Paleoproterozoic crustal evolution of the Hengshan–Wutai–Fuping region, North China Craton

An arguable point regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton（NCC）is whether the tectonic setting in the central belt during the mid-Paleoproterozoic（2.35-2.0 Ga）was dominated by an extensional regime or an oceanic subduction-arc regime.A review of the midPaleoproterozoic magmatism and sedimentation for the Hengshan-Wutai-Fuping region suggests that a back-arc extension regime was dominant in this region.This conclusion is consistent with the observation that the 2.35-2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks mainly comprise highly-fractioned calc-alkaline to alkaline（or A-type）granites,and that this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group.Although the final amalgamation of the NCC was believed to occur at ～1.85 Ga,recent zircon U-Pb age dating for mica schist in the Wutai Group suggests a collisional event may have occurred at ～1.95 Ga.The metamorphic ages of ～1.85 Ga,obtained mostly from the high-grade rocks using the zircon U-Pb approach,most probably indicate uplifting and cooling of these high-grade terranes.This is because（i）phase modeling suggests that newly-grown zircon grains in highgrade rocks with a melt phase cannot date the age of peak pressure and temperature stages,but the age of melt crystallization in cooling stages;（ii）the metamorphic P-T paths with isobaric cooling under 6-7 kb for the Hengshan and Fuping granulites suggest their prolonged stay in the middle-lower crust;and（iii）the obtained metamorphic age data show a continuous distribution from 1.95 to 1.80 Ga.Thus,an alternative tectonic scenario for the Hengshan-Wutai-Fuping region involves：（i）formation of a proto-NCC at ～2.5 Ga;（ii）back-arc extension during 2.35-2.0 Ga resulting in bimodal magmatism and sedimentation in rifting basins on an Archean basement;?     

华北克拉通新太古代早期构造热事件的响应:来自左权地区ca.2.7Ga TTG片麻岩的证据

新太古代早期是全球地质历史上一个重要的地壳生长时期,世界众多克拉通中广泛存在2. 7Ga左右的岩浆年龄记录。华北克拉通最主要的岩浆事件发生在新太古代晚期,这与世界其他克拉通广泛存在～2. 7Ga的构造热事件明显不同。但全岩Nd和锆石Hf同位素研究表明,华北克拉通～2. 5Ga的岩石主体来自于中太古代晚期-新太古代早期大陆物质的重熔或再造。因此,厘定～2. 7Ga地质事件在华北克拉通的空间分布对深入理解新太古代地壳形成与演化具有重要科学意义。华北克拉通已识别出的～2. 7Ga的花岗质岩石主要分布在胶东、鲁西、武川、赞皇和太华等少数杂岩区,中部带的恒山、阜平和中条杂岩中亦有零星出露。左权变质杂岩位于中部带中南段,赞皇杂岩西南,初步地球化学和锆石年代学研究表明,该地区有多种岩石类型记录了～2. 7Ga的年龄信息,包括副片麻岩、长英质浅色体、磁铁矿角闪片麻岩和TTG片麻岩。其中,TTG片麻岩的原岩为英云闪长岩,锆石发育明显的核边结构,核部具有清晰的岩浆环带,两个不同LA-ICP-MS实验室获得的不一致线上交点年龄分别为2727±14Ma和2731±12Ma,代表了TTG岩浆岩的结晶年龄。同时,左权变质岩石中较好地保存了新太古代晚期的岩浆和变质年龄记录,推测其所代表的构造热事件与华北克拉通～2. 5Ga所经历的大规模幔源岩浆的底侵作用有关。     

Neoarchean (2.5-2.8 Ga) crustal growth of the North China Craton revealed by zircon Hf isotope: A synthesis

The crustal growth of the North China Craton(NCC) during the Neoarchean time(2.5—2.8 Ga) is a hotly controversial topic,with some proposing thai the main crustal growth occurred in the late Neoarchean (2.5—2.6 Ga),in agreement with the time of the magmatism,whereas others suggest that the main crustal accretion took place during early Neoarchean time(2.7—2.8 Ga),consistent with the time of crustalformation of other cratons in the world.Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC.In this contribution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochronology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC.The results suggest that both 2.7—2.8 Ga and 2.5—2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested.The Eastern block is characterized by the main 2.7—2.8 Ga crustal growth with local new crustal-formation at 2.5—2.6 Ga,and the Yinshan block is characterized by～2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group.Detrital zircon data of the Khondalite Belt indicate that the main crustal growth period of the Western block is Paleoproterozoic involving some～2.6 Ga and minor Early- to Middle-Archean crustal components,and the crustal accretion in the Trans-North China Orogen(TNCO) has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy.Zircon Hf isotope compositions,coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO,and the evolution and tectonic division of the NCC is more complex than previously proposed,probably involving multi-stage crustal growth and subduction processes.However, there is no doubt that 2.7—2.8 Ga magmatism and crustal-formation are more widely distributed than previously considered,which is further supported by the data of zircons from Precambrian lower crustal rocks, overlying sedimentary cover,modern river sediments and Late Neoarchean syenogranites.     

