40Ar/39Ar Ages and Geochemistry of Seamount Basalts from the Western Pacific Province

Seamounts are features generated by hot spots and associated intraplate volcanic activity. The geochemical characteristics of igneous rocks constituting seamounts provide evidence of important details of dynamic processes in the Earth, such as mantle magma source areas, and are key to understanding how mantle plume processes control the formation and evolution of seamounts and their resulting geochemical characteristics. The Pacific Ocean contains a large number of hitherto unstudied seamounts, whose ages and geochemical characteristics remain poorly known. This study presents the geochemical characteristics of six basalt samples from five seamounts in the Western Pacific and the 40Ar/9Ar ages of three samples are determined. The new analysis yielded 40Ar/39Ar ages for seamounts samples MP3D21, MP5D11, and MP5D15A of 95.43 ± 0.33, 62.4 ± 0.26, and 99.03 ± 0.4 Ma, respectively. The geochemical profiles of seamounts samples MP3D04, MP3D21, MP5D11, MP5D15A, MPID201, and MPID202 are consistent with alkaline basalts, as evidence by alkali-rich, silicon-poor compositions along with high titanium concentrations. The primitive mantle normalized rare-earth elements and trace elements spider pattern are similar to those of ocean island basalts. The Ta/Hf and Nb/Zr ratios and La/Zr-Nb/Zr discriminant diagrams indicate that the six seamounts formed from magma that originated in the deep mantle.

Petrogenesis and Tectonic Setting of Early Cretaceous Intrusive Rocks in the Northern Ulanhot Area, Central and Southern Great Xing’an Range, NE China

Abundant Early Cretaceous magmatism is conserved in the central and southern Great Xing’an Range (GXR) and has significant geodynamic implications for the study of the Late Mesozoic tectonic framework of northeast China. In this study, we provide new high-precision U–Pb zircon geochronology, whole-rock geochemistry, and zircon Hf isotopic data for representative intrusive rocks from the northern part of the Ulanhot area to illustrate the petrogenesis types and magma source of these rocks and evaluate the tectonic setting of the central-southern GXR. Laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U–Pb dating showed that magmatism in the Ulanhot area (monzonite porphyry: 128.07 ± 0.62 Ma, quartz monzonite porphyry: 127.47 ± 0.36, quartz porphyry: 124.85 ± 0.34, and granite porphyry: 124.15 ± 0.31 Ma) occurred during the Early Cretaceous. Geochemically, monzonite porphyry belongs to the metaluminous and alkaline series rocks and is characterized by high Al2O3 (average 17.74 wt.%) and TiO2 (average 0.88 wt.%) and low Ni (average 4.63 ppm), Cr (average 6.69 ppm), Mg# (average 31.11), Y (average 15.16 ppm), and Yb (average 1.62 ppm) content with enrichment in Ba, K, Pb, Sr, Zr, and Hf and depletion in Ti, Nb, and Ta. The granitic rocks (e.g., quartz monzonite porphyry, quartz porphyry, and granite porphyry) pertain to the category of high-K calc-alkaline rocks and are characterized by high SiO2 content (>66 wt.%) and low MgO (average 0.69 wt.%), Mg# (average 31.49 ppm), Ni (average 2.78 ppm), and Cr (average 8.10 ppm) content, showing an affinity to I-type granite accompanied by Nb, Ta, P, and Ti depletion and negative Eu anomalies (δEu = 0.57–0.96; average 0.82). The Hf isotopic data suggest that these rocks were the product of the partial melting of juvenile crustal rocks. Notably, fractionation crystallization plays a crucial role in the process of magma emplacement. Combining our study with published ones, we proposed that the Early Cretaceous intrusive rocks in the Ulanhot area were formed in an extensional tectonic background and compactly related to the subduction of the Paleo-Pacific Ocean plate.

