Rb depletion in biotites and whole rocks across an amphibolite to granulite facies transition zone, Tamil Nadu, South India

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Relatively low concentrations of Rb and high K/Rb ratios are characteristic of many granulite facies terranes. This depletion in Rb has been attributed to both the removal of a partial melt and exchange with a metamorphic fluid phase. These models have been tested using Rb concentrations in biotites and whole rocks from intermediate and felsic gneisses collected along a traverse from just north of Krishnagiri to just north of Salem in Tamil Nadu State, South India. Along this traverse, the northern amphibolite-facies zone gives way to a clinopyroxene zone in which clinopyroxene appears in intermediate and felsic gneisses. Further south is the lowland charnockite zone characterised by the presence of orthopyroxene and the scarcity of clinopyroxene in intermediate to felsic gneisses. The abundance of orthopyroxene increases southwards and it is the dominant ferromagnesium silicate in the highland charnockite zone. There is a good correlation between Rb in biotite and whole-rock Rb in samples collected throughout the traverse. Intermediate and felsic gneisses in the northern portion of this traverse have relatively high modal abundances of biotite, low Ti concentrations in the biotites, high whole-rock Rb concentrations, low K/Rb ratios and high Rb concentrations within the biotites. Ti concentrations in the biotites increase southward into the clinopyroxene zone and then remain relatively constant. High K/Rb ratios first appear at the southern boundary of the clinopyroxene zone. In the lowland and highland charnockite zones, the majority of the rocks have relatively low Rb concentrations and high K/Rb ratios. Low Rb concentrations in biotites (at or near the detection limit of 65 ppm) first appear in the lowland charnockite zone and persist into the highland charnockite zone. A smaller group of rocks in the highland charnockite zone contain biotites with moderate Rb concentrations. Most of these rocks also contain anomalously high biotite concentrations and low K/Rb ratios. It is not possible to produce Rb-depleted rocks containing low-Rb biotites by simple, equilibrium, partial-melting models beginning with a rock with the average composition of the amphibolite-facies gneisses (70 ppm) and using distribution coefficients from the literature. It is possible to produce such Rb-depleted rocks if the melt is removed in batches; however, it requires large amounts of partial melting (>40%). Low-Rb biotites can be produced by both equilibrium and batch melting if the original rocks have low whole-rock concentrations of Rb (10–20 ppm) to begin with. Attempts to model Rb depletion by exchange with a metamorphic fluid phase are hampered by a general lack of experimentally determined mineral/fluid distribution coefficients. Recent, experimentally determined exchange coefficients between phlogopite and a (K,Rb)Cl-rich supercritical brine at high pressure and temperature indicate that it would require a fluid/rock mass ratio of approximately 0.125 to produce Rb-depleted rocks with low-Rb biotites by exchange with a KCl-rich fluid. Alternatively, biotites low in Rb may have formed shortly after the peak of metamorphism in the rocks after they were depleted in Rb but while they were still under relatively high-grade conditions.