M.E. Bickford

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500‐Ma Adjustment in Indian Proterozoic Stratigraphy

The Chattisgarh Basin of east central India and many unmetamorphosed Proterozoic sedimentary basins of Peninsular India have been considered mostly Neoproterozoic (1000–545 Ma) in age. A newly recognized succession of rhyolitic ignimbrite, ash beds, and volcaniclastic sandstones near the top of the ∼2.2‐km‐thick sedimentary fill of the Chattisgarh Basin is a chronostratigraphic marker. Euhedral igneous zircons from these units give U‐Pb SHRIMP ages of 990–1020 Ma, indicating that the basin fill beneath this marker horizon is pre‐Neoproterozoic. On the basis of newly reported zircon ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Paleoproterozoic rocks of central Colorado: Accreted arcs or extended older crust?

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SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley : Implications for the tectonic history of the Superior Province

\end{document} Ma from the basal part of the Vindhyan Basin and accepting the consensus that all virtually undeformed and unmetamorphosed craton‐interior Proterozoic sedimentary basins in peninsular India are approximately coeval, we conclude that these basins are approximately Mesoproterozoic (1600–1000 Ma) in age. The reassigned age for these rocks (1650 to 900 or possibly ∼1000 Ma), up to 500 Ma in variance with the current notion (∼1100 to ∼518 Ma; Naqvi 2005), calls for a thorough rethinking of contemporary models concerning tectonics, sedimentation, and other geological activity that affected the Indian shield in the Proterozoic Era.

Granite emplacement during tectonic exhumation: The Adirondack example

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.

Does the arc accretion model adequately explain the Paleoproterozoic evolution of southern Laurentia?: An expanded interpretation

Paleoproterozoic (1770–1735 Ma) bimodal volcanic rocks in central Colorado have been considered to represent southward growth of Laurentia by arc accretion. Although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs, there has been little evidence for pre–1800 Ma crust south of the Wyoming craton other than the 1840 ± 1 Ma Elves Chasm pluton in the Upper Granite Gorge of the Grand Canyon, Arizona. We report SHRIMP U-Pb ages of inherited zircons from metarhyolites and plutons in central Colorado that are latest Archean–earliest Proterozoic (2520–2000 Ma) and Trans-Hudson–Penokean (1878–1814 Ma). Associated quartzites contain detrital zircons with mean ages of 1735 Ma, indicating only local derivation. A meta-arkose, however, contains detrital zircons of Trans-Hudson–Penokean and Archean ages. We believe it likely that the 1900–1800 Ma Trans-Hudson–Penokean orogens, including Archean enclaves, extended farther south and west than is currently thought, and were the source of the bimodal volcanic rocks and associated plutons during the period 1770–1700 Ma.

Crustal evolution of southern Laurentia during the Paleoproterozoic: Insights from zircon Hf isotopic studies of ca. 1.75 Ga rocks in central Colorado

Anatectic leucogranites are common in metapelites within both the highlands and lowlands terranes of the Adirondack Mountains of northern New York State. The formation of these igneous bodies, which are folded in the lowlands and commonly mylonitized in the highlands, has been widely considered an event accompanying the ca. 1050 Ma Ottawan orogeny, during which metamorphic grade reached granulite facies in the highlands, while the lowlands experienced amphibolite facies metamorphism. Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons separated from leucosomes and melanosomes in both the southern highlands and the lowlands indicate that primary anatexis occurred ca. 1180–1160 Ma, and is thus a manifestation of heating during the earlier Shawinigan orogeny (ca. 1210– 1160 Ma) and associated anorthosite-mangerite-charnockite-granite (AMCG) magmatism (ca. 1165–1150 Ma). The absence of Ottawan overgrowths on Shawinigan zircons in these leucosomes suggests that by Ottawan time the rocks were too dry for further melting or zircon growth to occur. However, electron microprobe analyses of monazites from the southern highlands reveal multiple age zones, including cores with ages of ca. 1170–1180 Ma, consistent with primary growth during Shawinigan orogenesis, complex zones formed ca. 1140–1155 Ma during AMCG magmatism, and ca. 1050–1020 Ma formed during Ottawan orogenesis and high-grade metamorphism. Throughout the Adirondacks, leucosomes and melanosomes contain older, ca. 1320 Ma, zircons that are considered to be remnant detrital zircons derived from arc rocks of the Elzevirian terrane. The apparent absence of Archean detrital zircons suggests that the protoliths of the metapelites were deposited in restricted basins that did not receive detritus from the Superior craton.

Timing of anatexis in the eastern Adirondack Highlands: Implications for tectonic evolution during ca. 1050 Ma Ottawan orogenesis

Interest in Paleoarchean to early Meso-archean crust in North America has been sparked by the recent identification of ca. 3800–3500 Ma rocks on the northern margin of the Superior craton in the Assean Lake region of northern Manitoba and the Porpoise Cove terrane in northern Quebec. It has long been known that similarly ancient gneisses are exposed on the southern margin of the Superior craton in the Minnesota River Valley and in northern Michigan, but the ages of these rocks have been poorly constrained, because methods applied in the 1960s through late 1970s were inadequate to unravel the complexities of their thermotectonic history. Rocks exposed in the Minnesota River Valley include a complex of mig-matitic granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton Gneiss and the Montevideo Gneiss. The complex of ancient gneisses is intruded by a major younger, weakly deformed granite body, the Sacred Heart granite. Regional geophysical anomalies that extend across the Minnesota River Valley have been interpreted as defining boundaries between distinct blocks containing the various gneissic units. New sensitive high-resolution ion micro-probe (SHRIMP) U-Pb data from complex zircons yielded the following ages: Montevideo Gneiss near Montevideo, 3485 ± 10 Ma, granodiorite intrusion, 3385 ± 8 Ma; Montevideo Gneiss at Granite Falls, 3497 ± 9 Ma, metamorphic event, 3300–3350 Ma, mafic intrusion, 3141 ± 2 Ma, metamorphic overprint (rims), 2606 ± 4 Ma; Morton Gneiss: 3524 ± 9 Ma, granodiorite intrusion, 3370 ± 8 Ma, metamorphic overprints (growth of rims), 3140 ± 2 Ma and 2595 ± 4 Ma; biotite-garnet paragneiss, 2619 ± 20 Ma; and Sacred Heart granite, 2604 ± 4 Ma. Zircons from a cordierite-bearing feldspar-biotite schist overlying the Morton Gneiss yielded well-defined age peaks at 3520, 3480, 3380, and 3140 Ma, showing detrital input from most of the older rock units; 2600 Ma rims on these zircons indicate metamorphism at this time. Zircons from a hypersthene-bearing biotite-garnet paragneiss, overlying the Montevideo Gneiss near Granite Falls, yielded ca. 2600 Ma ages, indicating zircon growth during high-grade metamorphism at this time. Despite some differences in the intensity of the 2600 Ma event between the Morton and Montevideo blocks, both blocks display similar thermochronologic relationships and ages, suggesting that their boundary is not a fundamental suture between two distinct Paleoarchean terranes. Previously obtained zircon age data from the tonalitic gneiss at Watersmeet Dome in northern Michigan indicated formation at ca. 3500 Ma, whereas a granite body near Thayer was dated at 2745 ± 65 Ma and leucogranite dikes are ca. 2600 Ma. Thus, these rocks and those in the Minnesota River Valley were formed in the late Paleoarchean and show a history of igneous activity and metamorphism in the Mesoarchean and Neoarchean. The occurrence of ancient crustal rocks on both the northern and southern margins of the ca. 2900–2700 Superior craton suggests that they are remnants of once more-extensive Paleoarchean crust that existed prior to formation of the Neoarchean Superior craton.

Geochronology and geochemistry of Putnam-Nashoba terrane metavolcanic and plutonic rocks, eastern Massachusetts: Constraints on the early Paleozoic evolution of eastern North America

Sensitive high-resolution ion microprobe U-Pb zircon ages for late to posttectonic leucogranites fix the timing of extensional collapse of a portion of the Mesoproterozoic Grenville orogen of eastern North America. Plutons of Lyon Mountain Granite (LMG) were emplaced within the Carthage Colton shear zone synchronously with formation of extensional mylonite at 1045‐1037 Ma. Leucogranite melts were generated in the hot granulite facies core of the Adirondack Highlands‐Central Granulite terrane that served as the lower plate for down-to-the-northwest extension. The LMG suite is associated with hightemperature hydrothermal magnetite deposits in the Adirondack Highlands, and widespread Cl 1 CO2 hydrothermal alteration of upper-plate rocks is localized along the Carthage Colton shear zone where LMG granites are present. The relationships between melt generation, granite intrusion, high strain rates, extensional collapse, and hightemperature hydrothermal activity provide a framework for understanding midcrustal processes in modern and ancient orogenic belts.