Myrmekite — one hundred years later

Myrmekite, first detected by Michel-Lévy in 1875 and named by Sederholm in 1899, is an intergrowth between vermicular quartz and (sodic) plagioclase situated next to potash feldspar. In felsic plutonic rocks it occurs as: rims bordering granular plagioclase, intergranular blebs set between adjacent microperthite crystals, lobes associated with muscovite in deformed alkali feldspar megacrysts or as bulbous growths at their margins, and rims on plagioclase inclusions held within orthocalse megacrysts. A literature review based largely on papers published in the past quarter century shows that hypotheses for myrmekite genesis fall mainly into five categories: simultaneous or direct crystallization, replacement of potash feldspar by plagioclase, replacement of plagioclase by potash feldspar, solid-state exsolution, and recrystallizing quartz involved with blastic plagioclase.     

蠕英石的成因

蠕英石常见于中酸性侵入岩和片麻岩中。蠕英石石英含量与蠕英石斜长石An值或岩石中Ca含量呈正比关系。条纹钠长石、甚至钾长石的残余体出现在蠕英石中,说明蠕英石是含钙的销质气液对钾长石发生交代作用的产物。尽管有多种形态和产状的蠕英石,其成因却都是相同的。岩石受变动后形成的蠕英石应晚于未变动时形成的蠕英石。     

Cooling process of a granitic body deduced from the extents of exsolution and deuteric sub-solidus reactions: Case study of the Okueyama granitic body, Kyushu, Japan

The variation in cooling processes with depth in a magma body is evaluated quantitatively by analysis of the extent of exsolution coarsening and deuteric coarsening as sub-solidus reactions. This method is applied to evaluation of the Okueyama granitic body of central Kyushu, Japan. Exsolution coarsening has produced microperthite textures in this body, while deuteric coarsening has resulted in patchperthite, myrmekite, and reaction rims, respectively. Through measurement of six textural parameters, including the width and spacing of lamellae and the thickness of myrmekite and reaction rims, the extent of these sub-solidus reactions is shown to increase systematically with depth in the granite body, indicating that the Okueyama cooled gradually from the roof. The hornblende–plagioclase and ternary feldspar thermometers indicate temperature a range of 710 to 620 °C for volume diffusion associated with exsolution coarsening, while deuteric coarsening is found to have occurred at temperatures below 500 °C on the basis of the ternary feldspar thermometer. The cooling period corresponding to exsolution coarsening is estimated using a one-dimensional heat transfer model, yielding periods of 820 y at the roof and 1390–1890 y at the base of the exposure (1000 m below the roof) depending on total depth of the original magma body.     

Petrogenesis of a porphyritic granite east of Oporto (Portugal)

A coarsely porphyritic granite of great areal extent in northern Portugal is described from the locality of Cete, cast of Oporto. Geological and petrographical evidence go to show that it originated by consolidation from a magma of much the same composition.The relative ages and modes of formation of the constituents are deduced from textural relationships. A paragenetical table is given. Initial magmatic conditions are demonstrated by the preferential distribution of the primary accessories in biotite, the earliest of the main constituents. Rare andalusite seems to be an early constituent. Intrusion is correlated with early crystallization. The magma had been emplaced before oligoclase started to separate. The microclineperthite megacrysts are shown to have largely crystallized from a magmatic fluid, ending on a replacement stage. Foliation and lineation, defined by planar and linear parallelism of the megacrysts, have formed in situ, not as flow structures due to intrusion. Orientation of older biotite and plagioclase inclusions in the megacrysts is described. Perthite is inferred to have formed by exsolution.The orthomagmatic stage passed into a final metasomatic period, with replacement textures. The main generation of quartz is partly magmatic, partly metasomatic. Primary plagioclase in contact with microcline suffered decalcification of its borders. There also occur albite fringes around primary plagioclase which were formed by albitization of surrounding microcline. Six different habits of quartz, representing two or three generations, and two types of myrmekite are distinguished. A new type of myrmekite quartz is described. The features of the metasomatic or endoblastic stage are discussed.With 17 Figures and 2 Tables... One of the fundamental branches of plutonic geology, ... is concerned with textures as indicating the time-relations of the rock components. H. H. Read (1949)     

Igneous origin of K-feldspar megacrysts in deformed granite of the Papoose Flat Pluton, California, USA

Many points of evidence, especially igneous microstructures and structures resulting from solid-state deformation, indicate that K-feldspar megacrysts in deformed granites of the Papoose Flat pluton are residual phenocrysts, not porphyroblasts. Evidence of an igneous origin includes features such as crystal shapes, simple twinning, zonally arranged euhedral plagioclase inclusions, oscillatory compositional zoning, and local occurrence in microgranitoid enclaves. Evidence of solid-state deformation of the megacrysts (which is consistent with their existence prior to the mylonitic deformation) includes marginal recrystallization and neocrystallization, microcline twinning, marginal replacement by myrmekite, and recrystallized/neocrystallized “tails”. Evidence of porphyroblastic growth, such as overgrown inclusion trails, is absent. This appears to be the situation in most felsic augen gneisses and mylonites.     

麻粒岩相条件下蠕状石成因的一种解释

关于蠕状石的成因，施工地质学家提出各种不同的成因假说，多数人认为是交代生成的。根据蠕状石总是产在麻粒岩相变质岩和相应的混合岩中，并常出现于斜长石，正长石和石英的接触部位，含蠕状石的斜长石的折射率低于不含蠕状石的斜工石以及近年来花岗质岩和石熔融实验研究获得的成果，笔者认为，蠕状石是在麻粒岩相变质条件下，由溶熔作用在变质岩中局部产生的花岗质熔浆于冷却结晶过程中最后共结产生的。     

Grain-size reduction of feldspars by fracturing and neocrystallization in a low-grade granitic mylonite and its rheological effect

Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite.     

An exsolution silica-pump model for the origin of myrmekite

Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi₂O₈] involves the exchange CaAlK_-1Si_-1, in which the AlSi_-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is pumped into the reaction volume from a silica reservoir, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.     

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。     

Corundum from basaltic terrains: a mineral inclusion approach to the enigma

 This paper investigates the origin of corundum megacrysts that occur in many basaltic terrains, and which are considered to be eroded from basaltic rocks. Geochemical data for over 80 primary mineral inclusions within corundum megacrysts are used to gain a new insight into the petrogenetic history of the corundum megacrysts. A wide spectrum of minerals is present as inclusions in the corundum; the most common are Nb−Ta oxides (such as titaniferous columbite and uranpyrochlore), alkali feldspar, low-Ca plagioclase (albite-oligoclase) and zircon. Rare inclusions include Fe,Cu-sulphide (low in Ni), cobalt-rich spinel, Th,Ce-rich phosphate and uraninite. The similar chemistry of some inclusion minerals from corundum occurring in widely separated areas suggests that the corundum megacrysts in basalts have a similar petrogenesis. Geochemical characteristics of the inclusions indicate a bimodal grouping, which is best explained by a mixing-hybridisation process. This study indicates that the corundum megacrysts are not cogenetic with their basaltic hosts but are crustal fragments accidentally incorporated into the erupting magma. It is suggested that interactions between a silicic component and an intruding carbonatitic or similar Si-poor magma is responsible for Al-oversaturation, resulting in locally distributed lenses of corundum-bearing rock of mixed paragenesis (“hybrid rock hypothesis”). Feldspar exsolution textures provide strong evidence that this hybridisation occurred at mid-crustal levels. Subsequent volcanic eruptions brought the corundum-bearing rocks (later disintegrated in the magma) up to the Earth's surface. This petrogenetic model for corundum megacrysts is experimentally testable. Received: 15 March 1995 / Accepted: 30 June 1995     

Petrological Implication of the Albite Rims in the Felsic Gneisses of the Fuping Complex

The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K-feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400–500°C the albite rim was derived from the K-feldspar composition adjustment in the late or post-magmatism stage. As the temperature decreased, the equilibrium between K-feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magma- or melt-injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium–high grade metamorphism might be subordinate. If present, the anatexis is water-present, but the rim texture cannot be taken as the symbol of anatexis.     

元古宙岩体型斜长岩的特征及研究现状

斜长岩是指斜长石含量>90%的岩浆岩,可分为6类。其中,岩体型斜长岩仅赋存于前寒武纪变质地体中,形成时代主要为元古宙（2.1~ 0.9 Ga）,代表地球演化史上很重要的构造-热事件。岩体呈穹隆状或层状产出,具典型堆晶结构,有含钾长石和斜长石出溶片晶的巨晶斜长石和富铝辉石。巨晶的出溶指示了岩浆由高压至低压的变压结晶过程,体现了斜长岩体深成、浅侵位的特点。关于斜长岩的源区,之前普遍认为源于幔源玄武质岩浆,而近10年来更趋向于源区为下地壳,母岩浆的成分为纹长苏长岩和铁闪长岩等新认识;其成因模式以底侵模式和地壳舌状物熔融模式最具代表性。岩体型斜长岩时空上常与奥长环斑花岗岩共生,构成AMCG（Anorthosite Mangerite Charnockite Granite）岩石组合,被认为属非造山岩浆作用的产物,可能代表大陆裂谷环境。然而,新近一些年龄结果显示,它们形成于造山作用的后期阶段,暗示岩体产出于碰撞后环境。斜长岩体中常赋存有Fe Ti V氧化物矿床,有的富含P及Cu,Ni硫化物等,属典型的岩浆矿床。对此,目前主要有结晶分异过程、早期堆晶过程及不混熔分离3种成因机制解释。由此对今后研究中值得关注的问题提出了一些看法。     

西秦岭糜署岭岩体镁铁质微粒包体的特征及成因

糜署岭岩体的岩石组合为灰白色似斑状二长花岗岩-花岗闪长岩-石英闪长岩，其中镁铁质微粒包体十分发育。包体密集成群、成带状分布，个体大小悬殊且有定向性。有的包体具有细粒边，有的见冷凝边；包体与寄主岩石在成分、色率以及组构上多呈弱过渡关系，界线模糊；包体中发育斜长石异常环带及针状磷灰石，常有寄主岩石中的钾长石巨晶(捕虏晶)。这些特征提供了基性岩浆与酸性岩浆混合的证据，也为探讨本区中生代壳幔混合作用和地壳增生提供了信息。     

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.     

Growth of myrmekite coronas by contact metamorphism of granitic mylonites in the aureole of Cima di Vila, Eastern Alps, Italy

In the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age were strongly deformed during eo‐Alpine regional metamorphism, with local development of ultramylonites. In the ultramylonite matrix, consisting of quartz, plagioclase, muscovite and biotite, microstructures show grain growth of quartz within quartz ribbons, and development of decussate arrangements of mica. These features indicate that dynamic recrystallization related to mylonite development was followed by extensive static growth during contact metamorphism. K‐feldspar porphyroclasts up to 1.5 cm are mantled by myrmekite that forms a continuous corona with thickness of about 1 mm. In both XZ and YZ sections, myrmekite tubules are undeformed, and symmetrically distributed in the corona, and oligoclase‐andesine hosts have random crystallographic orientation. Myrmekite development has been modelled from the P–T–t evolution of the ultramylonites, assuming that the development of the ultramylonites occurred during eo‐Alpine metamorphism at c. 450 °C, 7.5 kbar, followed by contact metamorphism at c. 530 °C, 2.75 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatite protolith indicate that the equilibrium assemblage changes from Qtz–Phe–Ab ± Zo ± Cpx ± Kfs during the ultramylonite stage to Qtz–Pl(An_30–40)–Ms–Kfs–Bt(Ann₅₅) during the contact metamorphic stage. The thermodynamic prediction of increasing plagioclase mode and anorthite content, change of white mica composition and growth of biotite, occurring during the end of the heating path, are in agreement with the observed microstructures and analysed phase compositions of ultramylonites. Along with microstructural evidence, this supports the model that K‐feldspar replacement by myrmekite took place under static conditions, and was coeval with the static growth accompanying contact metamorphism. Myrmekite associated with muscovite can develop under prograde (up‐temperature) conditions in granites involved in polymetamorphism.     

Weakening and strain localization produced by syn-deformational reaction of plagioclase

There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H₂OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H₂O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.     

Eclogitization of basites in early proterozoic shear zones in the area of the village of Gridino, western Belomorie

The considered part of ductile shear zones, which are widespread in the area of the village of Gridino in the central part of the Belomorian Mobile Belt, is accompanied by the high-temperature eclogitization of basites. The paper reports examples of eclogitization in rocks of various age, mostly in rocks of the Early Proterozoic lherzolite-gabbronorite and coronite gabbro complexes. The degrees of structural and mineralogical transformations in unequally deformed bodies of gabbronorites and olivine gabbronorites of the lherzolite-gabbronorite complex are correlated with the degrees of deformations of these rocks and their fluid recycling. The relatively weakly deformed rocks have massive and apomagmatic textures with garnet and omphacite reaction rims at boundaries between grains of magmatic plagioclase and pyroxenes. These rims are typical of the domainal equilibrium stage, which is an intermediate eclogitization stage between a magmatic rock and completely equilibrated eclogite. The enhancement of rock deformation results in the development of equilibrium eclogite with anisotropic metamorphic textures and structures. Textural transformations simultaneous with intense deformations are manifested involved the recrystallization of magmatic minerals and the development of linear fine-grained mineral aggregates of the Pl-Opx-Grt-Omp eclogite mineral assemblage and the systematically oriented growth of much larger mineral neoblasts in amphibole eclogite assemblages.     

Nucleation and growth of myrmekite during ductile shear deformation in metagranites

Myrmekite is extensively developed along strain gradients of continuous, lower amphibolite facies shear zones in metagranites of the Gran Paradiso unit (Western Alps). To evaluate the role of stress, strain energy and fluid phase in the formation of myrmekite, we studied a sample suite consisting of weakly deformed porphyric granites (WDGs), foliated granites (FGs) representative of intermediate strains, and mylonitic granites (MGs). In the protolith, most K‐feldspar is microcline with different sets of perthite lamellae and fractures. In the WDGs, abundant quartz‐oligoclase myrmekite developed inside K‐feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction. In the WDGs, stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between K‐feldspar and albite. In the FGs and MGs, K‐feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine‐grained recrystallized myrmekite and perthite aggregates. This was accompanied by a more pervasive fluid influx into the reaction surfaces, and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone. In the MGs, myrmekite forms complete rims along the outer boundary of the small K‐feldspar porphyroclasts, which are almost completely free of internal reaction interfaces. Therefore, we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress. Mass balance calculations indicate that, in Al–Si‐conservative conditions, myrmekite growth was associated with a volume loss of 8.5%. This resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite.     

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.     

Transformation of garnet megacrysts captured by alkali mafic magma

The results of research of symplectites from the Shavaryn-Tsaram (Hangaj plateau, Mongolia) and Bartoj (Dzhida basaltic field, Russia) alkali basaltic rocks are presented. The symplectite compositions and structures were studied, and the physical and chemical parameters at which primary megacrysts were transformed into secondary mineral assemblages were defined. It is established that both garnet megacrysts and garnet-clinopyroxene aggregates were formed at pressures of 10–13 kbar and temperatures over 1300°C. The transformation of garnet into minerals of the secondary assemblage is considered as solid state water assisted resorption of garnet at a depth corresponding to pressures of 4–8 kbar and temperatures ranging from 1000 to 1300°C. The kelyphitic rims on the garnet megacrysts resulted from melting of the megacrysts at the contact with the hosting alkali basaltic rock.