Mafic-felsic Magma Interaction Research Articles

Dynamics of a subvolcanic magma chamber inferred from viscous instabilities owing to mafic-felsic magma interactions

Dynamics of a subvolcanic magma chamber inferred from viscous instabilities owing to mafic-felsic magma interactions

Peritectic assemblage entrainment and mafic–felsic magma interaction in the Late Oligocene–Early Miocene Karadağ Pluton in the Biga Peninsula, northwest Turkey: petrogenesis and geodynamic implications

Peritectic assemblage entrainment and mafic–felsic magma interaction in the Late Oligocene–Early Miocene Karadağ Pluton in the Biga Peninsula, northwest Turkey: petrogenesis and geodynamic implications

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

The effects of mafic-felsic magma interaction on magma diversity: insights from an early Paleozoic hornblendite-quartz monzonite suite in the South China block

Understanding the mechanisms responsible for the interplay between mafic and felsic magmas is the key to retrieving information on their sources, and characterizing the exchange of mass between them. In order to characterize compositional and mineralogical changes in the mafic end-member during mafic-felsic magma interaction and to better understand the nature of early Paleozoic intracontinental magmatism in the South China Block (SCB), a detailed study was conducted on an early Paleozoic hornblendite-quartz monzonite suite in the SCB. The amphibole phenocrysts in the hornblendite are zoned with respect to major and trace elements. From the brown core to the light-green rim, these amphibole phenocrysts display significant increases in Si, Mg, and Mn, coupled with abrupt decreases in Al, Ti, Na, K, and most of the trace elements, but only minor variations in Ca, Fe, Co, and Ni. The light-green matrix amphiboles in the hornblendite have similar compositions to the outer rim of amphibole phenocrysts (except Na). It is important to note that the amphibole grains in the quartz monzonite have significantly higher rare-earth element (REE) contents than the amphibole grains in the hornblendite. There is convincing evidence to support a significant transfer of incompatible elements (e.g., K, Na, LILE, LREE, U, and Th) from the felsic magma to the mafic magma, such as (1) the absence of high-Ca plagioclase in hornblendite, with the majority of feldspar grains being albite (Ab96–97) and orthoclase (Or94–96), and (2) uniform Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr = 0.7081–0.7098; eNd(t) = −6.8 to −6.3; weighted mean zircon eHf(t) = −8.0 to −7.4) for the hornblendite and quartz monzonite samples. It is, therefore, suggested that during mafic-felsic magma interaction, water was transferred from the quartz monzonite magma to the coeval hornblendite magma and promoted the formation of the amphibole crystals in the latter. The incompatible elements transferred from the quartz monzonite magma to the hornblendite magma were mainly incorporated into the late-crystallized anhedral phases in the hornblendite (e.g., orthoclase, sodic plagioclase, quartz, zircon, and apatite). This study suggests that water, which behaves as a supercritical fluid in most mafic-felsic magmas, may play a key role in the exchange of mass between mafic and felsic magmas, and this may be extended to the petrogenesis of biotite-amphibole aggregations in intermediate-felsic magmas.

Tracing the styles of mafic-felsic magma interaction: A case study from the Ahvenisto igneous complex, Finland

Tracing the styles of mafic-felsic magma interaction: A case study from the Ahvenisto igneous complex, Finland

Field evidence, mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61 wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57 wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.

Geological Appraisal of Ladakh and Tirit Granitoids in the Indus-Shyok Suture Zones of Northwest Himalaya, India

