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Deep-crustal plutons, Geochemical evolution


Deep plutonic systems represent an important link between lower-crustal melt-generation sites and higher-level regions of magma accumulation, but models for these systems are limited by the relative scarcity of exposed weakly deformed, deep-crustal plutons. Exceptions include the ca. 92.3–89.7 Ma, dominantly tonalitic Tenpeak pluton and the smaller, nearby ca. 91 Ma Dirtyface pluton of the North Cascades (western North America), which represent deeply exposed crustal levels (∼25–35 km) of a Cordilleran arc. Initial subduction-driven magmatism in the Tenpeak pluton was marked by co-magmatic hydrous mafic and felsic magmas, which formed gabbro, diorite, tonalite, and hornblendite within a heterogeneous mafic complex. High-MgO, Ni and Cr tonalitic magmas (Schaefer Lake subtype) with (εNd)0 of +4.8 to +5.8 accompanied or shortly followed this magmatism, and typify the Dirtyface intrusion. This early magmatism formed moderately to steeply dipping sheets, which are best developed in the southwestern margin of the Tenpeak pluton and in an internal zone with host rock rafts. As the system evolved, a different source was tapped to produce typical calc-alkaline magmas (Indian Creek subtype) that are more isotopically evolved (initial εNd = +3.0 to +4.0). Magmas of this subtype formed bodies that are elliptical in map view and that truncated internal magmatic contacts and more strongly deformed tonalite, compatible with removal of older solidified and magmatic materials. The Schaefer Lake subtype terminated or was overwhelmed by the Indian Creek subtype in the youngest, high-volume magmas of the Tenpeak pluton.

Both plutons have moderately to steeply dipping contacts that define the shape of an elongate asymmetric funnel to wedge. During sheet emplacement, magma wedging isolated and rotated rafts and blocks of host meta-supracrustal rock. Vertical, mostly downward transport of host rock by ductile flow and at least modest stoping were important during emplacement of the larger bodies.

Only small ephemeral magma chambers formed in the early stages of pluton construction, but larger bodies (tens of cubic kilometers) probably remained mushy during crystallization of the relatively homogeneous younger tonalites. The juxtaposition of different magma subtypes, at least local mixing at the emplacement level, and removal and/or recycling of older magmas indicates that magmas from different sources utilized the same conduit for a protracted time interval. Large volumes of magma probably ascended through the system to form the larger and relatively more homogeneous intrusions in the shallow levels of the arc. This magma was likely filtered and homogenized by processes operating at the Tenpeak level. The end result was a deep- to shallow-crustal, steep, irregularly shaped magmatic system.

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Subjects - Topical (LCSH)

Batholith--Cascade Range; Magamtism--Cascade Range; Continental crust--Cascade Range

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Cascade Range (219 kB)
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This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License