The geology, petrology, geochemistry, mineralogy, and diapiric emplacement of the Duncan Hill pluton, North Cascades, Washington

Differential uplift (tilting) and erosion of the northern Cascade Range have exposed an oblique section through the Eocene Duncan Hill pluton, revealing continuous top-to-bottom exposure of the pluton and its associated magmatic rocks (Hopson et al., 1970). The work presented here (1) documents vertical, lateral, and temporal zoning within the pluton, (2) constrains the amount of post-emplacement tilting, and (3) proposes a petrogenetic model to explain the observed zoning patterns. A simple numerical model of the chemical evolution of the system was developed as a test of the petrogenetic model. The conclusions reached here are based on geologic mapping and field observations, petrographic examination of 211 thin sections, modal analyses of 56 samples, major- and trace-element analyses of 159 whole-rock samples, microprobe analyses of biotite, amphibole, pyroxene, opaque oxides, and plagioclase in 52 samples, and mathematical modelling.; The Duncan Hill pluton is divided into five zones on the basis of lithology, texture, structure, and whole-rock and mineral compositions. These parameters vary most strongly along the length of the body, corresponding to vertical zoning. The pluton grades from homogeneous granophyric leucogranite at the southeast end (zone I) through homogeneous granodiorite (zone II) to heterogeneous mafic tonalite and quartz diorite at the northwest end (zones III and IV). Zone V (adjacent to zone I) consists of heterogeneous dark appinitic rocks. Lateral zoning is pronounced in the deeper zones; the pluton margin and keel (zone IV) consists of migmatitic rocks containing a substantial mafic-ultramafic component, whereas the core (zone III) is mainly tonalitic to granodioritic. Temporal zoning is also displayed at all levels; intrusive sequences record progressive change of magma composition with time.; Evidence from (1) amphibole geabarometry, (2) rock type distributions, and (3) dike orientations indicate that the pluton has been tilted 10{dollar}spcirc{dollar}-13{dollar}spcirc{dollar} toward the southeast or east-southeast (azimuth 120{dollar}spcirc{dollar}-135{dollar}spcirc{dollar}). These results, taken with other constraints, indicate that the emplacement depth of the exposed pluton ranges from 9.9-13.5 km (northwest end) to 2.2-3.5 km (southeast end).; The petrogenetic model has five key features: (1) The magmatic system evolved from an extensive lower crustal zone of basalt injection and crustal anatexis which rose to a broad midcrustal protodiapiric antiform, and then to a much narrower diapir. (2) Magmas from crustal and mantle sources mixed during the protodiapiric and diapiric stages; the pluton crystallized from hybrid magma. (3) Three-dimensional zoning results mainly from melt extraction fractionation, which acted during diapiric ascent. (4) Separation of melt from hypersolidus mush was commonly incomplete; rocks on the "crystal" line of descent commonly retained a substantial fraction of melt when fractionation ceased. (5) Melt was progressively concentrated upward in the diapir, and crystalline material (both cumulate and restitic) was left behind; a trail of gneissic/migmatitic residue marks the passage of the diapir through the crust.