc-like magmatism in syn- to post-collisional setting: The Ediacaran Angra Fria Magmatic Complex (NW Namibia) and its cross-Atlantic correlatives in the south Brazilian Florianópolis Batholith

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian ‘Adamastor Rift’-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian ‘Adamastor Rift’.

Episodic mafic dike swarms (2.0 and 1.7 Ga) in the Quadrilátero Ferrífero Province, Brazil: Implications for the refinement of the São Francisco Craton magmatic barcode and the North China link

Late Archean to Paleoproterozoic dike swarms crosscut the basement and supracrustal sequences of the São Francisco Craton. Their spatial distribution and orientation patterns, as paleomagnetic signature, provide key evidence to the positioning of the cratonic area in ancient paleocontinental reconstructions through the refinement of the “magmatic barcode”. Zircon U–Pb data from dikes in the cratonic domain of the Quadrilátero Ferrífero Mineral Province, in the southern of São Francisco Craton, allowed the recognition of two magmatic stages: a Rhyacian-Orosirian pulse, belonging to the Paraopeba swarm, and a Statherian pulse that belongs to the Pará de Minas II swarm. The Ediacaran Brasiliano Orogeny later affected dikes from both swarms.The Paraopeba swarm consists of pulses related to the Rhyacian-Orosirian orogenic cycle, with dikes providing U–Pb zircon ages of 2019 + 23–11, 1944 ± 29 and 1918 ± 12 Ma. The zircon εHf values from these dikes are predominantly negative, suggesting reworking of preexisting crust, whereas the TDM ages vary from 4000 to 2500 Ma, which can be interpreted as a long period of crustal residence. The Statherian Pará de Minas II swarm, with a U–Pb zircon age of 1717 ± 9 Ma, is chronocorrelated to the lower stratigraphic sequence of the Espinhaço rift basin. Zircon grains from this dike have predominantly positive εHf values and TDM model ages between 3050 and 1950 Ma. Unlike the Paraopeba swarm, the Pará de Minas magmatism is marked by a striking input of juvenile mantle material with minor crustal contamination. The Ediacaran Brasiliano tectono-thermal event caused isotopic disturbance in both Paraopeba and Pará de Minas II swarms, resulting in lower intercepts at 595 ± 65 and 584 ± 27 Ma, respectively.Our data pinpoint dike emplacement related to tectonic events described in the southern of São Francisco. In the final stages of Rhyacian orogenic cycle, at ca. 2019–1918 Ma, data suggests an important crustal contamination. Thereafter, a crustal rifting event at ca. 1717 Ma involved dike swarm emplacement with magmatic source from a mantle plume. The Rhyacian-Orosirian ages obtained here are the first zircon U–Pb geochronological data presented for the Paraopeba dike swarm and allow for a refinement of the São Francisco Craton barcode, reinforcing previous suggestions of proximity with the North China Craton crosscut by the similarly aged Hengshan and Xiwangshan/Xuwujia dike swarms, whether or not those two cratonic pieces were part of the Columbia supercontinent.

Thermal Budgets of Magma Storage Constrained by Diffusion Chronometry: the Cerro Galán Ignimbrite

Abstract The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of the timing, frequency, and intensity of volcanic eruptions and provides important context for interpreting volcano monitoring data. Despite this, however, individual magmatic systems may exhibit a range of time–temperature paths, or thermal histories, that are the result of many complex and, in some cases, competing processes. This complexity contributes to an incomplete understanding of the long-term thermal evolution of magma stored within the Earth’s crust. Of recent interest to the volcanology community is the length of time large volumes of rheologically eruptible and geophysically detectable magma exist within the crust prior to their eruption. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to quantify the long-term thermal evolution of the 2.08 Ma, 630 km3 Cerro Galán Ignimbrite (CGI) in NW Argentina; one of the largest explosive volcanic eruptions in the recent geologic record. We find that diffusion of both Mg and Sr in plagioclase indicate that erupted magmatic material only spent decades to centuries at or above temperatures (~750°C) required to maintain significant volumes of stored eruptible magma. Calculated plagioclase equilibrium compositions reveal an array of liquids that is controlled overall by fractionation of plagioclase + biotite + sanidine, although high-resolution trace element transects record a diversity of fractionation pathways. Overall, we suggest that there is compelling evidence that the magma erupted from the CGI magmatic system spent most of its upper crustal residence in a largely uneruptible state and was rapidly remobilized shortly before eruption.

