Amphibole Fractionation Research Articles

Diverse intrusion modes during the construction of a high-silica magma reservoir: Evidence from La Obra–Cerro Blanco intrusive suite (central Chile)

Several conceptual models have been proposed for the amalgamation of granitoid plutons, which range from incremental growth to single-stage emplacement of these systems. This diversity of views has led to intense debate about the thermomechanical state of silicic intrusions and the magma differentiation paths within the crust. In this contribution, we present a comprehensive petrologic, geochronologic, and magnetic fabric data set from the La Obra–Cerro Blanco intrusive suite, which allows us to explore the petrogenesis and magma emplacement processes in the upper crust. This intrusive suite is composed of (1) a vertically zoned granitoid intrusion in spatial association with mafic layers and stocks and (2) a cupola-like high- silica granite. We interpret this intrusive suite as assembled by diverse but coexisting intrusion mechanisms over a time span of ~1.4 m.y. from 21.4 to 19.9 Ma. As indicated by the subhorizontal magnetic lineation, the first stage was dominated by horizontal emplacement of sheet-like intrusions of intermediate compositions, which became increasingly silicic after plagioclase and amphibole fractionation throughout the crustal column. The latest stage was instead dominated by cooling, crystallization, and differentiation of a thickened granitoid body and the formation of a high-silica magma chamber. The steep magnetic lineation and the abundance of aplite and rhyolitic dikes observed in the cupola-like, high-silica granites suggest that this portion acted as an evacuation channel of high-silica magma toward shallower levels, offering a rare opportunity to understand not only silicic magma accumulation and storage in the upper crust, but also the processes connecting the plutonic and volcanic environments.

The origin of Gangjiang adakite-like intrusions and associated porphyry Cu–Mo mineralization in the central Gangdese porphyry Cu belt, southern Tibet

The Gangjiang Cu–Mo deposit is located in the central Gangdese porphyry Cu belt (GPCB), south Tibet. Geological investigation reveals that the Gangjiang deposit is characterized by early potassic alteration, followed by late phyllic and argillic alteration. Late alteration has different degrees of overprinting over the potassic alteration zone. Copper-Mo orebodies are predominantly hosted in the potassic and phyllic alteration zones of the Miocene monzogranite porphyry (MGP) and granodiorite porphyry (GDP). In this study, we present the chemistry and Sr–Nd–Pb–Hf–O isotope compositions of fertile MGP and GDP, barren tonalite porphyrite (TP), and microgranular mafic enclaves (MMEs). Chemical compositions indicate that these intrusions are I-type granites that show high–K calc-alkaline signatures. The negative correlation between Dy/Yb and SiO2 indicates that the intrusions experienced fractionation of amphibole. They have high Sr/Y (>70) and La/Yb (>38) ratios, strong negative Nb–Ta–Ti anomalies, and positive Pb anomalies are typical for arc magma. The high Mg# of various porphyry intrusions (43–65) and MMEs (47–58) indicate the input of mafic components. Various intrusions have whole-rock 87Sr/86Sr(t=13Ma) ratios of 0.70529 to 0.70674, negative εNd(t=13Ma) (−5.0 to −1.2), positive zircon εHf(t) (1.2–9.8), and homogeneous δ18O (5.8–7.1 ‰, mean 6.6 ‰). The MMEs have similar Sr–Nd–Pb–Hf–O isotopic compositions with their host rocks. The features reveal that the Gangjiang adakite-like intrusions and MME samples were derived from the partial melting of the Lhasa Paleoproterozoic crust, with contributions from the juvenile mantle that has been metasomatized by subducted sediments-derived melts. Note, the MGP, GDP, and two types of MMEs have slightly lower δ18O values (down to 4.76 ‰) but higher εNd(t) (up to −1.2) than barren TP, indicating their formation involves more mantle components. Furthermore, the MMEs in the TP and MGP have high Cu contents (up to 1485 ppm), similar to the phenomenon occurs in the Qulong deposit. The Cu-rich MME is probably formed by Cu-refertilization by the underplate arc magma in the juvenile crust. The εHf(t) and εNd(t) values of fertile porphyries increased from the Jiru, Gangjiang, to Qulong deposit indicating increased mantle material additions from the west to the east section of the GPCB. These variations in mantle components have led to an increase in the size of the porphyry Cu deposits from Jiru to Gangjiang, and then to Qulong.

