ROCK LANDFORMS of Australia & NZ - METAMORPHIC LANDFORMS

This site will visit the landforms components of a metamorphic aureole, SE Queensland.
The aureole is a 'skin' of heat-altered rock (hornfels) that formed from phyllite around a granite (magma) intrusion.

Case Study: METAMORPHIC AUREOLE, BRISBANE Rock Types: metamorphics (phyllite, hornfels), and igneous (granite) Location: Mt Coot-tha, west of Brisbane GEOLOGICAL MAP
	SITES ON MAP: Site 1= Phyllite (base of Mt Coot-tha). - this is the original rock of the area (it is now mainly weathered and jointed). Site 2 = Hornfels (part way up mountain) - metamorphosed from phyllite (it is dark and tough) Site 3 = Granite (near summit of mountain) - this is the metamorphic heat source (it formed underground from intruded magma) (Note: this area is about half an hour's drive west of central Brisbane) KEY POINT: Geographical sequence of rocks associated with metamorphic aureole - phyllite (original rock), hornfels (altered rock), granite (heat source - intrusion)
West of Brisbane a contact metamorphic aureole (or sheath) is found. Three main rock types occur in outcrops at Mt Coot-tha: metamorphic phyllite (the country rock), metamorphic hornfels (the aureole) and granite (igneous intrusion). Key rock features are summarised below, and described in sequence in this case study. Site 1 The Bunya Phyllite, widespread across much of Nthn Brisbane, is the oldest rock of the area. It is found at the lowest elevation and often appears weathered, jointed and fractured. Site 2 Above the phyllite a metamorphic aureole of hornfels formed. It is believed that the hornfels derived from phyllite which had recrystallised through heat from the nearby intrusion. Site 3 The granite is an attractive pink-grey rock. Originally an intrusion, the granite now occupies the high ground, due to its resistant nature. The intruded granitic magma provided the heat source for the metamorphism of the surrounding rock. Granodiorite, a related rock, is also present. Animation: sedimentation, metamorphism & uplift, intrusion & re-metamorphism, erosion (This event sequence relates to part of the Brisbane region) BRISBANE METAMORPHICS TIMELINE - Key Dates top of page
Case Study: METAMORPHIC AUREOLE, BRISBANE (SITE: 1 on map - above) Rock Type: phyllite Features: Original country rock (now weathered and jointed)
KEY POINT: Original rock (phyllite) has suffered considerable fracturing, weathering, and erosion (it is less resistant than hornfels and granite, rocks that occupy higher areas)
The first of three sites on this metamorphic/igneous contact transect is found near the base of Mt Coot-tha. Here the original rock of the area, the metasedimentary rock, phyllite, is found. The phyllite has been jointed, fractured and weathered, and the roadside excavation has crumbled considerably. This site is lower than the more resistant granite intrusion and its hornfels aureole. This is the first stop (i.e. phyllite rock) of the transect sequence along the road (see geological map): (1) phyllite --> (2) hornfels --> (3) granite (where phyllite is the country rock, hornfels makes up the sheath, granite the heat source for metamorphism [of phyllite into hornfels].) Take care while crossing roads to inspect sites such as this. The crumbly nature of the slope makes it unsuitable for scrambling. From this point, drive up the Mt-Coot-tha road to observe other (signposted) sites of interest. top of page
Case Study: METAMORPHIC AUREOLE, BRISBANE ( SITE: 2 on map - above) Rock Type: hornfels Feature: part of the metamorphic aureole (band between granite and phyllite)
KEY POINT: Hornfels is tough - it resists weathering (metamorphosed from phyllite)
The exposure here is composed of the metamorphic rock hornfels. It originated from the fine-grained meta-sedimentary rock phyllite, which is found over much of the northern side of Brisbane. The phyllite was altered into hornfels as a results of heat from a granite intrusion (this process is referred to as contact metamorphism; heat rather than pressure was the main agent of recrystallisation). Fresh hornfels in this area is tough, fine-grained and of dark colour. It is quarried because of its durable properties. It is found locally in roadway and building retaining walls. The hornfels occurs in a relatively narrow band between the granite intrusion and the original phyllite. It is believed that the phyllite in close contact with the magma intrusion was recrystallised (metamorphosed) over 200 m.y. ago, forming a hornfels sheath around the underground granite blob. This is the second stop (i.e. hornfels rock) of the transect sequence along the road (see geological map): (1) phyllite --> (2) hornfels --> (3) granite (where phyllite is the country rock, hornfels makes up the sheath, granite the heat source for metamorphism [of phyllite into hornfels].) top of page
Case Study: METAMORPHIC AUREOLE, BRISBANE ( SITE: 3 on map - above) Rock Types: granite (left) / hornfels (right) Feature: contact metamorphic zone
KEY POINT: Contact zone between intrusion (granite - left) and metamorphosed rock (hornfels - right)
The granite of the Mt Coot-tha intrusion can be seen on the lower left of the picture. The contact zone can be identified in the field between this rock and the original phyllite, which was metamorphosed into hornfels through heat from the intruded magma; a process called contact metamorphism (see field sketch). The granite now forming the Mt Coot-tha summit was intruded as magma beneath the original phyllite. Heat flowing from this igneous feature brought about recrystallisation of phyllite into hornfels. Subsequent erosion removed overlying phyllite and some of the hornfels to expose the granite intrusion. The aureole passes around Mt Coot-tha as a band, identified by the location of hornfels. Close up the granite is pink-grey, and its constituent minerals of quartz, orthoclase, plagioclase and biotite can be seen. Cutting across the slope shown is a weathered dyke, probably related to the intrusion. This is the third stop (i.e. granite rock) of the transect sequence along the road (see geological map): (1) phyllite --> (2) hornfels --> (3) granite (where phyllite is the country rock, hornfels makes up the sheath, granite the heat source for metamorphism [of phyllite into hornfels].) top of page
(diagram from Willmott & Stevens - 1992) Intrusion of granite magma into phyllite bedrock. This caused a 'skin' of hornfels to form as phyllite was altered (metamorphosed) (240-220 million years ago)

to Metamorphic Page		Metamorphic Aureole, southeastern Qld The metamorphic aureole, Mt Coot-tha, west of Brisbane, is an excellent example of the results of contact metamorphism. During contact metamorphism, heat, rather than pressure, is the key cause of rock change (i.e. mineral recrystallisation). The heat source here was a granite (igneous) intrusion; heat from the intrusion flowed into the surrounding rock, altering it. The original rock of the area is phyllite - the phyllite in contact with heat from the intrusion was metamorphosed into hornfels. A hornfels 'skin' (aureole) thus formed around the intrusion. The three rock types are visible in the field. While not spectacular, they tell an important landscape story.
METAMORPHIC LANDFORMS		- Case Study (A) © C.Grant

Map of Metamorphic Sites	go to Metamorphic Landforms
Case Study (B) (auriole, SE Qld)	go to Main Landform Page

phyllite hornfels granite