Partial Melting Research Articles

Zircon Typology: a Synthesis. Applications to Magmatic, Metamorphic and Sedimentary Rocks

Abstract The study of more than 101 000 zircon crystals from 1105 different magmatic rocks (anatectic crustal, calc-alkaline, alkaline or tholeiitic) enables a general synthesis to be established which is useful for arguments in petrology. Populations are defined in typological distributions, mean points and typological evolutionary trends according to genetic groups. The overall logic obtained confirms the essential factors (temperature, chemistry) responsible for typological variations. The Hf contents of zircons, in substitution for Zr, make it possible to refine their origins (crustal, hybrid or mantle-derived), in particular by considering the early phase of crystallization, in equilibrium with the initial magma. They support the typological arguments and underline the role of the fluid phase in the magmatic evolution. Zircon crystals first formed in a given environment and subsequently immersed in a new medium of different chemical composition, temperature and fluid content show a spectacular readjustment of their typology. All the data obtained from the magmatic rocks are directly usable on ortho-derived metamorphic rocks with preserved zircon populations. For migmatites affected by partial melting, inherited zircon nuclei provide evidence (morphology, geochemistry notably Hf, Y) to constrain the nature of the protolith. Synthetic tables are proposed to help in the search for the origin of zircons found in sedimentary or volcano-sedimentary rocks, with a special mention for the most significant types and subtypes.

RocMLMs: Predicting Rock Properties Through Machine Learning Models

AbstractMineral phase transformations significantly alter the bulk density and elastic properties of mantle rocks and consequently have profound effects on mantle dynamics and seismic wave propagation. These changes in the physical properties of mantle rocks result from evolution in the equilibrium mineralogical composition, which can be predicted by the minimization of the Gibbs Free Energy with respect to pressure (P), temperature (T), and chemical composition (X). Thus, numerical models that simulate mantle convection and/or probe the elastic structure of the Earth's mantle must account for varying mineralogical compositions to be self‐consistent. Yet coupling Gibbs Free Energy minimization (GFEM) approaches with numerical geodynamic models is currently intractable for high‐resolution simulations because prediction speeds of widely‐used GFEM programs (100–102 ms) are impractical in many cases. As an alternative, this study introduces machine learning models (RocMLMs) that have been trained to predict thermodynamically self‐consistent rock properties at arbitrary PTX conditions between 1–28 GPa and 773–2,273 K, and dry mantle compositions ranging from fertile (lherzolitic) to refractory (harzburgitic) end‐members define9d with a large data set of published mantle compositions. RocMLMs are 101–103 times faster than GFEM calculations or GFEM‐based look‐up table approaches with equivalent accuracy. Depth profiles of RocMLMs predictions are nearly indistinguishable from reference models PREM and STW105, demonstrating good agreement between thermodynamic‐based predictions of density, Vp, and Vs and geophysical observations. RocMLMs are therefore capable, for the first time, of emulating dynamic evolution of density, Vp, and Vs due to partial melting and refertilization of dry mantle rocks in high‐resolution numerical geodynamic models.

Underplating-induced trans-crustal melting and maturation of Neoarchean continental crust in the North China Craton

A global change in granitoid compositions from early predominantly sodic tonalite-trondhjemite-granodiorites (TTGs) to later TTGs and more potassic granites occurred during the late Archean, coupled with a major period of crustal maturation. However, the detailed relationship between granitoid chemical evolution and the maturing crustal process remains enigmatic. Successive granitoid magmatism including late Neoarchean TTGs and high-K granites occurred in the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton and thus preserves crucial clues of the crustal maturation process. In this study, petrology, whole-rock geochemistry, and zircon U-Pb and Lu-Hf isotopes are reported for the late Neoarchean TTG gneisses, monzogranites, and minor metabasaltic to andesitic rocks from the WSP. The ca. 2560−2540 Ma TTG gneisses show low MgO, K2O/Na2O, but high (La/Yb)N, Sr/Y, and absence of Eu anomalies, indicating their derivation from partial melting of the thickened lower mafic crust. The ca. 2530−2500 Ma monzogranites are characterized by systematically high SiO2 and K2O/Na2O, but low MgO and Sr/Y, and moderately negative Eu anomalies, revealing they were formed by intracrustal reworking of local TTGs and sedimentary rocks in the middle to upper crust. Geochemical variations of these crustal-derived granitoids suggest that they were formed by melting at gradually higher crustal levels with the melt zone moved gradationally from the eclogite stability field into the plagioclase stability field. The ca. 2530−2500 Ma calc-alkaline metabasaltic to andesitic rocks sourced from metasomatized mantle outline roles of mantle-derived magma underplating in contributions of heating and trans-crustal melting magmatism. The long-term melting processes facilitated the upward movement of volatiles and heat-producing elements from deep to shallow crustal levels, and introduced K-enriched monzogranites into the upper crust, leaving a refractory, strengthening, and tectonically stable lower crust. Secular compositional evolution of crustal-derived granitoids reveals that continuous crustal reworking drove lithosphere differentiation and paved the way for the maturation of the Archean continental crust.

Petrogenesis and tectonic setting of mafic magmatism within the Laguna Amarga Metamorphic Complex, Andes of Catamarca, Argentina: Insights into the opening of the Cuyania/Precordillera terrane from the Ouachita rift

Neoproterozoic crystalline basement rocks are exposed as fault-bounded blocks over the high Andes of Catamarca. The crystalline basement is stratigraphically grouped into the unique Laguna Amarga Metamorphic Complex and represents the northern extension of the Cuyania/Precordillera terrane. Tabular bodies of meta-mafic rocks are widespread in the basement interspersed within a thick sequence of meta-sedimentary rocks derived from siliciclastic, calc-silicate, and limestone protoliths. Overall, the geochemical characteristics of meta-mafic rocks are in the compositional range of Normal-Mid Ocean Ridge Basalt (Normal-MORB), reflecting a common depleted-mantle source with varying degrees of partial melting. While preserving the typical bulk chemistry of MORB magmatism, some mafic magma underwent differentiation at emplacement, leading to the development of high-Ti mafic rocks. New U-Pb zircon geochronology reveals three distinct age peaks, with two coinciding with ages identified in the metasedimentary host rocks. The dominant Mesoproterozoic age cluster is linked to inherited zircon crystals assimilated within a single meta-mafic rock. In contrast, zircon ages from the late Ordovician to early Devonian are attributed to metamorphic overgrowths. Notably, the third age cluster, delineates a Late Neoproterozoic magmatic event, indicating the temporal span of mafic magmatism. The finding agrees with the best available age (576 ± 17 Ma) for mafic magmatism on the Precordillera Mafic-Ultramafic Belt. Stratigraphic relationships and geochemical fingerprints enable correlation among the meta-mafic rocks from Laguna Amarga, tracing a belt of mafic magmatism with an oceanic affinity that extends southward. Building upon previous works, this study reaffirms that the rift-drift transition of the Cuyania/Precordillera terrane, linked to the Ouachita rift opening from southeastern Grenville, evolved during the latest Neoproterozoic.

Controls on Corundum Formation: Metasomatism of Ultramafic Rock, Nattavit, South-East Greenland

Abstract The geotectonic setting for plumasite-type corundum occurrences is understudied, even though it is of importance for the understanding of trace-element patterns used for fingerprinting of ruby and sapphire. Mineral reactions related to metasomatism caused by pegmatite intrusion into ultramafic rock result in a characteristic trace element signature in corundum and thereby control its colour. The Nattivit area, Isertoq Terrane, South-East Greenland, provides a natural laboratory to investigate these mineral reactions and corundum trace element patterns given the excellent exposure of a typical plumasite-type occurrence where pegmatites intruded ultramafic rocks of different composition, namely lherzolite and dunite. The pegmatite dykes are 10 to 50 cm wide in the ultramafic rocks, whereas the adjacent alteration zone in the ultramafic rock reaches widths between 10 and 30 cm. Metasomatism resulted in desilication of the pegmatites and a decrease in Ca, Mg, K, Mn, Al and Fe away from the centre of the pegmatite dyke. Chromium, Ni, Mg, Fe, Sc, Co, V, Zn, Ti and Mn in the metasomatic reaction zones are predominantly derived from the ultramafic rock. We identify three zones with different mineral assemblages. In the lherzolite, tschermakite and biotite are formed in the centre of the reaction zone, which is followed by anorthite-rich plagioclase, hercynite, dolomite and ultimately pink corundum that occurs in the most heavily reacted part of the metasomatic reaction zone. The metasomatic reaction zones in the ultramafic rock include an intense reactionzone at direct contact to the pegmatite dyke with biotite and actinolite, and two alteration zones further away from the pegmatite dyke with enstatite, actinolite, anthophyllite, phlogopite, dolomite, sulphide, apatite and chrome-spinel. In the dunite, no biotite formed and hence, corundum contains more Fe, Mg and Ti. The plumasite-type corundum from Nattivit contains more Fe than Cr, which is typical of pink corundum hosted in mafic-ultramafic rocks. The corundum-forming reaction is dated from the pegmatite vein to 1843±4 Ma (U-Pb zircon age), which is coinciding with convergence of the Rae and North Atlantic cratons resulting in the Nagssugtoqidian Orogen. Only syn-tectonic, corundum-normative, peraluminous, calc-alkaline pegmatites of granitic to granodioritic composition that intrude into ultramafic rocks in the upper plate formed corundum in this area. The pegmatites are classified as muscovite class granitic pegmatites and intruded at upper amphibolite facies conditions. These pegmatites possible originated from partial melting of mafic granulite or a subducted oceanic plate. The formation of granitic pegmatites and related corundum mineralization in the upper plate of a collisional orogen described here is comparable to other corundum occurrences, e.g. Polar Urals, and thus is regarded as a typical geotectonic setting for plumasite-type corundum.

New constraints to subduction zone arc and backarc mantle temperatures: a test of the corner flow model

The heat source for the hot volcanic arc of subduction zones and the uniformly warm backarc is not well understood, particularly where backarc extension is absent. In the widely studied corner flow model, the traction of the underthrusting plate drags down the overlying viscous mantle, and the induced shallower seaward flow brings in heat from the landward and deeper asthenosphere. However, earlier comparisons with observations, especially backarc heat flow, gave poor agreement. There are several new constraints to the temperatures and structures above the underthrusting plate and in the backarc, especially those in western North America, for comparison with model predictions. Seismic receiver functions map horizontal boundaries, including the lithosphere-asthenosphere boundary (LAB). Seismic shear wave tomography gives mantle velocity-depth that provides an approximate proxy for temperature. Attenuation of seismic wave speed indicates high temperature or partial melting. The presence of recent backarc volcanics and their geochemistry give the depth and temperature of the LAB. From these constraints in western North America, the LAB reflects ponding of partial melt, is consistently at a depth of 65±3 km and temperature of 1350±25°C, and overlies nearly constant temperature (adiabatic) asthenosphere across the backarc from Mexico to Alaska. These depth and temperature estimates are greatly different from corner flow model predictions. An alternative hypothesis more consistent with these constraints is vigorous small-scale convection in the backarc asthenosphere. Whether or how this convection coexists with the large-scale flow patterns inferred from the interpretation of seismic anisotropy analyses, and to what degree it prevails in the arc-forearc region, deserve further research.

Gold Mineralization at the Syenite-Hosted Anwangshan Gold Deposit, Western Qinling Orogen, Central China

The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K2O = 10.13%, K2O/Na2O > 1) and high magnesium (Mg# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (εHf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ34S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ34S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ34S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO2–H2O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.

Serpentinised Dunites of the Neoarchean Shimoga Greenstone Belt, Western Dharwar Craton, India: Insights on Ni‐PGE Mineralisation and Genesis

