Deep Hot Zones Research Articles

Constraints on the source and petrogenesis of early Ediacaran shoshonitic mafic magmatism and high-K calc-alkaline granitoids in the Sergipano Orogenic System, Borborema Province, Brazil

The early Ediacaran Capela pluton (ca. 630 Ma) is intruded into the metasedimentary succession of the Sergipano Orogenic System along the oriental segment of the Macururé Domain, southeastern Borborema Province, NE Brazil. This body is an uncommon example of mantle-derived shoshonitic mafic rocks associated with high-K calc-alkaline granitoids emplaced in a pre- to early-collisional context. Here, we aim to unravel the petrogenesis of this plutonic association to determine the role of crustal contribution to magmatism and its geodynamical significance during the early stages of the Brasiliano/Pan-African Orogeny. The Capela pluton comprises cumulate hornblendites, gabbronorites, gabbros, diorites, granodiorites, quartz monzonites, and granites. Shoshonitic mafic rocks and high-K calc-alkaline granitoids show significant enrichments in LILE and LREE compared to HREE and HFSE. Ultramafic and mafic rocks display a conspicuous crustal signature, yielding subchondritic εNd(t) and zircon εHf(t) values, along with variable radiogenic Sr–Pb isotopic compositions. The dominant factors controlling the geochemical features of the mafic magmas were the enriched signature of the lithospheric mantle source and the degree of partial melting. Mantle source metasomatism is thought to have occurred by incorporating subducted sediments into the mantle wedge before the onset of the Brasiliano Orogeny. Geochemical modeling demonstrated that low partial melting degrees (5–10%) of phlogopite-bearing lithospheric mantle in the spinel stability field metasomatized by 5–10% subducted sediments may account for the main characteristics of the mafic rocks. Introducing sedimentary material into the mantle caused a decrease in Sm/Nd and Lu/Hf ratios while increasing the Rb/Sr, U/Pb, and Th/Pb ratios, which led to the time-integrated evolved isotopic signature observed in the mafic rocks. The coeval granitoids are metaluminous to slightly peraluminous and display Nd–Pb isotopic compositions similar to those of mafic–ultramafic rocks. The petrological and isotopic features of the granitoids are consistent with derivation from an ancient mafic lower crustal source. We argue that the ascent and emplacement of hydrous mantle-derived magmas as successive sills into the lower crust produced a deep hot zone beneath the Sergipano Orogenic System during the onset of the Brasiliano/Pan-African Orogeny. Aqueous fluids released from such potassic mafic magmas favored the partial melting of surrounding rocks, producing contemporary felsic magmatism.

Crystal entrainment from cool, low-silica rocks into hot, high-silica melts: diverse primary melt compositions at Taranaki volcano, New Zealand

The prevalence of antecrysts in arc volcanic rocks is widely accepted, yet the origin of their carrier melts remains debated. Crystal cargo in lava flows from Taranaki volcano, New Zealand, is dominated by plagioclase, clinopyroxene and amphibole. Except for some crystal rims, mineral phases are in disequilibrium with the melt they are entrained in. Major element chemistry reveals an almost complete compositional overlap between the crystals in the lava and those in xenoliths. The large volume fraction of crystals (35–55 vol%) exerts a strong control on whole-rock compositions, reducing silica by 5–11 wt% compared with the carrier melt. Yet there is no clear relationship between mineral proportion and bulk-rock compositions. Our data are inconsistent with extensive fractional crystallization, commonly invoked as a driver of magma evolution towards silica-rich compositions. Instead, high-temperature, aphyric carrier melts with varied compositions (55–68 wt% SiO 2 ) entrain crystal cargo while ascending through colder, low-silica rocks. Thus, some parental melts at Taranaki volcano are significantly more silica-rich than arc basalts commonly invoked as primary magmas. Further, thermometric and hygrometric constraints preclude a deep crustal hot zone for the source of these melts, which we argue are of subcrustal origin. Supplementary material: Supplementary figures and databases are available at https://doi.org/10.6084/m9.figshare.c.6406813

Lower crustal hot zones as zircon incubators: Inherited zircon antecryts in diorites from a mafic mush reservoir

