Hubbry Logo
search
logo
Felsic avatar
Felsic
Felsic
current hub
F

Felsic

logo
Community Hub0 Subscribers
Read side by side
Felsic
View on Wikipedia
from Wikipedia

In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.[1] It is contrasted with mafic rocks, which are richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Molten felsic magma and lava is more viscous than molten mafic magma and lava. Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Terminology

[edit]

Acid rock

[edit]

In modern usage, the term acid rock, although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO2 by weight) volcanic rock, such as rhyolite. Older, broader usage is now considered archaic.[citation needed] That usage, with the contrasting term "basic rock" (MgO, FeO, mafic), was based on an ancient concept, dating from the 19th century, that "silicic acid" (H4SiO4 or Si(OH)4) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from an acid-base perspective in aquatic chemistry considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by a tridimensional network of SiO44– tetrahedra connected to each other. Once released in water and hydrolyzed, these silica entities can indeed form silicic acid in aqueous solution.

Etymology

[edit]

The term "felsic" is a derivation of the words "feldspar" and "silica".[2] The similarity of the resulting term felsic to the German felsig, "rocky" (from Fels, "rock"), is accidental. Feldspar is from the German Feldspat, a compound of the German Feld, meaning field, plus spat[h], meaning mineral.[3]

Classification of felsic rocks

[edit]
A felsic volcanic lithic fragment, as seen in a petrographic microscope. Scale box is in millimeters.

In order for a rock (rather than a mineral) to be classified as felsic, it generally needs to contain more than 75% felsic minerals (namely quartz, orthoclase and plagioclase). Rocks with greater than 90% felsic minerals can also be called leucocratic,[4] from the Greek words for white and dominance.

Felsite is a petrologic field term used to refer to very fine-grained or aphanitic, light-colored volcanic rocks which might be later reclassified after a more detailed microscopic or chemical analysis.

In some cases, felsic volcanic rocks may contain phenocrysts of mafic minerals, usually hornblende, pyroxene or a feldspar mineral, and may need to be named after their phenocryst mineral, such as 'hornblende-bearing felsite'.

The chemical name of a felsic rock is given according to the TAS classification of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is metamorphic and has no definite volcanic protolith, it may be sufficient to simply call it a 'felsic schist'. There are examples known of highly sheared granites which can be mistaken for rhyolites.

For phaneritic felsic rocks, the QAPF diagram should be used, and a name given according to the granite nomenclature. Often the species of mafic minerals is included in the name, for instance, hornblende-bearing granite, pyroxene tonalite or augite megacrystic monzonite, because the term "granite" already assumes content with feldspar and quartz.

The rock texture thus determines the basic name of a felsic rock.

Close-up of granite from Yosemite National Park.
A specimen of rhyolite.
Rock texture Name of felsic rock
Pegmatitic Granite pegmatite
Coarse-grained (phaneritic) Granite
Coarse-grained and porphyritic Porphyritic granite
Fine-grained (aphanitic) Rhyolite
Fine-grained and porphyritic Porphyritic rhyolite
Pyroclastic Rhyolitic tuff or breccia
Vesicular Pumice
Amygdaloidal None
Vitreous (Glassy) Obsidian or porcellanite

See also

[edit]

Notes

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Felsic

View on Grokipedia
from Grokipedia
Felsic is a geochemical term used in geology to classify certain silicate minerals, magmas, and igneous rocks that are enriched in silica (SiO₂) and alumina (Al₂O₃), with silica contents typically exceeding 66 weight percent, resulting in light-colored compositions dominated by quartz and feldspar minerals.[1] The name "felsic" derives from "fel" (for feldspar) and "si" (for silica).[2] Felsic rocks form through the cooling and solidification of felsic magma, which originates primarily from partial melting of the continental crust, often in subduction zone settings where water from the subducting plate lowers the melting point of crustal rocks.[3][4] This magma is highly viscous due to its high silica content, leading to slow crystallization and, in extrusive cases, potentially explosive volcanic eruptions.[5] Intrusive felsic rocks cool slowly beneath the Earth's surface, developing coarse-grained textures, while extrusive varieties cool rapidly at the surface, forming fine-grained or glassy textures.[2] Common examples of felsic rocks include granite (intrusive, composed mainly of quartz, potassium feldspar, and plagioclase) and rhyolite (extrusive equivalent), as well as glassy forms like obsidian and vesicular pumice.[6][2] These rocks are characteristically light in color—ranging from white, pink, or light gray—due to their abundance of low-density, ferromagnesian-poor minerals such as quartz, potassium feldspar, and sodium-rich plagioclase, with lesser amounts of biotite or hornblende.[1] In contrast to denser, darker mafic rocks, felsic varieties are less common in oceanic settings but dominate continental crust, contributing significantly to its overall composition and influencing tectonic processes like crustal growth.[5]

