Abstract

The Black Range, located in southwestern New Mexico on the margin of the Basin and Range physiographic province, consists mainly of Tertiary volcanic rocks, which include mafic, intermediate, and silicic lavas, and silicic ash-flow tuffs. This report treats the petrology of basalt, basaltic andesite, and andesite. Basaltic rocks are alkalic and fall into two groups — late Oligocene and Miocene, which are quartz normative and contain quartz inclusions, and Pliocene basalts, which are SiO2 undersaturated and contain mafic and ultramafic inclusions. The andesite is Oligocene and Miocene in age and is potassic, having about 2.8 wt percent K2O at 60 wt percent SiO2; Oligocene latite is also present and has about 4.0 wt percent K2O at 62 wt percent SiO2. The Oligocene rocks have lower alkali and higher MgO and CaO contents than the Miocene rocks. The total alkali, FeO, and MgO proportions (AFM) of the middle Tertiary suite of basalt-andesite are like those of calc-alkalic suites. Clinopyroxene and orthopyroxene coexisting in basaltic andesite and andesite also suggest that these rocks are calc-alkalic. Microprobe analyses show that average plagioclase compositions in the basalt and andesite are labradorite and that the high alkali contents of these rocks are manifested in the mode by interstitial alkali feldspar.

Initial volcanism in early and middle Oligocene time was predominantly andesitic and was succeeded in late Oligocene and Miocene time by a suite of basalt, basaltic andesite, and andesite. On the basis of field relationships, the Oligocene andesite is interpreted as primary magmas, whereas younger andesite is differentiates of basaltic magma. Finally, only basaltic volcanism occurred in Pliocene time. The change in volcanism from intermediate to mafic is similar to the general trends of middle Cenozoic volcanism in the western United States. Compositional changes may have been related to physical changes in a subduction regime beneath New Mexico and progressive devolatilization.

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