赣东北前寒武纪变质地层钕模式年龄初步研究

 报道本区前寒武纪变质地层中一批钕模式年龄结果(T_DM=1437.23—2741.40Ma,平均为1865.27Ma)。在再沉积过程中,由于当时地壳活动强烈,常有来自上地幔的火山物质混入,从而降低了T_DM值。由此认为,含火山物质较少的沉积岩之钕模式年龄(T_DM=1924.52—2741.40Ma,平均T_DM=2272.81Ma,∑Nd=-10.15,(143)^Nd/¹⁴⁴Nd=0.5121125),才能相对代表陆源区大陆地壳的实际形成年龄。推测本区及外围还有一个尚未出露的晚太古代陆壳,这个陆壳正是元古宙的陆源区。     

赣中变质基度Nd模式年龄初步研究及基底地壳的形成时代

本文研究了赣东南变质基底的Ｎｄ模式年龄（ＴＤＭ＝１３６０－２１２４Ｍａ,平均为１７５５Ｍａ）,结合变质岩的Ｒｂ－Ｓｒ,Ｓｍ－Ｎｄ等时线年龄确认赣中地区存在前寒武纪地壳基底,推测本区还有一个尚未出露的古元古－新太古代的花岗质陆壳,这个陆壳正是元古代的陆源区。     

赣中变质基底Nd模式年龄初步研究及基底地壳的形成时代

本文研究了赣东南变质基底的Ｎｄ模式年龄（ＴＤＭ＝１３６０－２１２４Ｍａ,平均为１７５５Ｍａ）,结合变质岩的Ｒｂ－Ｓｒ,Ｓｍ－Ｎｄ等时线年龄确认赣中地区存在前寒武纪地壳基底,推测本区还有一个尚未出露的古元古－新太古代的花岗质陆壳,这个陆壳正是元古代的陆源区     

安徽南部燕山期中酸性侵入岩的源区锶、钕同位素制约

安徽南部燕山期中酸性侵入岩(年龄分别约为138Ma 和123Ma)的钕同位素初始比值∈_(Nd)(Ⅰ)值为—5——16.6,表明古老大陆地壳在这些岩石形成时起重要作用。江南的花岗闪长岩和花岗岩源区钕同位素组成与上溪群浅变质岩的原岩沉积物物源区相似。扬子系列的偏中性侵入岩则可能是幔源基性岩浆同化古老硅铝地壳物质的产物,但地壳物质占主导地位。     

扬子西北缘碧口块体花岗质岩体锆石U-Pb年龄、Hf同位素特征及其地质意义

对扬子西北缘碧口块体中白雀寺和大安花岗质岩体进行了锆石U-Pb定年及Hf同位素研究。结果表明,白雀寺岩体的年龄分2组:(855±6) Ma（n=23）和(917±14) Ma（n=3）。前者ε_Hf(t)为3.8~10.4,一阶段Hf模式年龄为1.00~1.27 Ga;后者ε_Hf(t)为2.0~8.1,一阶段Hf模式年龄为1.14~1.40 Ga。而大安岩体给出了6组不同的年龄：(854±10) Ma（n=3）、(801±7) Ma（n=3）、(702±10) Ma（n=3）、(565±6) Ma（n=1）、(246±7) Ma（n=1）和(207±2) Ma（n=1）。其ε_Hf(t)为-12.6~11.4（t=854 Ma）,一阶段和二阶段Hf模式年龄分别为0.95~1.90 Ga和1.02~2.55 Ga。白雀寺和大安岩体的侵位年龄分别为(855±6) Ma和(854±10) Ma,代表了扬子西北缘碧口块体内的一次重要岩浆事件,前者主体来源于中元古代新生陆壳源区,而后者主要来源于中元古代新生陆壳与晚太古代地壳物质的混合源区。此外,白雀寺岩体中(917±14) Ma的年龄代表新元古代早期的岩浆活动,而大安岩体中年轻的年龄可能记录着岩体后期所经历的复杂热改造事件。     

广西姑婆山花岗岩单颗粒锆石LA-ICP-MS U-Pb定年及全岩Sr-Nd同位素研究

LA-ICP-MS锆石U-Pb同位素年龄测定表明,组成广西姑婆山花岗岩的东、西岩体和里松岩体的年龄分别为160.8±1.6Ma、165.0±1.9Ma、163.0±1.3Ma,在误差范围内基本一致,说明整个姑婆山岩体是同一时代的产物,是燕山中期第一阶段华南大规模陆壳重熔型花岗岩浆活动的产物。姑婆山花岗岩中的各个岩体虽然形成于同一时代,但它们之间在岩石学、地球化学特征方面有一定的差异。除了主微量元素、稀土元素特征有所不同外,Rb-Sr、Sm-Nd同位素特征研究表明姑婆山西岩体的粗粒花岗岩、东岩体、里松岩体及其包体的平均(87Sr/86Sr)i=0.7064、εNd(t)平均为-3.03,反映它们的源区有较多地幔物质组分参与;而姑婆山西岩体的细粒花岗岩的(87Sr/86Sr)i=0.7173、εNd(t)平均为-5.00,具强烈的Eu亏损、高Rb/Sr值等特征,它的源区可能是由一个相对古老地壳组分和年轻地幔组分组成的混合源区。此外,姑婆山东岩体(GP-1)中发现的继承锆石的206Pb/238U年龄为806.4Ma,与杭州—诸广山—花山花岗岩带(HZH)上的赣北九岭堇青石花岗岩、广西英桥混合花岗岩的年龄相似,为HZH带新元古代的岩浆活动提供了锆石年代学方面的依据。     

新疆北部后碰撞幔源岩浆活动与陆壳纵向生长

新疆北部后碰撞幔源岩浆活动强烈。Nd、Sr和Pb同位素资料表明,在330～250Ma的后碰撞期间,有大量的幔源花岗岩类和少量的镁铁—超镁铁杂岩在上地壳侵位。与加里东、海西和喜马拉雅等造山带起源于再循环陆壳的花岗岩类不同,新疆北部后碰撞岩浆岩一般表现出ε_(Nd)(t)值高、(~(87)Sr/~(86)Sr)值相对较低、Nd和Pb模式年龄年轻等特点。阿尔泰山和天山的一些后碰撞花岗岩类可能具有陆壳源区的特点或表现出地壳物质对幔源岩浆及其分异产物有不同程度的混染,东、西准噶尔花岗岩类很少甚至没有受到陆壳物质混染。新疆北部后碰撞花岗岩类和镁铁—超镁铁杂岩主要是幔源岩浆及其分异产物在上地壳侵位的结果。这些幔源花岗岩类代表了新生的初始地壳,其时代可代表地壳形成时代。在后碰撞阶段,新疆北部的陆壳以纵向生长为特征。     

辽宁盘岭矿集区花岗岩锆石SHRIMP U-Pb年龄、Hf同位素组成及地质意义

辽宁盘岭矿集区产出在辽吉裂谷中段的草河口—草河城—黑峪一带,是一处典型的古元古代沉积盆地和多金属矿集区。矿区内发育有早白垩世的北大砬子岩体和古元古代高丽墩台岩体,与本区的矿化作用密切相关。北大砬子岩体锆石SHRIMP U-Pb年龄加权平均值为(123.9±2.2)Ma,高丽墩台岩体锆石SHRIMP U-Pb年龄加权平均值为(1 872±13)Ma。北大砬子岩体εHf(0)为负值,εHf(t)为负值(-16.86~-10.95),说明北大砬子岩体形成时壳源物质成分占主导地位,单阶段Hf模式年龄(TDM)平均为1 339 Ma,两阶段Hf模式年龄(TDMC)平均为2 043 Ma;高丽墩台岩体εHf(0)为负值,εHf(t)为正值(0.29~4.64),说明高丽墩台岩体形成时壳源物质成分占主导地位,并有地幔物质混染,单阶段Hf模式年龄(TDM)平均为2 171 Ma,两阶段Hf模式年龄(TDMC)平均为2 355 Ma。结合岩石地球化学特征得出:北大砬子岩体具有高钾钙碱性岩浆岩特征,为较高分异的过铝质I型花岗岩,是底侵岩浆提供热动力使下地壳物质部分熔融的产物,属于晚中生代大陆边缘岩浆弧环境,其与进入早白垩世古太平洋板块俯冲作用密切相关,岩浆活动直接影响盘岭矿集区内一期铜、铅、锌、钼等矿化作用;高丽墩台岩体具有低钾(拉斑)岩浆岩特征,为较高分异的过铝质I型花岗岩,是热的玄武质熔体加热下地壳使之熔融的产物,且可能有地幔物质贡献,岩体侵入诱发的一系列断裂(包括层间断裂)控制着矿集区内矿脉及岩脉的空间分布。     

