Abstract

The Virginia Dale ring-dike complex is a roughly circular granitic feature bout nine miles in diameter in the northern Front Range, straddling the Colorado-Wyoming boundary. The deeply eroded complex is a part of a large batholith of Sherman Granite, of Precambrian age, and discordantly intrudes metamorphic rocks. It consists of four main units: an outer granite ring dike, which merges to the north with the main Sherman batholith; a composite zone of metamorphic rocks and mafic igneous rocks; and two inner zones of quartz monzonite, comprising the core.

The outer ring dike, about a half mile wide, consists of coarse equigranular hornblende-biotite granite, and the two core zones consist of porphyritic biotite quartz monzonite. Both the granite and quartz monzonite are facies of Sherman Granite.

Between the outer ring and the core in the southern portion of the complex is a composite zone consisting of pre-Sherman metamorphic rocks, and of diorite, andesite, quartz monzonite, and hybrid rocks. These rocks show much evidence of brecciation, mechanical mixing accompanied by flow, and metasomatism. Field relations and petrographic analysis suggest that the diorites are of hybrid origin.

Chemical analyses, petrographic data, and trend analysis suggest that granite in the outer ring dike of the complex is of composition slightly different from that of granite outside the complex, and that it crystallized at higher water pressure. The quartz monzonite may have crystallized at even higher water pressure than that of granite and in addition was contaminated by mafic material. Relatively high magma volatile contents, as well as hybridization, seem to be characteristic of central complexes, even where erosion has been as deep as in the Virginia Dale complex.

Gravity data indicate that the core did not form by cauldron subsidence. Rather, it appears to be a dome emplaced along old ring fractures. The outer ring dike may be a collapse structure, although the batholith to which it is connected to the north may have been emplaced as large sheets.

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