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Granitoid Compositional Zoning by Side-wall Boundary Layer Differentiation: Evidence from the Palisade Crest Intrusive Suite, Central Sierra Nevada, California |
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| Authors: | SAWKA W N; CHAPPELL B W; KISTLER R W |
| Institution: | 1Department of Geology, The Australian National University Canberra, ACT 2601, Australia 2Earth Sciences Department, Lawrence Livermore National Laboratory Livermore, California 94550 3United States Geological Survey Menlo Park, California 94025 |
| Abstract: | Compositionally zoned plutons are an important feature of theSierra Nevada batholith, California. Two such plutons have beenexamined to determine the mechanism by which crystals separatefrom a magma. The Tinemaha pluton shows continuous compositionalvariation from 58 to 67% SiO2, whereas the McMurry Meadows plutonis bimodal, with an outer margin of mafic granodiorite (59–60%SiO2) and an inner core of granite (66–69% SiO2). Extremedifferentiates also occur as small isolated masses within thesuite and may contain up to 76% SiO2. Both plutons are uniformin strontium isotopic composition but are different from eachother, with initial 87Sr/86Sr values of 0?70719 and 0?70651respectively. The Tinemaha pluton is both horizontally and vertically( 1000 m) zoned, with fractionation occurring both inward fromthe contacts and upward. The vertical trends in relative mineralproportions are not consistent with crystal settling. Both thevertical and horizontal variations in the chemical compositionof 50 elements, in mineralogy, and in accessory mineral lightrare-earth element zoning, are all directly relatable to side-wallcrystallization which created a less-dense melt that buoyantlymoved upward along the wall towards the top of the magma chamber.The different rates for diffusive heat exchange and compositionaldiffusion within the magma initiated the double-diffusive gradientin the magma chamber. Compositional variations in the side-wallcrystal accumulation zone occur as boundary layer melts evolve,reflecting changes in the bulk convecting magma. The compositionalgap in the McMurry Meadows pluton is the result of a similarbut more efficient side-wall fractionation process, relatedto a higher proportion of melt to crystals in the initial magmaand a slower rate of side-wall solidification as a result ofthe thermal blanket created by the enclosing Tinemaha pluton. |
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