Gold and silver metallogeny of the South China Fold Belt: a consequence of multiple mineralizing events?

The South China Fold Belt is part of the South China Block that is interpreted to be the result of multiple tectonic and magmatic events that formed a collage of accreted Proterozoic and Phanerozoic terranes. The Jurassic to early Cretaceous Yanshanian period (180–90 Ma), a time of major tectono-thermal events that affected much of eastern and southeastern China, is of great metallogenic importance in the fold belt. This period is linked to subduction of the Pacific plate beneath the Eurasian continent, and is manifested by voluminous volcano-plutonic activity of predominantly calc-alkaline affinity.The distribution of gold and silver deposits in the South China Fold Belt indicates the presence of two distinct metallogenic provinces. A region of basement uplifts, which are controlled by shear zones and form Neoproterozoic inliers of metamorphosed iron-rich rock types, defines the first province. In this province, orogenic lodes and volcanic-related epithermal deposits represent the more significant precious-metal mineralization. The second province is essentially confined to a belt of Yanshanian felsic–intermediate volcanic and subvolcanic rocks that extends along most of the southeastern China coast in an area known as the Coastal Volcanic Belt. Deposits in the Coastal Volcanic Belt are silver- and/or copper-rich, volcanic-hosted and epithermal in character.The precious-metal metallogeny of the South China Fold Belt is interpreted to have developed in at least three stages: one as a result of collision events, during the Caledonian Orogeny (ca. 400 Ma), the second during the Indosinian Orogeny (ca. 200 Ma) and the third during or soon after the formation of the Yanshanian magmatic belt (Yanshanian Orogeny; 180–90 Ma). The latter was responsible for a hydrothermal event that affected large sections of the belt and its Proterozoic substrate. This may have resulted in the redistribution and enrichment of precious metals from preexisting orogenic gold lodes in Neoproterozoic basement rocks, which are now exposed as windows in zones of tectonic uplift. The Yanshanian hydrothermal activity was particularly widespread in the Coastal Volcanic Belt and resulted in the formation of both low- and high-sulfidation epithermal gold and silver, and locally copper and other base-metal mineralization. It is suggested that the Coastal Volcanic Belt has greater potential for world-class epithermal and porphyry deposits than previously realised.     

丹凤地区秦岭岩群片麻岩锆石U-Pb年龄:北秦岭地体中-新元古代岩浆作用和早古生代变质作用的记录

秦岭岩群被认为是出露于北秦岭地体内最古老的前寒武纪基底岩石,记录了北秦岭造山带的地壳形成和演化历史。本文报道丹凤-西峡地区五件秦岭岩群片麻岩锆石U-Pb年龄结果,限定其形成和变质时代,探讨北秦岭地体的构造归属。定年结果表明,岩浆成因锆石颗粒的年龄集中在1400～1600Ma左右和850～950Ma左右,记录两期主要岩浆活动。6粒锆石具有变质成因特征,低Th/U比值(0.03),206Pb/238U年龄变化在510～465Ma之间,加权平均值477±18Ma。这一古生代变质叠加时代与北秦岭地体南北缘高压变质作用时代基本一致,说明秦岭岩群遭受到北秦岭造山带俯冲-碰撞造山过程的变质作用。秦岭岩群主要形成于中元古代晚期至新元古代早期,基底岩石缺乏早元古代和太古代岩浆活动的记录。在岩浆作用时代上,北秦岭地体与广泛发育新元古代中-晚期岩浆作用的扬子陆块北缘有差别,也不同于晚太古代-早元古代的华北陆块南缘,可能是中-新元古代形成的独立微陆块。     

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.     

U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications

The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U-Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920-820 Ma) and Cambro-Ordovician (540-460 Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim-Kunlun terrane, South China block, the Aravalli-Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa-Qiangtang terranes were involved in these tectono-magmatic events. U-Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous-Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan sandstones and other rocks deposited in East Gondwana during the late Neoproterozoic and Paleozoic are all quite similar, making it difficult to use the age spectra for paleogeographic determinations. There is a suggestion in the data that the Qiangtang terrane may have been located further west along Gondwana’s northern boundary than the Lhasa terrane, but more refined spatial and temporal data are needed to verify this configuration.     

