Deep Hot Zones Research Articles

Generation of arc rhyodacites through cumulate-melt reactions in a deep crustal hot zone: Evidence from Nisyros volcano

The generation of continental crust of intermediate composition occurs predominantly in convergent margin settings, yet the mechanisms by which felsic, calc-alkaline arc magmas are generated remain poorly understood. Magma mixing appears to be a common process in voluminous intermediate arc rocks but the composition of the felsic mixing endmember is typically obscured by the mixing process. We investigate a suite of porphyritic (rhyo)dacitic magmas (65–72 wt.% SiO2) from Nisyros, a young stratovolcano in the Aegean arc, Greece. These magmas are not affected by shallow process such as hybridisation or crystal-melt segregation and thus offer a valuable insight into the origin of felsic melts at convergent margins. We find that the Nisyros (rhyo)dacites form through a reaction in which earlier-formed wehrlite cumulates in the deep arc crust react with melts to form amphibole. This implies that melt major element compositions are effectively buffered by a low-variance mineral assemblage to follow this peritectic boundary such that the silica content of melts extracted from the deep crustal hot zone is controlled by the amount of amphibole crystallised. The resorption of cumulates is pivotal in imparting a distinct trace element signature that is decoupled from major element systematics. For example, high compatible element contents and a strong amphibole signature (low Y and Dy/Yb) cannot be captured by simple crystallisation models and require cumulate resorption. Variable radiogenic isotope systematics indicate minor crustal contamination although assimilation is not proportional to silica content and hence not a main driving force behind the generation of felsic melts. Instead, the Nisyros (rhyo)dacites formed through melt-cumulate reaction processes prior to emplacement as mush bodies at shallow depth and partial eruption. Magma mixing only becomes an important process in the youngest unit on Nisyros. On a global scale, peritectic boundary melts are rarely sampled in the whole rock or melt inclusion record. Conversely, peritectic boundary melts do form a suitable felsic mixing endmember for the generation of voluminous “monotonous intermediate” magmas.

Fertilizing porphyry Cu deposits through deep crustal hot zone melting

Porphyry copper deposits are the world's largest source of copper, but their ultimate source and deep process are disputed. We compare the Sr/Y ratios and Nd isotopes of coeval ore-bearing and ore-barren intermediate rocks from collisional and subduction settings to unravel the sourced depth and nature. During the Miocene mineralization epoch in the Iranian plateau, the median Sr/Y ratios and the median εNd(T) values of ore-bearing rocks (56 ± 38, 1.1 ± 1.5) are slightly higher than those of the ore-barren rocks (30 ± 31, −0.3 ± 2.4), respectively. Such patterns also exist at mineralization epochs during the Miocene collision in the eastern Gangdese, and the early Cretaceous subduction in Southeast China. These results indicate that all ore-bearing and ore-barren rocks are from the thickened (>1.2 Gpa) juvenile dominated (>70% mantle contribution) lower crust, and the ore-bearing rocks are sourced from slightly deeper and more juvenile sources compared to the coeval ore-barren rocks. The subtle geochemical difference but contrasting ore-potentials indicate heterogeneous origins, and the ore-bearing rocks may be related to ore-enriched sources at the lower crust. A deep crustal hot zone with melting at different depths with heterogeneous sources can account for such phenomena. Delamination of a thickened orogenic root induces asthenosphere upwelling and generates abnormal heat flow in the lower crust. The porphyry mineralization is an indication of the balancing between the melting of thickened lower crust and mantle upwelling, which explains the rarity of mineralization compared to the prolonged orogenic magmatism.

Miocene porphyry copper deposits in the Eastern Tethyan orogenic belt: Using Sr, O isotopes and Sr/Y ratios to predict the source of ore-related and ore-barren magmas

There are many Miocene collision- and subduction-related porphyry Cu deposits in the Eastern Tethyan orogenic belt. However, the petrogenesis of the ore-related magmas and the mechanism of metal enrichment remain controversial. Here, we present a geochemical study and comparison of the major Miocene ore-related porphyries and coeval ore-barren magmatic rocks in the Urumieh-Dokhtar magmatic arc in Iran and the Chagai belt in Pakistan, and the Eastern Gangdese belt of the Himalaya. The results show that ore-related porphyries are characterized by relatively depleted mean Sr isotopic compositions (0.704217–0.706859 in the Urumieh-Dokhtar magmatic arc; 0.705052–0.706588 in the Eastern Gangdese belt; 0.705252–0.706708 in the Chagai belt), but higher maximum Sr/Y ratios (40–149 in Urumieh-Dokhtar; 79–178 in Eastern Gangdese; 52–178 in Chagai). In contrast, the coeval ore-barren magmatic rocks yield relatively enriched, mean Sr isotopic compositions (0.704722–0.707653 in Urumieh-Dokhtar; 0.704701–0.707802 in Eastern Gangdese; 0.705982–0.707856 in Chagai), but lower maximum Sr/Y (2–88 in Urumieh-Dokhtar; 61–136 in Eastern Gangdese; and 19–35 in Chagai). A deep crustal hot zone model provides a robust explanation of these data. Melts derived from a deeper melt source in juvenile mafic lower crust, where mantle-derived materials are common, have a greater potential to yield a higher tonnage of metallic Cu, such as the Sar-Cheshmeh deposit in Iran, which is derived from a deep melt zone, and characterized by a low radioactive mean Sr isotopic value of 0.704851, but a huge tonnage of metallic Cu of 7.2Mt and a high maximum Sr/Y value of 117 (representative of a deeper depth of the melt zone), whereas the Dalli deposit in Iran derived from a relatively shallow melt zone has a more radioactive mean Sr isotopic value of 0.706859, but a smaller amount of metallic Cu of 0.04Mt and a lower maximum Sr/Y ratio of 40. Conversely, re-melting the contact between newly-formed and pre-existing crust where mantle-derived materials are absent will produce ore-barren rocks. Additionally, the ore-barren rocks with low Sr/Y ratios (<20) should be derived from a shallower melt zone in the crust.

Late Cretaceous transition from subduction to collision along the Bangong-Nujiang Tethys: New volcanic constraints from central Tibet

This study deals with arc-type and subsequent bimodal volcanic rocks interbedded with (late) Cretaceous sedimentary formations near Gaize, central Tibet that shed new light on the Tethyan evolution along the Bangong-Nujiang suture. Unit I consists of trachyandesites interbedded with fine-grained sandstone, slate, and limestone. Zircon dating on a trachyandesite sample yields a 206Pb/238U age of 99±1Ma. The trachyandesites are characterized by strong enrichment in LILE but depletion in HFSE, low zircon saturation temperatures (TZr=642–727°C), and high oxygen fugacity (∆FMQ=−0.67–0.73), indicating their arc affinities. Unit II comprises a bimodal basalt-rhyolite suite interbedded with coarse-grained sandstone and conglomerate. Zircon dating on two rhyolitic samples yield 206Pb/238U ages of 97.1–87.0Ma. In contrast with Unit I trachyandesites, Unit II basalts and rhyolites exhibit OIB-like trace element patterns, high temperatures (T=1298–1379°C for basalts, TZr=855–930°C for rhyolites), and low oxygen fugacity (∆FMQ=−3.37~−0.43), suggesting that Unit II bimodal volcanic rocks probably formed in an extensional setting. The Sr-Nd isotopes of both Unit I (87Sr/86Sri=0.7052–0.7074, εNd(t)=−2.21–1.02) and Unit II (87Sr/86Sri=0.7057–0.7098, εNd(t)=−3.22–0.88) rocks are similar to mantle-wedge-derived volcanic rocks within the southern Qiangtang block. The parental magma of Unit I trachyandesites was formed by fluid induced melting of the mantle wedge above the subducted Bangong-Nujiang Tethyan slab, and contaminated by crustal materials during MASH process within a deep crustal hot zone; and Unit II bimodal volcanic rocks were derived by melting of upwelling asthenosphere and a mildly metasomatized mantle wedge during the Lhasa-Qiangtang collision. We propose that the transition from the Bangong-Nujiang Tethyan subduction to the Lhasa-Qiangtang collision occurred during the Late Cretaceous in central Tibet.

