Garnet-biotite thermometry, garnet-aluminum silicate-silica-plagioclase barometry, and EMP monazite geochronology of the early Proterozoic Ivanpah Mountains, eastern Mojave Desert, SE California.

摘要

Metamorphic fabric measurements indicate only one deformational fabric (NNE) is present in the Ivanpah Mountains, indicating that the area experienced one orogenic event during the Early Proterozoic. Peak P-T conditions, using garnet-Al2SiO5-silica-plagioclase barometry and garnet-biotite thermometry, were ∼4.0kb and >700°C, respectively. Peak metamorphic-T's were recorded by the largest garnet studied.; Electron Microprobe Analysis on monazite, using U-Th-Pb radioactive decay, suggest metamorphism occurred between 1752 +/- 86 Ma and 1689 +/- 34 Ma, similar to the age of the Yavapai terrane (∼1.70--1.69 Ma). These ages are constrained from six monazite grains included in three large garnets (2250--5000um). Matrix monazite from granitic rocks indicate magmatism may have occurred between 1559 +/- 31 Ma and 1416 +/- 28 Ma. Large garnets effectively shield monazite from Pb-loss when temperature is exceeded above what is considered its closure-T for radiogenic-Pb (∼700°C), thus petrographic location is also a function of closure-T of radiogenic-Pb in monazite.