Multi-stage rodingitization of ophiolitic bodies from Northern Apennines
(Italy): Constraints from petrography, geochemistry and
thermodynamic modelling
-
Abstract
The investigated mantle bodies from the External Ligurians (Groppo di Gorro and Mt. Rocchetta) show evidences
of a complex evolution determined by an early high temperature metasomatism, due to percolating melts of
asthenospheric origin, and a later metasomatism at relatively high temperature by hydrothermal fluids, with
formation of rodingites. At Groppo di Gorro, the serpentinization and chloritization processes obliterated totally
the pyroxenite protolith, whereas at Mt. Rocchetta relics of peridotite and pyroxenite protoliths were preserved
from serpentinization. The rodingite parageneses consist of diopside þ vesuvianite þ garnet þ calcite þ chlorite
at Groppo di Gorro and garnet þ diopside þ serpentine vesuvianite prehnite chlorite pumpellyite at Mt.
Rocchetta. Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures
(264–334 C at 500 bar and 300–380 C at 1 kbar). Garnet, the first phase of rodingite to form, consists of
abundant hydrogarnet component at Groppo di Gorro, whereas it is mainly composed of grossular and andradite
at Mt. Rocchetta. The last stage of rodingitization is characterized by the vesuvianite formation. Hydrogarnet
nucleation requires high Ca and low silica fluids, whereas the formation of vesuvianite does not need CO2-poor
fluids. The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal
fluids; the presence of andradite associated with serpentine and magnetite at Mt. Rocchetta suggests
Fe3þ-bearing fluids with fO2 slightly higher than iron-magnetite buffer. We propose that the formation of the
studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an oceancontinent
transitional setting and, locally, in an accretionary prism associated with intra-oceanic subduction.
-
-