ABSTRACTUltramafic rocks of the Archean greenstone belts worldwide are potential hosts for Cu‐Ni, precious metal deposits like platinum group elements (PGEs) and gold. This study highlights the geochemical evidence and genesis of Ni‐PGE mineralisation in the ultramafic rocks of Shimoga greenstone belt of the Western Dharwar Craton, India. Petrographically, the studied rocks are identified to be serpentinised dunites, while their geochemical signatures indicate komatiitic affinity. Presence of disseminated sulphides and pronounced serpentinisation in these rocks suggest a combination of Type II (disseminated sulphides) and Type IV (post‐magmatic alterations) komatiite‐related Ni–Cu‐PGE deposits. Chondritic Al2O3/TiO2 (9.65–16.38) ratios, superchondritic Gd/YbN (1.3–1.6), depleted high‐field‐strength elements (HFSEs) (Zr, Hf), enrichment of LREE over HREE and negative Nb–Ta anomalies reflect the generation of parental melts from plume‐sourced Al‐depleted komatiites with significant crustal contamination during their emplacement. The major, trace element and PGE relationships (FeO vs. MgO, Cu vs. Pd, Ni/Cu vs. Pd/Ir) infer the origin of sulphur undersaturated primary melts through moderate to higher degrees of partial melting followed by crustal assimilation that led to PGE enrichment. These observations suggest their formation from melts derived from greater depths of the upper mantle (> 400 km) at high pressure (> 10 GPa), wherein, the mantle residue retained majorite garnet. The high Ni (avg. Ni = 6511 ppm) substantiated by high Kambalda ratio ([Ni:Cr] × [Cu:Zn] = 5.6), Ni/Cr ratios (> 1) with high concentration of PGEs (avg. ∑PGE = 3078 ppb) confirm the fertile/mineralised nature of the komatiitic source and potential Ni‐PGE mineralization in the ultramafic rocks of the Shimoga greenstone belt.

Garnet clinopyroxenite formation via amphibole-dehydration in continental arcs: Evidence from Fe isotopes

Lower-crustal garnet clinopyroxenite (sometimes termed “arclogite”) fractionation in thick-crustal (>35 km) arc settings presents a compelling model to explain Fe-depletion trends, high oxygen fugacity, and evidence of recent delamination observed in many continental arcs. However, the origin of the garnet clinopyroxenites via igneous or metamorphic processes remains unclear. Due to the preferential incorporation of light Fe isotopes in garnet relative to clinopyroxene or amphibole, Fe isotopes are ideally suited for studying the effects of garnet fractionation on magmatic systems. Here, we present whole-rock and mineral Fe isotope data from a suite of lower to mid/upper-crustal Andean xenoliths from Mercaderes, Colombia. This data is combined with petrography, major and trace element mineral and whole-rock chemistry, geothermobarometry, and thermodynamic modeling to explore the xenoliths' petrogenesis and the Northern Andes' crustal structure. Whole-rock samples display a narrow range of Fe isotope compositions (δ56Fe = –0.02 to +0.11 ‰), which do not correlate with lithology, chemistry, or pressure-temperature conditions. This result is inconsistent with previous studies predicting the existence of an isotopically light Fe reservoir in the garnet-rich lower Andean crust. Through thermodynamic modeling, we show that the lack of isotopic fractionation in the Mercaderes xenoliths is more consistent with the suite representing a prograde metamorphic sequence, in which amphibole dehydration reactions drive metamorphism of mid/upper-crustal diorite protoliths. While our data do not preclude the presence of garnet clinopyroxenite cumulates at the base of the Andean crust, or that the delamination of such cumulates played an important role in the evolution of the Andes, they do indicate that not all garnet clinopyroxenites are cumulate in origin. Instead, the lower Andean crust represents an amalgamation of igneous and metamorphic rock, with metamorphism of mid-crustal lithologies and partial melting of mafic cumulate roots acting in tandem to drive densification and delamination of the lower crust in a self-feeding mechanism.

Crustal melting and continent uplift by mafic underplating at convergent boundaries.

The thick crust of the southern Tibetan and central Andean plateaus includes high-conductivity, low-velocity zones ascribed to partial melt. The melt origin and effect on plateau uplift remain speculative, in particular if plateau uplift happens before continental collision. The East Anatolian Plateau (EAP) has experienced similar, more recent uplift but its structure is largely unknown. Here we present an 80 km deep geophysical model across EAP, constrained by seismic receiver functions integrated with interpretation of gravity data and seismic tomographic, magnetotelluric, geothermal, and geochemical models. The results indicate a 20 km thick lower crustal layer and a 10 km thick mid-crustal layer, which both contain pockets of partial melt. We explain plateau uplift by isostatic equilibration following magmatism associated with roll-back and break-off of the Neo-Tethys slab. Our results suggest that crustal thickening by felsic melt and mafic underplate are important for plateau uplift in the EAP, Andes and Tibet.