Neoproterozoic tectonic synthesis of Uruguay

Neoproterozoic–lower Palaeozoic successions in the Brasiliano fold belts are described and a brief synthesis of these terranes is presented in order to erect a tectonic framework for this region. Tectonic events that occurred around the Río de La Plata craton were diachronous and reflected successive stages of the Brasiliano orogenic cycle. They took place in mobile belts that constituted part of the Gondwana supercontinent. The most thoroughly investigated Neoproterozoic sections are located in the eastern and southeastern regions of Uruguay. The Dom Feliciano Belt shows a tectonic evolution from back-arc to foreland basin characterized by fold-and-thrust, thick-skinned belts developed during the Brasiliano/Pan-African orogenic cycle. The most conspicuous features were late-tectonic high-K calc-alkaline granitoids, HT-LP metamorphism, significant displacements along shear zones, and post-tectonic granitoids. The final stage was characterized by post-collisional basins (molassic sequences) and extensional magmatism related to a phase of crustal stretching. Several lithotectonic units are present as basement inliers in the Dom Feliciano Belt: these include a low-to-medium metamorphic grade sequence (the Zanja del Tigre Formation), granitoids and gneisses (the Campanero Unit), high-grade basement of the Cerro Olivo Complex (Palaeoproterozoic or Neoproterozoic), and a low-metamorphic grade orogenic belt (the Rocha Formation). This paper provides a simplified tectonic map of eastern Uruguay, which we use to describe tectonic evolution from Precambrian to early Palaeozoic time.     

Rhyacian and Neoproterozoic magmatic associations of the Gurupi Belt,Brazil: Implications for the tectonic evolution,and regional correlations

The Gurupi Belt, in north-northeastern Brazil, is a mobile belt developed in the south-southwestern margin of the São Luís cratonic fragment and crops out as a tectonic and erosional window within the Phanerozoic cover. Field, petrographic, geochemical, geochronological, and Nd isotopic information (new and published) constrain the timing and types of magmatic associations present in the belt and the tectonic settings in which they formed. The Rhyacian was the main period of magmatic activity, which can be grouped into two main stages. (1) ~2185–2130 ​Ma: pre-collisional, juvenile, calc-alkaline magnesian and calcic ferroan granitoid suites, and minor calc-alkaline and tholeiitic mafic plutonism (now amphibolites), formed in intra-oceanic to transitional/continental arcs; and intra- or back-arc volcano-sedimentary basin. (2) ~2125–2070 ​Ma: syn- (two-mica granites) to late-collisional (potassic to shoshonitic granites and quartz-syenite) plutonic suites produced after crustal thickening and melting, with localized migmatization, that intruded during the compressive D₁ deformational phase and concomitantly with greenschist to amphibolite metamorphism. There is a zonation of the Rhyacian episodes, with intra-oceanic stages occurring to the northeast, and the continental arc and collisional phases occurring to the southwest, indicating the presence of an active continental margin to the southwest, and subduction from NE to SW (present-day configuration). This magmatic framework is a continuation to the south of what is described for the São Luís cratonic fragment to the north, and the orogenic scenario is identical to what is observed for the same period in the West African Craton (Eburnean/Birrimian orogen), which additionally supports previous geological correlations. In the Neoproterozoic, a few magmatic occurrences are recognized. An extensional event allowed the intrusion of an anorogenic, nepheline syenite at ca. 730 Ma, which was followed by the intrusion of a crustal, calc-alkaline microtonalite, of uncertain tectonic setting, at 624 ​Ma. Both intrusions underwent greenschist to amphibolite facies metamorphism between 580 Ma and 529 ​Ma. This metamorphic event is probably related to crustal thickening, which produced crustal melting and intrusion of two-mica granites between 595 Ma and 549 ​Ma. The absence of oceanic and arc-related assemblages, along with geophysical information about the basement of the Phanerozoic cover indicates an intracontinental setting for the Neoproterozoic–Early Cambrian evolution of the Gurupi Belt, with rifting and posterior closure of the basin, without oceanization. Rifting and closure correlate in time with the onset of Rodinia breakup and West Gondwana assembly, respectively, but we interpret the events in the Gurupi Belt as having no direct role in these two global supercontinent-related events, but, instead, as being related to orogenic events occurring in the periphery of the West African and Amazonian cratons at that time.     

