Rare-Earth Element Geochemistry of Eclogites from the Ultra-High Pressure Metamorphic Belt in Central China

Based on their REE contents and REE patterns, eclogites from the ultra\|high pressure metamorphic belt in central China may be roughly divided into six types including LREE\|rich, LREE\|rich+positive Eu anomaly, LREE\|rich+negative Eu anomaly, REE pattern\|smooth, MREE\|rich and HREE\|rich. The LREE\|rich, LREE\|rich+positive Eu anomaly and LREE\|rich+negative Eu anomaly types of eclogites are dominant. REE types of eclogites in different areas can be compared and the REE features of the same REE type of eclogites in different areas are similar. The results of reconstruction of the primary rocks show that the primary rocks of eclogites possibly are dominated by continental tholeiites which are the product of partial melting of relatively fertile mantle and the rocks of tholeiite crystallization\|differentiation. There is perfect evolution relationship among the primary rocks of the LREE\|rich, LREE\|rich+positive Eu anomaly and LREE\|rich+negative Eu anomaly types of eclogites and among those of the REE pattern smooth and MREE\|rich types of eclogites, the former three types were derived from continental settings and the latter two from nearly oceanic settings. Meanwhile, it is concluded that the mantle sources of primary rocks of the eclogites are inhomogeneous and the primary rocks of eclogites in this area appear to have undergone varying degree of crustal contamination.

The Dabieshan Coesite-bearing Eclogite Terrain-A Late Archaean Ultra - high- Pressure Metamorphic Belt

A U -Pb zircon age of 2774±24 Ma for eclogite from the Bixiling rock body of Anhui Province, central China, indicates that the Dabieshan coesite-bearing eclogite was probably formed in the Late Archaean. A phengite Ar-Ar isochron age of 662±13 Ma for the eclogite confines also an upper limit age of its subsequent retrograde metamorphism in the Precambrian. The results of isotopic dating for such type of eclogite coincide with the geological features of its restricted occurrence within the Archaean metamaorphic terrain composed of the Dabie Group. It is believed that the Dabieshan coesite-bearing eclogite terrain might be a Late Archaean ultra-high-pressure metamorphic belt. The Dabie Mountains area was the eastward extension of the southern Qinling structural belt during the Triassic. Both the Dabie Group and the coesite-bearing eclogite hosted therein underwent a late-stage dynamic metamorphic event. The present authors have obtained a muscovite Ar-Ar isochron age of 192.6±2.8 Ma from plagioclase gneiss and a hornblende Ar-Ar plateau age of 230.7±4.6 Ma for the low amphibolite in eclogite respectively, which represent the Indosinian reworking ages of the original metamorphic rocks of the Dabie Group gneiss and coesite-bearing eclogite.

Geochemical Characteristics of Pb Isotope of High-Pressure Metamorphic Rocks and Foliated Granites from HP Unit of Tongbai-Dabie Orogenic Belt

Whole-rock Pb isotopic compositions of the high-pressure (HP) metamorphic rocks, consisting of two-mica albite gneisses and eclogites, and foliated granites from the HP metamorphic unit of the Tongbai-Dabie orogenic belt are firstly reported in this paper. The results show that the HP metamorphic rocks in different parts of this orogenic belt have similar Pb isotopic compositions. The two- mica albite gneisses have 206 Pb/ 204 Pb=17.657-18.168, 207 Pb/ 204 Pb=15.318-15.573, 208 Pb/ 204 Pb=38.315-38.990, and the eclogites have 206 Pb/ 204 Pb=17.599-18.310, 207 Pb/ 204 Pb=15.465- 15.615 , 208 Pb/ 204 Pb=37.968-39.143. The HP metamorphic rocks are characterized by upper crustal Pb isotopic composition. Although the Pb isotopic composition of the HP metamorphic rocks partly overlaps that of the ultrahigh-pressure (UHP) metamorphic rocks, as a whole, the former is higher than the latter. The high radiogenic Pb isotopic composition for the HP metamorphic rocks confirms that the subducted Yangtze continental crust in the Tongbai-Dabie orogenic belt has the chemical structure of increasing radiogenic Pb isotopic composition from lower crust to upper crust. The foliated granites, intruded in the HP metamorphic rocks post the HP/UHP metamorphism, have 206 Pb/ 204 Pb=17.128-17.434, 207 Pb/ 204 Pb=15.313-15.422 and 208 Pb/ 204 Pb=37.631-38.122, which are obviously different from the Pb isotopic compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP unit. This shows that the foliated granites from the HP and UHP units have common magma source. Combined with the foliated granites having the geochemical characteristics of A-type granites, it is suggested that the magma for the foliated granites in the UHP and HP unit would be derived from the partial melting of the retrometamorphosed UHP metamorphic rocks exhumed into middle to lower crust, and partial magmas were intruded into the HP unit.

Fluid Activity and Tectonic Evolution in the Northern Qilian High-pressure Metamorphic Belt

The Northern Qilian high-pressure metamorphic belt has experienced multipledeformation-metamorphism, which consists of at least four stages. In 550.8-526 Ma, eclogites wereformed. High temperature and pressure caused the escape of a large quantity of gas-liquid fluidsfrom rocks while silicate melt was generated. In the late stage, small amounts of CO_2 and H_2Oinfiltrating along fractures were introduced. In the formation of glaucophane schist (447-362 Ma),devolatilization reactions were dominated during the subduction-uplift stage of the paleoplate. Inthe uplift-exhumation stage (400-380 Ma) the increase of internal space of fractures in the rocksfavoured fluid infiltration and concentration. These fluids participated in hydration reactions inthe retro-metamorphism. The fluids participating in the mineral reactions have the compositions ofCaCl_2-NaCl-H_2O. In subsequent thrusting (<380 Ma), the metamorphic terrain was uplifted to theshallower crust and ductile-shearing deformation took place, which caused mainly dehydrationreactions of minerals. In a near-surface environment the metamorphic terrain experienced brittledeformation, forming many accompanying fractures. Immiscible CO_2 and low-salinity aqueous fluidsoccurred in these secondary microfractures and were trapped and sealed. The thermodynamic conditionsof different deformation-metamorphic stages of the metamorphic terrain were calculated and thecorresponding P-T-t path was deduced, showing that the metamorphic terrain has experienced aclockwise path indicated by T- and P-rising, and T- and P-falling processes. This reveals that thesubduction zone has undergone multiple tectono-dynamic processes, i.e. initial deep burial,subsequent quick uplift and near-surface tectonism.

Tectonic Evolution of an Early Precambrian High-Pressure Granulite Belt in the North China Craton

A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is located in the central collision belt between the western block and eastern block, and represents the deep crustal structural level. The typical high-pressure granulite (HPG) outcrops are distributed in the Hengshan and Chengde areas. HPGs commonly occur as mafic xenoliths within ductile shear zones, and underwent multipile deformations. To the south, the Hengshan-Chengde HPGB is juxtaposed with the Wutai greenstone belt by several strike-slip shear zones. Preliminary isotopic age dating indicates that HPGs from North China were mainly generated at the end of the Neoarchaean, assocaited with tectonic assembly of the western and eastern blocks.

胶北平度大金埠含石墨麻粒岩变质演化和石墨结晶形成的研究

具有孔兹岩系特征的荆山群主要出露于胶-辽-吉带南段的胶北地体,且普遍经历了高角闪岩相-麻粒岩相变质作用。平度大金埠地区处于平度-莱西石墨成矿带,石墨矿赋存于古元古界荆山群陡崖组徐村段。通过对平度大金埠晶质石墨矿区含石墨泥质麻粒岩进行岩相学、矿物岩石学与地球化学、激光拉曼光谱、年代学等研究,发现大金埠含石墨泥质麻粒岩经历了三个变质演化阶段(M_(1)-M_(3)),且峰期达到高压-超高温变质作用条件。因受超高温变质作用影响,早期进变质阶段(M_(1))矿物信息保留不完整;峰期阶段(M_(2))以黑云母的消失为标志,矿物组合为Grt+Kfs+Pl+Rt+Qz+Liq,该阶段的温压范围为1010~1050℃、1.09~1.20GPa;峰后退变质阶段(M_(3)),黑云母重新出现,矿物组合为Bt+Grt+Kfs+Pl+Rt+Qz,该阶段的温压范围为810~843℃、0.68~0.74GPa,变质演化P-T轨迹具有顺时针特点。拉曼光谱分析显示样品中石墨的结晶程度普遍较高,其拉曼光谱特征与麻粒岩相变质条件下形成的石墨一致,高晶度石墨多结晶于峰期M_(2)向峰后M_(3)退变质作用阶段转变过程中。显然,平度大金埠晶质石墨矿区含石墨麻粒岩中石墨的结晶程度与其结晶时寄主麻粒岩的变质温度密切相关。年代学研究获得变质锆石1830±18Ma的加权平均年龄,与胶-辽-吉带及胶北地体区域上的深熔作用时代一致,代表了大金埠地区高结晶度石墨结晶形成的年龄。本次对石墨结晶形成与寄主变质岩及变质作用的关系探究,对优质晶质石墨矿资源的探矿、找矿具有重要指示意义。

