Distinction between Permian and post-Permian igneous rocks in the southern Sydney Basin, New South Wales, on the basis of major-element geochemistry

Permian and post-Permian igneous rocks of the southern Sydney Basin of New South Wales can be distinguished on the basis of major-element geochemistry. A data base of 102 analyses of these igneous rocks has been used to evaluate the effectiveness of various discriminants based on univariate and multivariate analysis. Inspection of histograms of compositional frequency for each major-element oxide is an efficient method of univariate analysis and assigns up to 92.2% of the rocks to the correct age group. Discriminant function analysis using up to 10 variables does not produce a more efficient discriminant than the simple binary plot of SiO ₂ versus TiO ₂,which assigns 95.1% of the rocks to the correct age group. No other binary or ternary diagram commonly used in petrology produces such an efficient discriminator.     

The relationship between formation structure and thickness in the Permo-Triassic succession of the Southern Coalfield, Sydney Basin, New South Wales, Australia

Using trend-surface analysis, the patterns of formation structure and thickness variation in the Permo-Triassic succession of the Southern Coalfield, Sydney Basin, are resolved into geologically meaningful large- and small-scale features. The structural trends correspond to the regional syncline of the southern Sydney Basin and the negative and positive residual domains to smaller-scale structures including the South Bulli and Douglas Park Synclines and the Bulli and Kemira Anticlines. The results of trend-surface analysis and simple linear regression reveal an inverse relationship between structural elevation (relative to sea level) and formation thickness on both a regional and a local scale. This relationship is strongest for the Bulli Coal and the Bald Hill Claystone. In the case of the Bulli Coal, the present-day structural trends and formation thickness trends show a strong inverse correlation and the residuals show a moderate inverse correlation; the correlations between the inferred Middle Triassic structure of the Bulli Coal and formation thickness are even stronger than those involving the present-day structure. These results indicate that structures which were active during the Permo-Triassic and which influenced sedimentation have been preserved in the present-day structure. The persistence of the patterns of structure and thickness variation upward through the Permo-Triassic sequence suggests that sedimentation was controlled by contemporaneous basement subsidence, rather than by short-lived compaction-induced subsidence patterns.     

The Permian faunas of northern New South Wales and the connection between the Sydney and Bowen basins

Permian sediments are continuous between the Sydney and Bowen Basins west of the Hunter‐Mooki fault system and its probable northern continuation, the Goondiwindi Fault. Both fault systems appear to have influenced sedimentation in Early Permian time. A disconformity between Lower Permian coal measures (dated by plant microfossils) and Upper Permian sandstones and shales (dated by marine macrofossils) is present in the northern extension of the Sydney Basin. This hiatus may be correlated with a similar break in sedimentation in the southeastern part of the Bowen Basin. It is probably related to a Mid‐Permian diastrophism which folded Lower Permian and older sediments east of the Mooki and Peel Faults. Marine connection between the Sydney and Bowen Basins appears to have been interrupted during the event so that the two basins may have been temporarily isolated. The difference in the fossil faunas of the Sydney and Bowen Basins may well reflect this isolation.     

Trend-surface analysis of structure and thickness of Bulli Seam, Sydney Basin, New South Wales

Methods are presented which permit the analysis of structural and thickness variations of a stratigraphic interval into a number of components and testing of the components for relationship between structure and thickness. The methods are tested first in the axial region of the South Bulli Syncline area where the Bulli Seam is known to be thicker. Maps of the residual values from the structural and thickness first-degree trends are similar and there is a significant inverse relationship between the residuals. Data from the area where the Bulli Seam occurs also give seam structural and thickness residuals which have a significant inverse relationship. Local exceptions to this relationship are present but in most instances are due to laccolithic doming or changes in lithology of the Bulli Seam. A number of the trend components of structural and thickness variation also show an inverse relationship. It is postulated that most of the present structures defined at the Bulli Seam horizon were active during deposition of the Bulli Seam, and within the limit of the conditions suitable for peat accumulation, controlled thickness of the Bulli Seam. Thickness changes in overlying units suggest that a major part of the warping of the Bulli Seam horizon had occurred by the end of the Triassic Period.     

