石榴石微量元素地球化学及其在沉积物源分析中的应用     

洪东铭  简星  黄鑫  张巍  马金戈 《地学前缘》2020,27(3):191-201

石榴石是沉积物中常见的重矿物,其可来源于多种岩石,而且不同类型母岩中石榴石具有多样的地球化学组成,因此碎屑石榴石的地球化学分析在沉积物源研究中应用广泛。通过电子探针分析可以容易地获得单颗粒碎屑石榴石的主量元素地球化学组成,可借此探讨其母岩类型,但也存在一定的局限性,比如中酸性火成岩和部分变沉积岩来源的石榴石通常都具有高Fe、Mn的特征,不易于区分。本文系统地收集了不同岩石类型的石榴石微量元素数据,尝试利用微量元素地球化学的差异性对碎屑石榴石物源分析进行补充。最终得出以下结论:(1)石榴石的稀土元素(REE)组成与钇(Y)元素指标可区分中酸性火成岩和变沉积岩来源的碎屑石榴石;(2)基性岩(橄榄岩、辉石岩)及所对应的变基性岩石(榴辉岩)中石榴石的微量元素地球化学组成相近,但部分橄榄岩来源的石榴石在镨/钬(Pr_N/Ho_N)值和重稀土总量(ΣHREE含量)上与辉石岩和榴辉岩的有显著差别,这一特点可运用于以基性岩母岩为主的碎屑沉积物源研究中;(3)夕卡岩中的石榴石在主量元素地球化学组成上表现为高度一致的高Ca特征,而稀土元素组成具有两种典型的分配模式,岩浆型(指示富铁、氧化环境)与热液型(指示富铝、还原环境)。综上所述,石榴石微量元素地球化学可以有效地运用于沉积物源分析研究中,是其主量元素物源分析方法的重要补充。  相似文献   

Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps     

Esther Schmädicke  Jürgen Gose 《Journal of Metamorphic Geology》2020,38(9):905-933

Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite-hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite-hosted eclogite in the Erzgebirge occurs in the same ultra-high pressure (UHP) unit as gneiss-hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H₂O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm⁻¹) and SW II (3,570–3,630 cm⁻¹) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H₂O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H₂O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H₂O-undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H₂O and CaO are positively correlated, pointing to the same degree of H₂O-undersaturation at peak metamorphism in all rock types. This ubiquitous water-deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite-hosted garnetite, representing metamorphosed rodingite.  相似文献   

Melt migration and melt-rock reaction in the Alpine-Apennine peridotites:Insights on mantle dynamics in extending lithosphere     

Elisabetta Rampone  Giulio Borghini  Valentin Basch 《地学前缘(英文版)》2020,11(1):151-166

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate.  相似文献   

New Reference Materials and Assessment of Matrix Effects for SIMS Measurements of Oxygen Isotopes in Garnet     

Alice Vho  Daniela Rubatto  Benita Putlitz  Anne‐Sophie Bouvier 《Geostandards and Geoanalytical Research》2020,44(3):459-471

Accurate ion microprobe analysis of oxygen isotope ratios in garnet requires appropriate reference materials to correct for instrumental mass fractionation that partly depends on the garnet chemistry (matrix effect). The matrix effect correlated with grossular, spessartine and andradite components was characterised for the Cameca IMS 1280HR at the SwissSIMS laboratory based on sixteen reference garnet samples. The correlations fit a second‐degree polynomial with maximum bias of ca. 4‰, 2‰ and 8‰, respectively. While the grossular composition range 0–25% is adequately covered by available reference materials, there is a paucity of them for intermediate compositions. We characterise three new garnet reference materials GRS2, GRS‐JH2 and CAP02 with a grossular content of 88.3 ± 1.2% (2s), 83.3 ± 0.8% and 32.5 ± 3.0%, respectively. Their micro scale homogeneity in oxygen isotope composition was evaluated by multiple SIMS sessions. The reference δ¹⁸O value was determined by CO₂ laser fluorination (δ¹⁸O_LF). GRS2 has δ¹⁸O_LF = 8.01 ± 0.10‰ (2s) and repeatability within each SIMS session of 0.30–0.60‰ (2s), GRS‐JH2 has δ¹⁸O_LF = 18.70 ± 0.08‰ and repeatability of 0.24–0.42‰ and CAP02 has δ¹⁸O_LF = 4.64 ± 0.16‰ and repeatability of 0.40–0.46‰.  相似文献   

华北地台北缘集宁地区古元古代片麻状石榴花岗岩的深熔成因及地质意义   总被引：3，自引：3，他引：0  

石强  董晓杰  徐仲元  关庆彬  李鹏川  张超  崔芳华 《岩石学报》2018,34(9):2754-2772

本文对华北克拉通北缘集宁地区空间上密切共生的片麻状石榴花岗岩和孔兹岩系富铝片麻岩的岩相学、地球化学及年代学特征进行了对比研究。SHRIMP锆石U-Pb定年方面,在富铝片麻岩中获得了1910±10Ma和1839±13Ma变质锆石年龄,在片麻状石榴花岗岩中获得了1919±17Ma的变质重结晶锆石年龄。在石榴花岗岩的石榴石包裹体中识别出与富铝片麻岩相对应的进变质阶段(M1)和峰期阶段(M2)的矿物组合,由此确认富铝片麻岩的变质作用和导致石榴花岗岩形成的深熔作用是同一构造热事件的产物。通过对二者变质作用演化及特征变质矿物的对比,认为深熔作用主要发生在峰期后等温降压阶段(M3),石榴花岗岩中的石榴石为深熔作用过程中的残留矿物相或转熔矿物相,而石榴花岗岩则是混合有大量残留矿物相的熔体结晶的产物。对片麻状石榴花岗岩和富铝片麻岩的地球化学组成特征进行了对比分析,片麻状石榴花岗岩既有一定的继承性,又有十分明显的变异性。变异性表现为:1)石榴花岗岩主量和微量元素含量分布极不均匀,微量元素含量普遍低于源岩(Cs、Rb、Th、U、Nb、Ta、Zr、Hf等);2)大离子亲石元素Cs和生热元素U、Th亏损明显,Sr相对富集;3)高场强元素Nb、Ta、P、Ti的明显亏损;4)铕异常变化大,存在铕富集型、铕平坦型和铕亏损型共存的稀土配分曲线的岩石,这是深熔成因石榴花岗岩最突出的表现,也可能是原地-半原地深熔花岗岩的主要地球化学标志。综合区域上的地质资料,认为深熔作用与碰撞后伸展构造背景下基性岩浆底侵事件有关。  相似文献   

西藏雅鲁藏布江缝合带中段日喀则地幔橄榄岩中发现金刚石等异常矿物   总被引：1，自引：0，他引：1  

徐向珍  杨经绥  熊发挥  巴登珠  张仲明  李源 《地质学报》2018,92(7):1389-1400

本文报道了雅鲁藏布江缝合带中段日喀则蛇绿岩地幔橄榄岩中发现与该带东西段蛇绿岩岩体中相似的金刚石和特殊地幔矿物群。日喀则地幔橄榄岩体以方辉橄榄岩为主,含少量二辉橄榄岩和纯橄岩,辉石岩和辉长岩呈脉状产在方辉橄榄岩中。通过重砂分选实验,在465kg的方辉橄榄岩大样中发现了30余种特殊矿物群,包括金刚石、自然金、自然铬、自然铜等自然元素矿物类;碳硅石等碳化物类;方铁矿、赤铁矿、磁铁矿、刚玉、铬尖晶石、金红石、锡石、黑钨矿等氧化物类;镍黄铁矿、方铅矿、辉钼矿、辉锑矿、闪锌矿、毒砂等硫化物类;Ag-Au等合金矿物类;橄榄石、辉石、锆石等硅酸盐岩类;白钨矿等钨酸盐岩类;萤石等氟化物类。金刚石和自然金、自然铬、自然铜连同碳硅石的发现表明岩体存在强还原环境,这套特殊矿物组合指示日喀则地幔橄榄岩可能与雅鲁藏布缝合带其他蛇绿岩一样,经历了深地幔演化过程。  相似文献   

新疆伊吾县宝山铁矿床矽卡岩成因研究     

刘洋  王得权  王辰  郑立宏  王瑞美 《矿床地质》2018,37(3):630-642

宝山铁矿床位于东准噶尔库兰卡孜干-北塔山-纸房-琼河坝岛弧带东段。宝山铁矿中矽卡岩与铁矿体密切共生,是重要的找矿标志。研究区矽卡岩中石榴子石、透辉石单矿物电子探针分析结果显示,石榴子石属于钙铁榴石-钙铝榴石系列,透辉石属于透辉石-钙铁辉石系列。矽卡岩中高场强元素(如Nb、Ta、Zr、Hf)相对亏损,稀土元素表现出LREE富集,HREE亏损的特点。∑REE为39.90×10~(-6)~178.43×10~(-6),∑LREE/∑HREE比值1.64~7.53,(La/Yb)N比值为1.32~10.10,轻、重稀土元素分异程度较弱,与玄武质凝灰岩具有相似的地球化学特征,这说明矽卡岩是玄武质凝灰岩受后期岩浆热液改造形成,随着温度和压力降低,磁铁矿沉淀形成铁矿体。  相似文献   

云南个旧锡铜矿集区石榴子石地球化学特征及成矿指示     

张银平  邵拥军  熊伊曲  席振铢  芦磊  张敏  毛禹杰 《矿床地质》2022,41(4):682-701

云南高松矿床中高峰山矿段和老厂矿床中竹叶山矿段分别是个旧超大型锡铜多金属矿集区发育的典型矽卡岩型锡、铜矿段。为查明个旧矿集区锡铜成矿环境的差异,文章选取上述2个矿段矽卡岩中的石榴子石为研究对象,根据产状和矿物共生关系将石榴子石分为与高峰山花岗岩有关早期（Grt-GS1）、晚期（Grt-GS2）、与竹叶山花岗岩有关（Grt-ZS）和与玄武岩有关（Grt-ZX）共4类。电子探针（EPMA）和激光剥蚀等离子质谱（LA-ICP-MS）分析表明：Grt-GS1(Adr_10.58-20.27Grs_38.62-48.28Spe_21.06-32.37Alm_11.88-18.03)成分复杂且变化范围较大,而Grt-GS2(Adr_48.09-69.73Grs_23.84-48.87Spe_3.29-9.58)和Grt-ZS (Adr_13.97-16.06Grs_78.08-79.71Spe_2.15-2.60)成分简单且变化小;稀土元素配分曲线均呈HREE富集、LREE亏损的“左倾型”,并具明显的负Eu异常。Grt-ZX成分简单且较均一(Adr_20.12-21.99Grs_70.84-71.44Pyr_6.29-7.92),稀土元素配分曲线呈HREE亏损、LREE富集的“右倾型”,呈明显的正Eu异常,且F、Cl、U、Th、Pb、V、Cr、Co、Ni、Zn、Ti等元素含量均高于其余3类石榴子石。所有石榴子石REE³⁺与Mg²⁺的正相关性表明,REE³⁺受[X²⁺]Ⅷ-1[REE³⁺]Ⅷ+1[Y³⁺]Ⅳ-1[Y²⁺]Ⅳ+1替换机制的控制,REE³⁺主要替代Mg²⁺进入石榴子石晶格。石榴子石的U、Sn含量及稀土元素配分特征指示：高峰山石榴子石（Grt-GS1和Grt-GS2）在演化过程中氧逸度明显升高、竹叶山石榴子石（Grt-ZS和Grt-ZX）形成的氧逸度总体高于高峰山,但Grt-ZX形成时氧逸度低于Grt-ZS。稀土元素特征显示：Grt-GS1、Grt-GS2与Grt-ZS均形成于弱酸性环境,而Grt-ZX形成于近中性且富F、Cl的环境下。∑REE³⁺与Y呈良好的正相关性和REE含量变化指示4类石榴子石均形成于封闭或近封闭的体系中,在接近平衡和低水岩比的条件中受扩散交代作用缓慢生长而形成。综合研究表明,玄武岩对成矿的影响在于改变了流体性质,为成矿提供了大量微量元素（如Co、Ni、V、Cr等）和有利于成矿离子运移和富集的卤族元素（如F、Cl）,而高峰山矿段锡富集的原因可能是在演化过程中水岩反应的加强和氧逸度的升高。  相似文献   

内蒙古北山地区芨芨台子-小黄山蛇绿岩构造属性及与成矿关系: 来自阿民乌素地幔橄榄岩印证     

董洪凯 《地质与勘探》2022,58(4):767-777

阿民乌素地幔橄榄岩属芨芨台子-小黄山蛇绿岩带一部分,该构造带南北两侧地质体无明显差异,不具分界断裂的构造特征。本文对阿民乌素地幔橄榄岩与月牙山地幔橄榄岩进行岩石化学、地球化学对比分析,为芨芨台子-小黄山蛇绿岩构造属性提供新依据,并对阿民乌素蛇绿岩成矿潜力进行分析研究。笔者所在团队在地幔橄榄岩上部的辉长岩中获得LA-ICP-MS锆石U-Pb同位素年龄值为462.5±3.2 Ma,属中奥陶世。其上发育奥陶纪-志留纪公婆泉组岛弧拉斑玄武岩。该期地幔橄榄岩轻重稀土之比LR/HR＝1.63~3.68, (La/Sm)N＝1.70~6.92,(Gd/Yb)N＝0.36~0.52,表明岩石轻稀土略富集,稀土配分曲线呈不规则“U”型,估算其为原始地幔橄榄岩经过10%~20%部分熔融的残留物。原始地幔标准化蛛网图富集高场强元素U、Zr、Hf、Yb和大离子亲石元素Rb、Sr,亏损高场强元素Nb、Ti和大离子亲石元素Ba。与月牙山地幔橄榄岩标准化蛛网图对比,最大区别在于阿民乌素地幔橄榄岩明显亏损高场强元素Ti。初步研究认为阿民乌素地幔橄榄岩属SSZ型、高压型蛇绿岩,是岛弧裂谷的产物。该橄榄岩具有形成大型铬铁矿的成矿构造背景,是寻找蛇绿岩型铬铁矿的有利部位。  相似文献   

Melt Pockets in Garnet Megacrysts from Cenozoic Alkali Basalts of the Shavaryn Tsaram Vicinity, Mongolia     

Anna ASEEVA  Oleg AVCHENKO  Alexander KARABTSOV  Alexander CHASHCHIN  Sergey VYSOTSKIY  Victoria YAKOVENKO  Natalia ZARUBINA  Natalia KHARITONOVA 《《地质学报》英文版》2022,96(1):111-122

Garnet megacryst with a multiphase inclusion from intraplate alkali basalts of the Shavaryn Tsaram(Tariat,Mongolia)was the object of the study.This unusual aggregate consists of porous glass,Ti-rich biotite,orthopyroxene,spinel,clinopyroxene,olivine,and ilmenite.Win TWQ 2.32 thermodynamic simulation of this system revealed a few intervals of equilibrium.Pressure and temperature adjustment reflected in the paragenetic minerals of the melt pocket.The capture of already crystallised garnet megacryst was at P=0.8-1 GPa and T=1120-1160℃.Mineral crystallisation inside the melt pocket,accompanied by external inputs,occurred at P=0.75-0.95 GPa;T=790-1120℃.Symplectite assemblage formed in the garnet megacryst due to decomposition at(P=0.55-0.7 GPa;T=850-930℃).The study of the oxygen isotope content in primary garnet and biotite of the melt pocket showed that the δ¹⁸O_VSMOW values are the same and correspond to that of typical mantle xenoliths.However,the chemical and microcomponent composition of the melt pocket minerals reveals a material that differs from basalts and peridotites.Thus,it has been revealed that the multiphase inclusion in the garnet megacryst formed not only on account of the garnet’s substance,but also due to the entrapped material of the Earth’s interior.  相似文献