广西大厂笼箱盖复式岩体的LA-MC-ICP-MS锆石U-Pb年龄及其地质意义

广西大厂矿田中部的笼箱盖岩体是一个多期次侵入的复式岩体,尽管前人对其中个别阶段岩体进行过年龄精测,但迄今还没有对多期岩体进行过系统的年代学精测。本文在运用阴极发光技术对岩体中锆石进行细致的内部结构分析的基础上,利用LA-MC-ICP-MS锆石U-Pb原位定年方法,系统测定了岩体中单颗粒锆石的²⁰⁶Pb/²³⁸U年龄,结果显示出,笼箱盖复式岩体形成经历了103.8~102Ma、96.6~93.86Ma、90.1~85.1Ma 3期活动,其中96.6~93.86Ma为主体形成时期,其第一阶段形成中细粒含斑黑云母花岗岩(96.6±2.5Ma)、细粒含斑的黑云母花岗岩(94.3±2.2Ma),第二阶段形成中细粒等粒状黑云母花岗岩(96.1±2.0Ma),第三阶段形成似斑状黑云母花岗岩(93.86±0.84Ma)。这些岩体之间界限清晰、无明显冷凝边,说明岩浆活动是一种连续的脉动过程。大厂矿田的锌铜矿体、锡多金属矿体的形成主要与笼箱盖复式岩体的第二期岩浆活动密切相关,成矿过程在短暂时间完成。笼箱盖复式岩体的形成时期与中国东部110~80Ma大规模成矿过程是吻合的,成岩成矿作用可能与岩石圈伸展作用有关。     

湘南尖峰岭岩体锆石U-Pb年龄、地球化学特征及成因

香花岭锡多金属矿田处于南岭西段中_晚侏罗世形成的花山_西山_香花岭_骑田岭NE向的富碱侵入岩带上,是湘南地区矿化类型最为完整的锡矿区。尖峰岭岩体为矿田内主要的成矿母岩之一。为了进一步明确尖峰岭黑云母花岗岩的岩石成因及成岩年龄,笔者对尖峰岭黑云母花岗岩开展了LA_ICP_MS锆石U_Pb测年及岩石地球化学特征分析。锆石LA_ICP_MS U_Pb测年获得206Pb/238U年龄介于(158.6±2.3)Ma~(164.8±4.5)Ma之间,加权平均年龄为(160.7±2.2)Ma(MSWD=0.8),这与香花岭锡多金属矿田的成矿时限(154~161 Ma)相吻合,表明岩体的侵位与区内多金属成矿作用均发生于中_晚侏罗世。尖峰岭岩体的主量、微量元素研究显示,尖峰岭黑云母花岗岩具有富硅、富碱及成矿元素,富LILE及Zr、Ga等部分高场强元素(HFSE),贫Ca、Mg、P、Eu特征,指示尖峰岭黑云母花岗岩属于过铝质A型花岗岩(A2型)。综合研究表明,尖峰岭黑云母花岗岩体与其周缘钨锡多金属矿床在时间、空间及成因上有密切关系。结合区域上已有的同位素年代学数据,香花岭矿区的成岩成矿作用与南岭西段同时代的其他钨锡多金属矿床形成于同一岩石圈伸展的地球动力学背景下。     

内蒙古道伦达坝铜钨锡矿床LA-ICP-MS锆石和锡石U-Pb年龄及其地质意义

道伦达坝矿床位于大兴安岭南段,是一个铜钨锡矿床,其铜、钨、锡储量均达中型。矿体呈脉状,主要产于二叠系砂板岩中的断裂破碎带中,华力西期黑云母花岗岩中的断裂破碎带中亦赋存有矿体。文章选取2件石英-萤石-白云母-电气石-锡石-黑钨矿阶段的矿石样品对其中的进行了LA-ICP-MS U-Pb定年,获得2件样品的~(207)Pb/~(206)Pb-~(238)U/~(206)Pb谐和年龄分别为(134.7±6.6)Ma(MSWD=1.4)和(136.8±7.4)Ma(MSWD=1.7),~(206)Pb/~(207)Pb-~(238)U/~(207)Pb等时线年龄分别为(132±12)Ma(MSWD=0.76)和(135±13)Ma(MSWD=0.9)。锡石定年结果表明,道伦达坝矿床形成于早白垩世。对矿区外围张家营子岩体中的斑状细粒花岗岩进行了LA-ICP-MS锆石U-Pb测年,获得的~(206)Pb/~(238)U加权平均年龄为(135±1)Ma(MSWD=1.3),该岩体的形成年龄与道伦达坝矿床的成矿年龄在误差范围内一致。本次定年结果表明道伦达坝矿床形成于早白垩世,与同期的花岗质岩浆活动有密切的成因联系,该矿床属于与花岗岩有关的岩浆热液脉型矿床。     

LA-ICPMS Zircon U-Pb Dating for Three Indosinian Granitic Plutons from Central Hunan and Western Guangdong Provinces and Its Petrogenetic Implications

1 Introduction Much attention has been paid to the structure, magmatism and sedimentation associated with the Indosinian Orogeny within the South China Block (SCB) since Deprat (1914) and Fromagat (1932) proposed the term “Indosinian movement” based on two unconformities between pre-Norian and pre-Rhaetian times during the early Triassic in Vietnam. However, this timing has been debated in terms of the Indosinian tectonic evolution within the SCB (Guo et al., 1983; Hsü et al., 1990…     

U–Pb dating of zircon and cassiterite from the Early Cretaceous Jiaojiguan iron-tin polymetallic deposit,implications for magmatism and metallogeny of the Tengchong area,western Yunnan,China

The newly discovered Jiaojiguan deposit, a medium-scale skarn iron-tin polymetallic deposit on the Sino-Burma boundary of Yunnan Province (SW China), is spatially associated with the biotite monzonitic granite. Here, we report new in situ zircon LA-MC-ICP-MS U–Pb ages, trace element and Hf isotope data from the granite, and U–Pb dating ages of cassiterite from the ore bodies. In this study, we obtain a weighted mean ²⁰⁶Pb/²³⁸U age of 124.1 ± 1.4 Ma for the zircon and a ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb intercept age of 123.8 ± 2.2 Ma for the cassiterite. The granite crystallized during the Early Cretaceous, with zircons exhibiting εHf(t) values from ?5.8 to ?0.6 and two-stage Hf model ages (T_DM2) of 1.21–1.54 Ga. The close temporal and spatial links between pluton emplacement and ore-forming events suggest that magmatic-hydrothermal events were the key factors that triggered the genesis of the iron-tin polymetallic deposits in the area. Regional geochronological data show that tin mineralization took place three times during the Cretaceous–Palaeogene in the Tengchong block due to re-melting of the underlying supposed Proterozoic (1.5 ± 0.5 Ga) Sn-rich strata/materials. Compared with those in the Bangong–Nujiang metallogenic belt (BNMB), we propose that the Cretaceous iron-tin polymetallic mineralization events in Tengchong–Baoshan closely resemble those of the Bangong–Nujiang belt in northern Tibet, both of which have experienced similar tectono-magmatic-metallogenic histories since the Mesozoic.     

The Mount Gharib A-type granite,Nubian Shield: petrogenesis and role of metasomatism at the source

The Mount Gharib peralkaline A-type complex (476±2 Ma), located in the Nubian Shield of Egypt, contains sodic-calcic to sodic amphiboles, accessory astrophyllite, zircon, fluorite, apatite, allanite, aenigmatite, elpidite(?) and ilmenite. This “within plate” hypersolvus suite is enriched in large-ion lithophile (LIL) and high field-strength (HFS) elements, and characterized by a fractionated REE pattern (Ce/Yb=49) and a significant negative Eu anomaly. A fine-grained acicular-amphibole-bearing roof facies shows further enrichment in the LIL and HFS elements. The suite was emplaced in a Pan-African granodiorite-adamellite host, which it locally metasomatized. The affected rocks contain hydrothermal albite, end-member arfvedsonite, astrophyllite, and levels of the LIL and HFS elements intermediate between those in the peralkaline granite and the roof facies. Trace element and isotopic modeling of this A-type granite, with its high initial ⁸⁷Sr/⁸⁶Sr value (0.7110), documents an active role of the lithosphere in magma generation. Lithospheric extension, expressed by regional dyke-swarms, was caused by cooling, fracturing and relaxation of the thin, newly formed Pan-African crust. Localized partial melting took place in an open system, possibly as a result of an influx of alkali-rich fluid derived from a sublithospheric source. Metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the A-type melt.     

Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central Australia

Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800–875 °C and >9–10 kbar) and subsequent decompression in a kilometre‐scale, E‐W striking zone of noncoaxial, high‐grade (c. 700–735 °C and 5.8–6.4 kbar) deformation. However, new SHRIMP U‐Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta‐volcaniclastics) yield ²⁰⁶Pb/²³⁸U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield ²⁰⁶Pb/²³⁸U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400–300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475–460 Ma & 400–300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300–1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed.     

Geochronology of a composite granitoid pluton: a high-precision ID-TIMS U–Pb zircon study of the Variscan Karkonosze Granite (SW Poland)

Resolving time differences between successive magmatic pulses in composite granitoid plutons is often a difficult task. High-precision CA-ID-TIMS zircon ages obtained from such a pluton, the Variscan Karkonosze Granite (NE part of the Bohemian Massif), provide evidence that the crystallization of the two main granite facies, porphyritic and equigranular, happened between 312.5 ± 0.3 and 312.2 ± 0.3 Ma, thus unresolvable at the 0.08–0.1 % precision level of a single ²⁰⁶Pb/²³⁸U age. This finding is at odds with most other previous dating attempts and asks for a re-evaluation of the previous scattered geochronological data. The main reasons for the scatter of the earlier dates obtained by various techniques can include analytical causes, the presence of older inheritance and disturbance of the U–Pb isotopic system, due to zircon metamictization (enhanced by high-U content in zircon) or late- and post-magmatic alteration.     

Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area

Abstract A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and ²⁰⁷Pb/²⁰⁶Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.     

Geochemical characteristics,cooling history and mineralization significance of Zhangtiantang pluton in South Jiangxi Province,P.R. China

The zircon SHRIMP dating of the Zhangtiantang granite gave an age of 159±7 Ma., which shows that the granite was produced at the early Late Jurassic. The Ar-Ar plateau ages of biotite and K-feldspar from the Zhangtiantang pluton are 153.2±1.1 Ma and 135.8±1.2 Ma, respectively. The Ar-Ar anti-isochrone ages of biotite and K-feldspar are 152.5±1.7Ma and 135.4±2.7Ma, respectively. The ages represent the isotopic closure ages of minerals in the pluton. The Zhangtiantang granites are regarded as peraluminous crust-derived type granites to possess the typical geochemical characteristics of calc-alkaline rocks on continental margin, with enriched Si, K, Al (average value of A/CNK as 1.18), HREE, Rb, U, and Th, heavily depleted V, Cr, Co, Ni, Ti, Nb-Ta, Zr, Sr, P, and Ba, strongly negative Eu and common corundum normative (average value of C as 1.84). The εNd_(t) values of the Zhangtiantang granite are −5.84 to −7.79, and t _2DM values are 1.69 to 1.83 Ga, which indicates partial melting of continental-crust metamorphic sedimentary rocks during the Middle Proterozoic. The cooling history of the Zhangtiantang granitic pluton indicates that the cooling velocity of pluton was faster (about 67°C/Ma) from zircon (158 Ma) to biotite (152 Ma), and was slower (about12°C/Ma) from biotite (152.5 Ma) to K-feldspar (135.8 Ma). It can be deduced that the temporal gap (about 10 Ma) between the granite formmation and W-Sn mineralization in South China may be related to ordinary magma-hydrothermal processes by the variational cooling curve of the pluton. The Zhangtiantang pluton was formed in a compressive setting, with differentiation evolution and mineralization occurring in a relative relaxation setting.     

Report on 34 Ga SHRIMP Zircon Age from the Yuntaishan Geopark in Jiaozuo, Henan Province

1 Introduction According to recent researches, the North China Craton consists of three parts: the eastern block, western block and central zone (Zhao, 2001; Wilde et al., 2002). Paleoarchean continental blocks and zircon residuals have only been found in a few regions, such as Anshan, East Liaoning (Liu et al., 1992; Song et al., 1996; Wan et al., 2002, 2005), Caozhuang, East Hebei (Liu et al., 1992) and Xinyang, West Henan (Zheng et al., 2004), which are mainly distributed in the east…     

Pb/Pb and U/Pb zircon dating of subvolcanic rhyolite as a time marker for Hercynian granite magmatism and Sn mineralisation in the Eibenstock granite, Erzgebirge, Germany: Considering effects of zircon alteration

Tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo- and exocontact of the Eibenstock granite, Erzgebirge, Germany. The same structures which cut the granite control the rhyolite emplacement and the location of ore-bearing greisen bodies. Albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock. Comprehensive zircon studies by scanning electron microscopy (secondary and backscattered electron as well as cathodoluminescence imaging; EDX measurements) reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity, including distortion of the U-Pb isotope system when affected by greisenisation. Single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a ²⁰⁷Pb/²⁰⁶Pb age of 290±5 Ma, whereas zircon from little altered granite gave a ²⁰⁷Pb/²⁰⁶Pb age of 320±8 Ma. The single evaporation age of zircon from rhyolite is confirmed by ²³⁸U/²⁰⁶Pb SHRIMP ages of 297±8 Ma. The significant time gap of at least 20 Ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the Eibenstock granite cannot be regarded as a source for tin-ore forming fluids as previously assumed.     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。     

中祁连西段黑沟梁子花岗岩的锆石U-Pb同位素年龄及成因

黑沟粱子花岗岩岩体为中祁连山带两段野马南山巨量花岗岩的一部分,出露于野马山南北缘,岩性为黑云母二长花岗岩。δ值、NK／A值、A／NCK值、Eu负异常、δEu值、ACF图解、⁸⁷Sr/⁸⁶Sr比值、εS_r、¹⁴⁷Sm／¹⁴⁴Nd比值、²⁰⁷Pb/²⁰⁴Pb比值、ENd值等岩石化学和同位素特征表明黑沟粱子花岗岩属钙碱性过铝质岩系,浆来源于地壳物质的重熔,具S型花岗岩特点。通过对黑沟梁子花岗岩的颗粒级锆石U-Pb同位素测年,获得²⁰⁶Pb/²³⁸U表面年龄统计权重平均值为(444±17)Ma。w(Nb)-w(Y)和w(Rb)一w(Y+Nd)图和R₁-R₂图解上两个样点均落入同碰撞花岗岩区,结合前人研究成果可以确定该岩体形成于板块碰撞阶段。该岩体的锆石U-Pb年龄的确定和成因的探讨对于深入研究祁连造山带的构造演化提供了一条重要的地质信息。     

Origin of Middle Cambrian and Late Silurian potassic granitoids from the western Kunlun orogen,northwest China: a magmatic response to the Proto-Tethys evolution

This paper presents new SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotopic data of two Early Paleozoic granitic plutons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U–Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513?±?7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6?±?6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072–0.7096, ε_Nd (T) of ?0.2 to ?1.6 and ε_Hf (T) (in-situ zircon) of ?1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A₁ subtype granite, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7077–0.7101, ε_Nd (T) of ?3.5 to ?4.0 and ε_Hf (T) (in-situ zircon) of ?3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (～513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (～421 Ma), most probably in Middle Silurian.     

Simultaneous horst-basin formation and magmatism during Late Variscan transtension: evidence from 40Ar/39Ar and 207Pb/206Pb geochronology in the Ruhla Crystalline Complex

40 Ar/³⁹Ar–mica and ²⁰⁷Pb/²⁰⁶Pb–zircon dates are presented and combined with existing P–T data and the sedimentary record. These data indicate that the RCC was faulted into three segments which underwent different exhumation histories during the Late Carboniferous/Early Permian. The eastern segment shows ⁴⁰Ar/³⁹Ar–biotite data of336 ±4 and 323±3 Ma. Furthermore, it is intruded by the Thuringian Hauptgranite dated at 337±4 Ma by the ²⁰⁷Pb/²⁰⁶Pb single zircon method. At approximately 300 Ma rocks of the eastern segment were finally exposed and, subsequently, subsided as part of the Oberhof pull-apart basin, filled by Late Carboniferous/Early Permian molasse sediments and volcanic rocks (296–285 Ma; Goll 1996). A similar Late Carboniferous evolution is inferred for the western segment, since it is also overlain by Upper Carboniferous volcanic rocks. In contrast to the eastern and western segments, distinctly younger intrusion and cooling ages were recorded for the central segment of the RCC (⁴⁰Ar/³⁹Ar muscovite: 311±3 Ma; ⁴⁰Ar/³⁹Ar biotite: 293–288±3 Ma) that was intruded by the Trusetal Granite, the Ruhla Granite and Brotterode Diorite (²⁰⁷Pb/²⁰⁶Pb single zircon: 298±2, 295±3, 289±4 Ma, respectively). These young data are unique in the MGCR and testify that plutonic activity and cooling of basement rocks took place simultaneously with basin formation and volcanism in the eastern and western segments. Overlying Upper Permian (Zechstein) and Triassic sediments indicate final exposure of the central segment by approximately 260 Ma, as a part of the Ruhla-Schleusingen Horst. Combination of these results with P–T data from the contact aureole of the Trusetal granite indicate that the central segment was unroofed by at least 8.5 km during the Late Carboniferous. The Late Carboniferous/Early Permian horst-basin formation, documented in the RCC, is due to dextral transtensional movements along the NW-trending Franconian fault system. It may have been enhanced by mantle upwelling widespread in Central Europe during the Early Permian that also caused intensive magmatism in the Thuringian Forest region. Received: 2 February 1999 / Accepted: 15 November 1999     

Chronology of multiphase emplacement of the Salmi rapakivi granite-anorthosite complex, Baltic Shield: implications for magmatic evolution

The U-Pb dating of 18 samples, representing the principal rock types of the 4000 km² Salmi anorthosite-rapakivi granite complex and its satellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield, reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years. From oldest to youngest they are: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 1543.4 Ma; (3) early wiborgite and pyterlite at 1540.6–1537.9 Ma; (4) biotite granite and more evolved granite at 1538.4–1535 Ma; (5) late pyterlite at 1535.2 Ma; (6) olivine gabbro and biotite-amphibole granite at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5–5.0 million years. Early wiborgite and pyterlite (3, above) and biotite granite (4, above) probably crystallized from multiple magma intrusions. Age differences of 3.4±1.5 million years between zircon and baddeleyite in olivine gabbro (6, above) are probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the Salmi complex. The ages and chemical features of early and late zircon populations, together with our modeling of magma crystallization and zircon growth, show that the duration of magma crystallization and Pb-diffusion in zircon was short lived and insignificant compared to the precision of dating of about ±1–2 million years. Hence, the range of U-Pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas. Average rate of magma emplacement was about 0.01 km³/year for the most voluminous phase of early biotite-amphibole rapakivi granite, and about 0.0024 km³/year for the Salmi complex as a whole. Compositional changes of the Salmi magmas over time are in agreement with the model of magmatism related to lithospheric extension. Received: 2 August 1996 / Accepted 19 December 1996     

Timing of the Yuchiling giant porphyry Mo system, and implications for ore genesis

The Yuchiling Mo deposit is a recently discovered giant porphyry system in the East Qinling Mo belt, China. Its apparent causative intrusion, i.e., the Yuchiling granite porphyry, is the youngest intrusion (phase 4) of the Heyu multiphase granite batholith, which was emplaced between 143 and 135 Ma. New robust constraints on the formation of the Yuchiling porphyry Mo system are provided by combined zircon U–Pb, biotite ⁴⁰Ar/³⁹Ar, and molybdenite Re–Os dating. Zircon grains from the Mo-mineralized granite porphyry yield weighted ²⁰⁶Pb/²³⁸U age of 134.0?±?1.4 Ma (n?=?19, 2σ error, MSWD?=?0.30). Magmatic biotite from the same sample yield a ⁴⁰Ar/³⁹Ar plateau age of 135.1?±?1.4 Ma (2σ error), and an inverse isochron age of 135.6?±?2.0 Ma (n?=?7, 2σ error, MSWD?=?10.8), which are effectively coincident with the zircon U–Pb age within analytical error. Three pulses of mineralization can be deduced from the molybdenite Re–Os ages, namely: ～141, ～137, and ～134 Ma, which agree well with the zircon U–Pb ages of granitic phases 1, 2, and the Yuchiling porphyry (phase 4), respectively. These well-constrained temporal correlations indicate that Mo mineralization was caused by pulses of granitic magmatism, and that the ore-forming magmatic-hydrothermal activity responsible for the Yuchiling porphyry Mo system lasted about 8 Ma. The Yuchiling Mo deposit represents a unique style of porphyry Mo system formed in a post-collision setting, and associated with F-rich, high-K calc-alkaline intrusions, which differ from convergent margin-associated porphyry Mo deposits.     

The age of camptonite dikes of the Agardag alkali-basalt complex (western Sangilen): results of Ar/Ar and U/Pb dating

A camptonite dike swarm (Agardag alkali-basalt complex) in the western part of the Sangilen Upland abounds in mantle xenoliths. Mineralogical, petrographic, and petrochemical studies show that the dikes are composed of lamprophyres of two groups, basic and ultrabasic. Ar/Ar dating of amphibole and phlogopite megacrysts gives an intrusion age for the dikes of 443.0 ± 1.3 Ma. ²⁰⁶Pb/²³⁸U dating of zircon from a glomeroporphyritic intergrowth in camptonite from one of the dikes yielded a core age of 489.0 ± 5.4 Ma. This corresponds to the time of formation of the Chzhargalanta granite–leucogranite complex (489.4 ± 2.6 Ma). The ²⁰⁶Pb/²³⁸U age of the zircon rim is 444.0 ± 7.5 Ma. The ages obtained by Ar/Ar dating of amphibole and biotite megacrysts and by U/Pb dating of the magmatic rim of zircon crystal from the camptonite coincide within the dating error, which indicates that the camptonite dikes formed in the Late Ordovician. These dikes are the oldest-known example of mantle-derived xenoliths in mafic volcanic rocks from an off-craton setting. These are samples of the Upper Ordovician lithospheric mantle.     

Petrogenesis of Late Cretaceous Jiangla’angzong I-Type Granite in Central Lhasa Terrane, Tibet, China: Constraints from Whole-Rock Geochemistry, Zircon U-Pb Geochronology, and Sr-Nd-Pb-Hf Isotopes

The Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA-ICP-MS zircon U-Pb analyses suggest that syenogranite has a weighted mean ~(206) Pb/~(238) U age of 86±1 Ma(mean square weighted deviation=0.37), which is in accordance with the muscovite Ar-Ar age(85±1 Ma) of Cu-Au ore-bearing skarns and the zircon U-Pb age(84±1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu–Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al-bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO_2(65.10–70.91 wt%), K_2O(3.44–5.17 wt%), and total K_2O+Na_2O(7.13–8.15 wt%), and moderate contents of A_(12)O_3(14.14–16.45 wt%) and CaO(2.33–4.11 wt%), with a Reitman index(σ43) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P_2O_5 contents show a negative correlation with SiO_2, whereas Pb contents show a positive correlation with SiO_2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc–alkaline metaluminous I–type granite. It is enriched in light rare earth elements(LREE) and large ion lithofile elements(LILE), and depleted in heavy rare earth elements(HREE) and high field strength elements(HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies(δCe=1.00–1.04). The relatively low initial 87 Sr/86 Sr ratios of 0.7106 to 0.7179, positive εHf(t) values of 1.0 to 4.1, and two-stage Hf model ages(TDM2) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The(~(143) Nd/~(144) Nd)_t values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its εNd(t) values range from-10.17 to-6.10, its(~(206) Pb/~(204) Pb)_t values range from 18.683 to 18.746, its(~(207) Pb/~(204) Pb)_t values range from 15.695 to 15.700, and its(~(208) Pb/~(204) Pb)_t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the postcollision extension of the Qiangtang and Lhasa terranes.