New Mexico Geological Society Annual Spring Meeting — Abstracts

New analyses of alkalic igneous rocks and carbonatites from the Lobo Hill alkalic complex near Moriarty, NM, provide new evidence for a widespread, Early Paleozoic, alkalic igneous event in New Mexico. The complex consists of brick-red fine-grained syenitic and carbonatitic dikes that intrude the Precambrian basement and clearly cross-cut the Precambrian foliations. Previous attempts at radiogenic isotope dating of the Lobo Hill complex were unsatisfactory; however, a new ⁴⁰Ar/³⁹Ar age determination on biotite from a syenite yields an age of 518 ± 5.7 Ma. Syenites have aphanitic porphyritic textures, with altered plagioclase phenocrysts set in felty to intergranular groundmasses; the rocks are almost devoid offerromagnesian minerals. Textures formed during rapid cooling indicate that the ⁴⁰Ar/³⁹Ar date can be interpreted as the age of emplacement and cooling at shallow depths. Na₂O and K₂O concentrations are highly variable. K₂O/Na₂O ratios vary from 0.34 to 2.96; two samples from the top of the complex have K₂O/Na₂O of ca. 130 as well as late-stage fluorite and high concentrations of Rb, Pb, Th, and Zn, and record metasomatism at the fringes of the system. Incompatible trace elements Y, Zr, and Nb increase with increasing SiO₂ in the suite while Ba and Sr decrease, suggesting that fractional crystallization of feldspar was the dominant differentiation process.

The Lobo Hill alkalic complex is one of several similar intrusions of Cambro-Ordovician age in New Mexico and Colorado. Although many of these complexes lack modern age determinations, the well-dated suites include the Florida Mountains alkali feldspar granite in southern New Mexico (503±10 Ma, U-Pb zircon, Evans and Clemons, 1988, AJS, 288:735-752; 491±5 Ma, ⁴⁰Ar/³⁹Ar hornblende, Ervin, 1998, MS thesis, NMSU), the Lemitar carbonatites (449±16, K-Ar biotite, McLemore, 1983, NMGS Guidebook, 34:235-240) in central New Mexico, and a volcanic rock from a depth of about 2800 m in an oil test well near Tularosa (541±21, K-Ar whole rock; Loring and Armstrong, 1980, Geology, 8:344-348). The chemistry of these rocks, and in some cases the short time interval between intrusion and deposition of the overlying Ordovician sedimentary rocks, suggests that they were all generated during continental rifting. Thus, we propose that an aulacogen, similar to the Southern Oklahoma aulacogen, existed in New Mexico during Cambrian and Early Ordovician time.