Appendix: Classification and Petrogenesis of K-Rich Rocks

Appendix: Classification and Petrogenesis of K-rich Rocks Classification and Nomenclature Potassium-rich rocks are found in several tectonic environments, including continental cratons, active subduction zones and post-collisional settings (e.g. Müller and Grove 2000). They are limited in abundance over a global scale, but are widespread in Italy. Potassium-rich rocks cover a very wide range of petrological and geochemical compositions, from alkaline to transitional, from strongly undersaturated to oversaturated in silica, from ultra- basic and basic to intermediate. Mineralogical composition is even more variable, being dependent not only on the chemical composition of magmas but also on the pressure-temperature conditions of crystallisation (Yoder 1986). This makes rocks with similar major and trace element compositions display sometimes different mineralogical compositions (heteromorsphism). In order to overcome this problem, it is appropriate to classify potassic rocks on the basis of chemical parameters. The K2O vs. SiO2 of Peccerillo and Taylor (1976; Fig. A.1) is useful to classify rocks from arc environments, including the mildly potassic shoshonites. The TAS diagram of Le Maitre (1989) is also useful for naming potassic and ultrapotassic rocks (Fig. A.2). In this book, nomenclature will follow, as close as possible, these diagrams. However, the extreme compositional variability which pertains to all the major petrological parameters in addition to silica and alkalis, makes K2O vs. SiO2 and TAS diagrams insufficient to express pet- rochemical affinities and variability of potassium-rich magmas. 318 Appendix: Classification and Petrogenesis of K-rich Rocks Fig. A.1. K2O vs. SiO2 classification grids for arc rocks. 1: arc tholeiitic series; 2a: calc-alkaline series; 2b: high-K calc-alkaline series; Field 3: shoshonitic series. Modified from Peccerillo and Taylor (1976). Fig. A.2. Total alkali vs. silica (TAS) classification diagram of volcanic rocks (modified from Le Maitre 1989). Rocks falling in the shaded area may be subdi- vided as shown in the inset table, on the basis of their sodic (Na2O-2.0 K2O) or potassic (Na2O-2.0 K2O) affinity. Classification and Nomenclature 319 In general, potassic rocks are defined as those that have K2O higher about 2 wt % and K2O/Na2O = 1.0-2.5 at a MgO > 3-4 wt % and SiO2 < 55-57 wt %. Ultrapotassic rocks have K2O > 3 wt % and K2O/Na2O > 2.5- 3.0 at a MgO > 3-4 wt % and SiO2 < 55-57 wt % (see Chap. 1, Fig. 1.3). The restriction in the MgO and SiO2 contents is required to exclude silicic rocks (e.g. dacite, rhyolite) whose high K2O and K2O/Na2O ratios relate to magma evolutionary processes and do not denote a potassic nature for their parental melts. The terms potassic or potassium-rich are loosely used to indicate both potassic and ultrapotassic rocks. Although there is a continuum in K2O enrichment in potassium-rich rocks, the following groups can be distinguished (e.g. Peccerillo and Tay- lor 1976; Morrison 1980; Foley et al. 1987): 1 - Shoshonites; 2 - Roman- type potassic series (KS); 3 - Roman-type high potassic series (HKS); 4 - Lamproites; 5 - Kamafugites. An additional group could be represented by Group-2 micaceous kimberlites, which have K2O contents and K2O/Na2O ratios comparable to those of some potassic rocks. Shoshonites are slightly silica-oversaturated to undersaturated rocks. They have K2O around 1.5-2.5 wt % (Peccerillo and Taylor 1976) and K2O/Na2O around unity in the mafic compositions. According to Morrison (1980), the shoshonitic suite should include only those rocks associated with calc-alkaline volcanics along active plate margins. However, Joplin (1968), who coined the term "shoshonitic suite", also included in this group moderately K-rich rocks occurring in some intraplate environments, where they represent the potassic equivalents of Na-transitional rocks. The shoshonitic suite includes mafic to silicic rocks. These fall in the shoshonitic basalts, shoshonite, latite, trachyte fields on the K2O vs. SiO2 diagram and in the K-trachybasalt, shoshonite, latite, trachyte fields of the TAS diagram. Some K-rich alkaline rhyolites may also belong to this suite. Roman-type potassic series (KS) has slightly higher K2O% (about 2.5-3.0 in the mafic range) and K2O/Na2O (1.5-2.5) than shoshonites. KS rocks are saturated to slightly undersaturated in silica. The entire rock suite includes trachybasalts, shoshonite, latite and trachyte. Roman-type high-potassium series (HKS) rocks are ultrapotassic (K2O > 3 wt %, K2O/Na2O > 2.5 in the mafic rocks) undersaturated in silica, rich in Al2O3 (12-20 wt % ca) and depleted in TiO2 (generally < 1.2 wt %). The mafic types have high CaO, typically around 10-13 wt % and Na2O around 2-3 wt %. HKS consists of the well known leucitite, leucite- tephrite, to leucite-phonolite series widely occurring in several Italian vol- canoes. Compositions of these rocks fall in the foidite, basanite-tephrite, phonotephrite, tephriphonolite and phonolite fields in the TAS diagram. 