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Sphene and zircon in the Highland Range volcanic sequence (Miocene, southern Nevada, USA): elemental partitioning, phase relations, and influence on evolution of silicic magma |
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| Authors: | Lindy L Colombini Calvin F Miller Guilherme A R Gualda Joseph L Wooden Jonathan S Miller |
| Institution: | 1. Department of Earth and Environmental Sciences, Vanderbilt University, PMB 351805, 2301 Vanderbilt Place, Nashville, TN, 37235-1805, USA 2. Stanford-USGS Micro Analysis Center, SHRIMP Lab, Stanford University, Green Earth Sciences Building, 367 Panama Street Room 89, Stanford, CA, 94305, USA 3. Department of Geology, San Jose State University, Duncan Hall 320, San Jose, CA, 95112-0102, USA |
| Abstract: | Sphene is prominent in Miocene plutonic rocks ranging from diorite to granite in southern Nevada, USA, but it is restricted to rhyolites in coeval volcanic sequences. In the Highland Range volcanic sequence, sphene appears as a phenocryst only in the most evolved rocks (72?C77 mass% SiO2; matrix glass 77?C78 mass% SiO2). Zr-in-sphene temperatures of crystallization are mostly restricted to 715 and 755°C, in contrast to zircon (710?C920°C, Ti-in-zircon thermometry). Sphene rim/glass Kds for rare earth elements are extremely high (La 120, Sm 1200, Gd 1300, Lu 240). Rare earth elements, especially the middle REE (MREE), decrease from centers to rims of sphene phenocrysts along with Zr, demonstrating the effect of progressive sphene fractionation. Whole rocks and glasses have MREE-depleted, U-shaped REE patterns as a consequence of sphene fractionation. Within the co-genetic, sphene-rich Searchlight pluton, only evolved leucogranites show comparable MREE depletion. These results indicate that sphene saturation in intruded and extruded magmas occurred only in highly evolved melts: abundant sphene in less silicic plutonic rocks represents a late-stage ??bloom?? in fractionated interstitial melt. |
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