EVALUATING SPATIOTEMPORAL AND GEOCHEMICAL VOLCANIC-PLUTONIC CONNECTIONS WITHIN THE IRON MOUNTAIN PENDANT, SIERRA NEVADA, CALIFORNIA

Whether coeval volcanic and plutonic rocks are compositionally complementary or equivalent provides information about the state and evolution of the magma chamber during eruption. However, limited areas expose both volcanic and plutonic levels. The Iron Mountain pendant (IMP) is comprised of metasedimentary units and rhyolite-andesite metavolcanic units. The IMP is intruded by Cretaceous plutons, including the hypabyssal 120.1 ± 2.2 Ma Star Lakes granodiorite-monzogranite porphyry. Preliminary map data shows that the units are spatially related, justifying exploration of potential volcanic-plutonic connections. U-Pb LA-ICP-MS zircon dating, XRF whole-rock geochemistry, and petrographic analysis were performed on samples collected during field mapping to test temporal compositional connections, illustrate compositional variability, and enable inter-unit comparisons.

Field observations of plutonic inclusions and mutually cross-cutting relationships indicated a possible connection between the porphyry and andesite. However, an andesite age of 155.6 ± 3.58 Ma (Bennett et al. GSA Abstract) rules out a genetic connection. Flow-banded rhyolite unconformably overlies the andesite unit. Rhyolite ages of 124.6 ± 0.83 Ma and 123.0 ± 0.62 Ma overlap with a previously unpublished age of 119.3 ± 1.6 Ma. Mineral identification in the rhyolite was challenging due to the aphanitic, recrystallized texture; however, both the Star Lakes porphyry and the rhyolite’s matrix contain an estimated 40-60% plagioclase and quartz. The porphyry contains ~30% biotite and hornblende, but the rhyolite contains <2% hornblende and no biotite. Two distinct populations exist between the porphyry (SiO₂ 65-70 wt.%) and rhyolite (SiO₂ 75-80 wt.%). Major elements (Al₂O₃, CaO, P₂O₅, MgO) vs SiO₂ show a decreasing linear trend between units. Although the metaandesite cannot be genetically related to the Star Lakes Porphyry, the metarhyolite unit may be a volcanic counterpart. A complementary magma system, where rhyolite melt is extracted from the underlying chamber, is viable as the composition of both units is distinct. Detailed mapping of the porphyry unit may identify compositional variations within the porphyry that more closely relate to the metarhyolite and further test a possible genetic connection.