Late Cretaceous volcanic arc magmatism in southeast Anatolian Orogenic Belt: Constraints from whole‐rock, mineral chemistry, Sr–Nd isotopes and U–Pb zircon ages of the Baskil Intrusive Complex (Malatya, Turkey)

The voluminous intrusive and extrusive magmatism was formed in the Elazığ‐Malatya region of the Southeast Anatolian Orogenic Belt (SAOB) as a consequence of the continental collision and/or convergent orogeny. The SAOB holds a great number of stratigraphic, metamorphic and tectonomagmatic units in the Elazığ‐Malatya region that are crucial for recognition of the geodynamic setting of southeast Anatolia during the Upper Cretaceous. The Baskil Intrusive Complex (BIC) is composed of mainly tonalitic/granodioritic and dioritic/gabbroic plutonic rocks, while its sub‐volcanic phase is represented by granophyre, aplite and microgabbro, which were intruded into the Upper Cretaceous supra‐subduction zone (SSZ)‐type İspendere ophiolites (SE Turkey). New U–Pb laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) zircon data provided crystallization ages ranging from 84.3 to 81.5 Ma for the BIC. Mafic and felsic rocks of the BIC have a low‐ to medium‐K calc‐alkaline, I‐type, metaluminous character, whereas extremely evolved felsic rocks of this complex show a peraluminous composition. The aluminium‐in‐hornblende barometery (~196 to 299 MPa) of magnesio‐hornblendes in the Baskil intrusive rocks indicates their emplacement in the shallow‐ to moderate crustal plutonic levels at a minimum of 7.3 km to a maximum of 11.1 km depth. The Baskil intrusive rocks present depletion in Ti, Nb, Ta, light REE with respect to primitive mantle, and relative enrichment in large‐ion lithophile element (Rb, Ba, Pb, Sr, Th), thus infer a pronounced subduction signature. Both mafic and felsic intrusives show low initial 87Sr/86Sr ratios (0.70357–0.70561) together with a high initial 143Nd/144Nd ratios (0.51279–0.51289), and high ƐNd(T) values (5.0–7.0) and relatively young Nd crustal residence model ages (TDM = 0.31–0.48 Ga). Based on our field, mineralogical and geochemical findings, we suggest that the BIC was originated from an amphibolitic source that interacted with subducted crust within SSZ and was changed by fluids during the early stages of the short‐lived volcanic arc magmatism that evolved at the fringe of the Tauride Platform.

Neodymium Isotope Constraints on the Origin of TTGs and High-K Granitoids in the Bundelkhand Craton, Central India: Implications for Archaean Crustal Evolution

Abstract The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica–high Mg (sanukitoids and hybrids) and high silica–low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid, and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages and geochemical signatures as well as εNd(t) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and preexisting crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.

Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps

ABSTRACT Continental flood basalt (CFB) provinces, while dominated by tholeiitic basalts and basaltic andesites, often also contain alkaline mafic to felsic lavas and intrusions. The tholeiitic and alkaline magmas may reflect different degrees of partial melting of the same mantle source, or the alkaline magmas may be derived from metasomatised, incompatible element-enriched mantle sources. The tholeiitic and alkaline suites, even if closely associated spatially or temporally, require independent magmatic plumbing systems. In the Saurashtra region of the northwestern Deccan Traps CFB province, India, tholeiitic lavas have been intruded by the ~66 Ma Mount Girnar plutonic complex, which comprises olivine gabbros (often with cumulate textures), diorites, and monzonites, profusely intruded by dykes and veins of foid-bearing syenites and lamprophyres. In the region surrounding the complex the tholeiitic lavas have been intruded by a large (12 km-diameter) silicic ring dyke, as well as tholeiitic dykes and sills. The region thus provides an excellent opportunity to study potential petrogenetic relationships between tholeiitic, alkaline, and silicic magmatism in a CFB province, evaluated here using field, petrographic, mineral chemical, and whole-rock geochemical (including Sr–Nd–Pb isotopic) data. Initial (at 65 Ma) Sr–Nd–Pb isotopic ratios of an olivine gabbro and diorites of the Girnar plutonic suite are in the ranges (87Sr/86Sr)t = 0.70499 to 0.70584, (143Nd/144Nd)t = 0.512675 to 0.512484 (εNdt = +2.4 to −1.4) and (206Pb/204Pb)t = 18.270–18.679. Foid-bearing syenites and lamprophyres have broadly similar isotopic ratios and marked enrichments in the most incompatible elements. Thermobarometric calculations indicate crystallisation of mineral phases in the Girnar plutonic suite at varied crustal pressures (0.02–0.9 GPa). Small but significant Sr–Nd–Pb isotopic variations within the plutonic suite rule out closed-system fractional crystallisation as a viable process, whereas a lack of correlation between isotopic ratio and degree of magmatic evolution (rock type) also negates any simple scheme of combined assimilation–fractional crystallisation. The circum-Girnar tholeiitic intrusions, hitherto practically unstudied, are low-Ti and moderately to fairly evolved (MgO = 8.0–3.9 wt %); olivine gabbro and picrite dykes with cumulus olivine show higher MgO (10.1–15.7 wt %), Ni (360–700 ppm), and Cr (410–1710 ppm) contents. The circum-Girnar tholeiitic intrusions have a large range of Sr–Nd–Pb isotopic ratios (e.g. εNdt = +4.2 to −18.7) indicating open-system processes. We infer that magmas of the alkaline Girnar plutonic suite were derived from enriched mantle, with only minor crustal residence or material input, possibly reflecting a very thin basement crust under the complex. In contrast, magmas forming the circum-Girnar tholeiitic intrusions were derived from depleted mantle (εNdt > +4.2) by high degrees of melting, and they experienced olivine fractionation or accumulation in crustal chambers and significant contamination by ancient granitic basement crust. These features probably reflect a much thicker crust surrounding the plutonic complex than directly under it. The circum-Girnar silicic ring dyke has Sr–Nd–Pb isotopic ratios suggesting an origin by anatexis of the basement crust. Based on a range of evidence, the tholeiitic and silicic circum-Girnar dykes and sills are petrogenetically and structurally unrelated to the alkaline Girnar plutonic suite.

