Dating Of Baddeleyite Research Articles

The origin of the Palaeoproterozoic AMCG complexes in the Ukrainian shield: New U-Pb ages and Hf isotopes in zircon

The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815Ma and continued until 1743Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.

The 2405 Ma doleritic dykes in the Karelian Craton: A fragment of a Paleoproterozoic large igneous province

New data on the age and composition of doleritic dykes of the Karelian Craton on the Fennoscandian Shield are reported. Based on the results of U–Pb dating of baddeleyite, a new age episode (2404 ± 5 Ma) in the formation of basic rocks on the Karelian Craton is established. Comparison of the composition of the studied dolerite with that of dykes of the same age from other Archean cratons worldwide shows their essential similarity and allows us to suggest their formation within a single large igneous province. The data obtained support the current models of supercontinental reconstructions for the period of 2400 Ma.

U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa.

Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U–Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U–Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.

Sensitive high-resolution ion microprobe U-Pb dating of baddeleyite and zircon from a monzonite porphyry in the Xiaoshan area, western Henan Province, China: Constraints on baddeleyite and zircon formation process

Baddeleyite and zircon, including zircon overgrowths on baddeleyite, co-occur in a granophyric monzonite porphyry intruding volcanic rocks of the Majiahe Formation of the Xiong’er Group in the Xiaoshan area of the western Henan Province (China). It is inferred that the low silica content of the initial magma resulted in the formation of euhedral baddeleyite. Sensitive high-reso­lution ion microprobe (SHRIMP) U-Pb dating of the baddeleyite yields a weighted mean 207 Pb/ 206 Pb age of 1779 ± 8 Ma, representing the crystallization of the porphyry. Survival of baddeleyite as the magma became silica saturated implies a rapid cooling process. Zircon grains in the porphyry are anhedral to needle shaped, and are often clustered together within K-feldspar stringers or along the interface of quartz and K-feldspar in the granophyric groundmass, suggesting undercooling likely due to rapid emplacement. These observations indicate that zircon did not crystallize until final emplacement. Our U-Pb analyses of zircon yield a weighed mean 207 Pb/ 206 Pb age of 1777 ± 8 Ma, similar to that of baddeleyite. Subsequent medium- to high-temperature hydrothermal alteration affected most minerals at subsolidus conditions. Amphibole-plagioclase thermobarometry indicates pressure of 2.4–4.3 kbar and temperature of 470–580 °C for this alteration stage. Related alteration, such as total albitization of plagioclase, crystallization of secondary apatite, and sporadic secondary zircon overgrowths on baddeleyite, indicates that the fluid phase was enriched in Si, Na, and halogens (e.g., F, Cl). In addition to direct replacement of baddeleyite, zirconium required for the development of zircon overgrowths may also have been available through the alteration of Zr-bearing matrix phases such as amphibole and ilmenite.

U-Pb baddeleyite dating of the Proterozoic Pará de Minas dyke swarm in the São Francisco craton (Brazil) – implications for tectonic correlation with the Siberian, Congo and North China cratons

U–Pb baddeleyite ages demonstrate the presence of three dyke generations within the NW-trending Pará de Minas swarm (São Francisco craton, South America), dated at ca. 1795 Ma (1798 ± 4, 1791 ± 7 and 1793 ± 18 Ma), ca. 1710 Ma (1702 ± 13 and 1717 ± 11 Ma) and 766 ± 36 Ma. These ages have implications for the São Francisco-Congo (SF-Congo) craton within the Rodinia and Columbia supercontinent configurations. At 1795 Ma, the Rio de la Plata craton (1790 ± 5 Ma Uruguayan dykes), Amazonian craton (1790–1780 Ma Avanavero LIP), Sarmatia (1789 ± 3 Ma Tomashgorod dyke), North Australia (1790 ± 4 Ma Hart dolerites) and the North China craton (1790–1770 Ma Taihang-Xiong’er event) could potentially have been “next-door neighbours” to the SF-Congo craton. Possible matches at 1710 Ma are the 1750–1700 Ma Bilyakchan-Ulkan complex and coeval dykes in the Siberian craton and the 1730 Ma Miyun dyke swarm in NCC. Given large uncertainties, the 766 ± 36 Ma age may be compared with the age of the 790 Ma Niquelândia complex in adjacent Goias state, the 799 Ma Gannakouriep dykes in the Kalahari craton, the 780 Ma Gunbarrel LIP of western Laurentia, the 760 Ma Mount Rogers LIP of eastern Laurentia and the 755 Ma Mundine Well event (western Australia). We propose a reconstruction in which the SF-Congo is attached to North China and Siberia, which existed from 1790 to 1380 Ma when SF-Congo and North China broke away from Siberia, and until 930 Ma when North China separated from SF-Congo.

