The Boulder Creek Batholith, Front Range, Colorado

Abstract

The Boulder Creek batholith is the best known of several large Precambrian batholiths of similar rock composition that crop out across central Colorado. The rocks in the batholith belong to the calc-alkaline series and range in composition from granodiorite through quartz diorite (tonalite) to gneissic aplite. Two rock types dominate: the Boulder Creek Granodiorite, the major rock unit, and a more leucocratic and slightly younger unit herein named Twin Spruce Quartz Monzonite. Besides mafic inclusions, which occur mainly in hornblende-bearing phases of the Boulder Creek Granodiorite, there are cogenetic older and younger lenses, dikes, and small plutons of hornblende diorite, hornblendite, gabbro, and pyroxenite. Pyroxenite is not found in the batholith. The Boulder Creek Granodiorite in the batholith represents essentially two contemporaneous magmas, a northern body occurring in the Gold Hill and Boulder quadrangles and a larger southern body exposed in the Blackhawk and the greater parts of the Tungsten and Eldorado Springs quadrangles. The two bodies are chemically and mineralogically distinct. The northern body is richer in CaO and poorer in K2O, is more mafic, and has a larger percentage of plagioclase than the southern body.A crude sequence of rock types occurs from west to east in the batholith accompanied by a change in plagioclase composition from calcic plagioclase on the west to sodic on the east. Ore minerals tend to decrease, and the ratio potassium feldspanplagioclase increases inward from the western contact of the batholith, indicating that the Boulder Creek batholith is similar to granodiorite batholiths the world over. Emplacement of the Boulder Creek batholith was contemporaneous with plastic deformation and high-grade regional metamorphism that folded the country rock and the batholith contact along west-northwest and north-northwest axes. Also, smaller satellitic granodiorite bodies tend to conform to the trends of foliation and fold axes in the country rock, suggesting that emplacement was controlled by preexisting structures in the country rock.On a gross scale, chemical equilibrium in the Boulder Creek Granodiorite is expressed by a near 1:1 ratio, or straight-line relationship in the distribution of iron, magnesium, and manganese in biotite and hornblende. General mineralogical trends in the Boulder Creek Granodiorite indicate that modal biotite, hornblende, and plagioclase tend to increase and quartz and microcline tend to decrease as CaO increases. These trends were not found in the Twin Spruce Quartz Monzonite.Differentiation is believed to have played a major role and assimilation a minor role in the development of the Boulder Creek batholith. The Boulder Creek Granodiorite is of probable mantle or lower crust origin, and, based on the scant data available, the Twin Spruce Quartz Monzonite may be of crustal origin, but the magma was extensively altered by contaminants of ambiguous origin. Mafic inclusions, possibly derived from a dioritic magma which was an early differentiate associa...

Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): implications for petrogenetic models

“…Figs. 11 (e and f)]. Wall rocks of the Pikes Peak batholith include extensive granodiorite and tonalite plutons of the Boulder Creek intrusive period (ca 1.7 Ga) with major element compositions similar to those used in the melting experiments (cf Gable, 1980) and ߳ Nd (1.08 Ga) values that overlap with those for the potassic granites (Fig. 8).…”

Geomorphic inferences from regolith thickness, chemical denudation and CRN erosion rates near the glacial limit, Boulder Creek catchment and vicinity, Colorado

“…Bedrock in and near the Boulder Creek catchment includes N 1.7 Ga Proterozoic gneisses and metasediments, Proterozoic granitic rocks of the 1.65 Ga Boulder Creek and 1.4 Ga Silver Plume batholiths and early Tertiary intrusive rocks of intermediate to silicic composition (Gable, 1980). Major primary and secondary minerals of the batholithic rocks, gneisses and metasediments include quartz, microcline, plagioclase, biotite, hornblende and mixed-layer smectite-illite, vermiculite, chlorite, and minor kaolinite and X-ray amorphous material (Table 1).…”

Eolian Additions to Late Quaternary Alpine Soils, Indian Peaks Wilderness Area, Colorado Front Range

“…In the Indian Peaks study area, biotite gneiss contains biotite, quartz, and plagioclase, with lesser amounts of sillimanite, Kfeldspar, garnet, and cordierite (Pearson, 1980). Boulder Creek Granodiorite contains plagioclase, quartz, K-feldspar, and biotite, with smaller amounts of hornblende, muscovite, sphene, zircon, and apatite (Gable, 1980). Based on X-ray diffraction analysis and microscopic examination, the sand fractions of the alpine soils we studied consist of quartz, K-feldspar, plagioclase, biotite, amphibole (probably hornblende), and sillimanite.…”