Granitic Melts Research Articles

Extremely fractionated magmas linked with W mineralization: Evidence from the LyangarW‐Modeposit, South Tianshan Orogenic Belt

AbstractTungsten (W) deposits are commonly related to the exsolution of magmatic‐hydrothermal fluids from high‐Si granites (SiO2 > 70%). However, whether the W‐related high‐Si granitic magma is produced via partial melting of metasedimentary source rocks or by high degree of fractional crystallization remains controversial. Here we present new geochronological and geochemical data on the intrusions associated with the Lyangar W‐Mo skarn deposit in the Southern Tianshan Orogenic Belt, Uzbekistan. Our new U–Pb zircon age data show that the major intrusion exposed in the region are ca. 280 Ma biotite gabbroic diorite and biotite granite and about 260 Ma porphyritic granite and muscovite porphyritic granite. The molybdenite grains in the skarn rocks and orebodies show weighted Re‐Os ages of 261.4 ± 7.8 Ma and 261.1 ± 3.8 Ma, respectively. In combination with the field contact, we confirm that the muscovite porphyritic granite is genetically related to the W mineralization. The gradual transition from the porphyritic granite to muscovite porphyritic granite, similar mineral assemblages and geochemical variations indicate that they are co‐magmatic, and that the porphyritic granite represents less evolved member. Rhyolite‐MELTS modeling further reinforces that the muscovite porphyritic granites can be produced by high degree of fractional crystallization (~33%, including ~1.2% biotite, ~27% plagioclase, ~2% alkali‐feldspar, ~0.21% Fe‐Ti oxides, and ~2.7% amphibole) of the porphyritic granite magma. On the basis of the positive ƐHf(t) values (+3.03 to +6.02), high‐SiO2contents and CIPW characters, the porphyritic granite is considered to have formed from dehydration melting at low pH2O of juvenile basaltic source rocks around 16 kbar and 850–1000°C. Our study demonstrates that extreme fractional crystallization of granitic magma plays a significant role in W enrichment in the granitic melt.

Slow crystal settling controls the diversity of high-silica granites of the Late Cretaceous Shengsi Pluton at northeastern tip of southeast China

The intrinsic mechanism on geochemical diversity of high-silica (>70 wt% SiO2) granites (HSGs) still remains unclear. In this paper, we focus on the magmatic differentiation of the Shengsi Pluton at the northeastern tip of SE China by an integrated study of geochronology, whole-rock geochemistry and mineral chemistry of zircon and apatite. The Shengsi granites crystallized at ca. 97 Ma, and can be divided into early medium-grained phase (MGP) and late fine-grained phase (FGP). The FGP granites have higher SiO2 (75.0–77.1 wt%) contents and lower amount of plagioclase, apatite and titanite. The MGP granites likely represent the primary granitic melts and they show transitional geochemical features between I- and A-type granites. In contrast, the FGP granites may represent the melts differentiated from the MGP magma mush and they are characterized by continuously varied to strongly depleted middle rare earth elements (MREEs). Geochemical modeling demonstrates that the varied REEs of the FGP granites are likely controlled by the crystallization/accumulation of titanite, plagioclase and minor zircon during magmatic differentiation. Such a crystal fractionation process in HSGs could be explained by slow “crystal settling” (mainly titanite) in crystal mush induced by dripping solid–liquid mixtures from the magma roof. This work exemplifies the effective crystal fractionation of >70 wt% SiO2 melts to generate >75 wt% SiO2 melts in the present coastal area of SE China, and it also highlights the role of slow settling of early crystallized minerals in crystal mush on geochemical diversity of HSGs.

First Age and Geochemical Data on Zircon from Riebeckite Granites of the Verkhnee Espe Rare Earth\u2013Rare Metal Deposit, East Kazakhstan

This paper is dedicated to the isotope-geochemical study of zircon from riebeckite granites of the Verkhnee Espe rare earth-rare metal deposit and the specification of its U–Pb age. Zircon from the Verkhnee Espe massif is peculiar in the high content of non-formula elements (up to 43000 ppm REE, up to 22000 Y, and others) and demonstrates a clearly expressed heterogeneous structure. The central and rim zones of the zircon show a “magmatic” rare-earth element (REE) distribution. The intermediate zones are characterized by a flattening of the REE patterns and an anomalous enrichment in REE, Y, Nb, and Ca. This compositional feature of the zircon may be caused by impact of fluid-saturated granite melts enriched in incompatible trace elements. The δ18О values in the zircon are 5.83–7.16‰, which generally corresponds to zircon formed from granitoid melts. The age of zircon from the Verkhnee Espe rare earth–rare metal deposit is 283 ± 3 Ma, which indicates that there is no significant age gap between granite crystallization, on the one hand, and metasomatic processes and ore generation, on the other.

