Isotope and chemical microsampling: Constraints on the history of an S-type rhyolite, San Vincenzo, Tuscany, Italy

Abstract

Fine scale heterogeneities in strontium isotope ratios, 40Ar-39Ar ages, and chemical com~sition have been determined for individual mineral grains and enclaves in the rhyolites of San Vincenzo, Italy. The rhyolites are peraluminous with phenocrysts of alkali feldspar, piagioclase, biotite, and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry. Group B lavas are distinguished from Group A in having higher MgO and CaO contents as well as containing two pyroxenes and chilled latite enclaves. The Group B lavas show textural evidence for d~sequilib~um~ such as resorption and sieve zones in feldspars. Laser fusion 40Ar-39Ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4.38 + 0.04 Ma, ~though small amounts of excess argon are present in subhedral and cloudy crystals. The groundmass in the Group A sample has a higher initial R7Sr/86Sr (0.725) than that of the Group B (0.7 13). The Sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within-grain zonation. The feldspars and coarse-grained biotites have initial 87Sr/8"Sr that are intermediate between those of the Group A and B glasses, with alkali feldspar generally having higher initial 87Sr/86Sr ratios than plagioclase. Chilled latite enclaves, which are present in only the Group B lavas, have relatively low initial 8'Sr/86Sr ratios in the range 0.7082-0.7088. The mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and, therefore, cannot be the source of the strontium isotope disequilibrium. Xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts, clinopyroxene-orthopyroxene clots, orthopyroxene-plagioclase clots, and plagioclase with extensive sieve textures. These grains all have relatively low initial 87Sr/86Sr ratios. Other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite, spinel, and sillimanite, and alkali feldspars with excess 4"Ar. The strontium isotope di~quilib~um in phenocrysts of plagioclase, alkali feldspar, and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration. A mode1 for the San Vincenzo magma chamber is proposed in which the Sr isotope evolution of the Group B rhyolites is due to magma mixing of a crustal melt with latite magma containing unradiogenic strontium. The variable initial strontium isotope compositions of Group A phenocrysts may be due to assimilation or crystallization along the interface between mingled Group A and B magmas.

Late Miocene remagnetization within the internal sector of the Northern Apennines, Italy

“…The Tuscan Unit tectonically overlies metamorphic rocks of the Monticiano -Roccastrada Unit (Middle Tuscan Ridge) and is tectonically overlain by the Ligurid Unit. The post-orogenic phase is characterized by extensive magmatism of Pliocene age, and in particular by the 4.4 Ma San Vincenzo rhyolite (Feldstein et al, 1994). Moderately negative inclinations, southerly declinations, high unblocking temperatures, and failure of the fold test also characterize the dominant component of magnetization of the Tuscan Cherts at Monti d'Oltreserchio and Campiglia in southern Tuscany (Fig.…”

Oxygen isotope heterogeneity of phenocrysts in rhyolite from San Vincenzo, Italy, by laser microprobe analysis.

“…The plagioclase in our sample occasionally has a glass margin of melting probably due to mixing with basic magma (Feldstein et al, 1994). We did not separate this glass margin from the non-al tered phenocryst.…”

“…In contrast, Holm and Munksgaard (1982) argued that such 180 enrich ment was caused by metasomatism, and they noted that the phenocrysts in the rhyolites were in iso topic equilibrium. Feldstein et al (1994) found Sr isotope disequilibrium in phenocrysts of plagio clase, feldspar, and biotite in the rhyolite of San Vincenzo. They attributed the grain-size Sr isotope disequilibrium to the systematic isotope heteroge neity in the magma chamber during crystallization.…”

“…This sample was glassy and absolutely fresh, and no oxidation or secondary mineral was found. Fine grained enclaves of latite composi tion (Ferrara et al, 1989), characterized by calcic plagioclase, magnesian biotite and minor amount of clinopyroxene, are ubiquitous in the studied lava, indicative of interaction of a second, hotter liquid with the rhyolite magma (Feldstein et al, 1994).…”

“…The studied sample was selected from the same specimen that of Group B studied by Feldstein et al (1994). This sample was glassy and absolutely fresh, and no oxidation or secondary mineral was found.…”