Plutonic Rock Types Research Articles

Classification of plutonic rock types using thin section images with deep transfer learning

Classification of rocks is one of the basic parts of geological research and is a difficult task due to the heterogeneous properties of rocks. This process is time consuming and requires sufficiently knowledgeable and experienced specialists in the field of petrography. This paper has a novelty in classifying plutonic rock types for the first time using thin section images; and proposes an approach that uses the deep learning method for automatic classification of 12 types of plutonic rocks. Convolutional neural network based DenseNet121, which is one of the deep learning architectures, is used to extract the features from thin section images of rocks; and the classification process is carried out with a single layer fully connected neural network. The deep learning model is trained and tested on 2400 images. AUC, accuracy, precision, recall and f1-score are used as performance measure. The proposed approach classifies plutonic rock images on the test set with an average accuracy of 97.52% and a maximum of 98.19%. Thus, the applied deep transfer learning is promising in geosciences and can be used to identify rock types quickly and accurately.

Petrogenesis of Late Cretaceous volcanic-plutonic complex from Xiaoxiong caldera in East Zhejiang

破火山内出露的火山岩与浅成侵入岩为硅质岩浆演化研究提供了一个重要窗口，从而备受关注。小雄破火山内的火山-侵入杂岩是中国东南沿海晚白垩世岩浆活动的典型代表，包括小雄组火山岩（K₂x）与两类侵入岩（花岗斑岩、正长斑岩）。本文以小雄火山-侵入杂岩为研究对象，开展了系统的锆石U-Pb年代学、岩石学和地球化学研究，旨在深入探讨破火山内火山岩与侵入岩之间的成因联系和岩浆演化过程。系统的LA-ICP-MS 锆石U-Pb年代学研究表明，小雄组火山岩形成于98~88Ma，并具有多期次喷发的特点，可分为下段、中段和上段，年龄分别为98~96Ma（K₂x¹）、95~92Ma（K₂x²）、~ 88Ma（K₂x³）。小雄花岗斑岩形成年龄为90Ma；正长斑岩形成稍晚，约88Ma。与下段流纹质玻屑凝灰岩的Nd-Hf同位素组成[ε_Nd（t）=-8.3~-7.2， ε_Hf（t）=-11.8~-7.2]相比，中段流纹岩要更为亏损[ε_Nd（t）=-5.84~-5.32， ε_Hf（t）=-10.1~-0.5]。研究表明，小雄组流纹质火山岩的母岩浆可能起源于发生在深部岩浆房中渐进的壳幔相互作用，中段流纹岩的源区混入了更多的亏损幔源组分。中段流纹岩与花岗斑岩具有相似的Nd-Hf同位素组成，以及 互补的微量元素地球化学特征，由发生在浅部岩浆房的分离结晶作用和堆晶作用所制约。值得注意的是，正长斑岩与花岗斑岩并不存在直接的成因演化关系，两者应是不同的起源。不同的正长斑岩岩株具有高度一致的结晶年龄、微量元素特征以及Nd-Hf同位素组成，以上特征均表明小雄破火山内的正长斑岩具有相同的起源。正长斑岩母岩浆起源于富集岩石圈地幔的部分熔融，岩浆源区混入了来自亏损的软流圈地幔组分，其地球化学成分变化主要受普通辉石+磷灰石+钛铁矿的分离结晶所控制。;The association of volcanic and shallow plutonic rocks in calderas may provide important clues to the geochemical evolution of silicic magma systems, and thus it has received a lot of attention. The Xiaoxiong caldera, mainly composed of volcanic rocks of the Xiaoxiong Formation (K₂x) and two types of plutonic rocks (granite-porphyry and syenite-porphyry), is a typical product of Late Cretaceous magmatism in SE China. In this paper, we conducted petrological and geochemical studies, as well as zircon dating and trace element analysis for the coexisting volcanic and plutonic rocks from the Xiaoxiong caldera, aiming to constrain the magmatic origin and evolution processes and evaluate the petrogenetic relationship between the volcanism and plutonism. LA-ICP-MS zircon U-Pb dating results revealed that the Xiaoxiong Formation was formed in 98~88Ma with characteristics of multi-staged eruptions. The ages of the Ⅰ-, Ⅱ-, Ⅲ-cycle volcanic rocks are constrained at 98~96Ma (K₂x¹), 95~92Ma (K₂x²) and ~88Ma (K₂x³), respectively. Meanwhile, zircon U-Pb dating provides crystallization ages of ca. 90Ma for granite-porphyry and ca. 88Ma for syenite-porphyry. However, in terms of Nd-Hf isotopic compositions, the rhyolites from the middle section [ε_Nd(t)=-8.3~-7.2, ε_Hf(t)=-11.8~-7.2] record increasingly depleted characteristics to the rhyolitic vitric tuffs from the lower section [ε_Nd(t)=-5.84~-5.32, ε_Hf(t)=-10.1~-0.5], implying a progressive crust-mantle interaction, and more juvenile, Nd-Hf isotopically depleted materials were involved into the source of these rhyolites. The rhyolites from the middle section and the granite-porphyry show cogenetic and complementary geochemical signatures. Therefore, we suggest that they could be closely linked by fractional crystallization and crystal accumulation in a shallow magma chamber, i.e., these rhyolites are formed by the extraction of interstitial melt from crystal mush, while the granite-porphyry represents the residual crystal mush. It is worth to note that there is no directly evolutional relationship between granite-porphyry and syenite-porphyry, and they are more likely to have different magmatic origins. In addition, the syenite-porphyries of different stocks in the Xiaoxiong caldera have a highly consistent crystallization ages, trace element characteristics and Nd-Hf isotope compositions, which indicates that they have the same magmatic origins. The syenite-porphyry was generated probably by pyroxene+apatite+ilmenite -dominated fractional crystallization from basaltic magma that was produced by magma mixing between melts derived from depleted asthenosphere and subduction-related enriched lithospheric mantle.

