Significance of Chloritoid-Bearing and Staurolite-Bearing Rocks in the Picuris Range, New Mexico
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Significance of chloritoid-bearing and staurolite-bearing rocks in the Picuris Range, New Mexico M. J. HOLD AW AY Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 ABSTRACT INTRODUCTION In the Picuris Range near Taos, New Mexico, the Precambrian The Picuris Range is a small segment of the southern Sangre de Ortega Quartzite and Rinconada Formation are folded together Cristo Range which projects westward into the Rio Grande de- and share a common history. The Ortega contains chloritoid-Al pression a few kilometres southwest of Taos, New Mexico (Fig. 1). silicate and essentially no staurolite, and the Rinconada contains Folded Precambrian metasedimentary rocks trend east-west, are lo- staurolite-almandine—bearing assemblages with minor graphite. Al cally intruded by Embudo granite, and are surrounded on all sides silicate-bearing Rinconada rocks locally contain Mn-rich garnet. by younger rocks (Montgomery, 1953). All the rocks of the area crystallized at temperatures a little above The metasedimentary rocks include a lower series: from oldest to the Al silicate triple point, as indicated by the paragenetic sequence youngest, Ortega Quartzite, schist and quartzite of the Rinconada kyanite —» andalusite —* sillimanite. Analysis of the assemblages in Formation, and slate and phyllite of the Pilar Formation; and an relation to chloritoid, staurolite, and Al silicate stability data, tak- upper series: Vadito Formation schist, amphibolite, metaconglom- ing into account the mineral compositions and the effect of graphite erate, and metafelsite (Nielsen, 1972). An unconformity separates on fluid composition, shows that all the rocks crystallized at 532 ± the two series, but the significance of this break is not clear. The 20 °C and about 3,700 b Pal, but PHJP<ot was 0.88 or less in the younger Vadito appears less recrystallized, and aluminous minerals Rinconada rocks and near 1 in the Ortega rocks. This small differ- are less abundant than in the lower series, but the differences may ence in PHío appears to be the most important factor in explaining only reflect the less mature sediment typical of the Vadito. The the juxtaposition of chloritoid and staurolite in the two units. study described here involves only the Ortega Quartzite and the 105° 45' Figure 1. Generalized geologic map of part of Picuris Mountains, based on map of Nielsen (1972). Localities men- tioned in Table 4 and text: 1, Rattlesnake Gulch; 2, La Sierrita; 3, Hondo Canyon; and 4, Agua Caliente Canyon. Faults are not shown. Triangles = pelitic en- claves in Ortega Quart- zite. Geological Society of America Bulletin, v. 89, p. 1404-1414, 5 figs., 12 tables, September 1978, Doc. no. 80912. 1404 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/9/1404/3418800/i0016-7606-89-9-1404.pdf by guest on 01 October 2021 CHLORITOID-BEARING AND STAUROLITE-BEARING ROCKS, PICURIS RANGE 1405 overlying schist and quartzite of the Rinconada Formation. The Structural formulae of silicates were determined on the basis of pilar rocks are principally graphite-muscovite phyllite and slate. the numbers of oxygens in the ideal anhydrous formulae. Ratios of Their fine grain size and lack of porphyroblasts appear to relate to divalent ions in the octahedral or eightfold positions were deter- rock composition and abundance of graphite and quartz. They con- mined by correcting Fe for estimated Fe+3 content. Chloritoid that tain no chlorite or other minerals that are restricted to low grade. crystallized with hematite was assumed to have 9% of its Fe in the Montgomery (1953) described the complex folding about east- Fe+3 state, whereas the chloritoid that grew in the absence of west axes. He also noted sillimanite in the Ortega and staurolite in hematite or reducing oxides was assumed to have 7% Fe+3. These the Rinconada and concluded that the Ortega was metamorphosed corrections are based on Halferdahl's (1961) new analyses. Stauro- to higher grade. In about 1965 Charles Naeser discovered lite, biotite, and almandine all grew with graphite and ilmenite. chloritoid in the Ortega Quartzite, indicating that it could not be These were given minimum corrections of 5%, 3%, and no correc- higher in grade, but could, in fact, be lower in grade than the Rin- tion, respectively, in accord with analyses given by Deer and others conada schists. (1962) for these minerals found in reducing assemblages. Inspec- Detailed structural analysis by Nielsen (1972), combined with tion of the structural formulae (Tables 5, 8, 9) shows that site oc- age dating by Fullagar and Shiver (1973) and Gresens (1975), cupancies are slightly improved if the above proportions of the Fe permits reconstruction of a tentative tectonic-metamorphic se- are added to the Al sites and subtracted from the Fe+2, Mg, Mn quence. During low-grade metamorphism there were two penetra- sites. The correction for ferric Fe produces a 1 to 2 mole percent tive events, one before and one after