Terminology and Definition

Core Definition

Felsic denotes a compositional category of igneous rocks and magmas characterized by elevated silica content, typically 65-75 wt% SiO₂, along with enrichment in feldspar minerals that contribute to their characteristically light-colored appearance.[7] This high silica proportion results in rocks dominated by lighter elements such as silicon, oxygen, aluminum, sodium, and potassium, contrasting with compositions richer in heavier elements like iron and magnesium.[8] In geochemical classification, felsic materials are distinguished from mafic (45–52 wt% SiO₂), intermediate (52–65 wt% SiO₂), and ultramafic (<45 wt% SiO₂) types using the total alkali-silica (TAS) diagram, which plots total alkalis (Na₂O + K₂O) against silica content to delineate fields such as rhyolite and dacite for felsic volcanic rocks. The TAS scheme, established as a standard for volcanic rock nomenclature, ensures consistent identification based on major element chemistry normalized to 100% on a volatile-free basis. Physically, felsic rocks exhibit low density, generally 2.6–2.8 g/cm³, owing to their silica-rich, low-iron mineralogy, and their melts display high viscosity due to extensive silica polymerization, which impedes flow compared to less siliceous magmas.[9] These rocks typically appear in light shades such as white, pink, or gray, reflecting the prevalence of pale minerals like quartz and feldspars.[8] The term "felsic" originated in 1912 as a portmanteau of "feldspar" and "silica," introduced to provide a descriptive, non-chemical alternative to the outdated "acidic" label for high-silica rocks and avoid misconceptions about their pH properties.[10] This nomenclature gained widespread adoption in the mid-20th century amid evolving geochemical understandings.[11]

Etymology and Naming

The term "felsic" is a portmanteau derived from "feldspar" and "silica," reflecting the predominance of these components in the rocks it describes.[12] It was coined in 1912 by American petrologists Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington in their paper modifying an earlier quantitative classification system for igneous rocks, where they proposed "felsic" to denote the collective group of modal feldspars, feldspathoids, and quartz.[13] This introduction served as a neutral, descriptive alternative to the longstanding but problematic designation "acidic" or "acid rock," which had been applied to high-silica igneous rocks since the 19th century based on analogies to acid-base chemistry in silicate melts—where silica acts as a network former akin to an acid. The "acid" label proved misleading, as it implied a direct relation to pH acidity in aqueous solutions, whereas felsic rocks exhibit high silica content (typically >65 wt% SiO₂) without necessarily producing acidic solutions upon weathering; instead, the term stemmed from melt chemistry, not proton donation.[14] Following its introduction, the term "felsic" gained traction in petrology literature during the mid-20th century, particularly from the 1950s onward, as quantitative geochemical analyses and modal classifications became more widespread. Its usage was formalized through the efforts of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks, established in 1970 under Albert Streckeisen, which integrated "felsic" into the QAPF (quartz-alkali feldspar-plagioclase-feldspathoid) modal classification scheme for both plutonic and volcanic rocks.[15] This standardization, detailed in IUGS recommendations from 1973 and refined in subsequent publications (e.g., 1976, 1989), defined felsic rocks as those dominated by >90% QAPF minerals in plutonic equivalents or analogous compositions in volcanics, emphasizing its role in distinguishing light-colored, silica-enriched lithologies.[15] In parallel with "felsic," the contrasting term "mafic" was introduced earlier by the same group of petrologists in 1903, derived from "magnesium" and "ferric" (iron) to describe iron- and magnesium-rich minerals and rocks, providing a balanced mnemonic pair for compositional spectra in igneous petrology. This duality—felsic for silica- and feldspar-dominated assemblages versus mafic for ferromagnesian ones—facilitated clearer, non-chemical nomenclature, influencing global standards and reducing reliance on outdated terms like "acidic" and "basic."[15]

