Orogenic to post-orogenic origin for the 1.76 Ga Horse Creek anorthosite complex, Wyoming, USA

The age and inferred tectonic setting of the 1.76 Ga Horse Creek anorthosite complex (HCAC) in the Laramie Mountains of southeastern Wyoming place important constraints on the origin of middle Proterozoic anorthosite complexes. The 100 km(2) HCAC consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite. U-Pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761.5+/-2 Ma and 1754.5+/-2.2 Ma, respectively. An additional period of zircon growth in the anorthosite occurred at 1753.4+/-2 Ma, as represented by a small population of anhedral zircon. We attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of Zr from ilmenite during reaction with plagioclase to form sphene. This reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite. The HCAC and the younger 1.43 Ga Laramie anorthosite complex to the north were intruded along or near a Paleoproterozoic suture zone, the Cheyenne belt, marking the boundary between Archean rocks of the Wyoming Province to the north and Proterozoic island are terranes to the south. We propose that the the HCAC was emplaced into young crust during or several million years after collision along the suture in an environment of late- to post-orogenic transtension. The presence of pre-existing crustal structures, especially Archean/Proterozoic boundaries, strongly influences the generation and emplacement of many middle Proterozoic anorthosite complexes.