Abstract:   The Catalina Intrusive Suite (”Catalina Granite”), forming the western end of the Santa Catalina Mountains north of Tucson, is apparently of mid-Tertiary age and post-dates the mylonitic deformation of older granites of the range.  Its semicircular outcrop  shape is bisected by the Pirate fault, though outliers are present in the Tortolita Mountains.  This shape and the presence of ring-dikes previously suggested concentric structure of otherwise homogeneous  elements.  This study however divides the Catalina IC into two units, the older of which  itself shows two domain.. The basal domain of the older unit , mostly of porphyritic coarse granite, contains two bands of problematic mafic segregations and dikes which suggests successive intrusions of  granite and mafics.   The upper domain of the older unit , mostly of quartz monzonite, shows gravity layering that dips NNE, like most of the Catalina IS’s country rocks.

The younger unit , of finer-grained leucocratic granite with a distinct xenolith assemblage, forms a nearly-continuous rim around  the older unit.  This rim may be considered a  three-dimensional carapace if some internal outcrops are part of a “lid”.  This rim/carapace is present only as sporadic thin rim-dikes on the north,  but thick on the east (the Reef of Rock ring-dike) and south.  That is, the Catalina IS is hinged on its northern margin.

The external shape of the Catalina IS  as a whole is insufficiently constrained in three dimensions from its outcrop pattern, despite excellent exposure with considerable topographic relief.  One possibility  is a NNE- tilted “bandshell”-shape (open to the Pirate fault), and if so the thick southern rim of younger leucogranite probably intruded a subsided southern margin of the older unit, in “trap-door” manner.

Despite excellent work now about 50 years old, recent work had lagged until Ducea  et al. (2020), and many features of the Catalina IS are still insufficiently known and deserve attention.  Among these are the age of the younger unit, and the rotation history of the older unit.