AN ERUPTED RECORD FROM THE MIOCENE SEARCHLIGHT PLUTON, NEVADA

The Miocene Searchlight pluton was exposed and tilted to near-vertical by regional extension in the Colorado River Extensional Corridor, allowing for the rare opportunity to study the entire ~10 km vertical dimension of the pluton, and Miocene volcanic units that it intrudes. These volcanic units are located in the northern Newberry Mountains and southern Highland Range, NV. Several lines of evidence, including geochronology, field relations, petrography, major and trace elements, and Sr and Nd isotopes, demonstrate that most of the overlying volcanic rocks were erupted from the Searchlight magma chamber.

Volcanic rocks similar in age to Searchlight pluton include trachydacite and trachyandesite flows (16.8 Ma; U/Pb zircon), (17.28+/-0.35 Ma; Ar/Ar whole-rock) [1] comprising the lower three-quarters of the section. These flows are overlain by rhyolite flows and tuffs (15.9 Ma; U/Pb zircon), (16.2Ma; Ar/Ar) [2]. Rhyolite dikes cut through upper Searchlight and the lower volcanic flows (74-77 wt.% SiO2) (15.6 Ma; Ar/Ar feldspar). The total thickness of the volcanic section is ~ 3 km. Quartz monzonite (16.7 Ma; U/Pb zircon) comprises the lower and the upper Searchlight pluton, and upper Searchlight includes distinctive porphyry that is similar texturally to overlying and age-correlative trachydacites. Middle Searchlight is granite (15.8 Ma; U/Pb zircon), and includes high-silica (77 wt.%) leucogranite.

Searchlight pluton and overlying volcanic units within the same age bracket exhibit striking mineralogical and geochemical similarities. Biotite is the most abundant mafic phase in the pluton and in volcanic units. Major elements for the pluton and volcanic units have coincident trends, and chondrite-normalized REE diagrams for plutonic units (other than cumulates) correspond strongly with one another. Rhyolite units exhibit LREE enrichments and deep negative Eu anomalies that are similar to granites. ε_Nd and ⁸⁷Sr/⁸⁶Sr values for the pluton and volcanic units also overlap substantially [3,4].

These data demonstrate that magmas, which eventually solidified to form Searchlight pluton, were also periodically vented allowing for comparisons between the temporal and dynamic evolution of the Searchlight magma system as it was constructed.

[1] Ruppert 1998, [2] Faulds 2002, [3] Bachl 1999, [4] Feuerbach 1998.