津巴布韦太古宙花岗-绿岩型金矿类型与资源潜力

津巴布韦太古宙花岗-绿岩型金矿床受构造的控制,金矿可分为褶皱控矿型、剪切带控矿型、层控型、深成花岗岩控矿型和构造-蚀变岩型等5类。津巴布韦花岗-绿岩型金矿多产于花岗质片麻岩与绿岩带的接触带上,金以自然金或金的硫化物形式产出。金矿成矿时代大致分为(2660±50)Ma和(2410±70)Ma(米德兰德绿岩带北部白钨矿Sm-Nd测年)2个时段,分别与新太古代TTG事件和大岩墙侵入相关。津巴布韦花岗-绿岩型金矿较多,但发现的中-大型绿岩型金矿较少,具有较好的金矿找矿前景。     

Gravity interpretation of the Dharwar greenstone-gneiss-granite terrain in the southern Indian shield and its geological implications

The Dharwar craton in the southern Indian shield has a wide distribution of volcano-sedimentary sequences surrounded by a vast gneissic complex, both of which have been intruded by younger granites. A gravity anomaly map of this craton, compiled from all the available data, is analysed here to study the structures and depths of the greenstone belts, the mode of granite emplacements and the greenstone-gneiss-granite associations in general. The anomaly map is a mosaic of well-defined gravity highs and lows characterizing the dense volcano-sedimentary sequences and exposed and/or concealed granites respectively. Gravity modelling indicates that the Shimoga belt has a limited depth range of only 3–4 km while the Chitradurga and Sandur belts have greater depths of over 10 km. The structures inferred for the Dharwar formations are alternating bands of synclines, filled with dense schistose rocks, separated by anticlinal ridges of gneisses and granites.     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996     

Sm-Nd, Lu-Hf and Pb-Pb signatures of gneisses and granitoids from the La Grande belt: extent of late Archean crustal recycling in the northeastern Superior Province, Canada

Sm-Nd, Lu-Hf and Pb-Pb isotopic signatures were determined for gneisses and felsic plutons of the La Grande granite-greenstone and the Nemiscau metasedimentary belts of Canada. The northern part of the La Grande belt exposes gneisses of the Langelier complex formed between ca. 2.8 and 2.9 Ga. The gneisses yielded εNd_(T) values between −2 and +1, εHf_(T) between −1 and +3, initial ²⁰⁷Pb/²⁰⁴Pb ratios of ∼14.9, higher than the value of 14.6 derived from the Stacey-Kramers growth curve, and crustal extraction ages ≥3.0 Ga. Modeling shows that the syn- to late-tectonic intrusions of granitoid rocks record at least 20%, and up to 40%, recycling of this gneissic basement. The southern part of the La Grande belt, where no basement gneisses are exposed, records lower proportions (5-20%) of this crustal end-member. In both parts of the La Grande belt, the post-tectonic plutons record the largest proportion of recycled crust, likely related to southeastward thrusting events in the Superior craton, crustal thickening and concomitant partial melting.The La Grande plutons have isotopic signatures more radiogenic than those of similar intrusions that formed concurrently, to the south, in the Abitibi greenstone belt. This reflects an ancient crust environment for the former vs. an oceanic setting for the latter. Mantle reservoirs with a protracted history of incompatible element depletion were present beneath the whole eastern Superior Province, in the late Archean.     

Upb zircon ages bearing on the nature of early archaean greenstone belt evolution,barberton mountainland,southern africa

UPb zircon systematics for granitic and gneissic clasts from the Moodies conglomerate of the Barberton Greenstone Belt, South Africa, exhibit discordancy patterns that suggest variable and multi-episodic lead loss and indicate crystallisation ages for the parent granitoids between 3.3 and 3.47 Ga. Metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2.8–2.9 Ga ago that is ascribed to a regional thermal event as also shown by published RbSr data for a nearby granitoid pluton. Since the Moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments, a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early Archaean granite-greenstone terrains. The maximum age of 3.3 Ga for the Moodies Group also shows that greenstone belt evolution in the Barberton region took place over a period of at least 150 Ma from about 3450 to 3300 Ma ago.The Kaap Valley and Stentor plutons intrude the lower part of the greenstone succession, and their diapiric emplacement caused large-scale deformation of earlier structures in the entire greenstone belt sequence at about 3.23–3.25 Ga ago.Detrital zircons in the sediments of the Fig Tree Group date the mean age of their source terrain at c. 3520 Ma which is also the maximum age of deposition for these strata. This age is indistinguishable from the oldest age reported from the Barberton greenstones and suggests the possibility that pre-greenstone granitoid crust may exist in the eastern Kaapvaal craton.     

Archaean stromatolites from the Ngesi Group,Belingwe greenstone belt,Zimbabwe; preservation and stable isotopes — preliminary results

The petrology and stable isotope chemistry of cyanobacterial stromatolites of Archaean age (2.7 Ga) from the Cheshire and Manjeri Formations of the Belingwe greenstone belt in Zimbabwe have been examined. Palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes. The Cheshire Formation shows little evidence of either anion or cation exchange during metamorphism, and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80°C. The Manjeri Formation, only slightly older, but overlain by a thick volcanic sequence, shows a low grade of metamorphism, and isotope ratios that are consistent with a metamorphic temperature of around 200°C.     

New Rb-Sr age determinations on the Archaean basement of Eastern Sierra Leone

The Archean basement of Sierra Leone is a typical example of granite-greenstone terrains found in ancient continental nucleii. Reconnaissance field mapping showed that the area can be subdivided into old gneiss, which predates the greenstone belts, and young granite which is later than the greenstone belts.New Rb-Sr whole-rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 ± 49 Ma and 2770 ± 137 Ma, which either reflect the time of formation of the original tonalites or their metamorphism. Three new Rb-Sr whole-rock age determination on young granites yield ages of 2786 ± 143 Ma, 2780 ± 79 Ma and 2770 ± 50 Ma, which are interpreted as the time of emplacement. The widespread occurrence of similar young granites, throughout the Archaean of West Africa, suggests that these results date a major event in the evolution of this segment of the crust.A published Pb-Pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000-2770 Ma. However, if the Rb-Sr ages of the old gneiss reported in this paper reflect the time of their formation, the age of the greenstone belts is tightly bracketed to ca. 2770 Ma. There is no isotopic evidence for rocks substantially older than 3000 Ma in the West African Archaean.     

An overview of the relationship between granitoid intrusions and gold mineralisation in the Archaean Murchison Province,Western Australia

In the Archaean Murchison Province of Western Australia, granitoid batholiths and plutons that intruded into the ca. 2.7–2.8 Ga and ca. 3.0 Ga greenstone belts can be divided into three major suites. Suite I is a ca. 2.69 Ga monzogranite-granodiorite suite, which was derived from anatexis of old continental crust and occurs as syn-tectonic composite batholiths over the entire province. Suite II is a trondhjemite-tonalite suite (termed I-type) derived from partial melting of subducted basaltic crust, which intruded as syn- to late-tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated. One of the Suite II trondhjemite plutons has a Pb−Pb isochron age of 2641±36 Ma, and one of the structurally youngest tonalite plutons has a minimum Pb−Pb isochron age of 2630.1±4.3 Ma. Suite III is a ca. 2.65–2.62 Ga A-type monzogranite-syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province. Gold deposits in the province are mostly hosted in brittle-ductile shear zones, and were formed at a late stage in the history of metamorphism, deformation and granitoid emplacement. At one locality, mineralisation has been dated at 2636.8±4.2 Ma through a pyritetitanite Pb−Pb isochron. Lead and Sr isotope studies of granitoids and gold deposits indicate that, although most gold deposits have initial Pb isotope compositions most closely similar to those of Suite II intrusions, both Suite I and Suite II intrusions or their source regions could have contributed solutes to the ore fluids. These preliminary data suggest that gold mineralisation in the Murchison Province was temporally and spatially associated with Suite II I-type granitoids in the northeastern part of the province. This association is consistent with the concept that Archaean gold mineralisation was related to convergent-style tectonic settings, as generation of both Suite II I-type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust, and old sialic crust or its anatectic products may also contribute solutes to the ore fluids. Integration of data from this study with other geological and radiogenic isotope constraints in the Yilgarn Block argue against direct derivation of gold ore fluids from specific I-type granitoid plutons, but favour a broad association with convergent tectonics and granitoid magmatism in the late Archaean.     

An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An₇₀) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth.     

