红旗岭镁铁-超镁铁岩侵入体及铜镍硫化物矿床的成岩成矿机制

红旗岭铜镍矿床地处华北地台与吉黑地槽系接触带--辉发河断裂北侧.区内出露30多个镁铁-超镁铁质岩体,其中1、7号超镁铁岩体中赋存铜镍硫化物矿(床)体.含矿岩体分相明显,各类岩石均具堆积结构.铜镍矿体呈似板状、脉状、透镜状及囊状赋存于超镁铁岩体底部橄榄辉岩相中.岩石学和地球化学研究表明,7号岩体形成以流动分异为主,1号岩体为重力分异;原始岩浆属拉斑玄武质,块状矿石系压滤作用产物,后续岩浆的补给和混合补充了成矿物质,硫化物不混溶程度受挥发分制约,矿床属岩浆深部熔离分异成因,成矿时代为印支期.     

熊耳山地区太古宙超镁铁质岩的地球化学特征及成因探讨

熊耳山地区太古宙超镁铁质岩在空间分布上具有不均一性、成带性和多方向性特征 ,岩石以橄榄岩、辉石岩、角闪石岩为主 ,蚀变强烈。岩石贫钙、镁 ,低铝、K2 O Na2 O,稀土总量较高 ,轻重稀土分馏显著。地球化学特征表明大部分超镁铁质岩体为岩浆侵入成因     

分离的大陆板块边缘镁铁－超镁铁岩系Th／Hf—Ta／Hf判别

本文在论述了Ｔｈ／Ｈｆ－Ｔａ／Ｈｆ关系判别岩浆岩源区大地构造环境机理的基础上，根据世界上典型大地构造环境区岩浆岩系的Ｔｈ、Ｔａ、Ｈｆ特征，提出了分离的大陆板块边缘镁铁－超镁铁岩的Ｔｈ／Ｈｆ—Ｔａ／Ｈｆ判别标志：Ｔｈ／Ｔａ＝１．８～４．０，Ｔａ／Ｈｆ＞０．１。     

铝对铬成矿地球化学的依存性和对抗性

基于岩浆铬矿床与超镁铁质岩石的关系，从元素亲合性和共生规律的角度揭示了铝对铬成矿制约的主导作用，分析了铝和铬亲石、亲氧的二重性及其晶体化学的依存关系。论证了超镁铁Ｓｉ－Ｏ体系中铝丰度过高或过低均不利于铬的成矿。揭示了制约元素分散富集的因素都是在一定条件下互为依存和转化的，是在动态中的矛盾的对立和统一。     

Metamorphic evolution of garnet-bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan

Garnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (> 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.     

Geochronology and petrogenesis of the post-orogenic Cu–Ni sulfide-bearing mafic–ultramafic complexes in Jilin Province, NE China

Northeastern (NE) China is a well-documented example of a collisional zone characterized by widespread post-orogenic granites and mafic–ultramafic complexes. Based on a study of the Hongqiling and Piaohechuan Cu–Ni sulfide-bearing mafic–ultramafic complexes in central Jilin province, we present geological, petrological, geochemical and geochronological data which indicates their post-orogenic origin.The Hongqiling complex comprises pyroxenite, olivine websterite, lherzolite, gabbro and leucogabbro. Zircon U–Pb SHRIMP analyses on a leucogabbro of the Hongqiling complex yield a weighted mean ²⁰⁶Pb–²³⁸U age of 216±5 Ma. The Piaohechuan complex is composed of gabbro, pyroxenite and dolerite, exposed as dikes. A plagioclase-bearing pyroxenite has a U–Pb zircon weighted mean ²⁰⁶Pb–²³⁸U age of 217±3 Ma, identical to that of the Hongqiling complex. These ages are coeval with the emplacement of A-type granites in the area, but slightly younger than the regional metamorphism (240 Ma) and syn-orogenic granitic magmatism (246±4 Ma). This suggests that these mafic–ultramafic complexes are post-orogenic in origin. The age data also indicated a short period of lithospheric stabilization of about 30 Ma after cessation of orogenic activity.Geochemical investigation indicates that the primary mafic magma was a lithospheric mantle-derived basalt resulting from the upwelling of asthenosphere due to lithospheric delamination during post-orogenic processes. The magmatic source was contaminated by a small amount of crustal material, and subsequent crystal fractionation resulted in the Cu–Ni mineralization.The widespread occurrence of mafic–ultramafic complexes in the Xing'an–Mongolian Orogenic Belt of NE China and in the Altay–Tianshan–Junggar Orogenic Belt of Northern Xinjiang indicates that mafic intrusions are an important magmatic suite that evolved during post-orogenic processes. Portions of this mafic magma could have underplated the lower crust, and served as the heat source for associated late-stage granitic magmas.     

苏鲁超高压变质带超镁铁岩深部碳酸盐交代作用的证据

中国大陆科学钻探工程第三先导孔(PP3钻孔),位于秦岭-大别-苏鲁超高压造山带的东部,赣榆岗上超镁铁岩体中。钻孔中超镁铁岩的岩性包括纯橄岩和石榴超镁铁岩等,该超镁铁岩的SiO_2含量均值在43.68%,变化范围为41.9%~47.11%;MgO均值在44.71%,变化范围为47.12%~48.62%;Cr_2O_3均值在0.39%,变化范围为0.30%~0.47%;CaO均值0.12%(变化范围为0.006%~0.34%);Na2O均值在0.05%(变化范围为0.005%~0.32%)。超镁铁质岩的Mg#(Mg/(Mg+Fe)×100)稳定在91.9~93.0之间,Cr#(Cr/(Cr+Fe)×100)均值在38,较高;变化在30~45之间,变化小。其中闪石化金云母超镁铁岩具有最低的值(Cr#=19)。Ni含量在2100×10~(-6)~2500×10~(-6),CaO含量均值为0.13%,Al_2O_3含量均值为0.41%。岩体成分均一,表现为高镁,低钙和低铝的特征。超镁铁岩的不相容元素和稀土元素总量很低,稀土元素总量均值在0.60×10~(-6)。(La/Yb)N比值在6.9~51.2,均值在16.1,重稀土元素严重亏损,是中国东部最亏损的地幔岩之一。超镁铁岩中橄榄石成分(Fo在88.7~93.1之间,Fa在6.8~11.1之间),从早期到晚期,岩石Fo值从93→91~92.4→88.7~89.1。铬尖晶石Cr#值从51到89变化,TiO_2和MnO_2值分别低于0.26%和0.46%,晚期铬尖晶石Cr#值增大,Ti含量减小。单斜辉石由透辉石(Wo_(45.9)8En_(47.89)Fs_(2.73)Ac_(3.39))和顽透辉石(Wo_(27.61)En_(68.78)Fs_(2.27)Ac_(1.34))两种,透辉石(Cpx_Ⅰ)在顽透辉石(Cpx_Ⅱ)中呈被交代的残余粒状。角闪石和金云母呈明显的条带状和脉状,局部发育。岩石具有弱的Na和K交代作用,较高的Sr和Ba等元素,亏损高场强元素(HSFE),以及特征的稀土元素和微量元素配分曲线;岩石中主要组成矿物橄榄石从早期到晚期,矿物Mg#指数下降(Mg#从93→88);单斜辉石中可看到透辉石被顽透辉石交代现象;次生角闪石和金云母的形成等特征都显示超镁铁岩经历了碳酸盐交代作用。在超镁铁岩矿物橄榄石中发现细小白云石和菱镁矿等碳酸盐矿物更有力证明了苏鲁超高压变质带超镁铁岩经历了深部碳酸盐交代作用。PP3钻孔超镁铁岩属于强烈亏损地幔岩区域,单斜辉石和角闪石等含水矿物在超镁铁岩中含量低,交代作用的范围和规模有限,且交代作用在动力学上是快速的和不平衡的过程。     

内蒙古东乌旗早二叠世超镁铁岩的发现及其构造意义

为研究东乌旗晚古生代地幔性质和造山演化特征,对在东乌旗西部首次发现的超镁铁岩—辉闪橄榄岩进行系统的岩相学和元素地球化学研究。东乌旗辉闪橄榄岩主要由橄榄石、角闪石、斜方辉石及少量斜长石、单斜辉石组成,SiO_2含量介于42.84%~43.96%、MgO(24.10%~26.10%)、Na_2O+K_2O(1.52%~2.32%,小于3.5%),低m/f(3.03~3.54)比值和高FeO~T(12.67%~14.33%)的含量、高Mg~#(76.42~79.20),属铁质超镁铁岩和拉斑玄武岩系列。岩石稀土总量较高(∑REE=39.57×10~(-6)~83.32×10~(-6)),轻稀土(LREE)相对于重稀土(HREE)明显富集[(La/Yb)_N=4.04~7.66],Eu异常不明显(δEu=0.90~0.95),稀土元素球粒陨石标准化配分模式表现为轻稀土相对富集的右倾型。岩石富集大离子亲石元素(LILE)Cs、Rb、Ba、Sr、K等,相对亏损高场强元素(HFSE)Nb、Ta,具明显的Nb、Ta、Ti负异常,而又有别于强烈Nb、Ta亏损的岛弧岩浆岩;相容元素Cr(769×10~(-6)~2480×10~(-6))、Ni(454×10~(-6)~901×10~(-6))含量较高;低Th/U(2.54~3.03)、Nb/U(3.40~12.85)比值和高La/Nb(2.60~3.63)、Ba/Nb(43.11~72.52)、Zr/Y(5.45~7.83)比值。综上,结合区域最新研究成果,我们认为辉闪橄榄岩来源于受俯冲流体交代的尖晶石相地幔橄榄岩部分熔融,上升过程中受地壳物质不同程度的混染,形成于早二叠世板内伸展构造体制,与古亚洲洋闭合之后板内非造山作用有关。这一认识填补了早二叠世幔源岩浆事件和非造山阶段超镁铁质岩石记录的空白。     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

扬子陆块西缘江浪穹窿超基性岩的成因:锆石U-Pb定年、岩石地球化学及Sr-Nd同位素

江浪穹窿位于扬子陆块西缘,本文作者在穹窿南部新发现一套侵入于二叠系及志留系的超基性岩,岩石主要由蛇纹石(约60%)、橄榄石(约30%)和少量磁铁矿(约5%)、角闪石(约5%)组成。为探讨超基性岩的成因,本文进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学及Sr-Nd同位素研究。定年结果表明,超基性岩中发育大量2427~430 Ma的捕获锆石,最年轻一组岩浆锆石²⁰⁶Pb/²³⁸U加权平均年龄为222.3±4.4 Ma(MSWD=1.9,n=6)。主微量元素分析显示岩石：(1)具有低的SiO₂含量(46.76%~39.07%)、高的Mg#值(82.3~74.0)与Cr、Co、Ni丰度;(2)稀土元素含量(ΣREE平均31.8 μg/g)与(La/Yb)_N值(5.26~1.38)偏低,稀土配分型式较为平坦,具有较弱的Ce负异常(Ce/Ce^*=0.80~0.67);(3)富集大离子亲石元素Rb、Ba和U,亏损高场强元素Zr和Hf;(4)(Th/Yb)_PM值(29.8~1.56)、(Th/Ta)_PM值(0.22~0.03)、(La/Nb)_PM值(1.91~0.39)及La/Sm值(5.88~1.11)较低。Sr-Nd同位素分析显示,超基性岩具有较低的(⁸⁷Sr/⁸⁶Sr)_i值(0.706872~0.702598)和高的ε_Nd(t)值(8.02~5.64),成分接近于亏损地幔和岛弧玄武岩,计算表明地壳物质的混染程度低于5%。结合前人研究成果,本文认为超基性岩结晶年龄为222.3 Ma,可能形成于古特提斯洋闭合阶段的岛弧背景;原始岩浆来自高度部分熔融的地幔源区,上升侵位过程中可能经历了铬铁矿与橄榄石的分离结晶作用。此外,捕获锆石的年龄谱反映江浪穹窿很可能存在太古宙—古元古代变质基底,并且具有Rodinia超大陆会聚—裂解以及泛非事件的地质年龄记录。