L. M. Heaman

The chemical composition of igneous zircon suites: implications for geochemical tracer studies

Abstract Minor and trace element data obtained for ultrapure zircon fractions isolated from a variety of igneous rocks indicate that zircon grains crystallizing in different magmatic environments have unique geochemical signatures. Zircon megacrysts found in kimberlites (mantle zircons?) are characterized by extremely low abundances of U ( 60). In addition, zircon crystallizing in mafic magmas have elevated Sc levels (86 to 230 ppm), strongly fractionated REE patterns, and generally high Th/U ratios (> 1) compared to other igneous zircon suites. The presence of diagnostic geochemical signatures in igneous zircon suites indicates that zircon can be used as a geochemical tracer and may be useful for identifying the geochemical nature of source regions in provenance studies of single detrital zircon grains from sedimentary rocks or for deciphering basement rock compositions, if not exposed, from a geochemical study of xenocrystic zircon grains.

UPb geochronology of layered mafic intrusions in the eastern Baltic Shield: implications for the timing and duration of Paleoproterozoic continental rifting

Abstract Paleoproterozoic layered mafic-ultramafic intrusions are widespread in the eastern Baltic (Fennoscandian) Shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts. UPb zircon/baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two Paleoproterozoic volcanic-sedimentary belts: the Pechenga-Imandra-Varzuga belt in the Kola Peninsula, and the Sumi-Sariola belt in Karelia. The layered intrusions in the Sumi-Sariola belt show an age progression from 2449.0 ± 1.1 Ma for the Burakovka complex, located at the southeastern edge of the belt, to 2442.1 ± 1.4 Ma and 2441.3 ± 1.2 Ma for the Lukkulaisvaara and Tsipringa intrusions of the Olanga complex in northern Karelia (northeastern part of the belt). These age results indicate a total duration of mafic magmatism along the ca. 800 km belt in Karelia to be less than 10 m.y. The Pechenga-Imandra-Varzuga belt, Kola Peninsula, contains two distinct age groups of layered mafic intrusions and related rocks. The younger of these overlaps with the ages obtained for the Olanga intrusions and includes the 2441.0 ± 1.6 Ma Imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442.2 ± 1.7 Ma. The older group of mafic layered intrusions in the Kola Peninsula includes the 2501.5 ± 1.7 Ma Pana tundra intrusion, the 2504.4 ± 1.5 Ma Monche pluton and the 2505.1 ± 1.6 Ma Generalskaya mountain intrusion. The temporal and spatial distribution of the 2.50-2.44 Ga rift-related igneous activity in the Baltic Shield is consistent both with the model of starting mantle plume, and with the model of passive rifting controlled by pre-existing lithospheric weaknesses.

Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex

The Coldwell Complex represents the largest alkaline intrusion associated with the Midcontinent Rift System in North America. This complex contains a plethora of rock types that have previously been subdivided into three intrusive centers. A detailed U-Pb zircon/baddeleyite age study of five samples indicates that the majority of the complex was emplaced into “cold” Archean crust at 1108±1 Ma and likely experienced a rapid cooling history. These data, combined with published U-Pb zircon/baddeleyite results for other rift related igneous activity, document the contemporaneous production and emplacement of tholeiitic and alkaline magmas at the onset of rifting. The Sr-Nd-Pb isotopic compositions of selected minerals from different phases of the complex display considerable scatter that is best explained by the presence of magmas with different initial isotopic compositions. The initial Sr and Nd isotopic compositions for clinopyroxene and plagioclase from one of the earliest gabbro phases (εNd=+0.5 to +1.6; εSr=+2.4 to +3.1) are identical to published data for primitive olivine tholeiites from the rift and indicate that the majority of magmas, both tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition. This very reproducible isotopic composition for rift magmatism can be explained by the dominance of a well-mixed mantle plume signature in magma genesis. The shift in isotopic compositions observed for the more evolved granite and syenite samples (εNd=−4.6 to −6.4; εSr=+10.2 to +13.8) combined with a less radiogenic Pb isotopic signature is consistent with derivation of these magmas from or interaction with an older granulite facies lower crust. The chondritic isotopic signature typical of most MRS volcanic and plutonic rocks is quite distinct from published results on associated carbonatites (εNd=+2.1 to +4.5; εSr=−8.0 to 2212;11.5) indicating the presence of at least two distinct subcontinental mantle isotopic reservoirs in this region.

Polymetamorphic evolution of the Lewisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile

U-Pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the Lewisian complex and bordering Laxford Front reveal complex discordance patterns indicating multiple Late Archean and Early Proterozoic crystallization, overgrowth and Pb-loss events. The earliest stages in the evolution of the complex remain poorly resolved. Zircon ages of ≥2710 Ma date high-grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting. A distinct event at 2490–2480 Ma, possibly initiated by metamorphism and deformation at high-grade conditions, caused amphibolitization of the granulites and emplacement of granitic pegmatites. This event can be correlated with development of Inverian shear zones and formation of granitoid layers along the Laxford Front. The emplacement of a younger generation of granitoid sheets during the Laxfordian event fromed hydrothermal titanite at ≥1754 Ma in gneisses south of the Laxford Bridge and partially reset older titanite at Scourie. Growth of secondary titanite and rutile also occurred during subsequent low-grade metamorphism at 1690–1670 Ma.

1.08 Ga diabase sills in the Pahrump Group, California: Implications for development of the Cordilleran miogeocline

U-Pb baddeleyite ages of 1087 ±3 and 1069 ±3 Ma for twodiabase sills that intrude the Crystal Spring Formation of the Pahrump Group, southeastern California, provide a minimum estimate for the depositional age of the Crystal Spring Formation. The new ages high-light the potential range in deposition ages for the Pahrump Group, from ≥1087 Ma to possibly as young as 600 Ma for Kingston Peak glaciogenic sediments. Consequently, there probably are significant disconformities within the Pahrump Group that separate unrelated, tectonically distinct episodes of basin subsidence. The Crystal Spring and Beck Spring Formations are considered to be platform-cover rocks preserved as a result of block faulting during deposition of the upper Kingston Peak Formation. The age of the Pahrump sills is in excellent agreement with the age of sills that intrude the Apache Group (central Arizona) and the Unkar Group (Grand Canyon) and supports earlier correlations of those units.

Precise U-Pb zircon ages for the Molson dyke swarm and the Fox River sill: Constraints for Early Proterozoic crustal evolution in northeastern Manitoba, Canada

Precise U-Pb zircon ages have been obtained for samples from the Molson dyke swarm and the Fox River sill in NE Manitoba, Canada. The ages determined for the Cross Lake and Cuthbert Lake dykes are 1,883.7−1.5+1.7and 1,883±2 Ma, respectively, and are in excellent agreement with the 1,882.9−1.4+1.5Ma age obtained for the Fox River sill. These results support the contention that the emplacement of the Fox River sill and the Molson Dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities. The timing of Early Proterozoic mafic magmatism in the western Superior Province appears to be synchronous with igneous activity in other parts of the Circum-Superior Belt and in the Trans Hudson orogen to the west. The emplacement of the Molson dyke swarm at 1,883 Ma indicates a 700 Ma interval of quiescence between the final igneous activity that is recorded in the Archean basement and dyke intrusion. The presence of deformed equivalents of Molson dykes in the Thompson Nickel Belt indicates that the intense deformation in this belt occurred sometime after 1,883 Ma.

U-Pb zircon geochronology of Precambrian tin-bearing continental-type acid magmatism in central Brazil

Abstract U-Pb geochronological data for alkali-rich granitic and associated rhyolitic rocks from two different tin-bearing sub-provinces of the Goias tin province, in central Brazil, reveal two distinct episodes of mid-Proterozoic continental acid magmatism at ca. 1770 Ma and 1600 Ma ago. U-Pb zircon data for the Sucuri and Soledade granites of the eastern Rio Parana Sub-Province (RPS) define upper intercept ages of 1767±10 Ma and 1769±2 Ma, respectively. These ages are indistinguishable from a U-Pb age of a rhyolite from the basal portion of the Arai Group (1771±2 Ma). These results attest to the contemporaneity of the intrusion of the granites and the crystallization of the rhyolitic lavas in the Rio Parana Sub-Province. U-Pb isotopic results from zircons from the Serra da Mesa granite in the western Rio Tocantins Sub-Province (RTS) indicate that the granitic igneous activity in that area is ca. 150 Ma younger than that in the RPS. The inheritance pattern observed in the zircons from the Arai rhyolite suggests that the sialic basement in the RPS region may include an early Proterozoic (ca. 2.2 Ga) component. Inheritance of possibly Archaean age was also found in zircons from the Sucuri granite. Lower intercept age of 658 Ma for the Soledade granite zircons indicate Pb loss during the late Proterozoic Brasiliano (Pan-African) thermo-tectonic event (650-550 Ma). Similar associations of tin-bearing, alkali-rich granites with rhyolites and succeeding continental sediments are known in other parts of the Precambrian shield of Brazil. This indicates that extensive areas of this shield experienced extensional tectonics in intracontinental settings between ca. 1700 and 1900 Ma ago.

