CAINOZOIC VOLCANOES ON THE AUSTRALIAN PLATE Excursion designed and delivered by Patrice Rey

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1 1 CAINOZOIC VOLCANOES ON THE AUSTRALIAN PLATE Excursion designed and delivered by Patrice Rey During the Cenozoic, eastern Australia experienced widespread volcanic activity that led to the formation of chains of volcanoes made of basalts, lesser amounts of intermediate rocks, and a variety of felsic extrusives and subvolcanic intrusives. From the border between Queensland and New South Wales to Bass Strait, the radiometric age of these volcanoes, and their counterparts in the adjoining Tasman Sea, display a progressive decrease from 28 to 10 million years. They are linked to a static, long-lasting, mantle plume that impinged on the base of the Australian Plate as it moved northwards away from Antartica (Wellman and McDougall, 1974; Johnson et al. 1989). Mount Canobolas, near the township of Orange, is an extinct volcano which last erupted 11 Ma ago. The eruptions took place from around 30 vents within 30 km of the current summit proudly standing at a breathtaking ca.1400m above sea level (taken at low tide). The main vent erupted over the entire life of the volcano while most of the other vents were only active for short periods. While the upper part of the volcanic structure has now been removed by erosion, the flanks of the shield can still be seen. The objective of the exercise is to document the local geology of Mount Canobolas Volcanoe. To achieve this aim, students will describe the various extrusive and intrusive making the Mount Canobolas and their spatial and temporal relationships. Students will work in groups of 3 and will be provided with a number of tasks to complete. At complete, students will be able to: Recognise and describe a number of igneous rocks (rhyolite, trachyte, basalt, andesite, tuffs). Recognise minerals in hand specimen (quartz, plagioclase, hornblende, feldspar). Say: As Sir Thomas Mitchell, I too climbed Mount Canobolas! NB1: ( NB stands for Nota bene, Latin expression which means Take Note ): Cainozoic and Cenozoic are two different yet equally valid spelling. NB2: Cainozoic is the Tertiary period of the Earth history spanning 54Ma to 8Ma. NB3: Ma stands for Million Years as an age. Myr is used for Million Years as a duration. Example: This Volcano is 20 Ma old. Its volcanic activity lasted for 5 Myr. NB4: Canobolas derives from an Aborigene world Coonoobooloo which means two shoulders.

2 2 Let s the game begin Thomas Livingstone Mitchell was born in Scotland in He joined the army and became a surveyor. A talented artist and writer, he was also a geologist and botanist. The books he wrote about his journeys were very popular. He married in 1818, and had 12 children, 6 girls and 6 boys. In 1837 Mitchell was knighted and became Sir Thomas Mitchell. In 1855 Lieutenant Colonel Sir Thomas Mitchell, as he had by then become, developed pneumonia and died in Sydney on the 5 October. In 1827, Major Thomas Mitchell arrived in Australia to become the Surveyor-General of the colony of New South Wales. He held the position for 27 years, and was responsible for the placement of roads, bridges and towns. Most considered him very arrogant and many suffered under his temper, which at times bordered on the explosive (just take a look at his picture pretty scary!). He led four expeditions of exploration and carried out most of the surveys of Eastern Australia, which led to new grazing lands being established in southern Victoria. Surveyor- General Mitchell explored northern and western New South Wales in 1831 and In 1835, he climbed Mount Canobolas and it is believed that he produced rough geological map of the area (Exercise 1). Unfortunately, the legend of the map was lost 170 years later here you are following in Sir Mitchell s footstep. Let s mark a pose and reflect on this historic moment. let s move on. Your mission, which is far from being impossible, consists to cruise around, observe, describe and ultimately provide a proper legends to Mitchell s geological maps.

3 3 Exercise 1: Sir Mitchell Challenge: Geological sketch of the Mount Canobolas summit. The triangle on the map bellow shows the location of the trigonometric station on Mount Canobolas. Four rock formations have been reported on Sir Mitchell s map. 1/ For 10 marks: Observe and describe the morphology of outcrops representative of each rock formation. Describe any relevant geometrical features (thickness of the beds, dikes and sills, cross cutting relationship if any), and describe any particular fabric (flow, jointing ) that can be seen. 2/ For 10 marks: At the scale of the hand specimen, and when necessary with the help of your hand lens, describe the color and texture of the rock, assess its grain-size and make an inventory of its mineralogy. Describe any fabrics and any other relevant features. 3/ For 5 marks: Give a name to each rock formation and justify your answer. 4/ For 5 marks: Check the stones used to build the wall around the trigonometric station. Describe the rock, (color, texture, mineralogy etc). Propose a name of that rock. Is this rock found around Old Man Canobolas?

4 4 Exercise 2: The cross-section controversy: Branagan/Packham vs NSW National Park. The geological cross-section, produced for the New South Wales National Park and Wildlife Service, on display at the Mount Canobolas information booth differs significantly from the one published in the 2000 edition of the the Branagan & Packham book Field Geology of New South Wales. 1/ For 5 marks: Analyse both x-sections and describe in what they differ. 2/ For 10 marks: Walk to Young Man Canobolas and describe the local geology in a brief report including sketches. 3/ For 15 marks: Now that you know the local geology of both Young and the Old Man Canobolas, draw your own cross-section and decide for yourself which of the above cross-section makes more sense.

5 5 Exercise 3: Another lost legend... The map below is what is left of an early sketchy map of the local geology of the Pinnacles, one of the many volcanic vents (~50 or so) of Mount Canobolas. 1/ For 20 marks: Construct a legend for the map and document cross-cutting relationships. 2/ For 10 marks: Look at the following photographs, name and point their possible location on the map. a> b> c> d>

6 6 IGNEOUS ROCKS: TEXTURES AND CLASSIFICATIONS INTRODUCTION Igneous (derived from the Latin ignius for fire ) rocks form by the cooling and the crystallization of molten rock. Based on their textures petrologists have divided igneous rocks into two broad categories. Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava (flows and fragmented debris called pyroclasts). Plutonic rocks (also called intrusive igneous rocks) are those that have solidified below ground; plutonic comes from Pluto, the Greek God of the underworld, not to be confused with the Walt Disney cartoon character who is neither Greek nor a God. The initial distinction between volcanic and plutonic rocks is made based on texture (finegrained volcanic vs. coarse-grained plutonic). Volcanic and plutonic rocks are divided further based on chemistry and mineral composition. COMMON TEXTURES OF IGNEOUS ROCKS Phaneritic: Coarse-grained groundmass in igneous rock, constituent minerals are discernible with the naked eye. Phaneritic textures are indicative of slow cooling away from the surface. The hot environment allow minerals to grow. Below is an example of phaneritic texture. The rock is a granite. Aphanitic: Very fine-grained groundmass in igneous rock, constituent minerals are not visible to the naked eye. Fine-grained textures generally indicate magmas that rapidly cool at or near the Earth's surface. Fast cooling prevents crystals from growing very large. The cutoff between fine- (aphanitic) and coarse-grained (phaneritic) textures is about 1 mm. If you plan on taking more geology classes, it is worth memorizing the word aphanitic (meaning grains too small to easily see). Porphyritic: Texture of igneous rock containing large crystals (phenocrysts) in a groundmass (matrix) of smaller crystals or glass. This texture suggests a two-stage cooling history. The phenocrysts crystallize first in hot magma environment. In a second stage, the magma cools

7 7 down and crystallizes rapidly to form a fine grained matrix. The picture on the right is an andesite with amphibole phenocrysts. Pyroclastic: A pyroclastic texture is defined by a mixture of rock fragments, pumice, and volcanic ash. The ash is very fine grained, so only the rock fragments and pumice are identifyable. A rock with a pyroclastic texture is termed a tuff if the largest fragments are less than 5 cm long; a volcanic brecchia has larger rock fragments and/or pumice. The non-genetic classification of pyroclastic rocks is based on the abundance and nature of the pyroclastes (blocks, lapillis, clasts).

