hide
Free keywords:
-
Abstract:
In rocks exhumed from depth corresponding to (U)HP conditions, fluid-induced dissolution– precipitation processes have significant bearing on element and isotope transport, on mineral reactions and hence on rheology. The U-Th-Pb system in geochronometers of such rocks — in this case, titanite and monazite — responds to fluid-mineral interaction; this response is dependent on the composition of the interacting fluid and on the deformation mechanism activated in the hosting rock. Tracking and dating the fluid-mineral-deformation histories using simultaneously measured in-situ U-Th-Pb and trace element data (Laser Ablation Split Stream ICPMS; LASS) and also Sr isotope data, is exemplified by the study of (A) pre- and post-UHP titanite from calc-silicates (Dabie UHP unit) and (B) monazite from relict granulites (Rhodope UHP domain). A) A pre-UHP titanite megacryst (XAl 0.18-0.19) has been partly replaced in a series of reactions that gave chemically and/or Sr-isotopically distinct product minerals. During stage I (subduction associated with rock deformation), rutile formed at the rim of the titanite. Following stage I, rutile back-reacted to form titanite by dissolution-precipitation reactions (stage II). The new titanite has relatively high XAl (0.21-0.27) and low HFSE contents. It is in turn replaced during stage III by titanite, showing low XAl (0.09-0.15) and relative HFSE enrichment. Channelized fluid infiltration (stage IV) resulted in titanite replacement by allanite. These four stages record in a single titanite the mineral-fluid reaction history from deep subduction to exhumation. Variable Sr isotopic composition demonstrates that fluids from different sources were present during the various stages of fluid-rock interaction. In rim portions completely replaced during the dissolution precipitation reaction stages, U-Th-Pb dating yield ages of c. 224 Ma (stage II) and c. 214 Ma (stage III). In the calc-silicate host rock pervasive fluid infiltration during exhumation has not been accompanied by ductile deformation. B) A HP granulite with the pre-deformative granulite mineral assemblage garnet, rutile, kyanite, K-feldspar, plagioclase, quartz has been pervasively deformed at high dislocation creep strain at high temperature and pressure conditions. The granulite is a relict within migmatitic rocks, migmatized in the presence of fluid during decompression at c. 42 Ma (Liati et al., 2011). Rutile and biotite yield U-Pb and Rb-Sr ages of 39–36 Ma, probably reflecting cooling following the migmatitization. Monazite in the HP granulite reacted with fluid by fluid-induced dissolution– precipitation reactions, resulting in resetting of U-Th-Pb ages at c. 54 Ma. Trace-element data are used to link the ages to the fluid-mineral-reaction-history and to identify monazite domains that have been completely reset during deformation.
