Detrital zircon U-Pb geochronology of stromatolitic carbonates from the greenstone belts of Dharwar Craton and Cuddapah basin of Peninsular India

Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin.Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics.In this study,we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition.The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426±26 Ma to 2650±38 Ma whereas the Sandur stromatolites gave an age of 3508±29 Ma to 2926±36 Ma suggesting Paleo-to Neoarchean provenance.The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761±31 Ma to1672±38 Ma suggesting Neoarchean to Mesoproterozoic provenance.The Rare Earth Element(REE)patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements(LREE)and enrichment in heavy rare earth elements(HREE)with pronounced positive Ce and negative Eu anomalies,typical of magmatic zircons.The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites.The 3508±29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton.The overall detrital zircon ages(3.5-2.7 Ga)obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin(2.7-1.6 Ga)collectively reflect on^800-900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.     

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.     

U-Pb SHRIMP ages of detrital zircons from Hiriyur Formation in Chitradurga greenstone belt and its implication to the Neoarchean evolution of Dharwar craton,south India

We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70–2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area.     

The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF₁, DhF_1a and DhF₂) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF_*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF₁ structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF₂) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ～2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.     

Petrography and provenance of silicified early Archaean volcaniclastic sandstones, eastern Pilbara Block, Western Australia

Intense post-depositional alteration has profoundly affected sandstones in the volcanic portions of Early Archaean (3·5–3·3 Ga) greenstone belts. The mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism, but grain textures are commonly well preserved. Consequently, microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes. Silicified Early Archaean volcaniclastic sandstones assigned to the Panorama Formation and Duffer Formation, Warrawoona Group, eastern Pilbara Block, Western Australia, were originally composed of volcanic (VRF) and sedimentary (SRF) rock fragments, volcanic quartz, feldspar, traces of ferromagnesian minerals and pumice. Only volcanic megaquartz remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) and sericite. In most areas, the sandstones were composed of dacitic to rhyolitic VRFs, now totally replaced by sericite-poor GMC and recognized by preserved microporphyritic textures. In a few areas, quartz-poor dacitic to andesitic(?) VRFs dominated the detrital assemblage. Minor SRFs and mafic VRFs, now replaced by GMC, are recognized on the basis of colour, internal structures, and internal textures, including skeletal, possible spinifex textures. Detrital feldspar is represented by blocky, sericite-rich grain pseudomorphs. A semi-quantitative point-count scheme, developed for the analysis of heavily altered sandstones, indicates the following primary detrital-mode ranges for Panorama arenites: quartz, 0–28%; feldspar, 0–28%, VRFs, 58–86%, and SRFs 0–25%. In about half the point-counted samples, feldspar could not be distinguished from rock fragments. In such cases, both were counted as one grain type, Lv', which makes up from 84 to 100% of the framework modes of these rocks. These sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments, but locally including minor mafic, ultramafic, and sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from plutonic or metamorphic sources. The Panorama modal assemblage represents a provenance that is lithologically more restricted than that of Archaean greywackes and other siliciclastic units common in the sedimentary portions of these same Early Archaean greenstone belts and younger greenstone belts (3·0–2·7 Ga).     

Geochronology of an archaean tonalitic gneiss dome in Northern Finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone

Archaean gneiss-greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late Archaean greenstone assemblages were deposited on older tonalitic crust.We report here well defined basement-cover relationships from a late Archaean greenstone belt in Lapland, north of the Polar Circle. The basal greenstone sequence contains quartzite, schist, komatiitic volcanics and an unusual volcanic conglomerate with well preserved granite pebbles of an older basement. These rocks surround a gneiss dome composed of foliated tonalite which shows a polyphase deformation pattern not seen in the neighbouring greenstones.Zircon fractions of the gneisses plot on two discordia lines and give upper intercept ages with concordia at 3,069±16 Ma and 3,110±17 Ma respectively. One fraction contains metamict zircons with components at least 3,135 Ma old. These are the oldest reliable ages yet reported from the Archaean of the Baltic Shield. Rb-Sr whole-rock dating of the tonalitic gneiss yielded an isochron age of 2,729±122 Ma and an I_Sr of 0.703±0.001. This is interpreted to reflect a resetting event during which the gneisses may have acquired their present tectonic fabric.Rb-Sr model age calculations yield mantle values for I_Sr at about 2,950±115 Ma and suggest that the tonalite was intruded into the crust as juvenile material at about 3.1 Ga ago as reflected by the zircon ages. It was subsequently deformed and isotopically reset at about 2.7 Ga ago, prior to greenstone deposition.Comparison with tonalitic gneisses of eastern Karelia displays significant differences and suggests that the Archaean of Finland may contain several generations of pre-greenstone granitoid rocks.     

Mafic and ultramafic magmatism and associated mineralization in the Dharwar craton,southern India

Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in nature, are preserved in the Sargur Group which is more than 3.3–3.4 Ga old, the youngest being manifested by 63–76 Ma old mafic dyke magmatism, possibly related to Deccan volcanism. In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties, the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga. Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8 and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga. From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids, derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures. The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous kimberlite/lamproite intrusions in the EDC.     

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.     

Terrigenous zircon of archean greenstone belts as a source of information on the early earth’s crust: Azov and Dnieper domains,Ukrainian shield

Zircons from the metasedimentary rocks of the Mesoarchean greenstone belts of the Azov and Middle Dnieper blocks of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. Detrital zircons from the metasediments of the Belozerskaya Formation of the greenstone belts of the Middle Dnieper block are usually dated within 3000–3100 Ma, while individual grains have an age of 3200–3300 Ma. This indicates that the sediments were derived mainly from proximal volcanic source with minor contribution of the basement material (Aul’skaya Group). The metasediments of the Soroki greenstone structure of the Azov block contain mainly zircons with ages within 3500–3600 Ma, except for scarce grains having the ages older than 3700 Ma. Zircon cores are overgrown by granulitic rims dated at approximately 3300 Ma. A wide scatter in ages and Th/U ratios in the zircons indicate that they were derived from rocks of different composition and age. Obtained data suggest significantly wider distribution of the Paleoarchean crust within the Azov block of the Ukrainian shield than was previously assumed.     

Rb-Sr and Sm-Nd ages and isotopic geochemistry of Archaean granodioritic gneisses from Eastern Finland

Twenty granodioritic rocks and one amphibolitic enclave of the “basement” of the Suomussalmi-Kuhmo Archaean (2.65 Ga) greenstone belts (central-eastern Finland), have been chosen together with one greenstone sample for Rb-Sr and Sm-Nd geochronological and isotopic studies.The granitoïd rocks are subdivided into three groups: two generations of grey gneisses and a post-belt augen gneiss. The Rb-Sr ages of the first and second generation of grey gneisses are 2.86 ± 0.09 and 2.62 ± 0.07 Ga, respectively. These results are corroborated by Sm-Nd data. The post-belt augen gneiss gives an age of 2.51 ± 0.11 Ga. The results show that the two generations of grey gneisses, the greenstone belts and the post-greenstone augen gneiss, were developed over a period > 350 Ma. The two generations of grey gneisses show identical I_Sr values (0.7023 ± 8 and 0.7024 ± 6) which contrast with that of the augen gneiss (0.7049 ± 8). The low I_Sr and the near-chondritic ?_T^CHUR values indicate that the grey gneisses cannot derived from much older continental materials. Trace element studies suggest that these grey gneisses have had a multi-stage development. The augen gneiss with a moderately high I_Sr is likely to be derived from a granodiorite originated by partial melting of older sialic crust. The more probable parent rock seems to be the first generation grey gneisses. The I_Sr and average Rb/Sr values preclude the greenstone belt and the second generation of grey gneisses as the protolith.     

额尔古纳地块基底地质构造

额尔古纳地块是额尔古纳-马门-加格达奇拼合地块中的典型代表.研究表明,其基底由前中元古代绿岩及与之伴生的花岗质杂岩组成,它们具有地壳早期演化的地质构造特征.绿岩带为典型的变质基性-酸性火山岩及部分变质沉积岩系构成的火山-沉积建造,火山岩以拉斑玄武岩为主,向上过渡为钙碱性火山岩系列,表现为双峰态型特点.花岗岩类为TTG岩系及石英二长岩-花岗岩组合.花岗岩-绿岩地体内各岩石类型的岩石地球化学特征与国外太古宙及我国华北陆台花岗岩-绿岩带内同类岩石极为相似.双峰态型火山岩及绿岩建造组合,以及类似于TH2、FII型的变质基性火山岩和长英质火山岩特征,结合高铝型英云闪长岩-奥长花岗岩组合,指示了研究区绿岩带的形成环境类似于大陆边缘弧后裂谷型火山-沉积盆地.     