8 8 CLASSIFICATION OF IGNEOUS ROCKS Most geologists have accepted the IUGS (International Union of the Geological Sciences) classification of igneous rocks as the standard. This classification is based on a combination of mineralogy, chemistry, and texture. Texture is used to subdivide igneous rocks into two major groups: (1) the plutonic rocks, with mineral grain sizes that are visible to the naked eye, (2) the volcanic rocks, which are usually too fine-grained or glassy for their mineral composition to be observed without the use of a petrographic microscope. The texture is largely the result of the depth of origin of the rock (volcanic at or near the surface, and plutonic at depth). Let's first examine the classification of plutonic rocks. In 1973, the IUGS suggested the use of the modal composition for all plutonic igneous rocks with a color index less than 90. A second scheme was proposed for those plutonic ultramafic rocks with a color index greater than 90. The three components, Q (quartz) + A (alkali (Na-K) feldspar) + P (plagioclase), are recalculated from the mode to sum to 100 percent. Each component is represented by the corners of the equilateral triangle, the length of whose sides are divided into 100 equal parts. Any composition plotting at a corner, therefore, has a mode of 100 percent of the corresponding component. Owing to the aphanitic texture of volcanic rocks, their modes cannot be readily determined; consequently, a chemical classification based on normative mineralogy is used. The boundaries between the different field in the volcanic rocks AQP triangle correspond to that of the igneous rocks.

9 9 The major division of volcanic rocks is based on the alkali (soda + potash) and silica contents, which yield three groups, the subalkaline, alkaline, and strongly alkaline rocks. Furthermore, as they are so common, the subalkaline rocks have two divisions based mainly on the iron content with the iron-rich group called the tholeiitic series and the iron-poor group called calc-alkaline. The former group is most commonly found along the oceanic ridges and on the ocean floor and is usually restricted to mafic igneous rocks like basalt and gabbro; the latter group is characteristic of the volcanic regions of the continental margins (convergent, or destructive, plate boundaries) and is comprised of a much more diverse suite of rocks. Chemically the subalkaline rocks are saturated with respect to silica. This chemical property is reflected in the mode of the mafic members that have two pyroxenes, hypersthene and augite [Ca(Mg, Fe)Si 2 O 6 ], and perhaps quartz. Plagioclase is common in phenocrysts, but it can also occur in the matrix along with the pyroxenes. Hornblende and biotite phenocrysts are common in calc-alkaline andesites and dacites but are lacking in the tholeiites. Dacites and rhyolites commonly have phenocrysts of plagioclase, alkali feldspar (usually sanidine), and quartz in a glassy matrix. Hornblende and plagioclase phenocrysts are more widespread in dacites than in rhyolites, which have more biotite and alkali feldspar. The alkaline rocks typically are chemically undersaturated with respect to silica; hence, they have only one pyroxene, the calcium-rich augite) and lack quartz but often have a feldspathoid mineral, nepheline. Microscopic examination of alkali olivine basalts (the most common alkaline rock) usually reveals phenocrysts of olivine, one pyroxene (augite), plagioclase and perhaps nepheline.

10 10 A Field Classification: Now that we have completely confused you, let's look at a much simpler classification. We call this a field classification because it requires little detailed knowledge of rocks and can be easily applied to any igneous rock we might pick up while on a field trip. It utilizes texture, mineralogy and color. The latter is a particularly unreliable property, but the classification realizes that certain fine-grained (aphanitic) igneous rocks contain no visible mineral grains and in their absence color is the only other available property. Students the thus cautioned to use color only as a last resort. To employ this classification we must first determine the rock's texture. You might remember we have five basic textures; phaneritic (coarse), aphanitic (fine), vesicular, glassy and fragmental (our classification doesn't bother with the latter because we often term all fragmental igneous rocks tuffs). Examine your rock and determine which textural group it belows to. If it is glassy, vesicular or fragmental you cannot determine mineralogy and hence the name is simply obsidian for a glass, tuff for a fragmental or pumice/scoria for a vesicular rock (the latter are differentiated on the basis or color and size of the vesicles or holes). For the phaneritic and some aphanitic rocks you must determine the mineralogy. Often it is only necessary to identify one or two key minerals, not all of the minerals in the rock. For instance quartz and potassium feldspar (k-feldspar) are restricted to granites and rhyolites. Amphibole is only abundant in diorite or andesite, although minor amounts can be present in granite. How am I getting these names? Let's take an example. I pick up my first specimen and notice that it is distinctly coarse grained (phaneritic). This means that it must be one of the rocks in the row labeled coarse (i.e., granite, diorite, gabbro or peridotite). I next place the rock under a binocular microscope and identify the minerals plagioclase and pyroxene. I go to the bottom row of the chart (Minerals Present) and look for a match with my mineralogy. I find it in the third column (Ca-play, pyroxene) and read the name (gabbro) from the coarse row on the chart. Pretty simple!! Relax, when you actually begin your igneous rock identification we will walk you through it step by step. But remember to refer to the above classification diagram often as an aid.