Results of dating of thorianite and baddeleyite from carbonatites of the Guli massif, Russia

The isotopic–geochronological features of thorianite and baddeleyite from carbonatites of the Guli massif, located within Maimecha–Kotui province in the north of the Siberian Platform, are characterized for the first time. The economic complex platinum-group element (PGE) and gold placer deposits are closely related to the Guli massif. Similar geochronological data for thorianite (250.1 ± 2.9 Ma, MSWD = 0.09, n = 36) and baddeleyite (250.8 ± 1.2 Ma, MSWD = 0.2, n = 6) obtained by two different methods indicate that carbonatites were formed close to the Permian–Triassic boundary and are synchronous with tholeiitic flood basalts of the Siberian Platform.

Quadrupole LA-ICP-MS U/Pb geochronology of baddeleyite single crystals

We present a straightforward method for accurate and precise U/Pb dating of baddeleyite using quadrupole LA-ICP-MS.

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U-Pb (uranium-lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

Formation of mafic magmas through lower crustal AFC processes — An example from the Jinan gabbroic intrusion in the North China Block

The Jinan gabbroic intrusion from the North China Block (NCB) experienced complex differentiation from a reduced and Si-undersaturated magma to an oxidized and Si-saturated melt, with a mineral assemblage of olivine+pyroxene+plagioclase+biotite+ilmenite. New dating of baddeleyite and zircon yield a consistent U–Pb age range from 130 to 134Ma, confirming emplacement during the early Cretaceous. In situ oxygen analyses on olivine show relatively homogeneous O isotopic compositions, and the olivine separates give a limited δ18O range from 6.0 to 6.1‰, higher than that of olivine from normal mantle peridotites. The zircons have even higher δ18O (6.9–7.3‰) than olivine and clinopyroxene separates (δ18O=6.2–6.9‰) and show clear O isotope disequilibrium with olivine/clinopyroxene. The δ18O of the primary magma for olivine and clinopyroxene crystallization was increased by 1.2–1.4‰ to form the evolved melt for zircon crystallization. This δ18O variation points to assimilation or contamination by high-δ18O crustal components during magmatic evolution. Systematic decreases in whole-rock εNd(t) and εHf(t) are accompanied by increases of δ18O, Ba/Nb, Ba/Th and Zr/Nb and decrease of Th/La, along with a change in oxygen fugacity of the magma. Modeling results suggest that the Jinan gabbroic intrusion experienced assimilation-fractional crystallization (AFC) with felsic material at lower crustal levels. Combined geochemical and Sr–Nd–Hf–O isotopic features suggest that the primary melts were derived from the NCB lithospheric mantle that had been further metasomatized by a small volume of carbonatitic melt, possibly derived from the subducted Yangtze crust.A compilation of the published mineral O isotope data shows that most Mesozoic mafic intrusions in NCB exhibit O isotope disequilibrium between zircon and olivine/clinopyroxene, with higher δ18O of zircon than olivine and clinopyroxene, indicating their crystallization from 18O-richer differentiated melts due to crustal contamination or assimilation. Therefore, δ18O of zircon as a tracer of mafic magmas should be used with caution, particularly for primary Si-undersaturated magmas.