Abstract Continental arcs are key sites of granitic magmatism, yet details of the origins of these magmas, including the role and contribution of mafic magma, the timing and location of initial zircon formation and how zircon isotopic signatures relate to granite formation, remain as challenges. Here we use U–Pb dating, trace elements and Hf isotopic systematics of zircon in mafic microgranular enclaves (MMEs), from the convergent plate margin Satkatbong diorite (SKD) in Korea to understand lower arc magmatism and zircon production. The host granitic body and MMEs display similar major element evolutionary trends and similar ranges of Sr, Nd and Hf isotopes, implying a cognatic relationship. Zircons show a large variability in ε Hf ( t ) ( c. 6 units) and age (>30 Ma). We propose that the SKD and MMEs originated from the same, long-lasting, lower crustal mush reservoir, enabling long and variable residence times for zircons. Prolonged zircon ages, combined with the Hf isotope variability within a single pluton (SKD and its MME), indicate that not all zircons were instantaneously crystallized in a rapidly cooling shallow magma chamber but were continuously formed in a long-lasting hot source. A low-melt-fraction mush type reservoir in a deep crustal hot zone provides a viable model for the source setting. Continuous replenishment of mafic magmas acts as the main re-activator of the reservoir, and provide a critical role in spawning zircons that record a long age span, because (1) the magma adds Zr into the reservoir, enabling it to reach zircon saturation and (2) the generated zircon grains are transported upward as antecrysts by flow inside of the reservoir. This means that antecrysts with different ages may mix with each other in the ascending magma body. The significance of this model is that a conclusive time of intrusion cannot be constrained by such zircon ages, as these antecrysts constitute inherited grains.

Caledonian hot zone magmatism in the ‘Newer Granites’: insight from the Cluanie and Clunes plutons, Northern Scottish Highlands

Scottish ‘Newer Granites’ record the evolution of the Caledonides resulting from Iapetus subduction and slab breakoff during the Silurian–Devonian Scandian Orogeny, but relationships between geodynamics, petrogenesis and emplacement are incomplete. Laser ablation U–Pb results from magmatic zircons at the Cluanie Pluton (Northern Highlands) identify clusters of concordant Silurian data points. A cluster with a weighted mean 206 Pb/ 238 U age of 431.6 ± 1.3 Ma (2 σ confidence interval, n = 6) records emplacement whilst older points (clustered at 441.8 ± 2.3 Ma, n = 9) record deep crustal hot zone magmatism prior to ascent. The Cluanie Pluton, and its neighbour the c. 428 Ma Clunes tonalite, have adakite-like high Na, Sr/Y, La/Yb and low Mg, Ni and Cr characteristics, and lack mafic facies common in other ‘Newer Granites’. These geochemical signatures indicate the tapping of batches of homogenized, evolved magma from the deeper crust. The emplacement age of the Cluanie Pluton confirms volumetrically modest subduction-related magmatism occurred beneath the Northern Highlands before slab breakoff, probably restricted by crustal thickening during the c. 450 Ma Grampian 2 event. Extensive new in situ geochemical–geochronological studies for this terrane may further substantiate the deep crustal hot zone model and the association between Caledonian magmatism and potentially metallogenesis. The term ‘Newer Granites’ is outdated as it ignores geochronology, petrography and tectonic setting. Hence, ‘Caledonian intrusions’ would be a more appropriate generic term to cover those bodies related to either Iapetus subduction or to slab breakoff. Supplementary material: Full geochronological, whole rock geochemical and geobarometric data are available at https://doi.org/10.6084/m9.figshare.c.6305927

Sources of Neoproterozoic post-collisional granites from the Porto Alegre region, eastern Dom Feliciano Belt, Brazil: Sr-Nd-Pb isotopic constraints

The Dom Feliciano Belt, southernmost Brazil and Uruguay, is a Neoproterozoic to Cambrian mobile belt that resulted from the convergence of the Kalahari, Congo, and Río de la Plata cratons. The present paper focuses on post-collisional granites located in the eastern sector of this belt, in the Porto Alegre region, RS, Brazil. The studied rocks yield crystallization ages of ca. 600 Ma and intrude subduction-related, calc-alkaline gneisses from the host basement. This work presents novel isotopic data (Sr-Nd-Pb) for representative samples of the Viamão, Porto Alegre, and Itapuã intrusive suites. For the Viamão Intrusive Suite, the results (87Sr/86Srt = 0.7089–0.7168; εNdt=−6.0 to −6.7; TDM = 1.4–1.5 Ga; 206Pb/204Pb = 18.20–19.71) suggest mixing of mantle (DM) and Paleoproterozoic crust-derived sources (Arroio dos Ratos Complex, a remnant of the basement). For the Porto Alegre Intrusive Suite, the results (87Sr/86Srt = 0.7090–0.7437; εNdt=−4.2 to −8.3; TDM = 1.3–1.8 Ga; 206Pb/204Pb = 18.89–22.19) suggest mixing of mantle (HIMU) and Tonian-Cryogenian crust-derived components (Chácara das Pedras Gneiss, a remnant of the basement). In turn, the results for the Itapuã Intrusive Suite (87Sr/86Srt = 0.7116–0.8733; εNdt=−1.6 to −6.0; TDM = 1.2–1.6 Ga; 206Pb/204Pb = 18.44–22.20) suggest mixing of the mantle (possibly HIMU) and a Nd-depleted, crust-derived component (in comparison to the source of the Porto Alegre Intrusive Suite). All the studied suites are interpreted as being formed in a Neoproterozoic post-collisional setting. The mantle-derived signatures (DM and HIMU) are interpreted as the result of the partial melting of remnants of former oceanic slabs related to previous subduction events. In turn, the crust-derived signatures are interpreted as the result of assimilation of basement rocks. We suggest that the post-collisional magmatism of this sector of the Dom Feliciano Belt is associated with a deep hot zone.

