DECIPHERING THE TECTONIC SETTING OF THE PECOS MAFIC INTRUSIVE COMPLEX USING TRACE ELEMENT CONCENTRATIONS IN ORTHOPYROXENE

The Mesoproterozoic Pecos Mafic Intrusive Complex (PMIC) located in west Texas is composed of layered mafic and ultramafic rocks. The PMIC is characterized by cycles of magma recharge and mixing marked by up-hole reversals in orthopyroxene Mg# and fluctuating olivine abundance. Mesoproterozoic magmatism in western Texas may be associated with continental arc magmatism, back-arc magmatism, collision of a continental indentor, or midcontinental rifting. Previously reported whole-rock geochemistry and mineral compositions suggest that the PMIC is associated with midcontinent rifting.

Orthopyroxene in rock chips collected from the three lowest accessible cycles of the PMIC were analyzed using laser-ablation inductively-coupled mass spectrometry. Concentrations of Ni, Cr, and Mn in orthopyroxene fluctuate in concordance with previously-determined cycle boundaries. A bimodality in Cr concentration of orthopyroxene at each depth interval is related to rock type. Orthopyroxene analyzed from orthopyroxenite chips have Cr content between 3000 to 5000 ppm whereas orthopyroxene from norite chips have Cr content <1000 ppm. This suggests that the orthopyroxenite layers formed prior to the formation of the norite layers.

Rare earth element concentrations of the melt from which the orthopyroxene crystallized were calculated for each depth interval. The normalized rare earth element (REE) patterns for the melt at each depth interval are similar, with light rare earth element values between 100 to 1000 times chondritic values and heavy rare earth element values ranging from 1 to 10 times chondritic values. These REE patterns have steep, negative slopes that differ from accepted patterns for mid ocean ridge basalts and continental flood basalts. The REE patterns and noritic nature of PMIC magmas may result from crustal assimilation, partial melting of a LREE-enriched depleted mantle, or magmatism associated with a continental arc. The Mesoproterozoic tectonic setting of PMIC magmatism is therefore still unclear.