Petrogenetic evolution of the Jalapa del Marqués pluton: Miocene arc magmatism in southern Mexico and its tectonic implications

Arc-related plutonic rocks in southern Mexico occur along the Pacific coast in the Sierra Madre del Sur (SMS) igneous province. These rocks record the migration of the Caribbean–North America–Farallon/Cocos junction towards the east between the Late Cretaceous and Miocene. However, little is known about the petrogenesis of igneous rocks that were emplaced during the latest expressions of magmatism in the SMS (i.e., early–middle Miocene). Based on fieldwork, petrography, whole-rock and mineral geochemistry, thermobarometric calculations, and zircon UPb dating, we document the petrogenetic evolution of the Jalapa del Marqués pluton and adjoining intrusions, which are the youngest so far dated in the SMS and are located ∼150 km inland from the present-day trench. This subduction-related magmatic complex consists of granites, granodiorites, diorites, dacite porphyries, and intermediate-to-felsic dikes, whose geochemical features suggest early evolutionary processes driven by fractionation of amphibole ± clinopyroxene (moderate–high pressures) or plagioclase (low pressures). Pulses of evolved magmas were episodically injected into upper-crustal reservoirs (7–14 km) over ∼9 Myr, starting at 22.14 ± 0.65 Ma (2σ)—earlier than previously thought (∼15.7 Ma)—and producing subsequent pulses at 19.45 ± 0.36 Ma, from 16.36 ± 0.37 to 15.8 ± 0.34 Ma, and from 13.9 ± 0.5 to 13.3 ± 0.2 Ma. These ages correlate with the onset and early stages of volcanic centers in the Trans-Mexican Volcanic Belt (TMVB), some of which were located up to 500 km landward from the present-day trench. The large difference in the distance-to-trench between coeval magmatism in the Jalapa del Marqués pluton and the TMVB, indicates a variation in the subduction angle between two segments of the Farallon/Cocos Plate that were likely separated by a slab-tear. The eastward migration of this slab-tear (along with the triple junction) terminated the magmatic activity in the SMS by switching the locus of magmatism to the TMVB. This probably occurred shortly after the studied magmatic complex was assembled.

Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe2+-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential.

Late Paleozoic potassic igneous rocks of the Kensu and Dzholkolot plutons in the eastern Kyrgyz Tien Shan: Petrology, geochemistry, U-Pb zircon geochronology, and related skarn-porphyry W-Mo-Cu-Au mineralization

The Kensu and Dzholkolot multiphase intrusions are situated in the eastern part of the “Main Structural Line of Tien Shan”, or the “Nikolaev Line”. The plutons comprise mafic to intermediate (monzogabbro, monzonite, syenite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) members, together with the interim (camptonite) and final monzodiorite-porphyry dikes. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type affinity, emplaced in post-collisional setting. Magmatic evolution included a generation of shoshonitic magma by a low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal?) magma chamber. This was followed by a generation of mantle-induced granitic magmas in a crustal protolith, with possible mixing/mingling of various magmas. The rocks crystallized at shallow levels, in an oxidized environment, under decreasing pressure and temperature toward the younger (granitic) intrusion phases.Isotopic U-Pb (LA-ICP-MS) zircon dating of the igneous rocks indicates their Late Carboniferous age (ca. 325–302 Ma). The magma emplacement included a number of intrusion phases from monzogabbro (321 ± 4 Ma), monzonite (319 ± 4 Ma), and camptonite (306 ± 4 Ma), to quartz syenite (305.5 ± 2 Ma to 302 ± 3.7 Ma), quartz monzonite (305 ± 3 Ma) and subsequent granitic phases. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks supporting rather post-collisional setting of the plutons are in agreement with a “scissor-like” closure of the Turkestan paleoocean, starting from the east. Zircon xenocrysts dated at ca. 1.9 Ga suggest the presence of an underlying old continental crust.The skarn-porphyry W-Mo(−Cu-Au) mineralization complements the group of similar deposits associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be related to the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, together with more common fertilization in Cu and Au, with its subsequent involvement in the magma generation in post-collisional setting. The endowment in W and Mo can also reflect a greater involvement of an ancient continental crust as the magma source(s). The oxidized paragenesis of hydrothermal alteration assemblages and mineralization corresponds to the respective affinities of the igneous rocks.

Critical differences between typical arc magmas and giant porphyry Cu ± Au systems: Implications for exploration

Abstract Porphyry Cu, and porphyry Cu-Au deposits, are associated with arc magmatism and their ore-forming systems generally follow the magmatic evolution of typical arcs. However, most arc magmas are barren and giant economic porphyry Cu ± Au deposits are rare. In this study, we model variations in rare earth element concentrations in the evolving arc magmas and giant porphyry Cu ± Au systems to quantify the percentage of the fractionating minerals required to produce the observed changes. We find that, during the andesitic stage of fractionation, ore-forming systems in thick crusts fractionate ~35% more amphibole than an average of thick arc magma systems (the thick-crust reference suite) and that ore-forming systems in thin crusts fractionate twice as much amphibole as their equivalent thin-arc magma reference suite. Thick-crust ore-forming suites also fractionate ~50% less plagioclase, and thin-crust ore systems ~40% less plagioclase, than their associated reference suites during the same andesitic stage of fractionation. Taken together, these observations imply that ore-producing magmas are appreciably wetter than their associated barren reference suites. Our modelling also shows that 80% more amphibole is required to reproduce the andesite stage of fractionation in the thick-crust reference suite than in its thin-crust equivalent, suggesting that magmas produced under thick crusts are wetter than those produced under thin crusts. On the other hand, the chalcophile element contents of the thick- and thin-crust ore-forming systems are similar to and higher than those of the thick- and thin-crust reference suites, respectively. Therefore, we suggest that the high water content plays a critical role in the formation of giant porphyry Cu ore in thick crusts, whereas both high chalcophile contents and high water contents are required to form giant porphyry Cu-Au deposits in thin crusts. The high fraction of amphibole fractionation in giant economic porphyry suites, compared with their relevant reference suites, results in lower Y in the ore-associated suites and this difference increases with fractionation. As a consequence, plots of Y against MgO can be used to identify porphyries that have economic potential and are preferred to Sr/Y plots because they are less affected by the intense alteration associated with giant porphyry Cu ± Au deposits.