Crustal evolution of Archaean granitoids in the Murchison Province, Western Australia

Four suites of granitoids intruded the supracrustal greenstone sequence in the Murchison Province of the Archaean Yilgarn Craton during a 300 million year period. The earliest granitoid suite intruded the base of the developing greenstone sequence as a series of thin subhorizontal tabular plutons of monzogranite and granodiorite at 2.9Ga. This suite has been deformed and metamorphosed, and is now a pegmatite-banded gneiss. At about 2.7 Ga, thick, subhorizontal, tabular plutons of monzogranite intruded the base of the greenstone sequence. This suite, which now forms much of the regions between greenstone belts, was folded and recrystallized during regional deformation and metamorphism. Two distinct but contemporaneous suites of post-folding granitoids intruded the greenstone belts at 2.6 Ga, largely post-dating regional metamorphism. One suite of post-folding granitoids comprises tonalite, trondhjemite, granodiorite and monzogranite plutons, confined mainly to the north of the Province. The other suite comprises quartz-rich monzogranite and syenogranite plutons, confined mainly to the south of the Province.Pegmatite-banded gneiss, recrystallized monzogranite, and the northern suite of post-folding granitoids were all derived by partial-melting of mafic crustal rocks. Most post-folding granitoids from the southern suite were derived by partial-melting of siliceous crustal material at least as old as basal greenstones. The modes and sites of intrusion of all granitoid plutons were controlled by active tectonic processes or by structural features of the crust. Widespread 2.6 Ga Rb---Sr ages of pegmatite-banded gneiss and recrystallized monzogranite reflect post-metamorphic cooling which was contemporaneous with intrusion of post-folding granitoids.     

The gold content of some Archaean rocks and their possible relationship to epigenetic gold-quartz vein deposits

Gold mineralization in Archaean granite-greenstone environments, especially gold-quartz veins, contributes considerably to the world's gold production. The formation of epigenetic gold mineralization in greenstone belts is generally explained by the metamorphic secretion theory. This theory is based on the assumption that the source of the gold may be komatiitic or tholeiitic lavas, pyritic chemical or clastic sediments and even granitic rocks from which, as a result of regional metamorphic overprinting, gold was extracted and concentrated in suitable structures.It has been shown that in proposed potential source rocks, gold is predominantly associated with sulfide minerals and thus relatively easily accessible to secretion and reconstitution processes.A large number of various rock types originating from granite-greenstone terranes of the Kaapvaal and the Rhodesian cratons were geochemically investigated, and the following ranges for gold determined:volcanic rocks (komatiitic and tholeiitic): 0.1–372 ppbgranitic rocks of the basement: 0.3–7.8 ppbiron-rich chemical sediments: 1.0–667 ppbStatistical treatment of the data reveals that volcanic rocks as well as iron-rich chemical sediments are favorable sources for epigenetic gold mineralization formed by metamorphic secretion, while the granitic rocks make less suitable primary gold sources. This finding explains the close spatial relationship which is common between gold-quartz veins and greenstone belts. The conspicuous abundance of epigenetic gold mineralization in the Archaean, however, is attributed to the unique geologic and metamorphic history of the granite-greenstone terranes.     

The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration

The Lega Dembi deposit is the largest gold producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that follows the structural contact between underlying feldspathic gneisses and the volcanosedimentary sequence of the Megado belt. This contact also marks the northernmost extension of the regional-scale, sinistral strike-slip Lega Dembi-Aflata shear zone. Mineralization and intense quartz-veining is best developed in graphite-rich sediments within an area not more than 80 m away from this tectonic contact. Hydrothermal wall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with galena. The variable deformation of the gold-quartz veins suggests a syn-kinematic timing for the gold mineralization during transcurrent shearing in a dilational segment of the shear zone. In addition to the structural control, lithological control on gold deposition is indicated by the almost exclusive occurrence of the gold mineralization in graphite-rich metasediments. This close relationship suggests that gold precipitation was the result of chemical reduction of regional ore-bearing fluids. Temperature conditions of mineralization are constrained by the actinolite-biotite alteration assemblage and by arsenopyrite chemistry, which indicate that ore deposition occurred at or close to peak metamorphic conditions at upper-greenschist to lower-amphibolite metamorphic grades. Rb-Sr dating of sericite indicates an age of about 545 Ma. for hydrothermal alteration and, thus, for gold mineralization. The style of gold mineralization, structural pattern and lithological assemblages at Lega Dembi are very similar to lode-gold deposits most commonly reported from Archaean granite-greenstone terranes. These similarities may open new perspectives for the exploration of lode-gold deposits, which has previously primarily focused on Archaean greenstone belts rather than Proterozoic or even Phanerozoic meta-volcanosedimentary belts. Received: 26 July 1996 / Accepted: 8 January 1997     

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism     

Integrated potential-field and seismic constraints on the structure of the Archean metasedimentary English River belt,Western Superior craton,Canada

The English River Subprovince is a prominent belt of metasedimentary rocks in the Archean Western Superior Province. The structure of its western half was investigated by using techniques of enhancement and automatic interpretation of magnetic data, and integration of magnetic-derived information with seismic and gravity data. The results indicate that a suite of exposed felsic plutons that intruded the belt at ca. 2698 Ma extends under most of the metasedimentary rocks that are exposed at the surface. The thickness of the metasedimentary rocks is interpreted to be less than 1 km in areas where it is underlain by the members of this intrusive suite. In other areas, the metasedimentary rocks attain thicknesses of 3–4 km and appear to be underlain by rocks similar to the gneissic rocks that are exposed in the adjacent metaplutonic Winnipeg River Subprovince. The integration of enhanced magnetic data with gravity data indicates that the large gravity anomaly that extends along the English River belt correlates well spatially and morphologically with the extensive suite of felsic intrusions that underlies the belt, suggesting that the crustal component of the gravity anomaly is related to this suite of intrusions. We interpret the source of the gravity anomaly as a dense unit comprising anhydrous mineral assemblages that formed within these felsic intrusions in response to low-pressure, high-temperature metamorphism that affected the belt at ca. 2691 Ma. On the basis of geochronological, geological and geophysical constraints, we propose that this metamorphic episode is linked to the continuation of magmatism at depth after the emplacement of the ca. 2698 Ma felsic plutons, being ultimately related to the advection of mantle heat into the crust during a period of regional extension.     

A model of ocean-crust accretion for the Superior province, Canada

One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.

In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.

The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences.     

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement.Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton.Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.     

Archaean crustal thickness of greenstone granite belts of South India

Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb?Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.     

Gravity anomalies and the Indian lithosphere: Review and analysis of existing gravity data

The upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the Indian Peninsula and the Himalaya. Analysis of the gravity field over the Dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth's crust during Archaean times. Precambrian mountain ranges such as the Aravallies, Vindhyans, Satpura and Eastern Ghats are located peripheral to Archaean cratons. Most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density. These mountain ranges with the exception of the Eastern Ghats do not appear to be locally compensated. Regional compensation seems to prevail over all these areas. Eastern Ghats ranges are also underlain by a crust of higher than normal density relative to the Dharwar and Bastar cratons and exist with a sharp contact with the cratons in the West. Isostatic compensation in the Eastern Ghats appears to have been achieved by thickening of the underlying crust. The Himalaya has attained a fairly high degree of isostatic compensation.     

Provenance of zircon xenocrysts in the Neoproterozoic Brauna Kimberlite Field,São Francisco Craton,Brazil: Evidence for a thick Palaeoproterozoic lithosphere beneath the Serrinha block

The 642 Ma-old Brauna Kimberlite Field is located on the northeastern sector of the São Francisco Craton (Serrinha block) and is one of the rare Neoproterozoic kimberlitic events in South America. Zircon xenocrysts from the volumetric most important kimberlite pipes Brauna 03, Brauna 07 and Brauna 04 were used as a tool to identify different components of the lithosphere beneath the northeast region of the São Francisco craton. A composite kimberlite sample of eight representative and different drill holes and three samples of the host rocks (Nordestina granodiorite) were sampled for SHRIMP geochronology. The results were compared with precise U–Pb age data for the regional rocks, i.e. the Archaean basement and the Palaeoproterozoic Rio Itapicuru greenstone belt. Samples from the Nordestina granodiorite gave three different ages: 2155 Ma in the western part of the batholith, 2139 Ma in its central part, and 2132 Ma in its eastern part. Zircon ²⁰⁷Pb/²⁰⁶Pb ages of the Brauna kimberlite zircon grains spread over the timespan 2107–2223 Ma and indicate four age groups at 2105 ± 3 Ma, 2138 ± 7 Ma, 2166 ± 5 Ma, and 2198 ± 4 Ma. Source rocks for the three former age groups can be found in the Rio Itapicuru greenstone belt, including zircon xenocrysts from the Nordestina granodiorite, whereas the latter age group has not yet been reported in the Serrinha block. The new zircon ages show that only rocks of the Palaeoproterozoic Rio Itapicuru greenstone belt and of a hidden 2.17–2.20 Ga crust were sampled by the kimberlite magma during its ascent through the lithosphere. It is proposed that there is none or a few Archaean crust beneath the kimberlite emplacement area, hence implying a thick Palaeoproterozoic lithosphere for this portion of the São Francisco craton.