胶北新太古代两类片麻岩的岩石地球化学特征和成因指示

在胶北三口探矿深钻中识别出两类新太古代片麻岩,一种是TTG片麻岩,数量较多;另一类是石榴黑云斜长片麻岩,数量相对较少,以夹层状发育于TTG片麻岩中.已有年代学资料显示,两种片麻岩原岩均形成于～2.5Ga.TTG片麻岩SiO2含量为53.95％ ～ 75.56％,Na2O为1.51％～5.94％,Na2O/K2O＞1;贫Fe2O3T(2.22％～9.54％)和MgO (0.53％ ～4.87％),Al2O3多大于15％(10.45％ ～ 17.64％),A/CNK=1.18～1.75,说明其属于高铝TTG岩系.富集大离子亲石元素(如Rb、Ba、Sr),亏损高场强元素(如Nb、Ta、Zr、Hf),Sr含量高(204×10-6～2906×10-6),Sr/Y比值高(7.30～355).∑REE含量(40.4×10-6～ 512×10-6)相对较高,轻重稀土强烈分异((La/Yb)N=11.5 ～ 121),Eu无异常或弱正异常(δEu=0.79～1.89).以上地球化学性质表明,胶北TTG片麻岩是含水玄武质岩石高压下部分熔融形成的,残留相主要是石榴石、角闪石和含Ti矿物,没有斜长石.全岩Nd和锆石Hf同位素数据显示其模式年龄与形成年龄相近,表明岩石源区可能是来源于亏损地幔的2.57～2.64Ga的初生玄武质地壳.另外,胶北TTG片麻岩高的Mg#值(40 ～67)和Cr(147×10-6～371×10-6)、Ni(6.68×10-6～ 156 × 10-6)含量,表示与地幔楔发生了反应,结合Nb、Ta负异常以及源区为初生的玄武质地壳等特征,说明研究区TTG形成于与俯冲有关的岛弧环境,而非加厚的下地壳.DF值和K-A关系图显示,石榴黑云斜长片麻岩原岩是泥质-粉砂质沉积岩.Cr/Zr比值(0.90 ～1.99,平均为1.62)和Th/Sc比值(平均为0.30)与太古宙沉积岩相应值相近(分别为1.44和0.4),稀土配分型式与太古宙沉积岩类似,表明源区物质以长英质为主.绝大多数样品显示轻稀土富集、重稀土平坦的特征,总稀土含量平均为176×10-6,(La/Yb)N比值平均为14.7,LREE/HREE平均为3.3,Eu无异常或微弱的负异常(δEu=0.69～0.92,平均为0.84),这些特征值表明原岩形成的大地构造背景为岛弧或主动大陆边缘.Grt-Bi地质温度计得到的变质温度为549 ～663℃,其中黑云母化学成分与角闪岩相中黑云母特征相同,共同指示变质程度为角闪岩相,可能代表了～2.5Ga麻粒岩相-高角闪岩相变质作用的退变质阶段.石榴黑云斜长片麻岩原岩与TTG属同一时代(～2.5Ga)的产物,形成的构造背景类似(岛弧或主动大陆边缘),说明当时该区有俯冲过程发生;TTG岩浆事件和～2.5Ga的变质事件大体同时,但变质事件较岩浆事件稍晚,可以用俯冲后的碰撞来解释.因此,研究区在～2.5Ga可能发生过一次重要的俯冲-碰撞拼合事件:TTG和石榴黑云斜长片麻岩原岩形成于俯冲阶段,后期发生碰撞引发麻粒岩相-高角闪岩相的区域变质作用.     

冀西北晚太古代岩石单元的地质关系: 对早期陆壳形成的指示

冀西北地区出露晚太古代桑干杂岩和崇礼杂岩构成的灰色片麻岩(TTG片麻岩)-麻粒岩地体和红旗营子群角闪岩相变质火山沉积岩系.本文对尚义地区红旗营子群中变质火山岩中的变玄武安山岩(角闪斜长片麻岩)进行了锆石LAMC-ICPMS定年,获得207 Pb/206Pb加权平均年龄2528±12Ma(2σ,MSWD=0.1,n=34),这一年龄代表了该岩石的形成时代.虽然这一年龄非常接近灰色片麻岩的形成时代,野外地质关系指示红旗营子群的形成早于灰色片麻岩及其中的闪长质侵入体.在灰色片麻岩分布区,红旗营子群和一些变质辉长质岩石呈岩片、透镜体或者“岩墙”状分布.变质辉长质透镜体或者“岩墙”状岩片以周边发育串珠状小透镜体,以及不见细粒边部和岩墙“手指”可以与该区分布的早元古代变质基性岩墙相区别.这些变辉长质岩石可能是红旗营子群火山岩系根部或者岩浆房的残留体,也可能部分是更早期地壳的残片.红旗营子群火山岩系的岩石化学特征可以和灰色片麻岩及闪长质片麻岩岩体分别进行对比.地(桑干杂岩和崇礼杂岩)组成了该地区晚太古代大陆二维地壳结构.认识这种地壳结构在空间上的延续性,并识别相关地质单元在时代和物质上的演化关系,可以揭示华北克拉通晚太古代大陆地壳的形成机制.     

