Garnet-orthopyroxene-plagioclase-quartz barometry: refinement and application to the English River subprovince and the Minnesota River valley

Recent experimental, theoretical, and thermodynamic studies permit better calibration of two reactions for geobarometry: grossular+pyrope+quartz=anorthite+enstatite grossular+almandine+quartz=anorthite+ferrosilite If both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet-pyroxene-plagioclase-quartz assemblage. Application to a variety of high-grade terrains generally yielded excellent results. However, poor results have been obtained for high-Fe rocks which can be traced to errors in activity models for garnet and/or pyroxene. Either a two-site ideal mixing model for orthopyroxene (cf. Wood and Banno 1973) underestimates enstatite activity for high-Fe orthopyroxenes or the Ganguly and Saxena (1984) model overestimates pyrope activity in low-Mg garnets. Application of both barometers to a variety of high-grade terrains gives the following average pressures:

Regional-scale hydrothermal alteration in the Central Blake River Group,western Abitibi subprovince,Canada: implications for VMS prospectivity

The Late Archean Blake River Group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the Abitibi greenstone belt. It contains a number of silicic volcanic centers of different size, including the large Noranda volcanic complex, which is host to 17 past-producing volcanogenic massive sulfide deposits. The Noranda complex consists of a 7- to 9-km-thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism. Massive sulfide formation coincided with a period of intense magmatic activity (cycle III) and the formation of the Noranda cauldron. Hydrothermal alteration in these rocks is interpreted to reflect large-scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex. The alteration includes abundant albite, chlorite, epidote and quartz (silicification), which exhibit broad stratigraphic and structural control and correlate with previously mapped whole-rock oxygen isotope zonation. The Mine Sequence volcanic rocks are characterized by abundant iron-rich chlorite (Fe/Fe+Mg >0.5), hydrothermal amphibole (ferroactinolite) and coarse-grained epidote of clinozoisite composition (<10 wt% Fe ₂O ₃). Volcanic rocks of the pre-cauldron sequences, which contain only subeconomic stringer mineralization, are characterized by less abundant chlorite and mainly fine-grained epidote (>10 wt% Fe ₂O ₃) lacking the clinozoisite solid solution. Alteration in the Mine Sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits (as far as several tens of kilometers) and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale. The lack of similar alteration in the pre-cauldron sequences is consistent with limited ¹⁸O-depletion and suggests that the early history of the volcanic complex did not support large-scale, high-temperature fluid flow in these rocks. Comparisons with a much smaller, barren volcanic complex in nearby Ben Nevis township reveal important differences in the alteration mineralogy between volcanoes of different size, with implications for area selection during regional-scale mineral exploration. The Ben Nevis Complex consists of a 3- to 4-km-thick succession of mafic, intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion. Alteration of the volcanic rocks comprises mainly low-temperature assemblages of prehnite, pumpellyite, magnesium-rich chlorite (Fe/Fe+Mg <0.5), iron-rich epidote (>10 wt% Fe ₂O ₃) and calcite. Actinolite ± magnetite alteration occurs proximal to the intrusive core of the complex, but the limited extent of this alteration indicates only local high-temperature fluid circulation adjacent to the intrusion. A distal zone of carbonate alteration is located 4–6 km from the center of the volcano. Although iron-bearing carbonates are present locally within this zone, the absence of siderite argues against a high-temperature origin for this alteration. These observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit.     

金红石和榍石Zr温度计在新疆西南天山榴辉岩中的应用

应用LA-ICP-MS方法对新疆西南天山高压-超高压变质带中的榴辉岩及其高压脉体中的金红石和榍石进行了Zr含量的检测和Zr温度计的计算。榴辉岩中位于石榴石幔部且与绿辉石共生的金红石包体Zr含量都集中于10~20μg/g;而基质金红石的Zr含量为30~50μg/g,高于包体金红石。榍石均为金红石退变质的产物,且各样品间的榍石Zr含量较均一,都集中在3~5μg/g之间。脉体金红石Zr含量则与榴辉岩中基质金红石的Zr含量相当甚至偏高一些,为30~60μg/g。金红石和榍石的Zr温度计研究也表明,榴辉岩石榴石中的金红石包体生长于压力峰期阶段,温压条件为480~540℃、2.7~3.0 GPa;基质金红石随温度增加达到退变质再平衡,记录了温度峰期的条件,约530~590℃、2.4~2.7 GPa;榴辉岩中高压脉体中的金红石则生长于退变质榴辉岩相阶段,金红石Zr温度计给出结果为540~580℃、1.5~2.1 GPa,记录了近等温降压的过程;榴辉岩中的榍石在1.0 GPa左右达到平衡,榍石Zr温度计给出的温度为540~560℃,记录了进一步的近等温降压的过程。根据以上4个阶段的分析结果,得出一个较完整的顺时针p-T轨迹,且与相平衡模拟所限定的p-T轨迹相一致。金红石的Zr含量可以作为压力的指示,表明压力校正在金红石Zr温度计中起到了重要作用。在对金红石和榍石Zr温度计进行应用时,要结合细致的岩相学观察,综合考虑压力、活度、扩散速率、退变质作用和流体影响等方面的因素,才能得到比较精确的温压估算结果和pT轨迹。     

The significance of garnet and cordierite from the Sioux Lookout region of the English River gneiss belt,Northern Ontario

Garnet and cordierite bearing Archean gneisses are a common rock-type in the English River gneiss belt of the Superior Province, northern Ontario. Bulk compositions of such gneisses are found to be depleted in potassium with respect to the garnet and/or cordierite-free biotite gneisses. The mineral assemblages of the garnet and cordierite gneisses indicate that they have equilibrated at higher metamorphic grades than the garnet and/or cordierite-free biotite gneisses. These, observations suggest that anatexis has occurred in the garnetiferous regions of the gneiss belt resulting in a granite melt which had migrated from the source rocks, thereby depleting the garnetiferous restite in potassium.     

Isotope hydrology of the Chalk River Laboratories site,Ontario, Canada

This paper presents results of hydrochemical and isotopic analyses of groundwater (fracture water) and porewater, and physical property and water content measurements of bedrock core at the Chalk River Laboratories (CRL) site in Ontario. Density and water contents were determined and water-loss porosity values were calculated for core samples. Average and standard deviations of density and water-loss porosity of 50 core samples from four boreholes are 2.73 ± 12 g/cc and 1.32 ± 1.24 percent. Respective median values are 2.68 and 0.83 indicating a positive skewness in the distributions. Groundwater samples from four deep boreholes were analyzed for strontium (⁸⁷Sr/⁸⁶Sr) and uranium (²³⁴U/²³⁸U) isotope ratios. Oxygen and hydrogen isotope analyses and selected solute concentrations determined by CRL are included for comparison. Groundwater from borehole CRG-1 in a zone between approximately +60 and −240 m elevation is relatively depleted in δ¹⁸O and δ²H perhaps reflecting a slug of water recharged during colder climatic conditions. Porewater was extracted from core samples by centrifugation and analyzed for major dissolved ions and for strontium and uranium isotopes. On average, the extracted water contains 15 times larger concentration of solutes than the groundwater. ²³⁴U/²³⁸U and correlation of ⁸⁷Sr/⁸⁶Sr with Rb/Sr values indicate that the porewater may be substantially older than the groundwater. Results of this study show that the Precambrian gneisses at Chalk River are similar in physical properties and hydrochemical aspects to crystalline rocks being considered for the construction of nuclear waste repositories in other regions.     

Rural community well-being: models and application to changes in the tobacco-belt in Ontario, Canada

In this paper we develop a model of changes in rural community well-being. The model conceptualizes four interrelated dimensions of rural community well-being: physical, psychological, social, and economic. The model recognizes that a range of external forces (political, economic, etc.) exert pressures on rural communities, of which changes in well-being are one outcome. The paper then applies the model to changes which occurred in the tobacco growing region of southern Ontario, Canada. It is argued that this region was impacted by a variety of forces, some general to farming, others specific to tobacco farming, particularly between 1979 and the early 1990s. The empirical application is based on a personally administered farm survey (n=63) conducted between July 1996 and January 1997 and agricultural census data for the years 1981, 1986, 1991, and 1996. The paper concludes by suggesting that the model provides a useful framework for analysing the forces and changing conditions of other rural communities, not only in Canada, but beyond.     

