Geological and structural setting of the Kanchenjunga area

As concerning previous geologic studies in far east Nepal, the regional antiformal shape of the LHS beneath the GHS in the Kanchenjunga area could be already envisaged in the large-scale map drawn by Shresta et al. (1984). In the 1990’s, Schelling and Arita (1991) and Schelling (1992) significantly improved the tectonostratigraphy of the area: in particular, these authors placed the MCT between peculiar augen-gneiss (reported as the Sisne Khola Augen Gneiss) located at the upper structural levels of the LHS, and strongly-foliated garnet-biotite schists and gneisses (reported as the Junbesi Paragneiss) located at the base of the GHS. Furthermore, Schelling (1992) suggested that the MCTZ accommodated about 150 km of crustal shortening during thrusting of the GHS over the LHS.

More recently, Goscombe and Hand (2000) and Goscombe et al. (2006) placed the MCT at the base of the augen gneiss and envisaged at higher structural levels, within the GHS, the High Himal Thrust, a major structure controlling the metamorphic and deformational architecture of the orogen. The most recent paper about the Kanchenjunga area is a petrologic study of Imayama et al. (2010), which provides a metamorphic P-T profile across the western portion of the area described in this paper.

As concerning our contribution to clarifying the geologic setting of far east Nepal, a new geologic map of the western flank of the Kangchenjunga massif, based on field investigations and structural-petrographic studies and preliminary presented by Mosca et al. (2011) is reported in Fig. 2. In the map, the lithologies exposed in the Kanchenjunga area are ascribed to two tectonostratigraphic domains, showing at a regional scale both compositional layering and composite foliations dipping mainly to the N-NE and to the W. These are the LHS and the GHS, the latter being subdivided into the structurally lower Inverted Metamorphic Sequence (IMS) and in the structurally upper Higher Himalayan Crystallines (HHC). In particular, the IMS (described in detail below) identifies an heterogeneous and strongly mylonitic group of lithologies characterized by an increase in metamorphic grade towards upper structural levels and resting between typical schists of the LHS at its bottom and peculiar Grt + Kfs + Ky + Sil anatectic paragneiss of the HHC at its top (i.e. Barun Gneiss; Groppo et al., 2012).

In the following, the mapped lithologies ascribed to the LHS, IMS and HHC are described from the lower to the upper structural levels (Fig. 2); Figs. 3 to 5 deal in particular with their main representative mesostructures. Microstructures of these rocks are shown by Figs. 6 to 9 in the next section, where petrography and mineral chemistry of the samples used in the “Average P-T” calculations will be detailed and discussed exhaustively.

Lesser Himalayan Sequence (LHS)

The LHS mainly consists of grey to pale-green fine-grained quartz-sericite schists, slates and phyllites (Fig. 3a), showing m-scale intercalations of either massive quartzites (±Grt ±Ctd) or chlorite-sericite schists. Cm-scale intercalations of garnet-amphibole quartzites are also observed, usually in the uppermost structural levels of the LHS, while graphite-rich schists occur locally towards the lower structural levels.

Figure 3. Representative mesostructures of the LHS

Representative mesostructures of the LHS

(a) Pale-green fine-grained slates showing relationships between S2LHS foliation and later S3LHS cleavage. (b) Quartz ribbons stretched along the main S2LHS foliation in the uppermost structural levels of the LHS. (c), (d) and (e) Decimetric to metric thick shear zones in the uppermost structural levels of the LHS, characterized by high concentration of quart in form of ribbons and/or boudins and deformed by a later folding event. (f) Detail of south-directed thrusting within the LHS.

The LHS shows widespread occurrence of a transpositive foliation, here labelled S2LHS, defined by Qtz + Wm ± Chl ± Bt ± Grt (Figs. 3 and 6), and representing usually the most pervasive foliation recognizable and traceable at the outcrop-scale (see Fig. 2).

An older S1LHS foliation, usually preserved in microlithons, is defined by Qtz + Wm ± Chl and results parallel to lithological banding;

The S2LHS foliation corresponds to the axial plane foliation of asymmetrical tight south-verging F2LHS folds deforming the older S1LHS (and the lithological banding). Phyllosilicates defining the S2LHS occur in different proportions; usually muscovite is more abundant in the micaceous levels, whereas biotite and/or chlorite prevail in the quartz-rich domains. A L2LHS stretching lineation is widespread on the S2LHS surface, and is defined by aligned Wm, Bt and Qtz. L2LHS plunges to the N, thus resulting always almost parallel to the S2LHS dip (Fig. 2).

The S2LHS surface results more pervasively developed in the uppermost structural levels of the LHS, where the older planar fabrics are intensively transposed: outcrops show widespread occurrence of tight intrafolial folds and isolated fold hinges, and the lithological/S1LHS surfaces merge in the composite S2LHS surface. Approximately 500-800 m beneath the contact with the mylonitic augen-gneisses at the base of the IMS (as defined below), the LHS schists becomes thinly foliated (with phyllonitic appearance) due to a very pervasive mylonitic S2LHS foliation (Fig. 3b, d, e): in this portions, the older S1LHS is usually preserved in microlithons and locally occurs as spiral-shaped inclusion trails of quartz in garnet porphyroblasts (see in the following Figs. 6a, b, e, f).

At the outcrop scale, a partitioning of deformation within the LHS is indicated by dm to m thick shear zones, parallel to the mylonitic S2LHS foliation and locally characterized by high concentration of quartz in form of ribbons and/or boudins (Figs 3c, d, e). In these levels, quartz veins are stretched and rotated due to complex folding of the foliation, with axial planes both at high and low angles to shear planes (roughly corresponding to the S2LHS mylonitic foliation). Mica fish, S-C fabrics and internal thrusting (Fig. 3f) show consistent top-to-south shear movements.