Geology and Age Constraints on the Origin of the Intrusion-Related, Sheeted Vein-Type Åkerberg Gold Deposit, Skellefte District, Sweden

The Early Proterozoic (~1.9 Ga) Skellefte mining district in northern Sweden hosts abundant base metal deposits, but there are also gold-only deposits. The Åkerberg gold ore is unusual given the noted lack of alteration, a scarcity of sulfides and gold associated with thin (mm-cm wide) parallel quartz veins hosted in a gabbro. The gold content is positively correlated with the density of quartz veins, but gold often occurs between veins and also in parts of the gabbro where there is no veining. The gabbro is intruded by a granodiorite and associated pegmatite bodies, and U-Pb dating of zircon and baddeleyite suggest that these lithologies developed close in time at around 1.88 Ga ago. There are no primary inclusions in quartz veins, but different types of secondary aqueous inclusions occur. The Åkerberg ore is interpreted as a sheeted vein complex, with veins constrained to tensional cracks induced when a granodioritic magma intruded the competent, sheet-like gabbro intrusion. It is suggested that unmixing of the felsic magma also produced pegmatite bodies and a gel-like melt which invaded fractures in the gabbro and deposited silica. In a comparison, the Åkerberg ore shares many characteristics with the intrusion-related style of gold mineralizations.

U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent

The Tandilia Terrane (southernmost fringe of the Rio de la Plata Craton) is an igneous and metamorphic complex produced by an accretionary orogeny (2.25–2.02Ga). Calc-alkaline acidic dykes with E–W strike and a major shear zone with similar orientation are related with the late orogeny stage, as supported by field relations. In a previous study the acid dykes gave 40Ar–39Ar ages of 2007±24Ma to 2020±24Ma. A N and NW trending tholeiitic dyke swarm (Tandil swarm) is also present in the Tandilia Terrane. One sample from the NW-trending subset previously gave a U–Pb (ID-TIMS) baddeleyite age of 1588±11Ma. New precise U–Pb (ID-TIMS) baddeleyite dating of both N- and NW-trending Tandil dykes yielded crystallization ages of 1589±3Ma, 1588±3Ma and 1588±3Ma. Significantly older tholeiitic dykes known as the Florida swarm occur in the Northern Rio de la Plata Craton, for which a U–Pb (ID-TIMS) baddeleyite age of 1790±5Ma was previously reported. Consequently intermittent rifting (1.79, 1.59Ga) took place after tectonic stabilization of the late Paleoproterozoic lithosphere (proto-Rio de la Plata Craton). The available geochemical data for the 1.59Ga Tandil dykes define low- and high-TiO2 trends, although, only the low-TiO2 subgroup is firmly dated. Both the Tandil and Florida dykes have geochemical and Nd–Sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects.The Tandil dykes may be linked with the 1.57±0.02Ga Capivarita anorthosite which occurs to the east of the northern part of the craton. Correlatives on other crustal blocks may include those in Baltica such as bimodal rock association (including the Breven-Hällefors and Åland-Åboland diabase dykes) and in the reconstructed Gawler Craton/NW Laurentia dolerites, bimodal magmatism and IOCG deposits. Contemporary within-plate bimodal associations are also present in the SW Amazonian Craton.Paleomagnetic data for the 1790Ma Florida dykes allows three possible reconstruction scenarios for the position of Rio de la Plata Craton: i) in the southern hemisphere linked to the Pampia terrane, and the Amazonia+Rio Apa landmass as close as they are in present day; ii) in the northern hemisphere, as a nearest neighbor to the reconstructed NW Laurentia and Gawler blocks (proto-Australia); or iii) in the northern hemisphere near the boundary of Amazonia (proto-Amazonian Craton) and Baltica. Positions 2 and 3 are consistent with all three criteria: paleomagnetic poles and proximity to matching ca. 1790Ma and 1590Ma mafic magmatism. Our data are consistent with the idea that the Columbia supercontinent started major break up at 1.59Ga.

Results of U-Pb dating of zircon and baddeleyite from the Noril’sk-1 ultramafic-mafic intrusion ( Russia)

We present for the first time the mineralogical and isotope-geochemical particularities of zircon and baddeleyite from various rocks of the economic ore-bearing Noril’sk-1 intrusion located in the northwestern part of the Siberian platform. The ultramafic-mafic Noril’sk-1 intrusion hosts one of the world’s major economic platinum-group-element (PGE)-Cu-Ni sulphide deposits. A detailed study of crystal morphology and internal structure identify four zircon populations, characterized by different U-Pb (SHRIMP-II) ages. The U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span (from 290 ± 2.8 to 226.7 ± 0.9 Ma). The established U-Pb ages imply that crystallization of baddeleyite and zircon populations corresponds to several stages of protracted evolution of the ore-forming magmatic system (290 ± 2.8, 261.3 ± 1.6, 245.7 ± 1.1, 236.5 ± 1.8, and 226.7 ± 0.9 Ma, respectively) that served as the favorable factor for accumulation of magmas and ores of unique scales and concentrations.

