Crustal Magma Research Articles

Differentiation of magma composition: Reactivation of mush and melt reaction in a magma chamber

Understanding the storage and evolution of magma in the Earth's crust is of great significance for the study of igneous rocks. In recent years, the low melt state is considered to be the normal state in a magma chamber for long periods of time, while the high melt state is relatively short-lived and closely related to magma eruption. Under this condition, the effect of fractional crystallization at a low melt state has been questioned. The separation of minerals is seriously restricted under the condition of low temperature and low melt. It is necessary to recognize the differentiation and evolution of magma. Therefore, based on the whole rock geochemistry, trace element composition of zircons, and thermodynamic simulation of Mesozoic trachytic volcanic rock samples from a typical volcanic basin in the Suolun area, we evaluated the reaction process in the magma chamber. The results showed that the trachytic volcanic rock samples exhibited obvious characteristics of K-feldspar accumulation, which could not be explained by simple fractional crystallization. The results of the thermodynamic simulation indicated that the differentiation of magma in the study area is mainly the result of a crystal-melt reaction in the magma chamber as oppose to the traditional view of separation crystallization. This process may be universal in low temperature magma chambers dominated by low melt. This conclusion provides a new perspective for us to understand the formation of magma, and it can help us understand the formation of deposits which related to magmatic evolution, such as rare metal granite deposits.

Structural Characteristics of Moho Surface Based on Time Series Function of Natural Earthquakes

Remote sensing is a non-contact, long-distance detection technology. The reflection characteristics of a seismic wave can be used to detect remote and non-contact targets. Based on the reflection characteristics of a seismic wave, the underground structure in Tengchong Volcanic Area is explored. In order to further study the deep structure and magmatic activity of the crust in the volcanic area, we carried out a one-year mobile seismic observation. In this paper, nine broadband seismic stations were set up in the Tengchong Volcanic Area, and 3350 receiver function waveforms were collected. The crustal thickness, average wave velocity ratio, and Poisson’s ratio below these stations were calculated by the receiver function method, and the velocity structure near the Moho below these stations was evaluated. Combined with topographic data from SRTM3 (Shuttle Radar Topography Mission 3), this study reveals the dynamic relationship among crustal structure, crustal magmatism, and regional tectonic movement. Mantle upwelling plays an important role on the Moho uplift in the northern Tengchong Volcanic Area, and there are interconnected intracrustal magma chambers in the upper platform. The evaluation results of the Moho transition zone also indicate that the Dayingjiang fault is closely related to the tectonic activity of the Tengchong Volcanic fault.

Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.

Generation and structural modification of the giant Kengdenongshe VMS-type Au-Ag-Pb-Zn polymetallic deposit in the East Kunlun Orogen, East Tethys: Constraints from geology, fluid inclusions, noble gas and stable isotopes

