New insight into the history of clastic dikes in the western San Juan Mountains, southwestern Colorado
D.A. Gonzales and J.L. Koch


Clastic dikes crosscut Paleozoic to Mesozoic strata in the western San Juan Mountains near Placerville and Ouray. These dikes trend W-NW for hundreds to thousands of meters and vary from tabular masses several meters thick to zones of bifurcating veins and breccia cemented by calcite. Clastic dikes near Ouray are exposed adjacent to, and cross cut, ~65 Ma granodiorite dikes and stocks and are overlain by ~30 Ma volcanic rocks. Near Placerville, clastic dikes crop out along fractures in Permian to Jurassic strata and have similar trends as some late Mesozoic to Cenozoic mafic to intermediate dikes, but are nowhere in direct contact with post-70 Ma intrusive rocks and lava flows. Clastic dikes exposed in the region are matrix- to clast-supported with angular to subrounded pebble- to boulder-sized fragments that are dominated by Proterozoic igneous and metamorphic rocks. A U-Pb detrital zircon-age analysis on a sample of clastic dike near Placerville yields zircon populations that are consistent with reworking of Proterozoic basement (1800 to 1400 Ma) and Paleozoic sedimentary rocks (500 to 350 Ma). Previous and current field investigations, and new fluid inclusion data, lend evidence that clastic dikes in the western San Juan Mountains formed by release of CO2-charged volatiles at depths greater than 500 m where Proterozoic basement was fragmented, entrained, and transported to higher stratigraphic levels. Herein, we propose that the clastic dikes formed by release of volatiles along deep fractures, from degassing of plutons or mantle melts after 70 Ma.


  1. Gonzales, D.A.; Koch, J.L., 2017, New insight into the history of clastic dikes in the western San Juan Mountains, southwestern Colorado, in: The Geology of the Ouray-Silverton Area, Karlstrom, Karl E.; Gonzales, David A.; Zimmerer, Matthew J.; Heizler, Matthew; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 68th Field Conference, pp. 141-147.

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