This paper presents a plate-scale model for the Precambrian growth and evolution of the North American continent. The core of the North American continent (Canadian shield) came together in the Paleoproterozoic (2.0–1.8 Ga) by plate collisions of Archean continents (Slave with Rae-Hearne, then Rae-Hearne with Superior) as well as smaller Archean continental fragments (Wyoming, Medicine Hat, Sask, Marshfield, Nain cratons). The resulting Trans-Hudson orogen was a collisional belt similar in scale to the modern Himalayas. It contains mainly reworked Archean crust, but remnants of juvenile volcanic belts are preserved between Archean masses. The thick, buoyant, and compositionally depleted mantle lithosphere that now underlies North America, although dominantly of Archean age, took its present shape by processes of collisional orogenesis and likely has a scale of mantle heterogeneity similar to that exhibited in the overlying crust.

In marked contrast, lithosphere of southern North America (much of the conti nental United States) was built by progressive addition of a series of dominantly juvenile vol canic arcs and oceanic terranes accreted along a long-lived southern (present coordinates) plate margin. Early juvenile additions (Pembine-Wausau, Elves Chasmarcs) formed at the same time (1.84–1.82 Ga) the core was assembling. Following final assembly of the Archean and Paleoproterozoic core of North America by 1.8 Ga, major accretionary provinces (defined mainly by isotopic model ages) were added by arc-continent accretion, analogous to present-day convergence between Australia and Indonesia. Also similar to Indonesia, some accreted terranes contain older continental crustal material [Archean(?) Mojavia], but the extent and geometry of older crust are not well known. Accretionary provinces are composed of numerous 10 to 100 km scale terranes or blocks, separated by shear zones, some of which had compound histories as terrane sutures and later crustal-assembly structures. Major northeast-trending provinces are the Yavapai province (1.80–1.70 Ga), welded to North America during the 1.71–1.68 Ga Yavapai orogeny; the Mazatzal province (1.70–1.65 Ga), added during the 1.65–1.60 Ga Mazatzal orogeny; the Granite-Rhyolite province (1.50–1.30 Ga), added during the 1.45–1.30 Ga tectonic event associated with A-type intracratonic magmatism; and the Llano-Grenville province (1.30–1.00 Ga), added during the 1.30–0.95 Ga broader Grenville orogeny. During each episode of addition of juvenile lithosphere, the transformation of juvenile crust into stable continental lithosphere was facilitated by voluminous granitoid plutonism that stitched new and existing orogenic boundaries. Slab roll back created transient extensional basins (1.70 and 1.65 Ga) in which Paleoproterozoic quartzite-rhyolite successions were deposited, then thrust imbricated as basins were inverted. The lithospheric collage that formed from dominantly juvenile terrane accretion and stabilization (1.8–1.0 Ga) makes up about half of the present-day North American continent. Throughout (and as a result of) this long-lived convergent cycle, mantle lithosphere below the accretionary provinces was more hydrous, fertile, and relatively weak compared to mantle lithosphere under the Archean core.

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Tectonic model for the Proterozoic growth of North America

The apatite-group minerals of the general formula, M10(ZO4)6X2 (M = Ca, Sr, Pb, Na…, Z = P, As, Si, V…, and X = F, OH, Cl…), are remarkably tolerant to structural distortion and chemical substitution, and consequently are extremely diverse in composition (e.g., Kreidler and Hummel 1970; McConnell 1973; Roy et al. 1978; Elliott 1994). Of particular interest is that a number of important geological, environmental/paleoenvironmental, and technological applications of the apatite-group minerals are directly linked to their chemical compositions. It is therefore fundamentally important to understand the substitution mechanisms and other intrinsic and external factors that control the compositional variation in apatites.

The minerals of the apatite group are listed in Table 1⇓, and representative compositions of selected apatite-group minerals are given in Table 2⇓. Also, more than 100 compounds with the apatite structure have been synthesized (Table 3⇓). Phosphate apatites, particularly fluorapatite and hydroxylapatite, are by far the most common in nature and are often synonymous with “apatite(s)”. For example, fluorapatite is a ubiquitous accessory phase in igneous, metamorphic, and sedimentary rocks and a major constituent in phosphorites and certain carbonatites and anorthosites (McConnell 1973; Dymek and Owens 2001). Of particular importance in biological systems, hydroxylapatite and fluorapatite (and their carbonate-bearing varieties) are important mineral components of bones, teeth and fossils (McConnell 1973; Wright et al. 1984; Grandjean-Lecuyer et al. 1993; Elliott 1994; Wilson et al. 1999; Suetsugu et al. 2000; Ivanova et al. 2001).

View this table:

 Table 1. 
Summary of the apatite-group minerals



View this table:

 Table 2. 
Representative compositions and unit-cell parameters of selected apatite-group minerals.



View this table:

 Table 3. 
Formulas of selected synthetic compounds with the apatite structure.



Following Fleischer and Mandarino (1995), Table 1⇑ also includes melanocerite-(Ce), tritomite-(Ce), and tritomite-(Y), the compositions …

Compositions of the Apatite-Group Minerals: Substitution Mechanisms and Controlling Factors

GSA Today: v. 16, no. 2, doi: 10.1130/1052-5173(2006)016 2.0.CO;2 INTRODUCTION Forty years ago, the eastern margin of North America inspired Tuzo Wilson (1966) to ask, “Did the Atlantic close and then re-open?” The Wilson cycle of closing and opening of ocean basins incorporates the cyclic assembly and breakup of supercontinents. Alternate processes of extension and compression of continental margins suggest an important potential for tectonic inheritance and overprinting. Now, we recognize a succession of two complete Wilson cycles in eastern North America: closing of an ocean and assembly of the Rodinia supercontinent, breakup of Rodinia and opening of the Iapetus Ocean, closing of Iapetus and assembly of the Pangaea supercontinent, and breakup of Pangaea and opening of the Atlantic Ocean (Fig. 1). Precambrian rocks of cratonic North America indicate less well-defined, earlier cycles. Tectonic inheritance at a range of scales has been recognized in the successive continental margins preserved within the crust of present eastern North America, posing several fundamental questions. Does each episode of supercontinent assembly and breakup adapt to the tectonic framework of a preexisting continental margin and, in turn, leave a mold for the next episode? Is tectonic inheritance through successive Wilson cycles a first-order constraint on the processes through which continental crust is accumulated and continental fabrics evolve? Does tectonic inheritance in the shallow crustal structures reflect a pervasive fabric of the deeper lithosphere?

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Tectonic inheritance at a continental margin

The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium

The rare earth element (REE) geochemistry of various phases from the active TAG hydrothermal mound has been examined and related to their mineralogy and fluid chemistry. The mound deposits range from black and white smoker chimneys, massive anhydrite/sulphide mixtures, oxides, and ochres. All phases, except black smoker chimney anhydrite, demonstrate a positive Eu anomaly when normalised to chondrite REE values. REE substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all REE other than Eu. Precipitation of anhydrite within the TAG mound is the major mechanism for removal of REE during mound circulation and 0.15-0.35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys. Oxides from the mound fall into three different categories with distinct REE patterns: oxide rims on sulphides, atacamite-bearing oxides, and silica-rich Fe-oxides and ochres. The oxide rim phases contain sulphide and seawater derived REEs whereas the atacamite-bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from, or alteration by, a modified hydrothermal fluid.

Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, 26°N Mid-Atlantic Ridge

We report field, petrographic, mineralogical, and geochemical data for two P-rich rock types of the anorthosite suite—Fe-Ti oxide-apatite rock (or nelsonite) and Fe-Ti oxide-apatite gabbronorite (or oxide-apatite gabbronorite). These rocks are take from the Château-Richer, Labrieville, and St. Urbain massifs in Quebec; from the Roseland massif in Virginia; and from the Carthage massif in New York. Our data set includes X-ray fluorescence (XRF) and instrumental neutron activation (INA) analyses of whole-rock samples, as well as electron-microprobe (EMP) and INA analyses of separated apatites.

At St. Urbain and Labrieville, nelsonite forms a small capping on massive hemoilmenite deposits fully contained within anorthosite, whereas at Roseland, nelsonite occurs as dikelike bodies within country rock. All three nelsonite occurrences consist almost entirely of ilmenite + apatite (but do not have a 2:1 oxide/apatite ratio), with only trace amounts of other minerals. All oxide-apatite gabbronorites form sill-like layers within or near the margins of anorthosite massifs and contain ilmenite ± magnetite, apatite, orthopyroxene + clinopyroxene, and plagioclase.

All apatites are F rich and Cl poor, consistent with high-temperature magmatic crystallization. Concentrations of apatite rare earth elements (REE) are highly variable (e.g., LaN ≈ 700–2,000× chondrites), but patterns show a consistent fractionation (e.g., LaN/LuN ≈ 10–20) with small to moderate negative Eu anomalies. Interestingly, the Labrieville nelsonite and its apatite lack an Eu anomaly, consistent with crystallization in a highly oxidizing environment.

Apatites in both rock types are enriched in REE relative to whole-rock samples (by factors of 3 to 10), but REE patterns of apatites and whole rocks are parallel in each case. Mass-balance calculations indicate that for nelsonite, 100 percent of the REE is contained in apatite; for oxide-apatite gabbronorites, ~100 percent of the light REE, ~90 percent of the middle to heavy REE, and ~75 percent of Eu are in apatite. These results can be explained satisfactorily by assuming that the rocks are cumulate in origin. If, on the other hand, nelsonites and oxide-apatite gabbronorites represent liquids, apatite DREE values must vary by more than a factor of 3.

Our field, mineralogical, and geochemical observations indicate that most nelsonites and oxide-apatite gabbronorites are not related directly. Indeed, apatites from oxide-apatite gabbronorites contain lower Sr and MgO but higher REE than those in nelsonites, indicating that nelsonites are the more primitive rock type. Nelsonites probably formed as cumulates primarily within larger deposits of Fe-Ti oxide (locally mobilized as dikelike bodies). oxide-apatite gabbronorites also represent cumulates, but these are related spatially and genetically to associated jotunites and mangerites and appear to form at several stages in the evolution of a particular anorthosite complex.

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Petrogenesis of Apatite-Rich Rocks (Nelsonites and Oxide-Apatite Gabbronorites) Associated with Massif Anorthosites

We report whole-rock, major- and trace-element compositions (obtained by XRF and INA methods) for the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, SW Greenland. In a previous petrologic study on the same sample suite, we documented differences in texture, mineralogy, and mineral compositions between these two anorthosite bodies. Chemical analyses confirm differences in composition between the two bodies, but these differences cannot be explained by variations in metamorphic conditions, and point towards differences in the nature of their protoliths. Analyzed Nordland samples are anorthosites and leucogabbros with 88–98% normative plagioclase, whereas those from Buksefjorden include anorthosite, leucogabbro, and gabbro with ~55–95% normative plagioclase. Two or more compositional groupings can be recognized at each site, which correspond to differences in color and mineralogy of the hand samples. Samples from Buksefjorden are mainly quartz-normative, whereas those from Nordland are olivine (± nepheline) normative. Other differences include higher Ni/Co ratio and REE contents in the granulite-facies anorthosites from Nordland. REE pattern shapes are similar, however, being moderately fractionated at ~0.5–10× chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50–150× chondrites, suggesting unfractionated (but evolved) parental magmas. Olivine must have been present in the protoliths of the Nordland rocks compared with Buksefjorden. Otherwise, the protoliths contained plagioclase with variable An-content (~An62–An92) and a mafic component with variable Fe/Mg (mg ~0.3–0.8). This mafic component was either hornblende or a combination of ortho- and clinopyroxene in fixed proportions, plus a small amount of magnetite. Mixing calculations demonstrate that some Buksefjorden anorthosites contain two varieties of plagioclase: a calcic type that may correspond to cumulus crystals, and a sodic-type that may correspond to a trapped-melt component. On plots of normative whole-rock An versus mg, compositions of the Buksefjorden and Nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions (Kiglapait, Skaergaard) and from the generally flats trends of plagioclase-rich cumulate rocks (St. Urbain and Stillwater anorthosites). This difference provides further evidence for the distinctive nature of Archean calcic-anorthosite complexes compared with other types of mafic intrusions. Moreover, this distribution of data points is consistent with the assembly of the protolith of the SW Greenland anorthosites mainly as mixtures of plagioclase and hornblende. Finally, the field for the Buksefjorden and Nordland anorthosites overlaps only slightly with that for the Fiskenaesset Complex, thus extending the known range of compositions for Archean anorthosites in West Greenland.

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

The Labrieville massif (>1010 Ma) is an eroded dome consisting mineral—plagioclase feldspar. The circumstances whereby such nearly monomineralic plutons can be produced of three zones: (1) a foliated inner core of pink anorthosite (>An30Or13, >2100 ppm Sr, >1300 ppm Ba) with minor have puzzled geologists for more than a century, and massif anorthosites remain petrologic enigmas. In adleuconorite; (2) a foliated outer core of pink anorthosite (>An35Or10, >1900 ppm Sr, >850 ppm Ba) with more abundant leuconorite; dition to this fundamental aspect of the ‘anorthosite problem’ (see Bowen, 1917; Scoates, 2000), the petro(3) a border of foliated to massive green leucogabbro. Antiperthitic plagioclase, orthopyroxene, hemoilmenite, and small amounts of genesis of massif anorthosites was apparently restricted in time and space. Specifically, virtually all of the world’s biotite are present in all rocks. The border zone contains additional clinopyroxene, magnetite, and apatite. Labrieville is more alkalic supply of massif anorthosite was formed during the Proterozoic, and much of it is found in the Grenville and Srand Ba-rich than almost all other massif anorthosites. The widespread presence of biotite and the high hematite content of Province of Quebec and Labrador (Kranck, 1961; Anderson, 1969; Ashwal, 1993). hemoilmenite suggest crystallization from a relatively oxidizing and water-bearing magma. Collectively, these features imply that Despite their similar plagioclase-rich nature, it is now apparent that massif anorthosites worldwide are minLabrieville originated from a magma that differed considerably from typical tholeiitic liquids. Majorand trace-element variations suggest eralogically and compositionally diverse (see Xue & that all rock types represent mixtures of cumulus minerals, with Morse, 1993), implying involvement of a wide range of negligible trapped melt. The compositionally distinct zones imply parental magma compositions or petrogenetic processes differentiation during crystallization. Although plagioclase comin their formation. However, comprehensive studies on positions become more evolved inward, other indicators (e.g. XMg individual anorthosite complexes that can elucidate their in pyroxene) suggest differentiation outward from core to border. We petrogenesis—combining field and petrographic obfavor the latter interpretation, and suggest that the increase in servations with mineral-chemical data, and with complete plagioclase An content reflects a decline in pressure during crysmajorand trace-element data on a wide range of rock tallization. types—are few in number. Admittedly, the immense size of certain massifs (e.g. Lac St. Jean Complex, Woussen et al., 1988) and the remote or inhospitable locations of

