A TEXTURAL INVESTIGATION OF THE GOLDIE LAMPROPHYRE, FREMONT COUNTY, COLORADO

Orbicular structures are classically defined as concentrically zoned, spheroidal to sub-spheroidal textures that grow in igneous systems and commonly contain mineralogically distinct layers. The Goldie lamprophyre is an early Cambrian syenitic lamprophyre dike of the McClure Mountain-Iron Mountain alkaline complex in South-Central Colorado which, in at least one outcrop, contains three textural variants that include porphyritic, banded phase, and spheroidal to sub-spheroidal features that were originally described as orbicular. The textural variants of the lamprophyre are described using optical and electron microscopy, textural relationships are constrained by EBSD, and rock and mineral compositions are used to establish relationships between texture-variants and the suite of lamprophyre dikes mapped in the area.

The lamprophyre host is a grey, fine-grained rock with plagioclase and sanidine phenocrysts that occur as tabular, 0.5-3 cm long single crystals, or, glomerocrystic splays of lath-like crystals. The spherical textures do not to express radial crystal growth, but they are commonly concentrically zoned and many have shells with several commonalities that include: a light-colored gradational boundary around the mantle marked by an increase in grain size; a finely laminated aphanitic layer with a dark rim marked by a high abundance of oxide phases; and a rim that is gray in color with a gradational boundary into surrounding lamprophyre. It is common to see inclusions of the tabular plagioclase and sanidine phenocrysts in the spherical textures. The spherical structures range in diameter from between 5 and 65 mm. Although they do not express the typical radial-crystal growth patterns commonly attributed to orbicular textures, their concentric zoning suggests sequential growth from core to rim or vice-versa. Some examples are distorted and slightly elongate, and the inclusions of plagioclase and sanidine laths suggest that the spherical structures may represent a second magma that was mixed with the lamprophyre. Observations made on field relationships between the outcrop and adjacent lamprophyre dikes suggest that intrusive geometries suitable for mixing to occur may be present.