Pb isotope insight into the formation of the Earth's first stable continents

The formation of stable buoyant continental crust during the Archaean Eon was fundamental in establishing the planet's geochemical reservoirs. However, the processes that created Earth's first continents and the timescales over which they formed are debated. Here, we report the Pb isotope compositions of K-feldspar grains from 52 Paleoarchaean to Neoarchaean granites from the Pilbara Craton in Western Australia, one of the world's oldest and best-preserved granite–greenstone terranes. The Pb isotope composition of the Pilbara K-feldspars is variable, implying the granites were derived from crustal precursors of different age and/or variable time-integrated 238U/204Pb and 232Th/204Pb compositions. Trends to sub-mantle 207Pb/206Pb ratios preclude the influence of 4.3 Ga crustal precursors. In order to estimate crustal residence times we derive equations to calculate source model ages in a linearized Pb isotope evolution system. The best agreement between the feldspar Pb two-stage source model ages and those derived from zircon initial Hf isotope compositions requires crustal precursors that separated from a chondritic mantle source between 3.2 and 3.8 Ga, and rapidly differentiated to continental crust with 238U/204Pb and 232Th/238U ratios of ∼14 and 4.2–4.5, respectively. The preservation of Pb isotope variability in the Pilbara Paleoarchaean granites indicates their early continental source rocks were preserved for up to 500 Ma after their formation. The apparent longevity of these early continental nuclei is consistent with the incipient development of buoyant melt-depleted cratonic lithosphere during the Eoarchaean to Paleoarchaean.

Isotopic modelling of Archean crustal evolution from comagmatic zircon–apatite pairs

The composition of Earth's early crust is challenging to assess as only a fragmented record remains. As a consequence, deciphering the composition of early crust as a means to understand early processes on our planet often relies on the isotopic composition of resistive minerals. Here we present a new tool for investigating igneous petrogenesis and crustal evolution by combining 87Sr/86Sr measurements of apatite inclusions with U–Pb and Hf isotope analysis of their host zircon crystals. This approach takes advantage of the complementary inverse fractionation behaviour of Rb/Sr and Lu/Hf, and the cogenetic host/inclusion relationship, to link the three isotopic systems and construct an evolution model in triple isotope space. By applying this triple isotope system modelling to Mesoarchean igneous rocks from the Akia terrane in SW Greenland, we reconstruct a bulk SiO2 composition of 63-68 wt% for the precursor source crust and infer an average crustal residence time of 300–350 Ma. Our modelling implicates involvement of intermediate to felsic Paleo-Eoarchean crust in the genesis of voluminous 3.0 Ga magmatic rocks of the Akia terrane, one of the largest components of the Archean North Atlantic Craton. A further outcome of this approach is the ability to place more robust empirical constraints on the Lu/Hf ratio of the precursor crust by comparison of the modelling output to a global whole-rock dataset. The approach in this work provides a template for further application to ancient rocks in order to better understand the evolution of early Earth.

