PETROGRAPHY AND GEOCHEMISTRY OF MAFIC-INTERMEDIATE DIKES FROM THE NORTHERN SAWATCH RANGE, COLORADO

The Homestake shear zone (HSZ) is a subvertical, NE-striking structure that originated during Paleoproterozoic continental assembly and was reactivated under lower temperatures during Mesoproterozoic intracratonic transpression. Original geologic mapping (Tweto, 1974) differentiated numerous lithologically distinct dikes that are spatially associated with the shear zone. On the basis of field relations and lithology the dikes can be differentiated into several sets; we focused on metalamprophyres and hornblende diorites hosted within biotite gneiss and migmatite. The hornblende diorites are locally folded and include folded leucosomes that suggest they participated in an early phase of high-temperature melting. This high-temperature melting is consistent with an early 1.7 Ga deformation event in the HSZ. The metalamprophyre dikes crosscut the 1.7 Ga fabric in the host gneiss and show a distinct foliation sub-parallel to the dike walls. In order to better understand the geochemistry and petrography of the dikes, we analyzed 8 metalamprophyres and 4 hornblende diorites using EPMA, and used whole-rock XRF to determine major and trace elements. We also analyzed an additional suite of 27 REE’s and trace elements for each lithology using ICP-MS. The metalamprophyres average of 54.27% SiO₂, 12.58% Al₂O₃, 8.97% FeO, 8.63% MgO, 1.49% Na₂O, and 0.64% P₂O₅. In contrast, the hornblende diorites average 52.37% SiO₂, 16.3% Al₂O₃, 11.01% FeO, 6.94% MgO, 0.60% Na₂O, and 1.01% P₂O₅. The metalamprohpyres thus have greater abundances of MgO and Na₂O while the hornblende diorites have greater abundances of Al₂O₃, FeO, and P₂O₅. Some trace elements showed significant differences; metalamprophyres average of 241 ppm Ni, 580 ppm Cr, and 377 ppm Zr whereas the hornblende diorites average 128 ppm Ni, 177 ppm Cr, and 662 ppm Zr. These results show that the dikes have both distinctly different field relations and geochemistry; we suggest the hornblende diorites may be Paleoproterozoic and the metalamprophyres are younger and possibly related to a widespread Mesoproterozoic tectonothermal event that broadly coincides with reactivation of the HSZ.