emplacement of the Embudo reduction in ratios involving Fe in the numerator. granite (1,673 m.y. ago). The first event produced a schistosity that nearly parallels bedding planes; the second event imposed an axial PETROLOGY AND MINERAL CHEMISTRY plane schistosity that locally transposes the earlier surface. In thin- section analysis of samples, evidence of these two events cannot be For the purpose of this report, quartzite and aluminous musco- clearly separated. Combined, the events produced the observed pre- vite quartzite are grouped as Ortega Quartzite. With rare ex- ferred orientation in most of the muscovite and in some of the bio- ceptions, the first staurolite-bearing or biotite-bearing rocks mark tite, kyanite, and chloritoid. One or, perhaps, two later events the bottom of the Rinconada Formation, which contains more (1,325 and 1,257 m.y. ago) were primarily thermal. Some of the pelitic schists than quartzites. This contact is easily seen in the field kyanite, chloritoid, and muscovite probably recrystallized during and has considerable petrologic significance. In two localities en- the later event(s), and staurolite, almandine, andalusite, sillimanite, claves of pelitic composition occur within the Ortega Quartzite and additional biotite grew at this stage. The thermal event(s) pro- (Fig. 1). Mineral abbreviations and formulae are given in Table 1, duced coarse, undeformed mineral growth in all units, and and mineral assemblages are given in Tables 2 and 3. presumably all minerals with the exception of kyanite approached equilibrium at this stage. Two later mild events occurred, the first Ortega Quartzite associated with limited retrogression of biotite and staurolite to chlorite and almandine to iron oxides. Quartzite. Most of the Ortega Quartzite is massive blue-gray This report summarizes the petrology and mineral chemistry of cross-bedded quartzite. In places it is red from hematite. Locally the the Ortega and Rinconada Formations and explains the immediate quartzite is rich in Al, K, and Fe, leading to development of miner- juxtaposition of chloritoid-Al silicate in the Ortega and als useful as metamorphic grade indicators. Aluminous layers may staurolite-almandine in the Rinconada. Ganguly (1969) and Richardson (1968) showed that in the simple system Al203-Fe0- Si02 these two assemblages are separated by two chemical reac- TABLE 1. MINERAL ABBREVIATIONS AND FORMULAE tions. Albee (1972, Fig. 14) showed that in pelitic compositions the Mineral Abbreviation Formula assemblage Fe chloritoid-Al silicate-muscovite (representative of Ortega) must pass through five successive topology changes before Quartz Qz Si02 staurolite-almandine-biotite-muscovite (representative of Rin- Kyanite Ky Al2Si05 conada) becomes stable. Clearly, if the two units crystallized at the Andalusite And Al2Si05 Sillimanite Sill Al Si0 same pressure and temperature, factors such as fluid composition 2 5 Muscovite Mus KAl3Si3O10(OH)2 and bulk-rock composition are responsible for the close proximity Paragonite Par NaAl3Si3O10(OH)2 of these assemblages in the Picuris Mountains. Biotite Biot K0.84(Fe, Mg)2.52Al1.72Si2.74O10(OH)2 Specimens have been collected from the Hondo Canyon area, Almandine Aim (Fe, Mg)3Al2Si3Oi2 Spessartite- Agua Caliente Canyon, Rattlesnake Gulch, and the area north of almandine Spes (Fe, Mn, Mg)3Al2Si3012 t La Sierrita (Fig. 1). About 110 thin sections have been studied in Staurolite St (Fe, Mg)2Al9Si4022.5(0H)2 detail. Minerals in 16 specimens have been analyzed by electron Chloritoid Ctd (Fe, Mg)Al2Si05(0H)2 microprobe (ARL-EMX, University of Texas at Dallas), using Plagioclase Plag (Na, Ca)(Al, Si)4Os Mn ferrogedrite Ged (Fe, Mn, Mg) Al Si 0 (0H) natural silicate standards. For each mineral except muscovite, two 5 4 6 22 2 Rutile Rut Ti02 different grains were analyzed at six closely spaced points on each. Hematite Hem Fe203 For muscovite, one analysis of six points was done. Analytical data Ilmenite Urn FeTi03 were converted to chemical analyses using the data reduction pro- Magnetite Mag Fe304 gram of J. Rucklidge and E. L. Gasparrini (unpub. data). Most Graphite Gra C analyses total to between 98% and 102% when water of hydration Guidotti and others' (1975) average. is taken into account. Accuracy of major elements is estimated to be + Griffin and Ribbe (1973). Oxygen has been increased by 0.5 to produce ±2% to 3% of the amount present, with A1 and Si the least accu- electrostatic balance. Note that this formula is 47/46 times the structural rate. formula. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/9/1404/3418800/i0016-7606-89-9-1404.pdf by guest on 01 October 2021 1406 M. J. HOLDAWAY be traced locally; there are kyanite-rich rocks in Hondo Canyon Minerals were analyzed from four specimens of quartzite (Table and andalusite-rich layers in Rattlesnake Gulch. 4). Chloritoid (Table 5) is Fe-rich and has a higher Fe+2/(Fe+2 + The aluminous quartzite contains muscovite and various combi- Mg) ratio when it occurs with hematite than when hematite is ab- nations of A1 silicate, chloritoid, and hematite (Table 2).