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
Authors
Otto I. Lang and David A. Gonzales, CPG-11266 Department of Geosciences, Fort Lewis College, Durango, Colorado
Otto Lang grew up in southeastern Idaho and is an undergraduate geology major at Fort Lewis College in Durango, Colorado. He is interested in geochemistry, volcanology, and igneous petrology. Otto hopes to pursue these fields in graduate school next year. In addition to a passion for geology, he also enjoys skiing, mountaineering, and traveling.
Abstract
Plutonic rocks were emplaced in the Rico Mountains in southwestern Colorado at ~68 Ma and ~4 Ma. The Late Cretaceous plutons were emplaced during subduction related to the Laramide orogeny whereas the Pliocene magmas formed in a period of incipient extension and bimodal magmatism attributed to slab rollback after 25 Ma.
Bulk-rock (Nd and Sr) and zircon (Hf) isotope data were employed to gain insight into the evolution of melt sources over
time. Nd(t) of -0.6 to -1.7, 87Sr/86Sr(t) ratios of 0.704525 to 0.705631 Hf (t)Nd(t) of -6.3 to -6.5, 87Sr/86Sr(t) ratios Hf (t) zircon signatures of -4.7 to 2.8.
The isotopic signatures, and the presence of inherited Proterozoic zircons in the ~68 Ma plutonic rocks is consistent with partial melting of the 1.8 to 1.3 Ga lithospheric mantle or lower crust. The ~4 Ma melts originated from melting of a more “evolved” crustal source in a period of elevated thermal gradients and emplacement of mantle magmas into the upper crust. The results reveal a shift in melt sources over time. This shift probably influenced the types of mineral deposits that formed in the region from 75 to 4 Ma.
Keywords: Isotope geochemistry, magmatism, igneous petrology, San Juan Mountains Introduction
Latest Mesozoic to Cenozoic magmatism on the Colorado Plateau and Southern Rocky Mountains (Fig. 1) is attributed to shallow subduction of the Farallon plate during the Laramide orogeny, followed by slab rollback and regional Neogene extension (e.g., Coney and Reynolds, 1977; Humphreys, 1995; Humphreys et al., 2003; Chapin et al., 2004; Smith et al., 2004; Farmer et al., 2008; Chapin, 2012). The western San Juan Mountains in southwestern Colorado expose 75 to 4 Ma felsic to mafic plutonic rocks (Gonzales, 2015; 2017) (Fig. 1). These plutonic rocks provide an opportunity to explore the temporal and spatial shifts of magmatism over the past 75 Ma, and offer insight into the timing and composition of magmatic events that influenced mountain building, mineralization, and geothermal activity in the region. Previous studies, for the most part, did not address the sources of magmas involved in these plutonic events.
In this investigation, isotopic signatures (Sr, Nd, and Hf) for the two generations of plutonic rocks in the Rico Mountains (Pratt, 1968; Pratt et al., 1969) were employed to investigate trends in magma sources over time. The Rico Mountains are an ideal location to address temporal variations in magma sources given that plutons were emplaced at the start (~68
50 TPG •
Jan.Feb.Mar 2019
Ma) and end (~4 Ma) of magmatism in the region (e.g., Tweto and Sims, 1963; Armstrong, 1969; Cunningham et al., 1994; Gonzales, 2015; 2017) and are in close spatial proximity. Our investigation reveals a shift in magma sources from ~68 to ~4 Ma and provides insight into melt production during and after the Laramide orogeny.
Geologic Setting
The Rico Mountains are located within the western San Juan Mountains along the southwestern edge of the southern Rocky Mountains (Fig. 1 inset). These rugged mountains expose Proterozoic basement overlain by Paleozoic to Late Cenozoic sedimentary rocks (Pratt, 1968; Pratt et al., 1969). The stratigraphic section is intruded by numerous felsic to mafic dikes, sills, and stocks (Cross and Spencer, 1900; Pratt et al., 1969; Gonzales, 2015) (Fig. 1).
The ~68 Ma intrusive rocks in the Rico Mountain (Gonzales, 2017) are part of a generation of plutons that formed from 75 to 60 Ma during the Laramide orogeny (e.g., Cunningham et al., 1994; Humphreys, 1995; Chapin et al., 2004; Chapin, 2012; Gonzales, 2015). These rocks are dominantly calc- alkaline (Gonzales, 2017), porphyritic monzonite to diorite (Cross and Spencer, 1900; Pratt, 1968; Pratt et al., 1969), and distinguished by 3 mm to 1 cm phenocrysts of hornblende and
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