Geochronologic evidence of upper-crustal in situ differentiation: Silicic magmatism at the Organ caldera complex, New Mexico

The combination of new 40 Ar/ 39 Ar and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U/Pb zircon ages with published geochemistry of the volcanic and plutonic rocks of the Organ caldera complex (New Mexico) provides a framework for understanding the origin of these silicic magmas and the time scales of caldera magmatism. The Organ caldera complex erupted three ignimbrites: the 36.45 ± 0.08 Ma Cueva Tuff, the 36.23 ± 0.14 Ma Achenback Park Tuff, and the 36.03 ± 0.16 Ma Squaw Mountain Tuff. The ignimbrite sequence is zoned from a crystal-poor, high-SiO 2 rhyolite at the base to a crystal-rich, low-SiO 2 rhyolite at the top. The ignimbrite sequence is intruded by the zoned Organ Needle pluton, which has previously been interpreted to be the nonerupted silicic cap and less-differentiated residual crystal mush of the caldera-forming magma chamber. The geochronology of the Organ Needle pluton indicates that these silicic magmas were generated via shallow-crustal in situ differentiation. U/Pb zircon and many 40 Ar/ 39 Ar biotite ages of the different phases of the Organ Needle pluton are temporally indistinguishable from the Squaw Mountain Tuff eruption age, indicating that this pluton was emplaced and rapidly cooled during or shortly after the youngest caldera eruption. New ages also suggest that Organ caldera magmatism was characterized by protracted emplacement of magmas following caldera collapse. Volcanism continued after the caldera eruptions until at least 35.7 Ma. Three silicic postcaldera plutons were emplaced between 36.0 and 34.3 Ma. Multiple diffusion domain thermal modeling of plutonic K-feldspar suggests reheating events, possibly related to postcaldera magmatism, at 34 Ma, 32–30 Ma, and as young as 26 Ma. Geochronology, geochemistry, and field-based observations of the Organ Needle pluton and caldera-forming ignimbrites support the hypothesis that some plutonic rocks are the nonerupted, geochemically complementary residues of large-volume silicic eruptions.