长英质片麻岩中堇青石的一种可能的形成机制——以南极拉斯曼丘陵高级区为例

本文通过对南极拉斯曼丘陵长英质片麻岩变质过程中堇青石与其它矿物之间结构关系的研究 ,识别出明显不同的两种组合Pl Kfs Qtz(Grt)和Crd Opq Spl±Qtz,认为区内高级变质作用向深熔作用转化过程中发生了长英质组分和镁铁质组分的分凝。分凝出的长英质熔体与堇青石的形成没有直接关系 ;镁铁质组分较富Mg、Fe ,贫Si、Ca ,当镁铁质组分达到一定的富集程度时即形成堇青石。时间上 ,堇青石形成于降压过程中发生的深熔作用的晚期     

Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai

Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called Older Granites of Egypt.²⁰⁷Pb/²⁰⁶Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma. A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feiran in south-west Sinai suggest emplacement of the protolith at 796 ± 6 Ma and this is comparable with ages for granitoids in north-east Sinai and southern Israel. None of the above gneisses is derived from remelting of older continental crust, but they are interpreted as reflecting subduction-related calc-alkaline magmatism during early Pan-african magmatic arc formation.     

对“托格拉克布拉克杂岩”的认识

新疆库鲁克塔格地区辛格尔南的达格拉克布拉克一带的片麻岩系是新疆最古老的变质岩系 ,是太古宙高级变质区的产物 ,其中达格拉克布拉克片麻岩套为具有太古宙特殊色彩的 TTG岩系 ,并经过了多次变形变质作用 ,为一套无序的变质深成岩 ,具中压相系角闪岩相区域中高温变质作用 ,具有太古宙典型的穹盆构造变形特征 ,是塔里木古老地块的古陆核     

吉南夹皮沟金矿带鞍山群的解体及主要地质事件序列的建立

位于华北地台最北缘的夹皮沟金矿带太古代鞍山群主要由两类双峰式灰色片麻岩组成,其原岩均为ＴＴＧ类古侵入体夹拉斑玄武岩质暗色包体,岩石化学上与国外许多太古代高变地体中的灰色片麻岩相似,因而不能作为地层来对待。原“鞍山群”可重新划分为白山镇片麻岩、夹皮沟片麻岩和北西向糜棱岩带三大岩石单元,两类灰色片麻岩之间有一些相似处,但也有许多重要差别,可能代表了不同的构造地体。就现有同位素年代数据和野外产状建立了本区主要地质事件的序列。上述岩石单元的划分及主要地质事件序列的建立对本区构造属性的确定及地壳演化的研究有重要的意义。     

西秦岭天水市元龙地区新元古代花岗质片麻岩锆石LA-ICP-MSU-Pb定年及其地质意义

西秦岭元龙花岗质片麻岩位于天水市伯阳-元龙之间的渭河断裂北侧.花岗质片麻岩中锆石Th/U比值较高,阴极发光图像显示锆石内部发育振荡环带,具岩浆成因特点.LA-ICP-MS的锆石U-Pb同位素206Pb/238U加权平均年龄为(924.2±2.7)Ma(MSWD=0.67),表明花岗岩岩体形成于新元古代,反映晋宁期西秦岭北缘曾存在一次俯冲碰撞事件,与Rodinia超大陆的汇聚事件一致.     

Petrological and Geochemical Characteristics of Archean Gneisses and Granitoids from Bastar Craton, Central India – Implication for Subduction Related Magmatism

The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.     

Raman spectra of polycrystalline microdiamond inclusions in zircons,and ultrahigh-pressure metamorphism of a quartzofeldspathic rock from the Erzgebirge terrane,Germany

ABSTRACT

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm^?1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm^?1. These data combined with previously reported diamond band data (1331–1337 cm^?1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disordered sp³-carbon, but show downshifting of the Raman band from the ideal value (1332 cm^?1), and have a broader diamond band (FWHM >3 cm^?1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm^?1. Our study together with previously reported C-isotopic compositions (δ¹³C, ?17 to ?27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of the P–T time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.     

关于三阶段铅演化体系：以大厂锡矿床及Lewisian片麻岩为例

以大厂锡-多金属矿床、Lewisian片麻岩为例,讨论了成矿/成岩过程中铅的三阶段演化轨迹。在206Pb/204Pb对207Pb/204Pb图解上,大厂硫化物铅的数据点构成了Pb-Pb等时线,该等时线给出了1.508Ga的矿化源岩年龄,说明成矿物质主要来自中元古代基底岩石——四堡群。四堡群本身又可能来自太古宙(2.86Ga)老陆壳,故大厂硫化物中的痕量铅应该具有三阶段的演化史。用三阶段铅演化模式解释大厂硫化物铅同位素资料,结果表明,太古宙老陆壳从上地幔源区分离出来时,其μ2=11.93,大厂矿床矿化源岩的μ3=9.76。不同变质程度的Lewisian杂岩全岩铅具有不同的演化历史:北部区角闪岩相片麻岩具有两阶段的演化史,中心区麻粒岩相片麻岩具有三阶段的演化史。北部区片麻岩两阶段Pb-Pb等时线给出了2.951Ga的成岩年龄,其μ1=7.81。中心区片麻岩三阶段Pb-Pb等时线给出了2.68Ga的变质年龄,其火成源岩的μ2=3.51。在解释异常铅等时线时,对其演化阶段级次的判别具有十分重要意义,它直接涉及不同演化阶段U-Pb体系μ值的计算。资料证明,如果把三次等时线当作二次等时线加以解释,则会导致一系列错误结论的产生。在一般情况下,用三阶段铅演化模式解释某组铅同位素资料时,只是假设第二阶段U-Pb体系是处于封闭状态的,但无任何直接证据。Lewisian北部区片麻岩两阶段铅演化体系与中心区片麻岩三阶段铅演化体系共存的情况为解决上述问题提供了极好的资料。