Caminito de la escuela

7.2.1 Summary of geochemistry

The GRV and HS are a silicic-dominated SLIP characterised by high K₂O, REE

(except Eu), Y, HFSE, Ga/Al and K/Na, and moderate to high Fe#, and by low CaO, Sr, Ba, Ni, and Cr. In primitive mantle-normalised plots, strong negative anomalies of Sr, Ba and Eu are shown in rhyolites. The GRV are metaluminous to peraluminous (chapter 4, Table 7.2), and alkali-calcic to calc-alkalic (Peacock, 1931; Frost et al., 2001).

Quartz-hosted melt inclusions from the lower GRV (this study) and upper GRV (Bath, 2005) have generally similar compositions to the rhyolites, but have lower contents of compatible elements (Sr, Ba, Eu, and Ti), and locally higher incompatible elements (Th, Nb, and Rb). Melt inclusions have high concentrations of F (up to 1.3 wt.%), moderate Cl (up to 0.4 wt.%) and low S and P (<a few hundreds ppm). Water contents calculated by difference method (Devine et al., 1995) is low for felsic magmas (average ~1-3 wt.% in homogenised inclusions). The water-poor magma composition is also indicated by the mostly anhydrous phenocryst parageneses (feldspar, ±quartz, ±clinopyroxene, Fe- Ti oxide, apatite in intermediate to felsic rocks). Good correlation between Cl – a highly volatile element (Carroll and Webster, 1994) – and incompatible elements (e.g. Pb, U, Th) in melt inclusions is compatible with volatile-undersaturation of the magma.

Comparison of whole-rock and melt inclusion data indicates that, despite weak to moderate alteration of feldspar and pyroxene, major elements have only partially been

modifi ed in whole-rock compositions (Na in particular). Mobility of some trace elements

(Pb, U, and Sn) is suggested by erratic distributions and variable depletion of whole-rock analyses in comparison with melt inclusions.

Fluorine and F/Cl of the melt are typically high (F ≤1.3 wt.%, Cl ≤0.2-0.4 wt.%),

and inferred water content is low, as suggested by high melt inclusion microprobe totals and anhydrous parageneses. Feldspar crystallised between hyper- and sub-solvus conditions, originating either one alkali-feldspar or two-feldspar assemblages.

The passage between the lower and upper GRV is marked by an increase of REE and HFSE (Th, Zr, Hf) content, and Fe# (Table 7.1). Plots of Ga/Al, HFSE, and Fe# indicate that the long-term compositional evolution of the GRV marked an evolution from

transitional to A-type.

7.2.2 Halogen content of magmas and implications on the geodynamic setting of the Gawler SLIP

Abundances of F and Cl of magmas vary signifi cantly according to the tectonic

environment. Arc magmas typically have low F/Cl, as shown by degassing volcanos and melt inclusion data (e.g. Aiuppa, 2009; Dunbar et al., 1989; Symonds et al., 1990; Wallace, 2005; Witter et al., 2005). This is believed to be the consequence of Cl introduced by recycling of subducted sediments and dehydration of the subducting slab into the mantle wedge (Aiuppa et al., 2009). Furthermore, arc magmas tend to have high water contents

and high oxygen fugacity, both characteristics that do not fi t the Gawler SLIP and other

intracontinental, “A-type” magmas.

Conversely, magmas produced in extensional and continental settings are known to be water-poor, high in F/Cl and low in oxygen fugacity (Aiuppa et al., 2009). For example, high F/Cl ratios are typical of the Proterozoic rapakivi granites of Finland and

Table 7.2. Main chemical and petrographic characteristics of the GRV and HS at Kokatha

Lower GRV Upper GRV Hiltaba Suite (Kokatha)

Paragenesis feld ±Qtz ±Cpx +Zrn +Fe-Ti

ox +Ap feld ±Cpx ±Qtz +Zrn +Fe-Ti ox +Ap (5) feld +Qtz +Fe-Ti ox +Ap +Fl +Zrn +Bt (8)

ASI metaluminous to

peraluminous (increasing with increasing SiO2)

metaluminous to peraluminous (increasing with increasing SiO2) (1, 2, 3)

metaluminous to peraluminous (3)

FeO# magnesian to ferroan (Frost

et al., 2001) ferroan (Frost et al., 2001) (1, 2, 3) magnesian to ferroan (Frost et al., 2001) (3) K2O high- to ultra-K, K/Na >1 high- to ultra-K, K/Na >1 (1,

2, 3) high- to ultra-K, K/Na >1 (3)

Tot Alkali silica-undersaturated to saturated (TAS, Le Bas et al., 1986)

silica-undersaturated to saturated (TAS, Le Bas et al., 1986) (1, 2, 3)

silica-undersaturated to saturated (TAS, Le Bas et al., 1986) (3)

Alkali versus CaO Calc-alkalic to alkali-calcic (Peacock, 1931)

REE and HFSE moderate high (WPG of Pearce, 1984;

A-type of Whalen et al., 1987) (1, 2, 3)

high (WPG of Pearce, 1984; A-type of Whalen et al., 1987) (3)

Other trace elements (primitive mantle-normalised)

Sr, Ba, Ti, P, ±Eu, Nb, Ta negative spikes; Th, ±Pb, ±Sn positive spikes

Sr, Ba, Ti, P, ±Eu, Nb, Ta negative spikes, Pb positive spikes (1, 2, 3)

Sr, Ba, Ti, P, ±Eu, Nb, Ta negative spikes

Volatile

components high F (≤1.3 wt%), moderate Cl (≤0.2 wt%), low H2O (≤2 wt% by diff erence)

high F (≤1.3 wt%), moderate Cl (≤0.4 wt%), low H2O (≤2 wt% by diff erence and paragenesis) (6, 7)

Temperature ≤950°C (Zrn saturation) ≤1050°C (Zrn saturation) ≤°C (Zrn saturation) 900-1100°C (two pyroxene)

(7) Emplacement

mechanism felsic (+mafi c) lavas, felsic ignimbrites felsic lavas

References: (1) Giles, 1988; (2) Stewart, 1994; (3) PIRSA, unpublished dataset; (4) Giles, 1988; (5) Blissett, 1993; (6) Bath, 2005; (7) Creaser and White, 1991; (8) Flint, 1993

Cenozoic topaz rhyolites of the US (e.g. Christiansen et al., 2007). Ratios of F/Cl >1 have been measured in continental peralkaline rhyolites (Bailey, 1980), F/Cl >3 have

been measured in topaz rhyolites (Christiansen et al., 1983), and F/Cl ≥10 have been

measured in tin- and topaz-rhyolites and tin-granites (Webster et al., 2004).

Hypotheses for the high halogen contents and high F/Cl ratios in these rocks may involve source rock characteristics, magma fractionation, degassing during eruption, or a combination of these.

In the Gawler SLIP, melt inclusions indicate that high halogens and high F/Cl are primary characteristics of the magma, and are not related to preferential devolatilisation of Cl during eruption/emplacement (e.g. Christiansen et al., 1986; Webster, 1992). Thus the hypothesis of a degassing-related depletion of Cl (hypothesis 1) can be discarded.

Concentrations of F and Cl of igneous rocks are also controlled by fractionation processes (e.g. Christiansen, 2007). Fluorine mostly behaves as an incompatible element during crystal fractionation, and tends to partition in the silicate melt during separation of

a fl uid phase (Carroll and Webster, 1994; Webster, 1990). Thus, F will concentrate in the

melt with progressing crystallisation and, because of its high solubility in silicate melts (Dolejs and Baker, 2007), it can reach high abundances (e.g. tin- and topaz-rhyolites,

and pegmatites; Webster and Duffi eld, 1994; Thomas et al., 2005). Chlorine is also

incompatible with respect to most crystal phases, but has a preference for the fl uid phase

in equilibrium with the melt (e.g. Webster and Holloway, 1990). The highest values of Cl concentration (>1 wt.%) occur in intermediate to felsic alkaline rocks (e.g. Lowenstern, 1994), with the exception of the even higher values of kimberlites (Kamenetsky et al., 2007).

This fractionation-induced enrichment in halogens is also valid for the Gawler SLIP in general, as shown by melt inclusion data. In the Gawler SLIP magmas, Cl was more incompatible than F, as shown by correlations with other incompatible trace elements (cf. Cl vs Pb plot, Fig. 4.15). During fractionation, F content was partially buffered by

crystallisation of F-apatite, ±fl uorite, and possibly other F-bearing magmatic phases.

Therefore increase of F/Cl cannot be a result of protracted fractionation. This requires that the magma in its less evolved stages of fractionation, and possibly the source rocks, had a high F/Cl ratio.

This introduces the problem of how a high-F and high-F/Cl source rock can be produced in the crust. Christiansen (2007) pointed out that, during the process of emplacement and crystallisation of magmas, F is preferentially retained by phosphates

and silicates, whereas Cl is lost to a high extent in hydrothermal fl uids. Further, preferential

degassing of Cl over F is observed in modern volcanic systems (e.g. Nicotra et al., 2010),

in agreement with higher distribution coeffi cient between gas phase and melt for Cl than

that for F (e.g., Kilinc & Burnham, 1972; Webster & Holloway, 1988). Thus, even magmas with a low F/Cl may crystallise to form rocks with higher F/Cl ratio values. Remelting of these rocks can produce magmas with high F/Cl; and the halogen concentrations of these magmas could then be enhanced by crystallisation.