Hydrogeological conditions of the Pre-Yenisei petroleum subprovince

The study presents results of a hydrogeological and hydrogeological research conducted on the Mesozoic–Cenozoic sedimentary cover and Precambrian–Paleozoic platform deposits of the Pre-Yenisei petroleum subprovince. The hydrogeological structure of the study area is found to be of a transition type from the West Siberian to Tunguska artesian basins, with its own set of pertinent parameters, such as groundwater depths, rock permeabilities, water chemistry and gas compositions, gas saturation, vertical zoning, etc. The upper part of the sedimentary section is known to be flushed with the infiltration waters to depths of 2–2.5 km. The deeper intervals contain the older sedimentary waters with the moderate metamorphic grade. The results of the study provide implications for the petroleum potential of the area of interest.     

Late-Neoproterozoic ferroan granitoids of the Transversal subprovince,Borborema Province,NE Brazil: petrogenesis and geodynamic implications

ABSTRACT

Ferroan granites (585–530 Ma) have been described in the Transversal subprovince of the Borborema Province (BP) and in Pan-African counterparts. They comprise two groups: Group 1 – slightly peraluminous to metaluminous, alkali-calcic rocks, with low Fe# mica and crystallized under intermediate fO₂ (Aroeiras Complex and Serra Branca – Coxixola dike swarms); Group 2 metaluminous to slightly peraluminous, alkalic to alkali-calcic rocks, with high Fe# mica and crystallized under low fO₂ (Queimadas and Prata intrusions). Group 1 marks transition from collision to transcurrence (ca. 585 Ma), or from transcurrence to uplift and transtension (ca. 545 Ma). Group 2 – represents granitoids intruded during extensional tectonics in transcurrent setting (ca. 550 Ma), or coeval with deposition of transtensional intracratonic basins (ca. 530 Ma). Hf and Nd model ages are older than 2.0 Ga, suggesting that the ferroan granitoids involved partial melting of Paleoproterozoic rocks. The data presented in this paper show that the ferroan magmatism was widespread in the BP and its counterparts in Africa in pre-drift reconstructions.     

The isotope geochemistry of fracture calcites from the Chalk River area,Ontario, Canada

Calcite is a common fracture inflilling mineral in the Grenville gneisses of the Chalk River area, Ontario, Canada. It exhibits a variety of occurrences and textures which suggests calcite has precipitated under different hydrogeochemical conditions that may be identified through a detailed chemical and isotopic investigation of the calcite and associated infilling minerals.The δ¹⁸O of these calcites range over 20%. but the δ¹³C varies over a narrow range of 5%.. None of the calcites analyzed is in isotopic equilibrium with both the δ¹⁸O and δ¹³C of the present day ground water. The lightest δ¹⁸O calcites (near 0%. SMOW) are present in sealed fractures and are sometimes associated with laumontite. This suggests that these light calcites formed from hydrothermal solutions (at temperatures less than about 300°C) shortly after the period of metamorphism that formed the gneisses. This interpretation is supported by relatively unradiogenic⁸⁷Sr/⁸⁶Sr ratios near 0.709 and δ¹³C values of −5 to −6%..Most of the Chalk River calcites, however, are considerably heavier in¹⁸O and lighter in¹³C than the hydrothermal end member. This may be the result of low temperature recrystallization of the hydrothermal calcites by meteoric waters under variable water/rock ratios. The¹³C contents and⁸⁷Sr/⁸⁶Sr ratios of these younger, low temperature calcites appear to be partially buffered by the isotopic composition of the original hydrothermal calcite.Pyrite is often associated with the fracture calcites. These pyrites display a wide range in δ³⁴S values of about 70%., which suggests that sulphide precipitation occurred under semi-closed conditions. These data indicate that fracture permeability has been a major control on the isotopic composition of fracture minerals since formation of the gneiss.     

Application of selected geothermometers to exploration of low-enthalpy thermal water: the Sudetic Geothermal Region in Poland

The most important intakes of thermal waters within the Sudetic Geothermal Region occur in three separate hydrogeothermal systems: (1) Lądek, (2) Duszniki and (3) Cieplice. All these waters are of meteoric origin and circulate in crystalline rocks to different depths. Their outflow temperatures are between less than 20°C and to about 87°C. To evaluate the geothermal fields in the light of their prospectiveness, to further exploration of thermal energy resources, we took an effort to apply selected isotopic and chemical geothermometers to assess the maximum possible temperatures, which may be found in the reservoirs. The only chemical geothermometers which give a reliable range of reservoir temperatures are SiO₂ (chalcedony), Na–Ka–Ca and partly Na–K ones. The oxygen isotopic geothermometer in the SO₄–H₂O system gives a real range of estimated reservoir temperatures only for deeply circulating waters in the Cieplice area. On the other hand, in the case of CO₂ rich waters in the Duszniki area, where outflow temperatures do not exceed 30°C, application of chemical or isotopic temperature indicators always leads to erroneous results due to the lack of equilibrium in the thermodynamic system of water–rock interaction.     