Relation to Acid Rocks

The term "acid rock" emerged in 19th-century geology to denote igneous rocks with high silica content, rooted in early chemical theories that viewed silica as occurring primarily in the form of silicic acid within magmas.[16] This nomenclature, introduced by chemists like Robert Bunsen, categorized rocks as acid (high SiO₂), basic (low SiO₂), or intermediate based on their presumed reaction behaviors analogous to acids and bases.[1] The terminology gained systematic structure through the 1903 quantitative classification by Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry S. Washington (CIPW system), which divided igneous rocks into acid, intermediate, and basic series using chemical analyses and normative mineral calculations to reflect silica saturation levels.[17] However, the "acid" label proved misleading, as it evoked pH connotations irrelevant to solid rock chemistry, prompting a shift toward more neutral descriptors focused on silica content and mineralogy.[1] The term "felsic" largely supplanted "acid" in the mid-20th century to promote terminological clarity and avoid aqueous solution analogies, emphasizing instead the enrichment in feldspar and silica characteristic of these rocks. This replacement aligned with broader efforts to refine petrographic language, as seen in the etymology of "felsic" itself, derived from feldspar and silica as a direct counter to the limitations of "acid rock." Despite the transition, "acid rock" endures in specific modern applications, notably volcanology, where it describes high-silica (typically >65% SiO₂) lavas and eruptions like those producing rhyolite, highlighting viscous, explosive behaviors.[18] The International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks endorses this selective persistence but deems "acid" obsolete for general rock classification, favoring terms like "felsic" in comprehensive schemes such as the total alkali-silica (TAS) diagram.[19] Terminology shifts are illustrated in geological textbooks: mid-20th-century works, such as those from the 1960s referencing CIPW norms, routinely applied "acid rocks" to high-silica compositions, while post-2000 texts, including practical guides on igneous processes, standardize "felsic" for its precision and avoidance of outdated chemical implications.[1][20]

Composition and Characteristics

Mineralogical Components

Felsic rocks are defined by their mineral assemblage dominated by silica-rich phases, with quartz typically comprising 20-60 vol% of the modal composition, providing structural stability and contributing to the rock's light color and hardness. Alkali feldspar, including varieties such as orthoclase and sanidine, forms the primary component at 35-60 vol%, while plagioclase feldspar, ranging from oligoclase to albite compositions, accounts for 10-30 vol%, influencing the rock's overall alkalinity and sodic character. These proportions are determined through modal analysis, which quantifies volume percentages of visible minerals in thin sections or hand samples.[21][22][23] Accessory minerals play a subordinate but texturally significant role, with micas such as muscovite and biotite constituting 5-10 vol%, imparting a flaky or schistose appearance in some varieties, and hornblende appearing as minor prismatic crystals. Trace phases like zircon and apatite occur in less than 1 vol%, often as euhedral inclusions that aid in geochronology and phosphate content, respectively, without substantially altering the dominant felsic framework.[7][24] In plutonic felsic rocks, such as granites, the slow cooling rates promote coarse-grained, phaneritic textures where interlocking crystals of quartz and feldspars are visible to the naked eye, fostering equigranular or porphyritic fabrics. Volcanic equivalents, like rhyolites, exhibit contrasting textures due to rapid surface cooling, resulting in aphanitic groundmasses that may be glassy (as in obsidian) or porphyritic with larger phenocrysts of quartz and feldspar embedded in a fine matrix. These textural differences highlight the control of cooling environment on mineral growth and rock fabric.[25][7] The relative abundances of these minerals in felsic rocks directly reflect fractional crystallization during magma evolution, where progressive removal of denser mafic phases concentrates residual silica and alkalies into quartz and feldspars; modal analysis via the QAPF diagram provides quantitative estimates, placing felsic fields in regions where quartz exceeds 20 vol% of the total Q + A + P components, with alkali feldspar often dominating over plagioclase.[26][27]

Geochemical Properties

Felsic rocks are defined by their high silica content, typically ranging from 68 to 77 wt% SiO₂, which distinguishes them from more mafic compositions. Average major oxide compositions include approximately 70 wt% SiO₂, 14.5 wt% Al₂O₃, 7.6 wt% combined Na₂O and K₂O (with Na₂O around 3.6 wt% and K₂O around 4.0 wt%), and low levels of FeO (about 2.6 wt%), MgO (1.0 wt%), and CaO (2.5 wt%). These proportions reflect the enrichment in light elements and silica typical of felsic magmas, derived from partial melting of crustal materials.[28][29] Geochemical classification of felsic rocks often employs indices such as the silica saturation index, which assesses the degree of silica oversaturation based on the presence of quartz and the stability of silica-deficient minerals, and the aluminum saturation index (ASI), calculated as the molar ratio Al₂O₃ / (CaO + Na₂O + K₂O). An ASI greater than 1 indicates peraluminous compositions with excess alumina, favoring minerals like muscovite or cordierite, while values less than 1 denote metaluminous types balanced by calcium and other cations. These indices provide insights into the magma's differentiation and source characteristics without relying on modal mineralogy.[1][30] Analytical methods for determining these compositions include X-ray fluorescence (XRF) spectrometry for major elements, which offers precise whole-rock analysis through fusion of powdered samples into glass beads, and inductively coupled plasma (ICP) techniques, such as ICP-optical emission spectrometry (OES) or mass spectrometry (MS), for trace elements at parts-per-million levels. These techniques ensure accurate oxide quantification after sample preparation involving acid digestion or fusion to minimize matrix effects.[31][32] Compositional variations exist between continental and oceanic felsic rocks, with continental examples often showing elevated K₂O due to crustal assimilation, particularly in A-type granites that exhibit high K₂O/Na₂O ratios (up to 2.8) and alkaline affinities in anorogenic settings. Oceanic felsics, such as those in island arcs, tend toward lower potassium and more calcic trends influenced by slab-derived fluids.[33][34]

Formation Processes

Magmatic Origins

Felsic magmas primarily originate from partial melting of the continental crust, often involving protoliths such as amphibolites or greywackes at temperatures between 700°C and 900°C under mid- to lower-crustal pressures.[35] This process is facilitated in collisional orogens where tectonic thickening elevates temperatures sufficiently to induce melting without requiring excessively high heat inputs.[35] Alternatively, in subduction settings, fluids derived from the dehydrating slab can infiltrate the overlying mantle wedge or crust, lowering the solidus temperature and promoting partial melting to generate felsic compositions.[36] Hydrous conditions play a critical role in these melting processes, with water contents in the resulting melts typically ranging from 2 to 6 wt%, which depresses the melting point by tens to hundreds of degrees compared to anhydrous systems.[37] Dehydration reactions of hydrous minerals, such as biotite (around 800–850°C) or hornblende (around 900°C), drive fluid-absent melting in the lower crust, releasing water that further enhances melt production.[38] These conditions are common in amphibolite-facies rocks, where the breakdown of these minerals generates silica-rich melts.[39] A key mechanism is crustal anatexis, involving incongruent melting of metasedimentary rocks, which produces peraluminous leucogranitic melts through the partial breakdown of muscovite and biotite without complete equilibration.[40] This process yields high-silica, alkali-enriched magmas characteristic of felsic compositions.[37] Isotopic signatures provide strong evidence for crustal derivation, with ⁸⁷Sr/⁸⁶Sr ratios exceeding 0.710—contrasting with mantle values below 0.705—indicating minimal mantle input, as observed in Himalayan leucogranites sourced from thickened metasediments.[41]

Crystallization and Differentiation

Felsic magmas can evolve through fractional crystallization from a more mafic parent magma, a process where early-formed mafic minerals such as olivine and pyroxene are removed, progressively enriching the residual melt in silica (SiO₂) and alkalis (Na₂O and K₂O). This differentiation occurs as the magma cools, with denser mafic crystals settling to the chamber floor or being filtered out, leaving a more viscous, silica-rich liquid that ultimately solidifies into felsic rocks like granite or rhyolite. The process is well-illustrated by Bowen's reaction series, where early high-temperature phases give way to late-stage felsic minerals like quartz and potassium feldspar, driving the compositional shift toward felsic end-members.[42][43] Assimilation and contamination further modify felsic magma compositions by incorporating surrounding crustal material, particularly during prolonged residence in shallow chambers where heat from the magma melts wall rocks, adding incompatible elements and radiogenic isotopes to the melt. This interaction is quantitatively modeled using energy-constrained assimilation-fractional crystallization (EC-AFC), which accounts for the heat budget required for melting country rock while simultaneous crystallization removes phases, ensuring energy balance in the system. For instance, in felsic systems like the Skye igneous complex, EC-AFC simulations demonstrate how limited assimilation (e.g., 10-20% of the magma mass) can significantly alter trace element ratios without excessive crustal input.[44][45] Liquid immiscibility represents a rarer mechanism in highly evolved felsic systems, where the melt separates into conjugate felsic (silica-rich) and mafic (iron- or phosphorus-enriched) liquids due to thermodynamic instability, potentially contributing to the final stages of differentiation. This process is evidenced in certain granitic intrusions and volcanic glasses, though it is less common than fractionation owing to the narrow temperature-composition window required, often below 800°C.[46][47] Crystallization of felsic magmas typically occurs over a temperature range of 800–650°C, with quartz and alkali feldspars forming as late-crystallizing phases near the solidus. Phase equilibria in the quaternary Qz-Ab-Or-H₂O system, a key model for felsic compositions, reveal a ternary eutectic at approximately 650°C under water-saturated conditions, where the melt fully crystallizes into quartz, albite, and orthoclase assemblages. These diagrams highlight how water content lowers the liquidus, facilitating the sequential precipitation of minerals that define felsic textures.[42][48][49]

Types and Classification

Plutonic Felsic Rocks

Plutonic felsic rocks are intrusive igneous rocks that form from the slow crystallization of silica-rich magma at depth, resulting in coarse-grained textures and compositions dominated by quartz, feldspars, and minor mafic minerals. These rocks are classified using the International Union of Geological Sciences (IUGS) QAPF modal diagram, which applies to plutonic rocks where the combined volume percentages of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F) exceed 90% of the total rock volume, placing them firmly in the felsic field with high silica content typically above 65 wt%.[50][51] In this system, the felsic plutonic field is defined by quartz contents between 20% and 60%, with the precise rock name determined by the relative proportions of alkali feldspar and plagioclase.[52] The primary types of plutonic felsic rocks include granite, granodiorite, and pegmatite. Granite is characterized by phaneritic, often equigranular textures with visible interlocking crystals, containing 20–60% quartz and alkali feldspar comprising more than 35% of the total feldspar content.[50][53] Granodiorite, a close relative, shares the 20–60% quartz range but features a higher proportion of plagioclase feldspar, exceeding 65% of total feldspar, often accompanied by biotite and hornblende for a slightly darker appearance.[52][50] Pegmatite represents a late-stage, exceptionally coarse-grained variant, typically of granitic composition, formed from volatile-rich residual melts with crystals larger than 2.5 cm, sometimes reaching meters in size due to enhanced ionic mobility in low-viscosity fluids.[54][50] Diagnostic textures in these rocks reflect their slow cooling history and magmatic processes. Graphic intergrowths, resembling cuneiform writing, occur in granites and pegmatites where quartz and alkali feldspar crystallize interlocked in a symbiotic manner, indicating near-solidus temperatures around 650–700°C.[50] Xenoliths, angular fragments of assimilated country rock, are common inclusions in granites, evidencing magma-wall rock interaction during emplacement.[50] These phaneritic textures, with grain sizes from millimeters to centimeters in granite and granodiorite, contrast with the aphanitic varieties of their volcanic equivalents like rhyolite.[50] Plutonic felsic rocks are emplaced at crustal depths of 5–20 km, where pressures range from 1.5–5 kbar, allowing for the formation of large intrusive bodies such as stocks and batholiths.[50] Cooling occurs gradually over timescales of 10^5 to 10^6 years, enabling full crystallization and development of their characteristic coarse textures through conductive and convective heat loss.[55][50]

Volcanic Felsic Rocks

Volcanic felsic rocks form through the rapid extrusion and cooling of silica-rich magmas at Earth's surface, resulting in a variety of textures distinct from their intrusive counterparts. These rocks typically exhibit fine-grained or glassy matrices due to quick quenching, often containing phenocrysts of quartz, feldspars such as sanidine or plagioclase, and minor biotite or hornblende.[56] Unlike slower-cooling plutonic equivalents like granite, volcanic felsic rocks are prone to explosive eruptions owing to their high gas content and viscosity, posing significant hazards through pyroclastic flows, ash falls, and caldera collapses.[57] The primary types of volcanic felsic rocks include rhyolite, obsidian, pumice, and ignimbrite. Rhyolite, the most common, appears as porphyritic varieties with visible phenocrysts in a finer aphanitic groundmass or as aphanitic flows lacking large crystals; it represents the crystallized extrusive form of felsic magma.[56] Obsidian is a natural volcanic glass, dark and glossy, formed by extremely rapid cooling that prevents crystallization, often associated with rhyolitic compositions and exhibiting conchoidal fracture. Pumice consists of highly vesicular, frothy material, light-colored and low-density, produced when gas expansion during eruption creates voids in the cooling rhyolitic or dacitic foam; it commonly floats on water due to its porosity exceeding 75%.[58] Ignimbrite, or welded tuff, arises from hot pyroclastic density currents that deposit and partially fuse ash and pumice fragments, forming layered, welded sheets with fiamme—flattened pumice inclusions—evident in hand specimens.[57] Classification of volcanic felsic rocks follows the total alkali-silica (TAS) diagram for chemical composition and the QAPF (quartz-alkali feldspar-plagioclase-feldspathoid) scheme adapted for modal mineralogy, but emphasizes textural variations such as vitrophyre (glassy with phenocrysts) or porphyritic forms.[59] Rhyolite specifically requires silica content greater than 69 wt% SiO₂, distinguishing it from less siliceous dacites, while all felsic volcanics share high alkali (Na₂O + K₂O > 5 wt%) and low iron-magnesium contents.[56] These systems align with plutonic classifications but highlight extrusive textures like flow banding in obsidian or welding in ignimbrites, aiding identification in the field. Eruption dynamics of felsic magmas are dominated by their high viscosity, typically exceeding 10⁶ Pa·s, which traps volatiles and promotes explosive Plinian-style eruptions characterized by towering ash columns reaching tens of kilometers.[60] This viscosity, driven by polymerized silica networks and crystal content, hinders gas escape, leading to rapid pressure buildup and fragmentation; such events often culminate in caldera formation as magma chambers evacuate, as seen in supervolcanic systems.[61] The resulting hazards include widespread tephra dispersal and devastating ignimbrite flows traveling at speeds over 100 km/h, burying landscapes under meters of hot debris.[57] Cooling rates for volcanic felsic rocks are exceptionally rapid compared to intrusive settings, ranging from hours for glassy obsidian to days for thin flows and up to years for thick rhyolite domes or ignimbrite sheets, preserving delicate textures and phenocrysts.[62] In rhyolite flows, this allows sanidine phenocrysts to form during late-stage crystallization while the groundmass remains aphanitic or glassy, recording eruption temperatures around 700–800°C before quenching.[63] Such swift cooling limits devitrification in obsidian and welding in pumice-rich ignimbrites, influencing their durability and use in archaeological contexts.[64]

Geological Occurrence

Major Formations

Felsic rocks form extensive batholiths and volcanic complexes worldwide, with prominent continental examples including the Sierra Nevada Batholith in the United States, which covers approximately 70,000 km² and consists primarily of granitic intrusions emplaced between 80 and 120 million years ago during the Cretaceous period.[65][66] Another key continental formation is the Caledonian granites of the Scottish Highlands, dating to around 400 million years ago in the Silurian-Devonian period and representing a major component of the exposed Precambrian and Paleozoic crust in the region.[67][68] In oceanic and arc settings, the Coastal Batholith of Peru exemplifies large-scale felsic magmatism, stretching over 1,600 km along the Andean margin with significant rhyodacite components intruded between approximately 100 and 40 million years ago in the Late Cretaceous to Eocene.[69][70] Similarly, the Taupo Volcanic Zone in New Zealand hosts multiple rhyolitic calderas, such as the 35-km-wide Taupo Caldera, with activity spanning the last 2 million years and cumulative rhyolitic eruptions exceeding 10,000 km³ since 1 million years ago.[71][72] Super eruptions highlight the explosive potential of felsic magmatism, as seen in the Yellowstone Caldera system in the United States, where the Lava Creek Tuff eruption approximately 640,000 years ago produced about 1,000 km³ of rhyolitic tuffs.[73][74] The Fish Canyon Tuff in Colorado represents an even larger event, with an estimated volume of 5,000 km³ of crystal-rich rhyolitic ignimbrite erupted around 28 million years ago from the La Garita Caldera.[75] The age distribution of major felsic formations shows peaks in the Precambrian, particularly in the Superior Province of Canada where Archean granites and gneisses formed around 2.7 billion years ago, and in Phanerozoic continental arcs driven by subduction processes.[76][77]

