MONAZITE AGES DETERMINED BY ELECTRON MICROPROBE ANALYSIS OF SCHIST FROM THE RINCON RANGE, SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO
Adam S. READ1, Nelia W. DUNBAR1, and Michael J. JERCINOVIC2 1New Mexico Bureau of Mines & Mineral Resources, New Mexico Institute of Mining and Technology Socorro, NM 2University of Massachusetts, Amherst, MA
In-situ dating of the REE-phosphate mineral, monazite, can be accomplished by analysis of 5 micron spots on polished sections using the electron microprobe. This non-destructive technique requires an accurate determination of the concentration of Th, U, and Pb in the monazite followed by an iterative calculation of the age based on the decay of both Th and U to form radiogenic Pb. This technique has the advantage of direct thin-section analysis, retaining structural context. In addition, the high spatial resolution and rapid element mapping capability of the microprobe can reveal details of complex age zoning with the added benefit of being relatively quick and inexpensive. Multiple analyses are also possible within chemical or age domains that can be easily mapped by the microprobe. The repeated age determinations and domainal analysis provide the statistics required for the assessment of true analytical error. We have applied this technique to Proterozoic monazites within a quartz-sillimanite-muscovite schist from the Rincon Range north of Mora, New Mexico. The monazites had been dated by conventional isotopic means (~1.42 Ga), but these rocks were thought to be polymetamorphic, with an earlier tectonometamorphic event at ~1.6 Ga related to the Mazatzal Orogeny. Our results show age domains within the analyzed Rincon Range monazites that generally agree with the isotopically-derived age for this sample. However, we did not see any evidence for relict ~1.6 Ga monazite cores. This may be due to sufficiently high temperatures at ~1.4 Ga to recrystallize monazite, pervasive refabrication, and/or episodic fluid flux. Alternatively, these results could suggest that 1.6 Ga and 1.4 Ga polymetamorphism is not the correct interpretation. In either case, the complex age domains in these monazites, their chemical zoning, and their generally embayed and anhedral crystal morphology may collectively imply slow cooling at ~1.4 Ga punctuated by repeated magmatic-metamorphic pulses during the ~1.4 Ga tectonometamorphic event. In the Rincon Range, this could have lasted from perhaps 1470 Ma until 1330 Ma.