Abstract: New field observations on granitoids and associated lithounits in some parts of Indus-Shyok Suture Zones have been documented in order to re-establish the geological relationships between various volcano-plutonic magmatic lithounits. Careful examination of outcrops and contact relationships between the various lithounits have pin-pointed the sequence of geological events. Field features of granitoids exposed along Leh-Saboo-Khardung_La suggest multiple pulses of mafic-felsic magma interactions (mingling to mixing) with almost 25% of the mafic to hybrid magma input in the evolution of the eastern part of Ladakh batholith. Along Khardung_La-Shyok-Diskit, thick sequence of volcanic lithounits is exposed, which dominantly consist of massive basaltic andesite, porphyritic andesite, dacite and rhyolite forming Khardung Formation. On the other hand Shyok Formation, dipping opposite to the Khardung Formation, composed predominantly of meta-sedimentary lithounits and subordinate amount of volcanic materials at present exposed level. Spectacular intrusive contacts of Ladakh granitoids with metavolcanics and meta-sedimentary country rocks of Shyok Formation near Diskit can be observed, which are manifested by ubiquitous xenoliths near the marginal parts. Although the nature of granitoid melt invasion into country-rocks was relatively winty, granitoid melt has produced leucogranite-pegmatite system because of devolatization and decompression effects. Frequent xenoliths of porphyritic andesite and dacite roof pendants are being reported in Tirit granitoids, which strongly suggest sub-volcanic emplacement of granitoid melt, extensive assimilation and roof collapse of overlying volcanic materials. It is more likely that the xenoliths hosted in Tirit granitoids belong to Shyok volcanics. It is suggested that multiple pulses of coeval mafic and felsic magmatism occurred extensively and emplaced at differential crustal levels.

Magma mixing/mingling in Salmas granodiorite, NW Iran: evidence from mafic microgranular enclaves

The interaction of mafic–intermediate and felsic rocks of the Salmas plutonic rocks produced mixed rocks (granodiorites) which contain mafic microgranular enclaves (MMEs). Enclaves ranging from a few millimeters to centimeters in size, and from ellipsoidal to rounded in shape. Based on both field observation and mineralogical compositions, MMEs are composed of quartz diorite whereas the felsic host rocks comprise mainly granodiorite. MMEs are characterized by a microporphiritic texture and revealed some types of microscopic textures, e.g., prismatic-cellular plagioclase with spike zones and rounded plagioclase megacrysts, blade-shaped biotite and acicular apatite. The host rocks show textures such as oscillatory- and reversely zoned plagioclase with spike zone. Compositions of plagioclases (An41 to An48) of MMEs are similar to those of host rocks (An38 to An45) which suggest partial to complete equilibration during mafic–felsic magma interactions. The individual petrographic and microstructural textures and mineral chemistry similarities between the MMEs and their host rocks and diorites indicate that the enclaves are of mixed origin and most probably formed by interaction of lower crust magma (granitic melt) and evolved mantle-derived magmas (diorites).

Geothermobarometry of Askaoun Pluton in Ouzellarh-Sirwa Promontory (Central Anti-Atlas; Morocco)

Rocks of the late Neoproterozoic Askaoun pluton (558 ± 2 Ma) located in the Ouzellarh-Sirwa promontory (Anti-Atlas) include magmatic microgranular enclaves (MMEs) ranging from rounded to ovoid in shape, dark and fine grained and generally 5 to 10 cm in size, some reaching a size of 50 cm. They are composed of microdiorite, quartz microdiorite and micromonzodiorite, whereas the felsic host rocks comprise mainly quartz-diorite and amphibole-biotite granodiorite based on mineralogical compositions. The mineral assemblage is similar to those described in their hosting granitoids but with different proportions. In this study composition of minerals is used to describe the nature of the magma and estimate the pressure, temperature and oxygen fugacity at which Askaoun pluton is emplaced. Based on chemistry of biotite Askaoun pluton formed from calc-alkaline magma. Compositions of plagioclase (An5 - An29); hornblende (Mg ≠= 0.59 - 0.65) and biotite (Mg ≠= 0.49 - 0.55) of MMEs are slightly distinct or similar to those of host rocks (An7-40; hbl Mg ≠= 0.64 - 0.69; Bi Mg ≠= 0.49 - 0.50) which suggest partial to complete equilibration during mafic-felsic magma interaction. The coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the assemblage titanite-magnetite-quartz were used to constrain the P, T and fO2 during the crystallization of the parent magmas. The Askaoun pluton was emplaced at temperature ca. 504°C - 633°C and at pressure ca. 0.9 - 4.66 ± 0.6 Kbars (average depth = 6.5 km) from a highly oxidized magma (log fO2 = ?24.8 to ?19.2).

Granite genesis and mafic-felsic magma interaction in the lower crust

Field observations from an exposed section of deep continental crust, the Athabasca granulite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafic magmas. The 1.9 Ga Chipman mafic dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of ∼40 km. The Fehr granite shows evidence for extensive partial melting and generation of granitic leucosome. Mafic dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafic magma promotes extensive partial melting of fertile granitoids, which in turn filter and entrap later mafic dikes and sills. The result is almost inevitable mingling and hybridization (i.e., contamination) of mafic and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.

Mineralogy, whole-rock and Sr–Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88–83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An 33–An 60), hornblendes (Mg#=0.77–1.0) and biotites (Mg#=0.61–0.63) of MMEs are slightly distinct or similar to those of host rocks (An 12–57; hbl Mg#=0.63–1.0; Bi Mg#=0.50–0.69), which suggest partial to complete equilibration during mafic–felsic magma interactions. The felsic host rocks have SiO 2 between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La cn/Lu cn=1.5–7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46–1.1). Initial ε Nd(i) values vary between −3.1 and 1.6, initial 87Sr/ 86Sr values between 0.7056 and 0.7067. Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22–52, low contents of SiO 2 (53–63 wt%), low ASI (0.7–1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb) cn=1.4–3.9; (Tb/Yb) cn=0.9–1.5] and show small negative Eu anomalies (Eu/Eu*=0.63–1.01). Isotope signatures of these rocks ( 87Sr/ 86Sr (i)=0.7054–0.7055; ε Nd(i)=–1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO 2, ASI; high Mg#, CaO, Al 2O 3, TiO 2, P 2O 5, Sr and Nb concentrations compared to dioritic and tonalitic enclaves. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3–3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.

Mineralogy and geochemistry of microgranular enclaves in Palaeoproterozoic Malanjkhand granitoids, central India: evidence of magma mixing, mingling, and chemical equilibration

Palaeoproterozoic (ca 2,480 Ma) felsic magmatism of Malanjkhand region of central Indian Precambrian shield, referred to as Malanjkhand granitoids (MG), contain xenoliths of country rocks and mesocratic to melanocratic, fine-grained porphyritic microgranular enclaves (ME). The shape of ME is spheroidal, ellipsoidal, discoidal, elongated, and lenticular, varying in size from a few centimeters to about 2 m across. The contact of ME with the host MG is commonly sharp, crenulate, and occasionally diffuse, which we attribute to the undercooling and disaggregation of ME globules within the cooler host MG. The ME as well as MG show hypidiomorphic texture with common mineral Hbl-Bt-Kfs-Pl-Qtz assemblage, but differ in modal proportions. The variation in minerals' composition, presence of apatite needles, elongated biotites, resorbed plagiclase, ocellar quartz, and other mafic–felsic xenocrysts strongly oppose the restite and cognate origins of ME. Compositions of plagioclases (An3–An29), amphiboles (Mg/Mg+Fe2+=0.55–0.69), and biotites (Mg/Mg+Fe2+=0.46–0.60) of ME are slightly distinct or similar to those of MG, which suggest partial to complete equilibration during mafic–felsic magma interactions. Al-in-amphibole estimates the MG pluton emplacement at ca 3.4 ± 0.5 kbar, and therefore, magma mixing and mingling must have occurred at or below this level. The $$\hbox{Fe}\rightleftharpoons\hbox{Mg}$$ substitution in biotites of ME and MG largely suggests subduction-related, calc–alkaline metaluminous (I-type) nature of felsic melts. Most major and trace elements against SiO2 produce near linear variation trends for ME and MG, probably generated by the mixing of mafic and felsic magmas in various proportions. Trace including rare earth elements patterns of ME–MG pairs, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. The available evidence supports the model of ME origin that coeval mafic (enclave) and felsic (MG) magmas produced a hybrid (ME) magma layer, which injected into cooler, partly crystalline MG, and dispersed, mingled, and undercooled as ME globules in a convectively dynamic magma chamber.

Field Evidence of Magma Mixing in Plutonic Rock from the Benom Complex, Central Belt of Peninsular Malaysia

The a lka lic seri es of the Benom Complex, Pahang, consists of monzonite, syenite and gabbro ic rock. The contact between monzonite and gabbroic rock suggest that both magmas are mixed. Ev idence of the mix ing can be seen at the new roadcut outcrop a long the Benta township by-pass such as dispersion of K-fe ldspar from monzonite in the gabbroic rock. The end stage of this dispersion is the free sw imming of K-fe ldspar individual crystals in the gabbroic material. Thi s structure is interp reted as the result of mechanical transfer during the mafic-fe lsic magma interaction and mix ing event. All these fea tu res suggest an origin for the alka lic in termediate rocks of the Benom Complex invo lvi ng a magma mix ing process ..

Petrology and Isotope Geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic-Felsic Magma Interactions During the Construction of Shallow-level Calc-alkaline Plutons

Petrology and Isotope Geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic-Felsic Magma Interactions During the Construction of Shallow-level Calc-alkaline Plutons

Field Evidence of Magma Mixing from Microgranular Enclaves Hosted in Palaeoproterozoic Malanjkhand Granitoids, Central India

The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (~2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (±Mo±Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.

Mafic-felsic magma interaction at Satsuma-Iwojima volcano, Japan: Evidence from mafic inclusions in rhyolites

Geochemical and petrographic studies of the rhyolites and mafic inclusions from Satsuma-Iwojima volcano were carried out in order to investigate evolution of a silicic, bimodal magma system during the post-caldera stage. Abundant mafic inclusions, which are fine-grained with vesicles in their cores, are present in the Showa-Iwojima rhyolitic lava. Inclusions with similar textures are found in Iwodake volcanic bombs but are less common than in the Showa-Iwojima lava. The major and trace element compositions of the inclusions plot along mixing lines connecting the host rhyolites with spatially and temporally associated basaltic to basaltic andesite magmas. Plagioclase phenocrysts in the inclusions have a large variation in core compositions (An42 to An96), and exhibit various zoning profiles and reaction textures, indicating they coexisted with melts ranging from basaltic to rhyolitic composition. Pyroxenes also exhibit a wide range in composition and a variety of zoning patterns consistent with multiple sources. These results suggest that a stratified magma chamber exists beneath the volcano, consisting of a lower basaltic layer, an upper rhyolitic layer and an episodically-present, thin middle layer of andesite. Variations in the chemistry of the Iwodake and Showa-Iwojima mafic inclusions suggest that multiple injections of very similar basaltic magma have occurred since the growth of the Iwodake dome. More extensive textural disequilibrium shows that the Showa-Iwojima rhyolites formed through more extensive interaction with mafic magma. The mafic-felsic interaction is consistent with degassing model of a magma chamber estimated by other researchers, which consists of degassing of upper rhyolitic magma by convection in a conduit and supply of a CO2-rich volatile phase from underlying basaltic magma to the rhyolitic magma.

Mafic microgranular enclaves from the Xanthi pluton (Northern Greece): An example of mafic-felsic magma interaction

Petrographic, geochemical and isotopic data are reported for the mafic microgranular enclaves occurring in a calc-alkaline Oligocene pluton outcropping near Xanthi town in northern Greece. The intrusion is composed of dominant granodiorites with minor tonalites and quartz diorites. Mafic enclaves occur in large swarms concentrated near the margins of the pluton. They have fine-grained porphyritic textures and contain the same minerals as their granodioritic and tonalitic host rocks (Pl, Qtz, Bt, Hb1, Kfs ± Ap, Zrn, Mt, Spn) but in totally different modal proportions. Chemical compositions range from mafic to intermediate and are characterized by higher abundance of Na2O) and lower contents of CaO and Sr with respect to an average basaltic magma with a similar SiO2 content. The initial Sr isotopic ratios, ranging from 0.7061 to 0.7063, are similar to those of the enclosing rocks (0.7062-0.7063). Field evidence, as well as petrographic and geochemical data, suggests that the analyzed enclaves derived from disruption of synplutonic intrusions of cogenetic mafic magma. The Xanthi pluton could represent ascent of a zoned magma body, with a silicic cap (granodiorite, tonalite, quartz diorite) having intruded first followed by the deeper more mafic levels as syn-intrusive dikes.

Plagioclase mantles on sanidine in silicic lavas, Clear Lake, California: Implications for the origin of rapakivi texture

At Clear Lake, California, mixed dacite lavas contain sanidine mantled by oligoclase that closely resemble rapakivi feldspars in granites. Resorption and mantling of sanidine by plagioclase is one of many disequilibrium textures in these lavas resulting from maficfelsic magma interaction. Mantling of sanidine by plagioclase may occur directly in mixed magmas due to shifts in composition and temperature, or indirectly in crystal-rich silicic magmas influenced by thermal under-plating, or mafic-felsic hybridization in adjacent regions of the magmatic system. Other silicic laves containing mantled sanidine also show evidence for mafic-felsic magma interaction. Textural and compositional similarities between rapakivi texture in extrusive and intrusive rocks suggest that they have a common origin. Moreover, modification of early-formed textures due to protracted crystallization, subsolidus reactions, and metasomatic-hydrothermal alteration in the plutonic environment accounts for most of the observed differences between extrusive and intrusive examples. The association of coeval mafic rocks and evidence for mafic-felsic magma interaction in many plutons containing rapakivi exture confirm that magma mixing plays a key role in its formation in some intrusive texamples. Evidence for a compositional control and multiple episodes of mantle formation are also compatible with repeated cycles of recharge and mixing. Regardless of the exact cause of K-feldspar instability, textural evidence from volcanic rocks and experiments indicates that development of rapakivi texture is controlled by cation diffusion in a dissolution boundary layer developed on K-feldspar. Mantles appear to form in a two-stage process that involves initial epitaxial nucleation of plagioclase on sanidine, followed by simultaneous dissolution of sanidine and inward growth of plagioclase.

Granitoid episodes and mafic-felsic magma interaction in the Svecofennian of the Fennoscandian Shield, with main emphasis on the ∼1.8 Ga plutonics

Three major episodes of felsic plutonism can be identified in the Svecofennian of the Fennoscandian Shield. Each of these episodes is associated with coeval (but not comagmatic) mafic mantle-derived plutonism. The first episode comprises the early Svecofennian (∼1.90-1.86 Ga) granitoids, which formed the first continental crust, together with associated supracrustals, in most of the Svecofennian by a separation from the mantle at that time. They are dominated by tonalitic-granodioritic compositions and contain frequent examples of mafic-felsic magmatic interaction. The second episode (∼1.83-1.77 Ga), which is of main concern here, includes the Transscandinavian Igneous Belt (TIB), a huge N-S trending batholith of granitoids, mafics and some associated volcanics, separating the Svecofennian from the Southwest Scandinavian Gneiss Domain, and the smaller massifs and plutons of the Within Svecofennian Episode 2 (WSE2) granites. The TIB contains several examples of extensive homogenized hybrids of mafic-felsic magma-mixing, related to large aeromagnetic anomalies. The hybrids generally grade into the coarse porphyritic TIB-granites, and have intermediate mineralogy and chemistry. The geochemistry of the hybrid rocks is consistent with mixing trends in the range between ≈55% SiO 2 and 65–70% SiO 2 , which represent the approximate end members. Mixing and mingling on a local scale is also commonly observed. The WSE2-granites have a more limited felsic composition, higher contents of incompatible trace elements, and usually lack evidence of mafic-felsic interaction, consistent with a generation by a lower degree of partial melting of the same composite, early Svecofennian crust compared to the TIB-granites. The third episode corresponds to the rapakivi granites (∼1.65-1.56 Ga). These are intimately associated with coeval mafic magmatism. Interaction between magmas has been identified in composite dikes, but is also proposed to have formed the dark, olivine- and pyroxene-bearing, more mafic rapakivi varieties, which are very similar to the homogenized hybrids of the TIB.