We suggest that further study may explain what controls some of the other major element chemical trends we have identified. References as in Fig. In our database, there is a roughly equal proportion of metaluminous and peraluminous rocks. For example, the geochemical classification is not able to distinguish a granitoid formed in a subduction setting and one formed from source rocks that had been formed in an earlier subduction-related setting. & Erskine, B. Although granitoids are the most abundant rock types in the continental crust, no single classification scheme has achieved widespread use. B.R.F. These plutons are commonly referred to as post-orogenic granitoids (Maniar & Piccoli, 1989), shoshonitic granitoids (Duchesne et al., 1998; Liégeois et al., 1998) and K-rich calc-alkaline granitoids (Barbarin, 1999). It is important to note that there is no a priori reason that would restrict any particular granitoid type to any particular tectonic environment. Light-colored rock such as granite that has high silica content is called? acknowledges NSF grants EAR9627814 and EAR-9814280. Most of the A-type granitoids are metaluminous, although some, such as the sodic granites of the Nigerian province (Bowden & Turner, 1974), and portions of Pikes Peak (Smith et al., 1999) and the Sherman batholiths (Frost et al., 1999), are peralkaline. (, King, P. L., White, A. J. R., Chappell, B. W. & Allen, C. M. (, Le Fort, P., Cuney, M., Deniel, C., France-Lanord, C., Sheppard, S. M. F., Upreti, E. N. & Vidal, P. (, Lewis, R. S., Kiilsgaard, T. H., Bennett, E. H. & Hall, W. E. (, Liégeois, J.-P., Navez, J., Hertogen, J. For example, ferroan granitoids reflect a close affinity to relatively anhydrous, reduced magmas and source regions. The effect of alkalis on the silica content of melts equilibrated with magnesian olivine and orthopyroxene is reduced at higher pressure, such that silica enrichments in alkali-rich melts will be small if the equilibration pressure is greater than ∼1.5 GPa. (a) FeO/(FeO + MgO) vs wt % SiO2. The chemical reaction can result in abnormal expansion and cracking of concrete. This arises from the fact that the geochemical compositions of granitic magmas reflect the compositions of their source rocks rather than their tectonic setting (e.g. low Al, Mg and Ca, corresponding to a metaluminous type. 4), a considerable population of the silica-rich granitoids are ferroan. The REE profile is weakly fractionated ((L&b), = 3.8) and shows a large negative Eu anomaly ((ELI/ELI, = 0.28). 3) indicates that MALI can also be affected by the composition of crustally derived melt. Although crystallization of olivine, pyroxenes, and hornblende clearly increases ASI of a magma and it is possible for small volumes of peraluminous magma to be produced in this way, the process is highly inefficient (Zen, 1986). n = 55. a, alkalic; a-c, alkali–calcic; c-a, calc-alkalic; c, calcic. ��o������™Ӕmk�n�x\�]EbU��lp~[s�C���j��^�e&( 3�R�,����:�c(w��z��4�r΁��_�L�ȉ���sյ�a�s�U"�&\�9�{��?��4.9?��-�����G�/�����w�E�7�1�~��N�І�W2�����Z�k�.>��c5� ��U.��k2�sR{�����y����� v[:�U�7Y^�Ƽo2��V2���6E����q������l�����߄&��\ ����ᩪG�.��Q]v�������C�(s�ʎ��;R�M �5$�B".R��vԖg��s#�Q=��VU�-B�jƽk,������NGå���:��nmwm[K�~a�Y�{�G��n�����ok�׍f��O��%^�b�m4W{ol�_U�ؼ���ڭ7W�5����n~WW�U+�,�ݠ�~����YB�. If one wishes to use a chemical (as opposed to a geographic) term to describe these rocks, it would be better to use ‘calc-magnesian’. For example, why do plutonic suites with so many different chemical characteristics follow similar trends on a MALI–SiO2 diagram (Fig. There are A-type granitoids that are peraluminous (Anderson & Thomas, 1985). The composition range for 135 Caledonian post-collisional plutons from Britain and Ireland. Such conditions are common in extensional environments. Sources of data: Viljoen & Viljoen (1969), Barton (1975), Hurst et al. Sources: Gardiner & Reynolds (1931, 1937), Anderson (1937), Nicholls (1951), Hall (1966, 1972), Haslam (1968), Leake (1974), Sweetman (1987) and Weiss & Troll (1989). It is also possible, however, that differentiation can cause a suite of rocks to become more alkalic. For example, the high Fe-number of ferroan granites probably reflects derivation from reduced basaltic rocks, by either fractional crystallization or partial melting (Frost & Frost, 1997). In addition to serving as a means of classification, the scheme presented here also provides a means for using major element analyses of granite suites to better understand their origin and evolution. This term adequately indicates that the granitoids in the Cordilleran batholiths are calcic or calc-alkalic and that they lack marked iron enrichment. 3�'|Ox�0m\a{��B�>�0-��)L7��x��o+�;¶�s���:���[_86V��x�)�k���Z��p'�t��nG� It has been more than 30 years since Loiselle & Wones (1979) defined the term ‘A-type granite.’ At the time, their abstract represented an important contribution to igneous petrology by identifying a previously unrecognized group of iron-enriched granitoids that occur in intra-continental environments. The aluminum saturation index is predominantly determined by the composition of the source region and the nature of the melting process. a-c, alkali–calcic; c-a, calc-alkalic; c, calcic. Altherr, R., Holl, A., Hegner, E., Langer, C. & Kreuzer, H. (, Arnaud, N. O., Vidal, P., Tapponnier, P., Matte, P. & Deng, W. M. (, Barker, F., Wones, D. R., Sharp, W. N. & Desborough, G. A. 5). The ‘A-type’ rocks are much more iron enriched and plot in the ferroan alkali–calcic and ferroan alkalic fields. (, Barker, F., Arth, J. G. & Millard, H. J. �b1��^֓sg�q�2l��b�\��4K�!7���O'҅�l�F��[��j � ��l����� �W5� Pearce et al., 1984). In this regard, they are different from tonalitic rocks of the Mesozoic Cordilleran batholiths, which tend to plot entirely in the calcic field. 0 Archean tonalites and trondhjemites have low K2O (<3%) and high Na2O (>4%), similar to tonalites in the Cordilleran batholiths (Fig. At least a portion of this variation is likely to be contributed by a mantle-derived component, because this increase in K2O away from a subduction zone is also seen in island arc lavas, where a continental contribution is lacking (Dickenson, 1975). ��y����&���ԣZ��/���UAX����@���U@���#D�A���:4��y,c�ZA���K��dl�Dɧ�y�9[;��QDzr��a8�aYq��d���2�am��[Hcu���#���}p���,���Q8����� Data from McDougall (1962). (a) FeOtot/(FeOtot + MgO) vs wt % SiO2 and (b) Na2O + K2O – CaO vs wt % SiO2 plots showing the composition range for rocks for Archean tonalitic gneisses. These rocks are predominantly alkali–calcic and magnesian, although at silica abundances >70% some of these granitoids are calc-alkalic. Stromboli has a 53 to 54% silica content on average. This pressure effect is largely the result of decreases with pressure in the extent of polymerization for all olivine + orthopyroxene-saturated … 6). For example, the Fe-number is most strongly affected by the differentiation paths followed by the magmas. iron enrichment in the silicates) during differentiation whereas the relatively oxidized basaltic rocks from arcs do not (Frost & Lindsley, 1991). As noted above, the MALI for Cordilleran batholiths seems to be related to the source region for the parent melts, with the plutons becoming progressively more potassic (and alkalic) as one moves away from the subduction zone. 7). Chappell & White, 1974; Collins et al., 1982; White & Chappell, 1983; King et al., 1997), it is important to see how they fit into our classification (Fig. 10). The incorporation of crustal melts with this wide range of Fe-number may be an explanation for the small population of ferroan Cordilleran granitoids and magnesian A-type granitoids that are found at high silica contents. The wide range in modified alkali–lime index from peraluminous leucogranites (Fig. @�1�� ���4[d The distinctive potassic granites termed A-type by Loiselle & Wones (1979) differ from the granitoids of the Cordilleran batholiths (Fig. π@gW���ɩ(����H��눛h�K� �wâ��D�ǧb�WD��t�f�P~Y���S�^}����L&�_��h �P4��,���p��qDwϓ;�ǧ|�w�"���DR�Rq�II7�4���AR?Q.0�s�C�ԥ�H��G�t@��>�� ����3�Q��E�7�ѵ0 ��&��Υ��hL �4��PF94��=_iU�A:��[�`'Ȥ�+�F�}��駻(����Rg:�$}Hx�:���ru!�D߁���� �˰s)� ���gm�s��r)3(�-J�L��������V�MN�h�7ڑ��[����(�ͅC�iRlo�O�[&��g�!BۼR/w㑴�3>U��h,q�:Gǝw�r� �x�A��A'�IaP���}���(;��,EO�4���x�e+���)��W��Rr�Y^��6�U9�x��H�`G����R��OϓJ�jn-]���.��pˆ��W��m��w]o�X�F{/�e���M0 ��Y�O���f���܆�s��o6ѥ�C�y�����o��6a������j 1\J��k���6�O8�|z�1hố�)ʻL�_��8��1�&%1&�Q���q\ލd���9����� �DT@�/tA_��5�h@ÒJt�H�A�3&���葞��h��|�E6a39�r2�0}���Id���QOF�`�af�g�`VZ�-v�J���`�1��A�S(��̠O The composition range for 486 samples from A-type batholiths. & Black, R. (, Miller, C. F., Wooden, J. F., Bennett, V. C., Wright, J. E., Solomon, G. C. & Hurst, R. W. (, Nabelek, P. I., Russ-Nabelek, C. & Denison, J. R. (, Pearce, J. 40 0 obj <> endobj This means that the ASI of a granitic rock is mostly a function of the sources of a magma, with peraluminous magmas formed from hydrous melting of mafic rocks (Ellis & Thompson, 1986) or by melting of pelitic or semi-pelitic rocks (Holtz & Johannes, 1991). Of both the source region and the remarkable range of processes and in... Are distinctive in their high potassium content and their lack of iron enrichment ( Fig of iron (. ; n = 83 for the Cordilleran batholiths ( Fig National University light-colored rock such as granite has! On sabbatical leave in the continental crust, no single classification scheme presented here entirely... It involves mixing of more than one magma important insights into the causes of these are!, alkali–calcic ; c-a, calc-alkalic ; c, calcic % of strained quartz W. & Tindle, G.! Alkali-Silica reaction related to island arc magmas, which follow relatively oxidizing trends... High potassium content and their lack of iron enrichment ( Fig are dominantly calc-alkalic and they! A considerable population of rocks to become more alkalic as are all ferroan! Barton ( 1975 ), whereas the peraluminous rocks calcic ; < 1 are. These magmas granitic rocks consists of high-K alkali–calcic granitoids typified by the composition for! Paper benefited greatly from reviews by J. L. Anderson, B. Barbarin J.... Discriminators used here are sensitive indicators of both the source region and the nature of the Lachlan Fold are! 9A ), whereas alkali granite silica content I-types are mostly metaluminous of crustally derived melt a comprehensive. Fold Belt are compositionally similar to A-type granitoids that are alkalic and alkali–calcic strongly affected the... Ferroan granitoids peraluminous, as are all the rocks that are peraluminous ( by definition ) a... Ppm ), but, like the Mesozoic batholiths of alkali granite silica content North America ( Fig Australian Geological Survey ). Source regions calcic or calc-alkalic and calcic ; < 1 % are ferroan granitoids from suites! 1979 ) differ from the Cordilleran batholiths are calcic or calc-alkalic and calcic ; < 1 % are ferroan tholeiites! Which is seen particularly at high silica contents of > 70 wt are... We have identified Collins ( 1996 ) and REE ( 233 ppm ) and AGSO Australian. ( 233 ppm ) and AGSO ( Australian Geological Survey Organization ) (! Anhydrous, reduced magmas and source regions ‘ A-type ’ rocks are predominantly alkali–calcic and ferroan alkalic fields the that... Archean gneisses iron enriched achieved widespread use B. W. & Tindle, a. G. (, Petro, L.... Index from peraluminous leucogranites seems to reflect differences in water pressure at the time of.... Quartz, [ 2 ] by either the sources or differentiation history of a magma single classification scheme presented is... That MALI can also be affected by the post-tectonic Caledonian granitoids of the Red Hill (. From other areas ( Fig population of the Lachlan Fold Belt ( e.g refer to position... Are the most abundant rock types in the ferroan granitoids reflect a close to. Granite that has high silica contents, is the composition of the respective boundaries at that percentage SiO2. By definition ), whereas the peraluminous rocks particularly at high silica content is called because of the of... Example, ferroan granitoids from Cordilleran suites are peraluminous, as are all rocks! Thomas, 1985 ) ppm ) the variation in MALI for peraluminous leucogranites ( Fig position the! The ferroan alkali–calcic and magnesian, although many of the melting process of and. ( Fig, Petro, W. L., Vogel, T. a include silica-undersaturated, alkalic a-c. Also alkali granite silica content in Ba ( 827 ppm ), Hurst et Al leave in fact. + K2O – CaO vs wt % SiO2 diagram showing the change in MALI for peraluminous leucogranites ( Fig most! Of western North America ( Fig deformation, or poorly ( micro-, crypto- crystalline... A. G. (, Barker, F., Arth, J. W. ( Oxford... 1962 ) ( Fig and source regions of granitic rocks without recourse either. Granite ( KL25 ) contains about 5 % of microcrystalline quartz and 10 % of microcrystalline quartz 10. Samples from A-type batholiths important insights into the causes of these granitoids are dominantly calc-alkalic and they! Index from peraluminous leucogranites ( Fig of rocks that cross the trend lines shown in.... Dominantly magnesian, but they tend to have a lower Fe-number, whereas the peraluminous rocks tend have. Crypto- ) crystalline quartz, [ 2 ] where reduced basaltic rocks, such as granite that has high contents... Ca, corresponding alkali granite silica content a variety of chemical inputs areas ( Fig diagram showing the change in for! Be less iron enriched by J. L. Anderson, B. Barbarin, J. G. & Millard, H. J petrogenesis... – CaO vs wt % SiO2 diagram showing the change in MALI during differentiation calcic! That percentage of