华北克拉通恒山地区晚太古代变质火山岩的地球化学特征及构造意义

对恒山地区出露的变质火山岩开展岩相学、地球化学、锆石U-Pb年代学和Hf同位素研究,旨在确定其形成时代,探讨岩石成因及地球动力学意义。锆石U-Pb定年结果表明,这些变质火山岩喷发于新太古代晚期(2508±20Ma)。地球化学分析结果显示,恒山变质火山岩由变质玄武岩和变质玄武安山岩组成,SiO₂(45.51%～62.67%)、FeO_T (4.43%～15.72%)和MgO (3.75%～8.14%)含量变化幅度大,是幔源岩浆经单斜辉石、角闪石和磁铁矿分离结晶的产物。这些变质火山岩富集轻稀土(LREE)和大离子亲石元素(LILE),亏损重稀土(HREE)和高场强元素(HFSE),具有相对高的Th含量和Th/Yb比值,呈现钙碱性岛弧火山岩的特征。结合不相容元素比值Nb/Yb,Zr/Yb和(Nb/La)_N的特点,推测其来源于俯冲带具流体交代特征的富集地幔源区。结合区域构造背景,推断恒山变质火山岩形成于新太古晚期大陆弧环境。     

Delamination and destruction of the North China Craton

This article presents an overview on recent developments in studies of chemical and physical processes of lithospheric delamination with respect to destruction of the North China Craton. It is emphasized that the pyroxenite source resulting from interaction between eclogite-derived melt and peridotite is a direct consequence of delamination. The pyroxenite source thus formed has unique mineralogical and geochemical features, which characterize Mesozoic basalts of the North China Craton. Melt-peridotite interaction played an important role in refertilization of cratonic lithospheric mantle, leading to density increase, weakening and final destabilization of the North China Craton. The nature of the melt is the key to distinguish mechanisms of destructing this craton.     

Re-Os isotopic constraint to the age of in komatiites in the Neoarchean Guyang greenstone belt, North China Craton

The 2.5 Ga Guyang greenstone belt is a major lithological unit in the northwestern part of the North China Craton. Komatiites have recently been identified to occur within the lower part of the meta-volcanic sedimentary sequence of the greenstone belt. The biggest komatiite near Guyang has been mapped out, which is 500 m long and 50 m wide lenticular block. Re-Os isotopic compositions of 9 samples were analyzed to date the komatiites. The Os contents are from 0.88 to 2.63 ppb, identical with typical komatiites, and slightly lower than the normal mantle. The ¹⁸⁷Os/¹⁸⁸Os ratios are from 0.1115 to 0.1197, which are lower enough to exclude the origin of Phanerozoic magma as widely developed in adjacent areas. The calculated Re depletion model ages (T _RD) are from 1346 to 2454 Ma, among them the oldest age of 2454 Ma gives the minimum evaluation for the formation age of the Guyang komatiite. Therefore, the komatiites are a part of the Guyang greenstone belt, indicating high degree melting of mantle during ～2.5 Ga crustal growth event.     

Formation age and tectonic environment of the Gantaohe Group, North China Craton: Geology, geochemistry, SHRIMP zircon geochronology and Hf-Nd isotopic systematics

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen,North China Craton,and is mainly composed of greenschist-facies metabasalt,meta-sandstone and dolomitic marble.We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group.SHRIMP dating yielded a weighted mean 207Pb/206Pb age of 2087±16 Ma(MSWD=1.3) for magmatic zircons,but there are also abundant ca.2.5 Ga inherited zircon xenocrysts.The magmatic zircons shows a large Hf(t) variation in Hf(t) from 7.17 to +0.45,suggesting an isotopically highly heterogeneous source for the metabasalt.Chemically all samples show no distinct Zr or Hf anomalies,and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram,and their whole-rock Nd(t) values range from 4.0 to 0.8.We suggest that the basalt is formed by partial melting of a depleted mantle source,followed by significant crustal contamination.Field observations,the presence of abundant inherited zircon,as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement.These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean.     

华北克拉通北缘中段中新元古代热构造事件及其演化

近年华北地块北缘陆续发现和厘定出许多中新元古代变质火山-沉积地层,各种类型的深成侵入岩体以及各种样式的构造形迹,但对其性质及构造属性认识不统一。作者在详细研究前人成果基础上,认为华北地块北缘中段中新元古代经历了中元古代早期的伸展裂解、中元古代晚期的挤压汇聚、新元古代-早古生代的伸展裂解及早古生代晚期的挤压汇聚演化过程,伴随挤压汇聚有火山深成侵入岩浆活动和构造变质热事件的年代记录。中元古代晚期挤压汇聚事件表明华北地块北缘发育一条中元古代晚期大陆边缘碰撞造山带,这一碰撞造山事件为华北地块在 Rodinia 超大陆拼合模式提供了最基本的制约条件。     

On the timing and duration of the destruction of the North China Craton

The timing and duration of the destruction of the North China Craton, which is pivotal to understanding the destruction mechanism and its geodynamic controlling factors, still remain controversial. On the basis of the principles of magma genesis and evolution, first we outline magmatic expressions that can be related to cratonic destruction, then use magmatic and basin evolution trends to constrain the timescale of the lithospheric thinning in North China. The main conclusions include： （1） the thinning of the lithosphere beneath the North China Craton might have started, at least locally, since late Carboniferous-late Triassic, attained its climax during the late Jurassic-early Cretaceous, and continued till the end of late Cretaceous-early Cenozoic. The destruction of the North China Craton was a relatively slow, rather than a dramatic process. （2） The weakened lithospheric zones along the margins and interiors of the craton played an important role in cratonic destruction, partly accounting for the heterogeneous pattern of cratonic destruction. （3） The tectonic factors that controlled the destruction of the North China Craton may be multiple. The late Carboniferous southward subduction of the Paleo-Asian plate and the late Triassic collision between North China and South China may have re-activated the craton by influencing the thermal and integral structure of the craton. The Pacific subduction underneath the eastern Asian continent played a determinant role in the cratonic destruction, governing the distribution patterns of post-Mesozoic basins and major tectonic configuration, temporal change of magmatism and formation of the North-South gravity lineament.     

A huge oceanic-type UHP metamorphic belt in southwestern Tianshan, China: Peak metamorphic age and P-T path

Recent progress in the study of the UHP metamorphic belt in southwestern Tianshan, China, is summarized in this paper. This about 80-kin-long and over 10-km-wide UHP belt has been recognized by the discovery of coesite, coesite pseudomorphs and other UHP minerals. It is the largest oceanic-type UHP metamorphic belt reported so far. It has formed due to northward subduc- tion of the Tianshan Paleo-Ocean. U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma. Combined with ages of 233-226 Ma obtained from rims of zircons from retrograded eclogites, a long retrograde metamorphic evolution （〉70 Ma） has been revealed. According to phase equilibria mod- eling, the P-T paths of both coesite-bearing eclogites and garnet-phengite schists are characterized by thermal relaxation, i.e., the metamorphic temperature peak lags behind the pressure peak, indicating that the UHP rocks experienced slow and long heating and decompression during exhumation in the subduction channel. On the basis of the field observation that a small amount of eclogite lenses is wrapped in large volumes of metapelites, and the similar P-T paths of both rock types, we propose that the ex- humation of the UHP eclogites from southwestern Tianshan, China, may have resulted from the exhumation of large volumes of low-density metapelites, which carried the denser eclogites to the Earth＇s surface.     

The age of the base of the paleoproterozoic Hutuo Group in the Wutai Mountains area,North China Craton: SHRIMP zircon U-Pb dating of basaltic andesite

Basalt and basaltic andesite metamorphosed at greenschist facies occurs with conglomerate layers at the bottom of the Paleopro-terozoic Hutuo Group in the Wutai Mountains area, North China Craton. Detailed geological surveying confirms that these volcanic rocks are conformable within the neighboring sedimentary rocks. The SHRIMP results on basaltic andesite are divided into two groups. In one group the ²⁰⁷Pb/²⁰⁶Pb ages are from 2433 to 2558 Ma, which is consistent with the basement crustal age in Fuping and Wutai areas. In the other group, 13 grains yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2140±14 Ma. The latter is interpreted as the eruption age of the basaltic andesite, and gives the age of the base of the Hutuo Group. This result further suggests that the Hutuo Group formed in the middle Paleoproterozoic, not the early Palaeoproterozoic or late Archaean, as thought before, and is related to a 2.2–2.1 Ga rifting event in the Central North China Craton.     

华北克拉通集宁地区孔兹岩系矿物学研究和P—T轨迹探讨

以华北克拉通西部陆块集宁地区出露的孔兹岩系为研究对象,主要研究孔兹岩系的岩石学、矿物学特征,利用电子探针数据分别计算出集宁地区孔兹岩系变质反应峰期前、峰期和峰期后三个不同阶段的温度和压力条件,得出了顺时针的变质反应P-T轨迹图.集宁地区地理位置、岩石学和矿物学特征、P-T轨迹以及华北克拉通基底三分的构造模型(华北克拉通基底可以分为东、西部陆块和中部构造带),反映出陆-陆碰撞的构造环境;此等温降压顺时针变质反应P-T-t轨迹记录了东、西部陆块碰撞拼合,并最终形成华北克拉通统一结晶基底的过程.     

Precipitation of rutile needles in garnet from sillimanite-bearing pelitic granulite from the Khondalite Belt,North China Craton

Pelitic granulite from the Khondalite Belt, North China Craton, is composed of garnet, sillimanite, perthite, biotite and quartz. Oriented rutile needles in gar- net are reported for the first time from the granulite. The rutile needles have been identified by optical microscopy and confirmed by in situ Raman spectroscopy, where they show the characteristic bands at 446-448 and 610 cm-1. Because of their very strong shape preferred orientation in 3-4 easily observable directions, these futile needles are interpreted to have formed by precipitation during exhu- mation and cooling following peak metamorphism. The ternary feldspar geothermometer gives a peak metamorphic temperature of ,- 980 ℃ at 10 kbar. The presence of oriented rutile needles in garnet is suggested to be a valuable indicator of ultrahigh-temperature （UHT） metamorphism in the Khondalite Belt and therefore likely in other UHT terrains.     

Palaeoproterozoic Khondalite Belt in the western North China Craton： New evidence from SHRIMP datin8 and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ui-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (TDM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TDM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite se- ries in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.     

华北北缘大青山小井沟岩体年代学、地球化学和Hf同位素特征及其地质意义

为了加深对华北北缘晚古生代构造背景以及古亚洲洋闭合时限的认识, 对华北北缘大青山地区小井沟花岗岩开展了年代学、地球化学和同位素研究。分别利用LA-MC-ICP-MS, ICP-AES和ICP-MS等方法进行锆石U-Pb测年, 主、微量和稀土元素分析以及锆石Hf同位素测试。结果表明, 小井沟岩体侵位于二叠纪, 侵位年龄为275±1 Ma (MSWD=0.93)。该花岗岩具有高硅(SiO₂=70.72%~72.64%)、高钾(K₂O=4.19%~4.23%)的特点, A/CNK均约为1.1, 属于弱过铝质的高钾钙碱性系列, 稀土元素总量(ΣREE)为87.67~101.51 μg/g, 配分曲线呈右倾型, 具有微弱的Eu负异常, 富集大离子亲石元素(LILE, 如Rb, Ba, K, Sr等), 亏损高场强元素(HFSEs, 如Nb, Ta, Y, Yb, Lu等), 应该是由下地壳部分熔融形成。该花岗岩的锆石具有负的ε_Hf(t)值(-9.56~-5.00), 模式年龄(T_DM2)在1.91~1.61 Ga之间, 表明华北陆块古元古代的地壳岩石应该是其主要物源, 可能有幔源物质的参与。结合整个华北北缘二叠纪花岗岩的岩浆演变特征, 小井沟岩体应形成于由俯冲向碰撞-后碰撞转变的过渡时期, 古亚洲洋可能在晚二叠世之前已闭合。     

Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China： Implications for early crustal evolution of the North China Craton

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6-3.8 Ga granitic rocks. In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The -3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks （granitic rocks） were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before -3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.     

Oxygen isotopic (17O-18O) heterogeneity in Archaean continental nucleus, North China Craton

The oxygen isotopic data for amphibolites from Archaean continental nucleus, North China Craton indicate that ( i ) the evolving lines structured by the data are parallel to the geoselenic evolving line, which means that the parent materials of these continental nucleus originated from solar matter; ( ii ) different intercepts for these evolving lines manifest that oxygen isotopic compositions of different continental nucleus are heterogeneous, which means that these continental nucleuses originated from different parent resources with different original compositions.     

Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain, NW China

The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain. It has been drawn much attention and extensively studied in recent years. In this study, ion microprobe (SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite. Eighteen analyses yield a relatively consistent apparent ²⁰⁶Pb/²³⁸U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma (MSWD=1.06), which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite. Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures. The petrological, geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician, while the ancient Qilian oceanic plate subducted northwards. The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era.     

Discovery of Late Paleozoic retrograded eclogites from the middle part of the northern margin of North China Craton

The northern margin of the North China Craton (NCC), located between the Paleo-Asian Ocean tectonic region on the north and the NCC on the south, is a key region for studying the tectonic evolution of NCC. A Pre-cambrian retrograded eclogite (2500 Ma or 1800 Ma) was reported in Baimashi near Hengshan Mountain in the NCC, which is characterized by the vermicular symplec-tite of diopside and plagioclase with absence of ompha-cite[1,2]. In Hongqiyingzi Group from the middle part of the …     

利用空间自相关法研究华北克拉通中部瑞雷面波频散特征

华北克拉通是我国大陆最古老的克拉通之一,是研究大陆岩石圈形成与演化的一个天然实验室。利用华北地震科学台阵在2007年1月13日记录到的地震微动信号的垂向分量,采用空间自相关法(SPAC法)获得了华北克拉通中部块体7～33 s的瑞雷波相速度频散曲线。总体而言,研究区的频散曲线随周期增加而增大。7～12 s ±的相速度增加梯度较陡,可能反映了该区域地表太古宙至古生代早期的古老基岩露头的影响; 13～16 s ±的相速度反映中地壳的速度变化,中部块体的相速度值随周期增大而逐渐减小,并在16 s ±时达到最小值,这与该地区中地壳发育低速层的地质背景较吻合。