Oxygen isotope geochemistry of the potassic igneous rocks from the Roccamonfina volcano, Roman comagmatic region, Italy

Abstract

18O/ 16O measurements were made on 80 samples of lavas, pyroclastics, and xenolithic nodules from the 0.05- to 1.54-m.y.-old Roccamonfina volcano. The δ 18O values of these magmas varied from +5.6 to +10.1 in the High-K Series (HKS) and +6.6 to +8.8 in the Low-K Series (LKS). The xenolithic nodules, many of which are HKS cumulates, crystallized from magmas with δ 18O= +7.8to+10.3. These data indicate that assimilation of high- 18O country rocks (pelitic schists, flysch, and some carbonates) was an important petrologic process during fractional crystallization of both the early, HKS magmas (leucite-bearing tephrites, leucitites, and phonolites) and the later, LKS magmas (trachybasalts, trachyandesites, and trachytes). Combined with 87Sr/ 86Sr, Pb isotope, and 143Nd/ 144Nd data obtained by other workers from the same outcrop localities, the 18O/ 16O data prove that: (1)| The assimilated country rocks had relatively high 87Sr/ 86Sr values (0.710–0.720), low 143Nd/ 144Nd values (ε Nd ≈ −10to−15), and 206Pb/ 204Pb values of about 18.70. (2)| The HKS and LKS represent completely separate trends of magmatic differentiation and different parent magmas. (3)| The HKS parent magma (leucite basanite?) probably had δ 18O≈ +6.0and 87Sr/ 86Sr≈ 0.708, while the LKS parent magma (trachybasalt?) had δ 18O≈ 6.5and 87Sr/ 86Sr≈ 0.706. (4)| The HKS and LKS parent magmas are only indirectly related genetically, perhpps in their source regions in the upper mantle, or during an earlier episode of wall rock-magma interaction in the lower crust. (5)| Although wall rock assimilation has played an important role in determining the isotopic and trace element characteristics of these magmas, the liquid line-of-descent and the changes in major element chemistry were dominated by fractional crystallization. (6)| The energy required to heat up and dissolve the assimilated rocks was provided by the latent heat associated with concurrent fractional crystallization of the cumulates (as represented by some of the plutonic nodules). (7)| The oxygen and strontium isotopic changes at Roccamonfina, although very significant, are much less than in the undersaturated magmas of the Roman Province further to the north. This is apparently because during and prior to the Quaternary potassic vulcanism, the latter terrane was invaded and heated by anatectic magmas associated with the Tuscan rhyolitic event, thus creating conditions favorable for much greater crustal assimilation.