Lower crustal control in the iron isotope systematics of plutonic xenoliths from Adak Island, Central Aleutians, with implications for arc magma geochemistry

We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc basalts and their crystallization products. The Fe isotope composition of ultramafic cumulate xenoliths increases from dunite (δ56Fe = –0.09 to –0.02 ‰) to clinopyroxenite (δ56Fe = +0.06 to +0.09 ‰), consistent with higher modal proportions of clinopyroxene (δ56Fe = –0.05 to +0.11 ‰) relative to olivine (δ56Fe = –0.10 to +0.06 ‰) in the latter. Mid-crustal cumulate amphibole gabbro and hornblendite cumulates also record heavier Fe isotope compositions (δ56Fe = +0.04 to +0.08 ‰) due to the abundance of isotopically heavy amphibole (δ56Fe = +0.07 to +0.09 ‰) and magnetite (δ56Fe = +0.11 to +0.13 ‰) in these rocks. High inter-mineral fractionations observed in spinel-olivine and spinel-clinopyroxene pairs (Δ56Fespl-ol = +0.12 to +0.28 and Δ56Fespl-cpx = +0.06 to +0.19) suggest that spinel is not recording equilibrium crystallization conditions for the ultramafic assemblages, likely due to subsolidus Fe-Mg exchange. Our data also include Fe isotope measurements of one mantle dunite (δ56Fe = +0.03 ± 0.05 ‰). Five Adagdak lavas, spanning from basalts to andesites, yield a narrow range of δ56Fe between +0.03 and +0.06 ‰. Our results highlight the potential of amphibole in driving the Fe isotope depletion trends observed in many erupted arc lavas, as amphibole hosts 28–99 % of the FeOT budget in the amphibole gabbro and hornblendite cumulates. This is also supported by single-crystal synchrotron Mössbauer spectroscopy of two amphibole grains, the first from an amphibole gabbro and the second from a hornblendite, which yield Fe3+/ΣFe ratios of 0.55 ± 0.06 and 0.58 ± 0.02, respectively. Water content and hydrogen isotope compositions determined by secondary-ion mass spectrometry from the same amphibole grains indicate partial dehydrogenation. Using Rayleigh fractionation modeling to account for oxidation during post-crystallization dehydrogenation, we calculate magmatic Fe3+/ΣFe ratios of 0.41 ± 0.04 for the amphibole gabbro and 0.30 ± 0.05 for the hornblendite. These data are then used to estimate an appropriate Fe force constant for Adagdak amphibole and quantitatively evaluate the effects of amphibole fractionation. Through a fractional crystallization model, we show how arc melts may experience periods of increasing δ56Fe during olivine-dominated fractionation, followed by decreasing δ56Fe once magnetite and amphibole saturate as cumulate phases. Notably, this dichotomy between fractionation of isotopically light versus heavy cumulate assemblages and its effects on the Fe isotope evolution of arc magmas is not captured by the Adagdak lava record, highlighting the utility of cumulates in chronicling the early isotopic evolution of magmatic systems.

Petrogenesis and zircon U-Pb ages of the Late Paleozoic high-potassic Sonkul and Kokturpak plutons in Kyrgyz Tien Shan, with implications for related skarn-porphyry W-Au(-Cu-Mo) mineralization

