Crustal evolution of the Shiwandashan area in South China: Zircon U-Pb-Hf isotopic records from granulite enclaves in Indo-Sinian granites

U-Pb dating coupled with Hf isotope analyses on zircon from metasedimentary granulite enclaves in the Jiuzhou peraluminous granite from the Shiwandashan area in southeastern Guangxi Province, South China are presented in this paper. The results show that the protoliths of these granulite enclaves were mainly composed of Neo-Mesoproterozoic (564–1061 Ma) clastic materials with a peak age at ~822 Ma. These materials were probably derived from the igneous rocks that were emplaced during the Neoproterozoic breakup of Rodinian Supercontinent. Subordinate sediments include the Paleoproterozoic (1778–2227 Ma) and even the Meso-Paleoarchean materials with the oldest U-Pb age at 3551±8 Ma, indicating the existence of ancient crustal rocks in the area and/or its surrounding regions. Younger grains include the early Mesozoic (234±2 Ma) magmatic zircon populations and the late Permian (253±3 Ma) metamorphic zircon populations. Further zircon Hf isotope analyses reveal that their protoliths were complex, containing both recycled crustal rocks and juvenile materials. Combined zircon U-Pb ages and Hf isotope compositions indicate that at ~253 Ma, the Shiwandashan area experienced an intensive thermal event that resulted in the granulite-facies metamorphism; and that crustal remelting occurred at ~234 Ma to form the S-type granitoids during the uplifting stage. The metasedimentary granulite enclaves are resitites of these granitoids.     

On the genetic classification and tectonic implications of the Early Yanshanian granitoids in the Nanling Range, South China

Early Yanshanian (Jurassic) granitoids are widespread in the Nanling Range of South China, and are associated with numerous non-ferrous and rare metal mineral deposits. These granitoids consist mainly of slightly peraluminous biotite monzogranites and K-feldspar granites that are closely associated in time and space with subordinate amphibole-bearing granites and muscoviteand garnet-bearing granites. In most previous studies, the biotite-bearing granites were classified as crustal transformation-type (corresponding to the S-type) granites which were interpreted to be derived from the regional Paleoproterozoic meta-sedimentary rocks. In this paper, we re-analyze the geochemical characteristics of a number of representative Early Yanshanian Nanling granitoids. There exists a clear negative correlation between SiO2 and P2O5 for the studied granitoids. The Early Yanshanian Nanling granitoid suites (including amphibole-bearing granodiorites→biotite monzogranites→K-feldspar granites→two-mica (muscovite) granites) are metaluminous to slightly peraluminous I-type or fractionated I-type granites. They were derived predominantly from Proterozoic igneous protoliths. The juvenile crust and/or newly mantle-derived materials might also have been involved in some of these granites. In addition, the Iand fractionated I-type granites are closely associated in time and space with minor amount of A-type felsic and mafic volcanic and intrusive rocks as well as alkaline rocks. All these rocks constitute a typical assemblage of anorogenic, intraplate magmatism, suggesting a dominant lithospheric extensional regime for the Nanling Range and neighboring region during the Early Yanshanian period.     

Nd isotopic characteristics of post-Archean sediments from the Eastern Nanling Range： Evidence for crustal evolution

A systematic Sm-Nd isotopic study was carried out for sediments and metasediments of different ages from Mesoproterozoic to early Mesozoic era in southwestern Fujian, Eastern Nanling Range. The results show that Nd model age (tDM) and εNd(t) value of most sediments are closely similar to those of Paleoproterozoic Mayuan Group, indicating that they may mainly be the recycling product of Paleoproterozoic crustal materials. However, the Nd model age significantly decreases with a corresponding increase in the εNd(t) value at Neoproterozoic (ca. 0.8-0.7 Ga) and Late Paleozoic (ca. 0.25 Ga), respectively. This is manifested by prominent vales and apexes on the diagrams of tDM VS. tStr. (stratum age) and εNd(t) VS. tstr. The decrease in tDM and the increase in εNd(t) are explained as a result of the significant incorporation of juvenile crustal materials that originated from depleted mantle due to strong lithospheric extension during both periods. It appears that tectonic magmatism in the Neoproterozoic and the Late Paleozoic is of prominent importance in affecting the geochemical nature of sediments in South China.     

Granitic magmatism related to early Paleozoic continental collision in North Qinling

Zircon U-Pb dating of early Paleozoic granitoids in North Qinling yields three age peaks of ~500, -452 and -420 Ma. They can be temporally correlated with high-pressure to ultrahigh-pressure metamorphism at ca. 500 Ma, retrograde granulite-facies meta- morphisms at ca. 450 Ma and amphibolite-facies metamorphism at ca. 420 Ma, respectively. The first episode of granitic magma- tism is considered to have resulted from continental collision, whereas the second and third episodes of magmatism are attributed to crustal uplifting. Combined with the regional geological setting and new results from high-pressure and ultrahigh-pressure metamorphic rocks, the ca. 500 Ma magmatism is interpreted as the result of partial melting of sedimentary rocks in accretionary wedge between the south Qinling microcontinent and the north Qinling belt including the southern margin of the North China Craton. The ca. 450 Ma intensive magmatism is ascribed to dehydration melting of deeply subducted continental crust at thick- ened conditions in response to slab breakoff, and the final magmatism in ca. 420 Ma is interpreted as the product of partial melt- ing during the tectonic transition from contraction to extension.     

东秦岭—大别地区中生代岩石圈拆沉的岩石学证据评述

岩石圈拆沉是碰撞造山带物质成分调整和构造演化的重要方式之一。构造地质研究、地球物理探测和地球化学分析等都已揭示东秦岭-大别造山带曾在中生代发生岩石圈拆沉,但有关中生代变质作用和岩浆活动与岩石圈拆沉的内在联系研究却较为薄弱。通过全面评述该区变质岩研究成果,作者认为：超高压变质岩的形成和剥露经历了240～200Ma的板片冷俯冲冷折返和196～163Ma的岩石圈拆沉热折返;热折返伴随了广泛而强烈的区域变质作用和中酸性岩浆活动,指示板片断离拆沉的发生;超高压变质岩p-T-t轨迹由两部分组成,即反映板片冷俯冲冷折返过程的发夹状曲线和指示板片断离拆沉热折返过程的新月形曲线。通过对花岗岩类同位素年龄统计和前人研究成果的评述,初步确定在200～100Ma之间发育大量花岗岩类,并集中在150～100Ma为主(即侏罗纪—白垩纪之交),高峰时间为130Ma左右;花岗岩类大量发育指示了岩石圈拆沉的存在,且滞后于根据变质岩研究所揭示的拆沉时间。羌塘地体、拉萨地体和西太平洋古陆在侏罗纪与欧亚大陆拼贴碰撞的远距离效应使东秦岭—大别造山带长期处于挤压环境,伸展作用被抑制;白垩纪的碰撞晚期伸展和西太平洋沟弧盆体系的远距离效应使东秦岭—大别地区的外部挤压消失,导致造山带岩石圈迅速强烈拆沉伸展和减压增温熔融,从而形成大规模早白垩世花岗岩类和中酸性火山岩。总之,东秦岭—大别地区岩石圈拆沉所导致的岩浆活动主要发生在J₃—K₁的挤压伸展转变期。     

Sm-Nd age dating of high-pressure granulites and amphibolite from Sanggan area, North China craton

The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856 ± 26) Ma for HP amphibolite. The Sm-Nd whole rock isochron of HP granulites give out an age of (1 870±150) Ma with Nd depleted mantle model age of (2 402—2 482) Ma. Considering the Nd isotope homogenization during the peak metamorphism of the HP granulite, Sm-Nd closure temperature and the retention of Nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we proposed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assembly of the North China craton.     

新疆南天山榆树沟层状杂岩体中的副麻粒岩——岩石学特征及其地球动力学意义

在前人定为“层状堆积杂岩体的辉长岩单元”中发现了一套副变质中－基性麻粒岩。它与正变质的基性麻粒岩成互层产出，二者又与层状变质橄榄岩共同组成了一个并非原生火成堆积岩的层状杂岩体。介绍了这套副变质麻粒岩的出露“层”序、主要岩石类型、原岩建造。认为其原岩是一套主要来源于洋盆内基性火山碎屑的深水沉积岩，其中碎屑锆石Ｕ－Ｐｂ年龄为４２１Ｍａ。这套岩石曾于中石炭世俯冲至中天山－哈萨克斯坦板块之下，同时遭受到麻粒岩相区域变质改造（３１５Ｍａ）；其后，又以高温韧性剪切方式，快速逆冲至南天山早古生代非岩浆型被动陆缘带中。     

Late Paleozoic magmatism in South China:Oceanic subduction or intracontinental orogeny?

A gneissic granite with an U-Pb age of 313±4 Ma was found in northeastern Fujian Province,South China.It is an S-type granite characterized by high K2O,Al2O3 and low SiO2,Na2O contents with high A/CNK ratio of 1.22 for the whole rock.Zircons with stubby morphology from the gneissic granite yield 206 Pb/238 U ages ranging from 326 to 301 Ma with a weighted average age of 313±4 Ma,and negative εHf(t) values from -8.35 to -1.74 with Hf model ages (TCDM) of 1.43 to 1.84 Ga.This S-type granite probably originated from late Paleoproterozoic crust in intracontinental orogeny.Integrated with previous results on paleogeographic reconstruction of South China,the nature of Paleozoic basins,Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks,we suggest the occurrence of a late Paleozoic orogeny in the eastern Cathaysia Block,South China.This orogenic cycle includes Late Carboniferous (340-310 Ma) orogeny (compression) episode and Early Permian (287-270 Ma) post-orogenic or intraplate extension episode.Therefore,the late Paleozoic magmatism in the southeastern South China probably occurred during the intraplate orogeny rather than the arc-related process.     

Ca. 2.5 Ga TTG rocks in the western Alxa Block and their implications

The Alxa Block is considered part of the North China Craton, but the unambiguous Archean basement has not been reported. In this study, we present the first evidence of the Neoarchean rocks in the Beidashan area of the western Alxa Block. The petrographic and geochemical data show that these rocks are granodiorite with TTG (tonalite-trondhjemite-granodiorite) characteristics. Zircon U-Pb dating gave an age of 2522±30 Ma for the magmatic core and 2496±11 Ma for the metamorphic recrystallized rim. The near-identical age between the Latest Neoarchean magmatism and the high-grade metamorphism shows that these features were related to the same Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event. The age-corrected Hf (t) value is mainly between 0.4 and 4.9. The two-stage zircon Hf model age ranges from 2.7 to 3.0 Ga, suggesting that the Mesoarchean- Neoarchean (2.7-3.0 Ga) juvenile crust was reworked at the end of the Neoarchean in the western Alxa Block. These data suggest that the western Alxa Block experienced a Mesoarchean-Neoarchean crust growth and Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event similar to the North China Craton.     

Zircon SHRIMP U-Pb dating for gabbro at Chaotiehe in the Haicheng area, eastern Liaoning

Chaotiehe gabbroic intrusion in the eastern part of Liaoning Province was dated by the zircon SHRIMP U-Pb technique. The results gave an emplacement age of 126±4 Ma, indicating that the intrusion thus formed during the lithospheric thinning of the North China Craton (NCC) in the Early Cretaceous rather than in a rifting setting during the Paleoproterozoic as previously thought. The gabbroic intrusion contains abundant old zircons with Paleoproterozoic (2.10–2.46 Ga and ca. 1.87 Ga) and Neoproterozoic (747–969 Ma) ages. The Paleoproterozoic zircons were probably derived from NCC itself, whereas the Neoproterozoic ones were likely from materials of the Yangtze Craton that had previously been subducted beneath NCC. These geochronological dates are of important implications for understanding the Mesozoic crustal evolution of NCC.     

内蒙古中部宝力高庙组长英质火山岩U-Pb-Hf同位素特征及其地质意义

锆石SHRIMP U-Pb定年结果显示: 内蒙古中部白音乌拉地区原宝力高庙组的流纹岩形成时代为300.0±2.9 Ma, 属晚石炭世; 青格勒宝拉格地区原宝力高庙组的凝灰岩结晶年龄为159.6±1.4 Ma, 并获得 3颗捕获锆石的年龄分别为291.8±3.4, 304.0±3.3和734.7±9.2 Ma, 应属于晚侏罗世满克头鄂博组。锆石LA-MC-ICP-MS Hf同位素分析显示: 流纹岩锆石ε_Hf(t) 值为+10.5~+12.9, T_DM^C值为493~645 Ma; 凝灰岩岩浆锆石ε_Hf(t)值为+10.1~+13.1, T_DM^C值为369~563 Ma。研究结果表明, 流纹岩源于晚古生代新生地壳的重熔并混入少量老地壳物质, 凝灰岩源于晚古生代地壳的熔融。Hf同位素特征显示晚古生代流纹岩和中生代凝灰岩源于相似的源区, 揭示了晚古生代的一次重要的增生事件, 并且在约160 Ma时期发生过地壳的再造。结合前人的研究成果表明, 兴蒙造山带在约300 Ma时处于古亚洲洋演化过程中岛弧向碰撞后伸展环境的转换时期, 在约160 Ma受到蒙古?鄂霍茨克构造域的影响。     

Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes,Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts.The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes(NHGD),and contains three types of amphibolites with distinct mineral assemblage,elemental and radiogenic isotope geochemistry,as well as various types of gneisses.SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.6±1.8 Ma,respectively,which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite(43.5±1.3 Ma).Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ～55 Ma,and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma.In addition,partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma,which led to the formation of high Sr/Y ratios two-mica granites.The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere.In large collisional belts,fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions,significantly different from those formed by decompressional melting during rapid exhumation.     

冀北赤城红旗营子杂岩的地质特征及意义

运用野外地质填图、岩石地球化学分析以及同位素年龄等方法探讨冀北赤城红旗营子杂岩的岩石组合、构造属性、变质演化以及时代归属等特征,为重新划分冀北赤城的红旗营子群提供依据。结果显示红旗营子杂岩由云州表壳岩及其内部的退变榴辉岩、蛇纹岩(蛇纹石化方辉橄榄岩)和蛇纹石化(橄榄)大理岩等透镜状构造岩块组成,它们一起经历了晚古生代(321~326 Ma B.P.)区域角闪岩相变质事件。其中,云州表壳岩的岩性主要为石榴黑云斜长片麻岩,原岩是形成于大陆岛弧环境的泥质(含泥质)砂岩,沉积作用发生在晚古生代(340~369Ma B.P.),属于晚古生代沉积-变质地层;石榴黑云斜长片麻岩之中的退变榴辉岩、蛇纹岩(蛇纹石化方辉橄榄岩)和蛇纹石化(橄榄)大理岩等构造岩块应属"外来"的洋壳残片,成因上可能与古亚洲洋的洋壳向南俯冲消减有关。赤城红旗营子杂岩的岩石组合、构造属性、变质演化以及同位素地质年代学等特征明显不同于冀北其他地区的红旗营子群,冀北红旗营子群需要重新划分。     

Zircon U-Pb dating confirms existence of a Caledonian scheelite-bearing aplitic vein in the Penggongmiao granite batholith, South Hunan

Granite at Penggongmiao is a large batholith in the Nanling Range, with an outcrop area of over 900 km 2 . There are many scheelite-quartz veins around the granite. LA-ICP-MS U-Pb dating was carried out for zircons from the granite. The middlecoarse-grained biotite granite has U-Pb ages of 435 to 436 Ma. Ages of 426.5±2.5 Ma were obtained for aplitic dyke cross-cutting the granite. The scheelite of magmatic origin in the aplite dyke was identified from petrographic investigation. This demonstrates that W-bearing granites of Early Paleozoic (corresponding to the Caledonian orogensis in the traditional sense) occur in the Nanling Range. This finding has important implications for the ore-forming potential of Paleozoic granites and on the extent of Mesozoic mineralization. Thus it merits performing an intensive study of Paleozoic granites in South China. The occurrences of aplite or microgranite may be an indicative of the Caledonian tungsten granites and associated W mineralization.     

A ∼2.5 Ga magmatic event at the northern margin of the Yangtze craton: Evidence from U-Pb dating and Hf isotope analysis of zircons from the Douling Complex in the South Qinling orogen

The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt. Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating, which yielded protolith emplacement ages of 2469 ± 22 Ma, 2479 ± 12 Ma, 2497 ± 21 Ma, 2501 ± 17 Ma and 2509 ± 14 Ma, respectively. An important peak age of ～2.48 Ga was also obtained for a metasedimentary rock in the same region. These discoveries suggest the occurrence of magmatic activity of 2.51–2.47 Ga at the northern margin of the Yangtze craton. The age-corrected ? _Hf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between ?5.5 and +0.3, and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga. Considering two important periods of ～3.3–3.2 Ga and ～2.95–2.90 Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo- to Mesoarchean crust at the northern margin of the Yangtze craton at ～2.5 Ga. In addition, metamorphic ages of 837 ± 8 Ma and 818 ± 10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock, indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic, although its geological meaning remains ambiguous.     

The Nd-Sr isotope study of Mesozoic granitoids in Jiangxi Province

Nd-Sr isotopic compositions of 12 Mesozoic granitoids from Jiangxi Province have been reported. They show low-εNd( - 13.8--8.3), high-/Sr(0.71069-0.73981) and old tDM (2087-1635 Ma). Comparison between the Sm-Nd isotopic compositions of the granitoids and those of the basement metamorphic rocks suggests that these granitoids were mainly derived from the metasedimentary rocks. There is an east-west trend Mesozoic granitoid zone with low-tDM from S. Jiangxi to S. Fujian. The origin of these granites was preliminarily discussed.     

Zircon isotopic ages from magnetite quartzites of the Jianping metamorphic complex, western Liaoning Province

Using the zircon Pb evaporation method and high precise ion microprobe analyses, zircon isotopic ages in supracrustal magnetite quartzites from Jianping metamorphic complex have been obtained. The results show that the deposition of the supracrustal rocks occurred around 2 552–2 520 Ma and the peak of granulite facies metamorphism reached at about 2 487 Ma; about (1 806±16) Ma, i.e. Mid-Proterozoic, the other possible metamorphic event with relatively low intensity happened in the study area. It is speculated that a likely tectonic setting where the Jianping metamorphic complex formed is an active continental margin that is involved in continental collision and crustal thickening shortly after its formation.     

Zircon chronology and REE geochemistry of granulite xenolith at Hannuoba

The lower crustal xenolith of mafic two-pyroxene granulite (the majority) and hypersthene granulite in the Cenozoic basalt at Hannuoba have the characteristics of igneous blastic structure and granulite facies metamorphic recrystallization. Study on the zircon chronology and REE geochemistry of granulite xenolith shows that the underplating of basic magma into the lower crust during late Mesozoic led to the formation of mafic accumulate, which further through metamorphism of granulite facies formed the high-density and high-velocity crustal bottom layer at the lower crust. This suggests that the underplating of mantle magma is the important way for the vertical overgrowth of continental crust since the Phanerozoic and provides new evidence for crust-mantle interaction.     

Multi-stage evolution of gneiss from North Dabie： evidence from zircon U-Pb chronology

TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultrahigh pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this region at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746~.31 Ma, consistent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concentrations give concordant ages of 212~21 and 120~11 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present resuits suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.