松辽盆地北部三叠纪重要火山-岩浆事件——对晚古生代地层时限的约束

对松辽盆地北部黑富地3井所钻遇的侵入岩进行同位素U-Pb年代学分析,以探讨其形成期次和构造演化特征,并对所侵入的地层进行时代约束.测试分析结果显示,锆石206Pb/238U年龄的加权平均值为245.5±1.9 Ma,表明侵入岩的结晶年龄为早中三叠世,指示松辽盆地北部发育早中三叠世岩浆热事件.同时存在两组捕获的锆石年龄,...     

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.     

A Back-arc Palaeotectonic Setting for the Augaro Neoproterozoic Magmatic Rocks of Western Eritrea

The Augaro volcano-sedimentary assemblages of western Eritrea are part of the Neoproterozoic, N-S trending belt of low-grade volcano-sedimentary and associated plutonic rocks. In contrast to the volcanic-dominated oceanic-arc assemblages in central Eritrea, the predominant rock types in the west are supracrustal sequences of sedimentary origin with subordinate volcanic rocks. These Augaro supracrustal rocks are overlain, unconformably, by a basin-fill metasedimentary succession known as the Gulgula Group. The Augaro metavolcanic rocks are tholeiitic and range in composition from basalt to basaltic andesite. Comparison of trace element characteristics and N-MORB-normalised spidergrams of these rocks with those of modern volcanic environments and age-comparable metavolcanic rocks of known tectonic association from the Arabian-Nubian Shield suggest that the volcanic assemblages from western Eritrea were generated in a back-arc tectonic setting.

Single zircon Pb-Pb evaporation and vapour-transfer U-Pb analyses of magmatic zircons from pre/syn-tectonic granites yield a mean ²⁰⁷Pb/²⁰⁶Pb age of 849±20 Ma and an upper concordia intercept age of 849±26 Ma. These ages are interpreted to represent the time of major magmatism in western Eritrea and are comparable to ages of early arc magmatism in central and northern Eritrea and in the southern Nubian Shield. Initial eNd values and initial Sr isotope ratios of whole-rock samples of magmatic rocks calculated for an age of 850 Ma range from +4.0 to +7.1 and 0.7026 to 0.7037, respectively. Single zircon ²⁰⁷Pb/²⁰⁶Pb ages, initial eNd value and Sr isotope ratio for a granitic clast in the Gulgula metaconglomerate suggest that the source area for the Gulgula metasedimentary rocks is similar to the surrounding Neoproterozoic rocks of western Eritrea.     

内蒙古达茂旗花岗岩类LA—ICP—MS锆石U—Pb年龄及其地质意义

对内蒙古达茂旗北部构造单元的巴特敖包岛弧带2个花岗闪长岩岩体样品和南部构造单元一个花岗岩岩体的2件样品进行了LA—ICP-MS锆石u—Pb定年。北部构造单元采自2个花岗闪长岩岩体的样品锆石^206Pb／^238U年龄分别为468Ma±2Ma和452Ma±2Ma,代表了这2个岩体的侵位时间,表明古亚洲洋向华北克拉通之下俯冲不晚于468Ma±2Ma;南部构造单元采自同一花岗岩岩体的2件样品获得的锆石~pb／z38U年龄分别为268Ma±2Ma和264Ma±2Ma,和该单元西部岩体已有的锆石年龄相符。这为华北克拉通北缘岩浆作用研究提供了新的年代学证据。     

内蒙古达茂旗花岗岩类LA-ICP-MS 锆石U-Pb年龄及其地质意义

对内蒙古达茂旗北部构造单元的巴特敖包岛弧带2个花岗闪长岩岩体样品和南部构造单元一个花岗岩岩体的2件样品进行了LA-ICP-MS锆石U-Pb定年。北部构造单元采自2个花岗闪长岩岩体的样品锆石206Pb/238U年龄分别为468Ma±2Ma和452Ma±3Ma,代表了这2个岩体的侵位时间,表明古亚洲洋向华北克拉通之下俯冲不晚于468Ma±2Ma;南部构造单元采自同一花岗岩岩体的2件样品获得的锆石206Pb/238U年龄分别为268Ma±2Ma和264Ma±2Ma,和该单元西部岩体已有的锆石年龄相符。这为华北克拉通北缘岩浆作用研究提供了新的年代学证据。     

