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.

Geochronological and geochemical constraints on the granitic gneiss in the Huozhou Complex: implications for the tectonic evolution of the Trans-North China Orogen

The Trans-North China Orogen is a major Neoarchean Paleoproterozoic collisional orogenic belt above the North China Craton, formed due to prolonged and complex processes. Even though many NeoarcheanPaleoproterozoic magmatic and metamorphic activities have been reported, due to the Huozhou Complex’s small outcropping range, little attention has been paid to the origin of various igneous rocks of the Huozhou Complex in the center of the Trans-North China Orogen. The Huozhou Complex, located south of the Luè liang, Wutai, and Hengshan complexes, is an important window into the Early Precambrian structure and evolution of the North China Craton. Its magma and metamorphism are crucial to understanding the development of the structural evolution of the Trans-North China Orogen. The Huozhou metamorphic complex area exposes a range of Precambrian metamorphic rocks, among which the most extensively dispersed is felsic biotite plagioclase gneiss. In this study comprehensive geological field survey, micropetrology,chronology, geochemistry, and Hf isotope analysis were carried out for the Qinggangping and Anziping gneiss in the north of the Huozhou Complex. The results show that the magmatic zircon age of the Qinggangping gneiss is2196 ± 14 Ma, and its protolith is I-type granite, formed by partial melting of igneous rocks in the absence of weathering. Its source is mainly the juvenile crust from depleted mantle dating 2431–2719 Ma, with a small amount of mantle-derived material. The Anziping gneiss has a metamorphic zircon age of 1931 ± 13 Ma with an S-type granite protolith belonging to peraluminous granite.The Anziping gneiss is formed by recycling pre-existing crustal components at 2613–2848 Ma. A minor quantity of mantle-derived magma is also introduced to the crust simultaneously. The samples of Qinggangping gneiss and Anziping gneiss show the characteristics of obvious negative Nb, Ti, and P elements in the spider diagram of primitive mantle standardization. This implies that the rocks have the characteristics of magmatic rocks in an island arc or subduction environment, which could have formed in the tectonic environment of the continental margin arc.

Petrogenesis of the Chaihulanzi Gneiss and its Tectonic Implications for the North China Craton

The Chaihulanzi area in eastern Inner Mongolia is tectonically situated on the northern margin of the North China Craton(NCC). The main Precambrian lithologies of the area have been referred to the Archean Jianping Group metasupracrustal sequences. Based on field observations, petrographic, whole-rock geochemical, and zircon U-Pb geochronological results, a magmatic origin for the units is proposed. Our results show that the Chaihulanzi gneisses are mainly of granitic, dioritic and granodioritic compositions, and show typical magmatic rock textures and mineral assemblages. The dioritic and granodioritic gneisses show Na-rich tonalite–trondhjemite–granodiorite(TTG)-like affinity with zircon U-Pb dates of ca. 2.57–2.59 Ga, representing a juvenile continental growth for the northeastern NCC. The granitic gneiss is indeed potassic granite and yielded a zircon U-Pb date of ~2.50 Ga, which is contemporaneous with the Jining–Jiaoliao microblock collision(2.53–2.49 Ga), implying another crustal growth event. The well-developed gneissosity in 2.57–2.59 Ga dioritic and granodioritic gneisses together with the 2.5 Ga potassic granite, which crosscuts the gneissosity implies a 2.57–2.50 Ga(mainly 2.53–2.51 Ga) collisional orogeny, probably related to the Jining–Jiaoliao microblock collision. Our new geology and chronological results provide new evidence for the early Precambrian tectonic evolution of the NCC.

