Water-enhanced plastic deformation in felsic rocks

Abstract

Using Fourier transform infrared spectroscopy (FTIR), we measured water contents in quartz and feldspar for four kinds of felsic rocks, i.e., undeformed granite, banded granitic gneiss, fine-grained felsic mylonite, and fine-grained quartz-mica schist, collected from Pengguan Complex and Kangding Complex in the Longmenshan tectonic zone, Sichuan, China. The absorbance spectra suggest that water in coarse-grained quartz and feldspar of undeformed granite and banded granitic gneiss occurs mainly as hydroxyl in crystal defects, and water in most of fine-grained quartz and feldspar of felsic mylonite is molecular water in inclusions and liquid-type water in grain boundaries, but in some cases it still occurs as hydroxyl in crystal defects. Water content of quartz in undeformed granite is 0.001 wt%–0.009 wt %, and that of feldspar 0.005 wt%–0.02 wt%. The banded granitic gneiss shows water contents of 0.002 wt%–0.011 wt% in quartz and 0.012 wt%–0.036 wt% in feldspar. Quartz ribbon and feldspar ribbon in fine-grained felsic mylonite show that their water contents are similar to those of coarse-grained quartz and feldspar in granite, 0.002 wt%–0.011 wt%, and 0.004 wt%–0.02 wt%, respectively. Water contents of fine-grained quartz and feldspar are respectively 0.004 wt%–0.02 wt% and 0.012 wt%–0.06 wt%. Water content of quartz in fine-grained quartz-mica schist is 0.007 wt%–0.15 wt%. Water-bearing minerals display much higher water contents than those of nominally anhydrous minerals, and the percentage of water-bearing minerals in felsic rocks increases with the strain of rocks. These new data indicate that hydroxyl in crystal defects has basically not been released during the shear deformation, and on the contrary, the increase in molecular water in inclusions and liquid-type water in grain boundaries as well as water-bearing minerals after shear deformation leads to a significant increase of the water content in deformed rocks. Based on data of creep tests, it is inferred here that the fine-grained mylonites with more water have much lower strength than that of the weakly deformed coarse-grained rocks in the middle crust, and this indicates that trace amount of water significantly helped develop the ductile shear zone.