LAYERED INTRUSIONS AS SUBDUCTION ZONE ANALOGUES: GEOCHEMICAL AND ISOTOPIC EVIDENCE OF FLUID CIRCULATION IN THE STILLWATER COMPLEX, MONTANA

Dehydration of subducting slabs is responsible for melt generation in the mantle and the creation of volcanic arcs at the surface. How slab fluids enter and interact with the mantle wedge is poorly constrained, due to lack of direct observation. We have suggested that layered intrusions can act as proxies for subduction zones, as contact metamorphism of the country rock into which the layered intrusion is emplaced releases fluids which migrate upward into the hot ultramafic intrusion, much like fluids released from subducting slabs migrate upward into the mantle wedge. The 2.06 Ga Bushveld Complex and the 2.7 Ga Stillwater Complex are layered intrusions for which there is evidence of fluid involvement during intrusion formation. Evidence for fluid infiltration at Bushveld is clearer than at Stillwater due to the higher initial H₂O content of the previously unmetamorphosed sedimentary Bushveld country rock. At Stillwater, smaller-scale hydrothermal features such as pegmatoids have often been overlooked in formation models. The trace element geochemistry and isotopic composition of Stillwater pegmatoids, nearby host rock, and underlying country rock are here characterized. Sr isotopes are combined with new Nd and Pb isotope analysis to trace fluid circulation from the country rock. Sr isotope results show that the isotopic age of the country rock hornfels is the same as the intrusive rocks as expected, but that initial Sr ratios are also similar (⁸⁷Sr/⁸⁶Sr_i,hornfels = 0.69354-0.70503; ⁸⁷Sr/⁸⁶Sr_i,Stillwater = 0.69816-0.70933). In addition, H isotopes from Stillwater Complex mineral separates and bulk country rock samples suggest fluid circulation. Country rock samples average δD = -48‰, while pegmatoid pyroxenes range from δD = -102‰ to -71‰ (Ultramafic Series) and δD = -68‰ to -60‰ (Lower Banded Series). The Ultramafic Series, which is stratigraphically below the Lower Banded Series, is more similar to the δD of the mantle (roughly -80‰), suggesting fluids were more influential higher in the complex. A layered intrusion where crustal fluids migrated upward from cold country rock into hot ultramafic rock is a prime location to observe the physical processes of fluid migration, for future application at subduction zones.