U-PB SHRIMP DATING OF ZIRCONS FROM CRUSTAL XENOLITHS: IMPLICATIONS FOR THE AGE OF THE CRUST AND TIMING OF RIFTING IN SOUTHERN NEW MEXICO

Crustal xenolith samples were collected from a ~2 Ma lava flow near Elephant Butte Reservoir in central New Mexico in a region known as Cutter Sag. This location is north of the Elephant Butte plug dome in an unnamed canyon with Cretaceous sedimentary rocks underlying the lava flow. The xenoliths have granitic compositions and consist of quartz, plagioclase, and K-feldspar with minor clinopyroxene, magnetite, and zircon. No hydrous minerals were observed, suggesting that the rocks reached granulite facies. Zircons from the samples were subhedral to rounded indicating recrystallization. Some of the crystals have normal oscillatory zonation whereas others have a more chaotic pattern of zonation also indicating recrystallization or alteration.

Samples were chosen for U-Pb SHRIMP dating at Stanford University. About 35 dates were obtained from sample 05AR-01. Core analyses had an average age of 1465 ± 36 Ma. The weighted average of seven rim ages is 9.1 ± 1.5 Ma. About 20 dates were obtained from sample 05AR-02. The cores had an average age of 1445 ± 40 Ma. Zircons from this sample had rim ages of 27 Ma, 21 Ma, 9 Ma, and 6 Ma. The rocks are originally Proterozoic in age and are part of a ~1.4 Ga granitic suite widely distributed in the southwest U.S. These rocks were were recrystallized in the Tertiary. The rim ages reflect Tertiary heating probably related to Rio Grande rifting between 27-6 Ma.