Early Earth zircons formed in residual granitic melts produced by tonalite differentiation

AbstractThe oldest geological materials on Earth are Hadean (>4 Ga) detrital zircon grains. Their chemistry and apparently low Ti-in-zircon temperatures (≤700 °C) are considered to be inconsistent with crystallization in a magma of the tonalite-trondhjemite-granodiorite (TTG) suite, although these are the dominant Archean (4.0–2.5 Ga) silicic rocks. Using a new data set of trace element contents in zircons from Paleoarchean Barberton TTGs (South Africa) and thermodynamic modeling, we show that these zircons have crystallized at near-solidus conditions from a compositionally uniform granitic melt. This melt is residual from the crystallization of a less evolved (tonalitic) parent and thereby shows major and trace element compositions different from bulk TTG rocks. A global compilation reveals that most Hadean detrital and Archean TTG-hosted grains share a peculiar zircon trace element signature that is distinct from the chemical trends defined by Phanerozoic zircons. Our model shows that the low Ti contents of early Earth zircons reflect crystallization at higher temperatures (720–800 °C) than initially inferred due to lower modeled TiO2 activity in the melt relative to previous estimates. We therefore propose that near-solidus zircon crystallization from a chemically evolved melt in a TTG-like magmatic environment was the dominant zircon-forming process on the early Earth.

«РУДНАЯ МАГМА». ЧТО ЭТО С ПОЗИЦИИ ГРАНИТНОЙ РУДНО-МАГМАТИЧЕСКОЙ СИСТЕМЫ?

The concept of “ore magma” remains an obscure hypothesis in ore formation. The article considers the process of natural overgrowth of trivial primary basaltic magma into an ore-bearing granite melting and further into the ore-forming “ore magma” as the concentration of volatile and ore components. The dark side of the problem lies in the fact that during the ore formation the “ore magma” liquates into contrasting phases and leaves practically no traces of itself (with rare exceptions). But the concept of the ore magma has received a logical scientific justification from the standpoint of ore-magmatic systems.

A simple and robust method for calculating temperatures of granitoid magmas

Calculating the temperatures of magmas from which granitoid rocks solidify is a key task of studying their petrogenesis, but few geothermometers are satisfactory. Zircon saturation thermometry has been the most widely used because it is conceptually simple and practically convenient, and because it is based on experimental calibrations with significant correlation of the calculated zircon saturation temperature (TZr) with zirconium (Zr) content in the granitic melt (i.e., TZr ∝ ZrMELT). However, application of this thermometry to natural rocks can be misleading, resulting in the calculated TZr having no geological significance. This thermometry requires Zr content and a compound bulk compositional parameter M of the melt as input variables. As the Zr and M information of the melt is not available, petrologists simply use bulk-rock Zr content (ZrBULK-ROCK) and M to calculate TZr. In the experimental calibration, TZr shows no correlation with M, thus the calculated TZr is only a function of ZrMELT. Because granitoid rocks represent cumulates or mixtures of melt with crystals before magma solidification and because significant amount Zr in the bulk-rock sample reside in zircon crystals of varying origin (liquidus, captured or inherited crystals) with unknown modal abundance, ZrBULK-ROCK cannot be equated with ZrMELT that is unknown. Hence, the calculated magma temperatures TZr using ZrBULK-ROCK have no significance in both theory and practice. As an alternative, we propose to use the empirical equation T_{SiO_{2}} (°C) = -14.16 × SiO2 + 1723 for granitoid studies, not to rely on exact values for individual samples but focus on the similarities and differences between samples and sample suites for comparison. This simple and robust thermometry is based on experimentally determined phase equilibria with T ∝ 1/SiO2.