Tectonic Settings

Felsic magmatism predominantly occurs in convergent plate margins, where subduction processes drive the generation of calc-alkaline felsic rocks through slab dehydration and subsequent crustal melting.[37] In these settings, hydrous fluids released from the dehydrating subducted oceanic slab flux the overlying mantle wedge, inducing partial melting that produces basaltic magmas; these then interact with the crust, leading to differentiation and the formation of felsic melts.[78] The majority of Phanerozoic granites, including I-type and S-type varieties, form in such convergent environments, reflecting the dominant role of subduction in continental crustal growth.[79] Explosive volcanism is common in these arc settings due to the volatile-rich nature of the magmas.[80] Intraplate tectonic settings, such as continental rifts and hotspots, are associated with A-type (alkaline) felsic rocks, which arise from partial melting of the lower crust or mantle in extensional environments lacking significant subduction influence.[81] These magmas often exhibit peralkaline compositions and are linked to lithospheric thinning and upwelling of asthenospheric material. A representative example is the peralkaline granites of the Oslo Rift, formed during Permo-Carboniferous extension within the stable Fennoscandian Shield.[82] In collisional orogens, felsic magmatism occurs syn- to post-collisionally, driven by melting of thickened continental crust due to radiogenic heating and tectonic decompression.[83] S-type leucogranites, derived primarily from metasedimentary sources, exemplify this process; the Miocene Himalayan leucogranites formed through partial melting of the overthickened (>50 km) crust in the Greater Himalayan Sequence following India-Asia collision.[83] Tectonic discrimination of felsic rocks relies on geochemical diagrams, such as those developed by Pearce et al., which use immobile trace elements like Rb, Y, and Nb to differentiate settings.[84] For instance, the Rb vs. (Y + Nb) plot distinguishes volcanic arc granites (VAG), characterized by low Y + Nb and moderate Rb from subduction-related origins, from within-plate granites (WPG), which show higher Y + Nb due to intraplate enrichment.[84] These diagrams aid in assigning ancient felsic suites to specific regimes without relying on field relations alone.[85]