Episodic crustal growth and reworking at the southeastern margin of the North China Craton: evidence from zircon U–Pb and Lu–Hf isotopes of Archean tonalite–trondhjemite–granodiorite gneisses in the Bengbu-Wuhe area

The cratonization history of the North China Craton(NCC)and the nature of tectonothermal events are still highly controversial.Tonalite-trondhjemite-granodiorite(TTG)gneisses,as the dominant lithological assemblages in Archean metamorphic terranes,can provide significant clues to the magmatic and metamorphic evolution of Precambrian crust.This study presents zircon laser-ablation inductively-coupled-plasma mass spectrometry U–Pb ages,trace-element,and in-situ LA-MC-ICPMS zircon Hf isotope data for the TTG gneisses from the Bengbu-Wuhe area on the southeastern margin of the NCC.Cathodoluminescence images and trace elements indicated that magmatic zircons display the characteristics of euhedral-subhedral crystals with oscillatory growth zoning structures,high RREE contents,marked Ce positive anomalies,and Pr–Eu negative anomalies.The metamorphic zircons display the spherical-oval crystals with distinct core-rim structures,high and homogeneous luminescent intensity,lower RREE,Nb,Ta,Hf contents,relative flat REE patterns,weak Ce positive anomalies,and Pr-Eu negative anomalies.The Ti–in–zircon geothermometer data indicate that the crystallization temperature of the TTG gneiss ranged from 754 to 868℃.Zircon U–Pb ages indicate that the TTG gneisses formed at 2.79–2.77 Ga and 2.50 Ga and underwent metamorphism at 2.57–2.52 Ga.The Hf isotopic data indicate that the magmatic zircons exhibit high,positive eHf(t)values close to those of the coeval depleted mantle,whereas the metamorphic zircons exhibit negative or nil eHf(t)values.This implies that the TTG gneisses were derived from the partial melting of the~2.9–2.6 Ga juvenile crustal sources mixed with~3.0–2.8 Ga ancient crustal materials.Combined with the regional tectonic evolution,we propose that the metamorphic basement at the southeastern margin of the NCC underwent episodic crustal growth at~2.7 and~2.5 Ga and subsequently underwent crustal reworking or re-melting of the ancient crust during the Neoarchean.The Neoarchean TTG gneisses might have been derived from the partial melting of lower crustal materials related to plate subduction.

Early Cretaceous A-type Granites in the Dandong Area,NE China and its Geodynamical Implications

A-type granites,as an important petrologic sign to build up environmental recognition,were mainly formed in extensional tectonic settings.A biotite syenogranite from the Fenghuangshan pluton in Dandong,Liaoning Province gave SHRIMP zircon U-Pb ages of 122.5±1.6 Ma,124.9±1.7 Ma and 126.9±1.1 Ma.The monzogranites have SHRIMP zircon U-Pb ages of 118.2±1.6 Ma,128.1±1.7 Ma and 131.6±1.9 Ma,giving an emplacement age for the pluton in the Early Cretaceous.SiO_(2) is 65.48-74.49 wt%,whereas that of K_(2)O is 4.16-6.44 wt%,and that of Na_(2)O is 2.99-4.70 wt%.They also contain 13.24-15.76 wt% of Al_(2)O_(3),with an A/CNK ratio of 0.92-1.10,averaging 1.02.The alkalinity rate(AR)ranges from 2.68 to 5.12,and this range is within the AR of peraluminous type rocks.The granites of Fenghuangshan pluton are characterized by high contents of Na and K and low contents of Ca(thermophile element)and Mg,which are features of A-type granites.The(Na_(2)O+K_(2)O)-Fe_(2)O_(3)^(*)×5-(CaO+MgO)×5 discrimination diagram also shows that Fenghuangshan pluton is an A2-type granite.The above granite has zircon ε_(Hf)(t)values ranging from-17.06 to-9.09,with single-stage Hf model ages(T_(DM))of 1141 Ma to 1498 Ma and two-stage Hf crustal model ages(T_(DM)^(C))of 1762 Ma to 2263Ma.A comprehensive analysis suggests that the Fenghuangshan pluton might have been formed from the lithospheric-plate sliding during the late stages of evolution of hot rift structures and might have been closely associated with the tectonic settings of Mesozoic Eurasia and the ancient Pacific Plate.

SHRIMP Dating and Recrystallization of Metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.

Archean (about 2500 Ma) anatexis in eastern North China Block

Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss.The elevatedεHf(t)values of the granites can be explained by the involvement of Hf-bearing minerals,such as orthopyroxene,amphibole,and biotite,in anatectic reactions in the granodioritic orthogneiss.Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above,it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism,and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism.This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India,Antarctica,northern and southern Australia,indicating a possible connection between these cratons during the Neoarchean.