Geochemical evidence from Lower Permian volcanic rocks of northeast New South Wales for asthenospheric upwelling following slab breakoff

Within the Daly River basin, Northern Territory, three erosion surfaces are described and their relationships to deep weathering are discussed. The Bradshaw surface is the highest and oldest surface recognized. It is of considerable perfection and forms main divides; it is associated with a deep lateritic profile with a strongly silicified horizon forming the lower part of the pallid zone and extending into rocks immediately beneath. The Maranboy surface now forms secondary divides, with related rock‐cut terraces, below the level of the Bradshaw surface. In most areas it was produced by the stripping of the upper, less silicified parts of the Bradshaw pallid zone. The Maranboy surface is associated with a lateritic weathering profile less deep than the Bradshaw profile and mainly developed in the Bradshaw weathering mantle. A younger erosion surface, the Tipperary surface, advanced by the removal of the Maranboy re‐weathered layer exposing the resistant Bradshaw silicified rock which commonly forms a base‐level of denudation. The Tipperary surface consists of broad plains, gently undulating terrain, and dissected headwater valley floors. It is relatively unweathered and carries depositional mantles which are attributed to climatically induced slope instability.

The ages of the erosion surfaces and the possibility of climatic changes in the area are also briefly discussed.     

东澳大利亚南悉尼盆地二叠系与地震沉积有关的软沉积变形构造

悉尼盆地位于澳大利亚东部,Lachlan褶皱带和New England褶皱带之间。悉尼盆地从晚石炭世末到中三叠世经历了弧后扩张到前陆盆地的不同阶段:弧后扩张阶段(石炭纪)、被动热沉降阶段(早、中二叠世Berry组)和挤压挠曲负载阶段(中二叠世Broughton组—三叠纪)。此时位于悉尼盆地东侧的New England褶皱带为岛弧背景。因此,二叠纪处于弧后盆地的南悉尼盆地受弧后扩张和东侧弧前海沟俯冲的影响地震活动强烈,发育一系列与地震有关的震积岩,形成多种类型的与地震活动有关的软沉积物变形构造。南悉尼盆地二叠系的软沉积物变形包括地裂缝、震褶层、液化脉、沙火山、负荷构造、火焰构造、枕状构造、球状构造、枕状层、滑塌构造、角砾岩化等。其中地裂缝、震褶层是地震颤动直接引起的断裂和褶皱;枕状层是地震颤动引起的砂层脱水、下沉形成的;液化脉、沙火山为液化的砂层穿入地震形成的裂隙形成的;负荷构造、火焰构造、枕状构造、球状构造是受地震颤动在砂、泥岩界面上由于砂层下沉、泥层上穿形成的;滑塌构造和角砾岩化是地震引起的重力滑塌或泥石流形成的。地裂缝、震褶层、液化脉、沙火山、负荷构造、火焰构造、枕状构造、球状构造、枕状层相当于原地震积岩,而滑塌构造和角砾岩化属于异地震积岩。     

Sedimentary environments and trace fossils of the Permian Snapper Point Formation,Southern Sydney Basin

The Lower Permian Snapper Point Formation at its type locality in the southern Sydney Basin is interpreted as a regressive sequence of a linear clastic shoreline. Lithologies, sedimentary structures, and palaeocurrent patterns suggest a prograding barrier‐beach environment. Barrier foot, bar nucleus, bar crest, and back‐bar are distinguished. Abundant trace fossils aid the recognition of minor facies. The thickness of sediments deposited in the protected inshore environment may be explained by progradation into rising relative sealevel, but rates of sealevel rise or land subsidence were ultimately exceeded by the rate of sediment supply. Up‐sequence changes in the character of the sedimentation units and biofacies may therefore reflect an evolution from a barrier profile to an open mainland beach.     

Sedimentation in the Narrabeen Group,southern Sydney basin,New South Wales

A study of the Narrabeen Group in the southern part of the Sydney Basin has provided a more complete understanding of the conditions of deposition in the area during the Lower Triassic. The sedimentology of the sequence indicates that several different types of fluvial deposits are represented, with an upward succession from piedmont conditions with braided streams to a swampy deltaic plain. This sequence is interpreted as the onshore portion of a slow marine transgression, probably brought about by declining erosive activity in the hinterland.

The palaeocurrent pattern shows that drainage was principally to the south‐southwest, parallel to the basin axis, and the lateral transition from quartzose to lithic sediments across the basin can be interpreted as a blending of sediment from western and northern source areas. In addition several influxes of material from a source east of the present coastline are indicated, with reworked volcanic debris being introduced into the south coast district. This southeasterly source area did not greatly impede the drainage of the basin during lower Narrabeen deposition, but was responsible for a major diversion of stream patterns as the topmost beds were laid down. This diversion was completed by a northeasterly drainage pattern during deposition of the overlying Hawkesbury Sandstone.     

南黄海盆地二叠系至三叠系沉积体系特征及其沉积演化

南黄海是下扬子板块主体,经过多年的油气勘探,从浅层陆相至深层海相地层仍未取得油气突破,总体勘探和认识程度都较低。根据南黄海地震和钻井资料,充分结合下扬子陆区成果认识,将南黄海二叠系龙潭组、大隆组和三叠系青龙组划分为6大沉积体系：河流沉积体系、三角洲沉积体系、潮坪沉积体系、开阔台地沉积体系、局限台地沉积体系、陆棚沉积体系,并分析了各沉积体系特征。根据二叠系龙潭组、大隆组和三叠系青龙组地震反射特征,可将其划分为4大地震相类型,分析了各地震相的沉积意义。钻井沉积相和地震相相结合,分析了沉积体系的分布特征,龙潭组、大隆组和青龙组沉积体系具有水体西北深、东南浅的分布特征。龙潭组至青龙组沉积时期,经历了两次大规模海侵和两次大规模海退,形成了海平面的上升-下降的旋回式变化,岩性组合上构成了潮坪砂泥岩-潮坪煤层-潮坪砂泥岩-陆棚页岩-开阔台地碳酸盐岩-局限台地碳酸盐岩的沉积演化序列。     

Mineralogy of gouge in north-northeast-striking faults,Sydney region,New South Wales

Gouges formed in north-northeast-striking fault zones of the Sydney region and associated host-rocks were investigated by XRD, SEM, TEM and optical microscopy in order to determine their mineralogy. XRD studies reveal that illite, illite–smectite, kaolinite, quartz and dickite are present in varying proportions. Kübler Indices (0.54–0.71) and low smectite contents in illite–smectite (<10% smectite) in most gouges and host-rocks, indicate the assemblages formed at temperatures between 120 and 150°C. Those at the Heathcote Road, Lucas Heights location formed at lower temperatures (<100°C). SEM images of the clays in host sublitharenites and gouges show a variety of sizes and habits that reflect variations in fluid temperature and rate of crystallisation. SEM studies also reveal that detrital quartz grains exhibit overgrowths and etch pits of varying density, size and shape that are more strongly developed in the gouges than in the host-rocks. These features are thought to be related to higher fluid/rock ratios brought about by major ingress of fluids into the fault zones. The mineral assemblage present and the features exhibited are believed to have formed in response to a thermal event associated with the early stages of the breakup of Gondwana.     

Cyclic sedimentation in the Permian coal measures of New South Wales

Statistical analysis of borehole sections through the Illawarra and Newcastle Coal Measures of the Sydney Basin shows that cyclic sedimentation is present. The composite sequence for the Southern Coalfield (Illawarra Coal Measures) is (in ascending order): sandstone—sandstone/siltstone alternations—shale—coal, whereas that for the Newcastle Coalfield is: shale—sandstone/siltstone alternations—sandstone, often conglomeratic, or conglomerate—sandstone/siltstone alternations—shale —coal.

The environment of deposition is discussed. It is suggested that in the Southern Coalfield cyclicity is due to sedimentational processes inherent in the deltaic and alluvial conditions envisaged during Permian times. Periodic influxes of glacial meltwaters, although not essential, are not ruled out.

In the Newcastle Coalfield, however, the composite sequence does not match easily the ideal cycles expected in deltaic and/or alluvial regimes. Contemporary volcanism and tectonism complicated matters and lack of sedimentological details makes it impossible at present to give preference to any one mechanism of cycle formation.     

Triassic palaeosols in the upper narrabeen group of New South Wales

Detailed study of palaeosols in the sea cliffs between Long Reef and Palm Beach, north of Sydney, has given a much clearer idea of the environment and climate of the area during the late Scythian to early Anisian portion of Triassic time.