中国新元古代大陆拼合与Rodinia超大陆

在对前人研究成果分析的基础上,结合对中祁连块块前寒武纪基底的研究,认为新元古代（1000－9000Ma）中国各主要克拉通地块（包括华夏地块、扬子地块、华北地块、阿拉善－祁连－柴达木地块、塔里木地块）曾经通过晋宁期碰撞拼合带发生过一次全面的多块体复杂拼贴。这次拼贴过程是全球性新元古代格林威尔碰撞造山作用和Rodinia超大陆形成过程的一个组成部分。新元古代拼合的古中国地块在当时位于Rodinia超大陆中北部澳大利亚、劳伦提亚和西伯利亚克拉通地块之同。中国的各主要克拉通地块只不过是巨型格林威尔碰撞带之中夹裹的一些小型地块,且都处于新元古代晚期Rodinia超大陆破裂的中心部位,这些正是中国大陆上克拉通地块活动性大的早期地质背景。     

The Cariris Velhos tectonic event in Northeast Brazil

The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the São Luis-West Africa craton to the north and the São Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Médio Coreaú, Ceará Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism, from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000–920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province.Within the Transversal Zone, the Alto Pajeú terrane locally includes some remnants of oceanic crust along with island arc and continental arc rock assemblages, but the dominant supracrustal rocks are mature and immature pelitic metasedimentary and metavolcaniclastic rocks. Contiguous and parallel to the Alto Pajeú terrane, the Riacho Gravatá subterrane consists mainly of low-grade metamorphic successions of metarhythmites, some of which are clearly turbiditic in origin, metaconglomerates, and sporadic marbles, along with interbedded metarhyolitic and metadacitic volcanic or metavolcaniclastic rocks. Both terrane and subterrane are cut by syn-contractional intrusive sheets of dominantly peraluminous high-K calc-alkaline, granititic to granodioritic metaplutonic rocks. The geochemical patterns of both supracrustal and intrusive rocks show similarities with associations of mature continental arc volcano-sedimentary sequences, but some subordinate intra-plate characteristics are also found.In both the Alto Pajeú and Riacho Gravatá terranes, TIMS and SHRIMP U–Pb isotopic data from zircons from both metavolcanic and metaplutonic rocks yield ages between 1.0 and 0.92 Ga, which define the time span for an event of orogenic character, the Cariris Velhos event. Less extensive occurrences of rocks of Cariris Velhos age are recognized mainly in the southernmost domains of the Province, as for example in the Poço Redondo-Marancó terrane, where arc-affinity migmatite-granitic and meta-volcano-sedimentary rocks show U–Pb ages (SHRIMP data) around 0.98–0.97 Ga. For all these domains, Sm–Nd data exhibit T_DM model ages between 1.9 and 1.1 Ga with corresponding slightly negative to slightly positive ε_Nd(t) values. These domains, along with the Borborema Province as a whole, were significantly affected by tectonic and magmatic events of the Brasiliano Cycle (0.7–0.5 Ga), so that it is possible that there are some other early Tonian rock assemblages which were completely masked and hidden by these later Brasiliano events.Cariris Velhos processes are younger than the majority of orogenic systems at the end of Mesoproterozoic Era and beginning of Neoproterozoic throughout the world, e.g. Irumide belt, Kibaride belt and Namaqua-Natal belt, and considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System. Therefore, they were probably not associated with the proposed assembly of Rodinia. We suggest, instead, that Cariris Velhos magmatism and tectonism could have been related to a continental margin magmatic arc, with possible back-arc associations, and that this margin may have been a short-lived (<100 m.y.) leading edge of the newly assembled Rodinia supercontinent.     

