Research - On the Central Arizona Sill Complex


The Central Arizona Sill Complex (CASC) consists of diabase sills intruded into undeformed upper Proterozoic sediments and, to a lesser extent, the metamorphosed basement complex upon which these sediments rest. It includes the previously named Sierra Ancha Sill Complex. The area of outcrop extends from the Mogollon Rim on the north to the latitude of Tucson, Arizona, on the south, and between longitudes of approximately 110 degrees, 20 minutes, west and 112 degrees west (that of Phoenix, Arizona), though by far the greater volume of sills lies between the twin cities of Globe and Miami and the Mogollon Rim.

Careful examination of major- and trace-element data shows that there is no detectable contamination of chilled marginal rocks by the hosts, and that liquid compositions are therefore obtainable for these rocks. A related observation is that the sills are internally differentiated, most probably by flow differentiation in the case of mafic minerals. Plagioclase is also probably flow differentiated in one of the sills examined, but in two others its manner of differentiation is enigmatic.

Three distinct igneous suites are recognized on geochemical grounds: A Tholeiitic suite and Alkaline suites numbers one and two. Published radiometric ages of 1.1 Ga. for these rocks can be related only to Alkaline suite #2. All three suites probably fractionated from mantle-derived magmas, to no less than approximately 55% liquid remaining, in magma chambers located at the base of the crust. The alkaline suites were probably emplaced relatively rapidly at shallow levels in the crust and were probably not significantly contaminated by the continental crust through which they passed. The Tholeiitic suite fractionated also at shallow levels in the crust, and may have been contaminated by that crust. It is possible, if not probable at least for the Alkaline suites, that source volumes in the mantle were influenced by some previous subduction or mantle metasomatic event, or that the primitive liquids were in some way similarly influenced.

Geochemically, the rocks of the CASC are most like continental flood basalts: In particular, the correspondence with certain rocks of the Karoo series is remarkable.