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McLEMORE, Virginia T.1, RÄMÖ, O. Tapani2, and KOSUNEN, Paula J.2
1New Mexico Bureau of Geology & Mineral Resources
2Geology Dept., P.O. Box 11, FIN-00014 Univ. Helsinki, Finland


The southern margin of Laurentia experienced prolonged and episodic convergent orogenesis, which ended at ca. 1.1 Ga following the culmination of the Grenville orogeny. The southwestern boundary of the Laurentia continent is not well known because of small outcrop area and, in many areas, intense alteration. In the southern Little Hatchet Mountains, in Hidalgo County, NM, relatively unaltered outcrops of hornblende granite, rapakivi granite, and diabase form a pluton of 5 by 4 km of presumed Proterozoic age. 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-most Little Hatchet Mountains is orange to red-brown and consists of plagioclase, K-feldspar, quartz, hornblende, and rare biotite. The rapakivi 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. The diabase synplutonic 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 synplutonic dikes and many of the enclaves show evidence for intensive mingling with the rapakivi granite host.

Proterozoic bimodal rapakivi granite–basalt/diabase/anorthosite complexes are typically found as discordant plutons in many Precambrian shield areas worldwide. Usually they post-date the formation of the enclosing metamorphic crust by some hundred million years and register extensional (or transtensional) events related to mafic underplating and associated thinning of crust. Inasmuch as the exact age of the Little Hatchet Mountains rapakivi suite is not yet known, its significance in terms of the tectonic evolution of soutwestern Laurentia margin remains to be established. It may, however, register an extensional event following accretion at ~1.4 Ga in the central Mazatzal province.