ISOTOPIC SIGNATURES
Table 3. Lutetium and Hafnium Isotope Data for Zircons from ~68 and ~4 Ma Plutonic Rocks in the Rico Mountains
Table 3. Lutetium and Hafnium Isotope Data for Zircons from ~68 and ~4 Ma Plutonic Rocks in the Rico Mountains (continued)
2015; Gonzales and Lake, 2017). Mantle magmas that invaded the upper crust after 25 Ma elevated thermal gradients (Armstrong, 1969) which led to production of small-volume crustal melts and bimodal magmatism in the western San Juan Mountains (Gonzales, 2015; Lake and Farmer, 2015; Gonzales and Lake, 2017) (Fig. 4). We propose that that the ~4 Ma plutons in the Rico Mountains crystallized from melts generated by partial melting of more evolved crustal rocks (Fig. 4). This is consistent with the mineral and chemical
Figure 4 - Simplified model to explain the variations in magma generation at ~68 and ~4 Ma in the Rico Mountains. The 68 Ma magmas formed as partial melts from the lower Proterozoic crust or CLM as invading hot fluids were released from subduction of the Farallon plate. Mantle melts invaded a zone of weak extension in the western San Juan Mountain from 25 to 4 Ma. The intrusion of melt increased the regional thermal gradient and contributed to partial melting of upper crustal rocks at ~4 Ma.
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