LAUSR

Abstract The Karaj Dam basement Sill is a 460 m–thick saucer shaped sill, situated in the Alborz Magmatic Belt, Northern Iran. The results of geochemical, textural and field relations reveal characteristics of a sill with a well-developed S-shaped compositional profile which could be subdivided into distinct parts and suggest that the sill was repeatedly split and reinjected with fresh magma in the upper half of the previous emplacements (over- to partly intra-accretion). Whole rock and mineral compositional profiles have recorded five to six discrete injections of magma, each of which individually show an upward increase in terms of primitivity which represents partial crystallization in feeder conduits. The first three small successive pulses of magma, emplaced in the basal 150 m of the floor sequence, were followed by voluminous fourth and fifth pulses in the upper portion of the sill. During final two pulses the system acts as a closed system for each independently, and evolves through compositional convection or compaction aided in-situ crystallization. Theoretical models for convection and compaction shows the significance of both processes. Considering the final porosity of 0.1  φ  V c V a  = 1 and 0.1 m/year), when the crystal mush varies between 1 to 0.01 m with the liquid viscosity ranging from 85 to 15 Pa·s. Our calculations further indicate that compaction driven velocity of liquid expulsion ( ω  −  w) hardly exceeds the V a in the basal 150 m of the floor sequence. The highest velocity is reached ( ω  −  w  = ~ 1 m/year) only if the crystal mush thickness is no less than 240 m with the porosity of 0.6 and the liquid viscosity of no more than 15 Pa·s. On the other hand, compaction is highly effective in the upper portion of the sill. Transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects, such as the depletion of the lower half of the sill and the enrichment of the sandwich zone in incompatible elements and modal granophyre. Crystallization of the fourth pulse of magma produced a peak in incompatible clement concentrations at a sandwich horizon located at the 410 m level, where the floor and roof of the sill appeared to converge at this stage. Subsequent to the time of this enriched zone crystallizing, the fifth pulse of magma was emplaced near this level and inflated the chamber vertically for ~ 200 m. Cumulus material containing interstitial melt and subsequent buoyancy driven upward transport of interstitial melt in this pulse concentrated incompatible elements at ~ 47 m below the main peak, at the stratigraphic height of 360–370 m. The boundaries between the successive pulses were cryptic and represent a gradational contact in terms of grain size, chemical composition and crystallization sequence. This implies a short time interval between the emplacements of the magma. This sill is comparable with Antarctica Beacon Sill and Hudson River Palisades Sill, and the recognition of evidence for reinjection and compaction in these macroscopically uniform sills, as well as the Karaj Dam basement Sill, suggests that these processes may be common in the construction of sills with a thickness of more than 100 m.