Magma Mixing Research Articles

Origin of Diorites and Coeval Mafic Microgranular Enclaves in the Liuba Region, South Qinling Orogen, Central China: Insights from Petrography, Zircon U-Pb Geochronology and Geochemistry

The formation of early Mesozoic granitoid plutons in the Qinling Orogen is widely regarded as a result of the collision and accretion between the Yangtze Block and the South Qinling Block during the early Mesozoic, but the specific magmatic process, source composition, tectonic environment and deep dynamic background remain controversial. This study reports the petrology, zircon U–Pb geochronology, and whole-rock geochemistry of diorites from the Liuba and Qingyangyi plutons in the South Qinling, to provide new evidence for understanding the final collision tectonic evolution process of Qinling Orogenic belt. The Liuba and Qingyangyi plutons, located in the central part of the South Qinling region, are primarily composed of quartz diorite and quartz monzodiorite, respectively. The results indicate that the weighted mean crystallization ages of the quartz diorite in the Liuba pluton range from 216.1 ± 0.8 Ma to 217.1 ± 1.3 Ma, with the weighted mean crystallization ages of its MMEs being 215.4 ± 1.0 Ma. The crystallization ages of the quartz monzodiorite in the Qingyangyi pluton range from 214.6 ± 0.9 Ma to 215.4 ± 0.9 Ma, suggesting that both plutons were formed in the late Triassic. The investigated plutons are characterized as right-leaning and have weak negative Eu anomalies on the chondrite-normalized REE patterns diagram. The large ion lithophile elements (LILE) Rb, Ba, Th and K are relatively enriched, while high-field strength elements (HFSE) Nb, Ta, Ti and P are strongly depleted. The formation of numerous MMEs in the Liuba pluton is the product of magmatic mixing. The Liuba and Qingyangyi plutons are the results of crust thickening and partial melting of lower crust caused by the comprehensive late Triassic collision between the Yangtze Block and the North China Block (NCB), and are the manifestation of magmatic intrusion along the South Qinling tectonic belt in the late Triassic period.

Petrogenesis of granitoids from silicic large igneous provinces (Central and North-East Asia)

Large granitoid provinces can be divided into areal and linear types, which differ significantly in the area and volume of granitoids in their composition. It is shown using the example of the largest granitoid provinces of Central and Northeast Asia (Angara-Vitim, Khangai, Kalba-Narym, Kolyma). It is assumed that these differences are due to the structure of pregranitic basement and degree of thermal impact on the lower and middle continental crust. An important factor in the formation of granitoid provinces is mantle mafic magmatism, the estimated scale of which correlates with the volumetric and areal characteristics of the granitoid provinces. The role of mafic magmatism is an additional input of heat from the fluids into the melting region of crustal protoliths, as well as a material contribution that is realized through various mechanisms of magma mixing. Mixing at the deep level is the most effective, resulting in the formation of significant volumes of increased basicity salic magmas. The petrogenetic role of contrasting magmas mixing at the mesoabyssal level of the earth's crust, as well as in hypabyssal conditions (mingling dikes), is not great, but these manifestations are the key argument in justifying the synchronicity of mafic and granitoid magmatism. Granitoids of Silicic Large Igneous Provinces (SLIPs) are characterized by a heterogeneous isotopic composition, generally corresponding to the parameters of the continental crust. The extremely high heterogeneity of spatially conjugate granitoids due to the mixing of silicic magmas formed through the melting of a small number of sources with contrasting isotopic compositions, including through mixing with magmas of mantle origin. Mafic rocks included in the granitoid provinces correspond to the isotopic composition of the enriched mantle (Angara-Vitim batholith) or indicate a significant contribution of contamination with continental crust material (Khangai area). The metallogeny of SLIPs is determined by the erosional section size and the crustal protoliths type, the metamorphism degree of which largely determines the initial fluid content of silicic magmas. The melting of highly metamorphosed ancient crustal protoliths produces relatively “dry” silicic melts, the melting of low-metamorphosed crustal sources leads to the formation of “aqueous” melts, the differentiation of which ends with pegmatite formation with rare metal mineralization. Non-subduction origin SLIPs formation is associated with the mantle plumes impact (in the form of synchronous basaltoid magmatism) on the heated crust of young orogenic regions, where tectonic processes ended no more than a few tens of Ma.

