Crystals whose fast directions of growth are oriented perpendicular to the contact will be able to continue to grow inward toward the melt. Anisotropy, therefore, refers to non-random properties. IUGS-recommended classification for classification of plutonic rocks in the field. The additional rock type anorthosite in the gabbro and diorite field is distinguished from these by having less than 10% dark-colored minerals. Their properties are isotropic, meaning the same in all directions. The Stephanian–Autunian volcanic rocks in the north-western part of the North German Basin are mainly limited to north–north-west- to north–north-east-trending grabens, and their thicknesses vary from a few metres to ∼100 m. In the north-east, they form continuous layers up to 2 km thick and may have covered an area of ∼180 000 km2. Volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet. Phenocrysts in more evolved volcanic rocks (dacites, rhyolites) may contain inclusions containing one or more of the following phases: fluid, vapor, hydrosaline melt, and daughter crystals. Together, the arc system was hundreds of kilometers wide and thousands of kilometers long. These elements correlate with each other (Figure 13f) and increase, along a very scattered trend, with decreasing Mg♯. (A) A rare view of any pegmatite dike shows the variable density of crystal nucleation at the contact surface with the host rock. Classifying Archaean remnant basins with respect to their precise tectonic setting is often problematic, but the selected depositional sequences contain features that enable basin distinction. Elsewhere in the foreland, Stephanian to Autunian magmatic activity occurred in much smaller volumes. (1989). Ex: Granite, Gabbro, Pegmatite are intrusive rocks. Bruce D. Marsh, in The Encyclopedia of Volcanoes (Second Edition), 2015. Inset after Yoon and Chough (1995). In the central and eastern North Sea, basalt, trachyandesite, and rhyolite flows and tuffs occur interbedded with Rotliegend-facies mudstones and sandstones. Accessory phases can strongly influence the partitioning of trace elements between cumulates and melts, and thus their accumulation can be readily identified using bulk-rock compositions. Only the Hwasan caldera has almost perfectly circular shape. The term originated from Pluto, the classical god of the underworld. The tabulation of radiometric ages of igneous rocks, which could possibly be subduction-related (Figure 66), in conjunction with flysch formation deposition, that may be interpreted as accretionary prism material, indicates that subduction must have ceased in the Palaeocene, about 63 Ma ago. Plutonic or intrusive rocks come into being when magma cools and crystallizes gradually inside the earth’s crust. Those that fit into the former category are known as intrusive (or plutonic) rocks, while those that fit into the latter are known as extrusive (or volcanic) rock. Rocks with M ≤90% (includes the majority of the rocks in Fig. The use of the name and concept goes back to the beginnings of the science of geology in the late 18th century and the then hotly debated theories of plutonism (or vulcanism), and neptunism regarding the origin of basalt. This can be refined later in the lab when an accurate knowledge of the mineral content is obtained. Further variations on this basic theme come from the geometry of the wall rock container holding the magma. Direct evidence for Paleoarchaean crust includes relatively large exposures of 3.4–3.2 Ga rocks in southwestern Montana (the MMP and Beartooth Mountains), northwestern Wyoming (Beartooth Mountains), and south-central Wyoming (Sacawee block of the Granite Mountains), and detrital zircons of this age that dominate the age spectra of Neoarchaean metasedimentary rocks throughout the province. Inclined walls enhance this variety, but increasing silica content stifles sorting and variety. A. Henk, M.J. Timmerman, in Encyclopedia of Geology, 2005. The middle depiction shows what can actually be expected under the initial conditions of emplacement of the classical chamber, which is similar to the Peneplain Sill in being nearly of a uniform composition. 5) serves to classify the majority of plutonic rocks. They are divided into plutonic and volcanic rocks. In 1968 the International Union of Geological Sciences set up a Subcommission on the Systematics of Igneous Rocks to examine the problems of classifying igneous rocks and to present recommendations on how they should be named. Anisotropy, therefore, refers to non-random properties. The beryl crystals that nucleated on the surface do not project to the opposite surface on the wall zone (i.e., the pegmatite was undersaturated with respect to beryl only a few cm into the dike). Because they lie equidistant from The Rajang Group flysch belt and the Meratus Mountains (Figure 66), Hamilton (1979) suggested that the volcano-plutonic arc may have been related to a trench in the neighbourhood of the Lupar Line, or alternatively within the Meratus Mountains. After that, the flysch strata must be interpreted as deposited in a basin which was not subducting. The prefix leuco- implies that the rock is lighter colored than is normal, and mela- that it is darker than normal. An integrated magmatic system is more aptly characterized as a mush column. 5). [Redrawn, with permission, from Le Maitre, R. W., et al. The voluminous calc-alkaline volcanism was immediately followed by granitic plutonism (Figure 9.3). a relatively coarse-grained (>3 mm) rock in which the individual crystals can be distinguished with the naked eye and which is presumed to have formed by slow cooling. [Redrawn, with permission, from Le Maitre, R. W., et al. The wider the variety of phenocryst assemblages, the greater is the crystal sorting and layering. Feldspars, which are nearly equidimensional in granites, acquire aspect ratios greater than 10 in pegmatites. B4.]. Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal) rocks and are usually coarse-grained. Paul J. Wallace, ... Erik H. Hauri, in The Encyclopedia of Volcanoes (Second Edition), 2015. The gap between the accretionary prism and the Schwaner Mountains is the natural place for shallow water fore-arc basin formation, and the Cenomanian to Turonian Selangkai Formation is ideally interpreted as occupying a fore-arc basin. L.A. Coogan, in Treatise on Geochemistry, 2007. Certain fields contain two names (e.g., gabbro and diorite). In the field, an accurate measure of the volume percent of each mineral present cannot be obtained and so a provisional name is identified, based on approximate amounts of the felsic minerals present. Archaean terranes are complex amalgamations of volcanic, sedimentary and plutonic rocks (e.g., section 2.4). Alternations of mineral layers have been replicated in experiments, however, in which a flux-bearing and H2O-undersaturated melt crystallized at a temperature far below its liquidus (e.g., Fig. Such rocks are called intrusive rocks or plutonic rocks (e.g. The Pb isotopic characteristics in consort with major and trace-element compositions are most compatible with derivation from a high-μ (238U/204Pb) mantle and/or mafic reservoir that formed initially in the Hadean by either metasomatic enrichment resulting from subduction-like processes or during an early Earth mantle-differentiation event, such as a large mantle upwelling. Border and wall zone, Case #1 pegmatite, Portland, Connecticut. Granites of this age also continue eastwards into Kalimantan (Figure 67). Geology of Japan after Teraoka and Okumura (2003). The monzogranite developed a system of rectangular joints. Some volcanic phenocrysts from basaltic rocks contain dense CO2-rich, primary fluid inclusions, indicating that the basaltic magma was CO2-saturated during crystallization. There is a general increase in Zr/Y with increasing concentrations of these elements in all the sample suites, as observed in clinopyroxene analyses (Figure 11b). A plutonic rock is also called an intrusive rock as it is derived from magma that intruded the rock layers but never reached the earth’s surface. More mafic magmatic activity resulted in the formation of dyke swarms and sills, such as the basaltic Whin Sill and the Midland Valley complexes in Great Britain, the basaltic dyke swarm in South Sweden, and the lamprophyre dyke swarms in the western Highlands of Scotland. We hypothesize that the c.3.3 Ga Wyoming Craton may have formed proximal to or been part of an Eoarchaean protocontinent associated with a similar mantle reservoir, comprised of other high-μ cratons, such as the Slave, Kaapvaal, Pilbara, and Karelia cratons and may have received detritus from older portions of this protocontinent, while forming largely from mantle-derived material. Charles S Hutchison, in Geology of North-West Borneo, 2005. 1 and 5). Generally light colored, coarse-grained plutons of these compositions are referred to as granitoids. Most plutons are thought to be Beryl shows up again in abundance in and along the quartz core of this body. Watch Queue Queue Likewise, elevated P/Y can be used to identify apatite accumulation but much of the existing P data is of insufficient quality to use this fingerprint. The IUGS scheme for plutonic rocks has three parts: Rocks whose total content (M) of mafic group minerals is ≥ 90% (includes those on the extreme left-hand side of Fig. Volcanic rocks form on the surface of a planet from molten rock after it has been ejected or extruded out of a volcano, a point in the crust which ha… Rocks that contain a feldspathoid mineral incorporate it in the rock name (e.g., nepheline syenite, leucite gabbro). Work began on the, This field classification is also used in informal discussion of the origin of common, Geology and Sedimentology of the Korean Peninsula, The Encyclopedia of Volcanoes (Second Edition), In the foreland, a variety of volcanic and, Archaean terranes are complex amalgamations of volcanic, sedimentary and, The presence of primary fluid inclusions in minerals from volcanic or, Kevin R. Chamberlain, Paul A. Mueller, in. Crustal growth mechanisms in the Paleoarchaean and Mesoarchaean appear to be characterized by episodic, broad magmatic blooms with coeval TTG and high-K plutonism at 3.5, 3.3, and 3.0–2.84 Ga. Granite is an example of a plutonic rock. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780124095489124893, URL: https://www.sciencedirect.com/science/article/pii/B0122274105005883, URL: https://www.sciencedirect.com/science/article/pii/B9780124055186000097, URL: https://www.sciencedirect.com/science/article/pii/B9780080437514002303, URL: https://www.sciencedirect.com/science/article/pii/B9780123859389000080, URL: https://www.sciencedirect.com/science/article/pii/B0123693969004743, URL: https://www.sciencedirect.com/science/article/pii/S0166263504800094, URL: https://www.sciencedirect.com/science/article/pii/B9780123859389000079, URL: https://www.sciencedirect.com/science/article/pii/B9780444639011000290, URL: https://www.sciencedirect.com/science/article/pii/B9780444519986500119, Geology and Landscape Evolution (Second Edition), 2018, Reference Module in Earth Systems and Environmental Sciences, Colin H. Donaldson, ... Michael John O'Mahony, in, Encyclopedia of Physical Science and Technology (Third Edition), By the mid-20th century it became clear that the basis of naming rocks from their physical and chemical properties was not uniformly applied and that two centuries of uncontrolled development of nomenclature had resulted in many multiple names for rocks with only minor, or even no, differences in characteristics. Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface. Simplified geological map of the Gyeongsang Arc System (Gyeongsang Volcanic Arc and Gyeongsang Backarc Basin). If magma does manage to reach the earth’s surface, it is no longer insulated by the rocks around it and will, therefore, cool rapidly. Evaluating the effect of grain size on mechanical parameters of rocks from different areas of Khyber Pakhtunkhwa, Pakistan (Comprised of course-grained plutonic rock, mostly feldspar and … Red circles denote the locations of light-blue beryl crystals at the contact. The subsequent main rift stage resulted in fissure eruptions of thick sequences of porphyritic trachyandesite flows that constitute the largest volume of volcanic rocks. The texture in which elongate crystals of feldspar, micas, beryl, tourmaline, and a few others (e.g., columbite) are oriented perpendicular to the substrate surface goes by several names in petrology, including stockscheider and comb layering, but the most widely used term today is unidirectional solidification texture, or UST. Work began on the plutonic rocks and recommendations for their classification were approved by the IUGS in 1972. Perhaps the best-known phaneritic rock is granite. At another extreme are lavas. Igneous rocks form in three main places: where lithospheric plates pull apart at mid- ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting. A knowledge of this, coupled with experimentally determined CO2 solubilities in basaltic melt, can be used to estimate the dissolved CO2 concentration of the basaltic magma at depths. The term plutonic rock is taken to mean an igneous rock with a phaneritic texture, i.e. Copyright © 2020 Elsevier B.V. or its licensors or contributors. The Lupar Formation, including the Pakong Mafic Complex, may also be accepted as accretionary prism material (Figure 66). Plutonic rocks from the _____ cooling of magma, result in the formation of relatively _____ crystals. Some intrusive rocks form tabular bodies. As a result, UST is an essentially one-dimensional growth that expands into two-dimensional surfaces as a few favorably oriented crystals compete successfully against the others along the growth front (London, 2008, 2009). The scheme adopted is another mineralogical one, again using the relative abundances of minerals and their grain size. At yet another extreme are large, long-lived, and heavily reinjected chambers. A pluton (pronounced "PLOO-tonn") is a deep-seated intrusion of igneous rock, a body that made its way into pre-existing rocks in a melted form several kilometers underground in the Earth's crust and then solidified.At that depth, the magma cooled and crystallized very slowly, allowing the mineral grains to grow large and tightly interlocked — typical of plutonic rocks. The lower SF, including any cumulates, was locally disrupted time and again by these injections. The two most incompatible elements for which there are extensive bulk-rock data are Zr and Y. Volcanic rocks were encountered in the drill core in the Kattegat, and seismic data suggest the presence of Early Permian volcanic edifices offshore from eastern Denmark. 4, middle and bottom triangles). These unconformably overlie Westphalian sediments and are interbedded with and overlain by Early Permian aeolian red sandstones. B.7.a) to define what is “normal.”. According to the trace element compositions and rare earth element patterns of the volcanic rocks, these rocks belong to the typical calc-alkaline volcanic rocks related to the subduction (Kim et al., 1991b; Park et al., 2006b; Yun et al., 2000). Since quartz and feldspathoids do not crystallize together from a magma, a rock will plot in either the QAP or the FAP triangle, or occasionally on the common side between plagioclase and alkali feldspar. However, it can be difficult to distinguish between primary fluid inclusions, trapped during igneous crystallization, and secondary inclusions formed by lower temperature hydrothermal processes. These rocks are rare on the surface, but make up peridotite, the rock of the upper mantle. A single diagram, the quartz–alkali feldspar–plagioclase–feldspathoid (or “foid”) double triangle (QAPF diagram) (Fig. Volcanic sequences are clearly the result of long periods of subaerial reinjection or eruption, which means they are actually magma chambers. This was accompanied by the intrusion of large amounts of syenitic magmas. This field classification is also used in informal discussion of the origin of common plutonic rocks, when petrologists will refer to just two broad groups of rocks— granitoids and gabbroids. During the initial stages, felsic intruded Westphalian sediments, accompanied or shortly followed by the extrusion of up to 1500 m of primitive, alkaline basalt to phonotephrite lavas in the southern parts of the rift. That isotropy arises from the randomness of their mineral distributions and from the absence of preferred orientation of those minerals. Plutonic rocks Plutonic rocks form when magma cools within the Earth's crust. A number of polygonal fields enable classification of samples. Despite the overlap between the concentrations of the compatible elements Cr and Ni in gabbros and basalts, there is little overlap in Zr and Y concentrations, indicating that incompatible elements are efficiently fractionated between the upper and lower oceanic crust (see Section 3.19.2.3.3.3). Magma chambers are thus integrated systems behaving in response to system size, nature of holding regime, and state of crystallinity of the magma itself. Le Maitre et al. The volcanic rocks reach thicknesses of up to 160 m in the central North Sea, and ∼680 m in the Horn Graben. The … Although pluton is a general term to describe an intrusive igneous body, there has been some confusion around the world as to the definition of a pluton. 5. It assumes that plutonic rocks contain crystals that are visible to the naked eye but did not specify an absolute lower crystal size for plutonic rocks. For medium-grained gabbroic rocks the term microgabbro may be used: dolerite and diabase, however, are synonyms of long-standing usage which the Subcommission recognized as so entrenched as to require retention. (B) The thin border (~ 4 mm) transitions sharply into coarse-grained UST with skeletal gray quartz and creamy K-feldspar. A review of the differences between intrusive and extrusive igneous rock textures and process. These sequence types record the principal stages during which basins formed, although there are numerous hybrid basin settings or subsettings (Ingersoll, 1988). From: Geology and Landscape Evolution (Second Edition), 2018, David London, in Reference Module in Earth Systems and Environmental Sciences, 2020. These formations covering small areas are called stocks. See more ideas about Geology, Geology rocks, Paleontology. On the figure above, the top row has both plutonic and volcanic igneous rocks arranged in a continuous spectrum from felsic on … In the foreland, a variety of volcanic and plutonic rocks are present in the Oslo Rift in south Norway, and large volumes of rhyolitic and andesitic volcanic rocks occur in northern Germany below younger cover and are known only from deep boreholes. This is because plutonic rocks are rocks formed when magma cools and solidifies below the earth's surface, and volcanic rocks are rocks formed when … It contains all the necessary elements, or foliated mudstones, mélange zones and ophiolite. The greater enrichment of Cr and Ni, for a given Mg♯, for samples from Hess Deep than in plutonic rocks from the other areas (Figures 13d and 13e) suggests greater reaction of olivine and clinopyroxene and/or spinel with interstitial melt in the Hess Deep rocks. At first sight the QAPF diagram is intimidating because of the large number of fields and names. In Scania in South Sweden, a north-west-trending swarm of subalkaline dolerite dykes may be related to a few isolated, north–north-west-trending dolerite dykes on the south-west coast and Bornholm Island, and to dolerite sills that intrude Cambrian shales in Västergötland. Ordinary plutonic rocks are mostly perceived as homogeneous in mineralogy and texture. Colin H. Donaldson, ... Michael John O'Mahony, in Encyclopedia of Physical Science and Technology (Third Edition), 2003. They have large crystals that are usually visible without a microscope. Plutonic rocks are also known as ‘intrusive igneous rocks’ because they form when magma squeezes into cracks and crevices, as if it is an ‘intruder’ who is invading the rocks. By continuing you agree to the use of cookies. Nov 6, 2016 - Explore Susanne Janecke's board "Geology: Plutonic Rocks", followed by 136 people on Pinterest. Archaean rocks from this region are characterized by extremely radiogenic initial Pb isotopic compositions, including samples with mantle-like Nd isotopic compositions and U–Pb ratios much below average continental crust. Recommendations about the nomenclature of volcanic rocks and exotic plutonic rocks were made and approved over the following 15 years and are collected in the Subcommission's principal report (Le Maitre et al., 1989), which included recommendations for the removal of the majority of the 1500 rock names that then existed. Figure 67. “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford, Fig. Yet, their story is still important. Injection of phenocryst-laden magma along steeply inclined walls, as mentioned already, promotes rapid sedimentation, avalanching, and sorting, leaving a telling sequence reflecting an intricate dynamic record that continues to evolve via diffusion until dropping temperature finally freezes in the record. In northern Britain, up to ∼100- to 240-m-thick sequences of alkaline basalt lavas and tuffs occur in the Mauchline and Thornhill basins. These rocks appear on the surface only after being uplifted and denuded. Intrusive, or plutonic, igneous rock forms when magma remains inside the Earth's crust where it cools and solidifies in chambers within pre-existing rock. The diagram shows that the “group” names peridotite, pyroxenite, and hornblendite may also be used for those ultramafic rocks that are respectively olivine-, pyroxene-, and hornblende-rich. The net result can be exotically layered sequences sandwiched within monotonous nonlayered sequences, or even a thick, coarsely layered sequence followed by a finely layered sequence, marking perhaps two distinct injection episodes, much as seen in volcanism itself. The Wyoming Craton is characterized by widespread Meso- to Neoarchaean plutonic and metasedimentary rocks that have Nd isotopic compositions that require the incorporation of Paleoarchaean to Hadean crust. The QAPF classification produces a general “root name” for a rock. Volcanic rocks are fine-grained and are found on most terrestrial planetary bodies in our solar system. That isotropy arises from the randomness of their mineral distributions and from the absence of preferred orientation of those minerals. The composition of the plagioclase must be used to distinguish gabbro (anorthite, An ≥ 50%) from diorite (An≤50%). Ordinary plutonic rocks are mostly perceived as homogeneous in mineralogy and texture. A 15–25 km thick, seismically fast, lower crustal layer (7.x layer) imaged beneath the northern two-thirds of the Wyoming Craton may serve as a proxy for the distribution of ancient crust. Watch Queue Queue. The distribution of extrusive volcanic rocks represents an occurrence of an arcuate volcanic arc associated with subduction of the Izanagi Plate. The presence of these inclusions also can be taken as evidence for vapor saturation during crystallization, but the multicomponent nature of such inclusions (H2O–CO2–NaCl) makes it difficult to infer the original dissolved volatile concentrations in the coexisting silicate melt. Plutonic rocks are characteristically coarse-grained. intrusive, or plutonic rocks. The environment of formation produces characteristic textures in igneous rocks which aid in their identification. Figure 9.4. Later, it would have migrated to the Lupar Formation outcrop. Note the subdivision of the backarc basin into three subbasins. They are interpreted as products of the end-stage of subduction, as the formerly subducting Proto South China Sea slab no longer subducts, but falls away with a steeping trajectory into the mantle under its own weight, a process colloquially known as ‘slab roll-back’ (Figure 67). The fast growth directions of crystals are those in which the crystal surface advances with the least amount of added material, i.e., directions of low atomic density through the crystals (London, 2008). The early trench would have been located within the Serabang Formation outcrop. In reality it is simple because only a small number of essential names are employed (gabbro, monzonite, syenite, diorite, granite, and anorthosite) which are combined (e.g., monzogabbro) or have a mineral prefix added (e.g., quartz syenite). Pluton, body of intrusive igneous rock the size, composition, shape, or exact type of which is in doubt; when such characteristics are known, more limiting terms can be used. They are simply the rocks formed through heating then followed by … The rock name can then be read from the polygonal field in which it plots. The granitic rocks comprise diorite/granodiorite with porphyritic texture and I-type characteristics, indicative of shallow-depth emplacement, either in the lower crust or in the upper mantle (Cho and Kwon, 1994; Hong, 1987; Jin, 1980; Jwa et al., 1990; Lee, 1992a). This issue, which is historically almost a philosophical matter in petrology, still plagues interpretations; for it is always easy to interpret a complex rock sequence as due to a specific string of injection events, which moves the problem to another venue. P.G. Such inclusions can be used to estimate the pressure of inclusion entrapment. These are perhaps end-members (see Figure 8.7), and the larger the body the higher the probability that it was formed over a significant period of time (say, hundreds to thousands of years) involving many injections of magma with varying amounts of phenocrysts. The lower depiction shows the result the serial emplacement of crystal-laden slurries, which, depending on the magnitude and frequency of delivery, can produce a highly varied final result. IUGS classification of plutonic rocks using the QAPF diagram. In this sense, igneous rocks are formed when molten rock (magma) solidifies either underneath the earth’s crust to form plutonic (intrusive) igneous rocks or on the surface of the earth to form volcanic (extrusive) igneous rocks. Two diagrams are employed, depending on whether hornblende is a significant constituent or not: the olivine–orthopyroxene–clinopyroxene triangle and the olivine–hornblende–total pyroxene (orthopyroxene + clinopyroxene) triangle Fig. Given the geochemical tools and physical insight now available on crystallizing systems, however, this question can be approached with far more objectivity and certainty than ever before. The lesson learned is that the three dynamic regimes of magma, characterized by large chambers, sills, and lava flows, each shows different aspects of the same spectrum of physical and chemical processes common to magma. Igneous rocks with Q ≥ 90% are unknown, so the term quartzolite is strictly speaking redundant. Sung Kwun Chough, in Geology and Sedimentology of the Korean Peninsula, 2013. Plutonic rocks form from igneous intrusions deep in the Earth's crust. But the small size of individual injections (i.e., flow thickness) promotes rapid cooling, thereby prohibiting crystal sorting, annealing, and extensive self-organization. “A Classification of Igneous Rocks and Glossary of Terms,” Blackwell, Oxford, Fig. They are the most common rocks on Earth and form the basis of our continents. Unidirectional solidification with elongation in the direction of growth arises even when a randomly oriented population of crystals nucleates on a substrate (Fig. One set, oriented roughly horizontally, resulted from the removal—by erosion—of the miles of overlying rock, called gneiss (pronounced “nice”). Granite). This video is unavailable. Plutonic rock formations covering over 100 sq km area are called batholiths. Ilmenite and titano-magnetite accumulation in a rock can be identified by enrichments in Ti over similarly incompatible elements. Previous comprehensive studies concern Archaean lithofacies (Eriksson et al., 1994), general Precambrian basin attributes (Eriksson et al., 2001b), clastic sedimentation patterns (Ojakangas, 1985), greenstone sedimentation (Lowe, 1994) and synrift and craton cover sequences (Eriksson and Fedo, 1994). Gabbros are extremely common rocks, with a variety of mineralogical characteristics that cannot all be represented on the QAPF diagram. Scarce Eoarchaean minerals consist of seven known occurrences of detrital zircons and xenocrystic zircon cores in Meso- to Neoarchaean rocks. In the case of pegmatites, zonation leads to textural and to chemical/mineralogical anisotropy. Five eruptive stages have been recognized, and the total volume has been estimated at ∼48 000 km3, approximately 70% of which are rhyolitic lavas and ignimbrites that predominate in the eastern part of the basin. The petrologist is then free to add qualifiers that may be a mineral present in small quantities (e.g., hornblende gabbro), a textural term (e.g., porphyritic granite), a chemical term (e.g., Sr-rich granite), a genetic term (e.g., contaminated diorite), a tectonic term (e.g., collision zone granite), or any other term that usefully describes special qualities that the rock possesses. They involve the "intrusion" or insertion of magma between other rocks, which then cools below the surface. Especially feldspathoid-rich plutonic rocks are sensibly accorded names reflecting the dominant mineral, for example, nephelinolite or leucitolite. This magma is under high pressure and takes a long time to cool, allowing time for the formation of large crystals. In spite of this, the thickness of highly fluid magma at any time may have been only a few hundred meters or less, as has been recognized by R. Hunter at Rum in piles of pieces of the lower SF dislodged by the injections.
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