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Abstract
Interleaved phyllosilicate grains (IPG) of various compositions are widespread in low-grade Verrucano metasediments of the northern Apennines (Italy). They are ellipsoidal or barrel-shaped, up to 300-400µm long and they are often kinked and folded; phyllosilicate packets occur as continuous lamellae or as wedge-shaped layers terminating inside the grain. Using electron microscopy techniques (SEM, TEM) six types of IPG have been distinguished on the basis of their mineralogical composition: (1) Chl+Ms±Kln; (2) Chl+Ms+Pg±Kln; (3) Ms+Prl±Pg; (4) Ms+Prl+Su; (5) Ms+Prl+Chl+Su; (6) Su+Ms. Types (1) and (2) are mainly composed of chlorite, with Ms and Pg as minor phases; Kln grows on Ms in highly weathered samples. Types (3), (4), (5), and (6) are composed of muscovite, with intergrown Prl, Chl, Su and new-formed muscovite. IPG show all kinds of contacts: from coherent grain boundaries with parallel basal planes and along-layer transitions to low- and high-angle grain boundaries.
IPG formed on pristine minerals such as chlorite and muscovite. The transformations took place during the prograde and retrograde metamorphic path of the rocks: they were facilitated by deformation and they occurred in equilibrium with a fluid phase, which allowed cation diffusion. Prograde reactions (Chl=Ms (or Pg); Ms=Prl; Ms=Chl) involve dehydration and sometimes a decrease in volume, whereas retrograde reactions (Ms=Kln; Ms=Su) involve hydration and an increase in volume. These transformations do not simply occur through an interchange of cations, but often involve deep structural changes: transitions from one phyllosilicate to another generally proceed through dissolution-recrystallization reactions. In conclusion, Verrucano IPG represent microstructural sites which have not completely equilibrated with the whole rock and whose mineral assemblage depends on the original composition of the microstructural sites.
Introduction
Composite grains of chlorite and muscovite have been frequently recognized in sedimentary and low-grade metamorphic rocks; they are commonly called stacks, intergrowths, or aggregates. Since Sorby (1853) first described them, these grains have been observed in pelitic and psammitic rocks of different age (Craig et al., 1982; van der Pluijm and Kaars-Sijpesteijn, 1984 and bibliography therein), and various hypotheses have been proposed for their origin: 1) stacks are detrital grains, i.e. clasts which may or may not be modified during weathering and transport (primary origin) (Beutner; 1978); 2) stacks originate from the mimetic growth of chlorite (with or without new-grown muscovite) on a detrital nucleus during diagenetic to metamorphic stages (primary-secondary origin) (Craig et al., 1982; Woodland, 1982; 1985; Dimberline, 1986; White et al.,1985; Milodowski and Zalasiewicz,1991; Li et al., 1994); 3) the aggregates are strain-controlled porphyroblasts formed entirely during metamorphism (secondary origin) (Weber, 1981). This diversity of opinions probably reflects the actual variability in the origin of the aggregates.
This study re-examines the fine-scale interleaved phyllosilicate grains (IPG from Franceschelli et al., 1991) of Verrucano rocks (northern Apennines) which constitute a quartz-arenite facies referred to the beginning of the Triassic rift process (Franceschelli et al., 1986 and bibliography therein). This group of formations experienced low-grade metamorphism (Franceschelli et al., 1986; Giorgetti, 1995) during the Apennine orogeny (Carmignani and Kligfield, 1990). Franceschelli et al. (1989; 1991) suggest that IPG originated during deformation and metamorphism and formed on a detrital precursor. The envisaged mechanism supports the idea that IPG result from an equilibrium process among detrital muscovite, the matrix mineral assemblage and an internally buffered fluid phase. The proposed model for IPG formation on detrital muscovite can also be extended to detrital chlorite.
Our purpose is to clarify the origin of the Verrucano IPG and their evolution during metamorphism using SEM-EDS and TEM. These methods allow detailed textural and chemical analyses.
Specimen description and analytical techniques
Samples examined for this study belong to the pyrophyllite+quartz zone (Franceschelli et al., 1986) and were collected from the Verrucano formations of the Monticiano-Roccastrada Unit which crops out along the mid-Tuscan ridge (Fig.1). The metapelites and quartzites consist of different modal proportions of detrital minerals and new-formed, metamorphic minerals (tab.1).
Detrital minerals include quartz, muscovite, chlorite, minor feldspars and rare carbonates. Syn- to post-tectonic metamorphic minerals (mainly phyllosilicates and chloritoid) crystallize in different microstructural sites: phyllosilicates such as Ms, Chl, Prl, Su (sudoite), Kln (mineral symbols from Kretz, 1983) constitute the rock matrix and grow as fine-grained (<10 µm) crystals underlying the main schistosity, or replace detrital muscovite or chlorite, forming finely intergrown aggregates (IPG; Franceschelli et al., 1991). IPG are ellipsoidal or barrel-shaped, up to 300-400 µm in length. They often maintain characteristics (such as microfolds, kinks, and pressure-solution effects at their borders) which suggest a detrital origin; furthermore, IPG are more abundant in coarse-grained samples (quartzites) than in fine-grained ones (metapelites).