Zircon U-Pb geochronology and Sr-Nd-Pb-Hf-O isotope signatures of Middle Eocene high-K plutons from the Eastern Pontides Orogenic Belt, NE Turkey: implications for parental magma sources and magmatic processes

ABSTRACT The Eastern Pontides Orogenic Belt (EPOB) in northeastern Turkey is a well-preserved continental magmatic arc in the Alpine-Himalayan Orogenic Belt and contains many Eocene I-type high-K plutons. Employing LA-ICP-MS U-Pb zircon geochronology, the crystallization ages of the investigated Yaylalıköy and Taşbaşı plutons were established to be Lutetian (Middle Eocene), ranging from 47 Ma to 46 Ma. These plutons exhibit a compositional spectrum ranging from diorite to granodiorite, characterized by I-type, metaluminous, and high-K geochemical signatures. Notably, they are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), displaying pronounced negative Eu anomalies (EuN/Eu* = 0.58–0.96). Isotopic signatures reveal homogeneous and moderately low initial 87 Sr/86 Sr(i) ratios (0.704643–0.705034) and positive εNd(i) values (0.24 to 1.67) for the investigated plutons. These isotopic signatures are consistent with mantle origin, as evidenced by their position within the mantle array on isotope ratio diagrams. Lead isotope compositions exhibit limited variations, with initial ratios of 206 Pb/204 Pb(i) (18.55–18.64), 207 Pb/204 Pb(i) (15.61–15.64), and 208 Pb/204 Pb(i) (38.46–38.75). Zircon εHf(i) values are positive (2.32–6.20), plotting between the depleted mantle and chondritic evolution lines. Additionally, these rocks display low zircon δ18 O values (+4.54 to + 6.53‰), comparable to mantle composition. The petrographic characteristics of the investigated plutons suggest that fractional crystallization with minimal assimilation and/or magma mixing played a dominant role during their solidification. The integration of geochemical and isotopic data, alongside regional geological constraints, advocates for enriched lithospheric mantle as the source for the parental magmas of the studied plutons. These magmas subsequently underwent differentiation within a deep-seated magma chamber, followed by emplacement into the crust.

KARAKTERISTIK BASAL DAERAH TANJUNG AGUNG

Research on crystalline rocks in Lampung has been carried out by various researchers, but this research still does not cover other areas, such as Tanjung Agung village, Katibung District, South Lampung. Therefore, this research aims to analyze the characteristics of basalt rocks in the Tanjung Agung area, South Lampung, which is important for expanding understanding of the geological diversity in the region. These rocks are found in the form of entablature-type columns formed from basalt lava flows. The methods used include literature studies, field observations, and laboratory analysis, especially petrographic analysis, to identify mineral content and rock structure to understand the crystallization process and formation of columnar joints. The results of the petrographic analysis show a porphyritic texture with plagioclase, pyroxene, and hornblende phenocrysts, as well as a finer groundmass. These rocks show gradual crystallization and textural variations such as trachytic, sieve, glomeroporphyritic, and zoning, which reflect processes of decompression, pressure reduction, and magma mixing. Based on the mineral composition, this rock is classified as basalt which undergoes a gradual crystallization process, namely the formation of large crystals (phenocrysts) at depth and the formation of a finer groundmass at the surface. This research also opens up opportunities for geochemical analysis to understand the origin and evolution of magma in the region. Thus, this research provides further insight into the dynamics of magmatism and rock formation in the Tanjung Agung area.

Apatite geochemical indicators for magma mixing and fractional crystallization in the origin of A-type granite

Geological and geochemical observations show that magma mixing and fractional crystallization are fundamental processes in the origin of A-type granites, with the dominant process determining their specific genesis. However, it is difficult to evidently distinguish such two processes using whole-rock geochemistry. Apatite has a long crystallization history and can precipitate from the melt during the whole magmatic evolution process, and its geochemical and Sr isotopic data would constrain the magma mixing and crystal fractionation processes. Here we present the integrated geochemical data and Sr isotopic compositions of apatite from mafic microgranular enclaves (MMEs) and their host A-type granites in the early Cretaceous Qianshan pluton to fingerprint apatite geochemical indicators for tracing magma mixing and fractional crystallization. The variable Sr isotopic compositions of apatites in the mafic microgranular enclaves (0.7097 to 0.7211) and the host biotite granite (0.7131 to 0.7171) suggest a magma mixing process, which cannot be revealed by consistent whole-rock Sr isotopic compositions. The trend between Eu/Eu* and rare earth elements + yttrium (REE + Y) contents and Sr contents in apatite among different samples mimics the magma mixing trend observed in whole-rock, suggesting that the broad range of Sr contents and abrupt increases in REE + Y contents can record the mixing process. However, the effect of fractional crystallization on the apatite composition depends on the partition behavior of elements in different minerals. Specifically, the continuous decreases in Eu/Eu* and Sr contents of apatites effectively indicate progressive feldspar crystallization, and the decreases in the (La/Sm)N and (La/Yb)N ratios of apatites suggest the crystallization of feldspar, biotite and apatite, while the increases of these ratios may trace the crystallization of minerals rich in MREE and/or HREE such as hornblende, zircon, and titanite. These observations confirm the sensitivity of apatite Sr isotopes and trace elements content, as well as REE patterns to magma mixing and fractional crystallization, thus providing valuable insights into the complicated magma evolution processes and petrogenesis of A-type granites. Furthermore, the universality of apatite in granitic rocks and the commonality of magmatic mixing and fractional crystallization in granite origin highlight the broader applicability of our study, offering a valuable perspective for understanding the origin of other granitoids.

