Ga Tonalite Research Articles

A review of the inferred geodynamic evolution of the Dharwar craton over the ca. 3.5–2.5 Ga period, and possible implications for global tectonics

The geological history and evolution of the Dharwar craton from ca. 3.5–2.5 Ga is reviewed and briefly compared with a second craton, Kaapvaal, to allow some speculation on the nature of global tectonic regimes in this period. The Dharwar craton is divided into western (WDC) and eastern (EDC) parts (separated possibly by the Closepet Granite Batholith), based on lithological differences and inferred metamorphic and magmatic genetic events. A tentative evolution of the WDC encompasses an early, ca. 3.5 Ga protocrust possibly forming the basement to the ca. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are interpreted as having formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex (PGC) was possibly sourced from beneath plateau remnants, and resulted in high-grade metamorphism of Sargur Group belts at ca. 3.13–2.96 Ga. At about 2.9–2.6 Ga, the Dharwar Supergroup formed, comprising lower Bababudan (largely braided fluvial and subaerial volcanic deposits) and upper Chitradurga (marine mixed clastic and chemical sedimentary rocks and subaqueous volcanics) groups. This supergroup is preserved in younger greenstone belts with two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga, the latter approximately coincident with ca. 2.6–2.5 Ga granitic magmatism which essentially completed cratonization in the WDC. The EDC comprises 2.7–2.55 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses and migmatites, approximately coeval greenstone belts (dominated by volcanic lithologies), with minor inferred remnants of ca. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoid intrusions (including the Closepet Batholith). An east-to-west accretion of EDC island arcs (or of an assembled arc – granitic terrane) onto the WDC is debated, with a postulate that the Closepet Granite accreted earlier onto the WDC as part of a “central Dharwar” terrane. A final voluminous granitic cratonization event is envisaged to have affected the entire, assembled Dharwar craton at ca. 2.5 Ga. When Dharwar evolution is compared with that of Kaapvaal, while possibly global magmatic events and freeboard–eustatic changes at ca. 2.7–2.5 Ga may be identified on both, the much earlier cratonization (by ca. 3.1 Ga) of Kaapvaal contrasts strongly with the ca. 2.5 Ga stabilization of Dharwar. From comparing only two cratons, it appears that genetic and chronologic relationships between mantle thermal and plate tectonic processes were complex on the Archaean Earth. The sizes of the Kaapvaal and Dharwar cratons might have been too limited yet to support effective thermal blanketing and thus accommodate Wilson Cycle onset. However, tectonically driven accretion and amalgamation appear to have predominated on both evolving cratons.

Formation age and metamorphic history of the Nuvvuagittuq Greenstone Belt

The Nuvvuagittuq Greenstone Belt (NGB) in Northern Quebec, Canada, is dominated by mafic and ultramafic rocks metamorphosed to at least upper amphibolite facies. Rare felsic intrusive rocks provide zircon ages of up to ∼3.8Ga (David et al., 2009; Cates and Mojzsis, 2007) establishing the minimum formation age of the NGB as Eoarchean. Primary U-rich minerals that may provide reliable formation ages for the dominant mafic lithology, called the Ujaraaluk unit, have yet to be found. Metamorphic zircons, rutiles and monazites are present in the unit and give variably discordant results with 207Pb/206Pb ages ranging from 2.8Ga to 2.5Ga. The younger ages overlap 2686±4Ma zircon ages for intruding pegmatites (David et al., 2009) and Sm–Nd ages for garnet formation in the Ujaraaluk rocks suggesting this era as the time of peak metamorphism and metasomatism in the NGB, coeval with regional metamorphism of the Superior craton. Sm–Nd data for Ujaraaluk whole rocks scatter about a Sm/Nd vs. 143Nd/144Nd correlation (MSWD=134) whose slope would correspond to 3.6±0.2Ga if interpreted as an isochron. This “isochron” is seen to consist of a series of younger ∼3.2–2.5Ga slopes for the different geochemical groups within the Ujaraaluk, emanating from a baseline distribution older than 4Ga. The 146Sm–142Nd chronometer is less affected by metamorphism at 2.7Ga because of 146Sm extinction prior to ∼4Ga. Expansion of the 142Nd dataset for the Ujaraaluk and associated ultramafic rocks continues to show a good correlation between Sm/Nd ratio and 142Nd/144Nd that corresponds to an age of 4388+15−17Ma. The dataset now includes samples with superchondritic Sm/Nd ratios that extend the correlation to 142Nd excesses of up to 8ppm compared to the terrestrial standard with a total range in 142Nd/144Nd of 26ppm. The upper Sm/Nd ratio end of the Ujaraaluk correlation is defined by rocks that are interpreted to be cumulates to compositionally related extrusive rocks indicating that this crystal fractionation had to occur while 146Sm decay was active, i.e. well before 4Ga. Intruding gabbros give 143Nd and 142Nd isochron ages of respectively 4115±100Ma and 4313+41−69Ma, also supporting an Hadean age for the gabbros and providing a minimum age for the intruded Ujaraaluk unit. 3.6Ga tonalites surrounding the NGB, 3.8Ga trondhjemitic intrusive veins, and a 2.7Ga pegmatite show a deficit in 142Nd compared to the terrestrial standard. These felsic rocks plot to the low Sm/Nd ratio side of the Ujaraaluk isochron and do not show a correlation between their Sm/Nd and 142Nd/144Nd ratios, which can be explained if they are melts of ancient LREE-enriched mafic rocks, such as the Ujaraaluk, with the melting occurring after 146Sm was extinct. A subset of least disturbed Ujaraaluk samples has coherent isotopic compositions for both short-lived and long-lived Nd isotopic systems giving 143Nd and 142Nd isochron ages overlapping within error of 4321±160Ma (MSWD=6.3) and 4406+14−17Ma (MSWD=1.0), respectively. This age represents our best age estimate for the Ujaraaluk unit. The NGB thus preserves over 1.6 billion years of early Earth history including an expanse of mafic crust formed in the Hadean.

Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust

To investigate formation of the Earth9s earliest continental crust, partial-melting experiments were conducted (at 900–1100 °C and 0.5–3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5–3.0 GPa and 950–1100 °C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth9s early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (>30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5–3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.

Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu–Hf isotope analysis of zircon

The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94–4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (> 4.03 Ga) crust. In this study, we have determined the Lu–Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu–Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have ε Hf( T) of − 2.4 ± 2.2 and − 3.2 ± 2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an ε Hf( T) of − 6.1 ± 2.5. This indicates that the granitoid formed by remelting of very old crust. The ε Hf( T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are − 1.2 ± 3.3 and − 3.3 ± 1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids.

Whole-rock geochemistry of some tonalite and high Mg/Fe gabbro, diorite, and granodiorite plutons (sanukitoid suites) in the Kuhmo district, eastern Finland

New whole-rock geochemical analyses (major and trace elements) of three tonalite plutons and high Mg/Fe granodiorites and diorites, termed sanukitoid suites, are presented from Neoarchaean Kuhmo district, eastern Finland. The 2.83 Ga and 2.78 Ga tonalites display only moderate fractionation of LREE/HREE thus demanding only minor or no residual garnet in their respective source regions. However, the more prominent HREE depletion in the 2.75 Ga tonalites requires more significant garnet residue. The relatively wide compo sitional variation in the tonalites implies that Archaean TTG (tonalite-trondhjemite-granodiorite) magmatism was derived in general from sources that exhibited considerable mineralogical heterogeneity. The ~2.70 – 2.74 Ga high-Mg/Fe granodiorites and diorites are characterised by strong fractionation of LREE/HREE, relatively high Ba and Sr, and elevated abundances of Cr and Ni as well systematic depletions in P and Ti, which are all typical features of Archaean sanukitoid suites. Some of the granodiorite plutons contain compositionally intermediate high-K enclaves, which share these geochemical characteristics, implying a contribution of mafic-intermediate LILE- and LREE- enriched source for the magma. The geochemical characteristics of the studied plutonic rocks demonstrate the role of contrasting sources for the tonalites and sanukitoid suites in the Kuhmo district.

Sedimentary and structural evidence for 2.7 Ga continental arc–oceanic-arc collision in the Savant–Sturgeon greenstone belt, western Superior Province, Canada

The western Superior Province sustained rapid crustal growth in the interval 2.72–2.68 Ga through amalgamation of microcontinental crustal blocks and juvenile oceanic terranes. Recent field, isotopic, and geophysical surveys provide insight on the nature, timing, and scale of this accretionary growth. However, few places offer the rich tectono-stratigraphic and structural detail with which to establish accretion of oceanic and continental blocks as does the Savant–Sturgeon area. Here, 3.4–2.8 Ga continental crust of the Winnipeg River terrane is juxtaposed with 2.775–2.718 Ga juvenile oceanic rocks of the western Wabigoon terrane across a 2.85–2.75 Ga, southwest-facing, continental margin sequence. The continental margin was reactivated at ~2.715 Ga with the establishment of an arc, recorded by 2.715–2.70 Ga tonalite and associated intermediate volcanic rocks. This magmatic activity is interpreted to reflect north- and east-dipping subduction that led to consumption of a small tract of oceanic crust between the Winnipeg River and western Wabigoon terranes, ultimately leading to their amalgamation after 2.703 Ga. The telescoped fore arc also includes continental-derived turbiditic wacke, siltstone, and iron formation (Warclub assemblage) that are in tectonic contact with diverse oceanic rocks of the western Wabigoon terrane. Collision is bracketed between 2.703 Ga (the maximum age of marine fore arc deposits) and ~2.696 Ga (the minimum age of a late-tectonic pluton). Effects include thrust stacking and the development of shallow-plunging folds and bedding-parallel fabrics (D1), overprinted by steeply plunging inclined folds, steep foliations, and shear zones (D2). Collectively, these structures have penetratively reworked the suture between the ancient fore-arc and oceanic rocks in the Savant–Sturgeon area.

Growth of Archaean crust in the Kuhmo district, eastern Finland: U–Pb and Sm–Nd isotope constraints on plutonic rocks

Neoarchaean plutonism in the Kuhmo district in eastern Finland, Karelia, includes tonalites, sanukitoids, leucocratic granodiorites, and leucogranites. A U–Pb (SIMS and TIMS) study of zircons and titanites places age constraints on this plutonic activity, and whole-rock Sm–Nd isotopes are used to characterise source variation. U–Pb results indicate that the tonalites were emplaced during at least three separate periods; >2.81, ∼2.78, and 2.76–2.74 Ga. The initial ɛ Nd values vary from +0.3 to +2.2, and suggest no major involvement of significantly older crust in the genesis of tonalites. The tonalites were followed by 2.74–2.70 Ga (possibly also 2.68 Ga) sanukitoids with initial ɛ Nd values of −0.7 to 1.2. Close temporal relations between the ∼2.74 Ga tonalites and sanukitoids indicate that their genesis may be linked. Subsequent leucocratic granodiorites and leucogranites were emplaced at 2.70–2.68 Ga and commonly contain inherited zircons. The initial ɛ Nd values of these granodiorites and granites vary from +1.1 to −3.4, indicative of multiple magma sources. The general decrease in initial ɛ Nd values with decreasing age demonstrate progressive recycling of pre-existing crust, thus stressing the importance of crustal recycling as a process in forming the leucocratic granitoid rocks. Overall, the Archaean plutonic rocks of the Kuhmo district are temporally similar to those in the adjacent Russian Karelia, suggesting that 2.83–2.68 Ga was one of the most extensive periods of crustal growth throughout the Karelia craton.

U-Pb ages (3.8–2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, Superior Craton, Canada

We report new U-Pb ages and Sm-Nd isotope data for an Eoarchean volcano-sedimentary sequence in the northeast Superior Province of northern Quebec, Canada. The Nuvvuagittuq supracrustal sequence is characterized by mafi c amphibolite rocks with rare felsic schists, accompanied by ultramafi c sills, oxide-rich and quartz-rich iron formations, and possible conglomeratic units, with peak metamorphism having reached upper amphibolite facies. The sequence crops out as an open, shallow, south-plunging synform with tonalites in the center and around the exterior margins of the fold structure. A 3817 ± 16 Ma age from a felsic schist is interpreted to represent a maximum age for the sequence, whereas a 3661 ± 4 Ma age from the enclosing tonalites suggests a minimum age. Detrital zircons from a conglomeratic unit yielded similar old ages of ca. 3.78 Ga. Younger ages of ca. 3.66 and 3.3 Ga from a conglomeratic unit and a felsic schist, and 2.7 Ga ages from an amphibolite gabbro and a pegmatite dike, underline the polymetamorphic history of the sequence. Initial Nd isotope compositions (eNd 3.8 = −0.5 to + 2.3) for mafi c amphibolites and felsic schists are indicative of a moderately depleted mantle. Sm-Nd analyses of a 2.7 Ga pegmatite dike and the ca. 3.66 Ga tonalites both yield Nd depleted mantle model ages (TDM) of ca. 3.9 Ga, indicating that they were derived from a crust of juvenile Eoarchean origin. We propose that fragments of Paleoarchean to Eoarchean terranes that rim the Superior Province represent remnants of a larger terrane(s) that was rifted and dismembered prior to the formation of the 3.0‐2.7 Ga Superior Craton.

