Jeffrey A. Grambling

Transcontinental Proterozoic provinces

Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Pressures and temperatures in Precambrian metamorphic rocks

ModelP-T estimates from nearly 100 Precambrian rocks have been derived by comparing published descriptions of metamorphic mineral assemblages to a standard set of mineral stability data. Pressures, temperatures and metamorphic geotherms show large variations. Nevertheless, modelP-T data reveal two distinct secular trends. Average metamorphic geotherms have declined through time, and average metamorphic pressures have increased through time. Metamorphic geotherms and pressures from Archean rocks average 54°C/km and 4.1 kbar, respectively. Corresponding figures for early Proterozoic terranes are 47°C/km and 4.3 kbar, and for late Proterozoic terranes 35°C/km and 6 kbar. These secular trends may reflect gradual cooling and thickening of the earths crust, at least in orogenic belts. There are exceptions to both trends, found in localities where mineral assemblages indicate high pressures (6–12 kbar) and low metamorphic geotherms (20–30°C/km). These assemblages formed in Archean crust that, at least locally, was thicker than 20 km during metamorphism.

Internally-consistent geothermometry and H2O barometry in metamorphic rocks: The example garnet-chlorite-quartz

Temperature and H2O activity can be determined with high precision using metamorphic mineral assemblages that define both a dehydration equilibrium and a temperature-sensitive cation-exchange equilibrium. Such determinations are obtained by applying the Gibbs method and then integrating two resulting differential equations, as illustrated here for the assemblage garnet-chlorite-quartz. The first equation, a geothermometer that monitors temperature based upon Fe−Mg exchange between garnet and chlorite, was calibrated using rocks at Pecos Baldy, New Mexico: 0=0.05 P(bars)−19.02 T(K)+4607 ln KD+24,156 with errors of ±8°C based upon analytical precision. The second equation monitors differences in the activity of water between specimens (1) and (2): 0=(0.1 XMg−chl, 1 − 2.05)(P2 − P1) +[−33.02+5.96 ln(XFe−chl, 1/Xalm, 1)][T2−T1 −2.67 RT1ln[a(H2O)2/a(H2O)1] +5.96 T1ln(XFe−chl, 2Xalm, 1/XFe−chl, 1Xalm, 2).For samples equilibrated at the same pressure and temperature, microprobe analytical errors of 1% limit precision to ±0.01 a(H2O). For samples equilibrated at the same pressure but variable temperature, uncertainty of ±8°C limits precision to ±0.06 a(H2O). Extreme presure sensitivity requires that the H2O-barometer be applied only to rocks where pressure gradients are absent or well-constrained. The geothermometer gives temperatures in agreement with two other garnet-chlorite geothermometers (Dickenson and Hewitt 1986; Ghent et al. 1987) and with garnet-biotite geothermometry (ferry and Spear 1978) over the temperature range 350–520°C. Application of the relative H2O barometer shows variations in the activity of water approaching 0.30 in several study areas. Either pelitic schists commonly equilibrate with a fluid that is not pure H2O, or some pelitic rocks undergo metamorphism in the absence of a free fluid phase.

40Ar/39Ar Evidence For 1.4 Ga Regional Metamorphism in New Mexico: Implications For Thermal Evolution of Lithosphere in the Southwestern Usa

40Ar/39Ar dates recorded by hornblende and muscovite from Proterozoic rocks in northern New Mexico indicate pervasive Mesoproterozoic metamorphism followed by a protracted uplift/cooling history. Most hornblendes yield 40Ar/39Ar dates of 1.43‐1.35 Ga, which record cooling through temperatures of ca. 500°C. This suggests that a regional amphibolite facies metamorphism (500‐550°C, 3.5‐4 kbars occurred at ca. 1.4 Ga. This metamorphism was superimposed on Paleoproterozoic amphibolite grade rocks at mid‐crustal depths of 10‐15 km. Contact metamorphic aureoles associated with 1.4 Ga plutons emplaced at 2.5‐4 kbars (8‐15 km) in the Sandia and Manzano Mountains record nearly concordant hornblende and muscovite 40Ar/39Ar dates of 1.4 Ga. These suggest relatively rapid cooling to ambient conditions of <300°C following the 1.4 Ga thermal event. Slightly younger (1.37‐1.31 Ga) dates are recorded by muscovite throughout northern New Mexico and by hornblende within high‐grade tectonic blocks. These are interpreted to record more protracted cooling following the 1.4 Ga metamorphism. Tectonic blocks with the highest metamorphic grades (both P and T) in northern New Mexico record the youngest hornblende and muscovite dates. Metamorphic grade decreases and 40Ar/39Ar mineral ages increase southward. This suggests differential unroofing (1.4‐1.0 Ga) of contrasting levels of a slowly cooled middle crust with a Mesoproterozoic (post‐1.4 Ga) thermal structure. Regional metamorphism at 1.4 Ga reflects fluid and magma advection into the crust during asthenospheric upwelling beneath southern Laurentia; 1.4 Ga metamorphism accompanied widespread but partitioned contractional deformation that affected interior Laurentia >1000 km from a probable compressive or transpressive plate margin.

A regional gradient in the composition of metamorphic fluids in pelitic schist, Pecos Baldy, New Mexico

AbstractTwo metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in

Crustal thickening during Proterozoic metamorphism and deformation in New Mexico

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Stratigraphic and structural relationships of multiply deformed Precambrian metamorphic rocks in the Rio Mora area, New Mexico

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