Petrogenesis and Tectonic Implications of the Paleoproterozoic A-Type Granites in the Xiong’ershan Area along the Southern Margin of the North China Craton

Paleoproterozoic A-type granites are widely distributed in the southern margin of the North China Craton(SNCC),providing important information for understanding the Paleoproterozoic tectonic regimes in this area.This paper reports newly obtained whole-rock compositions and zircon U-Pb ages for the Tieluping syenogranite porphyry(TLP)and Huoshenmiao alkali granite porphyry(HSM)in the SNCC.Zircons from the TLP and HSM have U-Pb ages of 1805±12 and 1792±14 Ma,respectively.These ages are taken to date the emplacement of these intrusions.They had high total alkali contents(K_(2)O+Na_(2)O>7.13 wt.%),with high 10000×Ga/Al ratios(3.06–3.41)and Zr+Y+Nb+Ce values(709 ppm–910 ppm)as well as high zircon saturation temperatures(864–970℃),indicative of A-type granite affinities.High Y/Nb(1.75–3.32),Ce/Nb(7.72–9.72),and Yb/Ta(2.89–5.60)ratios suggested that TLP and HSM belonged to the A2-type granite.The negative whole rockε_(Nd)(t)values(−8.4 to−6.6)and negative zirconε_(Hf)(t)values(−15.9 to−6.3)confirmed that TLP and HSM were likely generated by the partial melting of an ancient continental crust.TheεHf(t)(−7.4 to+4.0)values of inherited zircons in the TLP suggested that they were derived from the partial melting of Archean basement rocks.Considering the geochemical similarity of the 1.80 Ga A-type granitoids in the SNCC,we propose that the TLP and HSM were formed in a post-collisional regime that was likely associated with the break-off of the Paleoproterozoic subducted slab.Upwelling of the asthenosphere provided huge heat to generate the regional 1.80 Ga A-type granite in the SNCC.

Leaching behavior of Li and Ga from granitic rocks and sorption on kaolinite:Implications for their enrichment in the Jungar Coalfield, Ordos Basin

The granite collected from the Yinshan Mountain and kaolinite has been selected for the leaching and adsorption experiment,respectively,aiming to clarify the enrichment processes of Li and Ga during the deposition.Results suggest both Li and Ga could be leached out from granite by using different acid solutions of different p H and kaolinite can adsorb Li and Ga with varying degrees.Lithium and Ga had the highest leaching ratio when p H=1.Special geological events(e.g.volcanic eruptions and wildfires),which could result in very low p H values of water in peatland,may have accelerated the release of Li and Ga from the source rocks.Kaolinite has the highest adsorption fraction was obtained at p H=8.The different characteristics of Li and Ga displayed in the leaching and adsorption experiments probably result from the different occurrences and enrichment processes of Li and Ga in the coals.Lithium was probably enriched before the Li carriers(e.g.kaolinite)had been transported into paleomires because of its high leaching ratio and high adsorption fraction under neutral and alkaline conditions,whereas Ga was more likely concentrated by kaolinite and other carriers after it had been transported into the peat mires.

Crustal growth at ～2.5 Ga in the North China Craton:evidence from whole-rock Nd and zircon Hf isotopes in the Huai'an gneiss terrane

Huai'an 片麻岩岩层主要由 TTG 片麻岩和 dioritic 片麻岩组成。在 magmatic 锆石核心标明日期的 situ U-Pb 的激光显示这些片麻岩的 protolith 在 ∼2.5 Ga 被形成。TTG 片麻岩有积极 ε 2.7 ～ 4.3 的 Nd (t) 价值，和大多数 magmatic 锆石有积极 ε 2.0 ～ 8.3 的 Hf (t) 价值。这些积极 ε Nd (t) 和 ε Hf (t) 价值两个都类似于在 2.5 Ga 的同时期的弄空的披风的那些。而且，为 magmatic 锆石的 2.44 ～ 2.73 Ga 的年轻 Hf 模型年龄接近预定锆石生长。整个岩石的 ε Nd (t) 价值由于在它的来源的古老的外壳的参与比在 TTG 片麻岩在 dioritic 片麻岩(0.8 ～ 1.7 ) 是更低的。然而，从 dioritic 片麻岩的许多 magmatic 锆石有类似的 ε Hf (t) 价值(2.0 ～ 7.9 ) 到同时代的弄空的披风的；他们 2.49 ～ 2.75 Ga 的 Hf 模型年龄接近锆石的 U-Pb 年龄。最高的 ε Hf (t) 价值接近弄空的披风，和相对高的 ε Hf (t) 价值对应于相对年轻的 Hf 模型年龄。这些 Nd 和 Hf 同位素特征建议 Huai'an 片麻岩岩层的片麻岩的这二种类型在 ca 从少年外壳发源。2.5 Ga。