Late Cretaceous upper-crustal thermal structure of the Sevier hinterland: Implications for the geodynamics of the Nevadaplano

Abstract Crustal temperature conditions can strongly influence the evolution of deformation during orogenesis. The Sevier hinterland plateau in Nevada and western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of shallow-crustal metamorphism and granitic magmatism. Here, we investigate the thermal history of the Nevadaplano by measuring peak thermal field gradients attained in the upper 10–20 km of the crust along an east-west transect through nine ranges in eastern Nevada and western Utah, by integrating Raman spectroscopy of carbonaceous material thermometry and published conodont alteration indices with reconstructed cross sections. Thermal field gradients of 29 ± 3 °C/km were obtained in the House and Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7 °C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients typically between ~20–30 °C/km. The elevated thermal gradients in easternmost Nevada are interpreted to have been attained during ca. 70–90 Ma granitic magmatism and metamorphism and imply possible partial melting at ~18 km depths. Our data are compatible with published interpretations of Late Cretaceous lithospheric mantle delamination under the Sevier hinterland, which triggered lower-crustal anatexis and the resulting rise of granitic melts. The lack of evidence for structures that could have accommodated deep burial of rocks in the nearby Northern Snake Range metamorphic core complex, combined with thermal gradients from adjacent ranges that are ~1.5–3 times higher than those implied by thermobarometry in the Northern Snake Range, further highlights the debate over possible tectonic overpressure in Cordilleran core complexes. Cross-section retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.

Magmatic-hydrothermal tin deposits form in response to efficient tin extraction upon magma degassing

Most of the global Sn resources are from granite-related ore deposits, which form in response to cassiterite precipitation from hydrothermal fluids. However, the physical and chemical controls on the efficiency of Sn extraction from upper crustal plutons by exsolving magmatic fluids are still unclear. In this study, we determine the partition coefficient of Sn between aqueous fluids and granitic melts (DSnfluid/melt) at 800 °C, 150 MPa and the fO2 of the Ni-NiO buffer. To obtain equilibrium partition coefficients, a new experimental method has been used relying on local equilibrium between silicate melt and microscopic-sized fluid bubbles. The latter formed synthetic fluid inclusions in the quenched glasses, which in turn were analyzed by laser ablation inductively coupled plasma mass spectrometry along with the enclosing glass. The results show that at constant aluminum saturation index (ASI = 1.05–1.08) of the silicate melt, DSnfluid/melt increases from 1.9 to 35.0 as the total Cl concentration (mCltotal) in fluid increases from 1.0 to 16.6 mol/kg H2O. At a fixed mCltotal = 2 mol/kg H2O, DSnfluid/melt increases from 4.3 to 10.6 as the HCl concentration in the solution increases from 0.15 to 0.79 mol/kg H2O, which in turn is a function of the ASI of the melt (ASI = 1.06–1.29). Numerical modeling suggests that Sn is extracted by magmatic fluids from upper crustal plutons most efficiently at the late stage of crystallization and degassing. At a similar degree of crystallization, granitic magma with lower initial water concentration and higher ASI will separate a fluid phase with higher Sn concentration and thus has higher Sn mineralization potential. Due to the relatively high DSnfluid/melt value, fluids exsolved from highly evolved magmas can sequester enough Sn to form Sn deposits and the sub-solidus remobilization of Sn from granite bodies is not a pre-requisite for ore genesis.

Trace element fractionation between biotite, allanite, and granitic melt

The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with Dbiotite/melt of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with Dbiotite/melt < 0.9. This is likely one of the reasons why Cu is enriched together with Mo (Dbiotite/melt = 0.29) in porphyry deposits associated with intermediate to felsic plutons, while the other transition metals are not. Both Nb and Ta are mildly compatible in biotite with Dbiotite/melt being larger for Nb (3.69) than for Ta (1.89). Moderate (15—30%) biotite fractionation would be sufficient to reduce the Nb/Ta ratio from the chondritic value to the range observed in the continental crust. Moreover, the strong partitioning of Ti into biotite implies that already modest biotite fractionation suppresses the saturation of Ti-oxide phases and thereby indirectly facilitates the enrichment of Ta over Nb in the residual melt. The heavy alkalis, alkaline earths, and Pb are only mildly fractionated between biotite and melt (Dbiotite/melt = 3.8 for Rb, 0.6 for Cs, 0.6 for Sr, 1.8 for Ba, 0.7 for Pb). The rare earth elements are generally incompatible in biotite, with a minimum for Dbiotite/melt of 0.03–0.06 at Gd, Tb, and Dy, while both the light and heavy rare earths are less incompatible (e.g. Dbiotite/melt = 0.6 for La and 0.3 for Yb). This behavior probably reflects a partitioning into two sites, the K site for the light rare earths and the octahedral Mg site for the heavy rare earths. There is no obvious dependence of the rare earth partition coefficients on tetrahedral Al in the biotite, presumably because charge balancing by cation vacancies is possible. Allanite was found as run product in some experiments. For the light rare earths, Dallanite/melt is very high (e.g. 385 to 963 for Ce and Nd) and appears to increase with decreasing temperatures. However, the rather high solubility of allanite in the melts implies that it likely only crystallizes during the last stages of cooling of most magmas, except if the source magma is unusually enriched in rare earths.

