安徽庐枞盆地隐伏侵入岩的LA ICP MS定年及其构造意义

庐枞中生代火山盆地位于长江中下游断陷带内,地处扬子板块的北缘。庐枞盆地内火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山4组火山岩以及34个出露地表的侵入岩体。这些侵入岩体的时空格架和岩浆演化等相关研究工作已经广泛开展,但对于盆地新发现深部隐伏岩体的研究尚未开展,制约了对庐枞盆地岩浆作用及深部成矿预测的深入研究。本次工作在详细野外地质工作的基础上,系统对盆地北部清水塘、刘墩、乔冲、牛头山、李家庄5个代表性隐伏岩体开展岩石学、地球化学和年代学研究,通过侵入岩详细镜下鉴定和锆石LAICP-MS U-Pb定年等方法,确定其主要岩性为二长岩和正长岩,岩石全碱含量均较高,准铝质或过铝质;稀土含量中等,较强Eu亏损,呈略右倾的海鸥型稀土配分模式,成岩时代分别为133.2±1.4Ma、131.3±1.3Ma、131.0±1.4Ma、131.2±1.2Ma和130.9±1.2Ma,均形成于早白垩世。通过与庐枞盆地不同侵入岩的系统对比,提出这些岩体与庐枞盆地早期侵入岩时代接近,岩石学特征类似,应为同一岩浆作用的产物,形成于区域岩石圈伸展的构造环境。     

Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province,NW China

The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have ε_Nd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr₂O₃ contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO₂ and a volatile-rich condition.     

松辽盆地营城组中基性火山岩储层特征及成储机理

松辽盆地北部安达-汪家屯地区钻井大量揭示营城组中基性火山岩,并在其中获得了工业气流。TAS图解可划分出8种岩性,储层研究中归纳为6大类岩石类型,逾80%的有效储层分布于玄武岩和粗面岩之中。总结了4类11种储集空间类型及其特征,原生储集空间类型由喷发方式和喷发环境决定、其分布受火山岩岩相控制,次生储集空间发育程度受充填作用、溶解作用、风化淋滤作用和构造作用等影响。熔岩冷凝单元由单个或多个岩流单元组成、储层发育于其顶部和上部,每个冷凝单元构成相对独立的储层单元。火山岩旋回和期次顶部、近火山口和近源相带、靠近断裂等位置是储层发育的有利部位。玄武岩具有延伸范围广(10～22km)、储层厚度小(7～39m)等特点,储层勘探应优先寻找位于火山岩旋回和期次顶部的储层单元;粗面岩延伸范围小(3.1～6.3km)、储层厚度大(100～200m),储层勘探应以寻找具备有利烃源和盖层条件的储集岩体为目标。     

东秦岭大别山段中酸性小岩体成矿规律研究

东秦岭大别山段矿化中酸性小岩体集中分布在秦岭钼（钨）成矿带东段,是我国重要的钼（钨）矿带之一。矿化中酸性小岩体多是受北北西向和北北东向断裂构造控制的复式杂岩体。形成环境从浅成—超浅成—深成相,Mo元素丰度由高—低,分为浅成—超浅成斑岩钼（铜）矿化、中—深成斑岩—矽卡岩型钼（钨、铜）矿化和浅成斑岩型钼矿化三种类型。小岩体主导的钼（铜）矿床具有明显的金属（元素）或矿物组份分带,表现为各带间金属元素含量递变及不同矿物组合按序析出,成矿物质主要来源于下地壳。     

Hydrothermally induced diagenesis: Evidence from shallow marine-deltaic sediments,Wilhelmøya,Svalbard

Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelm?ya(Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent(～12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance(more than five times the thickness of the sill)from the intrusions. We present evidence that the sedimentary sequence can be divided into two units:(1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃,and(2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures(around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ～40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization(illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals.     

Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abundant glass is present along grain boundaries in coarse-grained,glass-bearing, crystalline gabbroic and peridotitic nodulesentrained and erupted in lavas from Iceland, Santorini and MaunaLoa (Hawaii), even when the total porosity is less than a fewvolume per cent. The glass films vary from a few microns toa few tens of microns thick, and are associated with stringsof small lensoid grain boundary pockets formed by impingementduring crystal growth. Additional porosity occurs as extensiveliquid-filled pockets adjacent to included grains within oikocrystsand as large triangular pockets formed by impingement of planar-sidedgrains. Interstitial material within glass films, and the irregularityof film thickness along a single grain boundary, suggest thatthe present pore structure is representative of the pore structurebefore entrainment and eruption. Pore geometry is consistentwith a dominant control by crystal growth during solidification,with little or no evidence for control by minimization of internalenergies driven by textural equilibration. Similarities betweenliquid distribution in the crystalline nodules and that of late-stage,interstitial phases in fully solidified mafic cumulates fromthe Rum and Skaergaard intrusions demonstrate that the crystallinenodules provide information about the latest stages of solidificationin slowly cooled mafic plutons. The highly permeable networkof intersecting liquid films, lenses and pockets may promotein situ crystallization in the solidifying mush, explainingthe common presence of adcumulates in such intrusions. KEY WORDS: textures; liquid distribution; mafic cumulates; crystalline nodules     

新疆北山盐滩断裂南一带高级变质岩系的发现、地质热力学特征及其大地构造意义

新疆北山盐滩断裂南一带从原划奥陶-志留系地层中解体出古老的角闪岩相-麻粒岩相变质岩和变质侵入岩体,主要岩性为黑云母斜长片麻岩、条带状混合岩、斜长角闪岩、及少量变粒岩、麻粒岩等。其中首次发现了代表中、下地壳深变质作用的中-高压基性麻粒岩。详细的野外调查、岩相学及矿物学研究表明,该基性麻粒岩主要由透辉石、紫苏辉石、褐色普通角闪石及拉长石组成。其演化经历了由麻粒岩相岩石经退变质反应而成为角闪岩相变质岩的过程。研究表明,该区麻粒岩中的单斜辉石及角闪石矿物学成因均属于变质成因,其中单斜辉石形成环境为高压型,而角闪石形成环境为中低压环境。麻粒岩相变质岩形成于约1.25GPa(42km埋深)以下的中高压-高压环境,其形成温度约841℃;而退变质作用下的角闪岩相变质岩应形成于650~657℃的温度范围之内,压力为0.460~0.495GPa之间,相应的代表埋深约为16.5km的中低压型环境。该区高级变质岩及其基性麻粒岩的发现,将对该区地壳成分、麻粒岩的成因与变质作用以及所处的大地构造背景演化等研究无疑将起到非常重要的作用。     

辽宁清原变质镁铁质火山岩Sm－Nd同位素年龄及其地质意义

在辽宁清原浑南群变质镁铁质火山岩中首次获得的Ｓｍ－Ｎｄ全岩等时线年龄为３０１８±１１０Ｍａ，证实了本区中太古代表壳岩的存在。浑南群为高角闪岩相至麻粒岩相。变质镁铁质火山岩的原岩相当于ＴＨ１型拉斑玄武岩，稀土配分为轻稀土亏损型模式。岩石的εＮｄ（ｔ）值为＋０．１９±０．１７，物源来自亏损相对较小的地幔。     

东昆仑夏日哈木铜镍矿镁铁质超镁铁质岩体岩相学、锆石U-Pb年代学、地球化学及其构造意义

文章报道了东昆仑夏日哈木铜镍矿成矿岩体的岩相学、锆石U-Pb年代学、全岩地球化学以及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及其形成的构造环境。夏日哈木Ⅰ号镁铁质超镁铁质岩体位于昆中基底隆起花岗岩带中段,北侧靠近昆北断裂。岩体走向NEE,剖面呈平缓的“岩盆状”,地表出露面积约0.7 km2。该杂岩体主要由辉长苏长岩、斜方辉石岩、橄榄辉石岩、斜长二辉橄榄岩和方辉橄榄岩组成,橄榄岩相和辉石岩相是主要的Cu、Ni赋矿岩相。镁铁质超镁铁质岩体主量元素具有低硅(w(SiO₂)=36.68%~52.58%)、低钛(w(TiO2)=0.13%~0.47%)、高镁(w(MgO)=10.91%~35.81%)、贫碱(w(K₂O+Na₂O)=0.26%~1.95%)的特征,属亚碱性系列岩石,m/f为3.88~6.29,属铁质超基性岩类(m/f=2~6.5)。岩石稀土元素球粒陨石标准化配分模式为轻稀土富集型,(La/Yb)N=1.44~2.98,Eu异常不明显,相似的稀土元素配分模式说明岩体的同源性。岩石富集大离子亲石元素(Rb、Th、U、K),相对亏损高场强元素(Nb、Ta)。岩体中存在新元古代花岗岩的捕虏体以及La/Yb、Ce/Yb、Th/Yb、Nb/La、La/Sm比值显示岩体经历了有限的地壳混染。辉长苏长岩锆石的LA-MC ICPMS、U-Pb年代学研究表明,岩体形成年龄为(423±1) Ma,MSWD=0.14,属晚志留世。锆石的176Hf/177Hf比值为0.282 628~0.282 833,相应的εHf(t)均为正值(4.0~10.9),Lu-Hf的单阶段模式年龄(tDM1)为610~875 Ma,平均值为788 Ma,大于锆石U-Pb年龄。研究认为,岩体的岩浆源区主要为亏损地幔,可能有早期流体交代的富集岩石圈地幔组分的加入和地壳物质的混染。结合区域构造演化,文章认为岩体形成于碰撞后伸展的构造环境,可能与俯冲板片的断离作用有关。岩浆演化过程中橄榄石和斜方辉石的分离结晶作用和地壳中硫的加入可能是促使岩浆体系达到硫饱和的主要机制。     

南秦岭西坝花岗质岩体矿物学特征及成岩意义

西坝花岗质岩体位于南秦岭构造-岩浆带中部,为南秦岭印支期花岗岩带五龙岩体群的重要组成部分,与双王金矿有密切的空间关系。该岩体主要由石英二长闪长岩和花岗闪长岩组成,岩体中发育较多的镁铁质包体。通过对岩体及镁铁质包体的系统的矿物学特征研究发现,本区花岗质岩体属于I型花岗岩类,成岩物质主要来源于地壳,并且有地幔物质的加入。岩体及镁铁质包体显示有多种壳-幔岩浆混合的岩相学证据:如包体的冷凝边、斜长石环带成分显示核部和边缘偏基性,而幔部偏酸性,包体中见大量的长针状磷灰石及角闪石,包体中有两种产状的角闪石等。根据矿物温度压力计计算,西坝岩体的结晶温度为646.72~703.84℃,压力为1.67×108~3.66×108Pa,平均为3.12×108Pa。岩体形成于相对高氧逸度(lgfO2为-16.16×105~-17.06×105Pa)、中等水逸度(fH2O为46.82×105~136.85×105Pa)和氢逸度(fH2为0.32×105~0.91×105Pa)环境。岩体的这种高氧逸度条件有利于金进入熔体相或流体相,是金矿形成的有利条件。