The Kengdenongshe giant Au-Ag-Pb-Zn polymetallic deposit is located in the East Kunlun Orogen (EKO). It contains about 42.2 t of Au, 608.6 t of Ag and 1.05 Mt of Pb and Zn with an average grade of Au 2.31 g/t, Ag 19.29 g/t and Pb + Zn 3.49 wt% (Pb: 1.23 wt%, Zn: 2.26 wt%). The NWW-trending ore bodies are predominantly hosted in Late Permian to Triassic rhyolitic tuff, which formed during Late Permian back-arc extension to Triassic arc-continental collision. The ore bodies are subdivided into Pb-Zn rich ore bodies on the top with high grades of Pb and Zn and low grades of Au and Ag, and Au-Ag rich ore bodies below with high grades of Au and Ag and low grades of Pb and Zn. The Pb-Zn rich ore bodies occur as vein, stockwork, and in breccia, and comprise quartz, pyrite, galena, sphalerite, and small amounts of chalcopyrite. The Au-Ag rich ore bodies consist of auriferous barite-sulfide-oxide veins and contain barite, pyrite (early strawberry and oolitic pyrite and later eu- to subhedral pyrite), galena, sphalerite, chalcopyrite, tetrahedrite and covellite. Gold is present as electrum, kustelite and native gold and silver is present as polybasite, pearceite, kongsbergite, and as minor native silver in microfractures in sulfides. The hydrothermal alteration minerals include, from bottom to top, quartz + barite + calcite around the Au-Ag rich orebodies, quartz + chlorite + epidote around the Pb-Zn rich orebodies, and quartz + K-feldspar within the tuff. Fluid inclusions from both the Pb-Zn rich and the Au-Ag rich orebodies consist of two phases (V–L-type) fluid inclusions of which the vapor phase has a size of 10–40 vol%. Fluid inclusions microthermometry reveal homogenization temperatures of fluid inclusions in Pb-Zn rich and Au-Ag rich ore bodies of 128–230 °C and 110–320 °C, with corresponding salinities of 0.7–9.9 and 0.2–18.3 wt% NaCl equivalent, respectively. H-O-S-Pb stable isotope and He-Ar noble gas isotope data indicate a mixed magmatic water-seawater source for both the Pb-Zn and Au-Ag rich ore bodies, and an additional meteoric water component for the Au-Ag rich ore bodies. The Pb-Zn and Au-Ag rich ore bodies share the same sulfur and lead sources, i.e. sulfur is derived from crustal magma and seawater/marine sulfate, and the lead originated from a mixed magmatic-ancient crustal sedimentary source. Collectively, the regional geology, mineralogy, alteration, and geochemistry indicate that the Kengdenongshe Au-Ag-Pb-Zn polymetallic deposit can be characterized as a VMS-type (volcanic-associated massive sulfide) deposit. Formation of the ore-hosting rhyolite tuff and mineralization are associated with Late Permian to Triassic marine volcanic exhalation. Middle to Late Triassic basin closure and arc-continent collision modified the deposit and resulted in the location inversion of the Pb-Zn and Au-Ag rich orebodies.

From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

Magmatic Evolution of Zoned and Unzoned Ignimbrites: Evidence for a Complex Crustal Architecture Feeding four Rapid-sequence, Caldera-forming Eruptions in the San Juan Mountains, Colorado

Abstract The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26·91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

Early Permian Granitic Magmatism in Middle Part of the Northern Margin of the North China Craton: Petrogenesis, Source, and Tectonic Setting

The late Palaeozoic was an important period of tectonic evolution for the northern margin of the North China Craton (NCC). The source(s) and tectonic setting of early Permian granitoid rocks emplaced along the northern margin of the NCC are still unclear. These granitoids formed between ~295.4–276.1 Ma (uncertainties ranging from ±1.5 to ±7.8 Ma) according to zircon laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb data. The Dadongou (DDG) pluton is an A1-type granite and the Dananfangzi (DNFZ) pluton is an A2-type granite. The Erdaowa (EDW), Lisicun (LSC), Wuhai (WH) and Gehuasitai (GHST) plutons are I-type granites. The Yuanbaoshan (YBS) dykes are diorite and syenodiorite. All the granitoids are enriched in large ion lithophile elements and light rare earth elements, depleted in high field strength elements and have negative εNd(t) and εHf(t) values. The A1-type granite was formed by the melting of the mafic crust. The A2-type granite was derived from partial melting of tonalite gneiss from the NCC crust and mantle materials. The EDW, LSC, WH and GHST granites mainly originated from partially melted granulite, with some mantle input. The YBS dykes are formed by the magma mixing of hot mantle melt and the relatively cold crustal magma. The northern margin of the NCC experienced anorogenic and collision tectonic stages, and the structural setting started to transform to post-collision at the later period of early Permian.

High-temperature melting of different crustal levels in the inner zone of the Emeishan large igneous province: Constraints from the Permian ferrosyenite and granite from the Panxi region