Exploring volcanic-intrusive connections and chemical differentiation of high silica magmas in the Early Cretaceous Yanbei caldera complex hosting a giant tin deposit, Southeast China

Study of the origin and chemical differentiation of silicic magmas can provide important insights into crustal evolution and rare metal metallogeny. Tin mineralization always tends to form in relatively reduced, highly fractionated granite systems. Recognition of distinctions between fertile and barren magmas is of enormous benefit to mineral exploration. The Yanbei caldera complex (YCC), is a typical inland volcanic-intrusive complex that also hosts a giant porphyry-type tin ore deposit in Southeast China. To study the origin and chemical differentiation of the YCC and constrain its relationship with tin mineralization, a comprehensive petrological, whole-rock major and trace element geochemical data, along with zircon UPb ages, trace element and Hf isotopic data of the Yanbei caldera complex (YCC) and the Xiaoji granite (XG) near the YCC are carried out. LA-ICP-MS zircon UPb dating indicates that the generation and evolution of the YCC and the XG took place in a short time (4 M.y.) of between 142.4 and 138.4 Ma. Distinct zircon Hf isotopic compositions of the volcanic and intrusive units from the YCC suggest that they were derived from different magma sources. The volcanic rocks and the XG have consistent and low zircon Hf isotopic compositions (εHf(t) = −14.9 ~ −9.0), implying that they were almost exclusively derived from melting of Paleoproterozoic crustal rocks. But the magma source of the intrusive units (the granite porphyry, GP and the biotite granite, BG) (εHf(t) = −6.0 ~ −0.8) contains a significant mantle-derived component input. The whole-rock and zircon compositions suggest that the compositionally zoned volcano of the YCC can be interpreted in terms of the “crystal mush model”. The rhyolite has the geochemical characteristics of highly evolved magmas which undergo crystal fractionation, while the dacite displays a complementary geochemical signature implying that it represents the residual crystal mush after extraction of the rhyolitic melts. The XG displays similar chronological, compositional, and isotopic features to the rhyolite, suggesting that it is the intrusive equivalent of the rhyolite. The intrusive units (GP and BG) of the YCC are highly-evolved granites with elevated tin contents, responsible for tin mineralization. By comparison of fertile and barren systems in the YCC, we suggest that simple crystal fractionation for the high-evolved/extracted granitic melt does not necessarily lead to tin mineralization. A tin-bearing and volatile-rich melt originated under high-temperature partial melting of crustal source induced by underplating/input of a hot mantle magma is an essential precondition for some magmatic-hydrothermal tin mineralization systems.

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland

Dehydration melting of metasupracrustal rocks at mid- to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ∼1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850 °C), at 20%–30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000–1 050 °C, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.

Rapid Ag diffusion in granitic melt: Implications for Ag mineralization and melt inclusion record

Element diffusion in silicate melts is an important process in the transport and enrichment of metals in magmatic and magmatic-hydrothermal systems, but the diffusivity of Ag, unlike other metals such as Cu, Mo, and W, is still unknown. In this study, Ag diffusion in granitic melts with up to 6.9 wt% H2O was investigated at 973–1673 K and 0.1–1 GPa in a piston cylinder apparatus and a cold seal vessel, using both a diffusion couple approach and an argentite (Ag2S) dissolution approach. Silver concentration profiles were analyzed by LA-ICP-MS on recovered samples. Silver solubility, determined by Ag concentration at melt-argentite interface, increases with increasing temperature. Silver diffusivity is found to increase with increasing temperature and water content but to decrease with pressure. At 1 GPa, Ag diffusivity in granitic melt can be described by the following expression:lnDAg,1GPa=-13.77±0.46-13341±534-700±39wTwhere D is Ag diffusivity in m2/s, w is the water content in wt%, and T is the temperature in K. The pressure effect over 0.1–1 GPa gives an activation volume of 11.3 cm3/mol for Ag diffusion. While not being on a par with the mobility of Na and Li, Ag diffusion is among the most rapid, comparable to Cu and faster than other species including H2O and Fe. This implies that Ag diffusion in melt is unlikely to be a rate-limiting step for Ag enrichment into exsolved fluid bubbles or sulfide blobs. Furthermore, melt inclusions in silicate minerals are expected to record Ag concentration in the bulk melt instead of an elevated concentration in the presence of an Ag-rich boundary layer. Our Ag diffusivity data also suggest that a few tens of hours are long enough to homogenize Ag concentration in melt in solubility or partition experiments at temperatures above 800 °C.

Quantifying F and Cl concentrations in granitic melts from apatite inclusions in zircon

Quantifying F and Cl concentrations in granitic melts from apatite inclusions in zircon

Study of the Behavior of Uranium, Niobium, and Tantalum in the Granite Melt–Fluoride Fluid System at 800–950°C, 2300 Bar

Study of the Behavior of Uranium, Niobium, and Tantalum in the Granite Melt–Fluoride Fluid System at 800–950°C, 2300 Bar

Insights into the tectonostratigraphic setting of the Southern Appalachians during the Blountian tectophase from an integrated geochemical analysis of magmatic phenocrysts in the Ordovician Deicke K-bentonite

We evaluate competing hypotheses regarding tectonic models of the early Taconic Orogeny in the Southern Appalachians during the Blountian tectophase with new geochemical data obtained from analyses of apatite and zircon phenocrysts, and melt and mineral inclusions therein, from the Ordovician Deicke K-bentonite (453.35 ± 0.10 Ma). Apatite geochemistry confirms that samples from different locations represent the same eruptive event, and zircon geochemistry and UPb geochronology (including several Grenville-age inherited detrital cores) confirm an evolved magma of continental-arc composition, with the trace-metal geochemistry of magmatic Taconic-age rims on Deicke zircons pointing to a provenance of continental crustal igneous rock melts, specifically a granitic melt with a high level of differentiation, rather than a melt that was mantle-derived, or strongly mantle-influenced. Melt inclusions within Deicke phenocrysts are dacitic (apatite) to rhyolitic (zircon), differences that imply an evolving Deicke magma during its eruption.Our data 1) indicate multiple crustal events, and 2) are tectono-chemical evidence supporting an arc-trench subduction system where subducting preexisting sedimentary rocks were subsequently incorporated into the melt during or prior to collision. Indonesia provides modern analogs: the Banda Arc system, where Australian continental crust is jamming the subduction zone in association with the development of a foreland basin and a sedimentary wedge; the New Guinea – western Melanesian region, with its orogenic highlands and associated foredeep; and the Sumatra –Toba system, where subduction-related explosive volcanism has generated enormous caldera-forming eruptions above continental crust basement. Thus, a model for closure of the Iapetus Ocean involving subduction of older passive margin sediments as part of an island-arc collision with the Laurentian passive margin is acceptable, but models based on a back-arc basin are not compatible with our data.

Potassium-rich granite melt inclusions in zircon from gabbro-hosted felsic stringers, Mid-Atlantic Ridge at 13°34′N: E-MORB connection

We present new data on crystallized micro-inclusions in zircon from clinopyroxene-bearing felsic stringers within gabbronorite of gabbro-peridotite oceanic core complex at Mid-Atlantic Ridge, 13°34′ N. A relatively large (about 49 × 22 μm) crystallized inclusion exposed in situ in thin section consists of the muscovite + biotite + plagioclase + K-feldspar + quartz assemblage. Smaller inclusions (≤20 μm) found in zircon separates consist of silicate minerals, interstitial water solution, and gas bubbles. At temperature close to 800 °C and estimated pressure about 1.5 kbar the inclusions completely homogenize into a K2O-rich (3.9 ± 0.1 wt%), H2O-bearing (about 4 wt%) granite (SiO2 70 ± 2 wt%) melt. Calculations employed DOMINO software show that complete closed-system crystallization of the analyzed melt exactly reproduces mineral assemblage of the large crystallized inclusion (+H2O) at about 1.5 kbar below 600 °C. Granitoid mineralogy of felsic stringers hosting zircon does not correspond to that of the crystallized melt inclusions, with a significantly lower K2O in the former. The K2O loss might result from interaction of the melt with pyroxenes of the host gabbro. This implies that considering felsic granitoid veins, often present in the oceanic lower crustal sections, as simply “frozen melt” is not always justified. Compositional variations of basalt lava glasses from the 13°25′–14° portion of the Mid-Atlantic Ridge show continuous geochemical trends from D-MORB to E-MORB with K2O enrichment up to 0.9 wt%, suggesting mixing from two sources. The most K2O-enriched E-MORB composition (K2O = 0.9 wt%, H2O = 0.7 wt%, K2O/TiO2 = 0.42, (La/Sm)N = 2.45) was assumed as a source of the granitoid inclusions. Computer simulation of equilibrium crystallization of this E-MORB composition returned a granite residual melt closely matching the average composition of the zircon-hosted melt inclusions at 820 °C, 1.5 kbar and melt fraction 5.2 vol%. The result suggests a direct link between the E-MORB magma occurring in the studied area, and the origin of the K-rich granite melt in zircon-hosted inclusions.