华北克拉通阜平杂岩新太古代-古元古代多期地质事件及其构造性质

本文收集了阜平杂岩新太古代早期-古元古代晚期基底岩石的岩石地球化学、锆石U-Pb年代学、同位素地球化学和变质作用资料,以期对阜平杂岩早寒武纪演化历史进行初步总结.阜平新太古代早期~2.7 Ga片麻岩原岩为英云闪长岩,具有TTG质片麻岩的地球化学特征;其锆石ε_Hf（t）具有较高的正值（+5.44~+7.50）,单阶段模式年龄为2 745~2 824 Ma,表明新太古代早期为阜平杂岩强烈的地壳生长时期.新太古代晚期片麻岩的时代集中于2 543~2 484 Ma,主要岩石类型为英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）,同时区域内还存在二长花岗岩.TTG质片麻岩的ε_Nd（t）值为-1.64~+0.96,单阶段模式年龄为2.76~3.04 Ga;锆石ε_Hf（t）值为-1.9~+7.91,单阶段和两阶段模式年龄分别为2 546~2 888 Ma和2 548~3 119 Ma.这些TTG岩石主要为新太古代早期岩石的部分熔融,并有少量中太古代地壳物质参与.近于同期具有岛弧性质的辉长岩和变质作用暗示阜平杂岩新太古代晚期可能经历了俯冲和弧-陆或陆-陆碰撞.古元古代中期（2.1~2.0 Ga）阜平地区花岗质岩浆活动强烈.该阶段花岗岩具有A型花岗岩特征,与同期的火山-沉积岩系形成于华北克拉通古元古代中期伸展的陆内裂谷环境中.阜平杂岩中基性麻粒岩包体记录的变质作用时代为1.89~1.85 Ga,并具有顺时针演化的P-T轨迹,其代表了古元古代晚期裂谷闭合的陆内造山过程,表明华北最终克拉通化.      

The generation and evolution of the Archean continental crust: The granitoid story in southeastern Brazil

The Archean Eon was a time of geodynamic changes. Direct evidence of these transitions come from igneous/metaigneous rocks, which dominate cratonic segments worldwide. New data for granitoids from an Archean basement inlier related to the Southern São Francisco Craton (SSFC), are integrated with geochronological, isotopic and geochemical data on Archean granitoids from the SSFC. The rocks are divided into three main geochemical groups with different ages: (1) TTG (3.02–2.77 Ga); (2) medium- to high-K granitoids (2.85–2.72 Ga); and (3) A-type granites (2.7–2.6 Ga). The juvenile to chondritic (Hf-Nd isotopes) TTG were divided into two sub-groups, TTG 1 (low-HREE) and 2 (high-HREE), derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts. The compositional diversity within the TTG is attributed to different pressures during partial melting, supported by a positive correlation of Dy/Yb and Sr/Zr, and batch melting calculations. The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs, indicating the presence of subduction processes concomitant with TTG emplacement. From ～2.85 Ga to 2.70 Ga, the dominant rocks were K-rich granitoids. These are modeled as crustal melts of TTG, during regional metamorphism indicative of crustal thickening. Their compositional diversity is linked to: (i) differences in source composition; (ii) distinct melt fractions during partial melting; and (iii) different residual mineralogies reflecting varying P–T conditions. Post-collisional (～2.7–2.6 Ga) A-type granites reflect rifting in that they were closely followed by extension-related dyke swarms, and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle. The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening, regional metamorphism and crustal melting, and post-collisional extension, similar to that seen in younger Wilson Cycles. It is compelling evidence that plate tectonics was active in this segment of Brazil from ～3 Ga.     

华北克拉通太古宙TTG岩石的时空分布、组成特征及形成演化:综述

华北克拉通具有3.8Ga以上的演化历史,TTG是其地质记录的最重要载体。华北克拉通太古宙(特别是中太古代以前)地质演化在很大程度上与TTG岩石密切相关。在华北克拉通,始太古代(3.6~4.0Ga)TTG岩石仅在鞍本地区被发现,但冀东地区已在多种变质碎屑沉积岩中发现大量3.6~3.88Ga碎屑锆石;古太古代(3.2~3.6Ga)TTG岩石在鞍本、冀东、信阳地区被识别出来;中太古代(2.8~3.2Ga)TTG岩石在鞍本、冀东、胶东、鲁山等地存在;可把新太古代(2.5~2.8Ga)进一步划分为早期和晚期两个阶段:新太古代早期(2.6~2.8Ga)TTG岩石已在10余个地区被发现,新太古代晚期(2.5~2.6Ga)TTG岩石几乎在每一个太古宙基底岩石出露区都存在。野外地质、锆石定年、元素地球化学和Nd-Hf同位素组成研究表明,中太古代以前TTG岩石局部存在,主要分布于Wan et al.(2015)所划分的三个古陆块中;新太古代TTG岩石广泛分布,是陆壳增生最重要时期岩浆作用的产物。TTG岩石类型随时代变化,3.1~3.8Ga和2.7~2.9Ga TTG岩石分别主要为奥长花岗岩和英云闪长岩;2.5~2.6Ga期间花岗闪长岩大规模出现,并有壳源花岗岩广泛分布,表明这时陆壳已有相当的成熟度。奥长花岗岩轻重稀土分异程度从弱到强的时间出现在~3.3Ga;2.5~3.3Ga的TTG岩石轻重稀土分异程度变化很大,表明其形成条件存在很大差异。TTG岩石主要为新生地壳,但也有相当部分为壳内再循环产物或形成过程中受到陆壳物质影响。华北克拉通中太古代以前的主要构造机制是板底垫托或地幔翻转作用,新太古代晚期板块构造体制可能已起作用。     

http://www.sciencedirect.com/science/article/pii/S1674987114000309

In the early 1980s, evidence that crustal rocks had reached temperatures 〉1000 ℃ at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic-metasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological ma- terials and increases in computing power opened up new fields of investigation. The robust quantifi- cation of pressure （P）, temperature （T） and time （t） that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen sig- nificant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics.     

Thickness and geothermal gradient of Neoarchean continental crust: Inference from the southeastern North China Craton

The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granodiorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at ~2.70 Ga and ~2.55 Ga which were sourced from partial melting of juvenile crustal components. The ~2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)_N = 39), whereas the ~2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)_N = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P–T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44–51 km with geothermal gradients of 17 to 20 °C/km for the ~2.70 Ga TTG gneisses whereas the ~2.55 Ga TTG gneisses show lesser crustal thicknesses of 35–43 km with geothermal gradients of 19 to 26 °C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (~41 km, ~20 °C/km) of the modern average continental crust, indicating that a modern-style plate tectonic regime may have played an important role in the formation and evolution of the Neoarchean continental crust in the NCC.     

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift,southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.     

http://www.sciencedirect.com/science/article/pii/S1674987114000322

The Central Hebei Basin （CHB） is one of the largest sedimentary basins in the North China Craton, extending in a northeast-southwest direction with an area of 〉350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi- mentary rocks recovered from drill holes that penetrated into the basement of the CHB, Two samples of gneissic granodiorite （XG1-1） and gneissic quartz diorite 048-1） have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41-2.51 and ～2.5 Ga, respec- tively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher REE contents and lower Eu/Eu＊ and （La/Yb）n values. Two metasedimentary samples （MG1, H5） mainly contain ～2,5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf （2.5 Ga） values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2,9 Ga, respectively, Therefore, ～2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.     

Neoarchean granite-greenstone belts and related ore mineralization in the North China Craton:An overview