显生宙碰撞造山带超高温变质作用的加热机制:来自二维数值模拟的约束

大量的岩石学证据表明:碰撞造山带中常发育900~1100℃的超高温变质作用。然而,碰撞造山带中如何出现如此极端的超高温条件仍然存在争议。为了更好地理解超高温变质作用加热机制和碰撞造山带中主要热源的相对贡献,我们建立了一系列高分辨率二维热-动力学模型,借此探讨了俯冲大陆岩石圈密度亏损程度、大陆地壳放射性生热率和大陆汇聚速率等因素对碰撞造山过程中超高温变质主要热源的影响。当大陆岩石圈密度亏损(Δρ=ρ软流圈地幔-ρ岩石圈地幔)大于50kg/m^(3)时,有利于发生大陆平板俯冲,软流圈地幔无法上涌为地壳物质提供热源;此时,具有较高放射性生热率(>3μW/m^(3))的地壳可以发生“浅俯冲-折返”型超高温变质作用。而当大陆岩石圈密度亏损小于10kg/m^(3)时,大陆上地壳在深俯冲阶段首先发生超高压榴辉岩相变质作用,随后伴随着大陆岩石圈地幔后撤和软流圈上涌,进而出现以异常高的地幔热流加热为主的“深俯冲-折返”型超高温变质作用。此外,较低的大陆汇聚速率(<1cm/yr)更有利于“深俯冲-折返”型超高温变质作用的产生。将数值模拟结果与特提斯构造域的变质岩石数据和地球物理观测进行对比,我们认为在现今板块构造体制下,由具有密度亏损程度较高的大陆岩石圈平俯冲有利于“浅俯冲-折返”型超高温变质作用的发生,而由密度亏损程度较低的大陆岩石圈俯冲可能导致“深俯冲-折返”型超高温变质作用的产生。

滇西南哀牢山构造带李子冲矿区综合物探的应用研究

云南省金平县李子冲铁铜矿区地处扬子板块与印支板块交接部位的哀牢山构造带南段,其外围已发现大量的铁铜矿床(点),是云南重要的铁铜矿产基地。但研究区高山植被覆盖严重,地形起伏较大,地质构造复杂,岩浆活动强烈,找矿困难。为克服单一勘探方法找矿的局限性,选择高精度磁法、双频激电法和EH4电磁测深法对矿区进行综合地球物理勘探,结合地质调查资料,对矿区构造、隐伏岩体及矿化体的深部延伸情况进行解译,并总结找矿信息。综合物探结果表明,矿化体主要在哀牢山群变质岩系产出,与基性侵入岩和花岗闪长岩体关系密切,并伴有多种矿化现象;铁矿化区显示高磁、低视电阻率、高视幅频率特征,铜矿化区则具有低磁、低视电阻率、高视幅频率特征。根据磁力线突变或弯曲变形的磁异常现象和低视电阻率的激电异常及EH4剖面测量解译结果,推测矿区可能存在形成时间较早的北西向张性断裂和形成时间较晚的北东向剪性断裂,近北西向断裂有利于成矿而近北东向断裂对矿体具有破坏作用。对矿区内激电异常与磁异常整体套合较好且EH4电磁测深揭露深部具有成矿潜力的位置进行工程验证,结果显示,矿体在深部具有一定程度的延伸,近北西向断裂构造及其次级构造是主要的赋矿区域,相对低缓的磁异常区域深部可能存在新的铁铜矿化体,有扩大储量的可能,这对下一步地质勘查工作具有指导意义。

大型带式焙烧球团高硫、高氮烟气超净装置的设计与应用

山东青岛邦拓400万吨/年带式焙烧球团烟气净化采用CFB脱硫+SCR脱硝干式超净工艺技术,在入口SO_(2)浓度超过4000 mg/Nm^(3),NO_(x)浓度超过600 mg/Nm^(3)的高硫、高氮工况下,实现了出口SO_(2)排放浓度小于20 mg/Nm^(3)、NO_(x)排放浓度小于35 mg/Nm^(3)、粉尘排放浓度小于5 mg/Nm^(3)的超低排放指标。此项目的成功应用可为大型带式焙烧球团烟气超低排放治理工艺选择提供更多借鉴和参考。

Study of the High Pressure Metamorphic Blueschist and Its Late Proterozoic Age in the Eastern Jiangnan Belt

Several blueschist outcrops, specially glaucophane-bearing ones, were found in Dexing, eastern Jiangnan belt. The latest researches suggest that the blueschists distributed in the Late Proterozoic ophiolite melange blocks of the northeastern Jiangxi fault zone between the Jiuling tectonic region and the Huaiyu tectonic region of volcanic are that is located near the frontier of Jiangxi, Zhejiang and Anhui provinces (Fig. 1).

Eclogites of the Dabie Region: Retrograde Metamorphismand Fluid Evolution

Based upon fluid effects, retrograde metamorphism of eclogites in the Dabieregion can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stageis marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and isthought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely tohave occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyaniteporphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals suchas phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such asamphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with loweramphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. Theproduct of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferentialshearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanyingvein rutile was precipitated from fluids of this stage after local transport and concentration, andmay hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins canbe used as an exploration indicator for dissemination-type rutile deposits.

Crustal structure of the eastern Dabie orogenic belt from the seismic tomography and wide-angle reflection studies

The studies of seismic tomography and wide-angle reflection have been carried out to reveal the velocity structUrebeneath the eastern Dabie orogenic belt. The result from the seismic tomography shows the high velocity bodiesmight be positioned to a depth of only about 1 .5 km below sea level within the Dabie ultra-high pressure metamorphic (UHPM) belt; the fan-profile shows the Shuihou-Wuhe fault, the demarcation between the South Dabieand the North Dabie, slopes to the south-west at a dip angle of about 45° in the bottom of upper crust. The wideangle reflection shows the middle crustal boundaries and the complex features from the lower crust.

High-Pressure Eclogite-Blueschist Metamorphic Belt and Closure of Paleo-Tethys Ocean in Central Qiangtang, Qinghai-Tibet Plateau

The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.

Ultra-hot Mesoproterozoic evolution of intracontinental central Australia

The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature （UHT） meta- morphism from ca. 1220 to ca. 1120 Ma. This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism. The tectonic setting for this thermal event （the Musgrave Orogeny） is thought to have been intracontinental and the lithospheric architecture of the region is suggested to have had a major influence on the thermal evolution. We use a series of two dimensional, fully coupled thermo-mechanical-petrological numerical models to investi- gate the plausibility of initiating and prolonging UHT conditions under model setup conditions appro- priate to the inferred tectonic setting and lithospheric architecture of the Musgrave Province. The results support the inferred tectonic framework for the Musgrave Orogeny, predicting periods of UHT meta- morphism of up to 70 Ma, accompanied by thin crust and extensive magmatism derived from both crustal and mantle sources. The results also appear to be critically dependent upon the specific location of the Mus^rave Province. constrained between thicker cratonic masses.

THE XIAOJIAO HIGH-GRADE ECLOGITE-TYPE RUTILE DEPOSIT IN JIANGSU, CHINA: GEOLOGY, GEOCHEMISTRY AND METALLOGENESIS

The Xiaojiao rutile deposit with stratiform, banded and lens-shaped rutile-bearing eclogite in the Sulu ultra-high pressure metamorphic zone represents the largest and richest primary rutile mineralization in the zone. Four kinds of rutile can be recognized. They are: (1) inclusions in garnet and omphacite; (2) intergranular infilling between garnet and omphacite; (3) altered remnant; and (4) hydrothermal infilling in veinlets. Electron microprobe data show that sodic metasomatism probably from metamorphic fluid could play an important role in rutile mineralization during high and ultra-high pressure metamorphism. The present paper concludes that according to geochemical characteristics, the eclogite originated from mafic and ultramafic rocks in the upper mantle and crust-mantle boundary and the Xiaojiao eclogite-type rutile deposit is genetically closely related to ultra-high pressure metamorphism.

桥上CRTSⅡ型板式无砟轨道后浇带维修对线-桥系统的影响

对于桥上CRTSⅡ型板式无砟轨道结构,采用超高压水射流法破除伤损后浇带后,线-桥系统发生内部应力重分布。本文建立了桥上CRTSⅡ型板式无砟轨道结构纵向力计算模型,分析不同温度荷载作用下破除后浇带混凝土对线-桥系统纵向相互作用及结构位移的影响,确定了破除伤损后浇带的合理温度范围。结果表明:破除底座后浇带混凝土后,剪力齿槽纵向反力随温度荷载增加而大幅增大,降温工况下后浇带处钢筋拉应力较小;当施工温度高于底座板锁定温度10℃以上时,桥墩纵向受力较大;在后浇带破除位置,底座板、轨道板和钢轨的纵向位移均会发生突变,其中底座板纵向位移受温度荷载的影响较大,升温、降温20℃时分别可达4.0、2.5mm。建议后浇带维修施工温度控制在不低于底座锁定温度15℃和不超过底座锁定温度10℃的范围内。

北大别变质带高温变质作用的证据和时限:龚家岭地区刚玉片麻岩及其围岩的记录

长期以来,大别造山带的超高压变质作用一直是研究的热点,而有关该造山带的高温或超高温变质作用的研究则相对有限,且主要集中于北大别变质带中残存的特征性岩石--榴辉岩和变基性岩,对于普遍发育的长英质和泥质片麻岩的变质属性和时限则极少涉及,制约了人们对该造山带形成与演化过程的充分解读。为此,本次研究以北大别龚家岭地区广泛出露的刚玉片麻岩和含辉石花岗闪长片麻岩、黑云角闪斜长片麻岩为研究对象,展开了详细的野外调查、岩相学、矿物化学分析、热力学评价和年代学研究。研究显示,这些片麻岩经历了麻粒岩相、角闪岩相和绿片岩相3个阶段的变质作用,温压条件分别为:(1)T=724℃~762℃和P=0.3~0.4 GPa;(2)T=586℃~595℃和P=0.3~0.4 GPa;(3)T=382℃~395℃和P=0.1~0.3 GPa,总体显示了近等压降温的特征。其中条件(1)基本达到了高温变质作用范畴,为北大别变质带中普遍发育的片麻岩的主期变质条件。锆石U-Pb定年结果显示,刚玉片麻岩具Group-Ⅰ和Group-Ⅱ两组锆石年龄,分别为709±19 Ma~677±19 Ma和207±2 Ma。含辉石花岗闪长片麻岩则记录了Group-Ⅰ、Group-Ⅱ和Group-Ⅲ等3组锆石年龄,分别为602±23 Ma~237±11 Ma、217±4 Ma和192±5 Ma。结合锆石CL图和Th/U比值分析,这两个片麻岩的Group-Ⅰ组锆石Th/U比均大于0.1(多数>0.4),均为岩浆继承锆石,该年龄反映了早期多岩浆事件。Group-Ⅱ组锆石的Th/U比均<0.1,显示了变质成因特征,其217~207 Ma年龄对应于主期麻粒岩相变质时限。Group-Ⅲ组锆石的Th/U比均<0.1,表现变质增生特征,其192 Ma年龄相当于角闪岩相变质时限。本次研究表明北大别变质带主体经历了浅层次的高温变质作用,缺乏高温超高压变质的“记录”。

胶北日庄石墨结晶与寄主麻粒岩的变质演化关系

日庄镇含石墨泥质麻粒岩出露于莱西石墨成矿带,详细的岩相学、地球化学、拉曼光谱、年代学等研究显示岩石经历了超高温变质作用。麻粒岩变质演化过程包括三个阶段,其中峰前M 1阶段矿物仅部分保留;峰期M 2阶段和峰后M 3阶段矿物组合分别为Grt+Qz+Pl+Kfs+Sil+Rt+Liq和Grt+Qz+Bt+Pl+Kfs+Sil+Rt+Liq;相平衡模拟结合二长石温度计和石英中Ti温度计研究结果显示峰期M 2阶段的温压条件为1025~1100℃、1.05~1.2GPa。研究样品中石墨结晶程度较高,石墨拉曼光谱特征与麻粒岩相变质条件下形成的石墨一致,岩相学特征指示高晶度石墨结晶于M 2向M 3转变过程中。麻粒岩中获得变质锆石年龄为~1.85Ga,该年龄是寄主岩石峰期变质作用后熔体冷却时间的记录。岩相学和年代学研究表明,优质高晶度石墨的形成和生长可能与胶-辽-吉活动带区域上碰撞折返过程中发生的深溶事件或混合岩化作用密切相关。

江苏省东海县毛北金红石矿区中-北矿段矿体地质特征及成因初探

通过对矿区进行勘查工作,对矿床地质条件、控矿因素及成因进行总结研究。矿区位于苏北超高压变质带,岩浆活动强烈,基底地层为海州群变质岩系。本次工作共发现十条工业矿体,矿床赋存于榴辉岩中,成矿期后矿床受断层切割,使岩(矿)体不连续,走向及倾向发生明显变化。根据赋矿榴辉岩岩体结构构造不同分为块状榴辉岩及云母榴辉岩,前者较后者含矿性相对较好。金红石矿物颗粒主要呈浸染状分布于石榴子石、绿辉石等矿物粒间,矿石中金红石含量区间在1%~6%。榴辉岩的成因已有定论,经历了四个变质阶段,而对榴辉岩相峰期变质的温度及压力的认识观点不一,但不可否认的是本矿床属深成岩浆变质矿床。

东昆仑大格勒一带基性二辉麻粒岩的变质演化及地质意义

基性麻粒岩作为高温变质作用产物,其矿物组合能够较好地记录区域地壳热流的峰值范围,是揭示造山带演化历史的重要窗口。本文报道了在东昆仑造山带东段大格勒地区金水口岩群副片麻岩中发现的基性二辉麻粒岩露头,它们呈块状-片麻状构造,斑状变晶-细粒变晶结构,主要矿物为斜长石、斜方辉石、单斜辉石、黑云母和普通角闪石。斜方辉石与单斜辉石和高钙斜长石(An=74~81)的平衡结构以及斜方辉石中包裹残余普通角闪石和黑云母的结构表明这些岩石经历了较充分的麻粒岩相结晶作用。填隙状生长的普通角闪石和黑云母可能代表了冷却至固相线附近矿物-残余熔体发生反应的产物。根据二辉石温度计和相平衡模拟计算,获得在0.4~0.7GPa压力条件下的麻粒岩相变质温度为840~910℃。详细的变斑晶斜长石和钛铁矿包体研究表明,麻粒岩相变质之前可能存在一期低温高压变质作用,形成的矿物组合为斜黝帘石+金红石+榍石+普通角闪石±阳起石+黑云母+石英,相平衡模拟确定该阶段的温压条件为0.85~1.2GPa、500~640℃。斜黝帘石在减压阶段的早期转变为低钙斜长石(An=46-51),同时角闪石转变为单斜辉石,可能代表了低压角闪岩相阶段。通过LA-ICPMS锆石U-Pb测年以及锆石包体分析,确定麻粒岩相变质年龄为411.4±2.9Ma(MSWD=1.05)。这些二辉麻粒岩是在金水口以外的东昆仑地区发现的首例晚古生代低压高温变基性岩,其与围岩片麻岩构成一个麻粒岩地体。它们反映了泥盆纪早期东昆仑造山带的异常高温事件,与后碰撞伸展阶段软流圈地幔上涌带来的区域高温热流有关。本研究对解析东昆仑造山带高级变质地体的精细结构以及古特提斯洋形成过程的深部动力学机制具有重要意义。