ch. 10 igneous rocks of the continental lithosphere

1. Ch. 10Igneous Rocks of the Continental Lithosphere

3. Continental Basalt Provinces

4. Continental Basalt Provinces Continental Flood Basalts CFB�s Eg Colombia Rv basalts - Enormous outpourings of tholeiitic basalt >100,000 km3 in little over 3 Ma Few if any phenocrysts � primary magma? Mg:Fe ratio too low for typical mantle melts � an atypical mantle source with 2 possible complications 1) east dipping subduction under the Cascades must have passed under the Columbia Rv Plateau and could have contributed subduction zone magmas; 2) The linear belt of volcanics in the Snake River province has been ascribed to the passage of a hot spot beneath North America � Yellowstone Hot Spot Currently thought that the subcontinental mantle, including lithosphere and asthenosphere, is heterogeneous, with depleted and enriched portions. Unlike oceanic lithosphere, continental lithosphere is not readily recycled into the mantle and instead is welded or attached to the overlying continental crust quite early. CFB�s originate from melting of enriched sublithospheric (asthenospheric) mantle of the type that also produces E-MORBs and OIBs.

6. Continental Basalt Provinces Layered Mafic Intrusions Increasing tectonic evidence suggesting they are related to periods of major crustal extension and rifting in continental lithosphere and may be the intrusive deep crustal feeders for the CFB�s also associated with rifting Bushveld, Stillwater (Montana), Skaergard (Iceland), Dufek (Antarctica) Overall basaltic in composition and subdivided into thick units ranging from ultramafic at the bottom to felsic at the top.

8. Continental Basalt Provinces Komatiites Rare rocks believed to represent ultramafic lava flows Occur in layered stratigraphic patterns with pillow morphologies at the top Odd rocks because ultramafics have liquidus T between 1400-1600? C at low P and are thus unlikely to occur as liquids even in the Earths interior Notable for their spinefex texture, after the large elongated bladelike phenocrysts of olivine in a groundmass of cpx To generate ultramafic melts in the Archean the mantle must have been much hotter at shallow depths as it would require ~80% partial melting Profound implications for plate tectonics � sea-floor spreading and subduction may have operated more effectively and rapidly in the Archean than today, allowing for efficient recycling of the early products of crust building which may be why the rocks from this earliest part of the Earths history are so rare

9. Continental Rifts Large scale continental extensional environments probably represent the initial rifting stages of single continental plates as they break up and drift apart. Include �failed rifts� Eg East African Rift system, which is volcanically active due to large scale extensional stress in the crust combined with normal faulting which provides pathways for magma to reach the surface. Continuous volcanism for the past 30 Ma. Dominantly alkalic. Nephelinites and alkali basalts occur early in the sequence and are followed by phonolites, trachytes and rhyolites. This increase in silica along with increasing Fe:Mg ratios in phenocrystic ferromagnesian minerals are key indicators of fractionation processes operating in deep magma chambers Late magmas are characterised by highly alkalic and silica-deficient lavas and even by carbonatites.

11. Chemistry, Petrography and Petrogenesis of Continental Rift Magmas The large diversity of igneous rocks at rifts raises many petrological questions. How are these magmas related to each other? How many are mantle derived and how many are the product of fractionation in mantle or deep crustal magma chambers? Has crustal melting or contamination played a role in magma evolution? The chemistry of the silica-undersaturated alkali basalts, basanites and nephelinites is similar to that of OIB�s ie a chemical signature of deep melting or enriched mantle, possibly asthenosphere. The interesting rocks of continental rifts are the more evolved ones eg phonolite, trachyte, rhyolite. The majority of these are peralkaline. As reflected by the presence of sodic px and amphibole eg aegerine, aegerine-augite, riebeckite and high Fe:Mg ratios in ferromagnesian minerals and the presence of fluorite. The highly alkaline nature and high conc of incompatible elements suggests small degrees of partial melting at great depth (60-100 km). The mantle beneath East Africa is not normal subcontinental lithosphere mantle but may be upward bulging asthenosphere. The continenal crust is thinned to ~20km

13. Carbonatites Rare igneous rocks containing >50% carbonate minerals (calcite, dolomite, magnesite and sodium carbonate). Rich in Na-rich pyroxenes and amphiboles + apatite, phlogopite, magnetite, fluorite, perovskite, monazite, pyrochlore and barite Usually accompanied by nepheline syenite Abundant fracturing by alkali-rich fluids resulting in metasomotism � referred to as fenetization Originate by small degrees of fractional melting of mantle peridotite which contains carbonate minerals or CO2-rich fluid phase and is important in the role of melting

15. Anorogenic Granites Granite related plutonic rocks with no obvious relationship to subduction or plate convergence A-type granites. However substantial overlap with I-type Usually either metaluminous or peralkaline, high Fe:Mg ratio in ferromagnesian minerals Characteristic occurrence on the liquidus of only a single alkali feldspar, rather than the separate crystallisation of a K-rich alkali feldspar and a Na-rich feldspar. Upon cooling, this single feldspar decomposes (exsolves) into the lamellar intergrowth called perthite with K-rich and Na-rich lamellae � referred to as hypersolvus crystallisation and is an important clue as to low P and shallow depth Simultaneous crystallization of 2 feldspars is called subsolvus texture

18. Anorthosites Plutonic rocks containing >90% plagioclase Lunar anorthosites Archean Megacrystic Anorthosites � plagioclase up to 1m across! Massif Anorthosites � includes the charnockite series (opx granite + fayalite, hedenburgite)

19. Kimberlites and Lamproites Volumetrically miniscule but economically important � diamonds Kimberlites are potassic ultramafic rocks that occur as small plugs or pipes Lamproites are ultramfic end members of a family of highly porphyritic ultrapotassic rocks Contain abundant xenoliths

20. Kimberlites and Lamproites

22. Study Exercises