The Late-Permian Emeishan mantle plume event caused large-scale flood basalts and related mafic-ultramafic layered mafic intrusions in the inner zone of the Emeishan large igneous province (ELIP). However, the related crustal magma response of this mantle plume event in the inner zone of the ELIP remains poorly understood. In this contribution, we report contemporaneous (~259 Ma) ferrosyenite and A-type granite in the inner zone of the ELIP, and their chronological and geochemical data suggest simultaneous crustal melting at different depths. The ferrosyenites display moderate SiO2 (60.14 to 63.58 wt%), high Fe2O3T (4.28 to 8.52 wt%) and alkaline (Na2O + K2O = 10.91 to 12.75 wt%) contents. In combination with their positive zircon εHf(t) values (+1.5 to +12.9), we propose that the ferrosyenites were formed by high temperature melting of Fe-rich refractory juvenile lower crust at H2O-poor condition, which was induced by underplating of the high-temperature mantle plume. Rhyolite–MELTS modeling results show that ~7% melting of ferrodiorite-like source at ~1120 °C and 8 kbar can produce similar compositions with least evolved ferrosyenite under a relatively dry condition. The Baima granites have high SiO2 (70.58 to 74.26 wt%) contents and 10000*Ga/Al ratios (3.00 to 3.28), indicating the affinity to A-type granites. Their evolved zircon εHf(t) values (−8.1 to −0.6) indicate an evolved crustal source region. Rcrust modeling indicates that ~9% partial melting of ancient granodiorite-like crustal material under low pressure condition (4 kbar) can produce the observed granitic compositions. Considering the close spatial and temporal relationship between ferrosyenites and A-type granites, we propose that the emplacement of ferrosyenitic magma resulted in low-degree partial melting at a relatively shallow crustal level, responsible for the formation of the A-type granites. In summary, this study highlights the significant impact on crustal magma response to the high-temperature mantle plume event in the inner zone of ELIP.

The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions.

Mineralogy Constraints on Magmatic Processes Controlling Adakitic Features of Early Permian High-magnesium Diorites in the Western Tianshan Orogenic Belt

AbstractWhole-rock geochemistry, usually changed by magmatic processes, might provide misleading information on the petrogenesis of adakites. The Heishantou porphyritic diorites in Nileke, Western Tianshan orogenic belt record complex magma chamber processes, such as magma replenishment, fractional crystallization and crustal contamination, and thus, provide ideal samples for tracing the magmatic processes that generate the typically high Sr and La contents and Sr/Y and La/Yb ratios of adakites. In situ clinopyroxene and amphibole compositions of Heishantou porphyritic diorites (271 ± 2 Ma) are characterized by low Yb and Y content with high Sr/Y and La/Yb ratios, from which calculated early magmas resemble typical adakites derived from partial melting of a subducted slab. But whole-rock composition shows low MgO, Ni and Cr, thus, the Heishantou diorites were previously regarded as the result of partial melting of thickened lower crust. Plagioclase phenocrysts exhibit complex compositional zoning due to magma replenishment, and the rims have higher 87Sr/86Sr ratio and Sr content than the cores, indicating crustal assimilation. The cores of zoned clinopyroxene phenocrysts have high Mg#, Cr and Ni but low Yb and Y with high Sr/Y and La/Yb ratios, which are consistent with the high Mg# of primary adakitic magmas. Magmatic processes have significantly changed the primary adakitic features of Yb, Y and Sr content, Sr/Y and La/Yb ratios, in addition to Mg# values. The Heishantou primitive high-Mg# adakite was derived from partial melting of a delaminated lower crust followed by storage, recharge, and assimilation in a crustal magma chamber. The Western Tianshan orogenic belt experienced a succession of lower crust delamination events in the Early Permian that involved melting of thickened lower crust, subsequent lithospheric detachment and asthenospheric upwelling.

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: REPLY

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: REPLY

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: COMMENT

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: COMMENT

Genesis and multi-stage evolution of crustal magma: A case study of the Late Jurassic Xurucuo plutonic complex in the South Lhasa terrane, Tibet

Genesis and evolution mechanisms of granites provide clues regarding the development of architecture and composition of continental crust. Lithologically zoned granitoid complexes and mafic microgranular enclaves (MMEs) are common and enlightening in understanding of genesis and, particularly the dynamic evolution of granitoid magmas, as well as mantle–crust interaction. The Late Jurassic Xurucuo plutonic complex (XPC) in the South Lhasa is diverse in lithology, being composed of zoned monzogranite, granodiorite, quartz diorite, diorite, and MME. Here, zircon UPb ages, oxygen and hafnium isotopes, and whole-rock and mineral geochemistries of the XPC were determined in order to reveal the genesis of the zoned pluton, and further, the dynamic processes of the crustal magmas. The XPC granitoids are high-K calc-alkaline and metaluminous to moderately peraluminous, I-type and arc-related magmatic rocks. These rocks were dated to be 159–154 Ma, with zircon δ18O values of 6.87–8.83‰ and negative εHf(t) values of −18.4 to −9.7. The concomitance of different lithologies and consistent O − Hf isotopic compositions suggest a prolonged cognate evolution of the magmas. The lithological and geochemical features of these granitoids indicate that they were generated from the reworking of crystal magma mush at shallow crustal levels aided by magma replenish, mixing and subsequent fractional crystallization. Combining our results with those of previous studies for the Late Jurassic magmatism in the Lhasa terrane, we propose an integrated model wherein mantle-derived magma intruded into crustal mush during the northward subduction of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane.

Periodic Mixing of Magmas Recorded by Oscillatory Zoning of the Clinopyroxene Macrocrysts from an Ultrapotassic Lamprophyre Dyke

Abstract Ultrapotassic rocks are volumetrically minor, but widely distributed in different geological settings. Extensive studies have concerned mantle melting processes that generated these rocks. However, crustal processes that they may have involved are poorly known. In this paper, we describe complex oscillatory zoning patterns of clinopyroxene (Cpx) macrocrysts from an ultrapotassic lamprophyre dyke in the Kyrgyz North Tianshan orogen. These macrocrysts commonly have a corroded or patchy-zoned core surrounded by a mantle with distinct oscillatory zoning, which is, in turn, surrounded by a euhedral rim. The oscillatory zoning of the mantle is composed of alternating coarse and fine layers with a clear resorption surface, or closely packed layers with a straight or wavy boundary in back-scattered electron images. High-amplitude oscillation of Mg#, Ti, Al, Cr and Sr across the layers of the mantle is attributed to magma mixing. Low-amplitude, high-frequency oscillation of Mg# across the closely packed layers was probably developed as a result of kinetic effects or crystal movement under thermal and chemical gradients. In addition, cryptic sector zoning of some macrocrysts clearly shows a Si- and Mg-rich hourglass sector and an Al- and Ti-rich prism sector. The sector zoning indicates crystallization of these macrocrysts under low degrees of undercooling, and the presence of concentric Cr-rich and Cr-poor layers within the same grain indicates that the growth process was disrupted by multiple magma recharging events. The cores of the macrocrysts have Mg# with three distinctive ranges: &amp;lt;84–90 (Core I), 74–84 (Core II) and 60–70 (Core III). The mantles have Mg# ranging from 64 to 90 without a distinct gap. The rims have a narrow range of Mg# from 76 to 80. The cores and mantles with high Mg# (≥85) have variable La/Yb from 1·8 to 5·0 and Dy/Yb from 2·3 to 4·6. The macrocrysts overall have variable 87Sr/86Sr from 0·7072 to 0·7084. Highly variable trace elements and 87Sr/86Sr within a single grain indicate that both primary and evolved magmas with different compositions were periodically recharged into the crustal magma reservoirs. Modelling results reveal that the melts in equilibrium with the Cpx macrocrysts may have been derived from the magma reservoirs at three different depths equivalent to crystallization pressures of ∼5·4, ∼3·3 and ∼1·6 kbar, respectively, making up a transcrustal magmatic system. The Cpx-laden melts in deep magma reservoirs may have been frequently transported to shallower reservoirs. Magma mixing in the shallower reservoirs led to heterogeneous magmas with different cooling rates and chemical compositions. Early crystallized Cpx crystals were overprinted with diverse zoning patterns during overgrowth and accumulation. Thus, the complex zoning patterns and compositions of the Cpx macrocrysts have important implications for a transcrustal magmatic system in the formation of ultrapotassic rocks.

Ps‐P Tomography of a Midcrustal Magma Reservoir Beneath Cleveland Volcano, Alaska

AbstractSeismic tomography of the crust is an essential tool for studying the three‐dimensional structure of magmatic plumbing systems feeding active volcanoes, but it is often limited in resolution by the absence of deep local seismicity. Teleseismic receiver functions can be used to illuminate local structural variations, but typically do not account for the effects of three‐dimensional velocity heterogeneities. Here we harness the complementary strengths of both techniques by processing Ps‐P delay times derived from teleseismic receiver functions in a tomographic S wave inversion. Using our inversion technique, we produce the first tomographic crustal velocity model beneath Cleveland Volcano, identifying a vertically extensive high VP/VS anomaly beneath the volcano that likely signifies a middle‐to‐lower crustal magma reservoir. The observation is the first of its kind in the central Aleutians, illustrating the potential of our technique to advance our understanding of crustal magmatic systems without broad seismic networks or distributed local seismicity.

Petrology and geochemistry of the Deccan basalts from the KBH-7 borehole, Koyna Seismic Zone (Western Ghats, India): Implications for nature of crustal contamination and sulfide saturation of magma

The Deccan basalt erupted at the K-T boundary and comprises multiple flow units with spatial variations in physiography and geochemistry. The Koyna borehole-7 (KBH-7 of MoES-BGRL) of the Koyna Seismic Zone is situated in the western part of the Deccan continental flood basalt province, where the lava pile is 1251 m thick. Thirty-one samples of four flow units (named FA, FB, FC, FD; 1248.20–1110.78 m) from the lower part and another four flow units (named FV, FW, FX, FY; 202.79–42.85 m) from the upper part of the KBH-7 borehole show significant petrographic, mineral chemical and bulk-rock geochemical variations. Individual flows are characterized by higher abundances of plagioclase and clinopyroxene phenocrysts in their lower than upper parts; conversely, their upper parts are relatively enriched in magnetite and ilmenite. In addition, the flows become amygduloidal near their tops. Seven samples from the lower part of the FB flow unit have relatively higher MgO (≈ 8.94–12.9 wt%) and lower TiO2 (≈ 1.43–1.67 wt%) contents compared to samples from other flow units and are identified as low-Ti high-Mg basalt. Clinopyroxene phenocrysts from low-Ti high-Mg basalts are more magnesian (Mg-number ≈ 62.4 to 82.3) and chromium-rich, and not in equilibrium with plagioclase phenocrysts in the rocks. The magnesian-rich clinopyroxene phenocrysts were probably concentrated in the flow by flow differentiation. The trace element geochemistry indicates that the flows in the KBH-7 borehole are crustally contaminated. Extremely low Nb/Ta (<10) values of a few basaltic samples indicate contamination by the Gondwana sediments (e.g., Gondwana greywacke Nb/Ta ≈ 3.8–12.8) along with the tonalitic crust (Nb/Ta ≈ 5.59–10.2). The major and trace element geochemical character of the KBH-7 basalts can be explained by assimilation and fractional crystallization (AFC) modeling using Archean tonalitic crust and Gondwana sediments as major contaminants. AFC modeling reveals that 6.5–14.5% partial melting of the primitive mantle followed by 64–85% AFC in the crustal magma chamber accounts for the variable geochemical character of the KBH-7 basalts. Despite assimilation by crustal components, the basalts do not show any signs of sulfide supersaturation or segregation of an immiscible sulfide liquid, unlike the Nadezhdinsky formation of the Siberian trap flood basalt that produced the Noril'sk Ni-sulfide deposits. The variable concentration of chalcophile elements (Ni ≈ 11–858 ppm, Cu ≈ 32–342 ppm) or ratios like Cu/Zr (> 1) and Ni/MgO (≈ 1.9–163) of the KBH-7 basalts also indicate the absence of an earlier sulfide liquation of the magma. This may be related to the absence of crustal sulfur from the tonalitic basement rocks or the Gondwana sediments (greywacke) in conjunction with high iron content (FeO(T) ≈ 11.9–15.9 wt%) of the basaltic magma.

Petrogenesis of the Late Triassic Mengsong strongly peraluminous granites in the southeastern Tibetan Plateau: highly fractionated from crystal mush

ABSTRACT The Triassic Lincang granites make up the largest peraluminous granitic batholith in the Changning-Menglian Paleo-Tethys orogenic belt, southeastern Tibetan Plateau. It is undoubtedly very important to investigate the role of crustal magmatism in the evolution of the Changning-Menglian Paleo-Tethys orogenic belt. The Late Triassic Mengsong pluton is composed of strongly peraluminous two-mica monzogranites, and is located in the southernmost part of the Lincang batholith. Despite all being peraluminous granites, they differ markedly in dimensions, petrographic and geochemical characteristics. Mengsong two-mica monzogranites have low contents of TiO2, FeOtot, MgO, and CaO, but high values of SiO2 and Na2O, compared to biotite monzogranites of the Lincang batholith. The Lincang biotite monzogranites show an enrichment in light rare earth elements (LREE) and weak Eu anomalies (δEu = 0.3–0.8). However, the Mengsong two-mica monzogranites show a clear tetrad effect and contain pronounced negative Eu anomalies (δEu = 0.1–0.3). Geochemical compositions and mineral textures of two-mica monzogranites indicate that they are of highly fractionated type affinity. The two-mica monzogranites are characterized by negative εHf (t) values (−9.3 to −1.7) and εNd (t) values (−9.6 to −7.9), while the biotite monzogranites have lower εHf (t) values (−15.5 to −6.1) and εNd (t) values (−14.5 to −11.5). Isotopic compositions suggest that they were cogenetic and derived from partial melting of the lower crust. Our petrogenetic and zircon U–Pb age data support models that relate the Lincang and Mengsong granites to post-collisional magmatism in the Changning-Menglian Paleo-Tethys orogenic belt during the Middle to Late Triassic. We interpret the Mengsong pluton to represent highly fractionated melts from crystal mush reservoirs and the Lincang granites to be complementary residual.

The importance of in situ crystallisation and loss of interstitial melt during formation of the Kærven Syenite Complex, Kangerlussuaq, East Greenland

The Kærven Syenite Complex (KSC) is one of the oldest felsic intrusions in the Tertiary East Greenland province. Here we update our previous description of the KSC and supply a greatly expanded and comprehensive geochemical dataset. New data allow us to present a more detailed petrogenetic model for the evolution of the KSC and to investigate the geochemical characteristics of igneous cumulates subjected to loss and, occasionally, replacement of residual liquid. The KSC comprises eleven mappable units that generally young westwards. Rock types range from quartz syenite to quartz alkali feldspar syenite and alkali feldspar granite. Individual intrusive units are relatively narrow and steep-sided and are collectively suggested to represent a ring dyke complex. Basement gneiss and gabbro host rocks have locally contaminated the oldest quartz syenite KSC unit, but most of the main part of the complex escaped significant influence from host rocks. A late suite of E–W to NE–SW striking peralkaline dykes of trachytic to phonolitic compositions intrude the KSC. Compositions of the KSC rocks span a considerable range in SiO2, 59–73 wt%. Concentrations of several elements vary widely for a given SiO2 (especially at SiO2 &lt; 66 wt%), and variation diagrams do not suggest a single model for the evolution of the units of the complex. A cumulative origin is envisaged for several KSC units. Geochemical modelling suggests that KSC magmas were derived from more than one primary magma, and that the complex evolved through a four-stage process: fractional crystallisation in precursory magma chambers was followed by final emplacement of each unit, establishment of a crystal/melt mush, expulsion of part of the residual melt and, finally, crystallisation of the remaining melt. Trace element disequilibria between alkali feldspar and host rocks in two closely associated quartz alkali feldspar syenite units indicate that highly evolved residual melt was replaced by a less evolved melt phase. Modelling of potential parent melt compositions to the Kærven magmas suggests an origin not in the Iceland plume asthenosphere, but rather in a moderately enriched source, possibly in the continental lithosphere. The course of melt evolution by fractional crystallisation is indicated to have taken place in magma chambers at depth, and repeated rise of magma into the upper crustal magma chambers and crystallisation there formed the KSC. Based on our survey of published geochemical data, the inferred parental magmas seem to have few equivalents in the North Atlantic Igneous Province and may have been generated mainly from melting of enriched dry lithospheric mantle of possibly Archaean age.

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

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

Petrologic imaging of the architecture of magma reservoirs feeding caldera-forming eruptions

Caldera footprints and erupted magma volumes provide a unique constraint on vertical dimensions of upper crustal magma reservoirs that feed explosive silicic eruptions. Here we define a Vertical Separation (VS) ratio in which we compare the geometric vertical extent with the range of depths indicated petrologically by melt inclusion water and CO2 saturation pressures for fifteen caldera-forming eruptions spanning ∼100 km3 to ∼103 km3 in volume. We supplement melt inclusion saturation pressures with rhyolite-MELTS barometry and plagioclase-melt hygrometry to generate a petrologic image of magma reservoir architecture. We find that pre-eruptive upper crustal magma reservoirs range from contiguous bodies (where petrologic and geometric estimates match closely) to vertically dispersed structures. Vertically dispersed pre-eruptive reservoirs are more common among intermediate-volume eruptions than among the smallest and largest caldera-forming eruptions. We infer that the architecture of magma reservoirs tracks the thermomechanical evolution of large volcanic systems.