Type of Culminating Activity
Master of Science in Geology
Walter S. Snyder, Ph.D.
The volcanics of the Early Triassic Koipato Formation of central Nevada unconformably overlie the Golconda Allochthon and, classically, this relationship has been used to define the timing of the Sonoma Orogeny as post-Middle Permian to earliest Triassic. However, the Koipato Formation represents a rather isolated magmatic succession, with other western U.S. Early Mesozoic igneous provinces determined to be younger or lacking rocks of Koipato age. This isolation, coupled with the fact that the Koipato Formation does not overlap the Golconda Allochthon, has left open two possible scenarios for its tectonic history: 1) the Koipato Formation represents the earliest, post-Sonoma Orogeny continental margin arc magmatism, which then quickly shifted the locus of magmatism to other locations, or 2) the Koipato Formation was part of an offshore island arc that was deposited on its subduction complex (the eventual Golconda Allochthon), and then this piggyback complex was thrust over the continental margin in post-Koipato time. The Koipato Formation, of central Nevada, is largely composed of Early Triassic, intermediate to felsic, intrusive and volcanic units with minor amounts of sedimentary and metasedimentary rocks, which have been classically subdivided into three units: (in ascending order) the Limerick Greenstone, Rochester Rhyolite, and Weaver Rhyolite. This stratigraphic scheme has been modified by research presented here. The focus of this research has been to help clarify the age and tectonic and magmatic frameworks of the Koipato Formation, in particular as it impacts the interpretations for the Sonoma Orogeny and the Early Mesozoic Cordilleran magmatic arcs.
New field evidence and geochronology presented in this study demonstrate that the Koipato Formation represents an intermediate to felsic volcanic sequence that documents a short-lived latest Permian to Early Triassic series of magmatic events. Geochronologic data identifies previously unrecognized unconformities within the Koipato Formation and helps to constrain these unconformities and the ones bounding the Koipato Formation.
Field evidence and U-Pb geochronology support the interpretation that the Rochester and lower Weaver Rhyolites are partly coeval units. Also, U-Pb geochronology has proven that the silicic intrusive units observed throughout the Humboldt Range are coeval to the older sequence of the Rochester and the lower Weaver Rhyolites and acted as feeders for these felsic volcanics. Finally, two phases of silicic volcanism are identified within the Koipato Formation, which are separated by a previously unidentified unconformity. This unconformity is documented to have a time span of <350,000 years and separates the older Rochester and Weaver Rhyolites in Troy Canyon and the East and Tobin Ranges from the young Rochester and lower Weaver Rhyolites of Limerick Canyon and the sedimentary and upper Weaver Rhyolites in Troy Canyon. Also, this unconformity records the erosion of the older phase of silicic volcanism from the west side of the Humboldt Range.
U-Pb geochronology shows that the Koipato Formation is predominately late Early Triassic (249.59 to 248.32 Ma), with the majority of volcanism lasting for ~1.2 Ma. The existence of ~254 Ma inherited zircons within the leucogranite of the Humboldt Range has been inferred to represent the earliest stages of Limerick Greenstone-type Koipato volcanism, which extends the age of Koipato Formation volcanism to the latest Permian. The unconformity between the Limerick Greenstone and Rochester Rhyolite identified by Wilkins (2010) in the East Range has been dated and spans a time gap of ~200,000 years in Troy Canyon and ~1 Ma in Limerick Canyon of the Humboldt Range. Also, this unconformity may have a slight angular component, but this could not be confirmed in the Humboldt Range. This unconformity documents that the transition from intermediate to felsic volcanism was associated with a pause in magmatism and perhaps tectonism. The unconformity between the Golconda Allochthon and the Koipato Formation has been constrained in this study to represent a time gap of ~15 to 6 Ma based on the age of Middle Permian for the youngest unit within the Golconda Allochthon and ~254 Ma from the inherited grains of the leucogranite intrusive. The unconformity between the overlying Prida Formation and the Koipato Formation represents a time gap of ~3 to7 Ma based on an Anisian age of the Prida Formation and the 248.32 Ma obtained from the youngest sample of the Koipato Formation. This time gap would be long enough to allow for a major change from the volcanic setting of the Koipato Formation to a carbonate platform, which is required for deposition of the Prida Formation.
Sr and Nd isotopic investigation of the Koipato Formation demonstrates that intermediate to felsic members exhibit uniformly high 87Sr/86Sr (0.7089 – 0.7126) and fairly negative εNd values (-9.73 – -12.89). These compositions require that the volcanics of the Koipato Formation were at least partially sourced from Precambrian continental crust material. Nd (TDM) isotopic evolution modeling for these samples yield mantle extraction model ages of the source continental crustal material between 1.7 and 2.4 Ga, and indicate that the Koipato Formation was erupted through Paleoproterozoic crust.
These data also imply that the underlying Golconda Allochthon was, at the time of Koipato magmatism, already overlying the continental margin, thus precluding the interpretation that the Koipato Formation and the Golconda Allochthon were emplaced piggyback onto the continental margin in post-Koipato time. These data, however, still leave open the possibility that final emplacement of the Golconda Allochthon, with the Koipato Formation on top, did not occur until a later time in the Mesozoic.
Vetz, Nicholas Quentin, "Geochronologic and Isotopic Investigation of the Koipato Formation, Northwestern Great Basin, Nevada: Implications for Late Permian-Early Triassic Tectonics along the Western U.S. Cordillera" (2011). Boise State University Theses and Dissertations. Paper 217.