Mineral Chemistry
Mineral compositions have been determined within selected micro-structural sites in order to reveal the transformation pathways accompanying fabric evolution, and to support estimates of PT conditions during successive foliation-forming episodes and the related mineral equilibrations. Compositional variations in minerals have been determined using a Jeol, JXA-8200 electron microprobe (WDS, accelerating voltage of 15 kV, beam current of 15 nA) operating at the Earth Sciences Department “A. Desio” of Milano University. Natural silicates have been used as standards and the results were processed for matrix effects using a conventional ZAF procedure. Proportional formulae have been calculated on the basis of: 23 oxygens for Amp, 6 oxygens for Cpx, 12 oxygens for Grt and Ep, 11 oxygens for micas, 28 oxygens for Chl, 8 oxygens for Fsp and 10 for Ttn. Fe3+ was determined for Amp, Grt and Cpx; cations are in atoms per formula unit (a.p.f.u.). Mineral compositions synthesised below, are detailed in Table 2 and in diagrams of Figures 10-14, which show the compositional trends for mineral phases such as Amp, Cpx, Grt and white micas that are significant for thermobarometric estimates.
Amphiboles are scattered in composition over the edenite-hornblende, glaucophane, and actinolite fields. Most of them have edenite-hornblende compositions, and only a few data cluster around the glaucophane end-member, as shown in Figures 10a and 10c, where the compositional gap described by Ungaretti et al. (1983) between Ca- and Na-amphiboles, is evident only in metagranitoids and eclogites. Glaucophane is the most common composition for syn-D1 and syn-D2 amphiboles and only in a few cases amphiboles show an edenitic composition in “grey-type” metagranitoids and paragneisses. Syn-D3 compositions range from edenitic to pargasitic, whereas in syn-D5 the edenite content decreases up to actinolite composition. A decrease in AlVI associated with an increase in AlIV mark the transition from syn-D2 to syn-D3 amphiboles, whereas a strong decrease in Altot characterises the amphibole re-equilibration during D5 (Figs. 10b and 10d). Ti content generally ranges between 0.01 and 0.04 a.p.f.u. and Na(B) in NaCa-amphibole varies from 1.48 to 0.06 a.p.f.u. from syn-D3 to syn-D5, respectively.
Table 2. Representative compositions of Amp, Cpx, Grt and Wm in micaschists (= Micas.), paragneisses (= Parag.), eclogites (= Eclog.), zoisitites (Zoisit.), metaquartzdiorite (= Qzdior.), grey-type metagranitoids (= “grey” Gran.) and green-type metagranitoids (= “green” Gran.).
Figure 10. Compositional range of amphiboles from metaintrusives (a, b) and metasediments (c, d).
Stars locate end-member composition (Act = actinolite; Brs = barroisite; Ed = edenite; Gln = glaucophane; Ktp = katophorite; Prg = pargasite; Ts = tschermakite; Win = winchite). Straight lines define the amphibole trends for high and intermediate pressure, from Vermont (Laird & Albee, 1981). Amphibole composition trends during Alpine evolution is shown by the grey arrows on the Al(VI) vs Al(IV) diagrams (b, d). Different colors identify rock types and simbols deformation stages: grey-type metagranitoids in red; green-type metagranitoids in orange; eclogites in green; metaquartzdiorites in violet; micaschists in light-blue; paragneisses in brown; D1 = full triangle; D2 = open triangle; D3 = full box; D4 = open box; D5 = open diamond. Full triangle identifies pre-D3 grains in eclogites.
Clinopyroxenes vary in composition from Jd to Di, in a range from Jd98, to Jd15 (Fig. 11). The highest acmitic values occur in Cpx from some “grey-type” metagranitoids. Jd-content in both grey and green metagranitoid types, is also controlled by the bulk chemistry as shown by the two clusters of pre-D3 and syn-D1 and D2 grains in Figure 11. In green-type metagranitoids and in eclogites a weak increase in Jd and a decrease in Acm contents characterise the transition from syn-D1 to syn-D2 Cpx. Similarly, a Jd increment characterises the transition from syn-D1 to syn-D2 Cpx compositions in paragneisses and micaschists. A higher Di content distinguishes post-D3 Cpx, both in eclogites and paragneisses. XMg varies from 0.01 to 0.62 with higher values in syn-D3 grains.
Garnets, as inferred microstructurally, grew during both Alpine and pre-Alpine metamorphic evolutions, and pre-Alpine relicts have been recognized in paragneisses (open circles in Fig. 12 and zoning profile in Fig. 13). The transition from pre-Alpine to Alpine syn D1-D2 garnets is characterised by an Alm, Sps and Adr decrease and a Grs and Prp increase. The transition from syn-D1 to syn-D2 Grt in paragneisses is marked by a Sps decrease associated with a Grs increase, while Grt occurring in micaschists (Fig. 12) shows an homogeneous composition with Alm 65-70, Prp 10-20 and Grs 15-25; also syn-D1 Grt megablasts in zoisitites (Fig. 13) show a quite uniform compositional profile. The strong heterogeneous composition of pre-D3 garnets in “grey-type” metagranitoids is due to their microstructural position. As already described, these Grt develop in coronas at the boundary between Pl and Bt sites and the highest Grs values occur in Grt growing on the Pl site from those replacing Bt, that are characterised by higher Alm+Prp contents (Figs. 12 and 13), as already described by Koons et al. (1987). Grt filling the veins at high angle with mylonitic foliation in D1 shear zones are characterised by very low amounts of Prp and Sps (<5%). In green-type metagranitoids and metaquartzdiorite, the Alm+Prp content decreases and Grs increases characterise the transition from syn-D1 to syn-D2 compositions. In eclogites, Alm ranges from 60 and 65%, Prp from 10 and 20%, Grs from 20 to 30 % and Sps is <5%.
Figure 11. Pyroxene compositions according to Morimoto (1988) for Cpx in metaintrusives and metasediments.
Different colors identify rock types and simbols deformation stages: grey-type metagranitoids in red; green-type metagranitoids in orange; eclogites in green; metaquartzdiorites in violet; micaschists in light-blue; paragneisses in brown; D1 = full triangle; D2 = open triangle; D3 = full box; D4 = open box; D5 = open diamond. In grey-type metagranitois full triangle identifies pre-D3 grains. See discussion in the text.
Figure 12. Garnet compositional variations in metasediments and metaintrusive rocks.
Different colors identify rock types and symbols deformation stages, respectively: grey-type metagranitoids in red; green-type metagranitoids in orange; eclogites in green; metaquartzdiorites in violet; micaschists in light-blue; paragneisses in brown; D1 = full triangle; D2 = open triangle; syn-D1 veins = yellow cross; D3 = full box; D4 = open box; D5 = open diamond. In grey-type metagranitois full triangle identifies pre-D3 grains. See discussion in the text.
Figure 13. Examples of compositional zoning in Grt from pargneisses, zoisitites and grey-type metagranitoids.
Paragneisses: zoning from pre-Alpine core (a: pre-D1) to Alpine rim (b: syn-D1-D2). Zoisitites: Rim-to-rim (a to b) compositional profile across a syn-D1 megablast. Grey-type metagranitoids: variations in composition across a Grt corona developed between the Pl (a) and Bt (b) site.
White micas consist of paragonite and phengite (Fig. 14) with Ca never exceeding 0.03 a.p.f.u.; K content in paragonite is up to 0.09 a.p.f.u., whereas Pg is ≤ 0.13 a.p.f.u. in phengite. Eclogites contain phengitic micas that show the highest Si4+ content (3.35–3.55 a.p.f.u.), with the highest values in syn-D2 grains. In metagranitoids, Si4+ varies from 3.2 to 3.5 a.p.f.u. from syn-D5 to syn-D1/D2 grains. In metasediments, the compositional variations in Ph are lower than those of metaintrusives (3.35<Si4+<3.45 a.p.f.u.). Mg values are 0.25-0.4 a.p.f.u. in Ph from metasediments and more scattered (0.2-0.8 a.p.f.u.) in those from metaintrusives, where a higher variation in the Fe3+ content is suggested by the deviation from the Ph-Lc trend in the diagram (Fig. 14) of Massonne & Schreyer (1987). Ti content does not exceed the value of 0.24 a.p.f.u.: in grey-type metagranitoids 0.10<Ti<0.24 a.p.f.u. where Ph replaces igneous Bt and Ti ≤ 0.10 a.p.f.u. in all the other micro-sites.
Figure 14. Phengite compositions in metasediments and intrusive rocks displayed in the ternary diagram of Massonne & Schreyer (1987) and in the Si/Mg (in a.p.f.u.) plot.
Stars locate end-member composition (Ph = phengite; Lc = leucophyllite). Different colors identify the rock types and symbols the deformation stages: grey-type metagranitoids in red; green-type metagranitoids in orange; eclogites in green; metaquartzdiorites in violet; micaschists in light-blue; paragneisses in brown; D1 = full triangle; D2 = open triangle; D3 = full box; D4 = open box; D5 = open diamond. In grey-type metagranitois full triangle identifies pre-D3 grains. In metagranitoids a higher content in Fe3+ is suggested by the deviation from the Ph-Lc tie-line.
Chlorite has XMg values between 0.73 and 0.48 and Si ranges from 2.78 to 2.98 a.p.f.u.; Mg-richer Chl occurs in metaquartzdiorites. Epidotes shows Fe2O3 up to 15.93 wt.% and MnO is lower than 0.21 wt.%; generally syn-D3 Ep are Czo and pre-D3 Zo, whereas greenschist-facies epidote are Fe-Ep. Alpine plagioclase has An < 0.05 and Or < 0.01 and titanite has Al2O3 ranging from 2.03 to 2.11 wt %.