Eric A.K. Middlemost’s research while affiliated with Tarbiat Modares University and other places

Pyroxenes provide an elegant key to understanding the tangled petrogenesis of the diverse midalkaline rocks of the Warrumbungle Volcano. These rocks evolved from two primary magmas, and the pyroxenes record the various stages in their origin and evolution. Trace-element abundance patterns for chromian diopside crystals from a spinel-lherzolite xenolith provide evidence of an early mantle-depletion event. Depleted spinel-lherzolite is an appropriate mantle source for the ITEP (incompatible-trace-element-poor) series of rocks. Subcalcic augite megacrysts are common in the basic rocks of this series. They crystallized in a high-pressure, intratelluric environment where they controlled the initial magmatic differentiation of the ITEP series. Aluminian augite occurs as megacrysts and also in gabbronorite xenoliths found in one of the more differentiated rocks of the ITEP series. This pyroxene crystallized in a lower-crustal magma chamber where its host magma was peraluminous and intermediate in composition. A few clinopyroxene (Cpx) megacrysts are Ferich and exceptionally enriched in rare-earth elements (REE). They crystallized from a moderately differentiated magma, locally contaminated by crustal material, or were later subjected to a metasomatic event. One of the nepheline-normative basic rocks of the ITER (incompatible-trace-element-rich) series contains small inclusions of K-rich omphacite. This phase contains one of the highest K contents (0.6-2.3 wt%) ever reported in a Cpx, and crystallized from a K-rich liquid at a greater depth than any of the other pyroxenes, probably deep within the upper mantle. Orthopyroxene (Opx) megacrysts are rare and belong to at least three geochemically distinct types: in association with chromian diopside, with subcalcic augite, and with aluminian augite megacrysts. Each type of Opx crystallized in a separate chemical and physical environment.

Pyroxenes provide an elegant key to understanding the tangled petrogenesis of the diverse midalkaline rocks of the Warrumbungle Volcano. These rocks evolved from two primary magmas, and the pyroxenes record the various stages in their origin and evolution. Trace-element abundance patterns for chromian diopside crystals from a spinel-lherzolite xenolith provide evidence of an early mantle-depletion event. Depleted spinel-lherzolite is an appropriate mantle source for the ITEP (incompatible-trace-element-poor) series of rocks. Subcalcic augite megacrysts are common in the basic rocks of this series. They crystallized in a high-pressure, intratelluric environment where they controlled the initial magmatic differentiation of the ITEP series. Aluminian augite occurs as megacrysts and also in gabbronorite xenoliths found in one of the more differentiated rocks of the ITEP series. This pyroxene crystallized in a lower-crustal magma chamber where its host magma was peraluminous and intermediate in composition. A few clinopyroxene (Cpx) megacrysts are Ferich and exceptionally enriched in rare-earth elements (REE). They crystallized from a moderately differentiated magma, locally contaminated by crustal material, or were later subjected to a metasomatic event. One of the nepheline-normative basic rocks of the ITER (incompatible-trace-elementrich) series contains small inclusions of K-rich omphacite. This phase contains one of the highest K contents (0.6–2.3 wt%) ever reported in a Cpx, and crystallized from a K-rich liquid at a greater depth than any of the other pyroxenes, probably deep within the upper mantle. Orthopyroxene (Opx) megacrysts are rare and belong to at least three geochemically distinct types: in association with chromian diopside, with subcalcic augite, and with aluminian augite megacrysts. Each type of Opx crystallized in a separate chemical and physical environment.

The main aim of igneous petrology is to develop a complete specification of the magma/igneous rock system. This paper is a sequel to an earlier essay (Middlemost, 1991) on the classification of igneous rocks and magmas. It explores the ways and means of developing a single, consistent method of naming all igneous materials. All modal classifications are fettered by problems arising from heteromorphism, extremes in grain size, and the presence of glass. Only chemical parameters can provide a reliable and straightforward method of classifying all the common igneous rocks and their magmas. This is undoubtedly true of glassy rocks and magmas. The potential of the TAS diagram in this new and enlarged role is evaluated, resulting in a modification of some boundaries recommended for the volcanic rocks.A new comprehensive chemical classification of the plutonic rocks is introduced. To keep it and the volcanic classification in tandem, several new terms are proposed. They include gabbroic diorite as a plutonic equivalent of basaltic andesite and peridotgabbro as a plutonic equivalent of picrobasalt. Peridotite is defined as the plutonic equivalent of picrite and by taking the idea of equivalence a step further, it is defined as a peridotgabbroic or gabbroic rock that contains more than 18% MgO and less than 2% total alkalis. The picrobasaltic and basaltic rocks that contained more than 18% MgO and more than 2% (Na2O + K2O) are called alkalic picrites. Their plutonic equivalents are named alkalic peridotites. A benefit of this new chemical classification of plutonic rocks is that it enables one to avoid the awkward term ultramafic. A single classification that links magmas, plutonic and volcanic rocks should be appreciated by all geochemists and petrologists who amass, and manipulate, large geochemical databases but are unwilling, or unable, to carry out quantitative modal analyses. This classification also enables geoscientists to focus on magma — the most important concept in igneous petrology.

The IUGS Subcommission on the Systematics of Igneous Rocks has recently published an excellent book on the classification of these rocks. This event has shifted the vexed question of classification towards the top of the agenda in igneous petrology. Over the years the Subcommission has used many different criteria to establish the positions of the boundaries between the various common igneous rocks. It now has to adopt a holistic approach and develop a comprehensive, coherent classification that is purged of all the minor anomalies that arise between the various classifications that it has approved. It is appreciated that the Subcommission's classification was never intended to have any genetic implications; however, it is suggested that an ideal classification should he presented in such a way that it is able to group rocks into an order that directs attention to petrogenetic relationships between individual rocks and larger groups of rocks. Unfortunately, many of the Subcommission's definitions are Earth chauvinistic; for example, igneous rocks are defined as being those rocks that solidified from a molten state either within or on the surface of the Earth. Nowhere in the book is it acknowledged that during the past 20 years, while the Subcommission has been framing its many recommendations, a whole new science of planetary petrology has subsumed classical petrology. In any new edition of the book, the Subcommission should acknowledge that rocks are essentially the solid materials of which planets, natural satellites and other broadly similar cosmic bodies are made.

The circa 2.06 Ga Mt Weld carbonatite complex of Western Australia intrudes an Archean greenstone sequence dominated by basic and ultrabasic metamorphosed igneous rocks. Carbonatites form the core of the complex and are surrounded by glimmerites. The dominant carbonatite is svite and is intruded by rauhaugites and carbonate-rich veins. The present investigation examines the mineral chemistry and petrology of the layered rauhaugites. They are essentially composed of ferroan dolomite, mica, magnetite and apatite, with accessory amounts of pyrochlore, ilmenite, sphalerite, baddeleyite, pyrite, galena and minerals enriched in the REE. The micas consist of titan-phlogopite, low-Ti phlogopite and tetraferriphlogopite. It is proposed that the parental magma of the Mt Weld complex was a potassic, aillikitic lamprophyre.Der circa 2,06 Ga alte Mt. Weld Karbonatit in West-Australien intrudiert eine archaische Grngestein-Sequenz, die von metamorphosierten basischen und ultrabasischen Magmatiten dominiert wird. Karbonatite bilden die Kernzone des Komplexes und werden von Glimmeriten umgeben. Das am weitesten verbreitete Gestein des Karbonatites ist Sbvit, der wiederum von Rauhaugiten und Karbonat-reichen Gngen intrudiert wird. Diese Untersuchung befat sich mit der Mineralchemie und Petrologie der geschichteten Rauhaugite. Sie sind im wesentlichen aus eisenreichem Dolomit, Glimmer, Magnetit, und Apatit zusammengesetzt; dazu kommen als Akzessorien Pyrochlor, Envenii, Zinkblende, Baddeleyit, Pyrit, Bleiglanz und an seltenen Erden angereicherte Minerale. Die Glimmer bestehen aus Titan-Phlogopit, Phlogopit mit niedrigen Titange halten sowie Tetraferriphlogopit. Ein Kali-reicher ailikitischer Lamprophyr ist als Ausgangsmagma fr den Mt. Weld-Komplex zu sehen.

Normative minerals are an integral part of most chemical classifications of volcanic rocks; however, when calculating the norms of these rocks the ratio has a powerful influence on the abundance and nature of these minerals. This arises because when the norms of the common, non-peralkaline rocks are computed at low ratios (< 1.1), every molecular unit of Fe2O3 uses a molecular unit of FeO to form magnetite, thus releasing a molecular unit of SiO2. At higher ratios rocks eventually contain an excess of Fe2O3 relative to FeO, and normative haematite forms. One of the basalts examined in the present study is nepheline and olivine normative when its ratio is 0.2, olivine and hypersthene normative when the ratio is 0.5, and hypersthene and quartz normative when the ratio is 1.2. The precise nature of such changes in both normal and peralkaline rocks is examined; and the concept of a set of standard ratios is evaluated.The following standard ratios are recommended: foidite 0.15–0.40, picrobasalt 0.15, basanite/tephrite 0.20–0.30, basalt 0.20, trachybasalt 0.30, basaltic andesite 0.30, phonotephrite 0.35, basaltic trachyandesite 0.35, andesite 0.35, tephriphonolite 0.40, trachyandesite 0.40, dacite 0.40, phonolite 0.50, trachyte/trachydacite 0.50 and rhyolite 0.50.After examining many erroneous, and a few bizzare, norms that have been published in recent times, it is recommended that a standard igneous norm (SIN) should be developed and used. This programme should use a set of standardized iron oxidation ratios, and also be able to correct the anomalies inherent in the original CIPW system of calculation.

The Derwent-Hunter Volcano (31°S, 156°E) is one of a 1400-km-long chain of Oligocene-Miocene undersea volcanoes that transect the Tasman abyssal plain. It developed some 15–16 Ma ago and the rocks it contains form an abbreviated differentiation sequence that range in composition from olivine tholeiite to either mugearite or basaltic andesite. These rocks generally contain normative hypersthene, but do not contain significant quantities of either normative quartz or normative nepheline; and their trace-element abundances generally lie in the area of overlap between the subalkali and alkali rock series. The more primitive rocks contain spinels (36% Cr2O3), olivines (Fo81–83), plagioclases (An63–70) and less commonly augite, set in a glassy mesostasis. Some specimens contain xenocrysts of anorthitic plagioclase (An85–94) and calcic salite (En36 Fs12 Wo52).

In northeast India, Early Cretaceous lamprophyres intrude both the Gondwana (Permian-Lower Cretaceous) sedimentary sequence and older cratonic blocks. Spessartite is the typical lamprophyre of the Proterozoic Meghalaya craton; whereas a minette-lamproite suite intrudes the Gondwana coal-bearing sequence. Cores from exploration boreholes, and rocks from deep coal mines, have provided excellent fresh samples. The petrography, mineral chemistry, geochemistry and isotope geochemistry of such samples have been examined. Essential characteristics of the minette-lamproite suite are: (a) depletion in Ca, Al and Na; (b) enrichment in K, Ti and other incompatible elements; and (c) presence of phlogopite megacrysts of possible mantle origin.Preliminary Sr, Pb and Nd isotopic data demonstrate significant differences between the minette-lamproite suite and the spessartites.It is concluded that the minette-lamproite suite evolved from a primary magma generated by the partial melting of a carbonate-bearing harzburgite that had been extensively metasomatised.

Late Proterozoic kimberlites occur in both southern and central India. They are similar in petrography and geochemistry to the kimberlites of southern Africa and Yakutia, U.S.S.R. These rocks contain low-CaO forsteritic olivines (Fo92), abundant phlogopites, magnesian ilmenites, aluminous—magnesian chromites, chrome-pyrope garnets surrounded by thick kelyphytic rims, perovskites, Fe-rich serpentines and occasionally economic quantities of diamond. It is propoed that a typical Indian kimberlite is the product of the mechanical mixing of a number of batches of kimberlitic (s.l.) magma. Such magmas are generally mobilized from depths of between 110 and 260 km. The batches of kimberlitic (s.s.) magma are believed to have equilibrated with the essential phases in the upper mantle at depths of at least 200 km.

The partly eroded remains of a Miocene compound shield volcano crop out near Orange, central New South Wales. A cluster of conical and domical landforms occupies the central elevated core of the volcano, and they are usually surrounded by flows of more basic lava that radiate out from this centre. At the present level of erosion, trachyte, and in particular ferroaugite trachyte, is the most abundant type of rock in the core of the complex, whereas hawaiite is the dominant type of rock in the outer part. Other rocks in the central area include mugearites, benmoreites, comendites, and a wide variety of pyroclastic rocks. The pyroxenes in the hawaiite‐trachyte suite range from Mg‐rich augite to ferrohedenbergite. Arfvedsonite is the characteristic mafic phase in the comendites. All the rocks of the complex are part of a single comagmatic suite. It is proposed that the magmas that formed most of the mugearites, benmoreites, trachytes and comendites evolved at relatively low pressures in a large compositionally zoned body of magma, in which the main processes of differentiation were crystal‐settling and volatile‐transfer.