Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 169

(1) Diopside-salite, and
(2) Salite-diopside.

Gaeta et al. (2006) investigated chemical and isotopic compositions of clino-
pyroxene crystals from juvenile scoria clasts, lava flows, and hypabyssal magmatic

ejecta representative of the whole eruptive history of the Alban Hills Volcanic
District. The Alban Hills is a Quaternary ultra-potassic district that was emplaced
into thick limestone units along the Tyrrhenian margin of Italy. Alban Hills vol-
canic products. Even the most differentiated rocks, are characterised by low SiO 2 .
They suggested that the low silica activity in evolving magmas can be ultimately
due to a decarbonation process occurring at the magma/limestone interface. They
proposed that, the differentiation process was initiated by crystallisation of clino-
pyroxene

magnetite coupled with assimilation of a small
amount of calcite and/or interaction with crustal CO 2 . By combining age, chemical
data, strontium and oxygen isotopic compositions, and REE content of clinopy-
roxene, they gave an insight into the evolution of primitive ultrapotassic magmas of
the Alban Hills Volcanic District over an elapsed period of about 600 kyr.
Geochemical studies of clinopyroxene crystals, consistent with data coming
from other Italian ultrapotassic magmas, indicate that Alban Hills primary magmas
were generated from a metasomatized lithospheric mantle source. In addition, their
study shows that the Sr 87 /Sr 86 and LREE/HREE of Alban Hills magmas continu-
ously decreased during the 600 ± 35 ka time interval of the Alban Hills eruptive
history, possibly re

± leucite ± apatite ±

ecting the progressive depletion of the metasomatized mantle

source of magmas.
According to them, there is no evidence of mixing process responsible for the
coexistence of both diopside and salite in the same lava flow. Variation in the

aqueous vapour pressure may be responsible for different crystallization sequences
of the phenocrysts. Salitic pyroxenes either reacted with liquid or are replaced by
olivine, phlogopite and Ti-magnetite. According to Aurisicchio et al. the predom-
inant role of water in the evolution of Alban Hills magma is supported by high F
content of the lavas erupted by the Alban Hills volcano. This possibly resulted in
the enhancement of water solubility in the magma. There is common occurrence of
mica in the groundmass of the rocks. The explosive character of the Alban Hills
volcanism also indicate the presence of an aqueous vapour phase.

4.8.4.6 The Phlegrean Field

The Phlegrean Field (Fig. 4.20 b, Di Girlamo et al. 1984) is constituted of a complex
volcanic system that developed over the last 50,000 years (Beccaluva et al. 1990).
There was first early submarine volcanic activity represented by eruption of tuffs,

latititic and trachytic lavas. Subaereal volcanism is associated with pyroclastic
products and lavas in different parts of the Phlegrean Fields. In the last phase there
was ejection of coarse-grained brecciated material (Breccia Museo) containing a
large variety of lava blocks including leucitic lavas and basement rocks, which have