Controls on the⁸⁷Sr/⁸⁶Sr Ratio 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 10–30 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.