CAMBRIAN-ORDOVICIAN ALKALINE ROCKS IN NEW MEXICO; EVIDENCE OF A PALEOZOIC AULACOGEN

Virginia McLemore, New Mexico Bureau of Geology and Mineral Resources,

November 2016

SUMMARY

 A widespread Cambrian-Ordovician alkaline magmatic event occurred throughout New Mexico and southern Colorado (FIg. 1, Table 1) and is evidenced by the intrusion of carbonatites, syenites, monzonites, and alkaline granites and associated K-metasomatism (episyenites). The term episyenite, as used by Leroy (1978), is used to describe rocks that were desilicated and metasomatized by hydrothermal solutions. These rocks now resemble syenites. The metasomatic episyenite consists of predominantly K-feldspar with accessory hematite, biotite, and plagioclase; quartz is rare to absent.

Only a few of these alkaline rocks have been well dated (Table 1; Loring and Armstrong, 1980; McLemore, 1983; Evans and Clemons, 1988; Ervin, 1998; http://geoinfo.nmt.edu/staff/mclemore/projects/documents/Riggins_SEG.pdf), but similar field relationships, textures, mineralogies, and geochemistries of these unmetamorphosed alkaline rocks suggest a common Cambrian-Ordovician age. The Cambrian-Ordovician magmatic event is well documented in southern Colorado and New Mexico (Fig. 1; McLemore, 1987b, 1989). Such alkaline magmatism is consistent with continental rift and aborted rift systems, although no Cambrian-Ordovician rift-basin sediments or rift-related structures have been observed for this time period in New Mexico (Fig. 2). Specific geographic boundaries of such a rift would be difficult to observe because of complex overprinting related to the ancestral Rocky Mountains orogeny, Laramide orogeny, and Rio Grande rifting and other tectonic events. Recognition of widespread Cambrian-Ordovician magmatic activity in New Mexico, evidence of relatively rapid uplift and erosion in the Florida Mountains (Evans and Clemons, 1988; Clemons, 1998; Ervin, 1998), and the presence of carbonatites (McLemore, 1983, 1987a) suggest that New Mexico was not a simple passive margin during the Cambrian-Ordovician; but rather experienced sufficient extension to perturb the mantle and initiate magmatism. Thus, we propose that an aulacogen, similar to the Southern Oklahoma aulacogen (Fig. 2; Lambert et al. 1988; McConnell and Gilbert, 1990), existed in New Mexico during Cambrian and Early Ordovician time. Additional studies are underway to fully characterize and evaluate the tectonic history during the Cambrian-Ordovician in New Mexico and southern Colorado.

TABLE 1-Known and possible Cambrian-Ordovician carbonatites and alkaline igneous rocks in New Mexico and southern Colorado. Locations are shown on Figure 1.

FIGURE 1: NMGS Carbonates_alkali NM, CO.jpg

FIGURE 2: tectonics(OK,TX,NM).jpg

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