Geoquímica de las rocas plutónicas del batolito cambro-ordovícico del Cordón de Lila y de la Sierra de Almeida, Región de Antofagasta, Chile

An Upper Cambrian-Ordovician (490-460 Ma) batholith crop out along the Cordón de Lila and Sierra de Almeida, in the Antofagasta region, northern Chile, which belongs to the Famatinian arc. Two plutonic rock types can be distinguished: granodiorites and monzogranites. The first lithology is positioned along an occidental fringe in as much the second is located along an oriental part of the batholith. The main geochemical difference between the two lithologies is the enrichment in silica and K of the second with respect of the first one. Most of the granodiorites are metaluminous or are located between the metaluminous and peraluminous fields. The monzogranites are mostly peraluminous. The chemical composition of the plutons (mayor and trace elements and rare earth elements) allow to infer a common origin for both groups, from a mafic magma that evolved by fractional crystallization. The difference in composition between both lithologies, specially the meta or peraluminous character, can be explained by means a different degree of interaction of the original magma with the felsic crustal rocks.

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.

Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.

Effect of compositional variations on log responses of igneous and metamorphic rocks. II: acid and intermediate rocks

Abstract An extensive data-set of petrophysical down-hole measurements exists for boreholes drilled into continental crystalline crust. We selected boreholes covering a range of different types of plutonic rocks and gneisses in amphibolite or high-grade metamorphic rocks. According to Serra’s concept of electrofacies, a specific set of log responses should characterize one rock type. Here, we concentrate on the detection of compositional variations between rock types. Bulk composition of the protoliths influences the mineralogical composition of the metamorphic rock, and we demonstrate how this impacts on the down-hole measurements. Integration of logging data with geochemical core data and mineralogical descriptions allows the calibration of the log responses to rock types. The relationship of the log responses with core data shows a remarkably good correlation, and diagnostic trends are detected. From the logs, potassium and neutron porosity are particularly helpful in distinguishing different types of gneisses and igneous rocks with respect to their protoliths. The proportions of amphibole/pyroxene, mica + K-feldspar and feldspar + quartz in the rocks seem to control the direction of correlation in a cross-plot, i.e. positive or negative, depending on increasing or decreasing mineral proportions. This is true for all boreholes, and a generalized classification scheme could be developed for these crystalline rocks.

Silica enrichment, graphic granite and aquamarine growth: a new exploration guide

Granitic pegmatites are traditionally known to contain graphic, perthitic and myrmekitic intergrowths related to quartz and K- and Na- feldspars. They are further considered to characterise the pegmatite types distinguishing them from the granites and other related plutonic rock types. Graphic granite is accepted also as a synonym to granitic pegmatite. Systematic studies, by the author and colleagues, on the granitic pegmatite gem deposits have permitted the definition of two aquamarine gem provinces in ENE Brazil, one in the NeoProterozoic and the other in the Archaean sequences. Potash feldspars in the pegmatites in the former show perthitic intergrowths, whereas in the latter graphic intergrowth dominates with anomalously coarse centimetric quartz along the cleavages of K-feldspar. Several granitic pegmatites hosted in Archaean complex, in Lages Pintadas Aquamarine Province, Santa Cruz, RN State, present this texture-structure. Graphic intergrowth is attributed to the eutectic crystallization, succeeded by hydrothermal fluids with silica enrichment permitting the growth through diffusion and nucleation of quartz and along cleavages of potash feldspar. In the Archaean terrain, the abundance of recycled chert forming metapsammitic migmatites traversed by numerous quartz veins and coarse graphic granites, has contributed to the growth of beryl and also the aquamarines.

Raman spectroscopic characterization of a Martian SNC meteorite: Zagami

To demonstrate the ability of Raman spectroscopy to determine the mineralogical character of a rock that originated on Mars, we analyzed a small slab of “normal Zagami” by point analyses and multipoint scans using laboratory spectrometers. Spectra of clinopyroxenes were dominant; their compositions were estimated from a calibration of Raman peak positions with Mg/(Mg+Fe) based on lunar pyroxenes of known composition, and these agree with compositions obtained by electron microprobe. A few spectra of orthopyroxene were observed. The broad spectrum of maskelynite was observed, but not that of plagioclase feldspar. Spectra of minor phosphates, magnetite, and pyrrhotite were obtained, as were spectra of an organic contaminant and of hematite, both apparently introduced during sample handling prior to Raman analysis. The modal analysis based on the multipoint scans agrees well with published values. If the spectra had been obtained on the surface of Mars by Raman spectroscopic analysis as a stand‐alone method and no other information about the sample was available (and by ignoring the spurious hematite and organic material), we could rule out sedimentary and plutonic rock types and conclude that the sample was a pyroxene‐phyric basalt.

Petrogénesis del plutón monzogranítico peralumínico de Santa Eufemia (Batolito de los Pedroches, Córdoba)

The Santa Eufemia pluton is part of the composite magmatic assoclatlon of Los Pedroches batholith, located in the southern branch of the Central Iberian Zone of the Hercynian orogenic belt of Spain. This high-Ievel postkynematic batholith consists of three main types of plutonic rocks: boitite±amphibole granodiorites, biotite-cordierite porphiritic monzogranites and cordierite leucogranites. The Santa Eufemia pluton consists of biotite-cordierite monzogranites, and in a lesser extent of cordierite leucogranites. Scarce enclaves of different type are found in the monzogranites, biotiteplagioclase- eordierite and fine grained porphiritic monzogranite enclaves being the most common. The petrographic, structural and geochemical (major and trace elements) data suggests a compiex evolutionary history of a metasedimentary-source monzogranitic magma with no genetical relationships with more basic magmas such as the one which is now represented by the granodiorite facies of the batholith. Non-minimum melt and reequilibrated source material (secondary restites) mixing during the ortomagmatic evolution of the magma accounts for the compositional heterogenities of the monzogranites. The ultimate product of this evolution is the segregation of a fluid-saturated residual liquid in the final level of intrusion of the magma, leading to the emplacement of leucogranite masses and dikes and the exsolution of a supercritical fluid phase related with the late magmatic recrystallization of sorne mineral phases (muscovite, albite, quartz, tourmaline, topaz), the geochemical removilization of sorne incompatible trace elements (Rb, Cs, Li, Be, Sn, Nb) due to fluid-rock interactions and the formation of periplutonic wolframite-arsenopyritequartz mineralizations.

Assimilation of peridotite in zoned calc-alkaline plutonic complexes: evidence from the Big Jim complex, Washington Cascades

The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist. Most of the major plutonic rock types (from websterite through hornblendite, gabbronorite, hornblende gabbro and diorite, to granodiorite) enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the Chiwaukum schist. The larger xenoliths preserve tectonite fabrics. All have Mg#'s (mole fraction MgO/(MgO+FeO*)) from 0.90 to 0.89, the same as those of Chiwaukum metaperidotites, and distinctly different from undeformed Big Jim dunite (Mg#'s 0.84 to 0.82) and websterite (0.82 to 0.78). Contact relations indicate widespread, stepwise replacement of harzburgite by pyroxenite, hornblendite, gabbro and diorite. Thermodynamic modelling using an expanded regular solution model for silicate liquids (Ghiorso 1985; Ghiorso and Carmichael 1985) predicts that reaction between olivine (Fo90) and a liquid with the composition of Big Jim diorite +1.5 wt% H2O, at 1,100° C and 3 kb, would produce websterite (Mg#'s 0.75 to 0.81) and dunite (0.79 to 0.82). This process is exothermic and results in a negative change in volume, since it increases total solid mass. Under conditions of decreasing temperature, modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the Big Jim complex where crystal fractionation alone fails. The Big Jim complex has affinities with other ultramafic to felsic plutonic complexes such as the Bear Mountain complex (Snoke et al. 1981, 1982) and the Emigrant Gap complex (James 1971). The latter have wehrlite and clinopyroxenite, rather than websterite, but both have concentric zoning, with olivine-bearing rock types surrounded by successively more felsic pyroxenite, gabbro and diorite. In general, concentrically zoned complexes of this type may form where magma reacts with mantle-derived wall rock or ultramafic cumulates. Assimilation of peridotite in fractionating magma may be important in subduction-related magmatic arcs.

Plutonic rock types series: discrimination of various granitoid series and related rocks

The Streckeisen QAP diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions. Three of them integrate varied compositions from basic to acid terms: (1) the calcalkaline series with its low K (or trondhiemite), medium K (or granodioritic), high K (or monzonitic) variants; (2) the alkaline series with its characteristic syenitic members; (3) the tholeiitic series, deprived of K feldspars. The fourth (4) corresponds to mobilizates associated with migmatites of varied composition but always quartz rich. The generalization in the QAPF system, allowing the plotting of undersaturated alkaline series, provides an excellent visual display of the major associations and trends for the plutonic rocks.

The geology and tectonic setting of the Velasco Alkaline Province, eastern Bolivia

The Velasco Alkaline Province of Mesozoic age is situated near to the Bolivian-Brazilian border, and is enclosed in Precambrian gneisses of the Brazilian Shield. It consists of an early volcanic suite, and a series of 14 interfering circular, ring and elliptical plutons, and is associated with a silicified carbonatite complex. The combined length of the province is nearly 80 km and it outcrops over an area of 440 km 2 . The plutonic rock types range from foyaite to aegirine granite but each individual pluton may be classified as either over- or under-saturated. The province was built up by a series of plutonic events which for the most part were aligned along a NE-trending lineament. Normal faulting was active during the development of the province. Although the Velasco Alkaline Province lies over 1500 km from the eastern continental margin it is deduced that it defines a plume-generated triple point initiated at the same time as the break-up of the South American-Africa plate.

Spectral reflectance and discrimination of plutonic rocks in the 0.45‐ to 2.45‐μm region

Visible and near‐infrared field spectral reflectance measurements of plutonic rocks were acquired in the 0.45‐ to 2.45‐μm region with a portable field reflectance spectrometer. These spectra were used to determine spectral signatures for the various rock types and to evaluate the separability of these rocks based on their spectral characteristics. A total of 135 samples were divided into 11 groups based on their mineralogy. These 11 groups approximately correspond to traditional rock classifications and include five granitic groups, three gabbroic groups, and three ultramafic groups. The positions, intensity, and presence of iron, CO3−2, and Al‐OH and Mg‐OH absorption bands varied among the 11 groups. Each rock group also had a range of albedos that is characteristic of the group. Stepwise linear discriminant analysis was performed on the spectral data to determine the separability of the 11 groups. Classification accuracy for 30 equally spaced wavelength bands between 0.45 and 2.45 μm was 78% with 10% serious misclassifications. The same analysis was repeated, limiting the spectral data to the wavelength regions corresponding to the proposed Landsat D thematic mapper scanner. Classification accuracy was 71% with 13% serious misclassifications. To assess the effects of weathering on spectral separability of plutonic rocks, a set of 47 spectra of weathered plutonic rocks was analyzed by using the classification functions developed in the preceding analyses. Classification accuracy of these spectra could not be as critically assessed because precise mineralogy was not known; however, serious classification errors totaled 17% with the 30‐channel data and 30% with the simulated Landsat D data. These results indicate that plutonic rock types can be successfully discriminated from one another with good results on the basis of their spectral characteristics in the 0.45‐ to 2.45‐μm region.

Rb-Sr isochron ages from east Milne Land, Scoresby Sund, East Greenland

A prominent NNE-SSW trending fault in central Scoresby Sund, separates the eastern part of Milne Land geologicaIly from the rest of the island. This east Milne Land fauIt block, comprises metasediments bounded by a variety of post-kinematic intrusions in which three major plutonic rock types are found. Field investigations indicate the oldest plutonic rock to be a granodiorite, followed by a coarse-grained mafie, quartz syenite, and the youngest bodies to be of granitie composition. In the southern part a large area is covered by Mesozoic sediments whieh eross the fault-line without displacement. Locally, outliers of Tertiary plateau basalts rest on the Mesozoie rocks (fig. 33). The geology has been outlined by Henriksen & Higgins (1971, 1973) and preliminary age determinations were published by Hansen et al. (1972) and Hansen & Steiger (1976). This more extensive study deals with Rb-Sr whole-roek and mineral analyses on samples from two post-kinematic intrusions and from the metasediments in the northern area.

Archean metallogeny in southern Africa

The Archean granite-greenstone terrain of southern Africa possesses a number of unique characteristics. Not only is the shield area one of the smallest in the world but it is also one of the oldest crustal fragments still preserved. Estimates of lithic proportions show that, in the southern African Archean, the granitic rocks outnumber those of the volcanic belts (greenstone belts) by approximately 6:1--in marked contrast to the situation on the Canadian Shield where granite: greenstone ratios have been estimated to be approximately 1:2.In order to establish further details of the lithological proportions, the southern African Archean greenstone belts are described in terms of a three-stage stratigraphic model involving a Lower Ultramafic Unit at the base, followed by a Mafic-to-Felsic Unit and a terminal Sedimentary Unit. Planimeter studies demonstrate that on average the southern African greenstone belts possess a relatively high proportion of ultramafic and mafic volcanic and plutonic rock types and a correspondingly lower proportion of rocks of intermediate to felsic composition. The relative abundances of the various volcanic components of the lower two stratigraphic units are tabulated, and the lithic proportions are compared with similar data available from greenstone belts of the Superior Province in Canada. Geochemical data illustrating the nature of Archean volcanism in southern Africa and elsewhere are made available for purposes of comparison.The distribution patterns of a wide variety of Archean mineralization types, including the deposits associated with the ancient granites and pegmatites, are illustrated by means of mineral distribution maps. A close genetic relationship exists between the various lithological subdivisions of the Archean greenstone belts and the mineralization. Important deposits of chrome, nickel-copper, gold, and chrysotile asbestos are associated with the ultramafic-mafic component of the volcanic pile, whereas antimony, gold, and lesser amounts of mercury, barite, corundum, copper-lead-zinc, and massive sulfide iron-formations occur throughout the greenstone piles and commonly occur as chemical sediments terminating cycles of volcanism. These well-banded rocks, which probably formed by fumarolic-exhalative processes, also contain significant stratiform gold mineralization.The Archean mineralization of southern Africa is briefly compared and contrasted with that of the Canadian Shield and elsewhere. It is concluded that the disparity of lithological types and proportions between the Canadian and southern African shields is, together with differences in age, the underlying cause of the variability in the mineralization thus far encountered in the two regions.

Mineralogy and petrology in the New Zealand Geological Survey 1865–1965

Abstract The development of mineralogy and petrology in the New Zealand Geological Survey falls into four phases: (1) 1865–92: a period of reconnaissance geology and petrology without the aid of the petrological microscope except for observations of Cox in 1880, (2) 1892–1905: commissioning of Sollas to study rocks from Coromandel Peninsula was the only activity, (3) 1905–38: petrology used almost entirely, and generally to a minor extent, as aid to 1-mile mapping, (4) 1938–65: petrology and mineralogy undertaken by a separate Petrology Section, with independent investigations as well as aiding mapping; building up of central rock and mineral collections, and steady growth of staff and equipment. Minerals first described from New Zealand are taranakite (1866), awaruite (1886), tuhualite (1932), hydrogrossular (1943), huttonite (1950), wairakite (1955), and wairauite (1964). Four meteorites are known—Wairarapa Valley (1863), Makarewa (1879), Mokoia (1908), and Morven (1925); only the Mokoia meteorite, a rare carbonaceous type, was seen to fall. Data on New Zealand mineralogy are summarised in six tables which show the development of quantitative techniques—chemical analysis (1866), microscopical examination (1880), refractive index (1935), mineragraphy (1940), X-ray examination (1945 examination overseas; 1955 in New Zealand), electron microscope (1955), and electron microprobe (1964, examinations overseas). Undoubtedly the development of precise methods in mineralogy over the last 10–15 years is one of the most dramatic features of the century and marks the change from a qualitative to a quantitative science. Three rocks first described from New Zealand have since been recognised throughout the world—dunite (1864), rodingite (1911), and ignimbrite (1932). Data on New Zealand volcanic, plutonic, sedimentary, metamorphic rocks, selected mineral deposits and specialised economic applications, and other matters are summarised in 18 tables. Features of volcanic petrology are: the recognition of “andesite” or “Marshall” line (1912), the postulation of Cainozoic volcanic provinces (1941, 1960), the recognition of ignimbrite sheets in the Central North Island (1932), and of the importance of plagioclase-hypersthene mineral assemblage in rhyolitic rocks of the North Island. Data on the sequence of volcanic eruptions are still meagre, but attention is drawn to the importance of detailed petrological and mineralogical comparison of the Alexandra and Egmont volcanism as an aid in predicting the future behaviour of Mt Egmont. New Zealand ultramafic rocks are of five types: the great Ultramafic Belt of the South Island, diapiric serpentinite, gabbro-peridotite complexes, metamorphic serpentinite, and dunite mylonite. Many ultramafic rocks display a low-temperature contact alteration, especially Ca-metaspmatism resulting in the formation of rodingite but recently (1965) a high-temperature contact has been recognised in the Red Hills Other types of plutonic rocks are less known, a result of inaccessibility in mountainous terrains and the difficulties of rock correlation.

Petrology and Age of Plutonic Rocks, Extreme Southeastern Maine

A detailed field and petrographic investigation was made of part of the plutonic belt in extreme southeastern Maine to determine the age and origin of the plutonic rocks. Work was concentrated in the Calais area, Maine, because of the diversity of plutonic rock types and the presence of sediments which could be dated by contained fossils. The area is underlain by metasedimentary, volcanic, and sedimentary rocks, and several plutonic rock types which range in age from Cambro-Ordovician to Late Devonian. The oldest rocks, the Cambro-Ordovician Charlotte Group, consist largely of isoclinally folded quartzites, phyllites, slates, and schists. The Silurian succession consists of two major groups: (1) a thick sequence of inter-layered shale and rhyolite extrusives of Middle Silurian age and (2) rhyolite and basalt extrusives of Late Silurian age. The youngest stratified unit, the Late Devonian Perry Formation, consists of a thick sequence of coarse clastic sediments and interbedded basalt flows. The plutonic rocks of the area are chiefly norites, gabbros, diorites, and granites. Structural and petrographic relationships suggest that the bodies of mafic rocks are remnants of a large stratiform body emplaced in sediments of the Charlotte Group prior to or during regional deformation of this group and that the present isolation of the masses is a result of transverse faulting and partial assimilation and replacement by younger plutonic bodies. All the diorites and granites were emplaced in the post-Silurian-pre-Upper Devonian interval, as they exhibit intrusive contacts toward rock of Late Silurian age but have contributed detritus to the Perry Formation. Petrographic characteristics indicate that all the granites and diorites are related and are probably of about the same age, although granites invariably show intrusive relationships toward diorites. Contact features establish that the Red Beach Granite is slightly younger than the other granites and diorites; mineralogical and textural features indicate that it formed under near-surface conditions. The presence of plutonic rocks in a region characterized by low-grade metamorphism suggests a magmatic derivation. Contact and petrographic characteristics confirm a magmatic origin, although, locally, granitization operated as a subordinate process. Distribution of the plutonic rocks in a linear belt suggests that emplacement occurred along a zone of thrust faulting traversed by numerous cross faults.

AVERAGE CHEMICAL COMPOSITIONS OF SOME IGNEOUS ROCKS

Average chemical compositions are given for the common plutonic rock types and their volcanic equivalents. An attempt has been made also to give the general average chemical compositions of silicic, intermediate, subsilicic and ultramafic igneous rocks