RARE-EARTH AND OTHER TRACE-ELEMENT MOBILITY DURING MYLONITIZATION - ACOMPARISON OF THE BREVARD AND HOPE-VALLEY SHEAR ZONES IN THE APPALACHIAN MOUNTAINS, USA

In progressing from a granitoid mylonite to an ultramylonite in the Br evard shear zone in North Carolina, Ca and LOI(H2O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al-Ti-Fe is ocon. The increase in Ca and LOI is related to a large increase in ret rograde epidote and muscovite in the ultramylonite, the decreases in K , Na, Si, Ba and Sr reflect the destruction of feldspars, and the decr ease in Mg is related to the destruction of biotite during mylonitizat ion. In an amphibolite facies fault zone separating grey and pink gran itic gneisses in the Hope Valley shear zone in New England, compositio nal similarity suggests the ultramylonite is composed chiefly of the p ink gneisses. Utilizing an Al-Ti-Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M-HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Pa are enriched, and Ca, Na, P, Sr and LREE are lost durin g deformation. In contrast to the Brevard mylonite, the Hope Valley my lonite appears to have increased in volume by about 70%, chiefly in re sponse to an introduction of quartz. Chondrite-normalized REE patterns of granitoids from both shear zones are LREE-enriched and have promin ent negative Eu anomalies. Although REE increase in abundance in the B revard ultramylonites (reflecting the volume loss), the shape of the R EE pattern remains unchanged. In contrast, REE and especially LREE dec rease in abundance with increasing deformation of the Hope Valley gnei sses. Mass balance calculations indicate that greater than or equal to 95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15-40% of the REE can be accounted for c ollectively by titanite, apatite and zircon. Possible sites for the re maining REE are allanite, fluorite or grain boundaries. Loss of LREE f rom the pink gneisses during deformation may have resulted from decrea ses in allanite and perhaps apatite or by leaching of REE from grain b oundaries by fluids moving through the shear zone. Among the element r atios most resistant to change during mylonitization in the Brevard sh ear zone are La/Yb, Eu/Eu, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U an d Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and H f/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoli ths of deformed rocks.