ABSTRACT The Sonkul and adjacent Kokturpak multiphase plutons are situated along the ‘Major Structural Line of Tien Shan’ (known also as the ‘Nikolaev Line’) that extends for over 500 km and divides the terranes of the Middle and Northern Tien Shan. The plutons comprise a broad variety of high-potassic igneous rocks including the early olivine gabbro, intermediate monzonite to quartz monzonite, and later granodiorite, monzogranite, and leucogranite-alaskite, in turn followed by the late mafic dikes. U-Pb dating of zircon grains yields ages of 299 ± 2 Ma (olivine gabbro), 300 ± 3 Ma to 298 ± 3 Ma (monzonite to quartz monzonite), 299 ± 3 Ma to 298 ± 4 Ma to 297 ± 4 Ma (granodiorite), and 289 ± 4 Ma to 285 ± 2 Ma (monzogranite). These consecutive intrusive phases appear to have been produced by partial re-melting of a partially crystallized (mush) magma batch at deeper levels, which is evident by the presence of zircon antecrysts dated at some 306–311 Ma (up to 323 Ma?). Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions emplaced in a transitional subduction-related to post-collisional setting. A low-degree partial melting of the metasomatically enriched upper mantle can be envisioned as the leading process of shoshonitic magma generation; this was followed by amphibole fractionation in a deep (lower crustal ?) magma chamber and possibly a mantle-induced generation of granitic magmas in the crustal protolith. Further mixing/mingling of the mafic magma and crustal granitic magma was followed by the magma fractionation and emplacement at shallow crustal levels. Assimilation of the old continental crust is consistent with the presence of ancient xenocrystic zircons (dated at ca. 1.5 to 2.5 Ga) in the studied rocks. The Late Carboniferous to Early Permian age and high-K calc-alkaline to shoshonitic composition are similar to those of highly mineralized intrusions of the other segments of the Middle Tien Shan. In this regard, several magmatic pulses of high-potassic intrusions and corresponding metallogenic pulses can be suggested in this region. Whereas the early (ca. 337 to 315 Ma) igneous suites accompanied by dominantly porphyry Cu-Au-Mo mineralization, the later (ca. 305 to 285 Ma) suites including the Sonkul and Kokturpak plutons are accompanied by W- and/or Au-dominant mineralization. This mineralization appears to pre-date more substantial intrusion-related to orogenic-type Au mineralization.

Late Paleozoic potassic igneous rocks of the Adyrtor intrusions in the eastern Kyrgyz Tien Shan: geochemistry, isotope U-Pb zircon geochronology, and related W-Mo(-Cu-Au) mineralization

ABSTRACT The Adyrtor multiphase pluton is situated in the easternmost part of the ‘Main Structural Line of Tien Shan’, or the ‘Nikolaev Line’, and intrudes the Paleoproterozoic metamorphic sequence representing the oldest (1.8–2.5 Ga) continental basement fragment in the Tien Shan orogenic belt. The intrusions comprise mafic to intermediate (monzodiorite, monzonite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) rocks. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type granite affinity, emplaced in post-collisional setting. Based on the isotopic U-Pb data, quartz syenite from the Adyrtor intrusions crystallized at 330.7 ± 4.3 Ma, and quartz monzonite – at 329.5 ± 5.8 Ma. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks support rather post-collisional setting of the plutons, principally due to a ‘scissor-like’ closure of the Turkestan paleoocean, starting from the east. Magmatic evolution included a generation of shoshonitic magma by low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal ?) magma chamber. Then, under the influence of mantle-supplied fluids and heat, granitic magmas in the crustal protolith could have been generated, with further mixing/mingling of the mantle-derived mafic (shoshonitic) magma and mantle-induced crustal granitic magma, followed by the magma fractionation and emplacement at shallower crustal levels. Assimilation of an old continental crust is also envisioned, consistent with the presence of the xenocrystic zircons dated at ca. 1.6 Ga to 2.5 Ga and the A-type granitoid affinity of the igneous rocks. The associated skarn-porphyry W-Mo(-Cu-Au) mineralization is consistent with post-collisional setting of the productive intrusions. This mineralization complements the group of similar deposits and occurrences associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be explained by the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, with its further involvement in the magma generation in post-collisional setting. Also, a stronger W-Mo mineralization can reflect a greater role of a crustal protolith in the magma generation, as revealed by the distinct A-type affinity of the igneous rocks.

Petrogenesis and tectonic implications of late Permian andesites from Rongzharinian, northern Qiangtang, Tibet

At convergent margins, high-magnesian andesitic rocks (HMAs) can offer vital information about the formation of the continental crust and the interactions between the deep crust and mantle. This study focuses on new identified HMAs from the Rongzharinian area, northern Qiangtang Terrane (NQT). We report the major, trace elemental, Sr-Nd-Hf isotopic data, and zircon U-Pb age of volcanic rocks with the aim of understanding their petrogenesis and tectonic setting. A concordant age of 257.7 ± 1.5 Ma for the Rongzharinian high-Mg andesites is determined by zircon U-Pb dating using LA-ICP-MS techniques. The Rongzharinian HMAs have high MgO contents (5.88–7.39 wt%), Mg# values (63–68), and Cr (143–185 ppm) and Ni (48–66 ppm) concentrations, close to primitive HMAs. They underwent variable degrees of fractionation of amphibole and minor clinopyroxene and negligible crustal contamination during magma evolution. REE modeling results show that the Rongzharinian HMAs can be generated by 4 %–7% melting of hydrous spinel lherzolite at relatively shallow depths. Geochemical data indicate that the Rongzharinian HMAs were dominantly derived from the mixed mantle wedge source composed of N-MORB-like and E-MORB-like components. Based on the regional tectonic evolution, we propose that the Late Permian rollback of the Paleo-Tethyan slab may induce upwelling of the asthenosphere mantle, resulting in melting of the mantle source and the generation of the Rongzharinian high-Mg andesites.

