Country Rock Displacement during Emplacement of the Joshua Flat Pluton, White-Inyo Mountains, California
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The Jurassic Inyo batholith is a 70 by 25 km, rectangular-shaped, NW-SE oriented body consisting of several distinct internal plutons, which range from monzonite to granodiorite. This batholith intruded upper Proterozoic to lower Paleozoic anchizonal to greenschist facies, multiply deformed metasediments during which a narrow (.004 to.16 batholith radii) structural aureole formed in the country rock. The structural characteristics of this aureole vary from rather discordant margins to regions dominated by complex ductile deformation.
The southern end of the batholith consists of the zoned Joshua Flat pluton with an outer monzonite displaying widespread cm- to dm-scale, margin-parallel compositional magmatic banding and magmatic foliations with foliation intensity and presence of this magmatic banding decreasing toward the center of the pluton. The magmatic foliation crosscuts compositional banding and thus formed later. A complex, but narrow, structural aureole is present in this region. A relatively abrupt transition occurs at the outer margin of the aureole where folded, but relatively flat-lying lower to upper Cambrian metasediments become steeply NE-dipping. Progressing inwards, bedding and bedding parallel cleavage show a fan-like, but largely inward-dipping (=dipping toward the pluton contact) structure. Stratigraphic units everywhere face inwards and are locally overturned near the batholith, an observation that has been used by others to argue for country rock return flow during emplacement. However, in the inner aureole, chocolate tablet boudins in competent layers, indicating thinning by ductile flattening, are associated with discrete ductile mylonite zones, asymmetric folds, and moderately plunging mineral lineations which support oblique sinistral shear in a SW domain and dextral shear in a SE domain.
Initial results point to a combination of downward displacement and oblique SE and SW ductile flow occurring during batholith emplacement. Our initial estimates of bulk shortening in the aureole indicate that at least 40% to 60% of the aureole is missing. Well preserved and undeflected regional markers outside the aureole indicate that the additional transfer of country rock is needed during emplacement and must involve a large amount of vertical displacement within the region now occupied by the batholith. Local discordant margins and rare xenoliths in the magma indicate that the process of stoping probably accounts for some of the missing aureole.
KeywordsGeological Society Country Rock Lower Cambrian Stratigraphic Unit Contact Aureole
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