PALEOPROTEROZOIC IGNEOUS ROCKS OF THE COLORADO FRONT RANGE AS INDICATORS OF SUBDUCTION ZONE PROCESSES DURING BASIN CLOSURE AND ACCRETION

Igneous rocks are common products of convergent tectonics such that understanding the timing and causes of their formation can provide vital insight to paleotectonic environments. The northern Colorado Front Range houses various enigmatic c. 1.8-1.7 Ga granodioritic and tonalitic igneous bodies. Bulk chemical analysis was performed on twelve granodioritic and thirteen tonalitic rock samples. U-Pb zircon dating via LA-ICP-MS of two granodioritic and two tonalitic samples was completed to gain insight on the tectonic relationship between these rocks.

While both rock suites exhibit peraluminous (ASI diagram), calc-alkaline (AFM diagram), and volcanic arc granite (Rb vs. Nb+Y diagram) chemistry, the granodioritic and tonalitic rocks are geochemically distinct with no intermediate chemistry present. Incompatible multi-element diagrams suggest the granodioritic rocks formed from typical subduction-related mantle flux melting, but that the tonalitic suite has characteristics of Tonalite-Trondhjemite-Granodiorite (TTG) suites, commonly associated with the Archean. Such TTG suites are often interpreted to have formed by the melting of a shallowly subducted slab. U-Pb Concordia diagram results were highly scattered due to inheritance and Pb-loss. However, when compiling results from both sets of samples, each had a cluster of concordant analyses younger than the c. 1751 Ma maximum deposition age of the country rock. ²⁰⁷Pb/²⁰⁶Pb weighted means of these concordant analyses provide intrusive ages of 1731 ±10 Ma for the granodioritic rocks and 1742 ±15 Ma for the tonalitic rocks.

Though not statistically robust, these results suggest that the tonalitic rocks are potentially c. 10 m.y. older than the granodioritic rocks. A possible tectonic model is that initial basin closure and accretion was associated with shallow subduction and generation of the tonalitic suite at c. 1740 Ma. Subsequently, the slab steepened and allowed for more typical arc-related magmatism, thus generating the granodioritic suite at c. 1730 Ma. Caveats to this model include the exact age relationship and whether the tonalitic suite was formed by the proposed processes.