Late asymmetric F3LHS folds and crenulations (Fig. 3a, d) with hinges trending from W-E to NW-SE, roughly orthogonal to the L2LHS, show south-vergence. The axial planar foliation of these folds (S3LHS) is only locally well developed and mainly defined by Wm ± Bt (Fig. 6d). More often, Wm + Bt ± Chl ± Ctd lepidoblasts statically overgrow the S2LHS, without a clear S3LHS development (Fig. 6c, f).

Later deformations include open to chevron folds and crenulations, characterized by N-S to NNE-SSW trending hinges and exhibiting usually sub-vertical axial planes.

Greater Himalayan Sequence

Inverted Metamorphic Sequence

Schist and phyllonites of the LHS are overlied by a peculiar package of two mica augen-gneisses (Fig. 4a), well recognizable in the field with thickness on the order of 1500-2000 m along the Tamor-Ghunsa Khola and <1500 m along the Simbuwa-Kabely Khola. Within this lithology, m thick intercalations of chloritic schists and small mafic enclaves elongated parallel to the main foliation are locally observed (Fig. 4b). On the basis of their petrography and structural position, these augen-gneisses (Sisne Khola Augen Gneiss of Schelling, 1992) have been correlated to the Ulleri Gneiss of central Nepal (e.g. Le Fort, 1975; Arita, 1983). Upwards, the augen-gneisses pass to a few km thick package of two micas + garnet (± St ± Ky ± Kfs) schists (Fig. 4c) and gneisses, locally anatectic (Fig. 4d) toward upper structural levels, hosting m thick bodies of calc-silicate granofels and quarzites interbedded within the main foliation.

Figure 4. Representative mesostructures of the IMS

Representative mesostructures of the IMS

(a) Typical appearance of the two micas mylonitic augen gneiss: the large K-feldspar porphyroclasts are enveloped by the main S2IMS foliation defined by muscovite and biotite. (b) Small mafic enclave elongated parallel to the main S2IMS mylonitic foliation. (c) Transposition of lithological boundaries by S2IMS mylonitic foliation. (d) anatectic gneiss in the upper structural levels of the IMS. (e) Detail of mylonite derived from augen-gneiss, showing quartz ribbons stretched along the main S2IMS mylonitic foliation. (f) Strongly foliated augen-gneiss with top-to-the-south sense of shear obtained from rotation of K-feldspar porfiroclasts. a, c and f modified from Mosca et al., 2011.

All the lithologies included in the IMS are highly deformed and have a strong mylonitic fabric (Fig. 4), thus making often difficult to identify and correlate the different planar fabrics among different outcrops. Moreover, at the upper structural levels of the IMS, partial melting locally affects some lithologies. In the anatectic rocks, the development of oriented structures is not only controlled by deformational events, but also by the production, extraction and crystallization of the melt (e.g. Solar, 2008); therefore the correlation between the main foliation recognised in the lower IMS and that recognised in the upper IMS is not trivial. For this reason, the neutral notation Sm (main schistosity) has been used locally for the anatectic samples (see in the following Fig. 9).

A dominant mylonitic foliation defined by Wm + Bt ± Ky ± St, here labelled S2IMS, characterizes the lower IMS (see also Fig. 7. This foliation causes the intense transposition and consequent parallelization of the original compositional layering (Fig. 4c) and of at least an older S1IMS metamorphic foliation (Fig. 9). The S1IMS is preserved in microlithons as well as in form of spiral-shaped inclusions trails of Qtz ± Ilm ± Rt in garnet porphyroblasts locally enveloped by the S2IMS (Fig. 9). The S2IMS foliation corresponds to the axial plane foliation of F2IMS isoclinal to asymmetric south-verging tight folds deforming S1IMS/lithological banding. The S2IMS dips to the north (Fig. 2) and contains a pervasive stretching lineation defined by elongated K-feldspar porphyroclasts in the augen-gneisses, and by the preferred alignment of minerals or mineral aggregates (Bt, Wm, St and Ky) in the schists. The orientation of the stretching lineation is always parallel or slightly oblique to the dip-direction of the S2IMS foliation (Fig. 2). Kinematic indicators such as rotated K-feldspar porphyroclasts showing both sigma-type and delta-type geometries, mica-fish and common S-C fabrics, uniformly define a consistent top-to-south sense of shear.

The S2IMS foliation is in turn deformed by later south-verging ENE-WSW trending folds and crenulation, thus suggesting a protracting history of the south-verging shearing in post-S2IMS time; this deformation is associated to a reactivation of the S2IMS foliation along the limbs of the asymmetric folds, evolving then in slip surfaces.

At the base of the IMS, meso- and microstructural observations suggest that the whole package of augen-gneisses was a site of high strain concentration. Augen-gneisses generally show a strongly mylonitic texture with large (often > 3 cm) K-feldspar porphyroclasts enveloped by the pervasive S2IMS mylonitic foliation defined by Wm + Bt. Quartz often occurs in mm to cm thick elongated ribbons and stretched bodies, parallel to the mylonitic foliation (Fig. 4e). Rare outcrops show less deformed portions, roughly preserved as macro-lithons. Within strongly foliated augen-gneisses (Fig. 4f), K-feldspar occurs as porphyroclasts widely dispersed through a fine-grained foliated matrix of feldspar, quartz and micas. In general, the presence of large K-feldspar porphyroclasts made the rock mechanically inhomogeneous, thus causing a local pronounced perturbation of the shear planes (S2IMS foliation) within the sheared rock bodies.