Plagioclase-hosted Crystallized Melt Inclusions within Hypabyssal Volcanic Rocks of the Torud-Ahmad Abad Magmatic Belt, Iran: Analysis of Origin and Fractionation Processes

Investigating crystal-rich clots hosted in phenocrysts and phenoclasts within Eocene subvolcanic rocks in Torud-Ahmad Abad, south-southeast of Shahrood (northern part of central Iran zone). These crystal-rich clots and clusters (alias stone inclusions, nanogranitoids, microcrystal clots) are interpreted as crystallized melt inclusions (cMIs) within phenocrysts and phenoclasts, highlighting plagioclase-hosted inclusions. Least-altered hypabyssal igneous rocks are trachy-andesitic, basaltic andesitic, and dacitic porphyries. These porphyries have porphyritic, glomeroporphyritic, granular, and trachytic textures with variable-sized phenocrysts of plagioclase (albite-labradorite), green hornblende (magnesio-hastingsite), and clinopyroxene (diopside-augite), with minor biotite, and Fe-Ti oxides; large plagioclase phenocrysts, exhibiting clear normal oscillatory zoning, were consistently utilized as plagioclase-hosted inclusions due to their abundance in rocks. MIs exhibit complete post-entrapment crystallization (PEC), generally with a slightly finer grain size than the igneous groundmass, i.e. no preserved glassy MI were observed in these phenocrysts, only cMIs. These variably sized, cryptocrystalline to microcrystalline clots in various phenocrysts seem to also represent primary igneous assemblages, manifested as clusters of microphenocrysts; these are referred to as crystal clots. SEM-EDS analyses determined the composition of crystal-rich clots in various plagioclase phenocrysts forming inclusions. The major-element composition of these crystal-rich clots in plagioclase are basalt, basaltic andesite, andesite, trachy-andesite, and trachyte that seem to be melt trapped during plagioclase phenocryst growth; these trapped interface melts then form microphenocryst assemblages that are preserved in phenocrysts, which are trapped when some process interferes with the growth of a phenocryst. These cMIs exhibit compositional variations from their bulk host rock, resulting from entrapment during magma mixing during plagioclase growth.

Insights on Arc Magmatic Systems Drawn from Natural Melt Inclusions and Crystallization Experiments at PH2O =800 MPa under Oxidizing Conditions

Abstract Whole rock compositions at Buldir Volcano, western Aleutian arc, record a strong, continuous trend of iron depletion with decreasing MgO, classically interpreted as a calc-alkaline liquid line of descent. In contrast, olivine-hosted melt inclusions have higher total iron (FeO*) than whole rocks and show little change in FeO* with decreasing MgO. To investigate this discrepancy and determine the conditions required for strong iron depletion, we conducted oxygen fugacity (ƒO2) buffered, water-saturated crystallization experiments at 800 MPa and ƒO2 = QFM+1.6 ± 0.4 (1$\sigma$) (where QFM refers to the quartz-fayalite-magnetite buffer) on a high-Al, basaltic starting material modeled after a Buldir lava. Experimental conditions were informed by olivine-hosted melt inclusions that record minimum entrapment pressures as high as 570 MPa, >6 wt.% H2O, and ƒO2 of QFM+1.4 (±0.2), making Buldir one of the most oxidized and wettest arc volcanoes documented globally. The experiments produce melts with Si-enrichment and Fe-depletion signatures characteristic of evolved, calc-alkaline magmas at the lowest MgO, although FeO* remains roughly constant over most of the experimental temperature range. Experiments saturate CrAl-spinel and olivine at 1160oC, followed by clinopyroxene and Al-spinel at 1085oC, hornblende at 1060oC, and, finally, plagioclase and magnetite between 1040oC and 960oC. Hornblende crystallization, not magnetite, generates the largest increase in SiO2 and largest decrease in FeO* in coexisting melts. Compositions of melt inclusions are consistent with experimental melts and reflect crystallization of a basaltic parent magma at high PH2O. In contrast, the whole rock compositional trends are influenced by magma mixing and phenocryst redistribution and accumulation. The crystallization experiments and natural liquids (melt inclusions and groundmass glass) from Buldir suggest that for an oxidized, hydrous primary basalt starting composition, significant Fe-depletion from the melt will not occur until intermediate to late stages of magma crystallization (< ~4.5 wt.% MgO). We conclude that the Buldir whole rock trend cannot be reproduced by crystallization at arc-relevant oxygen fugacities and is not a true liquid line of descent, warranting caution when interpreting volcanic trends globally.

Nephelinites from Burko volcano (Tanzania) record the phase relations among perovskite, magnetite, titanite and andradite in evolved alkaline and silica-undersaturated systems

Nephelinitic rocks from Burko volcano in the Northern Tanzanian Divergence Zone of the East African Rift System represent transitional compositions between primitive and evolved nephelinites which exhibit two distinct phase assemblages, allowing to constrain the magmatic history of such particular rock types. Detailed petrography, mineral compositions and whole rock geochemistry were used to reconstruct the crystallization conditions and the petrological evolution of the Burko rocks and to compare them to the nearby volcanoes of Oldoinyo Lengai and Sadiman. Burko samples report a characteristic mineralogy of intermediate nephelinites (Mg# of 60–40) which comprise nepheline-diopside-magnetite-perovskite assemblages. They evolved mainly via fractionation of clinopyroxene, apatite, magnetite, and perovskite/titanite to peralkaline nephelinites (Mg# of 40–25) comprising nepheline-aegirine-augite-titanite-andradite±K-feldspar assemblages. The presence of unexposed primitive olivine nephelinites is, however, indicated by rare forsterite antecrysts.Reworking of crystal mushes and/or magma mixing are evident from different xenocrysts, antecrysts and pyroxenitic–ijolitic inclusions, precluding simple fractional crystallization modelling. The evolution from diopside-bearing nephelinites towards aegirine-augite-bearing ones was accompanied by a decrease in temperature (from ~1100 to ~900 °C) and an increase of log(aSiO2) towards ~−0.5 and of fO2 (∆FMQ ~2.5 to ~3). Especially in the peralkaline nephelinites, late-stage enrichment of Sr, Ba and halogens is documented by Sr-and F-rich apatite, barytolamprophyllite, celsian, götzenite, and eudialyte. In comparison, evolved and mostly peralkaline nephelinites from the nearby Oldoinyo Lengai and Sadiman volcanoes contain similar mineral assemblages, indicating comparable formation processes, but slightly different melt evolution trajectories. The variations in nephelinite composition and phase assemblages are linked to a) slightly different parental melt compositions related to variable amounts of amphibole, mica and carbonate in the molten mantle veins and b) different crystallization conditions, especially redox conditions, during cooling.

Nucleation delay controlling the formation of mafic enclaves and banded pumice

The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.

First boron isotopes in the southern Jilin TTG series uncover a Neoarchean oceanic arc in the eastern North China Craton

The Neoarchean evolution of the eastern North China Craton (NCC) is still controversial. This study presents the first B isotopes, together with zircon U-Pb-Hf isotopes and whole-rock geochemical analyses, for the TTG and dioritic series in the Baishan area of the southern Jilin region. LA-ICP-MS zircon U-Pb results uncover the Neoarchean magmatic activities, including granodioritic gneisses (2648 ± 10 Ma and 2622 ± 8 Ma), and quartz dioritic gneiss (2539 ± 7 Ma). The 2.65–2.60 Ga TTG series exhibit intermediate calc-alkaline characteristics, with relatively lower Th/La ratios (0.11–0.41) and positive zircon εHf(t) values (+3.73-+7.93), suggesting that the TTG series were likely derived from partial melting of mafic lower crust. By contrast, the 2.54 Ga dioritic series show positive Zr, Hf and Eu anomalies, with relatively lower Nb/Zr ratios (0.013–0.028) and εHf(t) values (+2.00-+5.49), indicating that they were possibly produced by mixing of the mantle-derived magma and crustal melts. Importantly, the 2.65–2.60 Ga TTG series are characterized by positive whole-rock δ11B values of + 4.11-+15.08 ‰, resembling the Izu-Bonin-Mariana oceanic arc and South Sandwich Island arc volcanic rocks. The formation of these TTG rocks is attributed to 11B-rich fluids released by subducted oceanic slab and subsequent metasomatism of the subarc mantle wedge. Unlike the oceanic arc TTG series, the 2.54 Ga dioritic series exhibit lighter whole-rock δ11B values of − 4.23–−4.50 ‰, reflecting an arc-continental collision induced by slab breakoff and mantle-derived magma upwelling. Integrated with previous studies, it suggests that the subduction-collision process in the eastern NCC resulted from the co-evolution of oceanic arc and continental margin arc.

Neoproterozoic Tectonics of the Arabian-Nubian Shield: Insights from U–Pb Zircon Geochronology, Sr–Nd–Hf Isotopes, and Geochemistry of the Deki Amhare Complex Granitoids, Central Eritrea

The Deki Amhare complex is located in central Eritrea, within the Arabian–Nubian Shield (ANS). It consists of an inner core of monzogranite porphyry and diorite enclaves (MMEs), surrounded outwardly by granodiorite and quartz diorite. The zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions of the Deki Amhare complex granitoids were used to discuss the Neoproterozoic tectonics of the ANS. The Late Tonian granodiorite and quartz diorite are metaluminous and calc-alkaline to slightly high-K calc-alkaline I-type plutons, with ages of 811.2 ± 4.8 Ma and 811.6 ± 5.7 Ma, respectively. They exhibit positive εHf(t) (7.6–9.5) and εNd(t) (3.9–4.7) values and relatively low (87Sr/86Sr)i ratios (0.70374–0.70463), indicating that they derived from the partial melting of a metasomatized mantle wedge during intra-oceanic subduction. The Ediacaran monzogranite porphyry and MMEs are subalkaline to alkaline A2-type granitoids with ages of 620.0 ± 4.3 Ma and 614.8 ± 3.9 Ma. These display positive εHf(t) (5.3–8.7) and εNd(t) (4.2–4.7) values, as well as low (87Sr/86Sr)i ratios (0.70310–0.70480), implying that they formed through crust–mantle magma mixing related to post-collisional slab break-off. Based on these data, three stages of regional tectonic evolution can be described: (1) from ~1200 Ma to ~875 Ma, the mafic oceanic crust was derived from depleted mantle during the opening of the Mozambique Ocean; (2) from ~875 Ma to ~630 Ma, intra-oceanic subduction and arc formation occurred with the development of I-type batholiths; and (3) from ~630 Ma to ~600 Ma, crustal and lithospheric reworking took place post-collision, leading to the formation of A2-type granitoids.

Petrogenesis and high-precision U-Pb zircon geochronology of the Howley Islands intrusions, Central Newfoundland Appalachians: Hydrous magmatism of Emsian age (ca. 400 Ma) along a multi-million-ounce orogenic gold belt

The Howley Islands intrusions consist of three coarse-grained amphibole-phlogopite/biotite quartz gabbro dykes and one medium-grained amphibole-biotite quartz diorite body that cut rocks of the Exploits Subzone in central Newfoundland along strike from the multi-million-ounce Valentine gold deposits. The petrogeneses and ages of these rocks were investigated to better constrain the process evolution of the orogenic gold belt that extends for more than 200 km across central Newfoundland.The quartz gabbro dykes are composed of magnesio-ferri-hornblende-cummingtonite-phlogopite/biotite macrocrysts mantled by plagioclase (labradorite to oligoclase)-quartz coronas. The gabbros are LILE- and LREE-enriched, transitional arc-like rocks that formed from a different melt source and parental magma than the quartz diorite body. The quartz diorite is plagioclase-rich (50 modal % andesine), contains only trace cummingtonite, lacks phlogopite, and preserves rare diopside overgrown by magnesio-ferri-hornblende. This intrusion is more alkaline and OIB-like than the quartz gabbros and exhibits the influence of a deeper, more enriched mantle component, although both melts variably interacted with deep lithosphere. The quartz gabbro dykes and quartz diorite body may represent melts of the lower lithosphere and upper asthenosphere, respectively.The abundance of coarse- to medium-grained amphibole and phlogopite/biotite in the samples is consistent with crystallization of hydrous magmas and rapid, water-enhanced crystal growth, with cooling paths recorded by chemically zoned grains of magnesio-ferri-hornblende and plagioclase. One quartz gabbro displays reverse core to rim chemical zoning of plagioclase from andesine to labradorite, which may reflect decreasing pressure during magma ascent and crystallization, magma mixing of evolved and more primitive magmas, and/or fluctuations in H2O content. The presence of cummingtonite suggests crystallization at relatively low temperatures in shallow, low-pressure, upper crustal magma chambers. The quartz gabbros may represent melts equivalent to the nearby Howley Islands gabbro body, whereas the quartz diorite may represent a plagioclase cumulate along the margin of another melt chamber.U-Pb CA-ID-TIMS zircon geochronology yielded ages of ca. 400.3 Ma for the gabbro dykes and 399.9 Ma for the quartz diorite intrusion, within the ca. 410–377 Ma age range for mineralization of the nearby Valentine gold deposits. The ca. 400 Ma intrusions, when considered in conjunction with regional models, reflect melting and hydrous magmatism in the mantle wedge above a retreating Avalonian slab that was dehydrating during the Acadian orogenic cycle. The coincidence of Pridoli (ca. 422 Ma) to Emsian (ca. 400 Ma) bimodal magmatism and orogenic gold mineralization in central Newfoundland reflects more than twenty million years of high geothermal gradients and fluid flow, which when combined with structural focusing along faults, was apparently optimal for the formation of orogenic gold deposits of significant economic potential.