Geochemistry of the NeoArchaean high-Mg basalts, boninites and adakites from the Kushtagi–Hungund greenstone belt of the Eastern Dharwar Craton (EDC); implications for the tectonic setting

Volcanic rocks of the Kushtagi–Hungund greenstone belt from the Eastern Dharwar Craton (EDC) show extreme diversity due to the presence of high-Mg, high Fe-, high Mg–Fe basalts, high Mg-dacites and andesites (boninitic affinities), and sodic–plagioclase–quartz rich felsic rocks (adakites with Na 2O (∼4–6 wt.%). The adakites ( Drummond and Defant, 1990; Martin, 1999) and high-Mg-dacites–andesites are being reported for the first time from greenstone belts of the Dharwar craton. Trace elements in these different types of volcanic rocks have distinct characteristics. For example, the basalts have (1) Al 2O 3/TiO 2 ∼16 (PM value of 17), (2) (La/Yb) N ∼1–3, (3) low order negative anomalies of Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti*, (4) Zr/Nb ∼4–10 and (5) Ce/Nb ∼3–4. Two samples of andesites having Mg-enrichment and Al 2O 3/TiO 2 ratios of 31–35 are interpreted to have boninitic affinities. Other dacites and andesites are characterized by (1) high-Mg (8–10 wt.%), (2) high Al 2O 3/TiO 2=22–27, (3) Zr/Nb=8–14, (4) (La/Yb) N ∼2–7, (5) negative Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti* anomalies and (6) Sc/Lu=67–172. The adakites are characterized by (1) a narrow range of SiO 2 (67–70 wt.%), (2) enriched Na 2O (4–6 wt.%) with very high Na 2O/K 2O (2.5–7.7), (3) Low Mg# (0.38–0.48), (4) high (La/Yb) N (13–31) with fractionated patterns, no Eu anomaly and (Gd/Yb) N =2–4, (5) very high Al 2O 3/TiO 2 (50–80) and (6) enriched Ni, Cr and Co (total 50 ppm). These adakites resemble 3.0 Ga tonalite–trondhjemite gneisses of the Dharwar craton, which are interpreted as products of partial melting of the basaltic slab. The available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting.

3.56 Ga tonalite in the central part of the Bastar Craton, India: oldest Indian date

The lack of information regarding Early Achaean rocks from the Indian Peninsula is mainly due to the absence of high-quality U–Pb zircon ages. A tonalite sample from the TTG gneiss-supracrustal association from the central part of the Bastar craton in central India is dated at 3.56 Ga. This is the oldest protolith age so far recorded from India. These TTG gneisses have enclaves of metasedimentary and metavolcanic rocks that may provide important information on the first-billion year history of the Indian craton. This new date for tonalite from the Bastar craton is the first precise U–Pb zircon age from India's first U–Pb geochronology laboratory, located at the Geological Survey of India which hopefully will remedy the paucity of good geochronological data for older rocks of the Indian peninsula.

Xenon compositions of magmatic zircons in 3.64 and 3.81 Ga meta-granitoids from Greenland – a search for extinct 244Pu in ancient terrestrial rocks

In order to solve the fundamental problem as to whether the extinct radioactive isotope 244Pu was present in the early Earth, we have analysed xenon in magmatic zircons from three early Archaean meta-granitoids from Greenland. Two samples, a granite (sample G97/111) and a ferrogabbro/ferrodiorite (sample G97/112), are from the same ca 3.64 Ga composite intrusion at the mouth of Ameralik fjord. The third sample (sample G97/018) is a ca 3.81 Ga tonalite from a locality south of the Isua supracrustal belt. The 3.81 Ga tonalite (sample G97/018) and possibly the 3.64 Ga granite (sample G97/111) magmatic zircons appear to have small excesses of fission xenon different from the 238U-derived spontaneous fission xenon. This excess fission xenon is consistent with that expected for 244Pu-derived spontaneous fission, and it may indicate that these zircons incorporated 244Pu when they crystallised. If this is true, it suggests the existence of 244Pu early in the Earth’s history. Furthermore, our preliminary investigation supports views that the Earth’s primitive material had a near-chondritic 244Pu/ 238U abundance ratio and that fissiogenic xenon in the atmosphere is of plutonic origin.