Age range of formation of sedimentary-volcanogenic complex of the Vetreny Belt (the southeast of the Baltic Shield)

As a result of studying the Vetreny Belt greenstone structure (the southeast of the Baltic Shield), zircons from terrigenous deposits of the Toksha Formation, underlying the section of the sedimentary-volcanogenic complex, and zircons of the Vetreny Belt Formation, deposits of which crown the section, were dated. The results of analysis of age data of detrital zircons from quartzites of the Toksha Formation indicate that Mesoarchean greenstone complexes and paleo-Archean granitogneisses of the Vodlozero Block (Karelia) were the provenance area from which these zircons were derived. The occurrence of the youngest zircons with age of 2654.3 ± 38.5 Ma is evidence that the formation of the Vetreny Belt, including the Toksha Formation, began no earlier than this time. Zircons from volcanic rocks of the Vetreny Belt yielded the age of 2405 ± 5 Ma. Thus, the age interval of the formation of the sedimentary-volcanogenic complex of the Vetreny Belt ranges from 2654.3 ± 38.5 to 2405 ± 5 Ma.     

Origin of the Belomorian Chupa Gneiss in the Light of New Lithogeochemical Data

The stratotype Chupa Sequence, which makes up the synonymous tectonic nappe and is a member of oldest (Late Archean) rocks in the Belomorian Mobile Belt, is scrutinized. The nappe is mainly composed of the migmatized, coarse-grained garnet–biotite gneiss with kyanite. Relic formations are represented by lenses of the fine-grained (possibly primary) garnet–biotite gneiss. Fine-grained gneisses generally represent primary sedimentary formations, while metadacites are rare. The kyanite-bearing gneiss is presumably a restitic rock produced by migmatization. Chupa metasediments are represented by immature (low-differentiated) metagraywackes and are akin to metaterrigenous rocks from Late Archean greenstone belts of Karelia (Kostomuksha) and Canada (Quetico). The provenance of metasediments comprises the following three components: tonalite (dacite) 70%, mafic rocks 20%, and ultramafic rocks 10%. The Chupa metagraywacke presumably deposited in a fore-arc basin.     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

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.     

大兴安岭中段乐平统-中三叠统沉积物源分析:来自重矿物组合及碎屑锆石年代学证据

黑龙江西部龙江地区位于中亚造山带东段,黑河-贺根山缝合带与西拉木伦缝合带之间,地层记录了两大古板块之间古亚洲洋闭合过程的信息。本文对龙江地区乐平统林西组和下-中三叠统老龙头组的砂岩样品进行碎屑重矿物和碎屑锆石U-Pb同位素年代学研究。碎屑重矿物组合以锆石+磷灰石+金红石+角闪石+绿帘石+重晶石的组合为特征,表明物源主要来自于中酸性岩浆岩,并有少量变质岩及沉积岩组分。林西组样品最年轻的锆石年龄为278±3Ma,老龙头组样品最年轻的锆石年龄为247±3Ma、243±4Ma及237±3Ma,结合前人的研究,限定了林西组沉积于乐平世,老龙头组沉积于早三叠世-中三叠世。碎屑锆石年龄谱明显分为五组:237～258Ma、270～329Ma、357～558Ma、680～1633Ma及1893～1966Ma。其中237～258Ma的碎屑锆石主要来自与古亚洲洋洋壳消亡前的俯冲增生过程相关的火山活动,270～329Ma的碎屑锆石主要来自大石寨组火山岩及其同期侵入岩,357～558Ma的碎屑锆石来自早古生代-晚古生代早期岩浆弧,680～1633Ma的碎屑锆石可能来自兴安及额尔古纳地块的变质基底,而较古老的～1800Ma的锆石年龄暗示了华北克拉通基底的物源信息。通过研究发现林西组及老龙头组样品前30%年轻的碎屑锆石年龄与地层沉积年龄之差都小于100Ma,结合对砂岩碎屑组成、重矿物组合及盆地与火山弧位置关系的研究,认为研究区乐平世-中三叠世沉积盆地具有汇聚背景,为弧前盆地。     

