Mj. Bickle et al., Controls on the Sr-87/Sr-86 ratio of carbonates in the Garhwal Himalaya, headwaters of the Ganges, J GEOLOGY, 109(6), 2001, pp. 737-753
The episodic variation of the seawater Sr-87/Sr-86 ratio has been attribute
d 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 concentration
s of dissolved Sr concentrations with high Sr-87/Sr-86 ratios has raised th
e possibility that collisional orogens play a critical role in moderating t
he variations in seawater Sr-87/Sr-86 ratios. Here we describe Himalayan ca
rbonates and calc-silicates from Garhwal, the headwaters of the Ganges, wit
h extreme Sr-87/Sr-86 ratios (>1.0). Elevated Sr-isotope ratios result from
exchange with Rb-rich silicate material during both Himalayan and pre-Hima
layan metamorphic episodes, and the carbonates contribute a significant fra
ction to the Ganges Sr-87 flux. Particularly elevated Sr-87/Sr-86 ratios ar
e found in calc-silicates from the Deoban Formation of the Lesser Himalaya.
A detailed traverse of shales and calc-silicates from this unit confirms t
hat carbonate horizons have increased Sr-87/Sr-86 ratios as a result of iso
topic exchange over length scales of 10-30 cm. We conclude that metamorphis
m of carbonates may cause elevation of their Sr-87/Sr-86 ratios and that up
lift of metamorphosed carbonates may be a consequence of collisional orogen
s, which contributes to the elevation of seawater Sr-87/Sr-86 ratios.