Magma Evolution Processes Research Articles

Two distinct early Paleozoic subduction zones in the eastern Central Asian Orogenic Belt: Evidence of subduction recycling and arc evolution

This paper presents zircon U–Pb dating and Hf isotopic, and whole-rock geochemical and Sr–Nd isotopic data for early Paleozoic igneous rocks as well as zircon U–Pb dating for the sedimentary rocks of the Songnen Massif within the eastern Central Asian Orogenic Belt (CAOB), aiming to reveal arc magma evolution process. The zircon U–Pb dating results of the siltstone of the Xiaojingou Formation yielded 450 Ma, 471 Ma, 502 Ma, 773 Ma, 818 Ma and 949 Ma age peaks, as well as multi-episodic Paleo-proterozoic ages (2486-1612 Ma), implying its deposition time is younger than ∼ 450 Ma. The zircon U–Pb dating results reveal that the Songnen Massif exists three episodes of volcanic rocks of the middle Cambrian (502 Ma), Middle Ordovician (453-449 Ma) and the Silurian (435-420 Ma). The middle Cambrian basalts are low-K tholeiitic to calc-alkaline series, weakly depleted εHf(t) and εNd(t) values. They were derived from the weakly depleted mantle metasomatized by slab fluid in an immature island arc setting. The Middle Ordovician to Silurian volcanic rocks show SiO2 of 49.4–58.1 wt%, plotting into high-K calc-alkaline series, which are also featured by higher Th/Nd and (Hf/Sm)PM ratios, together with moderate depleted εNd(t) values, suggesting their derivation of the depleted mantle wedge metasomatized by sediment-derived melt. In contrast, coeval early Paleozoic volcanic rocks in the eastern Xing’an Massif with the MORB-like εNd(t) values are characterized by an origin of the depleted mantle wedge and the distinct interaction of oceanic crust melt and mantle peridotite. Combined with the variation of calculated crust thickness, we suggested that there was a continental arc in the eastern Songnen Massif, which developed from immaturity to maturity during middle Cambrian to Silurian, while the early Paleozoic island arc in the eastern Xing’an Massif is characterized by ocean ridge subduction process.

Fluid exsolution during disequilibrium crystallization in the border and wall zones of internally zoned pegmatites

Understanding processes that occur during pegmatite crystallization are important for understanding magma evolution and enrichment processes of e.g. strategic and rare metals in pegmatites. Several processes are involved in the formation of pegmatites, and among these the timing of fluid saturation and exsolution is a critical area of investigation. The primary question focuses on if the timing of these processes influences pegmatite crystallization and the concentration of elements. The understanding of the relationship between fluid dynamics and mineralization is essential to develop comprehensive models of pegmatite formation. The unidirectional tourmaline prisms and quartz-tourmaline intergrowths (QTIs), composed of a central tourmaline surrounded by skeletal tourmaline intergrown with quartz are anisotropic textures, linked to disequilibrium crystallization at high degrees of undercooling. This study aims to trace fluid saturation and evolution in the outer zones of internally zoned pegmatites and to provide more accurate temperature constraints for their crystallization, using the Emmons pegmatite (Maine, USA) as a case study. The methodology comprises detailed inclusion petrography, microthermometric analyses, Raman spectroscopy, and LA-ICP-MS analyses. The results indicate that localized fluid saturation occurs early during the magmatic stage of pegmatite crystallization. At this point, the crystallizing mineral assemblages are composed of feldspar, muscovite, quartz, tourmaline, and minor garnet. The exsolved fluid is similar throughout the host tourmalines and is characterized by an H2O-NaCl-CO2-(N2-CH4) composition. Temperatures, which are obtained from primary fluid inclusion isochores vary between 315 °C and 440 °C and confirm that the pegmatite crystallized at undercooled conditions. A decrease in temperature occurs on a centimeter scale between the central tourmaline and the skeletal tourmaline of the QTIs, which could be caused by a nucleation delay within individual boundary layers that are formed around the central tourmaline of the QTI. The higher concentration of water and other incompatible components, including fluxes, in the boundary layer led to fluid exsolution localized at the crystal-melt interface. The increased concentration of fluxes causes a nucleation delay, increasing the degree of undercooling and consequently leading to the skeletal morphologies seen in the QTIs. Furthermore, studied melt inclusions indicate that a compositionally modified melt was present during tourmaline crystallization, confirming the existence of a boundary layer at centimeter to decimeter scale.

Zircon U–Pb ages and O–Hf isotopes of Quaternary trachytes from the East Sea: Implications for the genesis of low-δ18O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts, including Dokdo (DD), Ulleungdo (UD), Simheungtack (ST), Anyongbok, and Isabu in the southwest of the East Sea back-arc basin. In this study, we present whole-rock geochemical, zircon U–Pb age, and in situ O–Hf isotope data for the submerged volcanic rocks from DD, UD, and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes. All samples used in this study were trachytes and exhibited ferroan, alkalic, and metaluminous to weakly peraluminous characteristics. They showed light rare earth element (REE)-enriched patterns with (La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot. In addition, they were enriched in large-ion lithophile elements and high field strength elements; they exhibited positive Pb anomalies and strongly negative Ba, Sr, P, and Ti anomalies. The zircons yielded a weighted-mean 206Pb/238U age of 2.61, 0.348–0.704, and 2.76–2.94 Ma for the DD, UD, and ST trachytes, respectively. All zircons exhibited lower δ18O values than normal depleted mantle values, regardless of the crystallization age and spatial distribution of volcanoes. The δ18O values showed no correlation with U contents or Th/U ratios, indicating that the low δ18O signatures were of primary magmatic origin. The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive εHf values. Binary O–Hf mixing modeling revealed that low-δ18O rocks with positive εHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ18O and positive εHf signatures and an enriched mantle source with normal δ18O and negative εHf values. The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting. In contrast, the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks (Triassic–Jurassic granitoids), resulting in increased δ18O values and decreased εHf values relative to those of the UD and ST volcanoes. Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Geochemical comparison between oceanic and continental arc volcanic rocks: Insights to arc magmatism

Compositional differences between continental and oceanic arc volcanic rocks have long been recognized; however, our understanding of them is incomplete. This study presents a comprehensive geochemical comparison, including major, trace and Sr–Nd–Pb isotope compositions, between global continental and oceanic arc volcanic rocks. Our data compilation reveals that volcanic rocks from a single oceanic arc occupy basaltic to basaltic–andesitic average lithochemical compositions, except for volcanic rocks from the Lesser Antilles arc, which occupies an andesitic average lithochemical composition, similar to the bulk composition of volcanic rocks from single continental arcs. The composition of the average continental arc volcanic rocks, the average Lesser Antilles arc volcanic rocks and the average oceanic arc volcanic rocks are different in many ways, which span the entire compositional range from basalts to rhyolites, and are reflected in major, trace element and Sr–Nd–Pb isotope compositions. Qualitative geochemical analyses and semi‐quantitative modelling suggest that the compositional differences in mafic arc volcanic rocks can be accounted for by differences in source mixing between mantle wedge peridotite and slab‐derived crustal components. Slab melts‐peridotite source mixing governs most of the lithochemical features of the continental arc and the Lesser Antilles arc mafic volcanic rocks, which is especially true for Sr–Nd–Pb isotopes. On the other hand, such source mixing occurred to a lesser extent in the petrogenesis of oceanic arc mafic volcanic rocks. Magma evolution processes such as fractional crystallization may further modify the lithochemical composition of the arc volcanic rocks, especially the felsic ones, but this is not the governing factor.

Basaltic Lava Characteristic in Goa Pandan Area, Sukadana, East Lampung: Inferences from Stratigraphy and Petrography Analysis

The presence of extensive basalt formations in East Lampung has its own attractions in the field of geology, particularly the presence of Goa Pandan as a lava cave and tourist attraction in East Lampung. The presence of basalt lava in the southern part of the South Sumatra Basin has drawn attention to the presence of this lava, but detailed research on the characteristics of the lava and its formation process is still very rare. This study aims to determine the eruption period of the basalt lava and magma evolution process based on the correlation between lava stratigraphy and petrography analysis. Field observations show a lava sequence that forms Goa Pandan. Each lava sequence is characterized by autobreccia and vesicular structures on the surface. In addition, the presence of columnar joints, sheeting joints, massive lava, and other additional structures indicate the characteristics of low-viscosity basalt lava. The presence of mineral structures and abundance under the microscope clearly shows the magma formation process when basalt lava flowed on the surface. Resorption-overgrowth of plagioclase and pyroxene minerals indicates an open system when basalt lava flowed on the surface. In addition, the presence of zoning and patching in plagioclase minerals indicates that magma variability is influenced by temperature. The documentation of this lava stratigraphy can serve as a basis for further understanding of magma characteristics and formation processes. There is still much geological work that can be done in the research area to get a detailed picture of the evolution process of magma and the presence of basalt lava in this area.

Magma evolution and growth process of the Ushikiri-yama granodiorite body a member of the Cretaceous Northern Kyushu batholith, SW Japan: Implication for the crustal forming and growth of the initial Cretaceous plutonic activity in northern Kyushu along the East Asian active continental margin

The ascending and emplacement processes of granitoid magma combined with magma genesis are essential in intrusive rocks of various scales. This study is performed to reconstruct and re-evaluate the formation of the Ushikiri-yama granodiorite body, a small-scale plutonic body in the Northern Kyushu batholith. The Ushikiri-yama granodiorite occurs as a laccolith, which can be classified into the south and north bodies. Both bodies are situated along the low-angle rigidity boundary of the host rocks between crystalline limestone and pelitic-psammitic metamorphic rocks. Magmatic foliation and lineation reveal that the granodiorite magma extends to the south and north from the central region of the Ushikiri-yama granodiorite to form a laccolith. The zircon UPb dating of the south and north bodies presents ages of 112.1 ± 0.8 and 110.8 ± 0.6 Ma, respectively, which indicates that the granodiorite magmas are active at different pulses. These ages correspond to the initial stage of the Northern Kyushu batholith. The Ushikiri-yama granodiorite magma ascends from the source region through the same conduit with at least two pulses and is located at the shallow crustal level, utilizing the low-angle rigidity boundary to form a small-scale laccolith as a pioneering magmatic activity of the Northern Kyushu batholith. As the northern region of Southwest Japan and the southern region of the Korean Peninsula during the Cretaceous are assumed to be situated within the extensional setting due to slab roll-back, the results of this study provide new insights into the complicated batholith formation processes and crustal evolution under the extensional regime.

Magmatic Evolution of Dago Volcano, West Java, Indonesia

Dago Volcano is a product of Miocene Sunda Arc volcanism located southeast of the capital city of Jakarta. The morphological change from flat lava flow to steeper lava morphology implies a process of magma evolution under Dago Volcano. This research provides an overview of the magma evolution that occurs on this volcano. The methods used include volcanostratigraphic analysis, petrographic analysis, mineral chemistry, and whole-rock geochemistry. The volcanostratigraphy of Dago Volcano is composed of two eruption centers and a flank eruption forming lava and cinder cones products. The mineralogical associations of Dago Volcano products include plagioclase, olivine, and clinopyroxene. The mineral textures of Dago edifices show zoning, sieve, and reaction rims textures. Geochemically, the Dago Volcano product has a magma affinity of med-K calc-alkaline with quite high levels of MgO, Ni, and Cr approaching the characteristics of primitive magma. The magma evolution process of Dago Volcano includes fractional crystallization and magma mixing which originates from the same magma source.

U–Pb geochronology and geochemistry of dikes in the Changbaishan Tianchi volcanic field (NE China) and their relations with the coeval Jingbohu and Longgang monogenetic volcanic fields