Late Mesoarchean crust growth event： evidence from the ca. 2.8 Ga granodioritic gneisses of the Xiaoqinling area, southern North China Craton

Recently, more attention has been paid to Precambrian magma events and crustal growth of the North China Craton(NCC), accompanying with controversy in activity stages and dynamic mechanism. In this study, we report the Archean(2802 ± 13 Ma) granodioritic gneisses in the Taihua Complex from the Xiaoqinling area, located in the southern margin of the NCC. The zircon Lu–Hf isotope analysis of the rocks showed ^(176)Hf/^(177)Hf ratios of0.280977–0.281228, corresponding to ε_(Hf)(t) values ranging from-3.5 to +6.6, and two–stage Hf model ages varying from 2836 to 3409 Ma. It was confirmed that late Mesoarchean(2.9–2.8 Ga) juvenile crust made contribution to the source material of these ca. 2.8 Ga granodioritic gneisses in the Xiaoqinling area. The whole rock geochemical data indicate that the granodioritic gneisses are high-K calc-alkaline series, probably generated at relatively high pressure and temperature, and formed under the continental arc setting. Statistically, we conclude that the magmatic activities during 2.9–2.7 Ga may represent the most intense crustal growth events in the NCC and these Archean rocks at different locations in southern NCC underwent a similar crustal evolution history.

Late Mesozoic magmatism and tectonic evolution in the Southern margin of the North China Craton

Late Mesozoic granitic magmatism(158–112 Ma) are widespread in the southern margin of the North China Craton(NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage(158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted(such as Y and Yb), belonging to typical I-type granite. The late stage(126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2 O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites.Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage(126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.

Metallogenic geodynamic background of Mesozoic gold deposits in granite-greenstone terrains of North China Craton

The spatial distribution map of 65 mid-large gold-deposits hosted in the granite-greenstone terrains of the North China Craton is first drawn. These gold deposits mainly concentrate in the Mesozoic remobilized Yinshan-Yanshan-Liaoning-Jilin intracontinental collisional orogenic belt, the northern Qinling and the Jiaodong Mesozoic collisional orogenic belts, and the Mesozoic intracontinental fault-magmatic belts developed along the Taihangshan and the Tan-Lu faults; their mineralizing time is predominantly Jurassic-Cretaceous, i.e. the Yanshanian. The metallogenic geodynamic background is exactly the compression-to-extension transition regime during continental collision.

Mineral Chemistry of Biotite and Its Petrogenesis Implications in ca.2.5 Ga Wangjiazhuang Granitic Pluton,North China Craton

The Wangjiazhuang granitic pluton is located in the central Zanhuang Domain,the central part of the North China Craton,which is mainly composed of biotite monzogranite with few mafic microgranular enclaves.Biotite is an important ferromagnesian mineral in most of the intermediatefelsic igneous rocks,and its mineral chemistry can record the properties of magma and the petrogenetic physicochemical conditions.In this study,we carried out a detailed petrographic study by electric probe microanalysis on biotite for the biotite monzogranite and mafic microgranular enclaves,to discuss the source,physicochemical conditions,and the magma mingling/mixing processes of the Wangjiazhuang granite.The results show significantly different chemical compositions from the biotite monzogranite and mafic microgranular enclaves.The crystallization of these biotite grains from the biotite monzogranite and mafic microgranular enclaves all occurred in low oxygen fugacity.The biotite grains in biotite monzogranite are rich in Fe,poor in Mg,which belong to siderophyllite.The ratios of[(Fe^(3+)+Fe^(2+))/(Fe^(3+)+Fe^(2+)+Mg2+)]are between 0.78 and 0.86.The average of FeOT(total FeO)/MgO of biotite grains in biotite monzogranite is 9.02.The MF values[2×Mg/(Fe^(2+)+Mg+Mn)]of biotite monzogranite are between 0.31 and 0.47,suggesting biotite monzogranite derived from crustal source rocks(metasedimentary rocks).The formation of granitic rocks including the Wangjiazhuang granite was related to the subduction event at ca.2.5 Ga which resulted in the melting event,and then induced the early partial melting of TTGs and metasedimentary rocks.The biotite in mafic microgranular enclaves varies from siderophyllite to ferrobiotite,and MF values range from 0.63 to 1.06,suggesting that magma of mafic microgranular enclaves had experienced magma mixing/mingling in various degrees.Biotite monzogranite and parts of mafic microgranular enclaves have a similar crystallized condition,while other mafic microgranular enclaves are different from biotite monzogranite.The differences between biotite monzogranite and mafic microgranular enclaves may be a consequence of continuous interaction between granitic and mafic magmas.

华北克拉通五台绿岩带锆石年代学及地质意义

五台花岗-绿岩带位于华北克拉通中部造山带的中心地区,保存有较完整的地质记录,并赋存条带状铁建造(BIFs)型铁矿,一直是研究的热点地区之一。然而,该绿岩带尚需详细年代学研究来重新厘定其岩石地层划分。本文对该区域三个组,即金岗库组、柏枝岩组和鸿门岩组中发育的变质火山岩开展了锆石U-Pb年代学和Lu-Hf同位素组成分析。锆石U-Pb定年结果显示,五台花岗-绿岩带五台群中的金岗库组、柏枝岩组和鸿门岩组的变质火山岩喷发年龄分别为2540±8 Ma、2524~2530 Ma和2513~2517 Ma,同时还记录了~2700 Ma的继承年龄和1930~1853 Ma的变质年龄。锆石Lu-Hf同位素分析结果显示它们的εHf(t)值分别为+0.4~+5.5、+1.2~+7.5和+2.2~+10.5,相应的亏损地幔模式年龄为2639~2845 Ma、2546~2788 Ma和2425~2752 Ma,εHf(t)值呈现大的变化且增加的趋势。结合前人的研究,本文确定了五台群中的金刚库组、庄旺-文溪组、柏枝岩组和鸿门岩组分别形成于~2540 Ma、~2530 Ma、~2525 Ma和~2515 Ma,它们记录了四期火山活动,说明五台绿岩带中的表壳岩系尽管经历了变形变质作用的改造依然保存了较完整的地层层序序列,并提出回撤-前进式俯冲-底冲演化模型。这些结果对理解五台花岗-绿岩带乃至华北克拉通新太古代-古元古代构造及演化具有重要意义。

阴山地块固阳地区新太古代TTG片麻岩成因及构造意义:锆石U-Pb年代学、地球化学和Lu-Hf同位素约束

作为太古宙基底保存较完整的地区,华北克拉通阴山地块出露大量的新太古代岩石组合,主要包括表壳岩、TTG片麻岩(英云闪长质片麻岩-奥长花岗质片麻岩-花岗闪长质片麻岩)、钾质花岗岩和闪长岩(赞岐岩)等。前人对阴山地块新太古代TTG片麻岩开展了一系列的工作,但有关其岩石成因、构造背景以及新太古代地壳生长机制仍然存在较大争议。本次工作在内蒙古固阳西部的花岗-绿岩带中新识别出一套TTG片麻岩组合,并对其进行了岩石学、同位素年代学和岩石地球化学的综合研究。LA-ICP-MS锆石U-Pb定年结果显示TTG片麻岩组合形成时代为2550~2524Ma;Lu-Hf同位素分析结果显示其εHf(t)值为+0.54~+4.96,t C DM模式年龄为2985~2715Ma,这与阴山地块目前最古老的奥长花岗质片麻岩(2.7Ga)的岩浆年龄近一致,说明其可能来源于2.7Ga新生镁铁质地壳的部分熔融。全岩地球化学分析结果显示,该TTG片麻岩组合具有较高的SiO 2(63.32%~70.80%)和Al 2O 3(15.19%~16.55%)含量,以及较高的Sr(410×10^(-6)~1532×10^(-6))和较低的Y(2.61×10^(-6)~14.04×10^(-6))含量,同时具有相对较高的(La/Yb)N(10.31~56.85)和Sr/Y(44.32~349.1)比值,并相对富集轻稀土元素(LREEs)和大离子亲石元素(LILEs),这些均与典型的埃达克质岩石地球化学特征相似。另外,该TTG片麻岩样品具有相对较低的MgO(0.60%~2.60%)、Ni(3.12×10^(-6)~11.12×10^(-6))、Cr(3.77×10^(-6)~18.11×10^(-6))含量和Mg#(34.88~53.45),说明其来源于加厚的大陆下地壳埃达克质岩浆。因此,本文认为在2.55~2.52Ga,固阳新太古代TTG片麻岩可能由加厚的镁铁质下地壳部分熔融形成的。结合前人的研究结果,进一步推断阴山地块在新太古代可能经历两期岩浆事件:第一期岩浆事件发生在2.7Ga,形成加厚的镁铁质地壳和少量的~2.7Ga TTG岩石;第二期岩浆事件发生在2.55~2.50Ga,形成阴山地块大面积的新太古代TTG岩石和其他花岗质岩石组合。综上,我们认为以地幔柱为主导的地球动力学机制可以较好地解释阴山地块加厚镁铁质下地壳部分熔融和TTG岩浆的形成过程。

山西云中山地区~2.53Ga碱性花岗岩的成因和地质意义

在山西云中山遨湖地区进行野外填图时,识别出一个新太古代碱性花岗岩岩体。该岩体东西向展布,长约3km,宽约1km,岩石类型为花岗闪长岩和正长花岗岩。岩石具有较高的SiO 2(66.33%~75.01%)、Na 2O(3.78%~7.62%)、碱(Na 2O+K 2O=7.89%~12.56%)和Al 2O 3(13.95%~18.08%)含量;贫CaO(0.14%~2.20%)和MgO(0.03%~0.23%);另具较高的AR(2.72~5.44)、里特曼指数(δ=2.22~6.66)和FeO T/MgO值(3.75~30.39);其稀土总量较低,富集轻稀土和大离子亲石元素(Rb、Th、K),相对亏损Nb、Ta、Sr、Ti、P等元素,与I型花岗岩的特征相似。上述全岩主、微量元素结果显示这套岩石与云中山地区新太古代的花岗闪长岩成分相似,指示碱性花岗岩的母岩浆来自新生地壳(花岗闪长岩)的部分熔融。相较于正长花岗岩,花岗闪长岩的SiO 2含量偏低,但Na 2O含量更高。因此,我们推测在岩石形成过程中经历了分离结晶作用,早期形成花岗闪长岩,晚期形成正长花岗岩。结合区域地质及前人的研究成果,我们认为该新太古代碱性花岗岩形成于板块俯冲碰撞之后的伸展环境中。此外,SHRIMP锆石U-Pb测年结果显示其侵位时代为~2527Ma。锆石原位Lu-Hf同位素分析显示该碱性花岗岩具有正的εHf(t)值(+3.5~+6.6),二阶段模式年龄(t DM2)为2520~2710Ma,在误差范围内与岩石的结晶年龄一致。云中山地区从超镁铁质岩-镁铁质岩、TTG向正长花岗岩的转变,记录着华北克拉通新太古代晚期由板块俯冲碰撞向碰撞后伸展的转变。新太古代碱性花岗岩的出现代表一次重要的热事件,标志着克拉通化的完成。

华北克拉通吉南夹皮沟新太古代石榴黑云斜长片麻岩的变质P-T-t演化及构造意义

华北克拉通东部陆块新太古代末期的构造体制是近年来备受学界关注的热点议题。本文对地处华北克拉通东部陆块东北部的吉南夹皮沟石榴黑云斜长片麻岩进行了详细的岩相学、矿物化学、变质相平衡模拟、传统地质温压计和锆石U-Pb年代学研究,以明确其变质P-T-t演化,探讨构造过程和动力学机制。石榴黑云斜长片麻岩主要由石榴石、黑云母、斜长石、角闪石、石英和少量钛铁矿组成,记录了逆时针P-T轨迹,可以划分为三个变质阶段。岩石第一阶段的矿物组合为黑云母+斜长石+角闪石+石英+钛铁矿(+熔体)(Bt 1+Pl 1+Amp 1+Qz+Ilm(+L)),记录的温压条件为790~854℃和0.55~0.88GPa,之后通过升压略降温到达压力峰期阶段,在这个过程中角闪石分解、石榴石生长、基质型斜长石的X_(An)值从核到边降低。第二阶段的矿物组合为石榴石+黑云母+斜长石+角闪石+石英+钛铁矿(+熔体)(Grt+Bt_(2)+Pl_(2)(核部)+Amp_(2)+Qz+Ilm(+L)),相平衡模拟获得该阶段的温压条件为670~745℃和1.10~1.24GPa,传统地质温压计的结果为707~771℃和0.99~1.18GPa,两者可相互佐证,可能代表了岩石的压力峰期阶段。第三阶段的矿物组合为黑云母+斜长石+石英+钛铁矿(Bt_(3)+Pl_(2)(边部)+Qz+Ilm),形成于压力峰期后的降温降压过程,石榴石边部分解形成斜长石,该类斜长石的X_(An)值从核到边逐渐升高。变质锆石U-Pb定年获得2483±8Ma(MSWD=3.2)的年龄,结合稀土配分模式和锆石Ti温度计计算结果,将该年龄解释为岩石压力峰期阶段前的变质时代。考虑到吉南变质地体新太古代末期岩浆活动与变质作用的时间重叠性、区域上的“穹窿-龙骨”构造样式、前人的变质作用资料以及石榴黑云斜长片麻岩记录的升压轨迹,本文认为吉南新太古代末期变质作用与重力凹沉构造有关。

华北克拉通中元古代A型花岗岩的成因及其构造意义

华北克拉通保存有与Columbia超大陆裂解相关的中元古代岩浆记录。本文综述了华北克拉通中元古代A型花岗岩的时空分布、岩石学和地球化学等特征,讨论了各期次A型花岗岩的成因及构造背景,论述了其对华北克拉通中元古代裂解事件的指示意义。华北克拉通主要发育4期A型花岗岩:1)~1.78 Ga A型花岗岩主要分布于南缘熊耳裂陷槽和晋陕地区;2)~1.70 Ga A型花岗岩主要分布于北缘渣尔泰—白云鄂博和燕辽裂陷槽;3)~1.60 Ga A型花岗岩主要分布于南缘熊耳裂陷槽;4)~1.32 Ga A型花岗岩主要分布于北缘渣尔泰—白云鄂博和燕辽裂陷槽。这4期A型花岗岩均来源于地壳物质的部分熔融,可能与同期伸展作用导致岩浆底侵提供热源有关。整体具有造山后伸展到陆内裂谷环境的演化序列,且存在明显的时空差异。华北克拉通中元古代A型花岗岩记录了4期主要的裂解事件,为华北克拉通参与了Columbia超大陆裂解提供了依据。

山西羊投崖岩体花岗斑岩锆石U-Pb年龄、Hf同位素组成和地球化学特征及其地质意义

为限定华北克拉通内的羊投崖岩体的岩石成因和构造背景,对羊投崖岩体中较新鲜的花岗斑岩开展地球化学、锆石年代学和Hf同位素研究。结果表明,羊投崖花岗斑岩形成于早白垩世(131.7±0.5Ma);花岗斑岩富硅,为高钾钙碱性系列弱过铝质花岗岩类;相对富集K、Pb、Rb等大离子亲石元素(LILEs)和轻稀土元素(LREE),相对亏损Nb、Ce、Zr、Ti等高场强元素(HFSEs)和重稀土元素(HREE);Eu呈负异常,ε_(Hf)(t)为–28.6~–19.7。研究表明,羊投崖花岗斑岩总体上为I型花岗岩,为古元古代–新太古代古老下地壳物质的部分熔融产物,但在成岩过程中有幔源物质参与,形成于早白垩世古太平洋板块向欧亚板块俯冲背景下的伸展构造环境,为中生代华北克拉通构造体制转折的产物。