Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c.   1.65  Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42  Ga and probably during collisional assembly of island arc crust into new (1.7–1.6  Ga) continental lithosphere. Supracrustal rocks record 650–750  °C, 6–8  kbar metamorphism (M₂); these high temperatures may have been reached during sandwiching between c.   1.65  Ga granitoids. However, the early history has been obscured by renewed tectonism at c.   1.4  Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4  kbar (M₃). Metamorphic temperature variations from uppermost-amphibolite to amphibolite facies rocks may be associated with c.   1.65 and/or 1.4  Ga plutonism, but not to a 1.4  Ga extensional shear zone as previously proposed. Syn- and post-1.4  Ga contraction is suggested by high- and low-temperature microstructures showing top-to-the-south-east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c.   1.65  Ga, but that the present fabric also records 1.4  Ga tectonism involving high- T  metamorphism and fabric reactivation.     

海南岛地壳生长和基底性质的Nd同位素制约

通过对海南岛花岗岩、变质沉积岩等地壳岩石SmNd同位素的详细剖析,探讨了海南岛的基底性质及其地壳生长史。研究表明:海南岛九所—陵水断裂的南、北经历了不同的构造演化史。断裂以北地壳岩石的ε_Nd(t)分布在3.3至-17.2之间,Nd模式年龄分布在1.2～3.2Ga之间,并在1.4～1.6Ga和2.0Ga形成统计峰值。断裂以北地壳主要以幕式生长形成,太古代时期形成若干陆核(距今3.1～3.2Ga或更早?),1.4～1.6Ga和1.8～2.0Ga为地壳生长的主要时期。断裂以北地壳岩石的SmNd同位素特征和地壳生长史与华夏古陆的相似,其基底是华夏古陆的一部分,断裂以南可能是亲东冈瓦纳(澳大利亚)的裂解块体。     

Paleoproterozoic juvenile magmatism within the northeastern sector of the São Francisco paleocontinent: Insights from the shoshonitic high Ba–Sr Montezuma granitoids

New,integrated petrographic,mineral chemistry,whole rock geochemical,zircon and titanite UPb geochronology,and zircon Hf isotopic data from the Montezuma granitoids,as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex,provides new insights into the late-to post-collisional evolution of the northeastern Sao Francisco paleocontinent.U-Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca.2.2 Ga to 1.8 Ga and comprise distinct groups.Group I have crystallization ages between ca.2.15 Ga and 2.05 Ga and are interpreted as inherited grains.Group II zircon dates vary from 2.04 Ga to1.9 Ga and corresponds to the crystallization of the Montezuma granitoids,which were constrained at ca.2.03 Ga by the titanite U-Pb age.Inverse age zoning is common within the ca.1.8 Ga Group III zircon ages,being related to fluid isotopic re-setting during the Espinhaco rifiting event.Zircon ε_(Hf)(t) analysis show dominantly positive values for both Group I(-4 to+9) and Ⅱ(-3 to+8) zircons and T_(DM2) model ages of 2.7-2.1 Ga and 2.5-1.95 Ga,respectively.Geochemically,the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids,enriched in LILES and LREE,with high to moderate Mg#and depleted in some of the HFSE.Their lithochemical signature,added to the juvenile signature of both inherited and crystallized zircons,allowed its classification as a shoshonitic high Ba-Sr granitoid related to a late-to post-collisional lithosphere delamination followed by asthenospheric upwelling.In this scenario,the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc,being these hybrid magma interpreted as the source of the Montezuma granitoids.The Corrego Tingui Complex host rocks are akin to a syn-to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks.The results presented in this study have revealed the occurrence of juvenile rocks,probably related to an island arc environment,that are exotic in relation to the Paleo-to Neoarchean crust from the Sao Francisco paleocontinent's core.     

Diversity of 1.8 Ga potassic granitoids along the edge of the Archaean craton in northern Scandinavia: a result of melt formation at various depths and from various sources

The edge of the Archaean craton in northern Scandinavia had been intensively reworked during the Svecofennian orogeny 1.93-1.86 Ga ago and was subsequently intruded by potassic granitoids of 1.79–1.80 Ga age. Despite similar or even identical ages and overlapping areas of occurrence, these rocks belong to two different groups, the Edefors and Lina granitoids, which have contrasting geochemistries and Sm---Nd isotopic characteristics. The Edefors granitoids range from syenites to granites, and are alkali-rich and distinctly metaluminous. They crystallized from dry magmas. This is indicated by the scarcity of pegmatites and aplites. The contacts to older rocks are often distinct, but gradual transitions to Lina-type granitoids are common. The Edefors granitoids have high contents of Zr but not of elements such as Y, REE, Ta and Nb, and have low Mg/Mg+Fe ratios. They also frequently have positive Eu anomalies, even in the quartz rich varieties. Initial ε_Nd values range from −2.1 to +1.4, indicating that the Edefors granitoids were formed by the mixing of mantle-derived magmas and continental crustal materials. The amount of crustal component was probably less than 35% in most cases. The Lina granitoids are accompanied by abundant pegmatites and aplites. Ghost structures and remnants of country rock are common. True granites predominate, but also quartz monzonites occur. The content of HFS elements is low and the Mg/Mg+Fe ratios are higher than in the Edefors granitoids. Initial ε_Nd values range from −9.3 to −3.7, reflecting a significant portion of Archaean Nd in the source materials. The Lina granitoids are largely the result of remobilisation of continental crust with a small input of juvenile material. However, the dominant source for these crustally derived granitoids are c. 1.9 Ga old granitoids. These carry a large proportion of Archaean Nd. The most probable environment of the formation of potassic migmatite granitoids, such as the Lina type, is a collision zone between two masses of felsic crust (e.g. arc-continent or continent-continent), but the details of such a collision in the Baltic Shield remain to evaluated. The formation of the Edefors granitoids could have been associated with an extensional zone developed due to delamination caused by separation of the down-dip oceanic lithosphere from the continental lithosphere.     

Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous igneous rocks from southern Brazil: A study of magmatic processes

Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We interpret the ε_Nd paths considering the secondary fractionation of ¹⁴⁷Sm/¹⁴⁴Nd due to major petrogenetic processes. The inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time. Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt. Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first component at the end of the Camboriú collisional orogeny (2.0 Ga).     

内蒙古中—东部地区基底岩石、花岗岩和玄武岩的Nd同位素组成及其对该区地壳演化的指示

对出露于内蒙古地区的华北地台北缘中段及兴蒙造山带内共21件不同岩性的样品进行Nd同位素研究。这些样品的Nd模式年龄值表明:兴蒙造山带与华北地台具完全不同的特征,兴蒙造山带以年轻的亏损地幔模式年龄为特征(tDM=0.4~1.1 Ga),普遍低于华北地台西段的tDM值(1.8~3.4 Ga)。锡林浩特地块作为独立块体具与兴蒙造山带不同的特征,锡林浩特地块的亏损地幔模式年龄介于兴蒙造山带年龄与华北地台年龄之间。Nd模式年龄计算结果表明内蒙古地区华北地台北缘的地壳增生事件主要集中于中元古代之前,而兴蒙造山带地壳增生事件自新元古代开始。通过对内蒙古地区华北地台北缘εNd(t)值随时间的变化分析可知,在中元古代及海西期均存在古老地壳的再循环及新地壳的增生事件。内蒙古兴蒙造山带地壳增生速率表明该区地壳主要增生事件发生于1 000~700 M a,其后形成的岩浆岩所反映的增生过程表明有古老地壳组分的参与。     

http://www.sciencedirect.com/science/article/pii/S1674987114000206

The North China Craton（NCC） has a complicated evolutionary history with multi-stage crustal growth,recording nearly all important geological events in the early geotectonic history of the Earth.Our studies propose that the NCC can be divided into six micro-blocks with ＆gt;～3.0-3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts（CRB）.The micro-blocks are also termed as highgrade regions（HGR） and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses,all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies.The micro-blocks are,in turn,from east to west,the Jiaoliao（JL）,Qianhuai（QH）,Ordos（ODS）,Ji’ning（JN） and Alashan（ALS） blocks,and Xuchang（XCH） in the south.Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean.Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca.2.9-2.7 and2.6-2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC.Hafnium isotopic model ages range from ca.3.8 to 2.5 Ga and mostly are in the range of 3.0-2.6 Ga with a peak at2.82 Ga.Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered,with a dominant ca.2.7 Ga magmatic zircon ages.Most of the ca.2.7 Ga TTG gneisses underwent metamorphism in 2.6-2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks.Abundant ca.2.6-2.5 Ga orthogneisses have Hf-in-zircon and Nd wholerock model ages mostly around 2.9-2.7 Ga and some around 2.6-2.5 Ga,indicating the timing of protolith formation or extraction of the protolith magma was from the mantle.Therefore,it is suggested that the 2.6-2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking（crustal melting）.As a distinct characte     

