Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite,Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes     

《International Geology Review》2012,54(8):1005-1019

ABSTRACT

The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H₂O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO₂ (45.5–48.7 wt.%) and Na₂O + K₂O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na₂O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H₂O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H₂O exerts a major control. Zircon δ¹⁸O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ¹⁸O values may be due to incorporation of large amounts of low-δ¹⁸O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H₂O content of the liquid that finally crystallize high-An plagioclases.  相似文献   

Mineral chemistry and geothermobarometry of Moshirabad pluton,Qorveh, Kurdistan,western Iran   总被引：1，自引：0，他引：1  

ALI A. SEPAHI  MOHAMMAD MAANIJOU  SEDDIGHEH SALAMI  SARA GARDIDEH  TAYEBEH KHAKSAR 《Island Arc》2012,21(3):170-187

The Moshirabad pluton is located southwest of the Sanandaj–Sirjan Metamorphic Belt, Qorveh, western Iran. The pluton is composed of diorite, monzodiorite, quartz diorite, quartz monzodiorite, tonalite, granodiorite, granite, aplite, and pegmatite. In this study 31 samples from various rocks were chosen for whole‐rock analyses and 15 samples from different lithologies were chosen for mineral chemical studies. The compositions of minerals are used to describe the nature of magma and estimate the pressure and temperature at which the Moshirabad pluton was emplaced. Feldspar compositions are near the binary systems in which plagioclase compositions range from An₅ to An₅₃ and alkali‐feldspar compositions range from Or₉₁ to Or₉₇. Mafic minerals in the plutonic rocks are biotite and hornblende. Based on the composition of biotites and whole‐rock chemistry, the Moshirabad pluton formed from a calc‐alkaline magma. Amphiboles are calcic amphiboles (magnesio‐hornblende or edenite). Temperatures of crystallization, calculated with the hornblende–plagioclase thermometer, range 550–750°C. These temperatures indicate that plutonic rocks have undergone some retrogressive changes in their mineral compositions. Aluminum‐in‐hornblende geobarometry indicates that the Moshirabad pluton was emplaced at pressures of 2.3–6.0 kbar, equal to depths of 7–20 km, but with consideration of regional geology, lower pressures than the above pressure range are more probable. Alteration of amphiboles can be the reason for some overestimation of pressures.  相似文献   

南太行山平顺闪长岩体斜长石捕虏晶特征及其对壳幔岩浆混合过程的记录     

张海东  刘建朝  彭素霞  张苏楠  王得权 《吉林大学学报(地球科学版)》2014,44(2):566-573

在南太行山平顺闪长岩体中偶见具有特殊环带结构的斜长石,其核部为钙含量高的基性斜长石（An>63）,幔部为富钠的斜长石,二者之间存在由绢云母和绿帘石组成的溶蚀带,且核、幔An值变化截然不同。根据斜长石的环带构造特征和成分分析,认为本区岩浆混合过程大致如下：起源于EMI型富集地幔的幔源岩浆底侵作用于下地壳,在高温下结晶出钙含量高的基性斜长石,且所携带的热使下地壳开始熔融形成壳源岩浆;壳、幔两种岩浆沿着太行山深大断裂快速上升侵位于地壳浅部,发生岩浆混合;与此同时,早期形成的基性斜长石由于压力突然减小发生溶蚀,形成形态不规则的溶蚀带;壳幔混合岩浆在基性斜长石基础上结晶生长富钠斜长石,形成具有特殊环带结构的斜长石。这为研究区内存在岩浆混合作用提供了最直接的证据。  相似文献   

攀西地区新街层状岩体粒间不混熔作用:来自斜长石环带结构的记录     

董欢  邢长明  王焰 《大地构造与成矿学》2017,41(2)

位于攀西地区的新街层状岩体赋含大量钒钛磁铁矿,是峨眉山大火成岩省的一部分。岩体下部带和中部带以单斜辉石岩为主,并伴生浸染状钒钛磁铁矿矿化;上部带以辉长岩为主,赋存厚层的钒钛磁铁矿矿体。之前研究认为厚层的钒钛磁铁矿矿体的形成与粒间不混熔的富Fe熔体有关,但对富Fe熔体的演化过程缺乏细致研究。本文通过对新街岩体上部带的富矿辉长岩层和上覆浅色辉长岩中斜长石环带结构和成分的研究,揭示了富Fe熔体的演化过程。在浅色辉长岩中保存的岩浆不混熔的直接证据表现为矿物粒间共轭的富Si交生体和富钛铁矿交生体代表的非反应结构。本次研究发现,与粒间富Si交生体接触的斜长石边部的FeO和TiO_2含量随斜长石牌号(An)值的降低而降低,而与粒间富钛铁矿交生体接触的斜长石边部的FeO和TiO_2含量随An值的降低而升高,说明斜长石的边部成分变化记录了粒间共轭的富Si和富Fe熔体的成分特征。在富矿辉长岩中,斜长石可分为初生和新生两种,初生斜长石的An值介于57~62,FeO含量为0.34%~0.50%,TiO_2含量为0.06%~0.13%,新生斜长石具有相对较高的An值(61~81)和FeO、TiO_2含量,二者的内部和边部还发育增生斜长石,其An值(~50)相对较低;在初生斜长石边部可见不连续的新生斜长石环带和增生斜长石边,造成其内部成分显著不均一,并发育复杂的环带结构。本文认为,初生斜长石是岩浆正常分离结晶作用的产物。在粒间熔体发生不混熔后,不混熔的富Fe熔体逐渐向岩浆房下方迁移并结晶出了一些相对高An值的新生斜长石,或沿一些初生斜长石边部生长形成不连续的高An环带。当富Fe熔体演化至晚期,由于矿物生长空间受限,仅在初生和新生斜长石局部形成了相对低An值的增生边、或沿颗粒裂隙进入斜长石内部形成增生斜长石核。  相似文献   

A 3100-year history of argon isotopic and compositional variation at El Chichón volcano     

D.A. Jones  P.W. Layer  R.J. Newberry 《Journal of Volcanology and Geothermal Research》2008

El Chichón volcano has produced at least 11 eruptive events in the past 8000 years, all of which display a relatively constant trachyandesitic composition. The constancy of the eruptive products attests to the tapping of a long-lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma. We have sampled eruptive products from eight of these events, subjecting their dominant phenocryst phases (plagioclase and hornblende) to argon isotopic analysis in order to investigate the dynamics of a steady state magmatic system. Plagioclase from the older eruptions, 1500 yr BP, 1600 yr BP, 2000 yr BP and 3100 yr BP, display variable enrichment of ⁴⁰Ar (excess argon), whereas hornblende from the same eruptions shows little or no enrichment. In contrast, both plagioclase and hornblende phenocrysts from the younger eruptions in 1982, 550 yr BP, 900 yr BP and 1250 yr BP have argon isotopic ratios near-atmospheric values. Isochron analysis of these mineral phases rules out xenocrystic contamination as the source of excess argon, while mafic enclaves and isotopic and compositional zoning evidence frequent recharge events, suggesting the source of this argon is most likely the same as the source of this basaltic magma; correlation with Cl points to melt/vapor inclusions as the primary host for Ar within the phenocrysts. Argon isotopic variations point to a disturbance of the system between 1500 yr BP and 1250 yr BP, while compositional evidence for a major mafic input is present in the 900 yr BP eruption, indicating a complex relationship between recharge and isotopic signature of eruptive products. The amount of excess argon within a plagioclase phenocryst are a function of variations in melt and vapor inclusion abundance, time elapsed between melt/vapor inclusion entrapment and eruption, variations in Ar abundance in melt (itself a function of vapor exsolution timing) and time variations in Ar isotopic composition of the melt; subdued behavior of hornblende is due to slower diffusion and minimal inclusions.  相似文献   

Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy     

U. ALTENBERGER 《Journal of Metamorphic Geology》1995,13(5):553-558

Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.
The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.  相似文献   

Experimental simulation of plagioclase diagenesis atP-T conditions of 3.5 km burial depth     

Stephen L. Karner  B. Charlotte Schreiber 《Pure and Applied Geophysics》1993,141(2-4):221-247

Dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments. The diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a Bridgeman-type pressure vessel. The experiments, each of two weeks duration, simulated pressure-temperature conditions approximating 3.5 km burial depth. By using a double-acting pore-fluid reservoir, solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz (90:10 wt% respectively).Scanning electron microscope analysis of the post-experiment samples reveals dissolution features and precipitated products. Dissolution voids of 10 microns occur typically in areas of maximum stress such as crack-tips and grain contacts. Dissolution on a larger scale is exemplified by topographical smoothing of grain su faces. The dissolved species are subsequently reprecipitated as Ca-enriched overgrowths (possibly zeolites) and clays. These precipitates are found individually on the scale of 10 microns and collectively as surface coatings on both feldspar and quartz grains. Atomic absorption spectroscopic analyses of the pore fluid suggest that the fluid chemistry is consistent with the observed experimental precipitates.These experiments show that clay coatings are unnecessary precursors to grain surface dissolution and that the diagenetic precipitation is not mineral selective. Also, the mass transfer of the dissolved species appears to be localized because grains displaying both dissolution and precipitation features are commonplace. Volume changes due to mineral transformation/alteration may increase secondary porosity if the dissolved species produced from dissolution are only partially involved in reprecipitation and the remaining dissolved material is flushed out by the pore fluids. However, if the mass transfer is primarily local then permeability would significantly decrease as precipitates may choke the pore throats.  相似文献   