Geochemical and isotopical variations within the Campanian Comagmatic Province: implications on magma source composition

A spatial variation in chemical and isotopical composition is observed between the volcanoes belonging to the Campanian Comagmatic Province. At a given MgO content, magmas from volcanic islands (Procida and Ischia) are enriched in Ti, Na, depleted in La, Ba, Rb, Sr, Th, K contents, and shows lower LREE/HFSE (e.g., La/Nb = = 1-2), lower Sr-Pb isotopic ratios and higher Nd isotopic ratios with respect to magmas from volcanoes locat- ed inland (Campi Flegrei and Somma-Vesuvius). The observed compositional variations are explained involving two different mantle sources in the genesis of the magmas erupted in this region: a deeper asthenospheric man- tle source, from which the Tyrrhenian magmas also derived and a lithospheric mantle source enriched by slab- derived fluids. The contribution of the enriched-lithospheric mantle became more pronounced moving from the Tyrrhenian abyssal plain through the Italian Peninsula where it dominates, likely in response to the thickening of the lithosphere observed under the Peninsula

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

Early Devonian Arc-Related Volcanic Rocks in the Haerdaban, North Margin of the Yili Block: Constraint on the Southward Subduction of the Junggar Ocean

The origin and tectonic implication of Early–Middle Devonian magmatism in the northern margin of YB (Yili Block) remain enigmatic and are important for understanding Late Paleozoic evolution of the Junggar Ocean and southern Kazakhstan Orocline. Here, we present the systematic study of whole-rock geochemical and Sr–Nd isotope features as well as U–Pb–Hf isotope characteristics of zircon crystals for newly identified Early Devonian volcanic rocks from the northern margin of YB. The volcanic rocks are composed of rhyolite, rhyolite porphyry, and rhyolitic tuff. Zircon U-Pb age dating indicates they were formed at ca. 407~418 Ma. They have high SiO2 (70.16–77.52 wt.%) and alkali (5.10–9.56 wt.%) contents, and high Zr + Nb + Ce + Y content (~456 ppm), indicative of A-type magma. Their relative depletion of Nb, Ta, and Ti, and enrichment of LILEs show arc affinity. Their low initial 87Sr/86Sr ratios (0.699708–0.709822) and negative εNd(t) values (−1.8 to −4.0) indicate a mainly continental magma source and their positive εHf(t)values (+6.13 to +14.81) are possibly due to the garnet effect. All these above reveal that volcanic rocks were generated by re-melting of lower crust under a high temperature condition, which was induced by long-lived heat accumulation with no or minimal basalt flux. Combined with active continental margin inference evidenced by contemporaneous sedimentary rocks, we attribute the generation of the volcanic rocks to a continental arc setting related to the southward subduction of Junggar oceanic crust. Thus, we infer the Early–Middle Devonian arc-related magmatic rocks in the northern margin of YB are eastward counterparts of the southern limb of the Devonian Volcanic Belt, which resulted from a relatively steady-state southward subduction.

Identifying crystal accumulation and melt extraction during formation of high-silica granite

High-silica (&lt;70 wt% SiO2) magmas are usually believed to form via shallow crustal–level fractional crystallization of intermediate magmas. However, the broad applicability of this model is controversial, because the required crystal-melt separation processes have rarely been documented globally up to now. The ca. 50 Ma Nyemo composite pluton of the Gangdese batholith belt in southern Tibet, which comprises intrusive rocks with intermediate- to high-silica compositions (65–78 wt%), offers a unique opportunity for substantiating the coexistence of extracted melts and complementary silicic cumulates in one of Earth’s most complete transcrustal silicic magmatic systems. The Nyemo pluton intrusive rocks exhibit similar zircon Hf isotopic compositional ranges (mean εHf(t) = +5.7 to +8.3), suggesting a common, non-radiogenic magma source with crustal assimilation in the deep crust. Yet, these rocks have distinct geochemical characteristics. High-silica miarolitic and rapakivi granites are strongly depleted in Ba, Sr, and Eu, and their zircon trace elements show extremely low Eu/Eu* and Dy/Yb. In contrast, monzogranite is relatively enriched in Ba and Sr with minor Eu anomalies, and the zircon trace elements are characterized by relatively high Eu/Eu* and Dy/Yb. Therefore, we propose that the high-silica granites represent highly fractionated melt extracted from a mush reservoir at unusually low storage pressure (~99–119 MPa), and that the monzogranite constitutes the complementary residual silicic cumulates.