Late crystallization of K-feldspar and the paradox of megacrystic granites

K-feldspar crystals >5 cm in greatest dimension are common in calc-alkaline granites and granodiorites worldwide. Such megacrysts are generally interpreted as having grown to large sizes early in a magma’s crystallization history while they were largely molten, owing to field relations such as megacryst alignment and megacryst-rich clusters and to crystallographic features such as zonally arranged inclusions and sawtooth Ba zoning. These features are consistent with early growth but do not require it. In contrast, experimental petrology, mineral compositions, and natural examples of partial melting of granite demonstrate that K-feldspar is typically the last major phase to crystallize and that most K-feldspar growth occurs after the magma crosses the rheologic lock-up threshold of ~50 % crystals. The near-absence of K-feldspar phenocrysts in dacite lavas and tuffs, even in highly crystalline ones, demonstrates that natural magmas do not precipitate significant K-feldspar while they are mobile. The highly potassic compositions of megacrysts (and indeed, of K-feldspar in non-megacrystic granites as well) require exsolution of albite component down to temperatures of ~400 °C. The low Ca contents of megacrysts cannot result from exsolution of anorthite and must represent recrystallization of the crystals at low temperature. These mineralogical and experimental constraints require that K-feldspar megacrysts indicate widespread and thorough recrystallization of the host granites and granodiorites.     

How late are K-feldspar megacrysts in granites?

Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains.     

The morphology and chemistry of K-feldspar megacrysts from Ikizdere Pluton: evidence for acid and basic magma interactions in granitoid rocks, NE-Turkey

Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with X_Mg[=Fe_tot/(Fe_tot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, X_Mg[=Mg/(Mg+Fe_tot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab₇₅₋₂₅An₆₅₋₃₅) and as minor quartz. The compositions of the KFMs range from Or₉₅Ab₅An₀ to Or₈₂Ab₁₇An₁. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma.     

Scale of pluton/wall rock interaction near May Lake,Yosemite National Park,CA, USA

Interaction of magma with wall rock is an important process in igneous petrology, but the mechanisms by which interactions occur are poorly known. The western outer granodiorite of the Cretaceous Tuolumne Intrusive Suite of Yosemite National Park, California, intruded a variety of metasedimentary and igneous wall rocks at 93.1 Ma. The May Lake metamorphic screen is a metasedimentary remnant whose contact zone exhibits a variety of interaction phenomena including xenolith incorporation, disaggregation, and partial melting. The chemical contrast of these metasedimentary rocks with the invading pluton provides an excellent measure of pluton/wall rock interactions. Wall rock xenoliths (mostly pelitic quartzite) are predominantly located in an elongate horizon surrounded by a hybridized fine-grained granodiorite. Initial Sr and Nd isotopic ratios of the hybridized granodiorite indicate significant local incorporation of crustal material. Major- and trace-element geochemical data indicate that contamination of the granodiorite occurred via selective assimilation of both high-K and low-K, high-silica partial melts derived from pelitic quartzite. Although the hybridized granodiorite shows significant amounts of contamination, adjacent to xenoliths the proportion of contamination is undetectable more than a meter away. These results indicate that the chemical and isotopic variability of the Tuolumne Intrusive Suite is not caused by magma contamination via in situ wall rock assimilation.     

Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano,Chile

K-feldspar megacrysts are common in granitoids, but relatively rare in chemically equivalent volcanic rocks. Dacites from Taapaca volcano have euhedral sanidine megacrysts up to 5 cm long. Small crystals, where present, are rounded. Growth of the megacrysts engulfed plagioclase and amphibole crystals. Crystal size distributions (CSD) of sanidine megacrysts are hump shaped. All these data show that megacrysts developed from the host magma by coarsening: this was enabled by the cycling of magma temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic overturns. Plagioclase crystals enclosed in the megacrysts are small and have short, steep, straight CSDs, which contrasts with the CSDs of plagioclase in the groundmass which are shallower and extend to larger sizes. This shows that plagioclase was also coarsened approximately synchronously with sanidine, in response to the same temperature conditions.     

Fabric studies within the Cascade Lake shear zone, Sierra Nevada, California

The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc.     

花岗岩中的钾长石巨晶：以南岭佛冈花岗质杂岩体中微斜长石巨晶为例

野外地质观察和岩石显微结构研究表明,佛冈花岗质杂岩体中微斜长石巨晶是岩浆结晶的产物,不是交代斑晶,也不是变斑晶,它们与基质中的微斜长石构成双峰式粒径,反映了岩浆的两阶段结晶历史,由于微斜长石是钾长石的低温变体,因此一种可能的机制是佛冈花岗质杂岩岩浆在侵位和基本固结后,冷却缓慢,使早结晶的正长石转变为微斜长石,此外,在新近的研究中,还发现了罕见的,但在佛冈花岗杂岩体中为数不少的“十字”贯穿式双晶微斜长石,以及微斜长石巨晶的“环斑”和“珠边”结构。     

K-feldspar megacrysts in granites — Phenocrysts,not porphyroblasts

K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts. Most of the microstructural, mineralogical and chemical evidence (e.g., shape, alignment, concentration, Ba content, zoning, inclusions, and twinning) favours a phenocryst origin. The main features that have been used to support a porphyroblast origin are occurrence of megacrysts: (1) across aplite vein boundaries, (2) in country rocks, and (3) in or across boundaries of microgranitoid enclaves (mafic inclusions). However, these features can be explained by the phenocryst hypothesis. In particular, megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave. All available evidence favours or is consistent with a phenocryst origin for K-feldspar megacrysts in granitoid rocks and their enclaves.The large size of the megacrysts is evidently due to nucleation difficulties for K-feldspar in granitic melts. Though K-feldspar is commonly the last mineral to begin crystallizing in granitic magmas, abundant melt is still present at that stage, allowing sufficient space for the megacrysts to grow. The reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called “megacrysts” until the magma contains such a high proportion of crystals that it cannot erupt.     

北京周口店岩体中钾长石巨晶的特征及成因

花岗岩类岩石中的钾长石巨晶,有的认为是斑晶,有的认为是变斑晶。本文通过对北京周口店岩体中钾长石巨晶的野外产状、显微结构、矿物化学等特征研究,证明巨晶是在岩浆结晶作用的早—中期阶段,直接从不饱和水的熔体中生长而成的斑晶。钾长石巨晶多见于较贫钾长石组分的花岗岩类岩石(石英二长岩-花岗闪长岩)中,其原因可能是,与花岗岩和白岗岩岩浆相比,这种成分的岩浆温度较高,粘度较小,组分扩散较快,钾长石的成核速率较小,而生长速度较大。周口店岩体中钾长石巨晶的大小和含量的变异,可能主要受冷凝梯度dT/dt的控制。     

Multi-batch, incremental assembly of a dynamic magma chamber: the case of the Peninsula pluton granite (Cape Granite Suite, South Africa)

The S-type Peninsula Pluton (South Africa) exhibits substantial compositional variability and hosts a large variety of mafic and felsic magmatic enclaves with contrasting textures and compositions. Moreover, the pluton is characterized by mechanical concentrations of K-feldspar megacrysts, cordierite and biotite, generating a complex array of magmatic structures including schlieren, pipes, and spectacular sheeted structures. Chemical evidence indicates that the pluton is constructed incrementally by rapid emplacement of numerous magma pulses. Field, and textural data suggest that magmatic structures form by local flow at the emplacement level of highly viscous crystal-rich magmas (i.e. crystallinity up to 50?vol.%) through magma mushes assembled from older batches. At the time of arrival of relatively late magma batches, some areas within the pluton had achieved crystal fractions that allowed the material to act as a solid, whilst maintaining enough melt to prevent formation of sharp intrusional contacts. Magmatic structures represent “snapshots” of processes that operate in multiphase crystal-rich mushes and their genesis is due to mechanical and thermal instabilities in the crystal-rich magma chamber that are triggered by the emplacement of pulses of new magma derived from the melting of a compositionally variable metasedimentary source.     

北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的岩石成因及其构造意义

文中研究了北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的年代学、地球化学和Sr-Nd同位素组成。毛藏寺岩体主要岩石类型为花岗闪长岩。锆石U Pb定年获得花岗闪长岩岩浆结晶年龄为（424±4） Ma。花岗闪长岩具有高的Mg#（约55）,K2O/Na2O=0.77~0.91,A/CNK=0.92~0.94,表明岩石属准铝质。在微量元素组成上,花岗闪长岩富集LILE、亏损HFSE,轻重稀土分异明显［(La/Yb)N=16.9~19.5］,具有弱的Eu负异常（Eu/Eu*=0.75~0.83）;花岗闪长岩具有ISr=0.706 3~0.706 5,εNd(t) =-1.5~-1.1,TDM=1.10~1.16 Ga。这些地球化学特征和Sr Nd同位素组成表明,花岗闪长岩岩浆源区为基性下地壳变玄武质岩石,但在成岩过程中有少量幔源物质的加入。黄羊河岩体主要由钾长花岗岩组成,其岩浆结晶年龄为（402±4） Ma。岩石富碱（K2O+Na2O=6.91‰~7.66%）,K2O/Na2O>1,A/CNK=0.97~1.05。钾长花岗岩富集LILE及HFSE,轻重稀土元素分馏中等［(La/Yb)N =10.6~17.8］,并具有明显的负Eu异常（Eu/Eu*=0.43~0.68）,表明钾长花岗岩具有铝质A型花岗岩的地球化学特征。钾长花岗岩具有ISr=0.710 3~0.711 3,εNd(t)=-6.7~-6.0,TDM=1.46~1.55 Ga,反映岩浆主要来自地壳中长英质物质的部分熔融。冷龙岭地区花岗岩类的岩石成因及其岩浆演化揭示了北祁连山造山带从加里东早期的挤压构造体制向加里东晚期的伸展构造体制的演化。这些花岗岩类形成于碰撞后构造背景,岩浆的产生可能与俯冲的北祁连洋板片的断离作用有密切联系。     

Kimberlites and megacrystic suite: Isotope-geochemical studies

Major and rare-earth element data on Cr-poor megacrystic suite from Yakutian kimberlites were generalized. Sr-Nd isotopes were studied in garnet, clinopyroxene, and phlogopite megacrysts as well as in garnet and clinopyroxene from deformed xenoliths. It was shown that Sr-Nd composition of these minerals is similar to that in the least altered diamondiferous kimberlites. The crystallization age of megacrystic minerals was determined by Rb-Sr isochron and Ar-Ar (for phlogopite megacrysts) methods. Obtained data indicate that crystallization of Cr-poor megacrystic suite began at the prekimberlitic stage and continued to the pipe emplacement. It was established that garnets from coarse-porphyric deformed lherzolites and megacrysts are similar in major and rare-earth element compositions and were derived from a common asthenospheric source. However, the distribution of incompatible elements and P-T estimates of crystallization cannot be explained by hypothesis of fractional crystallization of garnet megacrysts. It is suggested that megacrystic assemblage crystallized directly in asthenospheric melt. En route to the surface, this melt caused a metasomatic reworking of lithospheric mantle, on the one hand, and was enriched in Mg and Cr owing to the contamination by lithospheric material, on the other hand.     

微区-微量样品Rb-Sr同位素分析技术及其应用前景

利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始⁸⁷Sr/⁸⁶Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。     

Crystallization of Cr-poor and Cr-rich megacryst suites from the host kimberlite magma: implications for mantle structure and the generation of kimberlite magmas

Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.     

Crystallization and resorption in plutonic plagioclase: Implications on the evolution of granodiorite magma (Gęsiniec granodiorite, Strzelin Crystalline Massif, SW Poland)

Granodiorite from the Gęsiniec Intrusion, Strzelin Crystalline Massif, SW Poland contains complexly zoned plagioclases. Five chemically and structurally distinct zones can be correlated among crystals: ‘cores’ (25–35% An), inner mantles (40–45% An), outer mantles (40–25% An), resorption zones (35–50% An) and rims (35–30% An). Good structural and chemical (major and trace elements) correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale. Plagioclase, being the liquidus phase, records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes.

Crystallization began with the formation of inner mantles. The paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma. Normally zoned inner mantles were formed under increasing undercooling. Compositional trends in mantles suggest closed system crystallization.

The major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase. The injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast, kinetically controlled growth of complex multiple, oscillatory zonation within resorption zones. The ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles—the ‘cores’. The decompression range is estimated at a minimum of 2 kbar. Emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases. Melt inclusions crystallized completely during post-magmatic cooling.

The zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution. Zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur. Crystals probably did not travel between different regimes. Resorption occurred but as single albeit complex episodes. Good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects.     

Alkali feldspar megacryst growth: Geochemical modelling

Summary Alkali feldspar megacrysts from the porphyritic Karkonosze granite (Western Sudetes, Poland) were formed during magma mixing. Barium concentrations in zoned crystals, a sensitive indicator of feldspar migration between coeval magmas, serve to reconstruct the crystallization path of the megacrysts. Based on geochemical data, a double mixing model for the formation of the porphyritic granite and for megacryst growth is constructed. The feldspar growth model supports megacryst nucleation and early crystallization in a hybridized crustal magma of granodioritic composition. The growth model gives credibility of the choice of partition coefficients used in the modelling. Insights gained from mixing models based on whole rock composition and mineral zonation allow the recognition of various hybridization events that are reflected in a variety of megacryst crystallization paths within the pluton.     

Pb isotopic zoning of K-feldspar megacrysts determined by Laser Ablation Multi-Collector ICP-MS: Insights into granite petrogenesis

This study investigates Pb isotopic zoning in magmatic K-feldspar megacrysts from the Monte Capanne pluton (Elba, Italy) using Laser Ablation Multi-Collector-ICPMS. The studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal-derived magmas.The investigated K-feldspar megacrysts exhibit strong zoning in ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, correlated with lead elemental variations. We interpret these variations as reflecting growth zoning, as opposed to secondary diffusive exchange. Despite a great variety of zoning patterns, we were able to correlate different events of megacryst growth, reflecting crystallization in a dynamic magma system. Our two-step model includes (1) growth of a granitic magma chamber by addition of low ²⁰⁸Pb/²⁰⁶Pb magma to a high ²⁰⁸Pb/²⁰⁶Pb magma contaminated with crustal material (i.e., the megacryst cores) and (2) recharge by mantle-derived magma (i.e., the megacryst rims). We interpret the thorogenic nature of the megacryst rims to reflect the mantle-derived component involved in the mixing process. Taking account of other data from the Tuscan Magmatic Province, the mantle source is inferred to have been metasomatized by continental material during subduction. TIMS Sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled.     

Geochemical and isotopic (Nd–O) evidence bearing on the origin of late- to post-orogenic high-K granitoid rocks in the Western Superior Province: implications for late Archean tectonomagmatic processes

The granitoid rock dominated central Wabigoon subprovince of the Superior Province records low-K trondhjemite–tonalite–granodiorite (TTG) type magmatic episodes at <2.83–2.74 and 2.722–2.709 Ga, and high-K mafic to felsic plutonism at 2.690–2.685 Ga. High-K units consist of granite to granodiorite dykes and sills, a K-feldspar megacrystic granodiorite suite of sanukitoid affinity and a suite of mafic dykes and intrusions. Initial ϵ_Nd values (−3.1 to +3.3) indicate variable input to all units from light REE-enriched older crustal materials. The δ¹⁸O (VSMOW) range of felsic compositions (+7.1 to +8.9%) overlaps closely that of average upper Superior Province crust. The granite/granodiorite units probably received melt components derived from both older tonalitic crust and isotopically juvenile supracrustal material. The thermal flux for partial melting was provided by mafic components of the coeval megacrystic granodiorite suite. This latter suite likely formed by extensive crustal assimilation and fractionation of enriched-mantle-derived high-Mg dioritic magmas in a post-collisional setting, possibly resulting from slab breakoff or broader scale lithospheric delamination. A genetic link is inferred between mafic magmatism and the late- to post-tectonic high-K granitoid magmatism that typically represents the last stabilization event within Superior subprovinces. That crustal recycling processes played a major role in the petrogenesis of central Wabigoon high-K granitoid suites is consistent with other evidence that supports repeated and substantial continental recycling within this subprovince as far back as the Mesoarchean.     

Crustal Contamination of Picritic Magmas During Transport Through Dikes: the Expo Intrusive Suite, Cape Smith Fold Belt, New Quebec

The Proterozoic Expo Intrusive Suite comprises a series of maficto ultramafic intrusions crosscutting the Povungnituk Groupof the Cape Smith Fold Belt in New Quebec. The intrusions aremainly in the form of blade-shaped dikes that penetrate a sediment-richhorizon in the middle of the Beauparlant Formation and terminatedownward against massive basalts of the lower Beauparlant Formation.Significant accumulations of magmatic sulfide occur at the basalterminations of the dikes. At stratigraphic levels above theBeauparlant Formation the intrusions appear as broad dikes orsills within the Nuvilik Formation, below the mineralized lavaflows and subvolcanic intrusions of the Raglan Formation. TheExpo Intrusive Suite and the mineralized bodies of the RaglanFormation are probably coeval and comagmatic with the overlyingChukotat Group. Post-emplacement folding has exposed the ExpoIntrusive Suite over about 5 km of structural relief, revealingthe basal sulfide concentrations where dike segments terminateon the flanks of anticlines. The parent magma as preserved inchilled margins and narrow dikes was a picrite containing 17wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th,U and Nb relative to middle and heavy rare earth elements. Thecompositions of ultramafic cumulate rocks within the intrusionsare strongly enriched in Th, U and Nb relative to heavy rareearth elements, reflecting assimilation of the enclosing basaltsand metasediments. Modeling of the assimilation process suggeststhat the picritic magma was capable of assimilating masses ofbasalt or sediment up to 50% of the original mass of magma.Assimilation of 10% of a mixture of basalt and sediment causedthe magma to become sulfide-saturated, and was accompanied bythe crystallization of masses of ultramafic cumulates approximatelyequal to the mass of rock assimilated. The presence of dikeswhose chilled margins resemble uncontaminated primary magmasbut that contain abundant cumulates recording wholesale assimilationof host-rocks indicates that the process of assimilation andfractional crystallization required to produce continental tholeiitesfrom picritic parent magmas may not require the presence oflong-lived magma chambers, but can occur during transport alongdikes and reaction with wall-rocks. KEY WORDS: komatiite; Expo Intrusive Suite; assimilation; fractional crystallization; sulfide mineralization     

Magma mixing and mingling textures in granitoids: examples from the Galway Granite, Connemara, Ireland

Summary ?Many granitoid intrusions display textural evidence for the interaction of mafic and silicic magmas during their genesis. The ∼ 400 Ma Galway Granite exhibits excellent evidence for magma mixing and mingling both at outcrop/map scale (magma mingling and mixing zones), and at thin-section/crystal scale (mixing textures). These textures – quartz ocelli, rapakivi feldspars, acicular and mixed apatite morphologies, inclusion zones in feldspars, anorthite ‘spikes’ in plagioclase, sphene ocelli, K-feldspar megacrysts in mafic microgranular enclaves (MME), and mafic clots – constitute a textural assemblage whose origin can be explained in terms of magma mixing and mingling models. Furthermore, textures from this assemblage have been recorded throughout the Galway batholith indicating that magma mingling and mixing played a key role during its evolution. Received November 18, 2000; revised version accepted November 6, 2001