Crustal melting vs. fractionation of basaltic magmas: Part 2, Attempting to quantify mantle and crustal contributions in granitoids

Granitic melts may form either directly, by melting of pre-existing crustal rocks, or by fractionation of mafic to intermediate magmas, typically mantle-derived. Each model is applicable to distinct portions of the Earth at different times. Whenever there is an important flux of mafic magmas from the mantle, differentiation of basaltic magmas dominates. In contrast, locations with a lower mafic magma flux, elevated thermal gradients and/or a fertile, thick crust are dominated by crustal melting and reworking. This only partly overlaps with the dichotomy between magmatic arcs and collision zones, as places like e.g. inverted back-arcs also feature large-scale crustal melting, whereas post-collisional domains include a sizeable mantle-derived component. Petrogenesis of individual granitic suites probably cannot be accounted for by pure, end-member models, as these suites typically feature a fair proportion of hybrid or ambiguous granitoids. Thus, we explore various petrogenetic and geological scenarios leading to the formation of hybrid granitoids at various scales. Finally, we outline possibilities to quantify the respective contributions of crust and mantle involved in their formation – hopefully, paving the way for a better understanding and more rigorous discussion of the mechanisms of crustal growth and recycling.

Crustal melting vs. fractionation of basaltic magmas: Part 1, granites and paradigms

Granitoids are a major component of the continental crust. They play a pivotal role in its evolution, either by adding new material (continental growth), or by reworking older continental crust. These two roles correspond to two main ways of forming granitic magmas, either by partial melting of pre-existing crustal rocks yielding granitic melts directly, or by fractionation of mantle-derived mafic to intermediate magmas. Both models represent endmembers, or paradigms that have shaped the way the geological community envisions granitoids, their occurrence, features, formation and meaning for crustal evolution and differentiation of the whole planet.In this paper, we expose the two competing paradigms and their implications. We explore the evidence on which each model is based, and how each school of thought articulates a comprehensive view of granitic magmatism based on field geological, petrological, geochemical (including isotopes) and physical constraints; and how, in turn, each view shapes the thinking on crustal growth and evolution, and the interpretation of proxies such as age and Hf isotopic patterns in detrital zircon databases. We emphasize that both schools of thought build a different, but internally consistent view based on a large body of evidence, and we propose that each of them is, or has been, relevant to some portions of the Earth. Thus, the key question is not so much “which” model applies, but “where, when and to which extent”.

Diversity of granitic rocks constrained by disequilibrium melting and subsequent incremental emplacement and differentiation

Compositional diversity is a fundamental feature of Earth's granitoids and it has been prevailing on Earth since the early Archean. Although the exotic mechanisms, like magma mixing, crustal assimilation and contamination, could place constraints on the diversity, the compositional diversity of common huge batholiths in the field require intrinsic mechanisms running in the magma itself. This paper highlights the detailed processes from melting of a given source to subsequent incremental emplacement and differentiation control the diversity of granitic rocks as we observed on the Earth's surface. The diversity is firstly introduced from the significant heterogeneity of crustal rocks (in minerals and geochemistry) and the melting behaviors (congruent or incongruent melting), by which disequilibrium melting of crustal rocks could generate heterogeneous radiogenic isotopes. At the second stage, such heterogeneity inherited from source and melting behaviors in different melt batches can be reconciled to some extents during the incremental emplacement of melt batches, self-mixing and differentiation in magma chambers, which represent a homogenization process in magma system. The homogenization extent depends on temperature, residence time and other parameters of granitic melts. A large amount of published Hf isotopic data in zircon from granitic rocks provide us a way to assess the roles of disequilibrium melting and homogenization process. On the basis of the difference in zircon Hf isotope compositions and the diffusivity of Hf in granitic melts, we propose a new thermometry for granitic magmatism. Given a certain melt residence time, the compiled global Hf isotopic ratios in zircon from granitic rocks indicate that the magma temperatures of intermediate–felsic rocks decreased steadily throughout Earth's history, corresponding to the secular cooling of Earth's mantle.