Evaluation of CHIRPS and its application for drought monitoring over the Haihe River Basin,China

To establish the drought index objectively and reasonably and evaluate the hydrological drought accurately, firstly, the optimal distribution was selected from nine distributions (normal, lognormal, exponential, gamma, general extreme value, inverse Gaussian, logistic, log-logistic and Weibull), then the Optimal Standardized Streamflow Index (OSSI) was calculated based on the optimal distribution, and last, the spatiotemporal evolution of hydrological drought based on the OSSI series was investigated through the monthly streamflow data of seven hydrological stations during the period 1961–2011 in Luanhe River basin, China. Results suggest: (1) the general extreme value and log-logistic distributions performed prominently in fitting the monthly streamflow of Luanhe River basin. (2) The main periods of hydrological drought in Luanhe River basin were 148–169, 75–80, 42–45, 14–19 and 8–9 months. (3) The hydrological drought had an aggravating trend over the past 51 year and with the increase in timescale, the aggravating trend was more serious. (4) The lower the drought grade was, the broader the coverage area. As for the Luanhe River basin, the whole basin suffered the mild and more serious drought, while the severe and more serious drought only cover some areas. (5) With the increase in time step, the frequency distribution of mild droughts across the basin tended to be concentrated, the frequency of extreme droughts in middle and upper reaches tended to increase and the frequency in downstream tends to decrease. This research can provide powerful references for water resources planning and management and drought mitigation.     

Turbidites and iron formations, Beardmore-Geraldton, Ontario: application of a combined ramp/fan model to Archaean clastic and chemical sedimentation

Sandy turbidites, grain flows, conglomeratic mass-flows and oxide-facies iron formation are present in the late Archaean Beardmore-Geraldton terrain, a metasedimentary belt which extends for at least 80 km in an E-W direction. The marine portion of this basin contains four lithofacies associations (LA): (1) Thinbedded, iron formation-clastic sediment association. This association represents a continuum of deposit types containing iron formation; subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand/silt. (2) Thin-bedded, turbidite-dominated association. These sediments consist mostly of silt/sand beds which either show no vertical trends, or thin and fine upwards over a few metres. (3) Medium-bedded, turbidite-dominated association. Most of these sediments are medium to coarsegrained, vertically unstructured sand sequences with occasional structured intervals. (4) Thick-bedded association. This is dominated by poorly graded sands up to 7–8 m in thickness. Sand beds are characterised by a thin basal zone of coarse sand and pebbles, a large central interval containing a mixture of medium and coarse sand, and a thin upper zone of fine sand/silt. The overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars. We infer a nearshore break in slope, locally with large channels (LA 4) extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans (structured portions of LA 3). However, many deltaic surfaces probably were not tapped by major channels, but merged downslope into a submarine ramp. Sediment was transported across the ramp by slump events and sheet-like grain flows (unstructured portions of LA 3). Iron formation and LA 2 sediments probably accumulated both in upper-mid ramp areas with low sediment delivery rates, and distal to fan-ramp successions. As major streams on the braid plain changed position, associated submarine channels and slump-fed ramp deposits also would have shifted laterally. This produced overlap of different facies associations in both the fan and ramp environments, which may explain why observed vertical trends in bedding are limited to several metres. We suggest that on narrow, active Archaean cratonic margins, additional non-regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity, and seismic activity associated with the arc, also contributed to the lack of vertically structured sequences. Turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation.     

