PETROLOGY AND GEOCHEMISRY OF LATE-STAGE INTRUSIONS OF THE A-TYPE, MID-PROTEROZOIC PIKES PEAK BATHOLITH (CENTRAL COLORADO, USA):

PETROLOGY AND GEOCHEMISRY OF LATE-STAGE INTRUSIONS OF THE A-TYPE, MID-PROTEROZOIC PIKES PEAK BATHOLITH (CENTRAL COLORADO, USA): IMPLICATIONS FOR PETROGENETIC MODELS

SMITH, D. R., Department of Geosciences, Trinity University, 715 Stadium Drive, San Antonio, TX 78212-7200; NOBLETT, J., Department of Geology, Colorado College, Colorado Springs, CO 80903; WOBUS, R. A., Department of Geology, Williams College, 947 Main Street, Williamstown, MA 01267; UNRUH, D., United States Geological Survey, MS963, Box 25046, Federal Center, Denver, CO 80255; DOUGLAS, J., Department of Geosciences, Trinity University, 715 Stadium Drive, San Antonio, TX 78212-7200; BEANE, R., Department of Geology, Williams College, 947 Main Street, Williamstown, MA 01267; DAVIS, C., Washington & Lee University, Lexington, VA 24450; GOLDMAN, S., Department of Geology, Colorado College, Colorado Springs, CO 80903; KAY, G., Department of Geology, Colorado College, Colorado Springs, CO 80903; GUSTAVSON, B., Department of Geology, Carleton College, Northfield, MN 55057; SALTOUN, B., Department of Geology, College of Wooster, Wooster, OH 44691-2363; STEWART, J., Department of Geology, Beloit College, Beloit, WI 53511.

The ~1.08 Ga anorogenic; A-type Pikes Peak batholith (Front Range, central Colorado) is dominated by coarse-grained, biotite + emphibole syenogranites and minor monzogranites, collectively referred to as Pikes Peak granite (PPG). The batholith is also host to numerous small, late-stage plutons that have been subdivided into two groups (e.g. Wobus, 1976. Studies in Colorado Field Geology, Colorado School of Mines Professional Contributions, Colorado): (1) a sodic series (SiO₂=~44-78 wt%; K/Na=0.32-1.36) composed of gabbro, diabase, syenite/quartz syenite and fayalite and sodic amphibole granite; and (2) a potassic series (SiO₂=~70-77 wt%; K/Na=0.95-2.05), characteristics for the two series indicate different petrogenetic histories.

Potassic granites of the late-stage intrusions appear to represent crustal anatectic melts derived from tonalite sources, based on comparison of their major element compositions with experimental melt products. In addition, Nd isotopic characteristics of the potassic granites [Î_Nd(1.08 Ga)=-0.02 to –2.7] overlap with those for tonalities/granodiorites [ca 1.7 Ga Boulder Creek intrusions; [Î_Nd(1.08 Ga)=-2.4 to –3.6] exposed in the region. Some of the partial melts evolved by fractionation dominated by feldspar. The late-stage potassic granites share geochemical characteristics with most of the PPG, which is also interpreted to have an anatectic origin involving tonalitic crust. The origin of monzogranites associated with the PPG remains unclear, but mixing between granitic and mafic or intermediate magmas is a possibility.

Syenites and granites of the sodic series cannot be explained as crustal melts, but are interpreted as fractionation products of mantle-derived mafic magmas with minor crustal input. High temperature and low oxygen fugacity estimates (e.g. Frost et al., 1988. American Mineralogist 73, 727-740) support a basalt fractionation origin, as do [Î_Nd(1.08 Ga)=+2.2 to –0.7], which are higher than Î_Ndvalues for Colorado crust at 1.08 Ga (ca – 1.0 to –4.0). Enrichments in incompatible elements (e.g. rare earth elements, Rb, Y) and depletions in compatible elements (e.g. Cr, Sr, Ba) in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation. Accessory mineral fractionation, release of fluorine-rich volatiles and/or removal of pegmatitic fluids could have modified abundances of Ce, Nb, Zr, and Y in some sodic granitoids magmas.

Gabbros and mafic dikes associated with the sodic granitoids have Î_Nd(1.08 Ga) of –3.0 to +3.5, which are lower than depleted mantle at 1.08 Ga, and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction-related processes. However, it is difficult to characterize the mantle component in these magmas, because assimilation of crust during magma ascent could also result in their observed geochemical features.

The Pikes Peak batholith is composed of at least two petrogenetically different granite types, both of which exhibit geochemical characteristics typical of A-type granites. Models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central Colorado at ~1.1 Ga.