岩浆长期分离结晶对岩石锶同位素初始比值的影响

讨论了在岩浆长期分离结晶作用的模式,由于晚期残留熔体相对于早期结晶相其Rb/Sr比增高,当有一定滞留时间条件下,将导致晚期结晶相~(87)Sr/~(86)Sr初始比增高,因而在一个复式岩基内会形成具有不同锶同位素初始比的多个岩石单元。文中以安徽地洼阶段的黄山一狮子岭岩体为例,通过模拟计算,得到其初始岩浆(?)=0.7122,(Rb/Sr)0=3,该岩浆的80％分离结晶形成黄山岩体,其(?)=0.7143(t=130Ma),其余20％的残余熔体结晶后形成狮子岭岩体,其(?)=0.7180(t=123.1Ma)。     

北京云蒙山花岗岩基的岩石变形及其成因

云蒙山花刚岩基由１２个侵入体组成，岩基中的片麻状花岗岩是在１２６±２１Ｍａ时侵入的型花岗岩，根据岩基中花岗岩与围岩的变形循征，确定岩基的侵位机制为底辟定位型，花岗岩中的变形组构与环绕岩基的糜棱岩的岩基定位晚期、花岗岩浆完全结晶之前形成。由于云蒙山花岗岩基的强力定位，导致密怀变质核杂岩在燕山晚期再次发生伸展构造运动。     

黄山-太平复式花岗岩基黑云母化学成分与波谱学特征

本文讨论了取自黄山-太平花岗岩基-黑云母的化学成分和不同成因岩体的波谱学特征之间的关系。研究表明它们之间存在着密切关系,可能鉴定并建立黑云母中主要阳离子成分与吸收键特征以及黑云母的谱学特征,也可以用于区分不同成因的花岗岩类型。     

藏南冈底斯南缘程巴岩体高Sr/Y花岗闪长岩和包体形成机制及Sr-Nd-Hf同位素制约

程巴岩体位于藏南冈底斯岩基东段南缘,由花岗闪长岩、细粒闪长质包体等组成。测得的锆石U-Pb年龄可以代表岩石的形成年龄,即花岗闪长岩形成年龄为29.40±0.18Ma与29.42±0.25Ma,细粒闪长质捕虏体形成年龄为30.02±0.15Ma。花岗闪长岩具有较高的Si O2(65.2%~66.2%)、K2O(3.2%~4.0%),较低的铁(TFe O=3.2%~4.0%)和Mg O(约2%),同时具有高Sr(774×10-6~813×10-6)、低Y(9.9×10-6~11.2×10-6)、高Sr/Y值(63.4~82.2)等特征;闪长质包体表现出较低的Si O2(53%~56.1%)和K2O(1.5%~3.2%),较高的铁(TFe O=6.1%~8.1%)、Mg O(4.0%~6.2%)和Na2O/K2O≥2,同时具有负Eu异常(Eu/Eu*=0.432~0.804)。2种岩性都富集LREE及LILE,亏损HREE及HFSE,具有较高且一致的εHf(t)值(+1.1~+6.2)和全岩εNd(t)值(-2.9~-5.9)。以上数据表明,花岗闪长岩与细粒闪长质包体由同一岩浆分离结晶形成,花岗闪长岩经历磷灰石和角闪石的分离结晶,其高Sr/Y值为岩浆分离结晶的结果,并不代表原始岩浆组分。     

Petrology,geochemistry and thermobarometry of the northern area of the Flamenco pluton,Coastal Range batholith,northern Chile. A thermal approach to the emplacement processes in the Jurassic andean batholiths

The Flamenco pluton is part of a N–S alignment of Late Triassic to Early Jurassic intrusive belt comprising the westernmost part of the Coastal Range batholith in northern Chile. The Jurassic-Cretaceous voluminous magmatism related to subduction in the western active continental margin of Gondwana is emplaced in the predominantly metasedimentary Paleozoic host-rocks of the Las Tórtolas formation, which in the northern area of the Flamenco pluton present an intense deformation, including the Chañaral mélange.Geochemically, the Flamenco pluton shows a wide compositional variability (SiO₂ between 48wt % and 67wt %). Gabbros, Qtz-diorites and tonalites, mesocratic and leucocratic granodiorites are classified as calc-alkaline, calcic, magnesian and metaluminous magmatism. Flamenco granitoids define cotectic linear evolution trends, typical of magmatic fractionation processes. Geochemical trends are consistent with magmas evolved from undersaturated and low-pressure melts, even though the absence of transitional contacts between intrusive units precludes in-situ fractionation. Although some granodioritic samples show crossed geochemical trends that point to the compositional field of metasediments, and large euhedral prismatic pinnite-biotite crystals, typical Crd pseudomorph, are observed in contact magmatic facies, geochemical assimilation processes are short range, and the occurrence of host-rocks xenoliths is limited to a few meters from the pluton contact.A thermal approach to the emplacement process has been constrained through the thermobarometric results and a 2D thermo-numerical model of the contact aureole. Some Qtz-diorites and granodiorites located in the north area of the pluton exhibit granulitic textures as Hbl-Pl-Qtz triple junctions, poikiloblastic Kfs and Qtz recrystallization. The Hbl–Pl pairs have been used for the thermobarometric study of this metamorphic process, resulting granoblastic equilibrium temperatures between 770 and 790 °C, whereas Hbl–Pl pairs in domains that preserve the original igneous textures yield temperatures above 820 °C. This is characteristic of self-granulitization processes during the sequential emplacement of composite batholiths.In addition, the thermal modeling was used in order to compare the expected and observed thermal contact aureole of the intrusive body. Model P-T conditions have been established between 3 and 4 kbars (extracted from the thermobarometric results), and temperatures between 1159 °C (liquidus temperature for a tonalitic composition) and 992 °C (fixed at the rheological threshold of a 50% crystal fraction). The thermal modeling estimates a homogeneous contact aureole, where the established temperatures for the melting reactions in the host-rocks are located at distances between 200 and 650 m from the magma chamber boundary, whereas the temperatures for Crd stabilization extend 1500 m far from the contact in the case of the emplacement at liquidus temperatures and 4 kbars. According to field observations, the contact aureole presents a scarce development in the northern area of the Flamenco pluton, with few migmatite outcrops and less than 1 Km in thickness for Crd-schists. However, in the southern contact, partially melted rocks are described at distances up to 2 km from the Flamenco pluton boundary.The processes of self-granulitization and the differences between the observed and calculated (by the thermal modeling of one single pulse) contact aureole suggests a process of incremental emplacement for the Flamenco pluton, by accretion of magmatic pulses from north to south (in its current position), where the thermal maturity reached through the repeated magmatic intrusion generates a more extensive area of high-grade metamorphism.     

Petrogenesis and Geodynamic Implications of Early Cretaceous (~130 Ma) Magmatism in the Baingoin Batholith, Central Tibet: Products of Subducting Slab Rollback

The Baingoin batholith is one of the largest granitic plutons in the North Lhasa terrane. Its petrogenesis and tectonic setting have been studied for decades, but remain controversial. Here we report data on geochronology, geochemistry and isotopes of Early Cretaceous granitoids within the Baingoin batholith, which provide more evidence to uncover its petrogenesis and regional geodynamic processes. The Early Cretaceous magmatism yields ages of 134.4–132.0 Ma and can be divided into I-type, S-type and highly fractionated granites. The I- and S-type granites exhibit medium SiO2, high K2O/Na2O with negative εNd(t) and εHf(t) values, whereas, the albite granites have very high SiO2 (79.04%–80.40%), very low K2O/N2O, negative εNd(t) and a large variation in εHf(t). Our new data indicate that these granitoids are derived from unbalanced melting in a heterogeneous source area. The granodiorites involved had a hybrid origin from partial melting of basalt-derived and Al-rich rocks in the crust, the porphyritic monzogranites being derived from partial melting of pelitic rocks. The albite granites crystallized from residual melt separated from K-rich magma within the ‘mush’ process and underwent fractionation of K-feldspar. We believe that the Early Cretaceous magmatism formed in an extensional setting produced by the initial and continuous rollback of a northward-subducting slab of the NTO.     

秘鲁安托尼奥铁矿区花岗闪长岩地球化学特征及其地质意义

安托尼奥(Antonio)铁矿位于秘鲁中东部阿普里马克(Apurimac)始新世-渐新世斑岩-矽卡岩型铜-钼(铁-金)成矿带,矿体的空间展布受到安达胡拉斯-雅乌里(Andahuaylas-Yauri)岩基岩石与弗若巴木巴(Ferrobamba)组碳酸盐岩的接触带控制。矿区花岗闪长岩具有高硅和钠,低铁、镁和钙的特征,铝饱和指数(A/CNK)为0.92～0.99,富集Rb、Ba、K、Sr等大离子亲石元素和活泼不相容元素,亏损Nb、Ti、P等高场强(HFSE)元素,表现出准铝质I型花岗岩的特征。结合区域构造演化认为,花岗闪长岩为幔源镁铁质岩浆结晶分异,可能有俯冲洋壳与下地壳部分熔体物质参与,形成于纳兹卡(Nazca)大洋板片低角度俯冲到南美大陆板块之下的挤压背景向拉伸背景过渡时期。动力背景的转变有利于区域大量斑岩型及相伴生的热液型、矽卡岩型矿化的形成。结合矿区地球物理勘查结果,安托尼奥出露的多个铁矿体在矿区深部合并为一个矿体,向南西侧倾伏延深,并可能有其他类型矿体(矿石)出现。     

大板金矿“港湾成矿”模式探讨

自 8 0年代末 ,辽宁西部排山楼金矿发现以来 ,对于同韧性剪切带型金矿床的赋存规律一直引起广大地质工作者的重视。通过对大板、老虎沟、樱桃沟等金矿的初步研究表明 ,采用微观和宏观相互结合、相互映衬的指导思想 ,针对韧性剪切带动力机制进行思考 ,认为闾山造山带中北北东向和东西向两个方向的剪切带为一期韧性剪切构造作用的产物 ,形成时间为印支末期—燕山早期 ,其区域构造样式为共轭式韧性剪切带 ,呈网结状。含金矿液的沉淀作用一般发生于韧性剪切带的网结点处附近 ,较大型的金矿床应赋存于韧性剪切带中刚性体周围 ,尤其是首尾等“港湾”处 ,即“港湾成矿”。     

冈底斯曲水岩基岩浆混合:来自暗色岩浆包体角闪石显微结构的证据

冈底斯岩浆带位于拉萨地体南缘,其形成过程主要受中生代新特提斯洋板片俯冲和新生代印度-亚洲陆-陆碰撞控制,是揭示青藏高原形成过程和深化大陆动力学研究的天然实验室。曲水岩基位于冈底斯岩浆带中段,介于拉萨和曲水之间,主要由花岗闪长岩、花岗岩、闪长岩和辉长岩组成。岩基花岗质岩体中包含大量暗色岩浆包体,包体产出状态有同侵位岩墙、包体墙、包体群等,表明岩浆混杂与混合现象。前人关于曲水岩基做了大量研究工作,取得很多进展,比如,发现这些暗色岩浆包体与寄主岩具有相同的结晶时代,主要集中在55~45Ma。但是,关于曲水岩基形成在俯冲背景还是碰撞背景还存在着争论。这些广泛分布的暗色岩浆包体和寄主岩的关系,及其所代表的岩浆混合过程还需要精细的矿物学工作。因此,本文在综合分析野外岩性分布、暗色岩浆包体出露形态的基础上,重点选择花岗闪长质寄主岩和其中的暗色岩浆包体中的角闪石进行矿物显微结构和构造的分析,并结合电子探针数据,以探求曲水岩基的岩浆混合过程。我们初步认为曲水岩基的形成经历两期混合过程：早期的基性岩浆和酸性岩浆端元在深部的混合;晚期基性、酸性岩浆混合后的中性岩浆爆破、上升,并继续与酸性岩浆混合。此外,曲水岩基形成于俯冲到碰撞的转换过程,受控于俯冲板片作用所产生的弧型岩浆和板片回旋及稍后的断离所产生的地幔岩浆双重作用。     

滇西临沧花岗岩基新生代剥蚀冷却的裂变径迹证据

为揭示临沧花岗岩基的剥蚀冷却历史,探讨印藏碰撞对滇西的影响,对6块临沧花岗岩基样品进行锆石和磷灰石裂变径迹测定,并利用模拟退火法对其中5块样品的磷灰石裂变径迹数据进行非线性热史反演,估算了不同时期的剥蚀量和抬升量. 结果表明,岩基自印藏陆陆碰撞以来经历了两期冷却事件,早期冷却速率仅5～10 ℃/Ma,晚期冷却速率明显增高,特别是近3 Ma以来的冷却速率达到16～20 ℃/Ma;两期总剥蚀厚度可达3300～3500 m. 分析表明冷却事件与印藏碰撞关系密切,早期冷却是在印藏碰撞影响下,临沧岩基卷入逆冲推覆运动而遭遇抬升、剥蚀的结果;晚期冷却则是上新世以来,特别是3Ma以来岩基经受整体的强烈抬升、剥蚀的结果,该期构造抬升量约为672～1263 m;裂变径迹资料还揭示印藏碰撞先影响南部岩体,随后才波及到岩基中北段.