Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

The evolution of late-Hercynian granites and rhyolites documented by quartz – a review

Abstract: The potential of igneous quartz for providing a better understanding of magmatic processes is demonstrated by studying late-Hercynian rhyolites and granites from central and western Europe. Cathodoluminescence (CL) reveals growth patterns and alteration structures within igneous quartz reflecting the magma crystallisation history. The relatively stable and blue-dominant CL of zoned phenocrysts is principally related to variations in the Ti concentration, which is a function of the crystallisation temperature. The Al/Ti ratio of igneous quartz increases with progressive magma differentiation, as Ti is more compatible, compared to Al, Li, K, Ge, B, Fe, P during magma evolution. The red-dominant CL of the anhedral groundmass quartz in granite is unstable during electron bombardment and associated with OH- and H2O-bearing lattice defects. Thus, CL properties of quartz are different for rocks formed from H2O-poor and H2O-rich melts. Both groundmass and phenocrysts in granites are rich in alteration structures as a result of interaction with deuteric fluids during cooling, whereas phenocrysts in extrusive rocks do not usually contain such structures. The combined study of trace elements along with the analysis of quartz textures and melt inclusion inventories may reveal detailed PTX-paths of granite magmas. This study shows that quartz is a sensitive indicator for physico-chemical changes during the evolution of silicarich magmas. Common growth textures show a wide variety in quartz phenocrysts in rhyolites and some granites. This paper presents a classification of textures, which formed as a result of heterogeneous intra-granular lattice defects and impurities. The alternation of growth and resorption microtextures reflects stepwise adiabatic and non-adiabatic magma ascent, temporary storage of magma in reservoirs and mixing with more mafic, hotter magma. The anhedral groundmass quartz overgrowing early-magmatic phenocrysts in granites is free of growth zoning.

Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada

Abstract: Comb-layered quartz is a type of unidirectional solidification texture found at the roofs of shallow silicic intrusions that are often associated spatially with Mo and W mineralisation. The texture consists of multiple layers of euhedral, prismatic quartz crystals (Type I) that have grown on subplanar aplite substrates. The layers are separated by porphyritic aplite containing equant phenocrysts of quartz (Type II), which resemble quartz typical of volcanic rocks and porphyry intrusions. At Logtung, Type I quartz within comb layers is zoned with respect to a number of trace elements, including Al and K. Concentrations of these elements as well as Mn, Ti, Ge, Rb and H are anomalous and much higher than found in Type II quartz from Logtung or in igneous quartz reported elsewhere. The two populations appear to have formed under different conditions. The Type II quartz phenocrysts almost certainly grew from a high-silica melt between 600 and 800°C (as β-quartz); in contrast, the morphology of Type I quartz is consistent with precipitation from a hydrothermal solution, possibly as α-quartz grown below 600°C. The bulk compositions of comb-layered rocks, as well as the aplite interlayers, are consistent with the hypothesis that these textures did not precipitate solely from a crystallising silicate melt. Instead, Type I quartz may have grown from pockets of exsolved magmatic fluid located between the magma and its crystallised border. The Type II quartz represents pre-existing phenocrysts in the underlying magma; this magma was quenched to aplite during fracturing/degassing events. Renewed and repeated formation and disruption of the pockets of exsolved aqueous fluid accounts for the rhythmic banding of the rocks.

What Caused the Formation of the Giant Bingham Canyon Porphyry Cu-Mo-Au Deposit? Insights from Melt Inclusions and Magmatic Sulfides

Abstract: Porphyry Cu deposits are commonly thought to have formed by magmas that were unusually rich in metal and/or sulfur. In this study, we test this assumption by reconstructing the metal and sulfur content of an ore-related latite magma at Bingham Canyon and comparing it with that of intermediate magmas in several other arc magma systems. The ore-related latite magma at Bingham Canyon records strong evidence for magma mixing and has a major to trace element composition that can successfully be modeled by a mixture of ~40 wt % mafic magma, which was similar to the most mafic rock found at Bingham Canyon (a melanephelinite containing 45 wt % SiO 2 ), and ~60 wt % felsic magma of rhyolitic composition. Based on the modal abundance of 0.19 ± 0.01 vol % sulfides and laser ablation-inductively coupled plasma-mass spectrometry analyses of unaltered sulfide inclusions preserved within hornblende and plagioclase phenocrysts, the latite magma contained 50 to 90 ppm Cu, 0.8 to 2.0 ppb Au, 2 to 3 ppm Mo, and ≥0.12 to 0.14 wt % S. Whole-rock and melt and sulfide inclusion data suggest that the bulk of copper and Au in the latite magma was derived from the mafic end member, whereas significant amounts of sulfur were also provided by the felsic end member. A rough, independent estimate of the amount of Cu present in the mixed magma can be obtained by taking the Cu content of mafic, sulfide-undersaturated silicate melt inclusions and multiplying it with the mass fraction of mafic magma involved in the magma mixing. Applying this latter approach to two other porphyry Cu-mineralized magma systems (Santa Rita, USA; Bajo de la Alumbrera, Argentina) and several modern arc magma systems suggests that ore-forming intermediate magmas in mineralized systems were not unusually Cu rich. Whether or not they were unusually sulfur rich could not be answered with the available data. If the sulfur contents of mineralizing magmas prove to be normal, then the most distinctive feature of fertile magma systems may be the formation of large, long-lived magma chambers at 5- to 15-km depth and the development of vent structures that enable focused fluid flow.

Crystallization paths of leucite-bearing lavas: Examples from Italy

Abstract: The salic phases found in leucite-basanites, -trachytes, and -phonolites may be used to portray crystallization in the system NaAlSiO_4-KAlSiO_4-CaAl_2Si_2O_8-SiO_2, the phonolite pentahedron. Only two lavas have been found that contain the assemblage leucite-nepheline-plagioclase-sanidine and liquid, a natural pseudo-invariant assemblage (at 900° C±100) equivalent to the isobaric invariant point of the four component system. The diversity of phases in this group of lavas illustrates the role of halogens in controlling their crystallization paths. Thus the presence of F in the leucite-basanites has stabilized magnesian biotite and suppressed sanidine, as has been found in other basanitic lavas (Brown and Carmichael 1969). The presence of Cl in these same lavas has induced the crystallization of sodalite, which takes the place of nepheline in the groundmass. However in the leucite-trachytes, biotite has suppressed olivine and coexists with sanidine and leucite. The presence of S may produce hauyne at the expense of nepheline, and in general sulphate minerals, which include apatite, have the role in lavas of low silica activity that pyrrhotite plays in liquids of high silica activity. Both pyroxenes and titaniferous magnetites in this suite of lavas are very aluminous. Groundmass crystals of pyroxene may have one-fifth of Si replaced by Al. Other phases which occur occasionally are melanite garnet and a potassium-rich hastingsite, but neither ilmenite nor a sulphide mineral has been found. Phenocryst equilibration temperatures, derived from olivine and Sr-rich plagioclase, are generally in the range from 1,050° C to 1,150° C. The high content of incompatible elements (e.g., K, Ba, Rb, F, Sr, P) in these lavas suggests that they represent a small liquid fraction from a mantle source which possibly contains phlogopite.

Geochemical and Sr-Nd Isotopic Characteristics of Post-Collisional Calc-Alkaline Volcanics in the Eastern Pontides (NE Turkey)

Abstract: Major, trace element, K-Ar age and Sr-Nd isotopic data are presented for the Eocene Torul volcanics in the eastern Pontide orogenic belt (NE Turkey). The studied rocks are composed of basaltic andesitic, andesitic, trachyandesitic, and minor trachydacitic lavas associated with their pyroclastics. These rocks contain plagioclase (An2-44), hornblende (Mg#= 0.78-0.98), clinopyroxene (Wo43-46 En41-43 Fs10-15), biotite, quartz, and minor sanidine phenocrysts. K-Ar ages on hornblendes ages range from 43.99 (±2.59) to 33.45 (±2.32) Ma, within the Middle to Late Eocene. The volcanic rocks show calc-alkaline affinities and have medium to high K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depletion of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn= 4.0-9.8) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70457 and 0.70511 and initial 143Nd/144Nd values between 0.51264 and 0.51278. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination ± magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched upper mantle, previously modified by subduction- induced metasomatism in a post-collisional geodynamic setting.