Tectonic processes involving amalgamations of microblocks along zones of ocean closure represented by granite-greenstone belts(GGB) were fundamental in building the Earth's early continents. The crustal growth and cratonization of the North China Craton(NCC) are correlated to the amalgamation of microblocks welded by 2.75-2.6 Ga and ~2.5 Ga GGBs. The lithological assemblages in the GGBs are broadly represented by volcano-sedimentary sequences, subduction-collision related granitoids and bimodal volcanic rocks(basalt and dacite) interlayered with minor komatiites and calc-alkalic volcanic rocks(basalt, andesite and felsic rock). The geochemical features of meta-basalts in the major GGBs of the NCC display affinity with N-MORB, E-MORB, OIB and calc-alkaline basalt, suggesting that the microblocks were separated by oceanic realm. The granitoid rocks display arc signature with enrichment of LILE(K,Rb, Sr, Ba) and LREE, and depletion of HFSE(Nb, Ta, Th, U, Ti) and HREE, and fall in the VAG field. The major mineralization includes Neoarchean BIF-type iron and VMS-type Cu-Zb deposits and these,together with the associated supracrustal rocks possibly formed in back-arc basins or arc-related oceanic slab subduction setting with or without input from mantle plumes. The 2.75-2.60 Ga TTG rocks,komatiites, meta-basalts and metasedimentary rocks in the Yanlingguan GGB are correlated to the upwelling mantle plume with eruption close to the continental margin within an ocean basin. The volcanosedimentary rocks and granitoid rocks in the late Neoarchean GGBs display formation ages of 2.60-2.48 Ga, followed by metamorphism at 2.52-2.47 Ga, corresponding to a typical modern-style subduction-collision system operating at the dawn of Proterozoic. The late Neoarchean komatiite(Dongwufenzi GGB), sanukitoid(Dongwufenzi GGB and Western Shandong GGB), BIF(Zunhua GGB) and VMS deposit(Hongtoushan-Qingyuan-Helong GGB) have closer connection to a combined process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75-2.6 Ga GGB and ~2.5 Ga GGBs, the former involve plume-arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arcplume interaction.     

鲁西七星台地区新太古代基性岩浆作用:变质辉长岩的时代和组成

通过研究鲁西七星台地区新太古代变质辉长岩及相关岩石的锆石SHRIMP U-Pb年龄和地球化学组成.它们侵入新太古代表壳岩和TTG岩体.根据12个样品SHRIMP U-Pb锆石定年,可把形成时代划分为3期:2 662~>2 711 Ma、2 608~2 618 Ma和2 508~2 526 Ma.其他样品（进行地球化学分析）的形成时代是根据岩石空间分布、野外特征及与定年样品所代表岩石的关系来确定的.>2.65 Ga变质辉长岩既有来自于富集地幔源区也有来自亏损地幔源区.~2.6 Ga变质辉长岩具平坦型稀土模式,大离子亲石元素富集,Nb、Ta、P亏损,来自亏损地幔源区,可能遭受陆壳物质影响,~2.6 Ga变质辉石岩显示中稀土富集,与单斜辉石堆晶作用有关.~2.5 Ga变质辉长岩存在平坦型-轻微亏损轻稀土和轻稀土富集型两种类型稀土模式.与~2.5 Ga变质辉长岩相比,~2.5 Ga变质辉长闪长岩稀土含量更高,轻重稀土分异程度更高,大离子亲石元素更为富集,Nb、Ta亏损更为明显,是~2.5 Ga辉长质岩浆进一步结晶分异产物.结合前人研究,可得出如下结论.（1）七星台地区存在>2.65 Ga、~2.6 Ga和~2.5 Ga 3期变质辉长岩,其中~2.6 Ga变质辉长岩规模最大;（2）不同时代变质辉长岩地球化学组成特征不同,反映了源区组成和形成过程的复杂性;（3）鲁西地区在新太古代早期（>2.7~2.6 Ga）存在长期连续的基性岩浆作用,可能与地幔岩浆板底垫托有关;（4）在七星台地区首次发现~2.5 Ga辉长岩-辉长闪长岩,为鲁西地区A带广泛存在的~2.5 Ga深熔作用提供了热源来自地幔的直接证据.      

Lithosphere development in the Slave craton: a linked crustal and mantle perspective

The late tectonic evolution of the Slave craton involves extensive magmatism, deformation, and high temperature-low pressure (HT-LP) metamorphism. We argue that the nature of these tectonic events is difficult to reconcile with early, pre-2.7 Ga development and preservation of a thick tectosphere, and suggest that crust–mantle coupling and stabilization occurred only late in the orogenic development of the craton. The extent and repetitiveness of the tectonic reworking documented within the Mesoarchean basement complex of the western Slave, together with the development of large-volume, extensional mafic magmatism at 2.7 Ga within the basement complex argue against preservation of a widespread, thick, cool Mesoarchean tectosphere beneath the western Slave craton prior to Neoarchean tectonism. Broad-scale geological and geophysical features of the Slave craton, including orientation of an early F1 fold belt, distribution of ca. 2.63–2.62 Ga plutonic rocks, and the distribution of geochemical, petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an NE–SW structural grain to the craton. These trends are oblique to the earlier, ca. 2.7 Ga north–south trending boundary between Mesoarchean and Neoarchean crustal domains, and are interpreted to represent a younger structural feature imposed during northwest or southeast-vergent tectonism at ca. 2.64–2.61 Ga. Extensive plutonism, in part mantle-derived, crustal melting and associated HT-LP metamorphism argue for widespread mantle heat input to the crust, a feature most consistent with thin (<100 km) lithosphere at that time. We propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the D1 structural grain, producing the early 2.63–2.62 Ga arc-like plutonic rocks. Subsequent collision (external to the present craton boundaries) possibly accompanied by partial delamination of some of the underthrust lithosphere, produced widespread deformation (D2) and granite plutonism throughout the province at 2.6–2.58 Ga. An implication of this model is that diamond formation in the Slave should be Neoarchean in age.     

华北中部造山带左权变质杂岩ca. 2.5Ga和ca. 1.9Ga变质年龄记录及其地质意义

华北克拉通早前寒武纪基底由多个微陆块组成,其主期拼合时代是困扰地质学家的一个重大课题。华北中部造山带作为新太古代-古元古代一条重要的碰撞型造山带已得到广泛共识。大量高精度年代学资料显示,华北中部造山带至少记录了大约1. 85Ga、1. 95Ga和2. 5Ga的三组变质年龄信息。但目前,2. 5Ga左右的变质年龄仅在华北中部造山带中部的阜平和赞皇等少数杂岩区有零星报道。左权变质杂岩位于华北中部造山带中南段东侧、阜平杂岩以南,向东紧邻赞皇杂岩,是洞悉早前寒武纪时期华北克拉通基底形成及演化过程的一个重要窗口。杂岩区出露多种早寒武纪变质岩石,其中长英质黑云斜长片麻岩分布范围最广,局部暗色矿物富集;斜长角闪岩或角闪片麻岩多以透镜状或似层状方式产出于长英质片麻岩中;杂岩区南部发育多个小型磁铁矿矿床。本文对研究区多种类型岩石样品进行了细致的岩相学、锆石U-Pb年代学和锆石稀土元素研究,发现多数样品中发育变质成因锆石,记录至少两组变质年龄信息。第一组年龄(1903Ma)仅被个别角闪片麻岩样品保存,反映了杂岩区所经历的变质峰期或近峰期阶段的时代;第二组年龄(2483～2507Ma)分布广泛,所代表的变质事件发生在区域片麻理之前,与华北克拉通约25亿年时发生的大规模构造热事件有关。     

华北克拉通南缘太华杂岩组成及演化

太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值。本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期。而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期。2)中太古代-新太古代(2.91～2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期。提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85～2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在～2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表。3)太华杂岩在所谓的全球陆壳生长"沉寂期(2.45～2.20Ga)"岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关。4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97～1.80Ga),变质作用的时限跨度可达150Myr。5)提出华北克拉通南部曾经为一个统一基底,称之为"南部太古宙地块",此地块形成时间为新太古代末期(～2.5Ga)。该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(～2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30～1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97～1.80Ga)陆块最终拼合。     

Pe trology and SHRIMP zircon geochronology of granulites from Vesleknausen,Lützow-Holm Complex,East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the L＆#252;tzow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss （charnockite and minor biotite gneiss）, mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750e850 ？C, approximately 150 ？C lower than those estimated for met-asedimentary gneisses from Rundv？gshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ？ 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundv？gshetta-Vesleknausen-Strandnibba region in southwestern L＆#252;tzow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc mag-matism. The Neoarchean magmatic terrane was tectonically mingled with other fragments （such as metasedimentary units in northern L＆#252;tzow-Holm Bay） by subduction/collision events during the as-sembly of Gondwana supercontinent, and subsequently underwent w850 ？C granulite-facies meta-morphosed during Neoproterozoic to Cambrian final collisional event.     

Eastern Ancient Terrane of the North China Craton

Based on the spatial distribution of ancient rocks and zircons, three ancient terranes older than ca. 2.6 Ga have recently been identified in the North China Craton, namely the Eastern, Southern, and Central Ancient Terranes. The Eastern Ancient Terrane is the best studied and understood of the three ancient terranes. It has a long geological history back to ca. 3.8 Ga ago and includes the areas of Anshan-Benxi, eastern Hebei, eastern Shandong and western Shandong. In Anshan-Benxi, several different types of 3.8 Ga rocks were discovered together with 3.1-3.7 Ga rocks, whereas 2.9-3.0 Ga K-rich granites and 2.5 Ga syenogranite occur on larger scales. In eastern Hebei, 3.0-3.4 Ga rocks and older detrital and xenocrystic zircons were identified. In eastern Shandong, there are a large volumes of 2.7 Ga and 2.9 Ga rocks. In western Shandong, early Neoarchean(2.6-2.7 Ga) intrusive and supracrustal rocks are widely distributed. Whole-rock Nd and zircon Hf isotope data suggest that both mantle additions and crustal recycling played important roles within the Eastern Ancient Terrane during almost every tectono-magmatic event. Most BIFs in the North China Craton are late Neoarchean in age and are distributed on continental crust along the western margin of the Eastern Ancient Terrane, probably suggesting that a stable environment was one of the key factors for the formation of large-scale BIFs.     

Late Neoarchean synchronous TTG gneisses and potassic granitoids in southwestern Liaoning Province,North China Craton: Zircon U-Pb-Hf isotopes,geochemistry and tectonic implications

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50–2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (ε_Hf(t) = −3.73 to +6.42) as the associated potassic granitoids (ε_Hf(t) = −2.44 to +7.80), and both rock types yielded mean Hf crustal model ages of 2.8–2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton.     

Zircon U-Pb geochronology,geochemistry and Sr-Nd-Pb isotopes from the metamorphic basement in the Wuhe Complex:Implications for Neoarchean active continental margin along the southeastern North China Craton and constraints on the petrogenesis of Mesozoic g

We report zircon U-Pb geochronology,geochemistry and Sr-Nd-Pb isotope data from mafic granulites and garnet amphibolites of the Wuhe Complex in the southeastern margin of the North China Craton (NCC).In combination with previous data,our results demonstrate that these rocks represent fragments of the ancient lower crust,and have features similar to those of the granulite basement in the northern margin of the NCC.A detailed evaluation of the Pb isotope data shows that Pb isotopes cannot effectively distinguish the role of the Yangtze Craton basement from that of the NCC basement with regard to the source and generation of magmas,at least for southeastern NCC.The age data suggest that the protoliths of the granulites or amphibolites in the Wuhe Complex were most likely generated in Neoarchean and that these rocks were subjected to Paleoproterozoic(1.8-1.9 Ga) high-pressure granulite facies metamorphism. This study also shows that the Precambrian metamorphic basement in the southeastern margin of the NCC might have formed in a tectonic setting characterized by a late Neoarchean active continental margin.     

Metallogenesis of Precambrian gold deposits in the Wutai greenstone belt:Constrains on the tectonic evolution of the North China Craton

The Wutai greenstone belt in central North China Craton(NCC) hosts a number of Precambrian gold deposits and ore occurrences. Based on the host rock association, these can be divided into Banded Iron Formation(BIF), meta-volcano-sedimentary and meta-conglomerate types. The two former types formed during ~2.5-2.3 Ga and the third one at ~1.85 Ga. The characteristics of these Precambrian gold deposits are broadly similar with those of the orogenic gold deposits. Based on available geochronological data, here we reconstruct the major tectonic events and their relationship with gold mineralization in the Wutai-Hengshan-Fuping region during Neoarchean to Paleoproterozoic as follows.(1)~2.6-2.5 Ga: widespread intrusion of tonalite-trondhjemite-granodiorite(TTG) magmas in the Hengshan terrane and Fuping continental arc, formation of the Wutai volcanic arc in the southern margin of Hengshan terrane with granitoids emplacement, and the Hengshan-Wutai intra-oceanic arc accretion to the Fuping arc at the end of Neoarchean.(2) ~ 2.5-2.3 Ga: the subduction of Hengshan arc from north leading to persistent magmatism and orogenic gold mineralization.(3)~2.2-2.1 Ga:extension leading to the formation of graben structure in the Wutai and Fuping region, deposition of the Hutuo and Wanzi Group sediments, formation of placer gold through erosion of the orogenic gold deposits.(4)~2.2-2.0 Ga: widespread magmatism in the Wutai-Hengshan-Fuping region.(5)~1.95-1.8 Ga: regional metamorphism associated with collision of the Western and Eastern Blocks of the NCC and associated orogenic gold deposits. The multiple subduction-accretion-collision history and subsequent deep erosion has significantly affected most of the Precambrian gold deposits in the Wutai greenstone belt.