大别山北坡尖晶石橄榄岩中的高温石榴辉石岩包体：兼论上地幔部分熔融

出露于饶拔寨一带的含尖晶石橄榄岩体是大别山区最大的一个上地幔残片,固态侵位于北大别深成片麻岩中,呈无根侵入体的形式产出。岩体内的石榴辉石岩和榴辉岩包体,具高温成因的特点。属于上地幔部分熔融的产物,在Pl-CIPW值对Al_2O_3图解上落于拉斑玄武岩区;榴辉岩、石榴辉石岩和含尖晶石橄榄岩的REE配分形式相似,均属平坦型,只是尖晶石橄榄岩的∑REE较低。在Ni-Co-Sc三组分图解上,石榴辉石岩投绘于深位玄武岩熔体的趋势线附近,而尖晶石橄榄岩的投点则接近超镁铁质残余的趋势,清楚揭示出地幔部分熔融的迹象。尖晶石的Cr~#=12～21,而共生橄榄石Fo=92～93,说明部分熔融程度不高,估计约15％。尖晶石贫TiO_2暗示熔融过程氧逸度低。饶拔赛含尖晶石橄榄岩的出现表明;伴随超高压岩石单元的折返和隆升,会有大陆地幔残片被携带上来。软流圈的上涌或板片的断离使侵出的岩石圈板片得以维持较高的温度,这也就是石榴辉石岩早期深位退变质为麻粒岩相的原因,是后续的进一步抬升,才出现以韭角闪石 斜长石组合为特征的高角闪岩相的退变质作用。     

昌乐地区新生代碱性玄武岩中刚玉、尖晶石巨晶岩相学、矿物化学特征

山东昌乐蕴含中国最大的宝石级刚玉—蓝宝石矿床。昌乐刚玉巨晶与玄武岩界限截然,边缘圆化,沿巨晶溶蚀圆化的边缘发育富铝的尖晶石和钛磁铁矿反应环边,显示其为深源捕虏晶的岩相学特征。对刚玉等巨晶及其反应边矿物进行岩相学、电子探针和LA-ICP-MS(Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry)分析能有效揭示其成因及演化历程。昌乐刚玉2组样品(CL-1和CL-2)矿物化学和关键元素特征及Ga/Mg、Cr/Ga、Fe/Ti、Fe/Mg等关键元素比值显示为岩浆型来源,而CL-2组样品明显叠加了后期熔体交代作用的特征。反应边尖晶石具刚玉巨晶向尖晶石巨晶过渡的成分特征,关键元素特征指示两者具强烈亲缘性,尖晶石巨晶是刚玉经历复杂交代作用的产物。昌乐刚玉呈现碱性熔体中结晶的化学成分特征,极有可能源于交代富集的地幔岩低度熔融形成的富铝的碱性熔体。     

黔东施秉下翁哨地区金刚石指示矿物地球化学特征及其地质意义

对黔东施秉下翁哨地区金刚石指示矿物镁铝榴石、铬尖晶石分别进行了电子探针（EMPA）主量元素和LA- ICP- MS微区原位微量元素测试,结果表明：镁铝榴石主要属于G9型（约占87%）,次为G12型（约占11%）和G3型（约占2%）,源区为二辉橄榄岩和异剥橄榄岩,当其与橄榄石共生,获得平衡温度（tNi）为686~1225℃,压力（PCr）为1.9~4.0 GPa,少数落在形成金刚石的主要温压范围内（压力约4.0~6.5 GPa,温度约950~1300℃）;铬尖晶石主要来源于橄榄岩尖晶石捕虏晶（Xen）（约占90%）,少量为铬铁矿（Chr）（约占10%）,个别落在全球原生金刚石中铬铁矿包裹体的范围内。研究认为,恢复区域加里东期造山运动以来的古地貌,寻找金刚石指示矿物的母岩体/新岩体及开展其矿物间的对比研究工作,特别是对加里东期以来区域的地幔物质组成、岩石圈厚度和热状态等进行深入研究,将对扬子克拉通的演化、深部地幔结构剖析和金刚石成矿事件等具有重要的科学研究意义。     

