Hybridization between felsic and mafic magmas in calc-alkaline granitoids — a case study in northern Sardinia, Italy
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Felsic
Hornblende
Phenocryst
Silicic
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Silicic
Protolith
Mineral redox buffer
Amphibole
Volcanic belt
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A study of amphiboles and associated minerals in samples of Soufrière Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 µm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe–Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 ± 10°C, but was variably heated before eruption. The heating occurred within ∼30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855°C at a PH2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860°C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830–860°C. The average ascent rate of magma during much of the 1995–2002 eruption has been >0·02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.
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Igneous differentiation
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The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber.
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Fractional crystallization (geology)
Igneous differentiation
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The Cretaceous-Palaeogene alkaline province, mainly located in the Kırşehir Block, extends E-W for some 300 km in Central Eastern Anatolia. The Dumluca, Murmana, Karakeban and Çaltı plutons constitute the easternmost part of the province. These plutons have introsive contacts with the Cretaceous Divriği ophiolitic melange and, except Karakeban, are associated with huge iron deposits. The Ypresian-Lutetian sediments unconformubly overly them. The four studied plutons belong to two completely different types of magmatic assocsiation, based on petrographic and geochemical (mainly major elements and REE) fetures. The dominant association comprises three plutons (Dumluca, Murmana, Karakeban) and is rather uniformly represented within each of them. These three plutons have a bimodal character evidenced by the coexistence of two groups of rocks, one mafic and the other felsic. The dominant felsic group, mainly composed of Na-rich and Ca-poor quartz monzonites and monzonites (Murmana, Dumluca), may also include syenites, quartz syenites, adamellites and granites (Karakeban). This group displays a cafemic alkaline over staurated trend, usually magnesian (Dumluca and Murmana) and common (Karakeban). The mafic group is mainly made up Na-rich and Ca-poor gabbros/diorites, monzogabbros/monzodiorites, rather often silica-undersaturated. This group represents a cafemic alkaline saturated to undersaturated trend either ferriferous (Karakeban) or magnesian (Dumluca) or or common (Murmana). Mafic dykes, cutting through the felsic rocks, belong to the same mafic group. These three plutons represent a composite alkaline association and the same type of association characterizes also each individual pluton. Field, petrographic and geochemical data suggest that the felsic group has not been derived from the mafic one by crystal fractionation, but that the two coevial groups may have interacted with each other. The subordinate association is represented by Çaltı pluton. This homogenous pluton, tonalitic and gronodioritic in composition, corresponds to a cafemic calc-alkaline association,with slight magnesian affinity. Such a coexistence of two quite different magmatic associations in the Divriği region has already been reported in the western part of Kırşehir Block. It shows that the alkaline character is restricted to only part of the plutons located in this block and may suggest that several successive magmatic events, related to quite different geodynamic conditions, occured in this domain. In this framework, the Dumluca, Murmana and Karakeban plutons are thougth to be derived from two different magma sources. One of them is mantle related mafic magma generated in a post-collisional lithospheric attenuation environment which accordingly caused to melt the lower parts of the continental crust forming the felsic magma source. This collisional event is attributed to the juxtaposition of the Pontide and Anatolide plates in the pre-Maastrictian time. The Çaltı pluton, representing different mineralogy and chemistry, should be solidified from the collision related and calcalkaline another hybrid magma source due especially to intruding the already obducted Divriği ophiolitic melange. Isotopic and geochronological data would be particularly helpful to better constrain this magmatic history.
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Aztec Wash pluton, a 50 km[sup 2] intrusive complex in the northern Eldorado Mountains, was emplaced ca. 16 Ma (Faulds et al., 1990) during extension within the Colorado River Corridor. The pluton displays extreme compositional variability, ranging from olivine gabbro (ca. 50 wt% SiO[sub 2]) to highly evolved aplite (76% SiO[sub 2]). Most of the intrusion is medium grained, homogeneous granite (ca. 72% SiO[sub 2]), but 1/3 is highly heterogeneous and dominated by mafic to intermediate rocks; a 6 [times] 3km, N-S mafic zone almost bisects the pluton. Well-displayed magma mingling and late mafic and felsic dikes verify the coexistence of mafic and felsic melts. Hornblende barometry indicates that the entire exposed portion of Aztec Wash pluton was emplaced at very shallow depth (
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Magma chamber
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The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al2O3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer.
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Actinolite
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Field relations and geochemistry indicate that Aztec Wash pluton had a complex, open-system history. The tilted pluton represents a 2.5 km thick chamber that was recharged with both felsic and mafic magma. The lower portion is highly heterogeneous, with mafic sheets; cumulates; hybrid rocks; mafic, felsic, and composite dikes; and sheets and pods of granite (heterogeneous [H] zone). The upper part is granite that is generally homogeneous in texture and geochemistry (granite [G] zone). At the base of the G zone, a discontinuous zone (buffer [B] zone) records interaction between the G and H zones. Complexity of the H zone makes detailed reconstruction of magma chamber history difficult, and the relatively homogeneous G zone appears to offer few clues about the evolution of the pluton or the interaction between the felsic and underlying more mafic magmas. Accessory mineral textures, zoning, and assemblages in the G zone, however, are far from homogeneous and provide clear evidence for fluctuating conditions that elucidates magma chamber history.
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Abstract This chapter documents the fracture process associated with the early cooling stage of felsic magma. Characteristics of pre-exhumation joints include their spatial distribution in granite bodies, their fracture surface morphology, and geological and petrological evidence for the depth of fracture initiation. These characteristics allow inferences about the depth and the time of joint origin in the South Bohemian Pluton. The intrusion levels of currently exposed granites of the pluton were 7.4 km in the northern part and 14.3 km in the southern part. Within the northern Mrákotín Granite (Boršov) early NNE joints propagated while the granite was at a temperature near the solidus, and, in part, magma was still being injected, post-dated by thin granite dykes along NNE joints. Evidence for the pre-exhumation initiation of these joints comes from the geochronological dating of these late-granite dykes (1–2 cm thick) at 324.9 Ma in age, which were creating their own rupture in the rock. The timing of the pluton emplacement at 330–324 Ma and the cooling ages of 328–320 Ma have been given by previous studies. From fluid inclusions within the late-granite dykes that occupy joint surfaces, the trapping depth of the analysed inclusions was calculated to be 7.4 km. Near the solidus H 2 O separates during the crystallization of anhydrous phases. The associated increasing H 2 O pressure can initiate the first cracks and can generate a small portion of new granitic melt, which forces the cyclic fracture propagation together with mobile, low-viscosity ‘residual melt’ input into the fracture. The determination of the intrusion level and time at which the dykes began cooling provide evidence for the joint initiation at a depth of 7.4 km, which was connected with the level and process of final emplacement and early cooling of the Mrákotín Granite long before the main exhumation. At the earliest, the erosion of the upper rock pile, 7.4 km in thickness, started significantly after generation of the early joint sets. The NNE-trending joints are persistent in orientation throughout the South Bohemian Pluton, but the joint-surface morphology varies in all subplutons and occupies all sections of the stress intensity v. crack-propagation velocity curve (Wiederhorn-Bahat curve).
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