On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth’s earliest magmatic differentiation

We present whole-rock and zircon rare earth element (REE) data from two early Archaean gneisses (3.81 Ga and 3.64 Ga) from the Itsaq gneiss complex, south-west Greenland. Both gneisses represent extremely rare examples of unaltered, fresh and relatively undeformed igneous rocks of such antiquity. Cathodoluminescence imaging of their zircons indicates a single crystallisation episode with no evidence for either later metamorphic and/or anatectic reworking or inheritance of earlier grains. Uniform, single-population U/Pb age data confirm the structural simplicity of these zircons. One sample, a 3.64 Ga granodioritic gneiss from the Gothåbsfjord, yields a chondrite-normalised REE pattern with a positive slope from La to Lu as well as substantial positive Ce and slight negative Eu anomalies, features generally considered to be typical of igneous zircon. In contrast, the second sample, a 3.81 Ga tonalite from south of the Isua Greenstone Belt, has variable but generally much higher light REE abundances, with similar middle to heavy REE. Calculation of zircon/melt distribution coefficients ( D REE zircon/melt) from each sample yields markedly different values for the trivalent REE (i.e. Ce and Eu omitted) and simple application of one set of D REE zircon/melt to model the melt composition for the other sample yields concentrations that are in error by up to two orders of magnitude for the light REE (La–Nd). The observed light REE overabundance in the 3.81 Ga tonalite is a commonly observed feature in terrestrial zircons for which a number of explanations ranging from lattice strain to disequilibrium crystallisation have been proposed and are further investigated herein. Regardless of the cause of light REE overabundance, our study shows that simple application of zircon/melt distribution coefficients is not an unambiguous method for ascertaining original melt composition. In this context, recent studies that use REE data to claim that >4.3 Ga Hadean detrital zircons originally crystallised from an evolved magma, in turn suggesting the operation of geological processes in the early Earth analogous to those of the present day (e.g. subduction and melting of hydrated oceanic crust), must be regarded with caution. Indeed, comparison of terrestrial Hadean and >3.9 Ga lunar highland zircons shows remarkable similarities in the light REE, even though subduction processes that have been used to explain the terrestrial zircons have never operated on the Moon.

A Mainly Crustal Origin for Tonalitic Granitoid Rocks, Superior Province, Canada: Implications for Late Archean Tectonomagmatic Processes

The central Wabigoon subprovince of the Superior Province, like most plutonic domains within Archean cratons, is dominated by granitoid rocks of the tonalite–trondhjemite–granodiorite (TTG) series. Heterogeneous <2·83–2·74 Ga tonalite gneisses and foliated tonalite to granodiorite units, emplaced at 2·722–2·709 Ga, exhibit initial εNd values (–3·1 to +3·3) indicative of variable input from light rare earth element enriched older (3·2–3·4 Ga) crustal materials. Their δ18O (VSMOW) range (+7·1 to +8·9‰), which overlaps closely that of average upper Superior Province crust, indicates input from high-18O crustal materials. The preferred petrogenetic model for Wabigoon tonalitic rocks involves partial melting of overthickened amphibolite-dominated lower-crustal materials within a Cordilleran-type arc. Assimilation of >2·74 Ga tonalite gneiss crust by younger tonalite magmas was probably an important process. Unlike the model of TTGs representing direct partial melts of subducting slabs in an arc setting, this model requires no direct tie to subduction. Careful re-evaluation of the TTG classification is required for it can mistakingly ‘pigeon-hole’ temporally and genetically unrelated rocks and perhaps assume an incorrect petrogenetic or tectonic model. An important role of crustal recycling processes in central Wabigoon tonalite petrogenesis is in keeping with evidence that supports substantial rates of continental recycling as far back as the earliest Archean.

GEOLOGY OF THE SOUTHERN AMAZON CRATON IN SOUTHWESTERN MATO GROSSO, BRAZIL: A REVIEW

The southern Amazon Craton in Mato Grosso, is reviewed in light of more recent geological, petrological, structural and geochronological data. Jaurú and Rio Alegre terranes, the Santa Helena Granite Complex and the Aguapeí Belt are the major tectonic units of the area. The Jaurú terrain consists of MORB suite and arc basalts with minor chert and felsic volcanics, calc-alkaline tonalites, orthogneisses, and migmatites of ca. 1.78 Ga. Two well-marked granitic events occurred in this terrain between 1.55 to 1.48 Ga (the Cachoeirinha magmatic arc) and close to 1.0 Ga (the Guapé Intrusive Suite). The Rio Alegre terrain consists of mafic volcanics and ultramafic intrusives with minor chert and tonalite intrusions and its minimum age is given 1.55 Ga tonalites. The Santa Helena Granite Complex (1.45 Ga) is a NW-trending, A-type, hornblende-biotite syenogranite batholith with abundant xenoliths of older rocks and emplaced between the Rio Alegre and Jauru terranes. The Aguapeí Belt is a narrow NW-trending zone that follows the limits between the Rio Alegre terrain and the Santa Helena Complex. It consists of low-grade, folded shallow marine to fluvial sediments. The Jaurú Terrain is interpreted as representing an island arc and the Rio Alegre as ophiolite, both with an age between 1.9 Ga and 1.55 Ga. The Cachoeirinha magmatic arc of ca. 1.55 Ga is likely to result from the amalgamation of these two terranes. The suture between the Rio Alegre and Jaurú terranes was intruded by the Santa Helena Granite Complex at about 1.45 Ga. The youngest events (1.2 Ga to 1.0 Ga) are represented in the Aguapeí Belt, by extensional reactivation, intracontinental rifting, sedimentation, deformation, and low-grade metamorphism, and are correlated to the Sunsas Orogeny of eastern Bolivia.

Geology of the Southern Amazon craton in Southwestern Mato Grosso, Brazil: a review

The southern Amazon Craton in Mato Grosso, is reviewed in light of more recent geological, petrological, structural and geochronological data. Jauru and Rio Alegre terranes, the Santa Helena Granite Complex and the Aguapei Belt are the major tectonic units of the area. The Jauru terrain consists of MORB suite and arc basalts with minor chert and felsic volcanics, calc-alkaline tonalites, orthogneisses, and migmatites of ca. 1.78 Ga. Two well-marked granitic events occurred in this terrain between 1.55 to 1.48 Ga (the Cachoeirinha magmatic arc) and close to 1.0 Ga (the Guape Intrusive Suite). The Rio Alegre terrain consists of mafic volcanics and ultramafic intrusives with minor chert and tonalite intrusions and its minimum age is given 1.55 Ga tonalites. The Santa Helena Granite Complex (1.45 Ga) is a NW-trending, A-type, hornblende-biotite syenogranite batholith with abundant xenoliths of older rocks and emplaced between the Rio Alegre and Jauru terranes. The Aguapei Belt is a narrow NW-trending zone that follows the limits between the Rio Alegre terrain and the Santa Helena Complex. It consists of low-grade, folded shallow marine to fluvial sediments. The Jauru Terrain is interpreted as representing an island arc and the Rio Alegre as ophiolite, both with an age between 1.9 Ga and 1.55 Ga. The Cachoeirinha magmatic arc of ca. 1.55 Ga is likely to result from the amalgamation of these two terranes. The suture between the Rio Alegre and Jauru terranes was intruded by the Santa Helena Granite Complex at about 1.45 Ga. The youngest events (1.2 Ga to 1.0 Ga) are represented in the Aguapei Belt, by extensional reactivation, intracontinental rifting, sedimentation, deformation, and low-grade metamorphism, and are correlated to the Sunsas Orogeny of eastern Bolivia.

Distinguishing Grenvillian basement from pre-Taconian cover rocks in the Northern Appalachians

Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accu- rate structural interpretations. The Cavendish Formation in southeastern Ver- mont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimen- tary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex.

U–Pb geochronology of late Neoarchean tonalites in the Flin Flon Belt, Trans-Hudson Orogen: surprise at surface

The Flin Flon volcanic belt of the Trans-Hudson Orogen in northern Manitoba comprises magmatic rocks (1.88 – 1.90 Ga) generated in an intra-oceanic arc and identified as one of the best examples of juvenile Paleoproterozoic crust. In the Flin Flon – Athapapuskow Lake area, the belt consists of a series of fault-bounded blocks, each having distinct stratigraphic and magmatic affinities, juxtaposed to form an accretionary collage. Two tonalitic bodies within the Northeast Arm shear zone, a major deformation corridor separating two of the most important blocks in the area, have been dated by the U–Pb zircon method at [Formula: see text] and [Formula: see text]. These results indicate the presence of fragments of late Neoarchean crust within the Flin Flon belt, possibly related to ca. 2.5 Ga granitic bodies found in terranes beyond the western limit of the belt. The presence of ca. 2.5 Ga tonalites within the Flin Flon Belt, herein reported for the first time, has major implications for basement involvement in the early tectonic evolution of the Flin Flon Belt.