Sheeting and dyking emplacement of the Gunnison annular complex, SW Colorado

Emplacement of the Proterozoic Gunnison annular complex, Colorado, involved brittle failure and both subhorizontal sheeting and steeply-dipping dyking . The annular complex consists of a central diorite body (1730 Ma), surroun ded by a ring of metamorphosed supracrustal rocks, in turn ringed by tonali te and granodiorite (1721 Ma). The older central diorite was emplaced as si lls parallel to bedding, prior to regional deformation of the Gunnison volc anic are, similar to 1730-1710 m.y. ago. This central body was deformed dur ing regional shortening, into an upright bowl with inward-dipping walls. Th e surrounding country rocks were folded and locally transposed against the central body, forming an arcuate foliation, conforming roughly to the shape of the body. This foliation and bedding acted as mechanical planes of weak ness, which localized the syn-deformational emplacement of the outer ring i ntrusions, as multiple magma sheets parallel to steeply dipping bedding and foliation. Portions of the outer rings were also injected along concentric , inward-dipping, shear fractures generated under high magma pressure, a pr ocess similar to cone sheet emplacement in volcanic ring-dyke complexes. Mu ltiple mechanisms were therefore involved in the emplacement of this sheete d annular plutonic complex, which otherwise superficially resembles a subvo lcanic ring-dyke complex. (C) 2001 Elsevier Science Ltd. All rights reserve d.