The rise and fate of a long-lived deep crustal ‘hot zone’ beneath the neogene-quaternary Cordillera de San Buenaventura in the Southern Puna Plateau (NW Argentina)

ABSTRACT Arc magmatism plays a key-role in the growth and differentiation of continental crust. In particular, the prolonged magmatic flux events control the genesis of deep crustal hot zones where magma accumulation and fractionation favour the continental arc crust evolution and geochemical stratification. In this view long-lived magmatic systems located in arc domains, with their erupted products spanning from basalts to rhyolites are formidable archive for the understanding the evolution of transcrustal magmatic plumbing systems and the construction of a stratified continental crust. This study focuses on the long-lived (ca. 9 Ma) Miocene-Holocene Cordillera de San Buenaventura volcanic system in the Southern Puna Plateau (NW Argentina), a unique natural laboratory where exploring the building up of MASH-zones (or deep crustal ‘hot zones’) and related sustained volcanism. Synthesis of new and published data, together with new thermobarometry and fractional crystallization modelling, indicates a cyclic scenario with mantle melts undergoing fractional crystallization dominated incipient and waning stages alternate to major mafic magma recharge events during the building up phase of the MASH-zone. This magmatic scenario is also discussed in the light of the coeval geodynamic framework dominated by the subduction of the Nazca plate and the eastward migration of the frontal arc magmatism.

Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (>5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.

Petrogenesis of Early Cretaceous granitoids and mafic enclaves from the Jiaodong Peninsula, eastern China: Implications for crust-mantle interaction, tectonic evolution and gold mineralization

Early Cretaceous granitic plutons in the Jiaobei Terrane, Eastern China, contain abundant mafic enclaves (MMEs), which are important to understand the tectonic settings and genesis of gold mineralization in the Jiaodong Peninsula. Here, we combine petrology, geochronology, and isotopic geochemistry for MMEs and host granitoids to constrain their tectonic evolution and understand the genesis of coeval gold mineralization in the Jiaodong Peninsula during the Early Cretaceous. Zircon U-Pb dating reveals two magmatic events: (1) the earlier magmatic event (130–126 Ma), represented by Congjia MME (CJ-MMEs) and host granodiorite, and (2) the later magmatic event (121–116 Ma), including Gushan MME (GS-MMEs) and host granite. Field relationship, disequilibrium textures, and elemental and Sr-Nd-Hf isotopic compositions indicate that MMEs and host granitoids formed through mixing/mingling of mafic and felsic magmas. However, significant differences between CJ-MMEs and GS-MMEs indicate that the former has adakitic affinity and was produced by mixing of magmas from lithospheric mantle and mafic lower crust in deep hot zone, whereas the latter has arc-magmatic affinity, suggesting enriched lithospheric mantle source. Higher SiO2 and lower MgO contents in the host granitoids indicate that they were originated from reworking ancient NCC lower crust. Our results reveal crustal growth during the earlier magmatic event, followed by rapid crustal/lithospheric thinning in the later magmatic event, all triggered by ongoing rollback of subducting Paleo-pacific plate. These results combined with complex assemblages of sulfides and volatiles in GS-MMEs indicate that the lithospheric thinning, the rise of enriched lithospheric mantle-derived melts to shallow crustal levels, and crustal-mantle interactions around ∼ 120 ± 3 Ma, are the main factors for massive gold precipitation in the Jiaodong Peninsula.

Early carboniferous rifting of the Harlik arc in the Eastern Tianshan (NW China): Response to rollback in the southern Altaids?

