IGNEOUS ROCKS AND CONSTITUENT HORNBLENDES OF THE HENRY MOUNTAINS, UTAH

Abstract

The dominant intrusive igneous rock in the Henry Mountains is diorite porphyry. This rock is chemically and mineralogically montonous through the stocks, laccoliths, bysmaliths, and dikes. The aggregate volume exceeds 16 cubic miles. Locally the diorite porphyry is cut by and grades into monzonite porphyry which is more variable in composition but constitutes only 5 per cent of the exposed rock. Most hornblende phenocrysts in the diorite porphyry are also chemically alike, with molecular proportions of FeO/MgO of about 0.9 to 1.1. In one coarse-grained laccolith the large hornblende phenocrysts have a molecular proportion of FeO/MgO of about 0.4. This more magnesian hornblende is compositionally similar to most hornblendes from the hornblendite and amphibolite inclusions in the diorite porphyry. Except for relatively high Na content, the diorite porphyry of the Henry Mountains could have been derived by the tapping of the upper portions of a typical andesitic parent magma from which early formed crystals of hornblende and pyroxene have settled. The monzonitic variant appears to represent a differentiate of the diorite. The inclusions are interpreted to be early formed mafic hoods or crystal segregates from the parent magma. Possibly, however, they are xenoliths of amphibolite or basaltic crust.