冀北镇宁堡—赤城一带红旗营子杂岩变基性岩及其围岩锆石U-Pb年代学研究

红旗营子杂岩主要块体包括太古代火成岩(斜长角闪片麻岩、石英闪长岩-英云闪长岩-花岗闪长岩),晚古元古代或古生代变基性岩(退变榴辉岩和斜长角闪岩),晚古生代变表壳岩(石榴角闪黑云斜长片麻岩和蛇纹石化大理岩).本次研究的变基性岩(石榴斜长角闪岩)角闪岩相变质作用发生的时间为322.0 Ma-317.3 Ma,其围岩变表壳岩...     

川西新元古代灯杆坪花岗岩成因与岩浆演化:来自斜长石和黑云母矿物学依据

新元古代灯杆坪花岗岩体位于龙门山造山带中段, 处于青藏高原东缘与扬子地块西缘的过渡地带,是研究扬子西缘构造-岩浆事件的典型地区。通过对新元古代灯杆坪岩体开展野外地质调查、岩相学分析以及电子探针测试工作,重点对花岗岩中斜长石结构成因特征、黑云母成分特征、岩浆演化进行了分析讨论。结果表明,灯杆坪二长花岗岩中斜长石具有简单“反环带”结构,主要为更长石(An_12.51Ab_47.43Or_40.06),正长花岗岩中“熔蚀结构”显著,碱性长石居多,多为透长石(An_7.46Ab_42.39Or_50.16); 花岗岩体中的黑云母具有富铝、富铁、低镁、低钾的特征,属铁质黑云母。同时I_Fe值为0.47~0.50,表明该岩体中的黑云母未遭受后期流体的改造。黑云母地球化学特征分析结果表明,灯杆坪岩体属于过铝质花岗岩,其形成与壳源岩浆有关。结晶压力和深度分别为1.81×10 ⁵~2.23×10 ⁵ Pa、6.58~8.11 km,黑云母的结晶温度为750~786 ℃,并且有较高的氧逸度,属于中深成相。这一结果为扬子地块西缘新元古代800~740 Ma期间的强烈伸展-岩浆事件提供了新的证据。     

Chemical composition and petrogenesis of plagioclases in plagioclase-phyric basalts from the Southwest Indian Ridge(51°E)

Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts(PUBs)erupted on the Southwest Indian Ridge(SWIR)(51°E) to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems. The phenocryst cores are generally calcic(An_(74–82)) and are depleted in FeO and MgO. Whereas the phenocryst rims(An_(67–71)) and the plagioclase in the groundmass(An_(58–63)) are more sodic and have higher FeO and MgO contents than the phenocryst cores. The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt, but are likely to have crystallized from a more calcic melt. The enrichment in incompatible elements(FeO and MgO), as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass, are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized. Our results indicate that an evolved melt replenishing under the SWIR(51°E) drives the eruption over a short period of time.     

Plagioclase weathering in the groundwater system of a sandy,silicate aquifer

The weathering rate of plagioclase was estimated in the groundwater system of a sandy, silicate aquifer formed after the Wisconsin Glacial Stage. The study area is an isthmus lying between Crystal and Big Muskellunge Lakes in northern Wisconsin, USA. Plagioclase occupies 3% of the quartz and K‐feldspar dominated sediments. Groundwater in the study area is recharged in part by precipitation through the isthmus soils and in part by seepage from Crystal Lake, which is of low ionic strength and chemically in steady state. Water analysis revealed that the chemistry of groundwater recharged from Crystal Lake is regulated by mineral dissolution reactions. The rate constant for plagioclase was estimated using mass balances for sodium concentrations along a groundwater flowline from Crystal Lake. For this calculation, various kinds of hydrological/mineralogical information were used: groundwater flow path from oxygen isotope analysis, groundwater travel times from flow modelling, mineral composition from microprobe analysis and surface area of minerals from BET (Brunauer–Emmett–Teller) analysis. The overall range of the estimation was less than an order of magnitude (3·5 × 10^?16 to 3·4 × 10^?15 mol/m²/s). The result is up to three orders of magnitude slower than the previous field estimates, which applied geometric methods in measuring mineral surface areas. However, this result is somewhat higher than the estimates reported by other BET area‐based studies, which were undertaken on soil profiles having different hydrological conditions. This rate difference is interpreted as a result of higher mineral reactivity owing to younger sediment age. The rate difference is smaller when this result is compared with the estimates from the soils of similar age, indicating that the differences in hydrological condition are not sufficient to explain the weathering rate discrepancy between the laboratory and field studies, which is up to five orders of magnitude. Copyright © 2002 John Wiley & Sons, Ltd.     

Flow kinematics in the deeper part of a subduction channel, as inferred from inclusion trails in plagioclase porphyroblasts from the Sambagawa metamorphic rocks, south-west Japan

The subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the [010] and [001] directions, respectively, and their long‐axis orientations define an internal foliation S_i (best‐fit great circle) and lineation L_i (maximum on the S_i). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to L_i, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the L_i, respectively. However, the latter type commonly consists of composite trails, where the S_i is first rotated about an FIA perpendicular to the L_i (i.e. unique axis), and then about an FIA parallel to the L_i. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of S_i about an FIA nearly parallel to the L_i is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel.     

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The ‘asthenospherized’ part of the Ronda peridotitecontains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)_N vs (Sm/Yb)_N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)_N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the ‘layeredgranular’ subdomain and ‘plagioclase-tectonite’domain) are on average more fertile than the residual, ‘coarsegranular’ subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements     

内蒙古四子王旗大庙花岗岩体的成因与构造意义

内蒙古中部地区晚古生代花岗岩类侵入岩分布广泛,在空间上构成一条巨大的东西向花岗岩带,是研究花岗岩成因和地壳演化的天然实验室.四子王旗北大庙岩体作为一个典型代表,以花岗闪长岩为主,其内部普遍发育镁铁质微粒包体.对理解花岗岩石成因和演化有重要意义.时包体及其寄主岩进行岩相学研究及矿物化学分析,结果显示:包体具塑性外形及岩浆结构,存在多种不平衡矿物组合,是岩浆混合的重要证据;MME中斜长石斑晶边部与核部An值较低,幔部An值较高,黑云母斑晶MgO边部与核部较低,幔部较高,两种矿物相似的成分变化指示结晶环境的改变,从矿物学角度证实存在基性岩浆的反复注入.黑云母及角闪石化学成分上也显示出岩体形成于造山带背景下,岩浆具壳幔混合的特点.岩体形成温度为650℃～700℃左右,压力为1.5×105Pa左右,氧逸度lgfo2较高,约-10～-15.     

高温高压及干燥条件下斜长石和辉石断层泥的摩擦滑动研究

岩石摩擦滑动的力学行为能够很好地用速率和状态依赖性摩擦本构关系来描述。在速率和状态依赖性本构关系中,速度依赖性参数a-b是控制摩擦滑动稳定性的重要参数。且a-b<0是断层上不稳定滑动成核的必要条件。为了进一步研究控制摩擦滑动的矿物成分因素,在已有辉长岩摩擦滑动实验结果的基础上,对辉长岩中主要矿物(斜长石和辉石)在不同加载条件下进行了3个系列的摩擦滑动实验研究。通过对实验结果的分析得到如下结论:(1)斜长石断层泥在实验温度范围内(<617℃)显示速度强化,且在大于450℃的高温区其速度依赖性参数a-b随温度的增大而增加。辉石断层泥的实验显示其有利于速度弱化滑动行为的出现,是造成辉长岩在615℃出现速度弱化的原因。(2)斜长石与辉石断层泥的摩擦强度都没有随温度的增加而显著变化。辉长岩断层泥的摩擦强度(1.83mm滑动位移处的值)为斜长石与辉石断层泥摩擦强度按体积百分比加权的平均值(摩擦系数精确到2位有效数字一致)。这一结果表明,辉长岩断层泥的应力支承结构可以抽象为一种双组分并列单元模型。(3)尽管前人研究结果表明石英控制了花岗岩的摩擦特性,但是某种主要矿物对岩石整体摩擦滑动性质的控制作用并不具有普适性。对于中性岩、基性岩而言,不能以一种主要矿物(如斜长石等)来判断岩石整体的摩擦滑动稳定性,否则会导致错误结论。     

新疆且末阿尔金断裂南缘斜长片麻岩锆石U-Pb测年及其地质意义

在阿尔金地块南缘构造混杂岩带中,新发现了多个规模较大的斜长片麻岩块体。运用LA-ICP-MS锆石U-Pb定年分析方法,对阿尔金地块南缘且末大石门地区构造混杂岩带中的斜长片麻岩进行了年代学测定,并初步探讨其地质意义。30个锆石颗粒的28个有效测点,测出的年龄明显分为2个区间:①(1784±41)^(2150±34)Ma之间,加权平均值为(2130±27)Ma(MSWD=0.21,1σ);②(2324±32)^(2684±51)Ma之间,加权平均值为(2520±23)Ma(MSWD=0.06,1σ)。根据该次年龄测试结果和区域调查资料表明,阿尔金地块南缘且末大石门地区,在古元古代存在一期构造-热事件,该期事件与吕梁运动的时限相吻合。     

内蒙古月牙山蛇绿岩的岩石学、地球化学特征及其地质意义

内蒙古月牙山蛇绿岩分布于红柳河-洗肠井蛇绿岩带东部的月牙山一带,出露较完整,自下而上由超基性杂岩、辉长岩、层状玄武岩和枕状玄武岩及放射虫硅质岩组成。岩石学、岩石化学、地球化学资料显示,蛇绿岩套中的主要单元均源于幔源,接近于现代洋壳蛇绿岩特征。与蛇绿岩单元伴生的斜长花岗岩属于幔源型大洋斜长花岗岩,且为蛇绿岩组成的一部分;辉长岩、玄武岩均形成于正常洋中脊(N-MORB)环境。