Depositional age and provenance of high-grade paragneisses from the Mérida Andes, Venezuela: Implications for the Ediacaran–Cambrian tectonic setting of northwestern Gondwana

Isotopic, geochemical, and geochronological data are provided to constrain sedimentary sources and depositional ages of high-grade paragneisses of the Iglesias Complex in the Mérida Andes (Venezuela). U-Pb geochronology of detrital zircons suggests a maximum depositional age of 540–530 Ma, whereas age spectra reveal sources from Pan-African-Brasiliano belts, the Amazon Craton, and Oaxaquia-Putumayo basement. Reworking of such Gondwanan sources is also reflected in whole-rock Nd and Hf crustal residence ages of ~2.0–1.3 Ga. Post-depositional disturbance of the U-Pb system by metamorphism during the early Paleozoic and Permo–Triassic is assessed through in-situ analysis of zircon rims and discordant analyses. Field evidence, geochemical and isotopic signatures are consistent with pelitic-psammitic and volcanic-volcaniclastic compositions for sedimentary protoliths. The former show provenances from continental intermediate sources, whereas the latter are probably sourced in an immature volcanic arc, suggesting the initiation of subduction of the Iapetus oceanic crust beneath northwestern Gondwana around 530 Ma. Sedimentary precursors were likely deposited during the latest Ediacaran–earliest Cambrian in an extensive continental shelf, fed by detritus draining from the topographic highs of the Pan-African-Brasiliano belts, across the Amazon craton and into the Iapetus Ocean. Thus, the main direction of sediment flow was opposite to the present-day transport from the Andes Cordillera by the Pacific Ocean to the Amazon fan in the Atlantic Ocean. Chemical trends in gneisses suggest fluvial sedimentary protoliths and cold-climate weathering at high latitudes. Metasedimentary rocks in the Mérida Andes show similarities with equivalent units in Peri-Gondwanan crustal fragments such as the Acatlán Complex and the Santander Massif, but no correlation with Ediacaran metasedimentary rocks of the Maya Block. Ediacaran–Cambrian fossiliferous sedimentary rocks in the Eastern Venezuela Basin and the Colombian foreland basin are possible equivalents but these units remained unmetamorphosed, as its paleoposition was far from the subduction trench during the Early Ordovician climax of the Famatinian orogeny.

On the petrogenesis of Paleoarchean continental crust: U-Pb-Hf isotope and major-trace element constraints from the Bastar Craton, India

Combined geochronological and (isotope) geochemical investigations of tonalite-trondhjemite-granodiorite (TTG) suites, which dominate Archean cratons, provide constraints on the sources and petrogenetic processes that gave rise to the Earth's early continental crust. In situ U-Pb and bulk Lu-Hf analyses of single zircon grains from TTGs of the Paleoarchean Bastar Craton in central India date the emplacement of the igneous rock suite and trace the chemical evolution of its source rocks. Complementary whole rock Lu-Hf and major and trace element data constrain the petrogenesis of the TTGs. The rocks are variably enriched in fluid mobile elements, particularly K and Pb, and are characteristically depleted in heavy rare earth elements. Calculated initial Hf isotopic compositions of zircon and whole rocks cluster around the chondritic value at ca. 3.55 to 3.45 Ga. The high precision Hf-isotope data reveal a trend of increasing radiogenic 176Hf/177Hf with age, the slope of which implies a 176Lu/177Hf = 0.02, and thus derivation from a mafic protolith. Geochemical modeling indicates that the granitoids can be directly produced by melting of a hydrous basalt in the garnet stability field, followed by minor fractional crystallization. Variations in the modal abundance of garnet suggest the source rocks melted at different depths. Rocks with moderately to strongly depleted HREE patterns require >10% garnet in the residue, consistent with derivation from a garnet-amphibolite or garnet-pyroxenite source. Modeling of the Lu-Hf systematics further highlights the important role of melting as opposed to fractional crystallization for reproducing the primary chemical characteristics of TTGs.The global Hf-isotope record of Archean mantle-derived rocks is considerably more radiogenic than that of the bulk silicate Earth, providing unambiguous evidence that parts of the mantle were significantly depleted in incompatible elements since at least the early Archean. The differences between Hf isotopic compositions of the mafic (greenstone belts) and felsic (gneissic terranes) rocks can be accounted for by crustal residence times of several 100 million years for the protoliths of the felsic magmatic suites. Consequently, TTGs integrate the geologic history of their precursor(s), complicating their isotopic record. Therefore, the near-chondritic Hf isotopic composition of TTGs is not direct evidence for their derivation from a primitive mantle, but rather a consequence of their specific petrogenesis involving an aged, mafic precursor. This relationship introduces uncertainties into models of crustal growth and geodynamics that are based on the felsic record, especially where the whole rock context is absent. In contrast, high-precision Hf-isotope analyses of single zircon grains, combined with whole rock isotope and elemental constraints, can be used to reliably identify the sources and processes involved in the generation of Earth's oldest continental crust.

Magmatic Water Content and Crustal Evolution Control on Porphyry Systems: Insights from the Central Asian Orogenic Belt

Abstract Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e., Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for fO2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness.