Geochemical variation in the Penmaenmawr intrusion (North Wales)

To determine the pattern of geochemical variation within the Penmaenmawr intrusion, 41 rocks were analysed for major oxides and 15 minor elements. The results indicate pulsational emplacement of sheets of magma from a parental body at depth undergoing crystallization differentiation. These sheets now strike NE–SW and dip southeastward. The resultant intrusion shows a range of SiO₂ contents from 57·3% to 65·6% (H₂O-free). Geochemical evidence suggests that it was derived from an intermediate parental magma and is possibly related to parts of the Conwy Rhyolite Formation. © 1997 John Wiley & Sons, Ltd.     

西藏那曲地区嘎加组闪长玢岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

西藏那曲地区的中—晚三叠世嘎加组中存在一套闪长玢岩,对这套闪长玢岩进行了LA-ICP-MS锆石U-Pb同位素测定,获得的年龄为115.4±0.6Ma,属早白垩世,与区域上多尼组和则弄群时代一致。闪长玢岩SiO2含量均值为57.96%,富集Rb、Th元素,亏损Nb、P、Ti元素,与上地壳具有相似的蛛网图曲线。εHf(t)值在2.0～4.1之间,平均值为3.0,岩浆来自新生地壳。结合地球化学和构造环境综合分析,闪长玢岩可能形成于拉萨地块和羌塘地块碰撞环境,115Ma时研究区洋壳俯冲作用已结束,进入陆—陆碰撞的演化阶段,随着羌塘地块与拉萨地块的碰撞,地壳不断加厚,慢慢聚集在浅部岩浆房中的大量岩浆把已经加厚的地壳不断加热,从而引起上覆地壳的部分熔融。     

青藏高原南羌塘荣玛晚侏罗世闪长岩:班公湖——怒江特提斯洋向北俯冲产物

班公湖—怒江特提斯洋的演化一直以来备受关注,对于其俯冲极性存在不同认识。报道在青藏高原南羌塘荣玛地区新发现的晚侏罗世闪长岩年代学、地球化学和锆石Hf同位素特征。LA-MC-ICP-MS锆石U-Pb年龄结果表明,岩石形成于149.7±0.8Ma,时代为晚侏罗世。岩石呈富碱,低Mg O、TFe O、Ca O,富集轻稀土元素和Rb、Th、U、K等元素,亏损Ta、Nb、P、Ti等元素。锆石εHf(t)值为正值,并且具有较大的变化范围(+4.26~+9.29),指示岩浆来源于幔源和壳源岩浆的混合。结合区域地质背景认为,荣玛地区晚侏罗世闪长岩为班公湖—怒江特提斯洋向北俯冲所引起的幔源和壳源岩浆混合作用的产物。     

西准噶尔托里县布尔克斯台岩体岩浆混合成因的确认及其地质意义

布尔克斯台岩体中存在大量闪长质微细粒包体, 在阿达依金矿外围最为发育。包体带状分布, 大小不一, 具有细粒-微细粒结构和斑状结构等典型的岩浆结构, 与寄主岩石界线或截然或渐变过渡, 形态多为长条状、扁(椭)球状等, 显塑性流变特征, 长轴与达尔布特断裂延伸方向基本一致。镜下见混合带, 主要矿物边缘熔蚀呈不规则状, 斜长石异常环带发育, 且斜长石捕虏晶中包裹了许多细小的暗色矿物, 包体中发育独特的针状空心磷灰石。这些闪长质微细粒包体的岩相学特征佐证了布尔克斯台岩体为一岩浆混合花岗岩, 表明达尔布特构造-岩浆带发生过强烈的壳-幔岩浆混合作用。闪长质微细粒包体与阿达依金矿的密切共生关系, 为进一步研究该金矿及该构造-岩浆带的金矿成矿物质来源与成因提供了新的思路。     

大别山北麓的奥陶纪岩浆弧：侵入岩年代学和地球化学证据

采用颗粒级锆石U-Pb和角闪石的~(40)Ar-~(39)Ar定年方法,测得大别山北坡马畈闪长质杂岩体中石英闪长岩的结晶锆石年龄分别为(463.5±3.4)和(458.6±6.7)Ma,角闪石~(40)Ar-~(39)Ar坪年龄为(462.7±1.4)Ma。从镁铁质包体到闪长岩和花岗岩类,ε_(Nd)(t)介于4.3～1.8之间,且随着Mg~#值的减小和SiO_2及Nb含量增高,岩石的ε_(Nd)(t)值降低,结合低Nb/Zr比值和中等-高的Ba/Th比值等特点,认为有关岩石的形成与幔源岩浆受板片流体影响并发生地壳AFC(同化混染与分离结晶同时出现)作用有关,但它们亏损Nb等高场强元素的特征,并不是地壳物质影响所致,而反映了地幔岩浆源区的特点,表明岩浆形成于与俯冲有关的环境。这一研究进一步证明,在大别山北麓的北淮阳地区,曾经发育了奥陶纪岩浆弧,向西与北秦岭岩浆弧相接。从大别山北麓的马畈杂岩体到昆仑山的布青山英云闪长岩体,中央造山带内与俯冲有关的侵入岩侵位年龄有向西变新的趋势,可能意味着原特提斯秦岭洋在向华北地块南缘活动大陆边缘俯冲过程中,是从东向西逐渐关闭的。     

云南香格里拉浪丁岩体锆石U-Pb年龄及地球化学特征

香格里拉地区广泛分布印支期中-中酸性浅成-超浅成岩体,位于西斑岩带的浪丁岩体主要由角闪闪长玢岩、石英闪长玢岩、含辉石角闪闪长玢岩组成。锆石U-Pb年龄显示岩体侵位于206.4±3.9 Ma。其地球化学特征与埃达克岩相似：SiO2含量为56.66%~64.84%,平均60.42%;Eu异常不明显（Eu/Eu*为0.90~1.23,平均为1.08）;富集轻稀土元素和大离子亲石元素,亏损重稀土元素,贫高场强元素;高Sr（490~1165×10-6,平均842×10-6）、Sr/Y（37.4~77.2,平均56.9）、La/Yb（21.7~28.4,平均25.6）,低Y（13.1×10-6~17.4×10-6,平均14.6×10-6）、Yb（1.20×10-6~1.56×10-6,平均1.37×10-6）。年代学和地球化学研究结果显示,浪丁岩体形成于岛弧环境下,其形成与晚三叠世甘孜理塘洋壳板片的部分熔融有关,该地区的甘孜-理塘洋壳向西俯冲时,可能以较低的角度进入中咱地块之下。     

西藏卡贡地区晚寒武世错多勤石英闪长岩LA-ICP-MS锆石U-Pb年龄及其构造意义

对卡贡地区错多勤石英闪长岩进行了LA-ICP-MS锆石U-Pb测年工作,获得该岩体的206Pb/238U年龄加权平均值为495.9±3.2 Ma（MSWD=1.8,n=24）,属于晚寒武世的产物。岩石地球化学特征表明,岩石具有低钾富钠（Na2O/K2O=4.16~5.47）、低碱（K2O+Na2O=6.41%~6.67%）、低铝（Al2O3=14.64%~14.98%,A/CNK≈0.9）的特征,属于低钾钙碱性准铝质I型花岗岩。岩石稀土元素总体表现为轻稀土元素相对富集的左陡右缓的右倾型,相对亏损Nb、Sr、P、Ti等高场强元素,富集Rb、Th、U等大离子亲石元素,具有岛弧型花岗岩的特征。结合区域地质资料分析,该岩体与区内原特提斯洋的俯冲存在成因联系,错多勤石英闪长岩的发现有助于进一步探索原特提斯洋演化过程。     

Geochemical Characteristics of Intrusive Rocks, Southeastern Mindanao, Philippines: Implication to Metallogenesis of Porphyry Copper-gold Deposits

This paper describes petrologic and geochemical characteristics of intrusive rocks associated with porphyry copper deposits in south-eastern Mindanao, the Philippines, where the Kingking deposit is located. Intrusive rocks at the Kingking deposit, are classified into biotite-bearing diorite porphyry, hornblende diorite porphyry and diorite porphyry. Intrusive rocks in other areas in south-eastern Mindanao, including Batoto, Bukal, Maragusan, Masara, Amacan and Sumlog, are hornblende diorite porphyry and hornblende quartz diorite. They are adakitic in Sr/Y-Y diagram, but not in La/Yb-Yb diagram due to relatively low REE contents. The magmas are oxidizing as suggested by the high X_Mg of mafic silicate phenocrysts and SO₃ contents of microphenocrystic apatite. An Oligocene-Miocene diorite intrusive complex formed by calc-alkaline, hydrous, oxidizing magma is considered to be associated with porphyry-type copper-gold mineralization.     

Zircon U-Pb age,geochemical and Nd isotopic data of Middle Jurassic high-Mg dioritic dike in Liaodong Peninsula,NE China

Zircon LA-ICP-MS U-Pb age, geochemical and Nd isotopic data are presented for a newly recognized high-Mg dioritic dike from Haicheng, Liaodong Peninsula, NE China, to constrain its petrogenesis. The zircons from the high-Mg diorite exhibit striped absorption and oscillatory growth zoning in the cathodoluminescence （CL） images, and have high Th/U ratios （0. 05-0.9）, indicating a magma origin. Zircon LA-ICP-MS U-Pb dating indicates that 206pb/238U ages of 12 spots of zircons are between 167 Ma and 178 Ma, yielding a weighted mean 206pb/238U age of 172 ＋ 2 Ma （MSWD =4. 1 ）, which represents the forming age of the high-Mg dioritic dike, i. e. Middle Jurassic. Geochemically, the samples have SiO2 =55.4-60. 6 wt. % , Na20 =2. 2-2.76 wt. % , K20 = 1.32-2. 02 wt. % and （Na2O ＋ K2O） =3.82--4. 47 wt. %, belonging to sub-alkaline series and displaying a calc-alkaline evolutionary trend. They are characterized by high MgO （4. 75-6.85 wt. % ）, Mg# （55-61）, Cr（130-262 ppm）, Ni（63-130 ppm）, Sr（568-857 ppm）, and Ba（484-1 130 ppm） contents, with geochemical features analogous to those of high-Mg adakites. They show variable end （t） values （ - 1.3 to - 3.9） , with a weighted value of - 2. 7, which plot intermediately between the field of the ancient continental crust and the depleted mantle source, indicating that both the lower crust and mantle source are necessary for the generation of the parent magma of the Haicheng high-Mg diorites. The Haicheng high-Mg dioritic dike in the Liaodong Peninsula and the Jurassic magmatism in the eastern North China Craton formed under a continental crustal thickening setting that may be related to subduction of the Paleo-Pacific oceanic plate.     

Petrogenesis of Shangyu gabbro-diorites in western Shandong: Geochronological and geochemical evidence

Chronology and geochemistry of the Shangyu gabbro-diorite in western Shandong were studied to understand their petrogenesis and the nature of the Mesozoic lithospheric mantle. The Shangyu intru-sion is mainly composed of a suite of gabbro-diorite. Zircons from the intrusion display eu-hedral-subhedral in shape and have high Th/U ratios (1.23―2.87), implying their magmatic origin. LA-ICP-MS zircon U-Pb dating results for two samples indicate that they were formed in the Early Cre-taceous, yielding weighted mean 206Pb/238U ages of 129±1Ma and 134±2Ma, respectively. Except for early cumulate such as sample QT-19, their SiO2 and MgO contents range from 50.12% to 56.37% and from 3.52% to 6.37%, respectively. Moreover, the gabbro-diorites are characterized by high Mg# (0.54―0.63), enrichment in Na (Na2O/K2O ratios more than 1), Cr (73×10-6―217×10-6) and Ni (34×10-6―241×10-6), and intensive enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Their initial 87Sr/86Sr ratios and ε Nd(t) values range from 0.70962 to 0.71081 and from-16.60 to-13.04, respectively. Taken together with the Early Creta-ceous high-Mg diorites and the mantle xenoliths from the Tietonggou and Jinling as well as basalts from the Fangcheng and Feixian, it is suggested that the primary magma for the Shangyu gab-bro-diorites should be derived from the enriched lithospheric mantle intensively modified by conti-nental crust. The Sr-Nd-Pb isotopic compositions for the Early Cretaceous high-Mg diorites in western Shandong display a trend of spatial variations, i.e., initial 87Sr/86Sr, 207Pb/204Pb and 208Pb/204Pb ratios de-creasing and ε Nd(t) values increasing from southeast to northwest in western Shandong, which is con-sistent with the tectonic model that the Yangtze Craton subducted beneath the North China Craton oriented in north-west direction in the Early Mesozoic.