Assimilation And Fractional Crystallization Research Articles

Sr- and Nd-Isotope Geochemical Effects of Assimilation and Fractional Crystallization (AFC) Processes in the Heterogeneous Crust: The Example of Ferrobasites from the Ladoga Graben (Karelia, Russia)

Sr- and Nd-Isotope Geochemical Effects of Assimilation and Fractional Crystallization (AFC) Processes in the Heterogeneous Crust: The Example of Ferrobasites from the Ladoga Graben (Karelia, Russia)

Petrogenesis of the granitoids related to skarn-type mineralization in the Nyainqentanglha Metallogenic Belt, Tibet

The Nyainqentanglha Metallogenic Belt (NMB) is an economically important lead-zinc ore province located in the Central and Southern Lhasa subterrane, Tibet. The NMB consists mainly of skarn-type lead-zinc polymetallic deposits that form at the contact between Late Cretaceous-Eocene intrusive rocks and carbonatite/volcanic-sedimentary strata. These deposits are generally considered to be related to S-type granites formed by the Indo-Asian continental collision. However, the εHf(t) (ave. −1.6) of zircons from the mineralization-related granitoids indicates that it is crust-mantle mixing products. In addition, the volcanic rocks of the Dianzhong Formation, which have a close spatio-temporal relationship with the skarn-type deposits, typically show mantle-derived features. Therefore, the occurrence of AFC processes in mantle-derived material passing through the thick basement of the Lhasa Terrane may better explain the genesis of the mineralization-related granitoids. In this study, we present results on zircon U–Pb ages, major and trace elements of the granitoids associated with skarn-type mineralization from the Narusongduo district. LA-ICP-MS zircon U–Pb dating shows that the granite formed in the Paleocene (58.6 ± 0.5 Ma). Combined with published regional data, we propose the mineralization age of the skarn-type deposits in the NMB ranges from the Late Cretaceous to Eocene. Detailed petrology and geochemistry of mineralization-related granitoids are evaluated to constrain the magmatic evolution process. The granitoids have high contents of SiO2 (mean 72.8 %), K2O (mean 4.0 %), Rb (mean 186.9 ppm), DI (differentiation index) 84.7, A/CNK (mean 1.3), and low contents of MgO, TFe, suggesting that the granitoids have undergone strong differentiation. In addition, the continuous decrease of P2O5 with progressive differentiation and the lower average P2O5 abundance suggest that the mineralization-related granitoids belong to the I-type granite. The results suggest that the mineralization-related granitoids in the NMB originated from the mantle and is the fractionated I-type granite formed by the process of AFC (Assimilation and Fractional Crystallization). Considering that Pb and Zn often coexist in deposits, we suggest that magmatic differentiation may play an important role in the formation of the granite-related Pb and Zn mineralization.

Monazite record of assimilation and differentiation processes in the petrogenesis of Himalayan leucogranites

The Himalayan leucogranites have received particular attention as they record the continental crust-building processes of a collisional orogenic system. Deciphering the magmatic evolution processes and potential sources is essential for understanding the origin of the Himalayan leucogranites. In-situ monazite Th-Pb ages, trace element geochemistry, and Nd isotope ratios have been determined with whole-rock geochemical data for the leucogranites and their country rocks in the Yadong area, southern Tibet. Monazites in most of the studied leucogranites are marked by multiple age populations with variable REE ratios and wide ranges of Nd isotope ratios, reflecting a complex history of magmatic processes. Abundant inherited monazites are identified in the Yadong leucogranites, and most of them are excluded from the source regions due to their distinguished Nd isotope compositions from the magmatic monazites. The inherited monazites share close affinities in geochronological dates and geochemical compositions with those in the country rocks, strongly suggesting a major origin of the country-rock contamination. This highlights the contamination-assimilation processes in the genesis of the Yadong leucogranites. It is worth noting that xenocrystal monazites from the metasedimentary country rock are less preserved in the leucogranites than those from the granitoid country rock, which probably reflects the different assimilation reactions for different types of country rock. Assimilation of the metapelitic rocks released low content of xenocrystic monazite and high volume of partial melt into the leucogranites, whereas assimilation of the granitoid rocks released high content of xenocrystic monazite. In addition to the contamination-assimilation processes, the uniform Nd isotope data with large variations in REE ratios for magmatic monazites in some leucogranite samples suggest a subsequent closed-system fractional crystallization process during the magma evolution. Besides, some magmatic monazites in the muscovite granites show a significant tetrad effect in the Chondrite-normalized REE patterns, indicating that the magma system has evolved into an aqueous fluid-available environment. This supports an important role of the magmatic differentiation process in the formation of the Yadong leucogranites. Taken together, a petrogenic model involving AFC (assimilation and fractional crystallization) processes is proposed, and the geochemical variations of the Yadong leucogranites are mainly attributed to the post-source processes. It is suggested that the rock geochemistry of the Himalayan leucogranites often dose not directly reflect the nature and the petrogenetic processes in the source regions.

Absence of low-δ18O magmas despite widespread assimilation of altered crust in a large magmatic and hydrothermal province

Magmas with low δ18O values, which require a contribution of isotopically light oxygen from hydrothermally altered rocks, are globally rare despite the common association of large-scale magmatism with intense hydrothermal activity. Here we consider this paradox in the central Taupō Volcanic Zone (TVZ, New Zealand), one of the most intense regions of Quaternary silicic magmatism and hydrothermal activity on Earth. We present new data on phenocryst δ18O values from 62 magma batches erupted since ∼1.6 Ma, ranging from basalt to rhyolite, and match these against existing and new Sr and Nd isotopic data. Basalt δ18Omelt values (typically ≥+6.3 to +6.5‰) calculated from phenocrysts are mostly high relative to mid-ocean ridge basalts (MORB), whereas basaltic andesites extend to +7.1‰. Rhyolite δ18Omelt values are mainly +7.0 to +8.5‰, which is considered high-δ18O in most settings. However, assimilation and fractional crystallisation (AFC) models show a mismatch between the amount of greywacke basement assimilation required to explain the Sr-Nd isotopic ratios and the δ18O values of many TVZ rhyolites. We attribute this mismatch to shallow assimilation of low-δ18O altered rocks after prior assimilation of unaltered greywacke at deeper levels. We infer that interactions between large silicic magmatic systems and shallow altered crustal rocks are common and widespread in the TVZ, despite the absence of silicic magmas with low δ18O values (<∼+6.0‰). This absence reflects the combined influence of high δ18O values inherited from the slab in parental basalts, and the limited isotopic leverage available in the TVZ to drive large 18O depletions via hydrothermal alteration. The TVZ example suggests that large-scale interactions between magmas and altered host rocks are likely to be important in many silicic magmatic provinces and not restricted to the sparse occurrences of low-δ18O magmas. Spatial clustering of the lowest TVZ rhyolite δ18Omelt values (+6.3 to +7.0‰) occurs along a western corridor of presently weak hydrothermal activity, implying that hydrothermal activity was more intense there in the past, and may have represented a focus of heat flow similar to the present-day eastern TVZ margin. Magma δ18O values can thus be useful for tracing areas of past alteration even in settings where conventionally normal to high δ18O values are retained in erupted products.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.

Compositional Evolution of Polygenetic Fissure Volcanic Systems: Insights From the Latest Eruptions at Craters of the Moon Volcanic Field

AbstractThe Craters of the Moon (COM) Volcanic Field in Idaho is the largest mostly Holocene lava field in the conterminous U.S. and the site of the most recent volcanism in the Yellowstone‐Snake River Plain province. This prominent example of a polygenetic fissure field has produced over 60 eruptions over the past 15,000 years from the Great Rift fissure system. The eight most recent lava flows, known as Period A, were erupted between about 2,500 and 2,000 years ago. In this study, major and trace element and Pb‐Sr‐Nd isotope data were obtained, with several samples collected from each flow from widely spaced locations to evaluate heterogeneity in their source magmas. The results reveal three previously unrecognized compositional groups that may represent separate magma batches, and that assimilation and fractional crystallization (AFC) controlled the evolution of the entire Period A suite. The three oldest flows were periodic withdrawals from a body of highly evolved and poorly homogenized magma, and four later flows represent successive samplings from a stratified body of more primitive magma. These two flow groups are related via variable AFC modeled with Magma Chamber Simulator software, with Neogene rhyolite as a plausible crustal assimilant. A third compositional type that requires a different parent magma and AFC history is represented by a single flow erupted from vents a few km away from the others. The compositional evolution and variability observed in this sequence of flows provide new insights into how magmas are generated, modified, and delivered in such polygenetic fissure systems.

Paleoproterozoic Crust–Mantle Interaction in the Khondalite Belt, North China Craton: Constraints from Geochronology, Elements, and Hf-O-Sr-Nd Isotopes of the Layered Complex in the Jining Terrane

The Paleoproterozoic Khondalite Belt, located in the northwestern segment of North China Craton (NCC), is characterized by widespread high-temperature/ultrahigh-temperature (UHT) granulite/gneiss and large-scale magmatic activity. The tectonic evolution is still controversial. Here, we report new geochronological, elemental, and Hf-O-Sr-Nd isotopic data for a Paleoproterozoic layered complex in the Jining terrane to constrain the tectonic evolution of the Khondalite Belt. In situ zircon U-Pb dating indicates that the Sanchakou gabbros were emplaced between ~1.94 Ga and ~1.82 Ga, which might be the heat source of UHT metamorphism. The elemental and Hf-O-Sr-Nd isotopic analysis shows that the formation of Sanchakou gabbros is consistent with the assimilation and fractional crystallization (AFC) process. The magma originates from the 10%~20% partial melting of the spinel + garnet lherzolite mantle. The Sanchakou gabbros are magmatic crystallization products mixed with crustal wallrocks in the magma chamber. We have established a tectonic evolution model involving asthenosphere upwelling after the amalgamation of the Ordos and Yinshan Blocks at ~1.95 Ga.

Origin of silicic rocks of the Deccan Traps continental flood basalt province: Inferences from field observations, petrography, and geochemistry

The origin of silicic rocks (SiO2 > 65 wt%) in Continental Flood Basalt (CFB) provinces could be attributed to complex petrogenetic processes. The 65.5–66 Ma old Deccan Traps CFB contains eight sporadic but significant silicic rock exposures that are studied here in a comprehensive framework using field observations, petrography, major oxides (n = 56), and trace element chemistry. Rhyolite and granophyre, as well as subordinate felsite, ignimbrite, trachyte, pitchstone, and microgranite coexist with volcanic and plutonic mafic rocks such as basalt, basaltic andesite, and gabbro. Multiple isolated and circular/semi-circular hills and linear dykes of silicic rocks are present in the form of lavas with prominent flow folding, rheomorphic ignimbrite, and tuffs. The ‘Rheological Agpaitic Index’ (RAI) indicates that most of the silicic rocks in the Deccan Traps are effusive in nature, except for Rajpipla, Alech, Bombay, and Osham silicic rocks, which are marked by explosive volcanism. Thermodynamic-based Rhyolite-MELTS modelling suggests that the major oxide composition of Pavagadh and Barda basalt is a likely candidate for the parental melt composition of the silicic rocks of the Deccan Traps. Ba, Sr, P, Zr, and Ti anomalies are consistent with the fractionation of K-feldspar, plagioclase, apatite, zircon, and Fe-Ti oxides, respectively. Two broad REE patterns are noticed in the Deccan Traps silicic rocks: a flat pattern for Barda, Alech, and Chogat-Chamardi silicic rocks, and a steep REE pattern for Osham, Rajula, Pavagadh, Rajpipla, and Bombay silicic rocks. Trace element modelling reveals that 5–10 % partial melting of a spinel peridotite source could produce an REE pattern and abundances similar to the associated basalts. Further extensive fractional crystallization (60–90 %) of the parental mafic melt at a deeper depth (where spinel is stable) could produce the REE composition and pattern observed in most silicic rocks except for those of Barda, Alech, and Chogat-Chamardi, which require fractional crystallization of the same parental melt at a shallower depth (where spinel is not stable). The geochemical variability of Deccan Traps silicic rocks reveals an origin from a mantle-derived parental mafic melt that evolved via the assimilation and fractional crystallization (AFC) process to form the silicic exposures, which is typical of silicic volcanism in other global CFBs.

Middle-late Miocene rift-related magmatism in Guaymas, Sonora, Mexico at the eastern margin of the Gulf of California: Petrogenetic implications associated to the Pacific-north American plate boundary

Miocene volcanism in the Guaymas region at the eastern margin of the Gulf of California, in coastal Sonora, NW Mexico, was dominated by arc magmatism. However, the presence of rift-related magmatism is also exposed. Rocks associated with subduction of the Farallon plate beneath North America plate are of Oligocene-Miocene ages, while the magmatic rocks with ages between ca. 12.5 to ca. 6 Ma occur in a post-subduction tectonic setting. This period is sometimes referred to as the proto-Gulf phase. The proto-Gulf magmatic rocks are exposed as part of a volcanic igneous plumbing system, herein named Guaymas Group. The Guaymas Group has an age range between ∼12 and 10 Ma and is defined by two magmatic events: an intermediate compositional pulse (Unit 1) and a felsic pulse (Unit 2).The lithological units from the Guaymas Group have similar geochemical features, exhibiting LILE and LREE enrichment and Nb, Ta and Ti depletion. These rocks show a transitional to tholeiitic composition affinity that differs from the arc magmatism from coastal Sonora. The geochemical data suggest that assimilation and fractional crystallization (AFC) processes played an important role in the geochemical variations observed in volcanic and subvolcanic rocks. The geochemical characteristics of the Guaymas Group rocks also could indicate an origin by decompression melting of a mantle source metasomatized by subduction processes.Finally, considering the geological, geochronological and geochemical study, we suggest that the emplacement of the Guaymas Group rocks is contemporaneous with the oblique rifting environment associated with an incipient Pacific-North American plate boundary.

Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

The Oytag pluton in the West Kunlun orogenic belt of China represents the rare exposed Carboniferous granitoids associated with the early-stage evolution of the Paleo-Tethys Ocean, and its origin is controversial. New zircon U-Pb data suggest an emplacement age of 327–324 Ma for trondhjemite, which slightly predated tonalite (∼314 Ma). These results, along with available published ages, indicate several episodes of magmatism to build the Oytag pluton. Both trondhjemite and tonalite have positive whole-rock Nd isotopic compositions (ε Nd (t) = +4.56 to +8.01 for trondhjemite; ε Nd (t) = +4.68 to +6.15 for tonalite), which positively correlate with whole-rock MgO values. Given the presence of Ordovician inherited zircon grains, variations of whole-rock Nd isotopic compositions are attributed to addition of <10% crustal contaminants during assimilation and fractional crystallization (AFC) processes. Interestingly, trondhjemite and tonalite follow distinct AFC trends, consistent with their different ages. Zircons from two trondhjemite samples have overlapping U-Pb ages but show distinctly different trace elements and internal structures (sector-zoned vs. oscillatory-zoned). The sector-zoned zircons have a less evolved signature of lower Hf contents, higher Eu/Eu* and Th/U ratios than the oscillatory-zoned ones, indicating significant crystal fractionation processes for their parental melts. The more variable Hf contents of the oscillatory-zoned zircons than those of the sector-zoned ones reflect a more dynamic environment where they crystallized. We propose that the sector-zoned zircons were crystallized from locked, interstitial melt in a mush and the oscillatory-zoned zircons from extracted melt. In situ crystal-liquid segregation in a mush chamber thereby could be an efficient approach to the differentiation of Oytag trondhjemite. On the other hand, zircons from tonalite have similar Hf contents to the sector-zoned zircons from trondhjemite, but display slightly higher Eu/Eu* ratios and much higher Th/U ratios. This indicates that tonalite should have experienced much different AFC processes from trondhjemite. Both the episodic magmatism feature and the distinct differentiation processes argue for rises of different batches of magmatism to generate trondhjemite and tonalite in Oytag. Along with the large variations of Hf isotopes (5–6 epsilon units) in zircons, we propose a partial melting of juvenile mafic crust model for the Oytag pluton. Heat from upwelling of asthenosphere may have triggered the partial melting of wet, mafic underplates during the opening of a Paleo-Tethyan back-arc basin. • The Oytag pluton comprises predominant trondhjemite with minor tonalite. • Oytag trondhjemites (327–324 Ma) formed earlier than tonalites (∼314 Ma). • The Oytag pluton was built by different batches of magmas via partial melting.

The role of assimilation and fractional crystallization in the evolution of the Mars crust

Recent findings of evolved martian crustal materials by orbiters and rovers challenged the longstanding notion of an only-tholeiitic basaltic crust. Models exploring intraplate magmatic processes like fractional crystallization show that unrealistic amounts of solid accumulation are sometimes required for the formation of martian evolved materials, including evolved materials analyzed by the Curiosity rover at Gale crater. Assimilation and fractional crystallization (AFC) play an important role in the diversification of magmas on Earth, but has not been explored for its role in the generation of evolved martian compositions. In this study, we conduct a series of thermodynamic AFC models using the Magma Chamber Simulator (MCS) model interfacing with the Rhyolite-MELTS algorithm to better understand the controls on assimilation and whether it contributed to the evolution of the martian crust. We find that AFC models replicate evolved Gale crater targets effectively at different pressures and water contents. AFC likely contributed to martian crustal formation, particularly evolved compositions. Higher crustal temperatures likely lead to a higher volume of evolved magmas formed by AFC processes on early (rather than modern) Mars. AFC may have also occurred during the Amazonian, especially in the fractured upper layers of crust, although in lower volume than during the Noachian.

Newly discovered Early Carboniferous and Late Permian magmatic rocks in eastern Myanmar: Implications for the tectonic evolution of the eastern Paleo-Tethys

Eastern Myanmar is located at the junction of the Changning-Menglian and Chiang Rai-Chiang Mai zone and is a crucial region for constraining the evolution of the eastern Paleo-Tethys. This study presents new zircon U-Pb geochronological, mineral and whole-rock geochemical, and Sr-Nd-Hf-O isotopic data for magmatic rocks from the Sukhothai arc in eastern Myanmar. The rock suites analyzed include 360–355 Ma basaltic rocks and trondhjemitic dikes, and 257–254 Ma volcanic rocks and gabbroic cumulates. The basaltic rocks were derived from partial melting of mélange pair with peridotite and experienced assimilation and fractional crystallization (AFC). The trondhjemitic dikes were formed by partial melting of the basaltic rocks and experienced fractional crystallization at shallow depth. We suggest that andesite and dacite were derived from partial melting of depleted mantle wedge and underwent AFC process. The gabbroic cumulates are a crystallizing phase associated with the melts that produced the coeval volcanic rocks. We propose that eastern Myanmar, Central Tibet, SW Yunnan and Southeast Asia share a similar three-staged magmatic history, forming a ∼ 4000 km long magmatic belt. Stage I records the magmatic events related to subduction of the eastern Paleo-Tethys Ocean during the Early Carboniferous. A back-arc basin was opened during the Late Carboniferous, and extensive subduction-related magmatism was followed since the Permian. Stage II records the igneous rocks formed during the final amalgamation between the Indochina and Sibumasu Blocks during the Late Permian to Middle Triassic. Stage III is defined by the post-collisional magmatism distributed across the suture zone during the Late Triassic.

Magmatic evolution of the host magma of plutonic rocks in the Procellarum KREEP Terrane

The origin of the KREEP (K, Rare Earth Element and P)-rich component of the Mg-suite rocks of the Procellarum KREEP Terrane (PKT), one of three major lunar crustal terranes, is unclear. In an attempt to determine its origin, we estimated the composition of host magmas of plutonic rocks, including Mg-suite rocks and evolved rocks, from the PKT (PKT-host magmas) by using secondary ion mass spectrometry analyses of plagioclase. Calculated partition coefficients for Sr, Ba and Ti between plagioclase and melts, taking into account the anorthite content of plagioclase, temperature and bulk rock major-element compositions, were applied to determine parental magma compositions. The PKT-host magmas contained 160–360 ppm Sr, 520–7600 ppm Ba and 1.1–7.0 wt.% TiO2; most of them had higher Ti and Ba concentrations than KREEP basalts and high-K KREEP. We used phase relations based on the Rhyolite-MELTS algorithm to explore the evolution of the PKT-host magmas and KREEP basalts from two bulk silicate moon (BSM) starting compositions, a BSM with chondritic ratios of refractory elements, and a crustal-component-enriched BSM with non-chondritic ratios of refractory elements. We propose a three-stage evolution model. Stage-1: polybaric multi-step fractionation from a BSM magma to form ferroan anorthosite (FAN) crust and an evolved magma as the first KREEP-rich component (M0). Stage-2: assimilation and fractional crystallization (AFC) of M0, early cumulate and FAN associated with deep mantle overturn and impact events to form the PKT-host magmas, Mg-suite rocks and an evolved magma as the second KREEP-rich component (K0). Stage-3: further AFC cycles of K0, Mg-suite rocks or FAN associated with shallow mantle overturn and impact events to form KREEP basalts. This three-stage model for a crustal-component-enriched BSM with non-chondritic ratios of refractory elements (e.g., a sub-chondritic Ti/Ba ratio) reproduced the compositions of both the host magmas of FAN and the PKT-host magmas that fractionated to form the Mg-suite rocks and KREEP basalts of the PKT region. In particular, the model reproduced the high Ti and Ba contents of the PKT-host magmas we estimated from plagioclase composition. Variations of TiO2 and Ba contents (and hence Ti/Ba ratios) of the magmas were critical controls on their evolution.

Unraveling the petrogenesis of the Miocene La Peña alkaline intrusive complex, Mendoza, Argentina: Insights from the study of the disregarded late dykes

The La Peña Complex (LPC) is a silica-undersaturated alkaline potassic intrusive system, with a subduction-related signature, linked to the early Miocene retroarc magmatism of the Southern Central Andes, in the flat slab segment. The LPC is composed of several intrusions, predominantly plutonic (clinopyroxenite, malignite and syenite), cross cut by a voluminous swarm of radial and annular dikes with mostly volcanic-subvolcanic textures and variable compositions (foid-bearing alkali feldspar trachyte, trachyte, benmoreite, ledmorite, syenite, tephrite, tephriphonolite and alkaline lamprophyre). In the TAS classification these rocks plot in the alkaline series covering a wide spectrum of compositions following two different trends: 1) alkaline (potassic) strongly silica-undersaturated series, from tephrite, phonotephrite to tephra-phonolite, and 2) mid-alkaline, less silica-undersaturated series, ranging from basaltic trachyandesite to trachyandesite (benmoreite), and trachyte. Dikes from the alkaline series show higher K2O/Na2O ratios and Sr, La, Ce, contents compared to those from the mid-alkaline series. Rocks of the alkaline series are richer in K-feldspar, sodalite, leucite (pseudoleucite), biotite, potassic-ferro-pargasite and garnet than the less silica-undersaturated (trachytic) rocks, reflecting a stronger alkaline potassic affinity. A review of geochemical, isotopic and mineralogical data, and a new geochemical modeling performed on the LPC dikes, suggests that both trends represent separated magmatic series that evolved from two different parental magmas lodged ∼30 km deep in the crust. Our results suggest that the compositional variations observed in LPC dikes, cannot be explained by a simple magmatic evolution via fractional crystallization from a unique parental magma, and that an assimilation and fractional crystallization (AFC) process is required to explain some compositional differences. Our results suggest an upper crustal contaminant (evolved rocks) with a Grenvillian isotope signature. On the other hand, analyses of feldspar crystals from the tephriphonolitic dikes indicate local mixing effects, between an evolved tephriphonolitic melt and a less evolved and hotter mafic magma.The origin of both parental magmas could be explained by different melting degrees of the same mantle source, a phlogopite-bearing spinel lherzolite metasomatized by subduction derived fluids. We consider as a possible explanation that alkaline and coeval calc-alkaline magmatism in this part of the Andes, is due to local heterogeneities in the mantle source, and different degrees of partial melting Similar isotopic compositions of the LPC dikes, with those from other Miocene magmatic occurrences with arc-signature and similar age (e.g., Paramillos de Uspallata, Las Máquinas basalt, Abanico Fm and Farellones Fm) suggest an analogous mantle source for these rocks, from arc and retroarc in the Pampean flat slab regions. However, our results suggest that the isotopic trend contamination of LCP is different from that of Paramillos de Uspallata and other arc rocks of the Southern Volcanic Zone. The crustal contaminant of LPC possibly has another composition that those of Precordillera and Principal Cordillera Miocene rocks. The age of LPC rocks (∼19 Ma) and their arc-related signature agree with the eastward broadening of the arc magmatism between 17 and 19 Ma in this part of the flat slab. According to our interpretations, the LPC is a singular occurrence of two alkaline magmatic series on destructive plate margins, associated with calc-alkaline magmatism, occurring closely in time and space.

Linking ocean subduction with early Paleozoic intracontinental orogeny in South China: Insights from the Xiaying complex in eastern Guangxi Province

The tectonic setting of the early Paleozoic orogeny in the South China Block (SCB) has been hotly controversial. Although most studies have ascribed it to an intracontinental orogeny, there are various views on the geodynamic drivers of the intracontinental orogeny. This study reports on the Ordovician Xiaying igneous complex in the eastern part of Guangxi province, southern South China. This complex is composed of granodiorite, monzogranite and micro-granular diorite enclosed by granodiorite. The zircon U–Pb dating suggests that these three types of rocks emplaced synchronously at around 464–442 Ma, significantly older than those S- and I-type granites (450–400 Ma) distributed in the interior of the SCB. One monzogranite collected in the border of the complex has an older age (480 Ma). The Xiaying hornblende–bearing granodiorites have low SiO2 (61.8–68.7 wt%), P2O5 (0.07–0.10 wt%), Rb/Sr (0.52–1.30) and Rb/Ba (0.13–0.34) and high Fe2O3t (3.84–5.57 wt%), MgO (1.48–2.35 wt%), CaO (2.28–4.54 wt%) and Na2O/K2O (0.90–1.91), with metaluminous to weakly peraluminous features (most A/CNK = 0.87–1.07), showing typical I-type granitoid characteristics. The Nd–Hf isotopic model ages (ca 1.6–1.8 Ga) indicate that the Xiaying granodiorites were probably formed by partial melting of a late Paleoproterozoic basement in the deep crust. The micro–granular dioritic enclaves (MMEs) have similar formation ages and Nd–Hf isotopic compositions to the host granodiorites, and show geochemical evolution relation with granodiorites, suggesting that they resulted from the mixing of mafic and granodioritic magmas, which derived from a depleted mantle and a Paleoproterozoic crust, respectively. Based on their similar formation ages and zircon Hf-isotope compositions and the assimilation and fractional crystallization (AFC) modelling, it is suggested that major monzogranites were generated from the granodioritic magma through the AFC process. The rock associations and arc-related geochemical characteristics of the granodiorites and dioritic MMEs suggest that they probably formed in a subduction-related tectonic setting. Combined with the spatial distribution of the rock types and formation ages of these early Ordovician to early Devonian magmatic rocks in the SCB from southeast to northwest, it is inferred that there was an Ordovician northwestward subduction of the proto–South China Ocean occurring to the south of the SCB, which may connect with the Tam Ky–Phuoc Son Suture Zone in the eastern Indochina Block. It was the subduction of the proto–South China Ocean and subsequent collision between the southern SCB and an unknown continent that triggered early Ordovician to early Devonian intracontinental orogeny in the interior of the SCB.

Evidence of iron oxide-copper–gold mineralization in the Torud-Chahshirin Magmatic Belt, northern Iran: Insight from the Robaie area

The Torud-Chahshirin Magmatic Belt in northern Iran is defined by Tertiary porphyry, IOCG, Skarn, Manto, MVT, and Au, Ag and base metal-rich epithermal systems, and it is a part of the eastern Alborz Orogenic Belt of Iran. Mineralization in the Torud-Chahshirin Magmatic Belt occurs as veins, veinlet, and disseminated mineralization hosted by the Eocene-Oligocene volcanic, subvolcanic, and volcanoclastic rocks. Mineralization prospects of the Torud-Chahshirin Mgmatic Belt are spatially and temporally accompanied with magmatic rocks of early to middle Eocene (∼55–37 Ma.) calc-alkaline and early to late Oligocene (∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels. The Robaie area is located in the Torud-Chahshirin Magmatic Belt. The dominant stratigraphic unit is composed of Eocene volcanic rocks (trachyandesitic to andesitic in composition), which are intruded by subvolcanic rocks. The subvolcanic rocks have features typical of shoshonitic affinity and are metaluminous to weakly peraluminous. These rocks are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion of heavy rare-earth-elements (REEs, LaN/YbN ≈ 9.65–24.34), and high-field-strength-elements (HFSEs). Values of εNd(i) commonly show a small variation between +0.2 and +0.7, whilst 86Sr/87Sr (i) goes from 0.7056 to 0.7063. The whole set of evidence agrees with derivation of intrusive rocks of Robaie from primitive magma resulting from partial melting in a mantle wedge above a subduction zone. A sample from a dyke of biotite-hornblende diorite porphyry, with a negative εNd(i) value (−1.7), probably corresponds to a magma that underwent assimilation of crustal material. U-Pb data of zircons from dioritic stock and dyke show ages of 49.99 Ma and 50.43 Ma, respectively. Mineralization in Robaie is related to the biotite-hornblende diorite intrusions and is associated with hydrothermal alterations including argillic, argillic-silicification, argillic-sericitic, and propylitic alteration. The ore-bearing veins, veinlets, and disseminated mineralization are controlled by structures. There are two types of veins and veinlets mineralization in the Robaie area including: 1) quartz-chalcopyrite-pyrite-specularite ± gold veins, and 2) quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets. The highest amount of copper (>1%) and gold (>5 g/t) is associated with quartz-chalcopyrite-pyrite-specularite ± gold veins. Microthermometric study of fluid inclusions shows that homogenization temperatures are 181–422 °C and salinities of ore-forming fluids ranged from 11.1 to 17 wt% NaCl equivalent. The temperature and salinity of ore-forming fluids were lower in the formation of the quartz-chalcopyrite-pyrite-specularite ± gold veins than in the quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets, and fluid mixing was the most important mechanism for mineralization in the area. The mineralogy, alteration, geochemistry, fluid inclusion studies, and petrogenesis of subvolcanic rocks of the Robaie area indicate that the Robaie deposit could be classified as a member of the Iron Oxide-Copper-Gold (IOCG) deposits.

Three‐dimensional P‐wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

AbstractIn this study, we compiled and analyzed 69310 P‐wave travel‐time data from 6639 earthquake events. These events (M ≥ 2.0) occurred from 1980s to June 2019 and were recorded at 319 seismic stations (Chinese Earthquake Networks Center) in the study area. We adopted the double‐difference seismic tomographic method (tomoDD) to invert the 3‐D P‐wave velocity structure and constrain the crust‐upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt (MLYB). A 1‐D initial model extracted from wide‐angle seismic profiles was used in the seismic tomography, which greatly reduced the inversion residual. Our results indicate that reliable velocity structure of the uppermost mantle can be obtained when Pn is involved in the tomography. Our results show that: (1) the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning: a nearly E–W‐trending low‐velocity zone is present beneath the Dabie Orogen, in contrast to the mainly NE‐trending low‐velocity anomalies beneath the Jiangnan Orogen. They suggest the presence of thickened lower crust beneath the orogens in the study area. In contrast, the Yangtze and Cathaysia blocks are characterized by relatively high‐velocity anomalies; (2) both the ultra‐high‐pressure (UHP) metamorphic rocks in the Dabie Orogen and the low‐pressure metamorphic rocks in the Zhangbaling dome are characterized by high‐velocity anomalies. The upper crust in the Dabie Orogen is characterized by a low‐velocity belt, sandwiched between two high velocity zones in a horizontal direction, with discontinuous low‐velocity layers in the middle crust. The keel of the Dabie Orogen is mainly preserved beneath its northern section. We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen, which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low‐velocity layers observed there. Continuous deep‐level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks, forming the ‘sandwich’ velocity structure there; (3) high‐velocity updoming is widespread in the crust‐mantle transition zone beneath the MLYB. From the Anqing‐Guichi ore field northeastward to the Luzong, Tongling, Ningwu and Ningzhen orefields, high‐velocity anomalies in the crust‐mantle transition zone increase rapidly in size and are widely distributed. The updoming also exists in the crust‐mantle transition zone beneath the Jiurui and Edongnan orefields, but the high‐velocity anomalies are mainly stellate distributed. The updoming high‐velocity zone beneath the MLYB generally extends from the crust‐mantle transition zone to the middle crust, different from the velocity structure in the upper crust. The upper crust beneath the Early Cretaceous extension‐related Luzong and Ningwu volcanic basins is characterized by high velocity zones, in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression‐related Tongling ore field. The MLYB may have undergone a compressive‐to‐extensional transition during the Yanshanian (Jurassic–Cretaceous) period, during which extensive magmatism occurred. The near mantle–crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust. The magmas may have ascended through major crustal faults, undergoing AFC (assimilation and fractional crystallization) processes, became emplaced in the fault‐bounded basins or Paleozoic sequences, eventually forming the many Cu‐Fe polymetallic deposits there.

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

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

The Southeastern Saurashtra dyke swarm, Deccan Traps: Magmatic evolution of a tholeiitic basalt–basaltic andesite–andesite–rhyolite suite

The Deccan Traps continental flood basalt (CFB) province of India contains several dyke swarms, which are dominated by tholeiitic basalts and basaltic andesites. The Southeastern Saurashtra dyke swarm, containing mainly these rock types, also contains an andesite and several rhyolites. Based on petrographic, mineral chemical, and whole-rock major and trace element and Sr-Nd isotopic data, we discuss the magmatic evolution and pressure-temperature conditions of crystallisation of these dyke magmas. The tholeiitic basalts and basaltic andesites have low to moderate TiO2 contents. The andesite and the rhyolites have low CaO, MgO, Fe2O3t, TiO2 and P2O5, and high K2O, Rb, Ba and light rare earth element contents. Thermobarometric calculations for equilibrium mineral–whole-rock pairs indicate plagioclase crystallisation at 1200–1170 °C, overlapping with clinopyroxene crystallisation at 1181–1143 °C, and a pressure range of 0.1–3.6 kbar indicating crystallisation during magma ascent or storage at shallow crustal levels. Major and trace element modelling is consistent with the rhyolites being produced by advanced fractional crystallisation of basaltic parental magmas. However, a broad trend of increasingly radiogenic Sr and nonradiogenic Nd isotopic ratios, from the mafic rocks through the andesite to the rhyolites, suggests a combined assimilation and fractional crystallisation (AFC) process between basaltic magma and ancient granitic crust. The rhyolites contain the largest crustal contributions and display the most “enriched” Sr-Nd isotopic characteristics yet recorded from the Deccan Traps (εNdt = −20.6, (87Sr/86Sr)t = 0.74855). The widespread occurrence of mafic enclaves in the rhyolite dykes suggests that processes such as magma mixing may also be responsible for some of the andesitic rocks known in Saurashtra.

Paleo- to Mesoproterozoic magmatic and tectonic evolution of the southwestern Yangtze Block, south China: New constraints from ca. 1.7–1.5 Ga mafic rocks in the Huili-Dongchuan area

Paleo- to early Mesoproterozoic rocks in the Yangtze Block are extremely sparse and are predominately exposed along the southwestern margin. Their petrogenesis and tectonic significance remain unclear, resulting in a limited understanding of the tectonic evolution of the southwestern Yangtze Block during this period. Therefore, we report here detailed geochronological and geochemical analyses of newly discovered Paleo- to early Mesoproterozoic mafic intrusive rocks in the Huili-Dongchuan area, southwestern Yangtze Block and interpret their petrogenesis and tectonic significance. Secondary ion mass spectroscopy (SIMS) zircon and baddeleyite U-Pb dating yielded crystallization ages of ca. 1716–1714 Ma for gabbros and diabases (referred to as the 1.72–1.71 Ga group), ca. 1705–1700 Ma for gabbros and a diabase-porphyrite (the 1.70 Ga group), and ca. 1519–1504 Ma for a gabbro and a diabase (the 1.52–1.50 Ga group), which effectively constrain the minimum depositional age of the Tong'an and Dongchuan groups that they intrude to 1.72 Ga. The three groups (or episodes) of intrusive rocks differ from each other in their chemical compositions and isotopic characteristics, most likely because of varying degrees of melting in a convective upper mantle (probably the asthenosphere), which had been metasomatized by melts with enriched components input by a plume from the lower mantle. Modeling calculations suggest that fractional crystallization (FC) and assimilation and fractional crystallization (AFC) were the most important processes involved in modifying the chemical compositions of these rocks. These magmatic activities are interpreted to have been related to mantle plumes and continental rifting that occurred in the Huili-Dongchuan area near the southwestern margin of the Yangtze Block.