甘肃辉铜山铜矿床燕山期钾长花岗岩的发现及其地质意义

文章对辉铜山铜矿床内的钾长花岗岩进行了岩石化学分析、钕同位素测量和高精度的40 Ar/ 3 9Ar法年龄测定。结果表明 ,钾长花岗岩的形成时代为 (192± 3)Ma ,相当于燕山早期 ,它们具有A型花岗岩的特征 ;成岩物质来自地幔 ,并遭受了地壳物质的混染。岩浆侵位可能与本区在燕山早期发生的陆内伸展有关     

湖南铜山岭矿区花岗闪长岩岩石成因:岩石地球化学、U-Pb年代学及Hf同位素制约

铜山岭矿区位于南岭西段,铜铅锌矿体主要围绕Ⅰ号岩体分布。通过岩石地球化学特征研究揭示岩体为准铝质弱过铝质高钾钙碱性花岗闪长岩系列,轻重稀土元素分异强烈,多数投点落入Ⅰ型花岗岩区,少数点落入S型区。微量元素判别图反映构造环境为火山弧和同碰撞环境。岩体La-ICPMS锆石U-Pb年龄(Ⅰ号成岩年龄为166.64 Ma±0.40Ma,Ⅲ号成岩年龄为148.30Ma±0.35 Ma)和Hf同位素特征(Ⅰ号岩体εHf(t)在-15.86～-22.67之间,Ⅲ号岩体εHf(t)在-10.09～-11.00之间);Ⅰ号岩体可能混入新生地壳物质,而Ⅲ号岩体源区仅是壳源。两个岩体的Hf同位素模式年龄也不相同,Ⅰ,Ⅲ号岩体两阶段模式年龄分别为2 220Ma～2 647Ma和1 841Ma～1 898Ma,反映Ⅰ号岩体源区物质时代为太古至早元古代,而Ⅲ号岩体源区时代为早元古代。综上所述,Ⅰ号岩体是以古老地壳物质为主体混入新生地壳物质形成岩浆源区而熔融形成的,形成时代是在166Ma;Ⅲ岩体,是由于150Ma前后早期挤压增厚的下地壳发生熔融而形成的。     

Evolution of the early Earth: Constraints from Nd---Nd isotopic systematics

Combined ¹⁴⁷Sm---¹⁴³Nd and the now extinct [τ(1/2)₁₄₆=103×10⁶ yr] ¹⁴⁶Sm---¹⁴²Nd isotopic systematics are reported for early Archean gneisses from Greenland (Amîtsoq and Akilia associations), and Canada (Acasta gneiss). Using both field relationships and high resolution U---Pb SHRIMP ion-probe ages, it has been possible to identify the most ancient rocks in these terrains for isotopic analyses. Preliminary ¹⁴²Nd analyses of a still limited number of samples have failed to identify terrestrial ¹⁴²Nd anomalies. Effects, if present, are limited to < 10 ppm and we have thus been unable to confirm the +33±4 ppm ε₁₄₂ value claimed by Harper and Jacobsen (1992a, b) for a single sample. From the lack of ¹⁴⁶Sm---¹⁴²Nd effects we infer that large-scale fractionation events that may have occurred in the first 200 Ma of Earth history did not leave a significant nor widespread imprint on the early Archean mantle or crust. If a terrestrial magma ocean, with associated LREE fractionation, formed as a result of planetary accretion, then it had a lifetime of at most 250 m.y. before being remixed into the Earth's mantle.

The samples analysed in this study have a range of ε₁₄₃ values including highly positive values of up to +4.2. This requires that the earliest known Archean crust was differentiated from a reservoir that was strongly depleted in the LREE as compared with chondritic compositions. In the early Archean it is proposed that the depletions in LREE are a consequence of extraction of a limited fraction of the Earth's continental crust ( < 10%) from the upper 200 km of the mantle. A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir can be extended to account for both the present-day, as well as the evolving Nd isotopic composition of the Earth's crust and mantle. In contrast to previous models, the rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched component in the early Archean mantle.     