Paleomagnetic and geochronological evidence for large-scale post–1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons along the Limpopo belt

Proterozoic reconstructions of the Kaapvaal and Zimbabwe cratons have been limited by the scarcity of precisely dated paleomagnetic poles for the Zimbabwe craton. We present new U-Pb baddeleyite and apatite dates from two diabase sheets that have previously yielded paleomagnetic data from the Mashonaland igneous province in the Zimbabwe craton. Discordant baddeleyite analyses yield upper intercept dates of 1871.9 ± 2.2 and 1882.7 +1.6/–1.5 Ma. Apatite data from the same samples give less precise but statistically indistinguishable dates, providing direct constraints on the post-magmatic thermal history of the diabases. The new U-Pb dates and other recently published baddeleyite dates from the Mashonaland province are coeval with mafic magmatism in the adjacent Kaapvaal craton (1879–1872 Ma), but paleomagnetic poles from the two intraplate suites differ by 39°, suggesting that the two cratons underwent substantial relative motion after ca. 1.88 Ga. Paleomagnetic reconstructions are consistent with >2000 km of lateral displacement being accommodated in the Limpopo orogenic belt that separates the two cratons.

U–Pb dating of baddeleyite and zircon from the Shizhaigou diorite in the southern margin of North China Craton: Constrains on the timing and tectonic setting of the Paleoproterozoic Xiong'er group

The Xiong'er volcanism marks an important thermal event in the southern margin of the North China Craton during the Paleoproterozoic. Precise U–Pb ages of zircon and baddeleyite from the Shizhaigou diorite which intruded the Majiahe formation of the upper part of the Xiong'er Group were determined in this study by multi-collector SIMS. The mean 207Pb/ 206Pb ages are 1789 ± 3.5 Ma for baddeleyite and 1780 ± 11 Ma for zircon from Shizhaigou diorite, respectively. The major and trace elements and Sr–Nd isotopic compositions indicate an EM-I mantle type signature and crustal contamination features. Combining with previous chronological data, it is proposed that the age of the Xiong'er volcanism is between 1770 Ma and 1800 Ma with a peak at 1780 Ma. Our result supports the view that the Xiong'er volcanic rocks formed in an extensional rift environment.

Precise U–Pb dating of Paleoproterozoic mafic dyke swarms of the Dharwar craton, India: Implications for the existence of the Neoarchean supercraton Sclavia

We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177 Ma, and confirm a third at 2369–2365 Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4 ± 1.0, 2365.9 ± 1.5 and 2368.6 ± 1.3 Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365 Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth. A protracted episode of Pan-Dharwar mafic dyke swarm emplacement at ∼2.2 Ga is the most geographically widespread Paleoproterozoic mafic magmatic event currently recognized in the Dharwar craton. U–Pb baddeleyite dating of two dykes located south of the Cuddapah basin yield ages of 2220.5 ± 4.9 Ma (N–S trending Kandlamadugu dyke) and 2209.3 ± 2.8 Ma (NW–SE trending Somala dyke), indicating emplacement of two distinct subswarms in the region. Two dykes from the northern Dharwar craton (the E–W Bandepalem and NW–SE Dandeli dykes) separated by ∼550 km yield indistinguishable U–Pb ages of 2176.5 ± 3.7 (baddeleyite and zircon) and 2180.8 ± 0.9 Ma (baddeleyite), respectively. Collectively, these age data hint at the presence of a giant 2.18 Ga radiating mafic dyke swarm that spans >100,000 km 2 of the Dharwar craton, which has a focal point west of the present day Deccan Flood basalt province. The identification of Pan-Dharwar mafic dyke swarm emplacement at ∼2.2 Ga allows for new global correlations with contemporaneous mafic magmatic events previously recognized in many other Archean cratons worldwide (e.g., within Antarctica, Australia, Finland, Greenland, and North America). Increasing evidence suggests that this global mafic magmatism at ∼2.2 Ga was linked with intracontinental rifting, enhanced mantle plume activity, and the breakup of one or more Archean continents. We highlight the similarity in timing of Paleoproterozoic (2.23–2.17 Ga) mafic dyke swarm emplacement recorded in the Dharwar and Slave cratons and use the correlation of high precision U–Pb ages, geometry and regional patterns of mafic dyke swarm emplacement, and matching Archean geology in these cratons as reconstruction pins to constrain the configuration of the ∼2.2 Ga supercraton Sclavia.

Precise U–Pb and Pb–Pb dating of Phanerozoic baddeleyite by SIMS with oxygen flooding technique

Baddeleyite has long been recognized as one of the most important U-bearing minerals for dating silica undersaturated igneous rocks. Age determination of baddeleyite calls for analysis within small volumes using high-resolution secondary ion mass spectrometry (SIMS) because of its minuscule grain size as well as potential altered domains or micro-inclusions. However, precise SIMS U–Pb dating has been hampered for baddeleyite owing to crystal orientation effects that bias Pb/U ratio measured in baddeleyite. In this study we carried out a series of tests of U–Pb and Pb–Pb measurements on Phanerozoic baddeleyite using a multi-collector Cameca 1280 IMS with oxygen flooding technique. Our results demonstrate that the oxygen flooding can not only enhance secondary Pb+ ion yield by a fact of 7 for baddeleyite, but also depress the baddeleyite U/Pb orientation effect down to ∼2% (1 RSD). Therefore, Phanerozoic (as young as Cenozoic) baddeleyite can be precisely dated by SIMS Pb–Pb and/or U–Pb measurements with precision of 1–3% (2 RSE).

In situ U–Pb dating of micro-baddeleyite by secondary ion mass spectrometry

We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS) in situ analysis of ng-mass crystals that cannot be efficiently extracted by conventional mineral separation techniques. Average 207Pb/ 206Pb ages for Precambrian baddeleyite crystals are within < 0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. 206Pb/ 238U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO 2 +/U + calibration plot. Calibration is performed on separated baddeleyite crystals (∼ 100–200 μm in maximum dimension) mounted in random crystallographic orientation. 206Pb/ 238U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of 206Pb/ 238U between large crystals and micro-baddeleyite from FC4b is within ∼ 1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO 2 +/U +-based calibration to significantly reduce orientation dependent bias.

Baddeleyite: A promising geochronometer for alkaline and basic magmatism

The paper is devoted to the history of the discovery, petrological and mineralogical characterization, and U-Pb dating of baddeleyite ZrO2, which was separated from various Archean, Proterozoic, and Paleozoic rocks from Fennoscandia. The morphology of this mineral and its U-Pb age values were examined in the Archean carbonatites (2613 ± 18 Ma) of Siilinjarvi, Finland, and gabbronorite dikes (2738 ± 6 Ma) at the Kirovogorskoe deposit. U-Pb isochrons are reported for the baddeleyite-zircon pair obtained from the gabbronorites and anorthosites of the Proterozoic pyroxenite-gabbronorite-anorthosite association. The U-Pb baddeleyite dates for the early gabbronorite phase (2.5 Ga) and for a gabbronorite dike (late phase, 2.4 Ga) suggest that the basic magmatism evolved over a long time period (100 m.y.) in the Proterozoic. U-Pb dates are also reported for baddeleyite from the Paleozoic carbonatites of Kovdor, Sebljarv, and Vuorijarvi.

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions: Comment and reply: REPLY

Paleoproterozoic basins are notoriously difficult to date if they are poor in zircon-rich felsic-volcanic rocks. The detail with which basins are mapped often resolves the problem, as many contain thin zircon-rich ash beds. Dating detrital zircon is useful, but there are always questions of the

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions: Comment and reply: COMMENT

[Muller et al. (2005)][1] have presented important new age data for mafic intrusions in the Hamersley province of Western Australia, which they use to constrain the ages of the Ophthalmian orogeny and giant iron-ore deposits. Their 2208 ± 10 Ma age for dolerite sills intruded into the Turee Creek