K–Ar dating of fault gouge in the northern Sydney Basin, NSW, Australia—implications for the breakup of Gondwana

The occurrence of synkinematic and authigenic clay minerals is a common feature in fault gouges. Few attempts have been made to date fault gouges. We present the first age data in Australia for synkinematic illite–smectite growth in two fault zones of the northern Sydney Basin, NSW. The faults occur at Burwood Beach, NSW in the northern part of the Sydney Basin and are hosted by Early Permian siltstones, tuffs and coals of the Lambton Formation, Newcastle Coal Measures. The faults are 1.5 m apart, show normal displacement and trend N–S with steep easterly dips. Foliated gouge zones, comminution and dilational breccias are developed along both fault surfaces. K–Ar ages extracted from samples in the gouge and tuffs in the damage zones are 172 (6–10 μm) to 119 Ma (<0.4 μm), respectively. Older ages of 272–281 Ma for the coarse fractions (>2 μm), 237–245 Ma for the <2 μm fraction, 218 Ma for the <0.4 μm fraction and 196 Ma for the <0.1 μm fraction have been obtained from siltstones within and outside the damage zone. We believe the younger ages of 196–237 Ma indicate the time at which diagenetic illite–smectite formed and the 122–150 Ma dates from the <2 μm fraction represent the maximum age of gouge formation. The younger ages are thought to reflect the last slip event occurring on the faults, which is related to the rifting and dispersal of the eastern margin of the Australian continent.     

Fire in a Weichselia-dominated coastal ecosystem from the Lower Cretaceous (Barremian) of the Kurnub Group in NW Jordan

Large intervals of the Cretaceous are considered as a ‘high-fire’ period in Earth's history. However, so far most studies dealing in greater detail with the fossil evidence of palaeo-wildfires, i.e. fossil charcoal, originate from the northern hemisphere (i.e. North America, Europe, Asia) whereas there are large stratigraphic and geographic gaps on the Cretaceous southern continents. The present paper deals with the fossil contents (plant macro-remains, palynomorphs and charcoal) of a lignite lens from the lower part of the Lower Cretaceous Kurnub Group near King Talal Dam in Jordan. The data provide evidence for the repeated occurrence of palaeo-wildfires in coastal ecosystems on the northern margin of Gondwana during the Barremian. The fossil content of the lens indicates that the vegetation, which was repeatedly affected by fire, has been dominated by the matoniaceous tree fern Weichselia reticulata. Palynological data from the lignite, as well as the repeated occurrence of wildfires point to an at least seasonally dry (or at least less humid) climate during deposition of the lignite.     

K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais,Brazil)

K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K‐feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K‐feldspar, muscovite, biotite and illite) and different authigenesis of K‐feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K‐feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K‐feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

On the timing and causes of illite formation and remagnetization in the Cretaceous Marias River Shale, Disturbed Belt, Montana

In this study, we test a burial model for diagenetic illite and whether illitization is related to the acquisition of a chemical remanent magnetization (CRM). A prefolding, reversed polarity, secondary magnetization is found in carbonate concretions in diagenetically altered Upper Cretaceous rocks in the disturbed belt of Montana. Stratigraphically equivalent unaltered rocks to the east do not contain the magnetization. The magnetization could be a CRM or a thermoviscous remagnetization. The pole position is generally consistent with the Early Tertiary K–Ar ages of diagenetic illite–smectite formed under thrust loading, but it is not constrained well enough for a precise age comparison. If this magnetization is a CRM, it could be related to illitization. Alternatively, if the prefolding CRM had been acquired prior to thrust loading then the magnetization is not related to illitization. Geochemical studies are currently underway to test if the magnetization could be related to fluid alteration.     

Rate of strike-slip motion on the Amanos Fault (Karasu Valley, southern Turkey) constrained by K–Ar dating and geochemical analysis of Quaternary basalts

The left-lateral Amanos Fault follows a 200-km-long and up to 2-km-high escarpment that bounds the eastern margin of the Amanos mountain range and the western margin of the Karasu Valley in southern Turkey, just east of the northeastern corner of the Mediterranean Sea. Regional kinematic models have reached diverse conclusions as to the role of this fault in accommodating relative motion between either the African and Arabian, Turkish and African, or Turkish and Arabian plates. Local studies have tried to estimate its slip rate by K–Ar dating Quaternary basalts that erupted within the Amanos Mountains, flowed across it into the Karasu Valley, and have since become offset. However, these studies have yielded a wide range of results, ranging from 0.3 to 15 mm a⁻¹, which do not allow the overall role and significance of this fault in accommodating crustal deformation to be determined. We have used the Cassignol K–Ar method to date nine Quaternary basalt samples from the vicinity of the southern part of the Amanos Fault. These basalts exhibit a diverse chemistry, which we interpret as a consequence varying degrees of partial melting of their source combined with variable crustal contamination. This dating allows us to constrain the Quaternary slip rate on the Amanos fault to 1.0 to 1.6 mm a⁻¹. The dramatic discrepancies between past estimates of this slip rate are partly due to technical difficulties in K–Ar dating of young basalts by isotope dilution. In addition, previous studies at the key locality of Hacılar have unwittingly dated different, chemically distinct, flow units of different ages that are juxtaposed. This low slip rate indicates that, at present, the Amanos Fault takes up a small proportion of the relative motion between the African and Arabian plates, which is transferred southward to the Dead Sea Fault Zone. It also provides strong evidence against the long-standing view that its slip continues offshore to the southwest along a hypothetical left-lateral fault zone located south of Cyprus.     

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.     

西藏林子宗群火山岩氩-氩年代学研究

通过对西藏冈底斯南带林周盆地中 3个岩组样品的40 Ar 3 9Ar定年 ,初步建立了该剖面林子宗群火山岩形成的同位素年代时序格架 :底部典中组的下限年龄为 6 3.32± 0 .89Ma;中部年波组为 5 4.16± 0 .49Ma;上部帕那组下限年龄为 38.7± 0 .40Ma .。指出至少在林周盆地剖面 ,林子宗群火山岩的形成时代为早第三纪 ,而非晚白垩纪。     

秦岭岩群中两类斜长角闪岩的性质和时代及其地质意义

在北秦岭造山带核部秦岭岩群内发育两类不同产状的斜长角闪岩.一类与秦岭岩群中的大理岩紧密共生,呈规模较大的似层状或较小块体产于大理岩内,另一类则呈密集岩墙群型式侵入于秦岭岩群南段二云母石英片岩中.不同的产出状态表明两类斜长角闪岩的成因和时代存在显著差异,也具有不同的地质构造意义.地球化学上,两类不同产状的斜长角闪岩的原岩皆为玄武岩质的.侵入秦岭岩群二云母石英片岩中的斜长角闪岩墙群形成于板内拉张环境,SHRIMP锆石U-Pb测年揭示该类斜长角闪岩形成于晚奥陶世(449±11Ma),Sr-Nd同位素特征显示其岩浆源区为亏损地幔源区,40Ar/39Ar热年代学研究显示该类斜长角闪岩的角闪岩相变质作用发生于石炭纪末期(301.3±6.4Ma).地球化学和Sr-Nd同位素特征显示与秦岭岩群大理岩共生的似层状或块状斜长角闪岩的形成环境与侵入云母石英片岩中的斜长角闪岩墙群的存在显著差异,其形成于洋岛(OIB)或海山环境.40Ar/39Ar热年代学研究显示该类斜长角闪岩于晚二叠世(258.1±5.7Ma)发生了角闪岩相变质作用.不同性质、形成和变质时代、相似的变质作用等特点表明,秦岭岩群中的两类角闪岩分属不同性质的构造块体,秦岭岩群高级变地质地体可能是一个构造拼合地体.斜长角闪岩岩墙为晚奥陶世(449±11Ma)侵入秦岭岩群云英片岩中的基性岩墙群,是北秦岭晚加里东期后造山期热收缩而致的地壳伸展或岩圈拆离减薄的产物.与秦岭岩群大理岩共生的斜长角闪岩则可能是洋隆体的基性喷出岩+碳酸盐岩帽组合,是中二叠世(312～260Ma)期间构造移置而来的外来块体.     

滇西澜沧岩群碎屑锆石U-Pb定年及其地质意义

虽然前人对澜沧岩群做了大量的研究,但缺乏同位素年代学方面的研究。此次研究针对澜沧江南段菖蒲塘-大田山地区澜沧岩群绢白云母石英片岩进行碎屑锆石U-Pb定年,为澜沧岩群的形成时代增添年代学证据。极大多数(129粒)锆石阴极发光(CL)图像显示其有明显的结晶振荡环带,指示其为岩浆成因的碎屑锆石,129粒碎屑锆石具有多组峰值年龄,最年轻一组年龄加权平均值为452±26Ma,表明澜沧岩群最早沉积时限不早于452±26Ma;极少数(1粒)锆石具弱阴极发光,缺乏内部结构特点,认为其为变质重结晶锆石,变质结晶锆石U-Pb年龄为255±3Ma,与前人研究所得澜沧岩群变质时代基本吻合。综合分析,澜沧岩群的沉积时限不早于452±26Ma,在二叠纪末期可能发生变质作用。     