Heterogeneous Conservation of Zircon Xenocrysts in Late Jurassic Granitic Intrusions within the Neoproterozoic Jiuling Batholith, South China: a Magma Chamber Growth Model in Deep Crustal Hot Zones

Heterogeneous Conservation of Zircon Xenocrysts in Late Jurassic Granitic Intrusions within the Neoproterozoic Jiuling Batholith, South China: a Magma Chamber Growth Model in Deep Crustal Hot Zones

Seismic evidence for a possible deep crustal hot zone beneath Southwest Washington

Crustal pathways connecting deep sources of melt and the active volcanoes they supply are poorly understood. Beneath Mounts St. Helens, Adams, and Rainier these pathways connect subduction-induced ascending melts to shallow magma reservoirs. Petrogenetic modeling predicts that when these melts are emplaced as a succession of sills into the lower crust they generate deep crustal hot zones. While these zones are increasingly recognized as a primary site for silicic differentiation at a range of volcanic settings globally, imaging them remains challenging. Near Mount Rainier, ascending melt has previously been imaged ~28 km northwest of the volcano, while to the south, the volcano lies on the margin of a broad conductive region in the deep crust. Using 3D full-waveform tomography, we reveal an expansive low-velocity zone, which we interpret as a possible hot zone, linking ascending melts and shallow reservoirs. This hot zone may supply evolved magmas to Mounts St. Helens and Adams, and possibly Rainier, and could contain approximately twice the melt volume as the total eruptive products of all three volcanoes combined. Hot zones like this may be the primary reservoirs for arc volcanism, influencing compositional variations and spatial-segmentation along the entire 1100 km-long Cascades Arc.

Evidence for underplating in the genesis of the Variscan synorogenic Beja Layered Gabbroic Sequence (Portugal) and related mesocratic rocks

The Beja Layered Gabbroic Sequence (LGS) is a mafic, layered synorogenic intrusion that was emplaced at the SW border of the Ossa Morena Zone (OMZ) at ca. 350Ma during the early stages of the Variscan oblique continental collision. The LGS represents the primitive member of the Beja Igneous Complex (BIC), which records part of an important Variscan magmatic event to the north of the SW Iberian suture that led to the formation of several igneous complexes. Although LGS primary magmatic features are well-preserved from post-crystallization tectono-metamorphic events, the magma chamber processes were influenced by the Variscan regional stress field which affected also the development of the magmatic layering and associated foliation.The baric evolution recorded in LGS (from 9 to 4kbar) is compatible with early crustal underplating of juvenile basaltic magma at the Moho. Mild lower crustal contamination resulted in Nb-Ta-Rb-Th depletion and higher REE fractionation compared to MORB. Contamination proceeded mainly by assimilation-fractional-crystallization (AFC) at final level of emplacement.The obtained results suggest that the spatially associated BIC mesocratic rocks are genetically related to LGS, both deriving from an equivalent mantle source. It is proposed that the underplating of basaltic magmas at the lower crust gave rise to a deep crustal hot zone. This allows BIC evolution to be explained as a single, long-lived magmatic event of progressive geochemical and lithological diversification due to the involvement of distinct crustal components. The most important components were incorporated at depth by the reworking of OMZ lower crustal rocks, with the involvement of middle crustal rocks later on, and of upper crustal contamination during the final stages of emplacement.

Systematic mineralogical diversity in A-type granitic intrusions: Control of magmatic source and geological processes

The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were estimated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20%–40% partial melting of intracrustal feldspathic rocks, whereas the sodic granites are extreme fractionates (90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcaniclastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.

