TITANITE IN PEBBLE DIKES IN THE EAST TRAVERSE MOUNTAINS, CENTRAL UTAH, AND ITS IMPLICATIONS FOR INTRUSIVE AND HYDROTHERMAL SEQUENCING IN THE LITTLE COTTONWOOD INTRUSIVE COMPLEX

Titanite (Ttn) from pebble dikes in the East Traverse Mountains of central Utah can elucidate the crystallization history of a middle Cenozoic intrusive complex in the Wasatch Mountains. Based on textures, mineralogy, whole rock and titanite compositions, the clasts in the dikes were derived from the Little Cottonwood stock (~30 Ma), as well as the White Pine (~27.5 Ma) and Red Pine (~26 Ma) porphyries that crop out ~16 km to the east, where other pebble dikes cut the intrusion.

For example, electron microprobe analyses show that magmatic titanite from the LCS, WP, and RP groups are distinct based on Al₂0₃ concentrations: LCS 1.2-1.4 wt%, WP 1.4-1.6 wt%, and RP between 1.6-1.8 wt%.Using this criterion, most of the clasts in the East Traverse Mountains pebble dikes were derived from the in situ White Pine porphyry. Clasts of the LCS with their distinctive titanite compositions and textures are less abundant. These interpretations are supported by textural similarities and U-Pb zircon ages of the clasts. In addition, highly porphyritic and relatively unaltered Red Pine clasts are preserved in the pebble dikes; in situ exposures of this phase are rare and heavily altered.

Apparently, the pebble dikes formed contemporaneously with the younger Red Pine porphyry as it intruded the older solidified LCS and WP intrusions. The explosive emplacement of the pebble dikes involved hydrothermal fluids (either magmatic or meteoric or both) near the still hot intrusive complex. The pebble dikes were injected from the Red Pine porphyry and into the overlying White Pine and Little Cottonwood stock as well as the overlying sedimentary strata which are also represented by clasts in the dikes.

This interpretation reinforces the theory that the East Traverse Mountains were formed from a massive landslide off the Little Cottonwood stock. About 20 m.y. after dike emplacement and following a change from slab-roll back magmatism to Basin and Range extension, a mega-landslide transported the upper part of the intrusive complex including several of the pebble dikes away from the intrusive center, which subsequently underwent tectonic uplift, erosional denudation, and substantial glaciation.

Thus, these granitic clasts preserved in pebble dikes provide geochemical data for parts of the intrusive centers that have since been eroded away.