Evolution of the 3.65-2.58 Ga Mairi Gneiss Complex,Brazil:Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution,especially during the Archean.However,the scarcity of exposure to these rocks,the complexity of lithological relationships,and the high degree of superimposed deformation,especially with long-lived magmatism,make it difficult to study ancient rocks.Despite this complexity,exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust.Therefore,here we present field relationships,LA-ICP-SFMS zircon U-Pb ages,and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex.The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic,and felsic members ranging from TTG(Tonalite-Trondhjemite-Granodiorite)to granitic composition.Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean(ca.3.65–3.60 Ga),early Paleoarchean(ca.3.55–3.52 Ga),middle-late Paleoarchean(ca.3.49–3.33 Ga)and Neoarchean(ca.2.74–2.58 Ga),with no records of Mesoarchean rocks.Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time.In the Eoarchean,rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources,whereas mantle depletion played a role in the Paleoarchean,followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean.A different stage of crustal growth and recycling dominated the Neoarchean,probably owing to the thickening of the continental crust by collision,continental arc growth,and mantle differentiation.

Multiple thermal events recorded in the Acasta Gneiss Complex:Evidence from in-situ dating of zircon, titanite, and apatite

In-situ dating of the zircons, titanites, and apatites in the rock fragments of approximately 1 cm2in size from the Acasta Gneiss Complex was performed using secondary ion mass spectrometry or laser ablation inductively coupled plasma mass spectrometry to provide constraints on the thermal history of the Acasta Gneiss Complex. The zircons in these rock fragments typically exhibit multiple age populations, reflecting the presence of inherited zircons or the post-crystallization process of zircon overgrowth. Combined with previous studies, our zircon dating results reveal multiple magmatic events that occurred in the Acasta Gneiss Complex, specifically at >3.96, ~3.72, and ~3.57 Ga, respectively. A titanite Pb-Pb isochron age of2911±22 Ma(95% confidence level, MSWD=1.5) for sample AY199 is identified, consistent with the timing of the latest Archean magmatism in the Acasta Gneiss Complex. The titanite U-Pb ages for samples AC478 and AY066 are 1932±270 Ma(95% confidence level, MSWD=2.3) and 1813±45 Ma(95% confidence level, MSWD=2.3), respectively. The apatites in sample P090803-C exhibit a Pb-Pb isochron age of 1833±26 Ma(95% confidence level, MSWD=1.4). The apatite U-Pb ages for samples AC478, AY199, and AY066 are 1850±20 Ma(95% confidence level, MSWD=1.3), 1827±100 Ma(95% confidence level, MSWD=6.3), and 1807±58 Ma(95% confidence level, MSWD=3.9), respectively. Titanites in samples AC478 and AY066, as well as the apatites in all four investigated samples, show a uniform age(Pb-Pb or U-Pb age) of ~1.9–1.8 Ga,indicating U-Pb system reset in these minerals due to the Wopmay orogeny and documenting that the peak temperature condition associated with the Wopmay orogeny exceeded the apatite U-Pb closure temperature and approached or exceeded that of titanite.

Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex,Greenland

The Ujaragssuit Nunat layered(UNL)unit in the Itsaq Gneiss Complex,west Greenland,has been consid-ered to contain one of the oldest chromitites on Earth based on~ca.4.1 Ga Hadean whole rock Pt-Os model ages and ca.3.81 Ga zircon U-Pb age of the surrounding orthogneiss.This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit.In-situ U-Pb-Hf-O isotope measurements were made on the zircons.Zircons from both the chromitite and the intrusive granitoids show concordant U-Pb ages of ca.2.97-2.95 Ga.In contrast,Hf and 0 isotopic anal-yses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids.Zircons from granitoids yielded Th/U ratios higher than 0.2,initial Hf isotope ratios of 0.2805-0.2807(i.e,initial:Hf value of-11 to-5),andδ^(18)O values of mostly 6.0‰-7.0‰,which are typical for felsic igneous rocks in Archean continental crust.The least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078-0.28084(i.e.,initial:Hf value of-1.1 to-0.4),close the chondritic value at ca.3.0 Ga and the depleted mantle at ca.3.2 Ga.These zircons also haveδ^(18)O values of 4.2‰6.1‰which correspond to typical mantle values.The other chromitite zircons yielded Th/U ratios lower than 0.1,and Hf and 0 isotopic compositions ranging between the least altered zircons and the intrusive granitoid zir-cons.These results indicate that the zircons in the chromitites crystallized before or during the 2.97-2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent metasomatism.Furthermore,the present results suggest that zircons in the chromitites originally had depleted Hf iso-topic compositions at ca.3.2-3.0 Ga.This can be explained by two different models of the evolution of the UNL unit.One is that if the UNL unit was formed at>3.81 Ga as previously thought,with the zircons in the chromitites subsequently being precipitated by ca.3.2-2.95 Ga during metamorphism or metaso-matism.The other model is that the UNL unit itself was actually formed at ca.3.2-3.0 Ga,with zircon in the chromitite representing the crystallisation age of the unit,which was then tectonically incorporated into the ca.3.81 Ga orthogneiss prior to the 2.97-2.95 Ga granitoid intrusion event.In either case,our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2-2.95 Ga.Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

Study on fluid-rock equilibrium systems of Fuping gneiss complex, Taihang Mountains

The characters of fluid compositions and fluid-rock equilibrium system of the Archean Fuping gneiss complex, Taihang Mountains are first determined by fluid-rock equilibrium thermodynamics calculations and mi-crothermometry of fluid inclusions. These investigations suggest that the X(H2O) is generally less than 0.05 in fluids equilibrating with the granulite facies metamorphic mineral assemblage of Fuping gneiss complex, and fluid-rock interaction is chiefly controlled by CO2-rich fluid infiltration.

Geochemistry of Gneisses from Dabie Complex and Tongbai Complex in Qinling-Tongbai-Dabie Orogenic Belt:Implications for Location of Yangtze-Sino-Korean Suture

The Dabie complex (DC) and the Tongbai complex (TBC) are separately distributed in the middle and eastern parts of the Qinling-Tongbai-Dabie orogenic belt. In this study, the Dabie complex can be divided into two units: one is the complex with no high pressure and ultrahigh pressure metamorphic rocks (DC1), and the other is the complex containing coesite-bearing eclogite lenses or boudins (DC2). Gneisses are predominant in the TBC, DC1 and DC2. Major and trace element data of gneisses in the TBC, DC1 and DC2 show them to be the orthogneisses. The gneisses in the DC1 have higher incompatible element contents and higher ratios of w(K 2O)/w(Na 2O) and w(La) n/w(Yb) n than those in the DC2. However, no obvious differences arise in other element contents and the ratios of w(La)/ w(Nb), w(Nb)/w(Th), w(Nb)/w(Hf), w(Ba)/w(La), w(Sm)/w(Nd) and w(Th)/w(U) between the gneisses in the DC2 and those in the DC1. These observations suggest that the protoliths of the gneisses in the DC2 have affinities to those in the DC1. The difference between the DC1 and DC2 gneisses in incompatible element contents could reflect the difference in their partial melting extent. The TBC gneisses are geochemically similar to the DC1 gneisses, suggesting that the TBC and DC1 gneisses are the same lithologic unit in the Qinling-Tongbai-Dabie orogenic belt and that they have experienced similar formations and evolution histories. In the Qinling-Tongbai area, the TBC is part of the northern blocks of the Yangtze craton. Given the similarity of geochemical characteristics, the rock assemblage and the ages between the TBC and DC1 gneisses, we can infer that the Dabie complex also belongs to the northern blocks of the Yangtze craton. In terms of the distribution of eclogites and metamorphic facies, we propose that the collisional suture in the Dabie area is distributed along the Xiaotian-Mozitan fault, at the contact with the Shang-Dan-Tongbai fault to the west.

Petrological Implication of the Albite Rims in the Felsic Gneisses of the Fuping Complex

The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K-feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400- 500~C the albite rim was derived from the K-feldspar composition adjustment in the late or post- magmatism stage. As the temperature decreased, the equilibrium between K-feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magmaor melt-injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium-high grade metamorphism might be subordinate. If present, the anatexis is water-present, but the rim texture cannot be taken as the symbol of anatexis.

