U-Pb SHRIMP dating to determine the age of zircons from crustal exnoliths of the Elephant Butte volcanic field-implications for the timing of rifting in southern New Mexico

Three large xenolith samples were collected from the Elephant Butte volcanic field in the Spring of 2005. Samples were chosen for U-Pb SHRIMP dating at Stanford University. The xenoliths consist of quartz, plagioclase, and K-feldspar with minor clinopyroxene, magnetite, and zircon. This mineral assemblage suggests the protoliths were granitic rocks. No hydrous minerals were observed, suggesting that these rocks reached granulite facies. The zircons from the samples were subhedral to rounded indicating recrystallization has likely occurred. Some of the grains that were dated have normal zonation as where others have a spirally pattern of zonation also indicating recrystallization. 34 dates were obtained from sample 05-AR-01. Core analyses had an average age of 1465 ± 36 Ma. The weighted average of seven rim ages is 9.1 ± 1.5 Ma. 22 dates were obtained from sample 05-AR-02. The cores had an average age of 1445 ± 40 Ma. This sample also had rim ages of 21 Ma, 27 Ma, 9 Ma, and 6 Ma. This means that the rocks are originally Proterozoic in age but that they were recrystallized in the Tertiary. The core ages are similar to the age of the Proterozoic granites throughout New Mexico. The rims reflect Tertiary heating possibly related to Rio Grande rifting between 27-6 Ma.