ABSTRACT The plain around the intraplate Changbaishan Tianchi volcano in NE China includes monogenetic cones whose shallowest plumbing system consists of dikes, and three main phases of activity are recognized since 2.8 Ma. The monogenetic volcanism mainly developed in the shield-forming phase (2.8–0.3 Ma), but the age and geochemical features of many of these vents remain poorly known. Here, we present geochemical, Sr‒Nd‒Pb isotopic and zircon trace element data from two dikes feeding monogenetic vents located north and northwest of the Changbaishan Tianchi volcano. Our aim is to constrain the emplacement age, the magma evolution processes, and the mantle source. We compare our results with those from the Jingbohu and Longgang monogenetic vents located north and west of the Changbaishan Tianchi volcano to investigate possible mantle heterogeneities in the recent NE China volcanism. The two Changbaishan dikes have 230Th disequilibrium-corrected ages of 1.27 and 1.00 Ma and show a basaltic composition with a potassic affinity. The Mg# values are 35.48–34.27 and 66.09–65.79. The whole-rock compositions, trace element compositions, and Sr‒Nd variations are consistent with fractional crystallization and minor contamination with lower crustal material, indicating an OIB-like mantle source with an EMI component possibly related to fluids released from the underlying Pacific flat slab. The melting depth determined from Na/Ti and Sm/Yb is between 140 and 180 km, i.e. the asthenospheric mantle. The dikes are characterized by (K2O+Na2O)/δEu, La/Ba, Ba/Nb, La/Sm, and La/Nb values similar to those of the Jingbohu and Longgang basaltic rocks. We conclude that the NE China volcanism at approximately 1 Ma was related to a single, relatively homogeneous OIB-like mantle source modified by ascending slab-derived fluids. According to the available geophysical data, the ascending magmas stopped at the mantle/lower crust interface and experienced limited crustal contamination.

Conditions and Magmas Sources of Summit and Flank Eruptions of the Klyuchevskoy Volcano in 2020–2021: Isotope (Sr-Nd-Pb-O)-Geochemical Data

In 2021 a unique event occurred on Klyuchevskoy volcano (Kamchatka). After more than 30 years dominance of terminal eruptions a flank vent was formed. It was named after corresponding member of the Academy of Sciences USSR G.S. Gorshkov. The eruption began immediately after the end of the summit crater activation in 2020–2021 at an altitude of 2850 m in the northwestern part of the volcano, where no manifestations of flank volcanism were observed earlier. This paper presents geochemical and isotopic Sr-Nd-Pb-O data for lavas of the summit and flank eruptions of the Klyuchevskoy volcano in 2020–2021. A comparative petrographic analysis was carried out and the chemical composition of the Ol, Cpx, Pl phenocrysts of these lavas was analyzed. The lavas of both eruptions are alumina andesitic basalts of normal alkalinity. Variations of basic oxides in the lavas of the summit eruption and in the lavas of the G.S. Gorshkov vent are SiO2 53.1–53.2 wt. % and 51.6–53.2 wt. %, MgO 5.6 wt. % and 5.5–6.0 wt. %; respectively. Estimates of temperatures and pressures of plagioclase crystallization showed that for lavas of the summit eruption they were 1210–1118°С and 12.3–3.6 kbar, for lavas of the flank vent 1203–1119°С and 9.0–3.3 kbar. The content of the main elements, the proximity of the conditions of plagioclase generations, and the features of variations in the chemical composition of Ol, Cpx, and Pl phenocrysts in the lavas of both eruptions indicate the genetic relationship of the magmas that fed the summit and flank eruptions. The lavas of the summit eruptions of 2016, 2020–2021, as well as the lavas of the previous summit eruptions of the Klyuchevsky volcano, are characterized by highly sustained Sr-Nd-Pb isotopic characteristics: 87Sr/86Sr = 0.703625–0.703626, 143Nd/144Nd = 0.513085–0.513102, 206Pb/204Pb = 18.3148–18.3179). Isotopic ratios 207Pb/204Pb (15.5022–15.5107) and 208Pb/204Pb (37.9597–38.0143) are significantly higher for the lavas of the last summit and flank eruptions than for all Klyuchevskoy’s lavas of the past, which indicates more complex processes of magma evolution at crustal levels. The values of δ18O = 6.49–7.39 rel. units (SMOW) in the lavas of the considered eruptions are consistent with previously published data on the Klyuchevsky volcano. The lavas of the Gorshkov vent are enriched with Ba, Zr, Sr and other incompatible elements at constant values of MgO in comparison with the lavas of the last summit eruptions, which indicates differences in the ways of evolution of their magmas. Sharply increased values of the 87Sr/86Sr ratio (0.703673–0.703743) in the lavas of the G.S. Gorshkov vent, not previously observed in the lavas of the Klyuchevsky volcano, testifies to intensive assimilation of crustal matter by the initial melts in the northwestern part of the volcano.