Geochemical evolution of the Mangalwar Complex,Aravalli Craton,NW India:Insights from elemental and Nd-isotope geochemistry of the basement gneisses

The Banded Gneissic Complex(BGC) of the Aravalli Craton is divided into BGC-I and BGC-Ⅱ; the BGC-Ⅱ(central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and Nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO_2 gneisses(LSG) and high-SiO_2 gneisses(HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible(K, Sr, Ba, large ion lithophile elements) and compatible elements(MgO, Cr, and Ni). They display fractionated rare earth element patterns(avg.La_N/Yb_N=12.1)with small Eu-anomaly(δEu=0.9), and exhibit negative anomalies of Nb and Ti in primitive mantlenormalized multi-element diagram. In terms of Nd-isotope geochemistry, the LSG are characterized by_(εNd)(t)=4.2 and depleted mantle model age of 3.3 Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG:(1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge;(2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and(3)upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantlewedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg#(avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements(MgO=avg. 1.1 wt.%; Cr=avg. 8 ppm; Ni=avg. 6 ppm) but enriched in incompatible elements(Sr=avg. 239 ppm, Ba=avg. 469 ppm). Its_(εNd)(t) values vary from-9.5 to-5.4.These chemical features of the HSG are akin to potassic granitoids found elsewhere. In this backdrop, we propose that the HSG suite of the Mangalwar Complex was derived from re-melting(partial) of an older crust(TTG?) occurring within the BGC-Ⅱ.

Geochemical Characteristics and Genesis of the Granitic Gneisses from the Southeastern Dabie Mountains

The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.

东大别条带状片麻岩的构造变形特征及对大别杂岩流变机制的指示

大别造山带是由华北板块和扬子板块经过长期构造演化、最终碰撞拼合的产物。东大别造山带作为大别造山带和郯庐断裂带的过渡部位,对认识大别造山带的构造演化及其与郯庐断裂带的构造关系有重要的意义。本文对东大别造山带南北向霍山‒马垅剖面进行了野外构造观察和室内显微构造分析。东大别造山带发育地壳深部构造层次上的条带状片麻岩,通过角闪石‒斜长石温压计得到其形成条件为T=737±40℃,P=0.94±0.15 GPa,属于高角闪岩相‒低压麻粒岩相变质,形成深度约28km左右,处于中下地壳流变层。对条带状片麻岩及糜棱岩的应力进行恢复,结果显示东大别主要受到NE-SW向和NW-SE向两个方向上的挤压以及顺造山带的拉伸,可能代表受特提斯和太平洋构造体制共同作用的结果。大别造山带受特提斯构造体制影响的应力方向是NE-SW向,此应力可以分解成垂直于造山带挤压和平行于造山带向东牵引的力,并且南北向的空间缩短已经形成,应力已经作用到大别杂岩内部,大别杂岩需要在此应力的作用下由西向东发生塑性流动,进行应力释放,这也是塑性流动得以持续进行的动力。大别杂岩由西向东的塑性流动边界是晓天‒磨子潭剪切带与殷店‒马垅剪切带,开始于晚三叠世210±10 Ma,至少持续到早白垩世130±5 Ma。

河南省舞钢—鲁山地区石墨矿碳硫同位素特征及成因分析

华北克拉通南缘是我国晶质石墨矿重要成矿带之一。本文对位于华北克拉通南缘的舞钢—鲁山地区石墨矿进行了详细的岩相学、微量元素地球化学和碳同位素等方面的研究,探讨石墨矿的形成古环境、碳质来源和成矿机制。结果表明,该区太华杂岩中的石墨矿以片麻岩型石墨矿为主,石墨片径大于0.287 mm(50目)平均占比85.50%,为大鳞片晶质石墨矿。对比华北克拉通孔兹岩系内片麻岩型石墨的δ^(13)C值中位数,该类型碳同位素由北向南逐步降低。研究区石墨矿床主要形成于古元古代晚期2.3~2.15 Ga,变质程度为高角闪岩相,古沉积环境为富氧的海陆交互相,古水体盐度主要为咸水。石墨碳同位素组成相对稳定(δ^(13)C_(V-PDB)=-20.68‰~-28.41‰),是富含原生生物有机质在沉积富集后,变质过程中脱CH4热解为乙炔和氢,乙炔再裂解为单质碳和氢,有机物经变质结晶为石墨并聚集形成石墨矿床。石墨矿层较大的δ^(34)S波动(δ^(34)S_(V-CDT)=1.64‰~6.47‰)证明当时生物的硫呼吸已经开始,也支持了研究区石墨中碳主要来源于早期沉积物中有机质的结论。

内蒙古锡林格勒杂岩的地球化学研究:从Rodinia聚合到古亚洲洋闭合后碰撞造山的历史记录

内蒙古东部出露的锡林格勒杂岩主要由中元古代原岩为火成岩的黑云母片麻岩和斜长角闪岩透镜体组成。片麻岩显示轻稀土元素富集的中等分异特征 ,具明显的Eu负异常 ,其不相容元素特征与平均地壳类似但相对亏损Ba和Sr。斜长角闪岩构成一条Sm -Nd等时线 ,其年龄值为12 0 2± 6 5Ma ,εNd(1.2 0Ga)值为 6 .8。角闪岩的亏损地幔模式年龄 (tDM,1.2 6～ 1.32Ga)与其Sm -Nd同位素等时线年龄接近。角闪岩的Rb -Sr同位素等时线年龄为 393± 14Ma ,其87Sr/ 86 Sr初始比值为 0 .70 379± 0 .0 0 0 0 7,此Rb -Sr等时线年龄与本区出露的篮片岩Ar-Ar年龄 (～ 383Ma)一致 ,记录了中亚造山带东南部洋壳俯冲结束后由大陆碰撞造山引起的变质作用事件。黑云母片麻岩的Rb -Sr同位素等时线年龄为 2 79.5± 6 .4Ma ,87Sr/ 86 Sr初始比值为 0 .70 85 1± 0 .0 0 0 10 ,此Rb -Sr等时线年龄与本区广泛分布的早二叠世火山岩的Rb -Sr等时线年龄 (～ 2 77± 15Ma)一致 ,因此可能是由大规模岩浆活动诱发的变质作用的记录。锡林格勒杂岩因此不仅记录了Rodinia超大陆聚合的地质事件 ,并且通过对比研究本地区蛇绿岩和晚古生代岩浆岩的演化历史 ,可以恢复从Ro dinia超大陆聚合 (～ 1.2 0Ga)到古亚洲洋关闭 (～ 4 30Ma)、大陆碰撞造山 (～

辽西台里地区新太古代片麻岩杂岩的形成时代、岩石成因和地质意义

辽西台里地区片麻岩杂岩主要由片麻状花岗岩、黑云斜长片麻岩、角闪斜长片麻岩组成.锆石U-Pb定年结果表明,该套杂岩形成于2 510～2 559 Ma,与“绥中花岗岩”时代相同,同属华北克拉通新太古代结晶岩石.野外产状和岩石组构特征显示,本区片麻岩杂岩为一套同变形的深熔型混合岩.其中,角闪斜长片麻岩代表低度熔融的“古成体”,其间分布少量具“浅色体”特征的长英质岩脉;黑云斜长片麻岩为熔融程度较低的“残留体”;片麻状花岗岩为部分熔融程度最高的“新成体”.它们在矿物组成和地球化学上呈现良好的递变关系.结合前人Hf同位素研究资料,本文认为台里地区片麻岩杂岩可能记录了一期重要的陆壳再造事件.

鞍山地区营城子古太古代片麻岩杂岩的识别与锆石U-Pb年代学研究

鞍山地区分布有始太古代——新太古代花岗岩杂岩,是研究早期地壳物质组成及其演化的经典地区之一。通过大比例尺岩性调查工作,新近在营城子识别出一套片麻岩杂岩和奥长花岗岩组合,为本区古老岩系的对比、构造格局恢复、太古代地壳形成与演化研究提供了新的依据和线索。片麻岩杂岩主要由条带状黑云斜长片麻岩、(脉状)奥长花岗质片麻岩以及黑云母片岩组成,呈不同规模的包体产出于细粒、均匀块状的奥长花岗岩之中。片麻岩杂岩的岩石组合以及复杂多变、明显不均一的岩石组构特征表明具有深熔混合岩的成因特点。SIMS锆石U-Pb定年表明,片麻岩杂岩中条带状黑云斜长片麻岩和黑云母片岩形成年龄分别为3312±14Ma、3304±7Ma和3324±7Ma～3235±14Ma,明确反映古太古代热事件,此外个别样品中存在3.68～3.60Ga和～3.47Ga继承锆石。细粒黑云奥长花岗岩的年龄为3.14～3.13Ga,与全区中太古代岩浆热事件一致。区域对比分析表明:营城子片麻岩杂岩的岩石组合、产状关系和年代学特征与东山杂岩和深沟寺杂岩十分相似,为古太古代(3.33～3.24Ga)深熔作用的产物。营城子片麻岩杂岩是鞍山地区另一个保留有古老锆石信息和太古宙早期地壳岩石的地质体。

鞍山地区太古宙早期地壳生长及重熔——来自始—古太古代片麻岩杂岩的岩石学及年代学证据

本文报道了鞍山地区新厘定的一套始—古太古代片麻岩杂岩的岩石学特征及锆石U-Pb年代学数据,以探讨太古宙早期地壳的形成及演化过程。杂岩在全区广泛分布,野外以包体形式产于约3.1Ga细粒奥长花岗岩之中。主要包括始太古代奥长花岗岩/石英闪长岩和古太古代片麻岩杂岩两个岩石单元,前者呈透镜体产出于后者之中。始太古代奥长花岗岩/石英闪长岩部分遭受变质作用改造,但整体较好地保留了岩浆结构及构造,其年龄为3.77～3.81Ga,锆石εHf（t）值大于0,表明其为玄武质岩石经部分熔融形成的新生地壳。古太古代片麻岩杂岩由条带状片麻岩、浅色奥长花岗质片麻岩、黑云母片岩及少量斜长角闪岩等组成,岩石类型多样且组构复杂、不均一,受到塑性流变变形作用的改造,为地壳深部层次下变质-深熔作用的产物,各岩石锆石U-Pb年龄相近,为3.29～3.36Ga,锆石εHf（t）值小于0,表明其来源于始太古代古老地壳的重熔。杂岩记录了～3.8Ga、3.7～3.4Ga以及～3.3Ga等多期岩浆-构造热事件,其中～3.8Ga和～3.3Ga分别代表本区主要的两期地壳生长和地壳重熔事件。多期次地壳生长和重熔可能是早期地壳演化的主要机制及过程,这与全球多个太古宙地体类似。

库鲁克塔格地区新太古代深沟片麻杂岩特征

新疆库鲁克塔格地区深沟片麻杂岩是由新太古代阜平期表壳岩组合、灰色片麻岩及古元古代红色片麻岩组成的杂岩体。表壳岩组合为变火山－沉积岩、灰色片麻岩为太古宙ＴＴＧ岩浆岩组合，红色片麻岩为深沟变质核杂岩同构造侵入的“Ｉ”型花岗岩。深沟片麻杂岩组成内核，古元古界兴地塔格岩群组成滑脱系，古元古界南辛格尔塔格岩组组成盖层，它们构成一个完整的深沟变质核杂岩构造。灰色片麻岩、表壳岩组合的Ｓｍ－Ｎｄ全岩等时线年龄（２８３０．２±７９．７）Ｍａ，红色片麻岩锆石Ｕ－Ｐｂ年龄（２０５９±１４）Ｍａ。

内蒙古锡林浩特岩群岩相学、地球化学特征及构造环境分析

锡林浩特岩群是内蒙古中东部锡林郭勒微地块内原锡林郭勒杂岩的重要组成部分,由一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等变质岩组成,形成于中元古代[1]。岩群中片麻岩样品显示轻稀土元素富集的中等分异特征((La/Yb)N=4.71～7.97),斜长角闪岩具有平坦型((La/Yb)N=0.74～0.95)和轻稀土元素微弱富集型((La/Yb)N=1.06～3.63)两种稀土元素配分模式。在微量元素N-MORB标准化图解中,斜长角闪岩显示出洋中脊玄武岩和岛弧玄武岩的过渡类型特征。同时结合岩相学和岩石地球化学图解分析,推断片麻岩原岩为富含泥质的亚杂砂岩,而斜长角闪岩原岩为一套具有拉斑质到钙碱性过渡型特征的玄武岩;变质沉积岩形成于活动大陆边缘-大陆岛弧环境;变质基性火山岩形成于岛弧-弧后盆地。二者指示锡林浩特岩群形成于活动大陆边缘体系。

北大别片麻岩的超高压变质证据——来自锆石提供的信息

本文对北大别片麻岩锆石中矿物包体及年代学进行了研究,首次发现了北大别片麻岩的超高压变质作用证据。结合阴极发光图像和同位素定年,片麻岩锆石中矿物包体组合至少可分出三期:(1)原岩岩浆矿物组合,即斜长石、黑云母、石英和磷灰石;(2)超高压变质矿物组合,即金刚石、石榴子石和金红石等;(3)麻粒岩相退变质矿物组合,如透辉石等。其中,金刚石和石榴子石主要以包体形式被包裹于透辉石中,而透辉石是北大别麻粒岩相退变质阶段形成的代表性矿物。锆石SHRIMP U-Pb定年结果表明,北大别片麻岩的峰期变质时代和麻粒岩相退变质时代分别为218±3Ma和199±10Ma。这些证明北大别片麻岩,如同其中的榴辉岩一样,经过了印支期超高压变质作用。

秦岭杂岩牛角山花岗质片麻岩体锆石U-Pb同位素年龄及其地质意义

秦岭杂岩中发育的花岗质片麻岩体是目前秦岭造山带核部所识别的最古老的花岗质深成岩体。选取６组粒径晶形好、无磁性锆石进行ＵＰｂ同位素年龄测定，结果是（９５９±３．６）Ｍａ。这一成果对进一步认识该区晋宁期岩浆活动、构造背景及区域变形时代等具有一定意义。

河北平山县小觉地区阜平岩群黑云变粒岩——片麻岩深熔作用地球化学研究

本文重点对河北平山县小觉地区阜平岩群黑云变粒岩—片麻岩及相关岩石深熔作用的地球化学进行了研究。相对于熔融母岩,新生浅色体常量元素组成总体向低共熔区方向演化,稀土和高场强元素含量降低,Nd同位素组成也有不同程度的变化。本文对出现这些现象的原因进行了讨论。同时还讨论了黑云变粒岩—片麻岩地球化学组成的地质意义。

大别山北部漫水河灰色片麻岩的锆石U-Pb年龄及其地质意义

北大别杂岩主要由灰色片麻岩和斜长角闪岩组成。通过对大别山北部霍山漫水河花岗闪长片麻岩的锆石U-Pb年龄测定,得到中元古代、早三叠世和晚侏罗世3组年龄数据。中元古代的年龄老于灰色片麻岩集中在新元古代的锆石U-Pb上交点年龄,指示漫水河北部片麻岩中有古老锆石捕掳晶;早三叠世年龄指示该片麻岩中有印支期变质作用形成的锆石;晚侏罗世年龄表明灰色片麻岩中有燕山早期形成的岩浆锆石。根据北大别杂岩的构造背景、变质作用及地球化学特征,认为北大别杂岩中的部分灰色片麻岩形成于燕山早期的花岗闪长岩,它们可能源自或混染了经过早三叠世变质作用的扬子陆块俯冲基底。本文的结果表明,北大别杂岩既不是形成于白垩纪的岩浆杂岩,也不完全是形成于新元古代的TTG岩系,其中的部分灰色片麻岩为形成于燕山早期的花岗闪长岩,即在白垩纪大规模岩浆作用前的燕山早期,北大别曾有过一次岩浆作用。