皖南地区歙县岩群昌前岩组和昱岭关群井潭组凝灰岩SHRIMP锆石U-Pb年龄及其地质意义

江南造山带东北段歙县岩群和昱岭关群是皖南地区重要的前寒武纪地层单元。本文对皖南地区歙县岩群昌前岩组和昱岭关群井潭组凝灰岩样品中的锆石进行了测年研究,获得昌前岩组凝灰岩SHRIMP锆石U-Pb年龄为852±6 Ma和840±11 Ma,井潭组凝灰岩SHRIMP锆石U-Pb年龄为809±10 Ma,为皖南地区新元古代地层格架的建立和地层的对比增添了可靠的年代学依据。根据最新获得的锆石U-Pb年龄数据,确定歙县岩群的时代与江南造山带区域范围内梵净山群、四堡群、冷家溪群、双桥山群以及溪口岩群相当,其上覆的昱岭关群与下江群、丹洲群、板溪群、马涧桥组、登山群以及上墅组等地层单元可以对比。     

The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau,Yamaguchi Prefecture,Japan

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite‐series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO₂, but a compositional gap exists between 70.5 to 73.4 wt.% SiO₂ (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San‐yo Belt, which is genetically related to the ilmenite‐series granitoids of the Kanmon to Shunan stages. The aplite contains Cl‐rich apatite and REE‐rich monazite‐(Ce), allanite‐(Ce), xenotime and bastnäsite‐(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three‐phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two‐phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.     

Triassic paragonite- and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan

Paragonite- and garnet-bearing high-grade epidote-amphibolite (PGEA) in the Ise area of the Hida Mountains, Japan is characterized by the high-pressure (HP) epidote-amphibolite facies parageneses (M₁), garnet + hornblende + clinozoisite + paragonite + quartz + rutile. Paragonite and garnet of the peak M₁ stage are locally replaced by retrograde albite (+ oligoclase) and chlorite (M₂), respectively. Phase equilibria constrain peak metamorphic conditions of P = 1.1–1.4 GPa and T = 530–570 °C, and a decompressional P–T path for this rock. Mineral parageneses of prograde epidote-amphibolite facies are comparable to some HP rocks from the Hongan region of western Dabie, but differ from other HP mafic schists with cooling ages of c. 330 Ma in the Hida Mountains. New paragonite K–Ar dating for the PGEA yields a Triassic cooling event at 210 Ma that is coeval with regional cooling and exhumation of the Sulu–Dabie–Qinling (SDQ) belt. Both petrological and geochronological data of the Triassic HP epidote-amphibolite in Hida Mountains support our earlier hypothesis that the SDQ belt extends across the Korean Peninsula to SW Japan.     

华北克拉通胶东地区粉子山群碎屑锆石SHRIMPU-Pb定年

胶东是华北克拉通重要的变质基底出露区之一。对该区古元古代粉子山群和荆山群中碎屑锆石的年龄分布特征的研究,可以为胶东地区早前寒武纪演化提供重要依据。由于Pb丢失作用,前人仅在粉子山群中获得少量锆石谐和年龄。本文对粉子山群长石石英片岩中碎屑锆石进行了SHRIMP定年。52颗锆石的61个数据点分析,获得36个谐和年龄(不谐和度10%)。所有具谐和年龄的锆石都具有高的Th/U比值,大多显示明显的岩浆环带,原为岩浆成因。谐和锆石年龄分布在2033~3429Ma之间,主要峰值分别为~2.19Ga和~2.48Ga。该样品中还含有2颗年龄大于3.3Ga的锆石,位于谐和线上的2颗最年轻碎屑锆石的年龄为~2.08Ga。结合前人研究,可得出如下结论:1)粉子山群和荆山群原岩沉积时代可进一步限制在1.9~2.1Ga之间;2)粉子山群与荆山群的碎屑锆石年龄分布形式存在明显区别,指示其源区可能不同;3)胶东地区可能存在古太古代地壳物质。     

Radiochemistry of sediments from the southern Dead Sea, Jordan

 A sediment core from the southern Dead Sea was analyzed using gamma spectroscopy as well as ²¹⁰Pb dating in order to ascertain if any radioactive contamination could be detected and to determine the sedimentation rates in the area. Results of this study show no presence of man-made radionuclides in the area. Sedimentation rates were determined to be between 0.25 and 0.4 g/cm²/year. (∼0.5 cm/year), which is in line with what would be expected assuming carbonate layers are annual varves. Received: 31 January 1997 · Accepted: 11 March 1997     

西藏中冈底斯带北部早白垩世构造属性：来自则弄群火山岩锆石U-Pb年龄及地球化学的制约

中冈底斯带广泛分布则弄群火山岩,其成因机制一直存在争议。本文对出露于来多地区的则弄群火山岩进行了锆石U-Pb定年及全岩地球化学分析,旨在精确限其形成时代,探讨其成因机制。结果表明,来多地区的则弄群火山岩形成于124.0～114.7Ma之间;岩石SiO₂含量在54.53%～78.35%之间,铝饱和指数A/CNK为0.77～3.75,属偏铝质-过铝质岩石,轻稀土富集,负Eu异常明显,相对富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta等高场强元素,岩石具有弧火山岩的特征。综合分析认为,则弄群火山岩可能为班公湖-怒江缝合带南侧的狮泉河-永珠-纳木错-嘉黎蛇绿岩带所代表的Slainajap洋在早白垩世向南俯冲消减的产物。     

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.     

扬子西缘黄水河群玄武岩锆石SHRIMP U-Pb定年及其地质意义

分布于龙门山逆冲推覆带内的黄水河群夹片,为一套典型的浅变质(绿片岩)岩系,局部变质程度可达低角闪岩相,其中火山岩广泛发育.本文在野外地质调查基础之上,结合岩石学、岩相学分析,对干河坝组中采集的玄武岩锆石进行SHRIMP U-Pb测年,获得(799±8)Ma和(875±12)Ma两个206pb/238U加权平均年龄值,前者为玄武岩结晶(成岩)年龄,表明该玄武岩形成于新元古代中期,后者为捕获锆石年龄信息,代表晋宁运动的一次构造岩浆事件.结合区域上的成果资料,认为黄水河群与盐井群、苏雄组为晚青白口纪同时代沉积的地层.     

黑龙江佳木斯地块兴东群大盘道组碎屑锆石U-Pb定年及其地质意义

兴东群是佳木斯地块的结晶基底和多个铁、金、钨矿床(点)的含矿地层。为厘定其形成时代,揭示其地质和成矿意义,选取黑龙江省林口地区的兴东群大盘道组,在剖面实测、岩相学研究基础上,开展了代表性岩石的碎屑锆石U-Pb年代学研究。结果表明,所测大盘道组的主要岩石类型为大理岩与石英片岩,其原岩为灰岩及泥岩,变质程度可达角闪岩相,沉积于浅海陆棚相环境;石英片岩中碎屑锆石的LA-ICP-MS U-Pb年龄为(2338±20)Ma^751±18Ma,存在952 Ma、898 Ma、820 Ma及761 Ma 4个年龄峰值。结合羊鼻山地区大盘道组地层内有~520 Ma的花岗岩侵入以及该区域经历了~500 Ma的泛非期变质作用,确定大盘道组原岩沉积于新元古代—早古生代。本文测年结果显示有4个年龄峰值,其中898 Ma和761 Ma与前人报道的峰期年龄为898~891 Ma和757~751 Ma的两期岩浆事件相对应,另两组所测锆石也均为岩浆成因锆石,反映该区兴东群的物源复杂,且以岩浆岩为主。多期次岩浆作用使大盘道组富集Au、Cu、Pb、Zn、Fe等金属成矿元素,为羊鼻山铁钨矿床、孟家岗铁矿床和七星河金矿床等金属矿床形成提供了丰富的成矿物质基础。     

北秦岭东段二郎坪群火山岩锆石的LA-ICP-MS U-Pb定年及其地质意义

利用阴极发光（CL）技术和LA-ICP-MS原位分析方法,对北秦岭东段军马河、湾潭和湍河3个地区的二郎坪群基性火山岩进行了系统的锆石U Pb年代学研究。CL图像分析结果显示,三地火山岩中的锆石皆呈自形 半自形柱状形态和微弱的、宽的振荡环带结构。微量元素分析揭示,所有锆石都具有较高的稀土总量、重稀土和Th、U含量（ΣREE为(397.7~7 941.6)×10-6;ΣHREE为(376.7~7 774.2)×10-6;Th为(107.9~5 824.2)×10-6;U为(89.3~3 841.9)×10-6）以及轻稀土亏损、重稀土明显富集的左倾稀土配分曲线型式,所有锆石的Th/U比值皆大于0.4,显示典型岩浆锆石特征。LA-ICP-MS定年结果获得3地基性火山岩的形成年龄分别为(463±1.8) Ma、(475±1.5) Ma和(473±1.3) Ma,三者在误差范围内一致,表明北秦岭二郎坪群基性火山岩的形成时代为463~475 Ma。系统的岩石学和地球化学研究显示,二郎坪群火山岩具有弧后盆地火山岩的地球化学特征。研究获得的二郎坪基性火山岩463~475 Ma的结晶年龄明显晚于其南侧北秦岭超高压榴辉岩的原岩年龄(（791±6) Ma）以及变质年龄（(502±11) Ma）,而且榴辉岩的地球化学特征显示为板内玄武岩,也与二郎坪火山岩不同,因而指出北秦岭超高压榴辉岩与二郎坪火山岩没有直接成因联系。