ISOTOPIC SIGNATURES
Table 1. Location Information and General Features of ~68 Ma and ~Ma Plutonnic Rocks Collected in the Rico Mountains
Table 2. Strontium and Neodymium Whole-Rock Isotope Data for 68 AND 4 Ma Plutonic Rocks in the Rico Mountains
Cretaceous plutonic rocks is 1.74 whereas the mean value for Pliocene plutonic rocks is -2.26.
Discussion
This study advances our understanding of magma produc- tion in the western San Juan Mountains over the past 75 Ma. The variations in Sr, Nd, and Hf isotopic signatures (Figs. 2-3, Tables 2-3) for the ~68 and ~4 Ma plutonic rocks in the Rico Mountains argue for distinct magma sources.
The presence of inherited Proterozoic zircons in the ~68 Ma samples (Gonzales, 2015; 2017) indicate the incorporation of 1.8 to 1.3 Ga basement rocks in the magmas. The isotopic data (Tables 2-3) are consistent with melting of mafic Proterozoic
crust or continental lithospheric mantle (CLM) at ~68 Ma (Fig. 4). This is further supported by Nd time-evolution paths of 1.8 Ga mafic rocks in the region with signatures at ~68 Ma that are similar to those of Late Mesozoic plutonic rocks in the Rico Mountains (Fig. 2). A reasonable hypothesis is that subduction-related heat and fluids enabled melting of the CLM and lower crust (Fig. 4). This supports the idea proposed by Gonzales (2015) for the production and emplacement of calc- alkaline melts in the upper crust from 75 to 60 Ma.
Incipient extension in the western San Juan Mountains after 25 Ma was tied to slab rollback and formation of the Rio Grande rift (e.g., Lipman et al., 1970; Gonzales and Lake, 2017). Regional extension was accompanied by various stages of magmatism in the San Juan Mountains and surrounding region (Armstrong, 1969; Cunningham et al., 1994; Gonzales,
87Sr/86Sr(t) for plutonic rocks in the Rico Mountains. The isotopic evolution of
average crust in the region is from Gonzales and Lake (2017). CHUR stands for chondrite uniform reservoir, and MORB is the field for depleted mantle mid-ocean ridge basalts. The signature of regional continental lithospheric mantle in the western San Juan Mountains is identified as CLM. The area labeled Irving Formation is the isotopic trend of Paleoproterozoic mafic rocks in the area, time corrected to 68 Ma. The plot shows that ~68 Ma plutons were generated from a similar magma source as the CLM or melting of Proterozoic basement while ~4 Ma plutons were generated by melting of an upper crustal source.
52 TPG •
Jan.Feb.Mar 2019
~68 Ma and 4 Ma plutonic rocks in the Rico Mountains; the maxi- corner of the figure. DM represents the signature of the depleted mantle through time, and the CHUR denotes the chondrite uniform
Hf (t) values reveal a trend from less evolved sourc- es at ~68 Ma to more evolved crustal sources at ~4 Ma. These data reveal that Late Cretaceous plutons crystallized from a magmatic source close in composition to the CHUR and that, on average, Pliocene plutons were generated from a different and more evolved (i.e., upper crustal) source.
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