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Petrogenesis of the Labrieville Alkalic Anorthosite Massif, Grenville Province, Quebec

Calc-silicate xenoliths occur as part of a xenolith and autolith swarm in the Lower Zone of the Kiglapait Intrusion, Labrador. All xenoliths are lighter-colored than the local troctolite host rock, and are surrounded by a 1–2 cm thick reaction zone of clinopyroxenite. The typical mineral assemblage in calc-silicate xenoliths is diopside + forsterite + monticellite + spinel, and one also contains akermanite. Reaction zones are dominated by diopside and spinel. All clinopyroxene compositions are “fassaitic,” with high concentrations of Al 2 O 3 (4–12 wt%), TiO 2 (1–4 wt%), and calculated Fe 2 O 3 (Fe 3+ > Fe 2+ ). In addition, clinopyroxene grains locally display extreme zoning in Si, Al, Ti, and X Mg . Akermanite contains up to 1.5 wt% Na 2 O. Textures in the xenoliths are complex, with most containing highly irregular intergrowths of diopside, forsterite, and monticellite. Akermanite and forsterite are separated locally by a coarse symplectic intergrowth of diopside and monticellite, which resulted from a retrograde reaction. The dominant assemblage probably represents the production of monticellite from calcite + clinopyroxene + forsterite, until calcite was consumed. These reactions require metamorphic temperatures of ~900 °C and pressures ≤ 0.4 kbar, such that the xenoliths were probably derived from the roof of the magma chamber. Whole-rock compositions reflect either substantial chemical exchange with the Kiglapait magma, or a very impure dolomitic protolith. The clinopyroxenite reaction zones were produced primarily by assimilation of carbonate by the Kiglapait magma, in combination with the effects of magma undercooling near the xenoliths.

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High-temperature contact metamorphism of calc-silicate xenoliths in the Kiglapait Intrusion, Labrador

The Goochland terrane is an isolated block of Mesoproterozoic crust in the Piedmont Province of central Virginia. We report U-Pb zircon dates and whole-rock major and trace element data for the State Farm gneiss, one of the main units in the terrane, and additional results for a newly recognized suite of Neoproterozoic granitoids that intrude the gneiss. The State Farm gneiss ranges in bulk composition from quartz monzodiorite to granite, and three samples yield U-Pb zircon dates of 1046 +7/–6 Ma, 1039 +18/–11 Ma, and 1023 ± 10 Ma. We interpret these dates as igneous crystallization ages, which indicate a maximum emplacement interval of ca. 1057–1013 Ma for these samples. Neoproterozoic granitoids (including the Fine Creek Mills and Flat Rock granites) are more alkaline than the State Farm gneiss and display all compositional characteristics of A-type granites (e.g., high Fe/Mg, Ga, Ga/Al, Nb, Zn, Y, Zr). U-Pb zircon analyses from five separate bodies indicate crystallization ages from ca. 654 to 588 Ma, but all results are complicated by Mesoproterozoic (ca. 1 Ga) inheritance, coupled, in some cases, with secondary Pb loss. We interpret the ages and compositions of the younger rocks to reflect Neoproterozoic rifting of the Goochland terrane, but its location during rifting is uncertain. The terrane may have been separated from Laurentia during the Neoproterozoic breakup of Rodinia and later reattached.

Brent E. Owens

Papers

Petrogenesis of Apatite-Rich Rocks (Nelsonites and Oxide-Apatite Gabbronorites) Associated with Massif Anorthosites

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

Petrogenesis of the Labrieville Alkalic Anorthosite Massif, Grenville Province, Quebec

High-temperature contact metamorphism of calc-silicate xenoliths in the Kiglapait Intrusion, Labrador

Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia