Chemostratigraphy of Products of The Astroni Activity (4.1-3.8 Ka, Campi Flegrei, Italy)
Abstract
The Astroni volcano is a well-preserved tuff-ring in the NE sector of the Neapolitan Yellow Tuff (NYT, 12 ka BP) caldera, the youngest of the two major collapses which generated the nested Campi Flegrei caldera (CFc). The volcano has been characterised by a long-lasting activity in the age range 4.1-3.8 ka BP, generating 7 pyroclastic and 2 lava units. Thus the whole rock sequence is subdivided in Units named 1 through 9 upsection. Detailed petrographical, geochemical and isotopic investigations have been carried out on these rocks (pumice and scoria fragments, lavas) which have tex- tures variable from sub-aphyric to highly porphyritic to glomeroporphyritic. The phe- nocrysts are plagioclase, alkali-feldspar, Mg- to Fe-rich zoned clinopyroxene, biotite, opaques and apatite, in order of decreasing abundance. Their abundance is variable along the sequence: lower in Units 1 to 4, higher in Units 5 to 9. The groundmass ranges from hypocrystalline to hyalopilitic in pumice and scoria fragments, to felty in the lavas. The analysed rocks are nepheline normative trachyte to alkali-trachyte. Whole-rock major and trace element contents vary regularly with D.I. (71.5 and 81.0), suggesting fractional crystallization of mineral phases in a magma evolving from tra- chyte to alkali-trachyte. However, whole-rock 87Sr/86Sr values range from 0.70725 to 0.70755. It is noteworthy that the extreme detected values match those of the prod- ucts of the largest caldera-forming eruptions: the Campanian Ignimbrite (37 ka) and NYT, respectively. The 87Sr/86Sr ratio is variable with the chemical composition of the products, suggesting open-system evolution process acting in the magma reser- voir feeding the Astroni volcano. Furthermore, the chemostratigraphy of the Astroni sequence shows that activity started with extrusion of more evolved, alkali-trachytic magma (DI about 80-81; 87Sr/86Sr = 0.70755), becoming progressively less differ- entiated and less radiogenic up to emplacement of the lavas of Unit 6, likely due to ongoing mixing in the reservoir. The following activity was fed by magma with vari- able DI (72-80) and 87Sr/86Sr (0.70725-0.70747). Integration of the stratigraphical, petrologic and volcanological data has allowed us to formulate a coherent hypothesis on the state of reservoir before eruption, eruption dynamics and magma withdrawal.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.4194D