Jeffrey H. Tepper

Complex zoning in apatite from the Idaho Batholith; a record of magma mixing and intracrystalline trace element diffusion

Abstract Apatite crystals in a sample of biotite granodiorite from the Idaho batholith display complex chemical zoning, characterized by abrupt changes in REE+Y+Si content, more subtle variations in S, Na, La/Yb, and Mn contents, and petrographic evidence for multiple episodes of partial resorption. The zoning is attributed to changes in melt composition, resulting from magma mixing and differentiation, although the possibility that some rounded cores may be inherited cannot be disproved. High S contents in apatite cores and the presence of an included anhydrite grain indicate crystallization from an oxidized host magma. Divalent cations that occupy the apatite Ca sites (Sr, Mn, Fe) show evidence of having been redistributed between zones by intracrystalline diffusion, whereas cations that participate in coupled substitutions involving the tetrahedral site (Si, Y, REE, Na, S) were not readily redistributed. The main REE substitution in this case is REE3+ + Si4+ ↔ Ca2+ + P5+, and REE diffusion is rate-limited by slow Si diffusion. However, exchange of LREE (e.g., LaSm-1) on the Ca sites does not involve Si and proceeds more rapidly, resulting in homogenization of La/Sm between zones within individual crystals. Relative diffusion rates inferred from zoning profiles in this study are: Mn, Sr, Fe, and LaSm-1 are faster than Na, S, and LaYb-1, which are faster than Si. These data imply that REE patterns can be decoupled from REE abundances during diffusion, and that even apatite zones or cores that appear sharply bounded in backscattered electron images may not retain their original chemical or Sr-Nd isotopic traits.

Geochemistry of Mafic Enclaves and Host Granitoids from the Chilliwack Batholith, Washington: Chemical Exchange Processes between Coexisting Mafic and Felsic Magmas and Implications for the Interpretation of Enclave Chemical Traits

Mafic enclaves from three plutons in the Chilliwack batholith have been compared with contemporaneous mafic stocks in order to determine (1) the processes by which mafic and felsic magmas hybridize in the plutonic environment and (2) whether analysis of early‐formed enclave minerals, particularly apatite, can provide a means of seeing through hybridization effects and deciphering the original trace element characteristics of enclave magmas. Whole rock and mineral chemistry data reveal a two‐stage history of enclave hybridization. Stage 1, a diffusive exchange of trace elements between coexisting liquids, produced enclaves with distinctive concave‐upward rare earth element patterns that parallel those of the host granitoids but had minimal impact on the major elements, whose transfer is rate limited by the slow diffusion of Si. This stage probably occurred at a mafic‐felsic interface in a stratified magmatic system. Stage 2, a partial reequilibration of enclave minerals with a differentiated and hybridized interstitial melt, occurred after the enclaves were entrained in the host and partially crystallized. This process caused enclave and host minerals (amphibole, biotite, apatite) from each pluton to have similar major oxide chemistries but did not reequilibrate the trace elements. As a result of these hybridization processes, even early‐formed apatite crystals do not preserve information about the original trace element characteristics of enclave magmas in this case. However, the results of this study illustrate the potential of using enclave chemistry to constrain the nature and timing of mafic magma inputs into felsic magma bodies.

Rapid assembly and crystallization of a fossil large-volume silicic magma chamber

The rates at which large volumes of eruptible, silicic (>65 wt% SiO2) magma (magma chambers) are assembled, as well as their longevity in the upper crust, remain controversial. This controversy is due, in part, to a missing record of granitoid plutonic complexes that represent large fossil upper crustal magma chambers. We present new geologic mapping and high-precision U-Pb zircon geochronology from the Eocene Golden Horn batholith in Washington State, USA. These data reveal that the batholith was constructed as a series of sills over 739 ± 34 k.y. Topographic relief of >2 km permits volume estimates for 4 of the sills, the largest of which, a >424 km3 rapakivi granite, was emplaced over 26 ± 25 k.y. at a rate of ∼0.0125 km3/yr. This rate exceeds those needed to build large, silicic magma chambers in thermal models, and we suggest that that this unit may represent the first fossil magma chamber of this type recognized in the geologic record.

Petrology of the Grays River volcanics, southwest Washington: Plume-influenced slab window magmatism in the Cascadia forearc

The Grays River volcanics are part of the Coast Range basalt province and consist of ∼3500 m of tholeiitic basalt flows and volcaniclastic rocks that erupted in the Cascadia forearc from 42 to 37 Ma. Chemical and isotopic data, combined with migration of the location of magmatism through time, indicate that Grays River volcanics magmatism was related to subduction of a plume-influenced spreading ridge that produced a northward-migrating slab window. Involvement of a mantle plume source is indicated by ocean-island basalt (OIB)-like incompatible element enrichments and radiogenic Pb isotopic compositions (206Pb/204Pb > 19.3). These Pb isotope data are distinct from most Cascade arc rocks and from Cascadia sediment, but they overlap with compositions of other Coast Range basalts. A slab window setting accounts for the northward-younging age progression of the Grays River volcanics as well as geochemical traits, including low B/Be, that indicate the Grays River magmas ascended through the mantle wedge and subducting slab without acquiring an arc signature. Differentiation of Grays River magmas was dominated by clinopyroxene fractionation, which resulted in evolved compositions (Mg# = 59–32), low Sc contents, and Sr contents that increase with fractionation. Geochemical differences between the Grays River volcanics and other Cascadia forearc volcanic units that range from ca. 55 Ma (Crescent Basalts) to <3 Ma (Boring Lavas) were mainly caused by transient changes in tectonic setting (i.e., arrival of a mantle plume, ridge subduction) and do not record progressive chemical modification of the mantle wedge.

Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88–80) with relatively sodic plagioclase (An75–60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc.