Classification and Origin of Granites. A Multi-faceted Question

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2 Classification and Origin of Granites A Multi-faceted Question

3 What is a granite? IUGS classification Based on Modal Mineralogy Plutonic rock with less than 90% mafic minerals Alkali Granite Granite Quartz 20 - ~42% 20 - ~42% Plagioclase-Alkali Feldspar proportions Plagioclase <10% 10 65% Alkali feldspar >90% 90 35% This is a descriptive classification. No genetic interpretation

4 Hypersolvus versus Subsolvus Granites A Mineralogical Distinction Hypersolvus one feldspar, usually perthitic Subsolvus two feldspars Significance: difference in water pressure, temperature, and/or depth of crystallization. This distinction has petrogenetic implications (there is an interpretation assigned to the textures).

5 Peraluminous: Metaluminous: Subaluminous: Chemical Classification Al 2 O 3 > CaO+Na 2 O+K 2 O Corundum in the norm Characteristic mineralogy: muscovite, topaz, tourmaline, spessartine-almandine, corundum, andalusite, sillimanite Al 2 O 3 < CaO+Na 2 O+K 2 O and Al 2 O 3 > Na 2 O+K 2 O Anorthite prominent in the norm Characteristic mineralogy: biotite, hornblende Al 2 O 3 = Na 2 O+K 2 O Anorthite minimal in the norm Characteristic mineralogy: essential Al 2 O 3 minerals are feldspars Peralkaline: Na 2 O+K 2 O > Al 2 O 3 Acmite, sodium silicate, potassium silicate in the norm Characteristic mineralogy: aegirine, riebeckite, richterite

6 Classification Based on Tectonic Setting Pearce et al. (1984) tectonic discriminant diagrams. Note that Rb is a mobile element and diagrams using Rb are only appropriate when there is no possibility of hydrothermal or metamorphic redistribution of this element.

7 Classification Based on Source of Magmas The Alphabet Soup I & S Granites are ultimately the dregs of Petrogeny s Residua System. We can reach this point by extreme fractionation of a basalt magma, partial melting of appropriate source materials, or a variety of assimilation-fractional crystallization processes. The final product, by definition, is mineralogically a granite. The I & S classification (Chappell and White, 1974) only considers partial melting, and attempts to identify the partially melted source material as either meta-sedimentary or meta-igneous.

8 The Start of Granitology the single most important paper dealing with the petrogenesis of granites and related rocks. Experimental studies delineated a low temperature region in the system SiO 2 - Ab-Or (the major mineral components of granites). Granite compositions plot in this low temperature region The liquids can originate by fractional crystallization of more basic liquids (i.e., basalts) or by fractional melting of appropriate sedimentary and metamorphic rocks, p. 2. Subsequent studies of granites tended to de-emphasize fractional crystallization of basalts.

9 Geologic settings that can produce granitoid magmas Winter (2001) Source of the Heat Required for Melting 1) Slab melts and/or melts generated by metasomatism of the mantle wedge 2) Slab role-back and diapiric rise of asthenospheric mantle 3) Rifting and decompression melting 4) Mantle plume 5) Radioactive heating of a thick crustal sequence enriched in K, U, and Th Sources 1-4 involve basaltic magmas which serve as the heat source. These magmas can undergo complex interactions with the crust. Source 5 generates a purely crustal melt.

10 Fractional crystallization (FC) Assimilation-Fractional Crystallization (AFC) Melting + Assimilation + Storage + Homogenization (MASH) Magma mixing Progressive partial melting Restite unmixing Peritectic assemblage entrainment Petrogenetic Processes and the Formation of Granitoids Fractional crystallization of a basaltic magma leading to a granitic residuum. The classic Bowen estimate was that fractional crystallization of a basaltic magma would result in a 10% granitic residuum. Given the large volume of granitic rocks in the crust, where are the basaltic and intermediate rocks? Fractional crystallization can also play a role during differentiation of granitoid magmas. At high levels in the crust this process seems unlikely because of insufficient heat. Also little evidence of large scale melting of the country rock in granitioids. A deep-seated process proposed for granitoid magmas generated in subduction zones and continent-continent collisions. Envisions significant mixing of basaltic melts derived from the mantle with melts from the continental crust. Mixing of mafic and felsic magmas. Mafic magmas may be from the mantle or lower part of the crust. Felsic magmas formed in the crust by transfer of heat either from mafic magmas or regions of high heat-flow and high radioactive element content. This process seems to be important locally as there is abundant evidence of magma mixing in many high level plutons. Rising geotherms lead to progressive melting of meta-sedimentary and metaigneous protoliths. Classic Chappell and White model in which granitic magmas are a mixture of melt and material (restite) from the source region of the melt. Chemistry of granitic melts controlled by chemistry of the source, peritectic mineral entrainment (biotite for S-type granites and biotite + hornblende for I-type granites), and co-entrainment of accessory minerals (Clemens and Stevens, 2012, Lithos , ).

11 The I & S classification scheme, and the restite model, was developed in the Lachlan foldbelt of Australia. I-types arise from the partial melting of a meta-igneous source. S-types from the partial melting of a meta-sedimentary source. The classic Bowen and Tuttle explanation for the origin of granites (but note that Bowen and Tuttle suggested a second pathway involving fractional crystallization). Because the restite model has been widely used let s take a few minutes to explore the model in a bit more detail. We will use the Point Wolfe River pluton, New Brunswick, for our exploration. According to the restite model the chemical composition of a granite can be described as a mixture of partial melt and restite (fragments of the source material). Distinctive mineralogy and chemistry can be used to separate I and S types. More recent work (by Clemens and others) identifies the importance of peritectic assemblage entrainment (PAE) in the development of granitic magmas.

12 Major Elements reasonable fit to the restite model Trace elements don t fit the restite model

13 The major element data for the Point Wolf River pluton do agree (kind of) with the restite model, but the trace element data do not. This is a common observation and requires that the major elements and trace elements be decoupled. This is accommodated in the Peritectic Assemblage Entrainment model by invoking co-entrainment of accessory minerals which account for the trace element variations. What about trace elements that are incorporated in major minerals, i.e. Ba (alkali feldspar) and Sr (plagioclase and alkali feldspar)? In these cases the data are in better agreement with the restite model. But note there is an alternative interpretation fractional crystallization of plagioclase will lead to a significant decrease in Sr and a modest increase in Ba. This is what is observed and leads to an alternative conclusion that plagioclase fractionation played a major role in magma evolution.

14 What are the fundamental questions to be answered when studying a granitic sequence? In no particular order 1. What was the source of the melt crust, mantle, or both? 2. What processes occurred during the ascent and emplacement of the melt fractional crystallization, fractional crystallization + assimilation, magma mixing, peritectic reactions? 3. What was the source of the heat? 4. What was the tectonic environment? What are the tools we can use to answer these questions? Major and trace element rock geochemistry Mineral chemistry Stable isotopes Radioactive isotopes both whole-rock and mineral (zirconology, and other minerals) data Thermodynamic modeling

15 A convergent margin example the Avalonian terrane The Geologic Story: The Avalonian terrane was formed in the Neoproterozoic along the margin of Gondwana. It was subsequently rifted from the margin and formed a microcontinent that was trapped between Laurentia and Baltica during the amalgamation of Pangea. Subsequent rifting that led to the opening of the Atlantic Ocean split the terrane and it is now found in New Brunswick, Nova Scotia, Newfoundland, the British Isles and Scandinavia. Wikipedia

16 Based on zircon U-Pb dating, the Neoproterozoic volcanic and plutonic rocks of the Caledonia terrane were divided into the ca. 620 Ma Broad River Group, exposed mainly in the northeastern and southern parts of the terrane, and the ca Ma Coldbrook Group which forms most of the western and northern parts. Prior to the U-Pb zircon study all the rocks were thought to be of similar age. The recognition of two distinct igneous events was a major step in understanding the geologic history of the terrane. Study Area White et al. (2002) Fyfe et al. (2012)

17 Volcanic Plutonic

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20 On standard trace element discriminant diagrams the mafic volcanic and plutonic rocks plot in the arc-basalt fields. Pearce (2008) Wood (1980)

21 On the MALI diagram of Frost et al. (2008) the rocks plot in the alkali calcic to calcic fields. Note that the same volcanics (Broad River Group) that plotted in the peraluminous field, plot as calcic in this diagram. But look at the plotting index. Another clue to what really happened. The felsic volcanic and plutonic rocks range from slightly metaluminous to relatively strongly peraluminous. The volcanic rocks tend to be more peraluminous. However, the rocks have been metamorphosed to lower greenschist facies.

22 On the standard granitoid discriminant diagrams there is a hint that the younger volcanics and plutonics may be Within-plate (post-orogenic) compared to the older volcanics and plutonics. Pearce et al. (1984) Whalen (1987)

23 Nd isotopes provide an additional clue. For both the older and younger sequences the data fall between depleted mantle and the Proterozoic crust. The Nd isotope data are best explained by a model that has as one end member the SCLM (subcontinental lithospheric mantle) and as the other end member the crust. The amount of crustal input, based solely on isotopes, is estimated to be less than 20%. Samson et al. (2010)

24 The tectonic story for the Caledonia terrane, based on many lines of evidence including sedimentary sequences, is shown in the cartoon below. According to this story the older igneous sequences (620 Ma) were formed in a subduction setting while the younger igneous sequences were formed during slab break-off and extension ( Ma). The Nd isotope data tell us that the mafic melts originated in the subcontinental lithospheric mantle without significant input from either melts or fluids from the subducting slab. Both the older and younger sequences have similar chemical characteristics, despite an inferred change in tectonics. The only chemical hint of this change is the slightly within plate characteristics of the granitoids and felsic volcanics. Pollock et al. (2009) The origin of granites - A Multi-faceted Question!