Polished thin sections were prepared for optical observations, SEM study using back-scattered electron (BSE) imaging and X-ray energy dispersive (EDS) analyses. Analyses were performed with an EDAX 9100/70 attached to a Philips 515 scanning electron microscope. The operating conditions were: 15 kV, 20 µA emission current, and a 0.2 µm beam spot size. Natural minerals were used as standards. In particular, several analyses were performed on an homogeneous muscovite crystal during the entire working period of the instrument.
IPG for TEM observations were selected from polished sections using a petrographic microscope; the sections were glued onto Cu grids with a single central hole of 200-400 µm in diameter and thinned by argon ion mill (Gatan Dual Ion Milling 600 at Granada University). Two ion-milling conditions were used: 1) 6 kV, 1A, and 15° incident angle while perforating; 2) 6 kV, low-angle (12°) and low current (0.4A) final milling for ~4 hrs to clean the sample surface. Samples were analysed with two transmission electron microscopes: a Philips CM20, operating at 200 kV, with a LaB filament, and a point to point resolution of 2.7 Å (Granada University), and a Philips EM 400T, operating at 120 kV, with a W filament, and a point to point resolution of ~4 Å (Siena University).
Diffraction patterns were obtained from selected areas (SAED); high-resolution images (HRTEM) were obtained following the procedures suggested by Buseck et al. (1988) and Buseck (1992).
Both microscopes were equipped with a X-ray energy dispersive (EDS) EDAX DX4 which allowed semi-quantitative analyses of areas with a minimum size of 200 Å.
Optical, SEM, and TEM results
IPG (up to 300-400 µm in length) are always coarser than the matrix phyllosilicates, detrital quartz and feldspars. Most IPG are randomly oriented with respect to the bedding and the main schistosity and are sometimes strongly deformed. IPG are composed of different phyllosilicate packets from one to tens of microns thick; although the different phyllosilicates can sometimes be identified optically, they are more easily identified in BSE images. Most packets are intergrown parallel or subparallel to (001); some have a lenticular shape or grow as wedges, which terminate inside the stacks, forming semi-coherent boundaries with the neighboring phyllosilicates. New minerals in the stacks grow preferentially at grain boundaries, in extensional directions and along microfold hinges. The IPG are often weathered; they are coated by reddish hydroxides and Ti-Fe oxides grow along stack borders or between the phyllosilicate lamellae (Fig. 2).
SEM-EDS analyses allowed the identification of 6 types of IPG with different phyllosilicate assemblages (tab.1).
Types (1) and (2) are composed of chlorite packets, which are optically continuous and separated by mica lamellae; types (3), (4), (5), and (6) show complex textures, in which continuous muscovite packets are intersected and split by other phyllosilicates with variable orientations.
Type (1) and type (2) IPG
Type 1 stacks (M62) belong to highly weathered quartzites containing limonitic haloes along veins and fractures and euhedral crystals of siderite, which are almost totally replaced by calcite+dolomite+hematite. The IPG are composed of weathered chlorite with parallel or subparallel intergrown packets of muscovite and lamellae or wedges of kaolinite; hematite can grow at the IPG borders. Type 2 IPG (M669) are made up of chlorite (60 to 70%), muscovite, and minor paragonite. Micas preferentially grow at the stack borders or in packets with (001) parallel or subparallel to the chlorite basal plane. Type 1 and 2 IPG are always embedded in a matrix comprising the same phyllosilicate assemblage (tab.1).
Both IPG types show similar characteristics at the TEM scale, and they are summarized in table 1.
Chl is the most abundant phase and occurs in packets several hundred Å thick (Fig.3a). Electron diffraction patterns show a one-layer polytype with sharp 00l reflections; rows with k≠3n sometimes have weak satellites indicating a 56Å spacing. A 4-layer polytype may therefore be present together with the dominant one-layer polytype (Fig.3b). Lattice fringe images show packets with undeformed and defect-free 14 Å layers (Fig.3a).
Muscovite occurs as a two-layer polytype either as isolated layers, interstratified with chlorite (Fig.3a), or as packets of variable thickness (Fig.4a). Muscovite in M62 IPG occurs as defect-free packets, whereas in M669 IPG lattice fringe images show packets with a typical "mottled structure" (Fig.5a), and defects such as layer splitting or termination can be observed. The textural relationship between chlorite and mica is analogous in the two types of IPG. Chl and mica are generally coherently intergrown with parallel (001) planes. A zone of disordered, interstratified chlorite and mica packets is shown in figure 4a; rarely, low- or high-angle grain boundaries can occur. The lateral transformation from chlorite layers to muscovite (or paragonite) layers is evident in figure 3a. AEM analyses of these mixed-layer packets fall on line between the two end-member mica and chlorite (Fig.4b). The presence of smectite layers, which would result in a higher Si content (as described by Nieto et al., 1994), can therefore be ruled out.