Water content drives distinct evolution trajectory of Early Cretaceous granitoids in inland and coastal southeast China

Water plays a crucial role in determining the crystallization sequence of magma, which subsequently influences the chemical compositions of magmatic rocks across different tectonic settings. In this study, we compared the evolutionary features of granitic rocks along the coast and inland areas of southeast China, aiming to identify the key factors influencing their evolution. Our work indicates that multiple granitic intrusions formed between 126 and 142 Ma in the coastal region of southeastern China, which is consistent with the formation ages of large-scale granitoids in the inland Gan-Hang Belt. Isotopic characteristics suggest that the granitic rocks in southern Fujian originated from the melting of juvenile crust, while those in northern Fujian and eastern Zhejiang were formed from the partial melting of ancient crustal rocks, incorporating mafic magma evolved from the mantle. Most of the granitoids from the coastal region of southeastern China exhibit low zircon saturation temperatures (680–800 °C) and Zr/Sr ratios (<1), suggesting their origin from a cold, wet magma reservoir. The porphyritic quartz diorite and porphyritic monzogranite represent the residual cumulate rocks of this hydrous magma reservoir, whereas the granitic porphyry and high-silica equigranular alkali feldspar granite evolved from the felsic melts extracted from the same reservoir. In contrast, most of the Early Cretaceous granitoids in the Gan-Hang Belt, located in the inland areas of southeast China, display high zircon saturation temperatures (800–900 °C) and Zr/Sr ratios (>1), indicating their origin from hot, water-poor magma reservoirs. The porphyritic granites in this region represent residual cumulate rocks formed in water-poor magma reservoirs, whereas the high-silica equigranular granites evolved from hot felsic melts extracted from similar magma reservoirs. In the Early Cretaceous, the coastal region of southeastern China was closer to the Late Mesozoic paleo-Pacific subduction zone, where crystal-melt segregation within cold, wet magma reservoirs predominantly influenced magma evolution. Conversely, the granitoids in the Gan-Hang Belt in the inland region, located farther from the Late Mesozoic paleo-Pacific subduction zone, were associated with a rift tectonic setting and formed through crystal-melt segregation within hot, water-poor magma reservoirs. Our study underscores the critical role of water content in magma reservoirs in shaping the chemical composition of granitic rocks through crystal-melt segregation, thereby deepening our understanding of crustal formation processes across diverse tectonic environments.

New approach to constraining Cr-mineralization and forearc processes: U-Pb dating and geochemistry of titanites in chromitites of the Dun Mountain ophiolite belt (New Zealand)

Ophiolites provide important insights for understanding of subduction zones and forearc mantle-wedge geodynamics. Specifically, within the mantle section, podiform chromitites may record processes associated with development of highly depleted supra-subduction ophiolites, chromium mineralization and subsequent metasomatism. This study explores the first ever U-Pb isotope and trace-element data obtained from titanites in chromitites of the Dun Mountain ophiolite belt in New Zealand. Titanite is unusual in such geochemically depleted rocks and a low recovery rate attests to its rarity (ca. 190 grains from 100 kg of material from two chromitite samples). Results of U-Pb geochronology and geochemistry for the titanites constrain both the mid-Permian timing of chromitite crystallization (∼283 Ma) and later metasomatism (∼272 Ma). Older titanite grains are depleted in HFSE and have flat REE chondrite-normalized patterns reflecting a high-degree of partial melting in depths greater than the plagioclase stability field (>15 km). Younger titanites show enrichment in HFSE, depletion in LREE and negative Eu anomalies, revealing melt extraction at shallower depths (<15 km) that likely record refertilization of the mantle-wedge. Results highlight the potential of titanites in the acquisition of direct geochronological and geochemical constraints from rocks commonly devoid of minerals suitable for U-Pb dating.

Geochronology and petrogenesis of Early Mesozoic Rb-mineralized granitic rocks from the Liuzhangshan Rb, West Qinling Orogen Belt, China