Magma sources and petrogenesis of the early–middle Paleozoic backarc granitoids from the central part of the Qilian block,NW China

The petrology, geochemistry, geochronology, and Sr–Nd–Hf isotopes of the backarc granitoids from the central part of the Qilian block are studied in the present work. Both S- and I-type granitoids are present. In petrographic classification, they are granite, alkali feldspar granite, felsic granite, diorite, quartz diorite, granodiorite, and albite syenite. The SHRIMP ages are 402–447 Ma for the S-type and 419–451 Ma for the I-type granitoids. They are mostly high-K calc-alkaline granitoids. The S-type granitoids are weakly to strongly peraluminous and are characterized by negative Eu anomalies (Eu/Eu* = 0.18–0.79). The I-type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive Eu anomalies (Eu/Eu* = 0.71–1.16). The initial (⁸⁷Sr/⁸⁶Sr) values are 0.708848–0.713651 for the S-type and 0.704230–0.718108 for the I-type granitoids. The ε_Nd(450 Ma) values are − 8.9–−4.1 and − 9.7–+ 1.9 for the S-type and I-type granitoids, respectively. The T_DM values are 1.5–2.4 Ga for the S-type and 1.0–2.3 Ga for the I-type granitoids. For the Qilian block, the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north Qilian oceanic lithosphere and lasted approximately 50 million years. Partial melting of the source rocks consisting of the Neoproterozoic metasedimentary rocks of the Huangyuan Group and the intruding lower Paleozoic basaltic rocks could produce the S-type granitoid magmas. Partial melting of basaltic rocks mixed with lower continental crustal materials could produce the I-type granitoid magmas. Major crustal growth occurred in the late Archean and Meso-Paleoproterozoic time for the Qilian block. The magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1.0 Ga for the Qilian block.     

Nd isotopic constraints on crustal formation in the North China Craton

Recent tectonic analysis suggests that the North China Craton consists of two Archean continental blocks, called the Eastern and Western Blocks, separated by the Paleoproterozoic Trans-North China Orogen. Although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton, the available Nd isotopic data show some important differences in Nd model ages between the tectonic units. The Eastern Block shows two main Nd model age peaks, one between 3.6 and 3.2 Ga and the other between 3.0 and 2.6 Ga. Limited Nd isotopic data from the Western Block show a large range of model ages between 3.2 and 2.4 Ga. These differences are consistent with the recently-proposed model.The Nd isotopic data from mantle-derived mafic rocks indicate that the mantle beneath the North China Craton was depleted in the Archean, consistent with major crustal growth during this period. In the Paleoproterozoic, however, the mantle-derived mafic rocks show negative ε_Nd(t) values, implying crustal contamination. This may have resulted from subduction and collision between the Eastern and Western Block, implying that the mechanisms of crustal formation and evolution may have been different between the Archean and Paleoproterozoic.The North China Craton was re-activated by addition of mantle-derived magma into the lower crust in the late Mesozoic, resulting in rejuvenation of the lower crust. This indicates that underplating is also an important mechanism for continental addition, although in this case it may not equate to crustal growth, since it was preceded by removal of lithospheric mantle and possible some lower crust.     

Geochronology and Tectonic Evolution of Karimnagar and Bhopalpatnam Granulite Belts, Central India

The Karimnagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42±0.08 Ga and 2.47°0.03 Ga. Rims of monazite show mean age of 2.21±0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO₂* diagram delineating a clear isochron with age of 1.59±0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.     

基底岩系和花岗岩类Pb-Nd同位素组成限制祁连山带的构造属性

对扬子陆块的西北部边界至今尚未得到有效的限定.中央山系西段祁连山带基底岩系和花岗岩类的Pb-Nd同位素组成为限定扬子陆块的西北边界提供限制.祁连山带前寒武纪基底岩系的Nd同位素亏损地幔模式年龄(T_DM)主要分布于0.75-2.5 Ga之间, 峰值为2.1 Ga左右; 该带古生代花岗岩类的T_DM变化于1.07-2.14 Ga之间.由此表明, 祁连山带地壳增长主要发生于元古宙, 缺乏太古宙地壳增长的信息.祁连山带前寒武纪基底和花岗岩类全岩均以高放射成因的铅同位素组成为特征, 极大多数样品的206Pb/204Pb > 18.0, 207Pb/204Pb > 15.5, 208Pb/204Pb > 38.0.因此, 祁连山带地壳增长特征和铅同位素组成特征与华北陆块存在明显的差异, 而与扬子陆块一致, 从而表明祁连山带具有扬子型陆块的构造属性.因此, 扬子陆块的西北部边界扩大至祁连山带的北侧.自新元古代以来, 祁连山带经历了岩石圈裂解作用, 并有洋盆形成, 但这些构造事件均发生在扬子型陆块内部的地质背景.      

豫西地区秦岭造山带武当群Nd-Hf同位素组成及其物源特征

武当群变质沉积-火山岩组合是南秦岭地体中重要的基底岩石,其形成时代和地球化学特征可以为理解秦岭造山带的构造演化提供重要的证据.本文报道豫西地区武当群上部沉积岩和下部中-酸性火山岩Sm-Nd同位素和锆石Lu-Hf同位素组成,探讨火山岩成因和沉积岩物源的同位素特征.上部沉积岩的碎屑锆石初始ε_(Hf)值变化在-30～+10之间,对应的模式年龄值t_(DM2)在1.0Ga至3.2Ga之间,初始ε_(Nd)值在-4.0至-6.0之间.沉积物源表现为主要与扬子陆块有亲缘关系的地壳物质和近源的下部火山岩混合的特征.火山岩的锆石初始εHf值变化在-35～+15之间,对应的模式年龄值t_(DM2)在0.8Ga至3.5Ga之间,集中于1.5～1.8Ga和2.2～2.4Ga两个峰值.2个变质石英角斑岩样品初始ε_(Nd)值分别为-9.2和-10.7,而报道的湖北武当群的玄武-安山质熔岩的初始ε_(Nd)值以正值为主.因此,武当群不同类型的火山岩可能存在着成因差异.具有低初始ε_(Nd)值和ε_(Hf)值特征的火山岩可能由地壳物质的重熔而形成的;有些火山岩具有初始ε_(Hf)值变化范围较大(-35～+15)或正初始ε_(Nd)值的特点,可能是壳、幔物质混合成因,有显著的幔源或新生地壳物质的贡献.武当群Nd-Hf同位素组成和碎屑锆石年龄分布特征表明,与扬子陆块有亲缘关系的南秦岭地体在元古代期间可能经历多期地壳增生和再造作用.     

Episodic crustal growth in the Tanzania Craton: evidence from Nd isotope compositions

The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crust-mantle separation ages, and events marking juvenile crustal addition and crustal recycling. Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton, although a rock record of such antiquity is yet to be found there. The most significant period of juvenile crustal addition as well as crustal recycling is 2.7–2.6 Ga. The Nd isotopes of mafic samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton, with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle. The Nd isotope section reveals significant differences in Nd isotopes between the north craton and central craton; compared to the north craton, the central craton yields a Nd model age that is approximately 100 Ma older, and its ε_Nd(t) values are more negative, indicating that the two parts of the craton have different mantle source regions. Different types of granitoids are distributed in the Tanzania Craton, such as high-K and low-Al granite, calc-alkaline granite, peraluminous granite and transitional types of tonalite-trondhjemite-granodiorites (TTGs). Most of the granitoids formed later than the mafic rocks in syn-collision and post-collision events.     

The Tandilia System of Argentina as a southern extension of the R��o de la Plata craton: an overview

The southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in eastern Argentina. The geological evolution comprises mainly an igneous-metamorphic Paleoproterozoic basement named Buenos Aires Complex, which is covered by Neoproterozoic to Early Paleozoic sedimentary units which display subhorizontal bedding. The basement of calc-alkaline signature consists mainly of granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks, and granitoid plutons. Subordinate rock-types include schists, marbles, and dykes of acid and mafic composition. Tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids. The tectonic scenario seems related to juvenile accretion event (2.25?C2.12?Ga) along an active continental margin, followed by continental collision (2.1?C2.08?Ga) after U?CPb zircon data. The collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks. The tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution. The basement was preserved from younger orogenies such as those of the Brasiliano cycle. After a long paleoweathering process, the Sierras Bayas Group (c. 185?m thick) represents a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), superposed by Cerro Negro Formation (c. 150?C400?m thick, siliciclastics) assigned to Upper Neoproterozoic age. The final sedimentary transgression during Early Paleozoic was the Balcarce Formation (c. 90?C450?m thick) deposited over all the mentioned Precambrian units. Based on all the geological background, a tectonic evolution is offered.     

Zircon U-Pb and Lu-Hf isotope constraints on Archean crustal evolution in Southeastern Guyana Shield

The southeastern Guyana Shield,northeast Amazonian Craton,in the north of Brazil,is part of a widespread orogenic belt developed during the Transamazonian orogenic cycle(2.26-1.95 Ga)that includes a large Archean continental landmass strongly reworked during the Transamazonian orogeny,named Amapa Block.It consists mainly of a high-grade metamorphic granulitic-migmatitic-gneiss complex,of Meso-to Neoarchean age and Rhyacian granitoids and supracrustal sequences.For the first time,coupled U-Pb and Lu-Hf isotope data were obtained on zircon by LA-ICP-MS from five tectono-stratigraphic units of the Archean basement and one Paleoproterozoic intrusive rock,in order to investigate the main episodes of crustal growth and reworking.Whole-rock Sm-Nd isotope data were compared to the zircon Lu-Hf data.Three main magmatic episodes were defined by U-Pb zircon dating,two in the Mesoarchean(~3.19 Ga and 2.85 Ga)and one in the Neoarchean(~2.69-2.65 Ga).SubchondriticεHf(t)values obtained for almost all investigated units indicate that crustal reworking processes were predominant during the formation of rocks that today make up the Amapa Block.Hf-TDMC model ages,ranging from2.99 Ga to 3.97 Ga,indicate that at least two important periods of mantle extraction and continental crust formation occurred during the Archean in southeastern Guyana Shield,an older one in the Eoarchean(~4.0 Ga)and a younger one in the Mesoarchean(~3.0-3.1 Ga).The latter is recognized as an important period of crustal accretion worldwide.The recognition of an Eoarchean episode to the southeastern most part of the Guyana Shield is unprecedented and was not recorded by whole-rock Sm-Nd data,which were restricted to the Meso-Paleoarchean(2.83 Ga to 3.51 Ga).This finding reveals t hat continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the SmNd systematics.     

辽宁鞍山地区东山杂岩带3.3~3.1 Ga期间的岩浆作用——锆石SHRIMP U-Pb定年

报道了鞍山地区东山杂岩带奥长花岗岩和二长花岗岩的锆石SHRIMP U-Pb年龄。中粗粒奥长花岗岩中岩浆锆石的年龄为3329 Ma ± 22 Ma (MSWD=9.6),存在年龄为3687~3784 Ma的残余锆石。细粒奥长花岗岩和二长花岗岩中岩浆锆石的年龄分别为3141 Ma ± 8 Ma (MSWD=1.5)和3142 Ma ± 5 Ma (MSWD=0.35)。研究表明,约~3.3 Ga和3.1 Ga是鞍山地区2个重要的地壳演化阶段。     

Pb isotopic composition of late Archaean granites and the extent of recycling early Archaean crust in the Slave Province, northwest Canada

Initial Pb isotopic compositions have been determined for potassium feldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and central Slave Province of northwestern Canada to evaluate the extent of recycling of ancient crust within the province. Large differences in initial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the province have initial compositions only slightly more radiogenic than estimated mantle values (²⁰⁷Pb/²⁰⁴Pb 14.8–14.9), and were dominantly deruved from juvenile crustal sources. In contrast, plutons in the Point Lake and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions (²⁰⁷Pb/²⁰⁴Pb 15.1–15.2), and indicate significant recycling of pre-3.5 Ga crust. The Pb data support previous interpretations, based on Nd isotopes, for a major isotopic boundary in the central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indicate: (1) the age of the protolith to the granitoids in the Yellowknife area is younger than at Point Lake, but older than in the eastern Slave; or (2) the granitoids in the Yellowknife area contain a mixture of an older Point Lake-type component and younger crust. The absence of pre-3.2 Ga crust in the Yellowknife area and lack of evidence for pre-2.8 Ga inherited zircons in the Yellowknife granitoids favour the former possibility. Evidence for recycling of ancient crustal sources, such as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or derived, crust is not widespread in the province. The marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age, being younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).     

Paleoproterozoic granitoids from the northern limit of the Archean Amapá block (Brazil), southeastern Guyana Shield: Pb–Pb evaporation in zircons and Sm–Nd geochronology

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, T_DM and ε_Nd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean T_DM, inherited Pb–Pb zircon ages and strongly negative ε_Nd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.     

兴蒙造山带正ε(Nd,t)值花岗岩的成因和大陆地壳生长

大陆地壳的生长速率和地壳生长的位置均是地球科学中的最基本的问题。现有的许多大陆地壳生长模式认为 ,90 %的大陆地壳生长于 18亿年以前 ,显生宙以来的地壳生长不到整个地壳的 10 % ,主要位于活动大陆边缘。近年来在兴蒙造山带发现大量具有新生地壳来源性质的花岗岩产生于 50 0～ 10 0Ma ,对上述传统看法提出了挑战。现有的Nd同位素资料表明 ,兴蒙造山带的显生宙花岗岩 ,不论形成于什么时代和什么构造背景 ,也不论属于何种成因类型 ,几乎都具有正ε(Nd ,t)值和年轻的Nd模式年龄tDM 。从西往东 ,随着时代逐渐变新ε(Nd ,t)值有逐渐降低的趋势。花岗岩的tDM同由蛇绿岩和岛弧杂岩记录的古亚洲洋扩张的时间基本一致。只有一些在新元古代微陆块上的花岗岩才显示负ε(Nd ,t)值和较老的tDM,反映了其源岩包括前寒武纪地壳同地幔来源物质的不同程度混合。兴蒙造山带的花岗岩具有地幔来源的ε(Nd ,t)值 ,说明这些花岗岩中有一部分 (例如加里东期和海西早期 )可能同板块俯冲作用有关 ,花岗岩的来源是被交代的地幔楔。而大面积的晚古生代—中生代花岗岩则可能是由 80 0～6 0 0Ma前俯冲的洋壳形成的新生大陆地壳在拉伸体制下部分熔融而成。如果情况是这样 ,显生宙就曾发生过大规模的地壳生长。板内岩浆活动 ,特别是     

The generation and evolution of the Archean continental crust: The granitoid story in southeastern Brazil

The Archean Eon was a time of geodynamic changes. Direct evidence of these transitions come from igneous/metaigneous rocks, which dominate cratonic segments worldwide. New data for granitoids from an Archean basement inlier related to the Southern São Francisco Craton (SSFC), are integrated with geochronological, isotopic and geochemical data on Archean granitoids from the SSFC. The rocks are divided into three main geochemical groups with different ages: (1) TTG (3.02–2.77 Ga); (2) medium- to high-K granitoids (2.85–2.72 Ga); and (3) A-type granites (2.7–2.6 Ga). The juvenile to chondritic (Hf-Nd isotopes) TTG were divided into two sub-groups, TTG 1 (low-HREE) and 2 (high-HREE), derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts. The compositional diversity within the TTG is attributed to different pressures during partial melting, supported by a positive correlation of Dy/Yb and Sr/Zr, and batch melting calculations. The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs, indicating the presence of subduction processes concomitant with TTG emplacement. From ～2.85 Ga to 2.70 Ga, the dominant rocks were K-rich granitoids. These are modeled as crustal melts of TTG, during regional metamorphism indicative of crustal thickening. Their compositional diversity is linked to: (i) differences in source composition; (ii) distinct melt fractions during partial melting; and (iii) different residual mineralogies reflecting varying P–T conditions. Post-collisional (～2.7–2.6 Ga) A-type granites reflect rifting in that they were closely followed by extension-related dyke swarms, and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle. The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening, regional metamorphism and crustal melting, and post-collisional extension, similar to that seen in younger Wilson Cycles. It is compelling evidence that plate tectonics was active in this segment of Brazil from ～3 Ga.