Transcript Slide 1
Regional Metamorphism I. G. Kenyon Regional Metamorphism Associated with destructive plate margins during subduction and continental collision Occurs on a regional scale and involves thousands of cubic kilometres of rock Regional Metamorphism – Temperature/Pressure Ranges Regional Metamorphism Depth /km Pressure /MPa Temperature / °C Regional Metamorphic Rocks show Foliation Regional metamorphic rocks show foliation, a banding/layering/alignment of crystal long axes as they crystallised under directed (tectonic) stress Pink and blue mica crystals aligned parallel to each other produces the foliation Arrows indicate directed stress Regional Metamorphism of Argillaceous Rocks Argillaceous rocks are rich in clay minerals and include siltstone, mudstone, clay and shale Different metamorphic rocks are formed from argillaceous rocks under increasing temperature and pressure conditions Low Grade Regional Metamorphism - Slate Forms 5-15 km depth at high pressure, low temperature (<300°c) conditions Clay minerals recrystallize into chlorite and biotite mica and are coarser grained than the original clay minerals Chlorite and biotite mica are platy minerals have their long axes aligned and at right angles to the principal stress direction to form slaty cleavage Low Grade Regional Metamorphism - Slate Slaty cleavage direction Slate is coarser grained Principal stress directions and crystalline than the shale from which it has been derived but the crystals are too small to be seen with the naked eye Thin section of slate under the microscope, field of view 2.5mm Platy crystals of chlorite and biotite have their long axes aligned parallel and at right angles to the principal stress direction Low Grade Regional Metamorphism - Slate Relic bedding and laminations Slaty Cleavage Direction cuts across relict bedding At low grade, some relic sedimentary structures may be preserved such as bedding or lamination. Low Grade Regional Metamorphism - Slate Delabole Slate Quarry Delabole Butterfly (a brachiopod) Fossils may be present in slates but they will be deformed - stretched, elongated or compressed for example ‘the delabole butterfly’ a spirifer type of brachiopod Slate splits readily along its cleavage Medium Grade Regional Metamorphism - Schist Formed under higher temperatures 400 to 500 degrees centigrade and at depths of 15 to 25 km Higher temperature results in larger crystals 1 – 2mm and the growth of new minerals such as garnet along with quartz and micas Coarser foliation is known as Schistose Texture Garnet crystals occur as porphyroblasts up to 5mm in diameter and often distort the foliation Medium Grade Regional Metamorphism - Schist Garnet porphyroblast 8mm in diameter Principal Stress Directions Coarse foliation formed by aligned micas is known as Schistose Texture Growth of garnet has distorted the foliation High Grade Regional Metamorphism - Gneiss Formed under higher temperatures 500⁰c to 700⁰ c and at depths of 25 to 40 km Higher temperature results in larger crystals over 2mm and the growth of new minerals such as feldspar along with quartz and mica 1 cm Principal Stress Direction Gneissose banding Grey/white quartz Pink feldspar Minerals are segregated into discontinuous layers to produce a coarse foliation known as gneissose banding Mineral composition now similar to granite Black biotite mica Principal Stress Direction Very High Grade Regional Metamorphism - Migmatite Banded Gneissose Component Gradational junction between the gneiss and granite-no chilling or baking evident Granitic Component Temperatures above 700⁰ c and depths of 40 to 50 km results in the gneiss starting to melt The rock produced is a migmatite or ‘mixed rock’ comprising a banded gneissose component and a non-foliated granitic component Beyond 50 km depth and temperatures >800⁰c the migmatite melts completely to form magma, when cooled and solidified forms granite. That’s All Folks!