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

Abstract

A top-to-bottom cross section of the Searchlight pluton is exposed in a large 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 pluton 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 foliation that was subhorizontal prior to tilting. Contacts between the units are gradational over a few centimeters to about 20 m. Geometry, field relations, and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber. The upper unit formed as a solidification front that migrated downward from the roof, and the middle unit granite and the lower unit represent complementary segregated melt and crystal-rich cumulate. Thus, the dominant part of the pluton appears to have solidified in monotonic fashion from more or less 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 formed as quenched mafic melts. They are isotopically distinct from the main sequence and represent discrete injections into the magma chamber. The mafic magmas were contaminated by, but did not strongly affect, the main-sequence magma. The Nd and Sr isotopic compositions of the main-sequence magma are intermediate between those of regional crust and enriched mantle-derived basalts like those represented by the mafic pods. This suggests that the principal magma was a hybrid with about 60% mantle component, and thus the pluton represents both addition to and internal reorganization of the crust