Geochemical characteristics of zircons in the Ashizuri A‐type granitoids: An additional granite topology tool for detrital zircon studies

Abstract

Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I-, S-, M-, and A-types). To clarify geochemical characteristics of zircons in A-type granites, trace element compositions of zircons extracted from the A-type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A-type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A-type granite. Based on compilations of available data, zircons from A-type granites can be clearly discriminated from other-types of granites within Nb/Sr-Eu anomaly, U/Sr-Eu anomaly, Nb/Sr-U/Sr, and Nb/Sr-Ta/Sr cross-plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A-type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A-type granites, implying the existence of A-type magmatism in the Earth's earliest history. K E Y W O R D SAshizuri, A-type granite, HFSE, REE, zircon | INTRODUCTIONZircons are often used as geochronological and geochemical indicators for constraining the nature of their host rocks. Recent developments of analytical methods enable us to extract information concerning the host rock from a single zircon grain. As one such example, contribution rates of mature sediments into granitoid magma through geologic time have been examined based on compilations of oxygen isotope ratios of zircons extracted from granites (Valley et al., 2005) and of detrital zircons in river mouth sands (Iizuka et al., 2013). Belousova, Griffin, O'Reilly, and Fisher (2002) examined trace element contents in zircons from a variety of rock types and proposed some discrimination diagrams, but found that most rocks are inseparable except for peculiar rocks (kimberlite, carbonatite, and nepheline-syenite). Basically, most detrital zircons that accumulate at river mouths are derived primarily from granitoids (Hawkesworth, Cawood, Kemp, Storey, & Dhuime, 2009), and their U-Pb age populations reflect the age variation of the hinterlan...

O Batólito Catolé do Rocha (RN-PB): Um magmatismo granítico do tipo-A2 reduzido no Domínio Rio Piranhas-Seridó, Província Borborema, Nordeste do Brasil

“…Nb abundances are up to 2.6 ppm. The U/Th (~0.5-0.6) and Nb/Sr (~7.1) ratios are equivalent to those found in zircon crystals of A-type granites by Breiter et al (2014) and Sawaki et al (2017), respectively.…”

“…Zircon trace element composition has been used as a key in granite typological studies (e.g., Wang et al, 2012;Breiter et al, 2014;Sawaki et al, 2017). In A-type granites, zircon is characterized by relatively high Ce, Nb, Ta, U, Th, Hf and Y and low Sr and Eu contents, among other elements (Breiter et al, 2014;Sawaki et al, 2017). The analyzed CRB zircon (n = 2, facies I) register high Hf (up to 7,158 ppm), Y (up to 1634 ppm), Th (up to 214 ppm), and U (up to 120 ppm), and low Sr contents (0.4 ppm) ( Figure 8A).…”

A novel approach to identifying mantle-equilibrated zircon by using trace element chemistry

“…The distribution of Eu/Eu* broadly correlates to types of parental magma. Zircon from S-and A-type granitoids have a distinctly lower Eu/Eu* < 0.1 than zircon from I-type granitoids (Sawaki et al, 2017;. Therefore, mantleequilibrated zircon are expected to have Eu/Eu* between 0.1 and 0.…”

A Novel Approach to Identifying Mantle‐Equilibrated Zircon by Using Trace Element Chemistry

“…The distribution of Eu/Eu* broadly correlates to types of parental magma. Zircon from S‐ and A‐type granitoids have a distinctly lower Eu/Eu* < 0.1 than zircon from I‐type granitoids (Sawaki et al., 2017; Wang et al., 2012). Therefore, mantle‐equilibrated zircon are expected to have Eu/Eu* between 0.1 and 0.6.…”