The Moses Rock Dike: Geology, Petrology and Mode of Emplacement of a Kimberlite-Bearing Breccia Dike, San Juan County, Utah

Citation

McGetchin, Thomas Richard (1968) The Moses Rock Dike: Geology, Petrology and Mode of Emplacement of a Kimberlite-Bearing Breccia Dike, San Juan County, Utah. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/km6d-5q43. https://resolver.caltech.edu/CaltechETD:etd-06182009-081030

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Moses Rock dike is a well-exposed, four-mile long, kimberlite-bearing breccia intrusion in the east central Colorado Plateau, one of eight known kimberlite-bearing diatremes in the province. The dike occurs in gently dipping beds of the Permian Cutler Formation, 2 miles west of the Comb Ridge monocline in eastern Monument Valley, Utah. Contacts are little altered and the wall rocks generally undeformed. The present erosion surface is probably about 5,000 feet below the surface at the time of emplacement. By volume, the breccia in the dike consists of Cutler formation blocks, 72%; limestone fragments from underlying Paleozoic formations, 13%; crystalline rock fragments, 3%;and kimberlite, 12%. Essentially undiluted kimberlite occurs only locally, and occupies only about l% of the exposed parts of the dike. Mineral constituents of kimberlite are generally dispersed through the unconsolidated breccias. The breccias, including kimberlite, were probably emplaced as a fluidized solid-volatile system. This conclusion is based on the following observations: (1) No silicate melt was intruded at the level present erosion surface, (2) the breccias are particulate on all scales, (3) the particle size frequency distributions of the breccias are like those produced in comminution processes, (4) different types of breccias are intricately mixed, and (5) the mineral constituents of the kimberlite are commonly highly diluted with rock debris. Relationships of the breccia units suggests that flow of the fluidized system was concentrated in channels, now occupied by breccias that contain the largest upward displaced fragments and the largest crystalline rock fragments. Apparently the dike was emplaced along a fissure on which channels soon developed. A local joint system parallel to the contact, which cross-cuts regional joints, apparently played a key role in the dike formation and brecciation process. Crystalline rock and mineral fragments found in the dike range from acid to ultramafic types and are believed to represent rocks derived from the vent walls during the eruption. On the basis of the relative size and abundance of the xenoliths, it is inferred that metabasalt, granite and granite gneiss are abundant in the upper part of the crust, along the dikewalls; diorite, gabbro, mafic amphibolite constitute intermediate crystal layers; and mafic granulite and possibly hydrated ultramafic rocks constitute the lower crust. The suite of presumed crustal rocks is predominantly metavolcanic or metaplutonic, not metasedimentary. Dense and ultramafic fragments possibly derived from the mantle include antigorite-tremolite schist, jadeite-rich clinopyroxenite, eclogite, spinel-websterite, and spinel-lherzolite. The presence of garnet-periodotite at depth is inferred from the suite of mineral inclusions observed within pyropic garnets. Kimberlite of the Moses Rock dike is believed to be derived mechanically from physically disaggregated spinel and garnet peridotite in the mantle. All other rocks are believed to be accidental inclusions from the vent walls. Tentative P-T assignments to kimberlite clinopyroxenes based on their compositions suggests they are derived from various depths ranging from 50 to about 150 kilometers where the indicated temperatures are modest, about [...] C. Titanoclinohumite observed in kimberlite and as inclusions in pyropes may contain most of the water in the upper mantle. The Mohorovicic discontinuity apparently occurs in a petrologically complex region and may coincide with phase and compositional transitions, including hydration. A compositional transition between spinel and garnet periodotite with increasing depth in the mantle is consistent with the observations. The variety of ultramafic types and the complexity of the textures in the xenoliths suggest the mantle may be as complicated as the crust in composition and history. Numerical hydrodynamic models of eruption show that flow velocities are probably controlled by viscous losses and expansion of a volatile phase near the surface. Field observations of the largest blocks transported upward in the dike suggest flow velocities of 10 to 50 m/sec at the level of the present surface. Upward extrapolation by use of theoretical models suggests velocities of about 400 m/sec for the erupting fluidized system as it reached the earth's surface. The Moses Rock dike probably formed by eruption of kimberlite from a large reservoir in the mantle. The eruption was driven by volatiles, apparently mostly [...]. The kimberlite consists of physically disrupted rock from the reservoir environment.

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