Over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the Jemez volcanic field. Volcanism had begun in the area by about 16.5 Ma with episodic eruptions of alkaline basalts. By 13 Ma, alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite. High‐silica rhyolite, derived from melts of lower crust, also was erupting by about 13 Ma. Basalt and high‐silica rhyolite continued to be erupted until about 7 and 6 Ma, respectively, but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 Ma volumetrically overshadowed all other eruptive products between 10 and 7 Ma. From 7 to 3 Ma the dominant erupted lithology was dacite, which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high‐silica rhyolites. Less than 4–3 Ma volcanism was dominated by eruption of rhyolitic tuffs. Field relations, geochemistry, and dates specifically indicate the following with regards to stratigraphie relations: (1) distinctions among basalt of Chamisa Mesa, Paliza Canyon Formation basalts, and Lobato Basalt for other than geographic reasons are artificial; basaltic volcanism was continuous in volcanic field from >13 to 7 Ma, (2) Canovas Canyon and Bearhead rhyolites form a continuum of high‐silica rhyolite volcanism from >13 to 6 Ma, (3) hypabyssal and volcanic rocks of the Cochiti mining district probably represent the exhumed interior of a Keres Group volcano(s), (4) temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations, and (5) the Tewa Group formation Cerro Rubio Quartz Latite may more appropriately be considered part of the Tschicoma Formation of the Polvadera Group. Preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field's history.
The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9‐km‐diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K‐Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling‐related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles‐Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge‐shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo‐Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.
The Cerro Toledo Rhyolite is a group of high‐silica rhyolite domes and tephras that range in age from 1.45 to 1.12 Ma. The unit crops out in the Jemez Mountains of northern New Mexico and lies stratigraphically between the compositionally zoned upper and lower members of the Bandelier Tuff. The Cerro Toledo Rhyolite provides an exceptional opportunity to study the origin of compositional zonation in silicic magma chambers because it allows us to follow the restoration of the zonation with time between these two large caldera‐forming ignimbrite eruptions. Based upon stratigraphic, geochronologic, and geochemical evidence, we have correlated different Cerro Toledo Rhyolite tephra units with groups of domes. Early Cerro Toledo Rhyolite domes appear to be located generally along the Toledo caldera ring fracture, whereas younger domes tend to cluster in the Toledo embayment. The Cerro Toledo Rhyolite appears to have tapped the most fractionated liquids at or near the top of the Bandelier magma chamber and records the restoration of compositional gradients over a period of 0.33 m.y. after the Lower Bandelier ignimbrite eruption. Cl, Rb, Cs, heavy rare earth elements, Y, Nb, Th, and U increase in concentration in progressively younger Cerro Toledo Rhyolite rocks. Because of depletions in K/Cs, Sr, Zr/Nb, and La/Yb over the same interval, we favor crystal fractionation of essentially quartz, alkali feldspar, zircon, and a light rare earth element enriched phase (probably allanite) as the primary mechanism by which the compositional gradients were reestablished. We believe diffusive processes did not play an important role because (1) certain cations of widely different valencies and diffusivities are not fractionated with respect to each other and (2) the observed chemical gradients conflict with those predicted by recent experimental Soret studies.
Enclaves of diverse origin are present in minor amounts in the coarse-grained biotite granites of the Cornubian batholith, southwest England. The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal variable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising from < 2 to 5 vol.% of the granites. Enclaves of igneous origin are also present in all major plutons except Carnmenellis, generally comprising < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular enclaves (ME) but absent in NIE, represent phenocrysts derived from the silicic end-member during magma mixing events rather than products of metasomatism as suggested previously. Although the composition of the mafic end-member (basaltic or lamprophyric) involved in the mixing process is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volcanics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat provides an explanation for large-volume crustal melting in regions of relatively thin crust such as southwest England.
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