Field, geochronological, geochemical and Sr-Nd isotopic analyses are applied to late Paleozoic gabbro-diorites and monzogranites in the Hulugou-Miaoergou regions, Harlik arc, in order to provide constraints on the tectonic evolution of the Eastern Tianshan orogen in the late Paleozoic. LA-ICP-MS zircon U-Pb ages show that the first pulse of gabbroic magmatism occurred at 348 ± 4 Ma, accompanied by simultaneous dioritic (342 ± 3 Ma) and monzogranitic (343 ± 5 Ma) magmatism, and the second pulse of gabbroic magmatism was at 334 ± 3 Ma. Most of the gabbros are medium- to high- K calc-alkaline in composition, and show enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta). In combination with their juvenile isotopic signature (initial ⁸⁷Sr/⁸⁶Sr = 0.70345–0.70380, εNd(t) = 4.5–6), the geochemical features suggest that both pulses of gabbroic magmatism were likely derived from partial melting of asthenosphere facilitated by a flux from a subducting slab. The diorites also display the arc-related geochemical characteristics and juvenile isotopic signature (initial ⁸⁷Sr/⁸⁶Sr = 0.70355–0.70358, εNd(t) = 4.3–4.7), coupled with their intimate relationship with the gabbros indicate that they formed by fractional crystallization of clinopyroxene, plagioclase and amphibole from the first pulse of gabbroic magmatism. The monzogranites have relatively high A/CNK values (1.04–1.1) and are weakly to moderately peraluminous. Considering the presence of zircon xenocrysts dated at 510 to 450 Ma in the monzogranites, and their moderate molar Al₂O₃/(MgO+FeO^T) and molar CaO/(MgO+FeO^T) ratios, partial melting of supracrustal rocks of probable late Ordovician age was most likely the cause of their genesis and heterogeneity. These three different magmas (gabbroic, dioritic and monzogranitic) were probably extracted from a deep crustal hot zone. The primitive basaltic magmas continuously intruded the deep crust where they solidified, fractionated, assimilated, and heated the crust, generating in turn the peraluminous and A-type granitoids, similar to the magmas in the Lachlan orogen that intruded during progressive slab rollback. Based on all available evidence, we propose that southward slab rollback of the subducting Paleo-Asian Ocean in the early Carboniferous was responsible for the progressive emplacement of these different magmas, which eventually resulted in rifting of the Harlik arc.

Arclogites in the Subarc Lower Crust: Effects of Crystallization, Partial Melting, and Retained Melt on the Foundering Ability of Residual Roots

ABSTRACTThick-crusted (>45 km) Cordilleran arcs exhibit cyclic processes including periods of magmatic quiescence interspersed with pulses of high-flux magmatism. Most models assume that during high-flux events, fractional crystallization and partial melting within the deep crustal hot zone generate a dense (>3.4 g/cm3) arclogitic subarc root that can readily founder into the mantle. Yet these models do not consider that: (1) the retention of low-density melt within the subarc root, and (2) the protolith lithology of the restitic portion of the subarc root may greatly impact the density evolution of the root and its susceptibility to foundering. In this article, we first address the effect of retained melt on the foundering ability of the subarc root by calculating the density and time for foundering of melt-bearing arclogitic residue at 1.5, 2, and 2.5 GPa. We find that melt volumes >10–18% are required to stabilize the root within the lower crust; melt volumes below this threshold lower the viscosity of the residue so much so as to decrease the time for foundering by an order of magnitude. We then constrain through phase equilibria modeling the effect of partial melting of different lower crustal protoliths on the density of the restitic subarc root. To do this, we model the density and mineralogical evolution of restites in equilibrium with their derivative melts during open-system, isobaric partial melting of typical crustal assemblages from 600–1100°C at 1.5 (~50 km) and 2 GPa (~65 km). In our models, typical end-member assemblages in a lower crustal hot zone include basalt, metapelite, and metagraywacke. We find that melt-depleted restites derived from basaltic compositions are amphibole-bearing arclogites with densities conducive to foundering, which at 2 GPa can occur even in the presence of the coexisting hydrous felsic residual melt. Foundering of the amphibole ± melt-bearing root may refertilize the mantle wedge and induce melting of the surrounding asthenosphere as well as the arclogitic mass. However, if temperatures in a 50-km deep subarc hot zone are not sufficient to drive melt depletion of basaltic restites, these dense (3.1–3.3 g/cm3) residues are gravitationally stable, increasing the density of the lower crust and lowering the elevation of the arc. In comparison, partial melting of metasedimentary country rock produces alkali feldspar-rich residues that never achieve densities conducive to foundering. Thus, if high-flux events are driven by the influx of melt-fertile lithosphere beneath the arc as envisioned by the Cordilleran cycle model, then partial melting of the metasedimentary portion will generate low-density residues that remain in the lower crust and contribute to the thickness, geochemistry, and seismic structure of the bulk arc.