浙江江山寒武系碎屑锆石U-Pb年龄与沉积指向构造及其古地理意义

浙江江山北部处于紧邻江山-绍兴断裂带的扬子地块边缘;寒武系杨柳岗组陆源碎屑岩夹层中的碎屑锆石 LA-ICP-MS U-Pb定年结果表明,年龄范围为515~3340 Ma,主峰年龄828 Ma,其次为727 Ma及916 Ma;与邻区的对比结果 显示,研究区碎屑锆石年谱与华夏地块闽西北-赣中东部以及扬子地块赣东北-皖南地区可对比,尤其与前者最为相近, 而与华夏地块闽西南-赣南地区差异显著。杨柳岗组及其上覆华严寺组同沉积褶皱以及下伏大陈岭组单向斜层理特征表 明,沉积基底向北西倾斜,沉积物搬运方向由南东向北西。综合这两方面推测：在杨柳岗组沉积时期,研究区具有向北西 倾斜的沉积基底,向东南方向应与剥蚀区相连,陆源碎屑物应源自其东南方向的华夏地块东部;剥蚀区可能位于现今包括 江绍构造带在内的浙江中部,向北西方向主要提供前寒武系陈蔡群和相当于河上镇群以及少量双溪坞群或者三者剥蚀物再 沉积地层的剥蚀物质。     

Lithofacies associations and structural evolution of the Archean Rio das Velhas greenstone belt,Quadrilátero Ferrífero,Brazil: A review of the setting of gold deposits

The Rio das Velhas greenstone belt is located in the Quadrilátero Ferrífero region, in the southern extremity of the São Francisco Craton, central-southern part of the State of Minas Gerais, SE Brazil. The metavolcano–sedimentary rocks of the Rio das Velhas Supergroup in this region are subdivided into the Nova Lima and Maquiné Groups. The former occurs at the base of the sequence, and contains the major Au deposits of the region. New geochronological data, along with a review of geochemical data for volcanic and sedimentary rocks, suggest at least two generations of greenstone belts, dated at 2900 and 2780 Ma. Seven lithofacies associations are identified, from bottom to top, encompassing (1) mafic–ultramafic volcanic; (2) volcano–chemical–sedimentary; (3) clastic–chemical–sedimentary, (4) volcaniclastic association with four lithofacies: monomictic and polymictic breccias, conglomerate–graywacke, graywacke–sandstone, graywacke–argillite; (5) resedimented association, including three sequences of graywacke–argillite, in the north and eastern, at greenschist facies and in the south, at amphibolite metamorphic facies; (6) coastal association with four lithofacies: sandstone with medium- to large-scale cross-bedding, sandstone with ripple marks, sandstone with herringbone cross-bedding, sandstone–siltstone; (7) non-marine association with the lithofacies: conglomerate–sandstone, coarse-grained sandstone, fine- to medium-grained sandstone. Four generations of structures are recognized: the first and second are Archean and compressional, driven from NNE to SSW; the third is extensional and attributed to the Paleoproterozoic Transamazonian Orogenic Cycle; and the fourth is compressional, driven from E to W, is related to the Neoproterozoic Brasiliano Orogenic Cycle. Gold deposits in the Rio das Velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along Archean thrust shear zones of the second generation of structures.Sedimentation occurred during four episodes. Cycle 1 is interpreted to have occurred between 2800 and 2780 Ma, based on the ages of the mafic and felsic volcanism, and comprises predominantly chemical sedimentary rocks intercalated with mafic–ultramafic volcanic flows. It includes the volcano–chemical–sedimentary lithofacies association and part of the mafic–ultramafic volcanic association. The cycle is related to the initial extensional stage of the greenstone belt formation, with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains. Cycle 2 encompasses the clastic–chemical–sedimentary association and distal turbidites of the resedimented association, in the eastern sector of the Quadrilátero Ferrífero. It was deposited in the initial stages of the felsic volcanism. Cycle 2 includes the coastal and resedimented associations in the southern sector, in advanced stages of subduction. In this southern sedimentary cycle it is also possible to recognize a stable shelf environment. Following the felsic volcanism, Cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations, largely in the northern sector of the area. The characteristics of both associations indicate a submarine fan environment transitional to non-marine successions related to felsic volcanic edifices and related to the formation of island arcs. Cycle 4 is made up of clastic sedimentary rocks belonging to the non-marine lithofacies association. They are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles.     

