1100 MA RAPAKIVI GRANITES, LITTLE HATCHET MOUNTAINS, SOUTHWESTERN NEW MEXICO

The southern margin of Laurentia experienced prolonged and episodic convergent orogenesis, which ended at ~1100 Ma following the culmination of the Grenville orogeny. The southwestern boundary of the Laurentia continent is not well known because of small outcrop area and, locally, intense alteration. In the southern Little Hatchet Mountains, Hidalgo Co, NM, relatively unaltered outcrops of hornblende granite, rapakivi granite, and diabase form a pluton of 5 by 4 km. The diabase occurs as isolated enclaves, swarms of enclaves, and synplutonic dikes within the rapakivi granite. The coarse-grained hornblende granite, found in the southern Little Hatchet Mountains is orange to red-brown and consists of plagioclase, K-feldspar, quartz, hornblende, and rare biotite. The rapakivi biotite granite, found north of the hornblende granite is pink-gray to red-orange and medium to coarse grained and is characterized by large K-feldspar phenocrysts, many of which are mantled by plagioclase forming the rapakivi texture and has a U-Pb date of 1066 ± 8 Ma. The synplutonic diabase dikes and enclaves are dark gray, fine to medium grained, and porphyritic with dark mica, K-feldspar, and hornblende phenocrysts. The crenulate to cuspate margins of the dikes and many of the enclaves show evidence for intensive mingling with the rapakivi granite host.

The initial εc values of the Little Hatchet granites are higher (εNd +0.5 and +1.1) than the modeled evolution of country rocks or the regional Paleoproterozoic basement at 1066 Ma (εNd ~ -0.5). The granites also have significantly lower depleted mantle model ages (TDM ~1450 Ma) than the Paleoproterozoic rocks exposed at the current erosion level in southern New Mexico (TDM ~1700-1800 Ma). This suggests that the granites were generated by partial melting of a lower crustal source that was extracted from the mantle during the ~1450 Ma “anorogenic” granite event rather than during the Paleoproterozoic Mazatzal orogeny (1700-1650 Ma).