Petrogenesis of Garnet Lherzolite and Megacrystalline Nodules from the Thumb, Navajo Volcanic Field

Garnet Iherzolite and megacrystalline nodules from The Thumbcomprise the deepest mantle sample recovered from the diatremesand intrusions of the Four Corners area. Discrepancies betweenvarious geothermometers applied to these nodules are believedto reflect zoning of Fe in garnets as well as problems in calibrationof the geothermometers. Because the pyroxenes are not zoned,the pyroxene solvus method probably provides more reliable temperatureestimates for this nodule suite than methods based on partitioningof Fe and Mg with garnet. The preferred T-P range of the nodules,950–1230 °C and 32–40 kb, is thought to reflecta localized perturbation of the ambient geotherm by a diapiricor igneous intrusion. Garnet Iherzolites with coarse texture show a trend from fertileto refractory compositions consistent with variable depletionby partial melting. These nodules are interpreted as previouslydepleted mantle rocks, most of which were relatively littleaffected by the thermal perturbation. Garnet Iherzolites withsheared texture are enriched in Fe and Ti relative to the coarseIherzolites and are suggested to have formed by deformationand metasomatism of the coarse lherzolites during intrusionof magma related to the thermal perturbation. The texturallydiverse ‘megacrystalline’ nodules are interpretedas precipitates from this liquid. They are similar to discretenodules from kimberlite pipes, but have unique characteristicsof their own. The above relationships are thought to recordthe process of deep-seated intrusion and partial crystallizationof evolved liquids, possibly related to the host lamprophyre,shortly before the time of eruption.