Lenses ~50 Å thick without resolved basal planes are present within the muscovite packets: they are probably paragonite, as electron diffraction patterns display split pairs of 00l reflections with 10.0 Å and 9.6 Å lattice spacing corresponding to Ms and Pg (Fig. 5b). In addition, AEM analyses indicate the presence of muscovite and paragonite. Paragonite can also occur in muscovite-free packets or with subordinate potassium mica. Lattice fringe images are difficult to obtain because paragonite is easily damaged by the electron beam.
Muscovite and paragonite are parallel (Fig.5b) intergrown or form low-angle grain boundaries. Neither compositionally intermediate sodium potassium mica nor basal reflections with an intermediate periodicity are recorded. These features suggest the presence of two single phases rather than an intermediate phase, as described by Jiang and Peacor (1993).
In M62 IPG, kaolinite is present as a one-layer polytype and the presence of dickite or nacrite (two-layer polytypes) can be ruled out. SAED patterns show different degrees of stacking order: reflections may be sharp or weakly streaked along c (Fig. 6a). Kaolinite occur in packets of 7 Å-layers intercalated with muscovite packets (Fig.6b).
Muscovite-kaolinite have either parallel basal planes or slightly different orientation (Fig. 6a). The lateral transition from 10 Å-Ms layers to 7 Å-Kln layers occurs without the formation of an intermediate phase (Fig.6c). Furthermore, relics of 10 Å-layers can be observed inside kaolinite packets, causing deformation contrast of 7 Å-layers (Fig.6b).
Type (3), type (4), type (5) , and type (6) IPG
Type 3 IPG (M7) belong to quartzites with a muscovite, pyrophyllite, ±sudoite, and paragonite (recognized only at the TEM scale) matrix. The IPG generally contain more than 60% muscovite; pyrophyllite grows in packets or wedges inside the Ms crystal whereas paragonite is present in small (<10 µm) lenticular inclusions.
Type 4 IPG (M29) are rare and consist of intimately intergrown Ms+Prl+Su with parallel or subparallel basal planes. The matrix contains only Ms+Prl and rare Pg.
Type 5 (M8) is the most complex type of IPG. These stacks, which may be embedded in a Ms+Prl or Ms+Prl+Su matrix, are characterized by an extremely variable relative abundance of the four phyllosilicates. Chlorite is generally present in IPG as weathered, discontinuous packets surrounded by pyrophyllite and sudoite. Textural relationships between chlorite and muscovite are variable; hematite is present at the border of or inside the IPG. Type (3) and (4) IPG consist of stacks formed on muscovite. Type (5) IPG might actually have formed on an older muscovite or chlorite.
Type 6 IPG are mainly composed of sudoite and only occur in the M39 quartzite where they coexist with a Ms+Su+Prl matrix.
The similar features and relationships among the intergrown phases observed in the different IPG belonging to types (3), (4), and (5) allow for a summary description (tab.1).
Muscovite is characterized by two distinct microstructures: i) lattice fringe images show that Ms is present either as several hundred Ångstrom thick, defect-free packets with straight lattice fringes or as ii) small discrete packets with slightly different orientations along c (Fig.7a-b). Packets with bent, split or interrupted basal planes are characterized by a higher dislocation density. Chemical analyses in different muscovite packets of a single IPG reveal variable compositions; given that the analized areas are structurally homogeneous, we can exclude that this variability is due to contamination by other phases.
Pyrophyllite occurs both as the 2M polytype (Fig.8a) and as the 1T polytype. Prl packets are several hundred Ångstrom thick, often undeformed and defect-free (Fig.8b). Pyrophyllite is easily distinguished from muscovite for its different contrast, lack of "mottled structure", and its lattice fringe spacing (9.2 Å Prl; 10 Å Ms).
Sudoite electron diffraction patterns show both a one-layer and a two-layer polytype (IPG type (4), (5), and (6)). Both the relative intensity of 00l reflections in SAED and the chemical analyses allow sudoite to be distinguished from trioctahedral chlorite. Sudoite forms packets can be parallel intergrown with Ms (Fig.9a-b) or wedge-shaped and terminating inside a muscovite packet (Fig.10).
A one layer polytype of chlorite occurs as a minor phase in type (5) IPG. In some SAED patterns, satellite reflections are visible along c, both in 00l row and in rows with k≠3n. These satellite reflections indicate an ordered stacking sequence: long-period polytypes with 4- or 6-layer periodicity can be recognized.