Magmatic Evolution and Plumbing System of Gede-Salak Volcano, Banten, Indonesia

The study of individual volcanoes in northwest Java has been largely overlooked. In this study, an investigation of the magma evolution and plumbing system of Gede-Salak Volcano was conducted. A geological survey determined the lava unit and volcanism. The whole-rock geochemistry is utilized to determine the magma type and evolution. Mineral chemistry based on microprobe analysis revealed the magmatic process and phenocryst origin. Geothermobarometry is employed to estimate the temperature and pressure. The volcanism comprised the eruption of lava flows, sector collapse, and the eruption of lava domes. The magma evolution consists of two magma types: type A (lava flow and peripheral dome) and type B (summit dome). The processes identified are amphibole fractionation, magma mixing, and crust assimilation. Phenocryst textures and chemistry implied open-system processes inthe plumbing system involving three magma series, namely the felsic, intermediate, and mafic series. Magma type A resulted from multiple mafic recharges on the felsic series, while type B resulted from the mixing of intermediate and mafic series. The felsic and intermediate phenocryst crystallization occurred at 933-948°C and 1010-1011°C in the mid-crust at 14 km to 17 km depth. Meanwhile, the mafic series reside in the lower crust at 21 km depth and of 1065-1087°C temperature. Keywords: Gede-Salak, magma evolution, magma plumbing, geothermobarometry, mineral chemistry

Corrente Canoa pluton: A less-evolved portion of the Statherian Borrachudos Suite (Guanhães basement inlier, Araçuaí orogen)

We bring new geophysical, geochemical, petrographic and geochronological data for the Corrente Canoa pluton, an igneous body within the Guanhães basement inlier, Araçuaí orogen. Our results demonstrate that this unit was crystallized in 1703.1 ± 4.3 Ma (U–Pb, LA-ICP-MS, zircons) and is part of the anorogenic Borrachudos Suite, integrating the Statherian Silicic Large Igneous Province emplaced at the São Francisco-Congo Paleocontinent in the context of an extensive rifting event. The Corrente Canoa pluton presents a distinctive geophysical and geochemical signature, which sets it apart from the other plutons in the suite. In airborne gammaspectrometry maps, its area of occurrence is depleted in Th and U. Geochemically, it presents a negative correlation between SiO2 and TiO2. Al2O3, MgO, CaO e FOt, which suggests fractionation of plagioclase, pyroxene, amphibole, biotite and Fe–Ti oxides along the crystallization process. K-feldspar and pyroxene controlled the fractional crystallization, and the latter is increased in the Corrente Canoa pluton, indicating that it is less-evolved. The difference between Eu/Eu × of the Corrente Canoa pluton (1.01 av.) and the rest of the Borrachudos Suite (0.32 av.) also attest its lower degree of fractionation. Zr saturation thermometry indicates an average crystallization temperature for the Corrente Canoa pluton of 888 °C, a result higher than the average for the rest of the Borrachudos Suite (846 °C). Altogether, these results suggest that the Corrente Canoa pluton was crystallized in a deeper portion of the crust. Based on this evidence, we argue that the genesis of the Corrente Canoa pluton and their differences to the other plutons of the Borrachudos Suite are better explained by fractional crystallization processes rather than the metasomatism suggested by previous studies. Fractional crystallization not only clarifies the distinctions between these plutons but also enhances the understanding of the entire differentiation process within the Borrachudos Suite. Additionally, we discuss possible explanations for the juxtaposition of the Current Canoa and Morro do Urubu plutons, based on their differences in age and petrogenesis. The distinct characteristics of the Corrente Canoa pluton points to the need of a thorough cartographic review in the Guanhães block, because at least part of what is now mapped as basement units might be in fact rocks of the Borrachudos Suite, but with a different geophysical and geochemical signature than usual for this suite.

Petrology and Geochemistry of Adak Island Plutonic Xenoliths: Implications for Primitive Magma Generation and Crustal Differentiation in the Aleutian Island Arc

