土耳其Pütürge变质地体晚白垩世岩浆作用与变质作用的年代学研究

Pütürge变质地体位于新特提斯构造带南部的土耳其Anatolia逆冲推覆构造带内,形成于欧亚板决与阿拉伯板块之间晚白垩纪碰撞造山事件.Pütürge变质地体主要由变质泥质片岩及片麻岩、花岗质片麻岩、石英岩、角闪岩和大理岩组成,发育类似巴罗型递增变质带的变质带序列,变质程度达高绿片岩相至低角闪岩相.此前该变质地体一直缺乏精确的年代学约束,为此我们采用了二次离子质谱锆石U-Pb测年方法和黑云母40Ar/39 Ar测年方法,对该变质地体进行了年代学研究.结果表明,区内花岗片麻岩原岩形成于84.2±1.1Ma,变质泥质片麻岩中黑云母40Ar/39 Ar年龄所代表的变质时代为83.21±0.1Ma.这说明早白垩世期间岩浆侵入事件不久,Pütürge变质地体就发生了区域变质作用.     

Paleomagnetism of the Cretaceous Morelos and Mezcala Formations, southern Mexico

A paleomagnetic study of platform-facies carbonate rocks of the mid-Cretaceous Morelos Formation and deep-water carbonate rocks of the overlying Upper Cretaceous Mezcala Formation, sampled at Zopilote canyon, in Guerrero State, southern Mexico, indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the Late Cretaceous Laramide orogeny. The remanence carrier is interpreted to be magnetite, although other mineral phases of high coercivity carry recent secondary overprints. The overall mean is of Dec=323.1° and Inc=36.5° (k=162.7; α₉₅=2.7°; N=18 sites; 64% unfolding). Comparison with the North America reference direction indicates that this area has experienced a small, yet statistically significant, counterclockwise direction of 19.2±4.0°. Similar rotations are documented in other localities from southern Mexico; rotations are linked to mid-Tertiary deformation associated with the left-lateral strike-slip fault system that accommodated motion of the Chortis and Xolapa blocks.     

敦密断裂带白垩纪两期重要的变形事件

本文报道了敦密断裂带糜棱岩中黑云母~(40)Ar/~(39)Ar定年结果和大规模走滑-逆冲断裂的几何学、运动学特征及其形成时代,以便揭示断裂带两期变形事件的构造属性。黑龙江省密山市花岗质糜棱岩中黑云母~(40)Ar/~(39)Ar加权平均年龄为132.2±1.2Ma,它是敦密断裂带经历伸展事件的冷却年龄,也是东北亚大陆边缘在早白垩世Hauterivian期-Albian期发生强烈区域伸展作用的产物。密山市至辽宁省清原县系列大型走滑-逆冲断层和断层相关褶皱揭示出在晚白垩世晚期-末期发生右旋走滑-逆冲事件,该事件规模大,影响范围广,导致整个断裂带遭受到强烈改造,形成对冲式断裂系统。将研究区走滑-逆冲断裂与山东省郯庐断裂带中段挤压构造对比,认为郯庐断裂带北段和中段在晚白垩世末期都发生了强烈的走滑-逆冲事件,它们具有相同的构造特征和构造属性。     

New Discovery of the Early Cretaceous Volcanic Rocks on the Barton Peninsula, King George Island,Antarctica and Its Geological Significance

Ages determined with the 40Ar/39Ar isotopic system affirms the Early Cretaceous volcanic activity in the Barton Peninsula, King George Island, Antarctica. Two specimens of basaltic andesite collected from the lowermost volcanic sequence of the peninsula were irradiated and analyzed in different experiments, yielding an identical age spectrum, and two magmatic thermal events of the Early Cretaceous (120.4± 1.6 Ma, 119± 1 Ma) and Early Tertiary (53.1± 1.5 Ma, 52± 1 Ma) are distinguished. The former is interpreted to represent the primary cooling age of basaltic andesite, whereas the latter is the thermal reset age caused by the intrusion of granitic pluton. These new ages clearly indicate that volcanism was active during the Early Cretaceous on the Barton Peninsula and that intensive hydrothermal alteration and mineralization of Mesozoic volcanic rocks resulted from Tertiary magmatism.     

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three ⁴⁰Ar/³⁹Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.     

Age and origin of charoitite,Malyy Murun massif,Siberia, Russia

Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by ⁴⁰Ar/³⁹Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation ⁴⁰Ar/³⁹Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of published K–Ar ages (112 ± 5 Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 ± 3 Ma to 115.7 ± 4.3 Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.     

40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit,Tanami region,northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a ⁴⁰Ar/³⁹Ar plateau age of 1785 ± 32 Ma (2σ including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the ⁴⁰K decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 ±12 Ma (2σ including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite ⁴⁰Ar/³⁹Ar age indicates the timing of gold mineralisation. Thesericite age accords with a limited dataset of ²⁰⁷Pb/²⁰⁶Pb xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.     

