Controls on the 87Sr/86Sr ratios of carbonates in the Garhwal Himalaya, Headwaters of the Ganges

Abstract

The episodic variation of the seawater 87Sr/86Sr ratio has been attributed to either variations in the Sr flux or the Sr-isotopic composition of the riverine-dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentrations of dissolved Sr concentrations with high 87Sr/86Sr ratios has raised the possibility that collisional orogens play a critical role in moderating the variations in seawater 87Sr/86Sr ratios. Here we describe Himalayan carbonates and calc-silicates from Garhwal, the headwaters of the Ganges, with extreme 87Sr/86Sr ratios (>1.0). Elevated Sr-isotope ratios result from exchange with Rb-rich silicate material during both Himalayan and pre-Himalayan metamorphic episodes, and the carbonates contribute a significant fraction to the Ganges 87Sr flux. Particularly elevated 87Sr/86Sr ratios are found in calc-silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc-silicates from this unit confirms that carbonate horizons have increased 87Sr/86Sr ratios as a result of isotopic exchange over length scales of 1030 cm. We conclude that metamorphism of carbonates may cause elevation of their 87Sr/86Sr ratios and that uplift of metamorphosed carbonates may be a consequence of collisional orogens, which contributes to the elevation of seawater 87Sr/86Sr ratios