Magma-reservoir crystallization processes: small-scale dikes in cumulate gabbros, Mauna Kea Volcano, Hawaii

 Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale (<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene, and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo_71–66, cpx-Mg # 79–77, An_72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo₆₈ vs Fo₇₁₎. Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO₂<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage liquids (e.g., <5 wt.% MgO, based on <Fo₇₀) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo₇₀) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification. Received: 25 February 1997 / Accepted: 14 June 1997     

The Eocene corundum-bearing rocks in the Gangdese arc,south Tibet:Implications for tectonic evolution of the Himalayan orogen

The genesis of Liangguo corundum deposit in the southern Gangdese magmatic arc, east-central Himalaya, remains unknown. The present study shows that the corundum-bearing rocks occur as lenses with variable sizes in the Eocene gabbro that intruded into marble. These corundum-bearing rocks have highly variable mineral assemblage and mode. The corundum-rich rocks are characterized by containing abundant corundum, and minor spinel, ilmenite and magnetite, whereas the corundum-poor and corundum-free rocks have variable contents of spinel, plagioclase, sillimanite, cordierite, ilmenite and magnetite. The host gabbro shows variable degrees of hydration and carbonization. The corundum grains are mostly black, and rarely blue, and have minor Fe O and TiO_2. The spinel is hercynite, with high Fe O and low Mg O contents. The corundum-bearing rocks have variable but high Al_2O_3, FeO and TiO_2, and low SiO_2 contents. Inherited magmatic and altered zircons of the corundum-bearing rocks have similar U e Pb ages(~47 Ma) to the magmatic zircons of the host gabbro, indicating corundum-bearing rock formation immediately after the gabbro intrusion. We considered that emplacement of gabbro induced the contact metamorphism of the country-rock marble and the formation of silica-poor fluid. The channeled infiltration of generated fluid in turn resulted in the hydrothermal metasomatism of the gabbro, which characterized by considerable loss of Si from the gabbro and strong residual enrichment of Al. The metasomatic alteration probably formed under Pe T conditions of ~2.2 -2.8 kbar and ~650 -700℃. We speculate that SiO_2, CaO and Na_2O were mobile, and Al_2O_3, FeO, TiO_2 and high field strength elements remained immobile during the metasomatic process of the gabbro. The Liangguo corundum deposit, together with metamorphic corundum deposits in Central and Southeast Asia, were related to the Cenozoic Himalayan orogeny, and therefore are plate tectonic indicators.     

新疆北部卡拉麦里蛇绿岩中辉长岩的锆石U-Pb年龄及其构造意义

新疆东准噶尔卡拉麦里蛇绿岩是一套由以镁铁-超镁铁岩为主的构造岩块和不同岩性的基质组成的蛇绿混杂带,被认为是哈萨克斯坦板块和西伯利亚板块之间的缝合带。由于其特殊的大地构造位置,该蛇绿岩对恢复区域构造演化具有重要意义。然而,在过去20多年里,有关其形成及就位时代的争议一直很大,其原因在于不同学者采用来自化石资料和同位素年龄方面的不同证据。本文选择该蛇绿岩套中保存相对完整的辉长岩进行了LA-ICP-MS锆石U-Pb年龄测定,获得了3个峰值年龄:406±1.8 Ma,514±5.1Ma和332±6.4 Ma。结合该区野外地质特征和前人资料分析,本文认为除514±5.1 Ma的年龄可能来自古老围岩中的残余锆石之外,另外两组年龄分别与蛇绿岩的形成和就位有关,前者代表卡拉麦里蛇绿岩的形成时间,后者则代表卡拉麦里蛇绿岩的构造侵位时间,即卡拉麦里洋盆形成于泥盆纪初期,最终闭合于早石炭世早期。     

基于GEOROC数据库的全球辉长岩大数据的大地构造环境智能判别研究

辉长岩是化学成分与玄武岩类似的侵入岩,前人认为它的形成过程太复杂,对应的岩浆可能经过了分离结晶作用、混染作用等,不能用Pearce判别图来判断岩浆岩形成的构造环境。本文利用GEOROC数据库的资料对辉长岩进行大数据挖掘。首先根据前人成果,将GEOROC数据库的辉长岩形成的大地构造环境分为大陆玄武岩环境、汇聚边界环境、板内火山岩环境和大洋岛弧玄武岩环境等4类;然后在数据清洗基础上,利用Python语言,依托sklearn库,实现支持向量机、K近邻和随机森林等3种机器学习算法,获得3种对应的分类器结果输出。对辉长岩的构造环境进行智能判别结果显示,随机森林方法效果最好,判断准确率可达97%,利用辉长岩的地球化学大数据来判断岩浆岩的构造环境是完全可行的。     

甘肃天水地区百花岩浆杂岩的锆石LA-ICP-MSU-Pb定年及其地质意义

摘〓要：[HT5K]利用阴极发光和LA ICP MS微区定年分析方法,对甘肃天水地区百花岩浆杂岩中的辉长岩进行了单颗粒锆石U Pb同位素分析。百花岩浆杂岩体主要由辉石岩—辉长（闪长）岩—闪长岩—石英闪长岩组成,构成较完整的同源岩浆演化序列,地球化学研究表明其形成于岛弧构造环境。对百花岩浆杂岩中辉长岩的LA ICP MS锆石U Pb同位素定年结果表明,其同位素年龄为(449.7±3.1)Ma（MSWD=1.9）,形成于晚奥陶世,表明西秦岭北带岛弧型岩浆岩的形成时代为晚奥陶世,结合区域地质资料,揭示出以西秦岭北缘关子镇蛇绿岩为代表的古洋盆的俯冲作用及产生岛弧型岩浆活动的时限为中晚奥陶世。      

新疆塔城别斯托别苏长辉长岩的岩石学和锆石SHRIMP年代学研究

塔城市南约30km的别斯托别苏长辉长岩侵入石炭纪二长花岗岩中,二者的接触部位多被农田覆盖。二长花岗岩内部存在大量辉长岩、闪长岩、花岗闪长岩、石英闪长岩、石英斑岩等包体（但没有苏长辉长岩包体）。苏长辉长岩主要由透辉石-普通辉石、顽火辉石、基性斜长石（An：51 ~ 70）和少量钛铁矿-磁铁矿组成。从苏长辉长岩中分选出的锆石CL图像显示典型的扇形结构（锆石中存在辉石和斜长石包体）。对两个锆石样品进行的SHRIMP年代学研究得到平均年龄分别为：273.3 1.8Ma（MSWD = 1.18,n = 20）和272.5 2.8Ma（MSWD = 0.75,n = 12）。这两个年龄在误差范围内完全一致,表明别斯托别苏长辉长岩的形成时代为早二叠世晚期。在该苏长辉长岩与花岗岩接触带附近观察到的铜矿化现象为找矿勘探提供了方向。     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

The role of dextral transpressional faulting in the evolution of an early Carboniferous mafic–felsic plutonic and volcanic complex: Cobequid Highlands, Nova Scotia, Canada

The Wentworth plutonic complex, consisting of gabbro and granite, was emplaced in the earliest Carboniferous in the Cobequid shear zone of the northern Appalachians. The plutonic complex is coeval with a 5-km-thick pile of volcanic rocks. Early alkalic A-type granite correlates with thick felsic pyroclastics and minor basalt, which are overlain by 1.5-km-thick basalts that correlate with a large gabbro pluton that is intruded, in turn, by late granites. The basalt and gabbro are Fe-rich tholeiites. The geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma. The Wentworth plutonic complex lies on the north side of the dextral Rockland Brook fault, near the western tip of wedge-shaped basement block of the Avalon terrane. Field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike–slip faults. Dykes parallel to the mylonitic foliation in the Rockland Brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction. The plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone-bounded blocks, while the sinking of those blocks finally provided the space for mafic magma emplacement. Dyking was active over at least a 10-Ma time period. The overall location of plutonism in the Cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the Magdalen basin and the E–W transpressional contact of the Avalon and Meguma terranes. Magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge-shaped basement blocks, whose movement controlled the localisation of the voluminous magmatic activity.     

The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke

The Coldwell alkaline complex is a large (> 350 km²) gabbro and syenite intrusion on the north shore of Lake Superior. It was emplaced at 1108 Ma during early magmatic activity associated with the formation of the Mid-Continent Rift of North America. The eastern gabbro forms a partial ring dyke on the outer margin of the complex and consists of at least three discrete intrusions. The largest of these is the layered gabbro that comprises a 300 m thick fine- to medium-grained basal unit overlain by up to 1100 m of variably massive to layered gabbroic cumulates which vary from olivine gabbro to anorthosite. Several xenoliths of Archaean metamorphic rocks that range in size from 10's to 100's of meters are present in the central part of the intrusion. Within discrete horizons in the layered gabbro are many centimeter- to meter-scale, gabbroic xenoliths. The main cumulus minerals, in order of crystallization, are plagioclase, olivine and clinopyroxene ± Fe-Ti oxides. Biotite and Fe-Ti-oxide are the dominant intercumulus phases. Orthopyroxene occurs not as a cumulus phase but as peritectic overgrowths on cumulus olivine. A detailed petrographic and mineral chemical study of samples from two stratigraphically controlled traverses through the layered gabbro indicates that the stratigraphy cannot be correlated along the 33 km strike of the ring dyke. Mineral compositions show both normal and reversed fractionation trends. These patterns are interpreted to record at least three separate intrusions of magma into restricted dilatant zones within the ring dyke possibly associated with ongoing caldera collapse. Calculations of parental melt composition using mineral — melt equilibria show that even the most primitive gabbros crystallized from an evolved magma with mg# of 0.42-0.49. The presence of orthopyroxene overgrowths on cumulus olivine suggests rising silica activity in the melt during crystallization and implies a subalkaline parentage for the layered gabbro.     

新疆哈拉达拉辉长岩体中磁铁矿脉特征及其地质意义

哈拉达拉岩体是特克斯岩带中规模最大的层状辉长岩体，在岩体ＮＷ向断裂中发现磁铁矿脉，系统研究其地质产出特征，稀土元素特征和磁铁矿成分，结构，物性等矿物标型特征，认为磁铁矿脉是岩浆是结晶的产物，为贯入式钒钛磁铁矿脉，与攀枝花，力马河辉长岩体特征对比，认为该岩体具有形成攀枝花式铁矿的成矿远景。