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ISOTOPIC SIGNATURES


University of Colorado using thermal ionization mass spectrometry (TIMS) with a Finnigan-MAT 6-collector solid source mass spectrometer. Details on the preparation of samples for analyses are reported in Farmer et al., (1991) and Mueller et al., (2017 a, b). A discussion of analytical errors associated with these analyses are presented in Gonzales and Lake (2017).


Figure 1 - Generalized geologic map of the Rico quadrangle modified from Pratt et al., (1969). The map shows the distribution of Late Cretaceous and Pliocene plutonic rocks. The locations where rock samples were collected for this study are noted by EM (Elliot Mountain), EX (Expectation Mountain), SC (Scotch Creek), CP(Calico Peak), and PR (Priest Creek). The inset shows major geologic and physiographic provinces: NVF (Navajo volcanic field), SJB (San Juan Basin), and SRV (Southern Rocky Mountain volcanic field).


plagioclase set in a fine-grained matrix of quartz and plagioclase.


The ~4 Ma plutonic rocks are the youngest felsic plutons in the western San Juan Mountains. They are distinctly porphyritic alkaline (Gonzales, 2017) granodiorites and monzonites with 3 mm to 5 cm phenocrysts of quartz, orthoclase, sodic plagioclase ± biotite in a fine-grained, felty matrix.


The Rico Mountains host extensive mineralization that formed largely from the interactions of plutons with Paleozoic to Mesozoic strata, causing thermal metamorphism and metasoma- tism. Skarn and epithermal vein depos- its were mined for zinc, lead, copper, silver, and gold; porphyry Mo deposits are hosted by ~4 Ma plutons near Calico Peak and in the subsurface in the eastern part of the Rico Mountains (e.g. Cross and Spencer, 1900; Pratt et al., 1969; McKnight, 1974; Barnes, 1985; Cunningham et al., 1994; Wareham et al., 1998;). The Rico Mountains also


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contain several thermal springs situated along deep-seated, west- to northwest- trending faults (Pratt et al., 1969; Sare et al., 2009; Easley and Morgan, 2013).


Methods


Representative samples of ~68 Ma and ~4 Ma plutonic rocks were col- lected for this study (Fig. 1, Table 1). Reconnaissance field surveys were conducted to assess outcrop features and obtain samples for isotopic analy- ses. Mineral assemblages and textural features were compiled from detailed petrographic studies to classify and characterize the rocks. Sr and Nd isoto- pic bulk-rock data were complemented with Lu-Hf isotope analyses on indi- vidual zircons (Tables 2-3). Collectively, the bulk-rock and zircon isotopic data provide a basis to assess the sources of magmas that produced the two genera- tions of plutons.


Sr and Nd isotopic data (Table 2) were obtained from bulk-rock samples at the


Zircons used for Lu-Hf analyses (Table 3) were extracted using stan- dard separation procedures described by Gonzales (2017). The mounted zir- cons were analyzed at the University of Arizona LaserChron Center through laser ablation and inductively coupled plasma mass spectrometry (LA-ICPMS) using a Nu Plasma HR ICP-MS, together with a New Wave 193 nm ArF laser abla- tion system. The zircons were analyzed for 176Hf/177Hf ratios at the same spots selected for U-Pb age determinations (Gonzales, 2017) to determine time- corrected ratios. Laser spots had a diameter of 40 microns and depth of ~40 microns. Eight standards were analyzed twice after every five analyses of zircon crystals in the samples. The standards ensured accurate Hf ratios, and were used to minimize the instrumental drift effects and assess the precision of the analytical results. The unknowns were adjusted to the measured standards and  from a filtered table of results (Table 3). A complete and detailed description of the analytical methods associated with the conducted analyses can be found online (University of Arizona, 2017).


Results


The measured 143Nd/144Nd ratios of the Laramide plutonic rocks (Tables 1-2) range from 0.512513 to 0.512567


  Nd(t) signatures of -0.6 to -1.7 (Fig. 2). Pliocene rocks yield 143Nd/144Nd ratios of 0.512300


    Nd(t) values of -6.3 to -6.5.


Differences in the Sr isotope signa- tures between the two generations of plutonic rocks are less distinct (Fig.


2). Time-corrected 87Sr/86Sr(t) ratios for the ~68 Ma plutonic rocks range from 0.704525 to 0.705631 whereas the ratios for ~4 Ma rocks range from 0.705736 to 0.706143 (Fig. 2, Table 2).


Age-corrected Hf(t) values are -4.7 to 2.8 for the Pliocene plutonic rocks, and


-7.5 to 6.7 for the Late Cretaceous rocks


to the relatively tightly clustered Hf (t) values of the ~4 Ma rocks (Fig. 3). The       


Hf (t) values for the ~68 Ma rocks are highly scattered and showed a greater range compared


Jan.Feb.Mar 2019 • TPG 51


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