铜官山岩体矿物学-矿物化学特征：岩浆结晶动力学意义   总被引：1，自引：0，他引：1  

赵乘乘  杨晓勇  冯敏  孙卫东  洪长春  孙健  胡俊杰  唐树平 《地质学报》2012,86(11):1748-1760

本文对皖南官山岩体开展详细地显微镜观察鉴定,利用电子探针和LA-ICP-MS技术对岩浆岩典型矿物斜长石、角闪石和榍石进行了主量和微量元素测定。显微镜鉴定表明,铜官山岩体中存在着大量的岩浆不平衡结构：如斜长石和角闪石嵌晶结构以及针状磷灰石等。这些现象的存在表明铜官山岩体在形成过程中曾发生过一次或多次岩浆混合作用。电子探针分析结果显示,斜长石的成分环带是震荡环带,而大尺度的震荡环带可能代表了大规模的岩浆混合作用;角闪石成分TiO2-Al2O3图解、CaO/NaO2-Al2O3/TiO2图解和Mg-(Fe2++Fe3+)- LiNaKCa角闪石成因矿物族三角图解指示铜官山岩体中角闪石很可能为壳-幔混合成因。LA-ICP-MS技术对主要造岩矿物的微量和稀土元素分析表明,角闪石很可能为幔源或壳幔混合源,斜长石可能为不同分异程度岩浆的混合形成。本研究比较明确地反映了铜官山岩体的形成过程中岩浆来源和结晶动力学过程,即壳幔源区的混合交代作用,与前人通过元素-同位素手段获得的信息比较吻合。  相似文献   

下地壳含水性的演化 ——来自不同时代麻粒岩中长石水含量的证据   总被引：1，自引：1，他引：0  

杨贵才  杨晓志  夏群科 《岩石矿物学杂志》2012,31(4):565-577

利用傅里叶变换红外光谱(FTIR)和电子探针(EMP)分析了早古生代的松树沟麻粒岩和桐柏麻粒岩地体以及古元古代的莒南麻粒岩包体(其寄主岩石为新生代玄武岩)中长石的水含量和化学成分。结果显示,麻粒岩中的长石均含有以OH和H2O形式存在的结构水;3个地点的长石水含量分别为465×10-6~733×10-6、210×10-6~993×10-6和717×10-6~1 239×10-6。对比前人报道的中生代(道县和汉诺坝包体)和古元古代(女山包体、汉诺坝地体)的麻粒岩研究结果,发现早古生代样品和古元古代样品中长石的水含量都比中生代样品明显的高,而早古生代和古元古代样品之间却没有差别,指示了中国东部下地壳在中生代之前更加富水。  相似文献   

班公湖—怒江缝合带西段酸性侵入岩LA-ICP-MS锆石U-Pb年龄与地球化学特征     

张向飞  李佑国  曹晓民  曲晓明  辛洪波  王健  杜德道 《地质通报》2014,33(7):984-994

针对班公湖—怒江中特提斯洋盆闭合的时间问题,选择西藏日土县班公湖畔蛇绿岩中的小型酸性侵入岩体为研究对象,探讨其岩石成因和构造环境。研究区酸性岩体的岩性为斜长花岗斑岩。地球化学分析显示,岩石SiO2含量介于65.78%~68.76%之间,(K2O+Na2O)含量为6.39%~6.86%;具有轻稀土元素富集、分馏程度高、Eu正异常不明显的特点;富集大离子不相容元素Sr、Ba、Rb,亏损高场强元素Ta、Ti、Nb,并有Ta、Nb元素谷形,显示出典型的岛弧岩浆岩特征。2个斜长花岗斑岩体的LA-ICP-MS锆石206Pb/238U年龄加权平均值分别为96.08Ma±0.22Ma和96.05Ma±0.30Ma,均属于中生代晚白垩世。  相似文献