References

  1. General Classification of Igneous Rocks - Tulane University
  2. None
  3. https://web.crc.losrios.edu/~jacksoh/lectures/magmapt2.html
  4. Characteristics of Igneous Rocks – Geology 101 for Lehman ...
  5. [PDF] Magmas and Igneous Rocks - Tulane University
  6. Igneous Rocks Classified by Composition
  7. felsic – An Introduction to Geology - OpenGeology
  8. Volcanoes - Geography 101 Online
  9. Felsic - Etymology, Origin & Meaning
  10. Definition of felsic - Mindat
  11. felsic, adj. meanings, etymology and more - Oxford English Dictionary
  12. classification of igneous rocks - jstor
  13. Felsic Rock - an overview | ScienceDirect Topics
  14. [PDF] igneous rocks: a classification and glossary of terms
  15. The Origin of Igneous Rocks (Chapter 4) - A Brief History of Geology
  16. A Quantitative Chemico-Mineralogical Classification and ...
  17. Acid Volcanics - an overview | ScienceDirect Topics
  18. [PDF] IGNEOUS ROCKS: A Classification and Glossary of Terms
  19. [PDF] Igneous Rocks and Processes: A Practical Guide
  20. ALEX STREKEISEN-Granite-
  21. Identifying & Interpreting Plutonic Rocks
  22. [PDF] The Sandia granite--Single or multiple plutons?
  23. https://geo.libretexts.org/Bookshelves/Geology/Book%3A_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher
  24. 2 Igneous Rocks – Open Petrology - OpenGeology
  25. Igneous Rocks - Geology (U.S. National Park Service)
  26. (PDF) The IUGS systematics of igneous rocks - ResearchGate
  27. Fractional Crystallization - an overview | ScienceDirect Topics
  28. Average Rock Compositions
  29. [PDF] Igneous Rocks - SOEST Hawaii
  30. [PDF] Aluminum in zircon as evidence for peraluminous and ... - UCLA SIMS
  31. [PDF] Report (pdf)
  32. [PDF] Analysis of Major and Trace Elements - Ocean Drilling Program
  33. [PDF] Geochemical, modal, and geochronologic data for 1.4 Ga A-type ...
  34. [PDF] CHAPTER E: GEOCHEMICAL INTERPRETATION OF SAMPLES OF ...
  35. A new concept for the genesis of felsic magma - Nature
  36. When the Continental Crust Melts | Elements | GeoScienceWorld
  37. Subduction zone fluids and arc magmas conducted by lithospheric ...
  38. Dehydration Melting - an overview | ScienceDirect Topics
  39. Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O ...
  40. Himalayan Leucogranites: A Geochemical Perspective | Elements
  41. Himalayan-like Crustal Melting and Differentiation in the Southern ...
  42. Magmas and Igneous Rocks - Tulane University
  43. 7. 4.2 Crystallization of Magma - Maricopa Open Digital Press
  44. [PDF] Energy-Constrained Recharge, Assimilation, and Fractional ...
  45. [PDF] Geochemical Modeling of Assimilation in Magmatic Systems
  46. Silicate liquid immiscibility in magmas and in the system K2O-FeO ...
  47. Magmatic Differentiation - Tulane University
  48. Timescales of Quartz Crystallization and the Longevity of the Bishop ...
  49. Ternary Phase Diagrams - Tulane University
  50. 7 Plutons and Plutonic Rocks – Open Petrology - OpenGeology
  51. IUGS Chart - Plutonic QAP
  52. [PDF] DRAFT Geologic Map Unit Classification, ver. 6.1
  53. Classification of plutonic rocks - Geology is the Way
  54. Granitic pegmatite - ALEX STREKEISEN
  55. [PDF] How Fast Do Rocks Form? - DigitalCommons@Cedarville
  56. Glossary - Volcanoes of the United States [USGS]
  57. Glossary of Volcanic Terms - Volcanoes, Craters & Lava Flows (U.S. ...
  58. Definitions | Volcano World | Oregon State University
  59. USGS: Volcano Hazards Program Glossary - Basalt
  60. [PDF] Shear viscosity of rhyolite-vapor emulsions at magmatic ...
  61. Magma Melts and Eruption Types - Grand Canyon-Parashant ...
  62. [PDF] Field-Based Description of Rhyolite Lava Flows of the Calico Hills ...
  63. Post-eruptive mobility of lithium in volcanic rocks - PMC
  64. Hotter Side of Obsidian - Volcano World - Oregon State University
  65. [PDF] Disruption of regional primary structure of the Sierra Nevada ...
  66. [PDF] AGE DETERMINATIONS OF TEE ROCKS OF THE BATHOLITHS OF ...
  67. Scotland's Geology
  68. Geochronology and geodynamics of Scottish granitoids from the late ...
  69. The Coastal Batholith of central Peru - GeoScienceWorld
  70. The Peruvian Coastal Batholith is located in the western Cordillera....
  71. Caldera volcanoes of the Taupo Volcanic Zone, New Zealand - Wilson
  72. Taupo - Global Volcanism Program
  73. Age of the Lava Creek supereruption and magma chamber ...
  74. Fish Canyon Magma Body, San Juan Volcanic Field, Colorado
  75. Comparative orotomy of the Archean Superior and Phanerozoic ...
  76. Three billion years of crustal evolution in eastern Canada ...
  77. Origins of felsic magmas in Japanese subduction zone ...
  78. I-type and S-type granites in the Earth's earliest continental crust
  79. [PDF] Subduction Factory: How it operates in the evolving earth
  80. A-type granites in space and time - ScienceDirect.com
  81. The Oslo Rift: a review - ResearchGate
  82. Himalayan Leucogranites: Field Relationships and Tectonics
  83. Trace Element Discrimination Diagrams for the Tectonic ...
  84. An evaluation of the Rb vs. (Y + Nb) discrimination diagram to infer ...
User Avatar
No comments yet.