ND ISOTOPES OF GRANITE MAGMATISM, HIDALGO AND GRANT COUNTIES, NEW MEXICO

New U-Pb zircon data for granitoid rocks in the northern Burro Mountains, southwestern New Mexico (McLemore et al., this issue) have revealed two distinct magmatic events at 1460 and 1220 Ma. The former registers the ~1.4 Ga mid-continental “anorogenic” event whereas the latter is not as well documented in southernmost Laurentia. We have determined the Nd isotopic composition of Proterozoic and Tertiary granites in the Burro Mountains (Grant County) and the Little Hatchet Mountains (Hidalgo County) farther to the south. The 1460 Ma suite in the northern Burro Mountains has epsilon-Nd (at 1460 Ma) values of +2.3 to +2.8 and is similar to granites from the central (+2.5, +2.6) and southern (+2.7) Burro Mountains. The 1220 Ma suite in the northern Burro Mountains yields epsilon-Nd (at 1220 Ma) values of +1.3 to +2.2; these are similar to those of Precambrian granites in the Little Hatchet Mountains (+1.7, 2.4). Our data imply two (agewise) slightly different protoliths for the 1460 Ma and 1220 Ma suites and suggest that the Precambrian granites in the Little Hatchet Mountains may be part of the younger suite. Tertiary granites from the western Burro Mountains (33.4 Ma) and Little Hatchet Mountains (32.5 Ma) have epsilon-Nd (at 0 Ma) values of -7.3 and -6.5, respectively. They thus contain slightly more radiogenic Nd than the Precambrian granites (epsilon-Nd [at 0 Ma] values of -16 to -9) and a major cratonic source component.