320 Appendix: Classification and Petrogenesis of K-rich Rocks Lamproites are ultrapotassic, slightly undersaturated to oversaturated in silica and typically have low Al2O3 (< 10-11 wt %), Na2O (< 1.5-2 wt %) and CaO (< 6-7 wt %) in the mafic compositions. SiO2 is variable, from less than 45% to about 60 wt%. This variation does not seem to depend on magma evolution processes but probably reflects different pressure of genesis in the upper mantle. In fact, Mg# is generally high to very high in most low-silica and high-silica lamproites, sometimes reaching 75-80. The mineralogy of lamproites consists of highly magnesian olivine, Al-poor diopsidic pyroxene, phlogopite, sanidine, K-richterite, leucite and several 2+ uncommon phases such as perovskite [(Ca,Na,Fe ,Ce)(Ti,Nb)O3], jeppeite 3+ [(K,Ba)2(Ti,Fe )6O13], wadeite (K4Zr2Si6O18), shcherbakovite 3+ [(Ba,K)(K,Na)Na(Ti,Fe,Nb,Zr)2Si4O14] priderite [(K,Ba)(TiFe )8O16] and armacolite [(Mg,Fe)Ti2O5]. Lamproitic rocks take special names, which are not reported on the TAS diagram. Kamafugites owe their name to the katungite-mafurite-ugandite series of eastern Africa. They are ultrapotassic and share with lamproites low Al2O3 and Na2O and high MgO abundances. However, they are rich in CaO (up to 18 wt %) and strongly undersaturated in silica. Typical minerals include melilite, leucite, kalsilite, Mg-rich olivine, diopside, monticel- lite, phlogopite and perovskite. Most kamafugites fall in the foidite field in the TAS diagram, but special names are often used in the nomenclature of kamafugites. 8.1. Petrogenesis of Potassium-rich Magmas The genesis of potassic and ultrapotassic magmas has been a much debated issue since early times of igneous petrology (see Peccerillo 1992 for a re- view). Modern hypotheses suggest genesis by various degrees of partial melting in anomalous mantle sources, heterogeneously enriched in incom- patible elements and with variable isotopic signatures. High potassium contents require that one or more minerals rich in this element, such as phlogopite or K-richterite, is present in the mantle and melts preferentially during magma formation (e.g. Edgar et al. 1976; Gupta and Fyfe 2003). These are not normal mantle phases and their occurrence suggests metasomatic enrichment in potassium. This can be accomplished by several processes such as addition to the lithospheric mantle of fluids or melts coming from the deep mantle (e.g. from the asthenosphere), addition of crustal material from subducting slabs, etc. The variable isotopic and trace element compositions of world-wide potassic and ultrapotassic magmas Petrogenesis of Potassium-rich Magmas 321 suggest that different amounts and types of metasomatising agents acted on the sources of potassic magmas in various areas. The variable concentrations in potassic magmas of some major oxides such as CaO, Na2O, Al2O3, require that the peridotite contained different amounts of phases that hosted these elements. Therefore, the low CaO and Al2O3 contents of lamproites indicate that clinopyroxene was scarce or ab- sent in the source of these magmas. In contrast, the high concentrations of these oxides in Roman-type KS and HKS rocks and in kamafugites suggest that clinopyroxene was the main phase of the mantle sources of these magmas. In summary, lamproites are believed to derive from a peridotite which contained phlogopite- and/or K-richterite but was depleted in clinopyroxene (i.e. phlogopite-bearing harzburgite), whereas Roman-type rocks and kamafugites derive from cpx-bearing rocks such as phlogopite- lherzolite, to phlogopite-pyroxenite. The scarcity of clinopyroxene in the mantle source of lamproites can be related to old, pre-enrichment melting events and extraction of basaltic magmas, which had generated preferential melting and removal of clinopyroxene. Clinopyroxene-rich mineralogy of the mantle sources of kamafugites and Roman-type KS and HKS are most likely related to metasomatic events by Ca-rich melts (e.g. carbonatites) or by carbonate-rich sediments. 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