Amphibole Ar/Ar Geochronology from the Okcheon Metamorphic Belt, South Korea and its Tectonic Implications

An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show ⁴⁰Ar/³⁹Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in ⁴⁰Ar/³⁹Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified ⁴⁰Ar/³⁹Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These ⁴⁰Ar/³⁹Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of ⁴⁰Ar/³⁹Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.     

The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina

New structural field data at various scale and ⁴⁰Ar–³⁹Ar geochronological results, from the basement rocks in the Truong Son belt and Kontum Massif of Vietnam, confirm that ductile deformation and high-temperature metamorphism were caused by the Early Triassic event of the Indosinian Orogeny in the range of 250–240 Ma. A compilation of isotopic data obtained in other countries along the Sibumasu–Indochina boundary broadly indicates same interval of ages. This tectonothermal event is interpreted as the result of a synchronous oblique collision of Indochina with both Sibumasu and South China, inducing dextral and sinistral shearing along E–W to NW–SE and N–S fault zones, respectively. The collision along Song Ma follows the northwards subduction of Indochina beneath South China and the subsequent development of the Song Da zone which in turn was affected by the Late Triassic Indosinian phase of shortening. Within the Indochina plate, internal collisions occurred coevally in the Early Triassic, as along the Poko suture, at the western border of the Kontum Massif.     

大别-苏鲁造山带在朝鲜半岛的延伸方式——基于~(40)Ar/~(39)Ar构造年代学的约束

本文结合野外构造变形特征观测,在朝鲜半岛的不同构造单元采集14件糜棱岩和片麻岩样品进行~(40)Ar/~(39)Ar年代学分析,在此基础上通过对比朝鲜半岛与大别造山带不同构造单元的变形特征,探讨大别苏鲁构造带在朝鲜半岛的东延特征,取得如下认识:朝鲜半岛中部的主要构造带在中生代经历了碰撞阶段(~210Ma)、逆冲推覆(200~150Ma)、造山后伸展阶段(140~90Ma)三个主要的构造过程;从变形期次和变形特征看,临津江构造带与大别造山带的北淮阳构造带、苏鲁构造带北部威海地区具有可比性,沃川构造带与南大别构造带有相似之处;在朝鲜半岛,自临津江带至沃川带构成了较完整的中生代碰撞造山带,即大别-苏鲁造山带的东延部分,原认为的"京畿地块"应属造山带的一部分。     

西藏阿里地区冈底斯花岗岩体的热年代学特征及其构造意义

冈底斯花岗岩带是沿雅鲁藏布江北侧近东西向展布的一条长约2 500 km、宽100～300 km的巨型岩浆岩带。在阿里冈底斯山主峰岗仁波齐峰附近,冈底斯花岗岩体受到多期断裂活动的影响,特别受喜马拉雅大反向断裂和喀喇昆仑断裂活动的改造,造成断裂和岩体的关系出现很多复杂的情况,一些研究者把在北阿伊拉日居山分布的32～25 Ma的花岗岩作为喀喇昆仑断裂活动引起的同构造花岗岩,并把此年龄段归结为断层活动年龄,从而引起了极大的争论。本文的锆石U-Pb年龄指示了岗仁波齐峰地区的冈底斯花岗岩是由110 Ma、60 Ma和50 Ma的3期花岗岩组成,而韧性剪切带内的锆石年龄与附近未变形岩石内的锆石年龄一致,表明锆石的形态并未受到断裂活动的影响。韧性剪切带内云母的氩氩年龄为12 Ma左右,而周围未变形花岗岩的云母氩氩年龄在60～50 Ma左右,由此表明喀喇昆仑断裂在岗仁波齐峰地区是12 Ma开始活动的。由于研究区内韧性剪切带中的变形花岗岩并没有记录32～25 Ma这期热事件,由此排除了断裂在狮泉河-门士一线是32～25 Ma开始活动的可能性。     

Subduction-related Late Cretaceous high-K volcanism in the Central Pontides orogenic belt: constraints on geodynamic implications

Mineral chemistry, major and trace elements, ⁴⁰Ar/³⁹Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. ⁴⁰Ar/³⁹Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (La_N/Lu_N = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial ⁸⁷Sr/⁸⁶Sr values vary between .70615 and .70796, whereas initial ¹⁴³Nd/¹⁴⁴Nd values change between .51228 and .51249. Initial ²⁰⁶Pb/²⁰⁴Pb values vary between 18.001 and 18.349, ²⁰⁷Pb/²⁰⁴Pb values between 15.611 and 15.629 and ²⁰⁸Pb/²⁰⁴Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.     

Formation Age and Evolution Time Span of the Koktokay No. 3 Pegmatite,Altai, NW China: Evidence from U–Pb Zircon and 40Ar–39Ar Muscovite Ages

The Koktokay No. 3 pegmatite is the largest Li–Be–Nb–Ta–Cs pegmatitic rare‐metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones). However, the formation age and evolution time span have been controversial. Here, we present the results of LA‐ICP–MS zircon U–Pb dating and muscovite ⁴⁰Ar–³⁹Ar dating. Four groups of zircon U–Pb ages (～210 Ma, ～193–198 Ma, ～186–187 Ma and ～172 Ma) for Zones II, V, VI, VII, and VIII, and a weighed mean ²⁰⁶Pb/²³⁸U age of 965 ± 11 Ma for Zone IV are identified. Also, Zones II, IV, and VI have muscovite ⁴⁰Ar–³⁹Ar plateau ages of 179.7 ± 1.1 Ma, 182.1 ± 1.0 Ma, and 181.8 ± 1.1 Ma, respectively. Considering previous U–Pb age studies (Zones I, V, and VII), the ages of emplacement, Li mineralization peak, hydrothermal stage of the No. 3 pegmatite are in ranges of 193–198 Ma, 184–187 Ma and 172–175 Ma, with weighted mean ²⁰⁶Pb–²³⁸U ages of 194.8 ± 2.3 Ma, 186.6 ± 1.3 Ma and 173.1 ± 3.9 Ma, respectively. The No. 3 pegmatite formed in the early Jurassic. The results of xenocrysts suggest that there is another pegmatite forming event of around 210 Ma in the mining district and the old zircon U–Pb ages imply that Neoproterozoic crustal rocks pertain to sources of the No. 3 pegmatite. Including the previous muscovite ⁴⁰Ar–³⁹Ar age studies (Zones I and V), a cooling age range of 177–182 Ma is considered as the time of hydrothermal stage and end of formation. The evolution process of the No. 3 pegmatite lasted 16 Ma. Therein, the magmatic stage continued for 9–11 Myr and the magmatic–hydrothermal transition and hydrothermal stages were sustained at 5–7 Ma. These time spans are long because of huge scale, cupola shape, large formation depth, and complex internal zoning patterns and formation processes. Considering some pegmatite dikes in the Chinese Altai, there is an early Jurassic pegmatite forming event.     

Late Cretaceous blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications

A blueschist facies tectonic sliver, 9 km long and 1 km wide, crops out within the Miocene clastic rocks bounded by the strands of the North Anatolian Fault zone in southern Thrace, NW Turkey. Two types of blueschist facies rock assemblages occur in the sliver: (i) A serpentinite body with numerous dykes of incipient blueschist facies metadiabase (ii) a well‐foliated and thoroughly recrystallized rock assemblage consisting of blueschist, marble and metachert. Both are partially enveloped by an Upper Eocene wildflysch, which includes olistoliths of serpentinite–metadiabase, Upper Cretaceous and Palaeogene pelagic limestone, Upper Eocene reefal limestone, radiolarian chert, quartzite and minor greenschist. Field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the Miocene clastic rocks in a transpressional strike–slip setting, and represents an uplifted part of the pre‐Eocene basement. The blueschists are represented by lawsonite–glaucophane‐bearing assemblages equilibrated at 270–310 °C and ～0.8 GPa. The metadiabase dykes in the serpentinite, on the other hand, are represented by pumpellyite–glaucophane–lawsonite‐assemblages that most probably equilibrated below 290 °C and at 0.75 GPa. One metadiabase olistolith in the Upper Eocene flysch sequence contains the mineral assemblage epidote + pumpellyite + glaucophane, recording P–T conditions of 290–350 °C and 0.65–0.78 GPa, indicative of slightly lower depths and different thermal setting. Timing of the blueschist facies metamorphism is constrained to c. 86 Ma (Coniacian/Santonian) by Rb–Sr phengite–whole rock and incremental ⁴⁰Ar–³⁹Ar phengite dating on blueschists. The activity of the strike–slip fault post‐dates the blueschist facies metamorphism and exhumation, and is only responsible for the present outcrop pattern and post‐Miocene exhumation (～2 km). The high‐P/T metamorphic rocks of southern Thrace and the Biga Peninsula are located to the southeast of the Circum Rhodope Belt and indicate Late Cretaceous subduction and accretion under the northern continent, i.e. the Rhodope Massif, enveloped by the Circum Rhodope Belt. The Late Cretaceous is therefore a time of continued accretionary growth of this continental domain.     

Diachronous Palaeoproterozoic deformation and metamorphism in the Committee Bay belt,Rae Province,Nunavut: insights from 40Ar–39Ar cooling ages and thermal modelling

Regional‐scale ⁴⁰Ar–³⁹Ar data presented in this paper reveal significant across‐strike and along‐strike age differences in the Committee Bay belt (CBb), Rae Province, Nunavut, Canada, that complement variations in observed monazite ages. ⁴⁰Ar–³⁹Ar hornblende ages are c. 1795, 1775, and 1750 Ma in the western, eastern and central parts of the Prince Albert Group (PAG) domain respectively. The migmatite domain and Walker Lake intrusive complex are characterized by c. 1750–1730 ⁴⁰Ar–³⁹Ar hornblende ages without significant along‐strike variation. The ⁴⁰Ar–³⁹Ar data provide important constraints on the cooling history and on thermal modelling that elucidates the controls on diachroneity and metamorphic patterns within the belt. In the western CBb, prograde monazite growth occurred 26 ± 10 Myr earlier in the migmatite domain (1864 ± 9 Ma; peak P–T = 5 kbar?700 °C) than in the PAG domain (1838 ± 5 Ma; peak P–T = 5 kbar?580 °C). Calculations indicate that this earlier monazite growth results from tectonic thickening of higher heat productivity Archean lithologies in the migmatite domain, which undergoes more rapid prograde heating than the less radiogenetic and lower grade rocks of the PAG domain. Granite generation via biotite dehydration melting at 800 °C and 20 km depth is predicted to occur c. 1835 Ma, in agreement with geochronological constraints. The tectonic burial of crustal domains with contrasting radiogenic properties also explains the general congruence of lower to upper amphibolite facies metamorphic zones generated during the two main orogenic cycles (i.e. M₂–D₁ and M₃–D₂). The modelled timing of prograde monazite growth in the migmatite domain suggests that D₂ tectonic thickening began at 1872 ± 9 Ma, some 8 ± 3 Myr before monzazite growth, coeval with the inferred time of collision of the Meta Incognita terrane with the southern Rae Province. Along‐strike diachroneity, reflected in 25 Myr younger monazite and ⁴⁰Ar–³⁹Ar hornblende ages in the eastern relative to the western PAG domain, cannot be accounted for by heat productivity contrasts along the belt. Instead the younger deformation and metamorphism in the eastern CBb was driven by its proximity to the eastern promontory of the Superior Province which collided with the Rae Province at c. 1820 Ma. The ⁴⁰Ar–³⁹Ar data presented here support the interpretation that the youngest monazite in the CBb crystallized at c. 1790 Ma in the central CBb when this part of the belt was downfolded into a gentle synformal structure while the western part of the belt cooled through ⁴⁰Ar–³⁹Ar hornblende closure. The results of this study illustrate the important influence of contrasting rock properties on the thermal evolution of orogenic belts and on the temporal record of this evolution.     

福建龙海—漳浦沿海地区火山岩激光~(40)Ar/~(39)Ar年龄

福建东南沿海龙海—漳浦地区是新生代佛昙群玄武岩的分布区之一。对该区域火山岩而言,前人的研究主要集中在岩石地球化学特征、形成演化方面,对年代学研究较少,玄武岩喷发期次划分仍以20世纪80、90年代测定的K-Ar结果为依据,或通过下覆地层孢粉组合时代推断而来。为了更精确地测定该地区火山作用的时代及进一步确定其喷发期次,选取龙海—漳浦地区4个玄武岩样品,利用激光~(40)Ar/~(39)Ar测年方法进行精细定年。样品年龄为10.1～14.8Ma,明确了龙海-漳浦新生代玄武岩在中新世中晚期存在一次喷发期次。     

滇西澜沧江构造带中段新生代多期构造变形特征及时代约束

澜沧江构造带是青藏高原东南缘保山-羌塘地块与兰坪-思茅地块之间的大型走滑剪切带。构造地质学、岩石学和40Ar-39Ar年代学研究结果表明构造带中段剪切带内部存在早期斜向挤出和晚期水平走滑的两期线理及早期指示右行韧性剪切变形、后期指示左行脆韧性剪切变形的构造指向。糜棱岩中石英晶格优选方位以中温(450~600℃)柱面a轴底面滑移系为主,叠加中低温(300~550℃)底面a轴滑移系;剪切带内云母片岩和花岗质糜棱岩中黑云母40Ar-39Ar坪年龄和等时线年龄都分布于15~17Ma,反映剪切带隆升过程中脆韧性左行剪切变形阶段的时代。结合前人成果进行分析认为新生代早期保山地块和兰坪-思茅地块向南南东挤出的同时沿澜沧江构造带中段发生大规模右行斜向走滑韧性剪切作用,后期保山地块南部沿北东向畹町和南汀河左行断裂带相对中北段向北东运动,致使隆升到中上构造层次的韧性剪切带发生左行脆韧性变形。     

U-Pb and Ar/Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, ⁴⁰Ar/³⁹Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The ⁴⁰Ar/³⁹Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite ⁴⁰Ar/³⁹Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.