REE distribution and petrogenesis of the Spanish peaks igneous complex, Colorado

Jahn, Bor -ming, Sun, S.S. and Nesbitt, R.W. (1979) REE distribution and petrogenesis of the Spanish peaks igneous complex, Colorado. Contributions to Mineralogy and Petrology, 70 (3), 281-298. (doi:10.1007/BF00375358).

Abstract

The petrogenesis of the igneous complex in the Spanish Peaks region, Colorado, is reevaluated with Sr isotopic data (published and new) and new trace element data of REE and other transition metals. These data indicate that the diverse rock types ranging from ultrabasic lamprophyres to granitic porphyry of the igneous complex have independent origins.

The lamprophyric magmas could be derived from the upper mantle by small degrees of partial melting of garnet peridotite source(s). Limited intra-type crystal fractionation is responsible for the small differences in REE and other elemental abundances. The intermediate syenodiorites were also derived by partial melting of some mantle sources with garnet as a residual phase. However, the major and trace element data suggest that the source for the syenodiorites is distinctly different from that of the lamprophyres. The narrow range of K/Rb ratios observed in minettes (449–464), camptonites (384–450) and syenodiorites (370–460) suggest that amphibole fractionation is not important in the generation of these intra- and intertype rocks.

The granitic rocks were derived by melting of a lower crust whose composition varied from that of a granodioritic granulite to an amphibolite. It is suggested that the high alkali contents of the granitic porphyries are due to the introduction of alkalis into the source granulites shortly prior to the melting.

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