A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

角闪斜长片麻岩流变力学特性研究

为了解小湾水电站枢纽区角闪斜长片麻岩的流变力学特性,采用岩石全自动三轴流变伺服仪对角闪斜长片麻岩进行三轴流变力学试验。试验结果表明,强度较高的角闪斜长片麻岩会发生流变现象,尤其在高应力水平条件下,其流变特性明显;外荷载超过岩样的长期强度时,随着时间的逐渐增加试样变形经历典型流变3个阶段,并最终加速流变破裂。依据岩样稳态流变阶段的流变速率与应力水平的关系,提出了一种确定长期强度的方法。在Burgers流变模型中加入统计损伤,并用Mohr-Coulomb准则确定的岩石长期强度作为开始进入非线性加速流变阶段的阀值,得到相应的非线性损伤流变模型。利用岩石全程三轴流变试验结果,采用优化后的算法,对非线性损伤流变模型相应的参数进行了辨识,结果表明,建立的岩石统计损伤非线性流变变模型与流变试验结果吻合较为理想,可以较为准确地反映角闪斜长片麻岩的流变力学特征     

变形角闪质岩石流动变形及其意义——以辽东古元古宙褶皱带变形斜长角闪岩为例

通过显微和亚微分析揭示出辽东地区3种变形特点的角闪质岩石（片麻状斜长角闪岩、条带状斜长角闪岩和糜棱状斜长角闪岩）具有差异的显微及亚微构造。通过地质温压计计算,3种斜长角闪岩及其原岩的形成与变形温压条件具有从升温升压经降温升压到降温降压的变化规律（片麻状斜长角闪岩：597.0 ℃,0.536 GPa;条带状斜长角闪岩：617.3 ℃,0.455 GPa;糜棱状斜长角闪岩：558.8 ℃,0.514 GPa）。从其趋势和规律,结合显微构造分析,3种岩石是同一期变形不同阶段的产物。在递进变形作用过程中,塑性变形是斜长角闪岩流动和蠕变的主要机制。结合前人的实验变形结果,修正了大陆岩石圈结构模型。     

40Ar/39Ar traverse — Grenville Front Tectonic Zone to Britt Domain,Grenville Province,Ontario, Canada*

Abstract ⁴⁰Ar/³⁹Ar data (on hornblende, muscovite and K-feldspar) are presented for samples from the western Grenville Province taken along a 140-km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area. In regions where comparisons are possible, muscovite and (large domain) K-feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c.2°C Myr^-1, is appropriate for exhumation controlled by post-orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25-Myr offset in both hornblende and muscovite/K-feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen. The argon data from the GFTZ are interpreted in the context of an asymmetric crustal-scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short-lived thermal event associated with the development of the crustal-scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly.     

Intercalibration of the Hb3gr Ar/Ar dating standard

The ⁴⁰Ar/³⁹Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the ⁴⁰Ar^?/³⁹Ar_K (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is R_FCs^Hb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. ⁴⁰Ar-³⁹Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for ⁴⁰Ar/³⁹Ar geochronology.     

福建同安角闪辉长岩的矿物化学、40Ar-39Ar年龄及地质意义

福建同安角闪辉长岩的岩石化学成分具有低MgO高铝玄武岩的特征,其中的角闪石是富钙钙镁闪石,斜长石是An=91～95的钙长石.在基性的弧岩浆中,低MgO高铝玄武岩浆富水是导致结晶出高钙斜长石的关键因素.钙镁闪石的40Ar-39Ar年龄为129.62±0.15 Ma,说明同安角闪辉长岩是东南沿海早白垩世"初始引张阶段"岩浆活动的产物.     

钙碱性火成岩的角闪石全铝压力计 ——回顾、评价和应用实例

定量地获得岩体结晶时的压力,对于探讨其剥蚀深度及了解造山带的构造热演化史等方面有重要意义。对于钙碱性侵入岩而言,可以应用角闪石全铝压力计对钙碱性火成岩结晶压力进行估计。对于不同角闪石压力计公式的选择,前提是满足它们各自的适用条件,否则所得结果没有意义。角闪石全铝压力计不能用于估算A型花岗岩的结晶压力,更不能用于计算变质岩的变质压力。对于结晶温度低于750℃的钙碱性中酸性岩体而言,Anderson和Smith(1995)的温度校正意义不大。Ridolfi等(2010)提出的指数关系角闪石压力计公式适用于钙碱性火山岩中角闪石斑晶的结晶压力估算。Uchida等(2007)的黑云母全铝压力计缺乏理论和实验基础,不建议使用。应用角闪石全铝压力计所得到薛家石梁—黑山寨复式岩体侵位深度的区域变化,指示了云蒙山变质核杂岩下盘的掀斜;低角度正断层(拆离断层)可以通过高角度正断层的掀斜旋转而形成。     

黑龙江省伊春市牛奶沟钼矿地质特征及找矿标志

通过对伊春市牛奶沟钼矿地质特征及其矿体产出规律的分析,总结了侵入岩、构造对成矿的控制作用,为本矿区及周边地区进一步找矿提供了依据。     

广东佛冈花岗杂岩体中乌石闪长岩-角闪辉长岩的结晶条件

佛冈花岗杂岩体是南岭地区最大的晚中生代岩基,乌石闪长岩角闪辉长岩是佛冈钙碱性系列花岗岩中最基性的端员。乌石闪长岩角闪辉长岩和周围佛冈黑云母花岗岩、含角闪石花岗闪长岩同属于高钾钙碱性系列岩石。本文系统地总结并对比了多种角闪石压力计和角闪石斜长石温度计,针对中酸性钙碱性岩浆,提出了改进的角闪石Al_Ti温压曲线格子,并将其运用到乌石闪长岩角闪辉长岩的结晶条件估算中。估算结果显示,乌石闪长岩角闪辉长岩中角闪石结晶温度为82 0±2 0℃,压力为70～130MPa,XH2 O为0 .7±0 .1,其结晶条件代表岩体侵位时的物理环境。花岗闪长岩中的角闪石结晶温度约为82 0℃,压力约为2 6 0MPa ,是在较深的岩浆房中结晶的。     

Ar/Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event

The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and ⁴⁰Ar/³⁹Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe³⁺ content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe³⁺/(Fe³⁺ + Fe²⁺) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss.

⁴⁰Ar/³⁹Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina.