The Dharwar craton, southern India, and its Late Archaean plate tectonic setting: current interpretations and controversies

In spite of detailed geological investigations of the Dharwar craton since the 1890s, its principal lithological units, structure and chronology remain contentious. Important new work on lithostratigraphy, basin development, structure, geochemistry and geochronology has led to wide-ranging speculation on the Late Archaean plate tectonic setting. Much of the speculation is based on uniformitarian models which contrast with a recent proposal that the evolution of the craton was controlled by gravity-driven processes with no crustal shortening.     

Rare earth element geochemistry and Rb-Sr geochronology of Archaean stromatolitic cherts of the Dharwar craton, south India

Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their Al/Al + Fe ratios but depleted in Fe₂C₃ and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 ± 159 Ma and initial Sr ratio of 0.7128 ± 0.0012 (2σ). While this age is strikingly close to that of regional metamorphism in the Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time of regional metamorphism.     

Geochemical systematics of the Mauranipur-Babina greenstone belt,Bundelkhand Craton,Central India:Insights on Neoarchean mantle plume-arc accretion and crustal evolution

The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts,felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations.The intrusive and extrusive komatiitic basalts are characterized by low SiO_2(39-53 wt.%), high MgO(18-25 wt.%).moderately high Fe_2O_3(7.1-11.6 wt.%), Al_2O_3(4.5-12.0 wt.%), and TiO_2(0.4-1.23 wt.%)with super to subchondritic(Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO_2, TiO_2, MgO, Fe_2O_3, Al_2O_3 and CaO. At similar SiO_2 content of type Ⅰ and Ⅲ basalts, the type II basalts show slightly high Al_2O_3 and Fe_2O_3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type Ⅰ and Ⅱ basalts. TypeⅢ basalts show high Zr/Nb ratios(9.8-10.4), TiO_2(1.97-2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO_2(55-64 wt.%), moderate TiO_2(0.4-0.7 wt.%), slightly low Al_2O_3(7-11.9 wt.%), medium to high MgO(3-8 wt.%) and CaO contents(10-17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt-andesite dacite-rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta,Nb/Th, high MgO contents(20 wt.%), CaO/Al_2O_3 and(Gd/Yb)_N 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subductionmetasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.     

Chromite and PGE deposits of mesoarchaean ultramafic-mafic suites within the greenstone belts of the Singhbhum craton,India: Implications for mantle heterogeneity and tectonic setting

The Archaean cratonic nuclei of the continents are important as they contain the most significant evidences for the evolution of Earth e.g. the greenstone sequences. In the Indian Shield, one of the important cratons is the Singhbhum craton, where nearly 95% of the Indian chromite deposits and only PGE deposits are located which are hosted within Mesoarchaean ultramafic-mafic rock sequences. The ultramafic units occur as sill like intrusions within the Iron Ore Group (IOG) greenstone belts and often associated with gabbroic intrusions. In the Nuasahi and Sukinda mining districts of these occurrences, detailed petrological, geochemical and isotopic studies have been carried out in the last decades. Petrological and geochemical studies indicate a supra-subduction zone (SSZ) tectonic settings in Archaean for the origin of these ultramafic-mafic sequences. The Os isotopic and platinum group element (PGE) geochemical studies of chromites from the two mining districts indicate presence of a subchondritic source mantle domain beneath and within the Singhbhum craton similar to the Zimbabwean craton of southern African continent. The Os model age calculation indicates melt extraction from a subcontinental lithospheric mantle (SCLM) before 3.7 Ga which is similar to the other ancient cratons. As a whole the study supports the premise that India was part of the African continent in pre-Gondwana times and even in early Archaean and suggest possible amalgamation and building up of a supercontinent during late Archaean. However, in comparison with other occurrences, the Singhbhum craton of the Indian Shield and the Zimbabwean craton in southern Africa are characterized by the presence of subchondritic lithospheric mantle domains within the SCLM, which were developed prior to 3.7 Ga.     

Two types of Archaean supracrustal belts in the Bundelkhand craton,India: Geology,geochemistry, age and implication for craton crustal evolution

Groundwater sampling was carried out in fast growing Vijayawada urban agglomeration in Andhra Pradesh state with a view to generate base line data and to assess groundwater quality, its variations vis-à-vis urbanization and hydrochemical characteristics. The groundwaters are found to be fresh to brackish, hard to very hard, often enriched with nitrate, phosphate,and faecal colliform indicating anthropogenic influence on groundwater on account of urbanization. Manganese and iron of geogenic origin are also found to be in high concentrations in some localities. The quality deterioration is more in shallow aquifers, as also in core urban and red soil covered areas. Spatial variations in groundwater quality are discernible, with high concentrations of SO₄^-4–,NO₃^- in core urban area suggesting the impact of urbanization. A wide range of chemical constituents indicate water rock interactions as influenced by anthropogenic activities controlling the urban aquifers. The order of abundance of cations is Na>Mg>Ca>K, while the order of abundance of anions is Cl>HCO₃>SO₄>NO₃. The baseline study suggests the need to protect the groundwater resource through sound environmental protection measures for the welfare of the inhabitants in the city around which the new capital is proposed to be constructed for newly formed Andhra Pradesh state in India.     

新太古代清原绿岩带下甸子BIF铁矿地质特征及含黄铁矿条带BIF的成因探讨

新太古代清原绿岩带位于华北克拉通北缘,该绿岩带中发育独特的VMS和BIF(Algoma型)组合,其中下甸子铁矿是此类BIF的典型代表。下甸子BIF赋存于绿岩带南天门组下部,围岩及夹层为斜长角闪岩及少量石榴云母片岩。矿体夹层斜长角闪岩中锆石的SIMS年代学分析获得了2497.8±7.4Ma的变质年龄,而原位氧同位素分析显示变质锆石的δ18O值为5.3‰~6.2‰,与现代地幔基本一致,表明在变质过程中其锆石的氧同位素体系保持稳定。BIF矿石类型主要有硅酸盐型和含黄铁矿条带型两种,前者的矿物组合为石英、磁铁矿和铁阳起石,而后者的矿物组合为石英、磁铁矿、阳起石、黄铁矿和少量方解石。大部分矿石的Al_2O_3、TiO_2和HFSE(如,Zr、Hf、Th、U等)含量极低,说明其未受到碎屑物质混染,PAAS标准化后,两类矿石稀土元素显示与海水类似的特征,即La、Y的正异常和LREE相对于HREE的亏损;同时,显著的正Eu异常指示成矿过程中有海底高温热液组分的参与;此外,所有样品均无明显的Ce异常,表明其沉淀自还原的海水中。通过与华北地区其他Algoma型BIF对比发现,下甸子两类矿石均具有较高的CaO/(CaO+MgO)值以及接近球粒陨石的Y/Ho值,表明其可能沉淀环境与海底热液喷口较近,且热液组分(以高温热液为主,可能有少量低温热液)的贡献较大。相比于硅酸盐型矿石,含黄铁矿条带型矿石的HREE含量较低、Eu正异常和LREE含量偏高,这可能与其沉淀过程中海底的局部热液的脉动式活动有关,其中黄铁矿条带可能为热液喷流沉积成因。BIF围岩斜长角闪岩的地球化学特征分析显示,其原岩玄武质岩石的岩浆可能来自亏损地幔,但在上升过程中受到了少量陆壳物质的混染,结合前人对清原绿岩带表壳岩系和TTG的年代学及地球化学研究,推测下甸子BIF可能形成于晚太古代洋陆俯冲过程中的火山弧或弧后盆地环境中。     

印度达尔瓦尔克拉通绿岩带BIF型铁矿地质特征及成因分析

印度铁矿储量约占全球的7%,矿石类型以前寒武纪BIF铁矿为主,其中产于绿岩带以及绿片岩相岩石中的BIF型铁矿是印度最重要的铁矿类型。南印度地盾达尔瓦尔克拉通发育众多绿岩带,绿岩带中发育大规模BIF铁矿,BIF铁矿属于不同的地层序列,有不同的岩石组合关系。笔者对吉德勒杜尔加绿岩带和库斯赫塔吉绿岩带BIF地球化学分析表明,根据Al2O3含量,BIF分为页岩BIF（Al2O3≥2%）和石英岩BIF（Al2O3≤2%）,BIF呈石英氧化物相,碳酸盐相和硫化物相BIF主量和微量元素表明BIF为陆源碎屑沉积和火山碎屑沉积共同作用形成;稀土元素表明BIF铁矿呈Ce负异常和Eu正异常。达尔瓦尔克拉通测年数据表明,经过2.7~2.65 Ga和2.58~2.54 Ga两期主要的火山作用,2.7~2.6 Ga和2.58~2.52 Ga 2个阶段的大陆增生作用,形成了达尔瓦尔克拉通和绿岩带。BIF成矿来源上,AMOR的高温热液提供大量的Fe和SiO2,海洋中生物光合作用提供了O2,在化学沉积和碎屑沉积共同作用下,形成了BIF铁矿。     

Geology and geochemistry of giant quartz veins from the Bundelkhand Craton,central India and their implications

Giant quartz veins (GQVs; earlier referred to as ‘quartz reefs’) occurring in the Archean Bundelkhand Craton (29,000 km²) represent a gigantic Precambrian (∼2.15 Ga) silica-rich fluid activity in the central Indian shield. These veins form a striking curvilinear feature with positive relief having a preferred orientation NE-SW to NNE-SSW in the Bundelkhand Craton. Their outcrop widths vary from ≤1 to 70m and pervasively extend over tens of kilometers along the strike over the entire craton. Numerous younger thin quartz veins with somewhat similar orientation cut across the giant quartz veins. They show imprints of strong brittle to ductile-brittle deformation, and in places are associated with base metal and gold incidences, and pyrophyllite-diaspore mineralization. The geochemistry of giant quartz veins were studied. Apart from presenting new data on the geology and geochemistry of these veins, an attempt has been made to resolve the long standing debate on their origin, in favour of an emplacement due to tectonically controlled polyphase hydrothermal fluid activity.     

恒山绿岩带的构造特征—晚太古代大陆裂谷作用证据

恒山太古宙绿岩带呈断片或复式向形穿插于片麻岩基底内，绿岩带地层连续性较好，可以恢复出完整的沉积－火山地层序列。主要经历二期变形，早期逆冲褶皱变形和晚期右旋走滑变形。为绿帘角闪岩相－角闪岩相变质。构造恢复表明，绿岩带的形成明显晚于基底，并整合其上，且在其演化过程中对基底产生叠加影响，最合适的构造环境为大陆裂谷，在大陆裂谷闭合过程中。绿岩带及克拉通基底曾发生双向的逆冲作用，基底发生重新活动，并且受到开裂时形成的正断层的控制。当裂谷闭合到一定程度，由于受北部刚性基底的制约，横向挤压不再能调节压应力时，发生了右旋走滑变形，并使早先构造线向ＮＥＥ－ＥＷ向偏转。恒山绿岩带对应于五台山区、太行山北段、灵丘南山等地的五台群，有力地证明绿岩带形成于太古代末期－早元古代陆内裂谷环境，代表华北克技通演化过程中的重要事件，标志着晚太古代华北克拉通已达到相当大的规模，并发生ＮＥ向的刚性破裂。     

Paleomagnetism and geochronology of the Gwalior Sills,Bundelkhand craton,Northern India Block: New constraints on Greater India assembly

We present an updated paleomagnetic pole from the Gwalior Sills in the Bundelkhand craton within the Northern India Block (NIB). Geochronological results from baddeleyite grains from one of the sills yielded an age of 1719 ± 7 Ma which together with a previously published age indicates the emplacement of sills between 1712 and 1756 Ma (∼1730 Ma). The paleomagnetic pole calculated from additional sites in this study, combined with previous studies, falls at 13.5°N, 173.7°E (A95 = 3.6°, K = 98) indicating near equatorial latitudes for northern India. Limestone sampled a few meters above the contact with the sill exhibits similar directions consistent with having been baked by the sill. The pole does not resemble any younger poles from Peninsular India and receives a reliability score of R = 5. Dykes in the Singhbhum craton are slightly older (1765 Ma) and indicate low paleolatitudes for the Southern Indian Block (SIB). Although the Gwalior and Singhbhum poles data indicate low latitudes for both the NIB and SIB, they are statistically different and indicate that a rotation of at least 65° is required to bring the poles into accord. We propose that the NIB and SIB were in proximity but were separated by an ocean basin. We propose the name Gotosindhu (‘Ancient Sea’) for the body of water separating the NIB and SIB. We also review previous models for the assembly of the Columbia supercontinent during this time and critically examine the position of the NIB/SIB in those reconstructions.     

Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates

U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east.     

The Wide Spread 2 Ga Dyke Activity in the Indian Shield -Evidences from Bundelkhand Mafic Dyke Swarm, Central India and Their Tectonic Implications

The mafic dyke swarms are important feature of the Proterozoic and in parts of some stabilised cratonic areas. The early Proterozoic Bundelkhand massif of Central India is extensively intruded by suites of NW-SE and NE-SW trending mafic and ultramafic dykes. These dykes are mostly dolerites with subordinate pyroxenite, or lamproites, moreover, geochemical signatures of the two compositional types are different for the NW-SE and NE-SW trending suites. ⁴⁰Ar/³⁹Ar age determinations of the dolerite dykes suggest two phases of dyke activity at c.2150Ma and c.2000 Ma in this region. The dolerites are typically tholeiites and quartz normative types represented by Group I and Group II, whilst the ultramafics are komatiite or basaltic komatiite in composition and show an olivine-normative character. Rare earth element (REE) patterns show some enrichment of LREE and exhibit both positive and negative Eu anomalies. Most of the tholeiites display incompatible elements patterns indicative of an enriched mantle source, whilst those of the ultramafics indicate a depleted source. The 2 Ga event is a global event and well documented in various parts of Singhbhum, Aravalli terrane, Tamilnadu, Andhra Pradesh and Kerala regions of Indian Peninsular Shield and many parts of globe. The genesis of these dyke swarms clearly constitutes a major thermal event affecting the Earth's mantle during that period.     

