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Our Mineral Museum, bookstore, and offices will be closed for holidays from December 22rd through January 2cd.Publications purchased online while we are closed will be processed when we open on January 2cd.The complex age domains, chemical zoning, and generally anhedral crystal morphology may collectively imply slow cooling at ~1.4 Ga and/or repeated magmatic-metamorphic pulses during the ~1.4 Ga tectonometamorphic event that in the Rincon Range may have lasted from perhaps 1460 Ma until 1380 Ma.The best hope for finding older monazites in Proterozoic rocks from this and other ranges is likely to be within the cores of M1 garnets and other porphyroblasts.In-situ dating of monazite by measuring Y, U, Th and Pb allows the linking of microstructure and geochronology data, thus providing a new type of information for geologists.Electron Probe Microanalysis outperforms standard mass spectrometry methods by offering high spatial resolution: with spot size less than 1 µm, the lateral resolution is 10 to 100 times better than with isotopic methods. The quantified high resolution X-ray maps reveal the spatial variation of the geochemically important elements in order to guide further quantitative analysis. Several generations of monazite growth are represented, of foremost interest is the high Y rim and fracture filling monazite (ca.~1800 Ma) which has penetrated much older monazite (ca~2400 Ma).
Both the oldest and youngest points appear to be outliers and should probably be ignored in any case.
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The outcrop it was taken from is part of a screen of supracrustal rocks intruded by the1.65 Ga Guadalupita pluton.
For further background information, see an abstract of Read et al., 1999 which summarizes the geologic setting of the monazites in this sample that have been dated isotopically at ~1421 Ma.