Water availability controls crustal melting temperatures

Although the capacity for water to lower the solidus during crustal melting is well recognised, a modern consensus is that most granitic magmas are generated under fluid-absent melting conditions at temperatures ≥850 °C, and that water has little influence on crustal melting temperatures. By contrast, we show that granite magmas produced in different tectonic settings equilibrated in the crust at significantly different temperatures, ranging between 700 and 1000 °C, depending on water availability during melting. On a molecular scale, the process that lowers the solidus is similar to hydroxylation, i.e. the depolymerization of the aluminosilicate framework melt by complexing of OH− ions with the melt network formers, Si4+ and Al3+. On a larger scale, the process is “water fluxing”, which is consistent with the growing realisation that many granitic magma reservoirs form under protracted, cool-storage conditions, and consistent with the typically low (100–200 ppm) Zr content (hence low zircon saturation temperatures) of granitic magmas. Global compilations of common igneous and sedimentary rocks also show that Zr contents are consistently in the range of 100–200 ppm, indicating that average Zr contents do not change significantly during crustal differentiation. Such low average contents are most easily reconciled with models of low-T silicic melt generation and entrainment of refractory or antecrystic zircon in the magma. Low-K, I-type (Cordilleran) granitic magmas are cooler and much more voluminous than A-type magmas because they are hydrous, although transiently heated by mafic magma infusions. In turn, water availability is tectonically controlled, occurring in most suprasubduction (SSZ) environments where mantle-derived water drives melting of orogenic crust already elevated in heat flux (>75 mWm−2), typical of upper amphibolite facies conditions. However, within intraplate environments, where melting proceeds by high-T, hydrate-breakdown melting, crustal temperatures are not buffered during water fluxing and rise to granulite facies and ultrahigh temperature (UHT) conditions, particularly for refractory protoliths, producing ferroan, alkalic A-type granites and charnockites in deep crustal hot zones (DCHZ). The combination of tectonic setting, contrasting source rocks, and variation in water availability, controls the compositional diversity of granitic magmas.

Architecture of volcanic plumbing systems inferred from thermobarometry: A case study from the Miocene Gutâi Volcanic Zone in the Eastern Carpathians, Romania

The Gutâi Volcanic Zone (GVZ) in the Eastern Carpathians records one of the most complex and long-lasting periods of Miocene volcanic activity (15.4–7.0 Ma) in the Carpathian–Pannonian region. Two types of volcanism are documented: 1) caldera-related acidic volcanism (ca. 15.4 Ma); and 2) multiphase intermediate volcanism (13.4–7.0 Ma). Two composite volcanoes (Mogoşa - G5, Igniş - G7), six extrusive domes (Dăneşti - G2, Poiana Cremenei - G3, Breze - G6, Pleşca Mare - G9, Gutâi - G10 and Laleaua Albă - G11) and an intrusive complex (Firiza - G12) were selected for analysis to unravel the architecture of the volcanic plumbing systems (VPS) based on the pressure-temperature (P-T) conditions during crystallisation of amphiboles and clinopyroxenes. Using the reliable geothermobarometers for amphiboles, and for clinopyroxenes, P–T conditions were calculated for basalts, basaltic andesites, andesites, dacites/rhyolites, and mafic microgranular enclaves (MMEs) hosted by dacites. The calculated pressures vary from 0.4 to 1.8 kbar for low-Al amphiboles (7.5–9.3 wt% Al2O3, magnesio-ferri-hornblende) and from 6 to 9 kbar for high-Al amphiboles (11.2–15.6 wt% Al2O3, magnesio-hastingsite, ferri-sadanagaite and pargasite). The calculated temperatures vary between 800 and 860 °C for low-Al amphiboles and between 950 and 1010 °C for high-Al amphiboles. The P–T data acquired for the clinopyroxenes (augite and diopside with 62–85 Mg#) are 3.5–8.5 kbar (predominantly >5 kbar) and 1050–1150 °C. A difference of 4–5 kbar is observed between the pressures recorded by low-Al amphiboles and clinopyroxenes in the G2 dacites and G3, G6 and G10 high-silica andesites. The “at equilibrium” high-Al amphiboles and clinopyroxenes from the G5, G11 and G12 samples record similar high pressures (6.0–8.5 kbar) but differences in temperature of 90–140 °C. The different depths constrained by the P–T data for the crystallisation of the amphiboles and clinopyroxenes indicate the presence of magma storage reservoirs scattered throughout the entire thickness of crust, from very deep locations (27–33 km) near the lower crust–lithospheric mantle boundary (MOHO = 33–35 km), up to shallow levels in the upper crust (2–5 km). Most of the volcanic structures show complex, multi-level interconnected VPS. The presence of numerous very deep magmatic reservoirs in several volcanic structures across the entire time-interval of the volcanism (which have high isotopic signatures, such as 87Sr/86Sr values of 0.7070–0.7076, for even the most basic rocks) provides evidence for a main, deep magma reservoir (“deep hot zone”) that acted as a “MASH zone” at the crust–mantle boundary beneath the GVZ. The architecture of the magmatic plumbing systems controlled the geological evolution of the studied volcanic structures. The processes of assimilation fractional crystallisation and magma mingling and mixing were also controlled by the complex VPS, as recorded by a wide range of volcanological, mineralogical–petrographical, and geochemical features. Overall, the GVZ records a complex evolution of volcanic activity that reflects the intricate magmatic plumbing systems. The VPS models and their inferred architectures provide a better understanding of the complex and long-lasting volcanism in the GVZ, and are relevant to other areas of extinct volcanism globally.

Evolution of deep crustal hot zones constrained by the diversity of Late Mesozoic magmatic rocks in SE China

The formation of widespread Late Mesozoic magmatic rocks (dominantly felsic rocks) in SE China needs a physical and chemical balance between crustal and mantle melting, which could be reconciled by complicated transcrustal magmatic systems in deep crustal hot zones with time. After the relative magmatic quiescence at ca. 210–180 Ma, magmatic activities were mainly developed in the interior of SE China with an areal distribution and subsequently concentrated in the present coastal area with an overall linear pattern after ca. 140 Ma except the Lower Yangtze region. The spatiotemporal change requires a unique crust-mantle geodynamic coupling with the evolution of deep crustal hot zones from intracontinental to continent marginal setting. Correspondingly, the magmatic rocks show a compositional change from bimodal distribution in Early and Middle Jurassic to unimodal and weakly bimodal pattern in Late Jurassic and Early Cretaceous. In general, most of the Late Mesozoic intermediate to felsic magmatic rocks show higher Mg# than typical rifting-related rocks at given SiO2, which requires the addition of water and possible mass transfer and mixing of underplated hydrous basaltic magma in the deep crustal hot zones. The occurrence of high-Sr/Y and La/Yb rocks and possibly thickened crust (ca. 40 km) before ca. 140 Ma, combined with the current thin crust thickness (ca. 31-33 km) as revealed by seismic data, indicates a widespread crustal thinning after ca. 140 Ma. The relatively thickened crust before ca. 140 Ma is beneficial for melting of underlying old crust that is composed of dominantly Neoproterozoic and Paleoproterozoic components, the formation of porphyritic Cu deposits, the subsequent magmatic differentiation and associated giant W, Sn, Nb, Ta and Li ore deposits. In contrast, the deep crustal hot zones progressively migrated to the present coastal area after ca. 140 Ma, driving the increasing amounts of intermediate rocks and intermediate-felsic volcanic rocks. In general, the diversity and compositional change of Late Mesozoic magmatic rocks and associated ore deposits in SE China may be controlled by the evolution of deep crustal hot zones in different tectonic settings in response to the subduction of Paleo-Pacific Plate.

Prolonged magmatism and growth of the Iran-Anatolia Cadomian continental arc segment in Northern Gondwana

Much of the crust of Iran and Anatolia, including their oldest exposed rocks, formed during an episode of intense convergent margin (arc) magmatism as a result of subduction of oceanic lithosphere beneath northern Gondwana from ca 620 Ma to ca 500 Ma, the Cadomian crust-forming event. Most igneous rocks formed between ca 570 and 525 Ma. Cadomian crust is well-known from western and southern Europe and from eastern North America but is much less well-known from Iran and Anatolia. We use published age and compositional data and contribute new data in order to better understand this ancient magmatic system. Cadomian magmatism included calc-alkaline igneous rocks of arc affinity in the main arc and alkalic igneous rocks that formed in a back-arc setting; these igneous rocks are associated with sedimentary rocks. Geochemical and isotopic modelling reveals that basaltic magmas were the main input, that these formed by partial melting in the upper mantle, and that basaltic magmas evolved further in deep crustal hot zones to form granitic magmas through a combination of assimilating older continental crust and fractional crystalization of basaltic magmas.

Beyond crystal mushes: evidence for uptake of high-T pyroxene antecrysts from mid- to upper crustal andesites into tephras from the Central Plateau, New Zealand

ABSTRACT Pyroxene crystals from recent Central Plateau tephras are used to deduce their formation conditions through two-pyroxene thermobarometry. Crystals return pseudo-pressures and pseudo-temperatures that are artefacts of uptake of antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded magmas. MELTS modelling of tephra glass compositions shows that pseudo-PT conditions are reproduced at oxygen fugacities above the nickel-nickel oxide buffer (NNO+1, NNO+2), mid- to upper crustal pressures (100–400 MPa), and temperatures between c. 900°C and >1100°C. Modelled crystals from the deep crust (800 MPa) are restricted to clinopyroxenes. However, these display chemical equilibrium with shallow orthopyroxenes at higher pseudo-PT conditions than observed in Central Plateau pyroxenes. The data indicate uptake of high-temperature pyroxenes at mid- to shallow crustal levels into ascending andesitic melts and thus preclude the presence of long-lived crustal mush zones (<1000°C) as a source for the crystal cargo of the Central Plateau tephras studied here. Further, the apparent absence of deep crustal pyroxene antecrysts does not preclude models of arc andesite genesis without a ‘deep crustal hot zone’ beneath the Central Plateau. Generation and ascent of primary andesites from a heterogeneous mantle wedge is therefore a possible scenario at the southern Hikurangi margin.

