CRUSTAL HISTORY OF BIMODAL PALEOPROTEROZOIC SUITE IN CENTRAL COLORADO: NEW HF ISOTOPIC DATA

Paleoproterozoic bimodal volcanic rocks and associated granitic plutons in the Gunnison-Salida region of central Colorado have been proposed to represent arcs accreted to the Archean Wyoming Province during ca. 1.75 Ga growth of Laurentia. This hypothesis has recently been challenged because the rocks contain inherited ca. 1.85 Ga zircons, have Nd isotopic compositions indicative of involvement of an older crustal component in their petrogenesis, and exhibit the bimodal character suggestive of rifting and melting of pre-existing continental crust. To further investigate the involvement of older crust, we used a Laser-Ablation-MC-ICP-MS to measure the isotopic composition of Hf in igneous zircons from rhyolites and granites, and in a suite of detrital zircons from a Proterozoic quartzite. U-Pb ages of the igneous zircons were measured using an ion probe and range from ca. 1.785 Ga to ca. 1.664 Ga, encompassing samples from the Dubois Succession, the Cochetopa Succession, and a suite of younger granite intrusions. The detrital zircons have U-Pb ages tightly grouped around 1.74 +/-0.25 Ga and are considered a representative sample of zircons derived from Paleoproterozoic igneous rocks in the area. Hf isotopic data were obtained by ablating the same parts of the grains measured for U-Pb systematics in both igneous and detrital zircons. Using measured Lu/Hf ratios these data yielded calculated Lu-Hf TDM ranging from 2.12 Ga to 1.58 Ga, but averaging 1.83 Ga. More critically, 34 of 113 analyses yielded TDM > 1.84 Ga and 61 TDM > 1.80 Ga. Further, the average eHf (t) = 8.16 is distinctly less than the depleted mantle value of 10.2 at the typical crystallization age of ca. 1.75 Ga. These results are consistent with the presence of inherited ca. 1.85 Ga zircons and previously measured Nd isotopic compositions and indicate derivation of these Paleoproterozoic zircons and rocks from pre-existing crust or lithosphere of Trans-Hudson or Penokean age. The presence and involvement of older crust/lithosphere is a critical observation because it requires a modification of widely held views on the extent of the Tran-Hudson orogen and/or the geologic history of the accreted terranes of southwestern Laurentia.