PLUTON EMPLACEMENT IN ARCS - A CRUSTAL-SCALE EXCHANGE PROCESS

Buddington (1959) pointed out that the construction of large crustal m agma chambers involves complex internal processes as well as multiple country rock material transfer processes (MTPs), which reflect large h orizontal, vertical and temporal gradients in physical conditions. Thu s, we have attempted to determine the relative importance of different magmatic and country rock MTPs at various crustal depths, and whether country rock MTPs largely transport material vertically or horizontal ly, rather than seeking a single model of magma ascent and emplacement . Partially preserved roofs of nine plutons and in some cases roof-wal l transitions with roof emplacement depths of 1.5-11 km were mapped. D uring emplacement, these roofs were not deformed in a ductile manner, detached or extended by faults, or significantly uplifted. Instead, sh arp, irregular, discordant contacts are the rule with sloped blocks of ten preserved immediately below the roof, even at depths of 10 km. The upper portions of these magma chambers are varied, sometimes preservi ng the crests of more evolved magmas or local zones of volatile-rich p hases and complex zones of dyking and magma mingling. Magmatic structu res near roofs display a wide variety of patterns and generally formed after emplacement. Transitions from gently dipping roofs to steep wal ls are abrupt. Ar shallow crustal levels, steep wall contacts have sha rp, discordant, stepped patterns with locally preserved stoped blocks indicating that the chamber grew sideways in part by stoping. Around d eeper plutons, an abrupt transition (sometimes within hundreds of metr es) occurs in the country rock from discordant, brittle roofs to moder ately concordant, walls deformed in a ductile manner defining narrow s tructural aureoles. Brittle or ductile faults are not present at roof- wall joins. Near steep wall contacts at shallow to mid-crustal depths (5-15 km), vertical and horizontal deflections of pre-emplacement mark ers (e.g. bedding, faults, dykes), and ductile strains in narrow aureo les (0.1-0.3 body radii) give a complete range of bulk strain values t hat account for 0-100% of the needed space, but average around 30%, or less, particularly for larger batholiths. A lack of far-field deflect ion of these same markers rules out significant horizontal displacemen t outside the aureoles and requires that any near-field lateral shorte ning is accommodated by vertical flow. Lateral variations from ductile (inner aureole) to brittle (outer aureole) MTPs are typically observe d. Compositional zoning is widespread within these magma bodies and is thought to represent separately evolved pulses that travelled up the same magma plumbing system. Magmatic foliations and lineations commonl y cross-cut contacts between pulses and reflect the strain caused eith er by the late flow of melt or regional deformation. Country rocks nea r the few examined mid- to deep crustal walls (10-30 km) are extensive ly deformed, with both discordant and concordant contacts present; how ever, the distinction between regional and emplacement-related deforma tion is less clear than for shallower plutons. Internal sheeting is mo re common, although elliptical masses are present. Lateral composition al variations are as large as vertical variations at shallower depths and occur over shorter distances. Magmatic foliations and lineations o ften reflect regional deformation rather than emplacement processes. T he lack of evidence for horizontal displacement outside the narrow, sh allow to mid-crustal aureoles and the lack of lateral or upwards displ acement of pluton roofs indicate that during emplacement most country rock is transported downwards in the region now occupied by the magma body and its aureole. The internal sheeting and zoning indicate that d uring the downwards flow of country rock, multiple pulses of magma tra velled up the same magma system. If these relationships are widespread in arcs, magma emplacement is the driving mechanism for a huge crusta l-scale exchange process.