ABSTRACT The newly discovered Liuzhangshan (LZS) Rb deposit is a large granite-type deposit in the West Qinling Orogenic Belt (WQOB), spatially associated with biotite monzogranite. Previous research has focused on the geological characteristics and metallogenic conditions of the LZS Rb deposit; however, its geochronology and petrogenesis remain unclear. In this study, monazite U-Pb dating of biotite monzogranite in the LZS deposit yielded a U-Pb age of 200.8 ± 0.9 Ma via laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Combined with the age of magmatic rocks in the WQOB, we propose a potential mineralization event related to Rb around 200 Ma in the WQOB. The biotite monzogranite belongs to weakly peraluminous, high-K calc-alkaline serious, containing high SiO2, Na2O, K2O, and low FeO, MgO, CaO, and P2O5. Compared with primitive mantle and relatively unfractionated granodiorites found in the WQOB, the biotite monzogranite enriched in Rb, Th, U, and Pb, deleted in Ba, Sr, and Ti, and showed negative Eu anomalies, a high Rb/Sr ratio, and a low K/Rb ratio, an ∑REE of 31.3 ppm, an LREE/HREE ratio of 6.48, a LaN/YbN ratio of 2.59, an Mg# of 4.15, and a crystallization temperature of 760°C, the biotite monzogranite also had a high differentiation index (DI = 98.2), apparent tetrad effect (TE 1,3 = 1.12 ~ 1.15), characteristic of highly fractionated I-type granite. Monzoite from the biotite monzogranite in the LZS deposit has negative εNd(t) values ranging from −12.45 to −11.17, and TDM1 of 1569 ~ 2018 Ma, indicating that the LZS biotite monzogranite originated from the partial melting of a Mesoproterozoic ancient basement material, with a minor contribution from mantle-derived melts. Highly differentiated, low Sr and Yb contents, Sr/Yb ratios, and low crystallization temperature magma facilitated the enrichment of Rb.

The lithosphere-asthenosphere boundary and lithosphere replacement as informed by mantle xenoliths and their host lavas

Alkaline lavas hosting mantle xenoliths occur in the hot back arc of the Cordillera of western Canada and Alaska and provide constraints on lithosphere thickness (within 5 to 10 km) and controls on the lithosphere-asthenosphere boundary (LAB). Such information is complementary to geophysical investigations of lithosphere strength and deformation in similar continental regions. Barometry of spinel lherzolite xenoliths and alkaline lavas from the Cordillera show LAB depths from 50 to 80 km, and marked thinning of the lithosphere at the Cordillera-craton transition coincident with the Rocky Mountain Trench-Tintina Fault. Similar barometric data globally for continental intraplate lavas show a relatively common lava equilibration depth of 65 to 80 km, that mostly coincide with LAB depths observed by seismology, and with the spinel- to garnet peridotite transition. The LAB is inferred as accumulated partial melt, modulated in depth by mantle solidus depression caused by varying H2O storage capacity across the garnet-spinel peridotite phase change. This mechanism does not explain cases where LAB depths are &lt; 65 km, which instead require locally hotter asthenosphere. In many regions preserving a shallow LAB, foundering and wholesale replacement of mantle lithosphere is invoked. With the exception of North China, mantle xenoliths sampled from continental margins record cooling timescales that appear far longer than required for foundering and replacement in the past 50 Myr. Either cooling histories cannot be adequately modelled using closure/diffusion arguments or wholesale replacement of mantle lithosphere is a dubious process.

Unravelling different mechanisms of metasomatism and geodynamic evolution of pyroxenite-veined subcontinental lithospheric mantle beneath the Central European Variscides

Abstract Pyroxenite-veined garnet peridotites from the Gföhl Unit of the Moldanubian Zone in the Bohemian Massif provide direct constraints on diverse mechanisms of mantle metasomatism and refertilization driven by a single pulse of melt beneath the Central European Variscides. Here, we provide a detailed study on an intriguing example of this rock association where the garnet peridotites show a fertile character (high Al2O3, CaO, TiO2), corresponding to the subcontinental lithospheric mantle (SCLM). By contrast, their conspicuous LREE depletion and Sr–Nd isotopic signatures (87Sr/86Sr338 ≤ 0.7028; εNd338 ~7.3) are typical of depleted mantle residue after melt extraction. Such signatures reflect transformation of an original refractory protolith (likely harzburgite) to fertile lherzolite through percolation of primitive tholeiitic melts, parental to garnet pyroxenite in veins. The SCLM refertilization is further documented by the whole-rock positive correlation between incompatible elements (Zr, Yb, Sc, V), and trace element composition of clinopyroxene (high Ti/Eu and Ti/Nb) and garnet (elevated ∑REE, Zr, Ti). Trace element and Sr–Nd isotopic systematics of pyroxenites (87Sr/86Sr338 ~0.7025–0.7029; εNd338 ≤ 7.9) correspond to a source of melt similar to the depleted MORB mantle (DMM). Three mechanisms of metasomatism related to the interaction of this melt with the host peridotites were distinguished: (i) stealth metasomatism, reflected by extensive clinopyroxene and garnet crystallization in lherzolite adjacent to pyroxenite veins, (ii) cryptic metasomatism, recorded by lower Mg# values of orthopyroxene and olivine in lherzolite, and (iii) modal metasomatism, resulting in crystallization of amphibole and phlogopite in lherzolite close to the veins. The percolating basaltic melt was hydrous, moderately enriched in fluid-mobile elements (Cs, Rb, Ba, Pb, U, Li). Immiscible liquids, dense Ti-Mg-Fe-rich oxide melt and C-O-H fluid, trapped and crystallized as mono/multiphase solid inclusions in garnet, likely separated from a basaltic melt upon cooling. The lherzolite-pyroxenite interface reveals strong micro-scale element fractionation due to differentiation of a basaltic melt within the percolation channel. Volatile-bearing liquids that segregated from the melts migrating through wall-rock peridotites most likely caused chromatographic enrichment in highly incompatible elements (e.g. LREE) in distal peridotites relative to the LREE-depleted lherzolites adjacent to the veins. The DMM-like affinity of pyroxenites and pressure-temperature estimates for lherzolite (3.9–5.4 GPa/1010–1200 °C) and pyroxenites (2.8–4.2 GPa/860–1020 °C) point towards exhumation-driven SCLM refertilization. This was linked to decompression-induced partial melting of upwelling asthenosphere producing basaltic melts penetrating through and metasomatizing the SCLM beneath the Variscan orogenic belt in Central Europe.

Petrogenesis and tectonic setting of the proto-tethyan dachaidan ophiolite, North China: insights from clinopyroxene chemistry and whole-rock Sr–Nd–Pb and zircon Hf isotopes

IntroductionThe Dachaidan ophiolites outcrop within an ultrahigh-pressure metamorphic belt along the northern margin of the Qaidam Basin. However, their age, source, and tectonic setting remain still in debate.MethodIn this study, we investigated the geochemistry and geochronology of the Dachaidan ophiolitic gabbros.Results and DiscussionZircon U–Pb dating yielded a crystallization age of 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma for the gabbro. The gabbros have low SiO2 contents (47.15–50.10 wt.%) and high MgO contents (6.35–9.04 wt.%) and Mg# values (55–74). The total rare earth element (∑REE) contents are 8.35–28.07 ppm, lower than those of normal-type mid-ocean ridge basalts (MORBs), and the gabbros exhibit light REE depletion or flat REE patterns, with small positive Eu anomalies (Eu/Eu* = 1.06–1.40). Trace element patterns are depleted to enriched in Nb and Ta, similar to island arc rocks and MORB. Clinopyroxene thermobarometry indicates the parental magma of the gabbros formed by high-temperature (1,318°C–1,363°C) and medium-pressure (1.27–1.64 GPa) partial melting in a mantle wedge. The gabbros have depleted Sr–Nd–Pb-Hf isotopic compositions, with (87Sr/86Sr)i = 0.704586–0.707441, εNd(t) = 4.7–6.6, and zircon εHf(t) = 7.6–11.4. The age-corrected Pb isotope ratios of these volcanic rocks are variable, with 206Pb/204Pb(t) = 18.085–18.253, 207Pb/204Pb(t) = 15.595–15.614, and 208Pb/204Pb(t) = 37.880–38.148, which are similar to the isotopic compositions of typical Indian MORBs. The source of the Dachaidan ophiolite is inferred to have been depleted mantle. The Dachaidan ophiolite likely formed in a forearc oceanic setting along the northern margin of the Qaidam Basin, during the initial subduction of an oceanic plate.

Serpentinised Mantle Section of Neoproterozoic Ophiolite at Al‐Barramiya District, North Arabian‐Nubian Shield: Tectono‐Magmatic Evolution and Metamorphism

ABSTRACTThis research focuses on the field observations, petrography, mineral chemistry and geochemistry of the serpentinised peridotite of Al‐Barramiya ophiolitic sequence to place constraints on their magmatic history and their geodynamic evolution. Al‐Barramiya ophiolitic rocks are a dismembered ophiolite which was strongly deformed and metamorphosed under greenschist to lower amphibolite facies. They comprise a mantle section dominated by highly serpentinised peridotite with less metapyroxenite and chromitite, as well as a crustal portion represented by metagabbros. Along shear zones, the ophiolite sequence was affected by several types of alteration. Extensive carbonate alteration is common in the ultramafic section, resulted in talc carbonates, listvenites and magnesite, while rodingitisation is common in the metagabbro resulted in rodingite. Despite the extensive serpentinisation, some fresh relics of primary mantle minerals such as Cr‐spinel, olivine and pyroxenes are preserved sporadically in the serpentinised peridotite. Few Cr‐spinel crystals are sometimes surrounded by subhedral flakes of Cr‐chlorite (kämmererite) that was formed due to replacement of Cr‐spinel during later alteration or regional metamorphism. The serpentinite samples are depleted in the total REE (0.56–1.19 ppm) with slightly negative to slightly positive Eu anomalies (0.89–1.28). The fresh cores of Cr‐spinel have Cr# mostly &gt; 60, and the relics of pyroxenes and olivine are Mg‐rich suggesting that the Al‐Barramiya serpentinites are residual to high degrees of melt extraction. The estimated degrees of partial melting range between 18.2% and 20.7%. All the mineralogical and geochemical characteristics of the ultramafic section of the Al‐Barramiya ophiolites are most consistent with its formation in a fore‐arc setting.

Leucogranite intrusions in Eastern Kazakhstan: age, composition and mechanisms of formation

There are several large intrusions composed of 70–75% leucogranites within Eastern Kazakhstan. Information on the age of the intrusions is provided, the features of the composition of the rocks are considered, and conclusions about the petrogenetic mechanisms of forming of leucogranite magmas are drawn. Two stages of leucogranite intrusive magmatism have been established – in the Early Permian and in the Early Triassic. Their formation occurred in an intraplate geodynamic setting. Leucogranite magmas were formed as a result of partial melting of crustal substrates under the thermal influence of mafic magmas. Geochemical differences in leucogranites were due to different compositions of crustal substrates and different degrees of their melting.

Late Paleozoic slab rollback events in the southeastern part of the Central Asian orogenic belt: Implications for Paleo-Asian Ocean subduction and continental crust growth

The Central Asian orogenic belt is considered to be the largest Phanerozoic accretive orogenic belt on Earth. The late Paleozoic magmatic rocks in central Inner Mongolia are crucial for understanding continental crust growth and the tectonic evolution of the southeastern part of the Central Asian orogenic belt. We present comprehensive geochemical, isotopic, and geochronological data from three late Paleozoic magmatic units in the Mandula area, west of the Solonker suture zone. Zircon U-Pb dating indicates that these rocks formed during the late Carboniferous (316−304 Ma). The Mandula high-Mg diorites exhibit high MgO (3.9−6.5 wt%), high Mg# (61−69), and depleted Nd-Hf isotopic compositions, generated through interaction between a metasomatized mantle and slab melts with the overlying sediments. The Mandula granodiorites display adakite geochemical characteristics with high Sr/Y mass ratios (29−52), high MgO (1.7−2.2 wt%), and high Mg# (52−54), formed by partial melting of the oceanic slab with the addition of overlying sediment. Mafic microgranular enclaves have consistent ages, Sr-Nd-Hf isotope compositions, and hornblende crystallization temperature-pressure conditions with their host granodiorite, formed from a cognate magma associated with the host granodiorites through cumulate. We propose that two phases of slab rollback took place during the late Paleozoic southward subduction-accretion of the Paleo-Asian Ocean. The first phase corresponded to the transformation of low- to medium-angle slab subduction, while the second phase led to subduction-related extension. Considering the tectonic-magmatic evolution, crustal maturity, and thickness variations in the late Paleozoic southeastern part of the Central Asian orogenic belt, we propose that prolonged subduction and slab rollback promoted continental crust growth. The Central Asian orogenic belt coincides temporally and spatially with the Phanerozoic Pangea cycle, suggesting that continuous subduction and supercontinent amalgamation significantly contributed to continental crust growth.

Petrogenesis of Montana, USA Sapphires Inferred from Oxygen Isotopes and Zircon Inclusions

Abstract Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e., crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U-Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ18O values, from -3 to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0 to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4 to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 3‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U-Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U-Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.