Hydration vs. oxidation: Modelling implications for Fe–Ti oxide crystallisation in mafic intrusions, with specific reference to the Panzhihua intrusion, SW China     

Geoffrey H.Howarth  Stephen A.Prevec 《地学前缘(英文版)》2013,4(5):555-569

Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion:（1） low-Ti,high CaO and low H₂O（0.5 wt.%） parent magma（equivalent to Emeishan low-Ti basalt） at FMQ;and（2） high-Ti,low CaO and higher H₂O（>1.5 wt.%） parent magma（equivalent to Emeishan high-Ti basalt） at FMQ + 1.5.Modelling of these parent magma compositions produces significantly different results. We present here detailed f（O₂） and H₂O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultra-mafic -mafic layered intrusions.Modelling is consistent with numerous experimental studies on ferro-basaltic magmas from other localities（e.g.Skaergaard intrusion）.Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe-Ti oxides ore layers.We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H₂O contents（0.5-1 wt.%） but at the lower end of TiO₂ content for typical high-Ti basalts（2.5 wt.%TiO₂）. Distinct silicate disequilibrium textures in the Fe-Ti oxide ore layers suggest that an influx of H₂O may be responsible for changing the crystallisation path.An increase in H₂O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid.Continued cooling of the magma with high H₂O then results in precipitation of Mt-Uv alone. The H₂O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable.H₂O alters the crystallisation sequence of the basaltic magmas so that at high H₂O and f（O₂） Mt -Uv crystallises earlier than plagioclase and clinopyroxene.Furthermore,the addition of H₂O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe-Ti oxide ore layers.  相似文献   

东天山香山铜镍-钛铁成矿与两类岩浆演化关系探讨   总被引：1，自引：0，他引：1  

王亚磊  张照伟  尹希文  尤敏鑫 《岩石学报》2018,34(8):2245-2261

香山矿床是东天山唯一一个铜镍矿体与钛铁矿体共存的复合型矿床,目前普遍被认为是同源岩浆高度演化的产物。但为何区内众多同时代镁铁-超镁铁质岩体中只有香山岩体产出有大型钛铁矿床?这一问题仍有待进一步探讨。本文通过系统对比香山与其相邻的黄山东、黄山和黄山南等典型铜镍矿的地球化学特征、母岩浆性质及岩浆演化过程,认为香山岩体母岩浆相对其相邻典型铜镍矿床并不富集Fe和Ti,且其母岩浆早期演化过程及分异演化程度相似。模拟计算表明:岩浆体系早期都处于较还原的环境(f_(O_2)QFM+1),而且都发生了大量橄榄石、斜方辉石和单斜辉石的分离结晶和硫化物的熔离,由此消耗了岩浆体系中大量的Fe,导致残余岩浆中Fe含量降低;另外在岩浆演化的各阶段,尤其到岩浆演化后期,大量角闪石的结晶进一步降低了残余岩浆中的Fe和Ti的含量,不能造成Fe和Ti的逐渐富集,据此我们认为同源岩浆演化不具备形成钛铁矿床的物质基础。通过初步对比东天山地区典型铜镍矿床和钛铁矿床与该地区早二叠世两类玄武岩的地球化学特征及演化趋势,我们初步推测东天山地区铜镍成矿与钛铁成矿是两类不同性质岩浆独立演化的产物,铜镍成矿可能与拉斑玄武质岩浆演化密切相关,而钛铁矿则可能与碱性玄武岩演化密切相关。  相似文献   

Triassic High-Pressure Metamorphic Belt in the Central Qiangtang Terrane,Northern Tibetan Plateau     

ZHAI Qingguo  BOR-Ming Jahn  ZHANG Ruyuan  LI Cai  WANG Jun 《《地质学报》英文版》2013,87(Z1):98-98

  相似文献   

大别山西部河南罗山熊店加里东期榴辉岩锆石特征及SHRIMP分析结果   总被引：33，自引：7，他引：26  

简平  杨巍然 《地质学报》2000,74(3):259-264

熊店榴辉岩产于大别山西部苏家河构造混杂岩带内,是典型的高压一超高压中温榴辉岩。作者应用岩学结合阴极发光和扫描电镜方法,较系统地研究了榴辉岩中锆石在岩石中的赋存状态、内部结构及表面特征。锆石主要产于石榴子石等变质矿物内,与主晶界面清晰,具有多晶面等变质锆石特有形貌特征,内部均或发育变质生长结构,从而说明它们是变质作用的产物。ＳＨＲＩＭＰ分析表明,锆石＾２０６Ｐｂ／＾２３８Ｕ年龄为３３５～４２４Ｍａ,  相似文献   

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes   总被引：8，自引：1，他引：7  

Brueckner  & Medaris 《Journal of Metamorphic Geology》2000,18(2):123-133.

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.  相似文献   

A re-evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe   总被引：1，自引：0，他引：1  

S. Wallis  & M. Aoya 《Journal of Metamorphic Geology》2000,18(6):653-664

Known eclogite occurrences in the Sanbagawa metamorphic belt of SW Japan are dominantly in metagabbro bodies which have complex polyphase metamorphic histories. These bodies are generally described as tectonic blocks and their relationship to the Sanbagawa metamorphism is unclear. New findings of foliated eclogite in the Seba and Kotsu areas show that eclogite facies metamorphism is much more widespread than generally thought. Evidence that the foliated eclogite units originated as lavas or sediments implies that these units can be treated as a high-grade part of the subduction-related Sanbagawa metamorphism. Although separated by an along-strike distance of 80 km, the Seba and Kotsu eclogites have very similar garnet and omphacite compositions, suggesting that they were formed under similar metamorphic conditions. However, differences in the associated retrograde assemblages (epidote–amphibolite in the Seba unit and epidote–blueschist in the Kotsu unit) suggest contrasting P – T  paths. In both units, the eclogite rocks occupy the highest structural level of the Sanbagawa belt and overlie rocks metamorphosed at lower pressure. The lower boundary to the eclogite units is therefore a major tectonic discontinuity locally decorated with lenses of exotic material. These features can help trace the boundary into other areas. The previously known outcrops of eclogite show enough similarities with the newly found areas to suggest that all the eclogite facies rocks in the Sanbagawa belt constitute a single nappe that lies at the highest structural levels of the orogen.  相似文献   

安徽大别山北部榴辉岩的分布及主要特征   总被引：1，自引：0，他引：1  

周存亭  高天山  汤加富  沈荷生  胡云琴 《地质通报》2000,19(3)

安徽大别山北部新发现的榴辉岩主要产于花岗质片麻岩及基性—超基性岩中 ,与大别山南部的榴辉岩在产出围岩、矿物组成及地球化学特征等方面均十分相似。现今榴辉岩南多北少的分布 ,主要是在中生代热隆构造体制下剧烈差异抬升剥蚀 ,中生代花岗岩大面积侵位所致。  相似文献   

榴辉岩中石英出溶体的拉曼光谱学研究及其构造意义   总被引：2，自引：0，他引：2  

王璐  金振民  何谋春 《地球科学》2003,28(2):143-150

利用拉曼光谱学和费氏台方法对若干地区超高压榴辉岩中柯石英和榴辉岩中石英出溶体的超显微构造特征进行了研究.研究结果表明, 石榴石(或绿辉石) 中柯石英向石英相变是一个连续变化过程.绿辉石中石英出溶体和主晶具有一定的晶体学拓扑关系, 即石英出溶体长轴有两个展布方向: 平行于绿辉石(10 0) 裂理面和平行于绿辉石(- 10 1) 面.目前, 部分学者推断榴辉岩绿辉石中石英出溶体可作为超高压变质作用的标志.笔者认为, 棒状定向石英在绿辉石中出溶的温压条件和出溶机理尚未查明.因此, 需加强过量二氧化硅单斜辉石出溶机理的超高压实验研究, 从而为证实石英出溶体作为超高压标志提供实验定量约束.   相似文献   

Overview of the geology, petrology and tectonic framework of the high-pressure–ultrahigh-pressure metamorphic belt of the Kokchetav Massif, Kazakhstan     

S. Maruyama  C. D. Parkinson. 《Island Arc》2000,9(3):439-455

Abstract High‐ to ultrahigh‐pressure metamorphic (HP–UHPM) rocks crop out over 150 km along an east–west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet‐like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north‐dipping, fault‐bounded orogen‐parallel units (I–IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high‐pressure amphibolite through quartz–eclogite and coesite–eclogite to diamond–eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780–1000°C, P = 37–60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730–750°C, P = 11–14 kbar; Unit IV: T = 530°C, P = 7.5–9 kbar) and bottom (Unit I: T = 570–680°C; P = 7–13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top‐to‐the‐north) and top (top‐to‐the‐south) of the pile. The orogen‐scale architecture of the massif is sandwich‐like, with the HP–UHPM nappe group juxtaposed across large‐scale subhorizontal faults, against underlying low P–T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP–UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower‐grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30–40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab‐breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP–UHPM rocks.  相似文献   

Thermobaric structure of the Kokchetav ultrahigh-pressure–high-pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan     

Tsutomu Ota  Masaru Terabayashi  Christopher D. Parkinson and  Hideki Masago 《Island Arc》2000,9(3):328-357

Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO₂ (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.  相似文献