11 11 COMMON MINERALS OF IGNEOUS ROCKS To correctly classify many igneous rocks it is first necessary to identify the constituent minerals that make up the rock. Piece of cake you say, I saw most of these minerals when I did the Minerals Exercise or I have them in my mineral collection. Well, its not quite that easy. The mineral grains in rocks often look a bit different than the larger mineral specimens you see in lab or museum collections. The following section is meant to assist you in recognizing common rockforming minerals in igneous rocks. Refer back to it often as you attempt to classify your rock specimens. Plagioclase: the white or chalky looking grain is the common feldspar. Plagioclase is the most common mineral in igneous rocks. The chalky appearance is a result of weathering of plagioclase to clay and this can often be used to aid in identification. Most plagioclase appears frosty white to gray-white in igneous rocks, but in gabbro it can be dark gray to blue-gray. If you examine plagioclase with a hand lens you can often see the stair-step like cleavage and possibly striations (parallel grooves) on some cleavage faces. Some potassium feldspar is white like plagioclase, but is usually a safe bet to identify any frosty white grains in igneous rocks as plagioclase. Expect to find plagioclase in most phaneritic igneous rocks and often as phenocryts in aphanitic rocks. Quartz: the dark gray, glassy grain is quartz. Quartz is also a very common mineral in some igneous rocks. In igneous rocks it is often medium to dark gray and has a rather amorphous shape. If you look at it with a hand lens you will notice the glassy appearance and lack of any smooth cleavage surfaces. You will also find quartz grains resist scratching with a nail or pocket knife, You can expect to find abundant quartz in granite and as phenocryts in the volcanic rock rhyolite. In some other common igneous rocks you may find a few scattered grains of quartz, but it is often conspicuous by its absence. Once recognized, quartz is rarely confused with any other common rock-forming mineral.

12 12 Potassium Feldspar (K-feldpar, potash feldspar, orthoclase): commonly slightly pinkish grains with the stair-step cleavage characteristic of feldspars. All K-feldspar is not pink, microcline is usually white. How does one distinguish white potassium feldspar from plagioclase? Plagioclase has striations, potassium feldspar does not. In most cases any white feldspar is identified as plagioclase and any pink feldspar as orthoclase. Expect to find orthoclase as a common constituent of granite and matrix material in rhyolite. In the latter rock the orthoclase is too fine-grained to be seen even with a binocular microscope, but its presence gives most rhyolites a distinct pinkish cast. Muscovite: Muscovite is not a common mineral in igneous rocks, but rather an accessory that occurs in small amounts. It is shiny and silvery, but oxidizes to look almost golden. Muscovite has excellent cleavage and will scratch easily. If you suspect muscovite is present, try taking a nail to it. It should flake off the rock. Muscovite occurs in some granites and occasionally in diorite. Unlike, its close cousin, biotite, it rarely occurs as phenocrysts in volcanic rocks. Biotite: Biotite occurs in small amounts in many igneous rocks. It is black, shiny and often occurs in small hexagonal (6-sided) books. Unfortunately, it is often confused with amphibole and pyroxene. Like muscovite, it is soft and has good cleavage. Try scratching the black grains with a nail or knife. Biotite will flake off easily. Biotite is differentiated from amphibole by shape of the crystals (hexagonal for biotite and elongated or needle-like for amphibole) and by hardness (biotite is soft, amphibole is hard). It is differentiated from pyroxene by hardness, color (biotite is black and pyroxene dark green) and occurrence (biotite is found in light-colored igneous rocks like granites, diorites and rhyolites while pyroxene occurs in dark-colored rocks like gabbro and basalt). Expect to find biotite as a common accessory in granite, and as phenocrysts in some rhyolites.

13 13 Amphibole: Amphiboles appear as elongated, black grains are amphibole. Amphibole is a rather common mineral in all igneous rocks, however, it is only abundant in the intermediate igneous rocks. It occurs as slender needle-like crystals. It has good cleavage in 2 directions (with an 120º angle) and hence has a stair-step appearance under a binocular microscope. It is often confused with pyroxene which has also 2 cleavages but closer to 90º. Not all grains of amphibole will be oriented so you can see the elongation of the crystals. Its a good guess that if you see a few crystals that have the "classic" amphibole shape, the other black grains are also amphibole. Biotite and amphibole do occur together in igneous rocks, but the association is not all that common. Amphibole is very commom in diorite, less so in granite or gabbro. It also is a common and diagnostic phenocryst in andesite. Pyroxene: Pyroxene is common only in mafic igneous rocks. It occurs as short, stubby, dark green crystals. It has poor cleavage in 2 directions (at 90º) and cleavage surfaces are often hard to see with even a binocular microscope. It is often confused with amphibole. Amphibole is darker and occurs in needle-like crystals rather than the stubby shape of pyroxene. Association is the best guide for the identification of pyroxene. It is usually restricted to dark-colored rocks such as gabbro or basalt.

14 Olivine: Olivine is common only in ultramafic igneous rocks like dunite and peridotite. It occurs as small, light green, glassy crystals. It has no cleavage. The texture of olivine in igneous rocks is often termed sugary. Although olivine occurs in gabbro and basalt, it is far more common in peridotite and dunite. Because of the light green color and sugary texture it is rarely confused with other rock-forming minerals. 14

15 15 COMMON VOLCANIC ROCKS The silica content alone is enough to make the distinction between basalt, andesite, dacite, and rhyolite. Because of different silica content, these rock types have contrasting viscosity (resistance to flow), which impact on the explosiveness of the volcanoes. Silica rich lavas such as rhyolites have a higher viscosity and therefore a much-reduced mobility compared to that of basalts (cf. Figure below). The consequence is that felsic volcanoes are more explosive than mafic volcanoes. It is therefore important from an environmental protection point of view to be able to recognize felsic volcanoes. Rhyolite: a light-colored rock with silica (SiO 2 ) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 C to 850 C. Comendite: A variety of quartz-bearaing alkali rhyolite. Leucocratic (a fancy word that means pale) volcanic rock made of phenocrysts of quartz, alkali feldspar, aegerine (a clino-pyroxene), alkali amphibole, some biotite in a groundmass of quartz and alkali feldspar. Trachyte: Fine grained extrusive alkaline rock, sometimes porphyritic, approximately silicasaturated, and with a wide compositional range grading into rhyolites. Main components are alkali feldspar and minor mafic minerals, sometimes with a small amount of quartz. Often

16 16 trachytes show a preferential alignment of feldspar microlites bending around phenocrysts when they are present. Solvsbergite: Fine grained crystalline trachyte, rarely porphyritic; contains feldspar, pyroxene or amphibole, rare quartz. Dacite: Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68% silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 C and 1000 C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gaspoor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome. Andesite: Grey to black volcanic rock with between about 52 and 63 weight % silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100 C. Basalt: a hard, black volcanic rock with less than about 52 weight % silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move >20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 C to 1250 C.

17 17 References: Nb: Take a couple of minute to learn how to properly reference all papers mentioned in the text of the booklet. Remember, properly referencing your report is the best protection against plagiarism. Branagan, D. and G. Packham, Field Geology of New South Whales. Third Edition, Published by NSW Department of Mineral Resources. 418p. Johnson, R.W., O Reilly, S.Y., Lister, G.S., Etheridge, M.A., Knutson, S., Sun, S.S., Ewart, A., Green, D.H., McDonough, W.F., and Wellman, P., Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press. 408 p. LeMaitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyre, J., Le Bas, M.J., Sabine, P.A., Schmid, R., Sörensen, H., Streckeisen, A., Woolley, A.R., and Zanettin, B., Eds. (1989). A classification of igneous rocks and glossary of terms: Recommendations of the International Union of Geological Sciences - Subcommision on the Systematics of Igneous Rocks. Blackwell Science Publications, Oxford, U. K. 193 p. Wellman, P. and McDougall, I., Cenozoic igneous activity in Eastern Australia. Tectonophysics, 23,