Chemical trends from north (amphibolite facies) to south (granulite facies) along a 95 km traverse in Tamil Nadu, Southern India, include: whole-rock depletion of Rb, Cs, Th and U, enrichment in Ti and F, and depletion in Fe and Mn in biotite and amphibole; increases in Al and decreases in Mn in orthopyroxene; enrichment of fluorapatite in F. K-feldspar blebs are widespread along quartz–plagioclase grain boundaries, and could indicate either partial melting or metasomatism. In the northernmost portion of the traverse the principal rare earth element (REE)-bearing minerals are allanite and titanite. South of a clinopyroxene isograd, monazite grains independent of fluorapatite are the major REE- and Th-bearing phase. Further south independent monazite is rare but Th-free monazite inclusions are common in fluorapatite. During prograde metamorphism, independent monazite was replaced by REE-rich fluorapatite in which the monazite inclusions later formed. The loss of independent monazite was accompanied by a loss of whole-rock Th and possibly a small depletion in light REE. Most mineralogical features along the traverse can be accounted for by progressive dehydration and oxidation reactions. Trace-element depletion is best explained by the action of an externally derived low H₂O activity brine migrating from a source at greater depth, possibly preceded or accompanied by partial melting.

A localized dehydration zone, Söndrum stone quarry, Halmstad, SW Sweden, consists of a central, 1 m wide granitic pegmatoid dyke, on either side of which extends a 2·5–3 m wide dehydration zone (650–700°C; 800 MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet) overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet). Whole-rock chemistry indicates that dehydration of the granitic gneiss was predominantly isochemical. Exceptions include [Y + heavy rare earth elements (HREE)], Ba, Sr, and F, which are markedly depleted throughout the dehydration zone. Systematic trends in the silicate and fluorapatite mineral chemistry across the dehydration zone include depletion in Fe, (Y + HREE), Na, K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusion chemistry is similar in all three zones and indicates the presence of a fluid containing CO₂, NaCl, and H₂O components. Water activities in the dehydration zone average 0·36, or XH₂O = 0·25. All lines of evidence suggest that the formation of the dehydration zone was due to advective transport of a CO₂-rich fluid with a minor NaCl brine component originating from a tectonic fracture. Fluid infiltration resulted in the localized partial breakdown of biotite and amphiboles to pyroxenes releasing Ti and Ca, which were partitioned into the remaining biotite and amphibole, as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, and F. At some later stage, H₂O-rich fluids (H₂O activity >0·8) gave rise to localized partial melting and the probable injection of a granitic melt into the tectonic fracture, which resulted in the biotite and amphibole recording a diffusion profile for F across the dehydration zone into the granitic gneiss as well as a diffusion profile in Fe, Mn, and Mg for all Fe–Mg silicate minerals within 100 cm of the pegmatoid dyke.