大别山东部花岗片麻岩的锆石U-Pb年龄

花岗片麻岩是大别山区除超高压变质杂岩外的另一种重要岩石类型,本文测得南大别二长花岗片麻岩中单颗粒锆石U-Pb不一致线的上交点年龄为789±43 Ma.位于五河一水吼韧性剪切带南缘的糜棱岩化二长花岗片麻岩中锆石的²⁰⁷Pb/²⁰⁶Pb表面年龄接近一致年龄,为715～777 Ma,平均757 Ma,U-Pb不一致线的上交点年龄为815±70 Ma,下交点年龄为482±167 Ma.北大别石英二长片麻岩中锆石²⁰⁷Pb/206表面年龄的变化范围较广,介于594～700 Ma,平均为649 Ma,U-Pb不一致线的上、下交点年龄分别为814±97 Ma和477±77 Ma.这说明这些正片麻岩的侵位时间范围大致为750～850 Ma的晚元古代;南、北大别正片麻岩中的锆石年代学显示它们可能具有相似的形成与演化历史;约480 Ma前后的加里东运动对大别山地区可能也有影响.     

内蒙古达茂旗北部闪长岩锆石SHRIMP U-Pb、角闪石~(40)Ar/~(39)Ar年代学及其地质意义

内蒙古达茂旗北部的早古生代闪长岩侵入体产于包尔汉图-白乃庙岛弧带的西部,采用SHRIMP锆石U-Pb定年及角闪石40Ar/39Ar测年对其进行了精确的年代学研究。两件闪长岩样品分别获得的SHRIMP锆石U-Pb年龄为453±3Ma和446.8±5.3Ma,角闪石40Ar/39Ar坪年龄为459.2±2.4Ma和442.9±4.2Ma。这为研究该时期弧岩浆作用提供了新的年代学证据,并表明该岩浆侵位后,经历了结晶并快速冷却的过程,可能揭示了本区岛弧带和华北板块碰撞的构造意义。     

SOVETSKAYA GEOLOGIYA (SOVIET GEOLOGY) IN COVER-TO-COVER TRANSLATION, 1960, NOS. 11 AND 12

The Río Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial ⁸⁷Sr/⁸⁶Sr ratios of ～ 0.703 and a single-stage model μ₁ value of ～ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupés and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks from the southern part of the RNJP yielded SHRIMP U-Pb concordant ages of 1750 ± 24 Ma and 1570 ± 17 Ma and a Pb-Pb whole-rock isochron age of 1717 ± 120 Ma. These new U-Pb and Pb-Pb results confirm the previous Rb-Sr and Pb-Pb geochronological evidence that the main magmatic episodes within the RNJP occurred between 1.8 and 1.55 Ga, and suggest that this crustal province constitutes a segment of continental crust newly added to the Amazonian Craton at the end of the Early Proterozoic. In the area of the RNJP, there are several anorogenic rapakivi-type granite plutons. Because of the absence of recognized Archean material within the basement rocks, it is reasonable to consider the Early to Middle Proterozoic continental crust as the magmatic source for the rapakivi granite intrusions.     

安徽东至兆吉口铅锌矿区岩浆岩锆石U-Pb年龄及其地质意义

通过对安徽省东至县兆吉口铅锌矿区岩浆岩的LA-ICP MS锆石U-Pb年龄测定,获得矿区北部戴村花岗闪长岩体的年龄为145.5±1.3 Ma,花岗斑岩脉的年龄为143.5±4.3 Ma,细晶闪长岩脉的年龄为129.0±2.3 Ma和128.4±2.7 Ma,前两者为同一期岩浆作用的产物,后者为赋矿岩石之一,铅锌矿化与该期岩浆作用关系密切。矿区所有中酸性侵入岩体和岩脉中均发育大量继承锆石,反映有大量成岩物质来源于古老地壳基底。继承锆石核的同位素年龄集中于890～740 Ma,揭示出该区晋宁期华夏板块与扬子板块之间的构造-岩浆事件,少量大于1 000 Ma甚至2 500 Ma的锆石年龄数据反映该区可能存在早元古代甚至太古代古老陆壳基底。     

U–Pb zircon and microfabric data of (meta) granitoids of western Cameroon: Constraints on the timing of pluton emplacement and deformation in the Pan-African belt of central Africa

Striking characteristics of the western Neoproterozoic belt of Cameroon (NFBC) are the large volume of granitoids and crustal-scale shear zones. New structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the Atlantic Ocean.

Three different complexes have been identified in the study area: the migmatitic complex of Foumbot (MCF), the metagranitoid complex of Bangwa (BC), and the Batié pluton (BP). The MCF was intruded by the BC, while the BP cuts through the BC. U–Pb zircon dating of metaleucogranite and metagranodiorite of the BC yielded concordant to subconcordant ages of 638 ± 2 Ma and 637 ± 5 Ma, respectively. A concordant U–Pb zircon age of 602 ± 1.4 Ma has been obtained from porphyrogranite of the BP. These ages are interpreted as emplacement ages. Continuous deformation from magmatic to solid-state flow along the BP margins and the (sub)parallelism of the steep solid-state foliation in the BP margins with the foliation in the surrounding BC and MFC suggest synkinematic emplacement of the BP along crustal-scale NNE to ENE-trending strike–slip shear zones. Subhorizontal foliations in migmatitic-gneiss xenoliths found in the BC suggest that the major transcurrent motion was preceded by thrusting.

The new data confirm previous assumptions that the western NFBC is equivalent to parts of the Borborema province of Brazil. There are geochronological correlations between the studied (meta)granitoids and Brasiliano pre- to syn-transcurrent granitoids of the Borborema province.     

鲁东地区侵入岩形成时代和期次划分——锆石SHRIMPU—Pb年龄的证据

鲁东地区前寒武纪侵入岩其侵入活动集中于新太古代和新元古代,均遭受不同程度的区域变质和韧性剪切带的改造,形成一套花岗质片麻岩类。新太古代早期TTG质花岗岩分布于胶北隆起区莱州一栖霞一带,经受了区域变质作用,形成一套灰色片麻岩,锆石内核SHRIMPU—Pb年龄2726—2707Ma基本代表岩浆结晶年龄,锆石变质边为2500Ma;古元古代侵入岩——莱州基性一超基性岩组合形成于1900～1850Ma左右,锆石SHRIMPU—Pb年龄（1852±9）Ma,（1868±11）Ma;新元古代侵入岩主要为花岗闪长质-二长花岗质片麻岩,出露于胶南-威海造山带,锆石U～Pb定年结果集中为892～605Ma,其中荣成超单元锆石SHRIMPU—Pb年龄720～780Ma。中生代侵入岩以花岗岩类为主,集中出露于半岛北部和东南沿海一带。燕山早期侵入岩文登超单元、垛崮山超单元、玲珑超单元锆石SHRIMPU—Pb同位素年龄值集中在167～150Ma;燕山晚期侵入岩大店超单元锆石SHRIMPU～Pb年龄120±4Ma,崂山超单元锆石SHRIMPU～Pb同位素年龄值为（120±2）～（114±2）Ma。     

塔里木盆地沙雅隆起北部三道桥地区前震旦纪基底岩浆岩特征与锆石U-Pb年龄研究

对塔里木盆地北部三道桥地区前震旦纪基底的岩浆岩进行岩石学分析及时代限定研究。研究区前震旦纪基底的岩浆岩主要由钾长花岗岩、石英二长岩及角闪二辉岩组成。钾长花岗岩局部受断裂影响,发生碎裂化作用,形成碎裂化钾长花岗岩。通过对Qg2、Qg3、S53及Qg4井4口钻井的4个岩心样品进行LA-ICP-MS锆石U-Pb年代学研究,获得钾长花岗岩、石英二长岩及碎裂钾长花岗岩年龄分别为1846±1Ma、1791±1Ma、1824±2Ma,代表了古元古代岩浆岩的变质年龄;角闪二辉岩年龄为879±4Ma,代表了中新元古代火山侵入年龄。说明塔里木盆地沙雅隆起北部存在前震旦纪古老基底,这些结果也为塔里木克拉通的前寒武纪构造演化提供了新的限定。     

Early Neoarchean Magmatic and Paleoproterozoic Metamorphic Events in the Northern North China Craton: SHRIMP Zircon Dating and Hf Isotopes of Archean Rocks from the Miyun Area, Beijing

The Miyun area of Beijing is located in the northern part of the North China Craton(NCC)and includes a variety of Archean granitoids and metamorphic rocks.Magmatic domains in zircon from a tonalite reveal Early Neoarchean(2752±7 Ma) ages show a small range in ε_(Hf)(t) from 3.1 to 7.4and t_(DM1)(Hf) from 2742 to 2823 Ma,similar to their U-Pb ages,indicating derivation from a depleted mantle source only a short time prior to crystallization.SHRIMP zircon ages of granite,gneiss,amphibolite and hornblendite in the Miyun area reveal restricted emplacement ages from 2594 to2496 Ma.They also record metamorphic events at ca.2.50 Ga,2.44 Ga and 1.82 Ga,showing a similar evolutionary history to the widely distributed Late Neoarchean rocks in the NCC.Positive ε_(Hf)(t) values of 1.5 to 5.9,with model ages younger than 3.0 Ga for magmatic zircon domains from these Late Neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern NCC during the Neoarchean.Late Paleoproterozoic metamorphism developed widely in the NCC,not only in the Trans-North China Orogen,but also in areas of Eastern and Western Blocks,which suggest that the late Paleoproterozoic was the assembly of different micro-continents,which resulted in the final consolidation to form the NCC,and related to the development of the Paleo-Mesoproterozoic Columbia or Nuna supercontinent.     

阿拉善北大山乌布日布特地区晚古生代侵入岩的地球化学、年龄、Hf同位素特征及意义

内蒙古阿拉善西部的北大山北部出露一些古生代侵入岩,这些侵入岩的时空分布和源区物质组成特征对于确定北大山北部的构造属性以及探讨阿拉善西部的构造划分和构造演化具有重要意义。本文对北大山北部乌布日布特地区腰泉和沙枣泉两个花岗质岩体开展了详细的年代学、地球化学和锆石Hf同位素研究。锆石LA?ICP?MS U?Pb测年结果显示,腰泉黑云母正长花岗岩年龄为261.8±1.2 Ma,沙枣泉黑云母花岗岩年龄为303.1±1.6 Ma,均形成于晚古生代。腰泉黑云母正长花岗岩与沙枣泉黑云母花岗岩的稀土元素组成相似,均表现出轻稀土元素相对富集,重稀土元素相对亏损的特征,并显示有微弱—中等的Eu负异常。腰泉黑云母正长花岗岩与沙枣泉黑云母花岗岩均具有正的ε_Hf(t)值,腰泉黑云母正长花岗岩的ε_Hf(t)值为+1.93~+7.40,沙枣泉黑云母花岗岩的εHf(t)值为+0.52~+10.00,这与北大山北部其它古生代酸性侵入岩的锆石Hf同位素相似,表明北大山北部酸性岩浆的深部源区以新生地壳物质为主。北大山北部深部地壳物质组成特征与阿拉善中东部宗乃山—沙拉扎山地区相似,而与北大山南部和阿拉善中东部雅布赖—诺尔公—红古尔玉林地区具有明显差别。结合区域研究资料,推测北大山北部可能不属于阿拉善地块,而与阿拉善中东部的宗乃山—沙拉扎山地区具有相同的构造属性,是中亚造山带的组成部分。     

Magma sources and gold mineralisation in the Mount Leyshon and Tuckers Igneous Complexes, Queensland, Australia: U-Pb and Hf isotope evidence

In situ LAM-ICPMS U-Pb, Hf-isotope and trace-element analyses of zircon have been used to evaluate the relative contributions of juvenile mantle and crustal sources to the intrusive rocks of the mafic to intermediate, gold-poor Tuckers Igneous Complex (TIC), and the spatially and temporally related, felsic Mount Leyshon Igneous Complex (MLIC), which hosts a gold-rich porphyry system.

The TIC intrusions range in age from 304.2 ± 9.1 Ma to 288.5 ± 6.4 Ma, and the MLIC intrusions from 291.0 ± 4.8 Ma to 288 ± 6 Ma. Cross-cutting relationships define the intrusion sequence from oldest to youngest; Diorite, Monzodiorite, Mafic Granodiorite and Biotite Microgranite within the TIC; Early Dyke, Southern Porphyry and Late Dyke within the MLIC.

Zircons from the earliest rock type within each complex have a wide range in _Hf (5.2 to 14.8 for the TIC Diorite, 2.0 to 12.4 for the MLIC Early Dykes) suggesting the mixing of juvenile and crustal magmas. This interpretation is supported by trace-element data that show the presence of two distinct zircon populations in the MLIC Early Dyke. The later intrusive rocks have narrower ranges in _Hf (typically < 4 _Hf units) and trace-element patterns of zircon. This homogeneity suggests derivation from magmas produced by further mixing and fractional crystallisation of the TIC Diorite and the MLIC Early Dyke magmas respectively. A greater crustal contribution to the gold-rich MLIC is inferred from the range of median _Hf (3.2 to 4.5 for the MLIC, 5.4 to 8.7 for the TIC). We suggest that the MLIC was derived by melting of more felsic crustal rocks, and with less input from juvenile mantle, then the TIC; it was not derived by fractional crystallisation of an intermediate to mafic TIC-like magma. Modelling of Hf isotope data yields a mean model age of 1040 ± 10 Ma (at ¹⁷⁶Lu/¹⁷⁷Hf = 0.015) for the crustal component in both complexes.

Gold was precipitated in the MLIC Breccia during the emplacement of the Late Dykes. The isotopically homogenous nature of the Late Dykes suggests that no additional juvenile-mantle input was involved at the mineralisation stage. This supports a model in which gold and other metals were indigenous to the Late Dykes magma and were concentrated by magma differentiation and fluid-evolution processes.     

Petrogenesis of U- and Mo-bearing A2-type granite of the Gattar batholith in the Arabian Nubian Shield,Northeastern Desert,Egypt: Evidence for the favorability of host rocks for the origin of associated ore deposits

Hydrothermal ore deposits associated with granitic rocks are the result of combinations and series of successive processes, events and conditions in a magma system and at post-magmatic stage. The metal content of a granitic magma as well as fractional crystallization, metal–ligand complexation, emplacement mechanisms and tectonic processes influence the metal precipitation and content of a hydrothermal ore deposit. The present study of the Gattar batholith evaluates how these processes contribute to the generation of an ore deposit. Gattar batholith in the north Eastern Desert of Egypt as part of the northern region of the Arabian–Nubian Shield (ANS) is geographically, mineralogically and geochemically divided into two distinctive areas. The southern part mainly consists of syenogranite while the northern part is dominated by highly evolved alkali-feldspar granite. Uranium and Mo mineralization occurring here is mostly limited to the margin of the highly evolved alkali-feldspar granites. New U–Pb zircon geochronology within this study indicates an age of ~ 620–600 Ma, although high common Pb and discordant age data of many zircon grains reflect alteration by F-rich fluids. Some zircons with distinct older U/Pb ages were considered to be inherited from the wall rocks representing juvenile crust of the ANS. The two different granitic rocks are comagmatic in origin and have A-type characteristics generated by partial melting of lower juvenile crust of the ANS. Geochemical data are in good agreement with a magmatic origin of the alkali-feldspar granites, suggesting fractional crystallization of a syenogranitic source as the most favorable process of their formation. Petrography, geochemistry, and geochronology studies of the Gattar granites along with field observations provide evidence for multiple intrusions as a result of successive magma pulses with different chemical composition. This led to the formation of reverse zoning within the intrusive complex. This kind of magma emplacement promotes the escape of exsolved metal-bearing volatile-rich fluids into the boundary of the Hammamat Sedimentary Rocks (HSR) and Gattar granite at the northern part of the batholith. Build-up of volatiles is a common phenomenon in granitic rocks and typically results in enhanced contents of F, alkalis and ore metals released at subsolidus stage.     

青海南山中三叠世中酸性侵入岩的地球化学、锆石U-Pb年代学特征及构造意义

青海南山位于秦祁昆造山带结合部,大地构造位置特殊,构造演化复杂。对不同期次岩浆岩组合开展岩石地球化学和锆石U-Pb年代学研究,可为区域构造单元划分、构造阶段厘定和深部过程等提供丰富信息。对青海南山地区一些中酸性侵入岩开展岩石地球化学和锆石U-Pb年代学研究,结果如下:切如闪长岩体两个样品206Pb/238U加权平均年龄237.9±2.3Ma(MSWD=2.1)和241.0±3.2Ma(MSWD=2.4),黑马河含黑云角闪二长花岗岩体两个样品206Pb/238U加权平均年龄242.7±1.5Ma(MSWD=0.61)和239.30±2.0Ma(MSWD=1.7),表明这些侵入体的结晶年龄集中于237~242Ma,属中三叠世;地球化学上属钠质、富镁、铝不饱和的中钾钙碱性-高钾钙碱性岩石,稀土元素配分模式表现为中等程度右倾、中等程度的负铕异常。年代学和地球化学研究表明青海南山地区中酸性侵入岩形成于后碰撞环境。     

Age and emplacement of late-Variscan granites of the western Bohemian Massif with main focus on the Hauzenberg granitoids (European Variscides, Germany)

The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late- to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium- to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a ²⁰⁷Pb–²⁰⁶Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west- to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at > 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer.     

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。     

Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon

Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old ²⁰⁷Pb/²⁰⁶Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young ²⁰⁷Pb/²⁰⁶Pb ages, followed by a 4 1/2 h HF dissolution step in an oven at 190°C that liberates the excessively old Pb. The final residue yields in most cases U–Pb data that are consistent with the results obtained by the normal selection, abrasion, and total dissolution procedure. In these examples, the too-old ages cannot be easily explained by the presence of an inherited core component but are more likely to indicate segregation of Pb in zircon sub-domains during thermal annealing episodes early in its history, as has been proposed by other workers. Aside from shedding light on these particular aspects of zircon U–Pb systematics, the combined results also provide geologically relevant information concerning the regional evolution of the western Superior Province in Ontario. An age of 2718±3 Ma for a gabbroic unit from the Quetico Subprovince shows that this was coeval to 2722–2718 Ma ultramafic to felsic igneous rocks in the adjacent Shebandowan greenstone belt, including a gabbro body dated in this study at 2725+17/−11 Ma. These age relationships suggest that volcanic units of the Shebandowan greenstone belt were tectonically imbricated in younger sedimentary rocks of the Quetico basin during late Archean convergence. The other three samples represent felsic intrusive units from Geraldton in the Wabigoon subprovince. An age of 2699±1 Ma for an Au-mineralized feldspar porphyry dyke and identical ages of 2690±1 Ma for two phases of the syn-tectonic Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt.