THE PALEOPROTEROZOIC MAZATZAL PROVINCE OF SOUTHERN NEW MEXICO: INSIGHT FROM HF ISOTOPE GEOCHEMISTRY OF ZIRCONS, U-PB ZIRCON DATING, AND ND WHOLE-ROCK GEOCHEMISTRY

We conducted geologic mapping in the Salinas Peak area of the San Andres Mountains and analyzed zircons for U-Pb ages and Hf isotopic compositions to test models for the formation of the southern Mazatzal province during the Paleoproterozoic. U-Pb zircon data seem to indicate three episodes of magmatism: ~1680 Ma, ~1650 Ma and ~1630 Ma. Salinas Peak consist of strongly deformed gneiss, schist, and amphibolite cut by younger granite dikes all of which lie nonconformably below the Cambro-Ordovician Bliss Sandstone. Foliations generally strike to the NW or W and dip (shallowly) to the S. Fold axes trend SW. The granites are unfoliated. An orthogneiss has a U-Pb zircon age of 1674 ± 16 Ma (all ± are 2σ) and a U-Pb titanite metamorphic age of 1410 ± 14 Ma that we infer occurred during reheating during the ~1.4 Ga magmatic event. Subhorizontal, apparently undeformed granite intrusions from this area have ages of 1672 ± 13 Ma and 1671 ± 18 Ma. Undeformed S-type plutonic rocks from Mud Mountain and the San Andres Mountains have ages of 1651 ± 21 Ma and 1647 ± 19 Ma respectively. A 1631 ± 12 Ma granite is also present in the San Andres Mtns. Deformation in the area may have occurred between the ~1680 Ma and ~1650 Ma magmatism events.

Zircons from intrusive igneous and metasedimentary rocks were analyzed for Hf isotopes and five samples were analyzed for whole-rock Nd isotopes. The 1674 orthogneiss from the Salinas peak area has ε_Hf(t) values between 2.2 and 6.9 (all uncertainties ± ~1). Granites, gneisses, and an amphibolite that have ages associated with the 1655 Ma period of magmatism generally have zircon ε_Hf(t) values between 5.9 to 11.8. Most of the ~1630 Ma granites have ε_Hf(t) values ranging from 5.9 to 12.2. Whole rock Nd geochemistry data was collected for granites from Mud Mountain, the Caballo Mountains, the Kingston District, the San Andres Mountains and one gneiss located in the San Andres Mountains. These samples have ε_Nd(t) initial values ranging from 1.2 to -2.0. Thus, the Mazatzal province in southern New Mexico is characterized by igneous rocks with felsic compositions, several with S-type mineralogy and geochemistry, widely ranging Hf isotopic signatures of Mazatzal age zircons, and initial Nd isotopic compositions near Bulk Earth or slightly negative. Crustal melting is a likely source for the majority of the ~1650 Ma rocks.