Mineral chemistry and petrogenesis of granular ejecta from the Alban Hills volcano (Central Italy) |
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| Authors: | M Federico A Peccerillo |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università degli Studi “La Sapienza”, Roma, Italy, IT;(2) Dipartimento di Scienze della Terra, Università di Perugia, Perugia, Italy, IT |
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| Abstract: | Summary A suite of lithics (ejecta) collected from the latest erupted pyroclastic products of the Alban Hills volcano (Central Italy)
has been studied to determine their mineralogical composition and to investigate their genesis. The ejecta commonly have granular
texture and consist of coarse-grained crystals often associated with a fine- to medium-grained matrix. The mineralogical composition
is variable and consists of both typical igneous minerals and contact metamorphic phases. Garnet, clinopyroxene K-feldspar
are almost ubiquitous, whereas leucite, wollastonite, sodalite-group minerals, phlogopite, nepheline and phillipsite are present
in most of the ejecta; minor and accessory phases include cuspidine, amphibole, pyrrhotite, magnetite, apatite, uranpyrochlore,
sphene, kalsilite, and melilite; anorthite, zircon and fluorine-bearing Ca, Zr silicate phases, larnite, and baryte are found
sporadically. Ca, REE, Th silicophosphates occur in many samples generally disseminated along interstices and fractures of
main minerals. Calcite is present as discrete crystals sometimes enclosed in other minerals, as granules in the fine-grained
matrix and as late microcrystalline veins. It shows high oxygen and low carbon isotope ratios with δ18O = + 17.96 to + 27.19, and δ13C = −4.74 to −19.57.
Clinopyroxene ranges from diopside to compositions strongly enriched with both Ca-Tschermak’s and esseneite components. Feldspars
are generally potassic even though Ba and Sr are found in significant concentrations in some samples. K-feldspars from wollastonite-bearing
ejecta are often rimmed with elongated felty crystals identified by X-ray diffraction analysis as leucite. These feldspars
show a depletion in Si, and enrichment in Al and K from core to rim. Significant compositional variations are also shown by
various other phases such as nepheline, apatite, Ca, REE, Th silicophosphate.
The occurrence of igneous and contact metamorphic minerals, as well as the chemical variations of clinopyroxenes and feldspars
in the investigated ejecta reveal complex genetic processes related to the interaction between potassic magma and wall rocks.
The Ca-rich composition of most phases points to a carbonate nature for the wall rocks. Textural evidence suggests that coarse-grained
rocks formed at the margin of the magma chamber were invaded by a late, volatile rich potassic liquid which crystallized as
a fine-grained matrix and produced disaggregation and reaction of early formed minerals. Fluid phases percolating through
the rocks generated infiltration metasomatism and deposited some uncommon phases rich in Ca, REE, Th, U, which are found along
cracks and at the margins of early crystallized minerals.
Overall, the all spectrum of the minerals found in this study are also typical of carbonatitic rocks. Their presence in the
Alban Hills ejecta demonstrates that their genesis can be related to interaction between ultrapotassic melts and carbonate
wall rocks, in addition to precipitation from carbonatitic melts.
Received February 20, 2001; revised version accepted September 23, 2001 |
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