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The interaction of basite and granite magmas in subvolcanic conditions

Dokukina K.A.

Geological Institute of RAS, Moscow, Russia

dokukina@mail.ru

 

The interaction of mafic and felsic magmas were examined in subvolcanic hypabyssal condition of Tastau volcanoplutonic ring complex which is in the Zaisan Magmatic Province of Eastern Kazakhstan. The Zaisan Magmatic Province consists of voluminous volcanic and plutonic rocks emplaced during regional extension affecting the Zaisan orogen. Extension affected crust previously thickened by the Late Carboniferous – Permian collision of the Kazakhstan with the Siberian craton. Coeval mafic-felsic magmatism in the Zaisan Magmatic Province mostly consists of anatectic acidic magmas with associated mantle-derived magmas having high-K calc-alkaline. Age of the Zaisan magmatism is Permian – Early Triassic (248 – 293 Ma). The hypabyssal Tastau volcanoplutonic ring complex is the largest multiphase intrusive bodies which represent a volcanic root system. Intrusive age of Tastau complex is 242±20 Ma (U-Pb, zircon from synplutonic granite). The complex intruded low-grade sedimentary rocks, which comprises sandstone and siltstone of a greywacke composition. The volcanoplutonic complex has a form of an ellipse (13-18 km). Arc-shaped belts of composition dykes are intrude in the host rocks.

A wide variety of magmatic rocks is represented within the Tastau volcanoplutonic complex: leucogranite, granite, granosyenite, gabbronorite and gabbro-diorite and calcium basite. Tastau magmatic rocks represent the continued calc-alkaline series and they are characterized by variations in chemical compositions (46 < SiO2 < 78 wt. %). Unhybrid gabbronorite have the most magnesia content (#Mg=0.50–0.55). Synplutonic mafic dikes and enclave are a subalcaline gabbro, a monzonite, a syenite and a quartz syenite (SiO2 = 46.2–62.8, Al2O3 = 15.8–19.6, TiO2 = 0.75–2.22, FeOtot = 4.7–11.5, MgO = 1.8–5.4, CaO = 2–7 wt.%) and high content of alkalines (Na2O+K2O = 5.2–9.3 wt.%). Mafic enclaves are depleted relatively the unhybrid gabbro in Sr, Ca; and rich Rb, K. The felsic rocks are granite and felsite, which are depleted Ba, Sr, and rich U. All rocks have a similar positive chondrite-normalized REE patterns and negative europium anomaly (mafic enclaves (Ce/Yb)N=4.99–5.54, Eu/Eu*=0.68–0.78; granite (Ce/Yb)N=5.48–8.59, Eu/Eu*=0.24–0.64; felsite (Ce/Yb)N=2.50, Eu/Eu*=0.006), except the unhybrid gabbro (Ce/Yb)N=5.3, Eu/Eu*=1.02). Compositions of the Tastau magmatic rocks hit the intraplate magmatism area and the active continental margins.

The studied mafic rocks of the Tastau complex have similar REE patterns, and this suggests their origin from a common mantle source. The most primitive mafic member is olivine gabbronorite rich in Mg and TR. Felsic rocks have a slight negative Eu anomaly and an average Sr content suggesting the presence of residual plagioclase in the source. The high K content of granitoids (up to 5 wt.% K2O) is due to biotite or alkali feldspar (or both) participating in the partial melting of crustal rocks (Lindline et al., 2000). Biotite and amphibole crystallizing at the late stage show that granitic rocks crystallized from water-saturated granitic magma with 2.5 to 5.0 wt.% H2O (Maaløe and Wyllie, 1975; Naney, 1983). Melt inclusions in the felsic rocks of the Tastau series were studied in (Titov et al., 2001). The data show that magma formed in the lower crust due to the melting of a metapelitic source at a pressure about 10 kbar and temperature above 1000°C. The results suggest that the formation of the gabbro–granite association agrees with the model involving the rise of hot mantle melts, crust heating, and anatexis of lower crustal rocks (Huppert and Sparks, 1988; Lindline, 2000; and others). According to field and petrographic data, mafic and felsic rocks interacted intensively to produce composite and hybrid rocks of the gabbro–diorite series. Mafic inclusions in composite zones are rounded or pillow-like in shape, with fine-grained margins, crenate boundaries, and a magmatic texture. These characteristics clearly show their igneous origin: they are drops of mafic magma injected into completely or partially molten granitic magma. Mingling was accompanied by mixing, as evidenced by thin sections and the continuous series of synplutonic rocks of composition intermediate between mafic and felsic.

Magma mixing. Geochemical data suggest that the intermediate synplutonic intrusions of the Tastau complex consist of hybrid magma derived from mantle and granitic melt resultant from metapelite anatexis. The chemical interaction model was tested using a “mixing test” (Fourcade and Allègre, 1981). Samples of unhybridized olivine gabbro-norite and the most felsic leucogranites in the dike swarm were chosen as mixing end-members. The test was conducted for mafic inclusions and intermediate dikes. The test showed that the chemical interaction model provided a good explanation for available data. The proportion of felsitic magma was estimated from major elements at 37 and 52%. Later on, calculation results were tested with regard to trace and rare elements. Model results agree well with the modal composition of intermediate hybrid rocks.

Magma mingling. The diversity of synplutonic rocks, disequilibrium phenocrysts, along with the texture and composition of hybrid rocks, show a temperature and viscosity gradient when granitic and mafic magmas interact. We offer a model where near-liquidus basaltic magma is injected into subliquidus granitic magma and disintegrates there below the actual subvolcanic level. Basaltic magma inside viscous granitic material loses its intrusive power, is quenched and set in motion together with granitic magma. Magmatic flow and evolution in the granitic chamber with mafic components may have taken place under turbulent flow conditions. This conclusion is confirmed by field observations of flow textures and deformed mafic inclusions in granitic magma. The formation of diverse hybrid rocks and the acquisition of similar trace and rare element content by all the rocks must have taken place in an open system with crystal, fluid, and ion exchange. These processes must have preceded the quenching of mafic inclusions. Rock texture suggests rapid cooling of mafic globules, so that diffusion exchange between the magmas may have been insignificant. If so, compositional similarity between the inclusions and host magma was probably due to the transport and exchange of residual fluid at temperatures between the solidus of both magmas and subsolidus reactions. However, rapid turbulent flow in the magma chamber may have resulted in the destruction of quenched mafic fragments and their spread as clasts and crystals enriching granitic magma with femic components. The high rate of processes in the magma chamber may have been necessary for overcoming density, viscosity, and temperature contrast between two magmas to enable chemical interaction and produce much hybrid magma.

Magmas may have exchanged xenocrysts in a deep (relative to the actual level) magma chamber at thermal equilibrium, when dispersed near-liquidus basalt intruded molten granitic magma with scarce xenocrysts. This process may have given rise to large synplutonic bodies of monzonites and granosyenites, which were homogenization products of basaltic and granitic magmas. In (Frost and Mahood, 1987; Sparks and Marshall, 1986), the volume of hybrid magma was demonstrated to depend on temperature and the volume of interacting magmas. Mafic and granitic magmas in the Tastau complex may have coexisted at thermal equilibrium for some time. Large bodies of the synplutonic monzonite and granosyenite probably are homogenization product of mafic and felsic magmas.

 

References:

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Frost T.P., Mahood G.A.,Styles of mafic–felsic magma interaction: the Lamarck granodiorite, Sierra Nevada, California, USA // Geol. Soc. Am. Bull. 1987. Vol. 99. P. 272–291.

Huppert H.E., Sparks S.J. The generation of granitic magmas by intrusion of basalt into continental crust. // J. Petrol. 1988. Vol. 29 (3). P. 599–624.

Lindline J., Crawford W.A., Crawford M.L., Omar G.I. Post-accretion magmatism within the Kuiu–Etolin Igneous Belt, Southeastern Alaska // Can. Mineral. 2000. Vol. 38. P. 951–974.

Maaløe S., Wyllie P.J. Water content of a granite magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions // Contrib. Mineral. Petrol. 1975. Vol. 52. P. 175–191.

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Titov A.V., Khromykh S.V., Vladimirov A.G., Pospelova L.N. // Melt inclusions in garnet and quartz from dacite porphyries of the Aktobe Structure, Kazakhstan: estimation of generation conditions and composition of primary melts. Dokl. Earth Sci. 2001. Vol. 377 (2). P. 229–232.