甘肃北山红石山地区晚石炭世英云闪长岩锆石U- Pb年龄、地球化学特征及构造意义

笔者等以出露于红石山—百合山蛇绿混杂岩带南侧的英云闪长岩体为研究对象,通过野外地质调查和岩石学、地球化学、锆石LA-ICP-MS U-Pb年代学研究,探讨其成因、构造环境及红石山洋俯冲消亡过程。岩石地球化学研究表明,红石山南英云闪长岩体富硅(SiO₂=63.95%～66.22%)、高铝(Al₂O₃=15.31%～16.61%)、富钠(Na₂O=3.73%～4.33%)、低钾(K₂O=1.00%～1.85%),相对高锶(Sr=246.0×10^-6～417.0×10^-6)、低钇(Y=8.83×10^-6～11.80×10^-6),富集Rb、Sr等大离子亲石元素和LREE,亏损Nb、Ta、Ti、P等高场强元素,HREE相对亏损,无明显Eu异常。岩石学和岩石地球化学特征表明,为俯冲洋壳脱水熔融的产物。锆石LA-ICP-MS U-Pb测年表明,红石山南英云闪长岩体的形成年龄为306.9～309.9 Ma,...     

内蒙古阿拉善右旗塔木素地区酸性侵入岩锆石U-Pb年龄与地球化学特征

内蒙古阿拉善地块北缘塔木素地区酸性侵入岩发育,主要岩性为黑云母花岗岩、花岗闪长岩、二长花岗岩、英云闪长岩,其中花岗闪长岩、英云闪长岩、二长花岗岩构成塔木素地区侵入岩的主体,岩体侵入晚石炭世—早二叠世阿木山组,又被白垩系、侏罗系不整合覆盖,区域上呈近东西向展布。采用LA-ICP-MS技术,测得内蒙古阿拉善右旗塔木素地区的塔木素二长花岗岩和笋布尔英云闪长岩中锆石的206Pb/238U年龄分别为264.2±3.3Ma和273.0±3.3Ma,这2个年龄被解释为各自的侵位年龄,即二者均形成于中二叠世。花岗闪长岩、二长花岗岩和英云闪长岩在Si O2-K2O图解中落入钙碱系列和高钾钙碱性系列,A/CNK值分别为0.874~1.01、1.06~1.54和0.91~1.03;Ba、Rb、Th、K等大离子亲石元素富集,Nb、Ta、P、Ti等高场强元素相对亏损;稀土元素总量分别介于156.99×10-6~212.87×10-6、126.92×10-6~212.98×10-6和128.13×10-6~214.5×10-6之间,均呈负Eu异常;在构造环境Rb-(Yb+Nb)和Rb-(Yb+Ta)判别图解中均落入火山弧花岗岩区。结合区域背景分析,阿拉善北缘在早二叠世时可能存在与弧物质有关的岩浆活动。     

Sr–Nd isotopic systematics and geochemistry of intermediate plutonic rocks from Ikoma Mountains, Southwest Japan: evidence for a sequence of Mesozoic magmatic activity in the Ryoke Belt

Abstract The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activity in this area is divided into two periods, early–middle Jurassic and late Cretaceous, based on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite and Fukihata tonalite. Rb–Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial ⁸⁷Sr/⁸⁶Sr ratios are as follows: the Kyuanji quartz diorite has an age of 161.0 ± 17.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70727 ± 0.00007, while the Fukihata tonalite has an age of 121.4 ± 24.6 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70753 ± 0.00020. Our chronological results indicate that the Kyuanji quartz diorite belongs to the Jurassic mafic rocks, such as the Ikoma gabbroic mass, while the Fukihata tonalite belongs to the early Cretaceous granitic rocks. Both these intermediate plutonic rocks have different chemical characteristics and were derived from different magmas.     

