Geochemistry of the Sierra Tinaja Pinta intrusion group.

Abstract

Rocks from four hypabyssal plutons in the Sierra Tinaja Pinta Mountains east of El Paso, and just west of the Guadalupe Mountains show a narrow compositional range in the major elements but are diverse with respect to in the trace elements. The rocks are divided into four groups I-IV based on natural breaks evident in variation diagrams. It is suggested that the rocks may be related by a liquid line of descent which is dominated for most elements by fractionation of alkali feldspar. Group I consists of one sample, low in SiO2, which bears an uncertain relationship to the other three groups. Modal mineralogy implies that initial compositions of the main II group were first modified by fractionation of an assemblage of alkali feldspar, clinopyroxene, biotite, opaques, and apatite. Major and trace element constraints require slightly different assemblages and isotopic evidence Barker, 1977 requires some assimilation, probably of Precambrian rhyolite. The CaO and MgO contents of rocks preclude assimilation of a significant amount of carbonate. This first stage produced the bulk of the diversity in the Mayfield Valley laccolith. The second main stage III-IV compositions form the most evolved group II compositions to the most evolved rocks found Miller Mountain, Th = about 45, La = 222, Ba = 330. Stage II fractionation assemblage contained alkali feldspar, biotite and amphibole. Although biotite is rare in the last two groups of rocks, it is necessary to accomodate the trace elements.

33 samples from this reference