Abstract Deep crustal cumulates in arcs offer a window into the chemistry and crystallization conditions (P–T–H2O–fO2) of primitive basalts in the upper mantle and lower crust and can be studied in ancient exhumed terranes or in xenoliths erupted in young arc lavas. Here, we expand on previous studies and thoroughly characterize the extensive xenolith suites erupted from the Mt. Moffett and Mt. Adagdak volcanic centers (Adak Island, Central Aleutians), which range from primitive ultramafic cumulates to more evolved amphibole gabbros and hornblendites. We present detailed petrography as well as in situ trace and major element mineral chemistry. We use these data to calculate pressure, temperature, and fO2 estimates for the xenoliths, and compare these findings to experimental results to understand the crystallization sequence and P–T–H2O–fO2 under which the cumulates formed. The Moffett crystallization sequence is defined by early amphibole fractionation and an abrupt shift in oxide compositions from chromite to magnetite, while the Adagdak suite is characterized by simultaneous saturation of amphibole+plagioclase and oxide compositions that become increasingly aluminous before magnetite saturation. Olivine–spinel oxybarometry of the Adagdak xenoliths indicates that they are oxidized relative to mid-ocean ridge basalt (MORB:FMQ +0.1 to +2.1). Highly fractionated REE and elevated Sr/Y ratios are observed in clinopyroxene from the most primitive cumulates, consistent with a contribution from a basaltic eclogite melt. This basaltic eclogite melt is hypothesized to come from partial melting of the slab or through melting of basalt introduced into the subarc mantle through forearc subduction erosion. These signatures are greatly diminished in the more evolved lithologies, which can be explained through fractionation of plagioclase and amphibole. Our findings support the presence of a complex magmatic plumbing system beneath Adak, with Mt. Moffett and Mt. Adagdak volcanic centers tapping compositionally distinct sources. More broadly, our results are consistent with studies suggesting that low-degree basaltic eclogite melts through slab melting or forearc subduction erosion contribute to arc magmas in the Aleutians, although the associated geochemical signatures are easily obscured by differentiation in the crust.

Arc adakitic-like signatures caused by amphibole fractionation: An example from 20 Ma granitoids, Charco Rico district, Panama

The Charco Rico prospective comprises a segment of the Sierra de Majé massif in Central Panama, an intrusive complex constructed during a lull in volcanic activity between 40 and 16 Ma at the 75–0 Ma Central American Volcanic arc system (CAVAS). Charco Rico represents an intermediate suite (55–65 wt% SiO2) but with inclusion of related Majé intrusives, range from gabbro to granodiorite. Zircon UPb dating of Charco Rico granitoids yield LA-ICP-MS ages of 20.2 ± 0.9 to 19.1 ± 1.5 Ma. Their formation is consistent with maximum partial melting of ∼10% of a Sr-enriched (i.e., amphibolite) N-MORB like source followed by fractional crystallization of hornblende at about 0.8 GPa (∼29 km depth). Zirconium saturation thermometry shows a temperature of magma crystallization of 700–800 °C which essentially is a record of amphibole crystallization temperature. Whole rock chemistry of the Charco Rico and Majé intrusives and other 32–20 Ma arc-related suites in Panama record chemical parameters similar to adakite, e.g., Sr/Y that ranges to 80, high LREE/HREE ratios, and depletion in HREE (Yb, Lu). A fundamental chemical attribute of these suites, however, is depletion of MREE, and particularly Ho, relative to LREE and HREE, and the absence of Eu-anomalies. These features, together with positive and negative correlations of La/Yb and Dy/Yb with SiO2, respectively, infer a dominant role of amphibole fractionation from basaltic magma and/or residue during partial melting of older calc-alkaline bodies in the lower crust. 176Hf/177Hf ratios in zircon grains range from 0.282925 to 0.283192 and record εHft between +5.8 and + 15.3, which are indicative of a depleted mantle (juvenile) source and/or recycling (partial melting) of older juvenile crust. TDM incubation ages range to higher than 300 Ma, which may reflect old basement recycling. Partial melting of the depleted mantle ± lower crust may have been facilitated by a combination of hot asthenospheric upwelling during slab rollback, slab tear or lithospheric delamination, and simultaneous thinning of the continental crust under extension. Whole rock chemistry and reported cumulate hornblende in host granites with adakitic-like signatures, infer that hornblende differentiation controlled the magma chemistry and migration to high-SiO2 rocks with ‘adakite’ signatures.

Porphyry Copper Deposit Formation: Identifying Garnet and Amphibole Fractionation With REE Pattern Curvature Modeling

AbstractPorphyry copper ore‐forming intrusions are distinguished from barren arc magmas by high oxidation state and “adakitic” high Sr/Y and La/Yb. Controversy over petrogenesis of adakites has centered on ambiguities in interpretation of their steep rare earth element (REE) patterns, and on whether garnet participates in their petrogenesis. Lambda (λ) coefficients deconvolute subtle differences in REE pattern curvature, providing a more quantitative method to explore mineral fractionation processes. Here, we use trace element and numerical λ coefficient modeling to assess the relative influence of amphibole, garnet, and plagioclase in the petrogenesis of porphyry ore‐forming intrusions. We find that garnet‐fractionation trends are not evident in REE patterns of many adakitic porphyry‐forming intrusions. Instead, porphyry‐forming intrusions in the western Pacific and Eocene porphyry‐forming intrusions in northern Chile have REE patterns consistent with a garnet‐free, amphibole‐dominated cumulate. Traditional element ratios such at Dy/Dy*, LaN/YbN, and DyN/YbN are poor discriminants for garnet or amphibole fractionation.

Petrology and geochronology of Cretaceous−Eocene plutonic rocks in northeastern Washington, USA: Crustal thickening, slab rollback, and origin of the Challis episode

Cretaceous through Eocene plutonic rocks in northeastern Washington, USA, document a 60 m.y. history of crustal thickening and subsequent collapse and extension in response to two terrane-accretion events. Rocks emplaced 113−53 Ma have increasing La/Yb ratios reflecting orogenic plateau development after arrival of the Insular terrane by 100 Ma. Plutons emplaced 52−45 Ma (the Challis episode) document collapse of this plateau and define a SW-younging age progression attributed to breakoff and rollback of the Farallon slab following accretion of the Siletzia terrane at ca. 50 Ma. All of the rocks have chemical traits of arc magmas, likely inherited from their lower-crustal sources, but low B/Be ratios and the lack of evidence for amphibole fractionation indicate the Eocene magmas formed under drier conditions than are typical of active subduction settings. These magmas also originated at greater depth (eclogitic vs. gabbroic source) and were emplaced more shallowly than the earlier ones. All rocks have overlapping Sr-Nd and O isotopic data, indicating significant contributions from older continental crust, and depleted mantle Nd model ages become older toward the east, defining three regions that correspond with previously inferred lower-crustal domains. Farallon slab rollback also drove extension (core complex formation, dike swarms) and crustal uplift, which, along with voluminous magmatism, define the Challis episode. This tectonic model is further supported by seismic tomography, which has identified remnants of a detached slab in the upper mantle beneath the region.

Crustal control on the petrogenesis of adakite-like rocks

Adakite-like rocks have attracted extensive interest as they record magmatic processes and mantle-crustal material recycling, and because they are associated with major porphyry Cu deposits. However, their petrogenesis is still under debate. In this study, we compiled ca. 7000 sets of whole-rock geochemical and Nd isotopic data of Phanerozoic adakite-like rocks worldwide from published literature, with the aim to characterize geochemically adakite-like rocks from subduction, collision, and post-collision settings. Statistical analysis highlights the different lanthanides + Y patterns from various types of juvenile and mature crust. Modelling of the statistically-processed data suggest that pure fractionation of amphibole and/or garnet cannot account for the lanthanides + Y patterns of adakite-like rocks, whereas partial melting of mafic protoliths is a possible mechanism. This study supports a adakite-like magma generation model by partial melting of the eclogitic lower crust. The melt was then mixed with depleted mantle-derived basaltic melt, and subsequently fractionated. Compared with the adakite-like magma formation from the juvenile crust, that from the mature crust is featured by higher degree of partial melting, greater depths and higher water content. Additionally, secular peaks of La/Yb value (which reflects the depth of magma source) of adakite-like rocks worldwide suggest that the period of magmatic activity in deep-level magma reservoirs could be used as the indicator of either the final stages of the Phanerozoic orogenesis, or craton activation by subduction.

Geochemistry and origin of felsites and associated A-type granites at Wadi Atalla area, Central Eastern Desert, Egypt: Implication for genesis of Neoproterozoic post-collisional highly silicic melts

Alkaline volcanics (felsites) and A-type granites are spatially and temporally associated in the Wadi Atalla area, Central Eastern Desert, Egypt. These rocks represent final Neoproterozoic magmatic activity in the Northern Nubian Shield. Wadi Atalla felsites form NW-trending elongate mass consisting of silicic volcanics, which is intruded by A-type granites of Um Had and Um Effein plutons. Field relations indicate that both felsites and A-type granites were emplaced in a series of magmatic pulses. Felsites include tuffaceous, porphyritic and hypabyssal varieties, while A-type granites comprise syenogranite and alkali feldspar granite. Geochemically, they are highly silicic rocks and display a narrow range of SiO2. They have high-K alkaline nature with metaluminous to peraluminous character and were erupted in a post-collisional tectonic setting. Mineral chemistry of biotite supports the alkaline and A-type characters of the studied rocks. Atalla felsites and associated A-type granites show light REE enrichment relative to heavy REE [(La/Lu)n = 2.67–5.72] with pronounced negative Eu anomalies (Eu/Eu* = 0.15–0.52). Compositional similarities between Atalla felsites and A-type granites in the studied area suggest that the intrusive and extrusive events are broadly related. The systematic variation of major- and trace-element contents of the felsites and A-type granites indicates derivation from similar sources through partial melting of juvenile crustal rocks, followed by extensive fractional crystallization. The main fractionated phases are feldspars, with minor role of fractionation of amphibole, biotite, apatite and FeTi oxides. Multiple saturation calculations indicate water-saturated storage and final equilibration conditions for the felsites ranging from 788 °C at 90 MPa to 740 °C at 255 MPa. The parental magmas of the felsites and A-type granites were linked to lithospheric delamination and upwelling of asthenospheric mantle material. This process led to generation of mantle melts that supplied heat to melt the juvenile crust of the Arabian-Nubian Shield (ANS), along with crustal uplift recorded by intersecting strike-slip faults and shear zones. Atalla felsites represent a distinctive post-collisional alkaline volcanic phase in the Nubian Shield that has been emplaced in an extensional tectonic regime, during a phase of fracturing and crustal uplift, which followed the end of the Pan-African orogeny. This volcanic phase is younger than the Dokhan volcanics in the Eastern Desert and older than the typical Katherina volcanics in southern Sinai.

Crustal thickness of the Jiaodong Peninsula in the Mesozoic: Implications for the destruction of the North China Craton

The North China Craton underwent extensive and widespread crustal reworking (or decratonization) during the Mesozoic. However, how the decratonization operated is not well understood. Zircon compositions are widely used by the scientific community to reconstruct crustal thicknesses. In this study, we sampled 13 magmatic rocks in the Jiaodong Peninsula and used zircon Eu/Eu* to constrain the crustal thickness of the Jiaodong area and reveal decratonization processes in the Mesozoic time. The reconstructed crustal thickness using zircon Eu/Eu* is approximately 70 km in the Jurassic, and this value is 89 km at around 130 Ma, after which the crustal thickness drops to 30–40 km at ca. 110 Ma. These results are generally compatible with or slightly higher than the calculation results using a whole-rock La/Yb proxy for the Jurassic and ∼130 Ma rocks. Crustal thickness estimated using a whole-rock La/Yb proxy for the ∼110 Ma rocks is thicker than 70 km, which is not consistent with the geological facts and the result given by zircon proxy. The whole-rock proxy failed in estimating crustal thickness because of amphibole fractionation for the ∼110 Ma rocks. The crustal thickening from Jurassic to ∼130 Ma was probably related to the westward subduction of the Paleo-Pacific slab. The thinning of the crust from 130 to 110 Ma is not a rapid process but occurs more slowly than expected, which might be explained by the chemical erosion process rather than a mechanical delamination model. The chemical erosion was most likely induced by a rollback of the subducting slab and an upwelling of the asthenosphere.

Shallow subduction of Indian slab and tectono-magmatic control on post-collisional porphyry mineralization in southeastern Tibet

The 1500-km long Gangdese belt in southern Tibet shows uneven distribution of giant to large porphyry deposits over 45 Mt Cu mainly in the middle-east segment between longitude 87–92°E. However, the western (80–86°E) and eastern end (92–96°E) is generally barren. The recent discovery of Demingding, Cuibaizi and Tangbula porphyry deposits in the east of 92°E has attracted great attention on their origin. Recent high-resolution seismic imaging shows the torn Indian lithosphere subducts with diverse angle. In contrast to the middle-east segment with moderate angle Indian plate subduction, the southeastern Tibet is characterized with a shallow dip of subduction (eastern of Comei rift), where Demingding, Cuibaizi and Tangbula deposits are located. To reveal the formation mechanism of these deposits as well as the Indian slab motion pattern beneath them, we compiled reported whole rock major and trace elements, Sr and Nd isotope signature, zircon trace element, U-Pb ages and Hf isotope data of Miocene ore-related adakitic rocks from Demingding and Tangbula to the north, and Oligocene ore-barren adakitic rocks from Linchi to the south. There is a clear negative correlation between zircon U-Pb ages of rocks and their distance to Indus-Yarlung Zangbo suture: Linchi is around 20 km away from the suture, with an average U-Pb age of 27.1 Ma, while Demingding and Tangbula rocks are 50 and 90 km away from the suture, with ages of 20.3 and 19.7 Ma respectively. Granitoids from Linchi have less compatible element Ni and Cr contents of 4.07 and 5.84 ppm on average respectively, while those of Demingding (20.1 and 43.4 ppm) and Tangbula (5.52 and 7.90 ppm) are much higher. Ni and Cr as compatible elements are concentrated in mafic minerals, and are thus proper indicators for mantle imprint indicators. In addition, as shown by Sr-Nd isotopes, Linchi granitoids have features of ancient crust [(87Sr/86Sr)i = 0.7059 to 0.7085, εNd(t) = − 6.20 to −2.90], while Demingding and Tangbula granitoids show less radiogenic features [(87Sr/86Sr)i = 0.7059 to 0.7068, εNd(t) = − 3.27 to −1.70], which are more similar to coexisting juvenile arc. Demingding and Tangbula granitoids also show signature of high fO2 and hydrous melt condition, as indicated by calculated fO2 of ΔFMQ + 2, and amphibole fractionation indicated by decreasing Dy/Yb with SiO2, since amphibole favors MREE (Dy) rather than HREE (Yb). Combining above, our work shows that the Indian slab constantly subducted shallowly beneath these deposits, given no mantle-like signature of near-suture Oligocene adakitic rocks. We suggest the cause to the formation of Demingding, Tangbula and Cuibaizi deposits is the reactivation of pre-enriched arc root materials, triggered by constant northward migration of Indian slab. This is supported by similar isotope signature between Miocene Demingding and Tangbula ore-related porphyries and coexisting arc, and absence of coeval pre-collisional porphyry deposits. The essential components for these porphyry deposits, high fO2 and water content, were also likely contributed from the pre-enriched juvenile arc root.