Tectonic implication of Lower Cretaceous chromian spinel-bearing sandstones in Japan and Korea

Abstract In Japan and Korea, some Lower Cretaceous terrigenous clastic rocks yield detrital chromian spinels. These chromian spinels are divided into two groups: low-Ti and high-Ti. The Sanchu Group and the Yuno Formation in Japan have both groups, whereas the Nagashiba Formation in Japan and the Jinju Formation in Korea have only the low-Ti spinels. High-Ti spinels are thought to have originated in intraplate-type basalt. Low-Ti spinels (higher than 0.6 Cr#) were probably derived from peridotites, which are highly correlated with an arc setting derivation and possibly with a forearc setting derivation. Low-Ti spinels are seen in the Sanchu Group, the Nagashiba Formation and the Jinju Formation. Low-Ti spinels from the Yuno Formation are characterized by low Cr# (less than 0.6) and these chromian spinels appear to have been derived from oceanic mantle-type peridotite, including backarc. According to maps reconstructing the pre-Sea of Japan configuration of the Japanese Islands and the Korean Peninsula, the Korean Cretaceous basin was comparatively close to the Southwest Japan depositional basins. It is possible that these Lower Cretaceous systems were sediments mainly in the forearc and partly in the backarc regions. The peridotite might have infiltrated along major tectonic zones such as the Kurosegawa Tectonic Zone (= serpentinite melange zone) in which left lateral movement prevailed during the Early Cretaceous.     

北祁连拉硐超基性岩体铬尖晶石特征研究及找矿意义

拉硐超基性岩体位于青海省祁连县城东北约15 km处,面积约7．6 km2,岩性主要由蛇纹石化橄榄岩、蛇纹石化辉榄岩、蛇纹岩及纯橄岩组成,共发现157个铬铁矿小矿体,铬铁矿体成群出现,依据野外铬铁矿体分布范围,自西向东划分为10个矿带,通过对其中铬尖晶石取样分析研究,并与区内其他含矿岩体中铬尖晶石样品化验分析结果对比,确定该类岩体不同岩性组合带的找矿方向。     

Systematic variations in xenocryst mineral composition at the province scale, Buffalo Hills kimberlites, Alberta, Canada

The Buffalo Hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to Proterozoic crystalline basement termed the Buffalo Head Terrane in north-central Alberta, Canada. The kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond. The xenocryst assemblage in the province is atypical for diamondiferous kimberlite, including an overall paucity of mantle indicator minerals and the near-absence of compositionally subcalcic peridotitic garnet (G10). The most diamond-rich bodies are distinguished by the presence of slightly subcalcic, chromium-rich garnet and the general absence of picroilmenite, with the majority forming a small cluster in the northwestern part of the province. Barren and near-barren pipes tend to occur to the south, with increasing proximity to the basement structure known as the Peace River Arch. Niobian picroilmenite, compositionally restricted low-to moderate-Cr peridotitic garnet, and megacrystal titanian pyrope occur in kimberlites closest to the arch. Major element data for clinopyroxene and trace element data for garnet from diamond-rich and diamond-poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond, and diamond source rocks proximal to the arch were the most affected.     