A lithofacies description of Jurassic sediments in the south of the Predyenisei petroleum subprovince,West Siberia

We have studied Jurassic sections in the Predyenisei subprovince of the West Siberian petroleum basin, which were penetrated in the Vostok-1, Vostok-3, and Vostok-4 stratigraphic wells. The Urman, Togur, Ilan, Peshkovo, Tyumen’, Naunak, and Mar’yanovka Formations are described from a detailed comprehensive core analysis and log data. The depositional environment for these sediments was predominantly continental. There is evidence for short transgressions in the Ilan (Lower Toarcian) and Peshkovo (Upper Toarcian) Formations, as well as the Upper Urman (Upper Pliensbachian) and the Upper Tyumen’ (Bajocian) Subformations. In the Upper Naunak Subformation (Oxfordian), there was a change of facies from continental to littoral continental and littoral marine. The Mar’yanovka Formation developed in normal marine shallow- or moderately deep-water environments. Although good reservoirs are common throughout the Jurassic section in the southeast of West Siberia, only small, lithologically screened deposits are predicted here.     

Late quaternary stratigraphy and geochemistry of the R47 section,Rouge River Basin,south-central Ontario,Canada: Correlation with Scarborough Bluffs

Quaternary glacial diamictons in section R47, overlying Ordovician shale in the Rouge River Basin of south-central Ontario, are considered to be representative of older and younger Wisconsinan glacial diamictons in the field area. Field characteristics, clast composition, mineralogy, particle size and geochemistry permit delineation of two units within the sequence and subdivision of deposits that correlate with Sunnybrook and Halton diamictons of Wisconsinan age. No Wisconsinan interstadial deposits are present in the section; however, outwash sandy gravel capping the Halton diamicton was probably emplaced by a high-energy stream draining into Lake Iroquois just prior to the incision of the Rouge Valley in the later Wisconsinan (ca. 11 000-12 000 yr BP). A buried palaeosol, which developed in this sandy gravel and in overlying aeolian sediment, shows partial leaching of carbonates and a slight increase of clay in the palaeosol solum. The main clay-mineral transformations in the buried palaeosol appear to involve the degradation of illite and illite-smectite accompanied by the production of vermiculite and minor amounts of chlorite. The ground soil capping the buried palaeosol is formed in colluvium emplaced 200 ± 80 yr BP following a local forest fire; the ground soil Ahk horizon gives an age of 4000 yr in the future, probably as a result of nuclear bomb testing effects on modern radiocarbon. The presence of an A/C profile in the ground soil system, although thin, indicates that surface soils may form rapidly. The geochemistry of the two Wisconsinan diamictons shows similar calcite and SiO₂-corrected mean element concentrations, with slightly elevated levels of Ca, Sr, Hf and Lu in the younger deposit. Both diamictons are geochemically quite different from the shale bedrock in the area, indicating that the bulk of transported sediment came from outside the area.     

Floodplain formation processes and archaeological implications at the Grand Banks site,lower Grand River,southern, Ontario

Processes of floodplain development and the record of Princess Point cultural occupation (A.D. 500–1000) were examined at the Grand Banks site in the lower Grand River of southern Ontario. The Princess Point Complex of the early Late Woodland is significant because it represents the first shift to horticulture in this region in which inhabitants made significant use of floodplains. The floodplain of the lower Grand River has been constructed primarily via vertical accretion of sediment in a low energy environment conducive to limited erosion and slow burial of middle and late Holocene sediments. At this site, cultural materials are preferentially preserved in two buried soils each corresponding to relatively stable periods of valley infilling at or before 3200 B.P. and 1500 B.P. (¹⁴C years). Initial formation of the floodplain and subsequent stability of the floodplain surface can be tied to middle Holocene, and later, base-level fluctuations in Lake Erie. Understanding floodplain development is crucial in determining the linkages between settlement pattern and chronology, and, conversely, the archaeological record in floodplain settings provides important contemporary data for modeling floodplain geomorphological processes. © 1997 John Wiley & Sons, Inc.     

Oxygen isotope fractionations involving pyroxenes: The calibration of mineral-pair geothermometers

Oxygen isotope fractionations between wollastonite, diopside, jadeite, hedenbergite and water have been experimentally studied at high pressures (1<- PH₂O ≥ 24 kbar) and temperatures (400/dg ≤ T <- 800/dgC) using the three-isotope method (Matsuhisa et al., 1978). Initial ${}^{18}\text{O}{}^{16}\text{O}$ fractionations were made close to equilibrium and initial ${}^{17}\text{O}{}^{16}\text{O}$ ratios were well removed from equilibrium, allowing accurate determinations of the equilibrium ${}^{18}\text{O}{}^{16}\text{O}$ fractionations and of the extent of isotopic exchange. Scanning electron microscope and rate studies show that the wollastonite-water and diopside-water exchange reactions occur largely by solution-precipitation (Ostwald Ripening) mechanisms. Equilibrium ${}^{18}\text{O}{}^{16}\text{O}$ fractionations between water and the minerals wollastonite, diopside, and hedenbergite are in close agreement with one another, whereas significantly more positive fractionations are found for jadeite-water. These isotopic substitution effects can be ascribed to replacement of SiOM bonds (M is a divalent metal cation in octahedral coordination) by higher frequency SiOAl bonds. The fractionations determined in this study can be combined with quartz- and feldspar-water data of Matsuhisa et al. (1979) and revised magnetite-water data of O'NEIL (1963), to provide a coherent set of mineral-pair fractionations satisfactorily represented by straight lines through the origin on a conventional graph of In /ga versus T^?2. Mineral-water data, on the other hand, cannot readily be fitted to the simple relationship suggested by Bottinga and Javoy (1973). Coefficients “A” for the mineral-pair fractionations 1000 ln α = A × 10⁶T^?2 are:

     

19.

大别造山带黄镇榴辉岩矿物不同类型地质温度计应用和对比   总被引：4，自引：4，他引：4  

高天山  郑永飞陈仁旭 《岩石学报》2006,22(7):1957-1968

常用于测定榴辉岩形成温度的有石榴石-绿辉石Fe-Mg配分温度计和石英-矿物对氧同位素温度计。最近的自然观察和实验测定发现，金红石中的Zr含量与温度之间存在线性关系，因此能够用于变质岩测温。本文首次将这三种温度计用于同一产地榴辉岩及其中的石英脉。对大别造山带黄镇低温超高压榴辉岩中金红石Zr含量的温度计算得到，产于矿物内部金红石Zr含量温度明显地高于粒间金红石Zr含量温度，产于矿物石榴石、绿辉石和黝帘石内部金红石Zr含量温度主要集中在528～589℃之间，而产于粒间金红石的温度主要集中在465～528℃之间。榴辉岩中金红石Zr含量最高的产于石榴石中，但是所计算的温度503～589℃仍然不同程度地低于榴辉岩形成温度670℃。石英脉中金红石Zr含量温度主要集中在465～528℃之间。石英-耐熔矿物对氧同位素温度主要集中在650～695℃之间，表明耐熔矿物石榴石、锆石和蓝晶石在该区榴辉岩中相对其它矿物来说保存最好，退变质作用最弱，因此其氧同位素温度与峰期超高压榴辉岩相变质奈件基本一致。而石英．易熔矿物对温度主要集中在450～510℃之间，与易熔矿物绿辉石、钠云母、斜黝帘石／黝帘石在榴辉岩中蚀变强烈一致，反映了角闪岩相退变质阶段的流体活动。石榴石-单斜辉石Fe-Mg配分温度结果分为三组：795～863℃、629～679℃和468～572℃，其中后两组与金红石Zr含量和石英-矿物对氧同位素测温结果具有可比较性，指示了榴辉岩相变质和角闪岩相退变质过程中的Fe-Mg交换平衡，而第一组温度明显高于已知的榴辉岩相变质温度，表明绿辉石后成合晶导致了部分石榴石与单斜辉石之间的Fe-Mg不平衡。榴辉岩折返过程中的流体活动可能是导致矿物之间元素和同位素扩散交换再平衡或不平衡的基本原因。粒内金红石Zr含量温度仍然不同程度地低于榴辉岩形成温度，可能说明其在进变质过程中形成后相对“孤立”，即使在峰期榴辉岩相条件下也不能与锆石之间达到Zr配分再平衡。粒间金红石Zr含量降低可能与金红石重结晶有关，结果导致它们与锆石之间的Zr配分平衡遭到破坏。     

20.

Sm---Nd age and REE geochemistry of Proterozoic arc-related igneous rocks in the Richtersveld subprovince, Namaqua Mobile Belt, Southern Africa     

D.L. Reid 《Journal of African Earth Sciences》1997,24(4)

Igneous rocks of the Richtersveld Subprovince in South Africa and Namibia form a Palaeo-Mesoproterozoic arc complex, dominated by extrusive andesites and their plutonic equivalents (diorites, tonalites), although the whole spectrum of rock types from ultramafic to felsic is present. Sm---Nd isotope data indicate an extrusion age of 2000 Ma, thereby confirming previously published Rb---Sr and U---Th---Pb ages. Initial Nd, Sr and Pb isotope ratios indicate a juvenile crust-producing episode at this time, with rapid recycling of young crust to generate rhyolitic and granitic components within the arc complex. The Richtersveld Subprovince represents the oldest crust in the western Namaqua Province, which was subsequently recycled during post 2.0 Ga tectonothermal events, especially the 1200–1000 Ma Kibaran episode.     

	Ab	Jd	An	Di	Wo	Mt
Q	0.50	1.09	1.59	2.08	2.20	6.11
Ab		0.59	1.09	1.58	1.70	5.61
Jd			0.50	0.99	1.11	5.02
An				0.49	0.61	4.52
Di					0.12	4.03
Wo						3.91

大别造山带黄镇榴辉岩矿物不同类型地质温度计应用和对比

常用于测定榴辉岩形成温度的有石榴石-绿辉石Fe-Mg配分温度计和石英-矿物对氧同位素温度计。最近的自然观察和实验测定发现，金红石中的Zr含量与温度之间存在线性关系，因此能够用于变质岩测温。本文首次将这三种温度计用于同一产地榴辉岩及其中的石英脉。对大别造山带黄镇低温超高压榴辉岩中金红石Zr含量的温度计算得到，产于矿物内部金红石Zr含量温度明显地高于粒间金红石Zr含量温度，产于矿物石榴石、绿辉石和黝帘石内部金红石Zr含量温度主要集中在528～589℃之间，而产于粒间金红石的温度主要集中在465～528℃之间。榴辉岩中金红石Zr含量最高的产于石榴石中，但是所计算的温度503～589℃仍然不同程度地低于榴辉岩形成温度670℃。石英脉中金红石Zr含量温度主要集中在465～528℃之间。石英-耐熔矿物对氧同位素温度主要集中在650～695℃之间，表明耐熔矿物石榴石、锆石和蓝晶石在该区榴辉岩中相对其它矿物来说保存最好，退变质作用最弱，因此其氧同位素温度与峰期超高压榴辉岩相变质奈件基本一致。而石英．易熔矿物对温度主要集中在450～510℃之间，与易熔矿物绿辉石、钠云母、斜黝帘石／黝帘石在榴辉岩中蚀变强烈一致，反映了角闪岩相退变质阶段的流体活动。石榴石-单斜辉石Fe-Mg配分温度结果分为三组：795～863℃、629～679℃和468～572℃，其中后两组与金红石Zr含量和石英-矿物对氧同位素测温结果具有可比较性，指示了榴辉岩相变质和角闪岩相退变质过程中的Fe-Mg交换平衡，而第一组温度明显高于已知的榴辉岩相变质温度，表明绿辉石后成合晶导致了部分石榴石与单斜辉石之间的Fe-Mg不平衡。榴辉岩折返过程中的流体活动可能是导致矿物之间元素和同位素扩散交换再平衡或不平衡的基本原因。粒内金红石Zr含量温度仍然不同程度地低于榴辉岩形成温度，可能说明其在进变质过程中形成后相对“孤立”，即使在峰期榴辉岩相条件下也不能与锆石之间达到Zr配分再平衡。粒间金红石Zr含量降低可能与金红石重结晶有关，结果导致它们与锆石之间的Zr配分平衡遭到破坏。     

Sm---Nd age and REE geochemistry of Proterozoic arc-related igneous rocks in the Richtersveld subprovince, Namaqua Mobile Belt, Southern Africa

Igneous rocks of the Richtersveld Subprovince in South Africa and Namibia form a Palaeo-Mesoproterozoic arc complex, dominated by extrusive andesites and their plutonic equivalents (diorites, tonalites), although the whole spectrum of rock types from ultramafic to felsic is present. Sm---Nd isotope data indicate an extrusion age of 2000 Ma, thereby confirming previously published Rb---Sr and U---Th---Pb ages. Initial Nd, Sr and Pb isotope ratios indicate a juvenile crust-producing episode at this time, with rapid recycling of young crust to generate rhyolitic and granitic components within the arc complex. The Richtersveld Subprovince represents the oldest crust in the western Namaqua Province, which was subsequently recycled during post 2.0 Ga tectonothermal events, especially the 1200–1000 Ma Kibaran episode.