前寒武纪重大地质事件

本文概括性地阐述我国前寒武纪冥古宙、太古宙、元古宙三大地史阶段的重大地质事件,粗略勾绘前寒武纪地球演化的轨迹,期望了解我国与全球变化的异同,进一步突出我国前寒武纪三大地史阶段中新太古代超级地质事件及元古宙时期中国大陆块体对哥伦比亚及罗迪尼亚两个超大陆形成与破裂的地质响应。冥古宙是地球最早期的地史阶段,从太阳系形成的4 567 Ma至地球上最老的4 030 Ma的Acasta片麻杂岩。碎屑锆石保存最好的地点是西澳的Mt. Narryer和Jack Hills。目前在中国大陆至少有7个地点发现具有罕见的约4.0 Ga的碎屑锆石,这些地点并不位于克拉通区,而是赋存于造山系新元古代至古生代以碎屑岩为主的地层中。太古宙(4 030~2 420 Ma)定义为从最古老的岩石出现(4 030 Ma Acasta片麻岩)至冰碛层首次广泛分布的寒冷期之间的一段地史。最古老的岩石为英云闪长片麻岩,构成加拿大西北斯拉夫克拉通4.03~3.94 Ga Acasta片麻岩的一部分。西南格陵兰Isua带保存全球有最老的表壳岩,形成于3 810 Ma。太古宙最重大的地质事件莫过于2 780~2 420 Ma时期的新太古代超级事件。值得指出的是华北克拉通最古老、也是中国最古老的岩石出露在中国辽宁鞍山地区,约3.80 Ga英云闪长岩奧长花岗质片麻岩和3.30 Ga的表壳岩已被识别。华北克拉通太古宙有与世界各地太古宙相似的演化历史和特点,包括花岗岩绿岩带及高级变质片麻岩带、广泛的英云闪长岩奧长花岗岩花岗闪长岩(TTG)片麻岩、古陆壳的出露(略老于3.8 Ga)、广泛分布的BIF等。我国太古宙花岗岩绿岩带虽然在华北克拉通分布较广,但与南非、格陵兰、加拿大、西澳等地经典的花岗岩绿岩带相比,时代偏新,仅以新太古代为主,规模偏小,缺少大面积分布的科马提岩,且变质程度偏高,主要为角闪岩相麻粒岩相变质。演化到元古宙(2 420~541 Ma),则进入成熟的、较冷的、刚性程度较高的地球,以现代样式板块构造、超大陆旋回和更复杂的疑源类(eukaryotic)生命的发育为特征。这种变化大致出现在2 420 Ma左右,与哈默斯利型BIF的消失及地史中首次广泛出现的冰川沉积物年代相近。古元古代早期十分重要的“休伦冰川事件”、指示大氧化事件的古老红层在我国尚未被发现,与Lomagundi Jatuli (LJE) δ13C的同位素漂移有关的关门山组古元古代沉积地层的同位素年代学依据不足;古元古代磷矿和具有巨大石油潜力的2.01 Ga Shunga事件也未能鉴别。但中国最大特色是发育了与哥伦比亚和罗迪尼亚超大陆汇聚与裂解有关的良好地质记录,特别是华北克拉通保存了古元古代与哥伦比亚超大陆汇聚有关的超高温、高压麻粒岩等变质及岩浆事件,1 780 Ma以后的中元古代又保存了与哥伦比亚超大陆裂解有关的裂谷沉积及岩浆活动;而在扬子和塔里木陆块区则保存了与新元古代早期与罗迪尼亚超大陆汇聚有关的蛇绿岩、混杂岩、洋内弧、俯冲增生杂岩及大陆边缘弧,在约800 Ma以后则发育了与罗迪尼亚超大陆裂解有关的沉积及岩浆活动的地质记录,为中国和全球地质学者研究这一时期地球系统变化和成矿作用提供了客观的野外实验室和良好的范例。