The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa

Chromium as Cr³⁺ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al₂O₃in the system and hence the concentration of Al₂O₃ in partialmelts. The effect of Cr₂O₃ on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr#_spis accompanied by increasing MgO and SiO₂, whereas CaO remainsalmost constant. Consequently, the CaO/Al₂O₃ ratio of the meltincreases with Cr#_sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al₂O₃ ratios. The concentrationof Cr₂O₃ in the melt remains low even at high Cr#_sp, which meansthat the strong effect of Cr₂O₃ on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr₂O₃ nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr₂O₃; CaO/Al₂O₃ of melt; effect of Cr₂O₃     

Using calculated chemical potential relationships to account for coronas around kyanite: an example from the Bohemian Massif

Corona textures around kyanite, involving for example zoned plagioclase separating kyanite from the matrix, reflect the instability of kyanite with the matrix on changing P–T conditions, commonly related to decompression. The chemical potential gradients set up between the kyanite and the matrix as a consequence of slow Al diffusion drive corona development, with the zoning of the plagioclase reflecting the gradients. Calculated mineral equilibria are used to account for corona textures involving plagioclase ± garnet around kyanite, and replacement of kyanite by plagioclase + spinel symplectite, in quartz + plagioclase + K‐feldspar + garnet + kyanite granulite facies gneiss from the Blanský les massif in the Bohemian massif, Czech Republic. In the garnet‐bearing coronas, a commonly discontinuous garnet layer lies between the kyanite and the continuous plagioclase layer in the corona, with both the garnet and the plagioclase appearing mainly to replace matrix rather than kyanite. The garnet layer commonly extends around kyanite from original matrix garnet adjacent to the kyanite. Where garnet is missing in the corona, the kyanite itself may be replaced by a spinel–plagioclase corona. In a local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in μ(Na₂O)–μ(CaO)–μ(K₂O)–μ(FeO)–μ(MgO)–μ(SiO₂) in the model chemical system, Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂ (NCKFMAS). The discontinuous nature of the garnet layer in coronas is accounted for by the effect of the adjacent original garnet on the chemical potential relationships. The replacement of kyanite by spinel + plagioclase appears to be metastable with respect to replacement by corundum + plagioclase, possibly reflecting the difficulty of nucleating corundum.     

Fe—Mn—Co—Cu体系尖晶石的结构和红外辐射特性

采用XRD、SEM／EDAX、IR等测试方法研究了Fe2O3-MnO2-Co2O3-CuO体系的结构及其形成过程，测试了该体系的红外辐射性能，分析了该体系的结构特征与其红外辐射特性的关系。研究结果表明，高温合成过程中过渡金属氧化物形成立方尖晶石固溶体。尖晶石结构的四面体位置主要分布有Fe^3 、Mn^2 、Cu^2 等离子，八面体位置主要被Mn3^ ,Co^3 ,Cu^2 ,Fe^2 ,Fe^3 等离子占据。体系组成的变化对过渡金属离子在四面体位置和八面体位置的分布状况产生影响。由于在四面体位置和八面体位置上均存在多种过渡金属离子，Fe-Mn-Co-Cu体系立方尖晶石固溶体在2．5－25μm波段内具有较高的红外辐射率。     

伊通上地幔剪切带捕虏体中富铝尖晶石的地球化学特征

伊通上地幔剪切带中辉石岩、二辉橄榄岩和易剥橄榄岩是3个主要地幔捕虏体,其中尖晶石的化学成分以富铝为特征。辉石岩中尖晶石在薄片下呈绿色,为铝含量高、铬含量低的铝尖晶石;与之相比,二辉橄榄岩和易剥橄榄岩中尖晶石在薄片下呈棕色,为铝含量相对较低、铬含量相对较高的富铝铬尖晶石。尖晶石的颜色与Al、Mg、Cr、Fe含量有关。在不同捕虏体中尖晶石成分具有不同的变化范围,尖晶石化学成分之间存在一定的相关性,Mg#与Cr#,Mg#与Fe2 和Cr与Al呈现负相关性,而Mg#与Al#呈现出正相关性,反映不同捕虏体中尖晶石具有同一来源和成因联系。在地幔岩石部分熔融和结晶过程中Cr和Al的分异明显。固溶体条件下基本服从八面体Cr Al替代关系,而不同地幔捕虏体中尖晶石的成分差异是岩浆部分熔融和结晶过程中元素分异、固溶体离子替代的结果。