Grenvillian magmatism in the eastern Grenville Province, Canada

Abstract U-Pb age determinations on six newly identified examples of Grenvillian plutonism from eastern Labrador have yielded the following ages: Gilbert Bay granite 1132−6+7 Ma; Second Choice Lake pegmatite 1003±6 Ma; Southwest Pond granite 963±6 Ma; Chateau Pond granite 964±2 Ma; Riviere Bujeault headwaters quartz syenite 964±5 Ma; Upper St Lewis River (west) granite 956±1 Ma. In addition, eight new K-Ar and Rb-Sr hornblende and biotite ages ranging from 953 Ma to 811 Ma are reported for three of these plutons. In conjunction with three previous U-Pb determinations, it is concluded that the U-Pb dates reflect two periods of Grenvillian plutonism that occurred in separate areas of eastern Labrador, and which were also characterized by distinct emplacement styles. North of the Mealy Mountains terrane boundary (Lake Melville terrane and its border regions with the Mealy Mountains and Hawke River terranes) plutonism occurred between ∼1130 and 1080 Ma and consisted of sporadic, minor granitic intrusions. These intrusions had little structural effect on the host rocks and are inferred to have been emplaced at a high structural level, a model consistent with previous suggestions for tectonic stacking in the region. South of the Mealy Mountains terrane boundary (Mealy Mountains and Pinware terranes) plutonism was brief (966 to 956 Ma), but widespread. Typical magmatic products were circular (in plan) plutons, up to 20 km in diameter, having monzonite, quartz syenite and granite compositions. These plutons exerted marked structural influence on their host rocks and are interpreted to have been emplaced at intermediate structural levels. In a broader regional context, by utilizing a previously demonstrated correlation between positive magnetic anomalies and Grenvillian plutons together with reconnaissance geological mapping, a belt of Grenvillian plutons is inferred to exist across the southern half of the eastern Grenville Province. This zone of plutonism serves to emphasize a distinct difference between an exterior (northern) thrust belt and an interior (southern) magmatic belt. A similar bipartite division is apparent in other parts of the Grenville-Sveconorwegian Orogen. The mineral geochronological data probably do not reflect the cooling histories of individual plutons; alternative explanations include either a slow regional cooling event or the distal effects of younger plutonism/metamorphism to the south, as yet unidentified.

Geochronological constraints on the emplacement history of an anorthosite — rapakivi granite suite: U−Pb zircon and baddeleyite study of the Korosten complex, Ukraine

U−Pb zircon/baddeleyite ages obtained for the Korosten anorthosite-rapakivi granite complex, Ukrainian shield, suggest that different magmatic phases were emplaced during a period of ca. 30 million years as a series of distinct igneous episodes. The earliest 1789.1±2.0 Ma anorthosites were followed by 1781.3±3.2 Ma dykes of plagiogranite porphyries. The emplacement of a major rapakivi granite phase took place at 1767.4±2.2 Ma, and was followed by emplacement of layered intrusions of anorthosites, gabbronorites, diabases and ultrabasic rocks between 1761 and 1758 Ma. The minimum duration of magmatism of about 30 million years, the 6–15 million years interval between igneous pulses, and alternation of discrete episodes of basic and felsie magmatism are common features of major anorthositemangerite-charnockite-rapakivi granite complexes. Temporal distribution of igneous activity in the Korosten complex shows that the gabbro-anorthosites and the granites are not comagmatic, although they are possibly cogenetic, and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years. The time gap of about 20–25 million years between early basic and later and more voluminous granitic magmatism, characteristic of the Korosten pluton, Wiborg and Salmi batholiths, probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust.

Timing and thermal influence of late orogenic extension in the lower crust: a UPb geochronological study from the southwest Grenville orogen, Canada

Abstract In the southwestern Grenville Province, the Central Gneiss Belt consists of a belt of parautochthonous rocks in the north and a collage of allochthonous lithotectonic domains in the south. Near Pointe-au-Baril, Ontario, the allochthon-parautochthon boundary is marked by the Shawanaga shear zone, a northwest-directed thrust zone between the Britt and Shawanaga domains that was reactivated during ductile, top-to-the-southeast extensional shearing. UPb ages of 1042+4/−2, 1019±4 and 988±2 Ma for granitic pegmatite dykes that are pre-kinematic, late syn-kinematic, and post-kinematic with respect to top-side-down displacement constrain major extensional transport on the Shawanaga shear zone to ca. 1020 Ma. However, nearly all the zircons in the dykes are inherited from a pre-Grenvillian, ca. 1460 Ma granitic host rock to the dykes. Recognition of the inherited nature of these grains comes from the multi-dyke dating approach employed here, and illustrates a potential pitfall in dating shear zone movement using only single dykes interpreted as syn-kinematic. UPb ages of metamorphic titanite from 14 samples collected in a transect across the Shawanaga shear zone span a 93 m.y. segment of concordia. These ages do not correlate with titanite grain size or vary systematically from domain to domain. However, when sample microtexture, strain state, structural position, titanite morphology, and pegmatite UPb data are considered together, the titanite ages can be reasonably inferred to date: 1. (1) regional metamorphism (1049-1045 Ma); 2. (2) cooling below the titanite isotopic closure temperature (∼600°C) during extensional unroofing (1028-1018 Ma); 3. (3) recrystallization-controlled titanite growth and (or) isotopic resetting in high-strain zones (1008-1000 Ma); 4. (4) post-kinematic recrystallization, most likely in the presence of a late fluid phase (967-956 Ma). Each titanite age group is made up of samples from both the Britt and Shawanaga domains, indicating that these processes were not restricted to a single domain but rather occurred within localized regimes, some which were clearly structurally controlled. The 93 m.y. spread of concordant titanite ages within a 20 × 20 km area demonstrates that cooling through isotopic closure is only one of several possibilities to be considered when interpreting metamorphic titanite ages in high-grade orogenic terranes.