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path crosses the beginning of melting and approaches the conditions suggested by the X Prp contents of garnet rims, as very incipient partial melting has occasionally been observed in the field. The mean peak conditions associated with this path can be estimated at c . 700°C and 8 kbar. 9 . Discussion The structure of the Mérida Massif is characterized by the presence of three main units separated by tectonic con- tacts. The lower and upper units have a continental or arc affinity, given the abundance of metasedimentary Figure 12. a) U-Pb Tera-Wasserburg plot for garnet from sample 117,350. b) 238 U- 232 Th plot for garnet from sample 117,350. 20 R. ARENAS ET AL. &KDSWHU