James M. McLelland

Long-lived (1.8-1.0 Ga) convergent orogen in southern Laurentia, its extensions to Australia and Baltica, and implications for refining Rodinia

Abstract Between 1.8 and 1.0 Ga (Grenville-age), a series of subparallel accretionary orogens were added progressively to the southern edge of Laurentia. These belts now extend from Greenland/Labrador to southern California and are truncated at late Precambrian passive margins, suggesting that they once extended farther. We propose that Australia and Baltica contain their continuations. Together they comprise a long-lived orogenic system, >10 000 km long, that preserves a record of 800 million years of convergent margin tectonism. This tectonism culminated during Grenvillian continent–continent collisions in the assembly of the supercontinent Rodinia. Our reconstruction of the Australia–western US part of this assembly (AUSWUS) differs from the SWEAT reconstruction in that Australia is adjacent to the southwestern US rather than to northern Canada. The AUSWUS reconstruction is supported by a distinctive ‘fingerprint’ of geologic similarities between Australia and the southwestern US from 1.8 to 1.0 Ga, by numerous possible piercing points, and by an arguably better agreement between 1.45 and 1.0 Ga paleomagnetic poles between Australia and Laurentia. Geologic and paleomagnetic data suggest that separation between Laurentia and Australia took place ∼800–755 Ma and between Laurentia and Baltica ∼610 Ma. The proposed association of Australia, Laurentia, and Baltica, and the long-lived convergent margin they expose, provide a set of testable implications for the tectonic evolution of these cratons, and an important constraint for Proterozoic plate reconstructions.

Oxygen isotope geochemistry of zircon

The high-temperature and small sample size of an I.R. laser system has allowed the first detailed study of oxygen isotope ratios in zircon. Low-magnetism zircons that have grown during metamorphism in the Adirondack Mts., N.Y. preserve primary δ18O values and low-magnetism igneous zircons are likewise primary, showing no significant affect due to subsequent granulite facies metamorphism. The measured fractionation between zircon and garnet is Δ(Gt-Zrc) = 0.0 ± 0.2‰(1σ) for most low-magnetism zircons in meta-igneous rocks. The consistency of this value indicates equilibration at temperatures of 700–1100°C and little or no change in the equilibrium fractionation over this temperature range. In contrast, detrital low-magnetism zircons in quartzite preserve igneous compositions, up to 4‰ out of equilibrium with host quartz, in spite of granulite facies metamorphism. The oxygen isotope composition of zircon can be linked to UPb ages and can ‘see though’ metamorphism, providing a new tool for deciphering complex igneous, metamorphic and hydrothermal histories. Zircons separated by magnetic susceptibility show a consistent correlation. Low-magnetism zircons have the lowest uranium contents, the most concordant UPb isotopic compositions, and primary δ18O values. In contrast, high-magnetism zircons are up to 2‰ lower in δ18O than low-magnetism zircons from the same rock. The resetting of oxygen isotope ratios in high-magnetism zircons is caused by radiation damage which creates microfractures and enhances isotopic exchange. Zircons from the metamorphosed anorthosite-mangerite-charnockite-granite (AMCG) suite of the Adirondacks have previously been dated (1125–1157 Ma) and classified as igneous, metamorphic or disturbed based on their physical and UPb isotopic characteristics. Low-magnetism zircons from the AMCG suite have high, nearly constant values of δ18O that average 8.1 ± 0.4‰(1σ) for samples ranging from 39 to 75 wt% SiO2. Only olivine metagabbros have lower average values (6.4‰), consistent with the hypothesis that they represent nearly pristine samples of the anorthosites parent magma. Whole-rock values of δ18O are also high in the AMCG suite and increase with SiO2 content, as predicted for a process of assimilation and fractional crystallization. Taken together, these data suggest that the elevated values of oxygen isotope ratios result from partial melting and contamination involving metasediments in the deep crust, before the crystallization of zircon. More normal values elsewhere in the Grenville Province record deep-seated, pre-1150 Ma regional differences.