湘东南万洋山岩体的锆石SHRIMP U-Pb年龄、成因及构造意义

本文对湘赣交界地区发育的万洋山岩体进行锆石SHRIMP U-Pb定年和岩石学、地球化学分析。该岩体由英云闪长岩、花岗闪长岩和二长花岗岩组成,本次主要讨论英云闪长岩及其中发育的石英闪长岩包体,并获得英云闪长岩的锆石U-Pb年龄为438.0±3.0 Ma,石英闪长岩包体的锆石U-Pb年龄为425.6±3.1 Ma,为晚志留世。英云闪长岩矿物组合为斜长石、钾长石、黑云母、石英以及少量的角闪石、磁铁矿和榍石;地球化学特征显示为低硅、准铝质及钙碱性的花岗岩,在岩石类型判别图解中为I型花岗岩。石英闪长岩包体为细粒结构,矿物组合为角闪石、斜长石、黑云母、石英及少量辉石,表明岩石包体是岩浆成因的;包体中存在异常共生矿物斜长石斑晶、针状磷灰石,CIPW标准矿物计算中未出现刚玉分子,地球化学组成显示其具有低硅、低碱、准铝质的钙碱性岩类特征;包体还表现为富Mg、Fe以及高Mg#值(45~50),显示出包体高镁、偏基性的特征;包体与寄主岩稀土元素配分模式图和微量元素蛛网图分布特征基本一致,表明二者在成因上相关联。石英闪长岩包体分异指数DI=45~48与辉长岩接近,SiO_2含量略高于辉长岩,表明石英闪长岩包体源于上地幔基性辉长质岩浆、经岩浆混合演化形成。英云闪长岩显示为岛弧岩浆岩、具有活动大陆边缘岩浆岩特征,结合英云闪长岩的岩石类型、岩石包体成因认为:万洋山岩体可能是扬子板块与华夏板块在俯冲消减的地球动力学背景下,软流圈地幔上涌,诱发岩石圈地幔和上覆的古老地壳物质重熔,形成以壳源为主、壳幔混合成因的花岗岩。     

Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene-plagioclase-quartz.
To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water-saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S-type tonalitic affinity and is located very close to the tie-line between the average melts produced in water-saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone.
The age of the crustal anatexis is determined by the Rb-Sr whole rock isochron method for garnet-cordierite-biotite gneiss (host rock), garnet-orthopyroxene-cordierite gneiss (restite) and S-type tonalite (melt). This gives an age of 56.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.705711. The S-type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).     

Post-emplacement fluids and pluton thermobarometry: Mount Stuart batholith,Washington Cascades

The effects of post-emplacement infiltration of externally derived, high-temperature fluids into arc-related batholiths are often not well characterized. Such infiltration can have far-reaching effects on the elemental and light isotopic chemistry of a batholith and on its mineral phases. At high temperature, fluid infiltration can be less easily detected, especially if widespread. The Mount Stuart batholith of the Washington Cascades is offered as an example of high-temperature infiltration of high δ¹⁸O fluids derived from its contact aureole. Some of the fluid infiltration coincided with and may have been partly derived from a kyanite-grade, post-emplacement metamorphic event that affected northern portions of the batholith. However, the effects of the fluid infiltration were far reaching and affected the entire margin of the batholith, including southerly portions that did not experience post-emplacement metamorphism. The result led to an oxygen isotopic zonation of the batholith, which is viewed as secondary in origin, with expected effects on mineral chemistry, including derived estimates of thermobarometry, a portion of which is also substantially subsolidus in origin. Our revised emplacement barometry of the Mount Stuart batholith, excluding areas affected by fluid infiltration, demonstrates that it was emplaced at ～350–400 MPa. Soon after emplacement, the batholith was tilted to the north by loading processes and subsequently was righted and unroofed during erosion in the Eocene. Its current near palaeohorizontal orientation has implications for palaeomagnetic studies supporting northward transport (the Baja–British Columbia hypothesis), but these results need further study, given the batholith's complex metamorphic and deformation history and the nature of its dominant magnetic mineralogy (pyrrhotite).     

西昆仑康西瓦北部冬巴克片麻状英云闪长岩锆石SHRIMP U—Pb测年

西昆仑冬巴克寒武纪片麻状英云闪长岩是1:25万康西瓦幅、岔路口幅等4幅区域地质调查项目,首次从原划古元古界埃连卡特岩群中解体出来的一套侵入岩。重点对该套侵入岩的岩石学和同位素年代学进行了研究,获得了高精度锆石SHRIMP U- Pb同位素年龄502Ma±9Ma,其形成时代为中寒武世。该同位素资料的获得,为进一步深入详细地研究塔里木陆块与西昆仑造山带的地质构造演化提供了新的实际资料。     

Chemical and physical weathering in a hot‐arid,tectonically active alluvial system of Anza Borrego Desert,California

Investigation of chemical and physical weathering of bedrock and alluvial sediment in the Anza Borrego Desert, California, sheds light on weathering processes in hot‐arid systems and clarifies interpretations of climate from alluvial sediment. All of the alluvial sediment in the study area emanates from Cretaceous tonalite of the Peninsular Range, enabling exploration of the effects of external variables – climate, transport distance and tectonics – on the physical and chemical properties of the sediment. Chemical weathering in this area is dominated by plagioclase alteration observed in both bedrock outcrops and sediment, evinced most clearly by changes in the Eu anomaly. Biotite chemical weathering, manifested by interlayer K⁺ loss, is not evident in bedrock, but clearly observed in the sediment. Despite the weak intensity of chemical weathering (Chemical Index of Alteration = 56 to 62), fine‐grained (<63 μm) sediment displays a clear weathering trend in A–CN–K space and contains up to 25% clay minerals. Physical abrasion and grain‐size reduction in biotite during transport predominates in the sediment, whereas physical (insolation) weathering affecting bedrock is inferred from estimates of differential thermal expansion of mineral phases in response to extreme temperature changes in the study area. Chemical alteration and Brunauer–Emmett–Teller surface area both increase within the active Elsinore fault zone at the distal end of the depositional transect, reflecting tectonic‐induced fracturing and associated accelerated weathering. Extensive fracturing, together with a more humid Pleistocene climate, probably facilitated in situ bedrock weathering, preceding arid alluvial deposition in the Holocene. This study demonstrates that both climate and tectonic processes can affect chemical and physical weathering, resulting in alteration of plagioclase, leaching of K⁺ from biotite in the sediment and formation of clay minerals, even in hot, arid systems.     

西藏朗县地区增生楔杂岩带90Ma岛弧型深成岩浆活动和意义

碰撞前增生楔带的岩浆活动在世界许多缝合带均较强烈。为证实雅鲁藏布江带是否存在碰撞前增生楔岩浆活动,本文报道了朗县英云闪长岩岩株的地质特征,以及岩石学、锆石U-Pb定年、全岩地球化学和Sr-Nd同位素数据。结果表明,朗县岩体变形较弱,侵位于强烈变形的蛇绿混杂岩中(蛇绿岩年龄(145.7±2.5)Ma),时代为(92.58±0.72)Ma(MSWD=1.8),岩石属于钙碱性系列,高镁、富钠,相容元素(Cr=71.48×10-6~86.74×10-6,Co=22.32×10-6~23.52×10-6,Ni=33.51×10-6~36.31×10-6)含量高,富含大离子亲石元素Rb、Ba和放射性生热元素Th、Pb、U、K、Sr等,明显亏损Nb、Ta、Zr、Hf(Ti)等非活动性元素,轻稀土明显富集,重稀土亏损(LaN/YbN=6.95~7.44),具有不明显的铕异常,δEu值为0.82~0.91,εNd(t)值变化于2.68~3.23。在以上基础上,结合前人研究成果,认为朗县英云闪长岩体形成于增生楔岛弧环境,与雅鲁藏布江洋俯冲-消减带向南迁移密切相关,是90 Ma的洋脊俯冲地球动力背景下亏损地幔和增生楔熔体混合的产物。     

Melting experiments on hydrous low-K tholeiite: Implications for the genesis of tonalitic crust in the Izu–Bonin – Mariana arc

A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P–T–H₂O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H₂O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm³) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm³). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C).