Mesoproterozoic Metamorphism and⁴⁰Ar/³⁹Ar Thermal History of the 1.4 Ga Priest Pluton, Manzano Mountains, New Mexico

Thermal, structural, and petrological evidence for 1400-Ma metamorphism and deformation in central New Mexico

“…We propose that the mineral assemblages produced by layers that differ in composition are internally buffering f0,. Liou et al!s (1974) experiments using the QFM buffer have shown that the lower and upper boundaries of the transition zone are 515 and 580" C, respectively, at 4 kbar, the pressure estimated for peak metamorphism in the Manzano Mountains (Thompson et al, 1996). Apted and Liou (1983) and Moody et al (1983) both demonstrated a decrease in the temperature at which these boundaries occur with increasing f0,.…”

“…Hornblende, muscovite, and biotite 40Ar/39Ar ages from various metamorphic and plutonic rocks all yield cooling ages of less than ca. 1435 Ma, with the majority of mica ages younger than 1400 Ma (Dallmeyer et al, 1990;Thompson et al, 1991Thompson et al, , 1996Heizler et al, 1997). The radiometric data most pertinent to this study are the 40Ar/ 39Ar age spectrum results of Thompson et al (1996).…”

“…Regional P-T conditions during the second phase of deformation at ca. 1400 Ma were estimated at 500-540' C and 4 kbar based on mineral assemblages, geothermometry, and geobarometry (Grambling, 1986; Grambling et al, 1988; Grambling et al, 1989; Thompson et al, 1996).…”

“…Concordance of the intrusion age and the hornblende argon apparent age suggests that the pluton: (1) was emplaced at or below the 400-500°C isotherm; and (2) cooled quickly, such that the hornblende began to retain radiogenic 40Ar soon after emplacement of the Priest pluton (Thompson et al, 1996).…”

“…Silver et al (1977) and Anderson (1989) described the plutonism as anorogenic due to a perceived lack of metamorphism and deformation associated with the granitoids. Specific links between metamorphic and structural features and 1400-Ma plutonism have been, and are being, investigated on both regional and local scales (Grambling, 1986;Grambling et al, 1988Grambling et al, , 1989Nyman et al, 1994;Kirby et al, 1995;Thompson et al, 1996;Ralser et al, 1997). These recent studies have challenged the anorogenic model on the basis of evidence of deformation and metamorphism within the contact aureoles of the plutons as well as structures recording deformation of the plutons themselves.…”