TESTING GEOCHEMICAL HOMOGENEITY OF INCREMENTALLY EMPLACED SILLS AND LACCOLITHS, HENRY MOUNTAINS, UTAH

The Henry Mountains of southern Utah are cored by five Oligocene-aged, laccolithic igneous intrusive centers. Detailed field and laboratory work demonstrates that each intrusive center grew in stages from multiple, sequentially intruded pulses of magma. Pulses tend to be most easily distinguishable in the field and in thin section by distinct porphyritic textures. Previous geochemical analysis of these intrusions demonstrated that: (1) whole-rock major and trace element abundances in igneous rock are very consistent throughout the five intrusive centers, and (2) whole-rock isotope data reveal distinctions between but not within intrusive centers. However, these conclusions were based on results from a very limited suite of elements from a relatively small number of samples. We compile all known existing geochemical data for Henry Mountains igneous rocks, including results from more than 150 new samples, to test whether incremental assembly is apparent.

The compiled data allow us to test geochemical homogeneity of incrementally assembled intrusions a wide range of spatial scales. At the broadest scale, major and trace element abundances are insufficient to distinguish between separate intrusive centers (each ~25 to 100 km³). However, a more limited isotopic dataset (e.g. Sr, Nd, Pb) suggests intrusive centers are distinguishable from one another. At the scale of an individual intrusive center, component intrusive pulses (~5 to 10 km³) are usually indistinguishable with whole-rock geochemistry. A limited single-crystal dataset suggests trace element abundances in plagioclase and hornblende may differ between distinct pulses. At the most detailed scale, individual small intrusions (<5 km3) are geochemically homogeneous. Taken together, these compiled results suggest magma generation processes and geochemical evolution were very consistent throughout the magmatism in the Henry Mountains, a period of several million years. Additionally, the smallest magma pulses distinguishable with geochemistry have a volume on the order of 5 km³.