LINKING METAMORPHIC MONAZITE GROWTH TO THE P-T PATH USING MAJOR AND TRACE ELEMENT ZONING IN GARNET AND MONAZITE FROM MIGMATITIC PELITES OF THE EAST HUMBOLDT RANGE, NEVADA

Major and trace element zoning in garnet and monazite from migmatitic metapelites from the East Humboldt Range, Nevada are used to link geochronologic and petrogenetic studies. Monazite core domains show either concentric or patchy zoning in Y, Ca, U and Th. Two–three core domains are common, showing small rimward/overgrowth decrease(s) in X[Y+HREE] from ~0.055 to ~0.040. Xenotime occurs as fine inclusions in high-Y garnet cores and tiny, sparse grains in reaction coronas around garnet. Theoretical models of monazite growth suggest that the observed shifts in monazite core domain composition are likely due to monazite Y concentration following the monazite–xenotime solvus during xenotime and/or allanite breakdown with monazite growth. Distinct high Y, low Th+U rims (X[Y+HREE] = ~0.08) are observed on grains in both leucosome and melanosome domains, and these are interpreted to represent monazite growth during retrograde leucosome crystallization. Garnet trace element zoning is characterized by HREE+Y enriched cores, where sparse xenotime inclusions are observed, with a mantle domain that is relatively depleted in HREE+Y. X-ray mapping shows that mantle zones are partially resorbed. Higher Y+HREE garnet rims overgrew the mantles, interpreted to represent partially preserved garnet growth during anatexis. The enriched garnet rims are significantly resorbed, with some samples preserving a corona texture interpreted to represent the retrograde net transfer reaction grt+Kfs+melt = bt+sil+pl+qtz. This reaction probably resulted in higher Y+HREE monazite rim growth during garnet breakdown. A P–T path for these rocks is characterized by a steepening prograde segment culminating in nearly isothermal loading to ~10 kbar, ~ 700ºC followed by decompression and partial melting to peak conditions of ~750ºC, ~7 kbar before cooling. Theoretical modeling of parageneses along this P–T path suggests that garnet and monazite growth were initially correlated during early prograde growth but became antithetic (i.e. monazite growth domains record P–T segments of garnet breakdown) above xenotime and staurolite stability. Direct association of monazite and garnet growth or consumption provides a petrogenetic framework within which to interpret growth ages of these minerals.