祁连造山带东段早古生代葫芦河群变质碎屑岩中碎屑锆石LA-ICP-MSU-Pb年龄:源区特征和沉积时代的限定

祁连造山带东段葫芦河群的形成时代长期存在争议。选择葫芦河群变质碎屑岩为研究对象,运用LA-ICP-MS锆石U-Pb同位素年代学方法,探讨葫芦河群的形成时代和物源特征。结果表明,葫芦河群的2个样品碎屑锆石同位素年龄数据以及侵入其中的花岗岩同位素年龄表明,葫芦河群沉积时代限定为447～434Ma,其主体形成时代为早志留世。葫芦河群变质碎屑锆石年龄谱明显分为4组:(1)震旦纪—早古生代年龄组,426～595Ma,峰值为479Ma;(2)新元古代年龄组,738～981 Ma,峰值为887 Ma;(3)中元古代年龄组,1000～1 913Ma,峰值为1499Ma;(4)古元古代—新太古代年龄组,2053～2 872Ma,峰值为2448Ma。其中,早古生代年龄组可进一步细分为426～493 Ma和527～595 Ma两个年龄段,峰值分别为445 Ma和559Ma,前者年龄段指示其物源可能以邻近地区的北祁连造山带和西秦岭北缘构造带为主,是加里东期中南祁连和西秦岭微地块分别向北俯冲、碰撞产生的一系列火成岩在造山剥蚀后的沉积响应;后者年龄段则与北祁连造山带和西秦岭北缘构造带中泛非造山事件中的岩浆活动有关。新元古代年龄组可细分为738～799Ma、839～862Ma和902～981Ma 3个年龄段,峰值分别为768Ma、848Ma和948Ma,以902～981Ma年龄组为主;第一年龄段(738～799Ma)与北祁连造山带新元古代晚期岩浆事件的年龄大致相对应,与Rodi-nia超大陆的裂解事件相关;第二年龄段和第三年龄段(839～862 Ma、902～981 Ma)与中祁连地区和西秦岭北缘的新元古代早期构造岩浆事件年龄大致相对应,与Rodinia超大陆汇聚事件及岛弧型岩浆作用相关。中元古代年龄组可细分为1 000～1 197Ma和1 243～1 913Ma 2个年龄段,峰值分别为1 036Ma和1 593Ma,其物源可能来自祁连造山带和华北板块基底岩系。古元古代—新太古代年龄组反映了物源来自北祁连造山带和西秦岭北缘构造带的结晶基底,部分物源也有可能来自于华北板块基底岩系。综合分析显示,葫芦河群碎屑沉积物质来源较为复杂,具有明显的多元性,存在祁连造山带、西秦岭北缘构造带和华北板块基底3个物源区,其中祁连造山带和西秦岭北缘构造带提供了大部分物源,而祁连造山带应为葫芦河群贡献最大的物源区。     

Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence(LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence(GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800-2500 Ma,1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population(～860-800 Ma) with a variably enriched radiogenic Hf isotope signature(eHf = 10 to-20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.     

Identifying Laurentian and SW Gondwana sources in the Neoproterozoic to Early Paleozoic metasedimentary rocks of the Sierras Pampeanas: Paleogeographic and tectonic implications

The provenance of Neoproterozoic to Early Paleozoic sedimentary rocks in the Sierras Pampeanas has been established using U–Pb SHRIMP age determination of detrital zircons in twelve metasedimentary samples, with supplementary Hf and O isotope analyses of selected samples. The detrital zircon age patterns show that the western and eastern sectors of the Sierras Pampeanas are derived from different sources, and were juxtaposed during the Early Cambrian ‘Pampean’ collision orogeny, thus defining initiation of the supercontinent stage of southwestern Gondwana. The Western Sierras Pampeanas (WSP), which extend northwards to the southern Puna (Antofalla) and the Arequipa Massif (Peru), constitute a single large continental basement of Paleoproterozoic age — the MARA block — that was reworked during the Grenvillian orogeny. The MARA block probably extends eastwards to include the Río Apa block (southern Brazil), but in this case without a Mesoproterozoic overprint. Detrital zircons from the WSP and Antofalla yield age peaks between 1330 and 1030 Ma, remarkably similar to the range of ages in the Grenville province of eastern Laurentia. The WSP Neoproterozoic sedimentary cover to this basement shows the same 1330–1030 component, but also includes important 1430–1380 Ma zircons whose juvenile Hf and O isotopic signatures strongly suggest derivation from the Grenville and the Southern Granite–Rhyolite provinces of eastern Laurentia. In contrast the Eastern Sierras Pampeanas metasedimentary rocks have a typically bimodal detrital zircon pattern with peaks at ca. 1000 and 600 Ma, which respectively indicate sources in the Natal–Namaqua belt and the East African orogen and/or the Dom Feliciano belt of SE Brazil and Uruguay. Sedimentary rocks in the Eastern Sierras Pampeanas and Patagonia deposited during the Late Early Cambrian–Early Ordovician interval, after the Pampean orogeny, have detrital patterns common to many sectors along the Terra Australis orogen, reflecting increasingly dominant input to the Paleozoic basins from the Neoproterozoic to Early Cambrian orogenic belts of the Gondwana margin.     

Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton

The Saghro Group (SG) is a folded, low-grade volcano-sedimentary series up to 8 km thick that crops out within and to the north of the Pan-African suture zone in the central and eastern Anti-Atlas. Here we describe the SG of the Ougnat inliers that are exposed in the easternmost Anti-Atlas beneath the unconformable, Late Ediacaran Ouarzazate Group (OZG) volcanic rocks. The Ougnat SG mostly consists of volcaniclastic greywackes accumulated in a peritidal-to-shallow basin. The basin infilling was deformed by NNE-trending, mostly upright folds with axial-planar slaty cleavage and low-grade metamorphism. The deformed SG rocks were intruded by the ～550 Ma Mellab hypovolcanic granodiorite. The latter also crosscuts the lowest OZG rocks that are dated to 574–571 Ma in the western Saghro region. The SG rocks that form the Siroua and Saghro inliers have an oldest age of 620–610 Ma and were folded at ～610–580 Ma at the onset of the Cadomian orogenic events. We show that the SG rocks are similar to the “Série verte” (SV) rocks that are exposed in the Ougarta and western Hoggar east of the Pan-African suture. We infer that the SG and SV rocks accumulated in a same, continuous basin that was bounding the West African Craton to the north and the east. This strongly subsiding basin formed close to a volcanic arc and was folded during the last Pan-African synmetamorphic events. Fold orientation and age of folding differ however along the edge of the West African Craton. The orogenic greywackes that form the remnants of the SG-SV basin thus constitute a precious record of the diachronic Cadomian event s.l. along the West African Craton northern margin.     

秦岭南缘勉略带构造属性及晚古生代地质背景:来自碎屑锆石U-Pb年代学的制约

勉略构造带作为秦岭造山带内重要的构造边界,关于其构造属性及晚古生代以来的地质背景,一直是学术界争论的焦点.碎屑锆石U-Pb年代学在限定地层单元的最大沉积年龄、研究区域构造岩浆事件及约束构造地质背景等方面行之有效.基于此,通过对勉略带内五郎坪北侧两河口变沉积地层和侵入其中的变形花岗岩脉体进行LA-ICP-MS锆石U-Pb...     

Gondwana fragments in the Eastern Alps: A travel story from U/Pb zircon data

We compare detrital U/Pb zircon age spectra of Carboniferous and Permian / Lower Triassic sedimentary rocks from different structural positions within the Austroalpine nappe pile with published ages of magmatic and metamorphic events in the Eastern Alps and the West Carpathians. Similarities between sink and possible sources are used to derive provenance of sediments and distinct frequency peaks in sink and source age pattern are used for paleogeographic plate tectonic reconstructions. From this, travel paths of Austroalpine and West Carpathian basement units are traced from the Late Neoproterozoic to the Jurassic. We place the ancestry of basement units on the northeastern Gondwana margin, next to Anatolia and the Iranian Luth-Tabas blocks. Late Cambrian rifting by retreat of the Cadomian Arc failed and continental slivers re-attached to Gondwana during a late Cambrian / early Ordovician orogenic event. In the Upper Ordovician crustal fragments of the Galatian superterrane rifted off Gondwana through retreat of the Rheic subduction. An Eo-Variscan orogenic event at ~390 Ma in the Austroalpine developed on the northern rim of Galatia, simultaneously with a passive margin evolution to the south of it. The climax of Variscan orogeny occurred already during a Meso-Variscan phase at ~350 Ma by double-sided subduction beneath Galatia fragments. The Neo-Variscan event at ~330 Ma was mild in eastern Austroalpine units. This orogenic phase was hot enough to deliver detrital white mica into adjacent basins but too cold to create significant volumes of magmatic or metamorphic zircon. Finally, the different zircon age spectra in today's adjacent Carboniferous to Lower Triassic sediments disprove original neighbourhood of basins. We propose lateral displacement of major Austroalpine and West-Carpathian units along transform faults transecting Apulia. The intracontinental transform system was released by opening of the Penninic Ocean and simultaneous closure of the Meliata Hallstatt Ocean as part of the Tethys.     

北祁连造山带嘉峪关北大草滩地区早志留世碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

以北祁连造山带嘉峪关北大草滩地区原划为中—下奥陶统的阴沟群碎屑岩为研究对象,进行碎屑锆石LA-ICP-MS UPb测年,探讨其形成时代、物源组成和构造背景。结果表明,碎屑岩沉积时代早于432.5Ma,为早志留世,该套沉积地层并不属于早—中奥陶世阴沟群。碎屑锆石U-Pb同位素年龄可明显分为4组:早古生代年龄组,434～521Ma,峰值为447Ma;新元古代年龄组,791～992Ma,峰值年龄966Ma;中古元古代年龄组,1017～1755Ma,并出现1120Ma、1278Ma、1427Ma和1648Ma多个峰值;古元古代早期—新太古代晚期年龄组,1879～2663Ma,并出现2089Ma、2428Ma和2543Ma多个峰值。综合分析显示,碎屑岩沉积物质来源于祁连造山带和阿拉善地块,祁连造山带早古生代岛弧型岩浆岩和新元古代岩浆岩,以及造山带结晶基底岩系为该套碎屑岩沉积提供了更重要的物源。     

塔里木盆地东南缘(阿尔金山)"变质基底"记录的多期构造热事件:锆石U-Pb年代学的制约

塔里木盆地东南缘的阿尔金山被认为是塔里木克拉通变质基底的主要出露地区之一。 本文通过阿尔金山北坡不整合在太古代-古元古代变质基底之上的安南坝群中的碎屑岩和中南阿尔金中深变质岩石(原定为阿尔金岩群)的锆石U-Pb年代学研究,来确定塔里木盆地东南缘变质基底的性质及所经历的多期构造热历史。研究结果显示,塔里木盆地东南缘的安南坝群中含砾砂岩的碎屑锆石年龄集中在1.92Ga左右,少量在2.0~2.4Ga,表明其碎屑物质主要来源于下伏的太古代-早元古代米兰岩群和相关的深成侵入体。在中阿尔金地块和南阿尔金俯冲碰撞杂岩带的深变质岩石中,锆石U-Pb年代学数据表明其记录有新元古代早期(920~940Ma)、新元古代晚期(760Ma左右)和早古生代(450~500Ma)三期构造热事件,新元古代早期的构造热事件与塔里木(或晋宁)造山作用有关,它普遍存在于塔里木盆地周缘的和南中国地块(扬子克拉通)的变质基底岩石中,与Rodinia超级大陆汇聚相关;新元古代晚期的构造热事件也同样广泛存在于塔里木盆地周缘和扬子克拉通之中,被认为与Rodinia超大陆的裂解作用有关。因此,在新元古代时期,阿尔金的地质演化历史与扬子克拉通非常相似,而与华北则有很大的不同,锆石U-Pb测定还表明中南阿尔金的深变质岩石普遍遭受了早古生代的变质作用的改造,显示它们普遍卷入了早古生代的碰撞造山事件之中,成为早古生代碰撞造山带的组成部分。     

祁连山西段玉石沟地区上二叠统砂岩碎屑锆石年代学及其地质意义

玉石沟地区位于青藏高原东北缘,大地构造属于北祁连造山带南缘,其石炭纪—三叠纪是上叠盆地发育时期,表现为浅海相、海陆交互相至陆相稳定型沉积建造。对玉石沟北部紫红色粗砂岩样品进行LA-ICP-MS碎屑锆石U-Pb测年,其年龄结果主要分布在4个区间:260～350 Ma(峰值314 Ma)、400～500 Ma(峰值445 Ma)、1 700～2 000 Ma、2 200～2 600 Ma,800～1 000 Ma锆石数仅有2颗,另有1颗锆石为2 056 Ma。锆石CL图像显示:260～350 Ma锆石中既有变质锆石也有岩浆锆石,可能响应南祁连地区石炭纪—早二叠世的热事件。400～500 Ma锆石以岩浆锆石为主,表明北祁连造山带和中祁连地块广泛发育的弧岩浆岩和同碰撞花岗岩提供了物源。1 700～2 000 Ma和2 200～2 600 Ma锆石主要为变质锆石,反映了祁连地块基底变质岩的年龄信息,推测来源于基底变质岩的剥露。800～1 000 Ma年龄区间的锆石数量稀少,可能反映新元古代侵入体在该组沉积时期尚未大规模剥露。砂岩中最年轻的锆石年龄为(289±2)Ma,限定了其沉积时代的下限为早二叠世,结合实测地层剖面上的岩石组合和层序变化,将之归属于上二叠统红泉组。碎屑锆石年龄结构表明玉石沟地区红泉组兼具北祁连造山带和中—南祁连地块的年龄信息,红泉组沉积物可能具有南、北两个物源区。     

Neoproterozoic (800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc–continent collision at the northeast Rodinia margin

