J. L. Wooden

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the fi eld defi ned by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fi ngerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid fi eld, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

“Fingerprinting” tectono-magmatic provenance using trace elements in igneous zircon

Over 5300 recent SHRIMP-RG analyses of trace elements (TE) in igneous zircon have been compiled and classified based on their original tectono-magmatic setting to empirically evaluate “geochemical fingerprints” unique to those settings. Immobile element geochemical fingerprints used for lavas are applied with the same rational to zircon, including consideration of mineral competition on zircon TE ratios, and new criteria for distinguishing mid-ocean ridge (MOR), magmatic arc, and ocean island (and other plume-influenced) settings are proposed. The elemental ratios in zircon effective for fingerprinting tectono-magmatic provenance are systematically related to lava composition from equivalent settings. Existing discrimination diagrams using zircon U/Yb versus Hf or Y do not distinguish TE-enriched ocean island settings (i.e., Iceland, Hawaii) from magmatic arc settings. However, bivariate diagrams with combined cation ratios involving U–Nb–Sc–Yb–Gd–Ce provide a more complete distinction of zircon from these settings. On diagrams of U/Yb versus Nb/Yb, most MOR, ocean island, and kimberlite zircon define a broad “mantle-zircon array”; arc zircon defines a parallel array offset to higher U/Yb. Distinctly low U/Yb ratios of MOR zircon (typically <0.1) mirror their parental magmas and long-term incompatible element depletion of the MORB mantle. Plume-influenced sources are distinguished from MOR by higher U/Yb, U/Nb, Nb/Yb, and Nb/Sc. For zircon with U/Ybxa0>xa00.1, high Sc/Yb separates arc settings from low-Sc/Yb plume-influenced sources. The slope of scandium enrichment trends in zircon differ between MOR and continental arc settings, likely reflecting the involvement of amphibole during melt differentiation. Scandium is thus also critical for discriminating provenance, but its behavior in zircon probably reflects contrasting melt fractionation trends between tholeiitic and calc-alkaline systems more than compositional differences in primitive magmas sourced at each tectono-magmatic source.

U-Pb zircon ages from the southwestern Karoo Basin, South Africa - Implications for the Permian-Triassic boundary

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin.

Sr, Nd, and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador - Implications for crustal contamination and basement mapping

Abstract We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025−0.7033, eNd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032−0.7066, eNd = −0.3 to −4.4 and variable, but generally unradiogenic 207Pb/204Pb and 206Pb/204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture.

3. 96 Ga zircons from an Archean quartzite, Beartooth Mountains, Montana

U-Pb isotopic systematics of detrital zircons incorporated in a middle Archean quartzite from the Beartooth Mountains, Montana, were investigated with the SHRIMP ion microprobe. These new data reveal an extended and previously unrecognized record of crustal evolution for the northern Wyoming province. Seventy-eight analyses of 67 grains yielded a range of {sup 207}Pb/{sup 206}Pb ages from 2.69 to 3.96 Ga. Concordant analyses from 43 separate grains defined a maximum age for the deposition of the quartzite of 3.30 Ga; other provenance ages extend to 3.96 Ga. Ages of 3.30 Ga indicate that older crustal components with ages up to 3.96 Ga, or detritus from them, were also in the provenance of this quartzite. This older age is equivalent to that of the oldest known rock from the Acasta gneisses of the Slave province and is exceeded only by the > 4.0 Ga age of detrital zircons of the Yilgarnmorexa0» block of Western Australia. These data support an increased probability for the survival of sialic crust created before the cessation of the late bombardment at 3.8 to 3.9 Ga.«xa0less

New Sr, Nd, and Pb isotopic data from plutons in the northern Great Basin: Implications for crustal structure and granite petrogenesis in the hinterland of the Sevier thrust belt

The influence of tectonic setting and age on the variation of isotopic signatures of granitic plutons in the northern Great Basin has, in general, not been apparent from previous investigations. Although Elison et al. pointed out isotopic differences between Jurassic and younger plutons near the 0.706 Sr isopleth, and Farmer and DePaolo noted that there might be a difference between Cenozoic vs. Mesozoic plutonic rocks in the eastern part of the northern Great Basin, neither of these studies revealed the remarkable correlation between isotopic signature, age, and tectonic setting shown by our expanded Sr, Nd, and Pb isotopic data base. Jurassic-Early Cretaceous plutons in the northern Great Basin have a limited range of Sr and Nd isotopic values that cluster near bulk earth. All but one of these plutons have ϵ Nd values less negative than -7 despite their location both to the west and east of the ϵ Nd = -7 line. Construction of Sr 0.706 and ϵ Nd = -7 isotopic boundaries is virtually impossible for plutons of this age range. In contrast, Upper Cretaceous peraluminous granites east of the ϵ Nd = -7 line have very negative ϵ Nd values and high initial Sr ratios, and they appear to represent essentially pure crustal melts. The data favor a model that equates generation of these plutons via crustal thickening associated with the Sevier thrust belt. Cenozoic plutons appear to be mixtures of mantle and crustal reservoirs, and their isotopic systematics, along with those of the Late Cretaceous age plutonic suite, define a previously unrecognized, approximately east-west-trending crustal boundary between predominantly Archean crust to the north and predominantly Proterozoic crust to the south. The isotopic data from the Jurassic-Early Cretaceous plutonic suite do not reflect the presence of this boundary, suggesting that the isotopic systematics of this plutonic suite may not have been controlled by the same variations in crustal and/or mantle lithospheric structure at depth.

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

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.

Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin

Earliest Cordilleran magnatism in the southwestern United States is recorded by a belt of Triassic plutons that inme Protemzoic basemint of the Mojave crustal province and its cratona!/miogeoclinal cover. The belt extends from the western Mojave Desert through the Trausverse Ranges to the Colorado River trough. Triassic plutons are predominantly alkali-calcic, Fe- and Sr- mriched quartz mzrdior/tes and monzonites. The northern part of the belt is comlx)sed of two older plutonic mites (241-231 Ma) which are high K to shoshonitic; the southern part of the belt is a younger (218-213 Ma), sodic-alkalic suite. The plutonic record in southern California suggests a short-lived, southward younging continental margin arc sang for eraplacement of Triassic plutons, Sulimx7fx7f on a continental margin modified by smistral tranqform faulting Triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous Middle to Late Jurassic Cordilleran arc magmatism.

Crustal contamination in the petrogenesis of a calc-alkalic rock series: Josephine Mountain intrusion, California

The Josephine Mountain intrusion is a Cretaceous calc-alkalictonalite-granite pluton emplaced at 22 km depth in a continental margin arc. Variable uplift of adjacent terranes in southern California since mid-Cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle-derived arc magmas in this region. The parental magma of the intrusion was high-alumina basalt whose isotopic signature ( 87 Sr/ 86 Sr = 0.7087; δ 18 O = 7.5; ϵ Nd = −10) cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust. Such a signature could have resulted from high-pressure fractionation of primary low-alumina basalt coupled with assimilation of felsic/pelitic lower crust, partial melting of enriched subcontinental mantle followed by high-pressure fractionation, or a combination of these processes. Tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite. Assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma, leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism.

Contrasts between SmNd whole-rock and UPb zircon systematics in the Tobacco Root batholith, Montana: implications for the determination of crustal age provinces

Abstract Proper documentation of the extent and age of crust in the western US is critical for constraining a variety of geologic problems ranging from the growth rate of continents to Precambrian continental reconstructions. The secondary isotopic systematics of granitoids have been one of the principal means used to characterize continental crust in areas where the basement is covered. In southwestern Montana and eastern Idaho a group of Late Mesozoic to Cenozoic, dioritic to quartz monzonitic batholiths (e.g., Tobacco Root, Idaho, Pioneer, Boulder, etc.) share a limited range of Paleoproterozoic Sm-Nd depleted mantle model ages. The Tobacco Root batholith (TRB) has a Nd isotopic composition (ϵNd = −17.9 to −19.1) and Smue5f8Nd model age (TDM = 1.63 to 1.90 Ga) typical of this group. The TRB, however, intruded Archean crust (∼3.3 Ga, ϵNd = ∼ −35), rather than the presumed Proterozoic crust intruded by the other plutons. The Archean heritage of the TRB is confirmed by the presence of premagmatic zircons which range from 2.2 to 3.0 Ga. The combination of U-Pb zircon and Nd model ages suggest that the batholith was derived from both Archean and Proterozoic crustal sources, as well as an ∼80 Ma mantle component. This contrasts with a sample from the northern Idaho batholith which exhibits concordancy between its Sm-Nd and premagmatic zircon systems at ∼1.74 Ga. These data point to the difficulties that can occur if crustal age provinces are defined solely on the basis of Nd model ages of younger plutons, particularly in areas such as the northwestern US where Archean and Proterozoic crust is poorly exposed and dispersed over a large area.