A COMPARISON OF ND, SR, AND HF ISOTOPIC SIGNATURES FOR LATE CRETACEOUS AND PLIOCENE PLUTONIC ROCKS IN THE RICO MOUNTAINS, COLORADO: INSIGHT INTO MAGMATIC SOURCES AT 68 AND 4 MA

The Rico Mountains host 68 and 4 Ma plutons emplaced as stocks, sills, and dikes. The 68 Ma plutons are alkaline to calc-alkaline hornblende diorites to monzonites and are associated with Fe-Cu skarn mineralization, whereas the 4 Ma plutons are alkaline monzonites and are related to porphyry Mo deposits. Chemical distinctions hint at different magma sources for the two generations of plutons. In this investigation, bulk-rock Nd and Sr data along with zircon Hf data were employed to test this idea, and constrain potential melt sources.

The absence of 28-20 Ma plutons, widespread elsewhere in the western San Juan Mountains, distinguishes the Rico Mountains. The 68 Ma plutons originated from subduction-driven magmatism during the Laramide orogeny. Pliocene magmas are bimodal, and were generated during a period when mantle melts invaded the crust in a zone of incipient “extension” related to slab rollback after 25 Ma.

Late Cretaceous plutonic rocks are characterized by ε_Nd of -0.6 to -1.7, ⁸⁷Sr/⁸⁶Sr _(i) ratios of 0.704825 to 0.705923, and ε_Hf zircon signatures of -7.5 to 6.7. In contrast, Pliocene intrusive rocks have ε_Nd values of -6.3 to -6.5, ⁸⁷Sr/⁸⁶Sr _(i) ratios of 0.704825 to 0.705923, and ε_Hf zircon signatures of -4.7 to 2.8. Collectively, the distinct Nd and Hf data for Late Cretaceous and Pliocene plutonic rocks argue for different melt sources with overall similar “crustal” ⁸⁷Sr/⁸⁶Sr ratios.

The results of this investigation reveal that there was an extensive shift in magma sources in the Rico Mountains from 68 to 4 Ma. The presence of inherited Proterozoic zircons in the 68 Ma plutonic rocks suggests the incorporation of 1.8-1.3 Ga basement rock. We argue that the 68 Ma plutonic rocks were formed by partial melting of the lithospheric mantle or lower crust. Data suggest the 4 Ma magmas melted a more “evolved” crustal source during a period of elevated thermal gradients and emplacement of mantle magmas into the upper crust. Whether this period of magmatism involved melting of 68 Ma plutonic rocks or Proterozoic basement is uncertain. The different melt sources at 68 and 4 Ma could provide insight into the influence of magmas on base- and precious-metal mineralization in the area.