The Middle Triassic Meiwu Batholith, West Qinling, Central China: Implications for the Evolution of Compositional Diversity in a Composite Batholith

An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.

Evolution of major and trace element composition during melt migration through crystalline mush: Implications for chemical differentiation in the crust

We present the first quantitative model of heat, mass and both major and trace element transport in a mush undergoing compaction that accounts for component transport and chemical reaction during melt migration and which is applicable to crustal systems. The model describes the phase behavior of binary systems (both eutectic and solid solution), with melt and solid compositions deter- mined from phase diagrams using the local temperature and bulk composition. Trace element concentration is also determined. The results demonstrate that component transport and chemical reaction generate compositional variation in both major and trace elements that is not captured by existing geochemical models. In particular, we find that, even for the simplest case of a homogenous, insulated column that is instantaneously melted then allowed to compact, component transport and reaction leads to spatial variations in major element composition that, in this case, produces melt that is more enriched in incompatible elements than predicted by batch melting. In deep crustal hot zones (DCHZ), created by the repeated intrusion of hot, mantle- derived magmas, buoyant melt migrating upwards accumulates in high porosity layers, but has a composition corresponding to only a small fraction of batch melting, because it has locally equilibrated with mush at low temperature; moreover, melt migration and chemical reaction in a layered protolith may lead to the rapid formation of high porosity melt layers at the interface between different rock compositions. In both of these cases, the melt in the high porosity layer(s) is less enriched in incompatible trace elements than predicted if it is assumed that melt with the same major element composition was produced by batch melting. This distinctive decoupling of major and trace element fractionation may be characteristic of magmas that originate in DCHZ. Application of the model to a number of crustal systems, including the Ivrea-Verbano zone, the Rum layered intrusion, and the Holyoke flood basalt, suggests that composi- tional heterogeneity can be explained by buoyancy-driven melt migration and compo- nent transport through a reactive crystalline mush.

Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon

Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of >11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4–6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H2O-rich (~6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H2O-saturated liquidus at ~4 km. H2O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently ‘hot’ and ‘cold’ granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons.

Melt Segregation in Deep Crustal Hot Zones: a Mechanism for Chemical Differentiation, Crustal Assimilation and the Formation of Evolved Magmas

Mantle-derived basaltic sills emplaced in the lower crust provide a mechanism for the generation of evolved magmas in deep crustal hot zones (DCHZ).This study uses numerical modelling to characterize the time required for evolved magma formation, the depth and temperature at which magma formation occurs, and the composition of the magma.The lower crust is assumed to comprise amphibolite. In an extension of previous DCHZ models, the new model couples heat transfer during the repetitive emplacement of sills with mass transfer via buoyancy-driven melt segregation along grain boundaries.The results shed light on the dynamics of DCHZ development and evolution.The DCHZ comprises a mush of crystals plus interstitial melt, except when a new influx of basaltic magma yields a short-lived (20^200 years) reservoir of melt plus suspended crystals (magma). Melt segregation and accumulation within the mush yields two contrasting modes of evolved magma formation, which operate over timescales of c. 10 kyr^1 Myr, depending upon emplacement rate and style. In one, favoured by emplacement via over-accretion, or emplacement at high rates, evolved magma forms in the crust overlying the intruded basalt sills, and is composed of crustal partial melt, and residual melt that has migrated upwards out of the crystallizing basalt. In the other, favoured by emplacement via underor intra-accretion, or by emplacement at lower rates, evolved magma forms in the intruded basalt, and the resulting magma is composed primarily of residual melt. In all cases, the upward migration of buoyant melt yields cooler and more evolved magmas, which are broadly granitic in composition. Chemical differentiation is therefore driven by melt migration, because the melt migrates through, and chemically equilibrates with, partially molten rock at progressively lower temperatures. Crustal assimilation occurs during partial melting, and mixing of crustal and residual melt occurs when residual melt migrates into the partially molten crust, yielding chemical signatures indicative of a mixed crustal and mantle origin. However, residual melt is volumetrically more significant than crustal melt, except at the highest emplacement rates. Contamination of crustal melt by residual melt from basalt crystallization appears to be an inevitable consequence of melt segregation in DCHZ, and can explain the mixed crust^mantle origin of many granites.