Magma Source and Petrogenesis of the Early Cretaceous Granites in The Liaodong Peninsula: Evidence from In Situ Apatite Sr-Nd and Zircon Hf-O Isotopes

Apatite Sr-Nd and zircon Hf-O isotopes are broadly used to trace magma sources and constrain magma evolution processes, further improving our understanding of the origin of granitoids. We present zircon U-Pb ages, whole-rock major and trace elements, and whole-rock Sr-Nd-Hf, zircon Hf-O, and apatite Sr-Nd isotopic data for the coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry in the Yushulinzi pluton in the Liaodong Peninsula, the eastern North China Craton, to establish their magma sources and petrogenesis. The coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry were formed contemporaneously, with zircon U-Pb ages of 123–119 Ma. They share enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions, and the coarse-grained quartz monzonite has crust-like δ18O values (5.7–6.7‰). The coarse-grained quartz monzonite and biotite monzogranite have variable apatite (87Sr/86Sr)i ratios and negative apatite εNd(t) values. These isotopic characteristics indicate that the different rock types in the Yushulinzi pluton were derived from the partial melting of ancient crustal material in the North China Craton. Their geochemical and petrographic characteristics indicate that the crystal-melt segregation model can be employed to elucidate the genetic links among different rock types, with the coarse-grained quartz monzonite representing crystal accumulation and the biotite monzogranite and granite porphyry representing interstitial melts extracted from a crystal-rich magma chamber. Furthermore, the variable apatite Sr isotopic compositions and subtle differences in the peak zircon εHf(t) values of the studied rock samples confirm the possibility of a contribution from shallow crustal components and materials with high εHf(t) values during magma evolution, which is not readily revealed by their whole-rock Sr-Nd-Hf isotopic compositions. These results demonstrate that in situ apatite Sr-Nd and zircon Hf-O isotopic analyses have the potential to provide distinctive insights into the magma sources and evolution of magmatic systems.

Chalcophile element geochemistry of the Bijigou layered intrusion, northern margin of the Yangtze Block: Implications for the processes of magma evolution and sulfide segregation

毕机沟层状岩体是扬子地块北缘汉南杂岩中最重要的镁铁-超镁铁质侵入体之一,主要由下部带超镁铁质岩、中部带辉长岩和上部带闪长岩组成。本文对该岩体下部带橄长岩、橄榄辉长岩和橄榄辉长苏长岩以及中部带粗粒辉长岩、磁铁辉长岩和角闪辉长岩进行全岩主微量元素及亲铜元素分析,全岩 (La/Sm)_N(0.43~2.89)和(Tb/Yb)_N (1.08~1.52)比值以及La/Yb-Sm/Yb图解均表明毕机沟层状岩体源区属于尖晶石二辉橄榄岩。Ni/Cu-Pd/Ir图解显示毕机沟层状岩体母岩浆主要为高镁玄武岩,模拟计算及高的S/Se比值显示其母岩浆经历了~5%的地壳混染并引进了外界硫,导致毕机沟母岩浆在深部发生早期硫化物熔离,造成其极高的Cu/Pd比值(5.21×10³~1.67×10⁶)和低的PGE含量。毕机沟母岩浆侵位到浅部岩浆房后,下部带极少量的硫化物熔离进一步导致残余岩浆亏损PGE,但S和Cu含量相对升高;中部带从下部带残余岩浆中结晶分异,具有更低的PGE含量和相对较高的S以及Cu含量。根据毕机沟层状岩体岩浆演化及硫化物熔离过程,推测岩体深部及附近具有寻找Cu-Ni-PGE矿床的潜力,查明其岩浆通道系统及岩浆运移方向对找矿勘查具有重要意义。

Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong (SE Tibet): Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (εHf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high εHf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon εHf and zircon δ18O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high εHf and low δ 18O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-εHf and low- δ 18O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.

Distinct magma evolution processes control the formation of porphyry Cu–Au deposits in thin and thick arcs

Porphyry Cu–Au deposits are the major global source of copper, an essential metal for the green transition, and a significant source of gold. They occur both in thick continental and thin oceanic arcs at depths between ∼1 and ∼6 km above parental magma chambers situated at ∼5–15 km depth. Although it is believed that the metal precipitation processes for these deposits are the same in thin and thick arcs, it is not clear why porphyry deposits in these two environments display different Cu and Au endowments and different Au/Cu ratios. Using mass balance petrological modeling, I argue that porphyry Cu–Au deposits in thick and thin arcs form by two distinct magmatic evolution precursors. In thick arcs porphyry Cu–Au deposits are tied to the deep crust build-up of large volumes of magmas, volatiles and metals. These large volumes of magmas are H2O-undersaturated and need to rise to shallow level to exsolve fluids and metals. In contrast, porphyry Cu–Au deposits in thin arcs form by little differentiated mantle-derived magmas that rise directly to shallow crustal levels where they exsolve fluids and metals. These two distinct magmatic pathways of porphyry Cu–Au generation are ultimately controlled by the different arc thicknesses in the two environments, even though the processes of metal precipitation in the actual deposits of the shallow crust are similar. The model discussed here may stimulate the development of new tools for the exploration of porphyry deposits in the distinct environments of thick and thin arcs.

Late mesozoic magmatism in the Gucheng district: Implications for REE metallogenesis in South China

Regolith-hosted rare earth element (REE) deposits are common in South China. The newly-discovered Gucheng REE deposit in the western part of Guangdong Province, is characterized by HREE enrichment (with ∑HREE oxides of 55%). Three types of granites have been identified in the Gucheng ore district, a REE-fertile coarse-grained biotite granite (CGBG), a weakly REE-fertile fine-grained biotite granite (FGBG) and the barren medium-grained biotite granite (MGBG). New LA-ICP-MS zircon U-Pb dating reveals that the MGBG (107.7 ± 0.4 Ma), CGBG (103.2 ± 0.6 Ma) and the FGBG (98.9 ± 0.7 Ma) were all formed in the Late Cretaceous.Mineralogical and geochemical data indicate that the CGBG is an unfractionated I-type granite, whereas the FGBG and MGBG are fractionated I-type granites. The CGBG and FGBG have (87Sr/86Sr)i and εNd(t) ranging from 0.7121 to 0.7140 and −8.6 to −8.0, and 0.7125 to 0.7131 and −8.2, respectively. The source of these granites was mainly composed of two end-members: one having 90% Paleoproterozoic basement and another 10% enriched mantle metasomatized by subduction-related melts. The barren MGBG is also derived from partial melting of a Paleoproterozoic source with a minor contribution of enriched mantle. The involvement of enriched mantle can provide REE into the magmas, which plays an important role in REE enrichment. Geochemical data show that allanite have been crystallized during magma evolution process of all the three types of granites. However, no allanite has been discovered in the MGBG, and the FGBG contains lower abundance of allanite than that of the CGBG, which is due to different degrees of fractionation. The crystallization and separation of allanite has greatly affected the LREE of the three types of granites, which further influences their potential for LREE mineralization. Lower K/Rb and Nb/Ta ratios of the CGBG, FGBG and MGBG indicate that these three types of granites were affected by hydrothermal events after their crystallization. Zircon geochemical data show that the MGBG was formed in a lower oxygen fugacity environment than the CGBG and FGBG. Above all, involvement of enriched mantle, a relatively high oxygen fugacity, crystallization and separation of allanite as well as involvement of hydrothermal events after magmatic crystallization are important factors controlling REE mineralization at Gucheng, of which the hydrothermal events may play the most important role.

Geochronology, geochemistry, and petrogenesis of Early Cretaceous highly differentiated I-type granites in the central Great Xing’an Range, northeastern China

In this study, we undertook zircon U–Pb dating and Hf isotopic and whole-rock geochemical analysis of Mesozoic granites from the Zhalantun and Arxan areas of the Great Xing’an Range, northeastern China. Zircon U–Pb dating of four samples indicates they formed in the Early Cretaceous (ca. 140.7–130.0 Ma). Geochemically, the samples have high silica (74.00–77.29 wt.%) and total alkali (8.52–8.90 wt.%) and low CaO (0.21–0.71 wt.%), MgO (0.06–0.48 wt.%), and P2O5 (0.01–0.06 wt.%) contents and exhibit the weakly peraluminous (A/CNK = 0.98–1.06) and high-K calc-alkaline characteristics, with highly differentiated I-type granite affinities (differentiation index = 93.19–96.93). The studied granites are enriched in light rare earth and large ion lithophile elements and depleted in heavy rare earth and high field strength elements. Their zircon εHf(t) values are mainly positive (+2.49 to +11.64) and yield two-stage Hf model ages of 1023–445 Ma. The geochemical and isotopic data indicate they might have been derived from the partial melting of medium- to high-K basaltic crust material during the Mesoproterozoic to Phanerozoic. They underwent fractional crystallization in the magma evolution process, and K-feldspar and plagioclase were the major crystallization phases. Combined with the temporal–spatial migration of the Mesozoic magmatic events in the whole northeastern China, we propose that the generation of Early Cretaceous highly differentiated I-type granites in the central Great Xing’an Range was associated with the delamination effect caused by the roll-back of the Paleo-Pacific plate.

Apatite U-Pb Dating and Composition Constraints for Magmatic–Hydrothermal Evolution in the Giant Renli Nb-Ta Deposit, South China

Apatite is a nearly ubiquitous accessory phase in igneous rocks that crystallizes during the entire magma evolution process and has great implications for geochronology and petrogenesis. Previous studies suggested that Nb-Ta mineralization in the giant Renli deposit was genetically related to Late Jurassic two-mica monzogranite or Early Cretaceous muscovite monzogranite. Moreover, the magmatic–hydrothermal evolution of these two stages is poorly understood. In our study, we confirm that the muscovite monzogranite, biotite monzogranite, and two-mica monzogranite are all spatially associated with Nb-Ta pegmatites. We present new apatite U-Pb ages to constrain the timing of Nb-Ta mineralization and related magmatism. The results show that apatite from the two-mica pegmatite yield a lower intercept age of 130 ± 2 Ma (2σ), and apatite grains from two two-mica pegmatite samples yield a lower intercept age of 135 ± 8 Ma (2σ) and 134 ± 3 Ma (2σ), respectively. Apatite and whole-rock geochemistry suggest the oxidation degree of the Nb-Ta mineralization increases from north (RL-6) to south (RL-16) in the giant Renli deposit. This study demonstrates that a combination of apatite composition and U-Pb ages can be used to constrain the magmatic–hydrothermal evolution of granite and pegmatite-type Nb-Ta deposits.

Role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Island Ophiolite suite of India

The Mesozoic‐age Andaman Island Ophiolite located in the southeastern fringe of the Indian plate margin is one of the well‐preserved supra‐subduction‐zone oceanic lithospheric slices. It is tectonically associated with the Indonesian arc system and is an extension of the Phanerozoic Indonesian‐Myanmar orogenic belt. Thus, it offers a unique opportunity to study oceanic crust formation and specify magma evolution processes in the Tethyan subduction system. The volcanics of the Andaman Islands ophiolite display a wide compositional variation. Rock types vary from basic (basalts) to intermediate (basaltic‐trachyandesites, basaltic andesites) to acidic differentiates like dacites and rhyolites. This contribution presents the petrochemistry of the intermediate rock types—basaltic‐trachyandesites, trachyandesites and acidic—dacites and rhyodacites of Andaman Island Ophiolite (India). This study examines the role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Ophiolite suite. Plagioclases and pyroxenes are the major mineral constituents of these rocks on petrographic observation. Plagioclases of the intermediate rocks are of albite composition Ab98‐99. The acidic rocks show compositional variation from albite to oligoclase (Ab83–99). Analysed pyroxenes of intermediate and acidic rocks are augite (Wo46–49En34–41Fs13–17). The studied calc‐alkaline rocks show variable abundance of Large‐Ion Lithophile Elements and display strong positive Pb and negative Ti anomalies. The chondrite‐normalized Rare Earth Elements display an enriched pattern with varying degrees of enrichment. Clinopyroxene thermobarometry reveals that the intermediate magma and acidic magma becomes saturated with clinopyroxene at 1,062–1,133°C and 1,016–1,079°C, respectively, at 2 kbar and H2O = 2 wt%. Petrochemical findings and model calculations using PELE are consistent with a derivation of intermediate rocks from the basaltic melt by fractional crystallization. In contrast, acidic rocks are the product of partial melting of hydrothermally altered oceanic crust. The association of basaltic and intermediate rocks suggests a relatively stable magma chamber to generate differentiated intermediate magmas by fractional crystallization.

Magma Evolution Processes in the Southern Okinawa Trough: Insights from Melt Inclusions

Magma Evolution Processes in the Southern Okinawa Trough: Insights from Melt Inclusions

Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China

Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.

Melt volatile budgets and magma evolution revealed by diverse apatite halogen and trace elements compositions: A case study at Pulang porphyry Cu-Au deposit, China

Apatites those shielded as inclusions in igneous minerals are avoided of diffusion effects associated with evolving melts and can record the original volatile and trace element evolutions of the magma. Volatile and trace element compositions of apatites enclosed in high-Al, medium-Al, low-Al amphibole, magmatic biotite, plagioclase, and groundmass from ores, quartz monzonite porphyry (QMP) and its mafic microgranular enclaves (MMEs), and pre-ore coarse-grained quartz diorite porphyry (CQD) were present to evaluate the magmatic processes in the formation of Pulang porphyry Cu-Au deposit. All apatite inclusions are magmatic fluorapatites and occur as early-crystallization phases (940–980 °C), followed by the crystallization sequence of high-Al amphibole, medium-Al amphibole, magmatic biotite, plagioclase, low-Al amphibole (altered medium-Al amphibole) with fluctuating ƒO2 conditions. Compared to the low SO3 (<0.27 wt%) concentrations of other apatites inclusions in fertile QMP, the biotite-hosted apatites retain an abnormal SO3 (0.15–1.01 wt%) and Cl (0.08–0.71 wt%) enrichment. Estimates of sulfur and chlorine concentrations using partitioning models for apatite in QMP return similar features, where the biotite-hosted apatites yield highly variable melt S (315–2025 ppm) and Cl (0.1–1.2 wt%) contents. The presence of biotite-hosted S-Cl enriched apatites with overgrowth texture, together with the resorbed medium-Al amphiboles in QMP, both suggest that they have crystallized from an extra sulfur supplement process caused by the injection of a series of S-Cl enriched magma, and even subsequent remelting of the preexisting dioritic batholith and its interior apatites. Such magma processes further account for the fluid exsolution and alteration from medium-Al amphiboles to low-Al ones. Whereas, those from the barren CQD systematically returns normal SO3 (0.18–0.70 wt%) and Cl (0.03–0.51 wt%) concentrations with estimates of melt S (335–1321 ppm) and Cl (0.1–0.5 wt%) contents exhibiting features of typical arc basalts (300–1000 ppm; 0.01–0.85 wt%, respectively). Although the acicular apatite of MMEs has normal SO3 contents (0.19–0.37 wt%), its abundance also confirm an extra sulfur supplement process. The altered apatites of both QMP and CQD showing pitted surface with visible voids and LREE-rich mineral inclusions return low SO3 (<0.06 wt%) and Cl (<0.22 wt%) concentrations, indicating that these elements were depleted during potassic and phyllic alteration.Given that Sr/Y ratios (2.21–3.62) of apatite at Pulang do not vary significantly with changing melt composition (Mg = 119.1–528.8 ppm), the Sr/Y and Eu/Eu* ratios of apatite from QMP have confirmed a water-rich, oxidized magma origin, whereas the CQD are originated from a water-poor, reduced source. Besides, the hydrothermal alteration possibly accounts for the abnormally increased Ce/Ce* ratios of altered apatites from CQD and decreased Sr/Y ratios of the low-Al amphibole-hosted apatite from QMP. Therefore, using trace elements as indicators for petrogenetic studies would be more robust for those apatites enclosed in igneous minerals. Our studies demonstrate that apatite inclusions can be linked to discrete periods in the crystallization history of its host phases, thus providing insight into the magma evolution process.