AN INVESTIGATION OF THE AGE AND PETROLOGY OF THE TRIMBLE GRANITE, NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

The age and magmatic history of the Trimble Granite stock, which is exposed in the core of the Needle Mountains Proterozoic complex, was poorly constrained. This led to misinterpretation of its timing of emplacement and correlation with other magmatic events in the region. A new U-Pb zircon analysis constrains the crystallization of the Trimble Granite at 1457 ± 17.8 Ma. This redefines the timing of pluton emplacement, which was previously reported at 1350 ± 50 Ma from a Rb/Sr isochron analysis. The U-Pb zircon data reveal that the Trimble Granite correlates with older phases of the ~1440 Ma Eolus Granite, which hosts the Trimble Granite. Inherited 1750-1700 Ma zircons in the Trimble Granite also show that there was recycling of 1800-1700 Ma juvenile crust during Mesoproterozoic magmatism.

The Trimble Granite stock is a biotite-muscovite granite that has a crude zonation. The marginal zone adjacent to the Eolus Granite is porphyritic with microcline phenocrysts up to 2 cm that are set in a medium- to coarse- grained groundmass. This zone transitions towards the core of the stock into a fine- to medium-grained equigranular granite. The muscovite-biotite granite of the Trimble is distinct from most phases of the Eolus Granite which are dominated by biotite-hornblende or biotite granite, and are distinctly porphyritic. Greater magmatic recycling of 1800-1700 Ma basement may have contributed to the more peraluminous character of the Trimble Granite. The age range and compositional diversity of different phases within the Eolus Granite confirm that this is a composite batholith that was assembled by emplacement of granitic to dioritic phases over approximately 25 million years.