Neoproterozoic to Early Cambrian history of an active plate margin in the Teplá–Barrandian unit—a correlation of U–Pb isotopic-dilution-TIMS ages (Bohemia, Czech Republic)

The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.     

太行山阜平杂岩中麻棚-赤瓦屋岩体的时代、侵位深度及构造意义

中生代岩浆活动在太行山北段形成了一系列花岗岩体,其中的麻棚-赤瓦屋岩体侵位于穹窿状的阜平杂岩中,具有贫硅、富碱、高铝的钙碱性I型花岗岩特征。岩体分带明显,由边部向核部可以划分为石英闪长岩、花岗闪长岩、似斑状花岗岩3个相带,它们的锆石LA-ICP-MS U-Pb年龄分别为126.4±2.4 Ma、125.4±2.0 Ma和126.2±2.0 Ma,在误差范围内一致,指示岩体在早白垩世期间快速侵位、结晶。各相带样品的角闪石全铝平均值分别为1.018、0.880和0.871,对应的角闪石平均结晶压力分别为0.183、0.118和0.114 GPa,均值为0.141 GPa,对应的侵位深度约5 km,明显小于区域内元古界至侏罗系的地层厚度,表明在岩体侵位之前,阜平杂岩就已经具有穹窿状构造特征。结合相关资料,估计太行山早白垩世以来的平均隆升速率为0.07~0.08 km/Ma。     

U---Pb SHRIMP and Sm---Nd age constraints on the timing and sources of the Pan-African Cape Granite Suite, South Africa

Geochemical and isotopic U---Pb SHRIMP and Sm---Nd studies in three granitic plutons from the Pan-African Neoproterozoic/Cambrian Saldania Mobile Belt, southwestern South Africa supports differences in the sources and timing of emplacement of the main plutons related to the Cape Granite Suite as established by previous researchers. The Phase I, early syn-tectonic Darling Batholith Granitoid reflects an overall peraluminous chemical signature compatible with derivation mainly from melts extracted from paraderived crustal sources (Sa1 association). The Phase II, late to post-tectonic Robertson Pluton, reflects affinities to the Australian l-type granites (1a association). Despite the good structural constraints on the syn- and post-tectonic origin, the U---Pb ages point to a broadly synchronous crystallisation episode at 547±6 Ma and 536±5 Ma, respectively. In addition to U---Pb, Nd isotopic studies were also carried out for both plutons, as well as for the Riviera Granite, another phase II (1a association) pluton. The initial ε_Nd (550 Ma) based on a depleted mantle model range from −3.5 (Darling), to −3.1 (Robertson) and to −2.6 (Riviera). The Nd mean crustal residence ages are 1559 Ma for Darling, 1626 Ma for Robertson and 1243 Ma for Riviera. Despite the small databank, a dominant Mesoproterozoic ( 1600 Ma) crust may be seen as the best candidate to explain the model T_DM ages obtained. All the data largely overlap with others recently obtained for other plutons within the Cape Granite Suite and cast doubts on the current correlation between Saldania and the southeastern Brazilian, Dom Feliciano Belts.     

Sequence of magma emplacement and sulfide saturation in the Gaojiacun–Lengshuiqing intrusive complex (SW China)

The Lengshuiqing area contains several small intrusions made up of peridotite ± quartz diorite ± granite spatially associated with the Gaojiacun pluton (gabbroids + peridotite + diorite). Ni–Cu sulfide ore occur at Lengshuiqing, hosted in peridotite. SHRIMP U–Pb zircon dating produced the ages of 803 ± 4.2 Ma (peridotite), 807 ± 2.6 Ma (oikocrystic hornblende gabbro), 809 ± 4.3 Ma (hornblende gabbronorites) for the Gaojiacun pluton and 807 ± 3.8 Ma (diorite, intrusion I), 817 ± 6.3 Ma (quartz diorite, intrusion II) and 817 ± 5 Ma (peridotite, intrusion 101) for Lengshuiqing. These ages suggest the emplacement of the Gaojiacun pluton later than the intrusions from Lengshuiqing. The olivine from Lengshuiqing does not contain sulfide inclusions and is relatively Ni-rich (1,150–1,550 ppm Ni), suggesting its crystallisation before the sulfide saturation that generated the Ni–Cu deposits. The olivine of the gabbros in the Gaojiacun pluton is Ni-poor (250–800 ppm), which indicates crystallisation from a severely metal-depleted magma after a sulfide saturation event. The olivine in the peridotites from the Gaojiacun pluton has 800–1,150 ppm Ni and contains sulfide inclusions. Moreover, geological evidence suggests the genesis of the peridotites from Gaojiacun in conduits that were ascending through the gabbroids. A sequence of at least three stages of magma emplacement is proposed: (1) Lengshuiqing; (2) gabbroids from Gaojiacun; (3) peridotites from Gaojiacun. Given the age differences, the intrusions at Lengshuiqing and the Gaojiacun pluton might have been produced by different magmatic events.     

Temporal relationship between granite cooling and hydrothermal uranium mineralization at Dalongshan in China: a combined radiometric and oxygen isotopic study

A combined study using multi-radiometric dating and oxygen isotopic geothermometry was carried out for Mesozoic quartz syenite, alkali-feldspar granite and associated hydrothermal uranium mineralization at Dalongshan in the Middle-Lower Yangtze valley of east-central China. Radiometric dating of the quartz syenite yields a whole-rock Rb–Sr isochron age of 135.6±4.3 Ma, a zircon U–Pb isochron age of 132.9±2.2 Ma, and K–Ar ages of 126±2, 118±3 and 94±4 Ma for hornblende, biotite and orthoclase, respectively. The alkali-feldspar granite yields a whole-rock Rb–Sr isochron age of 117.3±3.3 Ma, a zircon U–Pb isochron age of 114.7±2.1 Ma, and K–Ar ages of 112±2, 109±3 and 88±4 Ma for hornblende, biotite and orthoclase, respectively. Oxygen isotope thermometry for both granites gives temperatures of 685 to 720, 555 to 580, 435 to 460 and 320 to 330 °C, for hornblende, magnetite, biotite and orthoclase respectively, when paired with quartz. The systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 27.4 to 58.6 °C/Ma from 800 to 300 °C in the early stage, but slow cooling rates of 6.3 to 7.2 °C/Ma from 300 to 150 °C in the late stage. The regular sequence of oxygen isotope temperatures for the different quartz–mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and O isotopic systems during granite cooling. Pitchblende U–Pb isochron dating yields an uranium mineralization age of 106.4±2.9 Ma, which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization, but is close to the closure time of the K–Ar system in the biotite. This points to a close relationship between granite cooling and ore-forming process. It appears that hydrothermal mineralization took place in the stage of slow cooling of the granite, whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures. Therefore, the activity of the ore-forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 Ma behind the crystallization timing of associated granite.     

Geochemistry and geochronology of the Neoproterozoic Pan-African Transcaucasian Massif (Republic of Georgia) and implications for island arc evolution of the late Precambrian Arabian–Nubian Shield

The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNd_int = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between  750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia.     

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria

Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. ⁴⁰Ar/³⁹Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.     

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement.Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton.Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.     

Highly fractionated I-type granites in NE China (I): geochronology and petrogenesis

Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world's most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing'an, Lesser Xing'an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region, aiming to understand their role in the building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities which suggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram.

The emplacement of the Xinhuatun pluton took place at 184±4 Ma as revealed by zircon SHRIMP U–Pb data. This is also supported by the slightly younger Rb–Sr whole-rock (WR) isochron age of 173±3 Ma. A whole-rock (WR) Rb–Sr isochron age of 154±3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U–Pb age of 137±2 Ma and WR Rb–Sr isochron age of 143±5 Ma. Biotite-WR Rb–Sr isochrons and ⁴⁰Ar/³⁹Ar ages of feldspars allow us to estimate the cooling rate of each pluton.

Geochemical data suggest that the rocks are highly fractionated I-type granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite.     

Onset of seafloor spreading in the Iapetus Ocean at 608 Ma: precise age of the Sarek Dyke Swarm, northern Swedish Caledonides

The Sarek Dyke Swarm (SDS) crops out in the Sarektjåkkå Nappe (SN) of the Seve-Kalak Superterrane in the northern Swedish Caledonides. The SN has two main components: (1) a 4–5 km thick succession of rift-related sedimentary rocks, which is intruded by (2) a suite of tholeiitic dykes (the SDS) constituting 70–80% of the nappe. The nappe was deformed during Caledonian thrusting, but dykes and sedimentary rocks in the interior of the eastern parts of the SN are preserved in a pristine state. The tholeiitic dykes of the SDS commonly occur in sheeted dyke complexes, and up to 11 successive generations can be identified from crosscutting relations. The SN represents the fossil continent–ocean transition between the Baltic craton and the Iapetus Ocean, marking the initiation of seafloor spreading. Bubble-shaped pods and veinlets of diorite are present in the SDS sheeted dyke complexes. The pods are absent in the oldest dykes, but the younger a dyke, the more frequent the pods. The diorite pods are the equivalent of gabbro pegmatites, and both cogenetic and coeval with the dykes. The rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex, so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons. Six zircon fractions, from the diorite pods including two single grains, were analysed geochronologically by the U–Pb thermal ionization mass spectrometry method. The data yield a linear array of points that are 0.4–0.8% normally discordant, indicating a crystallization age of 608±1 Ma (²⁰⁷Pb/²⁰⁶Pb=607.9±0.7 Ma, MSWD=0.33). This age is inferred to date the onset of seafloor spreading in the Iapetus Ocean along the Baltoscandian margin.     

Late Neoproterozoic-Cambrian Felsic Magmatism Along Transcrustal Shear Zones in Southern India: U-Pb Electron Microprobe Ages and Implications for the Amalgamation of the Gondwana Supercontinent

We report U-Pb electron microprobe ages for zircon and monazite from two granitic plutons from southern India, the Vattamalai granite within the Palghat-Cauvery Shear Zone system and the Pathanapuram granite within the Achankovil Shear Zone. A zircon grain from the Vattamalai granite has a core age of 693±132 Ma and is surrounded by a thick overgrowth with an age of 504±104 Ma. Monazites from the Vattamalai granite show a small range of ages between 500-520 Ma. PbO vs. ThO₂* plots of the monazites define a precise isochron age of 517±6.7 Ma (MSWD = 0.25). The oldest zircons in the Pathanapuram pluton are in the range 961-1149 Ma, with younger overgrowths at ~540-560 Ma. Monazite cores from the granite lie in the range of 526-574 Ma, whereas rims and bright overgrowths range from 506-539 Ma. These monazites define two linear arrays in PbO vs. ThO₂* plots with cores yielding an isochron age of 550±25 Ma (MSWD = 0.58) and the rims defining an age of 515±15 Ma (MSWD = 0.68).The age data from the granite plutons indicate multiple thermal imprints in southern India with the latest orogeny during the Late Neoproterozoic-Cambrian (Pan-African). The older zircon cores up to 1149 Ma from the Pathanapuram pluton suggest inherited components of late Mesoproterozoic age, caught up within the granite magma. However, the dominant 570-520 Ma ages obtained from both zircons and monazites closely compare with similar ages for magmatism and metamorphism from throughout the East African Orogen. Late Neoproterozoic-Cambrian felsic magmatism occurred along both the Palghat-Cauvery Shear System and the Achankovil Shear Zone, indicating that these shears were active at this time and may have served as pathways for the emplacement of magmas generated at depth. The magmatism represents part of the various collisional-extensional episodes that marked the final amalgamation of the Gondwana supercontinent.     

Early Cretaceous U–Pb zircon ages for the Copiapó plutonic complex and implications for the IOCG mineralization at Candelaria, Atacama Region, Chile

Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike–sill system exposed in the Candelaria open pit have been dated with the U–Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz–monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu–Au mineralization (Re–Os molybdenite ages at ∼115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria–Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz–monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu–Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified ⁴⁰Ar/³⁹Ar and K/Ar systematic in some areas.     

中国东北完达山地区早白垩世同构造岩浆侵位——对晚中生代左行走滑作用的响应

分布于中国东北完达山地区的饶河花岗岩岩体中暗色矿物和斑晶钾长石定向排列,呈北北东走向,其中透镜状闪长质捕掳体近水平排列,局部具有左行剪切的特点。岩体中发育石香肠状石英脉,表明岩体在侵位过程中受到左行剪切作用的影响或制约。对出露的花岗岩进行LA-ICP-MS锆石U-Pb定年,获得年龄121±1Ma和119±1Ma,表明该岩浆流动形成于早白垩世。同时对围岩辉长岩、侵入岩体中的正长岩脉和辉绿岩脉进行锆石U-Pb年龄分析,分别获得160±1Ma、109±2Ma、124±1Ma的年龄结果。根据各样品中继承锆石的特征,围岩辉长岩的年龄数据很集中,不存在古老锆石的年龄信息。岩浆流动岩体及岩脉中都有太古宙、元古宙等各时代的锆石年龄数据,可能表明完达山地区在约120Ma之前已完成古太平洋板块的俯冲拼贴,饶河岩体形成于走滑环境下的陆内变形,为同构造侵入岩。     

太行山北段赤瓦屋铜钨矿化区花岗质岩石的锆石U-Pb年龄及其地质意义

太行山北段位于华北克拉通中部,以发育中生代中酸性岩浆岩及多个大中型斑岩-矽卡岩铜钼矿床和金矿床为主要特征,最近在南部赤瓦屋岩体内部发现新类型铜钨矿体。文章选择赤瓦屋岩体为对象,开展不同岩相详细的野外地质和锆石U-Pb测年工作,确定赤瓦屋岩体不同岩性的成岩时代,探讨其地质意义。研究表明,赤瓦屋岩体有边缘相石英闪长岩、边缘相花岗闪长岩和中心相斑状花岗闪长岩及晚期中酸性岩脉,其中铜钨矿化主要发育于中心相斑状花岗闪长岩。石英闪长岩、花岗闪长岩、斑状花岗闪长岩和花岗闪长斑岩脉的锆石U-Pb谐和年龄分别为(134±1)Ma、(133±1)Ma、(131±2)Ma和(128±1)Ma,表明赤瓦屋岩体不同岩相体形成于早白垩世(134~131Ma)。结合区域年代学资料,提出太行山北段晚中生代至少存在两期岩浆-成矿事件。     

10 km Minimum throw along the West Bohemian shear zone: Evidence for dramatic crustal thickening and high topography in the Bohemian Massif (European Variscides)

The West Bohemian shear zone (WBSZ) forms a steep collapse structure along which east-side-down normal movements led to the juxtaposition of the relatively cold Cadomian basement of the Tepla-Barrandian unit against high grade Moldanubian rocks. Synkinematic plutons straddle the WBSZ. The Mut3nin pluton intruded into Moldanubian crust at a depth of 23dž km as derived by using Al-in-hornblende barometry. The Tepla-Barrandian Babylon pluton intruded at <12 km depth as indicated by phengite barometry and petrogenetic considerations. Both emplacement depths, together with mineral cooling ages, result in a minimum vertical displacement of 10 km between 340 and 320 Ma. This large throw could be explained by over-thickened crust that was weakened from below. The alkaline signature of the Mut3nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 Ma. The cold Tepla-Barrandian upper crust sank into its weak, partly molten Moldanubian substratum, resulting in elevator-style movements, not only along the WBSZ, but also along the Hoher Bogen and Central Bohemian shear zone. All these ductile normal shear zones were active simultaneously during the Lower Carboniferous and dip steeply towards the Tepla-Barrandian unit that probably formed a highly elevated plateau at this time.     

U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of Early Indosinian intrusive rocks in West Qinling, central China: petrogenesis and tectonic implications

The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.     

Late Jurassic–Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton: SHRIMP Zircon U–Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing

The Yunmengshan Geopark in northern Beijing is located within the Yanshan range. It contains the Yunmengshan batholith, which is dominated by two plutons: the Yunmengshan gneissic granite and the Shicheng gneissic diorite. Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U–Pb ages from 145 to 141 Ma, whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma. Dikes that cut the Yunmengshan diorite record SHRIMP zircon U–Pb age of 162±2 and 156±4 Ma. The cumulative plots of zircons from the diorites show a peak age of 155 Ma, without inherited zircon cores, and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunmengshan granitic pluton, whose igneous zircons contain inherited zircon cores. The data presented here show that there were two pulses of magmatism: early diorites, followed c13 Ma later by true granites, which incorporated material from an older continental crust.     

Age of granite emplacement in the Norseman region of Western Australia

Ion probe U‐Th‐Pb dating of zircons from the Late Archaean granites of the Norseman region of the southeastern Yilgarn shows the existence of two distinct magmatic episodes. Large regional tonalite and granodiorite plutons were emplaced between 2685 and 2690 Ma, whereas large regional granite, and small tonalite and leucogranite plutons that intrude the greenstones have ages of 2660–2665 Ma. A small body of granite that intrudes the western edge of the greenstones has an inferred emplacement of 2672 ± 7Ma, and contains inherited zircon that is ～2800 Ma. The monzogranite core from a second pluton in a similar structural position also contains ～2800 Ma zircon; this age is similar to published Sm‐Nd and Rb‐Sr whole rock ages for banded gneisses associated with other members of this suite of domal plutons and is interpreted as representing the age of a significant component within the source region for these distinctive rocks.

Available geochemical and isotopic data are interpreted as indicating derivation of both the older granodiorite and younger granite suites through anatexis of pre‐existing crust of broadly andesitic composition, whereas both the domal granites and the small, late tonalite plutons could have been derived by anatexis of heterogeneous material similar to that represented by the banded gneisses.

If regional metamorphism was related to the emplacement of large volumes of felsic magma within the upper crust, as suggested by Binns et al. (1976), then the Norseman area has probably undergone two periods of regional metamorphism of comparable intensity at approximately 2660 and 2685 Ma.     

Timing of Cadomian and Variscan tectonothermal activity,La Hague and Alderney,North Armorican Massif: Evidence from 40Ar/39Ar mineral ages

The La Hague region of northwest France exposes Palaeo-Proterozoic Icartian gneisses which were reworked and intruded by calc-alkaline plutonic rocks during the Cadomian Orogeny (about 700–500 Ma). ⁴⁰Ar/³⁹Ar mineral cooling ages have been determined to clarify the timing of the regional metamorphism of orthogneisses and the emplacement of quartz diorite plutons in this region. Metamorphic amphiboles within Icartian gneisses display discordant ⁴⁰Ar/³⁹Ar apparent age spectra interpreted to result from limited Variscan (about 350–300 Ma) overprinting of intracrystalline argon systems which initially cooled through post-metamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Moulinet quartz diorites record isotope correlation ages of 599 ± 2 and 561 ± 2 Ma, respectively. Igneous hornblende and biotite from foliated quartz diorite on the nearby Channel Island of Alderney record isotope correlation ages of about 560 Ma. The results imply that metamorphic and plutonic events in the La Hague-Alderney region were approximately contemporaneous with those recorded on Guernsey and Sark, which are thus likely to have formed part of the same tectonic block during the Cadomian Orogeny.     

Tectonic evolution of a continental magmatic arc from transpression in the upper crust to exhumation of mid-crustal orogenic root recorded by episodically emplaced plutons: the Central Bohemian Plutonic Complex (Bohemian Massif)

The Central Bohemian Plutonic Complex (CBPC) consists of episodically emplaced plutons, the internal fabrics of which recorded tectonic evolution of a continental magmatic arc. The ~354–350 Ma calc-alkaline plutons were emplaced by multiple processes into the upper-crustal Teplá-Barrandian Unit, and their magmatic fabrics recorded increments of regional transpression. Multiple fabrics of the younger, ~346 Ma Blatná pluton recorded both regional transpression and the onset of exhumation of mid-crustal orogenic root (Moldanubian Unit). Continuous exhumation-related deformation during pluton cooling resulted in the development of a wide zone of sub-solidus deformation along the SE margin of the CBPC. Finally, syn-exhumation tabular durbachitic pluton of ultrapotassic composition was emplaced atop the intrusive sequence at ~343–340 Ma, and the ultrapotassic Tábor pluton intruded after exhumation of the orogenic root (~337 Ma). We suggest that the emplacement of plutons during regional transpression in the upper crust produced thermally softened domain which then accommodated the exhumation of the mid-crustal orogenic root, and that the complex nature of the Teplá-Barrandian/Moldanubian boundary is a result of regional transpression in the upper crust, the enhancement of regional deformation in overlapping structural aureoles, the subsequent exhumation of the orogenic root domain, and post-emplacement brittle faulting.