Construction of a pluton: Evidence from an exposed cross section of the Searchlight pluton, Eldorado Mountains, Nevada

A top-to-bottom cross section of the Searchlight pluton is exposed in a lar ge tilt block in the Colorado River extensional corridor of southern Nevada . Hornblende barometry and geologic relations indicate that the pluton was about 10 km thick, extending from approximately 3 to 13 km depth. The pluto n is stratified with about 2 km of fine-grained quartz monzonite below the roof, 2 km of granite in the center, and 6 km of coarser, more mafic quartz monzonite at the bottom. The lower unit has a pronounced magmatic foliatio n that was subhorizontal prior to tilting. Contacts between the units are g radational over a few centimeters to about 20 m. Geometry, field relations, and elemental and isotopic data suggest that the three units mark the term inal stages of evolution of magma that filled a very thick magma chamber. T he upper unit formed as a solidification front that migrated downward from the roof, and the middle unit granite and the lower unit represent compleme ntary segregated melt and crystal-rich cumulate. Thus, the dominant part of the pluton appears to have solidified in monotonic fashion from more or le ss uniform magma that was compositionally similar to the upper unit. The basal quartz monzonite and the lower part of the granite enclose widely scattered, synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale. These rocks are commonly fine grained and fo rmed as quenched mafic melts. They are isotopically distinct from the main sequence and represent discrete injections into the magma chamber. The mafi c magmas were contaminated by, but did not strongly affect, the main-sequen ce magma. The Nd and Sr isotopic compositions of the main-sequence magma are intermed iate between those of regional crust and enriched mantle-derived basalts li ke those represented by the mafic pods. This suggests that the principal ma gma was a hybrid with about 60% mantle component, and thus the pluton repre sents both addition to and internal reorganization of the crust.