SHRIMP U-PB ZIRCON AGES AND ND SIGNATURES FROM CENTRAL COLORADO FRONT RANGE MIGMATITES AND RELATED IGNEOUS ROCKS: IMPLICATIONS FOR THE TIMING AND ORIGIN OF CRUSTAL GROWTH

Eighteen SHRIMP U-Pb zircon ages were determined on various samples of the Paleoproterozoic metamorphic basement of the central Colorado Front Range. Ages were obtained from 6 migmatites, including 2 melt phases, one selvage, and three whole-rocks in order to delineate the timing of migmatite formation as well as ages of protoliths. Also analyzed were 4 samples from layered gneisses (suspected metavolcanics) and 7 metaplutonic units (diorite to granite). Pre-metamorphic zones in zircon from several migmatites and layered gneisses yielded similar ages to those from two metaplutonic rocks, indicating a magmatic event at 1774 ± 15 Ma. One migmatite yielded a pre-metamorphic age at 1750 ± 8 Ma and two metadioritic units yielded ages of 1706 ± 8 Ma. Two of the layered gneisses, one metapluton, and a migmatitic melt phase yielded ages at 1693 ± 5 Ma that we interpret as the age of regional high-grade metamorphism and localized partial crustal melting. A monzogranite was dated at 1678 ± 6 Ma and yielded zircon core ages between 1800 and 1880 Ma (Trans-Hudsonian), a few at 2170 Ma, as well as some at 2500, 2570, and 2620 Ma. Similar inheritance was found in two migmatites. Initial εNd values (εNdi) from some of the dated samples vary from +4 to +1 with Nd model ages (T_DM) between 1800 and 1900 Ma. In contrast, three of the migmatites yielded εNdi between -0.5 and -1.0 at 1693 Ma with T_DM between 2050 and 2200 Ma. The range of these radioisotopic results indicate that mainly depleted mantle sources produced the central Colorado Paleoproterozoic basement at 1775 Ma, but also that some sources were contaminated by minor amounts of older crust. We speculate that the 1775-Ma basement was created within a volcanic arc environment and that older material was introduced as subducted detritus eroded from the nearby Archean Superior and Wyoming cratons and the Trans-Hudsonian terrane that binds them, and subsequently incorporated into arc-related magmas. These new SHRIMP ages place the protoliths of the central Front Range high-grade gneisses among the oldest known rocks within the Paleoproterozoic southwest of North America. In Colorado, the combination of old (ca. 1770 to 1780 Ma) zircon ages and mostly depleted Nd signatures (>+1.0) has otherwise been documented only in the Green Mountain Magmatic Arc of S Wyoming - N Colorado and the Needle Mountains of SW Colorado.