Magnetotelluric evidence for the deep causes of different eruptive styles of Changbaishan Tianchi and Longgang volcanoes

Longgang Volcano (LGV) and Changbaishan Tianchi Volcano (CTV) share a common magmatic source at mantle depths. However, the two volcanoes have produced completely different types of eruptions. By performing 3D inversion of an MT dataset that completely covers the LGV and CTV, we have obtained high-resolution electrical resistivity images. The results reveal that the two volcanoes have distinct magmatic plumbing systems, and this is likely the reason for their different eruptive styles. Results from 3D modeling do not show a magma chamber in the shallow crust beneath LGV, interpreted as the rapid rise of the magma from the mantle is responsible for producing a series of densely distributed volcanic cones in the LGV field. In contrast, there is a magma chamber in the upper crust beneath the CTV, where the fractional crystallization and mixing of magma has occurred. This magma chamber has facilitated multiple centralized eruptions, and thereby has led to the formation of the large CTV volcanic cone. These results indicate that differences in their crustal structures may have controlled the different eruptive activities of the LGV and CTV in CVS, Northeast China.

Petrogenesis of episodic volcanic-intrusive rocks in SE China: Crystal-melt segregation and magma mixing

Crystal-melt segregation and magma mixing are key magmatic processes that generate compositional variabilities in igneous rocks. Distinguishing these processes can provide insight into the evolution of volcanic calderas and the driving force of plate subduction. Intensive episodic magmatism occurred in Southeast China during the Cretaceous, producing a series of volcanic-intrusive rocks during different magmatic stages in chain-like calderas. We conducted comprehensive petrological, geochemical, isotopic, and zircon trace element analyses on two representative calderas (Xiaoxiong and Shiniushan).The results show that the magmatism produced multistage volcanic and coexisting intrusive rocks regarding their crystallization ages (102–88 Ma) and NdHf isotopic compositions (zircon ɛHf(t) = −11.7 to −1.8, whole-rock εNd(t) = −8.3 to −2.4). The volcanic and intrusive rocks are related by fractional crystallization.In general, the erupted silicic rhyolitic rocks are characterized by distinctly negative Eu anomalies (δEu = 0.10–0.71) and depletions of Ba, Sr, P, and Ti, while the coexisting intrusive rocks show distinct or complementary geochemical signatures, such as neglectable Eu anomalies (δEu = 0.92–1.09) and positive Ba anomalies. Furthermore, the whole-rock samples from the calderas have variable δ41K values (from −0.14 ‰ to −0.59 ‰) that show strong correlations with indices of fractional crystallization such as Eu anomaly, Sr content, and K2O/Na2O ratios, indicating a key role of plagioclase during crystal-melt segregation. This is also supported by petrographic evidence for crystal accumulation such as the synneusis of plagioclase phenocrysts. Mafic microgranular enclaves in the volcanic-intrusive rocks indicate that magma recharge events occurred in the magma reservoir. Synthesizing these lines of evidence, we argue the multiple magmatism in plate subduction zone can be sustained by recharge events in the magma reservoir along with crystal-melt segregation processes. Particularly, the volcanic rocks tend to have isotopically lighter K than intrusive rocks, and strongly support that the volcanic rocks represent the extracted melt that is complementary to the cumulative intrusive rocks.

Conditions for formation and preservation of andesite-hosted mafic enclaves during the 2018 Lower East Rift Zone eruption of Kīlauea

Andesites erupted at Kīlauea in 2018 in the Lower East Rift Zone for the first time in the known geological record. The evolved lavas erupted at Fissure 17 of the 2018 eruption, ranging from andesites to basaltic andesites, contain abundant mafic enclaves both in the lava flows and the ejecta, which are unusual at Kīlauea and in Hawai'i in general. Textural observations indicate that the enclaves originate from incomplete mixing of two magmas rather than the incorporation of cold basaltic wall rock. We suggest, on the basis of bulk and mineral compositions, that the source of the mafic enclaves is the early 2018 evolved basalt magma (phase 1b) that erupted concomitantly at adjacent fissures, which mixed with the andesite to produce the range of basaltic andesite compositions observed at Fissure 17. The coexistence of homogenized basaltic andesites and mafic enclaves within the same magma require a mixing mechanism resulting in both complete homogenization and preservation of enclaves. We propose that the range of mixing and mingling processes may be explained by spatial and temporal variability in the mixing percentages of the phase 1b basalt and the andesite within the andesite magma chamber. Field observations, chemical compositions, and 2D thermal conduction models suggest that enclaves are preserved where the basalt contribution to mixing is less than roughly 40 %, as a result of microlite crystallization leading to rigidification of the enclave magma. Above this threshold, the mixed magmas became largely homogenized. The scarcity of mafic enclaves at Kīlauea and in the Hawai'i igneous record is likely explained by mixing between magmas that lack sufficient compositional and rheological contrasts to preserve them.

The role of magma recharge and mixing in producing compositional modality in post-collisional volcanic rocks, Konya Volcanic Field, Central Anatolia (Türkiye)

The Neogene Erenlerdağ-Alacadağ (ErAVC) and Sulutas (SVC) volcanic complexes in the Konya Volcanic Field, Türkiye have distinctly different unimodal and bimodal compositional variations, respectively. They occurred in graben-like extensional basins behind the retreating Cyprus subduction zone between the African and Eurasian plates. We here investigate their compositional modality by using new and published whole-rock major and trace element and Sr-Nd-Pb isotope data.Both complexes are characterized by basaltic to rhyodacitic high-K calc-alkaline rocks with the geochemical signatures of orogenic volcanism, except for minor alkaline rocks in the SVC. Mass-balance models suggest that major element variations can be largely explained by the fractional crystallization of amphibole, plagioclase, and Fe-Ti oxides. However, Sr-Nd-Pb isotopes show correlations with SiO2 indicating that open-system processes played a role in their differentiation. Modeling of AFC (Assimilation and Fractional Crystallization) involving a recharge situation shows that low degrees of crustal assimilation (rate of assimilation/rate of fractional crystallization, r < 0.2 and crust/magma ratio, ρ: 15–16 %) of lower and upper crust-like rocks was involved in the differentiation of the ErAVC and SVC, respectively. However, the modeling suggests that magma recharge (β: rate of magma recharge/rate of assimilation) was more efficient in the ErAVC (β: 3.45, % ∼52.5 rate of recharge) relative to that of the SVC (β: 2.15, % ∼36.55 rate of recharge). We conclude that for the ErAVC and SVC, different parental magmas derived from the subduction-modified mantle source followed distinct differentiation paths in the crust, and their compositional modality was mainly controlled by the magma recharge and mixing process.

Chemical and isotopic investigation of the I-type Bega Batholith, southeastern Australia: Implications for batholith compositional zoning and crustal evolution in accretionary orogens

Cordilleran granitic batholiths represent significant episodes of crustal growth and differentiation, and commonly display lateral isotopic and chemical variations. Establishing the tectono-magmatic processes responsible for generating this compositional asymmetry is important for understanding crustal evolutionary processes throughout the Phanerozoic. The Bega Batholith, an example of a ‘Cordilleran style’ granite batholith, is the largest I-type Siluro-Devonian granite complex in the Lachlan Fold Belt (LFB) of southeastern Australia and comprises seven granite supersuites that display systematic lateral isotopic and chemical asymmetry. From west to east towards the present-day continental margin, an increase in the content of Na2O, Sr, Al2O3, and P2O5, with concomitant decreases in CaO, Sc, Rb, and V are observed. In the same direction, whole-rock initial 87Sr/86Sr decreases from 0.7098 to 0.7039, εNd values increase from −8.3 to +4.4, and δ18O decreases from 10.2 ‰ to 7.9 ‰. Depleted-mantle model ages also decrease from ca. 1800 Ma in the west to 600 Ma in the east. Here, we address whether these chemical and isotopic variations were generated by interaction between two distinct components (mantle-derived magmas and supracrustal sources) or were alternatively produced by partial melting of infracrustal source rocks formed sequentially by much earlier episodes of crustal underplating. Combined whole-rock Nd-Sr-O isotopic and geochemical analyses indicate that several I-type supersuites exhibit chemical and isotopic correlations consistent with two-component magma mixing. This new evidence challenges the long-held view that I-type granites derive exclusively from the melting of infracrustal sources, and that granite terranes represent wholesale crustal reworking rather than new crustal growth. Our results show that the compositional zoning within the Bega Batholith is multifaceted. Firstly, the presence of two discrete mantle sources endows chemically and isotopically distinct eastern and western segments in the batholith. Secondly, within these compositionally distinct regions the lateral compositional changes across supersuites derives from mixing between mantle-derived and supracrustal sources. Finally, progressive extension within a developing back-arc environment regulates the ratio of crust-mantle contributions and compositional architecture of each I-type supersuite.

New bulk rock and age data on the changes in magma evolution during the Miocene, Galatean volcanic area, central Anatolia, Turkey: A review

The Galatean Volcanic Province (GVP) lies within the Sakarya tectonic belt in the northwest of central Anatolia, Turkey, south of the North Anatolian Fault, cropping out over 12,000 km2. It consists of Early Miocene intermediate to acid lavas, pyroclastic rocks, volcaniclastic deposits, and Late Miocene OIB-like basalts. Our study is based on new bulk geochemistry, SrNd isotopes, and 40Ar/39Ar dating of the volcanic rocks from the south-western part of GVP, but integrates all the available previous data to understand how magmas evolved in the post-collisional geodynamic condition in the GVP. The initial eruptions were rhyolitic, as domes or pyroclastic deposits (Group 1) and basaltic lavas (Group 2) during the Early Miocene. Group 2 is also represented by large volumes of basaltic-andesitic, trachyandesite-dacite/trachydacite lava flows, small intrusions, and pyroclastic deposits as generated between 21 and 14 Ma. Relatively low 87Sr/86Sr ratios (0.705–0.706) of the early rhyolites (Group 1), similar to all the rocks from Group (2), suggest the generation of hybrid melts with variable contributions of mantle-derived and crustal material. The volcanic activity ends with OIB-like basalts (11–7 Ma) showing the lowest 87Sr/86Sr (∼0.703) suggesting an asthenospheric origin.The geodynamic model, based on post-Cyprian slab rollback, results in long-term (22–13 Ma) post-collisional delamination/drip processes that support magmatism. This magmatism is generated in the lithospheric mantle, with the formation of basaltic melts and acid hybrid melts showing variable contributions of mantle-derived and crustal materials in a complex trans-crustal magma plumbing system. Complex mixing of various intra-crustal magmas and fractional crystallization processes generated a huge volume of volcanic rocks. The Late Miocene small-volume OIB basalts were asthenospheric melts that during the late stage recorded localized decompression melting processes.

Geochronology and Origin of Quaternary Dacites from the Daliuchong Volcano in the Tengchong Volcanic Field (TVF), SE Tibetan Plateau

Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O &lt; 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks.

Petrogenesis of Early Permian within-plate-type gabbros in the Western Qiangtang Terrane, Tibet: partial melting of lithospheric mantle metasomatized by subducted oceanic sediments on the northern margin of the Gondwana

ABSTRACT The extensively distributed Early Permian mafic rocks in the Qiangtang Terrane of the central Tibetan Plateau are the magmatic records of separation of this terrane from the Gondwana. Nevertheless, the origins and mantle source characteristics of these rocks continue to be subjects of debate. This study reports zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical and Sr-Nd-Hf isotopic data on gabbros exposed in the Zougou Region, western Qiangtang Terrane. Our zircon U-Pb isotope age suggests that the Zougou gabbros (ZGs) were formed at ca. 290 Ma, representing newly discovered Early Permian mafic rocks within the Qiangtang Terrane. The ZGs display geochemical affinities with within-plate-basalts given that they are tholeiitic and have high Zr/Y and Ti/V ratios. Compared to previously examined Early Permian mafic rocks in the western Qiangtang Terrane, the ZGs are characterized by their distinct, nonradiogenic Nd (εNd(t) = -0.54 to −2.18) isotopic signatures, which cannot be attributed to crustal contamination or assimilation. This assertion is supported by their uniform whole-rock εNd(t), εHf(t), and zircon εHf(t) values, as well as by magma mixing modelling. In conjunction with regional geology, we propose that the ZGs were derived from an isotopically enriched lithospheric mantle metasomatized by previous subduction events. Furthermore, the positive decoupling of Hf-Nd isotopes (ΔεHf(t) = +2.21 to + 2.72), negative Nb-Ta anomalies, and variable Ba/Th ratios within the ZGs indicate that the metasomatic agents were fluids released from oceanic sediments. Our study provides insights into the compositions of the lithospheric mantle beneath the Qiangtang Terrane during the Permian, which is pivotal to discriminating the role of asthenospheric and lithospheric components in the formation of the contemporaneous mafic rocks in response to Gondwana break-up.