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Anorthosite complexes commonly are spatially and temporally associated with A-type granite batholiths, but their genetic relationship, if any, has been difficult to determine. This study focuses on the northern portion of the 1.43–1.44 Ga Sherman batholith, which is located adjacent to the 1.44 Ga Laramie anorthosite complex, Laramie Mountains, Wyoming, USA. The northern Sherman batholith is an ideal place to resolve the relationship of A-type granite with anorthosite because of the age of the crust into which it is emplaced. Most A-type granites are intruded into crust only slightly older than the granites themselves, and there is little isotopic contrast between potential mantle and crustal source rocks. The northern Sherman batholith intrudes Archean crust that by 1.44 Ga had developed Nd, Sr and Pb isotopic compositions that differed greatly from contemporaneous mantle. The northern Sherman batholith is an iron-enriched, metaluminous, alkalic to alkali-calcic batholith. Most of the batholith is composed of fayalite granite, with minor volumes of monzodiorite and olivine gabbro. Hypabyssal dikes of these compositions crop out in the highest structural levels exposed in the area. The northern Sherman batholith is less differentiated and has assimilated less felsic crust than the southern portions, which intrude more fertile Proterozoic crust. Northern Sherman batholith rocks are compositionally and isotopically very similar to monzonitic and dioritic rocks of the Laramie anorthosite complex. None of these rocks have isotopic compositions similar to their Archean host rocks, precluding derivation solely from such sources. Instead, northern Sherman batholith granites, like Laramie anorthosite complex rocks, are related to mantle-derived tholeiitic magmas emplaced at the base of the crust. The tectonic environment and magmatic processes that lead to the formation of massif anorthosite are also conducive to the formation of voluminous A-type granite, of which the northern Sherman granite is a prime example. |
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