The Tuva-Mongolia Massif is a composite Precambrian terrane incorporated into the Palaeozoic Sayany-Baikalian belt. Its Neoproterozoic amalgamation history involves early (800 Ma) and late Baikalian (600–550 Ma) orogenic phases. Two palaeogeographic elements are identified in the early Baikalian stage — the Gargan microcontinent and the Dunzhugur oceanic arc. They are represented by the Gargan Glyba (Block) and the island-arc ophiolites overthrusting it. The Gargan Glyba is a two-layer platform comprising an Early Precambrian crystalline basement and a Neoproterozoic passive-margin sedimentary cover. The upper part comprises olistostromes deposited in a foreland basin during the early Baikalian orogeny. The Dunzhugur arc ophiolite form klippen fringing the Gargan Glyba, and show a comprehensive oceanic-arc ophiolite succession. The Dunzhugur arc faced the microcontinent, as shown by the occurrence of forearc complexes. The arc–continent collision followed a pattern similar to Phanerozoic collisions. When the marginal basin lithosphere had been completely subducted, the microcontinental edge partially underthrust the arc, and the forearc ophiolite overrode it. Continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin. Owing to the lithospheric break, a new subduction zone, inclined beneath the Gargan microcontinent, emerged. Initial melts of the newly-formed continental arc are represented by tonalites intruded into the Gargan microcontinent basement and its cover, and into the ophiolite nappe. The tonalite Rb–Sr mineral isochron age is 812±18 Ma, which is similar to a U–Pb zircon age of 785±11 Ma. A period of tonalite magmatism in Meso–Cenozoic orogenic belts is recognized some 1–10 m.y. after the collision. Accordingly, the Dunzhugur island arc–Gargan microcontinent collision is conventionally dated at around 800 Ma. It is highly probable that in the early Neoproterozoic, the Gargan continental block was part of the southern (in modern coordinates) margin of the Siberia craton. It is suggested that a chain of Precambrian massifs represents an elongate block separated from Siberia in the late Neoproterozoic. The Tuva-Mongolia Massif is situated in the northwest part of this chain. These events occurred on the NE Neoproterozoic margin of Rodinia, facing the World Ocean.     

Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil

Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic tectonic cycles of supercontinents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were formed during the Jequié Orogeny (ca 2600 Ma), but extensive arc accretion occurred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriú Orogeny. Both accretionary and collisional orogenies are also identified in the Neoproterozoic Brasiliano Cycle. These are the accretionary Passinho Orogeny (ca 900 Ma) and São Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base‐metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry‐copper‐type magmatic‐hydrothermal settings during the Neoproterozoic.     

Cratonic reactivation and orogeny: An example from the northern margin of the North China Craton

Reactivation of cratonic basement involves a number of processes including extension, compression, and/or lithospheric delamination. The northern margin of the North China Craton (NCC), adjacent to the Inner Mongolian Orogenic Belt, was reactivated in the Late Paleozoic to Early Mesozoic. During this period, the northern margin of the NCC underwent magmatism, N–S compression, regional exhumation, and uplift, including the formation of E–W-trending thick-skinned and thin-skinned south-verging folds and south-verging ductile shear zones. zircon U–Pb SHRIMP ages for mylonite protoliths in shear zones which show ages of 310–290 Ma (mid Carboniferous–Early Permian), constraining the earliest possible age of deformation. Muscovite within carbonate and quartz–feldspar–muscovite mylonites from the Kangbao–Weichang and Fengning–Longhua shear zones defines a stretching lineation and gives ⁴⁰Ar/³⁹Ar ages of 270–250 Ma, 250–230 Ma, 230–210 Ma, and 210–190 Ma. Deformation developed progressively from north to south between the Late Paleozoic and Triassic. Exhumation of lower crustal gneisses, high-pressure granulites, and granites occurred at the cratonic margin during post-ductile shearing (~ 220–210 Ma). An undeformed Early Jurassic (190–180 Ma) conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis. Deformation was induced by convergence between the southern Mongolia and North China cratonic blocks, and the location of this convergent belt controlled later deformation in the Yanshan Tectonic Province. This province formed as older E–W-trending Archean–Proterozoic sequences were reactivated along the northern margin of the NCC. This reactivation has features typical of cratonic basement reactivation: compression, crustal thickening, remelting of the mid to lower crust, and subsequent orogenesis adjacent to the orogenic belt.