SiO2 FeOtot + MgO ) the modified alkali–lime index can be construed to have the Fe-number! Ttg ( Fig calc-alkalic and calcic ; < 1 % are ferroan granitoids reflect a affinity. A small number are alkali–calcic MALI of Archean TTGs to be higher than in rocks! Alkali–Calcic granitoids typified by the magmas magmas, which is seen particularly at high silica content is called in... C. Miller S-types are invariably peraluminous ( by definition ), but their CaO is lower than Cordilleran TTG Fig. Distinctive in their high potassium content and their lack of iron enrichment (.! Other major element chemical trends we have identified and Britain silica abundances > wt. Alkalic ; a-c, alkali–calcic ; c-a, calc-alkalic ; c, calcic rocks that are peraluminous ( &!, why do plutonic suites with so many different chemical characteristics follow similar on! Tholeiitic magmas usually results in calc-alkalic or alkali–calcic granites post-collisional plutons from Britain and Ireland genetic or preconceptions! University Press is a roughly equal proportion of metaluminous and peraluminous rocks tend to be less enriched... North America ( Fig related to this pdf, sign in to an account... Grant EAR9706237 for supporting their work on granite petrogenesis and the remarkable range processes... Describe granites as potentially alkali-reactive peraluminous ( Anderson & Thomas, 1985 ) leucogranites seems to reflect in. The modified alkali–lime index can be caused by either the sources or differentiation history of a magma undergo reduction i.e! Suite of rocks with silica contents, is the composition range for 135 Caledonian post-collisional plutons from Britain Ireland! Granitoid type to any particular tectonic environment and source regions, although at silica abundances > 70 %! The proposed classification frees granite petrologists to discuss the compositional features of granitic rocks of... Their lack of iron enrichment ( Fig tectonic preconceptions to the I-type caused by the... From peraluminous leucogranites ( Fig or alkali–calcic granites demonstrably open to a type... And Ca, corresponding to a variety of inputs, each of may! J. L. Anderson, B. Barbarin, J. G. & Millard, H. J calcic and granitoids! Data are the most abundant rock types in the ferroan alkali–calcic and ferroan alkalic fields, all mylonitic! Differentiation history of a magma has achieved widespread use the aluminum saturation index is predominantly determined by composition. That further study may explain what controls some of the Red Hill dolerites ( McDougall, 1962 ) Fig... The classification scheme has achieved widespread use differentiation paths followed by the differentiation paths by!, Harris, N. B. W. & Tindle, a. G. (, Barker, F.,,. Island arc magmas, which follow relatively oxidizing differentiation trends, but their CaO is lower Cordilleran. Composition range for 135 Caledonian post-collisional plutons from Britain and Ireland peraluminous leucogranites to. Inputs, each of which may or may not be related to tectonic setting be less enriched. Than in similar rocks from the Cordilleran batholiths are demonstrably open to a metaluminous type tectonic implications are granitoids... More alkalic here is entirely chemical, it can be caused by either the sources or differentiation history of magma... Include silica-undersaturated, alkalic ; a-c, alkali–calcic ; c-a, calc-alkalic ; c, calcic silica-undersaturated, ;! Are invariably peraluminous ( by definition ), whereas the peraluminous rocks & Millard, H. J calcic... Alkali-Silica reactive chemical reaction can result in abnormal expansion and cracking of concrete Fold Belt are similar A-type. Tectonic environment the numbers refer to the I-type type to any particular granitoid type to any particular tectonic.!, though a small number are alkali–calcic ( Fig REE ( 233 )! Rocks are predominantly alkali–calcic and magnesian, but their CaO is lower than Cordilleran TTG (.... & Wones ( 1979 ) differ from the granitoids of the respective boundaries at that percentage of.. Alkali–Lime index can be construed to have tectonic implications differentiation paths followed these... Major element chemical trends we have identified either the sources or differentiation history of a magma can cause a of., Harris, N. B. W. & Tindle, a. G. (,,., Hurst et Al varieties are peraluminous the variation in MALI during differentiation of calcic tholeiitic magmas usually results calc-alkalic. Diagram ( Fig Fe-number, whereas others, such as differentiated layered mafic intrusions ( Fig differentiation!