Iron isotope fractionation in hydrous basaltic magmas in deep crustal hot zones

Mafic magmatism within deep crustal hot zones plays an important role in the crustal formation and evolution. High-pressure garnet crystallization drives magma differentiation and Fe isotope fractionation at the base of the lower crust. The ∼840 Ma intrusion and the associated ∼830 Ma dikes from the Tongde region in South China consist mainly of gabbro and diorite, which record complicated magmatic processes of the deep crustal zones and provide new insights into the Fe isotope fractionation. Rocks from the Tongde intrusion have high Sr/Y ratios (61–163) and variable δ57Fe values (+0.07 to +0.21‰ relative to IRMM-014) that initially increase then decrease with progressive magmatic differentiation. All samples do not display obvious imprint of deuteric fluid exsolution, and residual garnet in the mantle source did not fractionate the Fe isotope compositions of the primary magmas significantly. The variable Fe isotope signatures in this study can be reproduced by sequential fractional crystallization of olivine/pyroxene and garnet in the deep crust followed by amphibole saturation in the upper section. The host rocks of the intrusion underwent diffusive exchange with the dikes. Removal of both isotopically light- and heavy-Fe minerals well explains the small Fe isotope fractionation in mafic and intermediate rocks. The high-Sr/Y suites of the intrusion and dikes have an average δ57Fe of +0.11 ± 0.02‰ (2SD), similar to the value of arc crust at convergent continental margins. Therefore, garnet fractionation is an important process in arc roots, which drives the melts evolving to calc-alkaline series. Fractional crystallization of hydrous basaltic magmas under high pressures in deep crustal hot zones makes a significant contribution to the formation and evolution of continental crust.

Repeated magmatic buildup and deep “hot zones” in continental evolution: The Cadomian crust of Iran

The generation and differentiation of continental crust by arc magmatism is strongly influenced by episodes of high magmatic flux (“flare-ups”). Magmatic flare-ups encourage the development of deep crustal hot zones where magmatic differentiation and density stratification combine to form the upper felsic and lower mafic continental crust. Such processes, which are responsible for the construction of continental arc crust, are prolonged events, which build a ∼30-40 km arc crust over tens of million years (∼100 Myr). New zircon U-Pb data reveal that the construction of Cadomian crust from NE Iran occurred over ∼15 ± 0.3 Myr. However, compiled zircon U-Pb ages reveal a prolonged magmatic flare-up of ∼45 Myr; ∼570 to 525 Ma. Basement outcrops in NE Iran expose lower- and upper crust that show how magmatic-geochemical differentiation occurred deep beneath a Cadomian continental arc in a crustal hot zone. Isotopic data for igneous rocks produced during this 45 Myr episode reveal interactions between mantle-derived melts and old continental crust. Synthesis of new and published data indicates that this type of interaction is common during periods of high magmatic fluxes. Our results indicate that differentiation of mafic melts in the lower crust during prolonged magmatic flare-ups plays a key role in building a stratified continental crust.

Magmatic evolution of the Cerro Maricunga gold porphyry-epithermal system, Maricunga belt, N-Chile

The Cerro Maricunga gold project is located in the Maricunga gold belt in northern Chile, within the remnant edifice of the Tertiary Ojo de Maricunga volcano. The gold mineralization is hosted within a 23‒15 Ma old complex system of interwoven porphyry intrusions, flows, dikes and diverse breccia types, all forming part of a large diatreme system. In particular, phreatomagmatic and phreatic processes contributed to the formation of the synmineral diatreme complex. The rocks are calc-alkaline (daci)andesites and very rare dacites of the medium-K series with a high-K tendency and a conspicuous “adakitic” signature. Rare-earth element patterns reflect formation during a period of distinctly thickened continental crust with retention of heavy rare-earth elements by a garnet-bearing residue at a depth of ≥45 km. U-Pb zircon ages show a wide spectrum from Proterozoic, over Permian and Triassic ages to Paleocene, Oligocene, early and mid-Miocene ages. The Phanerozoic zircon ages indicate the assimilation of country rocks, whereas the Proterozoic age reflects the presence of a lower crustal Neoproterozoic basement that is also suggested by εNd values of −3.9 to −3.1 for bulk rock, i.e. mantle depletion model ages around 1 Ga. The last magmatic activity at the Ojo de Maricunga volcano was at ∼14.9 Ma. Stable isotope data on hydrothermal barite and supergene acanthite and gypsum (δ34Sacanthite: −0.6 to −0.4; δ34Ssulfate: 0.2 to 4.6; δ18Osulfate: 9.2 to 13.8) indicate an igneous origin of sulfur and arid conditions during mineral formation. The emplacement of the diatreme complex took place during the adjustment of the Chilean flat slab setting in a time of increased crustal thickness. Although the rocks at Ojo de Maricunga show a pronounced adakite-like signature, they formed in a common MASH zone or a deep hot zone within the lower crust. Gold mineralization took place at ∼17.9 Ma (Re-Os on molybdenite), syngenetic with diatreme formation and emplacement of hydrous magma. The thickening of continental crust was more advanced in the northern part of the Maricunga belt in the early to mid-Miocene than previously assumed, and then shifted south where several younger gold porphyries formed.

Interaction between mingling mafic and felsic magmas: Its roles in differentiation of a quartz monzonite and MMEs from eastern South China

Compositional profiles and mapping of selected mafic microgranular enclaves (MMEs) in Muchen quartz monzonite in eastern South China give constraints on the interaction between mingling mafic and felsic magmas. The intrusion is a typical I-type MME-bearing magnetite-series granitoid in western Pacific. The MMEs and host quartz monzonite are not deformed and have similar magnetic fabrics, which does not support the MMEs are restites or earlier solidified mafic rocks but implies mafic magma globules flowed with felsic magma. The two MMEs represent mafic magma interacting with felsic magma at early and late stage, respectively. The late-stage MME has a Hbl-Bt-Kfs-Pl-Mag assemblage. The early-stage MME has a Cpx-Bt-Kfs-Pl-Mag assemblage with a rim similar to the late-stage MME. Acicular apatite implies rapid cooling of the mafic magmas; however, the similar isotopic ratios and mafic silicate compositions of the MME and quartz monzonite indicate partial equilibrium during magma interactions. Al-in-hornblende estimates the pluton emplacement at ~3.1–3.6 km and therefore the magma mingling-mixing still worked at shallow levels. Most trace element Harker diagrams do not produce linear variation trends and magma mixing cannot solo explain such a pattern. Enrichments of Na2O, REE, Y, Nb, Ta, Ga, Fe3+ and depletions of K2O, Rb, Ba, Sr in the MMEs through diffusion caused noticeable chemical differentiation of both mafic and felsic magmas. Therefore, mass transfer during magma mingling is an important mechanism influencing petrography and chemical compositions of I-type granitoids. Such processes may also extensively occur in the deep hot zones of the continental arc environments.

Rochas intrusivas básicas no sudeste do Cinturão Dom Feliciano, RS: petrografia e geoquímica das associações I e II

Intrusões dioríticas, gabroicas e outras rochas máficas-ultramáficas registram tanto os processos de extração e transporte de magmas do manto, resultando na acresção vertical de material à crosta, quanto os processos de interação entre esses dois componentes. Recentemente, essas rochas têm sido apontadas como importantes marcadores petrogenéticos, quando associadas ao magmatismo intermediário a ácido, em ambientes orogênicos a pós-colisionais. Na porção sudeste do Cinturão Dom Feliciano, no Rio Grande do Sul, constituído majoritariamente por granitoides, ocorrem intrusões dioríticas e gabroicas de pequeno porte e caráter dominantemente básico, na região compreendida entre Pinheiro Machado e Pedro Osório. Este artigo apresenta a caracterização geológica, mineralógica, petrográfica e geoquímica dessas rochas, propondo assim a separação de duas associações principais. A Associação I (AI), constituída por rochas gabroicas, comumente cumuladas e por vezes estratificadas, engloba os corpos do Passo da Fabiana, Passo da Olaria, Arroio Santa Fé e Desvio Herval. A Associação II (AII), diorítica a gabroica, compreende rochas maciças a localmente foliadas, nas ocorrências do Alto Alegre, Passo dos Machados e Campo Bonito. As duas associações são subalcalinas, com afinidade cálcio-alcalina médio-K (AI) e médio a alto-K (AII), com altos teores de alumina (Al2O3 &gt; 17%), mesmo para os termos não cumulados. Diferenças quanto aos elementos maiores (CaO, Mg# e álcalis), menores e traços (P2O5, Zr, Nb, Y, total e padrões de ETR) referendam a separação proposta. As assinaturas magnéticas sugerem o controle no posicionamento das intrusões por descontinuidades litosféricas, que teriam possibilitado a ascensão e a colocação desses magmas em níveis superiores da crosta. Processos petrogenéticos envolvendo a participação de líquidos derivados do manto em reservatórios supracrustais e a sua evolução nas deep crustal hot zones são propostos.