内蒙古哈珠地区石炭纪白山组火山岩:北山北部晚古生代活动陆缘岩浆作用的产物

北山北部晚古生代岩浆事件的性质对该地区这一时期的构造演化研究具有重要意义.对内蒙古北山哈珠地区石炭纪白山组火山岩进行了系统的年代学、地球化学、Lu-Hf同位素测试,结果显示:白山组主要由玄武安山质、安山质、英安质、流纹质火山岩组成,LA-ICP-MS锆石U-Pb测年获得安山岩、英安岩、流纹岩的成岩年龄分别为325.6±1.4 Ma、313.5±3.4 Ma、314.7±1.7 Ma,时代为早石炭世晚期-晚石炭世.火山岩空间分布上表现出明显的"组成极性",区域上由北至南从中性火山岩（钙碱性系列）→酸性火山岩（高钾钙碱性系列）演化,K₂O含量与K₂O/Na₂O比值也呈相应的增加趋势,其中玄武安山岩、安山岩具有高Al₂O₃、低TiO₂以及低的Ni、Cr含量,所有样品普遍亏损Nb、Ta、P、Ti等高场强元素,呈轻稀土富集、重稀土亏损的右倾稀土元素配分模式,以上特征指示白山组火山岩形成于洋壳向南俯冲过程中的活动大陆边缘背景下.另外,白山组中安山岩具有高的ε_Hf（t）值（+7.0~+14.1）和年轻的平均地壳模式年龄T_DMC（437~891 Ma）,接近上地幔的Rb/Sr比值（0.01~0.15）,Nb/Ta比值（12.59~18.80）处于地壳平均值和地幔平均值之间;流纹岩具有相对较低的ε_Hf（t）值（4.3~8.2）和偏老的平均地壳模式年龄T_DMC（804~1 054 Ma）,Rb/Sr比值（0.80~1.73）远大于地壳平均值,Nb/Ta比值（10.66~13.08）接近地壳平均值反映岩浆源区向洋侧以新生地壳和地幔物质为主,向内陆一侧逐渐演化为更多陆壳（较老地壳）物质的加入.综合以上分析并结合前人资料,北山哈珠地区晚古生代石炭纪白山组火山岩是红石山洋向南侧马鬃山-旱山地块俯冲过程中活动陆缘岩浆作用的产物.      

Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.     

滇西花岗岩类 Pb、Sr 同位素组成特征及其基底时代和性质

澜沧江西侧的临沧花岗岩带和怒江以西的腾冲花岗岩带是滇西最主要的两个花岗岩带,它们形成于不同构造环境。Pb、Sr 同位素研究表明,腾冲花岗岩主要来源于1200-2000Ma 的上地壳(或未分异的地壳)物质,同位素组成变化范围较小,说明源区物质组分较均一。临沧带 Pb 同位素组成变化较大,其物源是上地壳、造山带和上地幔不均匀的混合物,基底时代为800-1600Ma。因此,临沧地区和腾冲地区的基底时代和性质不同,应视为两个不同的基底地体。     

Petrology and Geochemistry of the Volcanic Rocks Dredged from the Geophysicist Seamount in the Kurile Basin: Evidence for the Existence of Thinned Continental Crust

Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of ⁸⁷Sr/⁸⁶Sr ratios (0.70287-0.70652), varying ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin.     

Mixing of asthenospheric and lithospheric mantle-derived basalt magmas as shown by along-arc variation in Sr and Nd isotopic compositions of Early Miocene basalts from back-arc margin of the NE Japan arc

We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO₂ (> 1.5 wt.%; referred to as HT) and lower TiO₂ (< 1.5 wt.%; LT) basalts. HT basalt has higher Na₂O + K₂O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr (SrI) = 0.70389 to 0.70631, initial ¹⁴³Nd/¹⁴⁴Nd(NdI) = 0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na₂O + K₂O and K₂O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea.     

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf_{949 Ma}=0.28223±5 (_Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.     

大兴安岭北段新林地区晚石炭世花岗岩的岩石成因及地质意义

大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近,岩性主要为二长花岗岩,另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma,均为晚石炭世岩浆活动的产物。花岗岩具有富硅（w（SiO₂）为66.77%~75.85%）、富碱（w（Na₂O+K₂O）为7.41%~8.69%）、高铝（w（Al₂O₃）为12.90%~16.22%）,低MgO、CaO、TiO₂的特点,属于钙碱性系列;铝饱和指数（A/CNK）为1.06~1.44,为过铝质岩石;镜下未见原生白云母、堇青石、石榴石等富铝矿物,不同于富铝的S型花岗岩;而w（P₂O₅）与w（SiO₂）负相关,呈I型花岗岩特征;富集LREE和Ba、Rb、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,与后造山I型花岗岩特征相似,应形成于拉张的构造环境。花岗岩的⁸⁷Sr/⁸⁶Sr为0.712 938、¹⁴³Nd/¹⁴⁴Nd为0.512 386,（⁸⁷Sr/⁸⁶Sr）_i值为0.704 4,ε_Nd（t）值为-1.09,T_DM2=1 172 Ma,源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果,大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。