GREENSTONE ROCKS IN THE MIMBRES VALLEY, CENTRAL NEW MEXICO: GEOLOGICAL SIGNIFICANCE FOR ARCHAEOLOGICAL PROVENANCE

The Mimbres Valley in New Mexico was a center of the Mogollon culture which was prevalent throughout much of the American southwest from ~1200 BC to ~1200 AD. Within the Mogollon lithic toolkit (projectile points, scrapers and cutters) were hafted greenstone axe heads. Due to the relative dullness of the cutting edge and availability of sharper lithics it has been suggested that the axes were ceremonial items. Such individual items, which may have held significance only to particular Mimbres communities, could provide a tool for modeling intercommunity trade and interaction. If the provenance of greenstone materials can be identified, then models for exclusivity and specialization of greenstone tools and their trade between Mogollon communities can be developed. One of the most probable sources of greenstone rock in the Mimbres valley is a transect of ~1.65 Ga mafic to ultra-mafic rock near San Lorenzo. Samples have evidence of an igneous mineral assemblage that included plagioclase + clinopyroxene ± orthopyroxene ± olivine ± amphibole. Many samples also have localized schistose domains and evidence of hydrothermal veining. Metamorphic minerals include chlorite, epidote, amphibole and quartz.

Compositional analysis confirms that the lithologies are mafic to ultra-mafic and probably originated in the mantle. Major and trace element discrimination diagrams indicate the magmas were tholeiitic, and may have evolved in an arc environment. However, spider diagrams of chondrite and MORB normalized trace element concentrations show a spread of values between MORB and OIB, particularly for the more mobile large ion lithophile elements. This suggests that post-crystallization hydrothermal affects probably altered the concentrations of some trace elements. A consequence is that it is currently not possible to discriminate between MORB and OIB sources for the rocks on the basis of composition alone. On the other hand, the samples show a broad diversity of rock-forming mineral assemblages, textures and compositional variation and the dataset provides a baseline against which the properties of greenstone tools can be compared. It is proposed that geochemical and petrographic analysis of greenstone tools with a well constrained provenance should provide information on whether the raw material for tool fabrication was sourced in the Mimbres valley.