At the base of the exposed succession, kaolinitic clayey soils with red B horizons were formed on volcanogenic sediments derived from the old Gerringong volcanic ridge to the east. The red Bald Hill Claystone consists largely of soil material, both in place and resorted. These were grey‐brown podzolic soils (ferrods) formed on well‐drained sites under coniferous forest. Humic gleys (fibrists) within the overlying Garie Formation were probably lowland catenary equivalents formed under Dicroidium, Voltziopsis, and equisetalean swamp woodland.

These soils were drowned by the relative rise of the Narrabeen lagoonal complex and the concomitant southward growth of sediments of the Gosford Delta. These sediments formed the lower Newport Formation, of clean quartz‐lithic sandstone and grey kaolinite‐illite shale, derived from an extensive source land to the north and west. The most widespread soils in the delta were gleyed podzolic soils (aquods) under Dicroidium heath. Younger soils of a similar type (alluvial soils or aquents) also supported Dicroidium heath in low‐lying areas and Pleuromeia meadows in interdistributary bays. On slightly more elevated areas a more oxidized gleyed podzolic soil (ochrept) developed. On clayey levees, grey clay soils (fluvents) formed under a Dicroidium flora locally dominated by Taeniopteris lentriculiformis.

Few palaeosols are preserved in the upper Newport Formation and Hawkesbury Sandstone because of the more frequent channel reworking of the floodplain during their deposition.

The dominance of podzols is compatible with a cool temperate climate.     

Thermal properties and coal rank in rocks and coal seams of the southern Sydney Basin,new South Wales: A palaeogeothermal explanation of coalification

Measurements of thermal conductivity on 106 disc specimens of rocks from 275 m of the Permian to Triassic section of the Southern Coalfield of the southern Sydney Basin have been carried out in an effort to explain a high rank gradient in the Permian coals. The samples came from six diamond drill holes north and west of Wollongong, although one hole only provided specimens of a syenitic sill (n = 17, mean thermal conductivity = 2.36 W/m°K, s.d. 0.03). When combined with previously published data on chip specimens, with which there is good agreement, from a further four drill holes the mean thermal conductivity for the late Permian and early Triassic sandstones and shales is 3.20 W/m°K.Heat generation by Permian volcanic rocks below the coal measures (from about 1 to 3 μW/m³), and by basement granitic rocks, appears to be consistent with previously reported heat flow for the southern Sydney Basin (about 80 mW/m²). This heat flow is a relatively high value for the east coast of Australia. Younger (Mesozoic and Tertiary) intrusive and extrusive igneous rocks produce local coal-rank anomalies, but do not appear to have any regional effect.The rank of coals above the Permian volcanic rocks appears to be little affected by the presence of the igneous rocks and the coal-rank decreases towards the major area of vulcanicity. Organic matter in sedimentary rocks interbedded with the volcanic rocks is of relatively high rank but it appears that these thermal effects do not extend more than about 100 m above the base of the coal measures. The area of high rank north and northwest of Wollongong seems likely to be a regional effect associated with a combination of high heat generation in basement and the Permian rocks, and high heat flow from the basement. Greater cover on the coal measures, together with an increase in the proportion of rocks of relatively low thermal conductivity in the cover, may also influence the rank in the Permian coal.     

Generation and expulsion of oils from Permian coals of the Sydney Basin,Australia

Organic geochemical and petrological assessment of coals/coaly shales and fine grained sediments, coupled with organic geochemical analyses of oil samples, all from Permo–Triassic sections of the Southern Sydney Basin (Australia), have enabled identification of the source for the widely distributed oil shows and oil seeps in this region. The Permian coals have higher hydrogen indices, higher liptinite contents, and much higher total organic matter extract yields than the fine grained sediments. A variety of source specific parameters obtained from n-alkanes, regular isoprenoids, terpanes, steranes and diasteranes indicate that the oil shows and seeps were generated and expelled predominantly from higher plant derived organic matter deposited in oxic environments. The source and maturity related biomarkers and aromatic hydrocarbon distributions of the oils are similar to those of the coals. The oil-coal relationship also is demonstrated by similarities in the carbon isotopic composition of the total oils, coal extracts, and their individual n-alkanes. Extracts from the Permo–Triassic fine grained sediments, on the other hand, have organic geochemical signatures indicative of mixed terrestrial and prokaryotic organic matter deposited in suboxic environments, which are significantly different from both the oils and coal extracts. The molecular signatures indicating the presence of prokaryotic organic matter in some of the coal extracts and oils may be due to thin sections of possibly calcareous lithologies interbedded within the coal measures. The genetic relationship between the oils and coals provides new evidence for the generation and expulsion of oils from the Permian coals and raises the possibility for commercial oil accumulations in the Permian and Early Triassic sandstones, potentially in the deeper offshore part of the Sydney Basin.     

Subduction-related Late Permian shoshonites of the Sydney Basin, Australia

Summary The Late Permian shoshonitic province of the southern Sydney Basin consists of lavas and intrusions confined to a 140 km long coastal belt, but geophysical and sedimentological data indicate that the province once had a length of at least 340 km. Both petrographic and geochemical data indicate shoshonitic affinities for these Late Permian rocks and, although the compositional range is from 48.4 to 60.6% SiO₂, all units except an andesite lava have <55% SiO₂. Subduction-related attributes of the province include enrichment in Al₂O₃ and LILE (Rb, K and LREE), depletion in HFSE (Nb, Ta, Zr, Hf and Ti), low Nb/U, and Sr/Nd of 30–35. Initial (at 250 Ma)⁸⁷Sr/⁸⁶Sr ranges from 0.70294 to 0.70440 whereas _Nd values range from +5.11 to +2.14 and plot almost exclusively in the mantle array. Low MgO and mg-numbers (maximum 7.37% and 64.5, respectively) demonstrate that none of the shoshonites represent primary magmas in equilibrium with mantle peridotite. Isotopic data and elemental contents are not supportive of a model in which crustal contamination is the dominant process in magma petrogenesis, but do not exclude contamination with Sr-rich material having a low⁸⁷Sr/⁸⁶Sr and high _Nd values.A temporal correlation between igneous rocks from the southern Sydney Basin, Dampier Ridge, New England and Tasmania indicates a widespread magmatic event which was, at least in part, shoshonitic in character and which developed in response to subduction along the east coast of Gondwanaland. The geochemical similarity between the Dampier Ridge and Sydney Basin samples does not support a model for spatial compositional variation across a subduction-related magmatic belt but, coupled with the variations in isotopic ratios, implies heterogeneity in the source.

---

Spät-permische Schoschonite des Sidney Beckens in Australien

Zusammenfassung Die spät-permische Schoschonit Provinz des südlichen Sidney Beckens besteht aus Laven und Intrusionen, die in einem 140 km langen Gürtel längs der Küste auftreten. Geophysikalische und sedimentologische Daten weisen darauf hin, daß diese Provinz ursprünglich eine Länge von mindestens 340 km hatte. Sowohl petrographische wie geochemische Daten weisen auf schoschonitische Affinität dieser spät-permischen Gesteine hin. Obwohl die SiO₂ Gehalte von 48.4 bis 60.6% schwanken, haben alle Einheiten mit Ausnahme einer andesitischen Lava < 55%. Auf Subduktion weisen u.a. Anreicherungen an Al₂O₃ und LILE (Rb, K and LREE), die Verarmung an HFSE (Nb, Ta, Zr, Hf and Ti) niedrige Nb/U und Sr/Nd von ungefähr 30–35 hin. Die⁸⁷Sr/⁸⁶Sr Initiale, berechnet für 250 Ma, liegen zwischen 0.70294 und 0.70440 und die _Nd-Werte zwischen +5,11 und +2,14 und fallen fast ausschließlich in den Mantel-Bereich. Niedrige MgO und mg-Zahlen (Maximal 7,37% bzw. 64,5) zeigen, daß die Schoschonite nicht primäre Magmen, die mit Mantelperidotiten im Gleichgewicht sind, darstellen. Isotopendaten und Elementgehalte weisen nicht auf ein Modell hin, bei dem Krusten-Kontamination bei der Magmen-Genese eine wichtige Rolle spielte, aber eine Nichtkontamination mit Material, das niedrige⁸⁷Sr/⁸⁶Sr Werte und hohe _Nd Werte hat, ist nicht auszuschließen. Eine zeitliche Beziehung zwischen den magmatischen Gesteinen des südlichen Sydney-Beckens, dem Dampier Ridge, Neuengland und Tasmanien weist auf ein weit verbreitetetes magmatisches Ereignis hin, das zumindest teilweise schoschonitischen Charakter hatte und das auf Subduktion längs der Ostküste von Gondwanaland zurückführbar ist. Die geochemische Ähnlichkeit von Proben des Dampier Ridge und des Sydney Beckens weist nicht auf räumliche Variationen in der Zusammensetzung über einen subduktionsbezogenen magmatischen Gürtel hin, sondern läßt eher, zusammen mit den Variationen der Isotopendaten, heterogene Quellen für diese Gesteine vermuten.

---

---

With 10 Figures     

鄂尔多斯盆地南缘上二叠统——中下三叠统地球化学特征及其古气候、古环境指示意义

为探索陆相湖泊环境记录中二叠纪—三叠纪之交古气候演化的信息,以鄂尔多斯盆地南缘陕西铜川石川河剖面上二叠统石千峰组(P3s)和三叠系的刘家沟组(T1l)、和尚沟组(T1h)、纸坊组(T2z)为研究对象,对界面上下地层开展了系统的矿物学、岩石学和地球化学研究。通过主量和微量元素、碳氧同位素以及TOC测试对古盐度、古氧化还原环境和古气候演化规律进行分析。测试结果显示在二叠系—三叠系(P/T)界线附近主、微量元素和碳、氧同位素发生较明显的变化,气候环境代用化学指标的波动指示了从晚二叠世至早中三叠世鄂尔多斯盆地的古气候、古环境的变化。研究结果表明上二叠统石千峰组形成于河流-三角洲沉积环境,氧化程度相对低,古气候相对温暖湿润;下三叠统刘家沟组与和尚沟组的紫红色砂泥岩代表水体较浅的河流-三角洲相,强氧化环境,气候干旱炎热;而到中三叠统纸坊组沉积期,湖平面上升,元素的迁移作用加强,氧化程度变弱,气温降低,气候转为半干旱半湿润气候。碳同位素分析结果显示,鄂尔多斯盆地陆相P/T界线上δ^13CPDB存在显著负偏,与全球范围内的海相P/T界线具有一致性,说明在华北地台陆相P/T界线上同样存在气候突变和生物灭绝等重大地质事件的沉积记录,与海相地层可对比。     

豫西宜洛盆地上二叠统-下三叠统泥质岩地球化学特征与物源分析

宜洛盆地晚二叠世-三叠纪沉积充填演化是秦岭造山带与华北克拉通共同作用的结果,物源分析是揭示盆山耦合的重要手段之一。论文采用沉积地球化学分析的方法对宜洛盆地上二叠统-下三叠统泥质岩地球化学特征与物源进行了研究,结果表明:宜洛盆地泥质岩稀土元素球粒陨石标准化曲线显示轻稀土元素富集,重稀土亏损,为右倾式曲线,元素Eu中度负异常,元素Ce轻微亏损,符合上地壳稀土元素分布特征。泥质岩源区源岩属性判别图解显示,宜洛盆地物源以长英质岩为主,主要来自于上地壳,孙家沟组后期有少量古老基底杂岩混入。构造背景判别图解显示,研究区物源经历了被动大陆边缘为主(孙家沟组下段)-活动大陆边缘为主(孙家沟组上段和刘家沟组)-大陆岛弧为主(和尚沟组)的演化;华北克拉通和秦岭造山带是宜洛盆地重要物源区,华北地台北部内蒙古隆起可能提供了一定量的物源。孙家沟组早期以克拉通内部物源为主,孙家沟组土门段之后秦岭微地块物源供给明显,体现了华北克拉通南缘基底隆升,由被动大陆边缘向秦岭初始"弧-陆"碰撞隆升的构造演化过程。这对深入揭示南华北盆地与周围造山带之间的耦合关系具有重要意义。     

Myrmekite associated with alkali feldspar megacrysts in felsic rocks from New South Wales

Comparison of myrmekites in undeformed and deformed ‘porphyritic’ felsic rocks shows that they have different morphologies and spatial distributions within their megacrysts. In particular, plagioclase inclusions in the undeformed megacrysts carry myrmekite rims which may be explained by solid-state exsolution, as may the quartz blebs in associated perthitic plagioclase. Myrmekite at the rims of the megacrysts may be due to an interaction between exsolution and metasomatic processes. Stress concentrated at the margins of pre-existing megacrysts in the deformed rocks and the occurrence here of random non-coherent plagioclase nuclei will explain the preponderance of myrmekite at this location.