Geology,geochemistry and geochronology of the Archaean Peninsular Gneiss around Gorur,Hassan District,Karnataka, India

The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF₁). These have been over printed by near-coaxial open folding (DhF₁₂) and non-coaxial upright folding on almost N-S trend (DhF₂). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF₁ deformation. The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (Ce_N/Yb_N < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7). A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sr _i of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr _i = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.     

Geochronology (SHRIMP II) of zircons from Archean stratotectonic associations of Karelian greenstone belts: Significance for stratigraphic and geodynamic reconstructions

Individual grains of zircon from the Archean Kostomuksha, North Karelian and Matkalakhta greenstone belts, which are situated respectively in western, northern and eastern Karelia, are studied using the ion microprobe SHRIMP II. As a result, the oldest ²⁰⁷Pb/²⁰⁶Pb ages of 3151 ± 4.6 and 3329 ± 16 Ma are first determined for detrital zircons from northern and eastern Karelia. The ²⁰⁷Pb/²⁰⁶Pb ages estimated for two subsequent metamorphic events of Archean Eon in eastern Karelia correspond to 3.25 and 3.17–3.10 Ga. The age value of 2711 ± 9.6 Ma is determined for silicic volcano-plutonic complex and quartz stockwork in northern Karelia and the date 2821 ± 15 Ma for magmatic rocks of eastern Karelia. Silicic volcanics from an oceanic plateau section in the Kostomuksha belt are dated at 2791.7 ± 6.1 Ma for the first time in the Archean of Fennoscandia. The oldest detrital zircons from siliciclastic metasediments determine the stabilization time of Archean continental nuclei in East Fennoscandia. The younger generation of greenstone belts is exemplified in the Karelian craton by the Matkalakhta and Kostomuksha structures comprising rock associations less than 2.82 Ga old, mafic rocks of the Kontokki Group included. Geological history of these belts corresponds to geodynamic mesocycle 90–110 Ma long and to the Archean global epoch of metallogeny, which was responsible for origin of most valuable deposits of base and precious metals.     

A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the ⁴⁰Ar/³⁹Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.     

Geochemistry of the Neoarchaean mafic volcanic rocks of the Geita area, NW Tanzania: Implications for stratigraphical relationships in the Sukumaland greenstone belt

Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at 2823 Ma.These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.     

Geochemistry and geochronology of mafic rocks from the Vespor suite in the Juruena arc,Roosevelt-Juruena terrain,Brazil: Implications for Proterozoic crustal growth and geodynamic setting of the SW Amazonian craton

This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (La_n/Sm_n – 0.83 to 4.58) and (Nb_n/La_n – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Yb_n = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.     

Origin and Geological Significance of TTG Gneisses from the Maevatanana Greenstone Belt in North–Central Madagascar, and A Comparison with India

The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite(TTG) gneisses,and is important for its concentrations of various metal deposits(e.g.,chromium,nickle,iron,gold).In this paper we report on the petrography,and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area,as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses.The gneisses consist mainly of granitoid gneiss and biotite(±hornblende) plagiogneiss,and analysis of thin sections provides evidence of crushing,recrystallization,and metasomatism related to dynamic metamorphism.Samples have large variations in their major and trace element contents,with SiO_2 = 55.87-68.06 wt%,Al_2O_3 = 13.9-17.8 wt%,and Na_2O/K_2O =0.97-2.13.Geochemically,the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend,while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage.Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at2.5-2.4 Ga.Most ε_(Hf(t)) values of zircons from the biotite(± hornblende) plagiogneisses are positive,while most ε_(Hf(t)) values from the granitoid gneisses are negative,suggesting a degree of crustal contamination.Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca.3.4-2.6 Ga,representing a period of paleocontinent formation in the Mesoarchean.Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃.Based on these data,the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt(mostly lower amphibolite facies) in the Maevatanana area,and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent.The lithological similarity of Precambrian basement,the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India,which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics,and also important constraints on the correlation of the two continental fragments.