The Grenville Orogenic Cycle (ca. 1350-1000 Ma): an Adirondack perspective

Abstract The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups. The Adirondacks have not yet provided any record of events within the interval ca. 1125-1100 Ma, although there is evidence of contraction elsewhere in the southwestern Grenville Province at that time. At 1100-1090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the result of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following, at ca. 1090 Ma, the global-scale continental collision of the Ottawan Orogeny was initiated. Strong convergence, deformation, and metamorphism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event. During the waning stages of the Ottawan Orogeny, overthickened crust and lithosphere delaminated and the orogen underwent collapse. Large extensional faults such as the Carthage-Colton-Labelle shear zone developed and rapidly exhumed granulite facies rocks in the mobile core of the orogen which centers on the Adirondack-Morin terranes and extends southeastward into the New York-New Jersey Highlands. Extensional faulting along the Carthage-Colton mylonite zone dropped the amphibolite facies Lowlands down to the west and into juxtaposition with granulite facies rocks of the Highlands. UPb cooling ages from garnet, monazite, and titanite exhibit a sufficiently broad spectrum to accommodate an initial rapid rate of rebound-related cooling followed by a slower, erosion-controlled cooling history. During delamination, late- to post-tectonic granites of the Lyon Mt. Gneiss (ca. 1070-1045 Ma) were emplaced. The youngest member of this suite is an undeformed fayalite granite dated at ca. 1045 Ma which crosscuts all older rocks and fabric. High-potassium, post-tectonic granites of similar age are common in other parts of the southwestern Grenville Province. Renewed contraction and metamorphism at ca. 1030 Ma demonstrate that the Ottawan Orogen was still experiencing convergence well after the peak of orogeny. However, most of the manifestations involve reactivation of older thrust faults, including the Grenville Front Tectonic Zone. The intrusion of small bodies of anorthosite at ca. 1015 Ma (i.e., Labrieville) provide further evidence for the emplacement of these rocks within collisional orogens, albeit in their collapsing phase.

Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

U-Pb zircon geochronology of the Adirondack Mountains and implications for their geologic evolution

U-Pb zircon studies of metamorphosed igneous rocks in the Adirondack Mountains have yielded preliminary ages within the range 1420-990 Ma. Several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre-granulite facies dynamothermal metamorphic event are documented. This information is consistent with recent field and geochronological studies throughout the Grenville province and suggests that a complex sequence of events occurred in the Adirondack Mountains prior to the widespread deformation and metamorphism commonly attributed to the {approximately} 1100-1000 Ma Ottawan phase of the Grenvillian orogenic cycle.

Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes

The low abundance of igneous zircon in Proterozoic massif anorthosites has presented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks. Indirect dating that relies on zircon ages from associated mangerite-charnockite-granite granitoids assumes that they have a coeval relationship with anorthosite that requires documentation. SHRIMP (sensitive, high-resolution ion-microprobe) U-Pb zircon-dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolving problems with inheritance. Within the Adirondack Mountains, 10 samples of massif anorthosite have yielded more-than-sufficient quantities of igneous zircon to establish directly the ages of the region9s classic anorthosite occurrences (e.g., the Marcy and Oregon Dome massifs). In addition, a ferrogabbro, a ferrodiorite, and a coronitic olivine metagabbro, all crosscutting massif anorthosite, were dated. The average age of this suite of 13 anorthositic samples is 1154 ± 6 Ma (MSWD [mean square of weighted deviates] = 0.26, probability = 0.99). In addition, eight associated granitoids have been dated by SHRIMP techniques and complement another five previously dated by multi-grain thermal-ionization mass spectrometry (TIMS) methods. The 13 granitoids yield an average age of 1158 ± 5 (MSWD = 0.89, probability = 0.60) and are broadly coeval with the massif anorthosite. The overlapping ages provide evidence that these rocks constitute a single, composite anorthosite-mangerite-charnockite-granite (AMCG) suite intruded at ca. 1155 Ma, an age corresponding to the ages of major AMCG suites in the Grenville province in Canada (e.g., Morin and Lac St-Jean). Although rocks of the Adirondack AMCG suite are now documented as broadly coeval, it does not follow that the constituent AMCG lithologies were comagmatic. Field relationships and mineral disequilibria in transitional zones are inconsistent with derivation from a single parental magma. Moreover, the presence of older (ca. 1.2–1.3 Ga) inherited cores in some zircons from AMCG granitoids conflicts with derivation of these rocks from magmas that formed anorthosite, gabbro, or ferrodiorite, or jotunite, in which zircons are highly soluble. The slightly older ca. 1158 Ma average age of the mangeritic and charnockitic members of the AMCG suite is consistent with an origin as early lower-crustal anatectites that left behind pyroxene-plagioclase restites. This refractory material then reacted (by assimilation–fractional crystallization [AFC]) with ponded, mantle-derived gabbroic magmas to produce plagioclase-rich crystal mushes with crustal isotopic signatures, as proposed much earlier by R.F. Emslie. These magmas are considered to be parental to the Adirondack anorthosite, and upon ascent they were emplaced in proximity to still hot, earlier mangeritic and charnockitic bodies where they underwent further fractionation. The composite nature of the Marcy massif documents that this process was repeated in several sequential pulses.

