Phase disequilibria and crustal contributions in the magmatic mafic enclaves of san vincenzo volcanic district

S. Meli, G. Serri

In the San Vincenzo volcanic district, both sedimentary and magmatic enclaves sporadically occur within the anatectic rhyolites which build up the whole centre (Ferrara et al., 1989). Recently, Meli & Serri (1999) recognized two kinds of magmatic inclusions with “volcanic” textures; they originated through mingling of magmas with contrasting physico-chemical characters.

Type I inclusions (high-K calc-alkalic andesites): rounded, with sizes ranging from few mm to 50 cm; sharp and lobate contacts with the host lava are common. Sub-spherical vesicles are always present The enclaves are holocrystalline, sub-aphanitic, with Pl and very minor coronitic Opx phenocrysts and relics of iddingsitized Ol; embayed Qtz xenocrysts often occur. Pl always shows an inner spongy rim; clear outer rims are also present. Opx is corroded, and coronitic textures made up of Cpx are common; rare Cpx phenocrysts look strongly resorbed. The groundmass is mainly composed by Pl and Cpx, with minor opaque minerals and Bt.

Type II inclusions (shoshonitic latites): rounded to ovoidal, smaller than 15 cm across in size. Irregularly shaped vesicles are present, and diktytaxitic voids are also common. Lobate to cuspidate contacts with the host lava are always sharp. Latitic enclaves are more porphyritic than andesites. Pl, Bt, Cpx and Opx phenocrysts are present; iddingsitized Ol pseudomorphs and Qtz xenocrysts also occur. Plagioclase has a very thin spongy rim; clear outer rims are narrow and rare. Biotite occur both as embayed phenocrysts and euhedral microphenocrysts. Opx and Cpx form glomerocrysts; some Cpx crystals are zoned, others are coronitic upon Opx. The groundmass is holocrystalline, composed by Bt, Pl, Kfs and opaques.

On the basis of geochemical models, Meli & Serri (1999) have shown that type I and II enclaves evolved from different parental magmas.

EPMA analyses have been performed on phenocrysts and groundmasses of both andesites and latites, as well as of the enclosing rhyolites.

In the latites, Cpx has a homogeneous composition (En_49-51Fs_5-6Wo_44-45). On the contrary, Opx displays bimodal chemical features (En_47-48Fs_51-52Wo₁ and En_71-73Fs_24-26Wo₃), testifying to a contribution of two different populations to the overall Opx content. Taking into account the Mg_v of coexisting pyroxenes and liquids (Grove & Donnelly-Nolan, 1986), only the En-rich population can be in equilibrium with Cpx and the melt. It is worth noting that the Fs-rich Opx is still richer in iron than the Opx microlites occuring in the rhyolite groundmass. This indicates that the host magma cannot represent the source for these xenocrysts. Also the high-K andesites contain two different Opx populations (En_72-86Fs_12-22Wo_1-4 and En_59-64Fs_34-40Wo_1-2); the En-rich crystals are likely to be in equilibrium with the melt.

Plagioclase analyses in high-K andesitic enclaves revealed that the rims (clear + spongy) are more An-rich (Ab_27-39An_58-70Or_2-3) than the inner part of the phenocrysts (Ab_44-57An_38-52Or_4-5). Phenocryst cores display only a weak normal zoning. Groundmass plagioclase composition resembles that of the outer rims (Ab_30-44An_51-66Or_4-5). Plagioclase phenocrysts in latitic inclusions have similar, weakly zoned, compositions (Ab_47-53An_40-48Or_5-7 in the cores and Ab_51-61An_34-44Or_5-6 in the inner rims); groundmass plagioclase is richer in An (Ab_36-46An_49-60Or_4-5).

Ternary feldspars geothermometry in rhyolites (Green & Usdansky, 1986) yielded an equilibrium temperature of about 780°C at a pressure of 400 MPa. Opx-Cpx geothermometry (Wells, 1977), applied in latites to mineral couples which are thought to be in equilibrium, give a temperature of about 990°C, suggesting a temperature contrast of about 210°C between latitic and rhyolitic melts. The high-K andesites have a mineral assemblage which is not suitable for reliable geothermometry.

K-feldspar xenocrysts are very rare in the magmatic enclaves; they have been observerd only in the latites. Their composition is close to that of sanidine phenocrysts of the host lava.

Embayed biotite phenocrysts have a TiO₂ content higher than 6%, very similar to the Ti content of biotite crystals in metamorphic xenoliths. Euhedral Bt microphenocrysts have similar MgO and lower TiO₂, thus suggesting a xenocrystic origin for the embayed bigger crystals.

Mineral chemistry of latites and high-K andesites indicate that both enclaves contain xenocrysts, which not always come from the host rhyolite. Latitic enclaves have little interacted with the lava, whereas andesites seem to be relatively uncontaminated by the host. However, also these enclaves underwent crustal contamination: Fe-rich Opx and Qtz probably come from the crust somewhere underneath the volcanic district. Anyway, the overall xenocryst contribution to the enclaves is always less than 2% by volume. In the andesites, only Pl, Cpx and En-rich Opx can represent an equilibrium paragenesis. In the latites, Cpx, En-rich Opx, Pl and Bt microphenocrysts can represent an equilibrium paragenesis, while Fs-rich Opx, high-Ti Bt, Kfs and Qtz point to a crustal contamination process, in which the host rhyolite played a significant role.