VOLUME 40 NUMBER 2

Petrology, geochemistry, and geochronology of Proterozoic granitoid and related rocks of the northern Mummy Range, north-central Colorado

Thomas G. Plymate, Thomas D. Moeglin, and W. Randall Van Schmus

Detailed geologic mapping in the northern Mummy Range, north-central Colorado, reveals seven distinguishable igneous granitoid rock units intruded into a sequence of high-grade metamorphic host rocks. UPb dating of zircons from twelve samples indicates that these granitoids were emplaced during two separate periods of magmatism about 300 m.y. apart, one in the Paleoproterozoic and another in the Mesoproterozoic. Zircon dating confirms that all of this magmatic activity post-dated the peak of Paleoproterozoic metamorphismin this area. Based on field, geochemical and mineralogical analyses, four distinct Paleoproterozoic granitoid rock units are interpreted to represent three separate, but geochronologically indistinguishable, magmatic events at ~1.7Ga. The earliest igneous event is recorded by a body of leucocratic tonalite which yields a zircon age of 1702 6 Ma. The second event is recorded by small bodies of granodiorite which are mineralogically and geochemicallyvery similar to the approximately contemporaneous Boulder Creek Granodiorite exposed to the south. The third Paleoproterozoic igneous event produced foliated granitoid rocks that yield a composite zircon age of 1695 20 Ma and which comprise two distinct lithologies: a melanocratic roof facies and a strongly foliatedmonzogranite. This foliated Paleoproterozoic monzogranite, which is geochemically and mineralogically indistinguishable from the approximately contemporaneous Rawah batholith exposed to the northwest, is the most widespread rock unit exposed in the northern Mummy Range, accounting for over 65% of the area of the Comanche Peak quadrangle north of the Skin Gulch shear zone. The earliest Mesoproterozoic igneous event in the northern Mummy Range is represented by small bodies of coarse-grained quartz diorite. The second Mesoproterozoic event is represented by the strongly porphyriticgranodiorite of the Hagues Peak pluton. The third and final Mesoproterozoic igneous event in the area produced widespread bodies of equigranular Silver-Plume-type peraluminous monzogranite that yields a zirconage of 1393 25 Ma. Most of our zircon ages for the Mummy Range granitoids are consistent with the most recent synthesis of models for the tectonic evolution of the Proterozoic Colorado province. Our dates for certain individual samples of the Paleoproterozoic granodiorite and foliated Paleoproterozoic monzogranite suggest thatPaleoproterozoic syntectonic plutonism may have continued slightly longer in this area than previously recognized.The mineralogy and geochemistry of most of the Mummy Range granitoids are consistent with current tectonic syntheses of the region. The composition of the foliated Paleoproterozoic monzogranite in the Mummy Range suggests a source that included pre-existing continental crustal material, consistent with recent suggestions of >1.8-Ga crustal basement beneath parts of the Colorado province.

KEY WORDS: granitic rocks, Proterozoic, Mummy Range, Colorado.

C. D. Walcott definitely a Rocky Mountain geologist

Ellis L. Yochelson

KEY WORDS: Charles Doolittle Walcott, History of geology, biography, U.S. Geological Survey, Smithsonian Institution, Rocky Mountains, Great Basin.

Geometry, timing, and continuity of the Rock Springs uplift, Wyoming, and Douglas Creek arch, Colorado: Implications for uplift mechanisms in the Rocky Mountain foreland, U.S.A.

Selena Mederos, Basil Tikoff, and Viki Bankey

The Rock Springs uplift of Wyoming and the Douglas Creek arch of Colorado are intrabasinal, Laramide age basement uplifts within the Rocky Mountain foreland, and are currently separated by the eastwest-trending Uinta Mountains. The geometry, timing, and progressive development of these uplifts were investigate dusing a combined geophysical and geological approach. New gravity surveys were combined with existing regional data to provide a regional Bouguer gravity anomaly map of these two uplifts and the intervening Uinta uplift. The gravity data show a distinct and continuous northsouth-striking gravity high along the trend of the two uplifts that crosses the eastwest-trending Uinta uplift. The relatively constant amplitude(~40 mGal) of the gravity anomaly indicates that the inferred basement relief is similar for both arches (~4km). Sedimentation patterns indicate that the Rock Springs uplift and Douglas Creek arch formed simultaneously in the Late Cretaceous. The intrabasinal setting of the uplifts records aspects of foreland deformation that are overprinted or obscured in better-developed uplifts. On the local scale, neither the Rock Springs or Douglas Creek uplift apparently reactivates a pre-existing structure. On a regional scale, there is no change in structural style or timing of the two uplifts, despite their formation in different crustal provinces. The Rock Springs uplift occurs within the Archean Wyoming province north of the Cheyenne belt, whereas the Douglas Creek arch occurs in Proterozoic crust south of this boundary. Timing relations, available from the basinal stratigraphy, indicate the uplifts were initiated as broad arches in the Late Cretaceous before developing into more concentrated uplifts. Thus, large-scale folding, and not reactivation of pre-existing structures, may be the primary control on the initial pattern of north- to northwest-trending foreland deformation.

KEY WORDS: basement-involved uplift, Rocky Mountain foreland, Laramide orogeny, Bouguer gravity,Uinta Mountains, Wyoming, Colorado.

Rift and grain in basement: Thermally triggered snapshots of stress fields during erosional unroofing of the Rocky Mountains of Montana and Wyoming

Donald U. Wise

Even though rift and grain (R/G) are New England quarrymens terms for ease-of-fracture, somewhat similar structures appear in basement exposures throughout the Rocky Mountains of Montana and Wyoming. However, the nature, origin, and structural utility of these subtle features are generally unrecognized in the western uplifts. R/G represent directions of easy mesoscopic splitting produced by closely spaced microscopic fracture planes in quartz, now healed into thin zones of very tiny fluid inclusions. At hand-specimen to outcrop-scale these represent strength anisotropies that can exert major controls in the orientation of a host of younger types of joints and other fractures, one of many types of tectonic heredity. In the Rocky Mountain uplifts, strike of R/G can remain constant over 100s of km2, commonly with strikes parallel to adjacent mountain fronts. A late to post-Laramide age is indicated by dips remaining vertical even in strongly tilted mountain fronts. It is argued that these microfractures were produced by stresses generated by differential thermal contraction along the several crystallographic axes of quartz grains. Thermally induced grain-scale stresses directionally augmented by even weak regional stress fields produced the observed systematic regional patterns. Temperatures well above 150C have been documented for some microcrack origins, but these Rocky Mountain examples are limited by stratigraphic cover and association with eroding mountain fronts to formation at temperatures below 80C, possibly as low as 5060C, and depths as shallow as 12 km. Some of the R/G orientational domains represent older stress fields, but the mountain-front parallel ones are essentially snapshots of the last gasps of dying Laramide stress fields. The relationships suggests formation by volumetric expansion and interchange of σ1, σ2, and σ3 stress fields as decreasing confinement by adjacent basin fill allowed the uplifted mountain mass to begin gravitational spreading.

KEY WORDS: Laramide, rift, grain, joint, granite, basement, microfracture, microcrack, microjoint,Wyoming, Montana, Rocky Mountain, tectonic heredity




		VOLUME 40 NUMBER 2 Petrology, geochemistry, and geochronology of Proterozoic granitoid and related rocks of the northern Mummy Range, north-central Colorado Thomas G. Plymate, Thomas D. Moeglin, and W. Randall Van Schmus Detailed geologic mapping in the northern Mummy Range, north-central Colorado, reveals seven distinguishable igneous granitoid rock units intruded into a sequence of high-grade metamorphic host rocks. UPb dating of zircons from twelve samples indicates that these granitoids were emplaced during two separate periods of magmatism about 300 m.y. apart, one in the Paleoproterozoic and another in the Mesoproterozoic. Zircon dating confirms that all of this magmatic activity post-dated the peak of Paleoproterozoic metamorphismin this area. Based on field, geochemical and mineralogical analyses, four distinct Paleoproterozoic granitoid rock units are interpreted to represent three separate, but geochronologically indistinguishable, magmatic events at ~1.7Ga. The earliest igneous event is recorded by a body of leucocratic tonalite which yields a zircon age of 1702 6 Ma. The second event is recorded by small bodies of granodiorite which are mineralogically and geochemicallyvery similar to the approximately contemporaneous Boulder Creek Granodiorite exposed to the south. The third Paleoproterozoic igneous event produced foliated granitoid rocks that yield a composite zircon age of 1695 20 Ma and which comprise two distinct lithologies: a melanocratic roof facies and a strongly foliatedmonzogranite. This foliated Paleoproterozoic monzogranite, which is geochemically and mineralogically indistinguishable from the approximately contemporaneous Rawah batholith exposed to the northwest, is the most widespread rock unit exposed in the northern Mummy Range, accounting for over 65% of the area of the Comanche Peak quadrangle north of the Skin Gulch shear zone. The earliest Mesoproterozoic igneous event in the northern Mummy Range is represented by small bodies of coarse-grained quartz diorite. The second Mesoproterozoic event is represented by the strongly porphyriticgranodiorite of the Hagues Peak pluton. The third and final Mesoproterozoic igneous event in the area produced widespread bodies of equigranular Silver-Plume-type peraluminous monzogranite that yields a zirconage of 1393 25 Ma. Most of our zircon ages for the Mummy Range granitoids are consistent with the most recent synthesis of models for the tectonic evolution of the Proterozoic Colorado province. Our dates for certain individual samples of the Paleoproterozoic granodiorite and foliated Paleoproterozoic monzogranite suggest thatPaleoproterozoic syntectonic plutonism may have continued slightly longer in this area than previously recognized.The mineralogy and geochemistry of most of the Mummy Range granitoids are consistent with current tectonic syntheses of the region. The composition of the foliated Paleoproterozoic monzogranite in the Mummy Range suggests a source that included pre-existing continental crustal material, consistent with recent suggestions of >1.8-Ga crustal basement beneath parts of the Colorado province. KEY WORDS: granitic rocks, Proterozoic, Mummy Range, Colorado. C. D. Walcott definitely a Rocky Mountain geologist Ellis L. Yochelson KEY WORDS: Charles Doolittle Walcott, History of geology, biography, U.S. Geological Survey, Smithsonian Institution, Rocky Mountains, Great Basin. Geometry, timing, and continuity of the Rock Springs uplift, Wyoming, and Douglas Creek arch, Colorado: Implications for uplift mechanisms in the Rocky Mountain foreland, U.S.A. Selena Mederos, Basil Tikoff, and Viki Bankey The Rock Springs uplift of Wyoming and the Douglas Creek arch of Colorado are intrabasinal, Laramide age basement uplifts within the Rocky Mountain foreland, and are currently separated by the eastwest-trending Uinta Mountains. The geometry, timing, and progressive development of these uplifts were investigate dusing a combined geophysical and geological approach. New gravity surveys were combined with existing regional data to provide a regional Bouguer gravity anomaly map of these two uplifts and the intervening Uinta uplift. The gravity data show a distinct and continuous northsouth-striking gravity high along the trend of the two uplifts that crosses the eastwest-trending Uinta uplift. The relatively constant amplitude(~40 mGal) of the gravity anomaly indicates that the inferred basement relief is similar for both arches (~4km). Sedimentation patterns indicate that the Rock Springs uplift and Douglas Creek arch formed simultaneously in the Late Cretaceous. The intrabasinal setting of the uplifts records aspects of foreland deformation that are overprinted or obscured in better-developed uplifts. On the local scale, neither the Rock Springs or Douglas Creek uplift apparently reactivates a pre-existing structure. On a regional scale, there is no change in structural style or timing of the two uplifts, despite their formation in different crustal provinces. The Rock Springs uplift occurs within the Archean Wyoming province north of the Cheyenne belt, whereas the Douglas Creek arch occurs in Proterozoic crust south of this boundary. Timing relations, available from the basinal stratigraphy, indicate the uplifts were initiated as broad arches in the Late Cretaceous before developing into more concentrated uplifts. Thus, large-scale folding, and not reactivation of pre-existing structures, may be the primary control on the initial pattern of north- to northwest-trending foreland deformation. KEY WORDS: basement-involved uplift, Rocky Mountain foreland, Laramide orogeny, Bouguer gravity,Uinta Mountains, Wyoming, Colorado. Rift and grain in basement: Thermally triggered snapshots of stress fields during erosional unroofing of the Rocky Mountains of Montana and Wyoming Donald U. Wise Even though rift and grain (R/G) are New England quarrymens terms for ease-of-fracture, somewhat similar structures appear in basement exposures throughout the Rocky Mountains of Montana and Wyoming. However, the nature, origin, and structural utility of these subtle features are generally unrecognized in the western uplifts. R/G represent directions of easy mesoscopic splitting produced by closely spaced microscopic fracture planes in quartz, now healed into thin zones of very tiny fluid inclusions. At hand-specimen to outcrop-scale these represent strength anisotropies that can exert major controls in the orientation of a host of younger types of joints and other fractures, one of many types of tectonic heredity. In the Rocky Mountain uplifts, strike of R/G can remain constant over 100s of km2, commonly with strikes parallel to adjacent mountain fronts. A late to post-Laramide age is indicated by dips remaining vertical even in strongly tilted mountain fronts. It is argued that these microfractures were produced by stresses generated by differential thermal contraction along the several crystallographic axes of quartz grains. Thermally induced grain-scale stresses directionally augmented by even weak regional stress fields produced the observed systematic regional patterns. Temperatures well above 150C have been documented for some microcrack origins, but these Rocky Mountain examples are limited by stratigraphic cover and association with eroding mountain fronts to formation at temperatures below 80C, possibly as low as 5060C, and depths as shallow as 12 km. Some of the R/G orientational domains represent older stress fields, but the mountain-front parallel ones are essentially snapshots of the last gasps of dying Laramide stress fields. The relationships suggests formation by volumetric expansion and interchange of σ1, σ2, and σ3 stress fields as decreasing confinement by adjacent basin fill allowed the uplifted mountain mass to begin gravitational spreading. KEY WORDS: Laramide, rift, grain, joint, granite, basement, microfracture, microcrack, microjoint,Wyoming, Montana, Rocky Mountain, tectonic heredity