Rare earth element geochemistry of garnet lherzolite and megacrystalline nodules from minette of the Colorado plateau province |
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| Authors: | SN Ehrenberg |
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| Institution: | Department of Geology, Northern Illinois University, DeKalb, IL 60115 U.S.A. |
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| Abstract: | Mineral and whole-rock REE abundances in garnet lherzolite and megacrystalline nodules from The Thumb display broad correlations with major element compositions. Lherzolites with > 12 modal % clinopyroxene plus garnet (“high-Ca Al lherzolites”) have relatively flat chondrite-normalized whole-rock REE patterns. Lherzolites poor in clinopyroxene and garnet (“low-CaAl lherzolites”) have lower HREE in clinopyroxenes and garnets and higher whole-rock LREE/HREE. It is concluded that the low-CaAl lherzolites may have undergone LREE metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene. The high-CaAl lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very LREE-enriched liquids. The “megacrystalline” nodules are medium- to ultracoarse-grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites. The REE abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated REE, similar to minette or kimberlite.The patterns of correlation of REE and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern African kimberlites. Both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components: refractory garnet lherzolite enriched in LREE and depleted in HREE, fertile garnet lherzolite with generally chondritic REE abundances, and a suite of ultracoarse minerals precipitated from magma with extremely fractionated REE generally similar to the host magmas. |
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