Zircon U-Pb geochronology of the Ottawan Orogeny, Adirondack Highlands, New York: regional and tectonic implications

Abstract Both single and multigrain U-Pb zircon thermal ionization mass spectrometry (TIMS) as well as sensitive high resolution ion microprobe (SHRIMP ll) dating of two suites of Adirondack granites have yielded ages constraining the principal tectonomagmatic events of the Ottawan Orogeny to the interval ca 1090–1035 Ma. The earliest of these consists of mildly A-type hornblende granites of the Hawkeye granite suite, multigrain samples of which define a tight age cluster of ca 1103–1093 Ma. Assemblages and fabrics in this suite demonstrate that it experienced the high-grade effects of the Ottawan Orogeny, thereby fixing the maximum age of the latter at ca 1090 Ma. The second suite consists of Lyon Mt. Granitic Gneiss, six samples of which cluster tightly at ca 1060–1045 Ma. Fabrics associated with this suite indicate a late- to post-tectonic origin thus fixing the minimum age for the Ottawan Orogeny. Especially critical are two samples of ca 1047 Ma fayalite granite that are essentially undeformed and must post-date tectonism. In addition, an undeformed pegmatite dike yields an age of 1034±8 Ma confirming the termination of Ottawan orogenesis by that time. It is suggested that the genesis of the Hawkeye suite is related to athenospheric heating of the crust due to far-field effects of contemporaneous magmatic events at the Midcontinent rift. Lyon Mt. Granitic Gneiss is interpreted as the result of deep crustal melting following delamination of the overthickened Ottawan orogen. Together with the results of metamorphic investigations, these two suites define a counterclockwise P–T–t loop for the Adirondacks during the Ottawan Orogeny. Geochronological and tectonic investigations from the Grenville Province of Canada and the northern Blue Ridge Province of the Appalachians demonstrate the presence of strong Ottawan deformation, magmatism, and metamorphism in these areas and emphasize the large scale and marked intensity of this event. In Canada, the crust responded to the Ottawan collision principally by imbricating into large northwest-directed thrust slices rather than the fold nappes of the Adirondacks. This is consistent with the apparent absence in the Canadian foreland of Hawkeye age magmatism and resultant rheological weakening of the crust.

Composition and petrogenesis of oxide-, apatite-rich gabbronorites associated with Proterozoic anorthosite massifs: examples from the Adirondack Mountains, New York

Mafic dikes and sheets rich in Fe, Ti-oxides and apatite are commonly associated with Proterozoic massif anorthosites and are referred to as oxide-apatite gabbronorites (OAGN). Within the Adirondacks, field evidence indicates that during middle to late stages of anorthositic evolution, these bodies were emplaced as magmas with unspecified liquid-crystal ratios. Sixty whole rock analyses of Adirondack OAGN and related rocks define continuous oxide trends on Harker variation diagrams (SiO2=37–54%). Similar trends exist for Sr, Y, Nb, Zr, and REE and together suggest a common origin via fractional crystallization. A representative parental magma (plagioclase-rich crystal mush) has been chosen from this suite, and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determined in actual rocks. Least squares, mass balance calculations of major element trends indicate that removal of intermediate plagioclase (∼An40–50) plus lesser amounts of pyroxene account for the compositional variation of this suite and produce very low sums of the squares of the residuals (R2s>0.25). The extracted mineral phases correspond volumetrically and compositionally to those of the anorthositic suite, and the model succeeds in accounting for the observed OAGN trends. The major element model is utilized to calculate trace elejent concentrations for successive magmas, and these agree closely with observation. We conclude that, beginning with a plagioclase-rich crystal mush, the extraction of intermediate plagioclase (∼An40–50) drives residual magmas to increasingly Fe-, Ti-, and P-rich and SiO2-poor conditions characteristic of Fenner-type fractionation. The crystallization sequence is plagioclase→plagioclase+orthopyroxene→plagioclase+orthopyroxene (pigeonite)+augite. Fe, Ti-oxides begin to crystallize near the end of the sequence and are followed by apatite and fayalitic olivine which appears in place of pigeonite. Augitic pyroxene becomes the dominant ferromagnesian phase in late stages of fractionation. Resultant OAGN magmas are injected into congealed anorthosite by filter pressing of liquid-rich interstitial fractions. Varying compositions of the dikes reflect filter pressing at different stages during fractionation and thereby provide information on the fractionation history of Proterozoic massif anorthosites.

Isotopic Constraints on Emplacement Age of Anorthositic Rocks of the Marcy Masiff, Adirondack Mts., New York

The igneous emplacement age of anorthositic rocks comprising the Marcy massif have been constrained by fixing minimum and maximum ages for their intrusion. Minimum ages have been fixed by: (a) U-Pb dating of cores of air abraded zircons from the anorthosite (>1113 Ma), (b) U-Pb dating of baddeleyite within the anorthosite (>1087 Ma), (c) U-Pb dating of baddeleyite from an olivine metagabbro crosscutting Marcy massif anorthosite (>1109 Ma), (d) U-Pb zircon dating of jotunitic rocks crosscutting the Marcy anorthosite massif (