TESTING INCREMENTAL EMPLACEMENT OF A HYPABYSSAL SHEET INTRUSION: COPPER RIDGE LACCOLITH, HENRY MOUNTAINS, UTAH

Portions of many subvolcanic magma systems grow through injection of component pulses too frequent to be distinguished with radiometric geochronology. Evidence of these pulses in the rock record can be difficult to recognize, since differences between the separate injections in igneous texture, geochemistry, etc. may be subtle or non-existent. We present results from a detailed analysis of a hypabyssal sheet intrusion using four independent techniques to test for component magma injections: (1) crystal size distribution (CSD) of porphyry groundmass, (2) whole-rock major and trace element geochemistry, (3) anisotropy of magnetic susceptibility (AMS), and (4) paleomagnetic analysis. We apply these techniques to a suite of samples of porphyritic andesite collected from a well-exposed cross section through the ~400-m-thick Copper Ridge laccolith (CRL) in Utah’s Henry Mountains. The ~28 Ma laccolith was intruded at a depth of ~2 km as part of the larger Mount Ellen intrusive center, which was constructed from dozens of component intrusions similar to the CRL.

In the CRL, CSD results from porphyry groundmass demonstrate clear patterns in igneous texture that suggest the CRL grew through emplacement of magma injections with an approximate thickness of 100 m (i.e. ~4 injections in the 400-m-thick laccolith). Preliminary analysis of whole-rock geochemistry data suggests major elements are not useful to distinguish component sheets. However, trace element data suggest the presence of at least two magma batches with distinct rare earth element abundances. Preliminary analysis of AMS data shows that magnetic fabric orientations through the CRL cross section can be grouped as a function of elevation. Samples within a given elevation range have a similar fabric orientation, but the group of samples above or below has a distinct fabric orientation. Paleomagnetic data demonstrate the remanent magnetization direction varied by 139° during emplacement and cooling through the Curie temperature. If we interpret this as a record of paleosecular variation of the magnetic field, simple thermal models suggest the CRL must have grown through multiple injections of magma over several thousand years. Collectively, results from these techniques provide very strong evidence the CRL grew through injection of several component magma pulses over a period of several thousand years.