GROWTH OF PLUTONS BY INCREMENTAL EMPLACEMENT OF SHEETS IN CRYSTAL-RICH HOST: EVIDENCE FROM MIOCENE INTRUSIONS OF THE COLORADO RIVER REGION, NEVADA

Two million year lifetimes of Miocene plutons of southern NV are far longer than required for cooling and solidification of a single magma pulse on the scale of the intrusions. Field relations record evidence locally for multiple replenishments in all of the plutons, yet much or most of their exposed areas lack such features. These intrusions exemplify a conundrum: many plutons appear to require construction by abundant, increments to account for their longevity, yet in vast portions evidence for multiple injections is sparse or absent.

Two lines of evidence suggest a resolution for this conundrum for the southern NV examples that may apply more generally: (1) Ages and zoning in zircons commonly demonstrate protracted histories and major fluctuations in temperature and host melt chemistry, and different crystals in the same sample reveal different histories. This indicates repeated interaction between resident and replenishment magmas even in the absence of clear field evidence. (2) The youngest intrusive phases are fine-grained, initially subhorizontal, granite sheets that mark fresh input into the plutons. Some of these sheets are confined within older, coarser, less felsic granite layers (cumulate?) that are interleaved between mafic sheets. Contacts between the two granite facies are 'soft,' with the fine-grained sheets having entrained coarse xenocrysts from the cumulate. Where such clear intrusive contacts are absent but zircon indicates repeated replenishment, subtler features – horizons where schlieren or fractionated leucogranites are localized – appear to mark some boundaries between melt-rich and melt-poor zones.

We propose that replenishments form sheets that are guided by contrasts in strength within a melt-bearing but crystal-rich host. Ascending magma stalls and spreads within weak, relatively melt-rich zones. Contacts, initially ill-defined because of similarity between intruding crystal-poorer magma and crystal-richer host, may be obscured further by subsequent destabilization of the crystal-melt framework. Zircon data testify to stirring together of successive increments and thus to the ephemeral nature of the initial intrusive contacts. Only during thermally waning stages of their histories will plutons retain a clear record of discrete replenishments.