Zircon geochronology, elemental and Sr-Nd isotope geochemistry of two Variscan granitoids from the Odenwald-Spessart crystalline complex (mid-German crystalline rise)

The crystalline parts of the Bergstrasser (western) Odenwald and the southern Spessart expose Variscan I-type granitoids of the mid-German crystalline rise that formed during subduction of the Rheic ocean and collision of Avalonia and Armorica about 365 and 330 Ma ago. We present geochemical, Sr-Nd isotopic, single zircon 207Pb/206Pb evaporation and conventional U-Pb data from a diorite-granodiorite complex of the southern Spessart and from a flasergranitoid of the Bergstrasser Odenwald unit II. Both intrusions provide almost identical zircon ages (332.4 ± 1.6 Ma for Odenwald and 330.4 ± 2.0 Ma for Spessart). Lack of inherited or pre-magmatic zircon components connotes magma genesis in deep crustal hot zones despite low temperature estimates (758–786 °C) derived from zircon saturation thermometry. Investigated rock samples display normal- to high-K calc-alkaline metaluminous (Spessart) and weakly peraluminous (Odenwald) geochemical characteristics. The Spessart pluton has lower eNd(T) values (−2.3 to −3.0) and higher 87Sr/86Sri ratios (0.7060 to 0.7066) compared to Odenwald flasergranitoid (eNd(T) = −0.8 and 87Sr/86Sri = 0.7048). In terms of the tectonic setting, the diorite-granodiorite complex of the southern Spessart forms the continuation of the north Armorican arc segment exposed in the Bergstrasser Odenwald. Taking into account previously reported geochemical and isotopic results, it is concluded that the Spessart pluton does not match compositions of Odenwald unit II granitoids but likely represents the north-eastward extension of unit III.

Heat flow and deep temperatures in the Southeast Basin of France: Implications for local rheological contrasts

Abstract Triassic salt at 5–10 km depth may drive some of the recent tectonic features in southeastern France. We estimate the likely temperature range of the salt using two different approaches. The first of these, based on the extrapolation of deep temperatures obtained in oil exploration wells, predicts temperatures at a depth of 8 km to be in the range of 230–300°C. However, this prediction could be biased by a lack of deep measurements and problems related to lateral heat transfer caused by thermal conductivity contrasts. The second approach can overcome these problems by modelling the actual heat transfer for appropriate basin geometry, including temperature-dependent thermal properties, and a mantle heat flow of 35 mW.m−2. This latter value enables us to reproduce available temperature measurements and surface heat flow data. Here we evaluate the stationary temperature field along two sections constrained by seismic profiles, one at a local scale across the Vistrenque graben and the other at a more regional scale across the Southeast Basin. Our findings suggest that the temperatures in the deepest parts of the evaporitic layer (11 km depth) can reach up to 398°C, but can be as low as 150°C on the edge of the basin at the top of the salty layer. This temperature difference leads to important changes in salt viscosity. Results indicate that at a depth of 8 km, lateral viscosity contrasts within the evaporitic layer may reach 40. Such rheological contrasts might favour and amplify local subsidence, as seems to have been the case near the two Palaeogene half-grabens of Vistrenque and Valence, where deep hot zones are identified.

A cryptic record of magma mixing in diorites revealed by high-precision SIMS oxygen isotope analysis of zircons

High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ 18O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values ( δ 18O = 6.0 ± 0.6‰ (2 σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ 18O and an apparent bimodal distribution with modes of 6.6 ± 0.4‰ and 7.3 ± 0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ 18O with zircon growth has been observed. The δ 18O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ 18O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ 18O values (5.3 ± 0.6‰ (2 σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth. The δ 18O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9 ± 0.5‰ (2 σ), AD1: 11.7 ± 0.6‰ (2 σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ 18O melt. High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration.

The sources of granitic melt in Deep Hot Zones

AbstractA Deep Hot Zone develops when numerous mafic sills are repeatedly injected at Moho depth or are scattered in the lower crust. The melt generation is numerically modelled for mafic sill emplacement geometries by overaccretion, underaccretion or random emplacement, and for intrusion rates of 2, 5 and 10mm/yr. After an incubation period, melts are generated by incomplete crystallisation of the mafic magma and by partial melting of the crust. The first melts generated are residual from the mafic magmas that have low solidi due to concentration of H20 in the residual liquids. Once the solidus of the crust is reached, the ratio of crustal partial melt to residual melt increases to a maximum. If wet mafic magma, typical of arc environments, is injected in an amphibolitic crust, the residual melt is dominant over the partial melt, which implies that the generation of I-type granites is dominated by the crystallisation of mafic magma originated from the mantle and not by the partial melting of earlier underplated material. High ratios of crustal partial melt over residual melt are reached when sills are scattered in a metasedimentary crust, allowing the generation of S-type granites. The partial melting of a refractory granulitic crust intruded by dry, high-T mafic magma is limited and subordinate to the production of larger amount of residual melt in the mafic sills. Thus the generation of A-type granites by partial melting of a refractory crust would require a mechanism of selective extraction of the A-type melt.

Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longaví Volcano (36·2°S; Andean Southern Volcanic Zone, Central Chile)

SUMMARY AND IMPLICATIONS The two Holocene dacitic episodes at NLV produced themost evolved magma compositions that are preservedat this volcano, although occurrences of amphibole-richcumulate xenoliths in earlier main-cone lavas indicatethat magmas lying along similar liquid lines of descentwere being generated at depth by the NLV magmaticsystem prior to the Holocene (Selle¤s, 2006). The erupteddacites have adakitic signatures, high B and fluid-mobile/immobile trace element ratios, and amphibole-rich pheno-cryst assemblages, and they lack clinopyroxene. Quenchedmafic enclaves in dacitic rocks represent inputs of unu-sually water-rich magmas, which also have relatively lowconcentrations of most incompatible elements, high B,high mobile/immobile trace element ratios, and a highmodal proportion of amphibole phenocrysts.These simila-rities and their mingled association in the Lomas Limpiasdacite and Castillo Andesite strongly support a closegenetic link in the form of a parental role for the enclavemagmas.A polybaric, two-stage crystal extraction modelconstrained by experimentally determined phase relationsand natural mineral compositions reproduces whole-rockmajor and trace element trends and is a logical conse-quence of differentiation of these extremely water-richmagmas. Anomalously low incompatible trace elementconcentrations in dacites (except forboron) are not consis-tent with significant upper crustal assimilation, but theyare easily reconciled with volumetrically important frac-tionation of amphibole, which in turn is supported by thepresence of amphibole-rich cumulate xenoliths. The traceelement abundances in the early, high-pressure stage ofevolution have been adjusted by the fractionation of 1 5%garnet. Although modal garnet is not observed, experi-mental studies (Mu«ntener et al., 2001; Ulmer & Mu«ntener,2005; Alonso-Pe¤rez, 2006) have shown that it could bestable inwet mantle-derived melts at pressures correspond-ing to inferred lower crustal pressuresbelow NLV.The lackof erupted primitive melts precludes rigorous modeling ofthe early, deep fractionation, which is inferred to be thekey to the adakitic signature of the Holocene NLVmagmas. The deep hot zone model proposed by Annenet al. (2006) provides an adequate thermo-mechanicalframework for generation of magmas derived by fractiona-tion of high-pressure assemblages from hydrous, mantle-derived magmas.A crystal fractionation origin for adakite-like rocks hasbeen proposed in earlier studies for other arc settings,notably in the geodynamically complex Philippine arc(Castillo et al., 1999; Prouteau & Scaillet, 2003; Solidumet al., 2003; Castillo & Newhall, 2004; Macpherson et al.,2006), but also in the Ecuadorian Andes (Chiaradia et al.,2004) and the Archean of South Africa (Kleinhanns et al.,2003). All of these studies concur that amphibole plays apreponderant role in the generation of the adakitic signa-ture (with or without garnet), as well as limiting theamount of plagioclase being fractionated. Previous studiescall upon elevated magmatic water contents to explain theparticipation of amphibole-rich assemblages, althoughthe reasons why elevated water fluxes occurred at a givenplace and time remain largely unexplored. We proposethat this unique occurrence of adakitic magmas in theQuaternaryAndean SVZ is ultimately related to the posi-tion of Nevado de Longav|¤ above the down-dip projectionof the oceanic Mocha Fracture Zone, which could trans-port large amounts of water hosted in serpentinite bodies(e.g. Bonatti & Crane, 1984; Ulmer & Trommdorff, 1995;Kerrick, 2002; Macleod et al., 2002; Ranero et al., 2003;Ranero & Sallare's, 2004).

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantle-derived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies

Large 226Ra excesses in a number of mafic arc magmas, and geophysical observations of earthquake hypocenters locations, indicate that mafic melts can get transferred from source to surface within days to ∼1 kyr. Decreasing 226Ra– 230Th disequilibria with increasing SiO 2 in co-magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems, or less in open systems. However, the rapid decrease of global 238U– 230Th disequilibria with increasing SiO 2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238U excesses. In the case of rapid magmatic evolution, open system processes are frequently involved in the production of andesites and dacites, and the data imply that the open system component is close to 238U– 230Th secular equilibrium. MELTS modeling of magma evolution at Santorini in the Aegean arc provides corroborating evidence from an individual well-studied arc volcano, that closed system fractional crystallization fails to produce the more evolved compositions, and increasing 87Sr/ 86Sr ratios with increasing SiO 2 from andesites to rhyolites suggests assimilation of old crust is involved. These observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust. Melt production in deep crustal hot zones provides an explanation for low 238U– 230Th disequilibria of the evolved magmatic component, as significant accumulation of evolved melts through partial re-melting of previously intruded basalts requires >100 kyr incubation time at typical magmatic flux rates. Thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems. Further insights into magmatic evolution can be gained from dating of minerals from mafic, intermediate and felsic arc magmas. (a) Many U–Th mineral isochrons in mafic arc magmas yield ages significantly older than those obtained from Ra–Th mineral isochrons from the same samples. A crystal size distribution case study from Soufrière, St. Vincent, indicates that this may result from the abundance of cumulate xenocrysts within more mafic arc magmas. (b) The majority of U–Th mineral isochrons in intermediate arc magmas yield ages within error of eruption age, indicating that other dating techniques that may be more suitable for deciphering the shorter timescales yielded by these compositions. In a case study of crystals of remobilized andesites from the Soufrière Hills volcano, Montserrat, intra-crystalline trace element diffusion modeling yields crystallization timescales of the order of 10–1000 years. Varying temperature-time histories of individual crystals can be interpreted to reflect multi-stage magma ascent and the development of ephemeral subvolcanic magma reservoirs. (c) U–Th mineral isochrons from felsic arc magmas yield very variable ages from >200 ka to shortly prior to eruption. A case study from the Taupo Volcanic Zone has shown that both remobilization of previous intrusives and melting of old country rock are occurring, giving rise to a this range in ages. Both intermediate and felsic magmas therefore can be shown to have multi-stage ascent histories. Future work will have to address the reasons for the apparent inter-volcano variability in time-averaged subvolcanic magma volumes within the arc crust. These include differences in magma supply rate; thickness, composition and thermal structure of the crust; and magma composition and volatile content, which may vary between different tectonic settings.